| 1. |
- Lorentzon, Marcus, 1993-
(författare)
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Nanostructured TiN/ZrAlN and HfAlN Thin Films : Effect of Structure on Mechanical Properties
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transition metal nitrides are a remarkable group of ceramic materials that offer exceptional properties such as high hardness, low tribological wear, excellent thermal stability, and high oxidation resistance. Alloys such as TiN, CrN, VN, ZrN, and HfN have been identified as ideal candidates for protective coatings on cutting tool inserts in the metal processing industry. While TiAlN has been widely accepted, ZrAlN and HfAlN alloys have much unexplored potential. With a melting point of HfN at 3300 °C, approximately 400 °C higher than TiN, HfAlN shows great potential for age-hardening at even higher temperatures. These remarkable materials inspire us to push the limits of what is possible, and to continue to innovate materials science.The work performed in this thesis focuses on the development of hard coatings using ionassisted reactive magnetron sputtering. The coatings are based on group IV TM-Al-N, where TM is either Ti, Zr, or Hf. The aim is to enhance the performance of these ceramic coatings by simultaneously increasing their hardness and toughness. To achieve this, the growth mechanisms, structure, and mechanical properties of the films were studied in detail. The coatings were deposited onto single crystal Si(001) and MgO(001) substrates.The first study describes the development of a multilayer structure, consisting of alternating layers of TiN and Zr0.37Al0.63N1.09, with a bilayer period of 20 nm, with the aim of combining the unique properties of the constituent materials. Cubic rocksalt TiN is known for its high hardness and unfortunate brittleness. Hexagonal wurtzite Zr0.37Al0.63N1.09 is less hard, but also more ductile. The crystal structure of the multilayers varied depending on the substrate temperature during growth. At temperatures below ~350 °C, the ZrAlN layers grew near amorphous, while they were nanocrystalline between 500 °C and 800°C. At 900 °C, the ZrAlN segregated into a nanolabyrinthine structure consisting of w-AlN and c-ZrN. The hardness of the films increased significantly with increasing deposition temperature, from 24 GPa to 36 GPa. The films also showed superior fracture stress compared to the available literature, increasing from 6.1 to 7.7 GPa. The fracture toughness of the films was also improved compared to the binary constituents, up to 2.8 MPa√m. These findings illustrate the potential of combining diverse materials, to create new structures with enhanced properties and highlight the importance of optimizing the growth conditions to achieve the desired film functionality.In a second study, single-crystal Hf1-xAlxNy films were grown at high temperatures on MgO(001) substrates. Excess nitrogen in HfNy (y=1.22, 1.33) film created ordered nanosized domains of variations in the nitrogen composition, leading to the formation of a compositionally modulated superstructure. In Hf0.93Al0.07N1.15, the immiscibility of the constituents (c-HfN and c-AlN) causes the formation of a superstructure consisting of isostructural Al-rich and Hf-rich domains due to surface initiated spinodal decomposition. Micropillar compression tests reveal a ductile HfN1.22 and substantial strain hardening upon deformation. Hf0.93Al0.07N1.15 exhibited a brittle nature, although at a substantially increased yield stress in comparison, consistent with the improved hardness from 26 GPa to 40.5 GPa, measured by nanoindentation.
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| 2. |
- Thörnberg, Jimmy, et al.
(författare)
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Microstructure and materials properties of understoichiometric TiBx thin films grown by HiPIMS
- 2020
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Ingår i: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 404
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Tidskriftsartikel (refereegranskat)abstract
- TiBx thin films with a B content of 1.43 <= x <= 2.70 were synthesized using high-power impulse magnetron sputtering (HiPIMS) and direct-current magnetron sputtering (DCMS). HiPIMS allows compositions ranging from understoichiometric to overstoichiometric dense TiBx thin films with a B/Ti ratio between 1.43 and 2.06, while DCMS yields overstoichiometric TiBx films with a B/Ti ratio ranging from 2.20 to 2.70. Excess B in overstoichiometric TiBx thin films from DCMS results in a hardness up to 37.7 +/- 0.8 GPa, attributed to the formation of an amorphous B-rich tissue phase interlacing stoichiometric TiB2 columnar structures. We furthermore show that understoichiometric TiB1.43 thin films synthesized by HiPIMS, where the deficiency of B is found to be accommodated by Ti-rich planar defects, exhibit a superior hardness of 43.9 +/- 0.9 GPa. The apparent fracture toughness and thermal conductivity of understoichiometric TiB1.43 HiPIMS films are 4.2 +/- 0.1 MPa root m and 2.46 +/- 0.22 W/(m.K), respectively, as compared to corresponding values for overstoichiometric TiB2.70 DCMS film samples of 3.1 +/- 0.1 MPa root m and 4.52 +/- 0.45 W/(mK). This work increases the fundamental understanding of understoichiometric TiBx thin films and their materials properties, and shows that understoichiometric films have properties matching or going beyond those with excess B.
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| 3. |
- Azina, Clio, et al.
(författare)
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Yttrium incorporation in Cr2AlC : On the metastable phase formation and decomposition of (Cr,Y)(2)AlC MAX phase thin films
- 2023
-
Ingår i: Journal of The American Ceramic Society. - : John Wiley & Sons. - 0002-7820 .- 1551-2916. ; 106:4, s. 2652-2665
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Tidskriftsartikel (refereegranskat)abstract
- Herein we report on the synthesis of a metastable (Cr,Y)(2)AlC MAX phase solid solution by co-sputtering from a composite Cr-Al-C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760 degrees C led to the formation of phase pure (Cr,Y)(2)AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr2AlC, Y5Al3, and Cr-carbides. In contrast to pure Cr2AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by similar to 200 degrees C for 5 at.% Y compared to Cr2AlC. Calculations predicting the metastable phase formation of (Cr,Y)(2)AlC and its decomposition are in excellent agreement with the experimental findings.
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| 4. |
- Bakhit, Babak, 1983-, et al.
(författare)
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Age hardening in superhard ZrB2-rich Zr1-xTaxBy thin films
- 2021
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Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 191, s. 120-125
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Tidskriftsartikel (refereegranskat)abstract
- We recently showed that sputter-deposited Zr1-xTaxBy thin films have hexagonal AlB2-type columnar nanostructure in which column boundaries are B-rich for x < 0.2, while Ta-rich for x ≥ 0.2. As-deposited layers with x ≥ 0.2 exhibit higher hardness and, simultaneously, enhanced toughness. Here, we study the mechanical properties of ZrB2.4, Zr0.8Ta0.2B1.8, and Zr0.7Ta0.3B1.5 films annealed in Ar atmosphere as a function of annealing temperature Ta up to 1200 °C. In-situ and ex-situ nanoindentation analyses reveal that all films undergo age hardening up to Ta = 800 °C, with the highest hardness achieved for Zr0.8Ta0.2B1.8 (45.5±1.0 GPa). The age hardening, which occurs without any phase separation or decomposition, can be explained by point-defect recovery that enhances chemical bond density. Although hardness decreases at Ta > 800 °C due mainly to recrystallization, column coarsening, and planar defect annihilation, all layers show hardness values above 34 GPa over the entire Ta range.
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| 5. |
- Bakhit, Babak, 1983-, et al.
(författare)
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Dense Ti0.67Hf0.33B1.7 thin films grown by hybrid HfB2-HiPIMS/TiB2-DCMS co-sputtering without external heating
- 2021
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Ingår i: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 186
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Tidskriftsartikel (refereegranskat)abstract
- There is a need for developing synthesis techniques that allow the growth of high-quality functional films at low substrate temperatures to minimize energy consumption and enable coating temperature-sensitive substrates. A typical shortcoming of conventional low-temperature growth strategies is insufficient atomic mobility, which leads to porous microstructures with impurity incorporation due to atmosphere exposure, and, in turn, poor mechanical properties. Here, we report the synthesis of dense Ti0.67Hf0.33B1.7 thin films with a hardness of ∼41.0 GPa grown without external heating (substrate temperature below ∼100 °C) by hybrid high-power impulse and dc magnetron co-sputtering (HfB2-HiPIMS/TiB2-DCMS) in pure Ar on Al2O3(0001) substrates. A substrate bias potential of −300 V is synchronized to the target-ion-rich portion of each HiPIMS pulse. The limited atomic mobility inherent to such desired low-temperature deposition is compensated for by heavy-mass ion (Hf+) irradiation promoting the growth of dense Ti0.67Hf0.33B1.7.
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| 6. |
- Bakhit, Babak, 1983-
(författare)
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Multifunctional Transition-metal Diboride Coatings Synthesized by Magnetron sputtering with Synchronized Metal-ion Irradiation
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Refractory transition-metal diborides (TMB2), classified as ultra-high temperature ceramics, are promising materials for extreme thermal and chemical environments. There is a growing demand for employing TMB2 in high-temperature electrodes, advanced nuclear fission reactors, molten metal containment, refractory crucibles, thermocouple protection tubes in steel baths and aluminum reduction cells, reinforcement fibers, solar power, aerospace, and armor applications. Magnetron-sputter-deposited TMB2 have recently received increasing attention as the next class of hard ceramic protective thin films. These layers usually crystallize in a hexagonal AlB2 crystal structure (P6/mmm, SG-191) in which B atoms form graphite-like honeycomb sheets between hexagonal-close-packed TM layers. The strong covalent bonding between TM and B atoms as well as within the honeycomb B sheets provides high melting temperature, hardness, and stiffness, while metallic bonding within TM layers results in good electrical and thermal conductivities. However, sputter-deposited TMB2 films suffer from several critical issues such as boron overstoichiometry, high brittleness, and low oxidation resistance. All of these aspects are addressed in the thesis.In Paper 1, the common issue with sputter-deposited diboride thin films, i.e. the presence of excess B, is resolved by using high power impulse magnetron sputtering (HiPIMS). The B/Ti ratio in TiBx films, used as a model materials system, is controllably varied from 2.08 to 1.83 by adjusting the HiPIMS pulse length ton, while maintaining the average power and pulse frequency constant. As a result, the peak current density increases from 0.27 to 0.88 A/cm2, which leads to an increased gas rarefaction and, hence, higher metal-ion densities in the plasma. Film growth becomes then increasingly controlled by ionized target atoms, rather than neutral species. Since sputter-ejected Ti atoms have a higher probability of being ionized than B atoms, due to their lower first-ionization potential and larger ionization cross-section, the B/Ti ratio in the films decreases a function of target peak current.While TM diborides are inherently hard, that alone is not sufficient to prevent failure in applications involving high stresses, as hardness is typically accompanied by brittleness. In order to avoid brittle cracking, thin films must be both hard and relatively ductile, which is referred to as high toughness. In Paper 2, it is demonstrated that Zr1-xTaxBy thin films grown by hybrid high-power impulse and DC magnetron co-sputtering (Ta-HiPIMS/ZrB2-DCMS) with x ≥ 0.2 are not only hard, but also tough. The films with x ≥ 0.2 show a self-organized columnar core/shell nanostructure (see Paper 3), in which crystalline hexagonal Zr-rich stoichiometric Zr1-xTaxB2 cores are surrounded by narrow dense, disordered Ta-rich shells that are B-deficient.The disordered shells have the structural characteristics of metallic-glass thin films, which exhibit both high strength and toughness. Hence, such a nanostructure combines the benefits of crystalline diboride nanocolumns, providing the high hardness, with the dense metallic-glasslike shells, which give rise to enhanced toughness.The mechanical properties of Zr1-xTaxBy thin films annealed in Ar atmosphere are studied as a function of annealing temperature Ta up to 1200 °C in Paper 4. In-situ and ex-situ nanoindentation analyses reveal that all films undergo age hardening up to Ta = 800 °C, with the highest hardness achieved for Zr0.8Ta0.2B1.8 (45.5±1.0 GPa). The age hardening, which occurs without any phase separation or decomposition, can be explained by point-defect recovery that enhances chemical bond density. Although hardness decreases at Ta > 800 °C due mainly to recrystallization, column coarsening, and planar defect annihilation, all layers show hardness values above 34 GPa over the entire Ta range.The oxidation resistance of TiBx thin films is addressed in Paper 5. In general, TMB2 suffer from rapid high-temperature oxidation, which is a critical issue for many applications. In this study, it is demonstrated that alloying the films with Al significantly increases the oxidation resistance with only a slight decrease in hardness. Contrary to bulk TiB2 synthesized by powder metallurgy processes, the oxidation products of TiB2 thin films do not contain the B2O3 oxide scale, which is usually observed below 1000 °C in air, and merely consists of a TiO2 phase. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale, which considerably decreases the oxygen diffusion rate by suppressing the oxidecrystallites coarsening.To realize the goal of fully multifunctional diborides, Zr1-xCrxBy thin films grown by hybrid Cr-HiPIMS/ZrB2-DCMS co-sputtering are studied in Paper 6. These layers exhibit a unique combination of high hardness, toughness, wear, oxidation, and corrosion resistance.The last paper (Paper 7) addresses the issue of efficient energy and resource consumption in industrial processes, which United Nations defines as one of the sustainable development goals. The idea here is to replace the conventionally used thermal-energy flux from resistive heaters with the irradiation by high mass metal ions (Hf+), which results in more efficient energy transfer to the deposited layer. We deposited Ti0.67Hf0.33B1.7 films using hybrid HfB2-HiPIMS/TiB2-DCMS co-sputtering at substrate temperature not exceeding 100 °C. Results reveal that dense layers can be achieved with high hardness values (> 40 GPa) even though no external substrate heating was used during the process.
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| 7. |
- Bakhit, Babak, 1983-, et al.
(författare)
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Multifunctional ZrB2-rich Zr1-xCrxBy thin films with enhanced mechanical, oxidation, and corrosion properties
- 2021
-
Ingår i: Vacuum. - : Elsevier BV. - 0042-207X .- 1879-2715. ; 185
-
Tidskriftsartikel (refereegranskat)abstract
- Refractory transition-metal (TM) diborides have high melting points, excellent hardness, and good chemical stability. However, these properties are not sufficient for applications involving extreme environments that require high mechanical strength as well as oxidation and corrosion resistance. Here, we study the effect of Cr addition on the properties of ZrB2-rich Zr1-xCrxBy thin films grown by hybrid high-power impulse and dc magnetron co-sputtering (Cr-HiPIMS/ZrB2-DCMS) with a 100-V Cr-metal-ion synchronized potential. Cr metal fraction, x = Cr/(Zr+Cr), is increased from 0.23 to 0.44 by decreasing the power Pzrb2 applied to the DCMS ZrB2 target from 4000 to 2000 W, while the average power, pulse width, and frequency applied to the HiPIMS Cr target are maintained constant. In addition, y decreases from 2.18 to 1.11 as a function of Pzrb2, as a result of supplying Cr to the growing film and preferential B resputtering caused by the pulsed Cr-ion flux. ZrB2.18, Zr0.77Cr0.23B1.52, Zr0.71Cr0.29B1.42, and Zr0.68Cr0.32B1.38 2 films have hexagonal AlB2 crystal structure with a columnar nanostructure, while Zr0.64Cr0.36B1.30 and Zr0.56Cr0.44B1.11 are amorphous. All films show hardness above 30 GPa. Zr0.56Cr0.44B1.11 alloys exhibit much better toughness, wear, oxidation, and corrosion resistance than ZrB2.18. This combination of properties makes Zr0.56Cr0.44B1.11 ideal candidates for numerous strategic applications.
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| 8. |
- Bakhit, Babak, 1983-, et al.
(författare)
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Systematic compositional analysis of sputter-deposited boron-containing thin films
- 2021
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 39:6
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Tidskriftsartikel (refereegranskat)abstract
- Boron-containing materials exhibit a unique combination of ceramic and metallic properties that are sensitively dependent on their given chemical bonding and elemental compositions. However, determining the composition, let alone bonding, with sufficient accuracy is cumbersome with respect to boron, being a light element that bonds in various coordinations. Here, we report on the comprehensive compositional analysis of transition-metal diboride (TMBx) thin films (TM = Ti, Zr, and Hf) by energy-dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), time-of-flight elastic recoil detection analysis (ToF-ERDA), Rutherford backscattering spectrometry (RBS), and nuclear reaction analysis (NRA). The films are grown on Si and C substrates by dc magnetron sputtering from stoichiometric TMB2 targets and have hexagonal AlB2-type columnar structures. EDX considerably overestimates B/TM ratios, x, compared to the other techniques, particularly for ZrBx. The B concentrations obtained by XPS strongly depend on the energy of Ar+ ions used for removing surface oxides and contaminants prior to analyses and are more reliable for 0.5 keV Ar+. ToF-ERDA, RBS, and NRA yield consistent compositions in TiBx. They also prove TiBx and ZrBx films to be homogeneous with comparable B/TM ratios for each film. However, ToF-ERDA, employing a 36-MeV 127I8+ beam, exhibits challenges in depth resolution and quantification of HfBx due to plural and multiple scattering and associated energy loss straggling effects. Compared to ToF-ERDA, RBS (for the film grown on C substrates) and NRA provide more reliable B/Hf ratios. Overall, a combination of methods is recommended for accurately pinpointing the compositions of borides that contain heavy transition metals.
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| 9. |
- Bender, Johanna, 1975, et al.
(författare)
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Lipid cubic phases for improved topical drug delivery in photodynamic therapy.
- 2005
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Ingår i: Journal of Controlled Release. - : Elsevier BV. - 0168-3659 .- 1873-4995. ; 106:3, s. 350-360
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Tidskriftsartikel (refereegranskat)abstract
- We have evaluated the efficacy of lipid cubic phases, highly ordered self-assembly systems on the nanometer level, as drug delivery vehicles for in vivo topical administration of delta-aminolevulinic acid (ALA) and its methyl ester (m-ALA) on nude mice skin. ALA, a precursor of heme, induces the production of the photosensitizer protoporphyrin IX (PpIX) in living tissue. Measuring the PpIX fluorescence at the skin surface, after topical administration, makes indirect quantification of the penetration of ALA into the tissue possible. Cubic phases were formed of lipid (monoolein or phytantriol), water and drug. In some cases, propylene glycol was included in the cubic phase as well. The drug concentration was 3% (w/w, based on the total sample weight) in all investigated vehicles. When the formulations were applied for 1 h, the monoolein cubic systems and the three-component phytantriol sample showed higher fluorescence compared to the standard ointment during the 10 h of measurement. Both ALA and m-ALA yielded similar results, although the differences between the investigated vehicles were more pronounced when using m-ALA. For the 24-h applications, the monoolein cubic systems with m-ALA showed faster PpIX formation than the standard ointment, implying higher PpIX levels at short application times (less than 4 h). The systemic PpIX fluorescence of ALA was elevated by using the lipid cubic formulations. Notably, a small systemic effect was also observed for the monoolein cubic sample with m-ALA. These results imply improved PpIX formation when using the lipid cubic systems, most probably due to enhanced drug penetration.
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| 10. |
- Björk, Jonas, 1983-, et al.
(författare)
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The Role of Metal Adatoms in a Surface-Assisted Cyclodehydrogenation Reaction on a Gold Surface
- 2022
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Ingår i: Angewandte Chemie International Edition. - : Wiley-V C H Verlag GMBH. - 1433-7851 .- 1521-3773. ; 61:49
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Tidskriftsartikel (refereegranskat)abstract
- Dehydrogenation reactions are key steps in many metal-catalyzed chemical processes and in the on-surface synthesis of atomically precise nanomaterials. The principal role of the metal substrate in these reactions is undisputed, but the role of metal adatoms remains, to a large extent, unanswered, particularly on gold substrates. Here, we discuss their importance by studying the surface-assisted cyclodehydrogenation on Au(111) as an ideal model case. We choose a polymer theoretically predicted to give one of two cyclization products depending on the presence or absence of gold adatoms. Scanning probe microscopy experiments observe only the product associated with adatoms. We challenge the prevalent understanding of surface-assisted cyclodehydrogenation, unveiling the catalytic role of adatoms and their effect on regioselectivity. The study adds new perspectives to the understanding of metal catalysis and the design of on-surface synthesis protocols for novel carbon nanomaterials.
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| 11. |
- Carlsson, Adam
(författare)
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Computational prediction of novel MAB phases
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The synthesis procedure of any materials system is often considered a challenging task if performed without any prior knowledge. Theoretical models may thus be used as an external input and guide experimental efforts toward novel exotic materials which are most likely to be synthesizable. The aim of this work is to apply theoretical models and develop frameworks for reliable predictions of thermodynamically stable materials. The material in focus herein is the family of atomic layered boride-based materials referred to as MAB phases.The ground state energy of a material system may be obtained by applying firstprincipal calculations, such as density functional theory (DFT), which has thoroughly been used throughout this thesis. However, performing modern state-of-the-art quantum mechanical calculations, in general, relies on a pre-defined crystal structure which may be constructed based on an a priori known structure or obtained through the use of crystal structure prediction models. In this work, both approaches are explored. We herein perform a thermodynamical screening study to predict novel stable ternary boron-based materials by considering M2AB2, M3AB4, M4AB6, MAB and M4AB4 compositions in orthorhombic and hexagonal symmetries with inspiration from experimentally synthesized MAB phases. The considered atomic elements are M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, A = Al, Ga, In, and B is boron. Among the considered compounds, seven experimentally synthesized phases are verified as stable, and we predict the three hypothetical phases to be stable - Hf2InB2, Zr2InB2, and Mo4AlB4. Additionally, 23 phases of varying symmetries and compositions are predicted as close to stable or to be metastable.However, the assumption of assigning initial crystal structures based on neighbouring compounds may drastically limit the outcome of a screening study. State-of-the-art techniques to generate low energy crystal structures within the considered material phase space is thus explored. More specifically, the Mo-Sc-Al-B system is studied along the ternary joints of (MoxSc1-x)2AlB2 where 0 < x < 1 by using the cluster expansion (CE) and the crystal structure prediction (CSP) codes, CLEASE and USPEX, in analogy. Previous attempts to study the Mo-Sc-Al-B system has been limited by only considering either hexagonal or orthorhombic symmetries. We challenge such approaches by covering larger portions of the phase space efficiently by combining CSP and CE frameworks. The Mo4/3Sc2/3AlB2 (R ̅3m) phase, previously referred to as i-MAB, is verified stable in addition to Mo2/3Sc4/3AlB2 (R3).The suggested approach of combining CE and CSP frameworks for investigating multi-component systems consists of initially performing CSP searches on the systems of smaller order constituting the system in focus. In the pseudo-ternary (MoxSc1-x)2AlB2 system, this refers to performing CSP searches on the ternary Mo2AlB2 and Sc2AlB2 systems. In addition, we also consider the structures of experimentally known phases with similar compositions. The complete set of structures obtained either from CSP or public databases, was later used to design CE models where mixing tendencies in addition to stability determined which model to further study. The predicted low-energy structures of the CE model were relaxed and used as seed structures within a complete CSP search covering the (MoxSc1-x)2AlB2 system for 0 < x < 1. We demonstrate that the use of seed structures, obtained from CE models, efficiently improved the search for low-energy structures within a multi-component system. The suggested approach is yet to be tested on any other system but is applicable to any alternative multi-component system.
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| 12. |
- Carlsson, Adam, 1994-
(författare)
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Explorations of boron-based materials through theoretical simulations
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on boron-based materials, notable for their structural complexity and unique combination of ceramic and metallic properties. These properties typically result in materials with high mechanical strength, electrical conductivity, and melting points. Among these materials are MAB phases, a family of layered materials comprised of a transition metal (M), an A-element (typically an element from Group 13-14), and boron (B). The layered nature of these materials provides a pathway towards the realization of 2D materials, coined MBenes (or boridene), through chemical exfoliation.While the potential for discovering novel materials is immense, their realization often demands extensive experimental efforts. Theoretical models may here be used as a filter by guiding experimental endeavours. The work presented herein aims to leverage theoretical models and to develop frameworks suitable for reliable thermodynamical predictions in hope of the discovery of additional boron-based materials.First-principles calculations, particularly density functional theory (DFT), have extensively been employed in this thesis to determine the ground state energy of materials and predict their stability or tendency to decompose. However, first-principles calculations typically rely on a pre-defined crystal structure which may be constructed through a priori information or obtained through the use of crystal structure prediction (CSP) frameworks. We herein explore both of these approaches by i) systematically substituting elements in known low-energy structures, and ii) deriving novel low-energy structures by combining CSP with cluster expansion (CE) models.The first approach is herein exemplified when considering the low-energy structures of V3B2 (P4/mbm) and Cr5B3 (I4/mbm). These structures are comprised of two M-sites in addition to boron and thus form the general compositions M’2M’’B2 and M’4M’’B3, respectively. In a follow-up project, this approach was refined by probing the Materials Project database for additional binary boron-based materials with structures of this nature. The M-sites of these candidate structures were further populated with elements ranging from Group 2 to 14 with the aim of discovering novel ternary boron-based materials.Alternatively, a hybrid method of the two techniques is herein explored in which manually designed hexagonal structures were made based on orthorhombic low-energy counterpart structures. A set of structural polymorphs for the M2AB2, M3AB4, M4AB6, MAB, and M4AB4 compositions were studied with varying stacking sequences followed by the evaluation of their thermodynamical stability.The second approach requires little to no structural information but is typically limited to considering fewer material systems due to a higher computational cost. This approach is herein applied to study low-energy basins within the complex phase space of (MoxSc1-x)2AlB2 and (M’xM’’1-x)3AlB4 systems with the aim of finding novel quaternary boron-based materials. A framework, suitable for exploring chemical phase spaces of complex systems, was herein developed by combining CSP and CE models with DFT calculations. The suggested framework is initiated by performing CSP simulations on the n-1 dimensional systems. Identified low-energy structures are subsequently used as input lattices to construct CE models for the n-dimensional system. The low-energy basins found in the n-dimensional system may potentially be used as seed structures in a comprehensive CSP simulation or as input structures for high-throughput screening. This approach, not only provides an efficient pathway to identify low-energy basins of complex material systems, but also attempts to bridge the gap in materials discovery with or without prerequisite information.The aspiration of bridging the gap between state-of-the-art simulation techniques, whether reliant on a priori information or not, is rooted in the intention of enhancing the foundation of materials discovery. The refinement of these theoretical simulations serves to guide and augment experimental efforts for the synthesis of novel materials which is pivotal for addressing and achieving current and future sustainability goals.
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| 13. |
- Carlsson, Adam, et al.
(författare)
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Theoretical predictions of phase stability for orthorhombic and hexagonal ternary MAB phases
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - Cambridge, United Kingdom : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 24:18, s. 11249-11258
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Tidskriftsartikel (refereegranskat)abstract
- In the quest for finding novel thermodynamically stable, layered, MAB phases promising for synthesis, we herein explore the phase stability of ternary MAB phases by considering both orthorhombic and hexagonal crystal symmetries for various compositions (MAB, M2AB2, M3AB4, M4AB4, and M4AB6 where M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Co, A = Al, Ga, and In, and B is boron). The thermodynamic stability of seven previously synthesized MAB phases is confirmed, three additional phases are predicted to be stable, and 23 phases are found to be close to stable. Furthermore, the crystal symmetry preference for forming orthorhombic or hexagonal crystal structures is investigated where the considered Al-based MAB phases tend to favour orthorhombic structures whereas Ga- and In-based phases in general prefer hexagonal structures. The theoretically predicted stable MAB phases along with the structural preference is intended to both guide experimental efforts and to give an insight into the stability for different crystal symmetries of MAB phases.
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| 14. |
- Dahlqvist, Martin, 1982-, et al.
(författare)
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Chemical order or disorder - a theoretical stability expose for expanding the compositional space of quaternary metal borides
- 2022
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Ingår i: Materials Advances. - : Royal Society of Chemistry. - 2633-5409. ; 3:6, s. 2908-2917
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Tidskriftsartikel (refereegranskat)abstract
- Inspired by the recent discovery of Ti4MoSiB2, a quaternary phase with out-of-plane chemical order that we denote as o-MAB, we perform an extensive first-principles study to explore the attained chemical order and disorder (solid-solution) upon metal alloying of M(5)AB(2) (T2 phases), with M from Groups 3 to 9 and A = Al, Si, P, Ga, and Ge. We show that the attainable chemistries of T2 can be significantly expanded and predict 35 chemically ordered o-MAB phases and 121 solid solutions of an MM-4 AB(2) stoichiometry. The possibility of realizing o-MAB or solid solution MAB phases combined with multiple elemental combinations previously not observed in these borides suggests an increased property tuning potential. Furthermore, five ternary T2 phases, yet to be synthesized, are also predicted to be stable.
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| 15. |
- Downes, Marley, et al.
(författare)
-
M5X4: A Family of MXenes
- 2023
-
Ingår i: ACS Nano. - : AMER CHEMICAL SOC. - 1936-0851 .- 1936-086X. ; 17:17, s. 17158-17168
-
Tidskriftsartikel (refereegranskat)abstract
- MXenes are two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides typically synthesized from layered MAX-phase precursors. With over 50 experimentally reported MXenes and a near-infinite number of possible chemistries, MXenes make up the fastest-growing family of 2D materials. They offer a wide range of properties, which can be altered by their chemistry (M, X) and the number of metal layers in the structure, ranging from two in M2XTx to five in M5X4T x . Only one M5X4 MXene, Mo4VC4, has been reported. Herein, we report the synthesis and characterization of two M(5)AX(4) mixed transition metal MAX phases, Ti2.5Ta2.5AlC4 and Ti2.675Nb2.325AlC4, and their successful topochemical transformation into Ti2.5Ta2.5C4T x and Ti2.675Nb2.325C4Tx MXenes. The resulting MXenes were delaminated into single-layer flakes, analyzed structurally, and characterized for their thermal and optical properties. This establishes a family of M(5)AX(4) MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.
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| 16. |
- El Ghazaly, Ahmed
(författare)
-
Acoustic Platform for MXene Synthesis and Electrochemical Behaviour of i-MXenes in Aqueous Electrolytes
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Climate Change is believed to be the greatest global challenge and on its forefront is the topic of energy. While being of extreme importance, debates over energy have become a normality. The related field of material synthesis for energy storage applications has also been growing, as well as the demand for industrial electrification from renewable sources of energy. Water-based supercapacitors are a type of energy storage devices that can deliver high power densities while maintaining long term cyclability in an environmen-tally friendly media. However, their challenges include maintaining high per-formance in term of energy density, safety, and low cost of electrode manu-facturing. MXene is family of two-dimensional transition metal carbides/nitrides that are terminated with H, OH and F groups. The material demonstrates superior physical and chemical properties related to energy applications in compari-son to its 3D parent material, the MAX phase. Since its discovery in 2011, MXene, such as Ti3C2Tz, has been widely investigated in the field of energy storage due to its high conductivity (20,000 S.cm-1) and a volumetric capac-itance that can reach 900 Fcm-3. However, reported synthesis processes for MXene are fraught with hazardous procedures that are time consuming. The first section of this thesis presents a new innovative method for Ti3C2Tz MXene synthesis, in which MXene was synthesized in a few milliseconds with the assistance of 30 MHz frequency surface acoustic waves (SAW) and 0.05M of LiF. The aluminium element in the Ti3AlC2 MAX phase was etched by so called “localized HF”, and the powder was converted to 2D Ti3C2Tz. This method showed resulting MXene comparable to that of previ-ously reported synthesis techniques, as demonstrated by the material’s elec-trochemical performance. The second section of the thesis focuses on investigating the electrochemical performance of a comparatively new family of MXene, coined i-MXene, in aqueous electrolyte. i-MXene, reported in 2017, has the chemical formula Mo1.33CTz and is a product of chemical etching of the in-plane chemically ordered (Mo2/3Sc1/3)2AlC i-MAX phase. The Mo1.33CTz was studied in a sul-phuric acid electrolyte. This electrolyte sets a limit for the electrode potential window and capacitance, and therefore, post-synthesis treatment protocols was used to enhance the electrochemical performance. The Mo1.33CTz recorded a volumetric capacitance of 1050 Fcm-3 and1600 Fcm-3 for hydrogel treatment and heat-treated electrodes, respectively. Moreover, mixing Mo1.33CTz with MoS2 and graphene improved both the specific capacitance and the electrode stability even further. The electrochemical properties of Mo1.33CTz were thereafter explored in dif-ferent sulfate-based aqueous electrolytes with univalent (Li+, Na+, and K+) and divalent (Mg2+ Mn2+ or Zn2+) cations. Mo1.33CTz exhibited a wider po-tential window without degradation, expanding the previously reported limit in sulphuric acid for both symmetric and asymmetric devices. Lithium chlo-ride gave the best results, being an electrolyte based on a natural salt that has high solubility at room temperature. It presented a large potential window, -1.2 to +0.3V (vs. Ag/AgCl), and a volumetric capacitance of ~800 Fcm−3 at a scan rate of 2 mVs−1. In addition, the performance of a Mo1.33CTz //MnxOn asymmetric device was tested in 5M LiCl electrolyte. The results showed a potential window of 2 V, a volumetric energy density of 58 mWhcm-3, and a 100% columbic efficiency after 10,000 charge/discharge cycles. A cyclic sta-bility is crucial for practical applications, and altogether, the promising re-sults motivate further exploration of i-MXenes for energy storage and be-yond.
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| 17. |
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| 18. |
- Greczynski, Grzegorz, 1973-, et al.
(författare)
-
Gas rarefaction effects during high power pulsed magnetron sputtering of groups IVb and VIb transition metals in Ar
- 2017
-
Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Institute of Physics (AIP). - 0734-2101 .- 1520-8559. ; 35:6
-
Tidskriftsartikel (refereegranskat)abstract
- The authors use energy- and time-dependent mass spectrometry to analyze the evolution of metal- and gas-ion fluxes incident at the substrate during high-power pulsed magnetron sputtering (HiPIMS) of groups IVb and VIb transition-metal (TM) targets in Ar. For all TMs, the time-and energy-integrated metal/gas-ion ratio at the substrate plane NMe+/NAr+ increases with increasing peak target current density J(T,peak) due to rarefaction. In addition, NMe+/NAr+ exhibits a strong dependence on metal/gas-atom mass ratio m(Me)/m(g) and varies from similar to 1 for Ti (m(Ti)/m(Ar) = 1.20) to similar to 100 for W (m(W)/m(Ar) = 4.60), with J(T,peak) maintained constant at 1 A/cm(2). Time-resolved ion-energy distribution functions confirm that the degree of rarefaction scales with m(Me)/m(g): for heavier TMs, the original sputtered-atom Sigmund-Thompson energy distributions are preserved long after the HiPIMS pulse, which is in distinct contrast to lighter metals for which the energy distributions collapse into a narrow thermalized peak. Hence, precise timing of synchronous substrate-bias pulses, applied in order to reduce film stress while increasing densification, is critical for metal/gas combinations with m(Me)/m(g) near unity, while with m(Me)/m(g) amp;gt;amp;gt; 1, the width of the synchronous bias pulse is essentially controlled by the metal-ion time of flight. The good agreement between results obtained in an industrial system employing 440 cm(2) cathodes and a laboratory-scale system with a 20 cm(2) target is indicative of the fundamental nature of the phenomena.
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| 19. |
- Grossmann, Lukas, et al.
(författare)
-
Evolution of adsorption heights in the on-surface synthesis and decoupling of covalent organic networks on Ag(111) by normal-incidence X-ray standing wave
- 2022
-
Ingår i: Nanoscale Horizons. - : Royal Society of Chemistry. - 2055-6764 .- 2055-6756. ; 7:1, s. 51-62
-
Tidskriftsartikel (refereegranskat)abstract
- Structural characterization in on-surface synthesis is primarily carried out by Scanning Probe Microscopy (SPM) which provides high lateral resolution. Yet, important fresh perspectives on surface interactions and molecular conformations are gained from adsorption heights that remain largely inaccessible to SPM, but can be precisely measured with both elemental and chemical sensitivity by Normal-Incidence X-ray Standing Wave (NIXSW) analysis. Here, we study the evolution of adsorption heights in the on-surface synthesis and post-synthetic decoupling of porous covalent triazine-phenylene networks obtained from 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) precursors on Ag(111). Room temperature deposition of TBPT and mild annealing to ~150 C result in full debromination and formation of organometallic intermediates, where the monomers are linked into reticulated networks by C-Ag-C bonds. Topologically identical covalent networks comprised of triazine vertices that are interconnected by biphenyl units are obtained by a thermally activated chemical transformation of the organometallic intermediates. Exposure to iodine vapor facilitates decoupling by intercalation of an iodine monolayer between the covalent networks and the Ag(111) surface. Accordingly, Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and NIXSW experiments are carried out for three successive sample stages: organometallic intermediates, covalent networks directly on Ag(111) and after decoupling. NIXSW analysis facilitates the determination of adsorption heights of chemically distinct carbon species, i.e. in the phenyl and triazine rings, and also for the organometallic carbon atoms. Thereby, molecular conformations are assessed for each sample stage. The interpretation of experimental results is informed by Density Functional Theory (DFT) calculations, providing a consistent picture of adsorption heights and molecular deformations in the networks that result from the interplay between steric hindrance and surface interactions. Quantitative adsorption heights, i.e. vertical distances between adsorbates and surface, provide detailed insight into surface interactions, but are underexplored in on-surface synthesis. In particular, the direct comparison with an in situ prepared decoupled state unveils the surface influence on the network structure, and shows that iodine intercalation is a powerful decoupling strategy.
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| 20. |
- Grossmann, Lukas, et al.
(författare)
-
Steering Self-Assembly of Three-Dimensional Iptycenes on Au(111) by Tuning Molecule-Surface Interactions
- 2022
-
Ingår i: Angewandte Chemie International Edition. - : Wiley-V C H Verlag GMBH. - 1433-7851 .- 1521-3773. ; 61:25
-
Tidskriftsartikel (refereegranskat)abstract
- Self-assembly of three-dimensional molecules is scarcely studied on surfaces. Their modes of adsorption can exhibit far greater variability compared to (nearly) planar molecules that adsorb mostly flat on surfaces. This additional degree of freedom can have decisive consequences for the expression of intermolecular binding motifs, hence the formation of supramolecular structures. The determining molecule-surface interactions can be widely tuned, thereby providing a new powerful lever for crystal engineering in two dimensions. Here, we study the self-assembly of triptycene derivatives with anthracene blades on Au(111) by Scanning Tunneling Microscopy, Near Edge X-ray Absorption Fine Structure and Density Functional Theory. The impact of molecule-surface interactions was experimentally tested by comparing pristine with iodine-passivated Au(111) surfaces. Thereby, we observed a fundamental change of the adsorption mode that triggered self-assembly of an entirely different structure.
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| 21. |
- Halim, Joseph, 1985-
(författare)
-
Synthesis and transport properties of 2D transition metal carbides (MXenes)
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the isolation and characterization of graphene, there has been a growing interest in 2D materials owing to their unique properties compared to their 3D counterparts. Recently, a family of 2D materials of early transition metal carbides and nitrides, labelled MXenes, has been discovered (Ti2CTz, Ti3C2Tz, Mo2TiC2Tz, Ti3CNTz, Ta4C3Tz, Ti4N3Tz among many others), where T stands for surface-terminating groups (O, OH, and F). MXenes are mostly produced by selectively etching A layers (where A stands for group A elements, mostly groups 13 and 14) from the MAX phases. The latter are a family of layered ternary carbides and/or nitrides and have a general formula of Mn+1AXn (n = 1-3), where M is a transition metal and X is carbon and/or nitrogen. The produced MXenes have a conductive carbide core and a non-conductive O-, OH- and/or F-terminated surface, which allows them to work as electrodes for energy storage applications, such as Li-ion batteries and supercapacitors.Prior to this work, MXenes were produced in the form of flakes of lateral dimension of about 1 to 2 microns; such dimensions and form are not suitable for electronic characterization and applications. I have synthesized various MXenes (Ti3C2Tz, Ti2CTz and Nb2CTz) as epitaxial thin films, a more suitable form for electronic and photonic applications. These films were produced by HF, NH4HF2 or LiF + HCl etching of magnetron sputtered epitaxial Ti3AlC2, Ti2AlC, and Nb2AlC thin films. For transport properties of the Ti-based MXenes, Ti2CTz and Ti3C2Tz, changing n from 1 to 2 resulted in an increase in conductivity but had no effect on the transport mechanism (i.e. both Ti3C2Tx and Ti2CTx were metallic). In order to examine whether the electronic properties of MXenes differ when going from a few layers to a single flake, similar to graphene, the electrical characterization of a single Ti3C2Tz flake with a lateral size of about 10 μm was performed. These measurements, the first for MXene, demonstrated its metallic nature, along with determining the nature of the charge carriers and their mobility. This indicates that Ti3C2Tz is inherently of 2D nature independent of the number of stacked layers, unlike graphene, where the electronic properties change based on the number of stacked layers.Changing the transition metal from Ti to Nb, viz. comparing Ti2CTz and Nb2CTz thin films, the electronic properties and electronic conduction mechanism differ. Ti2CTz showed metallic-like behavior (resistivity increases with increasing temperature) unlike Nb2CTz where the conduction occurs via variable range hopping mechanism (VRH) - where resistivity decreases with increasing temperature.Furthermore, these studies show the synthesis of pure Mo2CTz in the form of single flakes and freestanding films made by filtering Mo2CTz colloidal suspensions. Electronic characterization of free-standing films made from delaminated Mo2CTz flakes was investigated, showing that a VRH mechanism prevails at low temperatures (7 to ≈ 60 K). Upon vacuum annealing, the room temperature, RT, conductivity of Mo2CTx increased by two orders of magnitude. The conduction mechanism was concluded to be VRH most likely dominated by hopping within each flake.Other Mo-based MXenes, Mo2TiC2Tz and Mo2Ti2C3Tz, showed VRH mechanism at low temperature. However, at higher temperatures up to RT, the transport mechanism was not clearly understood. Therefore, a part of this thesis was dedicated to further investigating the transport properties of Mo-based MXenes. This includes Mo2CTz, out-of-plane ordered Mo2TiC2Tz and Mo2Ti2C3Tz, and vacancy ordered Mo1.33CTz. Magneto-transport of free-standing thin films of the Mo-based MXenes were studied, showing that all Mo-based MXenes have two transport regimes: a VRH mechanism at lower temperatures and a thermally activated process at higher temperatures. All Mo-based MXenes except Mo1.33CTz show that the electrical transport is dominated by inter-flake transfer. As for Mo1.33CTz, the primary electrical transport mechanism is more likely to be intra-flake.The synthesis of vacancy ordered MXenes (Mo1.33CTz and W1.33CTz) raised the question of possible introduction of vacancies in all MXenes. Vacancy ordered MXenes are produced by selective etching of Al and (Sc or Y) atoms from the parent 3D MAX phases, such as (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering of Mo and Sc. However, not all quaternary parent MAX phases form the in-plane chemical ordering of the two M metals; thus the synthesis of the vacancy-ordered MXenes is restricted to a very limited number of MAX phases. I present a new method to obtain MXene flakes with disordered vacancies that may be generalized to all quaternary MAX phases. As proof of concept, I chose Nb-C MXene, as this 2D material has shown promise in several applications, including energy storage, photothermal cell ablation and photocatalysts for hydrogen evolution. Starting from synthetizing (Nb2/3Sc1/3)2AlC quaternary solid solution and etching both the Sc and Al atoms resulted in Nb1.33C material with a large number of vacancies and vacancy clusters. This method may be applicable to other quaternary or higher MAX phases wherein one of the transition metals is more reactive than the other, and it could be of vital importance in applications such as catalysis and energy storage.
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| 22. |
- Halim, Joseph, 1985-, et al.
(författare)
-
Synthesis of Two-Dimensional Nb1.33C (MXene) with Randomly Distributed Vacancies by Etching of the Quaternary Solid Solution (Nb2/3Sc1/3)2AlC MAX Phase
- 2018
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 1:6, s. 2455-2460
-
Tidskriftsartikel (refereegranskat)abstract
- Introducing point defects in two-dimensional (2D) materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase, wherein one of the transition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of solid solution (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases.
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| 23. |
- Helmer, Pernilla, 1992-
(författare)
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A Computational Venture into the Realm of Laminated Borides and their 2D Derivatives
- 2022
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Daily life in modern society is highly dependent on many different materials and techniques for manipulating them, and the technological forefront is constantly pushed further by new discoveries. Hence, materials science is a very important field of research. The field of 2D materials is a rather young subfield within materials science, sprung from the realisation of the first 2D material graphene. 2D materials have, due to their 2D morphology, a very high surface-to-weight ratio, which makes them clearly attractive for applications where the material surface is an important characteristic, such as for energy storage and catalysis.The family of 2D materials called MXenes contrast to other 2D materials through the methods used to synthesise them. Traditionally, 2D materials are mechanically exfoliated from a 3D bulk structure in which the 2D sheets are only kept together by weak van der Waals forces, while MXenes are instead chemically exfoliated by selectively etching the A element from a member of the MAX phase family. A MAX phase is a hexagonal nanolaminated crystal structure on the formula Mn+1AXn, with n = 1 – 4, where the M indicates one or several transition metals, A stands for an "A element", commonly a metalloid, and X stands for C or N. After etching away the A element from the MAX phase the Mn+1Xn-layers are left, making up the MXene. MXenes thus show an unusual structural and chemical diversity, and the composition spectra is even further expanded by atoms and small molecules, called surface terminations, attaching to the MXene surface upon etching. These terminations in turn also influence the properties of the MXene. Hence, the MXene family shows great potential for property tailoring towards many different applications.Besides MAX phases there are many other nanolaminated materials which can not be mechanically exfoliated like graphene, and the natural question arises: can other nanolaminated materials be etched into completely new 2D materials? This thesis is concerned with the so called MAB phases – a family of laminated materials similar to MAX phases, but with B instead of C or N – and their 2D derivatives from a computational perspective. More specifically, paper I concerns the quaternary out-of-plane-ordered MAB (o-MAB) phase Ti4MoSiB2 – which has been etched into a 2D titanium oxide – and its related ternary counterparts Mo5SiB2 and Ti5SiB2. In paper II the properties and possible termination configurations of a 2D MXene-analogue named boridene is studied.Both projects concern novel materials that have recently been experimentally realised, and the main aim of the first principles calculations presented here has been to complement and explain the experimental results. In paper I bonding characteristics of Ti4MoSiB2, Mo5SiB2 and Ti5SiB2 are studied, with the goal of better understanding why the two former are experimentally realisable while the latter has never been reported. In Ti4MoSiB2 Ti and Mo populate two symmetrically inequivalent lattice sites, and the bond between these two sites was found to display a large peak of bonding states just below the Fermi level. This peak is fully populated in Ti4MoSiB2 and Mo5SiB2, but only partially populated in Ti5SiB2, which was identified to be the key difference causing Ti5SiB2 to be unstable.Paper II instead focuses on the 2D material boridene, derived from a 3D MAB phase with in-plane ordering (i-MAB). The i-MAB phase is similar in structure to i-MAX phases, and the boridene show similar structure and properties as the corresponding i-MXene etched from i-MAX, including a high activity for the hydrogen evolution reaction (HER). The boridene surface was experimentally found to be terminated by O, F and OH species, and the first principles investigations were aimed at screening the possible termination compositions using dynamical stability analysis, and how the electronic properties of boridene are influenced by the terminations. It was found that the terminations are critical to the dynamical stability of boridene, while the specific composition is less important. For termination with only a single species, the material was predicted to be a small bandgap semiconductor with varying bandgap for different species, while for termination with mixed species, the material was found to be metallic.Hence, this thesis has slightly expanded the theoretical knowledge of MAB phases and their first 2D derivative, boridene, by detailed first principles characterisation. Hopefully, these studies can contribute in further development of the considered and related materials, and bring meaningful insight into the behaviour and properties of MAB phases and their 2D derivatives.
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| 24. |
- Helmer, Pernilla, et al.
(författare)
-
Investigation of 2D Boridene from First Principles and Experiments
- 2022
-
Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 32
-
Tidskriftsartikel (refereegranskat)abstract
- Recently, a 2D metal boride - boridene - has been experimentally realized in the form of single-layer molybdenum boride sheets with ordered metal vacancies, through selective etching of the nanolaminated 3D parent borides (Mo2/3Y1/3)(2)AlB2 and (Mo2/3Sc1/3)(2)AlB2. The chemical formula of the boridene was suggested to be Mo4/3B2-xTz, where T-z denotes surface terminations. Here, the termination composition and material properties of Mo4/3B2-xTz from both theoretical and experimental perspectives are investigated. Termination sites are considered theoretically for termination species T = O, OH, and F, and the energetically favored termination configuration is identified at z = 2 for both single species terminations and binary termination mixes of different stoichiometries in ordered and disordered configurations. Mo4/3B2-xTz is shown to be dynamically stable for multiple termination stoichiometries, displaying semiconducting, semimetallic, or metallic behavior depending on how different terminations are combined. The approximate chemical formula of a freestanding film of boridene is attained as Mo1.33B1.9O0.3(OH)(1.5)F-0.7 from X-ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive-free Mo4/3B2-xTz shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.
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| 25. |
- Helmer, Pernilla, 1992-
(författare)
-
Venturing Further into the Field of 2D Materials and their Laminated Parent Phases
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The field of 2D materials is a relatively young and rapidly growing area within materials science, which is concerned with atomically thin states of matter. Because of their intrinsic 2D morphology, 2D materials have exceptionally high surface to weight or surface to volume ratio. This renders them excellent candidates for surface-sensitive applications such as catalysis and energy storage, which can aid us in the transition to a more sustainable society. 2D materials are also interesting because they show properties intrinsically different from those of their 3D counterparts, expanding the attainable property space within materials science. A 2D material can be synthesised by either a bottom-up or top-down approach. The focus here is on the latter, where the 2D material is derived by either mechanical exfoliation or selective etching of a 3D nanolaminated parent phase. A 3D laminate can typically be assigned to one of two types, depending on the type of interlayer bonding: van der Waals (vdW) or chemical bonding. In a vdW bonded phase, the constituent layers are kept together into their 3D form by rather weak vdW forces, while in the latter type, the layers are bound more strongly by chemical interactions (i.e., covalent, ionic and metallic bonds). The first 2D materials were derived from vdW-phases, which can be exfoliated by mechanical methods. In a chemically bound laminated phase, the inter layer bonding is stronger, and more complex methods are required for exfoliation of these phases into 2D. This thesis concerns the computational study and development of novel 2D materials through exploration of 3D nanolaminated structures, assessment of their phase stability, and potential for conversion into 2D. The 2D derivatives are in turn studied through prediction of dynamical stability, termination configuration, and evaluation of electronic properties. Paper III and IV each addresses a family of van der Waals structures. The family of 3D materials studied in Paper III was chosen because it was recently demonstrated as possible to use for derivation of so called 2D MX-enes, while the 2D form of NbOCl2, from the family studied in Paper IV, has been shown to exhibit exciting optical properties. Both projects focus on identification of parent 3D materials, their exfoliation from 3D to 2D, and the electronic properties of the studied phases. In each project, a range of different chemical compositions is considered, chosen based on the experimentally known members of the respective families. A 3D structural ground state is predicted for each composition and prototype, and the dynamical stability with respect to lattice vibrations is established for each identified structure. To assure the experimental relevance of each considered 3D phase, the thermodynamical stability of each structure is assessed via the formation enthalpy with respect to competing phases, identifying seven stable structures in Paper III, and 17 in Paper IV, all of which are also found dynamically stable. Evaluation of the exfoliation energy for all these phases indicates that 3D to 2D conversion is possible. The electronic band structure and density of states were evaluated both for the 2D materials –being the primary focus in both projects – and their 3D parent phases. Al-though the bandgap for semiconducting phases is generally increased upon exfoliation, the electronic properties are mostly retained when exfoliating the vdW phases studied in this thesis. Paper I, II and V address chemically bonded 3D phases and their 2D derivatives. In these 3D phases, auxiliary atoms are interleaved between the 2D units, which needs to be selectively etched to form the corresponding 2D material. Additionally, new terminating species – so called terminations –may attach to the surfaces of the 2D units exposed during etching. Paper I presents an analysis of bonding characteristics in a group of nanolaminated 3D chemically bonded borides: Mo2SiB2, Ti4MoSiB2, and Ti5SiB2, out of which only the two former are observed experimentally. We identify a peak of antibonding states at the Fermi level for Ti5SiB2 as a reason why full elemental substitution of Mo is not achieved experimentally. Papers II and V instead focus on 2D materials derived from chemical 3D parent phases. They go beyond the 2D transition metal carbides and nitrides (MXenes), which until recently were the only 2D materials synthesised through selective etching. Paper II is a study of possible termination configurations on the first 2D boride Mo4/3B2−xTz – boridene – which is identified as being a conductor or small bandgap semiconductor, depending on the terminating species and specific configuration. In Paper V, a computational methodology for simulation of the selective etching process is employed to predict the possibility of etching Y from YM2X2, where the transition metal M and metalloid or nonmetal X are chosen to cover a large compositional space. This results in the prediction of 15 stable 2D structures, out of which nine are not previously investigated. All 2D structures are found to be either metallic or semimetallic. In this thesis, several different computational tools are used to predict and study laminated 3D phases and their corresponding 2D derivatives, assessing their properties considering both purely hypothetical and experimentally realised structures. Experimental relevance is central to all calculations, either by complementing already established experimental results, or by rigorous assessment of thermodynamical and dynamical stability to estimate the potential for experimental synthesis. The thesis expands our knowledge of 3D laminated phases and their 2D derivatives, and identifies several new phases which are likely possible to synthesise.
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| 26. |
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| 27. |
- Johansson, Erik, 1993-
(författare)
-
Theoretical studies of the coupling between electronic, vibrational, configurational and structural effects in metal borides
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis addresses theoretical studies of the coupling between electronic, vibrational, configurational and structural degrees of freedom in metal borides. The effect of external conditions of temperature, pressure and composition on the interplay between internal degrees of freedom is investigated. The importance of excitations and disorder of the above types is well-established and known to dictate key materials science concepts such as phase stability, mechanical and electronic properties. Their mutual coupling composes the next level in complexity in understanding what parameters are to be necessarily included in the theoretical modelling of the system. The main tool used for making such predictions herein is density functional theory. It allows us to capture said excitations and disorder, and give accurate results with reasonable computational efficiency.Metal borides are chosen because of technologically interesting combination of both ceramic and metallic properties, like high hardness, melting point, fracture toughness and electrical conductivity, as well as previous reports of interesting fundamental physical phenomena, like the conventional superconductivity of MgB2 and the apparent off-stoichiometry of AlB2. The theoretical approach is chosen because of its ability to controllably couple and decouple different degrees of freedom to study their combined or isolated effect on the desired materials property. The level of theoretical modeling can be adjusted to fit what is reasonable in terms of efficiency, and still well be used for predictions with quantitative or semi-quantitative accuracy.The configurational aspect of phase stability of binary boron compounds has been believed to be trivial to understand as they most often can be constructed by stacking alternating layers of metal and boron atoms. However, a closer inspection of AlB2, the very name-giver of one of the most usual crystal structures within metal borides, shows a surprising existence of ambiguity regarding both its stable composition and the configuration of metal-site vacancies. Here theoretical approaches are used to reveal the configurational thermodynamics of these vacancies, the origin of their stability and coupling to the electronic structure of the compound. Furthermore, we answer the question why ideal stoichiometric AlB2 is unfavourable, why the vacancy stability window is so narrow, and how different arrangements of vacancies couple with the vibrational degree of freedom, including thermal expansion.If a second metal species is introduced the configurational problem becomes more complex. The arrangement of atoms on the metal sublattice is dependent on the bonding chemistry between the metal atoms and temperature driven thermodynamic effects, like entropy and lattice dynamics. The presented work on Ti1-xAlxB2 is an example of this, where mixing and clustering thermodynamics is considered to further investigate the potential for age-hardening of this ternary diboride. In it, the effect of lattice dynamics and configurational clustering on phase stability is discussed.The discovery of MgB2 being a superconductor in 2001 sparked many fruitful experimental and theoretical studies on the topic. It is generally agreed that MgB2 is a two-gap superconductor which originate in the σ (2D character boron px and py) and π (3D character boron pz) bands, respectively. Superconductivity in itself arises mainly from strong coupling between the E2g phonon mode, corresponding to in-plane bond-stretching vibrations of boron atoms, and electrons on the sigma bands. In the work presented, we use the coupling to the global structural parameters and external pressure to compare different theoretical models, with and without explicit treatment of electron-phonon coupling, and their ability to predict the superconducting transition temperature Tc of distorted MgB2. Moreover, a way to experimentally realize such lattice distorted MgB2 through clever nanostructure design is theoretically explored. Epitaxially growing an alternating out-of-plane ordered Mg/M diboride, with M atoms that naturally have clustering tendencies with respect to Mg, is proposed to being able to provide the necessary lattice distortions of both a- and c-parameters that can lead to an increase in Tc.All of the beforementioned covered compounds (TiB2, AlB2, Ti1-xAlxB2 and MgB2) crystallize in the AlB2-type structure with alternating hexagonal and metal 2D layers, that is most common in the diboride family. However, metal borides can also crystallize with puckered boron layers, while preserving the flat hexagonal metal layers, as in the case for ReB2. It is known for its exceptionally high Vickers hardness that varies from 30.1 ± 1.30 to 48.0 ± 5.6 GPa depending on indentation load. While boron-rich transition metal borides often are considered as potential candidates for hard and incompressible materials, rhenium borides with boron content higher than ReB2 have not been experimentally realized. Theoretical work has proposed that ReB4 adopts the same crystal structure as superhard WB4. In the presented papers, we report the successful synthesis of two novel ReB3 and ReB4 phases at high pressure that remain stable when decompressed down to ambient conditions. First-principles calculations are employed to characterize the electronic, dynamic and thermodynamic stability properties of these phases. Furthermore, novel complex modular Re2B5 and Re3B7 structures are synthesized and characterized by hexagonal boron networks interconnected by short B2 dumbbells.The aim of the in-depth investigations contained in this thesis, using state-of-the-art simulation techniques in collaborations with experimental work, is to further the understanding of how the coupling between electrons, vibrations, atomic configuration, disorder and external conditions influences the properties of materials and to share the results with the scientific community.
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| 28. |
- Kashiwaya, Shun, et al.
(författare)
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Synthesis of goldene comprising single-atom layer gold
- 2024
-
Ingår i: Nature Synthesis. - : Nature Publishing Group. - 2731-0582.
-
Tidskriftsartikel (refereegranskat)abstract
- The synthesis of monolayer gold has so far been limited to free-standingseveral-atoms-thick layers, or monolayers confned on or inside templates.Here we report the exfoliation of single-atom-thick gold achieved throughwet-chemically etching away Ti3C2 from nanolaminated Ti3AuC2, initiallyformed by substituting Si in Ti3SiC2 with Au. Ti3SiC2 is a renown MAX phase,where M is a transition metal, A is a group A element, and X is C or N. Ourdeveloped synthetic route is by a facile, scalable and hydrofuoric acid-freemethod. The two-dimensional layers are termed goldene. Goldene layerswith roughly 9% lattice contraction compared to bulk gold are observedby electron microscopy. While ab initio molecular dynamics simulationsshow that two-dimensional goldene is inherently stable, experiments showsome curling and agglomeration, which can be mitigated by surfactants.X-ray photoelectron spectroscopy reveals an Au 4f binding energy increaseof 0.88 eV. Prospects for preparing goldene from other non-van der WaalsAu-intercalated phases, including developing etching schemes,are presented.
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| 29. |
- Koriukina, Tatiana, 1994-, et al.
(författare)
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On the Use of Ti3C2TX MXene as a Negative Electrode Material for Lithium-Ion Batteries
- 2022
-
Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 7:45, s. 41696-41710
-
Tidskriftsartikel (refereegranskat)abstract
- The pursuit of new and better battery materials has given rise to numerous studies of the possibilities to use two-dimensional negative electrode materials, such as MXenes, in lithium-ion batteries. Nevertheless, both the origin of the capacity and the reasons for significant variations in the capacity seen for different MXene electrodes still remain unclear, even for the most studied MXene: Ti3C2Tx. Herein, freestanding Ti3C2Tx MXene films, composed only of Ti3C2Tx MXene flakes, are studied as additive-free negative lithium-ion battery electrodes, employing lithium metal half-cells and a combination of chronopotentiometry, cyclic voltammetry, X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy experiments. The aim of this study is to identify the redox reactions responsible for the observed reversible and irreversible capacities of Ti3C2Tx- based lithium-ion batteries as well as the reasons for the significant capacity variation seen in the literature. The results demonstrate that the reversible capacity mainly stems from redox reactions involving the Tx-Ti-C titanium species situated on the surfaces of the MXene flakes, whereas the Ti-C titanium present in the core of the flakes remains electro-inactive. While a relatively low reversible capacity is obtained for electrodes composed of pristine Ti3C2Tx MXene flakes, significantly higher capacities are seen after having exposed the flakes to water and air prior to the manufacturing of the electrodes. This is ascribed to a change in the titanium oxidation state at the surfaces of the MXene flakes, resulting in increased concentrations of Ti(II), Ti(III), and Ti(IV) in the Tx-Ti-C surface species. The significant irreversible capacity seen in the first cycles is mainly attributed to the presence of residual water in the Ti3C2Tx electrodes. As the capacities of Ti3C2Tx MXene negative electrodes depend on the concentration of Ti(II), Ti(III), and Ti(IV) in the Tx-Ti-C surface species and the water content, different capacities can be expected when using different manufacturing, pretreatment, and drying procedures.
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| 30. |
- Li, Xuechao, et al.
(författare)
-
Pyridinic Nitrogen Modification for Selective Acetylenic Homocoupling on Au(111)
- 2023
-
Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 145:8, s. 4545-4552
-
Tidskriftsartikel (refereegranskat)abstract
- On-surface acetylenic homocoupling has been proposed to construct carbon nanostructures featuring sp hybrid-ization. However, the efficiency of linear acetylenic coupling is far from satisfactory, often resulting in undesired enyne products or cyclotrimerization products due to the lack of strategies to enhance chemical selectivity. Herein, we inspect the acetylenic homocou-pling reaction of polarized terminal alkynes (TAs) on Au(111) with bond-resolved scanning probe microscopy. The replacement of benzene with pyridine moieties significantly prohibits the cyclotrimerization pathway and facilitates the linear coupling to produce well-aligned N-doped graphdiyne nanowires. Combined with density functional theory calculations, we reveal that the pyridinic nitrogen modification substantially differentiates the coupling motifs at the initial C-C coupling stage (head-to-head vs head-to-tail), which is decisive for the preference of linear coupling over cyclotrimerization.
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| 31. |
- Liu, Yanfeng, et al.
(författare)
-
Mo1.33C MXene-Assisted PEDOT:PSS Hole Transport Layer for High-Performance Bulk-Heterojunction Polymer Solar Cells
- 2020
-
Ingår i: ACS APPLIED ELECTRONIC MATERIALS. - : AMER CHEMICAL SOC. - 2637-6113. ; 2:1, s. 163-169
-
Tidskriftsartikel (refereegranskat)abstract
- Here, we report the usage of two-dimensional MXene, Mo1.33C-assisted poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as an efficient hole transport layer (HTL) to construct high-efficiency polymer solar cells. The composite HTLs are prepared by mixing Mo1.33C and PEDOT:PSS aqueous solution. The conventional devices based on Mo1.33C:PEDOT:PSS exhibit an average power conversion efficiency (PCE) of 9.2%, which shows a 13% enhancement compared to the reference devices. According to the results from hole mobilities, charge extraction probabilities, steady-state photoluminescence, and atomic force microscopy, the enhanced PCE can be ascribed to the improved charge transport and extraction properties of the HTL, along with the morphological improvement of the active layer on top. This work clearly demonstrates the feasibility to combine advantages of Mo1.33C MXene and PEDOT:PSS as the promising HTL in organic photovoltaics.
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| 32. |
- Meshkian, Rahele, 1984-
(författare)
-
Synthesis and characterization of Mo- and W-based atomic laminates
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mn+1AXn (MAX) phases are inherently nanolaminated compounds based on transition metals (M), A group elements (A), and carbon or/and nitrogen (X), which exhibit a unique combination of ceramic and metallic properties. My thesis work has focused on exploring novel MAX phase chemistries, including elemental combinations beyond those traditionally used for MAX phases, and their graphene-analogous 2D counterpart, MXenes. The first part of the thesis investigates Mo-based MAX phases, which are among the least studied, despite indication of superconducting properties and potential for derivation of Mo-based MXenes. Initially, I performed theoretical calculations focused on evaluation of phase stability of the Mon+1GaCn MAX phases, and synthesized the predicted stable Mo2GaC in thin film form using DC magnetron sputtering. Close to phase pure epitaxial films were grown at ~590 °C, and electrical resistivity measurements using a four-point probe technique suggest a superconducting behavior with a critical temperature of ~7 K. The follow-up of this work, was synthesis of a new MAX related material, Mo2Ga2C, also by means of DC magnetron sputtering. The theoretical predictions as well as the materials characterization by X-ray diffraction and neutron powder diffraction, suggested a Ga bilayer interleaved between a set of Mo2C blocks, arranged in a simple hexagonal structure. It is known that selectively etching of the A-layer in a MAX phase, shown for A=Al, can lead to realization of a MXene. Hence, the next step in my research was to explore the possibility of etching of A=Ga in Mo2GaC as well as in Mo2Ga2C, targeting a Mo2C MXene, as motivated by theoretically proposed superior thermoelectric properties of this 2D material. While Mo2GaC did not allow removal of the A-layer, I showed that Mo2C MXene could be realized from etching Mo2Ga2C thin films, removing the Ga bilayer, in 50% hydrofluoric acid at a temperature of ~50 °C for a duration of ~3 h. Hence, the results did not only produce the first Mo-based MXene, it also showed that MXenes can be obtained for etching A-elements other than Al. This, in turn, increase the pathways for expanding the family of MXenes. I thereafter set out to explore the magnetic properties resulting from Mn-alloying of the non-magnetic Mo2GaC MAX phase. For that purpose, (Mo,Mn)2GaC was synthesized using a DC magnetron sputtering system with Ga and C as elemental targets and a 1:1 atomic ratio Mo:Mn compound target. Heteroepitaxial films on MgO(111) substrates were grown at ~530 °C, as confirmed by X-ray diffraction. Compositional analysis using energy dispersive X-ray spectroscopy showed a 2:1 ratio of the M- and A-elements and a 1:1 ratio for the Mo and Mn atoms in the film. Vibrating sample magnetometry was utilized to measure the magnetic behavior of the films, showing a magnetic response up to at least 300 K, and with a coercive field of 0.06 T, which is the highest reported for any MAX phase to date. The second part of my research has been dedicated to realizing new MAX phase related, chemically ordered compounds and their MXene derivatives, and to initiate exploration of their properties. Materials synthesis was performed by pressureless bulk sintering, and inspired by theoretical calculations we showed evidence for a new so called o-MAX phase, Mo2ScAlC2, with an out-of-plane chemically ordered structure. It is the first experimentally verified Sc-containing MAX phase, which makes its corresponding MXene, Mo2ScC2, also presented in this work, the first MXene including Sc. Moreover, I discovered two so called i-MAX phases including W, (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC, which display in-plane chemical ordering in the M-layer. Furthermore, both was shown to allow synthesis of their corresponding 2D counterpart; W1.33C MXene, with ordered vacancies. Initial test on these novel MXenes showed a high potential for hydrogen evolution reaction. Altogether, I have in my thesis work realized 6 novel MAX phases and related materials, and have shown evidence for 4 new MXenes. These materials inspire a wide range of future studies, with respect to fundamental properties as well as potential for future applications.
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| 33. |
- Nayak, Sanjay Kumar, et al.
(författare)
-
First-principles study on the superconductivity of doped zirconium diborides
- 2022
-
Ingår i: Physical Review Materials. - College Park, MD, United States : American Physical Society. - 2475-9953. ; 6:4
-
Tidskriftsartikel (refereegranskat)abstract
- Recent experiments [Barbero et al. Phys. Rev. B 95, 094505 (2017)] have established that bulk superconductivity (Tc ∼ 8.3-8.7 K) can be induced in AlB2-type ZrB2 and HfB2, highly covalent refractory ceramics, by vanadium (V) doping. These AlB2-structured phases provide an alternative to earlier diamon-like or diamond-based superconducting and superhard materials. However, the underlying mechanism for doping-induced superconductivity in these materials is yet to be addressed. In this paper, we have used first-principles calculations to probe electronic structure, lattice dynamics, and electron-phonon coupling (EPC) in V-doped ZrB2 and consequently examine the origin of the superconductivity. We find that, while doping-induced stress weakens the EPC, the concurrently induced charges strengthen it. The calculated critical transition temperature (Tc) in electron (and V)-doped ZrB2 is at least one order of magnitude lower than experiments, despite considering the weakest possible Coulomb repulsion between electrons in the Cooper pair, hinting a complex origin of superconductivity in it.
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| 34. |
- Niu, Kaifeng, 1994-
(författare)
-
Mechanistic investigations of chemical reactions on 2D MXenes and metal surfaces from first-principles
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Chemical reactions on surfaces play a central role both for our daily life and industrial purposes, including the storage and release of energy, as well as the formation of new materials. To achieve high efficiency, catalysis lies in the heart of chemical reactions as it plays a critical role in accelerating the chemical transformation to target products. However, environmental issues arise as the applications of catalytic technologies and current synthetic approaches such as pollution from undesirable byproducts and massive emission of carbon dioxides due to the usage of fossil fuels. This calls for developing improved strategies for fabricating new materials with highly efficient catalytic properties. In recent years, on-surface chemical reactions have also been used to synthesize new low-dimensional materials with atomic precision, by coupling molecules into nanostructures. It is crucial to not only obtain high activity for chemical reactions, but also achieve distinct selectivity towards desired products. For this purpose, understanding mechanisms of target chemical reactions and origins of catalysts’ activity are of great significance to facilitate chemical processes.In this thesis, three types of chemical reactions are investigated within the framework of density functional theory (DFT), in which chemical reactions relevant for both heterogeneous catalysis and electrochemical synthesis are considered on two-dimensional transition metal carbides (2D MXenes), and chemical reactions for synthesizing organic nanostructures are studied on metal surfaces. Focusing on one of the most fundamental chemical reaction, C(sp3)-H activation, we demonstrate that MXenes can serve as highly efficient heterogeneous catalysts and exhibit high activity. The thermally triggered C-H activations are shown to follow the “radical-like” mechanism on MXenes, in which O terminations serve as active sites. By adopting the hydrogen affinity (EH) as a descriptor, both the geometry configuration and the catalytic activity of MXenes can be quantitatively characterized.In the context of on-surface synthesis, we theoretically propose reaction mechanisms of two types of chemical reactions on surface. A new strategy for constructing C-C bonds via the desulfonylation reaction was achieved experimentally for the first time by collaborators. With DFT calculations, an observed discrepancy between Ag(111) and Au(111) is ascribed to interactions between surfaces and molecules. Secondly, the formation mechanism of the 2D biphenylene network (BPN), a recently realized carbon allotrope formed by intermolecular HF zipping on Au(111), has been computationally investigated.With the tool of DFT calculations, a single Ni atom catalyst supported by Ti3C2T2 MXenes for electrochemical nitrogen reduction has been theoretically proposed. Such single atom catalyst (SAC) is computationally screened from three aspects including stability, activity, and selectivity. Our theoretical results show that not only the catalytic performance of the Ni SAC predicted by screening criteria can be verified, but also a H rich environment can be beneficial for the electrochemical nitrogen reduction on such SACs.In summary, first-principles calculations have been performed to evaluate the catalytic performance of 2D MXenes towards C-H activation, unravel formation mechanisms of organic materials synthesized via on-surface reactions, and design effective catalysts towards the synthesis of ammonia. It is anticipated that this thesis can pave the way for the rational design of high-efficient catalysts for various reactions and shed lights on developing synthetic strategies of unprecedented organic materials.
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| 35. |
- Niu, Kaifeng, 1994-, et al.
(författare)
-
Unveiling the formation mechanism of the biphenylene network
- 2023
-
Ingår i: Nanoscale Horizons. - : ROYAL SOC CHEMISTRY. - 2055-6764 .- 2055-6756. ; 8:3, s. 368-376
-
Tidskriftsartikel (refereegranskat)abstract
- We have computationally studied the formation mechanism of the biphenylene network via the intermolecular HF zipping, as well as identified key intermediates experimentally, on the Au(111) surface. We elucidate that the zipping process consists of a series of defluorinations, dehydrogenations, and C–C coupling reactions. The Au substrate not only serves as the active site for defluorination and dehydrogenation, but also forms C–Au bonds that stabilize the defluorinated and dehydrogenated phenylene radicals, leading to "standing" benzyne groups. Despite that the C–C coupling between the "standing" benzyne groups is identified as the rate-limiting step, the limiting barrier can be reduced by the adjacent chemisorbed benzyne groups. The theoretically proposed mechanism is further supported by scanning tunneling microscopy experiments, in which the key intermediate state containing chemisorbed benzyne groups can be observed. This study provides a comprehensive understanding towards the on-surface intermolecular HF zipping, anticipated to be instructive for its future applications.
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| 36. |
- Omran, Meis, et al.
(författare)
-
Whole-Body MRI Surveillance : Baseline Findings in the Swedish Multicentre Hereditary TP53-Related Cancer Syndrome Study (SWEP53)
- 2022
-
Ingår i: Cancers. - : MDPI. - 2072-6694. ; 14:2
-
Tidskriftsartikel (refereegranskat)abstract
- A surveillance strategy of the heritable TP53-related cancer syndrome (hTP53rc), commonly referred to as the Li–Fraumeni syndrome (LFS), is studied in a prospective observational nationwide multi-centre study in Sweden (SWEP53). The aim of this sub-study is to evaluate whole-body MRI (WB-MRI) regarding the rate of malignant, indeterminate, and benign imaging findings and the associated further workup generated by the baseline examination. Individuals with hTP53rc were enrolled in a surveillance program including annual whole-body MRI (WB-MRI), brain-MRI, and in female carriers, dedicated breast MRI. A total of 68 adults ≥18 years old have been enrolled to date. Of these, 61 fulfilled the inclusion criteria for the baseline MRI scan. In total, 42 showed a normal scan, while 19 (31%) needed further workup, of whom three individuals (3/19 = 16%) were diagnosed with asymptomatic malignant tumours (thyroid cancer, disseminated upper GI cancer, and liver metastasis from a previous breast cancer). Forty-three participants were women, of whom 21 had performed risk-reducing mastectomy prior to inclusion. The remaining were monitored with breast MRI, and no breast tumours were detected on baseline MRI. WB-MRI has the potential to identify asymptomatic tumours in individuals with hTP53rc syndrome. The challenge is to adequately and efficiently investigate all indeterminate findings. Thus, a multidisciplinary team should be considered in surveillance programs for individuals with hTP53rc syndrome.
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| 37. |
- Palisaitis, Justinas, et al.
(författare)
-
On the nature of planar defects in transition metal diboride line compounds
- 2022
-
Ingår i: Materialia. - : Elsevier Science Ltd. - 2589-1529. ; 24
-
Tidskriftsartikel (refereegranskat)abstract
- Planar defect structures appearing in transition metal diboride (TMB2) thin films, grown by different magnetron sputtering-deposition approaches over a wide compositional and elemental range, were systematically investi-gated. Atomically resolved scanning transmission electron microscopy (STEM) imaging, electron energy loss spec-troscopy (EELS) elemental mapping, and first principles calculations have been applied to elucidate the atomic structures of the observed defects. Two distinct types of antiphase boundary (APB) defects reside on the {1(1) over bar 00} planes. These defects are without (named APB-1) or with (APB-2) local deviation from stoichiometry. APB-2 de-fects, in turn, appear in different variants. It is found that APB-2 defects are governed by the films composition, while APB-1 defects are endemic. The characteristic structures, interconnections, and circumstances leading to the formation of these APB-defects, together with their formation energies, are presented.
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| 38. |
- Palisaitis, Justinas, 1983-, et al.
(författare)
-
On the Structural Stability of MXene and the Role of Transition Metal Adatoms
- 2018
-
Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 10:23, s. 10850-10855
-
Tidskriftsartikel (refereegranskat)abstract
- In the present communication, the atomic structure and coordination of surface adsorbed species on Nb2C MXene is investigated over time. In particular, the influence of the Nb adatoms on the structural stability and oxidation behavior of the MXene is addressed. This investigation is based on plan-view geometry observations of single Nb2C MXene sheets by a combination of atomic-resolution scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and STEM image simulations.
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| 39. |
- Persson, Ingemar, 1985-
(författare)
-
Surface characterization of 2D transition metal carbides (MXenes)
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Research on two-dimensional (2D) materials is a rapidly growing field owing to the wide range of new interesting properties found in 2D structures that are vastly different from their three-dimensional (3D) analogues. In addition, 2D materials embodies a significant surface area that facilitates a high degree of surface reactions per unit volume or mass, that is imperative in many applications such as catalysis, energy storage, energy conversion, filtration, and single molecule sensing. MXenes constitute a family of 2D materials consisting of transition metal carbides and/or nitrides, which are typically formed after selective etching of their 3D parent MAX phases. The latter, are a family of nanolaminated compounds that typically follow the formula Mn+1AXn (n=1-3), where M is a transition metal, A is a group 13 or 14 element, and X is C and or N. Selective etching by aqueous F- containing acids removes the A layer leaving 2D Mn+1Xn slabs instantly terminated by a mix of O-, OH- and F-groups. The first and most investigated MXene is Ti3C2TX, where TX stands for surface termination, which has shown record properties in a range of applications (eg. electrode in Li-batteries, supercapacitors, sieving membrane, electromagnetic interference shielding, and carbon capture). Adding to that, over 30 different MXenes have been discovered since 2011, exhibiting alternative or superior properties. Most importantly, elegant routes for property design in the MXene family has been demonstrated, by means of either varying the chemistry in the Mn+1Xn compound, by alloying two M elements, or by changing the structure of the MXene by introducing vacancies.The present work has a led to an additional route for post synthesis property tuning in MXenes by manipulation of surface termination elements. This enables a unique toolbox for property tuning which is not available to other 2D materials and is highly beneficial for applications that is dependent on surface reactions. Furthermore, chemical and structural characterization of terminations on single sheets is essential to rule out the influence of intercalants or contamination that is typically present in multilayer MXene samples or thin films. For that purpose, a method for preparing isolated contamination free single sheets of MXene samples for transmission electron microscopy (TEM) characterization was established. In order to determine vacancy and termination sites, atomically resolved scanning (S)TEM imaging and image simulations was carried out. Two main processes were employed to substitute the termination elements.1) An initial thermal treatment in vacuum facilitates F desorption and it was shown that O-terminations rearranges on the evacuated sites. H2 gas exposure in a controlled environment demonstrated a removal of the remaining O-terminations. As a result, termination-free MXene is possible to realize under vacuum conditions.2) CO2 was introduced as a first non-inherent termination on MXene by in situ CO2 gas exposure at low temperatures. That was a first demonstration of Ti3C2TX as promising material for carbon capture. Additionally, O-saturated surfaces were demonstrated after introduction of O2 gas on the F-depleted Ti3C2TX MXene, which is highly relevant for hydrogen evolution reactions where fully O-terminated Ti3C2TX are predicted to improve efficiency.A Lewis acid melt synthesis method was used to realize the first MXene exclusively terminated with Cl. Moreover, this was the first report of a MXene directly synthesised with terminations other than O, OH, and F.Furthermore, we have expanded the space of property tuning by introduction of chemical ordering, by selective etching of Y in an alloyed (Mo2/3Y1/3)2CTX MXene. This either produced chemical ordering with one M (Mo) element and vacancies, or ordering between two M (Mo and Y) elements. This was further reported to significantly increase volumetric capacitance because of the increased number of active sites around vacancies, leading to an increasing charge density. As a final note, the stability of Nb2CTX MXene under ambient conditions was investigated. It was found that the surface Nb adatoms, present after etching, got oxidized over time which resulted in local clustering and effectively degraded the MXene.This work has demonstrated reproducible surface characterization methods for determining termination elements and sites in 2D MXenes, that is ultimately governing MXene properties. Most importantly, we report on a new approach for MXene property tuning as well as contributing to several existing property tuning approaches.
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| 40. |
- Petruhins, Andrejs, 1987-
(författare)
-
Synthesis and characterization of magnetic nanolaminated carbides
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- MAX phases are a group of nanolaminated ternary carbides and nitrides, with a composition expressed by the general formula Mn+1AXn (? = 1 − 3), where M is a transition metal, A is an A-group element, and X is carbon and/or nitrogen. MAX phases have attracted interest due to their unique combination of metallic and ceramic properties, related to their inherently laminated structure of a transition metal carbide (Mn+1Xn) layer interleaved by an A-group metal layer.This Thesis explores synthesis and characterization of magnetic MAX phases, where the A-group element is gallium (Ga). Due to the low melting point of Ga (T = 30 °C), conventional thin film synthesis methods become challenging, as the material is in liquid form at typical process temperatures. Development of existing methods has therefore been investigated, for reliable/reproducible synthesis routes, including sputtering from a liquid target, and resulting high quality material. Routes for minimizing trial-and-error procedures during optimization of thin film synthesis have also been studied, allowing faster identification of optimal deposition conditions and a simplified transfer of essential deposition parameters between different deposition systems.A large part of this Thesis is devoted towards synthesis of MAX phase thin films in the Cr-Mn-Ga-C system. First, through process development, thin films of Cr2GaC were deposited by magnetron sputtering. The films were epitaxial, however with small amount of impurity phase Cr3Ga, as confirmed by X-ray diffraction (XRD) measurements. The film structure was confirmed by scanning transmission electron microscopy (STEM) and the composition by energy dispersive X-ray spectroscopy (EDX) inside the TEM.Inspired by predictive ab initio calculations, the new MAX phase Mn2GaC was successfully synthesized in thin film form by magnetron sputtering. Structural parameters and magnetic properties were analysed. The material was found to have two magnetic transitions in the temperature range 3 K to 750 K, with a first order transition at around 214 K, going from non-collinear antiferromagnetic state at lower temperature to an antiferromagnetic state at higher temperature. The Neél temperature was determined to be 507 K, changing from an antiferromagnetic to a paramagnetic state. Above 800 K, Mn2GaC decomposes. Furthermore, magnetostrictive, magnetoresistive and magnetocaloric properties of the material were iv determined, among which a drastic change in lattice parameters upon the first magnetic transition was observed. This may be of interest for magnetocaloric applications.Synthesis of both Cr2GaC and Mn2GaC in thin film form opens the possibility to tune the magnetic properties through a solid solution on the transition metal site, by alloying the aforementioned Cr2GaC with Mn, realizing (Cr1-xMnx)2GaC. From a compound target with a Cr:Mn ratio of 1:1, thin films of (Cr0.5Mn0.5)2GaC were synthesized, confirmed by TEM-EDX. Optimized structure was obtained by deposition on MgO substrates at a deposition temperature of 600 ºC. The thin films were phase pure and of high structural quality, allowing magnetic measurements. Using vibrating sample magnetometry (VSM), it was found that (Cr0.5Mn0.5)2GaC has a ferromagnetic component in the temperature range from 30 K to 300 K, with the measured magnetic moment at high field decreasing by increasing temperature. The remanent moment and coercive field is small, 0.036 μB, and 12 mT at 30 K, respectively. Using ferromagnetic resonance spectroscopy, it was also found that the material has pure spin magnetism, as indicated by the determined spectroscopic splitting factor g = 2.00 and a negligible magnetocrystalline anisotropy energy.Fuelled by the recent discoveries of in-plane chemically ordered quaternary MAX phases, so called i-MAX phases, and guided by ab initio calculations, new members within this family, based on Cr and Mn, were synthesized by pressureless sintering methods, realizing (Cr2/3Sc1/3)2GaC and (Mn2/3Sc1/3)2GaC. Their structural properties were determined. Through these phases, the Mn content is the highest obtained in a bulk MAX phase to date.This work has further developed synthesis processes for sputtering from liquid material, for an optimized route to achieve thin films of controlled composition and a high structural quality. Furthermore, through this work, Mn has been added as a new element in the family of MAX phase elements. It has also been shown, that alloying with different content of Mn gives rise to varying magnetic properties in MAX phases. As a result of this Thesis, it is expected that the MAX phase family can be further expanded, with more members of new compositions and new properties.
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| 41. |
- Qin, Leiqiang, 1987-, et al.
(författare)
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Flexible Solid-State Asymmetric Supercapacitors with Enhanced Performance Enabled by Free-Standing MXene-Biopolymer Nanocomposites and Hierarchical Graphene-RuOx Paper Electrodes
- 2020
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Ingår i: Batteries & Supercaps. - : WILEY-V C H VERLAG GMBH. - 2566-6223. ; 3:7, s. 604-610
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional (2D) transition metal carbides and carbonitrides, called MXenes, with metallic conductivity and hydrophilic surfaces, show great promise as electrode materials for supercapacitors. A major drawback of 2D nanomaterials is the re-stacking of the nanosheets, which prevents full utilization of surface area and blocks the access of the electrolyte. In this study, a free-standing nanocomposite paper electrode is realized by combining Mo1.33C MXene and positively charged biopolymer lignin (the second most abundant biopolymer in nature, L-DEA). The self-assembled layered architecture with alternating polymer and MXene flakes increases the interlayer space to promote ion transport, and with combining charge storage capability of the lignin derivative and MXene in an interpenetrating MXene/L-DEA nanocomposite, which offers an impressive capacitance of 503.7 F g(-1). Moreover, we demonstrate flexible solid-state asymmetric supercapacitors (ASCs) using Mo1.33C@L-DEA as the negative electrode and electrochemically exfoliated graphene with ruthenium oxide (EG@RuOx) as the positive electrode. This asymmetric device operates at a voltage window of 1.35 V, which is about two times wider than that of a symmetric Mo1.33C@L-DEA based supercapacitor. Finally, the ASCs can deliver an energy density of 51.9 Wh kg(-1) at a power density of 338.5 W kg(-1), with 86 % capacitance retention after 10000 charge-discharge cycles.
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| 42. |
- Qin, Leiqiang, et al.
(författare)
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High-Performance Ultrathin Flexible Solid-State Supercapacitors Based on Solution Processable Mo1.33C MXene and PEDOT:PSS
- 2018
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 28:2
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Tidskriftsartikel (refereegranskat)abstract
- MXenes, a young family of 2D transition metal carbides/nitrides, show great potential in electrochemical energy storage applications. Herein, a high performance ultrathin flexible solid-state supercapacitor is demonstrated based on a Mo1.33C MXene with vacancy ordering in an aligned layer structure MXene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film posttreated with concentrated H2SO4. The flexible solid-state supercapacitor delivers a maximum capacitance of 568 F cm(-3), an ultrahigh energy density of 33.2 mWh cm(-3) and a power density of 19 470 mW cm(-3). The Mo1.33C MXene/PEDOT:PSS composite film shows a reduction in resistance upon H2SO4 treatment, a higher capacitance (1310 F cm(-3)) and improved rate capabilities than both pristine Mo1.33C MXene and the nontreated Mo1.33C/PEDOT:PSS composite films. The enhanced capacitance and stability are attributed to the synergistic effect of increased interlayer spacing between Mo1.33C MXene layers due to insertion of conductive PEDOT, and surface redox processes of the PEDOT and the MXene.
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| 43. |
- Qin, Leiqiang, 1987-, et al.
(författare)
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MXene-based multifunctional smart fibers for wearable and portable electronics
- 2022
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 10:23, s. 12544-12550
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Tidskriftsartikel (refereegranskat)abstract
- Fiber type devices are promising for applications in wearable and portable electronics. However, scalable fabrication of fiber electrodes with multifunctional performance for use in distinct fields remains challenging. Herein, high performance smart fibers based on Mo1.33C i-MXene nanosheets and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate hybrid paste are fabricated with an easily scalable spinning approach. The hybrid fibers produced by this method can be applied in both high-performance supercapacitors and electrochemical transistors (ECTs). When assembled into a fiber type asymmetric supercapacitor with reduced graphene oxide (rGO) fiber, a capacitance of 105 F g(-1) and an energy density of 37 mW h g(-1) were reached for a potential window of 1.6 V. The hybrid fiber based ECT shows high transconductance and fast response time. This work demonstrates the potential of i-MXene-based fiber electrodes for multifunctional applications, to aid in the development of the next-generation, high-performance wearable electronic devices.
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| 44. |
- Rosén, Johanna, 1975-
(författare)
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Characterization of metal plasma streams from arc discharges in a reactive environment
- 2002
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Alumina is a material used in many applications mainly due to its chemical inertness and wear resistance at high temperatures. Despite this, the understanding of alumina thin films is not satisfactory, as can be concluded from the diverging material properties reported in the literature. The objective of this study is to contribute towards a fundamental understanding of the correlation between synthesis conditions, thin film composition and microstructure. A detailed description of the plasma chemistry including ion charge states is essential to achieve this task. The growth technique under investigation is pulsed cathodic arc, and cathodic arc plasma under varying process conditions have been characterized.The temporal development of the plasma chemistry has been investigated. Pulse-to pulse fluctuations of the plasma chemistry in the presence of magnetic fields, for an aluminium arc in an oxygen environment, were found to increase as the magnetic field strength increased. These results are explained by a varying energy input during plasma production, as seen through a measured fluctuating cathode potential.Further investigations dealt with temporal development of the plasma chemistry and average charge state within the pulse, for a zirconium and chromium arc in a nitrogen environment. A large non-metal fraction was observed in the beginning of the plasma pulse, which can be explained by the formation and erosion of a compound layer at the cathode surface. As the pressure increased, the concentration of these ions was found to increase, both in amplitude as well as in width out in the pulse. The average charge state also showed a time and pressure dependence, the former most pronounced at lower pressures with higher charge state in the beginning of the pulse. With increasing pressure, the charge state decreased towards a steady state value, which is proposed to be due to a higher probability of collisions in the plasma with possible charge exchanges.
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| 45. |
- Rosén, Johanna, 1975-, et al.
(författare)
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In- and Out-of-Plane Ordered MAX Phases and Their MXene Derivatives
- 2019
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Ingår i: 2D Metal Carbides and Nitrides (MXenes). - Cham : Springer. - 9783030190262 - 9783030190255 - 9783030190286 ; , s. 37-52
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Bokkapitel (refereegranskat)abstract
- The family of MXenes has expanded since the discovery of chemical order in parent quaternary MAX phases, displaying either out-of-plane (o-MAX) or in-plane (i-MAX) order upon alloying. Through selective chemical etching of these materials, corresponding MXenes can be derived, with out-of-plane and in-plane ordering of elements, as well as with ordering of vacancies. Both o-MAX and i-MAX phases have increased the number of metals that can be incorporated in these laminated carbides and nitrides. Examples of realized MXenes with out-of-plane order are Mo2Ti2C3 and Mo2ScC2, and for in-plane ordering of vacancies, there are Mo1.33C and W1.33C. Their versatile chemistry shows a high promise for a range of applications, including energy storage and catalysis
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| 46. |
- Sanchez-Grande, Ana, et al.
(författare)
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Surface-Assisted Synthesis of N-Containing pi-Conjugated Polymers
- 2022
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Ingår i: Advanced Science. - : Wiley. - 2198-3844. ; 9:19
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Tidskriftsartikel (refereegranskat)abstract
- On-surface synthesis has recently emerged as a powerful strategy to design conjugated polymers previously precluded in conventional solution chemistry. Here, an N-containing pentacene-based precursor (tetraazapentacene) is ex-professo synthesized endowed with terminal dibromomethylene (:CBr2) groups to steer homocoupling via dehalogenation on metallic supports. Combined scanning probe microscopy investigations complemented by theoretical calculations reveal how the substrate selection drives different reaction mechanisms. On Ag(111) the dissociation of bromine atoms at room temperature triggers the homocoupling of tetraazapentacene units together with the binding of silver adatoms to the nitrogen atoms of the monomers giving rise to a N-containing conjugated coordination polymer (P1). Subsequently, P1 undergoes ladderization at 200 degrees C, affording a pyrrolopyrrole-bridged conjugated polymer (P2). On Au(111) the formation of the intermediate polymer P1 is not observed and, instead, after annealing at 100 degrees C, the conjugated ladder polymer P2 is obtained, revealing the crucial role of metal adatoms on Ag(111) as compared to Au(111). Finally, on Ag(100) the loss of :CBr2 groups affords the formation of tetraazapentacene monomers, which coexist with polymer P1. Our results contribute to introduce protocols for the synthesis of N-containing conjugated polymers, illustrating the selective role of the metallic support in the underlying reaction mechanisms.
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| 47. |
- Schnitter, Claudia, 1989-, et al.
(författare)
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Epitaxial Growth of Magnetron-Sputtered ZrB2 Films on Si(100) Substrates
- 2022
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley-V C H Verlag GMBH. - 1862-6300 .- 1862-6319. ; 219:21
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial growth of ZrB2 films on Si(100) substrates at 900 degrees C is demonstrated using direct-current magnetron sputter deposition from sintered ZrB2 targets. This case of epitaxial growth is structurally more challenging than on Si(111), 4 H-SiC(001), and Al2O3(001). From pole figure measurements, two epitaxial relationships are determined: A) in-plane: ZrB 2 [ 001 ] parallel to Si [ 110 ] and ZrB 2 [ 110 ] parallel to Si [ 110 ] , out-of-plane: ZrB 2 ( 100 ) parallel to Si ( 100 ) , and B) in-plane: ZrB 2 [ 1 2 over bar 1 ] parallel to Si [ 110 ] and the same multiply rotated 90 degrees around the 102 axis, out of plane: ZrB 2 ( 102 ) parallel to Si ( 100 ) . From full width at half maximum (FWHM) values from rocking curve measurements (omega-scans) of the 100 and 102 peaks, a measure of epitaxial quality for these two preferred orientations is obtained. Both omega-scans and theta/2 theta diffractograms show higher quality for the A-type with a FWHM value of 2.00 degrees compared with 4.97 degrees for the B-type. The film composition is found to be ZrB2.3 from time-of-flight elastic recoil detection analysis. The B-type crystallographic relationship ZrB 2 ( 102 ) parallel to Si ( 100 ) and ZrB 2 [ 1 2 over bar 0 ] parallel to Si [ 110 ] has not been previously reported.
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| 48. |
- Schnitter, Claudia, 1989-
(författare)
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Epitaxial thin films of group 4 transition metal diborides
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Group 4 transition metal diboride films were deposited epitaxially onto different substrates, typically at 900 °C, using direct-current magnetron sputtering from compound targets of ZrB2, TiB2, and HfB2. Epitaxial ZrB2 has been deposited on Al2O3(0001), 4H-SiC(0001), and Si(100) describing the epitaxial relationships on Si(100) for the first time, where two relationships were observed from pole-figure analysis, namely A) in-plane: ZrB2[001] ∥ Si[110] and ZrB2[110] ∥ Si[110], out-of-plane: ZrB2(100) ∥ Si(100), and B) in-plane: ZrB2[11] ∥ Si[110] and the same rotated 90° around the 102 axis, out-of- plane: ZrB2(102) ∥ Si(100). Composition analysis by time-of-flight elastic recoil detection analysis revealed typically B-rich ZrB2 and TiB2, and stoichiometric to Hf-rich HfB2. For ZrB2, the application of an additional external magnetic field during growth influenced the B-to-Zr ratio towards being stoichiometric. Rocking curve measurements of ZrB2 deposited onto Si(100) reveal a higher crystal order in the 100-oriented domains, compared to the 102-oriented domains. In ZrB2 films annealed to temperatures in the range of 1100-1500 °C, rocking curve measurements of the symmetric 001 reflection as well as the asymmetric 101 reflection reveal increased order with increased temperature. This phenomenon occurred at lower temperature when the annealing was performed in H2 compared to Ar.The morphology in plan-view transmission electron microscopy reflects the composition of the film: TiB2.5 has B-rich areas around the grain boundaries, forming an almost continuous network around the grains. ZrBx films with x between 2.0 and 2.3 also contain B-rich regions, though to a smaller extent and mostly in areas where more than two grains adjoin each other. In Hf-rich HfB1.8, no B-rich areas were observed. Scanning transmission electron micrographs and a combination of B electron energy loss spectroscopy and energy dispersive X-ray analysis Hf distribution maps revealed Hf-rich areas in a close-to single crystalline matrix. The hardness of epitaxial HfB2 films is reported to 33 and 36 GPa for films deposited onto Al2O3 and SiC, respectively. These values are slightly higher than reported for bulk HfB2. ZrB2 films on SiC decrease in hardness to 38, 37, and 30 GPa, upon annealing in Ar up to 1100, 1300, and 1500 °C, respectively. In summary, the knowledge is expanded about epitaxially grown group 4 transition metal diborides in terms of their Chemical composition, crystallographic orientations, microstructure, electrical and mechanical properties, as well as their response to heat treatment.
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| 49. |
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| 50. |
- Tao, Quanzheng, 1989-
(författare)
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Synthesis and characterization of two- and three-dimensional nanolaminated carbides
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focused towards the synthesis and characterization of novel nanolaminated materials in primarily bulk (powder) form. Of particular interest is magnetic materials, or laminates that can be used as precursor for two-dimensional (2D) materials. 2D materials typically display a large surface-to-volume ratio, and as such they are very promising for applications within energy storage and catalysis. A more recently discovered family of 2D transition metal carbides/nitrides, called MXenes, are currently attracting a lot of attention. MXenes are produced by selective etching of parent 3D nanolaminates, so called MAX phases, facilitating removal of selected atomic layers, and formation of 2D sheets.In my work on new nanolaminates as precursors for 2D materials, I have synthesized (Mo2/3Sc1/3)2AlC and have studied its crystal structure. It was found that Mo and Sc are chemically ordered in the metal layers, with the in-plane ordering motivating the notation i-MAX for this new type of MAX phase alloy. By selective etching of Sc and Al, we thereafter produced a 2D materials with ordered vacancies, Mo1.33C, and studied the electrochemical properties. It was found that the material displayed a high capacitance, ~1200 F cm-3, which is 65% higher that the counterpart without vacancies, Mo2C.I also synthesized a previously not known out-of-plane ordered Mo2ScAlC2 MAX phase. By selective etching of Al, we produced a 2D material, Mo2ScC2, which is correspondingly ordered in the out-of-plane direction. Another related laminated material was also discovered and synthesized, Sc2Al2C3, and its crystal structure was determined. The material is potentially useful for conversion into a 2D material. I have also shown that Sc2Al2C3 is an example of a series of materials with the same crystal structure, with Sc replaced by other metals.Magnetic materials are used in many applications, such as for data storage devices. In particular, layered magnetic materials are of interest due to their anisotropic structure and potential formation of interesting magnetic characteristics. I have been synthesizing and characterizing magnetic nanolaminates, starting with the (V,Mn)3GaC2 MAX phase in the form of an epitaxial thin film. Analysis of the magnetic behavior showed a ferromagnetic response above room temperature I thereafter showed that our previously discovered family of i-MAX phases could be expanded with a subclass of ordered nanolaminates based on rare earth (RE) elements, of the general formula (Mo2/3RE1/3)2AlC , where RE=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. I studied their crystal structure by scanning transmission microscopy (STEM), X-ray diffraction (XRD), and neutron diffraction. We found that these phases can crystalize in three different structures, of space group C2/m, C2/c, and Cmcm, respectively. The magnetic behavior was studied and the magnetic structure of two materials could be determined. We suggest that the complex behavior identified is due to competing magnetic interaction and frustration.I also synthesized another rare earth-based nanolaminate, Mo4Ce4Al7C3. The crystal structure was investigated by single crystal X-ray diffraction and STEM. Magnetization analysis reveal a ferromagnetic ground state below 10.5 K. X-ray absorption near-edge structure provide evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only one of the two Ce sites are magnetic.
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