| 421. |
- Gueorguiev, Gueorgui Kostov, 1968-, et al.
(author)
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Nano-wire formation by self-assembly of silicon-metal cage-like molecules
- 2008
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 458:1-3, s. 170-174
-
Journal article (peer-reviewed)abstract
- By using DFT calculations we show that nano-wires consisting of endohedral MSi12 cage-like molecules are stable especially for light transition metal elements (M = Fe, Ni, Co, Ti, V, and Cu). The nano-wire assemblies are stabilized by the metal atoms located along their principal axes and can be seen as close Si-based analogues of C nanotubes, but with hexagonal cross-section due to the D6h-symmetry of their MSi12 building blocks. Independently on M, with the increase in the length of a (MSi12)m nano-wire, its HOMO-LUMO gap decreases gradually. The metallic behavior of (MSi12)m defines them as possible conductive components for self-assembled nano-devices. © 2008 Elsevier B.V. All rights reserved.
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| 422. |
- Gueorguiev, Gueorgui Kostov, et al.
(author)
-
Silicon-metal clusters : Nano-templates for cluster assembled materials
- 2006
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 515:3, s. 1192-1196
-
Journal article (peer-reviewed)abstract
- The structure, cohesive energy and electronic properties of MSin clusters were studied by first-principles calculations as a function of size (n) and species (M). We investigated 168 different clusters, containing from 1 to 14 Si atoms together with one transition metal atom among 12 different elements: Ti, Zr, Hf, V, Nb, Ta, Ni, Pd, Pt, Cu, Ag, Au. Clusters with n = 7, 10, 12 appear as local maxima in cohesive energy, independently of the metal involved. This, together with our previous findings for MSin (containing 12 other transition metal elements), establishes a systematic behavior. For most metals, MSi12 and MSi10 (the smallest endohedral species) are highly symmetric and exhibit local (with respect to their neighbors in size) stability. Thus, besides practically all MSi12 clusters (exceptions being HfSi12, ZrSi12), also some MSi10 such as VSi10, NiSi10, PdSi10, NbSi10 and AgSi10, are promising candidates as building blocks for cluster assembled materials. Electronic properties of structurally equivalent clusters depend markedly on the transition metal involved, providing the means to tailor pre-defined properties when designing extended phases. © 2006 Elsevier B.V. All rights reserved.
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| 423. |
- Gunnarsson Sarius, Niklas, 1976-, et al.
(author)
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Influence of ultrasound and cathode rotation on the formation of intrinsic stress in Ni films during electrodeposition
- 2011
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In: Transactions of the Institute of Metal Finishing. - : Institute of Metal Finishing. - 0020-2967 .- 1745-9192. ; 89:3, s. 137-142
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Journal article (peer-reviewed)abstract
- The influence of 25 kHz ultrasound and cathode rotation during electroplating of Ni films on Si wafers has been studied with respect to intrinsic stress formation. Current densities from 1.6 A dm-2 up to 28.3 A dm-2 were used in an additive-free Ni sulphamate electrolyte. In general more efficient agitation by either ultrasound or cathode rotation was found to reduce intrinsic stress towards compressive levels compared to conventional agitation with an electrolyte circulation pump. Further more, intrinsic stresses become less dependent on changes in current density. The latter effect is most pronounced for ultrasonic agitation. Structure analysis of samples deposited by ultrasonic agitation show dense deposits with initially smaller grains at high ultrasonic effect. Locally increased temperature at the substrates surface could be an important effect of ultrasound agitation.
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| 424. |
- Gunnarsson Sarius, Niklas, 1976-, et al.
(author)
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Ni and Ti diffusion barrier layers between Ti-Si-C-Ag nanocomposite coatings and Cu-based substrates
- 2012
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In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 206:8-9, s. 2558-2565
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Journal article (peer-reviewed)abstract
- Sputtered Ni and Ti layers were investigated as substitutes for electroplated Ni as adiffusion barrier between Ti-Si-C and Ti-Si-C-Ag nanocomposite coatings and Cu orCuSn substrates. Samples were subjected to thermal annealing studies by exposure to400 ºC during 11 h. Dense diffusion barrier and coating hindered Cu from diffusing tothe surface. This condition was achieved for electroplated Ni in combination withmagnetron-sputtered Ti-Si-C and Ti-Si-C-Ag layers deposited at 230 ºC and 300 ºC,and sputtered Ti or Ni layers in combination with Ti-Si-C-Ag deposited at 300 ºC.
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| 425. |
- Haddad, Noël, et al.
(author)
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Dielectric properties of Ti2AlC and Ti2AlN MAX phases : The conductivity anisotropy
- 2008
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 104:2
-
Journal article (peer-reviewed)abstract
- The optical properties of Ti2AlN and Ti2AlC were determined in the 2-80 eV energy range by electron energy loss spectroscopy and in the visible-ultraviolet range, from 1.6 to 5.5 eV, by spectroscopic ellipsometry. Both experimental techniques are angular resolved and in very good agreement over their overlapping energy range. We observe a dependence of the dielectric function as a function of the crystallographic orientation of the crystals. In particular, we notice a shift of the energy position of the plasmon absorption of Ti2AlC with respect to Ti2AlN. Moreover, a drastic change is also observed in the shape of the dielectric function as a function of the composition (or valence electron concentration). The dielectric functions are fitted to an empirical semiclassic Drude-Lorentz model to obtain physical parameters such as the relaxation times. These microscopic parameters are then used in a macroscopic model to yield the transport properties such as the static conductivity as function of the crystal orientation. Ti 2AlN is found to be a better conductor than Ti2AlC in all orientations, which is consistent with experimental measurements. A comparison of the electrical and optical properties of these two compounds is made in terms of different electronic properties and interband-intraband transitions deduced from our model. © 2008 American Institute of Physics.
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| 426. |
- Halim, Joseph, 1985-
(author)
-
Synthesis and Characterization of 2D Nanocrystals and Thin Films of Transition Metal Carbides (MXenes)
- 2014
-
Licentiate thesis (other academic/artistic)abstract
- Two dimensional (2D) materials have received growing interest because of their unique properties compared to their bulk counterparts. Graphene is the archetype 2D solid, but other materials beyond graphene, such as MoS2 and BN have become potential candidates for several applications. Recently, a new family of 2D materials of early transition metal carbides and carbonitrides (Ti2CTx, Ti3C2Tx, Ti3CNTx, Ta4C3Tx, and more), labelled MXenes, has been discovered, where T stands for the surface-terminating groups.Before the present work, MXenes had only been synthesized in the form of exfoliated and delaminated powders, which is not suitable for electronic applications. In this thesis, I demonstrate the synthesis of MXenes as epitaxial thin films, a more suitable form for electronic and photonic applications. Results show that 2D epitaxial Ti3C2Tx films - produced by HF and NH4HF2 etching of magnetron sputter-grown Ti3AlC2 - exhibit metallic conductive behaviour down to 100 K and are 90% transparent to light in the visible-infrared range. The results from this work may open the door for MXenes as potential candidates for transparent conductive electrodes as well as in electronic, photonic and sensing applications.MXenes have been shown to intercalate cations and molecules between their layers that in turn can alter the surface termination groups. There is therefore a need to study the surface chemistries of synthetized MXenes to be able to study the effect of intercalation as well as altering the surface termination groups on the electronic structure and chemical states of the elements present in MXene layers. X-ray Photoelectron Spectroscopy (XPS) in-depth characterization was used to investigate surface chemistries of Ti3C2Tx and Ti2CTx. This thesis includes the discussion of the effect of Ar+ sputtering and the number of layers on the surface chemistry of MXenes. This study serves as a baseline for chemical modification and tailoring of the surface chemistry groups to potential uses and applications.New MXene phases, Nb2CTx and V2CTx, are shown in this thesis to be produced from HF chemical etching of Nb2AlC and V2AlC powders. Characterization of the produced MXenes was carried out using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Transmission Electron Microscope (TEM) and XPS. Nb2CTx and V2CTx showed promising performance as electrodes for Li-ion batteries.In this thesis, electrochemical etching was used in an attempt to produce 2D metal carbides (MXene) from their ternary metal carbides, Ti3SiC2, Ti3AlC2 and Ti2AlC MAX phases. MAX phases in the form of highly dense bulk produced by Hot Isostatic Press. Several etching solutions were used such as HF, NaCl and HCl. Unlike the HF chemical etching of MAX phases, which results in MXenes, the electrochemical etching resulted in Carbide Derived Carbon (CDC). Here, I show the characterization of the produced CDC using several techniques such as XRD, TEM, Raman spectroscopy, and XPS. Electrochemical characterization was performed in the form of cyclic voltammetry, which sheds light on the etching mechanism.
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| 427. |
- Halim, Joseph, 1985-
(author)
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Synthesis and transport properties of 2D transition metal carbides (MXenes)
- 2018
-
Doctoral thesis (other academic/artistic)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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| 428. |
- Halim, Joseph, et al.
(author)
-
Transparent Conductive Two-Dimensional Titanium Carbide Epitaxial Thin Films
- 2014
-
In: Chemistry of Materials. - : American Chemical Society. - 0897-4756 .- 1520-5002. ; 26:7, s. 2374-2381
-
Journal article (peer-reviewed)abstract
- Since the discovery of graphene, the quest for two-dimensional (2D) materials has intensified greatly. Recently, a new family of 2D transition metal carbides and carbonitrides (MXenes) was discovered that is both conducting and hydrophilic, an uncommon combination. To date MXenes have been produced as powders, flakes, and colloidal solutions. Herein, we report on the fabrication of similar to 1 x 1 cm(2) Ti3C2 films by selective etching of Al, from sputter-deposited epitaxial Ti3AlC2 films, in aqueous HF or NH4HF2. Films that were about 19 nm thick, etched with NH4HF2, transmit similar to 90% of the light in the visible-to-infrared range and exhibit metallic conductivity down to similar to 100 K. Below 100 K, the films resistivity increases with decreasing temperature and they exhibit negative magnetoresistance-both observations consistent with a weak localization phenomenon characteristic of many 2D defective solids. This advance opens the door for the use of MXenes in electronic, photonic, and sensing applications.
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| 429. |
- Halim, Joseph, et al.
(author)
-
X-ray Photoelectron Spectroscopy Characterization of Two-Dimensional Titanium Metal Carbides (MXenes)
- 2014
-
Other publication (other academic/artistic)abstract
- Herein, we report X-ray Photoelectron Spectroscopy (XPS) analysis for cold pressed exfoliated 2D nanocrystals of transition metal carbides, MXenes. MXenes are a recently discovered family of 2D materials produced by selective chemical etching of the A element from MAX phases which are ternary metal carbides and nitrides. The latter has the formula of Mn+1AXn, where M is an early transition metal, A is an A-group element, and X is C and/or N. This study is a comparison between two MXenes, Ti3C2Tx and Ti2CTx, where Tx stands for surface termination groups such as –O, –OH, and –F. Ti3C2Tx and Ti2CTx were prepared by immersion of Ti3AlC2 and Ti2AlC powders in 50% conc. HF. A thorough XPS analysis was performed through peak fitting of high resolution XPS spectra and valence band, VB, spectra analysis. The effect of Ar sputtering as well as the number of layers n was the primarily interest of this study. According to the peak fitting analysis, both phases contain the following species, Ti–C, C–C, Ti–F, Ti–O and Ti–OH resulting in the following chemical formulas: Ti3C2(OH)x(O)y(F)z and Ti2C(OH)x(O)y(F)z. Comparing the VB spectra with the DOS calculations show the valance band spectra is actually a mixture of MXene with various terminations of OH, O and F. Before Ar+ sputtering both phases show a large percentage of fluorinated-TiO2 which is due to MXene surface oxidation as well as CHx, C-O and COO groups arising from either surface contaminations or due to drying the etched powders in ethanol after washing the powder of the HF acid. According to the VB spectra, it is shown that the fluorinated TiO2 is actually a mixture of anatase and rutile. The number of layers, n, also plays a role; the lower n, the more the MXene is prone to oxidation.
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| 430. |
- Hallen, A, et al.
(author)
-
Damage evolution in Al-implanted 4H SiC
- 2000
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In: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 338-3, s. 869-872
-
Journal article (peer-reviewed)abstract
- The build-up of damage in 4H SiC epitaxial layers implanted with 100 or 180 keV Al ions in the dose range of 10(13) to 10(15) cm(-2) has been studied by transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy in the channeling mode (c-RBS). Implantations have been done at temperatures between room temperature and 800 degreesC and the samples have been analysed after implantation and after post implant anneals. In as implanted samples channeling results show that a major part of the damage can be avoided already at implantations at 200 degreesC, but complete removal of damage is not possible even at an implantation temperature of 800 degreesC. After post implant annealing at typically 1600 degreesC a distribution of planar faults are seen by TEM. The size is around 10 nm, but increases with increasing annealing temperature.
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