| 1. |
- Lu, Jun, 1962-, et al.
(author)
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Tin+1Cn MXenes with fully saturated and thermally stable Cl terminations
- 2019
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In: Nanoscale Advances. - : Royal Society of Chemistry. - 2516-0230. ; 1:9, s. 3680-3685
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Journal article (peer-reviewed)abstract
- MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the materials properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 degrees C. Thus, we present an attractive new termination that widely expands the MXenes functionalization space and enables new applications.
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| 2. |
- Ding, Haoming, et al.
(author)
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Synthesis of MAX phases Nb2CuC and Ti2(Al0.1Cu0.9)N by A-site replacement reaction in molten salts
- 2019
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In: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 7:12, s. 510-516
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Journal article (peer-reviewed)abstract
- New MAX phases Ti2(AlxCu1−x)N and Nb2CuC were synthesized by A-site replacement by reacting Ti2AlN and Nb2AlC, respectively, with CuCl2 or CuI molten salt. X-ray diffraction, scanning electron microscopy, and atomically resolved scanning transmission electron microscopy showed complete A-site replacement in Nb2AlC, which lead to the formation of Nb2CuC. However, the replacement of Al in Ti2AlN phase was only close to complete at Ti2(Al0.1Cu0.9)N. Density-functional theory calculations corroborated the structural stability of Nb2CuC and Ti2CuN phases. Moreover, the calculated cleavage energy in these Cu-containing MAX phases are weaker than in their Al-containing counterparts.The preparation of MAX phases Nb2CuC and Ti2(Al0.1Cu0.9)N were realized by A-site replacement in Ti2AlN and Nb2AlN, respectively.
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| 3. |
- Halim, Joseph, 1985-, et al.
(author)
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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
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In: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 1:6, s. 2455-2460
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Journal article (peer-reviewed)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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| 4. |
- Palisaitis, Justinas, 1983-, et al.
(author)
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On the Structural Stability of MXene and the Role of Transition Metal Adatoms
- 2018
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In: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 10:23, s. 10850-10855
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Journal article (peer-reviewed)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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| 5. |
- Bakhit, Babak, 1983-, et al.
(author)
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Age hardening in superhard ZrB2-rich Zr1-xTaxBy thin films
- 2021
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In: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 191, s. 120-125
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Journal article (peer-reviewed)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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| 6. |
- Bakhit, Babak, 1983-, et al.
(author)
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Self-organized columnar Zr0.7Ta0.3B1.5 core/shell-nanostructure thin films
- 2020
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In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 401
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Journal article (peer-reviewed)abstract
- We recently showed that Zr1−xTaxBy thin films have columnar nanostructure in which column boundaries are B-rich for x < 0.2, while Ta-rich for x ≥ 0.2. Layers with x ≥ 0.2 exhibit higher hardness and, simultaneously, enhanced toughness. Here, we determine the atomic-scale nanostructure of sputter-deposited columnar Zr0.7Ta0.3B1.5 thin films. The columns, 95 ± 17 Å, are core/shell nanostructures in which 80 ± 15-Å cores are crystalline hexagonal-AlB2-structure Zr-rich stoichiometric Zr1−xTaxB2. The shell structure is a narrow dense, disordered region that is Ta-rich and highly B-deficient. The cores are formed under intense ion mixing via preferential Ta segregation, due to the lower formation enthalpy of TaB2 than ZrB2, in response to the chemical driving force to form a stoichiometric compound. The films with unique combination of nanosized crystalline cores and dense metallic-glass-like shells provide excellent mechanical properties.
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| 7. |
- Chang, Jui-Che, et al.
(author)
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HiPIMS-grown AlN buffer for threading dislocation reduction in DC-magnetron sputtered GaN epifilm on sapphire substrate
- 2023
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In: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 217
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Journal article (peer-reviewed)abstract
- Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).
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| 8. |
- Dorri, Samira, 1988-, et al.
(author)
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Enhanced quality of single crystal CrBx/TiBy diboride superlattices by controlling boron stoichiometry during sputter deposition
- 2024
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In: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584.
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Journal article (peer-reviewed)abstract
- Single-crystal CrB2/TiB2 diboride superlattices with well-defined layers are promising candidates for neutron optics. However, excess B in sputter-deposited TiBy using a single TiB2 target deteriorates the structural quality of CrBx/TiBy (0001) superlattices. We study the influence of co-sputtering of TiB2 + Ti on the stoichiometry and crystalline quality of 300-nm-thick TiBy single layers and CrBx/TiBy (0001) superlattices on Al2O3(0001) substrates grown by DC magnetron sputter epitaxy at growth-temperatures TS ranging from 600 to 900 °C. By controlling the relative applied powers to the TiB2 and Ti magnetrons, y could be reduced from 3.3 to 0.9. TiB2.3 grown at 750 °C exhibited epitaxial domains about 10x larger than non-co-sputtered films. Close-to-stoichiometry CrB1.7/TiB2.3 superlattices with modulation periods Λ = 6 nm grown at 750 °C showed the highest single crystal quality and best layer definition. TiB2.3 layers display rough top interfaces indicating kinetically limited growth while CrB1.7 forms flat and abrupt top interfaces indicating epitaxial growth with high adatom mobility.
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| 9. |
- Folkenant, Matilda, et al.
(author)
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Structure and properties of Cr–C/Ag films deposited by magnetron sputtering
- 2015
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In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 281, s. 184-192
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Journal article (peer-reviewed)abstract
- Cr–C/Ag thin films with 0–14 at.% Ag have been deposited by magnetron sputtering from elemental targets. The samples were analyzed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to study their structure and chemical bonding. A complex nanocomposite structure consisting of three phases; nanocrystalline Ag, amorphous CrCx and amorphous carbon is reported. The carbon content in the amorphous carbide phase was determined to be 32–33 at.% C, independent of Ag content. Furthermore, SEM and XPS results showed higher amounts of Ag on the surface compared to the bulk. The hardness and Young's modulus were reduced from 12 to 8 GPa and from 270 to 170 GPa, respectively, with increasing Ag content. The contact resistance was found to decrease with Ag addition, with the most Ag rich sample approaching the values of an Ag reference sample. Initial tribological tests gave friction coefficients in the range of 0.3 to 0.5, with no clear trends. Annealing tests show that the material is stable after annealing at 500 °C for 1 h, but not after annealing at 800 °C for 1 h. In combination, these results suggest that sputtered Cr–C/Ag films could be potentially applicable for electric contact applications.
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| 10. |
- Persson, Per O. Å., 1971-
(author)
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Characterization of Process-related Defects in Silicon Carbide by Electron Microscopy
- 2001
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Doctoral thesis (other academic/artistic)abstract
- Silicon carbide (SiC) is a semiconducting material, which provides advantages compared to other available semiconducting materials. Attractive properties of SiC are the wide bandgap (2.2-3.3 eV), high electric breakdown field (3x106 Vcm-1), high thermal conductivity (5 Wcm-1 K-1) and the chemical stabi!ity. Compared to silicon the electric breakdown field strength is 10 times higher, the thermal conductivity 3 times higher and twice the saturated electron drift velocity. Put this together and the result is fast electronic devices with low losses. However, the successful development of a material for device applications depends very strongly on the processing technology. Starting in 1978, there has been a rapid development in SiC crystal growth. Because SiC is gaining a more important commercial position the demand for knowledge regarding device processing is increasing. The main contribution of the present work relates to the investigation of process-induced structural crystal defects. It is known that structural crystal defects strongly affect device performance and lifetime. For that matter, knowledge about defects, in what the structure is, its nature and origin, is essential in a device optimization scheme. For the growth of SiC theSiC(001)/Si(001) interface was investigated by Molecular dynamics simulation, HREM image simulation and HREM microscopy. Simulated and real images match qualitatively and supports an interface model. Further, transmission electron microscopy have been employed to study the crystallinity and determine phase composition after heat treatment of heavily doped 4H-SiC. A solubility limit of ∼2x 1020 A1/cm3 (1900°C) is extracted, after which A1 containing precipitates are shown to form. By electrochemically etching SiC we find that pores propagate through the etched layers. It is shown that these pores propagate first nearly parallel to the basal planes to gradually change direction and propagate in towards the c-axis. For the development of ion-implantation processes for SiC transmission electron microscopy(TEM) was used to investigate B, C, N, Si, Ar and A1 ion-implanted 4H-SiC epilayers and subsequent defect formation after high temperature annealing with respect to implantation induced damage and ion mass of implanted ions. During the annealing process extrinsic, interstitial type, dislocation loops are formed on the SiC basal plane and their depth distribution can be related to the implanted ions. The investigation reveals that in samples where the implanted and electrically activated ions are substituting for a position in the Si sublattice, generating an excess of interstitial Si, the dislocation loops are more readily formed than in implanted with a specie substituting for C, which generates excess C. It is further shown that for the A1 implanted samples the loops were found to be subject to a conservative bulk ripening process where the average loop radius of the loop distribution increased while simultaneously decreasing the average loop/area density. It was also found that the total interstitial population, bound by loops, converges to an estimated 16 % of the implanted dose. Finally I investigated triangular structural defects which are occasionally generated during long term operation of 4H-SiC pin diodes. These are known to negatively affect the forward and reverse leakage characteristics of the diode. The techniques used for characterizing the structure and formation mechanisms of such defects were synchrotron white beam X-raytopography, scanning electron microscopy, in situ cathodo luminescence and transmission electron microscopy. It is shown from high-resolution images and two-beam tilting experiments that the defect results from glide slip on the (0001) basal plane. The defect is found to consist of a stacking fault bound by two partial dislocations with Burgers vectors 1/3<10 1̅ 0> and l /3<01 1̅ 0>. The fault is a means for stress relaxation in the epilayer, near the contact layer using an existing dislocation as a nucleation source.
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| 11. |
- Petruhins, Andrejs, 1987-
(author)
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Synthesis and characterization of magnetic nanolaminated carbides
- 2018
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Doctoral thesis (other academic/artistic)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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| 12. |
- Thörnberg, Jimmy, et al.
(author)
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Microstructure and materials properties of understoichiometric TiBx thin films grown by HiPIMS
- 2020
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In: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 404
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Journal article (peer-reviewed)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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