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Surface modification is used to dramatically alter the thermal properties of a bulk metallic material. Thermal barrier coatings (TBCs) are typically applied using spray deposition or laser-based techniques to create a ceramic coating on a metal substrate. In this study, an effective TBC is created directly on a metallic substrate by inducing surface chemical reactions. Aluminum-zirconium (Al-Zr) substrates are used to induce surface-limited reactions that produce a 75-80% decrease in bulk thermal conductivity and diffusivity, respectively. The substrates are cylindrical disks 12.6 mm diameter and 2 mm thickness. Thermal properties are measured using laser flash analysis (LFA) at incrementally elevated temperatures. Focused ion beam (FIB) slicing of the substrate coupled with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) show that the substrate oxidized only along the outer 20 μm of the bulk surface. The layer thickness is significantly less than typical TBCs that can range from 50 to 300 μm yet the 20 μm coating still achieves a dramatic reduction in thermal transport properties. Additionally, thermal analysis reveals a sequence of exothermic reactions starting at 439 °C that include both intermetallic (i.e., ZrAl3) and oxidation (i.e., Al2O3 and ZrO) reactions suggesting continuous surface bonding at the coating-metal interface. The onset of exothermic activity coincides with the transition in thermal properties measured using LFA. These results show that surface oxidation reactions could be used to dramatically alter the thermal transport properties of a metal substrate.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe(2) crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T′ semimetallic phase at high temperatures. Alloying MoTe(2) with WTe(2) reduces the energy barrier between these two phases, while also allowing access to the T(d) Weyl semimetal phase. The Mo(1–x) W(x)Te(2) alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe(2)-WTe(2) system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk Mo(1–x)W(x)Te(2) alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T′, and T(d) structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe(2)–WTe(2) system, including single-phase 2H, 1T′, and T(d) regions, as well as a two-phase 1T′ + T(d) region. Disorder arising from compositional fluctuations in Mo(1–x)W(x)Te(2) alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T′-MoTe(2) mode and the enhancement of a double-resonance Raman process in 2H-Mo(1–x) W(x)Te(2) alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo(1–x)W(x)Te(2) alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

Atom probe tomography (APT)-based isotopic analyses are becoming increasingly attractive for analysis applications requiring small volumes of material and sub-micrometer length scales, such as isotope geochemistry, nuclear safety, and materials science. However, there is an open question within the atom probe community as to the reliability of atom probe isotopic and elemental analyses. Using our proposed analysis guidelines, in conjunction with an empirical calibration curve and a machine learning-based adaptive peak fitting algorithm, we demonstrate accurate and repeatable uranium isotopic analyses, via atom probe mass spectrometry, on U3O8 isotopic reference materials. By using isotopic reference materials, each measured isotopic abundance value could be directly compared to a known certified reference value to permit a quantitative statement of accuracy. The isotopic abundance measurements for 235U and 238U in each individual APT sample were consistently within ±1.5% relative to the known reference values. The accuracy and repeatability are approaching values consistent with measurements limited primarily by Poisson counting statistics, i.e., the number of uranium atoms recorded.

High-volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultrathin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor-phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metal–organic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)bis(dimethylamido)molybdenum and diethyl disulfide, we deposit MoS2 films from ∼1 nm to ∼25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-...

Atom probe tomography (APT) can theoretically deliver accurate chemical and isotopic analyses at a high level of sensitivity, precision, and spatial resolution. However, empirical APT data often contain significant biases that lead to erroneous chemical concentration and isotopic abundance measurements. The present study explores the accuracy of quantitative isotopic analyses performed via atom probe mass spectrometry. A machine learning-based adaptive peak fitting algorithm was developed to provide a reproducible and mathematically defensible means to determine peak shapes and intensities in the mass spectrum for specific ion species. The isotopic abundance measurements made with the atom probe are compared directly with the known isotopic abundance values for each of the materials. Even in the presence of exceedingly high numbers of multi-hit detection events (up to 80%), and in the absence of any deadtime corrections, our approach produced isotopic abundance measurements having an accuracy consistent with values limited predominantly by counting statistics.

We study the crystal symmetry of few-layer 1T′ MoTe2 using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T′ MoTe2 is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T′ MoTe2 and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation.

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo(0.91)W(0.09)Te(2), a semiconducting alloy of MoTe(2), using Raman, photoluminescence (PL), and optical absorption measurements. Mo(0.91)W(0.09)Te(2) transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe(2) counterpart. We apply tensile strain, for the first time, to monolayer MoTe(2) and Mo(0.91)W(0.09)Te(2) to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe(2) and Mo(0.91)W(0.09)Te(2) extend the range of band gaps of TMDC monolayers further into the near infrared, an important attribute for potential applications in optoelectronics.

High volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultra-thin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metalorganic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)-bis(dimethylamido)molybdenum and diethyl disulfide we deposit MoS2 films from ≈ 1 nm to ≈ 25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-level uniformity are demonstrated on 50 mm quartz wafers. Process chemistry and impurity incorporation from precursors are also discussed. This low-temperature and fast process highlights the opportunities presented by metalorganic reagents in the controlled synthesis of TMDs.

The structural polymorphism intrinsic to select transition metal dichalcogenides provides exciting opportunities for engineering novel devices. Of special interest are memory technologies that rely upon controlled changes in crystal phase, collectively known as phase change memories (PCMs). MoTe$_2$ is ideal for PCMs as the ground state energy difference between the hexagonal (2H, semiconducting) and monoclinic (1T’, metallic) phases is minimal. This energy difference can be made arbitrarily small by substituting W for Mo on the metal sublattice, thus improving PCM performance. Therefore, understanding the properties of Mo$_{1-x}$W$_x$Te$_2$ alloys across the entire compositional range is vital for the technological application of these versatile materials. We combine Raman spectroscopy with aberration-corrected scanning transmission electron microscopy and x-ray diffraction to explore the MoTe$_2$-WTe$_2$ alloy system. The results of these studies enable the construction of the complete alloy phase diagram, while polarization-resolved Raman measurements provide phonon mode and symmetry assignments for all compositions. Temperature-dependent Raman measurements indicate a transition from 1T’-MoTe$_2$ to a distorted orthorhombic phase (T$_d$) below 250 K and facilitate identification of the anharmonic contributions to the optical phonon modes in bulk MoTe$_2$ and Mo$_{1-x}W$_x$Te$_2$ alloys. We also identify a Raman-forbidden MoTe$_2$ mode that is activated by compositional disorder and find that the main WTe$_2$ Raman peak is asymmetric for x

We introduce a simple criterion to identify two-dimensional (2D) materials based on the comparison between experimental lattice constants and lattice constants mainly obtained from Materials-Project (MP) density functional theory (DFT) calculation repository. Specifically, if the relative difference between the two lattice constants for a specific material is greater than or equal to 5%, we predict them to be good candidates for 2D materials. We have predicted at least 1356 such 2D materials. For all the systems satisfying our criterion, we manually create single layer systems and calculate their energetics, structural, electronic, and elastic properties for both the bulk and the single layer cases. Currently the database consists of 1012 bulk and 430 single layer materials, of which 371 systems are common to bulk and single layer. The rest of calculations are underway. To validate our criterion, we calculated the exfoliation energy of the suggested layered materials, and we found that in 88.9% of the cases the currently accepted criterion for exfoliation was satisfied. Also, using molybdenum telluride as a test case, we performed X-ray diffraction and Raman scattering experiments to benchmark our calculations and understand their applicability and limitations. The data is publicly available at the website http://www.ctcms.nist.gov/~knc6/JVASP.html.

Molybdenum ditelluride (MoTe2), which can exist in a semiconducting prismatic hexagonal (2H) or a metallic distorted octahedral (1T') phases, is one of the very few materials that exhibit metal-semiconductor transition. Temperature-driven 2H – 1T’ phase transition in bulk MoTe2 occurs at high temperatures (above ~900 C) and it is usually accompanied by Te loss. The latter can exacerbate the control over reversibility of the phase transition. Here, we study effects of high-temperature annealing on phase transition in MoTe2 single crystals. First, MoTe2 were grown in sealed evacuated quartz ampoules from polycrystalline MoTe2 powder in an iodine-assisted chemical vapor transport process at 1000 C. The 2H and 1T’ phases were stabilized by controlling the cooling rate after the growth. In particular, slow cooling at 10 C/h rate yielded the 2H phase whereas the 1T’ phase was stabilized by ice-water quenching. Next, the phase conversion was achieved by annealing MoTe2 single crystals in vacuum-sealed ampoules at 1000 C with or without additional poly-MoTe2 powder followed by fast or slow cooling. Similarly to the CVT growth, slow cooling and quenching consistently produced 2H and 1T’ phases, respectively, regardless of the initial MoTe2 crystal structure. We will discuss structural and optical properties of the as-grown and phase-converted MoTe2 single crystals using TEM, SEM/EDS, XRD, XPS and Raman. Electrical characteristics of two-terminal devices made from metallic 1T’ and bottom-gated FETs made from 2H exfoliated crystals will also be presented.

We study the crystal symmetry of few-layer 1T' MoTe

The need for low-cost high-performance broadband photon detection with sensitivity in the near infrared (NIR) has driven interest in new materials that combine high absorption with traditional electronic infrastructure (CMOS) compatibility. Here, we demonstrate a facile, low-cost and scalable, catalyst-free one-step solution-processed approach to grow one-dimensional Sb2Se3 nanostructures directly on flexible substrates for high-performance NIR photodetectors. Structural characterization and compositional analyses reveal high-quality single-crystalline material with orthorhombic crystal structure and a near-stoichiometric Sb/Se atomic ratio. We measure a direct band gap of 1.12 eV, which is consistent with predictions from theoretical simulations, indicating strong NIR potential. The fabricated metal-semiconductor-metal photodetectors exhibit fast response (on the order of milliseconds) and high performance (responsivity ~ 0.27 A/W) as well as excellent mechanical flexibility and durability. The results demonstrate the potential of molecular-ink-based Sb2Se3 nanostructures for flexible electronic and broadband optoelectronic device applications.

We examine anharmonic contributions to the optical phonon modes in bulk $T_d$-MoTe$_2$ through temperature-dependent Raman spectroscopy. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature in agreement with the Gr\"{u}neisen model. However, below 100 K we observe nonlinear temperature-dependent frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near $T\approx 250$ K that correlate well with the $T_d \to 1T^\prime$ structural phase transition. These results suggest that phonon-phonon interactions can dominate anharmonic contributions at low temperatures in bulk $T_d$-MoTe$_2$, an experimental regime that is currently receiving attention in efforts to understand Weyl semimetals., Comment: 14 pages, 3 figures, 1 table

We study the symmetry properties of 1T' MoTe2 using polarized Raman and second harmonic generation. We find that while the inversion symmetry is broken for even numbers of layers, the Raman modes are nearly unchanged.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe$_2$ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T$^\prime$ semimetallic phase at high temperatures. Alloying MoTe$_2$ with WTe$_2$ reduces the energy barrier between these two phases, while also allowing access to the T$_d$ Weyl semimetal phase. The MoWTe$_2$ alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe$_2$-WTe$_2$ system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe$_2$ alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T$^\prime$, and T$_d$ structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe$_2$-WTe$_2$ system, including single-phase 2H, 1T$^\prime$, and T$_d$ regions, as well as a two-phase 1T$^\prime$ + T$_d$ region. Disorder arising from compositional fluctuations in MoWTe$_2$ alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T$^\prime$-MoTe$_2$ mode and the enhancement of a double-resonance Raman process in 2H-MoWTe$_2$ alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in MoWTe$_2$ alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.