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1. Phase transition of Al ₂ O ₃ -encapsulated MoTe ₂ via rapid thermal annealing

   https://doi.org/10.1063/5.0097844  

   Sengupta, Rohan; Dangi, Saroj; Krylyuk, Sergiy; Davydov, Albert V.; Pavlidis, Spyridon (July 2022, Applied Physics Letters)

   Among group VI transition metal dichalcogenides, MoTe 2 is predicted to have the smallest energy offset between semiconducting 2H and semimetallic 1T′ states. This makes it an attractive phase change material for both electronic and optoelectronic applications. Here, we report fast, nondestructive, and full phase change in Al 2 O 3 -encapsulated 2H-MoTe 2 thin films to 1T′-MoTe 2 using rapid thermal annealing at 900 °C. Phase change was confirmed using Raman spectroscopy after a short annealing duration of 10 s in both vacuum and nitrogen ambient. No thickness dependence of the transition temperatures was observed for flake thickness ranging from 1.5 to 8 nm. These results represent a major step forward in understanding the structural phase transition properties of MoTe 2 thin films using external heating and underline the importance of surface encapsulation for avoiding thin film degradation. 

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2. Competing Heat Carriers Leading to Distinctive Cation Concentration Dependent Thermal Conductivity of Amorphous Li _x S ( x  = 0–2) Batteries

   https://doi.org/10.1002/adfm.202214501  

   Gao, Yufei; Fan, Hongzhao; Zhou, Yanguang; Hu, Ming (April 2023, Advanced Functional Materials)

   Abstract Thermal transport in amorphous lithium‐sulfur (a‐Li_xS) is systematically investigated using molecular dynamics and the contributions from different types of heat carriers are quantitatively evaluated. In general, the thermal conductivity (TC) ofa‐Li_xS changes largely by varying the concentration (x) of Li ions ina‐Li_xS. Interestingly, the TC ofa‐Li_xS shows three distinct regimes of dependence on Li concentration. For low Li concentration (x = 0.4–1.2), the TC grows slowly, followed by a rapid increase in TC for medium Li concentration (x = 1.2–1.6), where the growth rate is three times that of the first regime, and finally, the TC is independent of Li concentration (x = 1.6–2.0). The TC enhancement in the first and second regimes is mainly attributed to propagating and non‐propagating vibrational modes ina‐Li_xS, respectively. In contrast, the stable thermal transport regime is governed by the competition between propagating and non‐propagating phonons. These investigations provide quantitative TC data of various polysulfides for shuttling analysis, and a fundamental understanding of the thermal transport mechanism of complexa‐Li_xS structures, which is beneficial for the rational design of thermal management of Li‐S batteries. 

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3. Superconducting properties in doped 2M-WS ₂ from first principles

   https://doi.org/10.1039/D2TC01173E  

   Paudyal, Hari; Margine, Elena R. (April 2022, Journal of Materials Chemistry C)

   A new member of the transition metal dichalcogenide (TMD) family, 2M-WS 2, has been recently discovered and shown to display superconductivity with a critical temperature (Tc) of 8.8 K, the highest Tc among superconducting TMDs at ambient pressure. Using first-principles calculations combined with the Migdal-Eliashberg formalism, we explore how the superconducting properties of 2M-WS 2 can be enhanced through doping. Mo, Nb, and Ta are used as dopants at the W sites, while Se is used at the S sites. We demonstrate that the monotonous decrease in the Tc observed experimentally for Mo and Se doping is due to the decrease in density of states at the Fermi level and the electron–phonon coupling of the low-energy phonons. In addition, we find that a noticeable increase in the electron–phonon coupling could be achieved when doping with Nb and Ta, leading to an enhancement of the Tc of up to 50% compared to the undoped compound. 

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4. Vapor–liquid assisted chemical vapor deposition of Cu ₂ X materials

   https://doi.org/10.1088/2053-1583/ac8435  

   Shehzad, M Arslan; Lee, Yea-Shine; Cheng, Matthew; Lebedev, Dmitry; Tyner, Alexander C; Das, Paul Masih; Gao, Zhangyuan; Goswami, Pallab; dos Reis, Roberto; Hersam, Mark C; et al (August 2022, 2D Materials)

   Abstract Transition metal dichalcogenides (TMDs) are known for their layered structure and tunable functional properties. However, a unified understanding on other transition metal chalcogenides (i.e. M 2 X) is still lacking. Here, the relatively new class of copper-based chalcogenides Cu 2 X (X = Te, Se, S) is thoroughly reported. Cu 2 X are synthesized by an unusual vapor–liquid assisted growth on a Al 2 O 3 /Cu/W stack. Liquid copper plays a significant role in synthesizing these layered systems, and sapphire assists with lateral growth and exfoliation. Similar to traditional TMDs, thickness dependent phonon signatures are observed, and high-resolution atomic images reveal the single phase Cu 2 Te that prefers to grow in lattice-matched layers. Charge transport measurements indicate a metallic nature at room temperature with a transition to a semiconducting nature at low temperatures accompanied by a phase transition, in agreement with band structure calculations. These findings establish a fundamental understanding and thrust Cu 2 Te as a flexible candidate for wide applications from photovoltaics and sensors to nanoelectronics. 

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5. X-ray Diffraction and Equation of State of the C-S-H Room-Temperature Superconductor

   Lamichhane, A.; Kumar, R.; Ahart, M.; Salke, N.; Dasenbrock-Gammon, N; Meng, Y.; Lavina, B.; Chariton, S.; Prakapenka, V. B.; Somayazulu, M.; et al (September 2021, Journal of chemical physics)

   null (Ed.)

   X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C-S-H) room temperature superconductor is derived from previously established van der Waals compounds found in the H2S-H2 and CH4-H2 systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique leading to the superconducting critical temperature Tc of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the Al2Cu-type determined for (H2S)2H2 and (CH4)2H2 that differs from those predicted and observed for the S-H system to these pressures. The formation and stability of the C-S-H compound can be understood in terms of the close similarity in effective volumes of the H2S and CH4 components, and denser carbon-bearing S-H phases may form at higher pressures. The results are crucial for understanding the very high superconducting Tc found in the C-S-H system at megabar pressures. 

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