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1. Emergent layer stacking arrangements in c-axis confined MoTe2

   https://doi.org/10.1038/s41467-023-40528-y  

   Hart, James L.; Bhatt, Lopa; Zhu, Yanbing; Han, Myung-Geun; Bianco, Elisabeth; Li, Shunran; Hynek, David J.; Schneeloch, John A.; Tao, Yu; Louca, Despina; et al (December 2023, Nature Communications)

   Abstract The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe₂possesses two stacking arrangements, the ferroelectric Weyl semimetal T_dphase and the higher-order topological insulator 1T′ phase. However, in thin flakes of MoTe₂, it is unclear if the layer stacking follows the T_d, 1T′, or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe₂layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T′ and T_ddomains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe₂stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe₂flakes. More broadly, this work suggestsc-axis confinement as a method to influence layer stacking in other 2D materials. 

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2. Ultrafast electron diffraction instrument for gas and condensed matter samples

   https://doi.org/10.1063/5.0146577  

   Wang, Yibo; Saha, Sajib Kumar; Li, Tianlin; Xiong, Yanwei; Wilkin, Kyle; Adhikari, Anil; Loes, Michael; Abourahma, Jehad; Hong, Xia; Adenwalla, Shireen; et al (May 2023, Review of Scientific Instruments)

   We report the modification of a gas phase ultrafast electron diffraction (UED) instrument that enables experiments with both gas and condensed matter targets, where a time-resolved experiment with sub-picosecond resolution is demonstrated with solid state samples. The instrument relies on a hybrid DC-RF acceleration structure to deliver femtosecond electron pulses on the target, which is synchronized with femtosecond laser pulses. The laser pulses and electron pulses are used to excite the sample and to probe the structural dynamics, respectively. The new system is added with capabilities to perform transmission UED on thin solid samples. It allows for cooling samples to cryogenic temperatures and to carry out time-resolved measurements. We tested the cooling capability by recording diffraction patterns of temperature dependent charge density waves in 1T-TaS2. The time-resolved capability is experimentally verified by capturing the dynamics in photoexcited single-crystal gold. 

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3. 1T Phase as an Efficient Hole Injection Layer to TMDs Transistors: A Universal Approach to Achieve p-type Contacts

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

   Chen, Zhongfang Hu (January 2018, 2D materials)

   Recently, the fabricated MoS2 field effect transistors (FETs) with 1T-MoS2 electrodes exhibit excellent performance with rather low contact resistance, as compared with those with metals deposited directly on 2H-MoS2 (Kappera et al 2014 Nat. Mater. 13 1128), but the reason for that remains elusive. By means of density functional theory calculations, we investigated the carrier injection at the 1T/2H MoS2 interface and found that although the Schottky barrier height (SBH) values of 1T/2H MoS2 interfaces can be tuned by controlling the stacking patterns, the p-type SBH values of 1T/2H MoS2 interfaces with different stackings are lower than their corresponding n-type SBH values, which demonstrated that the metallic 1T phase can be used as an efficient hole injection layer for 2H-MoS2. In addition, as compared to the n-type Au/MoS2 and Pd/MoS2 contacts, the p-type SBH values of 1T/2H MoS2 interfaces are much lower, which stem from the efficient hole injection between 1T-MoS2 and 2H-MoS2. This can explain the low contact resistance in the MoS2 FETs with 1T-MoS2 electrodes. Notably, the SBH values can be effectively modulated by an external electric field, and a significantly low p-type SBH value can be achieved under an appropriate electric field. We also demonstrated that this approach is also valid for WS2, WSe2 and MoSe2 systems, which indicates that the method can most likely be extended to other TMDs, and thus may open new promising avenues of contact engineering in these materials. 

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4. Dynamical criticality of spin-shear coupling in van der Waals antiferromagnets

   https://doi.org/10.1038/s41467-022-34376-5  

   Zhou, Faran; Hwangbo, Kyle; Zhang, Qi; Wang, Chong; Shen, Lingnan; Zhang, Jiawei; Jiang, Qianni; Zong, Alfred; Su, Yifan; Zajac, Marc; et al (December 2022, Nature Communications)

   Abstract The interplay between a multitude of electronic, spin, and lattice degrees of freedom underlies the complex phase diagrams of quantum materials. Layer stacking in van der Waals (vdW) heterostructures is responsible for exotic electronic and magnetic properties, which inspires stacking control of two-dimensional magnetism. Beyond the interplay between stacking order and interlayer magnetism, we discover a spin-shear coupling mechanism in which a subtle shear of the atomic layers can have a profound effect on the intralayer magnetic order in a family of vdW antiferromagnets. Using time-resolved X-ray diffraction and optical linear dichroism measurements, interlayer shear is identified as the primary structural degree of freedom that couples with magnetic order. The recovery times of both shear and magnetic order upon optical excitation diverge at the magnetic ordering temperature with the same critical exponent. The time-dependent Ginzburg-Landau theory shows that this concurrent critical slowing down arises from a linear coupling of the interlayer shear to the magnetic order, which is dictated by the broken mirror symmetry intrinsic to the monoclinic stacking. Our results highlight the importance of interlayer shear in ultrafast control of magnetic order via spin-mechanical coupling. 

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5. Atomically engineering activation sites onto metallic 1T-MoS2 catalysts for enhanced electrochemical hydrogen evolution

   Huang, Y.C.; Sun, Y.H.; Zheng, X. L.; Aoki, T.; Pattengale, Zhang; Wei, YG*; Gu, J.* (April 2019, Nature communications)

   Engineering catalytic sites at the atomic level provide an opportunity to understand the catalyst’s active sites, which is vital to the development of improved catalysts. Herein, we show a reliable and tunable polyoxometalate (POM) template-based synthetic strategy to atomically engineer metal doping sites onto metallic 1T-MoS2, using Anderson-type POMs (XMo6, X = FeIII, CoIII, or NiII) as precursors. Benefiting from the synergistic effect of doping metals into 1T-MoS2 and the possible tuning effect of the Ni-O-Mo bond, the optimized Ni and O incorporated 1T-MoS2 (NiO@1T-MoS2) catalyst excels in the hydrogen evolution reaction (HER). With a positive onset potential of ~ 0 V and a low overpotential of -46 mV in 1.0 M KOH, its results are comparable to 20% Pt/C. First-principles calculations reveal co-doping Ni and O into 1T-MoS2 assists the processes of both water dissociation and hydrogen generation from their intermediate states. This research will expand on the ability to improve the activities of various catalysts by precisely engineering atomic activation sites to achieve significant electronic modulations and improve atomic utilization efficiencies. 

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