Mechanical Properties of 2D LiInP₂Se₆: Implication for Semiconductor Applications

Metal phosphorus trichalcogenides (MPTCs) are emerging 2D semiconductor materials with unique functional properties that set them apart from other 2D systems. Despite the importance of their mechanical properties for improving the semiconductor device’s durability and performance, as well as for utilizing strain effects to customize material properties and create new functionality, our current understanding of MPTCs’ mechanical behavior is lacking and lags behind our knowledge of their other properties. Here, we use LiInP2Se6 as a model example of MPTCs and report the first experimental measurements of the elastic and plastic (fracture) properties along both in-plane and out-of-plane directions by atomic force microscopy and nanoindentation. Being a 2D material that is entirely inorganic, LiInP2Se6 surprisingly exhibits mechanical properties that resemble those of hybrid organic–inorganic materials rather than pure inorganic 2D materials. It has a soft crystal structure with low elastic moduli, a low difference in in-plane vs out-of-plane mechanical properties, and a combination of elastic and plastic characteristics of hybrid organic–inorganic materials. Our work provides the mechanical information critically needed to mitigate and/or harness the strain effects in LiInP2Se6-based semiconductor devices and sheds light on the mechanical behaviors of MPTCs with indispensable insights

Understanding the Dynamic Liquid-Assisted Chemical Vapor Deposition Growth of Copper Telluride and Its Low-Temperature Phase Transition

Copper telluride is an emerging layered material that has been shown to undergo phase transitions with slight modifications in its stoichiometry (Cu2–xTe) at high temperatures. Using Raman spectroscopy and X-ray diffraction, we complete the spectrum of temperatures and detect a low-temperature phase transition of copper telluride for the first time. Moreover, liquid-assisted chemical vapor deposition (CVD) growth is heavily explored for its potential to grow various crystals at a large scale. However, the role that the liquid precursor plays in these growths remains largely elusive, and a theoretical study is impeded by the bulk amorphous liquid precursor. Here, we experimentally demonstrate how the liquid precursor contributes to the morphological orientations of Cu0.664Te0.336 crystals. Based on this, we propose a growth process in which tellurium supersaturation of the liquid precursor yields nucleation sites both on the surface and internally. Etching of Cu0.664Te0.336 via H2 flow is also achieved during CVD to increase the density of exposed sites. Optimal parameters to control H2 flow to achieve layer-by-layer thinning in geometric crystals are also realized. Our study thereby enhances the understanding of temperature-dependent copper telluride phases and presents liquid-assisted growth as a platform ripe with opportunities for materials engineering

Nanotube Structure of AsPS_4–<i>x</i>Se_<i>x</i> (<i>x</i> = 0, 1)

Single-wall nanotubes of isostructural AsPS4–xSex (x = 0, 1) are grown from solid-state reaction of stoichiometric amounts of the elements. The structure of AsPS4 was determined using single-crystal X-ray diffraction and refined in space group P1̅. The infinite, single-walled AsPS4 nanotubes have an outer diameter of ≈1.1 nm and are built of corner-sharing PS4 tetrahedra and AsS3 trigonal pyramids. Each nanotube is nearly hexagonal, but the ≈3.4 Å distance between S atoms on adjacent nanotubes allows them to easily slide past one another, resulting in the loss of long-range order. Substituting S with Se disrupted the crystallization of the nanotubes, resulting in amorphous products that precluded the determination of the structure for AsPS3Se. 31P solid-state NMR spectroscopy indicated a single unique tetrahedral P environment in AsPS4 and five different P environments all with different degrees of Se substitution in AsPS3Se. Optical absorption spectroscopy revealed an energy band gap of 2.7 to 2.4 eV for AsPS4 and AsPS3Se, respectively. Individual AsPS4 microfibers showed a bulk conductivity of 3.2 × 10–6 S/cm and a negative photoconductivity effect under the illumination of light (3.06 eV) in ambient conditions. Thus, intrinsic conductivity originates from hopping through empty trap states along the length of the AsPS4 nanotubes

Mixed Metal Thiophosphate Fe_2–<i>x</i>Co_<i>x</i>P₂S₆: Role of Structural Evolution and Anisotropy

Metal chalcophosphates, M2P2Q6 (M = transition metals; Q = chalcogen), are notable among the van der Waals materials family for their potential magnetic ordering that can be tuned with an appropriate choice of the metal or chalcogen. However, there has not been a systematic investigation of the basic structural evolution in these systems with alloying of the crystal subunits due to the challenge in the diffusion process of mixing different metal cations in the octahedral sites of M2P2Q6 materials. In this work, the P2S5 flux method was used to enable the synthesis of a multilayered mixed metal thiophosphate Fe2–xCoxP2S6 (x = 0, 0.25, 1, 1.75, and 2) system. Here, we studied the structural, vibrational, and electronic fingerprints of this mixed M2P2Q6 system. Structural and elemental analyses indicate a homogeneous stoichiometry averaged through the sample over multiple layers of Fe2–xCoxP2S6 compounds. It was observed that there is a correlation between the intensity of specific phonon modes and the alloying concentration. The increasing Co alloying concentration shows direct relations to the in-plane [P2S6]4– and out-of-plane P–P dimer vibrations. Interestingly, an unusual nonlinear electronic structure dependence on the metal alloying ratio is found and confirmed by two distinct work functions within the Fe2–xCoxP2S6 system. We believe this work provides a fundamental structural framework for mixed metal thiophosphate systems, which may assist in future studies on electronic and magnetic applications of this emerging class of binary cation materials

Raman Shifts in Two-Dimensional van der Waals Magnets Reveal Magnetic Texture Evolution

Two-dimensional (2D) van der Waals magnets comprise rich physics that can be exploited for spintronic applications. We investigate the interplay between spin–phonon coupling and spin textures in a 2D van der Waals magnet by combining magneto-Raman spectroscopy with cryogenic Lorentz transmission electron microscopy. We find that when stable skyrmion bubbles are formed in the 2D magnet, a field-dependent Raman shift can be observed, and this shift is absent for the 2D magnet prepared in its ferromagnetic state. Correlating these observations with numerical simulations that take into account field-dependent magnetic textures and spin–-phonon coupling in the 2D magnet, we associate the Raman shift to field-induced modulations of the skyrmion bubbles and derive the existence of inhomogeneity in the skyrmion textures over the film thickness

Synthesizing Mono- and Bimetallic 2D Selenophosphates Using a P₂Se₅ Reactive Flux

We have developed a new method for synthesizing mono- and bimetallic two-dimensional selenophosphates by using a reactive flux technique with powder precursors dissolved in a P2Se5 melt. This method allows us to bypass the difficulties commonly encountered in synthesizing these materials. M2P2Se6 (M = Mn, Fe, Cr, Cd, Mg, and Zn) and bimetallic MM′P2Se6 (M = Cu, Ag; M′ = Cr, In) powders were synthesized over 60 h using a P2Se5 reactive flux. As a further demonstration of potential optimization, Mn2P2Se6 powder with minor MnSe impurity was synthesized in 30 min. Reactions of Mn and Cr in P2Se5 flux were analyzed in situ using variable-temperature powder X-ray diffraction to investigate the formation reaction pathways, suggesting single-step formation of Cr2P2Se6 at 775 °C and formation of Mn2P2Se6 at 575 °C through an intermediate MnSe phase. Follow-up experimental syntheses generated crystals of the monometallic family M2P2Se6 (M = Mn, Fe, and Cd) and bimetallic family MM′P2Se6 (M = Li, Cu, Ag; M′ = Cr, In) with area dimensions of several square millimeters using heating profiles of just over 60 h. The homogeneity of bimetallic selenophosphates MM′P2Se6 was confirmed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Rietveld refinement. The crystallinity of the selected materials was characterized by transmission electron microscopy and atomic force microscopy measurements. The work functions of flakes were determined and ranged from 5.25 to 5.72 eV