A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

We report the connection between the stacking order and magnetic properties of bilayer CrI3 using first-principles calculations. We show that the stacking order defines the magnetic ground state. By changing the interlayer stacking order, one can tune the interlayer exchange interaction between antiferromagnetic and ferromagnetic. To measure the predicted stacking-dependent magnetism, we propose using linear magnetoelectric effect. Our results not only gives a possible explanation for the observed antiferromagnetism in bilayer CrI3 but also have direct implications in heterostructures made of two-dimensional magnets.

Stacking-dependent magnetism in bilayer CrI3

MoSi2N4 is a recently developed 2D material that exhibits remarkable thermal, mechanical, electronic, and optical properties. We used first principles calculations to study the structural and electronic properties of organic molecule doped MoSi 2N4 monolayers. Effective p-doping was achieved by molecular doping with tetracyanoquinodimethane and tetracyanoethylene, while n-doping was achieved by molecular doping with tetrathiafulvalene. The doping gap of tetrathiafulvalene-doped MoSi2N4 was successfully modulated by the application of an external electric field, which resulted in effective n-doping. Furthermore, molecular doping injects additional carriers into the host, which is beneficial for enhancing the performance of MoSi2N4 in nanoelectronic devices. Our results demonstrate the importance of molecular doping in tuning the electronic properties of MoSi 2N4 and broadening its applications.

Tuning the electronic properties of MoSi2N4 by molecular doping: A first principles investigation

Tunable electronic and magnetic properties of MoSi2N4 monolayer via vacancy defects, atomic adsorption and atomic doping

Very recently, a novel two-dimension (2D) MXene, MoSi2N4, was successfully synthesized with excellent ambient stability, high carrier mobility, and moderate band gap (2020 Science 369 670). In this work, the intrinsic lattice thermal conductivity of monolayer MoSi2N4 is predicted by solving the phonon Boltzmann transport equation based on the first-principles calculations. Despite the heavy atomic mass of Mo and complex crystal structure, the monolayer MoSi2N4 unexpectedly exhibits a quite high lattice thermal conductivity over a wide temperature range between 300 to 800 K. At 300 K, its in-plane lattice thermal conductivity is 224 Wm−1 K−1. The detailed analysis indicates that the large group velocities and small anharmonicity are the main reasons for its high lattice thermal conductivity. We also calculate the lattice thermal conductivity of monolayer WSi2N4, which is only a little smaller than that of MoSi2N4. Our findings suggest that monolayer MoSi2N4 and WSi2N4 are potential 2D materials for thermal transport in future nano-electronic devices.

https://iopscience.iop.org/article/10.1088/1367-2630/abe8f7/pdf

High intrinsic lattice thermal conductivity in monolayer MoSi2N4

Janus two-dimensional (2D) materials have attracted much attention as they possess unique properties caused by their out-of-plane asymmetry, and have been achieved in many 2D families. In this work, Janus monolayers are predicted for a new 2D MA2Z4 family by means of first-principles calculations, of which MoSi2N4 and WSi2N4 have been synthesized experimentally (Science, 2020, 369, 670–674). The predicted MSiGeN4 (M = Mo and W) monolayers exhibit dynamical, thermodynamic and mechanical stability, and they are indirect band-gap semiconductors. The inclusion of spin–orbit coupling (SOC) gives rise to Rashba-type spin splitting, which is observed in the valence bands, which are different from the common conduction bands. The calculated results show valley polarization due to SOC, together with inversion symmetry breaking. It is found that MSiGeN4 (M = Mo and W) monolayers have much higher electron mobilities with respect to the hole mobilities. Both in-plane and much weaker out-of-plane piezoelectric polarizations can be observed when a uniaxial strain is applied in the basal plane. The values of the piezoelectric strain coefficient d11 of the Janus MSiGeN4 (M = Mo and W) monolayers fall in between those of the MSi2N4 (M = Mo and W) and MGe2N4 (M = Mo and W) monolayers, as expected. It is proved that strain can tune the positions of the valence band maximum (VBM) and the conduction band minimum (CBM), and can enhance the strength of the conduction band convergence caused by compressive strain. It is also found that tensile biaxial strain can enhance the d11 of the MSiGeN4 (M = Mo and W) monolayers, and the compressive strain can improve the d31 (absolute values). Our predicted MSiGeN4 (M = Mo and W) monolayers, as derivatives of the 2D MA2Z4 family, enrich the field of Janus 2D materials, and these results can motivate related experimental work.

Predicted septuple-atomic-layer Janus MSiGeN4 (M = Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities

The septuple-atomic-layer VSi2P4 with the same structure of experimentally synthesized MoSi2N4 is predicted to be a spin-gapless semiconductor (SGS) with the generalized gradient approximation (GGA). In this work, the biaxial strain is applied to tune the electronic properties of VSi2P4, and it spans a wide range of properties upon increasing the strain from a ferromagnetic metal (FMM) to SGS to a ferromagnetic semiconductor (FMS) to SGS to a ferromagnetic half-metal (FMHM). Due to broken inversion symmetry, the coexistence of ferromagnetism and piezoelectricity can be achieved in FMS VSi2P4 with the strain range of 0% to 4%. The calculated piezoelectric strain coefficients d11 for 1%, 2% and 3% strains are 4.61 pm V-1, 4.94 pm V-1 and 5.27 pm V-1, respectively, which are greater than or close to a typical value of 5 pm V-1 for bulk piezoelectric materials. Finally, similar to VSi2P4, the coexistence of piezoelectricity and ferromagnetism can be realized by strain in the VSi2N4 monolayer. Our works show that VSi2P4 in the FMS phase with intrinsic piezoelectric properties can have potential applications in spin electronic devices.

Coexistence of intrinsic piezoelectricity and ferromagnetism induced by small biaxial strain in septuple-atomic-layer VSi2P4.

Motived by experimentally synthesized $\mathrm{MoSi_2N_4}$ (\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020})), the intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W) are studied by density functional theory (DFT). Among the six monolayers, the $\mathrm{CrSi_2N_4}$ has the best piezoelectric strain coefficient $d_{11}$ of 1.24 pm/V, and the second is 1.15 pm/V for $\mathrm{MoSi_2N_4}$. Taking $\mathrm{MoSi_2N_4}$ as a example, strain engineering is applied to improve $d_{11}$. It is found that tensile biaxial strain can enhance $d_{11}$ of $\mathrm{MoSi_2N_4}$, and the $d_{11}$ at 4\% can improve by 107\% with respect to unstrained one. By replacing the N by P or As in $\mathrm{MoSi_2N_4}$, the $d_{11}$ can be raise substantially. For $\mathrm{MoSi_2P_4}$ and $\mathrm{MoSi_2As_4}$, the $d_{11}$ is as high as 4.93 pm/V and 6.23 pm/V, which is mainly due to smaller $C_{11}-C_{12}$ and very small minus or positive ionic contribution to piezoelectric stress coefficient $e_{11}$ with respect to $\mathrm{MoSi_2N_4}$. The discovery of this piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ enables active sensing, actuating and new electronic components for nanoscale devices, and is recommended for experimental exploration.

Intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W).

Structure effect on intrinsic piezoelectricity in septuple-atomic-layer MSi2N4 (M=Mo and W)

The septuple-atomic-layer $\mathrm{VSi_2P_4}$ with the same structure of experimentally synthesized $\mathrm{MoSi_2N_4}$ is predicted to be a spin-gapless semiconductor (SGS). In this work, the biaxial strain is applied to tune electronic properties of $\mathrm{VSi_2P_4}$, and it spans a wide range of properties upon the increasing strain from ferromagnetic metal (FMM) to SGS to ferromagnetic semiconductor (FMS) to SGS to ferromagnetic half-metal (FMHM). Due to broken inversion symmetry, the coexistence of ferromagnetism and piezoelectricity can be achieved in FMS $\mathrm{VSi_2P_4}$ with strain range of 0\% to 4\%. The calculated piezoelectric strain coefficients $d_{11}$ for 1\%, 2\% and 3\% strains are 4.61 pm/V, 4.94 pm/V and 5.27 pm/V, respectively, which are greater than or close to a typical value of 5 pm/V for bulk piezoelectric materials. Finally, similar to $\mathrm{VSi_2P_4}$, the coexistence of piezoelectricity and ferromagnetism can be realized by strain in the $\mathrm{VSi_2N_4}$ monolayer. Our works show that $\mathrm{VSi_2P_4}$ in FMS phase with intrinsic piezoelectric properties can have potential applications in spin electronic devices.

Wen-Qi Mu

Papers

Predicted septuple-atomic-layer Janus MSiGeN4 (M = Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities

Coexistence of intrinsic piezoelectricity and ferromagnetism induced by small biaxial strain in septuple-atomic-layer VSi2P4.

Intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W).

Structure effect on intrinsic piezoelectricity in septuple-atomic-layer MSi2N4 (M=Mo and W)

Coexistence of intrinsic piezoelectricity and ferromagnetism induced by small biaxial strain in septuple-atomic-layer $\mathrm{VSi_2P_4}$