[Paper Review] Vertical Strain-Induced Modification of the Electrical and Spin Properties of Monolayer MoSi2X4 (X= N, P, As and Sb)

Shoeib Babaee Touski, Nayereh Ghobadi|arXiv (Cornell University)|Jun 13, 2021

2D Materials and Applications9 citations

TL;DR

This study investigates vertical strain engineering in monolayer MoSi2X4 (X = N, P, As, Sb) to tune their electrical and spin properties. Using density functional theory with spin-orbit coupling, it shows that vertical strain induces a maximum bandgap at small tensile strains, reduces bandgaps under compressive strain, and enables control over large spin-splitting in both valence and conduction bands—especially in heavier chalcogenides like MoSi2Sb4, where spin-splitting reaches up to 220 meV at the valence band K-point.

ABSTRACT

In this work, the electrical and spin properties of monolayer MoSi2X4 (X= N, P, As, and Sb) under vertical strain are investigated. The band structures state that MoSi2N4 is an indirect semiconductor, whereas other compounds are direct semiconductors. The vertical strain has been selected to modify the electrical properties. The bandgap shows a maximum and decreases for both tensile and compressive strains. The valence band at K-point displays a large spin-splitting, whereas the conduction band has a negligible splitting. On the other hand, the second conduction band has a large spin-splitting and moves down under vertical strain which leads to a large spin-splitting in both conduction and valence bands edges. The projected density of states along with the projected band structure clarifies the origin of these large spin-splittings. These three spin-splittings can be controlled by vertical strain.

Motivation & Objective

• To explore the effects of vertical strain on the electronic and spin properties of monolayer MoSi2X4 (X = N, P, As, Sb).
• To identify how strain alters bandgap, band edge positions, and effective masses in these 2D semiconductors.
• To investigate the origin and tunability of spin-splitting at K-points in valence and conduction bands.
• To determine the transition strain and pressure at which the bandgap closes, indicating a semiconductor-to-metal transition.
• To analyze the orbital contributions (d-orbitals of Mo, p-orbitals of X and Si) to spin-splitting and band structure.

Proposed method

• Density functional theory (DFT) calculations using the SIESTA package with the PBE functional for exchange-correlation effects.
• Inclusion of spin-orbit coupling (SOC) to accurately model spin-splitting effects in spintronic properties.
• Application of vertical strain (from -22% to +22%) to systematically tune the interlayer spacing and electronic structure.
• Calculation of projected density of states (PDOS) and projected band structures to identify orbital contributions to bands.
• Analysis of bandgap evolution, effective mass, and spin-splitting at K-point in valence and conduction bands under strain.
• Use of charge density analysis to examine electron localization and bonding character in the strained systems.

Experimental results

Research questions

• RQ1 How does vertical strain affect the bandgap and band edge alignment in monolayer MoSi2X4 (X = N, P, As, Sb)?
• RQ2 What is the origin of large spin-splitting in the valence and conduction bands, and how do orbital contributions (Mo d, X p, Si p) influence it?
• RQ3 How does the spin-splitting at the K-point of the valence band and conduction bands evolve under compressive and tensile strain?
• RQ4 At what strain does the semiconductor-to-metal transition occur, and how does this transition strain vary with X-element?
• RQ5 Can vertical strain be used to control both electrical and spin properties simultaneously in these 2D materials?

Key findings

• MoSi2N4 exhibits an indirect bandgap from Γ (valence) to K (conduction), while MoSi2P4, MoSi2As4, and MoSi2Sb4 are direct bandgap semiconductors at K-point.
• The bandgap reaches a maximum at small tensile strains (~+5%) and decreases under both compressive and larger tensile strains, closing at compressive strains of approximately -10% for MoSi2P4, MoSi2As4, and MoSi2Sb4.
• MoSi2N4 shows a larger transition strain of -22% and a higher transition pressure of 25.3 GPa compared to 7.3–9.1 GPa for the other compounds.
• The valence band at the K-point exhibits large spin-splitting (up to 220 meV) in MoSi2Sb4, increasing with heavier X elements due to enhanced spin-orbit coupling.
• The second conduction band shows significant spin-splitting (up to 151 meV in MoSi2Sb4), which decreases under tensile strain and becomes the lowest conduction band at high tensile strain due to band inversion.
• Spin-splitting in the first conduction band (λK,C1) increases with strain, while that in the second conduction band (λK,C2) decreases, with the curves crossing at tensile strain, indicating a band order reversal.