Ultrahigh anisotropic carrier mobility in ZnSb monolayers functionalized with halogen atoms.

The experimental fabrication of novel two-dimensional ZnSb inspires us to explore the tunability of its fundamental physical properties. In this work, we present the density functional theory simulations on the mechanical, electronic and transport properties of the two-dimensional ZnSb monolayers functionalized with halogen atoms. It is found that the halogen atoms prefer to form ionic bonds with Sb atoms and these ZnSbX (X = Cl, Br and I) monolayers are very flexible with Young's moduli ranging from 24.02 N m-1 to 30.16 N m-1 along the armchair and zigzag directions. The pristine ZnSb monolayer sheet exhibits metallic phase while the functionalization can lead to a metal-to-semiconductor transition with band gaps as large as 0.55 eV. The transport study reveals a large tunability with the hole mobility reaching 43.44 × 103 cm2 V-1 s-1 along the armchair direction and the electron mobility as high as 36.99 × 103 cm2 V-1 s-1 along the zigzag direction. In contrast, the electron mobility along the armchair direction and the hole mobility along the zigzag direction are of relatively small magnitude. The ultrahigh carrier mobility together with the directional anisotropy can boost the separation of photo-excited electron-hole pairs. The finite band gaps and exceptional transport property of ZnSbX monolayers render them new materials with promising applications in flexible optoelectronic and nanoelectronic devices.

Direct band gap and anisotropic transport of ZnSb monolayers tuned by hydrogenation and strain.

Using first-principles density-functional theory simulations, we explore the effects of hydrogenation and strain on the mechanical, electronic and transport properties of two-dimensional ZnSb monolayers. We find that the fully hydrogenated ZnSb monolayer exhibits large mechanical anisotropy between armchair and zigzag directions and the biaxial tensile strain reduces the anisotropy. In addition, we find that the hydrogenation can induce a metal-to-semiconductor transition with a direct band gap of 1.12 (1.92) eV using the PBE (HSE) functional. With biaxial strains, the band gaps decrease monotonically and remain direct for strains smaller than 5%. Moreover, large transport anisotropy is demonstrated by computing the effective masses of charge carriers along the asymmetric armchair and zigzag directions. We further reveal that strain can significantly tune the effective masses and a 3% strain can even switch the effective transport direction for holes. Our simulations suggest that the hydrogenated ZnSb monolayer is a promising candidate for electronic and opto-electronic applications with controllable modification via strain engineering.

Ideal strength and strain engineering of the Rashba effect in two-dimensional BiTeBr.

Achieving a large and tunable Rashba effect is of great significance for advancing both the understanding and applications of spintronics. Two-dimensional nanostructures that are expected to sustain large mechanical deformation provide a great platform to study the strain effect on the tunability of the Rashba effect. Using first-principles density functional theory simulations, we investigate the mechanical stability and the effect of biaxial tensile strain on the Rashba parameters of the two-dimensional BiTeBr monolayer sheet. The mechanical stability is computed taking into account both elastic and dynamical aspects. The simulated stress-strain curve shows that the BiTeBr monolayer sheet can withstand a biaxial loading stress up to 4.36 GPa with a critical strain of 17%. Further phonon dispersion calculations indicate that the structure becomes unstable due to the existence of imaginary frequencies for strains beyond 7% limiting the maximum stress to 2.79 GPa. Moreover, the electronic structures and Rashba parameters are calculated as a function of strain from 0% to 7%. The band gaps remain indirect and exhibit a linear dependence on the strain. The decrease in band gaps with increasing strains can be attributed to the different bonding characters of near-gap states that the conduction band minimum is of anti-bonding nature while the valence band maximum is mainly non-bonding. The Rashba parameters can be tuned by 33.6% from 1.28 to 1.71 eV Å following a linear dependence on strain. This enhancement can be understood from the perspectives of enhanced charge transfer and charge distribution inhomogeneity induced by strain. The intuitive understanding could be used to understand and motivate the application of various methods that can lead to charge redistribution to tune the electronic properties of Rashba materials.