- Nano Express
- Open Access
Modulation of Electronic and Optical Anisotropy Properties of ML-GaS by Vertical Electric Field
© The Author(s). 2017
- Received: 12 March 2017
- Accepted: 1 June 2017
- Published: 14 June 2017
We investigate the electric-field-dependent optical properties and electronic behaviors of GaS monolayer by using the first-principles calculations. A reversal of the dipole transition from E//c to E⊥c anisotropy is found with a critical external electric field of about 5 V/nm. Decomposed projected band contributions exhibit asymmetric electronic structures in GaS interlayers under the external electric field, which explains the evolution of the absorption preference. Spatial distribution of the partial charge and charge density difference reveal that the strikingly reversed optical anisotropy in GaS ML is closely linked to the additional crystal field originated from the external electric field. These results pave the way for experimental research and provide a new perspective for the application of the monolayer GaS-based two-dimensional electronic and optoelectronic devices.
As a typical two-dimensional (2D) material, graphene has rather unique and exceptional properties , which enables its superior performance in transistors and as electrochemical electrodes . Nevertheless, for use in nanoelectronic devices, the lack of intrinsic band gap  essentially restricts its application in the traditional emitting devices. Even though with surface functionalization and external electric or strain field, very small band gap can be achieved [4–7]. In this context, the search of other 2D materials that may offer new opportunities for specific properties and applications is both of fundamental interest and technological significance.
Recently, a stable class of 2D metal dichalcogenide (MD) materials, GaX (X = S, Se), has attracted much attention due to their exotic physical and chemical properties, with great promise for applications in fields such as solar energy conversion and optoelectronics [8–11]. Layer GaX is constructed by four-atom planes covalently bonded in the sequence of X-Ga-Ga-X with a D3h symmetry. Advanced applications often require materials with tunable and reversible electronic properties which can be deliberately modulated by external control parameters. Strain engineering has been identified as one of the promising routes to tune the electronic behavior and the electron energy low-loss spectra of GaS monolayer (ML) and other 2D materials . As an alternative, an applied electric field or light offers a novel way to modify the electronic properties over a wide range [13, 14]. For example, a strong electric field perpendicular to the plane of bilayer graphene can induce a significant band gap [15, 16], and the bandgap can also be modulated for BN with two or more layers . However, the effects of the external electric field on the electronic structures of 2D GaS ML are still unclear. In addition, an intrinsic large negative crystal field which exists in GaS ML results in an optical anisotropy that the absorption coefficient for E⊥c is about 103 cm−1, a factor of 30 smaller than for E//c . For optic materials, light emission polarization is closely related to the near band-edge transitions, occurring between the bottom of the conduction band and the top of the valence band. By employing an external electric field, the band structure and thus the optical properties of GaS ML can be conveniently modulated to meet the multiple demands of device applications.
To address this issue, we perform a theoretical prediction on the modulation of optical and electronic anisotropy on GaS ML. Optical absorption spectra for both E⊥c and E//c directions are calculated under various external electric fields. Band structure and orbit contributions are analyzed to explain the dependence of the dipole transition on the external electric field. Spatial distribution of the partial charge and charge density difference are further simulated, which show the interlayer coupling and asymmetry electronic structure induced by the vertical external electric field, and reveal the physical mechanism for the modulation of the optical and electronic anisotropy of GaS ML. The present results are beneficial to supply the theoretical guidance on the tunable electronic and optoelectronic devices based on 2D GaS material.
We perform the density functional theory (DFT) calculations with the Vienna Ab-initio Simulation Package (VASP) code , using the projector-augmented wave pseudopotential method . Exchange and correlation effects are treated by Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) . Heyd-Scuseria-Ernzerhof (HSE) hybrid functional is used to provide quantitative estimates of the band gap . A slab model of the GaS consisting of four atom layers in the order S-Ga-Ga-S is employed, and a 15-Å vacuum layer along the z direction are adopted to eliminate the interactions between the slabs. The Brillouin zone is sampled according to Monkhorst–Pack method . A 27 × 27 × 1 k-point mesh is used to relax the single-layer GaS, and a cutoff energy of 450 eV is taken for expanding the wave functions into a plane-wave basis. The convergence for energy is chosen as 10-5 eV between two steps and the maximum Hellmann-Feyman force acting on each atom is less than 0.01 eV/Å upon ionic relaxation. Gaussian smearing is used to address how the partial occupancies are set for each wave function, and the width of smearing is 0.1 eV. The imaginary part of the dielectric function due to direction interband transitions is obtained using the Fermi golden rule . During the calculation, the spin-orbit coupling (SOC) splitting is neglected due to its tiny effects on electronic and optical properties.
In summary, based on the first-principles DFT simulations, we investigate the electric-field-dependent optical properties and electronic behaviors of GaS ML. Optical absorption spectra for both E⊥c and E//c directions are calculated under various external electric fields. A reversal of the dipole transition from E//c to E⊥c anisotropy is found with a critical external electric field of about 5 V/nm. The band structure calculations indicate a reduction of the band gap and a transition from indirect to direct bandgap in GaS ML with an increasing external vertical electric field. Decomposed projected band contributions exhibit the asymmetric electronic structures in GaS interlayers under the external electric field, which explains the evolution of the absorption preference. Spatial distribution of the partial charge and charge density difference suggest that the strikingly reversed optical anisotropy in GaS ML is closely linked to the additional crystal field which originated from the external electric field. These results not only reveal the modulation of the electronic structures and optical properties of GaS ML by the external electric field but also provide some references to its future application in 2D electronic and optoelectronic devices.
The work was supported by the National Natural Science Foundations of China (Nos. 61674124, 11604275, 11304257, and 61227009), the National Key Research and Development Program of China (No. 2016YFB0400801), the Natural Science Foundation of Fujian Province of China (Nos. 2014J01026, 2016J01037, and 2015J01028), and Fundamental Research Funds for the Central Universities (Nos. 20720160122, 20720150033, and 20720160044).
The National Natural Science Foundations of China (Nos. 61674124, 11604275, 11304257, and 61227009), the National Key Research and Development Program of China (No. 2016YFB0400801), the Natural Science Foundation of Fujian Province of China (No. 2014J01026, 2016J01037, and 2015J01028), and the Fundamental Research Funds for the Central Universities (Nos. 20720160122, 20720150033, and 20720160044) provided support in writing the manuscript.
FG drafted the manuscript. Prof. ZMW helped to guide the calculations. CMK, CJZ, TC, HL, CMZ, and MMF took part in the data analysis. Dr. YPW and Prof. JYK participated in the conception of the project, improved the manuscript, and coordinated between all the participants. All authors discussed the results and the implications of this manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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