Structural, electronic and optical properties of metalloid element (B, Si, Ge, As, Sb, and Te) doped g-ZnO monolayer: A DFT study

Abstract

Stable geometries, electronic structure, and optical properties of ZnO monolayer doped with metalloid element (M = B, Si, Ge, As, Sb, and Te) atom have been studied using density functional theory. It is found that among these elements Ge, As, and Sb can be effectively doped at Zn site in the ZnO monolayer with the formation energies ranging from −1.02 to −0.96 eV. Except B element, all the metalloid atoms prefer to protrude out of the plane of the ZnO monolayer. The nonmagnetic nature of the ZnO monolayer is retained with the doping of B, Si, Ge, As, and Sb atom, while Te atom induces the magnetism in ZnO monolayer (2 $\mu B$). While doping of Si, As, Sb, and Te in ZnO monolayer resulted in a red shift in the absorption spectra of doped ZnO monolayer and the blue shift is observed for B and Ge doped ZnO. The static dielectric constant for ZnO monolayer is 1.49. With the doping of these metalloid elements in ZnO monolayer, the dielectric constant can be tuned from 1.36 to 2.84. These results are potentially useful for optoelectronic applications and the development of optical nanostructures.

Introduction

As a wide and direct band gap semiconductor zinc oxide has received considerable interest because of its wide variety of practical applications, such as electronic devices, gas sensors, ferroelectric transparent thin-film transistors, and optical devices like liquid crystal display [1]. There are number of experimental and theoretical studies on ZnO nanostructures such as nanosheet, nanoparticles, nanowires, and nanorods [[1], [2], [3], [4]]. These structures have absorbed increasing attention because of their specific structures and unique properties as compared that of bulk and have wide applications in nanoscale optoelectronic devices [[1], [2], [3], [4]]. Recently, two dimensional (2D) materials are attracting increasing research interest due to their unique properties, such as an electron confinement of 2D materials without interlayer interactions, high surface to volume ratio, and high optical transparency [3]. In this context, ZnO monolayer (g-ZnO) is one that mimics graphene in many aspects with several interesting properties along with sufficient stability. Similar to graphene, the experimental evidences of two-monolayer-thick ZnO (0001) films grown on Ag substrates through surface X-ray diffraction and scanning tunneling microscopy [5] further increases the interest in designing optoelectronic devices using g-ZnO.

It is always desirable to control the material properties achieve the appropriate characteristics for device engineering. In this context, the introduction of substitutional doping, arises as a very powerful strategy. Previous investigations have shown that the electronic and magnetic properties of the ZnO monolayer doped with foreign atoms can be exploited for nanoelectronic and spintronic applications [3,[6], [7], [8]]. Tang et al. [6] reported that the semi-hydrogenation on Zn sites turns nonmagnetic g-ZnO to magnetic semiconductor, but semi-hydrogenation on O sites showed nonmagnetic (NM) metallic behavior. Guo et al. [7] demonstrated half metallic behavior of ZnO monolayer with doping of B or C atom while the semiconducting behavior with N-doping. Ren et al. [8] investigated the structural, electronic, and magnetic properties of the ZnO monolayer doped with transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu). The electronic structure of the ZnO monolayer is significantly changed after the TM incorporation; particularly the Cr-doped ZnO monolayer has shown a metallic behavior. Furthermore, the induced magnetism is observed with the doping of Cr, Mn, Fe, Co, Ni, and Cu in ZnO monolayer [8]. By considering the major role of ZnO nanostructures in optical devices specifically, few studies have been conducted to understand the optical properties of doped ZnO monolayer [3,[9], [10], [11], [12], [13], [14], [15]]. It seen that, the doping effectively modulates the shift in absorption spectra. The optical absorption peaks showed a blue shift with the doping of Li, Na, K, Be, Mg, Al, and Ga element in ZnO monolayer [10,11,14,16]. While doping of N, Cu, In, and Cd in g-ZnO resulted in a red-shift in the absorption spectrum [12,13,15] of g-ZnO monolayer.

From the earlier few reports [[17], [18], [19]], it is seen that, metalloid elements play vital role in various nanostructures to tune the electronic properties of the host material and can be used in different applications such as catalyst [17], hydrogen sensor [18] and optoelectronic devices [19]. The enhanced conductivity is found in SnO₂ and TiO₂ nanostructures with the doping of antimony and boron [20,21]. The band gap opening is observed with Si doping in graphene [22]. The enhanced refractive index and reflectivity of graphene sheet is observed with the doping of metalloid elements. The doped graphene can be used for the development of photonic and nanoelectronic devices [22]. Inspired by the earlier research on doped ZnO monolayer, the present study aims to understand the structural and optical properties of metalloid element (M)-doped ZnO monolayer using first-principles calculations. The metalloid elements are B, Si, Ge, As, Sb, and Te. To the best of our knowledge, there is no study on the optical properties of ZnO monolayer doped with metalloid elements. In the following section (Sec.) 2, we describe in brief the computational details, followed by discussion of our results in Sec. 3. In Sec. 4, conclusions are given.