Topical PerspectivesStructural, electronic and optical properties of metalloid element (B, Si, Ge, As, Sb, and Te) doped g-ZnO monolayer: A DFT study
Graphical abstract
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.
Section snippets
Computational details
The geometrical structure, electronic, and optical properties of pristine and metalloid doped ZnO monolayers are calculated within the framework of density functional theory (DFT) by using the Vienna Ab initio Simulation Package (VASP) [23]. The projector augmented wave (PAW) method is used to represent pseudopotentials [[24], [25], [26]]. In the calculations, the generalised gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE) [27] treats exchange correlation functional is used.
Results and discussion
Before beginning our discussion, we would like to note the difference in ionic radii and the valence electronic configuration of metalloid elements along with Zn, and O atoms. Fig. 1 presents the valence electronic configuration and ionic radii of the metalloid elements along with Zn and O atoms. The ionic radius of B, Si and As is smaller as compared to that of Zn atom while Ge, Sb and Te have larger ionic radius. All the elements have higher electronegativity as compared to that of Zn atom.
Conclusions
We investigated the electronic and optical properties of ZnO monolayer doped with B, Si, Ge, As, Sb, and Te using the first-principles calculations. The valency of dopant effectively modulate the structural and electronic properties of ZnO monolayer. We have observed that, with M doping in ZnO monolayer the band gap decreases as compared to that of pristine ZnO monolayer. Te doped system shows half-metallic character with strong spin polarization in Te gives 2 μB magnetic moment. Doping of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
SYW and MDD acknowledges the Center for Development of Advance Computing (CDAC), Pune and Bangalore and the Bioinformatics Resources and Applications Facility (BRAF) for providing the supercomputing facilities. We also grateful for financial assistance by Department of Science and Technology, Delhi and BCUD, S.P.P U. Pune.
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