Applied Surface Science 390 (2016) 377–384Contents lists available at ScienceDirectApplied Surface Sciencejou rn al h om ep age: www.elsevier.com/locate/apsuscQuasi-particle energies and optical excitations of ZnS monolayerhoneycomb structureMasoud ShahrokhiUniversite´? Paris-Est, Laboratoire Mode´?lisation et Simulation Multi Echelle, MSME UMR 8208 CNRS, 5 bd Descartes, F-77454 Marne-la-Valle´?e, Francea r t i c l e i n f oArticle history:Received 5 April 2016Received in revised form 5 May 2016Accepted 10 August 2016Available online 24 August 2016a b s t r a c tUsing ab-initio density functional theory calculations combined with many-body perturbation formal-ism we carried out the electronic structure and optical properties of 2D graphene-like ZnS structure.The electronic properties were analyzed at three levels of many-body GW approach (G 0 W 0 , GW 0 andGW) constructed over a Generalized Gradient Approximation functional. Our results indicate that ZnSsheet has a direct band gap at the ? -point. Also it is seen that inclusion of electron–electron interactiondoes not change the sort of direct semiconducting band gap in ZnS sheet. The optical properties andexcitonic effects of these materials are investigated using the Bethe-Salpeter equation (BSE) approach.The formation of fi rst exciton peaks at 3.86, 4.26, and 4.57 eV with large binding energy of 0.36, 0.49 and0.73 eV using G 0 W 0 + BSE, GW 0 + BSE and GW + BSE, respectively, was observed. We show that the opticalabsorption spectrum of 2D ZnS structure is dominated by strongly bound Frenkel excitons. The enhancedexcitonic effects in the ZnS monolayer sheet can be useful in designing optoelectronic applications.© 2016 Elsevier B.V. All rights reserved.1. IntroductionZn-VI-based semiconductors have captivated enormous sig-nif i cant amount of attention due to their applications inshort-wavelength light-emitting devices [1–3]. Due to these pow-erful applications, a number of studies have been devoted toexploring new possible structural phases for these semiconduc-tors especially in nanocrystalline form [4–11]. Among the familyof Zn–VI semiconductors, ZnS with two different crystal struc-tures (zinc blende and wurtzite) [4] is a commercially importantsemiconductor having a wide direct band gap (E g = 3.68 eV) [12],rendering it a very attractive material for optical application. ZnShas important applications in ultraviolet light-emitting diodes andinjection lasers [13], fl at panel displays [14], sensors[15], infraredoptical windows [16], photocatalysis and triboluminescence [17].In recent articles by Akhtar et al. [18–20], full-potential lin-earized augmented plane wave (FP-L/APW) method based onspin-polarize DFT was employed to investigate the electronic andmagnetic properties of transition metal atoms (TM) doped ZnS thinf i lm. Electronic band structures and density of states (DOS) of thesecompounds demonstrate 100% spin polarization (half metallicity)with ferromagnetic exchange interactions. These ZnS based halfmetallic ferromagnets have the potential applications in futureE-mail address: shahrokhimasoud37@gmail.comspintronic devices. Furthermore, the ZnS fi lms show high crys-tallinity, good adhesion and minimum ref l ection in the visibleregion and hence have potential applications in optoelectronic orsolar cell devices [21]. Several researchers have previously investi-gated the electronic, magnetic, optical and mechanical properties ofnanostructures of ZnS including nanotubes, nanowires, nanobelts,and monolayer hexagonal sheets [12–16,22,23]. It is found that theband gap of ZnS nanostructures is smaller than that of SiC [24],BN [25] and BeO [26–28] nanostructures. Theoretical studies pre-dict that the ZnS monolayers have planar and buckled structures[29]. Also, a few theoretical studies have reported the electronicband structure, real and imaginary parts of the dielectric func-tion and energy-loss spectra of monolayer 2D-graphene like ZnSsheet [3,29,30]. In all those theoretical studies, the electronic andoptical properties have been investigated using the density func-tional theory (DFT) [31] and random phase approximation (RPA)[32]. Concerning the calculation of the electronic and optical prop-erties of semiconductors and insulators, the DFT and RPA are notsuff i cient to yield results in quantitative agreement with experi-ments because they ignore many-body effects on electronic andoptical properties. The many-body GW approximation (GWA) pro-vides a correct description of the electronic structure around thegap, in contrast to the DFT, which leads to more accurate bandgaps in the semiconductors and insulators [33–35]. Moreover, theelectron–hole two-particle Green function in Bethe-Salpeter equa-http://dx.doi.org/10.1016/j.apsusc.2016.08.0550169-4332/© 2016 Elsevier B.V. All rights reserved.
