ZnS Monolayer Research Articles

Rare earth (Tm, Y, Gd, and Eu) doped ZnS monolayer: a comparative first-principles study

In this current study, we used the density functional theory method to examine the physical properties of ZnS nanosheets doped with Tm, Y, Gd, and Eu at a concentration of 6.25%. The non-magnetic phase is energetically stable when doped with Y and Tm. However, the ferromagnetic state is thermodynamically stable when doped with Eu and Gd, show negative formation energy. The optimised structure is a planar structure for all doped systems, with an increase in the lattice parameter and bond length. On doping, the Fermi level is pushed into the conduction band, narrowing the band gap and exhibiting typical n-type semiconducting behaviour. In a wider optical window, Tm and Y-doped systems have lower reflectance and more excellent transmittance than Gd and Eu-doped systems in the visible light spectrum. The electrical conductivity has been calculated using the BoltzTrap package. The electrical conductivity has been enhanced by doping, making it suitable for optoelectronic, solar cells, spintronics, and thermoelectrics applications.

Carrier mobility and optical properties of a type-II GaSe/ZnS heterostructure as a photocatalyst: a first-principles study.

In this paper, a new GaSe/ZnS van der Waals heterostructure (vdWH) was constructed and a systematic analysis of the electronic structure, interfacial properties, and transport and photocatalytic capacity of the GaSe/ZnS vdWH was performed by using first-principles calculations. It was found that the heterostructure exhibited excellent photocatalytic performance for water splitting. The direct band gap of the heterostructure calculated using the hybrid HSE06 functional was 2.19 eV, which had a good visible light absorption ability. The electronic structure of the type-II band arrangement effectively reduced the recombination of electron-hole pairs. The heterostructure also showed excellent transport ability, and the carrier mobility of electrons and holes along different directions was greatly improved. Additionally, as the electric field strength increased, the band gap width of the GaSe/ZnS vdWH narrowed and the heterostructure characteristics transitioned from semiconductor to metal properties, which were attributed to the appearance of near-free electronic (NFE) states induced by the strong electric field. Meanwhile, the optical absorption capacity of the heterostructure was greatly improved compared to the ZnS monolayer, reaching 1.44 × 105 cm-1 at an incident photon energy of 8.65 eV. Therefore, the GaSe/ZnS vdWH was proved to be an excellent photocatalytic material for water splitting in the present study.

A DFT insight into magnetic, optoelectronic and thermoelectric properties of h-Zn1-xCuxS monolayer

In this study, we used the density functional theory (DFT) method to analyze the effect of Cu insertion on the structural stability, electronic, magnetic, optical and thermoelectric properties of the hexagonal ZnS monolayer (h-ZnS-2D). Our findings show that Cu replaces zinc sites by giving a stable planar structure in Ferromagnetic (FM) phase with slight decrease in the lattice parameter. A p-type semiconductor with a direct band gap that decreases with increasing Cu concentration was obtained. The half-metallic character found for low Cu concentrations let consider the h-Zn1-xCuxS-2D for potential applications in spintronic devices. The optical properties show that the h-Zn1-xCuxS-2D monolayers have high absorption and low transmittance in the UV range. The improved figure of merit of h-Zn1-xCuxS-2D due to increased electrical conductivity and decreased thermal conductivity with Cu content, makes these nanostructures a promising absorber for thin-film solar cells and suitable for solar thermal energy conversion technologies.

First principles insight of silicene-ZnS-silicene trilayer heterostructure

Using Density Functional Theory (DFT), 2D hexagonal silicene-ZnS-silicene trilayer heterostructure was studied with van der Waals correction as implemented in Grimme’s method. Small lattice mismatch of about 0.77% only between silicene and ZnS monolayer suggest ease in formation of sandwiched heterostructure. The negative value of total energy at 298 K from MD simulation confirms its ground state stability. Unlike monolayer silicene, our trilayer heterostructure exhibits a direct band gap of 0.63 eV in its equilibrium state. Calculated elastic moduli predict that Si-ZnS-Si has an enhanced ability to resist tensile and shear deformation than the pristine silicene and ZnS monolayer. Due to strong van der Waal’s interaction between the layers, Si-ZnS-Si has much lower thermal coefficient of linear expansion and therefore is more stable against any thermally induced deformation. When a transverse external electric field is applied, we observe direct-to-indirect band gap transition. On increasing the electric field further, the heterostructure remains indirect band gap semiconductor until it abruptly transforms to metallic nature at 1.0 V Å−1. Theoretical prediction of heterostructure property presented in this work may provide valuable data for developing future nanoelectronic devices.

Influence of Group-IVA Doping on Electronic and Optical Properties of ZnS Monolayer: A First-Principles Study.

Element doping is a universal way to improve the electronic and optical properties of two-dimensional (2D) materials. Here, we investigate the influence of group-ⅣA element (C, Si, Ge, Sn, and Pb) doping on the electronic and optical properties of the ZnS monolayer with a tetragonal phase by using first-principles calculations. The results indicate that the doping atoms tend to form tetrahedral structures with neighboring S atoms. In these doped models, the formation energies are all negative, indicating that the formation processes of the doped models will release energy. The formation energy is smallest for C-doped ZnS and gradually increases with the metallicity of the doping element. The doped ZnS monolayer retains a direct band gap, with this band gap changing little in other element doping cases. Moreover, intermediate states are observed that are induced by the sp3 hybridization from the doping atoms and S atoms. Such intermediate states expand the optical absorption range into the visible spectrum. Our findings provide an in-depth understanding of the electronic and optical properties of the ZnS monolayer and the associated doping structures, which is helpful for application in optoelectronic devices.

Theoretical Study of ZnS Monolayer Adsorption Behavior for CO and HF Gas Molecules.

Adsorption of carbon monoxide (CO) and hydrogen fluoride (HF) gas molecules on a ZnS monolayer with weak van der Waals interactions is studied using the DFT + U method. From our calculation, the ZnS monolayer shows chemisorption with CO (E ads = -0.96 eV) and HF (E ads = -0.86 eV) gas molecules. Bader charge analysis shows that charge transfer is independent of the binding environment. A higher energy barrier for CO when migrating from one optimal site to another suggests that clustering may be avoided by the introduction of multiple CO molecules upon ZnS, while the diffusion energy barrier (DEB) for HF suggests that binding may occur more easily for HF gas upon the ZnS ML. Adsorption of the considered diatomic molecule also results in a significant variation in effective mass and therefore can be used to enhance the carrier mobility of the ZnS ML. Additionally, the calculation of recovery time shows that desirable sensing and desorption performance for CO and HF gas molecules can be achieved at room temperature (300 K).

Ab initio investigation of non-metal-doped ZnS monolayer

Ab initio investigation of non-metal-doped ZnS monolayer

INCREASING THE PHOTOLUMINESCENCE EFFICIENCY OF CdS NC GROWN IN A GELATINOUS ENVIRONMENT

The optical and luminescent properties of nanostructures of CdS / ZnS QDs and CdS QDs doped with lithium, obtained by the sol-gel technology in an aqueous solution of gelatin, have been studied. It was determined from the optical absorption spectra of the CdS / ZnS QD nanostructures that the thickness of the shell layer corresponds to the thickness of one ZnS monolayer. The luminescence spectra revealed a significant increase in the intensity of luminescence nanostructure. The absorption spectra of undoped and doped CdS NCs coincide, which indicates that there is no change in the NC size and the absence of the formation of a lithium-sulfur compound. The luminescence spectra of lithium-doped QDs demonstrate an increase in the luminescence intensity, which is associated with the passivation of surface states.

Study of structural, optical and morphological properties of ZnO/ZnS hetrostructures deposited by spray pyrolysis method

We have prepared ZnO/ZnS multi-layer thin films as a hetero-nanocomposite structure by the spray pyrolysis method. The crystalline structure of both ZnS/ZnO core-shell and ZnO/ZnS/ZnO films deposited on a heated glass substrate located on a hot plate at 450 °C was analyzed by X-ray diffraction, field emission scanning electron microscopy, photoluminescence (PL) and UV–Vis spectroscopy. The diffractogram shows that a new hydroxide phase; simonkolleite (Zn5(OH)8Cl2·H2O) is also formed due to the thermal decomposition of ZnS precursor during the deposition heating process. photocatalytic activities of simonkolleite nanostructures is one of its interesting properties in optical applications. Morphology investigation of the prepared nanocomposite revealed the formation of randomly arranged nanoplates. Changes in the absorption edge of ZnO, as well as the reduction in bandgap of the prepared multilayers samples compared to ZnS monolayer, are also discussed. PL intensity ratio of two well-known blue and green emission of ZnO in the multi-layer films show a significantly changed due to the passivation effect of the core/shell structure. We have also discussed the origin of the emission bands affected by surface defects in the interface between ZnO and ZnS layers.

Atomic Sulfur Passivation Improves the Photoelectrochemical Performance of ZnSe Nanorods.

We introduced atomic sulfur passivation to tune the surface sites of heavy metal-free ZnSe nanorods, with a Zn2+-rich termination surface, which are initially capped with organic ligands and under-coordinated with Se. The S2− ions from a sodium sulfide solution were used to partially substitute a 3-mercaptopropionic acid ligand, and to combine with under-coordinated Zn termination atoms to form a ZnS monolayer on the ZnSe surface. This treatment removed the surface traps from the ZnSe nanorods, and passivated defects formed during the previous ligand exchange process, without sacrificing the efficient hole transfer. As a result, without using any co-catalysts, the atomic sulfur passivation increased the photocurrent density of TiO2/ZnSe photoanodes from 273 to 325 μA/cm2. Notably, without using any sacrificial agents, the photocurrent density for sulfur-passivated TiO2/ZnSe nanorod-based photoanodes remained at almost 100% of its initial value after 300 s of continuous operation, while for the post-deposited ZnS passivation layer, or those based on ZnSe/ZnS core–shell nanorods, it declined by 28% and 25%, respectively. This work highlights the advantages of the proper passivation of II-VI semiconductor nanocrystals as an efficient approach to tackle the efficient charge transfer and stability of photoelectrochemical cells based thereon.

Enhanced optical and thermoelectric properties of ZnS monolayer and stacked bilayer compared with bulk

In this study, we report the structural, electronic, optical and thermoelectric properties for three different forms of ZnS : bulk (ZnS-B), stacked bilayer (ZnS-BL) and monolayer (ZnS-ML), using first-principles of full-potential density functional theory calculations and semiclassical Boltzmann transport theory. Our results show that ZnS-BL and ZnS-ML with relatively low formation energies of about 152 meV/atom and 209 meV/atom respectively could be exfoliated from ZnS bulk and stabilized in a structure like that of graphene. The optimized ZnS-ML gives a planar structure whereas AB stacking bilayer gives low buckling effect of about 0.003 Å. The calculated electronic band structures and optical properties using TB-mbj approximation of the above structures reveal that their relatively large and direct electronic band gap (4.31 eV for ZnS-ML and 3,94 for stacked ZnS-BL) and their optical performances like low reflectivity and high transparency in the visible light with a widening of the optical window make them a candidate for future opto-electronic devices. Based on electron transport coefficients, we found a larger power and merit factors values, for ZnS-ML and ZnS-BL than ZnS-B, which imply an inherent advantage in their electrical transport properties and make them a potential two-dimensional material comparable to other known good thermoelectric materials.

Effect of doping chromium on electronic and magnetic properties of ZnS monolayer: a DFT study

Density-functional theory has been applied to investigate the electronic and magnetic properties of the pure and chromium-doped ZnS nanosheets via examining a number of important indicators including the electronic band structure and density of states. It is found that the pure nanosheet is a semiconductor with a band gap of about 2.525 eV. Moreover, the effect of doping the chromium atom is manifested in the form of reduction in the value of the band gap to 1.85 eV, which results in half-metallic properties for the ZnS nanosheet. Most importantly, it is verified that the chromium-doped nanosheet has an intermediate band, which can be utilized in fabricating more advanced solar cells with a high rate of photon absorption and state-of-the-art lasers with a high quantum performance.

Study on defects in ZnS/YbF3 infrared coatings on silicon substrates

In order to reduce the numbers of defects, the influence of deposition parameters (i.e., deposition rate and substrate temperature) and deposition methods (i.e., thermal evaporation and electron beam evaporation) on the defect density of ZnS monolayer, YbF3 monolayer and ZnS/YbF3 multilayer coatings deposited on silicon substrates were investigated experimentally. The results show that deposition rate and substrate temperature have major influence on the defect density. For ZnS monolayer coatings, the optimum deposition rate is approximately 1.2nm/s, and the optimum substrate temperature is from 80°C to 120°C. Meanwhile, for YbF3 monolayer coatings, the optimum deposition rate is approximately 0.4nm/s, and the optimum substrate temperature is from 100°C to 150°C. Furthermore, the defect density of ZnS or YbF3 monolayer coatings deposited by thermal evaporation is much lower than that by electron beam evaporation. The defect density of ZnS/YbF3 multilayer coatings can be reduced significantly by optimizing the substrate temperature and deposition rate with the method of thermal evaporation.

First-principles calculation of ZnS monolayer on Cu(111) surface

First-principles calculation is carried out on the interface of the ZnS(001) monolayer and Cu(111) surface. It is found that the ZnS monolayer significantly reconstructs after geometry optimization. The out-of-plane S atom has a positive displacement in the z direction while other atoms (Zn and S) have small displacements on the ZnS monolayer. The interface stacking sequence has an influence on the flatness of the ZnS monolayer and the binding energy of the interface. There are two approaches for the ZnS monolayer to reach the lowest energy state which take place on the two kinds of S atoms in the ZnS monolayer and result in the bulging feature. The van der Waals (vdW) interaction exists between ZnS monolayer and Cu surface.

Investigation of the electronic and optical properties of ZnS monolayer nanosheet: first principles calculations

In this paper, some of the electronic and optical properties of ZnS monolayer nanosheet have been calculated and compared with the bulk ZnS. The calculations were performed based on the density functional theory framework using full-potential linear augmented plane wave method, in which TB-modified Becke–Johnson potential has been considered for exchange–correlation term. The results show that the structural change of ZnS, from bulk to monolayer nanosheet, has dramatically affected its electronic and optical properties. The shift of van Hove singularities, variation of density of states, the appearance of energy gap in the valence band of ZnS monolayer nanosheet, and the elimination of degeneracy of the p and d orbitals in S and Zn atoms are some of the important effects. The calculated band gap of 3.88 eV, for bulk ZnS, is in good agreement with the experimental values. Also, there is a direct energy gap of about 3.22 eV for ZnS monolayer nanosheet. Compared to bulk ZnS, the optical anisotropy of ZnS monolayer nanosheet has significantly increased. With the structural change from the bulk ZnS to ZnS monolayer nanosheet, plasmon energies of ZnS monolayer are situated in the lower energies. We have proposed an equation for variations of refractive index as a function of wavelength and temperature, based on the experimental and ground-state data of bulk ZnS.

Quasi-particle energies and optical excitations of ZnS monolayer honeycomb structure

Using ab-initio density functional theory calculations combined with many-body perturbation formalism 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 (G0W0, GW0 and GW) constructed over a Generalized Gradient Approximation functional. Our results indicate that ZnS sheet has a direct band gap at the Γ-point. Also it is seen that inclusion of electron–electron interaction does not change the sort of direct semiconducting band gap in ZnS sheet. The optical properties and excitonic effects of these materials are investigated using the Bethe-Salpeter equation (BSE) approach. The formation of first exciton peaks at 3.86, 4.26, and 4.57eV with large binding energy of 0.36, 0.49 and 0.73eV using G0W0+BSE, GW0+BSE and GW+BSE, respectively, was observed. We show that the optical absorption spectrum of 2D ZnS structure is dominated by strongly bound Frenkel excitons. The enhanced excitonic effects in the ZnS monolayer sheet can be useful in designing optoelectronic applications.

Novel band structures in silicene on monolayer zinc sulfide substrate

Opening a sizable band gap in the zero-gap silicene without lowering the carrier mobility is a key issue for its application in nanoelectronics. Based on first-principles calculations, we find that the interaction energies are in the range of −0.09‒0.3 eV per Si atom, indicating a weak interaction between silicene and ZnS monolayer and the ABZn stacking is the most stable pattern. The band gap of silicene can be effectively tuned ranging from 0.025 to 1.05 eV in silicene and ZnS heterobilayer (Si/ZnS HBL). An unexpected indirect–direct band gap crossover is also observed in HBLs, dependent on the stacking pattern, interlayer spacing and external strain effects on silicene. Interestingly, the characteristics of Dirac cone with a nearly linear band dispersion relation of silicene can be preserved in the ABS pattern which is a metastable state, accompanied by a small electron effective mass and thus the carrier mobility is expected not to degrade much. These provide a possible way to design effective FETs out of silicene on a ZnS monolayer.

Tailoring the structural and electronic properties of a graphene-like ZnS monolayer using biaxial strain

Our first-principles full-potential density functional theory calculations show that a ZnS monolayer (ML-ZnS), which is predicted to adopt a graphene-like planar honeycomb structure with a direct band gap, undergoes strain-induced modifications in its structure and band gap when subjected to in-plane homogeneous biaxial strain (δ). ML-ZnS gets buckled for compressive strain greater than 0.92% ; the buckling parameter Δ(= 0.00 Å for planar ML-ZnS) linearly increases with increasing compressive strain (Δ = 0.435 Å at δ = −5.25%). A tensile strain of 2.91% turns the direct ML-ZnS band gap into indirect. Within our considered strain values of |δ| < 6%, the band gap shows linearly decreasing (non-linearly increasing as well as decreasing) variation with tensile (compressive) strain. These predictions (based on our calculations with two atoms per unit cell) may be exploited in future for potential applications in strain sensors and other nano-devices such as nano-electromechanical systems and nano-optomechanical systems.

Method for determining the elemental composition and distribution in semiconductor core-shell quantum dots.

In the biological sciences, the use of core-shell quantum dots (QDs) has gained wide usage but analytical challenges still exist for characterizing the QD structure. The application of energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS) to bulk materials is relatively straightforward; however, for meaningful applications of surface science techniques to multilayer nanoparticles requires novel modifications and analysis methods. To experimentally characterize the elemental composition and distribution in CdSe/CdS/ZnS QDs, we first develop a XPS signal subtraction technique capable of separating the overlapped selenium 3s (core) and sulfur 2s (shell) peaks (both peaks have binding energies near 230 eV) with higher precision than is typically reported in the nanoparticle literature. This method is valid for any nanoparticle containing selenium and sulfur. Then we apply a correction formula to the XPS data and determine that the 2 nm stoichiometric CdSe core is surrounded by 2 CdS layers and a stoichimetric ZnS monolayer. These findings and the multiapproach methodology represent a significant advancement in the detailed surface science study of multilayer nanoparticles. In agreement with recent surprising findings, the time-of-flight secondary mass spectrometry measurements suggest that the surface sites of the QDs used in this study are primarily covered with a mixture of octadecylphosphonic acid and trioctylphophine oxide.

Highlyphotoluminescent ZnSe/ZnS quantum dots

ZnSe quantum dots have been synthesized using a simpleaqueous route. Coating ZnSe quantum dots with a ZnS monolayer,yields a remarkable enhancement in the PL quantum efficiency atroom temperature without affecting the spectral distribution.The results suggest that passivation of surface states, alongwith an increased localization of the hole in core ZnSe layer, gives rise to high luminescence quantum yield.