Pseudopotential plane-wave calculations for ZnS

Effect of vacancy defects on transport in all-phosphorene nanoribbon devices from first principles

Defects in experimentally manufactured phosphorene nanoribbons (PNRs) occur unavoidably, affecting the functionality of PNR-based devices. In this work, we theoretically propose and investigate an all-PNR devices with single-vacancy (SV) defects and double-vacancy (DV) defects along the zigzag direction, accounting for both hydrogen passivation and non-passivation scenarios. We discovered that, in the case of hydrogen passivation, the DV defect can introduce in-gap states, whereas the SV defect can result in p-type doping. The unpassivated hydrogen nanoribbon exhibits an edge state with a considerable influence on the transport properties, which also masks the effect of defects on transport; furthermore, it demonstrates the phenomenon of negative differential resistance, whose occurrence and characteristics depend less on the presence or absence of defects.

Influence of Defects on the Stability and Hydrogen-Sorption Behavior of Mg-Based Hydrides

This review deals with the destabilization methods for improvement of storage properties of metal hydrides. Both theoretical and experimental approaches were used to point out the influence of various types of defects on structure and stability of hydrides. As a case study, Mg, and Ni based hydrides has been investigated. Theoretical studies, mainly carried out within various implementations of DFT, are a powerful tool to study mostly MgH₂ based materials. By providing an insight on metal-hydrogen bonding that governs both thermodynamics and hydrogen kinetics, they allow us to describe phenomena to which experimental methods have a limited access or do not have it at all: to follow the hydrogen sorption reaction on a specific metal surface and hydrogen induced phase transformations, to describe structure of phase boundaries or to explain the impact of defects or various additives on MgH₂ stability and hydrogen sorption kinetics. In several cases theoretical calculations reveal themselves as being able to predict new properties of materials, including the ways to modify Mg or MgH₂ that would lead to better characteristics in terms of hydrogen storage. The influence of ion irradiation and mechanical milling with and without additives has been discussed. Ion irradiation is the way to introduce a well-defined concentration of defects (Frankel pairs) at the surface and sub-surface layers of a material. Defects at the surface play the main role in sorption reaction since they enhance the dissociation of hydrogen. On the other hand, ball-milling introduce defects through the entire sample volume, refine the structure and thus decrease the path for hydrogen diffusion. Two Severe Plastic Deformation techniques were used to better understand the hydrogenation/dehydrogenation kinetics of Mg- and Mg₂ Ni-based alloys: Equal-Angular-Channel-Pressing and Fast-Forging. Successive ECAP passes leads to refinement of the microstructure of AZ31 ingots and to instalment therein of high densities of defects. Depending on mode, number and temperature of ECAP passes, the H-sorption kinetics have been improved satisfactorily without any additive for mass H-storage applications considering the relative speed of the shaping procedure. A qualitative understanding of the kinetic advanced principles has been built. Fast-Forging was used for a "quasi-instantaneous" synthesis of Mg/Mg₂ Ni-based composites. Hydrogenation of the as-received almost bi-phased materials remains rather slow as generally observed elsewhere, whatever are multiple and different techniques used to deliver the composite alloys. However, our preliminary results suggest that a synergic hydrogenation / dehydrogenation process should assist hydrogen transfers from Mg/Mg₂ Ni on one side to MgH₂ /Mg₂ NiH₄ on the other side via the rather stable a-Mg₂ NiH_0.3 , acting as in-situ catalyser.

Interaction potential models for bulk ZnS, ZnS nanoparticle, and ZnS nanoparticle-PMMA from first-principles

An ab initio derived transferable polarizable force-field has been developed for Zinc sulphide (ZnS) nanoparticle (NP) and ZnS NP-PMMA nanocomposite. The structure and elastic constants of bulk ZnS using the new force-field are within a few percent of experimental observables. The new force-field show remarkable ability to reproduce structures and nucleation energies of nanoclusters (Zn1S1-Zn12S12) as validated with that of the density functional theory calculations. A qualitative agreement of the radial distribution functions of Zn-O, in a ZnS nanocluster-PMMA system, obtained using molecular mechanics molecular dynamics (MD) and ab initio MD (AIMD) simulations indicates that the ZnS-PMMA interaction through Zn-O bonding is explained satisfactorily by our force-field.

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley-Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.