Induced structural modifications in ZnS nanowires via physical state of catalyst: Highlights of 15R crystal phase

Abstract

Peculiar and unique growth mechanisms involved in semiconductor nanowires (NWs) pave the way to the achievement of new crystallographic phases and remarkable material properties, and hence, studying polytypism in semiconductor NWs arouses a strong interest for the next generation of electronic and photonic applications. In this context, the growth of ZnS nanowires has been investigated, as bulk ZnS compound exhibits numerous unstable polytypes at high temperatures, but their stable occurrence is highly anticipated in a nanowire due to its special quasi-dimensional shape and growth modes. In this work, the idea is to provide a change in the growth mechanism via the physical state of catalyst droplet (liquid or solid) and study the induced structural modifications in ZnS nanowires. The HRTEM images of VLS (via liquid alloyed catalyst) grown ZnS NWs show periodic stacking faults, which is precisely identified as a stacking sequence of cubic or hexagonal planes leading to an astonishing 15R crystal polymorph. This crystallographic phase is observed for the first time in nanowires. Contrastingly, NWs grown with VSS (via solid catalyst) show crystal polytypes of zinc blende (3C) and wurtzite (2H). We calculated and discussed the role of cohesive energies in the formation of such ZnS polytypes. Further, we presented the selection rules for the crystallization of such 15R structure in NWs and discussed the involved VLS and VSS growth mechanisms leading to the formation of different crystal phases.

MOCVD Growth and Structural Properties of ZnS Nanowires: A Case Study of Polytypism

“…In a recent work, we compared the structural properties and crystal phases of ZnS NWs whether grown on bare GaAs (111)B substrate, or ZnS (buffer)/GaAs (111)B pseudosubstrate [ 17 ]. To do so, we used transmission electron microscopy, and details are given in this article.…”

“…A TEM analysis on VSS ZnS NWs fabricated in the same MOCVD run (with VLS ZnS NWs analyzed in Figure 9 ) but grown via solid Au droplets can be found in Ref. [ 17 ]. Pure WZ and ZB domains are revealed, and they frequently switch between these two structures.…”

“…Interestingly, Engel et al [ 15 ] have calculated the total energies using the first principle pseudopotential approach for 3C, 2H, 4H, 6H, and 6L polytypes, and found that all these energies are very close in ZnS. Other simple calculations from cohesive energies [ 16 , 17 ] lead to the same conclusion that different crystal phases should easily occur in this material. Indeed, this is in agreement with past experiments where 4H, 12R, and 15R polytypes have been clearly identified [ 18 ].…”

“…Jiang et al [ 22 ] also described ordered stacking faults resulting in a polytype modulated nanostructure composed of 8H and 10H half-cells. Very recently, we reported on the identification of 15R polytype in VLS ZnS NWs grown on GaAs (111)B [ 17 ].…”

Regulated dynamics with two-monolayer steps in vapor-solid-solid growth of nanowires

“…The insertion of a II-VI buffer layer was exploited recently in order to guarantee a VSS growth of ZnS nanowires. 48 In the VSS case, ex-situ TEM reveals facetted nanoparticles, but also quasi-spherical shapes 49 which may suggest VLS, or even bulb-like shapes. 50 Simple models based on the diffusion along the nanowire sidewalls describe satisfactorily the measured growth rates, with a diffusion length of the order of 100 nm, 49 provided one takes into account a change in the shape of the nanoparticle (and of the nanoparticle-nanowire contact area) during the growth.…”