Growth and secondary engineering of II-VI semiconductor nanostructures and their optical properties

Abstract

The rational growth of well-defined 1D nanostructured materials is at the heart of building blocks for future nanodevices, since it is crucial to control over the material physical properties. Meanwhile, the capability of controlling the structural complexity of nanomaterials and their physical properties is presently limited in the one-step “thermal evaporation and condensation” growth. The secondary engineering can provide conceptually a new dimension to the rational design of nanostructured materials. In this work, a series of systematic studies towards the rational growth and engineering of 1D nanostructured II-VI compound semicondutors are introduced, e.g., ZnS and CdS in the morphology of nanowire and nanoribbon are presented because of their unique optical properties and wide potential applications in nanoscale photonic devices. In addition, a range of advanced techniques are adopted to characterize the intriguing physical properties of the resulting semiconductor nanostructures. Some efforts are also made towards the possibilities in the rational assembly of nanostructured materials into electrically-driven photonic devices based on nanostructured II-VI semiconductors. Firstly, the growth approach for the synthesis of hexagonal wurtzite 2H structured single-crystal nanowires and nanoribbons of group II-VI compound semiconductors like ZnS and CdS will be addressed by the one-step “thermal evaporation and condensation” growth mechanism. Particularly, heteroepitaxial growth of single-crystal ZnS nanowire arrays on CdS nanoribbon substrates has been demonstrated by the metal-catalyzed vapor-liquid-solid growth method. ZnS nanowire arrays are obtained, which are vertically or crosswise aligned to the surface of CdS nanoribbon substrates. The orientation dependence of ZnS nanowire arrays on the substrate indicates the importance of crystalline substrates on the epitaxial growth process of 1D nanostructures. Secondly, a conceptually new method is introduced for enabling the structural modification of pre-synthesized nanomaterials with highly defined hierarchical nanoarchitectures. Nanocantilever arrays are formed on the edge of the ±(001) planes of pre-synthesized ZnS nanoribbons via catalyst-assisted post-annealing treatment, which is associated with orientation-dependent chemical etching of the ±(001) polar surfaces of ZnS nanoribbons. Thirdly, the feasibility of doping nanostructured materials is demonstrated by postannealing treatment via thermal solid-state diffusion. Mn doping of ZnS nanoribbons is achieved by annealing the host in MnS powder, where Mn dopants occupy the Zn2+ cation sites producing deep-lying states in the band gap of ZnS. In contrast to doping by ion implantation where high-density defects are invariably induced, Mn doping by thermal annealing appears to have negligible deleterious effect on the crystal structure and luminescence properties. Fourthly, the optical properties of pure and doped nanomaterials are investigated. The intrinsic near-bandgap emission of ZnS nanostructures is around 335 nm, and the defect related luminescence is around 500 nm. Notably, room-temperature lasing emission from ZnS nanowire arrays is revealed according to a superlinear increase of emission intensity and the peak narrowing when the excitation power density is above a threshold value. Temperature-dependent optical properties of Mn-doped ZnS nanorribons are also discussed. A photoluminescence peak at 585 nm independent of the measuring temperature and excitation power is demonstrated in Mn-doped ZnS nanoribbons that is attributed to Mn2+ ion incorporated in ZnS, leading to an indirect excitation mechanism upon photo-excitation. Lastly, various device structures are explored, including crossed nanowire p-n junction and nanowire-thin film hybrid structure, in order to realize the electrically-driven nanophotonic devices based on nanostructured II-VI semiconductors by taking advantage of their direct bandgap character. It is demonstrated that the contacting point (or area) of the heterojunction formation is the obstacle to carrier injection in these structures.