Study of quasi-1D SnO2 nanowires

The descriptions of SnO2 nanowires growth procedures are getting more and more frequent in the current literature. However, studies on the growth mechanisms are still lacking. In particular, no investigation has been reported on the growth process when the growth mechanisms are not based, as in the case of whiskers, on vapour-liquid-solid (VLS) transitions. In this paper, a new procedure is reported by the authors for growing SnO2 nanowires, based on the presence of liquid-tin droplets on the substrate. The Sn vapour pressure developed by these droplets, which find themselves very close to the growing tip of the wire, gives rise to a sufficiently high supersaturation to enable the fast growth rate usually observed. The principal features and results of this new procedure, as well as possible growth mechanisms, are also discussed

Growth mechanism of aligned ZnO nanorods by vapour phase process

ZnO is an important and versatile functional semiconducting material. In recent years one-dimensional nanostructures (wires, tubes, tetrapods, etc.) have received increased attention not only for their specific properties but also for the fabrication of nanoscale devices. Among these structures the parallelly aligned, column-shaped nanorods, orthogonal to the growth substrate plane, are particularly interesting for many applications such as dye sensitized solar cells, transistors, nanogenerators, short-wavelength nanolasers, etc. Many different techniques have been reported for the growth of ZnO nanostructures but thermal evaporation turns out to be one of the most convenient when considering the high quality and purity of the grown crystals, the simplicity of the growth apparatus and the easiness in the scaling-up of the process. In this communication the authors report on a selective growth process as regards well-aligned ZnO nanorods arrays extended up to a few cm2. The method, which is based on thermal evaporation and controlled oxidation, includes nucleation and growth kinetics control, adjustment of local growth temperature, selection of appropriate source materials and chemical composition of the substrate. The optimized growth parameters allowed to obtain arrays of (0001)-oriented, vertically aligned single crystals with length and diameters within 1-3 microns and 20-10 nm respectively. It is in particular pointed out: (a) the formation of sub-micrometric metal Zn clusters during the metal source evaporation; (b) their adhesion to a ZnO buffer layer (about 300 nm thick) previously deposited on the substrate (glass, Si, ...), which enables to keep an appropriate cluster distribution while avoiding larger clusters/drops formation thanks to suitable "surface wettability" conditions; (c) the subsequent growth of ZnO nanorods owing to the contemporary presence of Zn vapours, oxygen and Zn clusters on the substrate, these last ones acting as selective nucleation points. On the ground of what above and consequent process re-adjustments, the here proposed method, with its elevated yields and reproducibility as well as low production costs, turns out to be especially well-suited for large-scale application requirements

Method for large area deposition of ZnO tetrapod nanostructures

Among several morphologies ZnO presents one quite characteristic nano-crystalline structure, usually reported as "tetrapod" (TP), which consists of four needle-shaped "legs" connected at one common end and respectively arranged as axes of a tetrahedron (Fig. 1). Using an appropriate vapor-solid process, ZnO TPs are produced in large quantity (tens of mg per run) and, when removed from the growth reactor, they appear well assembled like a light white-grey "sponge". These aggregate structures mainly consist of TP nanocrystals, though usually they might also include unreacted Zn metal particles, nanosized ZnO powders and/or partially oxidized ZnO1-x nanostructures/powders. In order to "purify" as-grown TPs from the other undesired structures, the authors propose a multi-step process summarized as following: (1) post growth thermal annealing in vacuum (evaporation of metal Zn) and, subsequently, in oxygen atmosphere (oxidation of not completely reacted particles of ZnO1-x); (2) suspension of all the reaction products in appropriate liquids, in which ZnO is insoluble, in order to decant and to separate TPs from spurious structures; (3) room-temperature deposition of purified ZnO TPs on proper substrates (glass, silicon, alumina, etc., depending on the final application), whose sizes can vary from a few square mm up to many square cm; (4) heating at moderate temperature under low vacuum to remove traces of organic solvent and to favour the sticking of ZnO TPs on the substrates. The described procedure is highly valuable as it allows the achievement of homogeneous distribution of purified ZnO nanostructures on large substrates and a room-temperature deposition process which avoids detrimental interaction of ZnO TPs with substrate material, or with metal contacts previously deposited on the substrates. In practice, the proposed method is a new way to prepare large area films of metal oxides nanostructures, ready for device production. Application of the process to gas sensor fabrication and hybrid compounds (ZnO-MeS) preparation is also reported

Near-infrared photoluminescence enhancement in Ge/CdS and Ge/ZnS core/shell nanocrystals: Utilizing IV/II-VI semiconductor epitaxy

Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II-VI, III-V and IV-VI semiconductor quantum dots. Here, we use relatively unexplored IV/II-VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II-VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II-VI nanocrystals are reproducibly 1-3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II-VI nanocrystals. We expect this synthetic IV/II-VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials