Synthesis And Photoelectric Properties Of Binary And Multi-element Metal Sulfides

Crystalline metal sulfides as a typical and important group of semiconductorshave attracted extensive investigation due to their unique optical, magnetic andelectrical properties and the intriguing prospects for developments in photovoltaicsolar cells, biological labeling and medical diagnostics, etc. However, the uniquephotoelectrical properties of colloidal semiconductor nanocrystals exhibit distinct size-,shape-and composition dependence. Thus, developing general and facile syntheticapproaches that yield metal sulfides nanocrystals with precise control over not only thesize and shape but also the chemical composition is highly desirable for bothfundamental studies and practical applications. At present, a wealth of methods hasbeen developed for the synthesis of metal sulfide nanocrystals, but these developmentsare mainly concentrated in the controlled synthesis of binary sulfide. It is not desirableto controllable synthesis of ternary sulfides or even multiple sulfides. Therefore,large-scale, low-cost and effective synthesis and assembling of the metal sulfides withdifferent morphologies have a special significance both from the point ofnanoscientific and nanotechnological views.In this paper, we mainly focus on the issues related to the facile synthesis,morphology control and growth mechanism of ternary and quaternary metal sulfides.Then we have characterized the photoelectric property, optical band gap andphotovoltaic performance of these compounds. Further more, this paper also studiedthe internal electrical properties of the binary ZnSe nanowires under applied strain.The main contents are as follows:(1) Single crystalline Cu9BiS6nanoplates and composite structure CuBiS2nanowires have been synthesized via a hydrothermal route by using different sovlents.The morphology of these nanoplates identified by electron microscope was thehexagonal and octagonal configurations respectively. And the CuBiS2nanowires wereproved to be the twin crystal. The microstructures and growth direction of these twokind of nanostructures were analysed carefully by TEM photos. Optical diffusereflectance measurement demonstrated that these Cu9BiS6nanoplates and CuBiS2nanowires have a band gap of1.25eV and1.1eV respectively, which matched thesolar spectrum very well, thus were confirmed to be the alternative photovoltagematerials. (2) Flower-like hierarchical nanostructures with Cu3BiS3nanosheets asbuilding blocks formed by in situ corrosion of matrix Cu3BiS3microspheres weresynthesized via a facile hydrothermal method. Their morphology, microstructure, andcrystalline phase were characterized by scanning electron microscopy (SEM),transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively.Pore-size distribution analysis indicated that both mesopores and micropores existed inthe as-obtained products, which are favourable for increasing of surface-to-volumeratio. The optical band-gap of the hierarchical Cu3BiS3nanostructures was estimatedto be~1.2eV by UV-vis spectroscopy. Electrochemical measurements were also usedto investigate the electrochemical Li+intercalation performance for the flower-likeCu3BiS3nanostructures. The results showed that the initial discharge capacity of thenanosheets based Cu3BiS3hierarchical structures is676mAhg-1which is lager thanthe thereotical value of Bi2S3(625mAhg-1), but it degraded quickly duringsubsequent cycles, and further improvement in cyclic stability is still needed forpractical application in lithium-ion batteries.(3) Cu2ZnSnS4nanoparticles have been prepared by wet chemical method. Thecrystal structure analyzed by XRD coincides to stannite with tetragonal symmetry andlattice parameters a=0.5427nm and c=1.0848nm. Morphology of these nanoparticlescharacterized by SEM coincides to the well-dispersed nanospheres with the diameterranging from5nm to10nm. The band gap has been estimated to be about1.5eVbased on the corresponding Uv-vis absorption spectra.(4) Controllable interfacial alloying is achieved at a Au-ZnSe nanowire (M-S)contact via in situ Joule heating inside transmission electron microscopy (TEM). TEMinspection reveals that the Au electrode is locally molten at the M-S contact and the tipof the ZnSe nanowire is covered by the Au melting. Experimental evidences confirmthat the alloying at the reversely biased M-S contact is due to the high resistance of theSchottky barrier at this M-S contact, coincident to cathode-control mode.Consequently, in situ Joule heating can be an effective method to improve theperformance of nanoelectronics based on a metal-semiconductor-metal nanostructure.(5) The effect of strain on the current carrying capacity of ZnSe nanowire hasbeen studied by in situ transmission electron microscopy (TEM). Under TEMinspection the strain can be created at the selected position in a single ZnSe nanowireby the compressive stress applied along its axial direction using a movable probeelectrode. The induced strain is controllable in the magnitude of curvature of the ZnSenanowire bent by careful manipulation of the movable probe electrode. In situ current-induced Joule heating has confirmed that the strained segment in a single ZnSenanowire exhibited better ability than the unstrained segments against Joule heating.Consequently, the current carrying capacity of the ZnSe nanowire can be effectivelyenhanced by intentionally created strain. The experimental results have also provedthat a significant increase of the electrical conductance and the thermal resistance canbe achieved simultaneously in a single nanowire by the intentionally designed andcreated strain.