Research area: Materials development
A major aspect of research in the Kuno Lab focuses on the synthesis and characterization of solution-based semiconductor nanowires (NWs). We have therefore developed low-temperature solution chemistry routes for such materials using low melting “catalyst” nanoparticles to induce the asymmetric growth and crystallization of common binary semiconductors. Representative systems include ZnSe, CdS, CdSe, CdTe, PbS and PbSe. The overarching challenge behind these studies involves learning to control crystallization at the molecular level in order to enable the direct bottom up growth of technologically relevant nanostructures.
Features that distinguish these solution-phase NW syntheses from analogous vapor-liquid-solid (VLS) growth approaches include NWs made at temperatures less than 400 °C, NWs whose surfaces are passivated with organic ligands and NWs whose optical properties exhibit quantum confinement effects. In this regard, resulting NW diameters are often below 10 nm and are routinely below twice the bulk exciton Bohr radius of many materials. Lengths also exceed 10 mm. Complementary high resolution transmission electron microscope images show that the resulting one-dimensional (1D) wires are highly crystalline despite their low growth temperatures.
A unique aspect of the developed synthesis is the ability to obtain branched NWs by varying growth conditions such as the reaction temperature, the initial metal-to-chalcogen precursor stoichiometry and the identity of the metal-coordinating surfactant. This has led to the development of CdSe and CdTe nanowires with characteristic tripod, v-shape, and y-shape morphologies. Hyperbranched wires with multiple branching points can also be made. These first examples of NW branching therefore suggest that future developments may lead to the eventual shape control of 1D nanomaterials.
In recent years, fundamental and industrial interest exists in developing nanostructured layered compounds. Motivation arises because reducing their dimensionality leads to corresponding changes of their already anisotropic physical and chemical properties. This opens up new opportunities for investigating the evolution of two dimensional (2D) optical and electrical properties in systems beyond graphene.
Our most recent synthetic efforts have therefore focused on the direct bottom up growth of 2D materials such as TiS2 nanosheets and potential oxysulfide nanobelts. The latter TiSxOy material is interesting because it potentially possesses photocatalytic properties intermediate between the limiting TiS2 and TiO2 cases.
We have simultaneously pursued the preparation of MoS2 and have demonstrated preliminary growth of amorphous nanosheets of this system. This is complemented by similar work on realizing amorphous TiS2 nanosheets. Both discoveries raise the question as to why one can obtain regular shapes in an amorphous material. Current synthetic efforts are focused on developing other 2D systems in a bid to discover general solution-phase pathways as well as synthetic strategies for producing 2D materials in high yield.