Seung Min Park

Polyhedral gold nanocrystals with decahedral, icosahedral, and truncated tetrahedral shapes are synthesized by a simple one-pot polyol process in the prescence of poly(vinyl pyrrolidone) (PVP). High PVP concentration up to 360 equiv of the gold precursor, HAuCl4, effectively stabilizes decahedral seeds to yield uniform decahedra with various edge sizes. Decreased PVP concentration subsequently leads to selective formation of icosahedra and truncated tetrahedra. This results from a combination between the relative energy difference of the polyhedral structures and the oxidative etching rate of the seeds by Cl-/O2 during the reaction. The distinct morphologies of gold nanocrystals exhibit characteristic extinction patterns in the UV−vis−NIR ranges, and these properties are successfully analyzed by the discrete dipole approximation (DDA) calculation. Most extinctions stem from the polar and azimuthal dipolar excitations, and azimuthal quardrupole resonance appears between two dipolar bands in the 88-nm decahedra. Given these shape- and size-dependent optical properties, gold nanocrystals hold considerable promise for biomedical and photonic applications.

Photocurrent of a single ZnO nanowire synthesized by a sol-gel route was investigated. In vacuum, the dark current was bigger but the photoresponse was slower than that in air, attributed to the release of the available charge carriers by the desorption of water molecules and the decrease of the exchange rates of molecular ions. Under the steady radiation of the ultraviolet light (λ=325nm), a gradual decrease of the photocurrent was noticeable, which can be explained in terms of the annihilation of the carriers by the replacement of hydroxyl groups (OH−) by O2−, resulting in the decrease of charge carriers.