Anvar A. Zakhidov

The Optical Properties of Porous Opal Crystals Infiltrated with Organic Molecules

A fluorescent dye and a photochromic dye were infiltrated into polycrystalline samples of porous opal in order to determine the effects of opal repeat dimension on the optical properties of the dyes. These opals consist of faulted face-centered-cubic arrays of monodispersed, nanoscale SiO2 spheres. The observed absorption and fluorescence of tris(8-hydroxyquinoline) aluminum (Alq3) depended on the lattice constant of the opal. The Alq3 exhibited a blue shift of the absorption edge and a decrease of fluorescence intensity with decreasing lattice constant. Also, the stability of the photochromic behavior of cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl) ethene (CMTE) increased remarkably inside the opal matrix. These characteristics are possibly due to the interaction between the nano-structure-confined dyes and the surfaces of the SiO2 spheres.

ELECTRICAL PROPERTIES OF A PERIODIC POROUS CARBON REPLICA OF OPAL

The electrical conductivity and magnetoconductance of pyrolized porous phenolic resin opal replicas have been studied as a function of their heat treatment temperature (HTT) up to 2380°C. Porous graphite can be formed by pyrolizing the phenolic resin opal replica. In the case of replicas with a low HTT, a crossover from Mott variable range hopping (VRH) to Efros and Shklovskii VRH upon decreasing the temperature has been experimentally observed. For those with a high HTT, the electrical properties are consistent with those of pyrolytic graphite. The occurrence of positive magnetoconductance indicates the existence of quantum effect at low temperatures.

Nanostructured thermoelectrics based on periodic composites from opals and opal replicas. I. Bi-infiltrated opals

We have developed a variety of templating processes for the fabrication of three-dimensionally periodic, nanostructured thermoelectrics from opal and inverse opal matrices. These opals and inverse opals are periodic at optical wavelengths and have extremely high interfacial area. It was hoped that scattering processes at the interface between opal and infiltrated thermoelectric material would increase the thermoelectric figure of merit (ZT) by having a greater effect on phonon-mediated (lattice) thermal conductivity than on electronic conductivity. We provide the first demonstration that this approach can increase ZT by evaluating a simple prototype system: bismuth infiltrated into porous SiO/sub 2/ opal. We find a larger fractional decrease in thermal conductivity than for electrical conductivity (relative to bulk polycrystalline Bi). Since the thermopower is little changed, the overall effect we observe is as much as a two-fold increase of ZT compared with that for polycrystalline bulk bismuth. However, the observed ZT is much smaller than for single crystal bismuth.

Electronic transport properties of Bi/sub x/Te/sub y/-infiltrated synthetic opals

We have demonstrated that crystalline Bi/sub x/Te/sub y/ can be grown directly inside the 100-200 nm diameter pores of a synthetic opal host using a metal-organic chemical vapor deposition (MOCVD). A series of Bi/sub x/Te/sub y/-infiltrated opal samples have been characterized by X-ray powder diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDXS). Thermoelectric power values in the infiltrated opal samples ranged from -70 /spl mu/V/K to +200 /spl mu/V/K at room temperature T=300 K for the Bi- and the Te-rich Bi/sub 2/Te/sub 3/ compositions, respectively. The n-type (Bi-rich) samples were found to exhibit metallic electrical resistivity R (dR/dT>0) from 10 to 300 K, while the p-type (Te-rich) samples demonstrated negative dR/dT.