Temperature-Dependent Electron Transport through Silver Nanocrystal Superlattices

Abstract

Temperature-dependent electron transport was measured through three-dimensional close-packed alkanethiol-stabilized silver nanocrystal arrays using interdigitated array electrodes. Nanocrystals ranging from 35 to 77 Å in diameter with Coulomb blockade energies well above kT were studied. The nanocrystal superlattices exhibit linear current−voltage behavior for temperatures as low as 70 K. Ordered face-centered cubic (fcc) superlattices exhibit a positive temperature coefficient of resistivity (TCR), characteristic of a metal, at temperatures above approximately 225 to 245 K, depending on the particle size. The values of the conductivity, on the order of 10-6 to 10-7 Ω-1 cm-1, however, are characteristic of semiconductors. Below the transition temperature, the TCR for the size-monodisperse nanocrystal arrays becomes negative, characteristic of an insulator and the conductance G, of the ordered arrays scales exponentially with temperature as G ∝ exp[−(To/T)ν]. The exponent ν, ranges from 0.67 to 1.34 for nanocrystals 77 Å to 35 Å in diameter, respectively, characteristic of a gap in the density of states in the overall electronic structure of the superlattice. We believe that electron transport occurs through a polaron hopping mechanism. In contrast to the organized superlattices, disordered close-packed nanocrystals exhibit insulating behavior at all temperatures studied due to (Anderson-type) disorder.