Temperature-dependent electron transport through silver nanocrystal superlattices

Temperature-dependent electron transport was measured through three-dimensional close-packed alkanethiolstabilized silver nanocrystal arrays using interdigitated array electrodes. Nanocrystals ranging from 35 to 77 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) Omega (-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 infinity exp[-(T-o/T)(v)]. The exponent v, ranges from 0.67 to 1.34 for nanocrystals 77 A to 35 A 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.