Temperature-dependent electron transport through silver nanocrystal superlattices

Temperature-dependent electron transport was measured through three-dimensi onal close-packed alkanethiolstabilized silver nanocrystal arrays using int erdigitated array electrodes. Nanocrystals ranging from 35 to 77 in diamete r with Coulomb blockade energies well above kT were studied. The nanocrysta l superlattices, exhibit linear current-voltage behavior for temperatures a s low as 70 K. Ordered face-centered cubic (fcc) superlattices exhibit a po sitive temperature coefficient of resistivity (TCR), characteristic of a me tal, at temperatures above approximately 225 to 245 K, depending on the par ticle 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 t he transition temperature, the TCR for the size-monodisperse nanocrystal ar rays 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 nanocrystal s 77 A to 35 A in diameter, respectively, characteristic of a gap in the de nsity 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 nanoc rystals exhibit insulating behavior at all temperatures studied due to (And erson-type) disorder.