Electronic transport in nanocrystal solids

Nanocrystal quantum dots are often called “artificial atoms”, and a long-term goal of the nanoscience community is to engineer completely new materials and states of matter using nanocrystals. The first step towards this goal is to understand how charges flow in solids made from these materials. The applied motivation for such fundamental studies is the direct relationship between improved electrical conduction and better device performance.

In bulk crystalline materials, electrons and holes (positively-charged electron vacancies) are delocalized throughout the crystal due to strong quantum coupling between neighboring atoms in the material. This is similar to how π electrons are delocalized in conjugated molecules, and it allows charges to flow rapidly through the solid. This regime is known as coherent or band transport. In a solid made of nanocrystals, small variations in the sizes between neighboring nanocrystals and the larger separations between nanocrystals compared to atoms in a solid disrupt the quantum coupling and introduce energy barriers between nanocrystals. These “bumps in the road” are overcome by the thermal energy or by tunneling, and this regime is known as incoherent or hopping transport. Research efforts in the group focus on elucidating the sizes of these energy barriers, reducing them, and addressing fundamental questions about the physical nature of hopping transport. This is accomplished by examining the effects of temperature, magnetic fields, surface chemistry and particle size on the electrical conductivity. Our workhorse experimental methods are cyclic voltammetry (electrochemistry) and spectroelectrochemistry on quantum dot films, and these measurements are complemented by field-effect transistor, photoconductivity and Hall effect techniques. The transport studies also make extensive use of analytical theory and simple numerical simulations.

Selected publications