Nonmonotonic Size-Dependent Carrier Mobility in PbSe Nanocrystal Arrays.

On the basis of a tight binding system-bath model, we investigated carrier mobility of PbSe nanocrystal (NC) arrays as a function of NC size and inter-NC separation. The size-dependent trend of calculated carrier mobilities are in excellent agreement with recent experimental measurements: electron mobility increased up to NC diameter of ∼6 nm and then decreased for larger NCs, whereas hole mobility showed a monotonic size-dependency. Carrier mobility increase was associated with reduced activation energy that governs charge-transfer processes. In contrast, the decrease in electron mobility for large NCs was found to be due to smaller electronic coupling. Control of inter-NC separation is crucial for mobility enhancement: the mobility may change by an order of magnitude when inter-NC separation varies by as little as 1 to 2 Å. We anticipate similar size-dependency of the mobility in other semiconductor NC arrays, although crossover diameter in which mobility reaches its maximum depends on the material.