Electronic transport in CdSe quantum dot arrays

Abstract

When an isolated piece of conductor is sufficiently small, the energy to add another electron can be larger than the available thermal energy, and the number of electrons on the island is well-defined. If the island is made still smaller, the quantum confinement energy also becomes large, the electronic states on the dot become discrete, and the island is called a quantum dot, or sometimes an artificial atom. Quantum dots have been a focus of active research for the past decade, both as model systems for exploring physics, and as the ultimate limit in size reduction for conventional transistors. Early quantum dots were made only by lithographic patterning, but more recently the solution-based synthesis of semiconductor nanocrystals, which are much smaller than the lithographic quantum dots, has been developed. The nanocrystals can range in size from 1.5 nm to 8 nm in diameter with a narrow size distribution, and they can form close-packed arrays when deposited from solution, with organic molecules that coat the nanocrystals serving as spacers. These quantum dot arrays have the potential to be model artificial solids, with tunable intersite coupling, site energies, and order. I present results for electronic transport measurements on large arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time and four orders of magnitude in current. Furthermore, we do not observe a steady-state dark current for fields up to 1x106 V/cm and times out to 5 x 104 seconds.

(cont.) Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field-scaling of the current at all times. We posit the existence of a narrow space charge region near the injecting contact, and provide a consistent interpretation of our results within this model. The observation of extremely long-lived current transients points to the importance of long-range Coulomb interactions between charges on different nanocrystals; in the picture we develop, the interactions within the narrow space charge region determine the current.