Size-Dependent\nCharging Energy Determines the Charge\nTransport in ZnO Quantum Dot Solids

Building up a solid-state\nmaterial from quantum dots (QD), which\nare often referred to as artificial atoms, offers the potential to\ncreate new materials with unprecedented macroscopic properties. The\ninvestigation of the electronic properties of such QD assemblies has\nattracted attention due to the increasing applications of QD solids\nin both electronics and optoelectronics. In the past, charge transport\nin QD assemblies has been explained by a variety of mutually exclusive\ntheories, with the Mott and Efros-Shklovskii variable range hopping\nmodels being most common. However, these theories fall short in explaining\nthe anomalous exponents of the temperature-dependent conductivity\n∝ exp (− (<i>T</i><sub>0</sub>/<i>T</i>)<sup>α</sup>) observed in various QD materials. Here, we measure\nthe temperature-dependent conductivity of semiconducting ZnO QDs under\ndifferent UV illumination intensity. Regulating the UV intensity allows\nus to systematically change the effective diameter of the ZnO QDs\nwithout having to rely on cumbersome size control by synthesis. Instead,\nthe UV level controls the width of the QD depletion shell and therefore\nthe size distribution in the overall material. We observe exponents\nthat systematically increase from α = 0.25 to α = 0.62\nwith increasing illumination intensity, which we interpret in terms\nof a charge transport being limited by the (size-dependent) charging\nenergy of the QDs.