(637d) Understanding Low-Voltage Electrophoretic Deposition of Non-Oxide Semiconductor Nanocrystals

Electrophoretic deposition (EPD) of colloidal nanocrystals has tremendous potential as a method for the directed assembly of functional semiconductors. However, to achieve this, it will be critical to understand what governs the threshold voltage for deposition and how the voltage can be significantly reduced, in order to prevent electrochemical damage to the accreting semiconductor. Herein we demonstrate that post-synthetic modification of the surface chemistry of all-inorganic copper-zinc-tin-sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages as low as 4V—a three-fold or greater improvement over previous examples of non-oxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs. Thus, both the electrophoretic mobility and electrochemical reactivity of these particles are increased, favoring deposition. In situimaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor; this, coupled with chronoamperometry, reveals the fundamental reaction and mass transport limitations of low-voltage EPD. At low voltage, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film. This knowledge is applied to deposit thick, uniform and crack-free films without sintering from stable, well-dispersed colloidal starting materials.