Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage

Abstract

For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that postsynthetic modification of the surface chemistry of all-inorganic copper zinc tin sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages of as low as 4 V, which is a 3-fold or greater reduction over previous examples of nonoxide 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 situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor, yielding fundamental insight into the reaction mechanism and mass transport in the low-voltage regime. A crossover from mass-transport-limited to reaction-rate-limited EPD is observed. Under the latter conditions, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film, despite the gravitational stability of the colloids in the absence of electric fields. This knowledge is applied to deposit thick, uniform, and crack-free films without sintering, from stable, well-dispersed colloidal starting materials.

On Cordelair–Greil Model about Electrophoretic Deposition

“…[33] Dillon et al found that ligands could greatly affect electrophoretic mobility and electrochemical reactivity of particles used for EPD. [34] The charged status of QDs could also be affected by ligands. [13] Therefore, the EPD processes are very sensitive to the changes in ligands.…”

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

“…Therefore, it is essential to control the experimental conditions and reduce the voltage so as to prevent undesired reactions, such as decomposition of NCs and solvents. 133 Besides, a disadvantage of EPD is that a voltage-limited maximum thickness exists due to the accumulation of deposited charged NCs that impose repulsive electric fields.…”

“…After SPILE, the NCs capped with inorganic anions are negatively charged, leading to the development of special NC-deposition methods based on electric fields. For example, electrophoretic deposition (EPD) technique has been applied to efficiently load negatively charged NCs onto cathodes , and NC/graphene composites have been obtained by capitalizing on the electrostatic interactions between NCs and positively charged graphene . The electrostatic charge of a NC can be determined from electrophoretic mobility measurements, and the result is usually represented by ζ-potential.…”

“…In addition, since IL-capped colloidal NCs contain surface charge, electrophoretic deposition (EPD) is a promising technique to efficiently load negatively charged NCs onto cathodes via applied electric field. , Technically, EPD is governed by a threshold voltage to overcome resistance between NCs and cathode (Figure b). Therefore, it is essential to control the experimental conditions and reduce the voltage so as to prevent undesired reactions, such as decomposition of NCs and solvents . Besides, a disadvantage of EPD is that a voltage-limited maximum thickness exists due to the accumulation of deposited charged NCs that impose repulsive electric fields.…”

“…During deposition, the space between the electrodes is filled with NC solution. Reprinted with permission from ref . Copyright 2018 American Chemical Society.…”