Tunable Band-Edge Potentials and Charge Storage in Colloidal Tin-Doped Indium Oxide (ITO) Nanocrystals

Abstract

Degenerately doped metal-oxide nanocrystals (NCs) show localized surface plasmon resonances (LSPRs) that are tunable via their tunable excess charge-carrier densities. Modulation of excess charge carriers has also been used to control magnetism in colloidal doped metal-oxide NCs. The addition of excess delocalized conduction-band (CB) electrons can be achieved through aliovalent doping or by postsynthetic techniques such as electrochemistry or photodoping. Here, we examine the influence of charge-compensating aliovalent dopants on the potentials of excess CB electrons in freestanding colloidal degenerately doped oxide NCs, both experimentally and through modeling. Taking Sn 4+ :In 2 O 3 (ITO) NCs as a model system, we use spectroelectrochemical techniques to examine differences between aliovalent doping and photodoping. We demonstrate that whereas photodoping introduces excess CB electrons by raising the Fermi level relative to the CB edge, aliovalent impurity substitution introduces excess CB electrons by stabilizing the CB edge relative to an externally defined Fermi level. Significant differences are thus observed electrochemically between spectroscopically similar delocalized CB electrons compensated by aliovalent dopants and those compensated by surface cations (e.g., protons) during photodoping. Theoretical modeling illustrates the very different potentials that arise from charge compensation via aliovalent substitution and surface charge compensation. Spectroelectrochemical titrations allow the ITO NC band-edge stabilization as a function of Sn 4+ doping to be quantified. Extremely large capacitances are observed in both In 2 O 3 and ITO NCs, making these NCs attractive for reversible charge-storage applications.

Investigating the Role of Surface Depletion in Governing Electron-Transfer Events in Colloidal Plasmonic Nanocrystals

“…33,47 However, <n max > in undoped ZnO NCs was found to be on the order of 10 20 cm −3 , which is an order of magnitude smaller than that for doped In 2 O 3 NCs, where aliovalent doping is expected to increase the electron density. 13,17,45 Very similar trends in electron density are obtained with size series of Sn:In 2 O 3 NCs with different Sn atom % (Figure S18). Analogous to the results of the size series, for the doping series, volumetric addition of CoCp* 2 leads to an increase in the electron density and a simultaneous increase in the volume fraction of the plasmonic core (Figure S19).…”

“…Photodoping studies on a doping series of Sn:In 2 O 3 NCs and other doped metal oxides have reported similar observations. 17,32,33,35,45 Here, we confirm and interpret these trends by analyzing the distribution of electrons between the core and depletion layer as chemical doping proceeds.…”

“…An increase in the Sn doping percentage increases the electron density by lowering the energy of the conduction band minimum while the Fermi level stays at the same chemical potential. 45 Since the absolute position of the Fermi level is not affected by the doping percentage, this thermodynamic limit is independent of the Sn doping percentage. 17,45 This limit is also evident from the observation that when the chemical potential of the electrons is raised by employing larger NCs having the same doping percentage, the maximum electron density that can be transferred to the NCs decreased further to 1.4 × 10 20 cm −3 for 21 nm (Figure S20).…”

“…45 Since the absolute position of the Fermi level is not affected by the doping percentage, this thermodynamic limit is independent of the Sn doping percentage. 17,45 This limit is also evident from the observation that when the chemical potential of the electrons is raised by employing larger NCs having the same doping percentage, the maximum electron density that can be transferred to the NCs decreased further to 1.4 × 10 20 cm −3 for 21 nm (Figure S20). Previous reports also suggest that when a reducing agent with a lower chemical potential was employed, a smaller electron density is transferred from the reducing agent to the NCs.…”

“…Specifically, considering changes in the extinction, peak energy, and line shape during the course of the chemical reduction, we can track changes in depletion width and electron concentration in the plasmonic core. In previous reports, “extra” electrons added by chemical or photochemical reduction have been counted by titrating with an oxidizing agent such as [FeCp 2 *] + , ,,, but this does not account for the distribution of those electrons within each NC or even among the ensemble. Polydispersity in the NC diameter and doping percentages, and surface depletion, all can significantly affect the LSPR peak energy .…”