Potential-Modulated Attenuated Total Reflectance Characterization of Charge Injection Processes in Monolayer-Tethered CdSe Nanocrystals

Abstract

Reversible electron injection into pyridine-capped CdSe nanocrystals (pyr-CdSe NCs), tethered to indium-tin oxide (ITO) substrates using mercaptoalkylcarboxylic acids, is characterized using attenuated total reflectance (ATR) spectroelectrochemistry on a planar waveguide. The sensitivity of this technique provides for characterization of redox processes in submonolayer films of pyr-CdSe NCs. Optically determined onset potentials for electron injection, measured as bleaching/recovery of the exciton absorption band, provide estimates for the conduction band edge (E CB ). Potential-modulated attenuated total reflectance (PM-ATR), in which the in-phase and out-of-phase reflectance response is measured as a function of modulation frequency, provides estimates for rates of electron injection. These apparent rate constants are found to be nearly independent of tether chain length, suggesting that communication between tethered NCs and electrochemically less active (i.e., less conductive) regions on the ITO surface is rate-limiting.

Spectroelectrochemistry of Quantum Dots

“…[81] A number of SEC studies of QDs are focused on charge injection into quantum confined band levels of easily reduced QDs with relatively positive conduction-band (CB) potentials (e. g., CdSe, CdS, ZnO, PbSe). [65,77,78,80,[82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98] Electron injection into the CBs of colloidal chalcogenide QDs bleaches the first excitonic absorption (1S h !1S e ), introduces new absorption due to intraconduction-band (1Se!1Pe) electronic excitation and diminishes excitonic PL quantum yield through electron-exciton Auger recombination. [65,[77][78][79][80][82][83][84][85][86]93,95,97,98] SEC can determine the types of surface states in QDs and imitate the evolution of surface defects, providing more dimensional tests for the study of photo-stability.…”

“…Saavedra et al determined the rate of electron injection with respect to the dependence of the linker employed to attach the NPs. [86] In this work, an ATR setup was combined with the modulation of the applied potential to study the charge injection processes of a CdSe submonolayer. [86] An interesting SEC approach is the coupling of a spectroscopic technique with in-situ potentiometry.…”

“…[86] In this work, an ATR setup was combined with the modulation of the applied potential to study the charge injection processes of a CdSe submonolayer. [86] An interesting SEC approach is the coupling of a spectroscopic technique with in-situ potentiometry. [109,110] Band-edge potentials can be in-situ quantified using this approach, without special apparatus or modification of NC surface chemistry.…”

Characterization of Charge-Transfer Kinetics at Organic/Electrode Interfaces Using Potential-modulated Attenuated Total Reflectance (PM-ATR) Spectroscopy

“…The sensitivity can be enhanced by using an ATR geometry, wherein the probe beam is internally reflected multiple times through a planar waveguide electrode. 13,15,18,20 Compared with transmission and external reflection ER spectroscopy, PM-ATR spectroscopy provides a sensitivity enhancement of ten to several thousand, depending on the geometry of the waveguide electrode. The ATR geometry also provides spatial selectivity because the evanescent wave penetrates only a fraction of the wavelength (λ) into the medium adjacent to the waveguide.…”

Electron-Transfer Rate in Potential-Modulated Redox Reactions with Electro-Active Optical Waveguides

“…Spectroelectrochemical techniques are particularly useful to mitigate those difficulties by focusing solely on the faradaic process and avoiding the strong non-faradaic electrical background. [10][11][12][13][14][15][16][17][18][19][20][21][22] Based on spectroscopic optical changes that typically accompany a redox event, it is usually possible to select the wavelength of a probing light beam to uniquely capture the optical changes associated with the faradaic process and is immune to the presence and motion of background ions in the electric double layer.…”