Ultrafast Electron Transfer Dynamics from Molecular Adsorbates to Semiconductor Nanocrystalline Thin Films

Abstract

Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)2(SCN)2 (dcbpy = 2,2‘-bipyridine-4,4‘-dicarboxylate) sensitized TiO2 thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO2. The injection process was characterized by a fast component, with a time constant of <100 fs, and a slower component that is sensitive to sample condition. Similar ultrafast electron injection times were measured in TiO2 films sensitized by Ru(dcbpy)2(X)2 (X2 = 2CN- and dcbpy). Electron injection in these systems was found to compete with the vibrational energy relaxation process within the excited state of the molecules, leading to an injection yield that depends on the excited-state redox potential of the adsorbate. The injection rate from Ru(dcbpy)2(SCN)2 to different semiconductors was found to obey the trend TiO2 > SnO2 > ZnO, indicating a strong dependence on the nature of the semiconductor. To understand these observations, various factors, such as electronic coupling, density of states, and driving force, that control the interfacial ET rate were examined separately. The effect of electronic coupling on the ET rate was studied in TiO2 sensitized by three adsorbates, Re(L n )(CO)3Cl [L n is a modified dcbpy ligand with n (=0, 1, 3) CH2 units between the bipyridine and carboxylate groups]. We found that the ET rate decreased with increasing number of CH2 units (or decreasing electronic coupling strength). The effect of driving force was investigated in Ru(dcbpy)2X2 (X2 = 2SCN-, 2CN-, and dcbpy) sensitized SnO2 thin films. In this case, we observed that the ET rate increased with the excited-state redox potential of the adsorbates, agreeing qualitatively with the theoretical prediction for a nonadiabatic interfacial ET process.

Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles

“…This work, however, provides a comprehensive examination of electron transfer rates between QDs and MOs which varies both the size of the QD and the species of MO systematically. A body of work exists which focuses on electron transfer reactions between organic dyes and semiconducting metal oxide nanoparticles (16,17). Generally, these studies involve the anchoring of an organic dye (donor) with known oxidation potential to a nanoparticulate metal oxide (acceptor) thin film and collecting transient absorption data at the picosecond or nanosecond scale.…”

“…Later, this model was extended to describe electron transfer from a single donating state to a continuum of accepting states, such as those present in the conduction band of a semiconductor (12). This model, which has been used to successfully describe the dependence of electron transfer rate on free energy driving force for systems of organic dyes coupled to various metal oxides (13)(14)(15)(16)(17)(18), has yet to be applied to a quantized semiconducting nanocrystal donor and nanoparticulate metal oxide acceptor (QD-MO) system. The functional form of this many-state Marcus model is as follows:…”

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

“…[18][19][20] This trend was attributed to the existence of a 4 continuum of conduction band states and their increasing density at higher energy in metal oxides, similar to previous reports for ET from molecules to oxides. 21,22 Previous studies of ET in QD-acceptor complexes have also reported faster ET rates at smaller QD size. 23 Unfortunately, the range of driving force was limited and a critical test of ET models has not been possible.…”

Comparison of Interfacial Electron Transfer through Carboxylate and Phosphonate Anchoring Groups†

“…24,31,32,35,39,48 In this model, shown in Figure 8, photoexcitation of the adsorbate prepares an unrelaxed excited state (S** or 1 MLCT * ). Electron injection from this state occurs with rate constant k 1 , which competes with intramolecular relaxation within the excited-state manifold (with rate constant k 2 ).…”