Characterization of Photoinduced Electron Tunneling in Gold/SAM/Q-CdSe Systems by Time-Resolved Photoelectrochemistry E. P. A. M. Bakkers,* ,† A. L. Roest, † A. W. Marsman, ‡ L. W. Jenneskens, ‡ L. I. de Jong-van Steensel, § J. J. Kelly, † and D. Vanmaekelbergh † Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht UniVersity, P.O. Box 80000, 3508 TA Utrecht, The Netherlands ReceiVed: January 24, 2000; In Final Form: May 21, 2000 Colloidal CdSe quantum dots were chemisorbed on a gold electrode using a variety of self-assembled monolayers (SAMs) consisting of dithiols and rigid disulfides. After absorption of a photon with an energy larger than the band gap, a long-lived excited state is formed in the quantum dot; this state can decay by electron tunneling via the gold. The rate of photoinduced tunneling was measured directly by intensity- modulated photocurrent spectroscopy (IMPS), and its distance dependence was studied using rigid SAMs separating the Q-CdSe and Au. The tunneling rate was found to depend exponentially on the distance, with a decay length of 2 Å. I. Introduction Semiconductor and metal quantum dots (QDs) have attracted much attention in the past decade because of their interesting size-dependent optical and electronic properties. Potential ap- plications of nanocrystallites are in opto-electrical or electrical devices such as light-emitting diodes or single-electron transistors. 1-6 Much effort has been devoted to the control and improvement of chemical and physical properties of individual particles and arrays of these dots. 7-11 The particles can be attached to metal surfaces using a self-assembled monolayer (SAM) of difunctional linking molecules to form (ordered) two- or three-dimensional structures. 12-16 Intramolecular electron transfer in large (bio)molecules and organized structures is an important topic in current research. The relationship between tunneling rates and distance is usually studied either by synthesizing a series of analogous organic molecules with different donor-acceptor distances or by using biochemical molecules with different intramolecular donor- acceptor spacings. 17-24 Electroactive SAMs with redox centers at a controlled distance from the electrode are also convenient systems because the transfer kinetics can be studied as a function of both overpotential and temperature. 25-28 It is, however, difficult to find a system in which the distance dependence of electron transfer can be studied, independent of changes in the Gibbs free energy (ΔG°), the reorganization energy (λ) of the reaction centers, or the conductivity of the molecule. Another problem is that the transfer rate is often determined indirectly by measurement of the emission decay; quenching mechanisms other than electron transfer might be introduced by a chemical change in the system. We have developed a direct method for studying the distance dependence of electron tunneling that avoids the problems described above. A monolayer of quantum dots is chemisorbed on an organic SAM on a gold electrode, forming a double- barrier tunnel junction. 29-32 The gold/SAM/QD system is used as a photoelectrode in a three-electrode photoelectrochemical cell. With intensity-modulated photocurrent spectroscopy (IMPS), we are able to measure the tunneling rates of photoexcited charge carriers between the particle and the gold electrode. 25-27 In this work, we have studied electron tunneling as a function of the distance between CdSe quantum dots and a gold electrode. To vary this distance, we have used two classes of spacers: rigid and nonrigid molecules. In addition, an ohmic contact was established by epitaxial electrodeposition of isolated QDs on gold. After absorption of a photon with an energy larger than the band gap, a long-lived excited state can be formed if the hole from the HOMO or the electron from the LUMO is trapped in a localized band-gap state (see Figure 1). Excited-state decay via electron tunneling to and from the metal can then compete with intraparticle decay. Process r 1 , electron tunneling from the particle to the gold, can occur only when the gold Fermi level (E F ) is below the electron level (E e ). Process r 2 , tunneling from the gold to the hole in the particle, can only occur when E F is above the hole level (E h ). Using time-resolved photoelectro- chemistry, one can measure the rates of individual electron- transfer steps and, by scanning E F , the electronic structure of a long-lived excited state can be revealed. We show that a long- lived excited state in Q-CdSe consists of a hole trapped in a deep band-gap state and an electron in the LUMO. The distance dependence of photoinduced electron transfer was measured in the range 3-12 Å. II. Experimental Section The chemicals used were of pro analysi (reagent-grade) quality from Merck. High-purity water (18 MΩ cm) was used to prepare the solutions. The experiments were performed at room temperature, unless otherwise stated. All manipulations involving bis(trimethylsilyl)selenium, (TMS) 2 Se, were carried out in a nitrogen-filled glovebox. Q-CdSe Suspension. The Q-CdSe suspension was prepared according to ref 33. A sample of 16.7 g of the surfactant bis- (2-ethylhexyl)sulfosuccinate (AOT, see Figure 2) was purified * Author to whom correspondence should be addressed. † Department of Chemistry and Physics of Condensed Matter. ‡ Department of Physical Organic Chemistry. § Department of Interfaces Chemistry. 7266 J. Phys. Chem. B 2000, 104, 7266-7272 10.1021/jp000286u CCC: $19.00 © 2000 American Chemical Society Published on Web 07/14/2000