Chemically Functional Semiconductor Nanocrystals: Electrochemistry and Self-Assembly on Surfaces
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Chemically Functional Semiconductor Nanocrystals: Electrochemistry and Self-Assembly on Surfaces Benjamin M. Hutchins, Andrew H. Latham, Mary Elizabeth Williams* Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 ABSTRACT Semiconductor nanocrystals (i.e., Quantum Dots, QDs) exhibit size-dependent emission properties and have synthetically adjustable ligand shells, making them interesting materials for applications ranging from luminescent displays to biomolecular tags. In this paper, the electrochemical properties of two types of nanocrystal are studied with an emphasis on the effect of core/shell vs core structures. The band gap energy of CdSe particles, measured using optical spectroscopy, was shown to increase slightly with the application of a ZnSe shell, as expected based on the increased energy required to transfer an electron through the shell material. The electrochemically determined band gaps are overestimated in the case of CdSe/ZnSe core/shell nanoparticles, reflecting the band gap of the ZnSe shell. Finally, QDs were self-assembled onto gold surfaces by electrostatic and covalent attachment, and their presence confirmed by fluorescence spectroscopy. The high intensity of emitted light shows that the QDs can be selfassembly onto metallic surfaces, without energy transfer quenching of the luminescence. INTRODUCTION The electronic properties of semiconductor nanocrystals (known as quantum dots, QDs) have been extensively examined over the past decade.1-3 Their size- and composition-dependent band gap energies provide a near continuous spectrum of narrow line-width emission sources. Only recently, however, have their bulk properties been investigated using electrochemical methods.4-6 Using cyclic voltammetry (CV), the bulk electronic properties of CdSe and CdS QDs have been examined and compared to well-understood UV-Visible absorption spectra; the difference in potential between the anodic and cathodic peak currents has been shown to correspond to the average band gap (i.e., the separation of the lowest unoccupied and highest occupied (LUMO and HOMO) energy levels) of the sample.6 This previous work showed that the electrochemically determined band gaps may slightly overestimate those determined spectroscopically. Application of an electrical potential to an electrode surface shifts the Fermi level of the electrode and thus that of any surface-adsorbed particles to higher energy (reduction) or lower Conduction Band EF Valence Band Energy / V ZnSe Shell
CdSe Core
Scheme 1. Energy level diagram depicting the relative energies of the valence and conduction bands of two semiconductors, ZnSe and CdSe, versus the experimentally adjustable Fermi level (EF) of the electrode.
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energy (oxidation), as shown in Scheme 1. During voltammetry of semiconductors, when the Fermi potential reaches the semiconductor band edge, an electron can be transferred between the particle and the electrode, causing current to flow. A current peak in the cyclic voltammogram thus c
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