This paper focuses on: (a) the influence of the surface properties of quantum dots in determining the electrical and optical properties of semiconductor quantum dots in colloidal suspensions, and (b) the use of chemically-directed assembly to integrate nanoscale quantum dots at densities approximating 10+17 cm-3.
Photoluminescence (PL) and absorption spectra of colloidal cadmium sulfide (CdS) quantum dots are analyzed to investigate the role of surface states in determining the electrical and optical properties of these semiconductor quantum dots; it is found that: (a) binding of selected amino-acid based biomolecules to the surface states of these quantum dots leads to quenching of the photoluminescence (PL) from these quantum dots, and (b) the wavelength for the onset of the optical absorption of such a colloidal suspension shifts as the concentration of the electrolyte is changed. Bandbending associated with the field caused by the intrinsic spontaneous polarization of these würtzite quantum dots leads to a shift in the absorption threshold as a function of the electrolytic concentration. Results presented in this talk will provide evidence that the quenching of the PL intensity of a suspension of colloidal cadmium sulfide (CdS) quantum dots may be accomplished by introducing selected biomolecules in the quantum dot suspension. These altered electronic and optical properties underlie a number of applications of these nanocrystals in sensing electrolytic concentrations, in tuning optical transition energies, as well as in the potential control of quantum dot blinking.
Layer-by-layer chemically-directed assembly of ensembles of semiconductor quantum dots will be discussed for the case where layers of quantum dots alternate between ZnS-coated CdSe and uncoated CdS. The chemically-directed linking is accomplished through the use of biomolecular links, specifically peptides. Ensembles of nanoscale quantum dots with biomolecular links facilitate the fabrication of systems containing 10+17 quantum dots per cm+3.
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