Matthew F. Doty

Research Interests

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Quantum dots (QDs) are often called �artificial atoms� because they locally confine single charges in discrete energy states analogous to the orbital energy levels of natural atoms. These artificial atoms are used in a variety of optoelectonic devices, including lasers, single photon sources, and optical and infrared detectors. QDs are also widely used in the biological sciences as fluorescent markers that enable the spatially resolved detection of target biomolecules. Recent advances in materials science and nanofabrication techniques have made it possible to controllably couple individual QDs to create �artificial molecules.� In natural molecules, the degree of quantum coupling is determined by the electronegativity of each atom and the equilibrium spacing between the atomic nuclei. In artificial molecules constructed of QDs the degree of quantum coupling can be engineered using precise control over the spatial positions and relative bandgaps of each QD. This control over quantum mechanical coupling at the level of single electrons and holes enables the design of novel materials with revolutionary properties.

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There are many possible applications for artificial molecules of QDs. Efforts in my group are presently focused on spintronics, quantum computing, and photovoltaics. In a spintronic device, the spin of a single electron or hole confined in a QD could be used as a medium for storing information. A computer based on single spins would require drastically less energy and generate far less waste heat than a conventional charge-based computer. If the spin states can be manipulated and forced to interact coherently, a quantum computer fundamentally faster than any possible conventional computer could be developed. In photovoltaics there are several different ways that quantum dots could be used to increase the efficiency of light absorption and current extraction. In intermediate band solar cells, for example, delocalized molecular states of coupled quantum dots are proposed as an intermediate band that can capture lower energy photons that would otherwise be lost. The dynamics of optical absorption and emission from and to these intermediate bands will determine whether an efficient intermediate band solar cell can be developed. In nanocomposite or organic polymer solar cells, QDs are incorporated into the matrix material (for example the polymer) to act as a dissociation site that separates the photogenerated electrons and holes. The spatial arrangement of the quantum dots determines the efficiency of both exciton dissociation and current extraction.

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Progress towards any of these possible applications requires answers to many fundamental questions about the coupling between QDs. What are the physical mechanisms of coupling? Do particles tunnel between dots or transfer via resonant energy transfer? How do the mechanisms of coupling depend on the material composition of the dots, their spatial separation, their energy levels, or the scaffold that connects the dots? What are the dynamics of interactions between electrons or holes confined within these dots? How can we tune the degree of coupling in situ to create active materials? Research in my group tries to answer these questions with the techniques of optical spectroscopy, including photoluminescence, absorption, and single-photon counting. A common technique is to isolate single pairs of quantum dots and perform measurements when this pair is populated with only a single electron and/or hole. Measurements of the coupling mechanisms for single particles in single pairs of dots enable us to develop a detailed understanding that is unachievable with ensemble measurements. We also investigate the use of nanofabrication techniques to control the coupling and material properties.