(Top) Three-dimensional energy confinement leads to discrete energy states in quantum dots. (Bottom) Schematic of quantum dot photoluminescence.

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. The size of the quantum dot, along with the material composition of the quantum dot and the confining barrier, determine the energy difference between allowed states of the conduction and valence band, as shown in the figure on the left. The ability to tune the energy levels is one of the features that make quantum dots very interesting for optoelectronic devices. One of the simplest optical properties is photoluminescence, which is schematically depicted in the bottom panel of the figure on the left. (1) A photon promotes an electron from the valence band to the conduction band, leaving behind a hole. (2) The electron and hole relax to the lowest allowed energy levels of the QD. (3) The electron and hole recombine to emit a lower-energy photon. Interactions with charges and spins already occupying the quantum dot lead to interesting physical properties and device opportunities.

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.

There are many possible applications for QDs and artificial molecules composed of QDs. Efforts in the Doty 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, many different approaches based on quantum dots have been proposed. Films of colloidal quantum dots, for example, have been proposed as an inexpensive way of producing materials with tailored bandgaps for effective optical absorption of solar photons. InAs quantum dots have been proposed for use in low-energy photon harvesting and multi-junction photovoltaics. We have also developed a new nanostructred material that takes advantage of the unique properties of quantum dots to enable efficient upconversion of two low energy photons into a single high energy photon, as shown in the figure. This upconversion approach could provide a pathway to more effective harvesting of the solar spectrum without significantly increasing the complexity of photovoltaic device design.

Schematic of one upconversion device based on an InAs quantum dot. Absorption of a mid-energy photon (1) promotes an electron to a confined state. Rapid escape of the hole (2) due to zero valence band offset achieved with the graded host composition prevents radiative recombination. Absorption of a low-energy photon (3) promotes the electron to a high-energy state from which
it escapes (4) to reach the recombination zone and emit a high-energy photon
(5).

Progress towards any of these possible applications requires answers to many fundamental questions about the properties of individual quantum dots and the interactions 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.