The Doty Group uses advanced optical spectroscopy techniques to probe and control photons, charges, and spins in nanostructured materials.Our goal is to understand how nanometer scale structure and composition can be engineered to create new materials with optimized properties.The potential applications for the materials we study range from next-generation computing devices to high-efficiency photovoltaics The challenge in studying nanostructured materials is that bulk measurements are sensitive only to the average nanoscale structure, and yet many interesting material properties depend on details of the structure and composition with nanometer lengths scales. To overcome this challenge we have developed expertise in the investigation of single nanostructures. While experimentally challenging, this approach allows us to quantify the relationship between structure on nanometer length scales and the behavior of single excitons, photons, charges and spins.
- 2012 UD College of Engineering Outstanding Junior Faculty Award
- 2010 DuPont Young Professor Award 2010 Veeco Research Collaboration Gift
- 2010 University of Delaware Research Foundation Strategic Initiatives Award
- 2009 NSF CAREER Award
- 2008 University of Delaware Research Foundation Award
Quantum Dots and Novel Nanostructures
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.
Quantum Dot Molecules
Quantum dot molecules are formed by coherent tunneling between two individual quantum dots. The coherent tunneling leads to the formation of delocalized states that are truly molecular in nature, with bonding and antibonding orbital states. Quantum dot molecules are analogous to the hydrogen molecule, with two “artificial atoms” (individual quantum dots) coupled together to form delocalized orbitals. Unlike traditional molecules, however, the degree of energy confinement in each quantum dot (analogous to electronegativity) and the degree of coupling (analogous to the separation between atoms) can each be individually tuned. Our prototype material is two InAs quantum dots stacked on top of each other and embedded in a GaAs matrix. By growing a Schottky diode structure with a doped substrate, we can apply electric fields along the growth direction that control both the total charge state of the quantum dot molecule and the relative energy levels of the two dots. The amount of energy level offset controls the degree of coupling and the formation of molecular states.
Energy Transfer in Nanostructured Materials
Nanostuctured materials allow us to tailor material properties by controlling composition and structure on nanometer length scales. Integrating these nanostructured materials into devices while preserving their unique and tunable features, however, is very challenging. One approach is to simply create a film or layer composed of many individual nanostructures and hope that the ensemble preserves the properties of the individual nanostructures. One of the fundamental obstacles to this approach is that nanoparticles are never identical. Consequently, the interactions and energy transfer in an ensemble of nanostructures can be very different than either isolated nanostructures or bulk crystals. Characterizing these interactions and energy transfer mechanisms is challenging because the energy transfer occurs on extremely short time scales. Similar questions about energy transfer and relaxation dynamics in nanostructures are important to the development of new devices that are centered around single nanostructures. We characterize energy transfer in nanostructured materials using ultrafast optical techniques, including time-resolved optical spectroscopy and transient absorption spectroscopy.
- E.Y. Chen, J. Zhang, D.G. Sellers, Y. Zhong, J.M.O. Zide, M.F. Doty. A kinetic rate model of novel upconversion nanostructures for high-efficiency photovoltaics. IEE Journ. of Photovoltaics. In press (2016)
- X. Zhou, M. Royo, W. Liu, J.H. Lee, G. J. Salamo, J. I. Climente, M.F. Doty. Diamagnetic and paramagnetic shifts in self-assembled InAs lateral quantum dot molecules. Physical Review B 91 205427 (2015) http://link.aps.org/doi/10.1103/PhysRevB.91.205427
- F. Xu, L.F. Gerlein, X. Ma, C.R. Haughn, M.F. Doty, S.G. Cloutier. Impact of Different Surface Ligands on the Optical Properties of PbS Quantum Dot Solids. materials, 8 1858 (2015) http://dx.doi.org/10.3390/ma8041858
- D.G. Sellers, S.J. Polly, Y. Zhong, S.M. Hubbard, J.M.O. Zide, M.F. Doty. New Nanostructured Materials for Efficient Photon Upconversion. IEEE Journ. of Photovoltaics. 5 224 (2015)