****NON-TECHNICAL ABSTRACT**** Semiconductor nanostructures known as quantum dots (QDs) can be considered artificial atoms. Two QDs close to each other may become quantum mechanically coupled, resembling an artificial molecule, or quantum dot molecule (QDM). The ability to control the quantum mechanical behavior of assemblies of QDMs is important for future technologies. In order to be of use in future technologies it is necessary to be able to increase the number of QDMs assembled together, just as scientists today assemble many molecules into materials. This Faculty Early Career Development award supports a project that seeks to understand and investigate the signatures and mechanisms of quantum mechanical coupling in two types of QDMs. The geometric configuration of the QDMs under study is one that may be useful for increasing the size of the assembly of QDMs. Therefore the project may lead to a significant impact on technologies ranging from quantum information to photovoltaics. This project includes a comprehensive educational plan consisting of: 1) hands-on research and curriculum development for k-12 teachers; 2) hands-on exploratory science experiences for k-12 students; and 3) the development of interdisciplinary courses on nanoscale materials aimed at advanced undergraduate students. This award is supported by the Division of Materials Research and the Division of Physics.
Quantum dots are at the forefront of research into quantum coupling because they can locally confine single charges in discrete energy states that are analogous to the orbital energy levels of natural atoms. Coupling between two quantum dots leads to delocalized ?molecular? electron and hole wave functions that are distributed over both dots and the barrier in between. Such quantum mechanically coupled quantum dots may be viewed as a quantum dot molecule (QDM). While vertically stacked QDs, forming a vertical QDM, have been an important configuration for studying spin interactions and effects, they are unlikely to be a practical architecture for future technology. This Faculty Early Career Development award supports a project that seeks to investigate and understand the signatures and mechanisms of quantum coupling in two types of potentially scalable architectures of QDMs. These are 1) lateral QDMs consisting of two laterally separated InAs QDs embedded in GaAs and 2) bio-molecular QDMs comprised of two colloidally grown QDs connected by active bio-molecular linkers. Time-resolved optical spectroscopy methods will be utilized to study the quantum mechanical coupling in these single QDMs. The understanding of the physics of this coupling may lead the ability to control the quantum mechanical coupling in ways that are scalable and thus relevant to future technologies such as quantum information technology and optoelectronic devices. This project includes a comprehensive educational plan involving k-12 teachers and students as well as undergraduate and graduate students. This award is supported by the Division of Materials Research and the Division of Physics.
Intellectual Merits Quantum dots are tiny semiconductors, typically about 10,000 times smaller than the diameter of a human hair. Because of their extremely small size, quantum dots can contain only a few electrons and these electrons can only have specific energies. This "ladder" of allowed energy states is analogous to the energy levels of electrons bound to atoms. As a result, quantum dots are often called "artificial atoms." Unlike natural atoms, the size, shape and composition of quantum dots can be changed to tune the "atomic" properties. When two or more "artificial atoms" are brought together and allowed to interact, they can form quantum dot "molecules" that have unique and tunable properties. This is quite similar to the way natural atoms form many different molecules with different properties, but the formation of "molecular" states in quantum dot molecules can be tuned by applied stimuli like electric or magnetic fields. The objective of this proposal was to investigate the mechanisms of quantum dot coupling in order to understand how unique "molecular" properties arise, how they could be engineered with structure and composition of the constituent "atoms" and how the "molecular" properties can be controlled or manipulated after the molecules are formed. Understanding these questions can help us design materials and devices with new functionality for applications in information processing, energy harvesting and other fields. One model system that we studied extensively was laterally-separated pairs of InAs / GaAs quantum dots. (See image 1). Our work provided the first conclusive evidence for the existence of molecular-like states and allowed us to investigate how the properties of the "molecule" change as we add additional electrons or apply magnetic fields. The second image shows a series of discrete spectral lines, each of which corresponds to a different number of electrons or a different spatial distribution of the electrons in the molecule. We have explored other quantum dot systems as well, including probing the mechanisms of energy transfer between "colloidal" quantum dots that are manufactured by a different process and therefore have very different properties. Broader Impacts In parallel with our research efforts, we have developed and disseminated a series of modules intended to support hands-on K-12 education. The modules were prepared by teachers who spent the summer working with our research group. This program seeded a college-wide effort that has created 10 different modules that have been used in more than 10 schools and at informal science education events reaching in excess of 10,000 students. The final image shows one of these modules, exploring ideas of solar energy, in use by local K-12 students. All modules are available for free loan through the UD College of Engineering Lending Library.
