Our group's background in inorganic chemistry, catalysis, materials science, and biotechnology allows for a unique perspective on the current limitations in energy conversion/storage and national security capabilities. The underlying technologies of these energy and sensor applications are very similar, and depend on the precision of which interfaces are constructed, accessible surface areas and reaction sites, as well as high fidelity microstructure control of packaging. These technologies can benefit from the integration and optimization of materials based on inorganic nanoparticles for lighter, cleaner, and more efficient energy conversion processes, increased sensitivities of chemical and biological sensors, and self-healing interfaces and processes. These nanomaterials posses a unique set of conditions that enable the tuning of optoelectronic properties and catalytic activity. First, each nanoparticle has a size around 1-10 nanometers, where each nanometer is one billionth of a meter. This allows each particle to posses a set, "quantized", number of atoms, each of which bring with it a set number of electrons, and orbitals. Second, this collection of atoms are confined to a 'box' which is only a few nanomaterials, which further adjusts the energy levels, pushing them to higher energies. This allows for the tuning of optical and electrical characteristics, such as light absorption and emission in quantum dots (see below). Third, each nanoparticle posses a high percentage of it’s atoms at the particle surface, leading to poor coordination, and "dangling" bonds. Moreover, these surface atoms are often packed in high-energy configurations, such as corners and edges (see image). It is at these high energy surfaces in which heterogeneous catalysis takes place, such as that found in a fuel cell. Taken together, the tuning of energy levels by quantization and confinement lead to novel optical and electronic properties of nanomaterials, while their energetic surfaces allow for novel catalysis to take place with higher efficiency and stability.
Synthesis & Fabrication: The synthesis of these nanomaterials is an interesting combination of wet-chemical synthesis and materials "solid-state" processing. For example, we employ methods that are similar to organic synthesis, but add in components such as high temperature processing, annealing, nucleation and growth, and quenching. Thus, in our studies, "nanofabrication" takes place not in expensive clean rooms or fabrication facilities, but instead in a beaker, schlenk line, and glove box. There, we utilize air-free inorganic and materials chemistry to carry out the synthesis of a number of nanoparticles, including semiconductive quantum dots. As mentioned above, the energies with nanoparticles are controlled by size. This is perhaps best shown by the light absorption and emission properties of CdSe quantum dots (see image). In addition to the metallic, or semiconductive solid cores, these nanoparticles also posses a ligand "shell" that surrounds and protects the particle. This ligand shell can be functionalized, and undergo classical chemical reactions. In addition, these ligands and their functionality can be used as anchor sites to attach additional nanoparticles, or biomaterials for self-assembly studies, as described next.
Biomimetic Assembly: Nature has given us a tremendous example of the potential to self-assemble an organized hierarchy of autonomous nano- and micro-components. These components posses functional; structure, reaction centers, and molecular recognition sites that signal and transduce all biochemical processes. This includes highly efficient energy transfer, ion transport, information storage, replication, and self-healing. Each of these natural biological and biochemical processes can be considered the ultimate state of the art, and a model for non-biological systems. One of our research pursuits is to impart our nanoparticles with these types of properties. Thus, we attempt to "mimic" these systems using inorganic nanomaterials that have their interfaces modified with both native and synthetic biomaterials. One interesting question to ask is: If coated with a particular biomaterial, can a nanoparticle take the properties of the biomaterial? That is to say, can we start to self-assemble our synthesized nanomaterials into structures that mimic cells, membranes, or DNA? And if so, will our abiotic materials have similar functionality as their biotic analogues? In order to do this however, requires multiple surface chemistry steps, as well as purification processes. However, when successful a number of "biomimetic" processes emerge, such as those discussed next.
Energy Transfer: These nanomaterials also have the propensity for energy transfer. To explore this phenomenon, we are currently interested in a number of fundamental studies. The first, is to employ quantum dots in novel Fluorescence (Forster) Resonance Energy Transfer (FRET). In FRET, an excited electron in a donor (D*) will transfer non-radiatively to an acceptor (A) due to virbrational overlap accompanying their coupled dipole-dipole interaction (through-space), which is enhanced during excitation. The transferred 'energy' (D*+A→A*+D) is then radiatively released from the excited A (A*), revealing emission (F) at lower energy. In our systems, the quantum dots are employed as both Donors & Acceptors, as well as in conjunction with molecular fluorophores. An interesting case emerges when D-to-A distances are tuned. The ability to tune these distances (r) however is often limited by synthetic chemistry. If however, the r is defined by the structural tunability of a biomaterial, then the systems become very interesting.
The second use of these quantum dots is to employ their light harvesting (absorbing) properties for future, third-generation, photovoltaic applications. These photovoltaics have the potential for decreased manufacture cost, higher conversion efficiencies, flexibility, and incorporate molecules and nanomaterials for current generation. One such "solar cell" is that of dye sensitized solar cells (DSSCs), which utilize molecular dye to absorb light, generating an excited electron, which is then transported to a semiconductive surface. In our systems, the quantum dot serves as the dye, and a metal oxide, such as titanium dioxide, is used to rectify electron transport. The advantage here is that the quantum dot above can be synthesized to absorb different wavelengths of light, as described above, allowing for more solar radiation to be converted to electricity. This project is in its infancy, however we are currently progressing in the preparation of all the necessary nanomaterials, with construction of a cell to begin soon.