Nanocrystals are extremely small pieces of semiconductor material with dimensions in the nanometer (1 billionth of a meter) range. Because of their small dimensions, the physical, optical, mechanical and electronic properties are strongly dependent on size. As a result, one can prepare nanocrystals of the same material that emit or absorb different colors of light simply by varying the size of crystal. These size-tunable optical properties of nanocrystals have drawn significant interest for their application in several next-generation electronic devices, in particular light emitting diodes and solar cells. The two salient features that all nanostructures offer are “size” and “interface”. Our research team examines the effects of both these fundamental aspects of nanomaterials on their structural, optical, electronic, thermal and thermoelectric properties and engineers these for potential applications.
Our research focuses broadly on how to develop artificial materials that do not exist in nature; how to derive novel properties from existing materials by tuning their size (nanostructuring) and adding trace amounts of intentional impurities (also known as doping); and finally how to incorporate these materials into functional devices that can harness new forms of energy or improve the efficiency of existing energy conversion devices. The main goal of our research lies in expanding the possible range of material properties with a major focus on translating fundamental scientific theories into tangible materials. We aim to discover, understand, design, and exploit the benefits of novel multifunctional hybrid nanomaterials that will potentially lead to major advances in energy conversion and storage. Our research is highly interdisciplinary, cutting across aspects derived from chemical engineering, materials science, chemistry, physics, and electrical engineering.
Synthesis of Novel Colloidal Inorganic and Hybrid Organic-Inorganic Nanostructured Materials
The nanoscale ranges from about 1 nanometer (1 billionth of a meter) to 100 nanometers corresponding to roughly 100 atoms to 10 million atoms. The upper and lower boundaries of the scale are not sharply defined, but are chosen such that one excludes individual atoms on the bottom end and micrometer-scale objects on the top end. The properties of semiconductors, most strikingly their optical and electronic properties, depend strongly on their size once one starts to venture into the nanoscale regime. For instance, by varying the size of cadmium sulfide from a nano to a macroscopic regime, one can increase its melting point by 1200 °C! Rapid advances in both experimental and theoretical methods have led to a much better understanding of the properties of nanocrystals than ever before. However, compared to bulk materials, the field of nanocrystals is still lagging far behind since it is not trivial to synthesize these nanomaterials with precise size-control at a large enough scale for wide spread deployment.
We explore multiple routes to obtaining a wide array of materials that have never been synthesized at the nanoscale. In the process, we derive novel optical, electronic and thermal properties from these nanomaterials that would have been possible if these were not at the nanoscale. We use extremely facile low temperature colloidal routes to generate both inorganic nanocrystals as well as hybrid organic-inorganic composites comprising of conductive polymers and a variety of inorganic nanostructures (ranging from wires to rods to spheres).
Impurity Doping in Nanostructures
Apart from varying the size of the semiconductor, another way to influence their properties is through the introduction of trace intentional impurities (or doping). These impurities (or dopants) possess the ability to modify strongly the optical and electronic properties of semiconductors by introducing charge carriers (e.g. the electron which carries electricity). Most semiconductors are worthless if they cannot be doped. Modern technology owes its existence, in large, to the fact that materials such as silicon can be doped. It is the ability to control precisely the type and number of carriers available in the semiconductor by doping that has expedited the advance in electronic and optoelectronic technology. Adding dopants to nanocrystals further extends their properties and possible applications. By controlling the type and quantity of dopants in the nanocrystals, one can expand the range of their properties substantially. This could advance many applications ranging from solar cells to bio-imaging to wavelength tuned lasers. Unfortunately, doping nanostructures is inherently difficult due to a multitude of problems which are typical at the nano-regime. We investigate and outline new techniques of doping that provide a unique roadmap of how to introduce impurities in various classes of nanostructures.
Energy Conversion Devices (Optoelectronic and Thermoelectric)
With all the excitement surrounding the potential use of hybrid nanostructures, however, successful incorporation of these materials into practical applications is challenging since (a) we do not fully understand their properties (b) we do not have a clear path of how to translate the observed properties into a functional device. We aim at understanding and controlling energy transport in these nanostructures by fabricating model device structures that enable us to study the fundamental transport properties (charge, heat, light and mass) and associated mechanisms. This insight helps us to rationally incorporate these tiny crystals into efficient and engineered energy conversion devices such as transistors, solar cells, light emitting diodes, thermoelectrics (that convert waste heat to electricity) etc.