389397 Engineered Nanomaterials for Energy Harvesting
Due to a continual rise in demand for energy worldwide, significant attention is being provided to new environmentally clean energy resources such as solar energy on one hand while large efforts are also concentrated on thermoelectric devices which convert thermal energy directly into electrical energy and hold promising contributions in waste heat recovery. For both approaches, strategies are underway to identify materials and device architectures that are inexpensive and scalable for widespread use yet efficient compared to existing technologies. Nanotechnology enables novel approaches to energy conversion that may provide both higher efficiencies and simpler manufacturing methods. I plan to develop a research program which will focus on tuning and improving existing properties and harnessing novel properties through nanostructuring, doping and interface engineering in these materials.
Electronic, optical, mechanical and thermal properties of metals and semiconductors (SCs) in nano-sized systems can be controlled appreciably by varying their size and give rise to potentially new phenomena not observed in their bulk counterparts. Since the properties of the material are directly related to its crystal structure, the ability to control the crystal phase could also lead to new behavior. Nanostructures offer a potential method to obtain additional phases that would not exist in the bulk. Nanostructures can also lead to shifts in phase-transition (PT) temperatures. This effect is not only interesting for fundamental reasons, but the ability to modify the PT temperature allows one to stabilize (or destabilize) desirable (or undesirable) phases, depending on the goals. Because the properties of the nanostructures change through the PT, this can have implications for potential applications.
The introduction of trace intentional impurities (or doping) is central to controlling the behavior of SC materials. Dopants possess the ability to strongly modify the optical, magnetic, and electronic properties of bulk SCs. Modern SC-based technology owes its existence, in large part, to the fact that these materials can be doped. It is the ability to control precisely the number of carriers available in the SC by doping, which has expedited the advance in SC-based electronic and optoelectronic technology. The advantage of doped SCs is that they provide the device engineer with a wide range of mobilities, so that materials are available with properties that meet specific requirements. Hence, it is natural to extend the versatility of nanostructures by adding dopants.
While interfaces are typically viewed as a dividing layer between two neighboring materials, the properties of the interface are in-fact, at times, radically different from the constituent materials. Nanomaterials, in particular, where nearly 20-30% of the total atoms are located at or very close to the surface, provide an ideal platform to study these effects. Rational engineering of the interfaces between two different components in a nano-composite, presents an opportunity for creating materials with novel properties that could not be achieved otherwise. Hybrid nanocrystal/conducting polymer materials have extensively been used in optoelectronic devices such as photovoltaics and light-emitting diodes, and recently for thermoelectric applications as well.
My PhD research focused mostly on zero-dimensional nanostructures (nanocrystals typically less than 10-nm in size). I developed a versatile synthesis that yields high quality silver chalcogenide nanocrystals (NCs) with size-dependent phase behavior, optical and transport properties. However, by changing the surface of the NCs, I was able to modify the phase behavior of these NCs which allowed us to completely tailor both the optical and electronic transport properties of the material. Additionally, I developed a facile approach to lightly dope SC NCs (CdSe and PbSe) with a controllable amount of electronically active impurities. The addition of even a single impurity per NC has a dramatic effect on their optical properties. Furthermore, studies of the electrical transport through the NC films show complex behavior in the Fermi level as a function of dopant concentration. The results demonstrate that dopant behavior in NCs is not as simple as one might expect. Thus, these experiments begin to reveal the properties of a novel class of NC materials in which both interface and impurities play a key role that may be important for future NC devices.
Multicomponent materials allow us to unite the benefits of both components into one hybrid material and design systems that do not necessarily conform to standard mixing theories for carrier transport. My postdoctoral research aims towards designing and characterizing novel solution processable organic/inorganic nanocomposites (inorganic component being one-dimensional materials or nanowires) and gaining a functional understanding and control of carrier transport at these soft/hard interfaces primarily for thermo-electric applications. We observe unusual thermo-electric transport in a high-performance organic/inorganic thermoelectric composite leading to an optimized power factor in the composites that exceed that of the individual components, suggesting non-effective medium behavior. We increase the conductivity in these hybrid systems further by doping the individual components separately. Finally, by changing the nature of the interface, we can completely switch the nature of charge carriers in these material systems which are particularly attractive for building real thermoelectric modules. My results underscore the importance of understanding and controlling interfacial phenomena in hybrid organic/inorganic systems, and provide a general route for enhancing carrier transport in hybrid materials and devices.
Further work will seek to enhance this interfacial phenomenon in layered two-dimensional materials (nano-sheets) where essentially the entire material is at the interface. In the process, we hope to gain a fundamental understanding of these hybrid systems that would provide a general and robust framework to guide materials design and allow us to tailor suitable material systems for energy applications. Future research plans will be discussed in addition to the above topics.