Tuning Material Properties through Nanostructuring, Doping and Interface Engineering

Material properties 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. Electronic, optical, mechanical and thermal properties of metals and semiconductors depend strongly on the crystallite size once one starts to venture into the nano-scale regime. The evolution of these size dependent fundamental properties can be attributed to the increased surface area in addition to quantum effects that become increasingly important with decreasing size. This change is quite impressive especially in the case of semiconductors (SCs). For instance, simply altering the size of CdSe nanocrystals (NCs) can tune its band gap between 1.7 and 3 eV and hence, the material can absorb and emit across the entire visible spectrum. It is fascinating that the properties in a material of a single chemical composition can be varied so significantly, simply by tailoring bulk material parameters such as size.

Although most SCs can exhibit multiple solid phases, the lowest energy phase is typically observed. Even if we consider metastable phases that can, under certain conditions, be kinetically trapped, the number of solid phases that are accessible at modest temperatures and pressures is limited in most semiconductors. 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. One possible route is to decrease the size of the semiconductor. It is well-known that the relative stability of bulk phases can be altered in nano-scale crystallites due to the presence of the particle surface. Thus, nanostructures offer a potential method to obtain additional semiconductor 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. Doping can expand the range of properties in SC nanostructures substantially; thus opening up a plethora of applications ranging from solar cells and bio-imaging to wavelength tuned lasers.

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). We developed a versatile synthesis that yields high quality silver chalcogenide NCs with size-dependent phase behavior, optical and transport properties. However, by changing the surface of the NCs, we were 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, we developed a facile approach to lightly dope SC NCs (CdSe and PbSe) with a controllable amount of electronically active impurities. We observe that 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 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 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 carriers in these material systems. Our 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. Future research plans will be discussed in addition to the above topics.