CEMS Faculty

The goal of research in the Norris group is to tailor the optical properties of materials by controlling size, shape, and structure. In addition to investigating fundamental principles, each of their projects is pursued with specific applications in mind. Future research will focus on two main areas: Doped Semiconductor Nanocrystals and Thermal Plasmonics. Applications include photovoltaic and thermophotovoltaic solar cells, respectively. Each area is motivated and described briefly below.

Doped Nanocrystals

Nanometer-scale semiconductor crystals, also known as nanocrystals or quantum dots, are being studied to understand the influence of size on the behavior of semiconductors. Nanocrystals exhibit size-dependent optical properties that have many applications, including light-emitting diodes, solar cells, and bio-imaging. These have motivated the development over the last two decades of advanced liquid-phase chemical syntheses. Particles are now routinely prepared that are highly crystalline, strongly fluorescent, and very uniform in size. They are colloidal in nature and have surfaces that are coated with surfactants to prevent aggregation. Thus, they can be easily manipulated and processed in solution.

However, despite recent advances in nanocrystal research, several critical hurdles remain. In particular, the addition of intentional impurities (or dopants) has been a long-standing challenge. Two general motivations exist to dope nanocrystals. First, the behavior of modern electronic devices depends, in large part, on the ability to add dopants to bulk semiconductors. The tremendous impact of impurities on the properties of bulk semiconductors provides a compelling incentive to consider the same situation in nanocrystals. Second, compared to the bulk, the presence of impurities inside nanocrystals can have an even greater influence. Squeezing impurity atoms into the finite volume of the nanocrystal can enhance their interactions with the semiconductor. Thus, new physical phenomena can result. The Norris group is investigating processes to create doped nanocrystals and is studying the new properties that result.

Thermal Plasmonics

Surface plasmons are special electromagnetic waves that can exist at a metal surface. They are restricted to propagate at the interface and have an intensity that decays both into the bulk metal and the surrounding medium. Because these waves combine both light and oscillating surface charges, they provide an opportunity to study interesting new optical behaviors. In particular, they allow light to be concentrated in nanometer-scale volumes –- an effect that has implications for applications ranging from solar cells to biological sensors. Although direct interaction with light on an exposed metal surface is forbidden if it is perfectly smooth, patterned metal surfaces allow this interaction. As a result, the field of plasmonics has arisen to study how man-made metallic structures can control the generation, propagation, and manipulation of surface plasmons.

Typically, an optical signal is utilized to create the plasmons. Once formed, they can then be manipulated on the device before re-radiating as an optical output. However, in many cases it may be more convenient to have a non-optical source for surface plasmons on the device. One possible route that is not generally recognized is thermal excitation. For example, plasmons can be created by heating either the entire device or a specific location on it. The Norris group is investigating this possibility of using heat to generate and manipulate surface plasmons. In particular, they are exploring whether surface plasmons can tailor the thermal emission of patterned refractory metals. In short, they are working to control the optical “glow” of a heated metal. Of particular interest is solar thermophotovoltaics. In this case, a material must absorb the broad band energy from the sun and then re-emit this energy as a narrow band of infrared light. Electricity is generated when this light is directed toward a conventional photocell that is matched to its wavelength. In principle, this approach can lead to efficient solar-to-electrical conversion but has been limited by the broad thermal emission spectrum of typical materials. Patterned metals provide new opportunities to tailor these spectra and obtain efficient thermophotovoltaic devices.