Sandra J. Rosenthal
In the Rosenthal group we study semiconducting nanocrystals, a novel material whose optical properties and electronic structure can be precisely tuned by controlling the size of the nanocrystal. We are specifically interested in two applications exploiting the properties of nanocrystals: the use of nanocrystals as the light harvesting element in photovolatic devices and the use of fluorescent nanocrystals as biological probes for membrane proteins involved in neuronal signaling. We have also recently begun a program to explore the possible use of nanocrystals as a white light emitter for implementation in solid state lighting.
Nanocrystals are an ideal light harvester in photovoltaic devices. The band gap can be exquisitely tuned by controlling the size of the nanocrystal,thus the proper choice of size and type of nanocrystal allows one to create a photovoltaic whose absorption spectrum matches the spectral distribution of sunlight. The nanocrystals absorb sunlight more strongly than dye molecules or the bulk semiconductor material, therefore high optical densities can be achieved while maintaining the requirement of thin films. Perfectly crystalline CdSe nanocrystals are also an artificial reaction center, separating the electron hole pair on a femtosecond timescale. Nanocrystals have an intrinsic dipole moment originating from the top and bottom terminating planes of Se and Cd. Carriers are rapidly localized to the surface of the crystal where they remain for 290ns before recombining. The size-tunable band gap, large absorption coefficients, intrinsic electron hole pair separation, long exciton lifetime, and chemical robustness make nanocrystals the ideal material for solar cells. The photovoltaic devices we make in our laboratory could eventually be fabricated inexpensively at low temperatures and can cover large areas.
Fluorescent nanocrystals have several advantages over organic dye molecules as fluorescent markers in biology. They are incredibly bright and do not photodegrade. They have narrow, guaussian emission spectra enabling the co-localization of several proteins simultaneously. Drug-conjugated nanocrystals attach to the protein in an extracellular fashion, enabling movies of protein trafficking. We are synthesizing drug-conjugated nanocrystals which have high affinities and selectivities for serotonin, dopamine, and norepinephrine receptor and transporter proteins. These are neurotransmitters which control critical behaviors such as mood, sleep, appetite, and aggression. With the drug-conjugated nanocrystals we will be able to map the distribution of these proteins and be able to determine mechanisms which regulate protein expression at the cell surface. These proteins are also drug targets for the serotonin selective reuptake inhibitors, atypical antipsychotics, and drugs of abuse. The drug-conjugated nanocrystals also form the basis of a high-throughput fluorescence assay for drug discovery.
In response to ever increasing energy demands and subsequent costs, a tremendous emphasis is being placed on energy saving, solid state lighting devices in the form of light emitting diodes, or LED's. Specifically, a need exists for pure white-light LED's as a more efficient replacement for conventional lighting sources. Switching to solid state lighting would reduce global electricity use by 50% and reduce power consumption by 760 GW in the United States alone over a 20 year period. The complications associated with design and fabrication of such devices have generated great interest in developing white-light phosphors that do not depend on complex doping schemes or combinations of materials. One proposed solution is to use a mixture of semiconductor nanocrystals as the intrinsic emitting layer for an LED device. Semiconductor nanocrystals exhibit high fluorescence quantum efficiencies and large molar absorptivities. However, they still suffer from the problem that simply mixing the traditional red, green, and blue colors to achieve white light results in a loss in total device efficiency due to self absorption for a device of more than a few monolayers. We have demonstrated white-light emission from ultra-small cadmium selenide (CdSe) nanocrystals. This raises the intriguing possibility of using these nanocrystals as a white-light phosphor. These ultra-small nanocrystals exhibit broadband emission (420 - 710 nm) throughout most of the visible light spectrum while not suffering from self absorption. This is the direct result of the extremely narrow size distribution and an unusually large (40-50 nm) Stokes shift making them ideal materials for devices currently under development and also an ideal platform to study the molecule-to-nanocrystal transition.
We also perform fundamental studies on semiconducting nanocrystals. We have pioneered the use of Rutherford backscattering spectroscopy and atomic number scanning transmission electron microscopy to determine atomic level constitution and structure of nanocrystals with unprecedented detail. We use ultrafast spectroscopy to map out the ultrafast carrier dynamics of electrons and holes inside the nanocrystals and to follow the charge transfer reactions of the nanocrystals inside the photovoltaics.