Research

Research Topics

Ferritin Band Gap Measurements

Ferritin is a spherical protein shell consisting of 24 subunits used for iron storage in the human body and many other animals. Iron is collected inside and stored as a ferrihydrite nanoparticle. Ferritin is particularly interesting for the study of nanoparticles because the amount of mineral inside can be changed, and thus the size of the nanoparticles can be controlled. In addition, the iron can be completely removed and replaced with many different minerals. This allows for fine tuning of the band gap and potential applications in things like solar cells and detectors. Six DNA bonding cites are also spread symmetrically along the outside of the ferritin, opening up many potential applications for microfabrication of nanoparticle systems.

In our lab we measure these band gaps through absorption spectroscopy. We use a Xenon arc lamp to provide a wide spectrum from the near UV into the infrared. A spectrometer is used to select out wavelengths. The light is focused down and passed through the sample in solution. By measuring the amount of light absorbed at differnent wavelengths, we are able to extrapolate both indirect and direct band gaps.

Ferritin's unique characteristics can be used to create efficient multi-junction solar cells. The ability to synthesize cores with a wide variety of band gaps makes it possible to cover a larger amount of the solar spectrum. This increase in captured energy means that limiting efficiencies will be much higher than in standard single-junction silicon cells. In addition, the components in a ferritin solar cell can be made from earth-abundant elements. We have studied titanium, cobalt, and manganese oxyhydroxide cores, which cover much of the solar spectrum. Future research will investigate lead sulfide to complete this collection.

Spin Lifetimes

P-type InGaAs quantum dots have been shown to have T1 spin lifetimes of up to 5 microseconds. We plan to verify and improve upon these past results using our time resolved pump-probe technique to directly observe the spin decays. In using this technique to analyze a previous sample of n-type quantum dots, we observed strange oscillatory behavior and not an exponential decay as would be expected. We chose to use pump and probe beams of the same wavelength in order to pump and probe the same set of dots. We believe that some kind of interaction between the probe beam and the sample may be responsible for this strange behavior.

With the p-type dots, we plan to use pump and probe beams that differ by one or two nanometers in order to eliminate interaction between the probe beam and the sample, without seriously detracting from the probe beam's ability to detect spin polarization. If this method proves successful, we will later revist the n-type quantum dots using this revised method.

Optical and TEM Characterization of Quantum Dot Chains

>We decided to further study a few of the quantum dot samples we received from our collaborator, Haeyeon Yang, specifically the morphology. These three samples have formed into quantum dot chains, which are quantum dots that have formed into rows of closely-spaced quantum dots. These quantum dot chains were grown using a variation of the Stranski-Krastanov method, which includes an annealing process. In agreement with our photoluminescence studies, our transmission electron microscopy studies suggest a critical annealing temperature, where the samples' morphologies differ significantly in samples above/below the annealing temperature.

Electron Spin Resonance

Optical methods are not the only options for studying spin. Microwave techniques in particular have been used for years to study the spin of electrons. The microwave studies of spin center around the principle of magnetic resonance: transitions between the two spin-split energy levels of the electrons can be induced when microwaves with the corresponding energy are applied. The resonant microwave frequency f is given by the condition: hf = ΔE = gμBB. When the spins are manipulated in a standard magnetic resonance experiment (also called "electron spin resonance," ESR), the widths of the resonance peaks depend on the spin lifetime and can be an additional method for obtaining T2*.

More important, however, is the potential for coherently rotating the spins in a controlled fashion via pulsed microwaves. As an intense resonant microwave field is applied, the electrons are driven back and forth between spin states; these are Rabi oscillations. By interrupting the oscillation cycle and applying pulses of varying strengths at various time delays, the electrons'spins can be rotated to arbitrary directions. In these resonance experiments, the microwaves are directly and controllably manipulating the electrons'spin. In the quantum computing context, these would be called one qubit operations, which are needed in all quantum computing implementations. Through clever pulse sequences, additional spin dynamic information can be obtained; the Hahn spin echo is one such sequence and is the usual method for determining the spin coherence time T2.

Dielectric Spectroscopy of Nickle Nanorod Infused Polymers

In a collaborative project with mechanical engineering department's David Fullwood group, we've been working to obtain dielectric spectra of polymer samples infused with with nickel nanostrands (NiNS). Our recent measurements have been focused on Sylgard samples with 7-13% volume of NiNS and observing how they behave under strain. The dielectric spectra follow the form of the Cole-Cole equation, and in turn tell us how the junction gap (distances between NiNS) changes with strain.

Pedagogy

The following are publications related to teaching physics.

This paper presents the physics behind classic musical scales so that teachers and students can more effectively learn concepts such as mathematical ratios, harmonic resonators, beats, and human perception as they are related to simple tones and sound. In conjunction with this we developed a free, open-source software called Temperament Studio which is designed to aid in the education process.

Research Opportunities

John Colton (Materials Physics )

2D metal-halide perovskites for solar applications
"2D hybrid organic-inorganic metal halide perovskites" are a recently discovered class of semiconductors being studied in the hopes of developing highly efficient, low-cost, stable solar cells. Metal and halogen (group VII) atoms bind together in 2D layers, which are then stacked together via organic linker molecules. We are studying these interesting and important materials through optical absorption, electric field-modulated absorption, photoluminescence (fluorescence), time-dependent photoluminescence on nanosecond time scales, and dielectric spectroscopies. This allows us to determine important properties of the electrons inside these materials, to make better photovoltaic materials.
Nanoparticles as temperature sensors
We're working with a mechanical engineering professor (Troy Munro) to use semiconductor nanoparticles as temperature sensors. The wavelengths of light present in the nanoparticles' photoluminescence (aka fluorescence), and the time it takes for the luminescence to be emitted after the electrons have been excited both depend on the temperature. By characterizing the nanoparticles’ photoluminescence spectrum in both wavelength and time as a function of temperature, we hope to be able to use the nanoparticles as non-invasive temperature sensors in e.g. medical applications. For example, one could use the optical emission from nanoparticles injected into tissue to monitor temperatures as focused ultrasound is used to heat up and destroy tumors.