Welcome

Welcome to the Colton group research homepage! The main research focus in our lab is the optical investigation of semiconductors and various semiconductor nanostructures. Some of the properties we measure are exciton binding energy, photoluminescence, spin lifetimes, and band gap energies. Electron spin lifetimes have particularly interesting applications to the field of quantum computing (see explanation below). For more information about our research, see the links to the left. Specifically, our research page describes several of the experiments we're currently working on. The basics of semiconductors page contains a brief introduction to the physics behind our experiments, and the optical experiments page describes some of the different techniques we use.

Colton Research Group, Dec 2024

Physics Department Colloquium (3 Nov 2023)

Here's a colloquium I gave in Nov 2023 which explains some of our recent research on a particular class of hybrid organic-inorganic semiconductors, namely metal-halide perovskites.

Optical Investigations of Nanostructures

Inside our research lab.

Our recent publications and research have been focused on materials for solar energy known as metal halide perovskites or hybrid organic-inorganic perovskites. These consist of a crystalline structure of inorganic layers made of metal and halogen (group VII) atoms which are a couple of nanometers thick, which are then attached to each other via organic linker molecules. These materials are of interest for their potential applications as light emmitters (LEDs) and solar cells. These perovskites have been improving in efficieny and show a lot of promise in those applications.

Analyzing the optical properties of the materials can help us understand what the electrons are doing inside the materials, which then can help advance the field and the effort to more efficient solar cells. We're studying properties such as the dielectric constant (which affects how much electrons inside the material are shielded from applied electric fields), and the binding energy between electrons and holes (a hole is the absence of an electron, and acts like a positive charge), and looking at how those properties change as the specific material compositions are varied.

Ferritin Nanoparticles

Our lab is currently investigating nanoparticle synthesis using the protein ferritin. Ferritin is a spherical protein that is especially useful for nanoparticle formation, because it offers a uniformly-sized shell in which to synthesize the nanoparticles. By varying the combinations of metals inside the ferritin shell, it is possible to create nanoparticles which have a variety of band gaps.

One of the primary applications for these nanoparticles is in multi-junction solar cells. Traditional single-junction solar cells have a maximum theoretical efficiency of only 33.7%; however, by using multiple layers of material, the maximum efficiency can be increased to over 80%. These layers need to have the proper band gaps in order to absorb the maximum amount of light, and we are searching for the proper combination of nanoparticles to use for a high-efficiency solar cell of this type.

Selected Publications

The physics of musical scales: Theory and experiment

By Dallin S. Durfee and John S. Colton

Abstract: The theory of musical scales involves mathematical ratios, harmonic resonators, beats, and human perception and provides an interesting application of the physics of waves and sound. We first review the history and physics of musical scales, with an emphasis on four historically important scales: twelve-tone equal temperament, Pythagorean, quarter-comma meantone, and Ptolemaic just intonation. We then present an easy way for students and teachers to directly experience the qualities of different scales using MIDI synthesis.

Evolution of Nano-junctions in Piezoresistive Nanostrand Composites

By R. Adam Bilodeau, David T. Fullwood, John Colton, Anton E. Bowden, and Tyler Park (et al.)

Abstract: When nickel nanostrands (NiNs) are embedded inside of highly flexible silicone, the silicone becomes an extremely piezoresistive sensor capable of measuring a large dynamic range of strains. These sensors experience an increase in conductance of several orders of magnitude when strained to 40% elongation. It has been hypothesized that this effect stems from a net change in average junction distance between the conductive particles when the overall material is strained. The quantum tunneling resistance across these gaps is highly sensitive to junction distance, resulting in the immense piezoresistive effect. In this paper, the average junction distance is monitored using dielectric spectroscopy while the material is strained. By incorporating new barrier height measurements of the base silicone material from a nanoindentation experiment, this experiment validates previous assumptions that, on average, the junctions between NiNs decrease while the sample is strained, instigating the large piezoresistive effect. The nature of the material’s response to strain is explored and discussed.

Assemblies composed of oligothiophene-ruthenium complexes bound to CdSe nanoparticles

By Nathan Bair, Jared M. Hancock, Cameron J. Simonsen, Scott W. Thalman, John S. Colton, Matthew C. Asplund, and Roger G. Harrison

Abstract: Molecular conjugates are important to link light sensitized materials to electron acceptors. We have synthesized oligothiophenes and oligothiophene-ruthenium complexes and bound them to CdSe nanoparticles. The absorption and fluorescence properties of the oligothiophenes bound to CdSe were measured. Steady-state luminescence and time correlated single photon counting were used to observe the effects on fluorescence and fluorescence lifetimes before and after binding. It was found that fluorescence of CdSe nanoparticles was quenched when they were bound to the oligothiophenes, and that the fluorescence of the oligothiophenes was also quenched. The fluorescence lifetimes of the quenched species were shortened and suggest electron transfer from oligothiophene to nanoparticle is on the order of one nanosecond. Orbital energy calculations predict that the Ru bound oligothiophenes have HOMO-LUMO energies of correct energy to allow electron and hole transfer. These experiments show that the oligothiophenes efficiently transfer optical energy between CdSe nanoparticles and could potentially be used as charge transfer junctions.

Non-native Co-, Mn-, and Ti-oxyhydroxide nanocrystals in ferritin for high efficiency solar energy conversion

By S. D. Erickson, T. J. Smith, L. M. Moses, R. K. Watt, and J. S. Colton

Abstract: Quantum dot solar cells seek to surpass the solar energy conversion efficiencies achieved by bulk semiconductors. This new field requires a broad selection of materials to achieve its full potential. The 12 nm spherical protein ferritin can be used as a template for uniform and controlled nanocrystal growth, and to then house the nanocrystals for use in solar energy conversion. In this study, precise band gaps of titanium, cobalt, and manganese oxyhydroxide nanocrystals within ferritin were measured, and a change in band gap due to quantum confinement effects was observed. The range of band gaps obtainable from these three types of nanocrystals is 2.19–2.29 eV, 1.93–2.15 eV, and 1.60–1.65 eV respectively. From these measured band gaps, theoretical efficiency limits for a multi-junction solar cell using these ferritin-enclosed nanocrystals are calculated and found to be 38.0% for unconcentrated sunlight and 44.9% for maximally concentrated sunlight. If a ferritin-based nanocrystal with a band gap similar to silicon can be found (i.e. 1.12 eV), the theoretical efficiency limits are raised to 51.3% and 63.1%, respectively. For a current matched cell, these latter efficiencies become 41.6% (with an operating voltage of 5.49 V), and 50.0% (with an operating voltage of 6.59 V), for unconcentrated and maximally concentrated sunlight respectively.

Tuning the Band Gap of Ferritin Nanoparticles by Co-Depositing Iron with Halides or Oxo-anions

By Trevor Smith, Stephen D. Erickson, Catalina Matias Orozco, Andrew Fluckiger, Lance M. Moses, John S. Colton, and Richard K. Watt

Abstract: Iron-containing ferritin has been used for light harvesting and as a photocatalyst. In this study, we test the hypothesis that changing the iron mineral core composition can alter the light harvesting and photocatalytic properties of ferritin, by co-depositing iron in the presence of halides or oxo-anions. This caused the anions to be incorporated into the iron mineral. We report that some of these new iron minerals possess different band gaps than the original ferrihydrite within ferritin. We found an increase in band gap of up to 0.288 eV or a decrease by as much as 0.104 eV, depending on the type of anion and amount of anions incorporated into the ferrihydrite mineral.

Annealing-induced change in quantum dot chain formation mechanism

By Tyler D. Park, John S. Colton, and Jeffrey K. Farrer (et al.)

Abstract: Self-assembled InGaAs quantum dot chains were grown using a modified Stranski-Krastanov method in which the InGaAs layer is deposited under a low growth temperature and high arsenic overpressure, which suppresses the formation of dots until a later annealing process. The dots are capped with a 100 nm GaAs layer. Three samples, having three different annealing temperatures of 460°C, 480°C, and 500°C, were studied by transmission electron microscopy.Results indicate two distinct types of dot formation processes: dots in the 460°C and 480°C samples form from platelet precursors in a one-to-one ratio whereas the dots in the sample annealed at 500°C form through the strain-driven self-assembly process, and then grow larger via an additional Ostwald ripening process whereby dots grow into larger dots at the expense of smaller seed islands. There are consequently significant morphological differences between the two types of dots, which explain many of the previously-reported differences in optical properties. Moreover, we also report evidence of indium segregation within the dots, with little or no indium intermixing between the dots and the surrounding GaAs barrier.