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

Characterization of Nickel Nanostrand Nanocomposites Through Dielectric Spectroscopy and Nanoindentation

By Michael Koecher, Tyler Park, David Fullwood, and John S. Colton (et al.)

Abstract: One particularly promising model of electrical properties of conductive nanocomposites involves a combined quantum tunneling/percolation approach. However, two key inputs to the modelthe polymer matrix barrier height and the average gap between conductive filler particlesare difficult to determine experimentally. This article demonstrates improved methods for determining barrier height in polymer materials via conductive nanoindentation, with barrier heights measured between 0.4 and 1.7 eV for five different polymers. By using dielectric spectroscopy techniques, combined with the barrier height measurements, the average junction gap was determined for the first time for nickel-nanostrand nanocomposites with six different polymer matrices; the values range from 1.31 to 3.28 nm. Using those measured values for barrier height and junction gap distances in a simple model, we have tested predictions for bulk resistivity of six polymers. The model worked well for four of the six, which suggests that for a given volume fraction of filler, knowledge of the barrier height and the junction distance may in many cases be sufficient to provide an estimate of the bulk resistivity of the polymer-nanostrand blend, an important parameter in nanocomposite engineering. (c) 2013 Society of Plastics Engineers

Long-lived electron spins in a modulation doped (100) GaAs quantum well

By J. S. Colton, D. Meyer, K. Clark, D. Craft, J. Cutler, T. Park, and P. White

Abstract: We have measured T-1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. T-1 lifetimes in excess of 1 mu s were measured at 1.5K and 5.5 T, exceeding the typical T-2* lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently localized subsets of electrons. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759320]

Universal scheme for measuring the electron T-1 in semiconductors and application to a lightly-doped n-GaAs sample

By J. S. Colton, K. Clark, D. Meyer, T. Park, D. Smith, and S. Thalman

Abstract: A new technique has been developed for measuring the T-1 spin lifetime of electrons, and should have near universal applicability among III-V semiconductors. The technique has been applied to a lightly doped GaAs sample with n = 3 x 10(14) cm(-3). Spin decays were measured for fields from 0.5 to 7 T, at temperatures of 1.5 and 5 K. The spin lifetimes were shorter than expected, possibly due to compensation in the sample. (C) 2011 Elsevier Ltd. All rights reserved.

Nuclear effects in Kerr rotation-detected magnetic resonance of electrons in GaAs

By B. Heaton, J. S. Colton, D. N. Jenson, and M. J. Johnson (et al.)

Abstract: We have studied nuclear effects on electron spins in lightly doped n-GaAs bulk and quantum well samples using optically detected magnetic resonance, with the optical detection done via Kerr rotation. The magnetic resonance signal allowed us to measure the electron T(2)* lifetimes and g-factors. We observed strong dynamic nuclear polarization, which was controllable via nuclear magnetic resonance techniques. We measured the nuclear spin relaxation time (T(1)) by monitoring the changing Overhauser field in time, and were able to perform combined electron and nuclear resonance. (C) 2009 Elsevier Ltd. All rights reserved.

Resonant microwave cavity for 8.5-12 GHz optically detected electron spin resonance with simultaneous nuclear magnetic resonance

By J. S. Colton (et al.)

Abstract: We present a newly developed microwave resonant cavity for use in optically detected magnetic resonance (ODMR) experiments. The cylindrical quasi-TE(011) mode cavity is designed to fit in a 1 in. magnet bore to allow the sample to be optically accessed and to have an adjustable resonant frequency between 8.5 and 12 GHz. The cavity uses cylinders of high dielectric material, so-called "dielectric resonators," in a double-stacked configuration to determine the resonant frequency. Wires in a pseudo-Helmholtz configuration are incorporated into the cavity to provide frequencies for simultaneous nuclear magnetic resonance (NMR). The system was tested by measuring cavity absorption as microwave frequencies were swept, by performing ODMR on a zinc-doped InP sample, and by performing optically detected NMR on a GaAs sample. The results confirm the suitability of the cavity for ODMR with simultaneous NMR.

Anomalous magnetic field dependence of the T-1 spin lifetime in a lightly doped GaAs sample

By J. S. Colton (et al.)

Abstract: The T-1 spin lifetime of a lightly doped n-type GaAs sample has been measured via time-resolved polarization spectroscopy under a number of temperature and magnetic field conditions. Lifetimes up to 19 mu s have been measured. The magnetic field dependence of T-1 shows a nonmonotonic behavior, where the spin lifetime first increases, then decreases, then increases again with field. The initial increase in T-1 is understood to be due to correlation between electrons localized on donors. The decrease in T-1 is likely due to phonon-related spin-orbit relaxation. The final increase in T-1 with B indicates a suppression of the spin-orbit relaxation that may involve a level-crossing related cusp in the Rashba or Dresselhaus contributions to relaxation, or may arise from an unknown source.