Dr. Laszlo Frazer: Splitting the Semiconductor Atom
Last updated November 2016.
An exciton in the semiconductor cuprous oxide (Cu2O) is a nanoscale atom-like structure. I discovered photoionization of excitons in cuprous oxide. In this process, the electron and the hole which constitute the exciton are separated, preventing exciton polariton formation. The charges subsequently recombine and decay through phonon-linked luminescence. Exciton photoionization can be used to measure exciton properties and to induce additional absorption for energy applications.
Harmonic generation and multiphoton absorption are important tools for making ultraviolet light sources, which will be critical components of future semiconductor fabrication processes. I characterised third harmonic generation in cuprous oxide to understand how it influences exciton preparation. Due to its centrosymmetry, cuprous oxide does not produce second harmonic. It does have a two photon absoprtion resonance at the orthoexciton polariton energy, which can be used to prepare excitons into a spectrally narrow state for well-controlled experiments.
I perform second order susceptibility characterization on novel semiconductors. With the help of synthesis collaborators, new structure/property relationships have been discovered in chalcogenide compounds and perovskite-like structures using the Kurtz-Perry method.
Cuprous oxide is typically p-type because it contains copper vacancies which serve as acceptors. Acceptor concentrations are manipulated to make transistors, control conductivity, and adjust optical properties. I developed means of controlling the copper vacancy concentration; the copper deficiency can be reduced by oxidizing cuprous oxide. This bizarre process relies on the existence of the cupric oxide phase. To a certain extent, oxygen vacancy concentrations can be independently manipulated as well. Vacancies are important to exciton transport because they serve as symmetry-breaking traps.
Cuprous oxide has a bandgap suitable for efficient photovoltaic and photocatalytic devices. I have developed photovoltaic devices for charge transport investigations.
Cuprous Oxide Crystal Synthesis
Cuprous oxide is a significant material in fields such as photovoltaics, photocatalysis, nonlinear optics, electronic structure, nuclear magnetic resonance, and nanotechnology. Cuprous oxide crystals can be difficult to obtain. I am happy to provide samples to collaborators. My synthesis process is based on thermal oxidation of high purity copper precursors, thermal oxidation, optical floating zone crystallization, and annealing protocols. The largest sample dimension available is 10 cm. A variety of defect concentrations can be produced. Synthesis parameters can be chosen to produce spectrally narrow, spatially homogeneous exciton luminescence.
My research uses electron beam lithography, sputtering, evaporation, etching, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy for device preparation and characterization. I synthesised two dimensional, freestanding nanoparticle sheets from colloidal solutions and characterised their mechanical properties.
Frazer, L., et al. (2017). Seeing the invisible plasma with transient phonons in cuprous oxide. Physical Chemistry Chemical Physics, 19, 1151-1157.
Frazer, L., et al. (2017). Vacancy Relaxation in Cuprous Oxide (Cu2-xO1-y). Journal of Luminescence, 183, 281-290. https://arxiv.org/abs/1611.08793
Frazer, L., et al. (2015). Cupric oxide inclusions in cuprous oxide crystals grown by the floating zone method. Science and Technology of Advanced Materials, 16(3), 034901.
Frazer, L., et al. (2015). Evaluation of defects in cuprous oxide through exciton luminescence imaging. Journal of Luminescence, 159, 294.
Frazer, L., et al. (2014). Photoionization cross section of 1s orthoexcitons in cuprous oxide. Physical Review B, 89(24), 245203.
Frazer, L., et al. (2014). Third-harmonic generation in cuprous oxide: efficiency determination. Optics Letters, 39(3), 618-621.
Frazer, L., et al. (2013). Unexpectedly slow two particle decay of ultra-dense excitons in cuprous oxide. Solid State Communications, 170, 34-38.
Frazer, L., et al. (2017). Optimizing the Efficiency of Solar Photon Upconversion. ACS Energy Letters, 2, 1346-1354.
Li, H., et al. (2017). Structure evolution and thermoelectric properties of carbonized polydopamine thin films. ACS Applied Materials & Interfaces, 9(8), 6655-6660.
Haynes, A. S., et al. (2017). Second Harmonic Generation Response of the Cubic Chalcogenides Ba(6-x)Srx[Ag(4-y)Sn(y/4)](SnS4)4. Journal of Solid State Chemistry, 248, 119-125.
Thenuwara, A. C., et al. (2016). Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation. Angewandte Chemie International Edition, 55(35), 10381-10385.
Chang, K. B., et al. (2016). Hydrothermal crystal growth, piezoelectricity, and triboluminescence of KNaNbOF5. Journal of Solid State Chemistry, 236, 78-82.
Thenuwara, A. C., et al. (2015). Copper intercalated birnessite as a water oxidation catalyst. Langmuir, 31(46), 12807-12813.
Stoumpos, C., et al. (2015). Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties. Journal of the American Chemical Society, 137(21), 6804-6819.
Lai, W. H., et al. (2015) Second Harmonic Generation Response Optimized at Various Optical Wavelength Ranges through a Series of Cubic Chalcogenides Ba6Ag2.67+4δSn4.33-δS16-xSex. Chemistry of Materials, 27(4), 1316-1326.
Mesbah, Adel, et al. (2015) The U5+ compound Ba9Ag10U4S24: Synthesis, structure, and electronic properties. Journal of Solid State Chemistry, 221, 398-404.
Okoniewski, S., et al. (2014). Optorheological thickening under the pulsed laser photocrosslinking of a polymer. Journal of Applied Polymer Science, 131(17), 40690.
Chang, K. B., et al. (2013). Removal of Copper Vacancies in Cuprous Oxide Single Crystals Grown by the Floating Zone Method. Crystal Growth & Design, 13(11), 4914-4922.
Jang, J. I., et al. (2012). Strong P-band emission and third harmonic generation from ZnO nanorods. Solid State Communications, 152(14), 1241-1243.
He, J., et al. (2010). Fabrication and Mechanical Properties of Large-Scale Freestanding Nanoparticle Membranes. Small, 6(13), 1449-1456.
Kramer, E. M., et al. (2007). Measurement of diffusion within the cell wall in living roots of Arabidopsis thaliana. Journal of Experimental Botany, 58(11), 3005-3015.
I am interested in using science based methods to teach diverse undergraduates to combine different ideas and apply quantitative reasoning to life decisions. Previously, I earned a teaching certificate in undergraduate instruction from Northwestern University and taught introductory physics at Oakton Community College. In my experience, education will set minds free.
For undergraduates seeking science and technology careers, experience directly doing science is crucial to advancement. I benefited from the research mentoring of Professor Heinrich Jaeger and Professor Eric Kramer during my undergraduate years. I have enjoyed mentoring students with a variety of backgrounds through their first scientific experiences. My students have been placed in Ph.D. programs at the University of Washington, the University of Colorado, and Northwestern University. They have also been co-authors on peer-reviewed publications.
Ph.D. in Physics and Astronomy, Northwestern University, 2014 Ketterson Group
M.S. in Physics and Astronomy, Northwestern University, 2011
A.B. in Physics, University of Chicago, 2009
A.A., Simon's Rock College, 2007 (I highly recommend Simon's Rock)