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Creators/Authors contains: "Bristow, Alan D."

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  1. Liquid-phase exfoliation (LPE) is a promising and scalable technique to produce low-cost dispersible nanosheets of graphene and nano-graphite for electronic, optoelectronics, and photonics applications. Fundamental information about how LPE affects the electrical properties is lacking. Here, a relationship is provided between the morphology of nano-graphite flakes resulting from LPE and cascade centrifugation to the charge-carrier transport properties. A range of process parameters, such as centrifuge force and exfoliation solvent, are employed, leading to a range of flake sizes. Morphology is characterized by scanning electron microscopy, atomic force microscopy and optical profilometry. Raman spectroscopy is used to confirm morphology, crystallite size, and chemical properties. Terahertz time-domain spectroscopy with a Drude-Smith conduction model provides the charge-carrier concentration and scattering times from AC conductivity. Carrier concentration increases with a reduction in flake area, potentially resulting from the introduction of electronic defect states at the edge of the nano-crystallites. Meanwhile, the carrier scattering time decreases with decreased flake size, similarly due to this self-doping that increases the carrier-carrier scattering. The approach and results serve as a foundation for understanding the processing-dependent electrical characteristics of LPE flakes and nanosheets.

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  2. Abstract

    A type-II InAs/AlAs$$_{0.16}$$0.16Sb$$_{0.84}$$0.84multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of$$>100$$>100meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap ($$E{_g}$$Eg) density of states with an Urbach tail below$$E{_g}$$Eg. As temperature increases, the long-lived decay times increase$$<Eg, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers$$>E{_g}$$>Eg. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

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  4. As optical two-dimensional coherent spectroscopy (2DCS) is extended to a broader range of applications, it is critical to improve the detection sensitivity of optical 2DCS. We developed a fast phase-cycling scheme in a non-collinear optical 2DCS implementation by using liquid crystal phase retarders to modulate the phases of two excitation pulses. The background in the signal can be eliminated by combining either two or four interferograms measured with a proper phase configuration. The effectiveness of this method was validated in optical 2DCS measurements of an atomic vapor. This fast phase-cycling scheme will enable optical 2DCS in novel emerging applications that require enhanced detection sensitivity.

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  5. Abstract

    Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

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