Investigations of the Electrical, Vibrational and Optical Properties of Graphene-Based Materials

Ting-Fung Chung, Purdue University

Abstract

Graphene and its hybrids have stimulated significant scientific interest owing to their unique properties and technological importance. In this dissertation, we investigate the vibrational, electrical, and optical properties of these remarkable low-dimensional materials by multiple methods including optical measurements (Raman spectroscopy and photoelectrical measurements) and electrical transport measurements (such as the temperature and magnetic field dependence studies). The materials studied have been synthesized or fabricated by methods including chemical vapor deposition (CVD) as well as mechanical exfoliation and transfer. Twisted bilayer graphene (tBLG) exhibits distinct physical properties compared to monolayer and Bernal-stacked bilayer graphene counterparts. In particular, the electronic structures of tBLG depend sensitively on both the interlayer coupling and the twist angle between the two graphene layers, creating low-energy van Hove singularities (vHss) in the density of states at the intersection of the two Dirac cones that are separated by a finite wavevector in tBLG. We have studied the interlayer coupling by measuring the low-energy Raman modes of tBLG over a wide range of angles (from 5 degrees to 30 degrees) using Raman spectroscopy. We find two new Raman modes below 100 cm–1 , which are assigned to a fundamental layer breathing mode and a torsion mode (tentative assignment), in a small range of angles (~10.5 degrees and ~12.5 degrees for 633 nm and 532 nm laser excitation, respectively) at which the intensity of the G Raman band is strongly enhanced due to the presence of vHss. Our results reveal the unique interlayer coupling in tBLG and the similar resonance enhancement of such low-energy Raman modes as in the G Raman band. The close relation between vHs and resonantly enhanced Raman modes in tBLG motivates us to investigate the influence of electrical doping on the electronic and vibrational properties of tBLG. In particular, we have studied by means of Raman spectroscopy the effect of a strong transverse electric field and doping on the resonantly enhanced G Raman band in tBLG at ~12.5 degrees (measured with a 532 nm laser). We observe a striking splitting of the G band and strong modulation of the Raman intensities when the carrier density is tuned away from the charge neutrality point or Dirac point (CNP or DP). We have also examined the electron-phonon coupling in the tBLG, where we find individual phonon self-energy renormalization of the upper and lower graphene layers. TBLG at small-twist-angle is predicted to undergo dramatic modification of the electronic band structure near DP due to the interlayer hybridization and superlattice potential, yielding distinctive transport features related to vHss, superlattice-induced mini-gaps (SMGs) located slightly away from the main DP, and magnetic-field-induced Hofstadter fractal energy spectrum. We have examined the effect of acoustic phonon scattering on electron transport at various carrier densities through temperature-dependent measurements. We find that the resistivity acquired at carrier densities between the CNP and SMG follows a power-law dependence on the temperature, ~Tβ. The evolution of temperature exponent β with carrier density shows a W-shaped dependence, with minima near the vHss and maxima toward the SMGs. We have also performed transport study at high magnetic fields on small-twist-angle tBLG, with particular emphasis on the quantum Hall effect and quantum oscillations near the CNP and SMG. We observe Landau level crossings in the massless Dirac spectrum emanating from the main DP but not in the parabolic energy band near the SMG. This stark difference is further sustained by the observation of pi to 2pi Berry phase transition in quantum oscillations when tuning the Fermi level across the vHs (situated between the CNP and SMG). Graphene-semiconductor (such as quantum dots (QDs)) hybrids are of great interest in harnessing novel photoelectrical and optoelectronic properties. Such hybrids exploit the high carrier mobility of graphene and superior optical properties of QDs. We have studied hybrid phototransistors comprising of CVD graphene and cadmium selenide (CdSe) QDs (named GQFETs), and observed both ambipolar (negative and positive) photoconductivity and persistent photoconductivity at room temperature. We have also demonstrated a suppression of the persistent photoconductivity effect by thermal treatment, which is useful in recovering the functionality of the GQFETs.

Degree

Ph.D.

Advisors

Chen, Purdue University.

Subject Area

Nanoscience|Condensed matter physics|Materials science

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