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

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 (θ) 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 θ (from 5◦ to 30◦) 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 θ (∼10.5◦ and ∼12.5◦ 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 transverse electric field and doping on the resonantly enhanced G Raman band in tBLG at θ ∼ 12.5◦ (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-θ 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 and superlattice-induced mini-gaps (SMGs) located slightly away from the main DP. 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 the 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-θ 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 SMGs. This stark difference is further sustained by the observation of π to 2π 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

Strain-stress study of AlxGa1−xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed

Highly sensitive transient absorption imaging of graphene and graphene oxide in living cells and circulating blood

We report a transient absorption (TA) imaging method for fast visualization and quantitative layer analysis of graphene and GO. Forward and backward imaging of graphene on various substrates under ambient condition was imaged with a speed of 2 μs per pixel. The TA intensity linearly increased with the layer number of graphene. Real-time TA imaging of GO in vitro with capability of quantitative analysis of intracellular concentration and ex vivo in circulating blood were demonstrated. These results suggest that TA microscopy is a valid tool for the study of graphene based materials

Synthetic Graphene Grown by Chemical Vapor Deposition on Copper Foils

The discovery of graphene, a single layer of covalently bonded carbon atoms, has attracted intense interests. Initial studies using mechanically exfoliated graphene unveiled its remarkable electronic, mechanical and thermal properties. There has been a growing need and rapid development in large-area deposition of graphene film and its applications. Chemical vapour deposition on copper has emerged as one of the most promising methods in obtaining large-scale graphene films with quality comparable to exfoliated graphene. In this chapter, we review the synthesis and characterizations of graphene grown on copper foil substrates by atmospheric pressure chemical vapour deposition. We also discuss potential applications of such large scale synthetic graphene.Comment: 23 pages, 4 figure

Hysteretic response of chemical vapor deposition graphene field effect transistors on SiC substrates

Graphene field effect transistors (GFETs) fabricated by chemical vapor deposition graphene deposited onto SiC substrates exhibit sensitivity to broadband visible light. The hysteretic nature of this GFET type was studied utilizing a new current-voltage measurement technique in conjunction with current-time measurements. This measurement method accounts for hysteretic changes in graphene response and enables transfer measurements that can be attributed to fixed gate voltages. Graphene hysteresis is shown to be consistent with electrochemical p-type doping, and current-time measurements clearly resolve a hole to electron to hole carrier transition in graphene with a single large change in gate voltage. (C) 2013 AIP Publishing LLC

Use of graphene as protection film in biological environments

Corrosion of metal in biomedical devices could cause serious health problems to patients. Currently ceramics coating materials used in metal implants can reduce corrosion to some extent with limitations. Here we proposed graphene as a biocompatible protective film for metal potentially for biomedical application. We confirmed graphene effectively inhibits Cu surface from corrosion in different biological aqueous environments. Results from cell viability tests suggested that graphene greatly eliminates the toxicity of Cu by inhibiting corrosion and reducing the concentration of Cu(2+) ions produced. We demonstrated that additional thiol derivatives assembled on graphene coated Cu surface can prominently enhance durability of sole graphene protection limited by the defects in graphene film. We also demonstrated that graphene coating reduced the immune response to metal in a clinical setting for the first time through the lymphocyte transformation test. Finally, an animal experiment showed the effective protection of graphene to Cu under in vivo condition. Our results open up the potential for using graphene coating to protect metal surface in biomedical application