Ultraviolet Raman Spectroscopy of Single and Multi-layer Graphene

We investigated Raman spectra of single-layer and multi-layer graphene under ultraviolet laser excitation at the wavelength of 325 nm. It was found that while the G peak of graphene remains pronounced in UV Raman spectra, the 2D band intensity undergoes severe quenching. The evolution of the ratio of the intensities of the G and 2D peaks, I(G)/I(2D), as the number of graphene layers n changes from n=1 to n=5, is different in UV Raman spectra from that in conventional visible Raman spectra excited at the 488 nm and 633 nm wavelengths. The 2D band under UV excitation shifts to larger wave numbers and is found near 2825 1/cm. The observed UV Raman features of graphene were explained by invoking the resonant scattering model. The obtained results contribute to the Raman nanometrology of graphene by providing an additional metric for determining the number of graphene layers and assessing its quality.Comment: 18 pages; 5 figures; submitted for publication on February 20, 200

Evolution of Microscopic Localization in Graphene in a Magnetic Field from Scattering Resonances to Quantum Dots

Graphene is a unique two-dimensional material with rich new physics and great promise for applications in electronic devices. Physical phenomena such as the half-integer quantum Hall effect and high carrier mobility are critically dependent on interactions with impurities/substrates and localization of Dirac fermions in realistic devices. We microscopically study these interactions using scanning tunneling spectroscopy (STS) of exfoliated graphene on a SiO2 substrate in an applied magnetic field. The magnetic field strongly affects the electronic behavior of the graphene; the states condense into welldefined Landau levels with a dramatic change in the character of localization. In zero magnetic field, we detect weakly localized states created by the substrate induced disorder potential. In strong magnetic field, the two-dimensional electron gas breaks into a network of interacting quantum dots formed at the potential hills and valleys of the disorder potential. Our results demonstrate how graphene properties are perturbed by the disorder potential; a finding that is essential for both the physics and applications of graphene.Comment: to be published in Nature Physic

Raman Nanometrology of Graphene

Graphene is a two-dimensional honey-comb lattice of carbon atoms with very unusual electron energy dispersion. Since its recent micromechanical isolation and measurements, graphene attracted tremendous attention of the physics and engineering communities. In addition to the wealth of two-dimensional electron gas physics revealed by graphene, it is a very promising material for the electronic and sensor applications. Graphene manifests extremely high charge carrier mobility at room temperature, which is far beyond the values achievable in the materials currently used in transistor designs. For this reason, graphene is considered for applications in the integrated circuits beyond the conventional silicon complementary metal-oxide semiconductor electronic technology. Some of the major problems associated with graphene research and applications are the difficulties of graphene identification, e.g. distinguishing it from other carbon materials, and verification of the number of graphene atomic layers. In this dissertation research we expand the use of the micro-Raman spectroscopy as the nanometrology tool for graphene material characterization. Raman spectroscopy is a non-invasive technique, which is widely used to characterize structural and electronic properties of carbon-based materials such as carbon nanotubes, diamond, graphite and diamond-like carbons. Graphene's Raman spectrum has clear signatures, which allow one to identify it and determine the number of atomic layers with high accuracy. The main results of this dissertation are (i) measurement of the temperature coefficients of the G and 2D peaks in Raman spectrum of graphene; (ii) the study of the effects of different substrates on Raman signatures of graphene; and (iii) the first investigation of the ultraviolet Raman spectrum of graphene. The obtained results are important for graphene identification and device applications since electric bias and gate voltages in graphene transistors result in the device self-heating while device fabrication often requires the use of various substrates with properties different from those of the standard silicon. In addition, graphene-device characterization is conducted at different temperatures. The measurements of temperature coefficients of graphene Raman peaks was also instrumental in the study of heat conduction in graphene and shed light on its anharmonic properties

Structural Stability of Functionalized Silicene Nanoribbons with Normal, Reconstructed, and Hybrid Edges

Silicene, a novel graphene-like material, has attracted a significant attention because of its potential applications for nanoelectronics. In this paper, we have theoretically investigated the structural stability of edge-hydrogenated and edge-fluorinated silicene nanoribbons (SiNRs) via first-principles calculations. Various edge forms of SiNRs including armchair edge, zigzag edge, Klein edge, reconstructed Klein edge, reconstructed pentagon-heptagon edge, and hybrid edges have been considered. It has been found that fully fluorinated Klein edge SiNRs, in which each edge Si atom is terminated by three fluorine atoms, are the most stable structure. We also discovered that a hybrid edge structure of trihydrogenated Klein edge and dihydrogenated zigzag edge can increase the nanoribbon’s stability up to that of dihydrogenated armchair edge SiNR, which is known as the most stable edge-hydrogenated structure. With the attractive properties of silicene for practical applications, the obtained results will advance experimental investigations toward the development of silicene based devices