Irradiation Of Graphene Research Articles

Electronic states bound by repulsive potentials in graphene irradiated by a circularly polarized electromagnetic field

In the framework of the Floquet theory of periodically driven quantum systems, it is demonstrated that irradiation of graphene by a circularly polarized electromagnetic field induces an attractive area in the core of repulsive potentials. Consequently, the quasi-stationary electron states bound by the repulsive potentials appear. The difference between such field-induced states in graphene and usual systems with the parabolic dispersion of electrons is discussed and possible manifestations of these states in electronic transport and optical spectra of graphene are considered.

Thermal conductivity of graphene with defects induced by electron beam irradiation.

We investigate the thermal conductivity of suspended graphene as a function of the density of defects, ND, introduced in a controllable way. High-quality graphene layers are synthesized using chemical vapor deposition, transferred onto a transmission electron microscopy grid, and suspended over ∼7.5 μm size square holes. Defects are induced by irradiation of graphene with the low-energy electron beam (20 keV) and quantified by the Raman D-to-G peak intensity ratio. As the defect density changes from 2.0 × 10(10) cm(-2) to 1.8 × 10(11) cm(-2) the thermal conductivity decreases from ∼(1.8 ± 0.2) × 10(3) W mK(-1) to ∼(4.0 ± 0.2) × 10(2) W mK(-1) near room temperature. At higher defect densities, the thermal conductivity reveals an intriguing saturation-type behavior at a relatively high value of ∼400 W mK(-1). The thermal conductivity dependence on the defect density is analyzed using the Boltzmann transport equation and molecular dynamics simulations. The results are important for understanding phonon - point defect scattering in two-dimensional systems and for practical applications of graphene in thermal management.

Bombarding Graphene with Oxygen Ions: Combining Effects of Incident Angle and Ion Energy To Control Defect Generation

Ion bombardment is a key physical process in the ion implantation and irradiation of graphene, with important implications for tuning graphene’s electronic properties and for understanding the material’s behavior in irradiative environment. Using molecular dynamics with a reactive force field, this work systematically investigates the influence of the incident angle on the generation of defects and vacancies during the bombardment process. It is found that larger incident angles (between the incident line and the surface of graphene) ranging from 70° to 90° are desired for substitution and single vacancy, whereas smaller incident angles ranging from 30° to 50° are favored for forming double vacancies, multiple vacancies, and in-plane disorder. Oxygen ions with the incident angle of 70° produce the highest probability of ion substitution, and the ions at 40–60 eV and 70° yield the highest quality of doping with minimum other defects. These results demonstrate that bombarding graphene along oblique directio...

Graphene defect formation by extreme ultraviolet generated photoelectrons

We have studied the effect of photoelectrons on defect formation in graphene during extreme ultraviolet (EUV) irradiation. Assuming the major role of these low energy electrons, we have mimicked the process by using low energy primary electrons. Graphene is irradiated by an electron beam with energy lower than 80 eV. After e-beam irradiation, it is found that the D peak, I(D), appears in the Raman spectrum, indicating defect formation in graphene. The evolution of I(D)/I(G) follows the amorphization trajectory with increasing irradiation dose, indicating that graphene goes through a transformation from microcrystalline to nanocrystalline and then further to amorphous carbon. Further, irradiation of graphene with increased water partial pressure does not significantly change the Raman spectra, which suggests that, in the extremely low energy range, e-beam induced chemical reactions between residual water and graphene are not the dominant mechanism driving defect formation in graphene. Single layer graphene, partially suspended over holes was irradiated with EUV radiation. By comparing with the Raman results from e-beam irradiation, it is concluded that the photoelectrons, especially those from the valence band, contribute to defect formation in graphene during irradiation.

Electron Beam Induced Deposition on graphene on silicon oxide and hexagonal boron nitride: A comparison of substrates

Graphene, a monolayer of carbon atoms arranged in a hexagonal lattice, is an interesting material because of its unique electronic properties. Electron Beam Induced Deposition (EBID) is an attractive direct-write technique to fabricate graphene devices for electronic applications, due to the combination of the high resolution and the versatility of the technique. We study electron induced damage to graphene at the typical EBID energy and dose range. In particular, we investigate the role of the substrate by carrying out electron exposure studies of graphene on two different substrates – silicon oxide and hexagonal boron nitride. Raman measurements reveal the emergence of a disorder D peak upon irradiation of graphene on silicon oxide, whereas graphene on boron nitride does not show this damage. Further, we pattern structures on graphene using EBID from the platinum precursor MeCpPtMe3 and analyze the changes in the Raman spectrum of graphene coming about due to electron stimulated effects in the presence of the precursor.

All-optical control of ultrafast photocurrents in unbiased graphene.

Graphene has recently become a unique playground for studying light-matter interaction effects in low-dimensional electronic systems. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. In particular, strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field effects and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain. The obtained all-optical control of the photocurrents opens new routes towards graphene based high-speed and broadband optoelectronic devices.

Formation of graphane and partially hydrogenated graphene by electron irradiation of adsorbates on graphene

We report the formation of graphane and partially hydrogenated graphene by electron irradiation of graphene having chemisorbed H2O and NH3. Graphane is formed by the irradiation of graphene that is mechanically exfoliated onto SiO2 substrates in air at a relative humidity of 50%. Partially hydrogenated graphene is formed by the irradiation of graphene that is heated to remove adsorbates and then exposed to H2O or NH3 gases. We propose that hydrogenation is due to H+ ions and H radicals produced by the fragmentation of H2O and NH3 adsorbates by impact with backscattered and secondary electrons. This effect could be used as a technique for writing graphane and partially hydrogenated graphene nanostructures on graphene using electron-beam lithography.

Defect Scattering in Graphene

Irradiation of graphene on SiO2 by 500 eV Ne and He ions creates defects that cause intervalley scattering as is evident from a significant Raman D band intensity. The defect scattering gives a conductivity proportional to charge carrier density, with mobility decreasing as the inverse of the ion dose. The mobility decrease is 4 times larger than for a similar concentration of singly charged impurities. The minimum conductivity decreases proportional to the mobility to values lower than 4e(2)/pih, the minimum theoretical value for graphene free of intervalley scattering. Defected graphene shows a diverging resistivity at low temperature, indicating insulating behavior. The results are best explained by ion-induced formation of lattice defects that result in midgap states.