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Register a new userEffect of annealing on Raman scattering spectra of monolayer graphene samples gradually disordered by ion irradiation
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The Raman scattering spectra (RS) of two series of monolayer graphene samples irradiated with various doses of C$^{+}$ and Xe$^{+}$ ions were measured after annealing in high vacuum, and in forming gas (95%Ar+5%H$_{2}$). It was found that these methods of annealing have dramatically different influence on the RS lines. Annealing in vacuum below 500$^{circ}$C leads to significant decrease of both D-line, associated with defects, and 2D-line, associated with the intact lattice structure, which can be explained by annealing-induced enhanced doping. Further annealing in vacuum up to 1000$^{circ}$C leads to significant increase of 2D-line together with continuous decrease of D-line, which gives evidence of partial removal of defects and recovery of the damaged lattice. Annealing in forming gas is less effective in this sense. The blue shift of all lines is observed after annealing. It is shown that below 500$^{circ}$C, the unintentional doping is the main mechanism of shift, while at higher annealing temperatures, the lattice strain dominates due to mismatch of the thermal expansion coefficient of graphene and the SiO$_{2}$ substrate. Inhomogeneous distribution of stress and doping across the samples leads to the correlated variation of the amplitude and the peak position of RS lines.
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Gradual localization of charge carriers was studied in a series of micro-size samples of monolayer graphene fabricated on the common large scale film and irradiated by different doses of C$^+$ ions with energy 35 keV. Measurements of the temperature dependence of conductivity and magnetoresistance in fields up to 4 T showed that at low disorder, the samples are in the regime of weak localization and antilocalization. Further increase of disorder leads to strong localization regime, when conductivity is described by the variable-range-hopping (VRH) mechanism. A crossover from the Mott regime to the Efros-Shklovskii regime of VRH is observed with decreasing temperature. Theoretical analysis of conductivity in both regimes showed a remarkably good agreement with experimental data.
Broadening of the Raman scattering (RS) spectra was studied in monolayer graphene samples irradiated with various dose of ions followed by annealing of radiation damage at different temperatures. It is shown that the width {Gamma} (full width at half maximum, FWHM) of three main RS lines (G-, D-, and 2D) increases linearly with increase of the density of irradiation-induced point defects N d as {Delta}{Gamma} = m N d . The slope m of the linear dependencies is the same for one-phonon emitting G-line and D-line, and almost double for two-phonon emitting 2D-line. It is also shown that the width of D-line {Gamma} D for all samples is larger than one half of the width of 2D-line {Gamma} 2D , which shows that in the case of D-line, elastic electron scattering on point defects leads to an additional decreasing the lifetime of the emitted phonon. Theoretical model of the width of D-line in disordered graphene is developed which explains the experimental observations and allows to determine the numerical coefficient in the in-plane transverse optic phonon dispersion in graphene.
The influence of long-term ageing (about one year) on the Raman scattering (RS) spectra and the temperature dependence of conductivity has been studied in two series of monolayer graphene samples irradiated by different doses of C$^{+}$ and Xe$^{+}$ ions. It is shown that the main result of ageing consists of changes in the intensity and position of D- and G- and 2D-lines in RS spectra and in an increase of the conductivity. The observed effects are explained in terms of an increase of the radius of the textquotedblleft activatedtextquotedblright{} area around structural defects.
Raman scattering (RS) spectra and current-voltage characteristics at room temperature were measured in six series of small samples fabricated by means of electron-beam lithography on the surface of a large size (5x5 mm) industrial monolayer graphene film. Samples were irradiated by different doses of C${}^+$ ion beam up to $10^{15}$ cm${}^{-2}$. It was observed that at the utmost degree of disorder, the Raman spectra lines disappear which is accompanied by the exponential increase of resistance and change in the current-voltage characteristics.These effects are explained by suggestion that highly disordered graphene film ceases to be a continuous and splits into separate fragments. The relationship between structure (intensity of RS lines) and sample resistance is defined. It is shown that the maximal resistance of the continuous film is of order of reciprocal value of the minimal graphene conductivity $pi h/4e^2approx 20$ kOhm.
We consider the nonlinear terahertz response of n-doped monolayer graphene at room temperature using a microscopic theory of carrier dynamics. Our tight-binding model treats the carrier-field interaction in the length gauge, includes phonon as well as short-range neutral-impurity scattering, and fully accounts for the intrinsic nonlinear response of graphene near the Dirac point. Treating each interaction microscopically allows us to separate contributions from current clipping, phonon creation, and elastic impurity scattering. Although neutral impurity scattering and phonon scattering are both highly energy-dependent, we find that they impact conduction-band electron dynamics very differently, and that together they can help explain experimental results concerning field-dependent terahertz transmission through graphene.
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