Reduction of graphite oxide to graphene with laser irradiation

Abstract

Reduction of graphite oxide (GO) to graphene induced by picosecond pulsed laser irradiation has been studied by Raman spectroscopy, scanning electron microscopy together with modeling of temperature dynamics in the materials. Dependence of the D, G, and 2D Raman band parameters on the laser pulse energy and the irradiation dose was evaluated. The exponential decline of the full width at half maximum of the Raman lines with increasing product of the pulse energy and irradiation dose was observed indicating ordering in the film and reduction in the number of graphene layers during the laser treatment. The minimum concentration of structural defects and the largest relative intensity of the 2D peak were found for the 50 mW mean laser power and the 30 mm/s scanning speed. Modeling of temperature dynamics revealed that the temperature of the GO film irradiated with a single laser pulse at a fluence of 0.04 J/cm² (50 mW) increased up to 1400 °C for a few nanoseconds, which was sufficient for the effective reduction of GO to graphene with successive laser pulses.

Introduction

Graphene – an allotrope with two-dimensional arrangement of carbon atoms – was obtained in 2004 [1]. Due to its unique structure, graphene has superior electronic and mechanical properties which can be useful for diverse applications [2]. Application of graphene in transistor contacts, bio-sensing, transparent electrodes [3], saturable absorbers [4] etc. has been verified in research laboratories. The graphene thin film technology has been recently developed to replace indium tin oxide (ITO) for solar panels, displays and LED lighting [5]. Gao et al. reported on experiments in production of micro-scale supercapacitors based on reduction of graphite oxide (GO) films [6]. Since the first graphene flakes were obtained by the micromechanical cleavage method, many different methods for this material fabrication have been developed. Graphene is produced by using pulsed laser deposition, chemical vapor deposition, epitaxial growth on silicon carbide or metal substrates, etc. [7]. There is still a need for new production methods which are distinguished by repeatability and can be implemented in mass production. One branch of graphene producing methods is based on the reduction of GO to graphene. Chemical, thermal and photo-induced reduction methods are implied for this purpose [3], [8], [9], [10], [11]. Several methods based on laser-induced reduction have been described recently [6], [12], [13], [14], [15], [16]. This type of procedures permits local reduction of the electrically and thermally insulating GO and formation of conductive graphene domains, which can be used in microelectronics for efficient heat removal or electrical connections. Laser radiation is also used for patterning and writing in graphene layer forming functional devices in thermally reduced GO [17] or CVD graphene [18] and graphene bilayers [19].

Raman spectroscopy is considered to be one of the most reliable and nondestructive methods for graphene allotrope detection [20], [21]. Raman spectra of carboneous materials show common features in the 800–2000 cm⁻¹ region. The G-peak ∼1560 cm⁻¹ corresponds to the E_2g phonon in the Brillouin zone center. The D-peak ∼1360 cm⁻¹ is due to the breathing modes of sp² atoms and requires a defect for its activation. Usually, as-prepared graphene without structural defects does not exhibit a strong D-peak because the corresponding Raman mode is active only on edges of flakes. The 2D peak (∼2700 cm⁻¹) – the second order of the D-peak – is the most intrinsic to graphene [22], [23]. This peak is always present in graphene Raman spectra even in the absence of D peak because no defects are required for its activation. The intensity ratio for the D, G and 2D Raman bands is often used as a criterion of the graphene phase quality. Minimum of I_D/I_G and maximum of I_2D/I_G correspond to the highest quality of graphene and, in our case, to the optimal conditions for the GO-to-graphene reduction [24]. Zhang et al. [12] inscribed microcircuits with the femtosecond laser into the GO layer spin-coated on glass. However, from Raman spectra of reduced GO there was no evidence of graphene formation as the 2D spectral peak (∼2700 cm⁻¹) was absent. Zhou et al. [13] proposed micro-structuring of GO with a continuous wave diode laser, but no clear evidence of graphene phase was confirmed by Raman spectra. Sokolov et al. [25] used continuous wave and pulsed laser excitation of graphene oxide in air and nitrogen atmosphere. Raman spectra revealed formation of graphene by appearance of the 2D-line. The results also showed the dependence of the process quality on the ambient atmosphere as a decrease of the D-peak intensity in Raman spectra was observed in experiments conducted in the nitrogen atmosphere. This band represents structural defects in the graphene layer.

In this paper we present results of our experiments in developing the process of GO reduction into graphene by laser irradiation. Experiments were conducted in the nitrogen atmosphere. Irradiated samples were analyzed by means of optical and scanning electron microscopy (SEM), and Raman spectroscopy. Simulation of the temperature dynamics was performed to get insight into the laser-induced thermal changes inside the GO layer. This method allowed creating electrically and thermally conductive graphene domains in the insulating GO substrate. Handling of a laser beam enabled precise writing of micro-channels which can be used in electronics as contacts or as a part of the heat removal system.