Photo-thermal oxidation of single layer graphene

Abstract

Controlled oxidation of graphene is important for electronic and sensing applications as it offers routes for hole doping and metal–insulator transition. Current methods of oxidation, however, disrupt the graphene lattice and yield pores having diameter > 20 nm for any oxidizing species. In this work, a method for photo-thermal oxidation of graphene is presented for the first time that suggests absence of pores in the graphene layer during atomic force microscopy. The mechanism of oxidation is studied on CVD (chemical vapor deposition)-grown graphene using in situ Raman spectroscopy. Analysis of the temporal evolution of Raman spectra in different oxidizing environments enabled extraction of the reaction energy of oxidation – providing fundamental insight into the oxidation process. Additionally, atomic force microscopy revealed clear phase contrast between the oxidized and un-oxidized domains which were randomly distributed across the graphene layer. This work will enable engineering of oxygen-related defects in graphene for electronic and sensing applications.

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Introduction

Chemical functionalization holds the key to enable applications of graphene in electronics, optoelectronics, sensing, energy harvesting and for studying electrochemistry.^1–14 Appropriate functionalization can create bandgaps in single-layer graphene (SLG) and induce p/n doping, both of which are needed for electronic (transistors) and optoelectronic (photodetectors) applications. Functionalization can also modify the sensitivity and selectivity of SLG in chemical and biological sensors¹⁵ and electrochemical properties in redox reactions.^12–14,16 It can also influence the rate of chemical reactions in energy harvesting devices like fuel cells.^6,17 A wide range of chemicals are used to functionalize SLG either via adsorption of molecules, atoms or ions at the surface of graphene or via replacement of sp²-bonded carbon atoms in the graphene lattice with atoms like oxygen,^{13,14,18–22} sulphur,^23–28 nitrogen,^{10,15,17,28–32} etc. Oxygen functionalization is of particular interest for SLG as it offers routes for hole doping,^19,20 metal–insulator transition^22,33 and unique electrochemical reactions^12–14,16 in a controlled manner. Oxygen species like molecular oxygen (O₂) can adsorb at the surface of SLG at or above 260 Torr pressure^19,20 and induce hole doping that can be probed by monitoring shifts in the Raman scattering modes. At elevated temperatures (i.e., >300 °C at 350 Torr O₂ pressure¹⁹), the reaction between oxygen species and carbon atoms takes place leading to a reconstruction of the graphene lattice by formation of epoxides, carbonyls and other bonds, in addition to vacancies. These defects increase intensities of the disorder-activated modes in the Raman spectra collected from SLG, reduce the conductivity (both thermal and electrical) of SLG and at a higher concentration result in nanoscale pits or pores of sizes greater than 20 nm in diameter.¹⁹ Pores are also formed in SLG during O₂-plasma treatment,^34–36 which is typically used to pattern graphene into nanostructures for high-performance electronics³⁷ and for electrical³⁸ and mechanical³⁶ characterization. These plasma treatments also leave dangling bonds with associated functionalization at the edges of the SLG patterns.

In addition to the methods discussed above, SLG can be oxidized at temperatures as low as 40–120 °C in presence of ozone.^18,21,22 Exposure of SLG to ozone forms defects mostly near the grain boundaries with associated changes in the electrical and optical properties. At low levels of ozone exposure (i.e. for smaller oxidation time and/or lower oxidation temperature), oxygen species are adsorbed in the sp³-hybridized defect sites²¹ and at temperatures less than 80 °C increase the electrical conductivity of SLG via hole doping.²² An exposure of SLG to ozone at a higher level, however, reduces the conductivity of SLG due to creation of additional defects such as vacancies, pores, epoxides and carbonyls.^21,22 Pore formation is therefore always associated with the oxidation of SLG, whereas a pore-free oxidation method is highly desirable for electronic and optoelectronic applications.^{18–22,39–41}

This paper presents a method to obtain pore-free oxidation of SLG by exposing chemical vapor deposition (CVD)-grown SLG to low partial pressures of O₂, p(O₂) at high temperatures and in presence of a high-power laser. Application of the method resulted oxidation in nanoscale domains randomly spread across the oxidized area. Ex situ Raman spectroscopy and Atomic Force Microscopy (AFM) showed clear signatures of oxidation and AFM study confirmed the absence of pores in the oxidized areas. The kinetics of oxidation was studied using in situ Raman spectroscopy^42–45 by analyzing the temporal evolution of defect-bands of the Raman spectra. Analysis of in situ Raman data enabled extraction of reaction energy for the incorporation of oxygen species into the SLG lattice. This provided fundamental insights into the oxidation process by identifying the mechanism of oxidation (or oxygen defect engineering) and the active oxygen species.

Experimental setup

Fig. 1 shows a schematic of the experimental setup used to study the oxidation of SLG in a controlled environment. SLG was first grown on Cu foil using a low-pressure CVD process and later transferred onto arrays of silicon micro-pillars using thermal release tape and by etching Cu using a Fe(NO₃)₃ solution. The diameter and height of each silicon micro-pillar was ∼10 μm. Section S1 of ESI† details the substrate preparation, the SLG growth and the transfer process. Silicon micro-pillars with the SLG were placed inside a vacuum chamber with a pressure range of ∼1 mTorr to 760 Torr. Each micro-pillar was later exposed to a high-power (0–6 W), 532 nm continuous-wave (CW) laser with a spot size of ∼7 μm in an O₂ or Ar ambient or in their mixtures. The small thermal mass of the pillar allowed its efficient heating within fractions of a second to temperatures up to 1000 °C (please see ref. 43 for details on temperature estimation at different laser powers from the Raman peaks of the Si pillars). The scattered light from the micro-pillars was analyzed using a Raman spectrometer for in situ Raman analysis at different temperatures over time. The thickness of SiO₂ (∼90 nm) on top of the micro-pillar ensured constructive interference of the acquired Raman spectrum and enhanced its intensity.⁴⁶ Oxidation was performed using 25% O₂ (balanced with Ar) at different temperatures (∼400–750 °C) and at p(O₂) (∼1 mTorr to 2.5 Torr). During post-processing, ex situ Raman analysis was performed using CW lasers having different wavelengths (λ = 514, 532 and 633 nm). All the in situ and ex situ Raman spectra were analyzed after background subtraction. AFM topographic scans were also performed on the micro-pillars before and after SLG oxidation. For AFM imaging, uncoated silicon AFM probes (Budget Sensors; resonant frequency ∼ 300 kHz; force constant ∼ 40 N m⁻¹; tip radius < 10 nm) were used to measure the variation in height and phase of the probe tip moved along the surface of SLG in tapping mode.

Fig. 1 The experimental setup used to oxidize single-layer graphene (SLG) and study the kinetics of oxidation. The SLG placed on micro-pillars are exposed to a 0–6 W continuous-wave (CW) laser (having wavelength, λ ∼ 532 nm and spot size ∼ 7 μm) in an oxidative environment prepared using a flow of 25% O₂ (balanced with Ar) at low pressure. The scattered light from a micro-pillar was analyzed using a Raman spectrometer to study the time evolution of oxidation.

Results and discussion

AFM topographic scan is typically used to study the quality of SLG, its thickness and chemical functionalization.^{19,22,36,47–49} Fig. 2a shows AFM height profiles of SLG on top of a silicon micro-pillar. At larger size scales, AFM height profile and scanning electron microscopy image (Fig. S2a and S3 of ESI†) suggest incomplete coverage of SLG over the surface of different micro-pillars. The thickness of SLG was ∼1 nm, which is typical for a monolayer graphene. The wrinkles and residues observed in the AFM scan are residuals from the transfer process that remained on the sample even after annealing at 400 °C in presence of forming gas (10% H₂; 90% Ar). After oxidation at p(O₂) ∼ 2.5 Torr and at temperature T ∼ 678 °C for time duration t ∼ 150 s, AFM height profile shows small changes (<1 nm) in the SLG surface (Fig. 2b). Moreover, contrary to the recent reports,^19,34–36 the oxidized areas show no pores.

Fig. 2 Variation in height across the surface of SLG (a) before and (b) after oxidation obtained using Atomic Force Microscopy (AFM). Representative cross-sections of each height profile are shown below. Oxidation in (b) is performed at T ∼ 678 °C using p(O₂) ∼ 2.5 Torr for t ∼ 150 s. AFM scan after oxidation show no pores. The corresponding phase profiles are shown in Fig. 3.

The phase profiles obtained from the tapping mode AFM scans on SLG surfaces before (Fig. 3a) and after (Fig. 3b) oxidation show significant differences. Oxidation results in domains with phase values ∼3–4° higher compared to its surroundings. Similar phase differences have recently been reported during reduction of graphene oxide (GO) between GO and reduced GO (rGO) domains.⁴⁹ In analogy, we attribute the domains with higher and lower phases as oxidized SLG (O-SLG) and pristine SLG domains. The O-SLG domains are hydrophilic compared to the SLG domains and have higher presence of hydroxyl functionalities that increases tip–surface interaction and hence the phase values during AFM scan in those regions.^48,50 Analysis of the phase profile suggests oxidation in ∼30% (obtained by analyzing Fig. 3b using WSXM software⁵¹) of the scanned areas. Oxidation also occurred in domains impartial to the regions that had higher transfer residues at the beginning of oxidation (Fig. S3 of ESI†). This potentially suggests nucleation of oxidation at random spots in the SLG that increased in areas over time in a correlated manner; i.e. once oxidation started at a random spot in the SLG, oxidation at nearby spots were more likely that resulted in O-SLG domains of different sizes.

Fig. 3 Variation in the phase of AFM tip while scanning across the surface of SLG (a) before and (b) after pore-free oxidation. Representative cross-sections of each phase profile are shown below. Oxidized (O-SLG) domains and non-oxidized (SLG) regions are marked in (b). Presence of O-SLG domains in the post-oxidation data suggests that the oxidation initiated at random spots and then proceeded in a correlated manner.

Similar to the phase profiles obtained from AFM scan, the O-SLG domains also show noticeable contrast during SEM imaging after SLG oxidation (Fig. 4 and S5 of ESI† contains a larger scale version). Here, SEM was performed using a low (∼1 kV) acceleration voltage to reduce overall charging and hence to increase the contrast. The darker regions in the SEM image indicate lightly charged areas, as expected for the non-conductive O-SLG domains; these areas correspond very well with those having higher phases in the AFM scan (note that SEM performed at a higher acceleration voltage charged the O-SLG domains and removed the contrast between the O-SLG and SLG regions). All these suggest that the oxidation induced metal–insulator transition in the O-SLG areas in a manner which is far beyond O₂ adsorption.

Fig. 4 Comparison of (a) AFM phase profile and (b) SEM image (performed using ∼1 kV acceleration voltage) taken at the surface of a micro-pillar, on which SLG is oxidized at T ∼ 653 °C using p(O₂) ∼ 2.5 Torr for t ∼ 120 s. The oxidized domains (O-SLG; having higher phases in the AFM scan) appear darker in the SEM image compared to the non-oxidized areas. This suggests O-SLG domains have lower conductivity.

Raman spectroscopy has historically played a powerful role in structural characterization of graphitic materials⁵² and is used here in ex situ and in situ forms to analyze SLG oxidation. Fig. 5 plots ex situ Raman spectra taken using λ = 514 nm CW laser (power ∼ 1 mW, spot size ∼ 1 μm) before and after SLG oxidation. The post-oxidation Raman spectrum was collected from an area which is larger than the O-SLG domains (shown in Fig. 2b and 3b) and, therefore, had contributions from both the O-SLG and the SLG regions. (Wavelength and temperature dependent Raman spectra collected from SLG before oxidation are presented in Fig. S6 of ESI.† The presented data covers a wider range of temperature compared to that available in graphene literature.^53–55) Oxidation incurred distinct blue-shifts (i.e. towards higher frequencies) in the graphitic G-band frequency (ω_G: from ∼1588 to ∼1595 cm⁻¹) and the second-order 2D-band frequency (ω_2D: from ∼2691 to ∼2705 cm⁻¹) and also reduced the 2D/G peak ratios from ∼1.6 to ∼1.24. Oxidation also increased the magnitude of defect-related D- and D′-bands with associated increase in peak frequencies (ω_D: from ∼1353 to 1358 cm⁻¹; ω_D′: from ∼1626 to ∼1630 cm⁻¹) and peak ratios (from ∼0.1 to ∼0.86 for the D/G ratio; from ∼2 to ∼3.73 for the D/D′ ratio). It also changed widths of G-band peak from ∼16.1 cm⁻¹ to ∼23.5 cm⁻¹, D′-band peak from ∼20.1 cm⁻¹ to 11.9 cm⁻¹, D-band peak from ∼33.5 cm⁻¹ to 30.5 cm⁻¹ and 2D-band peak from ∼36 cm⁻¹ to ∼47.8 cm⁻¹. Most of these are typically observed during SLG oxidation in various manners.^{18,19,21,22,34,36} (The in situ Raman data, however, shows a different trend, as discussed later.) Note that the 2D peak, which disappears after complete oxidation of SLG^{18,19,21,22,36} and is generally absent for GO,^56–60 remained in the post-oxidation Raman spectrum due to partial coverage of scanned areas by the O-SLG domains. The remaining Raman bands (the D + D′ band, which appeared at ∼2951 cm⁻¹ after oxidation and the G*-band, which showed a frequency shift from ∼2455 to ∼2471 cm⁻¹) were weak in intensity compared to the others and hence omitted from additional analysis.

Fig. 5 Ex situ Raman spectra taken at λ = 514 nm from SLG before and after oxidation. The post-oxidation Raman spectrum was collected from the oxidized area presented in Fig. 2b and 3b. Representative Raman-band peaks (D-, G-, D′-, G*-, 2D- and D + D′-band peaks) are marked for the post-oxidation spectrum. The horizontal dotted lines represent the peak frequency of the G- and 2D-bands before oxidation suggesting a blue shift (increase in frequency) of all the Raman-band peaks. The defect-related Raman bands (i.e. the D-, D′- and D + D′-bands) are more prominent in the post-oxidation Raman spectrum.

The observation of defect-related Raman peaks before oxidation is possibly related to the defects present at the edges of the SLG patches (see Fig. S2 of ESI†) after the incomplete SLG transfer to micro-pillars. These peaks become prominent after oxidation with a D/G peak ratio of ∼0.86. This value is lower than that observed for oxidized SLG in literature,^18,21,22,52 as the post-oxidation Raman spectrum (taken with spot size larger than the size of the O-SLG domains) had contributions from the oxidized and un-oxidized regions. The D/D′ peak ratio of ∼3.73 obtained after oxidation, on the other hand, does not depend on the fraction of O-SLG domains covering the Raman scanned area, as it is a ratio between the Raman signals coming only from the defective (oxidized) regions of SLG. Our recent theoretical analysis^61,62 also calculated a similar D/D′ peak ratio for the SLG functionalized with molecular O₂ – suggesting the presence of small (not leading to visible pores during AFM scan), broad types of vacancies in the O-SLG domains.

In situ Raman data collected during SLG oxidation provided a unique opportunity to understand the nature of oxidation over time. Fig. 6a plots the temporal evolution of the Raman spectrum in the D- and G-band regions from the SLG-covered micro-pillar during oxidation at p(O₂) ∼ 2.5 Torr, T ∼ 678 °C. At this high temperature, the Raman-bands were broadened such that the G-band overshadowed the D′-band. An estimation of the evolution in D′-band was, therefore, impossible. The intensities of the D- and G-band peaks were extracted from the in situ data to calculate the D/G peak ratio, which is plotted against t in Fig. 6b. The D/G peak ratio increased with oxidation up to ∼0.8 and then slightly dropped down. In literature, significant decrease in the D/G ratio was observed during defect formation in SLG using energetic oxygen,^21,22,34 hydrogen^21,63 or argon^52,64 species and was attributed to the formation of sp³-hybridized defects and vacancies before and after the inflection point, respectively. To confirm whether this applies to the samples studied here would require a thorough investigation of the atomic structure of the oxidized regions, which is beyond the scope of this article. Note that due to differences in the spot sizes for the ex situ (∼1 μm) and in situ (∼7 μm) Raman analysis and also due to partial coverage of scanned areas by the O-SLG domains, the in situ D/G peak ratio obtained at the end of oxidation (∼0.7 in Fig. 6b) is different from the ex situ value (∼0.86 in Fig. 5). The incomplete oxidation of SLG on the micro-pillars also explains the observation of a lower maximum for the D/G peak ratio in Fig. 6b compared to that (∼100/E_L⁻⁴ ∼ 3.4; where E_L is the laser energy) in literature.^{21,22,34,52,64} Results from a controlled experiment performed by exposing SLG to O₂ at a lower pressure of oxygen, p(O₂) ∼ 1.25 mTorr and at a lower temperature T ∼ 378 °C showed negligible change in the D/G peak ratio (Fig. 6c and d). A mild O₂ exposure, therefore, is not conducive to SLG oxidation or defect formation. AFM scans on this mildly exposed SLG also showed no signs of phase change.

Fig. 6 Cascade plots of time dependent D-band and G-band Raman spectra during oxidation of SLG (a) at p(O₂) ∼ 2.5 Torr, T ∼ 678 °C and (c) at p(O₂) ∼ 1.25 mTorr, T ∼ 378 °C. Time evolution of the D/G peak ratios from analysis of the in situ Raman spectra in (a) and (c) are shown in (b) and (d), respectively. The lines in (b) and (d) are guide to the eye only.

Irrespective of the level of oxidation, ω_G exhibited a continuous blue-shift, while ω_2D exhibited an initial blue-shift, followed by an eventual red-shift over time (Fig. 7). As blue- and red-shifts in ω_2D originates from p- and n-doping, respectively,⁶⁵ the initial blue-shift in ω_2D is attributed to p-doping of SLG via O₂ adsorption. The subsequent red-shift in ω_2D is attributed to the n-doping of SLG due to desorption of hydroxyl (–OH) groups during high temperature processing. The time at which the effect from –OH desorption was visible (i.e. at ∼40 s in Fig. 7a and at ∼10 s in Fig. 7b) decreased with the decrease in p(O₂). In order to confirm that the observed initial p-doping of the SLG was indeed due to O₂ adsorption, SLG was annealed at ∼300 °C in vacuum (0.1 mTorr). This resulted in a monotonous blue-shift in ω_G and only a red-shift in ω_2D (Fig. S7 of ESI†) due to –OH desorption. Note that although the in situ Raman data suggested an overall red-shift in ω_2D at the end of oxidation, the ex situ Raman data collected after oxidation exhibited a blue-shift in ω_2D compared to the pre-oxidized values (Fig. 5). This was due to re-adsorption of –OH groups (more due to the hydrophilic nature of O-SLG) once the sample was exposed to ambient conditions for the ex situ Raman scan.

Fig. 7 Time evolution of the G-, 2D-band peak frequencies (ω_G and ω_2D, respectively) with respect to their pre-oxidized values obtained from the analysis of in situ Raman measurements during oxidation of SLG (a) at p(O₂) ∼ 2.5 Torr, T ∼ 678 °C and (b) at p(O₂) ∼ 1.25 mTorr, T ∼ 378 °C. The lines are guide to the eye only.

The effect of temperature on the kinetics of SLG oxidation is presented next. The time dependence of the D/G ratio at different temperatures with a fixed p(O₂) ∼ 2.5 Torr is shown in Fig. 8a. For T < 600 °C, the D/G ratio continuously increased over time. At higher temperatures, the D/G ratio reached a maximum (which happened at smaller t for higher T) and then dropped down. This inflection in the D/G ratio is possibly due to the formation of different types of defects before (sp³-hybridized) and after (vacancies) the inflection point.²¹ The oxidation at a reduced oxygen pressures of p(O₂) ∼ 1.25 mTorr yielded slower kinetics and did not show significant increase in the D/G ratio for T < 600 °C (Fig. 8b). At higher temperatures, however, the D/G ratio increased over time and showed an inflection point around t ∼ 170 s at T ∼ 740 °C. This occurrence of the inflection point at a longer time compared to that in Fig. 8a suggests differences in the degrees of oxidation at different p(O₂).

Fig. 8 Time dependence of the D/G ratios measured at different T for (a) p(O₂) ∼ 2.5 Torr and (b) p(O₂) ∼ 1.25 mTorr. The lines correspond to fits using eqn (1). The fitting parameters for a few temperatures are listed in the tables below. The parameters at lower temperatures are not unique; hence those fitted lines are not shown and corresponding parameters are not tabulated.

For an analysis of the defect density in SLG, the phenomenological model⁶⁴ relating the D/G ratio with lattice defects in graphene is considered; hence, the following expression is used to analyze the variation in the D/G peak ratio:

where D/G|₀ is the D/G peak ratio before oxidation, r_S is the radius of the structurally-disordered region with contribution C_S towards the D-band, r_A is the radius of the activated region in proximity of a defect with contribution C_A towards the D-band, L_D is the average distance between the defects, and F_G is the fraction of SLG being oxidized. Empirical analysis^52,64 suggests the following values for the defect parameters: r_S ∼ 1 nm, r_A ∼ 3 nm, C_S ∼ 0.86, C_A ∼ 4.56 (these values though were obtained by analyzing graphene defects characterized with Raman spectroscopy using λ = 514 nm, they are assumed to be similar for the λ = 532 nm Raman data. A change in these defect parameters has insignificant effect on the following discussion). At short oxidation time when the defect density is lower (i.e. L_D ∼ ∞), eqn (1) can be approximated as

D/G|_t∼0 = D/G|₀ + F_GπP_T²t²[C_Sr_S² + C_A(r_A² − r_S²)]. (2)

To derive eqn (2), L_D is approximated as 1/(P_Tt), where t is the oxidation time and P_T is a proportionality constant that varies with oxidation temperature and p(O₂). The quadratic time dependence of (D/G|_t∼0 − D/G|₀) in eqn (2) is a consequence of the L_D ∼ 1/t assumption. Oxidation performed at a reduced p(O₂) has a slower time dynamics and therefore allows analysis of the initial phases of oxidation for justification of the L_D ∼ 1/t assumption. Fig. S8 of ESI† signifies a clear quadratic time dependence for (D/G|_t∼0 − D/G|₀) at p(O₂) ∼ 1.25 mTorr and at T ∼ 660–740 °C, therefore, justifies the L_D ∼ 1/t assumption used in eqn (2). A fit of all the oxidation data using eqn (1) with L_D replaced with 1/(P_Tt) allows extraction of the variations in P_T and F_G with temperature and p(O₂). The extracted values are tabulated at the bottom parts of Fig. 8. An increase in P_T with the increase in T and p(O₂) corroborates with the expected increase in the oxidation rate at these conditions. The values of F_G in Fig. 8a suggest O-SLG domains occupying ∼10–15% (=F_G × 100%) of the total scanned area by in situ Raman spectroscopy, which is less than the ∼30% coverage obtained from ex situ Raman scan in Fig. 3b for a similar oxidation condition. The larger spot size (∼7 μm for the full-width half maximum) and higher power (>1 W) of the incident laser beam used for in situ Raman spectroscopy can account for this difference. These conditions induced acquisition of Raman signals from a wider area of the micro-pillar, therefore, reduced the contribution from the O-SLG domains in the in situ Raman data. A higher oxidation temperature increased the coverage of O-SLG domains and hence F_G (Fig. 8b). The parameters extracted in Fig. 8 will allow control of graphene oxidation at the conditions for which eqn (1) is fitted for. A finer control on oxidation at any given time, temperature and pressure will require fitting of the measured D/G ratio with eqn (1) for a wider range of oxidation conditions, which will be a topic of future study.

The in situ Raman data presented in Fig. 8 additionally allows calculation of the reaction energy (E_R) for SLG oxidation and hence to obtain mechanistic insight into the oxidation process. To calculate E_R, the D/G peak ratio at different temperatures was determined at a fixed time (∼100 s). The difference between D/G|_{t=100 s} and D/G|_t=0 (≡Δ(D/G)) is plotted as a function of 1/k_BT (Fig. 9), where k_B is the Boltzmann's constant. E_R was then extracted from the slope of the Δ(D/G) vs. 1/k_BT curves; E_R varies from ∼0.8 ± 0.17 eV to ∼1.11 ± 0.21 eV for the two oxidation conditions (i.e., p(O₂) ∼ 2.5 Torr and 1.25 mTorr, respectively) studied here. These values have potential contributions from defect formation in the SLG lattice and from reactions of oxygen species with the SLG and its defects. In fact, the E_R values are similar to the energies obtained for reactions of singlet O₂ with carbon atoms in a graphitic lattice,^5,66 which suggests possible conversion of O₂ to an active oxygen species like singlet O₂ during the high-power laser exposure. The other possible sources of oxidation from O₂ and ozone are ruled out, as they react with graphitic carbons at temperatures much higher⁶⁷ and lower^18,22,33,68 than the values used here. To understand more on the active oxygen species for the observed pore-free oxidation, a separate set of experiments were carried out where SLG (transferred on to SiO₂ substrates) was exposed directly to O₂ at temperatures up to 650 °C and at p(O₂) ∼ 5 Torr in an environmental microscope stage (Linkam Scientific). This O₂ exposure did not yield any insignificant increase in the D/G peak ratio (Fig. S9 of ESI†). On the other hand, SLG exposure to ozone at temperatures < 200 °C and at pressures < 30 Torr yielded high D/G peak ratio with large pores (Fig. S10 of ESI†) potentially near the grain boundaries of SLG.²¹

Fig. 9 Arrhenius plots of the percentage change in the D/G ratios, Δ(D/G), over their pre-oxidized values, (D/G)|₀, as a function of 1/k_BT for (a) p(O₂) ∼ 2.5 Torr and (b) p(O₂) ∼ 1.25 mTorr. The extracted reaction energies (E_R) of SLG oxidation at different p(O₂) are noted in respective figures.

Photo-activation of O₂ from the ground triplet to the excited singlet states is well-known in literature.⁶⁹ This singlet state of O₂ is presumably the active oxygen species for the laser induced oxidation studied here. In addition to the photo-activated singlet O₂, however, temperature is also responsible for the observed SLG oxidation. To confirm this, we exposed the SLGs placed on planar SiO₂/Si substrates to photo-activated O₂ generated with a laser power up to ∼4 W. The temperatures of the exposed areas increased up to 200–300 °C, however, SLG did not show any sign of oxidation. (This last observation further rules out the possibility of photo-conversion of O₂ to ozone, as ozone normally oxidizes SLG at temperatures < 200 °C.^18,22,33,68) All these suggest that the photo-activated oxygen species (possibly singlets of O₂) results in pore-free oxidation of SLG via a thermal process with the reaction energies shown in Fig. 9.

Conclusions

We obtained pore-free oxidation of single-layer graphene by exposing the material to photo-excited oxygen species. Surface analysis using atomic force microscopy suggested that the oxidized areas were randomly distributed in nanoscale domains across the graphene layer. The mechanism of oxidation was also studied using in situ Raman spectroscopy to monitor the time evolution of different Raman peaks. The origin of in situ peak shifts (blue-shift for G-band and red shift for 2D-band) was attributed to desorption of –OH groups during the high temperature oxidation process. The re-adsorption of these –OH groups in ambient condition resulted in blue-shifts for both of the Raman G- and 2D-band peaks, as observed during ex situ Raman spectroscopy. Empirical analysis of the time evolution of oxygenated defects suggested quadratic time dependence for defect density at lower levels of oxidation and a systematic variation of the proportionality constant with temperature and oxidation pressure at different times. The analysis of the in situ Raman data at various conditions also enabled extraction of fundamental parameters such as reaction energy for the metal–insulator transition in single-layer graphene and identified photo-excited singlet O₂ as the active oxygen species. In comparison with the oxygen species such as oxygen plasma, ozone and high pressure O₂ explored in literature for graphene oxidation, the singlet O₂ at ≤2.5 Torr pressure reacted with graphene in a milder way and induced oxidation of graphene in a controlled and pore-free manner. Future exploration of such controlled chemical modification of graphene will enable defect engineering of graphene at different size scales and hence will greatly expand the use of graphene in many envisaged applications.

Acknowledgements

Financial support from National Research Council (Contract Number: FA9550-12-D-0001) and Air Force Office of Scientific Research (Program Manager: Dr Charles Lee; Task Number: 3002MR12P/14RX03COR) are acknowledged for preparation of this manuscript.

References

1. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
2. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197–200.
3. V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril and K. S. Kim, Chem. Rev., 2012, 112, 6156–6214.
4. T. Kuila, S. Bose, A. K. Mishra, P. Khanra, N. H. Kim and J. H. Lee, Prog. Mater. Sci., 2012, 57, 1061–1105.
5. D. W. Boukhvalov and M. I. Katsnelson, J. Phys.: Condens. Matter, 2009, 21, 344205.
6. L. Dai, Acc. Chem. Res., 2013, 46, 31–42.
7. J. M. Englert, C. Dotzer, G. Yang, M. Schmid, C. Papp, J. M. Gottfried, H.-P. Steinrueck, E. Spiecker, F. Hauke and A. Hirsch, Nat. Chem., 2011, 3, 279–286.
8. J. E. Johns and M. C. Hersam, Acc. Chem. Res., 2013, 46, 77–86.
9. A. Criado, M. Melchionna, S. Marchesan and M. Prato, Angew. Chem., Int. Ed., 2015, 54, 10734–10750.
10. F. M. Koehler, A. Jacobsen, K. Ensslin, C. Stampfer and W. J. Stark, Small, 2010, 6, 1125–1130.
11. S. Eigler and A. Hirsch, Angew. Chem., Int. Ed., 2014, 53, 7720–7738.
12. D. A. C. Brownson, D. K. Kampouris and C. E. Banks, Chem. Soc. Rev., 2012, 41, 6944–6976.
13. D. A. C. Brownson, A. C. Lacombe, M. Gomez-Mingot and C. E. Banks, RSC Adv., 2012, 2, 665–668.
14. W. Deng, X. Ji, M. Gomez-Mingot, F. Lu, Q. Chen and C. E. Banks, Chem. Commun., 2012, 48, 2770–2772.
15. R. Lv, Q. Li, A. R. Botello-Mendez, T. Hayashi, B. Wang, A. Berkdemir, Q. Hao, A. L. Elias, R. Cruz-Silva, H. R. Gutierrez, Y. A. Kim, H. Muramatsu, J. Zhu, M. Endo, H. Terrones, J.-C. Charlier, M. Pan and M. Terrones, Sci. Rep., 2012, 2, 586.
16. W. Lee, S. Suzuki and M. Miyayama, Electrochim. Acta, 2014, 142, 240–246.
17. L. Qu, Y. Liu, J.-B. Baek and L. Dai, ACS Nano, 2010, 4, 1321–1326.
18. S. Huh, J. Park, Y. S. Kim, K. S. Kim, B. H. Hong and J.-M. Nam, ACS Nano, 2011, 5, 9799–9806.
19. L. Liu, S. Ryu, M. R. Tomasik, E. Stolyarova, N. Jung, M. S. Hybertsen, M. L. Steigerwald, L. E. Brus and G. W. Flynn, Nano Lett., 2008, 8, 1965–1970.
20. S. Ryu, L. Liu, S. Berciaud, Y.-J. Yu, H. Liu, P. Kim, G. W. Flynn and L. E. Brus, Nano Lett., 2010, 10, 4944–4951.
21. M. Seifert, J. E. B. Vargas, M. Bobinger, M. Sachsenhauser, A. W. Cummings, S. Roche and J. A. Garrido, 2D Materials, 2015, 2, 024008.
22. J. Yuan, L.-P. Ma, S. Pei, J. Du, Y. Su, W. Ren and H.-M. Cheng, ACS Nano, 2013, 7, 4233–4241.
23. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. A. Chen and S. Huang, ACS Nano, 2012, 6, 205–211.
24. F. Shahzad, S. Yu, P. Kumar, J.-W. Lee, Y.-H. Kim, S. M. Hong and C. M. Koo, Compos. Struct., 2015, 133, 1267–1275.
25. R. K. Shervedani and A. Amini, Carbon, 2015, 93, 762–773.
26. P. A. Denis, Chem. Phys. Lett., 2010, 492, 251–257.
27. H. Gao, Z. Liu, L. Song, W. Guo, W. Gao, L. Ci, A. Rao, W. Quan, R. Vajtai and P. M. Ajayan, Nanotechnology, 2012, 23, 275605.
28. S. Kundu, R. M. Yadav, T. N. Narayanan, M. V. Shelke, R. Vajtai, P. M. Ajayan and V. K. Pillai, Nanoscale, 2015, 7, 11515–11519.
29. D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye and S. Knights, Energy Environ. Sci., 2011, 4, 760–764.
30. Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai and L. Qu, J. Am. Chem. Soc., 2012, 134, 15–18.
31. Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Muellen, J. Am. Chem. Soc., 2012, 134, 9082–9085.
32. R. Cruz-Silva, A. Morelos-Gomez, S. Vega-Diaz, F. Tristan-Lopez, A. L. Elias, N. Perea-Lopez, H. Muramatsu, T. Hayashi, K. Fujisawa, Y. A. Kim, M. Endo and M. Terrones, ACS Nano, 2013, 7, 2192–2204.
33. N. Leconte, J. Moser, P. Ordejon, H. Tao, A. Lherbier, A. Bachtold, F. Alsina, C. M. Sotomayor Torres, J.-C. Charlier and S. Roche, ACS Nano, 2010, 4, 4033–4038.
34. I. Childres, L. A. Jauregui, J. Tian and Y. P. Chen, New J. Phys., 2011, 13, 025008.
35. D. C. Kim, D.-Y. Jeon, H.-J. Chung, Y. Woo, J. K. Shin and S. Seo, Nanotechnology, 2009, 20, 375703.
36. A. Zandiatashbar, G.-H. Lee, S. J. An, S. Lee, N. Mathew, M. Terrones, T. Hayashi, C. R. Picu, J. Hone and N. Koratkar, Nat. Commun., 2014, 5, 3186.
37. M. Y. Han, B. Oezyilmaz, Y. Zhang and P. Kim, Phys. Rev. Lett., 2007, 98, 206805.
38. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First and W. A. de Heer, Science, 2006, 312, 1191–1196.
39. J. Aumanen, A. Johansson, J. Koivistoinen, P. Myllyperkio and M. Pettersson, Nanoscale, 2015, 7, 2851–2855.
40. Z. Xu and K. Xue, Nanotechnology, 2009, 21, 045704.
41. J.-A. Yan and M. Y. Chou, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 125403.
42. R. Rao, D. Liptak, T. Cherukuri, B. I. Yakobson and B. Maruyama, Nat. Mater., 2012, 11, 213–216.
43. P. Nikolaev, D. Hooper, N. Perea-Lopez, M. Terrones and B. Maruyama, ACS Nano, 2014, 8, 10214–10222.
44. R. Rao, A. E. Islam, N. Pierce, P. Nikolaev and B. Maruyama, Carbon, 2015, 95, 287–291.
45. R. Rao, N. Pierce, D. Liptak, D. Hooper, G. Sargent, S. L. Semiatin, S. Curtarolo, A. R. Harutyunyan and B. Maruyama, ACS Nano, 2013, 7, 1100–1107.
46. Y. Y. Wang, Z. H. Ni, Z. X. Shen, H. M. Wang and Y. H. Wu, Appl. Phys. Lett., 2008, 92, 043121.
47. A. Eckmann, A. Felten, A. Mishchenko, L. Britnell, R. Krupke, K. S. Novoselov and C. Casiraghi, Nano Lett., 2012, 12, 3925–3930.
48. J. I. Paredes, A. Martinez-Alonso and J. M. D. Tascon, Langmuir, 2003, 19, 7665–7668.
49. J. I. Paredes, S. Villar-Rodil, P. Solis-Fernandez, A. Martinez-Alonso and J. M. D. Tascon, Langmuir, 2009, 25, 5957–5968.
50. J. I. Paredes, A. Martinez-Alonso and J. M. D. Tascon, Langmuir, 2002, 18, 4314–4323.
51. I. Horcas, R. Fernandez, J. M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero and A. M. Baro, Rev. Sci. Instrum., 2007, 78, 013705.
52. L. G. Cancado, A. Jorio, E. H. Martins Ferreira, F. Stavale, C. A. Achete, R. B. Capaz, M. V. O. Moutinho, A. Lombardo, T. S. Kulmala and A. C. Ferrari, Nano Lett., 2011, 11, 3190–3196.
53. M. R. Joya, A. R. Zanatta and J. Barba-Ortega, Mod. Phys. Lett. B, 2013, 27, 1350203.
54. I. Calizo, I. Bejenari, M. Rahman, G. Liu and A. A. Balandin, J. Appl. Phys., 2009, 106, 043509.
55. I. Calizo, S. Ghosh, W. Bao, F. Miao, C. N. Lau and A. A. Balandin, Solid State Commun., 2009, 149, 1132–1135.
56. G. Eda and M. Chhowalla, Adv. Mater., 2010, 22, 2392–2415.
57. C. Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M. Burghard and K. Kern, Nano Lett., 2007, 7, 3499–3503.
58. K. P. Loh, Q. Bao, G. Eda and M. Chhowalla, Nat. Chem., 2010, 2, 1015–1024.
59. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
60. D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R. D. Piner, S. Stankovich, I. Jung, D. A. Field, C. A. Ventrice Jr and R. S. Ruoff, Carbon, 2009, 47, 145–152.
61. J. Jiang, R. Pachter, F. Mehmood, A. E. Islam, B. Maruyama and J. J. Boeckl, Carbon, 2015, 90, 53–62.
62. J. Jiang, R. Pachter, T. Demeritte, P. C. Ray, A. Islam, B. Maruyama and J. Boeckl, J. Phys. Chem. C, 2016, 120, 5371–5383.
63. Z. Luo, T. Yu, K.-J. Kim, Z. Ni, Y. You, S. Lim, Z. Shen, S. Wang and J. Lin, ACS Nano, 2009, 3, 1781–1788.
64. M. M. Lucchese, F. Stavale, E. H. M. Ferreira, C. Vilani, M. V. O. Moutinho, R. B. Capaz, C. A. Achete and A. Jorio, Carbon, 2010, 48, 1592–1597.
65. A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari and A. K. Sood, Nat. Nanotechnol., 2008, 3, 210–215.
66. S. P. Chan, G. Chen, X. G. Gong and Z. F. Liu, Phys. Rev. Lett., 2003, 90, 086403.
67. J. M. Carlsson, F. Hanke, S. Linic and M. Scheffler, Phys. Rev. Lett., 2009, 102, 166104.
68. G. Lee, B. Lee, J. Kim and K. Cho, J. Phys. Chem. C, 2009, 113, 14225–14229.
69. A. A. Krasnovsky Jr, Biochemistry, 2007, 72, 1065–1080.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05399h

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