How Reliable Are Raman Spectroscopy Measurements of Graphene

Article pubs.acs.org/JPCC

How Reliable Are Raman Spectroscopy Measurements of Graphene Oxide? Jeremy S. Mehta,† Austin C. Faucett,† Anju Sharma,‡ and Jeffrey M. Mativetsky*,† Department of Physics, Applied Physics and Astronomy, and ‡Small Scale Systems Integration and Packaging Center, Binghamton University, Binghamton, New York 13902, United States

J. Phys. Chem. C 2017.121:16584-16591. Downloaded from pubs.acs.org by MCMASTER UNIV on 06/15/18. For personal use only.

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ABSTRACT: Graphene oxide (GO) has garnered attention for its tunable chemical, electrical, and optical properties. An integral part of the efforts to manipulate and improve the performance of GO is the ability to reliably characterize its complex structure. Raman spectroscopy and confocal Raman mapping are widely used for insight into the extent of GO’s nanoscale graphene-like domains, the degree of lattice order, and its sheet stacking structure. It has also been reported, however, that laser sources, similar to those used for Raman spectroscopy, can be used to intentionally reduce and ablate GO. In light of this, it is unclear how invasive Raman measurements of GO are and how reliable published Raman data is. In this study, we employ Raman laser doses spanning 4 orders of magnitude to investigate the impact of Raman measurements on GO structure. We find that GO undergoes reduction at all practical laser doses, with the degree of reduction increasing with dose. Lattice damage and ablation dominate at high laser doses. Based on our findings, we encourage the use of a minimal laser dose (8 × 107 J/m2 or below) for Raman measurements of GO. Despite the resulting loss in signal, these conditions limit sample modification and measurement inaccuracies.

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INTRODUCTION Graphene oxide (GO) is a mechanically robust and flexible two-dimensional material with tunable chemical, electrical, and optical properties.1−8 GO’s versatility is afforded by its complex structure, characterized by nanoscale domains of sp2-bonded carbon interrupted by lattice defects and sp3-bonded oxygencontaining functional groups.9,10 This structure offers diverse options for chemical interfacing, including aromatic stacking, hydrogen bonding, and covalent attachment of functional moieties for sensing, drug delivery, and composite material development.11−14 Although pristine GO is electrically insulating, electrical conductivity (up to 3112 S/cm demonstrated so far15) can be recovered through reduction, which removes oxygen-containing functional groups and partially restores the underlying sp2 carbon lattice.16−19 Moreover, GO is optically transparent and exhibits fluorescence, making it appealing for biological labeling.20,21 Since the properties of GO are intimately linked with its chemical makeup and twodimensional structure, accurate tools are needed to elucidate the relationships between its structure and function and to further develop routes for GO synthesis, functionalization, and reduction. Raman spectroscopy and confocal Raman mapping are ubiquitous methods for examining GO’s sp2 domain size, lattice order, functionalization, and sheet-stacking arrangement2,18,22−27 Raman spectroscopy relies on the detection of laser light that is inelastically scattered from a sample. Although some studies note the need to limit laser intensity during Raman measurement of GO to avoid visible sample damage,28−30 few experimental details are generally provided. It has also been noted that the main Raman bands can decrease © 2017 American Chemical Society

in intensity due to excessive Raman laser exposure; however, the specific physical origins of these changes have not been investigated.27,31 This lack of discourse about appropriate measurement conditions is concerning since laser light can also be used to photoreduce31−35 and ablate GO.36,37 Moreover, the effects of laser-induced sample modification may not be obvious in the absence of complementary characterization of the laser-exposed sample regions. Unintended (and undetected) sample modification during Raman acquisition can lead to compromised Raman spectra and inconsistencies in the literature. The influence of laser-induced sample modification becomes even more critical when quantitatively modeling the relationships between Raman signal intensity and degree of functionalization or lattice order.27,38,39 At present, the extent to which GO Raman data are skewed by measurement-induced sample modification is unknown. In this report, we investigate the influence of laser dose on GO film integrity and establish guidelines for performing reliable Raman measurements of GO. We find, through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), that GO reduction takes place at all laser doses that are practical for Raman measurement. Furthermore, as laser dosage increases, the degree of reduction increases and ablation occurs. We determine that in order to preserve sample integrity and obtain minimally perturbed Raman data, Raman measurements of GO must be performed Received: May 10, 2017 Revised: July 11, 2017 Published: July 12, 2017 16584

DOI: 10.1021/acs.jpcc.7b04517 J. Phys. Chem. C 2017, 121, 16584−16591

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The Journal of Physical Chemistry C

Figure 1. (a) Normalized Raman spectra for a GO film on SiO2/Si acquired with a 1 s exposure at laser intensities ranging from 0.0001I0 to I0 (I0 = 8.02 × 1010 W/m2). (b) Optical microscope image of a GO film after a series of Raman maps were performed, each over a 7.2 μm × 7.2 μm area, at laser intensities between 0.001I0 and I0 and with acquisition times of 0.5, 1, and 5 s. (c) Corresponding “probe” map of the D-band (1357 cm−1) intensity, recorded with a minimally invasive measurement condition (laser intensity = 0.001I0, time = 1 s/spectrum).

with a laser dose of 8 × 107 J/m2 or below despite the compromise in signal-to-noise.

AFM topography was measured using an AIST-NT Combiscope 1000 AFM in tapping mode. Silicon probes (BudgetSensors Tap300-G) with a nominal spring constant of 40 N/m, a nominal resonance frequency of 300 kHz, and a probe radius of under 10 nm were employed. Film thickness was determined by measuring the depth of a scratch made near areas of interest.

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EXPERIMENTAL METHODS GO purchased from Graphene Supermarket was produced by oxidizing graphite crystals with a mixture of sulfuric acid, sodium nitrate, and potassium permanganate (the Hummers method). GO powder was dispersed in deionized water at a concentration of 0.5 mg/mL and stirred for at least 30 days. Centrifugation was used to help remove multilayer GO. SiO2coated Si substrates were cleaned by sonication in acetone, 2propanol, and distilled water for 5 min each and then treated with UV/ozone for 60 min. GO films (30 nm thick) were formed by drop-casting the GO suspension with a volume of 0.5 μL per square centimeter of substrate. Voltage-induced GO reduction40 was performed by applying 5 V across two thermally evaporated gold contacts separated by 500 μm. Raman spectroscopy was carried out using a Renishaw inVia confocal Raman microscope. Raman spectra were acquired using a 532 nm, 47.2 mW laser and a 50×, numerical aperture (NA) = 0.75 microscope objective, resulting in a 0.87 μm spot size. Laser power was measured using a powermeter, and spot size was estimated through the relationship d = 1.22λ/NA where d is the spot diameter and λ is the laser wavelength. To investigate the influence of laser dose on GO, Raman maps were performed with a 0.9 μm step size, an integration time of 1 s, and laser intensities ranging from 0.0001I0 to I0 (I0 = 8.02 × 1010 W/m2). For subsequent measurements under minimally invasive conditions (referred to hereafter as “probe” measurements), Raman maps were performed with a 0.9 μm step size, a 1 s integration time, and a laser intensity of 0.001I0. Similar tests performed with a 785 nm laser yielded a less favorable trade-off between signal level and sample damage. We therefore focused on the effects of 532 nm laser exposure. XPS was performed using a PHI 5000 VersaProbe instrument in ultrahigh vacuum (10−7 Pa). A 25 W monochromatic Al Kα (1486.6 eV) X-ray source was used to acquire spectra from 100 μm × 100 μm sample areas. XPS spectra were calibrated to the location of the sp2-bonded carbon peak (284.5 eV).

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RESULTS AND DISCUSSION To examine the influence of Raman measurements on GO, we recorded Raman spectra at a range of laser intensities, varying over 4 orders of magnitude. Figure 1a shows a series of Raman spectra acquired at separate locations of a GO sample with laser intensities between 0.0001I0 and I0 and an integration time of 1 s (I0 = 8.02 × 1010 W/m2). Prominent excitation bands are observed at 1357 and 1600 cm−1, corresponding to the D-band and G-band. The D-band is a product of phonon breathing modes incited by carbon vacancies.39,41 The G-band is activated by the degenerate E2g phonon modes of carbon sp2 bonds.42 Less intense second-order peaks occur at 2700, 2940, and 3160 cm−1, corresponding to the 2D-, D+G-, and C-band, respectively. The 2D-band is the most widely used secondorder band. This band is associated with the doubly degenerate in-plane optical phonon modes and is sensitive to changes in the thickness and stacking arrangement of GO layers.16,33,43,44 As shown in Figure 1a, increasing the incident Raman laser intensity results in smoother spectra, reflecting a higher signalto-noise ratio (S/N). Optical microscopy reveals, however, that substantial modifications are induced in the GO sample, indicated by a darkening in color, at the higher intensities and longer acquisition times (Figure 1b). These results highlight the need to carefully balance measurement conditions to provide adequate S/N while preserving sample integrity. Ignoring this fact can lead to measurements that modify the GO and thus produce spectra that are not representative of pristine GO. The laser-induced modifications during Raman acquisition are more readily observed by monitoring the Raman D-band after the initial modification. Since film modification occurs over the duration of the acquisition, initial Raman measurements capture the time-averaged state of the GO film. To 16585


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Figure 2. Initial Raman metrics, reflecting the time-averaged state of GO during acquisition at different laser intensities: (a) G-band full width at halfmaximum, (b) D-band to G-band intensity ratio, (c) 2D-band to G-band intensity ratio, and (d) G-band signal-to-noise ratio. Probe Raman metrics, reflecting the final state of GO after initial laser exposure: (e) G-band full width half at half-maximum, (f) D-band to G-band intensity ratio, (g) 2Dband to G-band intensity ratio, and (h) signal-to-noise of the G-band. Highlighted regions correspond to the transition between reduction- and ablation-dominated regimes. Red lines are included to guide the eye.

capture the final state, we perform a second set of Raman measurements with a minimally invasive, low laser dose condition (intensity = 0.001I0, time = 1 s/spectrum) that we refer to as a “probe” measurement. This probe measurement condition is later shown to induce minimal sample modification among practical Raman laser doses. As seen in Figure 1c, the Dband signal initially increases with laser intensity and exposure time. At even higher intensities and longer times, the D-band signal drops to near-zero values. It has previously been reported that the D-band decreases in intensity at high laser doses.27

This change in Raman signal intensity has been attributed to defunctionalization of GO. Our results, on the other hand, demonstrate two regimes of GO film modification: (1) an initial increase in D-signal, which can be associated with improved lattice order due to GO reduction,16,22 and (2), a sharp decrease in D-signal, which is symptomatic of material loss due to laser ablation. These two physical processes are confirmed through further Raman, XPS, and AFM data presented below. 16586


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Figure 3. (a) Deconvoluted C 1s core XPS spectra for pristine GO and GO exposed to 0.001I0 and I0 laser intensity for 1 s. (b) Carbon bond type and (c) oxygen concentration as a function of laser intensity. The dashed lines represent values for pristine GO.

Laser-induced reduction31 can occur through a photochemical process at energies exceeding 3.2 eV32 or through a photothermal process when the local temperature is raised high enough to trigger deoxidation.45 Since our light source energy (2.3 eV) is below 3.2 eV, we conclude that the observed reduction is thermal in origin. Further evidence of the thermal nature of the process is given by the formation of D-band intensity halos (high D-band intensity regions) which surround the edges of areas with near-zero D-band intensity. These halos are generated by heat transfer from the laser-exposed sample regions, with the size of the halo increasing with laser intensity and exposure time. For insight into the extent and nature of the modifications induced in GO during Raman measurements, we quantify the dependence of several key metrics on laser intensity (Figure 2). We first examine measurements performed using laser intensities between 0.001I0 and I0. These measurements, referred to as “initial” measurements, reflect the time-averaged state of the exposed sample region during a 1 s acquisition period. It can be seen in Figure 2a that the G-band full width at halfmaximum (fwhm) narrows with increasing acquisition laser intensity, by up to 11%, and then saturates above 0.1I0. The ratio of the D- and G-band intensity (ID/IG) steadily increases with intensity, by as much as 15%, followed by a slight decline above 0.5I0 (Figure 2b). The ratio of the 2D- and G-band intensity (I2D/IG) remains steady until about 0.05I0, after which the ratio drastically decreases with increasing laser intensity, by as much as 50% (Figure 2c). Finally, the signal-to-noise ratio (S/N) of the G-band was quantified by dividing the average Gband signal of 400 spectra taken at different locations across the uniform GO film by the standard deviation. Figure 2d shows the S/N of the G-band initially increasing with laser intensity but then decreasing at laser intensities beyond 0.05I0. Each of these data sets demonstrate that excessive laser exposure compromises the accuracy of Raman measurements of GO. Fortunately, the G-band fwhm, ID/IG, and I2D/IG exhibit changes of less than a few percent at laser intensities below

0.005I0, suggesting that measurement-induced errors can be mitigated. To investigate the final physical state of the samples after laser exposure, a second set of “probe” measurements was performed using a minimally invasive condition (intensity = 0.001I0, time = 1 s). These probe measurements reveal a nonmonotonic dependence of the G-band fwhm with two distinct regimes (Figure 2e): in the first regime, the G-band narrows with increasing laser intensity (from 86.5 to 77.7 cm−1); in the second regime, above 0.05I0, the G-band broadens (from 77.7 to 83.2 cm−1). The narrowing of the Gband at low laser intensities indicates an increased presence of low defect, unstrained sp2 clusters, which is characteristic of GO reduction.33,46,47 Conversely, the G-band broadening that occurs at high laser intensities results from bond angle contortions and bond length disorder.33,46,47 Evidently at high laser doses, the GO lattice degenerates. The probe ID/IG signal mirrors the G-band fwhm, with a nonmonotonic dependence on intensity and a transition between distinct regimes at 0.05I0 (Figure 2f). Below 0.05I0, ID/IG increases with laser intensity (from 0.87 to 0.95), while above 0.05I0, ID/IG decreases (from 0.95 to 0.90). ID/IG is indicative of the average separation, LD, between lattice defects and hence the sp2 domain size in GO.19 It should be noted that the range of G-band fwhm values (between 77 and as 89 cm−1) establishes that the GO used in this study is within the disordered regime23,25,39,42,46 in which LD increases with ID/IG according to (ID/IG) = (0.0055 Å−2)LD.242 Using this framework, we find that between 0.001I0 and 0.05I0, LD increases slightly from 1.25 to 1.32 nm. This partial recovery of the sp2 lattice is consistent with GO reduction.33 Beyond 0.05I0, LD decreases, reaching a value of 1.28 nm at I0. This data supports our findings from the G-band fwhm, that exposure to laser intensities below 0.05I0 induces GO reduction while exposure to laser intensities above 0.05I0 results in decreased lattice order. The weaker second-order band at 2700 cm−1, the 2D-band, is sensitive to GO stacking arrangement.9,18,48 Probe I2D/IG values remain steady until 0.01I0 and then decrease by as much as 50% 16587


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Figure 4. Topographic AFM images of a GO film exposed to a series of Raman measurements performed along a vertical line with a laser intensity of (a) 0.00I0, (b) 0.01I0, (c) 0.1I0, and (d) I0 for 1 s at each sample position. (e) Average height profile for each sample. A height of 0 nm indicates the height of the SiO2 surface.

(Figure 2g). The decrease in I2D/IG suggests a disturbance in vertical planar alignment between adjacent sheets of the GO film at high laser intensities,2,9,18 a finding that is in accordance with the concomitant decrease in lattice order found from the G-band fwhm and ID/IG. Further insight into the nature of the GO film modification taking place at high laser intensities is provided by our probe Gband S/N data. Although each of the probe measurements is performed with the same conditions (intensity = 0.001I0, time = 1 s), the GO regions exposed to higher initial laser intensities exhibit a much lower S/N (Figure 2h). Over the analyzed range of intensities, the probe S/N steadily decreases from 6.3 to 0.8. This result is in agreement with the probe D-band map (Figure 1c) that shows a lack of signal at sample regions exposed to high laser doses. These marked losses of signal imply that the sample regions exposed to high laser intensities have less material present due to ablation. Collectively, the probe Raman metrics point toward GO modification in the form of reduction at low laser intensities and a gradual transition (around 0.01I0− 0.05I0) to film degradation and ablation at higher intensities. For insight into the chemical makeup of GO after exposure to Raman measurements, we employed XPS (Figure 3). C 1s spectra were decomposed into four components: carbon− oxygen bonds C−O at 286.7 ± 0.2 eV, CO at 288.0 ± 0.2 eV, and COOH at 288.9 ± 0.2 eV, as well as a component composed of sp2- and sp3-bonded carbon, C−C at 284.6 ± 0.3 eV.20,49,50 The C−C contribution is frequently fit to a single component because of the close binding energies of sp2 and sp3 carbon.20,51−53 Pristine GO exhibits prominent C−O and C−C peaks along with smaller CO and COOH components. After exposure to even a low laser dose of 0.001I0 for 1 s, there is a notable increase in the C−C concentration and a decrease in C−O. At I0, the peaks associated with carbon−oxygen bonds are largely extinguished (Figure 3a). The dependence of carbon bond composition on laser intensity is shown in Figure 3b. Bond composition was quantified by calculating the ratio of individual constituent peak areas to the total C 1s carbon peak area. At a laser dose of 0.001I0 for 1 s, the condition used for minimally invasive probe Raman measurements, the changes to the sample are modest: the C−C concentration increases from 43.3% to 48.7%, and the

C−O concentration decreases from 45.7% to 42.1%. As the laser intensity is increased, C−O is effectively removed and C− C concentration steadily increases, both evidence of GO reduction.22,54 We thus find that reduction occurs at all laser intensities, with a steady increase in reduction level with laser dose. GO reduction level is often characterized by the overall oxygen concentration, with lower oxygen levels indicating a more reduced sample.22 Comparison of the C 1s and O 1s core level spectra show that for GO exposed to a 0.001I0 laser intensity for 1 s, the overall oxygen concentration decreases from 33.6% in pristine GO to 31.8%, followed by a more abrupt loss of oxygen, to 27.2% at 0.01I0 and 9.3% at I0. (Figure 3c). This final oxygen concentration is comparable to that found for hydrazine reduction (8−11%),18,19 indicating a substantial degree of reduction. These results clearly demonstrate the importance of minimizing laser exposure to limit GO modification during Raman measurement. Direct evidence of GO film thinning caused by Raman measurement was observed by AFM (Figure 4). A series of Raman spectra were recorded along a vertical line with a 0.9 μm spacing. These measurements are representative of a single scan line in a Raman map. At a laser intensity of 0.001I0 and a spectrum acquisition time of 1 s (Figure 4a), AFM shows minimal topographic change due to the Raman measurements. Closer examination of the average line profile (Figure 4e), however, reveals a decrease in film thickness of up to 17% along the laser-exposed line. It is well established that GO reduction leads to GO sheet and film thinning, by as much as 50%.23,26,55 The observed minor change in film thickness is consistent with our Raman and XPS data that demonstrate a small degree of reduction taking place at 0.001I0, although we cannot rule out the possibility that slight ablation simultaneously takes place as well. At higher Raman laser intensities (0.01I0, 0.1I0, and I0) deep troughs are formed in GO films with the respective troughs penetrating as deeply as 55%, 75%, and even 100% through the film thickness at the trough center. These results provide definitive evidence that Raman measurements performed using laser doses above 0.01I0 for 1 s result in severe damage to GO. The observed film thinning is beyond what can be ascribed to 16588


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The Journal of Physical Chemistry C photoreduction alone, providing direct evidence of film ablation.31,36 Overall, our data shows that GO reduction occurs during Raman measurements, even at low laser doses. As laser intensity is increased, reduction levels increase and ablation occurs. To record meaningful Raman measurements of GO, one must minimize sample modification while preserving a sufficient S/N. As shown in Figure 5, the S/N for the D, G, and

J/m2 and an average of 400 spectra recorded at different locations, with the G-band S/N increasing from 6.5 to 130. Raman mapping is a particularly useful capability for establishing structure−function relationships in nanomaterials.1,56,57 Figure 7 shows filaments of reduced GO surrounded

Figure 5. Raman signal-to-noise increases with laser intensity, up to about 0.05I0−0.1I0, and drops at higher laser intensity. Sample integrity (represented by the change in oxygen concentration, measured by XPS) steadily decreases with laser intensity. Figure 7. Optical image and Raman maps for a voltage-reduced GO filament recorded for 1 s at each pixel, (a) at 0.01I0 and (b) at 0.001I0 laser intensity.

2D peaks initially increases with Raman laser intensity, until 0.05I0 −0.1I0, and then decreases as ablation sets in. On the other hand, sample integrity, indicated in Figure 5 by the change in oxygen concentration found using XPS, continuously diminishes as laser intensity is increased. To preserve sample integrity, and extract minimally compromised Raman data, S/N must thus be sacrificed. Although some degree of sample modification is unavoidable, the effects can be mitigated. At 0.001I0 for 1 s (8.02 × 107 J/ m2), a small degree of reduction occurs (characterized by a small oxygen concentration loss, from 33.6% to 31.8%). Under these conditions, the S/N is high enough to reliably fit the resulting spectra: 6.5 for the G-band, 11.7 for the D-band, and 3.2 for the 2D-band. Based on our results, we recommend that Raman measurements of GO are performed with a laser dose of 8 × 107 J/m2 or below. For uniform samples, an effective strategy for minimizing laser exposure while boosting S/N is to average spectra from different sample locations. Figure 6 shows an individual spectrum recorded with a laser dose of 8.02 × 107

by GO. The filaments were produced by voltage-induced reduction, in which reduction is triggered by the application of a voltage across adjacent electrode pairs.40 The voltage-reduced filaments are characterized by a narrower G-band fwhm and an increased ID/IG, indicating improved sp2 network order and an increased sp2 domain size.41,46 The I2D/IG maps exhibit higher noise levels because they rely on a less prominent second-order band which is almost half as intense as the first order D- and Gbands. The Raman maps shown in Figure 7a, acquired with a 0.01I0 laser intensity for 1 s at each point, are typical of published Raman data for GO, with a strong signal (G-band S/N = 15.7). It is important to note, however, that under this condition, according to our XPS and AFM data, the GO experiences significant sample modification characterized by a significant loss of oxygen (from 33.6% to 27.2% concentration) and film thickness (up to 55% of total thickness). In contrast, although acquisition with a laser intensity of 0.001I0 (Figure 7b) results in a lower S/N (G-band S/N = 6.5), sample modification is substantially less pronounced: a small decrease in oxygen concentration (from 33.6% to 31.8%) and a maximum loss in film thickness of 17%. When performing Raman mapping of GO, it is therefore advisable to settle for a low S/N to limit laser-induced sample modification and measurement error.

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CONCLUSIONS This study addresses the need to identify and mitigate the physical changes that take place in GO during Raman spectroscopy measurements. Our experiments reveal two types of modification to GO caused by Raman measurements: reduction and ablation. Reduction occurs at all laser intensities

Figure 6. (a) Raman spectrum of GO acquired at 0.001I0 for 1 s and (b) an average Raman spectrum from similar measurements at 400 sample locations. 16589


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(7) Maher, E.-K. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326−1330. (8) Murray, I. P.; Lou, S. J.; Cote, L. J.; Loser, S.; Kadleck, C. J.; Xu, T.; Szarko, J. M.; Rolczynski, B. S.; Johns, J. E.; Huang, J.; et al. Graphene Oxide Interlayers for Robust, High-Efficiency Organic Photovoltaics. J. Phys. Chem. Lett. 2011, 2, 3006−3012. (9) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cançado, L. G.; Jorio, A.; Saito, R. Studying Disorder in Graphite-Based Systems by Raman Spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276−1291. (10) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2008, 2, 463−470. (11) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving through Graphene Oxide Membranes. Science 2014, 343, 752−754. (12) Wang, Y. X.; Chou, S. L.; Liu, H. K.; Dou, S. X. Reduced Graphene Oxide with Superior Cycling Stability and Rate Capability for Sodium Storage. Carbon 2013, 57, 202−208. (13) Liu, J.; Cui, L.; Losic, D. Graphene and Graphene Oxide as New Nanocarriers for Drug Delivery Applications. Acta Biomater. 2013, 9, 9243−9257. (14) Yun, J. M.; Yeo, J. S.; Kim, J.; Jeong, H. G.; Kim, D. Y.; Noh, Y. J.; Kim, S. S.; Ku, B. C.; Na, S. I. Solution-Processable Reduced Graphene Oxide as a Novel Alternative to PEDOT:PSS Hole Transport Layers for Highly Efficient and Stable Polymer Solar Cells. Adv. Mater. 2011, 23, 4923−4928. (15) Chen, Y.; Fu, K.; Zhu, S.; Luo, W.; Wang, Y.; Li, Y.; Hitz, E.; Yao, Y.; Dai, J.; Wan, J.; et al. Reduced Graphene Oxide Films with Ultrahigh Conductivity as Li-Ion Battery Current Collectors. Nano Lett. 2016, 16, 3616−3623. (16) Díez-Betriu, X.; Á lvarez-García, S.; Botas, C.; Á lvarez, P.; Sánchez-Marcos, J.; Prieto, C.; Menéndez, R.; de Andrés, A. Raman Spectroscopy for the Study of Reduction Mechanisms and Optimization of Conductivity in Graphene Oxide Thin Films. J. Mater. Chem. C 2013, 1, 6905. (17) Mativetsky, J. M.; Treossi, E.; Orgiu, E.; Melucci, M.; Veronese, G. P.; Samorì, P.; Palermo, V. Local Current Mapping and Patterning of Reduced Graphene Oxide. J. Am. Chem. Soc. 2010, 132, 14130− 14136. (18) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A.; et al. Chemical Analysis of Graphene Oxide Films after Heat and Chemical Treatments by X-Ray Photoelectron and Micro-Raman Spectroscopy. Carbon 2009, 47, 145−152. (19) Mattevi, C.; Eda, G.; Agnoli, S.; Miller, S.; Mkhoyan, K. A.; Celik, O.; Mastrogiovanni, D.; Granozzi, C.; Garfunkel, E.; Chhowalla, M. Evolution of Electrical, Chemical, and Structural Properties of Transparent and Conducting Chemically Derived Graphene Thin Films. Adv. Funct. Mater. 2009, 19, 2577−2583. (20) Shang, J.; Ma, L.; Li, J.; Ai, W.; Yu, T.; Gurzadyan, G. G. The Origin of Fluorescence from Graphene Oxide. Sci. Rep. 2012, 2, 792. (21) Dong, H.; Zhang, J.; Ju, H.; Lu, H.; Wang, S.; Jin, S.; Hao, K.; Du, H.; Zhang, X. Highly Sensitive Multiple MicroRNA Detection Based on Fluorescence Quenching of Graphene Oxide and Isothermal Strand- Displacement Polymerase Reaction. Anal. Chem. 2012, 84, 4587−4593. (22) Pei, S.; Cheng, H. M. The Reduction of Graphene Oxide. Carbon 2012, 50, 3210−3228. (23) Eigler, S.; Dotzer, C.; Hirsch, A. Visualization of Defect Densities in Reduced Graphene Oxide. Carbon 2012, 50, 3666−3673. (24) Eigler, S.; Grimm, S.; Enzelberger-Heim, M.; Müller, P.; Hirsch, A. Graphene Oxide: Efficiency of Reducing Agents. Chem. Commun. 2013, 49, 7391. (25) Bouša, M.; Frank, O.; Jirka, I.; Kavan, L. In Situ Raman Spectroelectrochemistry of Graphene Oxide. Phys. Status Solidi B 2013, 250, 2662−2667.

that are practical for Raman measurements, with the reduction level being commensurate with the laser dose. Evidence for reduction is provided by an increase in the Raman ID/IG ratio and a decrease in the G-band fwhm (indicating an increase in sp2 domain size and improved sp2 lattice order), a decrease in oxygen concentration and an increase in C−C concentration measured by XPS, and a decrease in GO film thickness measured by AFM. The second regime of GO modification, at higher laser doses, is dominated by structural damage and ablation. Raman ID/IG, I2D/IG, and G-fwhm indicate a decrease in a structural order. Concurrently, a drop in Raman S/N occurs due to loss of material. Finally, the clearest evidence of GO ablation is provided by AFM which shows deep troughs in GO films exposed to a high laser dose during Raman measurements. Even under the gentlest Raman measurement conditions, GO modification takes place. However, these effects can be mitigated by employing a laser dose of 8 × 107 J/m2 or less. At this laser level, the structural effects on GO are modest. Although Raman S/N can be substantially higher at higher laser doses, the degree of GO modification also increases substantially. Based on our study, we suggest that sample integrity should be prioritized over S/N to obtain accurate Raman data.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeffrey M. Mativetsky: 0000-0002-6574-9843 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CMMI-1537648) and made use of facilities acquired through a National Science Foundation Major Research Instrumentation grant (CMMI-1429176). This work also made use of the Analytical and Diagnostics Laboratory at Binghamton University’s Small Scale Systems Integration and Packaging Center. We thank Jaymes Flournoy and Kevin Silverstein for assistance with sample preparation.

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