Controlling the local chemical reactivity of graphene through spatial functionalization

Accepted Manuscript Controlling the local chemical reactivity of graphene through spatial function‐ alization Sandra C. Hernández, Francisco J. Bezares, Jeremy T. Robinson, Joshua D. Caldwell, Scott G. Walton PII: DOI: Reference:

S0008-6223(13)00288-1 http://dx.doi.org/10.1016/j.carbon.2013.03.059 CARBON 7940

To appear in:

Carbon

Received Date: Accepted Date:

20 December 2012 29 March 2013

Please cite this article as: Hernández, S.C., Bezares, F.J., Robinson, J.T., Caldwell, J.D., Walton, S.G., Controlling the local chemical reactivity of graphene through spatial functionalization, Carbon (2013), doi: http://dx.doi.org/ 10.1016/j.carbon.2013.03.059

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Controlling the local chemical reactivity of graphene through spatial functionalization

Sandra C. Hernández *a, Francisco J. Bezares b, Jeremy T. Robinson c, Joshua D. Caldwell c, Scott G. Walton d

a

NRC Postdoctoral Research Associate, residing at Naval Research Laboratory, 4555 Overlook Ave.,

SW Washington, DC 20375, USA b

ASEE Postdoctoral Research Associate, residing at Naval Research Laboratory, 4555 Overlook Ave.,

SW Washington, DC 20375, USA c

Electronics Science and Technology Division, Naval Research Laboratory, 4555 Overlook Ave., SW

Washington, DC 20375, USA d

Plasma Physics Division, Naval Research Laboratory, 4555 Overlook Ave., SW Washington, DC

20375, USA Corresponding Author * [email protected]. TEL: 001-202-767-0356

KEYWORDS: graphene, functionalization, chemical patterning, electrodeposition, electrical characterization, plasma functionalization, site specific deposition, electron transfer kinetics, electron beam plasmas

ABSTRACT: Spatial distribution of nitrogen functional groups on graphene by use of physical masks during plasma processing is demonstrated. Structural and chemical modifications are evidenced by micro-Raman and high-resolution x-ray photoelectron spectroscopy mapping, demonstrating a strong dependence on the processing conditions and physical mask configurations. The resulting nitrogen chemical moieties influence the electronic transport by affecting the mobility of carriers and consequently increasing the electron transfer kinetics, which provide localized chemical reactive sites. The chemical moieties demonstrate the ability to tailor the locality of the surface chemistry on graphene opening up a wide range of reactivity studies and synthesis capabilities, such as site-specific electrochemical deposition.

1. Introduction Graphene has attracted enormous attention due to its unique chemical, mechanical, optical, and physical characteristics, which together, make graphene an ideal candidate material for a broad range of nextgeneration technologies. Consequently, various forms of graphene (e.g., single-layer, bilayer, chemically-modified, etc.) are being vigorously investigated for applications in electronics,1-4 energy conversion,5-8 and sensing,9-12 among others. Equally important is the ability to further tailor these properties through modification of select attributes such as surface chemistry, number of layers, sheet width, and edge structures. Manipulating the surface chemistry of graphene is important since the chemical composition strongly impacts the electronic properties as well as chemical reactivity both globally and locally. Methods aimed at such chemical control include in-situ elemental doping of graphene,13, 14 wet14-21 and dry chemical22 treatments, and plasma processing23-25. Precise control of the surface chemistry of graphene can also allow for subsequent surface procedures focused on band gap engineering, device fabrication26,27 and sensor applications.9

Given the strong impact of adsorbates, global chemical modification provides opportunities towards greater control over the properties of graphene films. Control over the spatial distribution of these groups provides an even greater functionality in that the local graphene reactivity can be manipulated, opening up a wealth of opportunities in biosensing, plasmonics, catalysis, smart surfaces, and heterojunction devices. The ability to pattern graphene with functional groups has been demonstrated 22, 28, 29 however, many processes are reductive in nature.28, 30, 31 Given the nature of carbon chemistry, it is inherently difficult to completely remove functional groups, which can unintentionally alter the properties of graphene. It would therefore be advantageous to introduce functional groups in a controllable fashion – only where desired with high spatial resolution. Plasmas form the basis of fabrication technologies capable of modifying the surface of materials over large areas with precision down to the nanoscale.32

They are also compatible with a wide variety of

chemistries. Electron beam-generated plasmas in particular, are capable of introducing a variety of functional group types over a range of coverages with minimal damage33,34 to the carbon back bone because of their inherently low ion energies35 and as such, offer a unique approach to large-area, uniform processing of graphene films. This work demonstrates the ability to manipulate the chemistry of graphene while regulating the spatial distribution of nitrogen functional groups on the surface by use of physical masks during plasma processing. Spatial control over structural and chemical changes is characterized through micro (-Raman and high-resolution x-ray photoelectron spectroscopy (XPS) mapping and electrical measurements are used to determine how local changes in chemistry influence the electronic properties. Lastly, we show that the resulting chemical patterns can be used to manipulate the local surface reactivity of graphene, enabling programmable, site-specific electrochemical deposition of gold. 2. Experimental 2.1. Graphene Growth and Transfer. Graphene films were grown by low-pressure chemical vapor deposition on copper foils36,

37

then transferred to SiO2/Si substrates using conventional wet-

chemical techniques38. Briefly, Cu foils were heated to 1030°C in H2/Ar (PH25 mTorr) and then methane was introduced (PCH4 30-50 mTorr) for approximately 1 hour (Ptotal 35-55 mTorr). After growth, samples were coated with a thin PMMA layer and the Cu foil was etched in a Transene Cu etchant. The PMMA/graphene film was then transferred to a water bath (>18 hrs) before transferring onto a SiO2/Si substrate. The PMMA/graphene/substrate was heated on a hot plate to 150C for 15 minutes and then the PMMA was removed by soaking the sample in acetone. Finally, the graphene/SiO2/Si samples were thermally annealed at 300C in flowing Ar/H2 for approximately two hours. 2.2.Plasma Processing. Pulsed, electron-beam generated plasmas were produced by injecting a 2 keV electron beam into mixtures of argon and nitrogen. The beams are produced by applying a -2kV pulse to a linear hollow cathode (with pulse width of 2ms and the duty factor of 10%). The emergent beam passes through a slot in a grounded anode and terminates at a second grounded anode located further downstream. As the beam propagates through the background gas it excites, dissociates, and ionizes the gas to form a plasma. The electron beam is magnetically confined to minimize spreading, resulting in a sheet-like plasma. The system base pressure is maintained at ~1x10-6 Torr prior to processing by a turbo molecular pump. Nitrogen was introduced at 5% of the total flow rate (180 sccm) with balance being argon to achieve a total pressure of 90mTorr. Graphene samples are placed on a processing stage adjacent to the plasma at a distance of 2.5 cm from the electron-beam axis. All processing experiments were performed at room temperature. To achieve chemical patterns, stainless steel masks with various geometries and aperture openings (e.g., rectangles, circles and cross-bars) were used. 2.3.Surface Characterization. Ex-situ surface diagnostics were performed using an X-ray photoelectron spectroscopy (K-Alpha XPS) system to identify the presence or absence of chemical elements. XPS chemical maps of the functionalized graphene surfaces were obtained measuring the core-level, high-resolution spectra at each point. Two-dimensional XPS maps were formed using

either 400 m or 50 m step sizes, with a spot size of 400 m and 60 m, respectively. MicroRaman characterization was performed on a custom system. The 514 nm excitation was provided by a Coherent Innova 90-5 Argon-Ion laser that was focused on the sample through a 100×, 0.75 NA objective on a Mitutoyo microscope. The laser power at the sample surface was measured to be approximately 30 mW and the laser beam was defocused to laser spot-size of approximately 2 μm. Power dependence measurements from 3 to 45 mW were performed on the graphene samples used in this study to ensure that the given measurement conditions did not induce any damage to the graphene and is included in the supplemental information. The Raman scattered light was then collected through the objective and focused onto a thermoelectrically cooled, Ocean Optics QE65000 spectrometer via a 200μm optical fiber with an acquisition time of 30s. Electrical characterization was performed in a two point probe configuration using a Cascade Microtech probe station system in combination with a Keithley 2400 measurement system. Current-voltage characteristics were obtained by sweeping the source-drain voltage from -1.0 mV to 1.0 mV at a zero gate voltage. Scanning electron micrographs were obtained using a Carl Zeiss SMT Scanning Electron Microscope at a beam voltage of 5kV. 2.4. Surface Energy Estimation. Static contact angle measurements were performed using an automated digital goniometer (AST Producs, Inc.) equipped with a dispersing needle holder. Liquids with known surface properties (water, ethylene glycol, and diiodomethane) were placed on the graphene with a micro-syringe dedicated for each liquid. Once the micro-syringe is inserted in the needle holder, a 1L droplet is extruded while the graphene is raised perpendicular to the needle holder. Contact angles of both sides of three independent drops were averaged for each sample. The surface energy was estimated using the van Oss-Chaudhury-Good model39. 2.5. Electrochemical Analysis and Deposition. Linear sweep voltammetry and electrodeposition experiments were performed in a three electrode system using a platinum wire as the counter electrode, a Ag/AgCl (saturated KCl) reference electrode, and the graphene surface as the working

electrode. Electrochemical analysis and subsequent metal deposition were performed in a commercially available, ready-to-use electroplating solution from Technic Inc. (Techni-gold 25 ES), with neutral pH under potentiostatic mode using a CH Instruments Electrochemical Analyzer (CHI600b). Electrodeposition was performed at a fixed applied potential of -0.6 V vs. Ag/AgCl reference electrode, the experiments were terminated at a total charge density of 50 mCcm-2. 3. Results and Discussion To demonstrate the ability to introduce functional groups with spatial control, masks with bar structures, were place on the graphene substrates. These masks where physically mounted in “hard” (vertical spacing < 1 m) or “soft” (vertical spacing ~ 25 m) contact with the graphene/SiO2/Si substrate during plasma processing, illustrated in Fig. 1A. Initial characterization of the patterning process was accomplished using Raman and XPS spectroscopy of pristine and functionalized graphene. Figure 1B shows individual Raman spectra of areas that were covered and exposed to the plasma by use of a physical mask in hard contact. The covered region shows preservation of the “G” (1570 cm-1) and “2D” (2700 cm-1) graphene signature peaks, while the exposed area shows induced sp3 hybridization evident by the presence of a “D” (1350 cm-1) peak while reducing the “G” and “2D” peak intensities. Conjugation of the six member ring structure of graphene can become disrupted when functional groups are introduced to the carbon structure due to electron sharing or sp3-bond formation.40, 41 Notably, the conversion of the sp2 carbon hybridization to sp3 hybridization breaks symmetry, causing a breathing mode of the sixmembered sp2-carbon rings to be activated, which gives rise to a “disorder-induced” peak observed at ~1340 cm-1 (D peak) in the Raman spectrum. Therefore, the presence of a D peak in the exposed region is indicative of localization of defects induced by the incorporation of functional groups at those areas.

Figure 1. (A) Schematic representation of a the processing configuration, (B) Single point Raman spectra of covered (blue) and exposed (red) graphene regions, Micro-Raman maps of the D/G ratios of graphene (C) before and (D, E) after masked plasma functionalization for a sample where the mask is in (D) good physical (hard) contact and (E) is slightly elevated (soft contact) above the surface. (F) Line scans tracking the D peak intensity across a bar feature denoted by the dotted line for hard and soft contact modes. The scale bar in all maps corresponds to 100 m.

In Raman spectroscopy, the comparison of the graphene D to G ratio provides a measure of the electron correlation length, which in turn provides a good figure of merit for the quality of the graphene film42. The films used here show the initial graphene films are uniform and have a low defect density with the characteristic average D/G ratio < 0.0142, 43 as seen in the -Raman map in Fig 1C. By comparison, figure 1D-E shows -Raman maps of the same area after plasma processing using “hard” (Fig. 1D) and “soft” contact (Fig. 1E) modes, respectively, with a map step-size of 24 m and step size of 2m. After exposure to a N2/Ar plasma, a significant D peak arises only in the uncovered (exposed to the

plasma) mask regions which is attributed to the introduction of chemical functionalities at those locations.44 The domain size La can be estimated by the D/G ratio as described by Tuinstra and Koenig45 which yields an average domain size of 300 nm before functionalization and 1.7 nm after functionalization, comparable to previous reports of pristine46 and plasma treated graphene47. Figure 1F tracks the D peak intensity across a covered region for both contact modes, the actual feature width is indicated by the dashed gray lines. In the case of the hard-contact mode (magenta line), there is a relatively sharp increase in the D peak intensity going from covered (areas under the physical mask) to exposed regions (areas not covered by the mask). In contrast, the soft-contact mode (blue line) results in a diffused boundary due to plasma “leaking” under the mask. Accordingly, the intensity of the D peak is greater over the entire surface and the features are less defined. The resulting features are comparable to the mask dimensions, demonstrating good pattern transfer. To better understand feature transfer and edge resolution,, masks with holes of varying diameters, spacing and aspect ratios (defined here as the diameter to depth ratio) were used. Various aspect ratios can be obtained by varying either the diameter of the hole or the thickness of the mask. Figure 2A shows a -Raman map of chemical patterns produced using a 500 m thick mask, with 1000 m diameter holes and hole-to-hole spacing of 300 m, using hard-contact mode. The -Raman map of the D/G ratio (Fig. 2A) shows that the average feature sizes are comparable to the hole diameter of the mask. High-resolution N1s and C1s spectra (Fig. 2B) of an exposed region confirm the chemical modification of the graphene film to be due to the introduction of nitrogen functionalities. 21 De-convolution of the C1s shows that the subcomponents consist of five peaks C1 (284.5 eV), C2 (285.5 eV), C3 (286.6 eV), C4 (287.6 eV), and C5 (288.8 eV). The main peak (C1) is assigned to sp2 hybridized C atoms in graphene, C2 and C4 reflect different bonding structure of the C-N bonds, corresponding to N-sp2-C and N-sp3-C bonds, respectively48-51. C3 is assigned to C-O and C5 is assigned to C=O oxygen functionalities. The N1s spectrum shows “pyridinic” (N with two carbon bonds) component at 399.5 eV and “pyrrolic” bonding configuration (N incorporated in five membered heterocyclic ring) at 400.4 eV. The gra-

phene region covered by the physical mask (blue curves) maintains the C-C sp2 bonding with no identifiable chemical modifications, and no nitrogen peak in the N1s region. Spectra at the covered region are identical to spectra taken prior to functionalization (black curves).

Figure 2. Raman and XPS maps of graphene after functionalization using a hard-contact mask with circular holes. (A-D) are produced using a mask with 1000 m diameter holes, while (E-F) are produced using 500 m diameter holes. (A) Map of the D/G Raman peak intensities, (B) single point C1s and N1s XPS spectra of covered and exposed regions. XPS maps of the (C) sp2 C1s and (D) N1s corresponding to the dashed black boxed region of (A). (E) -Raman map of the D/G ratio after plasma functionalization and (F) high-resolution XPS maps of the (F) N1s intensity corresponding to the dashed black box region of (E), using a mask with smaller diameter holes. The scale bar on all maps corresponds to 500 m, and the gray dashed lines represent the estimated mask edge.

The spatial distribution of the nitrogen containing functionalities throughout the patterned surface was characterized using XPS chemical mapping. These maps were obtained by de-convoluting components of the C1s spectra and N1s spectra from the dashed region outlined in the -Raman map (200 m step size) in Fig. 2A. The spatial resolution of the XPS map (spot size 400 m, 400 m step size) is lower than -Raman map, however, the results also show that the structural changes are driven by the presence of chemical moieties. Indeed, Fig. 2C shows the C-C sp2 hybridization component (284.5 eV), where a clear variation in the sp2 intensity is observed over the patterned area and the area of highest intensities correspond to the center point of the covered area. The N1s component (400 eV) map, shown in Fig. 2D, is an inverse of the C1s, since the areas exposed to the plasma have nitrogen containing functionalities and areas that were covered with the mask, do not. Pattern resolution as a function of mask dimensions was explored using a mask of comparable thickness (400 m) but smaller hole diameters (D=500 m) and hole-to-hole spacing (300 m) compared to the mask used in Fig. 2A. The resultant patterned features (Fig. 2E) are of similar dimensions as the features of the mask. Here however, the D/G ratio and the nitrogen content at the center of the hole is lower than the large diameter hole used to produce the patterns in Fig. 2A. The N1s chemical map (spot size 100 m, step size 70 m) in Fig. 2F shows the addition of nitrogen groups in the exposed areas with resulting feature size of 500 m, agreeing with the dimensions of the Raman map. Thus, the structural changes observed with the Raman map indeed correlate to the chemical modifications via functionalization. It is worth noting that the cleanliness of the graphene surface is critical; residues left behind from the graphene-transfer process can shield the surface from functionalization, resulting in a distorted pattern. An example of such shielding can be seen in the bottom of Fig. 2E (blue regions, distorted circle).

Figure 3. (A) An illustration of the effects of plasma molding for various aspect ratios (AR) holes of (i) AR = 2.0, (ii) AR = 1.0, and (iii) AR = 0.5. The blue lines are equipotential lines and the red lines are ion trajectories. (B) Line scan tracking the D peak intensity across the different size hole features labeled 1-5. (C) -Raman map of the D-peak of graphene functionalized using a mask with a variety of hole diameters. The scale bars corresponds to 500 m, and the gray lines represent the estimated mask edge.

The gradient at the perimeter of each hole and the varying D-peak intensity between holes could be related to slight differences in mask contact, although the fact that the transferred patterns are consistent for most holes (Fig. 2A vs. 2E), suggests a plasma phenomenon. One of the unique features of plasmas is its ability to 'mold' over surface features, with this molding being proportional to the length of the plasma sheath. The plasma sheath defines an ion-rich region of strong electric fields that governs the interaction of the bulk plasma with a surface. Generally, when the sheath length is much smaller than the diameter of a hole, the plasma will fill the hole; if the sheath is much larger, the plasma will not respond to the presence of a hole, maintaining the same profile that it would if interacting with a flat surface.52 When a plasma interacts with a flat surface, the sheath fields provide an anisotropic flux of

energetic ions to the surface. In a regime where the sheath length is comparable to the hole diameter, the sheath bends to partially fill the hole53 and when the sheath bends, the flux of ions becomes divergent. This phenomena is illustrated in Fig. 3A by modeling an electric field in the presence of a conducting hole of various aspect ratios and calculating ion trajectories. For the plasmas used in this work, the sheath length is reasonably estimated to be 50-100 m (see Supplemental Material). Thus, for the holes in Fig. 2, the plasma is expected to fully penetrate the holes with diverging ion trajectories near the bottom edge of the hole, resulting in a lower flux at the perimeter. Moreover, as the hole diameter decreases, less ions can reach the bottom of the hole. This description is in good qualitative agreement with the observed difference in total D-peak and N1s intensities. A third mask was used which spans the regime were the sheath length is comparable to the hole opening and thus only partial plasma penetration is expected, shown in Fig. 3. Here a 90 m thick mask containing five different hole diameters of 230, 180, 140, 70 and 50 m (labeled 1-5) was used to produce the functionalized regions apparent in the -Raman maps presented in Fig. 3. Shown in Fig. 3B are the D-peak line scans tracking the spatial variation across the different diameter features. As the aspect ratio decreases, a decreasing flux that remains largely uniform along the graphene surface53 is expected. A low intensity but uniform D-band signature for the smallest feature size is consistent with this concept. Good definition and resolution is observed throughout the range of feature sizes showing that the patterned chemistry is still preserved even at the smallest feature size. These results demonstrate the ability to chemically pattern large areas of graphene with flexibility in both patterned geometries and feature size, where the smallest features achievable are dependent on the features of the physical mask. Additionally, this patterning process is additive in nature and as such only the patterned areas (particularly in hard contact mode) contain functional groups, leaving the un-functionalized areas pristine. Thus with careful mask design, this approach is amenable to a wide variety of feature sizes.

Figure 4. (A) Micro-Raman map of the D/G peak intensity ratio and (B) high resolution XPS maps of the N1s after plasma functionalization of graphene using a mask with parallel bars. (C) Line scans of the Raman D-peak intensity across the bar features. (D) electrical characteristics at each point (1, 2, 3, … 6) across the patterns, where the labeled dots in Fig.4A represent the approximate mid-point for the resistance measurements, with probe spacing of 400 m. (E) Electrical characteristics across a functionalized (1 to 6) and non-functionalized (3 to 7) regions, here the dots represent approximate tip locations, with probe spacing of 4 mm.

The patterning of functional groups and their influence on electrical properties of graphene are shown in Fig. 4. Patterned areas with nitrogen functional groups with line widths of about 1.5 mm were produced and Fig. 4A and B show the corresponding micro-Raman and XPS maps of these features, respec-

tively. The structural changes correlate well with the chemical changes, where the highest intensity of the D mode corresponds to the highest nitrogen (N1s) signal. A line-scan of the Raman D-peak intensity across masked and exposed is presented in Fig. 4C. The graphene surface was electrically probed across the patterned surface at six different points, labeled 1-6 in Fig. 4A, where the source and drain were 400 m apart for all points. The resultant current-voltage (I-V) characteristics are shown in Fig. 4D, and indicate that areas that were masked (points 1 and 6) show resistances of about 1-2 k11, 54-57 while areas that were fully functionalized (points 3 and 4) show a resistance of about 7 k The increase in resistance after nitrogen functionalization is expected44 as the addition of functional groups provides defects that give rise to electron scattering points that decrease the conductance in that region. Similarly, the electrical characteristics, shown in Fig. 4E, across a functionalized bar (from points 1 to 6) shows a higher resistance compared to the electrical characteristics across an un-functionalized bar (from points 3 to 7). Here the dots in Fig. 4A (1-6 and 3-7) represent the approximate tip locations and the distance between the source and drain was about 4 mm. Changes in surface reactivity can be assessed by water contact angle measurements since they provide information of a system’s surface energy. In this case, un-functionalized graphene58 had a water contact angle of 94.8° with a corresponding surface energy of 38 mJm -2. Nitrogen functionalization (9 at.%) made the surface hydrophilic, reducing the water contact angle to 44.5° and increasing the surface energy to 44.5 mJm-2, indicating that the introduction of N-functional groups increased the surface reactivity. Electrochemistry offers a facile approach to further differentiate chemically modified graphene through shifts in reactivity59, and can aid in verifying the spatial distribution of chemical functional groups. In particular, electrodeposition, a reductive reaction in the case of a metal cation, provides information about the reaction magnitude in the form of current and mapping by the deposited material. On carbon-based surfaces, preferential nucleation and growth on the step edges of graphite surfaces60-62 and defect sites of carbon nanotubes,60, 61, 63-65 has been observed. For carbon nanotubes, defects influence charge transport with increased scattering, effectively decreasing carrier mobility and thus the den-

sity of states at those locations.66, 67 As a consequence, the local work function decreases, reducing the barrier for electron transfer, which in the case of electrodeposition, translates to a reduction in overpotential - the difference between the metal redox potential and the applied potential at which deposition occurs. In chemically modified graphene, nitrogen functional groups should behave analogously to carbon nanotubes defect sites, lowering the work function and thus the overpotential required for electrodeposition with respect to the pristine graphene lattice. Differences in the chemical reactivity between the un-functionalized and nitrogen functionalized graphene surface are further observed by linear sweep voltammetry (LSV). Figure 5A shows the LSV curve of pristine and a nitrogen-functionalized graphene surfaces in a commercially available gold electrolyte using a three electrode configuration. The untreated graphene surface shows a very low cathodic current density reaching about -0.8 Acm-2 as the voltage is swept from its open circuit potential to 1.25 volts vs. Ag/AgCl reference electrode. In comparison, the nitrogen functionalized graphene surface showed a five-fold increase in the cathodic current density reaching -4.3 Acm-2. While the LSV results indicate an increased chemical reactivity, it is not clear if the origin of this behavior is solely due to a higher defect density of the graphene lattice or the chemical nature of nitrogen functionalities. The high resolution N1s spectrum reveals the presence of pyridine and pyrrolic C-N bonding configurations which, similar to its N-doped carbon nanotubes counter parts, can play a role in the oxygen reduction reaction.68, 69 However, it is unclear which C-N bonding configurations are responsible for the enhanced electrocatalytic activity70 in graphene. Additionally, it is important to note that despite the reduced overpotential, chemical modifications did not led to spontaneous electrodeposition of Au.71, 72 This phenomena requires the addition of chemical species with redox potentials substantially more negative than Au to behave as reducing agents to proceed without an externally applied bias.65

Figure 5. (A) Linear sweep voltammetry of functionalized and non-functionalized graphene surfaces, (B) SEM (in-lens) image following nitrogen functionalization of circular features, the black arrows point to the circular patterns of various diameters. (C) Optical image following electrodeposition of gold showing selective gold film growth on the plasma patterned areas indicated by the red dashed box of (B). (D) SEM image of the smallest circular feature showing the electrodeposition selectivity at the edge of the feature.

The excess overpotential required for the unfunctionalized graphene surface indicates slower kinetics compared to the functionalized surface, consistent with the role of defects on graphitic materials, and provides a route to selectively label chemically functionalized regions.73, 74,75 The SEM image in Fig. 5B of the nitrogen-patterned features using the same mask described in Fig. 3 indicates a local modulation of electronic properties. Here, the color difference between functionalized and pristine graphene is a result of the difference in work function,76 where the patterned areas are darker in color, indicating a lower work function compared to the unfunctionalized graphene areas. An identically patterned gra-

phene surface was used as the working electrode for electrodeposition of gold. Optical and SEM images (Fig. 5C & D) show a remarkable level of deposition selectivity at an applied potential of -0.6V vs. RE. The nitrogen functionalized areas had continuous gold coverage, while the non-functionalized graphene area showed almost no deposition. Additionally, the gold films exhibit surface morphology characteristic of electrodeposited gold thin films,77 which lead to the observed optical properties.78-80 Furthermore, XPS and Raman performed on patterned samples with larger features (see Supplemental Material) showed the characteristic Au signature on the films and good quality graphene on the nonfunctionalized graphene area, respectively, indicating no chemical oxidation of the pristine graphene area during the electrodeposition process. Defects such as tears, wrinkles and grain boundaries on the unfunctionalized regions exhibited little to no nucleation at applied potentials as negative as -0.6 V, even though the functionalized areas have complete Au coatings (Fig. 5C & D). At potentials more negative than -0.8 V, small Au nanoparticles begin to grow on physical defects present on unfunctionalized graphene whereas thicker Au films are observed on the functionalized areas (see Supplemental Material). Such selective deposition provides a route toward programmable, surface-driven kinetics and device architectures. 4. Conclusions Chemical patterning of graphene was demonstrated using physical masks during plasma functionalization to spatially control the introduction of nitrogen functional groups.

Micro-Raman and high-

resolution XPS maps indicate the two-dimensional distribution of the functionalities over large areas can be precisely controlled. The spatial distribution of functional groups was shown to influence the local electrical properties, influence local kinetics, and the discrete features were utilized as templates for site-specific metal deposition. The use of plasmas and physical masks provides considerable flexibility in terms of patterned geometries and choice of chemical functionalities. One can envision that advanced masking techniques can provide even smaller feature sizes and greater control over edge features. This approach is anticipated to reach nanometer feature sizes provided the mask thickness is appropriately

scaled to maintain aspect ratios greater than one, in order to achieve reasonable functional group densities. The plasma operating conditions can be controlled and various gas backgrounds (O2, SF6, N2, NH3, etc.) can be used to further manipulate the surface chemistry of the exposed regions, providing additional flexibility in the chemical patterning of graphene. These findings demonstrate the ability to tailor the locality of the surface chemistry on graphene surfaces opening up a wide range of reactivity studies and synthesis capabilities, such as site-specific electrochemical deposition described here.

ASSOCIATED CONTENT Supporting Information. Raman power dependent measurements, detailed information for the sheath length estimation and characterization of the electrodeposited material is included in the supplemental information. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT S.C.H. and F.J.B. are grateful for the NRL/NRC and NRL/ASEE Postdoctoral Research Associateships. We thank Anthony Knoll, Christopher Compton, and Nelson Garces for the design of the physical masks, George Gatling for writing the Labview program, James Culbertson for the use of his Raman spectral mapping analysis program, P. E. Sheehan for the use of his Potentiostat, N.V. Myung for the electrolyte, and David R. Boris for insightful plasma discussions. This work was supported by the Naval Research Laboratory Base Program.

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