Chemically Isolated Graphene Nanoribbons Reversibly Formed in

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Chemically Isolated Graphene Nanoribbons Reversibly Formed in Fluorographene Using Polymer Nanowire Masks Woo-Kyung Lee,† Jeremy T. Robinson,† Daniel Gunlycke,† Rory R. Stine,† Cy R. Tamanaha,† William P. King,‡ and Paul E. Sheehan*,† † ‡

U.S. Naval Research Laboratory, Washington, D.C. 20375, United States Department of Mechanical Science and Engineering University of Illinois at Urbana Champaign, Urbana Illinois 61801, United States

bS Supporting Information ABSTRACT: We demonstrated the fabrication of graphene nanoribbons (GNRs) as narrow as 35 nm created using scanning probe lithography to deposit a polymer mask1 3 and then fluorinating the sample to isolate the masked graphene from the surrounding wide band gap fluorographene. The polymer protected the GNR from atmospheric adsorbates while the adjacent fluorographene stably p-doped the GNRs which had electron mobilities of ∼2700 cm2/(V 3 s). Chemical isolation of the GNR enabled resetting the device to nearly pristine graphene. KEYWORDS: Graphene nanoribbons, thermal dip-pen nanolithography, fluorographene

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raphene nanoribbons (GNRs) are typically cut from larger sheets of graphene or carbon nanotubes4 7 using processing steps such as ultrasonication or acid treatment that can limit yield, introduce defects, and offer limited dimensional control and registration. While electron-beam lithography can control GNR width and placement,8 irreversible damage is introduced from residual resist, the interaction with high-energy electrons, and the production of rough edges that degrade the GNR electrical properties. Second, while shaping a GNR controls many of its electronic properties, it cannot control the doping. Thus an additional device fabrication challenge is the stable and controlled doping of GNRs. Because all the charge carriers in graphene are at the surface, it is readily doped by adsorbates, and so a common problem is p-type doping by adventitious water or oxygen.9,10 Intentional doping with chemisorbed and physisorbed species11 15 has also been explored. However, physisorbed approaches are often in flux under ambient conditions while chemisorption can degrade electron mobility. An ideal fabrication technique could create GNRs of the desired width in registry with other components, would avoid fabrication and doping processes that degrade the graphene’s high-quality electronic properties, and would protect the GNR to enable operation outside of vacuum. To address these challenges, thin polystyrene (PS) nanowire masks were written on single-layer graphene (SLG) devices using thermal dip-pen nanolithography (tDPN). In tDPN, heated scanning probes deposit solid inks, such as polymers, directly onto a surface2,3,16 and thereby avoid the deleterious effects of e-beam exposure. The PS mask could then be used either to isolate a GNR geometrically by etching away excess graphene or to isolate chemically the GNR by converting the exposed graphene into an insulator. Geometric isolation via oxygen-plasma processing is straightforward and produces high-quality devices.17 Chemical isolation, the focus of this work, was achieved by r 2011 American Chemical Society

exposing the unmasked graphene to XeF2 gas to convert it to insulating fluorographene (C4F, Figure 1).18 To enable testing at each stage of fabrication, a base device was first fabricated by transferring and patterning chemical vapor deposition grown graphene onto 100 nm SiO2/Si substrates.19 A hydrophobic monolayer of hexamethyldisilazane (HMDS) was deposited onto the SiO2 before graphene transfer to reduce adsorbate diffusion under the graphene.20 This step produced stable (i.e., nonshifting VDirac) graphene devices in ambient conditions.20 After optical lithography defined the source and drain electrodes, the base graphene devices were annealed (175 °C, >3 h) under flowing argon and then prescreened before tDPN to ensure ambipolarity and low voltage Dirac points (VDirac, or charge neutrality point). Polystyrene was chosen as the polymer mask due to its hydrophobicity, electrical insulation (10 16 S/m), and melt processability (Tm = 240 °C). It also does not dope graphene, as shown by an absence of a ΔVDirac shift when it was spin-coated onto the base devices (see SI 4 in the Supporting Information). Lastly, polymers deposited via tDPN are typically highly ordered due to thermal annealing and shearing during deposition2,16 and such ordering greatly enhances polystyrene’s impermeability to gas.21 The PS lines were written in registry with the base device at 250 °C with diamondtip heated probes,22 and their width was varied from 300 to 35 nm by increasing the writing speed up to 40 μm/s with faster speeds producing thinner lines. Initial devices without polymer masks had two probe resistances >1 GΩ after a relatively short 240 s exposure to XeF2 indicating that conduction occurs solely through the masked graphene. It should be noted that the highly Received: September 15, 2011 Revised: October 24, 2011 Published: November 03, 2011 5461

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Figure 3. Rsheet vs Vg characteristics of PS-GNR devices after fluorination and subsequent O2 plasma etch. (1) Initial device with the Dirac point at 10 V (μh = 908 cm2/(V 3 s)). (2) PS/GNR device after fluorination (μh = 858 cm2/(V 3 s)). (3) Additional O2 plasma etch of the same device (μh = 692 cm2/(V 3 s)). After etching background C4F, the Dirac point was recovered to the initial state. Note that exposure to the oxygen plasma degrades the mobility.

Figure 1. Schematic diagram of the masking and isolation process. (a) Base SLG FET device with source and drain on HMDS treated 100 nm SiO2 gate oxide. (b) tDPN of polystyrene on SLG across the electrodes. (c) PS/GNR structure by O2 plasma etch (geometric isolation). (d) PS/ GNR structure by XeF2 fluorination (chemical isolation). (e, f) AFM image of each PS/GNR structure with source and drain contacts.

Figure 4. Structure and energy diagram across the graphene ribbon. (a) The graphene ribbon is sandwiched between large areas of C4F. Schematic energy diagrams before (b) and after (c) the system has equilibrated. ED is the Dirac energy in graphene, EC (EV) the conduction (valence) band energy in C4F, and EF the Fermi level.

Figure 2. Sheet resistance vs Vg characteristics of PS/GNR device by chemical isolation. (a) The histogram of the Dirac point shift after PS/ GNR devices with a total of 34 devices. (b1) black, initial device (W = 12 μm; L = 4 μm; μh = 2675 cm2/(V 3 s)); (b2) red, PS/GNR device (μh = 2692 cm2/(V 3 s)) from Figure 1f after fluorination; (b3) gray, after defluorination by annealing in hydrazine for 24 h, the background C4F was converted to graphene and recovered conductivity.

ordered PS deposited via tDPN appears to adhere strongly to the SLG, and our initial efforts to remove it by wet sonication led to significant device degradation (see SI 1, Supporting Information). Figure 2 follows the electronic properties of one GNR device at each stage of fabrication. The base device had the typical Rsheet = 1.7 kΩ and a VDirac ≈ 0 in air due to the HMDS treatment. Next, a 78 nm wide PS line (Figure 1f) was deposited between the source and drain electrodes and the entire device was then fluorinated. Fluorination p-doped the GNR shifting

the VDirac to 30 V (red curve, Figure 2b). This robust doping (ΔVDirac = 30 ( 5.5 V) was consistently observed in a large number of devices regardless of GNR width (60 300 nm, Figure 2a). Importantly, fluorination does not damage the carrier mobility of the polystyrene coated GNR, which was measured in air to be 2692 cm2/(V 3 s) and is slightly higher than that of the initial device, 2675 cm2/(V 3 s) (see Supporting Information, experimental detail). Indeed, the ratio between the mobility of the initial and PS-GNR devices (μPS‑GNR/μinitial) was 0.94 ( 0.15. Finally, the current on/off ratios (Ion/Ioff ≈ 4 10) of all devices increased slightly after fluorination and are comparable to those from previous GNR structures.8,23 Fluorine acts as an electron acceptor and when exposed to the graphitic basal plane11,14 strongly p-dopes carbon nanotubes and graphene.24 Doping in the present system differed from previous reports in occurring laterally due to electron flow from the GNR into the contiguous C4F (Figure 4). Doping due to polystyrene fluorination was ruled out since only the top few nanometers of the ∼100 nm thick mask were fluorinated (see SI 5, Supporting Information). More clearly, when the C4F surrounding the GNRs was removed with an O2 plasma, VDirac returned nearly to that of the initial device (Figure 3). 5462

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Nano Letters Density functional theory (DFT) provided insight into how electrons are pulled laterally from the GNR into the surrounding C4F. Calculations yielded the reported values for the band gap of EFG = 3.0 eV for C4F18,25 and the work function of graphene of ΦG = 4.3 eV26 and also predicted a C4F work function ΦFG = 6.0 eV. Importantly, the difference in the two work functions, ΔΦ  ΦFG ΦG, exceeds half the band gap of C4F such that the Dirac point in the GNR starts above the conduction band edge of C4F (Figure 4). As the system equilibrates, electrons near the GNR’s Fermi level will therefore flow into extended states of C4F. This electron flow leads to an energy separation ΔEtheory ≈ ΔΦ EG/2 = 0.2 eV between the Dirac point and the Fermi level. For comparison, we can extract the experimental energy separation ΔEexp using the relationship ΔEexp ≈ eΔVDirac (Cox/CQ) by setting ΔVDirac = 30 V and using an oxide capacitance Cox = 0.027 μF/cm2. The quantum capacitance, CQ, is given by CQ ≈ ΔEexp8πe2 /(hνF)2 which is roughly ≈ΔEexp  24 μF/(eV 3 cm2) using the Fermi velocity vF ≈ 106 m/s for graphene. This yields an experimental value of ΔEexp ≈ 0.17 eV, which is a close match to ΔEtheory = 0.2 eV given the well-known tendency of DFT to underestimate the band gap. The Dirac point shift did not correlate with the ribbon width (see SI 8 in the Supporting Information) indicating that the dominant doping mechanism is not an edge effect but rather a bulk effect. In other words, most electrons leaving the ribbon during the fluorination process do not accumulate near the edges but instead propagate deep into the fluorinated regions. Furthermore, as the fluorinated region is much larger than the ribbon, it acts as large reservoirs that can absorb all the electrons leaving the ribbon. In 2008, Ruoff suggested that one might produce conducting graphene devices inside surrounding insulating forms of graphene,27 presented here as chemical isolation. Chemical isolation provides several advantages over geometric isolation. First, chemical isolation produces ribbon edges that are robust and chemically well-defined since the GNR edge terminates at sp3 carbons covalently bound to fluorine rather than at a collection of sp2 and sp3 hybridized carbons bound to a variety of oxygen-rich functional groups left by oxygen plasma etching or oxidative synthesis.5 These better defined edges appear to help retain carrier mobility. Second, chemical isolation within a larger graphene film significantly reduces adsorbate permeation under the active GNR which degrades its electronic performance. We found that the Dirac point in chemically isolated devices remained stable for several weeks in air while shifting quickly and significantly for geometrically isolated devices (see SI 3 in the Supporting Information). Finally, leaving the graphene skeleton in place enables reducing the film back to graphene, thereby resetting it for future use. We tested the possibility of resetting a device to its initial conductive state using a low temperature hydrazine vapor treatment which was previously shown to reduce fluorinated graphene.18 Indeed, treating the devices with hydrazine reduction at 100 °C for 24 h was found to work particularly well, restoring the ambipolarity, the 0 V Dirac point, and nearly all conductivity, with the final Rsheet only ∼3 times that of the initial device (Figure 2). For five similarly prepared devices (not shown), the recovered VDirac = 0 ( 5.4 V with an Rsheet at the Dirac point of 8.6 ( 2.2 kΩ (See Supporting Information). To the best of our knowledge, this constitutes the most effective conversion of any insulating chemically modified graphene back to graphene with previous attempts only restoring conductivity

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to within ∼100 times of the starting values.18 An important difference may be the use of HMDS between graphene and SiO2 which could serve as a diffusion barrier to XeF2 much as it does for H2O or O2. This would prevent the fluorination of the underside of the graphene which may be more difficult to remove with hydrazine. In summary, a method was developed that in two dry processing steps defined, doped, and encapsulated a GNR device. Scanning probe lithography enabled the arbitrary placement of GNR devices via the deposition of a polymer mask followed by chemical isolation. Fluorination retained starting carrier mobility and provided stable p-doping of 0.17 eV while the polymer mask encapsulated the GNR allowing stable performance outside of a vacuum. Defluorination of patterned devices restored the initial device’s electronic properties, and this reversibility would enable write erase rewrite type device fabrication based on polymer/ GNR devices. Finally, choosing other chemical functionalizations such as hydrogen28 or amines should enable tuned doping of the GNR.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details electrical measurement of other control devices, and XPS data of PS spincoated devices. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors are grateful for funding from the DARPA TBN program, ONR Nanoscale Electronics, NRL’s Nanoscience Institute, and the NSF Center for Nanoscale Chemical-ElectroMechanical Manufacturing Systems (Nano-CEMMS). D.G. thanks C. T. White for discussions. ’ REFERENCES (1) Lee, W. K.; Sheehan, P. E. Scanning 2008, 30, 172. (2) Lee, W. K.; Whitman, L. J.; Lee, J.; King, W. P.; Sheehan, P. E. Soft Matter 2008, 4, 1844. (3) Lee, W. K.; Dai, Z. T.; King, W. P.; Sheehan, P. E. Nano Lett. 2010, 10, 129. (4) Bai, J. W.; Duan, X. F.; Huang, Y. Nano Lett. 2009, 9, 2083. (5) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Nature 2009, 458, 872. (6) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Science 2008, 319, 1229. (7) Wang, X. R.; Dai, H. J. Nat. Chem. 2010, 2, 661. (8) Han, M. Y.; Ozyilmaz, B.; Zhang, Y. B.; Kim, P. Phys. Rev. Lett. 2007, 98. (9) Ryu, S.; Liu, L.; Berciaud, S.; Yu, Y. J.; Liu, H. T.; Kim, P.; Flynn, G. W.; Brus, L. E. Nano Lett. 2010, 10, 4944. (10) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652. (11) Coletti, C.; Riedl, C.; Lee, D. S.; Krauss, B.; Patthey, L.; von Klitzing, K.; Smet, J. H.; Starke, U. Phys. Rev. B 2010, 81, 235401. (12) Dong, X. C.; Fu, D. L.; Fang, W. J.; Shi, Y. M.; Chen, P.; Li, L. J. Small 2009, 5, 1422. (13) Farmer, D. B.; Golizadeh-Mojarad, R.; Perebeinos, V.; Lin, Y. M.; Tulevski, G. S.; Tsang, J. C.; Avouris, P. Nano Lett. 2009, 9, 388. 5463

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