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Partially Fluorinated Graphene: Structural and Electrical Characterization Lanxia Cheng,*,† Srikar Jandhyala,†,‡ Greg Mordi,†,§ Antonio T. Lucero,† Jie Huang,† Angelica Azcatl,† Rafik Addou,† Robert M. Wallace,† Luigi Colombo,∥ and Jiyoung Kim*,† †
Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States ∥ Texas Instruments, Dallas, Texas 75243, United States S Supporting Information *
ABSTRACT: Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF4 as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF4 plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C−F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C−F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C−F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C. KEYWORDS: fluorinated graphene, Raman, GFETs, XPS, CVD graphene, ionic bond
1. INTRODUCTION Fluorographene, a monolayer of fluorine atoms covalently bonded to a carbon sheet, has emerged as the most applicable two-dimensional (2D) material because of its superior structural and electronic properties.1−3 Unlike the amorphous graphene oxide featuring random C−O bonding configurations,4 fluorination preserves the hexagonal crystallinity of graphene upon changing the carbon local binding state from sp2 to sp3 hybridization,5,6 which accordingly opens a large band gap predicted to be greater than 6.3 eV.7 Experimental results have shown that fully fluorinated graphene (FG) becomes an insulator with a band gap of ∼3 eV and in-plane resistance up to 1 TΩ,3,8 whereas partially FG is a semiconductor possessing variable band gaps that are tunable by modulating the fluorine contents.7,9 A recent study by Ho et al.2 also demonstrated the potential utility of fluorographene as an ultrathin tunneling barrier for graphene nanoelectronics given its low dielectric constant of 1.2 and monolayer film thickness of 0.5 nm. Because fluorine is more electronegative than carbon and hydrogen, fluorographene has been shown to be thermally stable up to 400 °C and can be reversibly functionalized to restore its original conductivity to some extent after further annealing5,8,10 or chemical reduction,3 thus presenting a readily available way of tuning the optical, © 2016 American Chemical Society
magnetic, and electronic properties of graphene by controlling via plasma treatment time or annealing. These resulting fluorocarbon-based materials are envisioned and also experimentally demonstrated as a promising alternative for such applications as optoelectronics,11 gate dielectrics,2 coatings, and atomic layer deposition (ALD) film seeding.12 Several fluorination approaches have been explored experimentally through exposing graphene or graphite to fluorinating agents, including XeF23,8 or F2 gas,5 CHF3,13 CF4,14 or SF615 plasma, or laser-irradiated fluoropolymer.16 Owing to the difference in their chemical reactivity and the relevant fluorination parameters, these FG sheets show significant variations in resulting CFx (x ≤ 3) conformations and C/F ratios,1 which inevitably influence such electronic properties as resistivity,17 electronic density of states,7 local magnetic moments,9 and binding energy.18 In comparison to existing studies related to the understanding of structural and electronic properties of fully FGs that are characterized with stoichiometric CF0.25 or CF1,3 partially reacted FG sheets (CFx, where x < 1),7,9,18 especially at lower fluorine coverage, remain less Received: December 1, 2015 Accepted: January 28, 2016 Published: January 28, 2016 5002
DOI: 10.1021/acsami.5b11701 ACS Appl. Mater. Interfaces 2016, 8, 5002−5008
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ACS Applied Materials & Interfaces experimentally explored with regard to the C−F binding compositions, electrical behaviors, and thermal stabilities. In this work, we have conducted complementary investigations on the structural and composition changes of chemical vapor deposition (CVD) FG sheets produced by CF4 plasma using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition to the covalently bonded C−F components, ionically bonded components are also identified for the FG sheet prepared by brief exposure to CF4 plasma; the coexistence of these two bonding states contributes to the observation of two charge neutrality points according to our back-gated graphene field effect transistor measurements. Additionally, the Raman mapping images suggest that the fluorination is spatially inhomogeneously occurring on the CVD graphene surface as a result of the presence of various structural imperfections. In contrast to the fully FG sheet, which is thermally stable up to ∼300 °C, the partially FG shows initialization of fluorine desorption at ∼150 °C; such nonuniform defluorination can be attributed to the existence of various C−Fx bonding states and conformations generated from most fluorination processes. Our experimental findings reveal practical challenges of realizing FG electronics, owing to its low thermal stability and complication of the C−F bonding states.
Figure 1. (a) Raman spectra showing the effect of the fluorination time on transferred CVD graphene on SiO2/n+Si substrate at 40 W and partial pressure of 700 mTorr. The Raman spectra have been shifted in the y axis for clarity. (b) Normalized intensity of the D and 2D peaks with respect to the G peak as a function of the fluorination time. (c) STM image (10 × 10 nm) of CVD graphene after 300 s of fluorination showing preservation of 2D graphene lattice structures.
2. EXPERIMENTAL SECTION 2.1. Material Characterization. CVD graphene films were synthesized on Cu foil and then transferred to 90 nm SiO2/n+Si substrate followed with ultra-high vaccuum (UHV) (∼10−9 mbar) annealing. The exfoliated graphene was obtained using scotch tape exfoliation described elsewhere.19 Graphene fluorination was carried out in a Technics Series 85 reactive ion etching (RIE) system using CF4 as the plasma source at room temperature. The structure of the FG sheets produced was characterized with a Renishaw confocal Raman spectrometer (laser power, ∼0.45 mW; wavelength, 532 nm). Raman mapping was conducted using a 100× objective lens with spatial resolution of ∼300 nm.20 Temperature-dependent Raman studies were performed in a Linkam THMS 600 stage purged with continuous N2 gas flow, and Raman spectra were collected at intervals of 50 °C from room temperature to 600 °C. At each interval, the temperature was held for 30 min before collecting the Raman spectra collection. The composition of FG was studied by X-ray photoelectron spectroscopy (XPS) using a monochromatic Al Kα (hν = 1486.7 eV) X-ray source equipped with a seven-channel analyzer using a pass energy of 15.0 eV with all scans taken at 45° and spot size of 100 × 100 μm2 with respect to the sample described elsewhere.21 Scanning tunneling microscopy (STM)- and scanning tunneling spectroscopy (STS)-plotted dI/dV curves of the surface density of state were conducted with an Omicron variable temperature scanning probe microscope at room temperature as described elsewhere.22 2.2. Electrical Characterization. Back-gate graphene field effect transistors (GFETs) were prepared using photolithography for CVD graphene and E-beam lithography for exfoliated graphene.23 After patterning, metal contacts (Cr/Au) were deposited using the e-beam evaporator as top-gate, source, and drain contacts. Electrical measurements were carried out using a HP 4155A semiconductor analyzer. All temperature-dependent electrical measurements were conducted in vacuum after pumping down to 1 × 10−6 Torr for 10 h in a cryogenic probe station (Lakeshore) prior to cooling them down to 77 K using liquid N2. For each measured temperature, a waiting time of 30 min is held to allow for sufficient stabilization time.
and 700 mTorr. Under these conditions, the relatively slow chemical reactions between CFx (x ≤ 3) species and graphene allow for better control over the fluorination process, in contrast to the fast fluorination, which occurs at a low CF4 partial pressure, as shown in Figure S1 of the Supporting Information. After an initial CF4 plasma exposure, Raman characteristics of pristine graphene show considerable changes, as evidenced by the emergence of additional peaks, namely, the D peak at 1350 cm−1, D′ peak at 1620 cm−1, and D + G peak at 2950 cm−1. Both the G and 2D peaks also broaden cooccurrence with a significant increase of the D/G peak ratios. Raman features of graphene and its derivatives have been extensively studied,24 and the D peak requires the presence of defects, e.g., lattice dislocation, vacancies, etc., for its activation. The D′ peak usually appears on the high-frequency shoulder of the G peak and activates upon a symmetry break in the graphene plane. Therefore, the changes in our Raman spectra are most likely caused by the introduction of structural disordering in the graphene lattice, e.g., the transformation of sp2 carbon to sp3 hybridization as a result of the fluorine adsorption.7 Continuous fluorination leads to an overlap of D′ over G peak without the noticeable Raman frequency shift that is commonly observed in doped graphene.24 The Raman signatures of graphene fluorinated for duration up to 200 s exhibit a similar characteristic to that from highly disordered or nanostructured carbon-based materials,24 e.g., graphene oxide (GO), which is featured with broad D and G peaks with comparable intensity and a weak 2D peak. Increasingly, upon a further increasing plasma treatment time of up to 300 s, all Raman peak intensities become nearly invisible, despite the fact that the Raman D, G, and 2D peaks still exist after multiplying the peak intensity 50 times. This gradual evolution of the Raman frequencies for FG seen in our work agrees well with those performed by Nair et al.8 using XeF2. The disappearance of Raman peaks is commonly considered evidence of the formation of fully FG, in which the band gap opening makes it transparent to a green laser (532 nm, 2.33 eV).
3. RESULTS AND DISCUSSION 3.1. Structure and Composition of the FG Sheet. Figure 1a shows the gradual structural changes in transferred CVD graphene as a function of plasma exposure time at 40 W 5003
DOI: 10.1021/acsami.5b11701 ACS Appl. Mater. Interfaces 2016, 8, 5002−5008
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intensity, while concurrently, a new peak at ∼285.4 eV is identified and attributed to the sp3 C−C binding state. In addition, several peaks are developed at ∼286. 2, 287.5, 288.6, 290.1, and 292.1 eV and are each ascribed to various C−C and C−F binding compositions, including C−CF, CF, C−F2, etc. Because it is not easy to assign each individual component accurately given the complicated binding states between C−F bonds, each binding state in Figure 2a was tentatively analyzed and deconvoluted in reference to previous XPS studies.10,14 These XPS binding states become a more prominent characteristic in the C 1s region after increasing the plasma exposure time to 300 s. The reduction of the C−C binding state at ∼285.4 eV and growth of the FC−CF2 states at ∼288.6 eV, along with the existence of other C−F binding states, serve as apparent evidence of the formation of covalently bonded C− F compositions. In the case of the F 1s binding energy region, the observed peak is deconvoluted to two different binding energies at ∼688.9 and ∼686.7 eV after 100 s of CF4 plasma treatment. The higher binding energy peak is attributed to the formation of covalent bonding as a result of the chemisorption of fluorine radicals on carbon atoms.3 However, the emergence of the lower binding energy peak at ∼686.7 eV indicates the presence of ionic C−F states, which are also observed at a lower fluorine content of ∼2% after 10 s of fluorination (Figure S2 of the Supporting Information). Because fluorine is highly electronegative with a dipole moment of (4.0) with respect to (2.5) for the carbon atom,18 this high electronegativity difference can induce both ionic and covalent bonds between carbon and fluorine atoms.28,29 The XPS peak area ratio of fluorine/carbon is estimated to be ∼0.35 (i.e., 35% fluorine content or CF0.35), which is higher than that obtained from a single-side FG;3 the higher C−F ratio derived for our single side of fluorinated sample might be caused by additional fluorine adsorption on defects in CVD graphene, such as tears, wrinkles, or grain/ domain boundaries if introduced during transfer and plasma treatment processes, which leads to the formation of CFx (x ≥ 1−3) species.14 In agreement with the changes to C 1s bonding states, the peak at ∼686.7 eV is below the limit of detection, accompanying an increase of the C−F covalent binding state at 688.9 eV as fluorination proceeds over 300 s, which suggests that a full transformation of ionic to covalent C−F bonding characteristics is likely to occur with an increased fluorination time. Few experimental observations of the ionic C−F bond have been discussed, where the planarity of the carbon sheet is preserved as a result of the weak C−F polarization,10 and similar bonding states have been revealed in the case of an ozone functionalized graphene surface, where room-temperature ozone exposure leads to physically adsorbed oxygen species on carbon atoms via so-called ionic bonding, which results from partial charge transfer.30 The effects of such bonding on the electronic properties of graphene will be discussed in the following section. Figure 2c shows the peak area evolution of C 1s, Si 2p, and O 1s before and after fluorination. Both the Si 2p and O 1s signals increase after initial fluorination, followed by a decrease upon longer plasma exposure, while the C 1s signal decreases continuously as the fluorination proceeds. The variations of Si 2p and O 1s signals, along with the C 1s state, can be attributed to the peak attenuations caused by removal of PMMA residues, followed by the attachment of fluorine to carbon atoms.27 Our XPS results suggest that a preservation of the original graphene lattice is maintained during CF4 plasma treatments.
In Figure 1b, the normalized D peak intensity with respect to the G peak increases considerably after initial fluorination and then drops to saturation upon increasing fluorination time. The normalized 2D peak intensity shows an opposite trend and reaches saturation gradually. Even though Raman studies by Eckmann et al.25 have indicated that it is possible to distinguish the defect nature by analyzing the intensity ratio of D/D′ peaks, it is not applicable for a high defect concentration region observed in Raman spectra, where a high ID/IG peak ratio of ∼2.5 is obtained for 10 s of fluorination. However, on the basis of related Raman studies,8,25 our observation of variations in D and D′ peaks is most likely induced by an arising of sp3hybridized carbon atoms as a result of fluorine adsorption. Figure 1c, the STM image of the FG surface after 300 s of fluorination, shows the distorted sp2 carbon atomic arrangement, an apparent indication of fluorine adsorption to the pristine graphene lattices (Figure S1b of the Supporting Information), which accordingly opened the band gap of graphene, as seen from their relevant dI/dV curves plotted from STS measurement (Figure S1c of the Supporting Information). To understand the binding nature between fluorine and carbon atoms of graphene, XPS spectra of C 1s and F 1s core levels of transferred CVD graphene on SiO2/n+Si before and after fluorinations were studied in panels a and b of Figure 2.
Figure 2. XPS spectra of as-transferred CVD graphene on SiO2/n+Si and FG after 100 and 300 s of CF4 plasma exposure at 40 W and 700 mTorr. (a) C 1s and (b) F 1s core levels. (c) Variation of peak areas under C 1s, Si 2p, and O 1s peaks as a function of the fluorination time normalized with respect to their corresponding areas before fluorination.
Prior to fluorination, the asymmetric peak found at ∼284.5 eV is ascribed to the sp2-hybridized carbon in pristine graphene. Additionally, three weak deconvoluted peaks at ∼285.8, 287.1, and 289.3 eV are also identified, originating from the oxidized sp3 carbon species, e.g., C−OH and CO groups. The presence of these peaks likely arises from the poly(methyl methacrylate) (PMMA) residue left by graphene transfer and/ or by carbon contaminations as a result of exposure by air.26,27 Dramatic changes, in terms of C 1s and F 1s peak positions and intensity, were observed immediately after 100 s of CF4 plasma treatment. With regard to the C 1s core level, the binding energy at ∼284.5 eV shows a significant decrease in its 5004
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Figure 3. Temperature-dependent IDS−VBG curves of back-gated graphene (exfoliated) FETs for different fluorination times at (a) 10 s, (b) 30 s, and (c) 100 s. The channel dimensions: (a) 3 × 5 μm (L × W). Data were collected at VDS = 0.1 V, and IDS has been normalized with respect to the width of the graphene channel.
3.2. Electrical Behavior of the FG Sheet. Fluorographene has been studied as a promising tunneling dielectric candidate for future switching devices, such as negative differential resistance (NDR)31 and bistable field effect transistor (BiSFET)32 devices, which require a low dielectric constant while maintaining ultrathin dielectric films of ∼1−2 nm to confine the condensates between two graphene layers. However, the transport characteristic and conduction mechanisms in FG sheets obtained for low F coverage have rarely been studied thus far. Panels a−c of Figure 3 show the IDS−VBG curves of back-gate GFETs (exfoliated single-layer graphene) for varying fluorination times of 10, 30, and 100 s, leading to partially covered fluorine based on our related Raman and XPS analysis. In contrast to the transport characteristics of pristine graphene FETs before fluorination, shown in Figure S3 of the Supporting Information, the on/off ratios have increased from ∼10 for pristine to greater than 102−103 after fluorination. Furthermore, most of the IDS−VBG curves for fluorinated GFETs have displayed two minimum conductivity points (Dirac points), which are more noticeable at a lower temperature of 77 K for graphene devices after quick fluorination treatment. Because covalent bonding only causes a decrease in overall channel conductivity,30 the observation of dual Dirac points is a clear suggestion of the presence of a heterojunction, e.g., p−n junction, in FG devices. Highly pdoped epitaxial graphene has been reported by Andrew et al.,28 induced by fluorine intercalations, whereas, in this work, this phenomena might be attributed to the coexistence of ionic and covalent C−F bonding characteristics in low fluorine coverage devices, consistent with our related XPS results. This observation of dual Dirac points is determined to be an intrinsic electrical behavior of partially FG (refer Figure S4 of the Supporting Information). In Figure 4, a dual conduction mechanism is proposed to interpret the dual VDirac points observed in the temperature-dependent IDS−VBG curves of graphene FET fluorinated for 10 s. When there is no applied back-gate bias (VBG = 0 V, region II), the Fermi level (EF) is pushed into the valence band in the graphene region, where the fluorine atoms are ionically bonded, whereas, in the case of the graphene region with fluorine atoms covalently bonded, the Fermi level is aligned in between the band gap, as shown in the schematic in Figure 4. Here, a source−drain current is mainly controlled by covalent bonding, which has a band gap and, therefore, results in one minimum. In the case of VBG < 0 V (region I), EF is further pushed into the valence band for both ionic and covalent bonds, which results in an increase in conduction in the GFET. On the other hand, when VBG > 0 V (region IV), at some value of VBG, EF will be aligned in such a
Figure 4. Temperature-dependent IDS−VBG curves of a back-gated graphene FET after fluorinated for 10 s at 700 mTorr and 40 W. The insets are schematics of the simplified band structures of covalently FG with valence and conduction bands in a quadratic type relation at the band edges and semi-ionic C−F with no band gap shown by linear density of states. The dash line refers to the Fermi level (EF). The colored regions in the plot relate to the band structures shown above them.
way that it passes through the Dirac point (zero density of states) of the ionic bonds and results in the second minimum (at VBG = 35 V in this case). A further increase in VBG results in EF being moved into the conduction band of the ionic bonds, and the current levels increase again (region V). In the case of region III, conduction in the GFET will be a function of the density of states in the conduction band (which are increasing with increasing VBG) of covalent bonds and the density of states in the valence band (which are decreasing with increasing VBG) of ionic bonds. With an increasing fluorine plasma treatment time, we notice that the minima in region II starts to dominate and we only have one minimum because of the increase in covalent bonding, which controls the conduction in graphene. IDS−VDS curves of our back-gated GFET devices after fluorination for 10 s at VBG = 0 V are also studied; the related results are depicted in Figure S5 of the Supporting Information. It is found that, at lower drain biases (0.1 V), the carrier conduction follows the two-dimensional variable-range hopping (2D VRH) mechanism,5 while at higher drain biases (5 V), it is more field-driven. On the basis of the 2D VRH mechanism, the hopping parameters were extracted to be ∼40 000, ∼167 000, and ∼546 000 K for graphene fluorinated for 10, 30, and 100 s, respectively. Because electron transport in FG takes places via localized electron states,11 the decrease of these states induced by the increase in the percentage of covalent bonding between fluorine and graphene, therefore, accounts for the increased of resistance in the resulting film. 5005
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Figure 5. Raman D/G peak ratio mapping images of FG after annealing at 300 and 600 °C for varied fluorination times: (a−c) 100 s and (d−f) 300 s. The Raman mapping area is 25 × 25 μm and is collected with a spatial resolution of ∼300 nm.
3.3. Thermal Stability of the FG Sheet. Previous Raman studies on the thermal stability of fully FG suggested that fluorine desorption occurs at around ∼400 °C, leading to a reversible recovery of the graphene lattice;5,13 however, singlespot Raman spectra measurements were fairly dependent upon collection position and might not be sufficient to gain detailed insight on the structural evolution of CVD FG sheets at micrometer scales. Here, we combine both Raman spectra (Figure S6 of the Supporting Information) and mapping images to examine the annealing temperature-dependent structural changes of FG samples over an area of 25 × 25 μm with a spatial resolution of ∼300 nm. These FG sheets are annealed in a N2-purged Raman cell to maintain an inert atmosphere, and Raman mappings are collected after 30 min of annealing, followed by cooling to room temperature. As shown in panels a and d of Figure 5, fairly uniform fluorination is achieved on the flat regions of both CVD graphene sheets after initial CF4 plasma exposure for 100 and 300 s; the corresponding average ID/IG ratios are ∼1.8 and 1.6, which indicates that the degree of fluorination can be readily controlled by plasma exposure time over large areas. However, for the 100 s FG sheet, we noticed that an inhomogeneous fluorination occurs on multilayer CVD graphene features, such as folds, wrinkles, etc.; these regions are less susceptible to fluorination,13 thus giving rise to higher conductivity, as observed in the microscopic findings by Wang et al.14 Annealing the FG sheets leads to considerable changes in the relevant ID/IG ratios as a result of fluorine desorption; the average ID/IG ratio of a 100 s FG sheet decreases continuously from 1.6 to ∼0.7, as seen in panels b and c of Figure 5, whereas the ID/IG of the 300 s FG sheet, which has mainly covalent bond components, increases to ∼1.8 first and then decreases to ∼1.5 upon annealing up to 600 °C (panels e and f of Figure 5). Both samples show an initial fluorine desorption below 300 °C or even at 150 °C (Figure S6a of the Supporting Information) for the lower fluorine coverage FG, which contains both ionicand covalent-bonded fluorine. Additionally, in contrast to flat regions, the ID/IG ratio of these wrinkles/folds is relatively low, which could indicate better structural recovery of graphene upon fluorine desorption. However, our Raman investigations show no full recovery of graphene Raman features, which agrees with the findings revealed by previous Raman studies3,8 that the loss of skeletal carbon atoms during defluorination
leaves behind various C−F compositions, such as CF4, C2F4, C2F6, etc.,3 or the remains of fluorine on the carbon scaffold.8 Because CVD graphene is far from a perfect single-crystal structure and contains various defects, such as grain boundaries, vacancies, folds/wrinkles, etc., the resulting FG sheets are therefore more likely to possess variable C−Fx binding structures and conformations, such as armchair, zigzag, boat, etc., with different stabilization energies,33 which can contribute to the inhomogeneous defluorination, as seen in the Raman mapping images.
4. CONCLUSION In conclusion, using CF4 plasma as a fluorinating agent, we have demonstrated a controllable way of tuning the fluorine concentration in CVD graphene at a high CF4 gas partial pressure and low plasma power. In addition to the formation of covalently bonded C−F components that account for the increase in graphene sheet resistance at high fluorine contents; both XPS and electrical measurements have provided evidence of the formation of ionically bonded C−F components, which serve as the dominant contribution to the observation of dual Dirac points in a graphene device at low fluorine content. This ionic component is only present at a low fluorine content, while continuous fluorination results in a complete development of covalently bonded C−F structures. In contrast to reversible defluorination, Raman mapping studies reveal that fluorine desorption is fairly inhomogeneous, owing to the presence of various C−F bonding components, and the defluorination process can start at ∼150 °C for samples at a low fluorine content. The incomplete recovery of graphene Raman features after annealing to 600 °C in a N2 environment is likely caused by imperfection in the CVD graphene structures and loss of skeletal carbon atoms during annealing. Our findings have provided fundamental insight on the complex scenario of fluorinating CVD graphene for realistic nanoelectronic applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11701. 5006
DOI: 10.1021/acsami.5b11701 ACS Appl. Mater. Interfaces 2016, 8, 5002−5008
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Raman spectra showing the effect of CF4 partial pressure on the transferred CVD graphene on SiO2/n+Si substrate using plasma power of 40 W for 10 s, STS of CVD graphene before and after fluorination showing the presence of band gap in fluorinated graphene, and STM image of the lattice structure of CVD graphene before fluorination recorded at 0.1 V and 0.7 nA (Figure S1), XPS of C 1s and F 1s binding states after 10 s of fluorination on highly ordered pyrolytic graphite (HOPG) using CF4 plasma at 40 W and 700 mTorr (Figure S2), IDS−VBG curves and mobility of back-gated graphene devices as a function of the temperature for exfoliated graphene (Figure S3), Raman spectra of graphene after different fluorination times in facing down setup, schematics showing the setup for two different fluorination procedures, and transport properties (IDS−VBG curves) as a function of the temperature of back-gated (exfoliated) graphene FETs fluorinated for 100 s using face-up and face-down configurations (Figure S4), temperature-dependent IDS−VBG curves of a backgated graphene FET after fluorination for 10 s at 700 mTorr and 40 W and conductivity as a function of T−1/3 for different applied drain biases (Figure S5), temperature-dependent Raman spectra of CVD fluorinated graphene sheets produced from 100 and 300 s of fluorinating at 700 mTorr and 40 W using CF4 (Figure S6), and TEM cross-section images of fluorinated graphene (four layers) stacked vertically on SiO2/n+Si (Figure S7) (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Telephone: +1-972-883-6412. E-mail: lanxia.cheng@utdallas. edu. *Telephone: +1-972-883-6412. E-mail: jiyoung.kim@utdallas. edu. Present Addresses ‡
Srikar Jandhyala: Intel Corporation, Hillsboro, Oregon 97124, United States. § Greg Mordi: Samsung Austin Semiconductor, Austin, Texas 78754, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Southwest Academy on Nanoelectronics (SWAN) Center, a Semiconductor Research Corporation (SRC) center sponsored by the Nanoelectronics Research Initiative and National Institute of Standards and Technology (NIST). The authors acknowledge Prof. Yves J. Chabal and Dr. Jean-Francois Veyan for their homemade fluorination tool.
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