High-Density Amine-Terminated Monolayers Formed on Fluorinated

Letter pubs.acs.org/Langmuir

High-Density Amine-Terminated Monolayers Formed on Fluorinated CVD-Grown Graphene Rory Stine,† Jacob W. Ciszek,§ Daniel E. Barlow,‡ Woo-Kyung Lee,‡ Jeremy T. Robinson,‡ and Paul E. Sheehan*,‡ †

Nova Research, Alexandria, Virginia 22308, United States U.S. Naval Research Laboratory, Washington, D.C. 20375, United States

‡

ABSTRACT: There has been considerable interest in chemically functionalizing graphene films to control their electronic properties, to enhance their binding to other molecules for sensing, and to strengthen their interfaces with matrices in a composite material. Most reports to date have largely focused on noncovalent methods or the use of graphene oxide. Here, we present a method to activate CVD-grown graphene sheets using fluorination followed by reaction with ethylenediamine (EDA) to form covalent bonds. Reacted graphene was characterized via X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy as well as measurements of electrical properties. The functionalization results in stable, densely packed layers, and the unbound amine of EDA was shown to be active toward subsequent chemical reactions.

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ince its discovery,1 the excellent electrical and mechanical properties of graphene have driven intense interest in its potential applications.2−4 The desire to extend graphene’s capabilities through chemical functionalization has been well documented.5 Just as with carbon nanotubes before it,6 the incorporation of organic functionalities into the carbon structure of graphene presents many advantages, such as incorporation into polymer composites7−10 or the production of biomolecular sensors.11−13 To date, most methods for attaching molecules to graphene have focused on noncovalent π-bond stacking14 or have started with graphene oxide.15−18 Existing methods of covalent functionalization typically rely on reactive diazonium salts19 or azides20 to functionalize a relatively unreactive substrate: graphene. However, a recent report of highly fluorinated graphene sheets21 offers a pathway to the covalent functionalization of graphene with the ease of reaction analogous to thiol monolayers on gold22 or alkylsiloxane on glass.23 As was previously shown with carbon nanotubes,24 the fluorinated carbon backbone of these structures can serve as an activated substrate for a number of chemical reaction schemes. Unlike with graphene oxide, fluorination provides a single, reactive C−F bond instead of an ensemble of functional groups, thereby greatly simplifying the chemistry. Here, we report the functionalization of CVDgrown graphene sheets with amine-terminated molecules, offering a pathway to the incorporation of useful functional groups and patterned surface functionality.

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residue. Functionalizing graphene directly on Cu was not possible because of its reactivity with EDA and pyridine. Fluorination and Amine Reaction. Graphene was fluorinated using previously reported methods.21 Samples were exposed to a mixture of XeF2/N2 (1:35) for 900 s at 40 Torr under intense light exposure. After fluorination, samples were reacted in EDA containing 1% pyridine (by volume) for 3 h on a hot plate set to 170 °C.28 Samples were then immersed in ethanol for 2 min, rinsed with copious amounts of acetone and ethanol, and dried with N2. XPS analysis was performed using a monochromatic Al Kα source. ATR-FTIR spectra were acquired under nitrogen purging using a Thermo Scientific Nicolet 6700 spectrometer with an MCT-A detector and a Harrick Horizon ATR accessory. Raman spectra were obtained using a 514.5 nm excitation source. FET Fabrication and Electrical Characterization. Standard photolithography techniques were used to fabricate devices on transferred graphene on 100 nm SiO2/Si. Source-drain contacts consisted of Ti/Au (5/20 nm). After fabrication, devices were annealed under flowing Ar/H2 at 150 °C for several hours. The Dirac point of the graphene FETs (device width = 12 μm, channel length = 4 μm) at each surface modification step was characterized by a probe station with gate voltages on the back-gated, heavily doped Si substrate with a source-drain voltage (Vsd) of 0.1 V. Density and Stability Analysis. Density and stability calculations were based on the intensity of the F 1s and N 1s peaks for fluorinated and amine-reacted samples, respectively. The XPS peak intensity was determined by fitting the areas of the peaks of interest using commercially available Unifit software. For comparisons between samples, peaks were normalized by taking the peak intensities as a ratio with respect to the Si 2p peak from the SiO2 substrate.

METHODS

Graphene Growth and Transfer. Graphene films were grown using low-pressure CVD on Cu foil substrates25 in a folded “enclosure” design26 and transferred using previously reported methods.27 Samples were then annealed in Ar at 400 °C for 2 h to remove the PMMA © 2012 American Chemical Society

Received: March 13, 2012 Revised: May 1, 2012 Published: May 11, 2012 7957

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284.3 eV (for sp2 carbon) coincides with an increase in the sp3 species, both fluorinated and unfluorinated.29 The relative C−F atomic percentages determined from the corresponding C 1s peaks at 287 and 288.6 eV are roughly equivalent (within 5%) to the total atomic percentage of fluorine determined from the F 1s peak, indicating that the fluorine in the sample is covalently bound to the graphene sheet. Following the reaction with EDA, the intensity of the F 1s peak is reduced by ∼90% whereas a large N 1s peak at 399.5 eV appears. A strong Si 2p peak (not shown) from the underlying SiO2 substrate is still visible in the survey spectrum, indicating that the loss of the F 1s peak is due to fluorine removal, not simply the shielding of an underlayer by the addition of EDA. A shift in the C 1s spectra can also be seen after the amine reactions, with the sp2 carbon peak increasing in intensity. This indicates a partial reduction in the carbon lattice back to sp2 carbon, similar to the reduction seen when fluorinated graphene is exposed to hydrazine.21 However, sp3 carbon remains, corresponding to the EDA-functionalized sites. The shoulder seen in the N 1s region at 401.4 eV (∼10% of the N 1s signal) most likely results from a small amount of the terminal amines that have acquired a proton during the rinsing procedure.30 Control samples where unfluorinated graphene was exposed to room-temperature EDA showed no similar N 1s peaks. Unfluorinated graphene samples exposed to the EDA solution at 170 °C were stripped from the SiO2 substrate, making control experiments under identical conditions impossible. Figure 2 depicts an ATR-FTIR spectrum taken on a fluorinated graphene sample before (Figure 2a) and after (Figure 2b) reaction with EDA. The spectra in Figure 2a for fluorinated graphene were taken with reference to the pristine graphene sample prior to fluorination. That is, positive peaks indicate functionalities added to the sample after fluorination; negative peaks indicate functionalities that were removed. Similarly, the spectra in Figure 2b for EDA-reacted graphene were taken with reference to the fluorinated graphene sample. Notable aspects of the fluorinated graphene spectra in Figure 1a are the loss of C−H stretching peaks at 2915 and 2851 cm−1, most likely from the removal of adventitious carbon during XeF2 exposure, the addition of small C−F stretching peaks at 1209 and 1079 cm−1, and the shift in the C−C peak from 1594 to 1571 cm−1. The spectra in Figure 2b show the appearance of overlapping peaks at 3270 cm−1 and an additional peak at 3160 cm−1, associated with N−H stretching, as well as C−H stretching peaks at 2920 and 2880 cm−1 confirming the addition of EDA. An alternate assignment of the 3160 cm−1 peak may be a Fermi resonance between the N−H stretch and bend. Primary amines generally show two N−H stretching bands, whereas secondary amines show one. It is possible that the bands for the primary and secondary groups overlap in the spectrum. The presence of at least two N−H stretching peaks reveals that an unreacted primary amine group remains, verifying that the molecule does not attach through both amine groups. The absence of a water bending mode at 1640 cm−1 shows that these peaks cannot be attributed to an O−H stretch from adsorbed water. Additionally, we see the C−C peak shifts back to its original position at 1594 cm−1. This large peak obscures the area where an additional N−H scissor peak would appear at 1592 cm−1. However, if we take the same spectra referenced to the original pristine graphene sample (inset), thus removing the C−C peak, we can see the addition of this N−H scissor peak, further confirming the presence of a primary amine group. One final note is that the oppositely

Free Amine Functionalization. EDA-treated samples were exposed to a solution of 25% glutaraldehyde (GA) in water (by volume) for 2 h at room temperature. Samples were then rinsed with water and exposed to a solution of 10% trifluoroethylamine (TFEA) in water (by volume) for 1 h at room temperature. After reaction, the samples were rinsed with copious amounts of water and dried with N2. A control sample was measured by exposing an EDA-treated sample to the TFEA solution while omitting the GA solution.

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RESULTS AND DISCUSSION Figure 1 depicts a schematic outline (Figure 1a) and XPS spectra (Figure 1b) showing the sequential steps going from Cu-grown CVD graphene that has been transferred to a SiO2 substrate, its subsequent fluorination with XeF2 gas, and its reaction with EDA. After fluorination, the appearance of a large F 1s peak is evident, and a decrease in the main C 1s peak at

Figure 1. (a) Schematic showing graphene fluorination followed by the reaction of EDA to form a monolayer. (b) XPS spectra for graphene (top), fluorinated graphene (middle), and fluorinated graphene reacted with an EDA monolayer (bottom). Scales are held constant for a given element across all samples (columns) but are varied for each element in a given sample (rows) to give maximum peak clarity. Component peaks in the C 1s spectra are defined as sp2 carbon (red), sp3 carbon (blue), C−N and/or C−C−F (green), and C−F (purple). 7958

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1350 cm−1, the loss of the 2D peak at 2690 cm−1, and the appearance of the D′ and D + D′ peaks at 1620 and 2950 cm−1, respectively. Following the reaction with EDA, a slight increase in the 2D peak intensity and a decrease in the D + D′ peak intensity can be seen. (These areas of the spectra are multiplied by a factor of 3 because the peaks are miniscule.) This suggests the reduction of the fluorinated graphene and the reformation of sp2 carbon bonds, in agreement with the shift seen in the C 1s peak after the EDA reaction in Figure 1. Additionally, there appears to be both a slight increase in height and a narrowing of the D peak, though the implications of this change are unclear. Both the Raman and XPS data are consistent with the formation of small conjugated sp2 regions within the functionalized substrate. This is further confirmed by the electronic properties of the graphene shown in Figure 4, which depicts the

Figure 2. ATR-FTIR spectra of fluorinated graphene (a) before and (b) after reaction with EDA. The spectrum of the fluorinated graphene was taken with reference to the pristine graphene sample prior to fluorination. The spectrum of the EDA-reacted graphene was taken with reference to the fluorinated graphene. The inset shows a region of the EDA-reacted spectrum referenced to the pristine graphene, thus removing a large C−C peak that otherwise obscures the N−H scissor mode.

sloping baselines shown for the two spectra appear to be an effect of graphene fluorination because they were consistently observed. The Raman spectra shown in Figure 3 reveal an initial increase in disorder after fluorination followed by a shift back toward graphitic carbon after reaction with EDA. The characteristic disorder-induced peaks are apparent after fluorination,21 including the appearance of the D peak at

Figure 4. Sheet resistance vs gate voltage for graphene (top), fluorinated graphene (middle), and fluorinated graphene reacted with EDA (bottom).

graphene sheet resistance (Rsheet) versus the gate voltage (Vg). Prior to fluorination, graphene shows Rsheet = 1 kΩ, with the Dirac point (VD) at Vg = 5 V (Figure 4a). After fluorination, Rsheet increases more than 4 orders of magnitude (Rsheet = 60 MΩ) as disorder in the graphene sheet increases, and no measurable VD can be seen (Figure 4b). Following the reaction with EDA, Rsheet decreases to 1.1 MΩ, and a measurable VD can again be seen at Vg = −20 V (Figure 4c). The decrease in resistance again confirms the reduction of the fluorinated graphene and the partial reordering of the disrupted π-bond network, whereas the negative shift in VD indicates n-type doping of the graphene. This type of doping is consistent with the incorporation of nitrogen into the graphene,31 but determining the presence of nitrogen in the graphene lattice would require additional experiments beyond the scope of this work. Because of the well-known structure of graphene, estimates can be made of the density of the aminated molecules on the surface. An analysis of the atomic percentages obtained from XPS spectra of fluorinated graphene shows a stoichiometric ratio of CxF where x = 3.4 ± 2. Note that this is a slightly

Figure 3. Raman spectra of graphene (top), fluorinated graphene (middle), and fluorinated graphene reacted to form an EDA monolayer (bottom). Spectra are offset for clarity. For 2D and D + D′ regions on F graphene and EDA samples, the y scale is multiplied by 3 to enhance the peak resolution. 7959

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higher concentration of fluorine than found when fluorinating graphene on copper.21 After exposure to EDA, 30% of fluorinated sites react to form bonds with the EDA molecules with the remaining lost fluorine most likely associated with reformed CC bonds. This gives a density of 1 EDA molecule per 9 carbons or, given the graphene bond length of 1.42 Å,32 a footprint of 23.7 Å2 per molecule. The density of the formed monolayer corresponds to ∼89% of the packing density for thiol SAMs on gold,33 indicating a densely packed monolayer. Importantly, the density of functionalization can be controlled by changing the extent of fluorination in the starting material. For example, when fluorination is limited to C8F, reaction with EDA generates a lower coverage of roughly 1 molecule per 18 carbons for a density of 47.4 Å2 per molecule. It should be noted, however, that these numbers are only rough estimates based on spectroscopy data, and additional characterization would be required to confirm these figures. The stability of the monolayer was tested by heating the sample and monitoring the decline in the XPS N 1s peak. Baking EDA samples at 120 °C (above the boiling point of EDA) for 30 min in air resulted in a 12 ± 4% loss of EDA from the surface. Extended solvent exposure (ethanol, 3 days) resulted in a slightly higher loss of EDA (21 ± 4%). Baking the samples at higher temperature (150 °C) shows more significant desorption, with the EDA layer completely desorbed by 190 °C. Though this seems lower than the temperature that would be expected for the thermal desorption of a covalently bound monolayer, it is also undoubtedly much higher than what would be seen for a physisorbed material, and it is possible that reactions with oxygen or contaminants in air at elevated temperature are aiding the removal of the EDA. The removal of EDA from the surface did not result in any significant changes to either the conductivity of the samples or to the Raman spectra, indicating that the sites where EDA had been bound do not undergo further sp2 bonding, leaving the graphene plane disrupted. Beyond demonstrating the fluorine-activated reaction of graphene, a prime motivation for attaching EDA to graphene is the subsequent ability to react the resulting free (terminal) amine because the presence of an amine group offers a larger array of potential reactions under mild conditions. For instance, attachments to biomolecules are routinely made through amine-terminated surfaces.34,35 To test the reactivity of the free amine groups, we exposed the EDA monolayer first to GA and then to XPS label TFEA. The availability of the free amine from EDA for further functionalization was proven by the appearance of a significant F 1s peak in XPS that was more than 8 times larger than that of a control sample where GA exposure was omitted. During this particular reaction, there was a 37% loss of EDA from the surface, perhaps indicating that the bond between EDA and graphene is susceptible to hydrolization. In summary, a method for the covalent functionalization of fluorinated graphene with ethylenediamine has been explored. The reaction was confirmed via XPS, FTIR, and Raman spectroscopy, and the effect on the graphene films was characterized by both spectroscopic methods and the measurement of electrical properties. The layer packing density was estimated to be 23.7 Å2 per molecule, indicating a tightly packed monolayer. The monolayer appears to be stable under heating and solvent exposure and is capable of undergoing subsequent chemical reactions involving the terminal amine group.

Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department of Chemistry, Loyola University, Chicago, Illinois 60626, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by the NRL Nanoscience Institute and the Defense Threat Reduction Agency. This project received support from the Defense Threat Reduction Agency-Joint Science and Technology Office for Chemical and Biological Defense (contract no. CB3773)

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