Spectroscopic Investigation of Plasma-Fluorinated Monolayer

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Spectroscopic Investigation of Plasma-Fluorinated Monolayer Graphene and Application for Gas Sensing Hui Zhang, Liwei Fan, Huilong Dong, Pingping Zhang, Kaiqi Nie, Jun Zhong, Youyong Li, Jinghua Guo, and Xuhui Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11872 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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ACS Applied Materials & Interfaces

Spectroscopic Investigation of Plasma-fluorinated Monolayer Graphene and Application for Gas Sensing Hui Zhang,†,‡ Liwei Fan, †,‡ Huilong Dong,† Pingping Zhang,† Kaiqi Nie,†,§ Jun Zhong,† Youyong Li,† Jinghua Guo,§ Xuhui Sun*,† †Jiangsu Key Laboratory for Carbon Based Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China §Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

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ABSTRACT. A large-area monolayer fluorinated graphene (FG) is synthesized by a controllable SF6 plasma treatment. The functional groups of FG are elucidated by various spectroscopies, including Raman, X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). Raman results suggest that the defects are introduced into the monolayer graphene during the fluorination process. The fluorine content can be varied by the plasma treatment and can reach the maximum (~24.6 at% F) under 20 s plasma treatment as examined by XPS measurement. The angle dependent NEXAFS reveals that the fluorine atoms interact with the graphene matrix to form the covalent C-F bonds, which are perpendicular to the basal plane of FG. FG is applied as gas sensing material and owns much better performance for ammonia detection compared to the pristine graphene. Based on our DFT simulation results, the fast response/recovery behavior and high sensitivity of the FG gas sensor are attributed to enhanced physical absorption due to the C-F covalent bonds on the surface of FG.

KEYWORDS. graphene • fluorination • NEXAFS • ammonia • gas sensor


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1. INTRODUCTION Graphene has attracted intense research attention owing to its unique and extraordinary physical and chemical properties.1-2 However, the nature of pristine graphene with zero band gap limits its application in the electronic device field.3-4 To dope graphene with other heteroatoms (e.g., nitrogen, boron, phosphorus, halogen, etc.)5-8 becomes the most practicable, convenient and efficient approach to modulate the band structure and properties of graphene9 and thus extend the applications in electronics and sensors.6,10-11 Graphene halides,12-13 one class of graphene derivatives, including fluoride,14-15 chloride,16-17 bromide,18-19 and iodide,20-21 have recently been investigated for the modification of graphene. Normally, fluorination is a p-doping type covalent modification which could tailor the intrinsic properties of graphene.14 Simultaneously, fluorinated graphene (FG) could exhibit some novel properties. Walter et al.22 reported the unique luminescence of FG and Ho et al.23 utilized the FG as the high performance gate dielectric materials. In addition, Li et al.24 found that the friction between a nanoscale probe tip and a surface of graphene could be altered over a wide range by fluorination. Recently, FG has been synthesized by various methods: (1) mechanical exfoliation of graphite fluoride;2,25 (2) chemically exposure to the XeF2 vapor;14,26-27 (3) F-based gas plasma treatment.28-29 For instance, SF6 and CHF3 plasma treatment provide a highly stable covalent bond of C-F.30 Graphene has been utilized in chemical sensor applications,31-32 because of its extremely high surface to volume ratio as well as the high crystal quality and low resistance (typically few hundred ohms),33-34 especially in gas sensing, including H2,35 NH3,36 NO237 and so on.32,38 Nevertheless, it is worth noting that the pristine graphene has no dangling bonds (which is desirable for specific gas/vapor adsorption) on its surface to enhance the adsorption of target gas molecules. Therefore, graphene needs to be functionalized with metals, polymers, or other


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suitable modifiers.39 For examples, the reduced graphene oxide,36 N doped graphene,11 Al doped graphene,40 and other doped graphene41 have been developed for detecting various gases. NH3 is a common gas closely related to human activity and the NH3 gas sensor based on semiconductor metal oxide usually requires high operational temperature.42 Therefore, a type of sensitive NH3 gas sensor working at low/room temperature is preferable for the practical application. Here, we report SF6-based plasma treatment to fluoridate the CVD-grown monolayer graphene. The electronic structure of FG is investigated by Raman, X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). The maximum F content can even reach to ~24.6% atomic, very close to the theoretical calculation value of single layer FG.14 The FG based gas sensors for ammonia exhibit fast response behavior and high sensitivity. The fluorinated method opens a door to potential application of graphene based gas sensor.

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EXPERIMENTAL SECTION

2.1. Materials preparation. Monolayer graphene was grown by the chemical vapor deposition (CVD) method using Cu foil (25 µm thick, 99.8% pure, Alfa Aesar) as the catalyst and CH4 as the carbon source. In the growth procedure, 7 sccm CH4 and 40 sccm H2 were introduced into the tube furnace at 1000 °C for 10 min. After the growth, the transfer of pristine graphene SiO2/Si wafer was performed by a typical PMMA method.43 After PMMA was spin-coated on the surface of graphene/Cu, 1M FeCl3 solution was used to etch the Cu foil. After DI water washing and drying in vacuum, the pristine graphene was finally transferred onto the surface of 300 nm SiO2/Si wafer.


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The fluorination of graphene film was carried out in a reactive ion etching (RIE) system. The graphene samples were directly immersed in SF6 plasma with 5W forward power and 5 sccm gas feed rate for different reaction times. The plasma was ignited between two metallic parallel-plate electrodes and the sample was located at the center of the electrodes, ensuring the uniform plasma treatment of the graphene samples. After pumping the chamber to 1×10-5 Torr, the graphene was fluorinated under 5 sccm SF6 and 2 sccm He atmosphere at 37.5 mTorr. The DC bias was set at 13 V and the reflect power was 1W. 2.2. Characterization. Raman spectra were obtained on a Jobin-Yvon HR800 Raman spectrometer with the laser of 512 nm wavelength and a spot size of 1µm. To avoid the laserinduced sample damage, each spectrum was integrated for 2 s. At least three spectra from different spots were taken on each sample and averaged. AFM (Veeco Multi-Mode V) was performed to monitor the thickness of FG on SiO2 under tapping mode. The scanning rate was 0.998 Hz and the resonance vibration frequency was ~350 kHz. XPS studies were carried out in a Kratos AXIS UltraDLD ultrahigh vacuum (UHV) surface analysis system using a monochromatic Al Kα line. The analysis was performed with CASA XPS analysis package. All spectra were fit to a mixed Gaussian and Lorentzian line shape. C K-edge NEXAFS measurements were performed at BL8.0.1 (wet-RIXS endstation) in the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. All of the C K-edge NEXAFS spectra were collected in total electron yield (TEY) mode and calibrated to the π* peak at 285.5 eV of highly oriented pyrolytic graphite (HOPG). F K-edge NEXAFS measurements were performed at BL20A of the Taiwan Light Source (TLS). The F K-edge NEXAFS were collected in total electron yield (TEY) mode and calibrated to the main strong peak at 709.6 eV of Fe2O3 crystal.


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2.3. Computational details. Density functional theory (DFT) calculations were performed by (CASTEP) program.44 The Perdew-Burke-Ernzerhof generalized gradient approximation (GGAPBE)45 was used to describe the electronic exchange and correlation interaction. The dispersion interactions were added using the semi-empirical scheme proposed by Grimme.46 Ultrasoft pseudopotential was adopted for the spin-polarized computation and the plane-wave cutoff energy was set to be 500 eV. The primitive cells of CF0.25 and CF0.125 were obtained from previous theoretical reports.14,47 Graphene was also used as comparison. Supercells were used to simulate the adsorption of gas molecules, and the distance of vacuum slab was set 20 Å to eliminate the interactions between the periodical images. For our modeled 2 × 2 CF0.25, 2 × 2 CF0.125, and 4 × 4 graphene supercells, a Monkhorst–Pack grid48 of 2 × 2 × 1 was used for the Brillouin zone sampling during geometry optimizations, while the electronic properties were obtained by 4 × 4 × 1 k-points. The band gap of CF0.25 is 2.95 eV from our calculation, and CF0.125 is metallic. Both of our calculation results show good accordance with the reported values,47 validating the reliability of our parameters. 2.4. Gas sensor fabrication and measurement. The simple electro-resistor type gas sensor was fabricated to test the sensing behavior of FG. A thin layer of 5 nm Ti/80 nm Au electrodes were coated onto the fluorinated graphene (FG/SiO2) film by electron beam evaporation coating (PVD 75, Kurt J Lesker) with a homemade mask to control the gap of 50 µm that exposed the sensing material. Gas-sensing properties were measured by a home-built intelligent gas sensing analysis system, wherein I-V characteristics of the sensor were measured by a computercontrolled Keithley-2400 system. In a typical testing procedure, the device was placed in a gas chamber and the gas flow rates were manipulated by the needle valves and mass flow controller. The bias on the device was set as 1V. The chamber was purged with dry air throughout the


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measurements at a constant rate. The sensor resistance and sensitivity were collected and analyzed by this system in real time when the target gas was introduced into the test chamber at a constant flow rate. The relative variation of sensor resistance is defined as sensor response (S) given by, S = (Rg-Rref)/Rref×100% Here, Rg and Rref are the resistances with and without the target gas, respectively. Response time is defined as the time required for 90% of the total resistance change upon exposure to gas.

3. RESULTS AND DISCUSSIONS The schematic of synthesis and device fabrication process of FG is illustrated in Figure 1. The monolayer graphene was first grown on a Cu foil by the low pressure CVD method.49 After transferred on SiO2 substrate by PMMA-assistant method,43 the fluorination of single-layer graphene was performed in a RIE system using SF6 gas as the source of fluorine30,50 at different reaction times ranging from 10 s to 90 s. The graphene films after plasma treatment are denoted as FG-X (X means plasma treatment time, e.g. FG-20 is the graphene treated by SF6 plasma for 20 s). The two Au/Ti electrodes were deposited on the FG film to fabricate the electro-resistor type gas sensor in which the change of resistance with exposure to the target gas is measured directly. AFM image of FG-20 on SiO2 is displayed in Figure 2A. It shows the FG-20 film with the large size and uniform morphology. According to the statistical chart of thickness in the insert of Figure 2A, the thickness of FG-20 film is about 1.5 nm, slightly thicker than the average height (~0.9 nm) of the pristine monolayer graphene, indicating that the FG-20 film is uniform monolayer fluorinated graphene. The increased thickness can be interpreted as a result of the


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addition of fluorine groups as well as the corrugation induced by the covalent addition.51 FG-20 film still keeps the large area feature with some wrinkled region as observed from low-magnified AFM and optical microscopy images in Figure S1. The large-area FG film is apt to be directly fabricated to the gas sensor device. Raman spectroscopy has been widely used to investigate the electronic and structural properties of graphene and graphene derivatives.52-53 Compared with pristine graphene, the Raman spectra of FG show different profiles, as shown in Figure 2B. The disorder-induced D band (~1365 cm-1) appears in the FG and its intensity increases upon increasing treatment time, which means the defects increase under continuous plasma treatment. In addition, the two phonon double resonance 2D band around 2690 cm-1 are significantly weakened after plasma treatment and the G band broadens due to the presence of a defect-induced D’ shoulder peak at ~1620 cm-1. The peak D’ appears clearly when plasma treatment time increases. The peak at around 2948 cm-1 is assigned to the D+G band.54 The changes in Raman spectra of FG are ascribed to the chemical doping effects. Notably, the blue shifts of G band (from ~1582 to ~1587 cm-1) and 2D peaks (from ~2685 to ~2692 cm-1) occur once graphene films are fluorinated, which is related to the formation of covalent C-F bonds. This is consistent with the strong electron-drawing capability of fluorine which could induce the p-type doping effect.29 These observations in Raman spectra strongly suggest that covalent C-F bonds are formed and thus a high degree of structural disorder is introduced by the transformation of C-C bonds from sp2 to sp3 configuration.54-55 Here, we use the I(D)/I(G) and I(2D)/I(G) ratio to describe the effect of plasma treatment time. In Figure 2C, the I(D)/I(G) ratio increases and reaches at maximum (~2.9) at 50 s, following by the decreasing tendency again under continuous SF6 plasma treatment. From Figure 2D, the I(2D)/I(G) ratio shows opposite profile, which decreases before 50 s and goes up after 50 s.


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The FG-90 exhibits an obvious intensity recovery of 2D peak which has not been observed in the literature.50 It may be linked to the fact that the large ratio of C-F stretches tends to be broken after longer plasma treatment time. In the process of SF6 plasma treatment, the fluorine atoms attack the carbon planar matrix firstly to form the C-F bonds which introduce the defects into the matrix. After a critical treatment time (~50 s), further plasma bombardment produced subsequent F atoms stepwise break down the previously formed C-F bonds and release the F atoms from the matrix and then carbon sp3 hybridization reform C=C double bonds.27,51 This is also in accordance with the optical transparency of FG films as shown in Figure S2. The ultravioletvisible spectrum (UV-vis) reveals a transmittance from ~97.9% for FG-30 to ~98.8% for FG-50 at 550 nm with most defects, and then reduces to~ 97.6% for FG-70. That is allocated to the recovery phenomena of carbon planar matrix. All monolayer FG have the higher transparency than that of the pristine graphene (~ 97.5% at 550 nm). The suddenly decrease of ID/IG from 20 s to 30 s may be attributed to the dramatically fluorine desorption during the period (seen in Table S1) and may cause some C=C bonds reformation, which has been observed in the previous works.27,50 XPS results further confirm the covalent C-F bonds in fluorinated graphene. As shown in Figure S3A, the fluorine related peaks are found in the survey scan, which verify the successful fluorination in the FG. The high-resolution C 1s and F 1s XPS spectra probe the bond type of fluorine in FG. Figure 3A presents the C 1s high-resolution spectrum of FG-20, which can be deconvoluted into five peaks with the binding energies of 284.6 eV, 285.5 eV, 286.7 eV, 288.5 eV and 291.7 eV, corresponding to C sp2, C-CF, C-CF2, C-F and C-F2 bond, respectively.28,56-57 In contrast, the C-F2 peak in FG-90 almost disappears due to its low stability compared with other type bonds, as shown in Figure S3B. Therefore, further plasma treatment will break part of C-F2


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to expel some fluorine atoms from FG. The small ratio of C-CF/C-F in FG-90 is consistent with the recovery of 2D peak intensity in Raman spectra. Besides, the fluorine content in FG-90 is far less than in FG-20 from the survey scan in Figure S3A. The reduction of C-F2/C-F bonds and decreasing content of fluorine prove that the further plasma treatment recovers the planar properties of graphene partially. The high-resolution F 1s XPS spectrum of FG-20 shows a single symmetric peak at 688.6 eV in Figure 3B, which is assigned to C-F covalent bond.28 The missing of ionic feature in F 1s signal at higher energy position suggests that all fluorine form stabilized covalent bonds instead of physically absorption on the graphene surface. The results indicate that SF6 plasma treatment method could create the chemical functional modification of graphene. The concentration of fluorine and ratio of C/F can be calculated from XPS survey scan spectra based on the atomic sensitive factors.58 The concentration of fluorine and ratio of C/F responding with the plasma treated time are shown in the Figure 3C and Table S1. The concentration of F in FG-20 is the highest, up to C3.1F, very close to the most stable saturated theoretical calculation value of single layer FG.14 With further plasma treatment, the concentration of F decreases. FG90 remains the lowest F content (~C20.1F). The FG films present non-linear profile of F content in FG responding with the treatment time because the subsequent F atoms will induce reconstruction of the fully covered FG, as confirmed by XPS and Raman. The C-F bonds will be broken down and the unbounded F will be desorbed out of graphene. We have also used XPS to investigate the chemical stability of fluorinated species in FG. Figure 3D presents the change of fluorine concentration for FG-50 which was stored in dry air at room temperature after fluorination. The F atomic concentration of FG-50 dropped slightly from 13.16% to 10.55% after 25 days. It may be resulted from that some fluorine atoms run off slowly due to the oxygen-rich functionalities in the ambient.27


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NEXAFS is a powerful technique to investigate the unoccupied electronic structure and chemical bonding of CNTs, graphene and other carbonaceous nanomaterials.59-60 The angledependent NEXAFS is a usual technique to monitor the orientation of polymer/molecule on substrate.61 In principle, the resonance in electronic transitions from an initial carbon K- shell to unoccupied π* or σ* state will be strongly enhanced when the electric field vector of the incident linearly polarized synchrotron light is parallel to the direction of either π* or σ* orbital. Thus the strong orientation dependence of transition probabilities makes NEXAFS a sensitive method to probe the alignment and orientation of molecular layers and thin films in a relatively large area depending on the beam spot size.57,59 The detail configuration and the orientation of C-F bonds of FG on substrate were investigated by angle-dependent NEXAFS. As shown in Figure 4A, C K-edge NEXAFS of FG-20/SiO2 and FG-90/SiO2, measured at the incident angle of 15º, show similar spectral profile with pristine graphene/SiO2 with five main peaks labelled as A1, B1 C1, D1, and E1. The three peaks, A1, D1 and E1 have been widely studied in carbon materials.57,62 The strong peak at 285.5 eV (labelled A1) corresponds to the electronic transition from the C K-shell to the conduction π* states in the vicinity of the K point of the BZ.63 The sharp peak at 291.8 eV (labelled D1) is due to an excitonic state rather than the band structure.62 In turn, the broad peak at 292.7 eV, labelled E1, is caused by the transition from the C K-shell to the σ* states at the Γ point of the BZ.64 Peak C1 at about 288.7 eV can be assigned to fluoride group.65-67 Peak B1 is attributed to the C-H σ*, which exist in the CVD synthesized graphene.57 The NEXAFS spectra suggest that FG-20 and FG-90 have mostly aromatic carbon ring structure and the plasma fluorination process is highly nonintrusive doping technology. As shown in the inset of Figure 4A, the intensity ratio of A1/C1 in FG-90 is larger than that in FG-20 at the same angle incidence (15°), which indicates more of


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C sp2 (aromatic) species in FG-90. It confirms that long time plasma exposure will partially recover the planar properties of graphene, which is in agreement with the results of Raman and XPS. F K-edge NEXAFS spectra of FG-90 at different incident angles present a broaden peak centered at ~693 eV which is linked to the C-F bond,65 as depicted in Figure 4B. The fluorine atoms may not have an identical coordination, which reflects random chemical bonding of the C and F atoms. However, the signal of FG-90 in 0° is noisy and weak suggesting that most of fluorine atoms tend to be perpendicular to basal plane of FG. For the detailed characterization of the spectrum of free-electron states in FG, the C K-edge NEXAFS spectrum is compared with the F K-edge spectrum of FG-90 at the same energy scale, as shown in Figure 4C. These spectra are made coincidently in energy by calibrating the energy scale relative to the Fermi level: using C 1s binding energy (284.6 eV) for C K-edge NEXAFS and F 1s binding energy (688.6 eV) for F K edge NEXAFS. The C K-edge NEXAFS spectrum of FG-90 can be decomposed into two components corresponding to the electron transition from the 1s core level of the carbon atoms bonded only to carbon atoms and of those bonded to the fluorine atoms using the NEXAFS spectrum of the pristine graphene/HOPG as a reference.66 The peak B1/C1 in C K-edge almost matches with the B2/C2 in F K-edge spectrum, which corresponds to the carbon atoms bonded with fluorine atoms. Based on the characterization of FG, the fluorine atoms tend to form covalent bonds with the carbon matrix of graphene under the SF6 plasma treatment. In addition, most of fluorine atoms tend to be perpendicular to basal plane of FG, which supplies the C-F bonds for adsorbing the target gas molecules. Therefore, FG is a good candidate for sensitive gas sensor.


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Semiconductor-based resistive gas sensors, which show significant resistance change upon exposure to target gases, are among the most widely investigated candidates due to their simple structural configuration, high sensitivity and stability, low cost, and compatibility with microelectronic processes.68 The gas sensing characteristics of FG films were also investigated by measuring the resistance across the FG film under room temperature while exposing in the target gas. Figure 5A presents the response of intrinsic graphene based gas sensor for different concentration of NH3 gas in dry air at room temperature. The pristine graphene sensor exhibits the slow response behavior and the saturation cannot be reached even after exposure to the NH3 for more than 200 s. In addition, the graphene sensor cannot return to its baseline after a long time recovery process (>500 s). In the case of 100 ppm, the response(S) reached ~2.5% for exposing about 240 s, but even after 500 s recovery, the graphene sensor recovers only about ~66.7% of response. The problem with poor sensitivity and respond speed have also been reported in other literatures.34,69 Here, the fluorinated graphene C-F bonds is introduced to resolve the shortcomings for the development of graphene based gas sensor. As shown in Figure 5B, FG-20 based sensor exhibits much better sensitivity of 3.8% response toward the concentration of NH3 gas of 100 ppm at room temperature. Herein, the gas flow time was always set to 30 s, which was much shorter than graphene sensor. Notably, the recovery time of FG-20 sensor for 20 ppm is less than 200 s, while the recovery time of pristine graphene is longer than 500 s at same detecting level. The response (S) decreases for the lower concentration of NH3. However, the FG-20 sensor could still reach ~ 0.33% when the NH3 concentration was even lower to 2 ppm, which is a remarkable detecting level. Overall, the response speed and sensitivity of FG sensor are improved significantly compared with the pristine graphene based sensor. As far as we known, the effective resistive FG sensor in this


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work shows good performance among the graphene and modified graphene based gas sensors, as shown in Table S2. Obviously, the fluorination could greatly improve the respond speed and enhance the sensitivity at room temperature. Figure 5C depicts the successive five response and recovery cycles for 100 ppm NH3 in dry air under the same conditions. The FG-20 gas sensor shows a good repeatability of the same target gas absorbed on FG surface within 5% relative changes in the resistances. The FG-20 gas sensor also shows the similar response after 25 days, exhibiting the good stability of the sensor. The response will slight decrease with the longer time probably due to the lose of F atoms from FG surface as explained previously. Further study is needed to show whether the SF6 plasma treatment could refresh the FG sensor to recover the response. Furthermore, the sensitivities of FG gas sensors with different plasma treatment time were also investigated under 100 ppm NH3 at room temperature, as displayed in Figure 5D. The response profile is similar as the curve of fluorine concentration. The FG-20 with the highest F/C ratio (~24.6 at% F) owns the highest response. When the plasma exposure time is longer than 20 s, both the response and concentration of fluorine decrease with longer treatment time. The similar profiles are due to the C-F bonds in FG-X (X>20) will be broken down after fluorine coverage reaches maximum under 20 s plasma treatment time. Although FG-10 shows the higher F/C ratio (20.16 at% F) than FG30 (~18.74 at% F) from the XPS results, FG-30 with the lower F concentration shows the higher response. More defects in FG-30 may be responsible for the better performance. The selectivity of FG-20 sensor was also examined by flowing different gas/vapor, including acetone, ethanol and CO, as shown in Figure S4. FG-20 almost exhibits no response for other gas/vapor of 100 ppm at room temperature. Therefore, FG-20 sensor has high selectivity to detect ammonia under the practical conditions.


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The superior performance of FG can be attributed to the p-type doping effect of fluorination. In the pristine graphene sensor, NH3 is physisorbed onto the graphene with nitrogen on the center of six-membered ring of carbon matrix.11,70 To better understand the adsorption performance of gas molecules on FG and the absorption mechanism of FG, the DFT calculations were carried out. Compared with NH3 on graphene (Figure S5A), the adsorption energy of NH3 on FG is enhanced and the distance between NH3 and FG decreases slightly, as shown in Table 1. FG with CF0.25 ratio shows three times stronger adsorption energy than graphene. The most stable adsorption position of NH3 on FG is similar as graphene, as shown in Figure 6A. The Eads value of FG is ~0.35 eV for CF0.25. When the C/F ratio were set 0.125, the Eads did not change much with respect to CF0.25 due to the adsorption position is on the top of C atoms. The electronic density distribution plot (Figure 6B) indicates physi-sorption between NH3 and FG. Finally, the DOS of FG after NH3 adsorption shows the significant change, in comparison with NH3 on graphene. One sharp peak emerged in the valence state of FG due to the PDOS from N 1s orbitals after NH3 adsorption for FG. The NH3 adsorption will diminish the opening-up band gap of FG, as shown in Figure 6C and Figure 6D. Besides, the delocalization of FG will be broken at the site of NH3 adsorption from the HOMO and LUMO calculation, as shown in Figure S5C and S5D. The discrete delocalization of FG after NH3 adsorption will result in the increased resistance of FG during the ammonia gas detection. Based on DFT simulation results, we attribute the enhanced gas sensing performance of FG to the opening-up band gap and enhanced NH3 physical adsorption effect.

4. CONCLUSIONS


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The fluorination of monolayer CVD-graphene has been realized by SF6 plasma treatment. The different fluorine content in FG can be controlled by the plasma treatment time. XPS and NEXAFS results provide the state of C-F bonds in the FG. It is found that the defects increase and planar matrix is distorted in FG after the short plasma treatment time. However, the planar configuration of graphene can be recovered by further treatment. The reduction in fluorine content is contributed to the less-stabilized fluorine group that is broken down in the continuous fluorination treatment. The FG based gas sensor exhibits fast response/recovery behavior and high sensitivity of detecting 2 ppm ammonia and 100 ppm with ~3.8% response in 30 s at room temperature. The fluorine species and defects in FG play key role in the function of gas sensor. The excellent sensing performance of FG is attributed to the stronger physic adsorption between the FG and NH3. High performance of FG gas sensor for ammonia gas at room temperature explores the feasibility for practical commercial applications of the ammonia gas detection.

ASSOCIATED CONTENT Supporting Information. Additional characterization and performance of FG were listed. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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‡These authors contributed equally. ACKNOWLEDGMENT The work was supported by Natural Science Foundation of China (NSFC) (Grant No. 91333112, U1432249), the Priority Academic Program Development of Jiangsu Higher Education Institutions. This is also a project supported by Collaborative Innovation Center of Suzhou Nano Science & Technology and sponsored by Qing Lan Project. The work at Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Dr. Lee Jenn-Min and Dr. Chen Jin-Ming for their kind support of experiments at TLS.

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Figures and Captions

Figure 1. Schematic diagrams of the synthesis process for fluorinated graphene (FG) and fabrication of FG gas sensor. The monolayer graphene was first grown on a Cu foil, and then transferred on SiO2 substrate following by plasma treated at different reaction times ranging from 10s to 90s. After that, two Au/Ti electrodes were deposited on the FG film to fabricate the electro-resistor type gas sensor with 50 µm gap in which the change of resistance with exposure to the target gas could be measured directly.


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Figure 2. (A) AFM image of FG-20 with the average height of FG. (B) Raman spectra of monolayer graphene and fluorinated graphene (FG) with different plasma exposed time. (C) and (D) Peak ratios of I(D)/I(G) and I(2D)/I(G) peaks for the monolayer graphene as a function of exposure time of SF6 plasma.


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Figure 3. XPS investigation of FG sample. (A) C 1s high-resolution spectra of FG-20, the peaks at 288.5 eV and 291.7 eV are related to C-F groups. (B) F 1s XPS spectra, indicating the covalent bonding of C-F. (C) Response of the fluorine content at different plasma treatment time. (D) Stability of FG-50 stored in dry air over several days after fluorination, determined by XPS. The F atomic concentration is dropped slightly from 13.16% to 10.55% in 25 days.


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Figure 4. NEXAFS investigation of (A) C K-edge of FG-20, FG 90 at the incident angle of 15° and (B) F K-edge of FG-90 at the incident angles of 75°, 45° and 0°. (C) The comparison of C K-edge and F-edge NEXAFS for FG-90.


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Figure 5. Response of (A) the pristine monolayer graphene based gas sensor and (B) FG-20 devices for different NH3 gas concentration in dry air at room temperature. (C) Repeatability of FG-20 device for 100 ppm of NH3 at room temperature. (D) Fluorine concentration and response of FG sensors with different plasma treatment time for 100 ppm NH3 gas at room temperature.


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Figure 6. DFT calculations of NH3 adsorbed on FG. (A) the most stable adsorption configuration of NH3 on CF0.25 ; (B) the electronic density distribution of NH3 on CF0.25; density of state for FG and FG(C) after NH3 adsorption (D).


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Table 1. The adsorption energy (Eads) and height for NH3/CO on top of graphene and FG, including CF0.25 and CF0.125 configuration. CF0.25 CF0.125 Graphene Eads (eV)

height (A)

Eads (eV)

height (A)

Eads (eV)

height (A)

NH3

0.35

2.88

0.34

2.90

0.11

3.06

CO

0.14

3.13

0.12

3.13

0.08

3.13


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Table of Contents


33

Spectroscopic Investigation of Plasma-Fluorinated Monolayer

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