The Electrical Properties of Graphene Modified by Bromophenyl Groups

CARBON 50 (2012) 1517– 1522

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The electrical properties of graphene modiﬁed by bromophenyl groups derived from a diazonium compound

Xiaochen Dong a, Qing Long a, Ang Wei a, Wenjing Zhang c, Lain-Jong Li c, Peng Chen b,*, Wei Huang a,* a Key Laboratory for Organic Electronics and Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210046, China b School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457, Singapore c Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan

ARTICLE INFO ABSTRACT

Article history: Graphene field-effect transistors were fabricated with mechanically exfoliated single-layer Received 30 June 2011 graphene (SLG) and bilayer graphene (BLG) sheets and the functionalization effects of Accepted 15 November 2011 bromophenyl groups derived from a diazonium compound on its transfer properties were Available online 25 November 2011 explored. Spectroscopic and electrical studies reveal that the bromophenyl grafting imposes p-doping to both SLG and BLG. The modification of SLG by bromophenyl groups significantly reduces the hole carrier mobility and the saturation current in SLG transistors, suggesting an increase in both long-range impurity and short-range defect scattering. Unexpectedly, the bromophenyl group functionalization on BLG does not obviously increase both types of scattering, indicating that the BLG is relatively more resistant to charge- or defect-induced scattering. The results indicate that chemical modification is a simple approach to tailor the electrical properties of graphene sheets with different numbers of layers. 2011 Elsevier Ltd. All rights reserved.

1. Introduction diazonium salts [17–22]. Recently, experimental reports have shown that graphene can be chemically functionalized by Graphene is a one-atom-thick sheet of carbon atoms arranged 4-nitrobenzene diazonium salts, where the nitrophenyl in a honeycomb lattice [1]. It has drawn great attention in groups covalently attach on graphene and result in the sp2 many areas of science and technologies, due to its perfect to sp3 transition of graphene carbon atoms [23–26]. The conse- two-dimensional nanostructure and unique electrical proper- quence of nitrophenyl modification is to largely suppress the ties [2–6]. The electrical properties of graphene are known to minimum conductivity of graphene layers. By contrast, the be sensitive to the charge impurities, doping level and defects 4-bromophenyl cation, which binds to graphene surface [7–9]. Chemical doping is an effective method to introduce through partial charge transfer [9], introduces relatively stable charge impurities or defects to graphene layers, which in turn p-doping to single-layer graphene (SLG) without impairing the can modulate the electrical properties of graphene [10,11]. minimum conductivity of SLG. Therefore, bromophenyl mod- Previous work has shown that aryl diazonium treatment of ification is an appealing way to tailor the electrical properties single-wall carbon nanotubes (SWCNTs) [12–16] produces of graphene. In addition to SLG, it is worth discovering the functionalized SWCNTs. Likewise, substituted aryl groups effect of bromophenyl modification on bilayer graphene can be readily anchored onto other carbon surfaces such as (BLG) due to that the electronic structure of BLG is distinct graphite and glassy carbon by electrochemical reduction of from that of SLG. SLG exhibits a linear electronic dispersion

* Corresponding authors: Fax: +65 65141086. E-mail addresses: [email protected] (P. Chen), [email protected] (W. Huang). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.11.029 1518 CARBON 50 (2012) 1517– 1522

and a vanished density of states near the Fermi level, whereas BLG has a quadratic low-energy dispersion and finite density of states near the Fermi level. These differences render different electronic transport mechanisms in the linear and high conductivity regimes. However, little is known about the difference in disorder and charged impurities scattering between BLG and SLG. Here, we study the effects of bromophenyl modification on the electrical properties of both SLG and BLG, respectively. Spectroscopic and electrical studies reveal that the bromophenyl grafting imposes obvious p-doing for both SLG and BLG. The bromophenyl grafting modification of SLG significantly reduces the hole carrier mobility and saturation current of SLG transistors, suggesting the enhancement of both long-range impurity scattering and short-range defect scattering. By contrast, the bromophenyl grafting modification of BLG does not obviously lower the hole mobility or the plateau width around charge neutrality point, indicating that BLG is relatively more resistant to scattering from charges or defects brought in by bromophenyl modification.

2. Experimental Fig. 1 – (a) Schematic of the SLG device modified with 4-bromobenzene diazonium salts. (b) Optical micrograph The SLG and BLG films were obtained by mechanical exfoli- (top view) of the SLG device. (c) Raman spectra for the SLG ation of nature graphite flakes (NGS, Germany). The layer film before and after modified with diazonium salts. Inset numbers of the graphene films were determined firstly by shows the chemical structure of diazonium salt. The Raman spectroscopy. The graphene field-effect transistors excitation wavelength of Raman laser is 488 nm. were fabricated by evaporating Au electrodes directly on top of the selected, large-sized and mechanically-exfoliated by the e-beam during the metal evaporation. After the modi- graphene films using hard-masks, where no photoresist fication by diazonium salts, the Raman peak intensity ratio was used to ensure the electrical characteristics were not af- between 2D and G bands, I(2D)/I(G), is significantly decreased. fected by the photoresist or other chemicals necessary for Meanwhile, the frequencies of 2D and G bands are also the electrode patterning. The 4-bromobenzene diazonium slightly up-shifted, suggesting the p-doping in SLG film salts (Sigma–Aldrich, >99%) were directly used without fur- [10,29]. In addition, multiple peaks around the D band ap- ther purifications. For the modification of graphene with dia- peared after the diazonium modification. These peaks indi- zonium salts, the 4-bromobenzene diazonium solution (0.5 cated that the diazonium compounds are present on the or 1 mM in water) was directly dropped on graphene layers surface of graphene flim [9,30]. or devices surface for certain period of time. The substrates To examine whether the presence of graphene defects af- were then thoroughly rinsed with de-ionized water and fects the diazonium salt attachment on graphene, we per- blown dried with N2. Raman spectroscopy (WITec CRM200 formed the Raman spectroscopic mapping for the SLG confocal Raman microscopy system, laser wavelength device. Fig. 2(b) shows the Raman D-band mappings for the 488 nm and laser spot size about 0.3 mm) was adopted to SLG before and after diazonium modification, where the examine the structure of modified SLG and BLG. All electrical map is constructed by integrating the D-band intensity in measurements were performed in ambient condition using a the confocal Raman measurement, with a typical optical res- Keithley semiconductor parameter analyzer (model 4200- olution 0.3 mm. Fig. 2(a) and (c) respectively shows the SCS). Raman spectra taken at selected sites on the device before and after diazonium modification. Fig. 2(a) shows that defect 3. Results and discussion intensities at the location closer to the gold electrode is higher than that in the middle of the channel. As discussed before, The schematic of the graphene device modified with there is no D-band observed for this SLG film before source 4-bromobenzene diazonium salt and the optical micrograph and drain electrodes are deposited on it. We suspect that of graphene device are shown in Fig. 1(a) and (b), respectively. the defects are produced by the e-beam used for electrode Fig. 1(c) shows the Raman spectra of the SLG (taken at the de- deposition, and this observation is consistent with the fact vice center) before and after modified with the diazonium that lower defect density was found at the center of the chan- salt. Two pronounced peaks (G band at 1585 cmÀ1 and 2D nel which is far from the electrode area (the area directly ex- band at 2696 cmÀ1) are observed for pristine SLG film, consis- posed to e-beam). By comparing the Fig. 2(a) and (c), it is tent with the previously reported [27,28]. It is noted that ini- concluded that the attachment of 4-bromobenzene diazo- tially the D band (1354 cmÀ1) does not exist and it only nium salt is in favor of the disordered site on SLG surfaces. appears after the deposition of electrodes, which is likely This phenomenon is even more obvious for BLG. Fig. 3 shows attributed to the disorder on graphene basal plane caused the parallel Raman experimental results performed on a BLG CARBON 50 (2012) 1517– 1522 1519

Fig. 2 – (a) Raman spectra of a SLG device before diazonium modification at different locations (labeled as 1, 2, 3 and 4). (b) 2-Dimensional D-band intensity mappings for the SLG device before (Top) and after (Bottom) diazonium modification. The map is constructed by plotting the D-band intensity as the map height. S and D represent the source and drain electrodes of the device. (c) Raman spectra of the SLG device after modified with diazonium salts at the corresponding sites. device, and the results clearly show that the 4-bromobenzene cations are able to create short-range or long-range scatters diazonium salt cannot attach on the no defective sites. on graphene. It is observed from Fig. 4(a) that the saturated

Fig. 4(a) shows the conductivity verse gate voltage (Vg) be- hole conductivity for the SLG device decreases with the fore and after exposure to the diazonium salt for different increasing time of diazonium exposure, suggesting that the periods of time. The transfer curves (conductivity vs. Vg) shift diazonium cations introduce short-range disorders on SLG. to more positive gate voltage after exposure to diazonium Noted that experiments and theoretical calculations for SLG treatment, indicative of p-doping on SLG. And the diazonium have shown that the saturated conductivity is dominated by salt does not signiﬁcantly lower the conductivity at the charge short-range disorders and the linear conductivity is domi- neutrality point (VNP; also minimum conductivity point), nated by long-range charged impurities [7,31,32]. Fig. 4(b) sum- which suggests that this salt does not produce appreciable marizes the relation between the exposure time of diazonium sp3 hybridization (covalent attachment) on graphene surfaces. salt with the device hole mobility. The hole mobility from the

The observation agrees with the proposal that bromophenyl transfer curves is calculated on the basis of the slop DId/DVg cation binds to the graphene surface through partial charge ﬁtted to the linear regime with the equation l = DId/DVg · transfer but not covalent bonding [9]. These results are in clear (L/W) · (1/CoxVd), where Cox is the capacitance of the dielectric contrast to those obtained for nitrobenzene diazonium salts, layer; L and W are the channel length and width of the graph- where the nitrophenyl groups are found to covalently react ene device [33]. The hole mobility (also termed as constant with graphene surfaces [24,25] and result in obvious conduc- mobility) was extracted from the linear conductivity regime tivity decrease at VNP. Since the bromophenyl cations have (linear region in Fig. 4a). It is observed that the hole mobility been recognized to bind to the graphene surface through decreases obviously with the increase in exposure time of dia- non-covalent bonds, it is interesting to discuss the effects of zonium salts on SLG, presumably due to the increase of charge the presence of bromophenyl cation on SLG concerning carrier impurities on SLG [7,31,32]. Therefore, from the above results scattering. In particular, it is still unknown whether the we conclude that the modiﬁcation of SGL with the diazonium

Fig. 3 – (a) Raman spectra of BLG device before diazonium salt modification at different locations (labeled as 1, 2, 3, and 4). (b) 2-Dimensional D-band intensity mappings for the BLG device before (Top) and after (Bottom) diazonium modification. S and D represent the source and drain electrodes of the device. (c) Raman spectra of the BLG device after modified with diazonium at the corresponding sites. 1520 CARBON 50 (2012) 1517– 1522

Fig. 4 – (a) The conductivity (r) vs. gate voltage (Vg) curves for the SLG device before and after modiﬁed by diazonium salts with different reaction time. (b) The relationship between reaction time and hole mobility.

Fig. 5 – (a) The conductivity (r) vs. gate voltage (Vg) curves for the BLG device before and after modified by diazonium salts with different reaction time. (b) The relationship between reaction time of and hole mobility. salt can induce many long-range scatters (charge impurities) SLG and BLG could be attributed to the differences in their as well as short-range disorders on SLG. density-of-states and the screening effect by the additional For BLG devices, the p-doping from diazonium salts is also graphene layer in BLG [40]. In addition, it has been theoreti- observed similar to SLG. Fig. 5(a) plots the conductivity vs. Vg cally demonstrated that the effect of long-range scattering for a BLG device before and after modification with the diazo- (e.g., induced by charged impurities) on carrier mobility is nium salts and Fig. 5(b) shows the relation between the con- weakened in BLG in comparison with the case of SLG [36]. stant hole mobility and diazonium exposure time. The This is consistent with our observation that diazonium mod- results show that the hole mobility is nearly unchanged with ification significantly reduced the hole mobility of SLG, diazonium exposure time, and the change of plateau width of whereas it did not obviously affect that of BLG. It is also con- minimum conductivity is not obvious. The theoretical calcu- ceivable that, since diazonium molecules only attach on the lations have predicted that the plateau width of minimum top layer of BLG, the scattering effect from the charged diazo- conductivity increases with the density of charged impurities nium molecules may be effectively screened by the bottom in BLG [34–38]. And the constant mobility regime in BLG is layer of BLG. governed by both short-range disorders and charged impurities [34–38]. The small change of plateau width indicates that 4. Conclusions the modification of BLG by diazonium salts does not introduce significant amount of charged impurities. Furthermore, SLG and BLG devices using mechanically exfoliated graphene the unchanged mobility together with the unchanged plateau layers were fabricated and functionalized with width suggests that the diazonium modification does not en- 4-bromobenzene diazonium salts, respectively. The electrical hance the short-range disorders. measurements suggest that the modification shows obvi- BLG exhibits distinct electronic structure (thus properties) ously different effects on SLG and BLG devices. The in- to SLG, resulting from the interlayer coupling between the creases of long-range charged impurity scattering and two Bernal AB stacked graphene layers [39]. Consequently, it short-range defect scattering are evident in SLG. However, has been shown that the charge transport in BLG and SLG is these scattering effects are not apparent in BLG devices differently influenced by carrier density, temperature [40], although the BLGs have been found to similarly transfer chemical doping [33], impurities [41], plasma treatment [42]. electrons to diazonium salts (p-doping of BLG). The unex- The observed opposite changes in the carrier mobility in pected electrical properties of BLG after 4-bromobenzene CARBON 50 (2012) 1517– 1522 1521

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