Work-Function Decrease of Graphene Sheet. Using Alkali Metal Carbonates

Transcription

1 Supporting Information Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates Ki Chang Kwon and Kyoung Soon Choi School of Chemical Engineering and Materials Science, Chung-Ang University 221 Heukseok-dong, Dongjak-gu, Seoul , Republic of Korea Buem Joon Kim and Jong-Lam Lee* Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk , Republic of Korea Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk , Republic of Korea Soo Young Kim* School of Chemical Engineering and Materials Science, Chung-Ang University 221 Heukseok-dong, Dongjak-gu, Seoul , Republic of Korea 1

2 Figure SI1(a) shows the change of the sheet resistance of graphene with growth time and HNO 3 doping. The sheet resistance of P-G decreased from to Ω/sq as the growth time was increased from 5 to 60 min. The number of graphene layers was considered to have increased with increasing growth time, thereby decreasing the sheet resistance. It is reported that HNO 3 is one of p-type dopants in graphene. 1 For doping of graphene, P-G was placed in the plastic petri dishes with HNO 3 for 1 h. After HNO 3 doping, the sheet resistance of the graphene decreased to ca Ω/sq, which is higher than the reported value. 1 Therefore, 10 min was selected as the optimum growth time as this gave the lowest sheet resistance. The optimum growth temperature was 950 C as indicated by the lowest sheet resistance, as shown in Fig. SI1(b). The sheet resistance and transmittance at 550 nm of optimized P-G was 1100 Ω/sq and about 97 %, respectively. Figure SI1(c) shows the AFM image of P-G transferred on the SiO 2 /Si wafer. The nm thickness of the graphene film revealed that it consisted of 1-2 sp 2 -hybridized carbon layers. Some wrinkles and ripples were observed on the graphene sheet due to the wet transfer process or the grain boundaries of graphene domains by copper foil substrate. It is reported that grain boundary of copper foil affects the quality of graphene, especially for the mobility. 2 Therefore, the higher sheet resistance of graphene compared to the reported value is considered to come from some wrinkles and ripples. 2

3 [Figure SI1] (a) Change of sheet resistance with growth time and HNO 3 doping. After HNO 3 doping, the sheet resistance of the graphene decreased to ca Ω/sq. Therefore, 10 min was selected as the optimum growth time due to the lowest sheet resistance. (b) Change of sheet resistance with growth time and HNO 3 doping. The optimum growth temperature was 950 C because of the lowest sheet resistance. (c) AFM image of pristine graphene (P-G) sheet. The thickness of the graphene film is about 0.8 ~ 1 nm 3

4 The Raman spectra of n-doped graphene with alkali metal carbonate solutions are shown in Fig. SI2. The G band was shifted to lower wavenumbers of about 1.6, 5.5, 0.5, and 0.5 for Li 2 CO 3, K 2 CO 3, Rb 2 CO 3, and Cs 2 CO 3 -doped graphene, respectively. Electron and hole doping on graphene films have been reported to shift the G band to lower and higher wavenumbers, respectively. 3,4 Therefore, the large peak shift of the G band in graphene indicated the graphene s strongly electron doped states. Moreover, the absence of any discriminative peak shift in the D peak between doped graphene and P-G suggested that the doping method of soaking in alkali metal carbonate does not induce any defects on the graphene surface. [Figure SI2] Raman spectra of n-doped graphene with solutions of four alkali metal carbonates. The slight peak shift of the G band in graphene indicates the strongly electron doped states of graphene. 4

5 [Figure SI3] The UPS spectra of graphene doped with alkali metal carbonate at a concentration of 0.1 M (Li 2 CO 3 is 0.01 M). The onset point of the UPS spectra in pristine graphene (P-G) is ev, suggesting that the work function of P-G is 4.25 ev. The onset points of the UPS spectra were 17.45, 17.6, 17.8, and 17.4 ev for Li 2 CO 3, K 2 CO 3, Rb 2 CO 3 and Cs 2 CO 3 -doped graphene, respectively. These data indicated that the work functions of graphene doped with each metal are 3.75, 3.8, 3.6, and 3.8 ev, respectively. 5

6 [Figure SI4] The UPS spectra of graphene doped with alkali metal carbonate at a concentration of 0.5 M (in case of Li 2 CO 3 is 0.05 M). The onset point of the UPS spectra in pristine graphene (P-G) is ev, suggesting that the work function of P-G is 4.25 ev. The onset points of the UPS spectra were 17.3, 17.7, 17.4, and ev for Li 2 CO 3, K 2 CO 3, Rb 2 CO 3 and Cs 2 CO 3 -doped graphene, respectively. These data indicated that the work functions of graphene doped with each metal are 3.9, 3.5, 3.8, and 4.05 ev, respectively. 6

7 [Figure SI5] The UPS spectra of graphene doped with alkali metal carbonate at a concentration of 1 M (in case of Li 2 CO 3 is 0.1 M). The onset point of the UPS spectra in pristine graphene (P-G) is ev, suggesting that the work function of P-G is 4.25 ev. The onset points of the UPS spectra were 17.4, 17.3, 17.7, and 17.8 ev for Li 2 CO 3, K 2 CO 3, Rb 2 CO 3 and Cs 2 CO 3 -doped graphene, respectively. These data indicated that the work functions of graphene doped with each metal are 3.8, 3.7, 3.5, and 3.4 ev, respectively. 7

8 [Figure SI6] The XPS wide scan data of pristine and n-doped graphene film. Metal ion peaks in each solution were observed in the graphene film doped by alkali metal carbonate solution. This result was evidence for covalent bonding between the functionalized carbon atoms and alkali metal ions. 8

9 [Figure SI7] (a) The change of work function by Cs 2 CO 3 on pristine graphen (P-G) and O 2 plasma treated indium tin oxide (ITO). It is shown that work function decreased from 4.25 ev to 3.4 ev for P-G and from 4.9 ev to 3.7 ev for ITO. It is considered that work function lowering by metal carbonates on graphene and ITO comes from the interfacial monolayer of C-O-Cs complexes which are residues of metal carbonates. (b) The OM image of Cs doped ITO. Some residues of Cs atom were found on the surface of ITO. 9

10 References (1) Lee, S.; Yeo, J.-S.; Ji, Y.; Cho, C.; Kim, D.-Y.; Na, S.-I.; Lee, B. H.; Lee, T. Nanotechnology 2012, 23, (2) Huang, P. Y.;Ruiz-Vargas, C. S.;van der Zande, A. M.; Whitney, W. S.; Levendorf, M. P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y.; Park, J.; McEuen P. L.; Muller, D. A. Nature 2011, 469, (3) Pisana, S.; Lazzeri, M.; Casiraghi, C.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Mauri, F. Nat. Mater. 2007, 6, (4) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; Sood, A. K. Nat. Nanotech. 2008, 3,