Electrochemically Exfoliated Graphene as Solution-Processable, Highly-Conductive Electrodes for Organic Electronics

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1 Supporting Information Electrochemically Exfoliated Graphene as Solution-Processable, Highly-Conductive Electrodes for Organic Electronics Khaled Parvez, Rongjin Li, Sreenivasa Reddy Puniredd, Yenny Hernandez, Felix Hinkel, Suhao Wang, Xinliang Feng, *, Klaus Müllen * Max Planck Institute for Polymer Research, Ackermannweg 10, D Mainz, Germany School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, , Shanghai, P. R. China Experimental details: Electrochemical exfoliation process: Natural graphite flakes (Sigma Aldrich) were used as carbon source (working electrode) for electrochemical exfoliation. The graphite flakes were adhered on a conductive carbon tape to form a pellet and then immersed into the H 2 SO 4 solution (Sigma Aldrich; 95-97%). A Pt wire was used as a counter electrode. The electrochemical exfoliation of graphite was carried out by applying positive voltage (+10 V) on the working electrode. The EG was then collected with a polytetrafluoroethylene (PTFE) membrane filter (pore size 0.2 µm) and washed repeatedly with DI water by vacuum filtration. The resultant EG was dispersed in N,N'- dimethylformamide (DMF) by sonication at low power for 10 min. The dispersion was kept for 24 h for the precipitation of un-exfoliated graphite flakes and/or particles. The top part of the dispersion was used for further characterization and device fabrication. Electrical characterizations of EG: Thin EG films for field-effect transistors (FETs) and resistance measurements were prepared by Langmuir-Blodgett (LB) assembly. EG dispersions (0.25 mg/ml) in 1:3 DMF/chloroform mixture was carefully dropped on the S1

2 water surface using a glass syringe. Drop wise addition of 5 ml EG dispersion resulted in a faint black colored film on water surface. Afterwards, the film was compressed by LB trough barriers while the surface pressure was monitored by a tensiometer. The EG layer was collected by vertically dip-coating the silicon substrates with 300 nm SiO 2 layer. The samples were annealed at 200 C for 30 min to remove residual solvent. A 60 nm thick Au was evaporated on top of the EG films through a mask to formulate S/D electrodes. To fabricate FETs based on single EG sheet, 100 nm thick platinum (Pt) was deposited by focused ion-beam (FIB) to connect the isolated graphene sheet with Au electrodes. The sheet resistance (R s ) of the single EG sheet was measured by two point probe method on the same device using the Ohm s law (R s = RW/L, where R is the resistance at 0.5V; W and L are the graphene width and channel length, respectively). The field-effect mobility was extracted from the slope ( I d / V g ) of the linear regime of the transfer curves using the equation, µ = (L/WC i V d ) ( I d / V g ), where L and W are channel length and width and C i is the capacitance. 1 Organic field-effect transistors (OFETs) fabrication: Graphene based S/D electrodes were prepared by vacuum filtration of EG dispersion (~ 0.60 mg/ml in DMF) through a PTFE membrane and the subsequent transfer of the EG film on SiO 2 /Si substrate. The thickness of the EG film was adjusted by controlling the filtration volume. The substrate was then annealed at 200 C for 30 min to evaporate the solvent. A 60 nm thick aluminum (Al) layer was thermally evaporated on top of the dried EG film on SiO 2 /Si substrate through a patterned mask. The substrate was then exposed to an O 2 -plasma chamber (Plasma system 200) for 10 min with 200 sccm O 2 flow, 300 W radio frequency (RF) power. Finally, the patterned EG electrodes were obtained by wet etching Al in 10% S2

3 HNO 3 solution. For the fabrication of Au-based S/D electrodes, a 50 nm thick Au layer was thermally evaporated on the SiO 2 /Si substrates. The channel length of the fabricated EG electrodes was 50 µm (L/W = 1/18). The p-doped silicon (p-si) below 300 nm SiO 2 layer served as the gate (capacitance C i = 11 nf/cm 2 ). The patterned EG and/or Au electrodes were then treated with hexamethyldisilazane (HMDS). The organic semiconductor material 4H-cyclopenta[1,2-b; 5,4-b ]dithiophenebenzo[c][1,2,5]thiadiazole (CDT-BTZ-C16) (M n = 10,000 g/mol) was synthesized according to our previously published report. 2 Finally, CDT-BTZ-C16 (2 mg/ml in o- dicholorobenzene) was drop-casted on hexamethyldisilazane (HMDS) treated substrates and subsequently treated at 200 C for 1h in nitrogen atmosphere. Flexible OFETs based on EG S/D were prepared by patterning the electrodes on PTFE membrane. First, EG dispersion was filtered through the PTFE membrane and then dried at 150 C for 30 min. Instead of transferring onto the substrates, EG electrodes were patterned on PTFE membrane by the similar method described above (i.e. Al evaporation, plasma treatment, metal etching etc.). The patterned EG film (channel length, L = 50 µm) was then transferred to HMDS treated PET substrates by mechanical press. Top-gate, bottom contact FETs were thus constructed by spin coating CDT-BTZ-C16 (2 mg/ml in CHCl 3 ) on the substrate and subsequent annealed at 150 C for 30 min. Afterwards, a dielectric layer of ~560 nm thick PMMA (15,000 g/mol, 12 wt% in toluene) was deposited by spin-coating at 1000 rpm for 30s (C i = 5.06 nf/cm 2 ) and dried in a vacuum oven at 80 C for 4 h. Finally, 30 nm thick Au was evaporated on top of the PMMA layer and used as a gate electrode. S3

4 Hole mobilities (µ) of all OFETs were calculated in the saturation regime by the following equation: Where, C i is the capacitance per unit area of the gate dielectric and V T is the threshold voltage. Characterization: The dimensional size of EG sheets was investigated by tapping mode AFM (Dimension 3100CL) and HRTEM (Philips Tecnai F20). Raman spectra were recorded with a Bruker RFS 100/S spectrometer. The Chemical composition was analyzed by XPS with an Omicron Multiprobe spectrometer using Al Kα radiation. Kelvin probe force microscopy was carried out using a PPP-EFM (Nanosensors). The cantilevers with a nominal resonance frequency 70 khz and a Pt/Ir coated were used. The work function of the tip (ϕ tip = ± ev) was calibrated on freshly cleaved highly ordered pyrolytic graphite (HOPG). The sheet resistance of EG films was measured with a four-point probe system (Keithly 2700 Multimeter). All the FET measurements were carried out inside a dry nitrogen glovebox with a Keithly 4200 semiconductor parameter analyzer. S4

5 Figure S1: Photographs show large scale production of EG where, (left) graphite electrode used for elecrochemical process and (right) dispersed EG (0.70 mg/ml) after exfoliation. Figure S2: Photographs of graphite electrodes (a) before, (b) and (c) after electrochemcial process in pure H 2 SO 4 and CH 3 COOH : H 2 SO 4 (1:1) electrolytes, respectively. A bias voltage of + 10V was kept constant for 2h in each case. S5

6 Figure S3: AFM image of (a) EG sheets on SiO2 substrates prepared by LangmuirBlodgett (LB) assembly and, (b) bilayer EG. Inset of (b) is the height profile of EG sheet. Figure S4: TEM images of EG sheets. S6

7 Figure S5: XPS survey spectra of EG. Figure S6: Optical micropscopic images of the transferred EG film on PET substrates where, (a) and (b) are 15 nm; (c) and (d) are 25 nm thick films with different magnifications (50 and 100x), respectivcely. S7

8 Figure S7: Sheet resistance of nitric acid (65% HNO 3 ) treated EG films. Figure S8: SEM images of (a) transferred and (b) patterned EG film on SiO 2 /p-si substrates. S8

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10 Figure S10: (a) Transfer and (b) output curves of flexible OFETs with EG electrodes (L = 50 µm, W = 900 µm) at a source-drain bias V SD = -60 V. The field-effect mobility of the flexible OFET device was calculated to be cm 2 /Vs, with a current on/off ratio of 10 4 (Figure S6). The relatively low performance of the flexible device compared to the SiO 2 /Si based devices might be caused by the poor interface between organic semiconductor and PMMA dielectric layer. S10

11 References: 1. Su, C. Y.; Lu, A. Y.; Xu, Y.; Chen, F. R.; Khlobystov, A. N.; Li, L. J. High- Quality Thin Graphene Films from Fast Elecrochemical Exfoliation. ACS Nano 2011, 5, Zhang, M.; Tsao, H. N.; Pisula, W.; Yang, C.; Mishra, A. K.; Müllen, K. Field- Effect Transistors Based on a Benzothiadiazole-Cyclopentadithiophene Copolymer. J. Am. Chem. Soc. 2007, 129, S11