Supporting Information. Exfoliation of Non-Oxidized Graphene Flakes for Scalable Conductive Film

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1 Supporting Information Exfoliation of Non-Oxidized Graphene Flakes for Scalable Conductive Film Kwang Hyun Park, Bo Hyun Kim, Sung Ho Song, Jiyoung Kwon, Byung Seon Kong, Kisuk Kang, and Seokwoo Jeon *, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon , Republic of Korea. KCC Central Research Institute, Gyunggi-do, , Republic of Korea. Department of Materials Science and Engineering, Seoul National University (SNU), Seoul , Republic of Korea. AUTHOR ADDRESS Kwang Hyun Park: Seokwoo Jeon: 1

2 S1.0. Experimental S1.1. Materials S1.2. Methods S1.3. equipment & techniques S2.0. Characterizations S2.1. Dispersion of graphene flakes in solvents with varying polarity index S2.2. XPS depth profiling analysis of oxygen S2.3. Characterizations of potassium intercalated graphite compound S2.4. TGA analysis of graphite, EGIC, and ES S2.5. HR-TEM images of graphene flakes S2.6. XPS analysis of graphene flakes S2.7. AFM images of graphene flakes according to different thicknesses S3.0. Graphene based conducting film S3.1. Modification of glass surface S3.2. Optical and morphological characterizations of graphene thin film S3.3. Electrical characterizations of graphene thin film S3.4. Electrical conductivity of graphene thin film 2

3 S1.0. Experimental S1.1. Materials Three different components (ZnCl 2 ( 98%, Aldrich), KCl (99%, Aldrich), and NaCl (99%, Aldrich)) were selected in order to prepare the eutectic salt. Graphite powder was purchased from Bay Carbon, Inc. (SP-1 graphite powder). For the solvents Hexane, Toluene, Tetrahydrofuran (THF), Ethanol (EtOH), Pyridine, N-Methyl-2- pyrrolidone (NMP), and Dimethyl sulfoxide (DMSO) were chosen according to different polarities and were purchased from Sigma - Aldrich. S1.2. Methods All the chemicals were dried at 110 C for more than 12 hrs to remove residual moisture and impurities. Three components, KCl:NaCl:ZnCl 2 (0.2:0.2:0.6, in mole fraction), were mixed and kept at 250 C for 30 min in a Teflon vessel. Then 9 g of the eutectic salts (ES) was grinded and mechanically mixed with 1 g of graphite powder. All mixing processes were conducted in glove box under Ar atmosphere. Each of the mixed samples was heated up to 210, 250, 300, and 350 C with heating rate of 10 C/min in vacuum and kept for 10 hrs (Model 4540, Parr instrument Inc.). The prepared samples are named as EGIC (210 C), EGIC (250 C), EGIC (300 C), and EGIC (350 C) according to operation temperature. Then, 100 mg of each of the four EGICs was put into 100 ml of the selected solvents and the mixture was mildly sonicated for 30 min in order to improve the exfoliation of graphene flake and to enable a stable dispersion in solvents. To measure the yield of graphene flakes, the dispersed graphene flakes were filtered out by using an anodic aluminum oxide (AAO, 0.1 µm pores, Whatman) filter and washed with warm distilled water (ph2). We carefully avoid 3

4 sedimented graphene flake during filtration process. After drying for 24hrs, the yield of graphene flakes was discreetly measured on the basis of weight change. The KC 8 compound was prepared at 210 C for 24 hrs by direct evaporation of K metal. S1.3. equipment and techniques The structural property of all the samples was analyzed by X-ray diffraction (D/MAX-2500 (18kW) with Cu Kα radiation (λ=1.518 Å). The surface functional groups of the samples were measured by X-ray photoelectric spectroscopy (XPS, Sigma Probe). Besides, the atomic ratio between the elements was analyzed from a depth profile conducted by an argon ion beam (accelerating voltage: 4 kev, Ion beam current: 2 µa). Thermal gravimetric analysis (TGA, G 209 F3) of ES, pristine graphite, EGIC, was conducted up to 1000 C at a heating rate of 10 C/min under N 2 atmosphere. Raman spectroscopy of all the samples was measured using a high resolution dispersive Raman microscope (LabRAM HR UV/Vis/NIR, excitation at 514nm). Also, morphology of the graphene flake was analyzed by atomic force microscopy (AFM, SPA400, SII, Japan), field emission scanning electron microscope (FE-SEM, Philips XL30SFEG) and field emission transmission electron microscopy (FE-TEM, Tecnai G2 F30). A four point probe was used to measure sheet resistance of the graphene flake thin film. S2.0. Characterizations S2.1. Dispersion of graphene flakes in solvents with varying polarity index Fig. S2.1 shows digital images of the dispersed graphene flakes in various solvents showing different polarity indices, as a function of time (30 min, 24 hrs, and 6 4

5 months). Initially, the solvents except tetrahydrofuran (THF), toluene, and hexane show good dissolution. Up to 6 months, the pyridine shows the best dissolution of both eutectic salts (ES) and the graphene flakes. It represents that the EGIC synthesized by insertion of intercalants facilitates introduction of the solvents into them with entropy gain without any degradations of graphene flaks. Fig. S2.1. Dispersion characteristics of graphene flakes and ES with polarity indexes. S2.2. XPS depth profiling analysis of oxygen To examine the oxidation of in-plane graphene in the EGIC, XPS depth profiling analysis was conducted using an argon ion beam (accelerating voltage: 4 kev, Ion beam current: 2 µa). The EGIC mainly consists of carbon, potassium, sodium, zinc, and chloride elements. We observe that the atomic percentage of oxygen stays at 0% with no fluctuation during the 22 etching stages (etching depth: ~ 220 nm) whereas the atomic percentage of carbon is increased ~ 3 times. It indicates that the oxygen element 5

6 is not introduced in this EGIC system. Fig. S2.2. Depth profile of oxygen and carbon conducted by etching of argon ion beam (accelerating voltage: 4 kev, Ion beam current: 2 µa). S2.3. characterization of potassium intercalated graphite compound Fig S2.3 illustrates the structural property and chemical compositions of KC 8 and KC 24 formed by the insertion of potassium atoms in graphite crystal. Fig. S2.3a schematically illustrates stage1 (KC 8 ) of the potassium intercalated graphite compound. Fig. S2.3b presents that the color of KC 8 significantly changes in both optical and digital image (Inset). XRD patterns in Fig S2.3c show phase transitions caused by crystallographic alloying of potassium atom in graphite crystal. The main peaks (d 004 and d 008 ) of KC 8 are observed at 16.1º and 33.1º, respectively while the peaks (d 002 and d 003 ) of KC 24 are dominant at 19.9º and 30.3º, respectively. 1 Moreover, XPS depth profiles (Fig S2.3d) demonstrate that the KC 8 and KC 24 compounds form by intercalation of potassium atom. It confirms that our results are very consistent with previous works. 2 6

7 Fig. S2.3. Characterizations of KC 8 a, Schematic of KC 8. b, Optical microscopy (OM) and digital image (Inset). c, XRD patterns of KC 8 and KC 24. d, XPS spectra of KC 8 and KC 24. S2.4. TGA analysis of graphite, EGIC, and ES Thermal gravimetric analysis (TGA) of graphite, EGIC, and ES in Fig. S2.4 was carried out to study correlations between thermal stability and intercalation. When all the samples are heated up to 1000 ºC at N 2 atmosphere, the weight losses for graphite, EGIC, and ES are observed to be 8%, 79%, and 100%, respectively. The weight of the ES steeply decreases in the range of ºC. It indicates that the ES is remarkably declined the thermal stability above 400 ºC due to decomposition by itself. On the other hand, considering finally remaining quantity (~ 21%) and the weight loss 7

8 curve of the EGIC which contains ES 90 wt% and graphite 10 wt%, its thermal stability is significantly improved compared to those of graphite and ES. It indicates that the EGIC possesses stronger interactions in between graphene layers by introduction of intercalant (dominantly potassium) than original graphite and the ES. Fig Thermal gravimetric analysis (TGA) of graphite, EGIC, and ES. S2.5. HR-TEM images of graphene flakes Representative HR-TEM images and diffraction patterns of a single and few layers of graphene flakes are shown in Fig S2.5. The thicknesses of the graphene flakes can be clearly discriminated by the lattice fringe of graphene edges and the intensity identification of I (0-110)/ I (1-210). 8

9 Fig HR-TEM images and diffraction patterns of single layer and a few layers graphene flakes S2.6. XPS analysis of graphene flakes Fig. 2.6 shows that the detail elemental compositions of the graphene flake were analyzed by XPS. The graphene flake exfoliated from pristine graphite mainly consists of carbon (97.9% 96.6%) and oxygen (2.10 % 2.90%). The increased oxygen content of the graphene flake is mostly ascribed to air or moisture during the exfoliation process or adsorbed on the surface of the graphene flake. Other elements such as potassium (0%), sodium (0.18%), chloride (0.10%), and zinc (0.23%) are ignorable. 9

10 Fig Element compositions of graphene flakes analyzed by XPS S2.7. AFM images of graphene flakes according to different thicknesses Representative AFM images of the prepared graphene flakes are shown in Fig S2.7. The topology and friction profiles suggest that that the graphene flakes have different shapes, thicknesses, and sizes. The graphene flakes having layers (a), 3-4 layers (b), and 1-2 layers (c) show thicknesses of ~ 6.0, ~ 2.0, and 1.0 nm, respectively. 10

11 Fig Representative AFM images of graphene flakes a, layers. b, 3-4 layers. c, 1-2 layers S3.0. Graphene based conducting film S3.1. Surface modification of glass substrate To improve the adsorption affinity between the graphene flake and glass substrate, the glass surface was modified by 3-aminopropyltriethooxysilane (APTES). First, the glass surface was cleaned using ethanol and acetone for 30 min with mild sonication. After drying, the glass substrate was treated with oxygen plasma (100 W in 11

12 power, for 4 min at 30 mtorr with 100 sccm O 2 flow rate). Then, it was transferred to the desiccator and stored for 1 hrs to enable the self-assembly of APTES. The modified glass substrate was baked at 110 ºC for 1 hrs. S3.2. Optical and morphological characterizations of graphene thin film The optical and morphological properties of the graphene thin film are analyzed by a He-Ne laser (Uniphase 110p, λ= nm, and 1 mm beam spot size), FE-SEM, and AFM. Fig. S3.2a shows that the transparency of the graphene films is decreased with increasing film thickness. Also, the histogram shows a high uniformity of transmittance with less than 1% deviation within same sample regardless of the film thickness (Fig. S3.2b). To examine the surface morphology, representative SEM images of thin (~ 80% transmittance) and thick (~ 60 % transmittance) graphene films are shown in Fig. S3.2c where the graphene flakes are uniformly stacked on the substrate without considerable folds and wrinkles. These continuous connections offer a low sheet resistance through the electrical pathway. In addition, the surface roughness and thickness are measured by AFM (Fig. 3.2d). A 3D view and 2D topological image of AFM show the roughness of the graphene film (RMS: 15 nm, Maximum height from peak to valley: ~ 50 nm excluding thick graphite flakes) and the histogram is a statistical analysis of the thickness. The inset of histogram graph is the profile of the thickness (Film thickness: ~ 10 nm). 12

13 Fig. S3.2. Conducting graphene flake thin film a) Digital images with increasing thickness of graphene flake thin film (measuring point, red circle). b) Transparency statistics of graphene flake thin film analyzed by He-Ne laser (N i =12, ε=1%, respectively). c, d) Morphological characterizations measured by FE-SEM and AFM. S3.3. Electrical characterizations of graphene thin film The sheet resistance of graphene films is measured by a four-point probe (CMT-SR 1000N, AIT) with a separation distance of 1 mm between probe tips. Fig. S3.3a shows the four-point probe apparatus and representative sheet resistances of 13

14 graphene films having different transmittances (78% and 49%). Fig. S3.3b shows the average sheet resistance data measured from the thin films separately prepared from 22 different batches. The sheet resistance decreases from a few kω/ to a few hundred Ω/ with decreasing transparency of the graphene film. The error bar of each data point represents the standard deviation of resistance obtained from a statistic calculation with ~ 6 measuring points at each sample. The overall results for electrical properties shown in Fig. S3.3b support the results described in the main text. Fig. S3.3. Electrical characterizations of the conducting graphene thin film a) Digital images for Four-point probe (CMT-SR1000N, AIT) analysis and the sheet resistances measured on reprehensive graphene flake thin films (78% and 49%). b) Statistical data for sheet resistance and transparency at laser nm of graphene films. S3.4. Conductivity of graphene flake thin film 14

15 Fig. S3.4. Conductivity of graphene flake thin film with annealing treatment Reference (1) Charlier, A.; Charlier, M. F.; Fristot, D. J. Phys. Chem. Solids 1989, 50, (2) Viculis, L. M.; Mack, J. J.; Mayer, O. M.; Hahn, H. T.; Kaner, R. B. J. Mater. Chem. 2005, 15,