Supporting information. Uniform Graphene Quantum Dots Patterned from Selfassembled

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1 Supporting information Uniform Graphene Quantum Dots Patterned from Selfassembled Silica Nanodots Jinsup Lee,,, Kyungho Kim,, Woon Ik Park, Bo-Hyun Kim,, Jong Hyun Park, Tae-Heon Kim, Sungyool Bong, Chul-Hong Kim, GeeSung Chae, Myungchul Jun, Yongkee Hwang, Yeon Sik Jung,*, Seokwoo Jeon*,, Department of Material Science and Engineering, KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon , Republic of Korea Graphene Research Center (GRC), KAIST LG Display R&D Center, 1007, Deogeun-ri, Wollong-myeon, Paju-si, Gyeonggi- do, , Republic of Korea Experimental section 1. Graphene Growth and Transfer Graphene was grown on prepared 25 μm thick Cu foils (Alfa Aesar, item No 13382) in a tube furnace using the CVD method with H 2 (10 sccm) and CH 4 (60 sccm) at 1000 C for 20 min. Thereafter, the system was cooled rapidly to room temperature under H 2 flow (10 sccm). A supporting polymer (950k PMMA) was spin-coated on the graphene to prevent damage from the transfer process. Graphene

2 on the other side of the Cu foil was removed using O 2 plasma. Cu foil was dissolved in an aqueous solution (0.1 M ammonium persulfate), thereby detaching the graphene from the Cu foil. PMMA/graphene films were transferred and dried on an Si/SiO 2 (300nm) substrate and then cleaned with acetone to remove the PMMA supporting layer. 2. Fabrication of GQDs The graphene surface was treated with a hydroxy-terminated PDMS homopolymer (5 kg mol -1, Polymer Source, Inc., Canada) which was 2.0 wt% in Heptane. PDMS homopolymer was spun-cast onto the substrates and annealed at 130 for 2 h. After annealing, washing processes were employed to remove the uncrosslinked PDMS. To achieve a spherical morphology, PS-PDMS BCP (Polymer Source, Inc., Canada) at 28 kg mol -1 PDMS (PS contains 25 kg mol -1, and PDMS 3 kg mol -1 ) and a 10.7% PDMS volume fraction were then spin-coated onto the substrates. PS-PDMS BCP was dissolved in toluene (1.5 wt% polymer solutions). The polymer solution was spin coated onto brush treated graphene and thermally annealed at 150 for 2 hours. Following the self-assembly of the PS-PDMS BCP, the final structure was a monolayer of the PS matrix embedded with silica nanodots. The annealed films were treated with CF 4 plasma (50 W, s) followed by O 2 plasma (60 W, s) to remove the PDMS surface layer and to selectively eliminate the PS layer; this process left silica nanodots (oxidized PDMS) on the graphene. The formed silica dots were used as masks to etch below the PDMS brush layer by CF 4 plasma (50 W, 5 s) and graphene by O 2 plasma (O 2, 40W, 5-8 s). Consequently, the remaining silica dots and the PDMS brush above the GQDs were removed with buffer oxide etchant (BOE) treatment for 10 s. For solution type GQDs, as-prepared GQDs on SiO 2 substrate was soaking in 0.1 wt% Hydrofluoric acid solutions in 2 h. Then, GQD solutions were neutralized by NaOH. To prepare pure GQD solutions and remove unexpected residue or dust, the precipitate in stabilized GQD solutions was removed.

3 3. Characterization The patterned silica particles were observed by a field emission scanning electron microscope (FE- SEM, Hitachi S4800). All GQDs on the Si/SiO 2 substrate were analyzed using AFM (Park System XE150) in tapping mode with Al-coated Si cantilevers. To form an accurate AFM image, the substrate have to be changed Mica instead of SiO 2 /Si substrate. The preparation of GQDs is similar to that using SiO 2. The whole fabrication process was developed onto Mica substrate with the same procedure in Figure 1b. These GQDs on Mica was finely cleaned in a bath with toluene and ethanol several times to reduce the effect of residue. High-resolution transmission electron microscopy (TEM, FEI Company, a Tecnai G2 F30) images were taken at an acceleration voltage of 200 kv. The specimen was prepared by attaching epoxy to the GQD/SiO 2 /Si substrate in order to prevent damage to the GQDs. X-ray photoelectron spectroscopy (XPS. Thermo VG Scientific ESCA 2000) and Raman spectroscopy (SENTERRA, Bruker GmbH, 532 nm) were performed. The photoluminescence of the GQDs was measured with two types of instrument. First, the GQDs on the Si/SiO 2 substrate had very low quantity compared to other thick films of GQDs; accordingly, for the measurements of photoluminescence, a High resolution Micro PL system (Horiba Jobin Yvon LabRAM HR UV/Vis/NIR) was used. Second, for the PL emission spectra of the GQDs at different excitation wavelengths, the sample must be dispersed in solution. A dispersed graphene solution was created by substrate etching process using a buffered oxide etchant. After the substrate was fully etched in a plastic dish, the PH was adjusted to 7 with NaOH solution.

4 Figure S1. Raman spectra for GQDs with silica nanodots at different etching times using O 2 plasma. Figure S2. PL spectra of GQD after additional O 2 plasma treatment with PS-PDMS nanodots for functionalization of surface of (a) 10 nm (b) 20nm GQDs Figure S3. SEM image of large area distribution of GQD before (a) and after (b) graphene etching. The

5 size of GQD is 10 nm. Figure S3 show the SEM image of large area distribution of GQDs synthesized with or without silica nanodots. A light gray particle measured that the uniformity of silica nanodots and their 10 nm size. The result reveals that the round shape nanodots are arranged in hexagonal structure and the dot to dot distance is about 23 nm. An Effective concentration of GQDs in aqueous solution requires assumption of the size of GQDs and distance between nanodots on the substrate. According to SEM image as shown in Figure S3, the number of GQDs in unit area can be estimated as N GQD = 25 GQDs / (115 nm) 2. = 1.9 x 10-3 GQDs / nm 2 Each of the wafers has roughly 25 mm x 18 mm size and the total volume of GQD is estimated as T GQD = (1.9 x 10-3 GQDs / nm 2 ) x 450 mm 2 x nm 2 / mm 2 = 8.5 x GQDs / wafer. The total volume of HF solution when we used to etch SiO 2 substrate measured 50 ml. From this value and T GQD, an effective density of SWNTs can be estimated as D eff = (8.5 x GQDs / wafer) / (50 ml / wafer) = 1.7 GQDs / ml.

6 Figure S4. Raman spectra of each process after graphene etching by O 2 plasma at 20 nm GQD Figure S4 shows the results of polymer (ox-pdms) mask base fabricated GQD PL. As shown ox-pdms mask base fabrication of GQD was successful. Comparing with bare substrate (Graphene substrate and bare SiO 2 /Si substrate) at 392 nm wavelength, a peak is detected that is not shown on the bare GR sample. From this we can conclude that the 392 nm wavelength peak indicates the GQD peak. For further analysis we will explain the various effects on GQD.

7 Figure S5. (a) SEM image of air plasma treated graphene onto SiO 2 from 0 s to 30 min. (b) Raman spectra of air plasma treated graphene (surface treatment time: ts: 0s to 30 min from bottom to top; consistent with (a)). (c) PL spectra of air plasma treated graphene (surface treatment time: ts: 0s to 30 min from bottom to top; consistent with (b)). The morphology of graphene film was investigated by SEM as a function of air plasma treated time.

8 Figure S5a shows SEM images of air plasma treated graphene film without BCP patterning on the Si/SiO 2 substrate. The coverage area of graphene was remarkably decreased from 100% (0 s) to less than 1% (30 min) as increasing air plasma treatment time. The D/G intensity ratio of Raman spectrum increased to 2.5 until 1 min and then subsequently the intensity of G and 2D peak was disappeared due to the damage and defect caused by additional air plasma (Figure S5b). This implies that air plasma treatment inflicted a defect on the graphene and then totally tore the graphene structure after 1 min. Although the shape of Raman peak at 1min is similar to that of GQDs as mentioned at Figure 3c in the manuscript, there is no PL peak, suggesting that unlike the GQD sample as we fabricated in this manuscript the graphene treated by air plasma without BCP patterning was not formed sub 10 nm sized sp 2 cluster or domains. This result shows that the BCP pattering makes the sub-5 nm size sp 2 domains or clusters of graphene and protect them against the additional damages by air plasma (Figure S5c). Figure S6. (a) Raman spectra of NH 2 - and CH 3 - functionalized graphene and pristine graphene. (b) PL spectra of GQDs with the alkyl (CH 3 -) and amine (NH 2 -) end group. Figure S6a shows the results of Raman spectra of GQDs on SAM-modified substrates. For the functionalization substrates, two SAM molecules such as 3-aminopropyltriethoxy silane (APS) and octadecyltrichlorosilane (OTS) were attached to SiO 2 substrates via the fabrication process described in

9 previous reports. 1,2 The Raman spectra of the graphene on CH 3 - functionalized substrate (red line of Figure S6a) showed a slight blue shift of G peak (from 1586 to 1589 cm -1 ) and the graphene on NH 2 - functionalized substrate (blue line of Figure S6a) showed a large blue shift of G peak (from 1586 to 1596 cm -1 ). These results suggest that the doping effect of NH 2 - functionalized substrate is more efficient than CH 3 - functionalized substrate. Moreover, the intensity ratio, I 2D /I G decreased depending on doping level (from black line to blue line) as shown in a previous work. 1 PL measurement was also carried out to investigate the effect of substrate functionalization on GQDs as shown in Figure S6b. The PL peak positions of GQDs on pristine SiO 2 substrate, CH 3 - and NH 2 - modified GQDs were observed at 395 nm, 397 nm, and 405 nm, respectively. Similarly, the Raman spectra in Figure S6a indicate that an external doping effect was induced by impurities such as ammonium persulfate onto graphene during wet-transfer process and the PL shift from modified-surface is correlated to their quantity of electron drawing by SAM end group.

10 Figure S7. Narrow scanned XPS analysis results in C1s of (a) GQDs (b) GQDs with 10 min hydrazine treatment (c) GQDs with 20 s additional oxygen plasma treatment.

11 Figure S8. (a) Photo-image of dispersion GQDs. (b) photograph of the GQDs aqueous solution under exposing 355 nm laser. Inset: the corresponding PL spectrum under excited wavelength (355 nm). Reference (1) Park, J.; Lee, W. H.; Huh, S.; Sim, S. H.; et al., Kim, S. B.; Cho, K.; Hong, B. H.; Kim, K. S. J. Phys. Chem. Lett., 2011, 2, (2) Lee, W. H.; Park, J.; Kim, Y.; Kim, K. S.; Hong, B. H.; Cho, K. Adv. Mater., 2011, 23,