Sung Kim, Dong Hee Shin, Chang Oh Kim, Soo Seok Kang, Soong Sin Joo, Suk-Ho Choi,, Sung Won Hwang, and Cheolsoo Sone

Raman-scattering behaviors have been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The peak frequencies of D and 2D bands are almost irrespective of d, and the intensity of the D band is larger than that of the G band over almost full range of d. These results suggest that GQDs are defective, possibly resulting from the dominant contributions from the edge states at the periphery of GQDs. The G band shows a maximum peak frequency at d = 17 nm whilst the full-width half maximum of the G band and the peak-intensity ratio of the D to G bands are minimized at d = 17 nm. We propose that the abrupt changes in the Raman-scattering behaviors of GQDs at d = 17 nm originate from size-dependent edge-state variation of GQDs at d = ~17 nm as d increases.

Sung’s work, “Size-dependence of Raman scattering from graphene quantum dots: interplay between shape and thickness”, has been published for applied physics letters.

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Graphene has surprisingly powerful adhesion qualities that could make possible graphene-based mechanical devices such as gas separation

via Unexpected adhesion properties of graphene may lead to new nanotechnology devices.

Dong Hee Shin, Dong Yeol Shin, Sung Kim, and Suk-Ho Choi
Single-layer graphene layers were synthesized using chemical vapor deposition and subsequently doped chemically using AuCl3 and PEI [Poly (ethylene imine)] for p-type and n-type graphene, respectively. The undoped and doped graphene layers were transferred to 300 nm-thick thermal oxides grown on top of an n++ Si substrate used as the back gate contact. Optical microscopy and Raman spectroscopy were used to identify the single-layer graphene sheet, on top of which 50-nm-thick Ti/Pt films were grown and patterned to the source and drain electrodes of 0.2 mm length and 0.5 mm separation using a shadow mask in a RF magnetron sputtering. Figure 1 shows transfer characteristics (drain current Id vs back-gate voltage VG) for the graphene transistors before and after PEI doping. The threshold voltage for the graphene transistor before doping was at ~ 25 V, indicating that the pristine graphene film is slightly p-type. To study the influence of the electric field on Raman spectra of the graphene films, they were measured by varying VG  from -2 to 2 V in steps of 0.2 V under a 532 nm (2.33 eV) laser excitation. For undoped graphene, the Raman intensities of the D and G bands exhibited significantly-different dependences on VG. As VG  increased under positively biased, the G band intensity monotonically decreased, whilst the D band intensity monotonically increased. In contrast, both the D and G band intensities increased with increasing VG under negatively biased. As VG was varied from 0 to 2 V, the D and G bands were blue-shifted from 1333 to 1343 cm-1 and from 1569 to 1580 cm-1,  respectively. As VG was varied from 0 to -2 V, the D band was blue-shifted from 1569 to 1576 cm-1, whilst the G band was almost fixed at about 1336 cm-1. These results are compared with those of the p-/n-doped graphene and discussed based on possible physical mechanisms.



Seung Bum Yang, Dong Hee Shin, Sung Kim, and Suk-Ho Choi*
Large-scale single-layer graphene (SLG) layers were synthesized by using chemical vapor deposition, and some of them were doped through chemical processes using hydrazine and AuCl3 solutions for n- and p-type SLG layers, respectively. The undoped and n-/p-type SLG layers were transferred onto SiO2 (300nm)/highly n-doped Si substrates used as back gates. 50 nm-thick Ti/Pt films were grown and patterned to the source and drain electrodes of 0.2 mm length and 0.5 mm separation using a shadow mask in a RF magnetron sputtering. The SLG layers was characterized by measuring source-drain current (ISD) – gate voltage (VG) curves and temperature-dependent resistivity  ρ (VG, T). Figure 1 shows ISD-VG  characteristics of typical two transistors with undoped and n-type SLG layers. The ISD  currents increased almost linearly with VG on both sides of Dirac point and the ISD  – VG  curve of the n-type SLG was shifted to negative gate voltages, indicating n-doping. The  ρ (VG, T) curves were shown to be linear in the range of T below ~ 200 K. In contrast, the ρ-T curves at higher T were highly nonlinear and became significantly dependent on VG, thereby increasing the slope in the curve with decreasing VG. These results were compared with those of undoped and p-doped SLG layers and discussed based on possible physical mechanisms. 

Dong Yeol Shin, Dong Hee Shin, Chang Oh Kim, Sung Kim, and Suk-Ho Choi
100 nm ZnO films were deposited on p-type Si (100) wafers by RF magnetron sputtering and subsequently heated at 900oC for 3 min by rapid thermal annealing. Graphene layers were grown on copper foils by chemical vapor deposition, some of them were chemically doped using PEI [poly (ethylene imine)] or AuCl3 for n- or p-type graphene, respectively, and subsequently transferred on the surface of the ZnO films. Electrical properties of undoped and doped graphene were evaluated for back-gated graphene field-effect transistors. In the I – Vg curve of undoped graphene, the current increased linearly with Vg on both sides with respect to Dirac point. The I – Vg curve was shifted in the negative voltage direction by n-type doping, whilst it was shifted in the positive voltage direction by p-type doping. As shown in Fig. 1, Raman spectra of graphene on SiO2 films showed three intense features, D, G, and 2D peaks at  ~1350,  ~1580, and  ~2700 cm-1, respectively, uniquely characteristic of undoped graphene film. The G bands were red-shifted (blue-shifted) from 1583 to 1579 cm-1 (or to 1589 cm-1) by n- (or p-) type doping,  respectively. The 2D bands were blue-shifted from 2673 to 2677 cm-1 (or to 2688 cm-1) by n- (or p-) type doping. In the photoluminescence (PL) spectra, near-band-edge emission from ZnO was observed at 379 nm,  and its intensities are 3.9, 2.4, and 4.6 times enhanced in undoped, n-type, and p-type graphene on ZnO film, respectively, compared to the emission from the bare ZnO film. The enhanced PL was attributed to the resonant excitation of graphene plasmon, and physical mechanisms for explaining the doping dependences were discussed with reference to the I-V and Raman data.  


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