Hye Ri Kim

Roll-to-roll production of 30-inch graphene films for transparent electrodes

The outstanding electrical, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as approximately 125 ohms square(-1) with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as approximately 30 ohms square(-1) at approximately 90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.1 SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), 2 Department of Chemistry, 3 Department of Mechanical Engineering, 4 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea. 5 NanoCore & Department of Physics, National University of Singapore, Singapore 117576 & 117542, 6 Digital & IT Solution Division, Samsung Techwin, Seongnam 462-807, Korea, 7 Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565 & Faculty of Science and Engineering, Meijo University, Nagoya 468-8502, Japan.

Quasi-Periodic Nanoripples in Graphene Grown by Chemical Vapor Deposition and Its Impact on Charge Transport

The technical breakthrough in synthesizing graphene by chemical vapor deposition methods (CVD) has opened up enormous opportunities for large-scale device applications. To improve the electrical properties of CVD graphene grown on copper (Cu-CVD graphene), recent efforts have focused on increasing the grain size of such polycrystalline graphene films to 100 μm and larger. While an increase in grain size and, hence, a decrease of grain boundary density is expected to greatly enhance the device performance, here we show that the charge mobility and sheet resistance of Cu-CVD graphene is already limited within a single grain. We find that the current high-temperature growth and wet transfer methods of CVD graphene result in quasi-periodic nanoripple arrays (NRAs). Electron-flexural phonon scattering in such partially suspended graphene devices introduces anisotropic charge transport and sets limits to both the highest possible charge mobility and lowest possible sheet resistance values. Our findings provide guidance for further improving the CVD graphene growth and transfer process.

Wafer-scale graphene/ferroelectric hybrid devices for low-voltage electronics

Preparing graphene and its derivatives on functional substrates may open enormous opportunities for exploring the intrinsic electronic properties and new functionalities of graphene. However, efforts in replacing SiO2 have been greatly hampered by a very low sample yield of the exfoliation and related transferring methods. Here, we report a new route in exploring new graphene physics and functionalities by transferring large-scale chemical-vapor deposition single-layer and bilayer graphene to functional substrates. Using ferroelectric Pb(Zr0.3Ti0.7)O3 (PZT), we demonstrate ultra-low-voltage operation of graphene field effect transistors within ±1 V with maximum doping exceeding 1013 cm− 2 and on-off ratios larger than 10 times. After polarizing PZT, switching of graphene field effect transistors are characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics.

Optical Probing of the Electronic Interaction between Graphene and Hexagonal Boron Nitride

Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilized to detect a few percent decrease in the Fermi velocity (v(F)) of graphene caused by the vdW interaction with underlying hexagonal boron nitride (hBN). Our study also establishes Raman spectroscopic analysis which enables separation of the effects by the vdW interaction from those by mechanical strain or extra charge carriers. The analysis reveals that spectral features of graphene on hBN are mainly affected by change in v(F) and mechanical strain but not by charge doping, unlike graphene supported on SiO₂ substrates. Graphene on hBN was also found to be less susceptible to thermally induced hole doping.

Experimental evidence of warm electron populations in magnetron sputtering plasmas

This work report on the results obtained using the Langmuir probe (LP) measurements in high-power dc magnetron sputtering discharges. Data show clear evidence of two electron components, such as warm and bulk electrons, in the sputtering plasma in a magnetic trap. We have also used optical emission spectroscopy diagnostic method along with LP to investigate the plasma production. Data show that there is a presence of low-frequency oscillations in the 2–3 MHz range, which are expected to be generated by high-frequency waves. Analysis also suggests that the warm electrons, in the plasmas, can be formed due to the collisionless Landau damping of the bulk electrons.

Plasma Diagnostics and Characterizations of Al-Doped ZnO Films Deposited with Low Temperature Sputtering Process

Facing targets sputtering (FTS) is known to be one of the promising magnetron sputtering systems for low temperature processes, because of the significant reduction of damage in the film structure, by the suppression of impinging high energy particles. In this study, FTS was compared with conventional magnetron sputtering (CMS), by various plasma diagnostics methods and film analysis. The sputtering target used was ZnO:Al (2%). OES was used to study the emission spectroscopy of process plasma. Ion current densities and the temperatures on the substrate were measured, to compare FTS with CMS. A coplanar-type Fabry?Perot interferometer (FPI) was utilized to measure the gas temperatures of Zn atoms, by Doppler broadening of the corresponding emission lines. Gas temperatures of Zn of FTS were measured to be lower, in the range of 200?300 K, compared with those of CMS. Film analysis showed that quality films can be synthesized at low temperature with rf powered FTS.

Raman spectroscopy study on the reactions of UV-generated oxygen atoms with single-layer graphene on SiO 2 /Si substrates

Successful application of graphene requires development of various tools for its chemical modification. In this paper, we present a Raman spectroscopic investigation of the effects of UV light on single layer graphene with and without the presence of O2 molecules. The UV emission from a low pressure Hg lamp photolyzes O2 molecules into O atoms, which are known to form epoxy on the basal plane of graphene. The resulting surface epoxy groups were identified by the disorder-related Raman D band. It was also found that adhesive residues present in the graphene samples prepared by micro-mechanical exfoliation using adhesive tape severely interfere with the O atom reaction with graphene. The UV-induced reaction was also successfully applied to chemical vapor deposition-grown graphene. Since the current method can be readily carried out in ambient air only with UV light, it will be useful in modifying the surfaces of graphene and related materials.

Stable and high-quality Al-doped ZnO films with ICP-assisted facing targets sputtering at low temperature

FTS (facing targets sputtering) has been studied intensively for high-quality TCO films in low-temperature processes. In this study, we designed ICP-assisted FTS process for high-quality Al-doped ZnO film synthesis in a low temperature process. A one-turn ICP coil was installed a few cm above the upper target edge through which hydrogen was introduced and fully dissociated to the atomic radicals. The increase of ICP power caused heating and rarefaction of Ar gas and generated abundant hydrogen atoms and hydrogenated molecules. In FESEM analysis, the films synthesized with high ICP power showed high crystallinity. XPS was used to analyze the film structure. In O1s spectra, the low binding energy component located at ~530.3 ± 0.4 eV corresponding to O2− ions on the wurtzite structure of the hexagonal Zn2+ ion array increased with the ICP power, indicating good crystal quality. With increasing ICP power fixing while fixing the RF power at the cathode, the resistivity was observed to decrease to 5 × 10−4 Ω-cm. For thermal reliability tests, films were stored in an air-based chamber at 200 °C. The films synthesized without ICP showed rapid degradation in the electrical properties, while the films synthesized with high ICP power showed good stabilities with little change in the electrical properties after 30 h of storage in an oven. By adding hydrogen, the carrier concentration of the films increased, while the mobility did not change much. From these results, it is expected that hydrogen was incorporated into the film as a stable n-dopant by using an auxiliary ICP plasma source.