In this review, we summarize solution-processed oxide thin-film transistors (TFTs) researches based on our fulfillments. We describe the fundamental studies of precursor composition effects at the beginning in order to figure out the role of each component in oxide semiconductors, and then present low temperature process for the adoption of flexible devices. Moreover, channel engineering for high performance and reliability of solution-processed oxide TFTs and various coating methods: spin-coating, inkjet printing, and gravure printing are also presented. The last topic of this review is an overview of multi-functional solution-processed oxide TFTs for various applications such as photodetector, biosensor, and memory.

In this study, sulfur-doped graphene (S-graphene) was synthesized by thermal treatment of exfoliated graphene under CS2 gas flow. Its electrocatalytic activity as a metal-free catalyst was evaluated and compared with other doped-graphenes and commercial platinum nanoparticles loaded on carbon black (Pt/C) catalysts for oxygen reduction reaction (ORR) in fuel cell cathodes. The resultant S-graphene was shown to act as a viable catalyst for ORR and its limiting current density and durability were improved compared to those of the commercial Pt/C catalyst. The current density at -1.0 V for the commercial Pt/C catalyst, pristine graphene, nitrogen-doped graphene (N-graphene) and S-graphene was 4.7, 0.15, 6.26 and 6.99 mA cm(-2), respectively. The durability of S-graphene (70.3%) was much better compared to commercial Pt/C (37.2%) and N-graphene (67.9%). When S-graphene was used as a supporting material for Pt nanoparticles, its catalytic performance was significantly higher than other Pt catalysts supported on different doped graphenes. Here, we demonstrate that S-graphene can be used as a novel graphene-based efficient metal-free ORR catalyst in fuel cells.

We report the fabrication of nanocrystalline graphite films on sapphire substrates of various cutting directions by using solid carbon source molecular beam epitaxy. Raman spectra show a systematic change from amorphous carbon to nanocrystalline graphite with a cluster diameter of several nanometers, depending on the growth temperature. The symmetry of the substrate seems to have little effect on the film quality. Simulations suggest that the strong bonding between carbon and oxygen may lead to orientational disorders. Transport measurements show a Dirac-like peak and a carrier type change by the gate voltage.

We investigated the surface potential (V surf ) of exfoliated MoS 2 flakes on bare and Au-coated SiO 2 /Si substrates using Kelvin probe force microscopy. The V surf of MoS 2 single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS 2 / Au interface leading to a modified band alignment. V surf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ∼20 nm (∼30 layers) on the bare and ∼80 nm (∼120 layers) on the Au-coated substrates, respectively. This thickness dependence of V surf was attributed to electrostatic screening in the MoS 2 layers. Thus, a difference in the thickness at which the bulk V surf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS 2 flakes. This work provides important insights to help understand and control the electrical properties of metal/MoS 2 contacts.

Surface enhanced Raman spectroscopy (SERS) has been intensively investigated during the past decades for its enormous electromagnetic field enhancement near the nanoscale metallic surfaces. Chemical enhancement of SERS, however, remains rather elusive despite intensive research efforts, mainly due to the relatively complex enhancing factors and inconsistent experimental results. To study details of chemical enhancement mechanism, we prepared various low dimensional semiconductor substrates such as ZnO and GaN that were fabricated via metal organic chemical vapor deposition process. We used three kinds of molecules (4-MPY, 4-MBA, 4-ATP) as analytes to measure SERS spectra under non-plasmonic conditions to understand charge transfer mechanisms between a substrate and analyte molecules leading to chemical enhancement. We observed that there is a preferential route for charge transfer responsible for chemical enhancement, that is, there exists a dominant enhancement process in non-plasmonic SERS. To further confirm our idea of charge transfer mechanism, we used a combination of 2-dimensional transition metal dichalcogenide substrates and analyte molecules. We also observed significant enhancement of Raman signal from molecules adsorbed on 2-dimensional transition metal dichalcogenide surface that is completely consistent with our previous results. We also discuss crucial factors for increasing enhancement factors for chemical enhancement without involving plasmonic resonance.