Seiji Obata

Control of work function of graphene by plasma assisted nitrogen doping

Nitrogen doping is expected to provide several intriguing properties to graphene. Nitrogen plasma treatment to defect-free and defective highly oriented pyrolytic graphite (HOPG) samples causes doping of nitrogen atom into the graphene layer. Nitrogen atoms are initially doped at a graphitic site (inside the graphene) for the defect-free HOPG, while doping to a pyridinic or a pyrrolic site (edge of the graphene) is dominant for the defective HOPG. The work function of graphene correlates strongly with the site and amount of doped nitrogen. Nitrogen atoms doped at a graphitic site lower the work function, while nitrogen atoms at a pyridinic or a pyrrolic site increase the work function. Control of plasma treatment time and the amount of initial defect could change the work function of graphite from 4.3 eV to 5.4 eV, which would open a way to tailor the nature of graphene for various industrial applications.

The importance of spinning speed in fabrication of spin-coated organic thin film transistors: Film morphology and field effect mobility

We have investigated the film morphology and the field effect mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films which were formed by spin coating on the SiO2 substrate with solution-processed graphene electrodes. The domain size and the density of aggregates in the C8-BTBT film showed the same dependence on the spinning speed. These competitive two factors (domain size and density of aggregates) give an optimum spinning speed, at which the field effect mobility of C8-BTBT transistor showed a maximum (2.6 cm2/V s). This result indicates the importance of spinning speed in the fabrication of solution processed organic thin film transistors by spin coating.

Self-Aligned Growth of Organic Semiconductor Single Crystals by Electric Field

We proposed a novel but facile method for growing organic semiconductor single-crystals via solvent vapor annealing (SVA) under electric field. In the conventional SVA growth process, nuclei of crystals appeared anywhere on the substrate and their crystallographic axes were randomly distributed. We applied electric field during the SVA growth of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) on the SiO2/Si substrate on which a pair of electrodes had been deposited beforehand. Real-time observation of the SVA process revealed that rodlike single crystals grew with their long axes parallel to the electric field and bridged the prepatterned electrodes. As a result, C8-BTBT crystals automatically formed a field effect transistor (FET) structure and the mobility reached 1.9 cm(2)/(V s). Electric-field-assisted SVA proved a promising method for constructing high-mobility single-crystal FETs at the desired position by a low-cost solution process.

Electric-field-assisted position and orientation control of organic single crystals.

We have investigated the motion of growing pentacene single crystals in solution under various electric fields. The pentacene single crystals in 1,2,4-trichlorobenzene responded to the electric field as if they were positively charged. By optimizing the strength and frequency of an alternating electric field, the pentacene crystals automatically bridged the electrodes on SiO2. The pentacene crystal with a large aspect ratio tended to direct the [1̅10] orientation parallel to the conduction direction, which will be suitable from a viewpoint of anisotropy in mobility. The present result shows a possibility of controlling the position and orientation of organic single crystals by the use of an electric field, which leads to high throughput and low cost industrial manufacturing of the single crystal array from solution.

Conductive Atomic Force Microscopy of Chemically Synthesized Graphene Oxide and Interlayer Conduction

Graphene oxide, a chemically modified graphene, has been attracting wide attention because of promising adaptability to a wide variety of applications. However, the properties of graphene oxide its...

Reduction of a Single Layer Graphene Oxide Film on Pt(111)

Graphene oxide (GO) is a very attractive material from which graphene of mass quantity could be fabricated. To restore the outstanding properties of graphene, however, complete reduction of GO is necessary, which no one has ever achieved before. We examined annealing of GO which was placed on Si(100) and Pt(111) and observed the surface atomic structure by scanning tunneling microscopy. A honeycomb lattice with long range order appeared for GO on Pt(111), but not for GO on Si(100). Reduction of GO together with restoration of the graphene lattice might be realized by the catalytic property of Pt, which opens a new way to synthesize graphene.

Growth of N-doped graphene from nitrogen containing aromatic compounds: the effect of precursors on the doped site†

Doping of nitrogen into the graphene lattice tunes and tailors its electronic properties. To elucidate the doping mechanism and develop a method of controlling the doped structure, nitrogen doped graphene was synthesized from four kinds of nitrogen-containing aromatic compounds: quinoline, pyridine, pyrrole, and pyrimidine on Pt(111) at a variety of temperatures. The doped site and amount of nitrogen in the synthesized graphene depended considerably on the source molecule. Especially, nitrogen doped graphene with pyridinic-N and pyrrolic-N were predominantly obtained from quinoline and pyrrole sources. Comparison between the graphenes from different molecules indicated that structure and thermal stability of the source molecules together with their structural affinity to the honeycomb lattice determined the doping amount and the doped site.

Graphene oxide: A fertile nanosheet for various applications

Graphene oxide (GO) is chemically exfoliated graphene with various oxygen functional groups bound to its sp2 basal plane. GO is not only a precursor for graphene in large-scale production but provides a fertile platform for applications from electronics to biology owing to its outstanding characteristics. In this review, we introduce the preparation and reduction methods and discuss recent application examples on electrochemistry and biological sensors.

Growth temperature dependence of nitrogen doped graphene structure on Pt (111) and analysis of its reactivity with oxygen

Nitrogen doping is an effective method for modulating the electronic states and properties of graphene. In particular, chemical vapor deposition using nitrogen-containing organic molecules such as pyridine has been expected to be a facile way to control the doping site and amount of nitrogen. However, the atomic structure of nitrogen-doped graphene (NG) synthesized from such molecules has not been investigated. Furthermore, the nitrogen doping sites of NG synthesized at a high temperature of more than 1000 K have also not been measured. In this study, we carried out Scanning Tunneling Microscopy (STM) measurements on the structure of NG synthesized from pyridine, and elucidated the doping sites. Furthermore, we investigated how the doping of nitrogen atoms affects the reactivity with oxygen molecules to reveal the active site of a carbon alloy catalyst. We found that NG synthesized at 1150 K has nitrogen atoms doped into the pyridinic site, and these pyridinic sites enhance the reactivity to oxygen when comparing the defects with/without nitrogen. These findings will help with the synthesis of NG when controlling the doping sites and the development of a catalyst with high efficiency.

High degree reduction and restoration of graphene oxide on SiO2 at low temperature via remote Cu-assisted plasma treatment

A high throughput synthesis method of graphene has been required for a long time to apply graphene to industrial applications. Of the various synthesis methods, the chemical exfoliation of graphite via graphene oxide (GO) is advantageous as far as productivity is concerned; however, the quality of the graphene produced by this method is far inferior to that synthesized by other methods, such as chemical vapor deposition on metals. Developing an effective reduction and restoration method for GO on dielectric substrates has been therefore a key issue. Here, we present a method for changing GO deposited on a dielectric substrate into high crystallinity graphene at 550 °C; this method uses CH4/H2 plasma and a Cu catalyst. We found that Cu remotely catalyzed the high degree reduction and restoration of GO on SiO2 and the effect ranged over at least 8 mm. With this method, field-effect transistor devices can be fabricated without any post treatment such as a transfer process. This plasma treatment increased electron and hole mobilities of GO to 480 cm2 V-1 s-1 and 460 cm2 V-1 s-1 respectively; these values were more than 50 times greater than that of conventional reduced GO. Furthermore, the on-site conversion ensured that the shape of the GO sheets remained unchanged after the treatment. This plasma treatment realizes the high throughput synthesis of a desired shaped graphene on any substrate without any residue and damage being caused by the transfer process; as such, it expands the potential applicability of graphene.