AVS2010 Session GR-TuP: Graphene Focus Topic Poster Session

Oxidation and reduction are the basic and most important processes for the carbon-based materials as initial processes for producing very thin graphitic materials composed of a single or several graphene layers [1]. Recently, Kosynkin et al. have reported that the graphene nano-ribbons (GNRs) can be produced using an oxidative process, and the edges of these GNRs are functionalized by carboxylic acids even after the reduction treatment [2]. On the other hand, it has been well-known that a certain type of GNR with zigzag edges exhibits the so-called flat-band magnetism, which stems from the localized states at the edges [3,4]. In this study, we reveal the electronic and magnetic properties of zigzag GNRs functionalized by carboxyl groups using the first-principles total-energy calculations within the spin density functional theory.

The zigzag GNRs employed in this study have widths from N=2 to N=8. We have found that the ground state of the GNR with the zigzag edges functionalized by the carboxyl groups is ferromagnetic, if N is even. For the GNR with N=4, the ferromagnetic state is more stable by 17 meV per unit cell than the anti-ferromagnetic one. However, if N is odd, the ground state of the functionalized GNR becomes the anti-ferromagnetic: e.g., for N=3, the anti-ferromagnetic state is more stable by 12 meV per unit cell than the ferromagnetic one. We have also investigated the energy dispersions of the GNRs. In the anti-ferromagnetic states, the energy bands for both spins are degenerate, and their dispersions near the Fermi level are nearly-flat at the edge of the Brillouin zone, while for the ferromagnetic states, the up-spin and down-spin bands split from each other. The spin density for the ferromagnetic states is localized near the GNR edges. Thus, the onset of the finite magnetic moment of the functionalized GNR is due to the so-called flat-band magnetism.

[1] S. Horiuchi et al., Appl. Phys. Lett. 84, 2403 (2004)

[2] D. V. Kosynkin et al., Nature lett. 458, 872 (2009)

[3] M. Fujita et al., J. Phys. Soc. Jpn. 65, 1920 (1996)

Plasma in gas phase is widely used in many industrial fields such as electronic device manufacturing processes (plasma etching, sputtering, plasma-enhanced CVD, etc.), hard coating processes (ion plating, sputtering, etc.), surface treatment processes (low or atmospheric pressure plasma treatments, sputtering, plasma etching, etc.) and so on. Plasma in solid phase has been utilized finally for surface plasmon resonance (SPR) spectroscopy, nanoparticles, etc., and plasmonics is developing as a new research field. On the other hand, plasma in liquid phase is not generally well-known, although it has been partially utilized in water treatments and electrical discharge machining. The fundamentals of plasma in liquid phase have not been established, including its generation techniques, its state, and activated chemical species. However, it would be reasonable to expect a higher reaction rate under lower-temperature conditions, and the greater chemical reaction variability since the molecular density of liquid is much higher than that of gas phase. So we have named the plasma in liquid phase “solution plasma” because we make variety of plasma by choosing the combinations of solvents and solutes in solutions, and are developing solution plasma processing (SPP). In SPP, aqueous solutions, nonaqueous ones, liquid nitrogen, supercritical fluids, etc. can be utilized as solutions. Recently, we have investigated the features of SPP and the applications such as syntheses of nanoparticles and mesoporous silica, and surface modification of particles.

In this research, graphene sheet were modified by a glow discharge in solution. A pulsed power supply was used to generate discharges. The pulsed width was 2 micro seconds, the repetition frequency was 15 kHz. The electrode was tungsten wire in the diameter of 1 mm with electrode gap of 0.3 mm. ammonium aqueous solution was used as the medium around plasma. Graphene sheets were separated by oxidation. The grapheme sheets were added to the ammonium solution and irradiated by glow discharge in the solution. The solution and the productants after the discharge were analyzed by optical emission spectroscopy, IR spectroscopy, Uv-Vis spectroscopy, AFM, XRD and TEM. After the discharge, the graphene sheets were modified by amino functional groups. Moreover, the aminocaproic acid was grafted into the amino functional groups on graphene sheets. Finally, graphene sheets were solidified because the space of sheets was measured.

Constant current tunneling spectroscopy has been used to study the high energy unoccupied electronic structure of single layer and bilayer epitaxial graphene on the Si-terminated face of SiC(0001) prepared with several different sample-annealing conditions that give different thicknesses of graphene film coverage. We identify a series of intense peaks in vertical sample-tip spacing versus voltage as derived from image-potential states of epitaxial graphene. These peaks shift in energy position between single-layer and bilayer graphene in a manner somewhat like the known work function difference (of ~4.4 eV and ~4.6 eV for single-layer and bilayer graphene respectively). We compare the series of image-potential-like-state energies measured experimentally with simple models of the tunneling potential due to a sharp tip protrusion and a flatter average tip radius that shows a variability of the energy series with spatial variation of the potential. In addition, we argue that variability in peak positions for nearly all of the observed image potential derived states arises from the variations in the 3-D tip shape, which then determines the filed dependent tunneling potential.

Graphene is a superb structural material for NEMS resonators because of its unique electrical and mechanical properties. Excellent material properties of graphene such as good conductivity, single atomic thickness layers, large surface area, low mass density, and high Young’s modulus as compared to other materials allow optimization of the resonance response to approach the ultimate limit for two dimensional NEMSs. Single device resonator has been studied extensively in the past with very high Q-factor and resonant frequency. However, for device technology, arrays of resonators provide a wider range of applications which appear to be possible in graphene based resonators. Figure 1 summarizes the fabrication process of the graphene resonators and Figure 2 presents a 3D schematic of graphene resonator array, indicating key parts and the materials. We have grown the monolayer quality graphene using thermal chemical vapor deposition method on Ni coated substrates at 1000˚C using a mixture of argon, hydrogen and methane as precursor gases. It was observed that growth time and thickness of Ni films are key parameters for controlling the number of layers of graphene. Initial results suggest the formation of 1-5 layers of graphene which was estimated from AFM, TEM and 2D peak intensity of Raman spectra [Figure 3]. Hall Effect data show mobility ~ 1500 cm²/V and resistivity of 2X10⁻⁶ Ω·cm for 3 layer graphene. Graphene was released from Ni by wet chemical etching and the layers were transferred onto SiO₂/ Si substrate. In order to employ graphene as a major building block, NEMS resonator structures were created by transferring a patterned graphene layer onto a silicon-on-insulator (SOI) substrate to act as mechanical interconnects. The Si device layer in SOI substrate is patterned by deep reactive ion etching (DRIE) using SiO₂ as hard mask to create the resonator body. An ultra-thin layer of high-k dielectric material is uniformly deposited by atomic layer deposition (ALD) followed by deposition and doping of a poly-Si layer. A lithography process is done to create the shape of the double side electrodes. Thereafter, the graphene is transferred and attached to the resonator bodies to complete the array. Finally, the resonator array is released by removing the buried SiO₂ in the SOI substrate.