ICMCTF2012 Session TS4-1: Graphene and 2D Nanostructures

Carefully optimizing the growth of graphene on Ir(111) by scanning tunneling microscopy and low energy electron microscopy yields a virtually defect free, weakly bound epitaxial monolayer of macroscopic extension.

Graphene on Ir(111) can be used as a laboratory to construct new types of graphene based compound materials. Specifically, patterned adsorption of atoms and molecules takes place resulting in cluster superlattices with exciting magnetic and catalytic properties. Intercalation underneath the graphene allows one to manipulate the properties of graphene itself, e.g. its ability to adsorb atoms and molecules as well as its magnetism.

Graphene, a two dimensional crystalline sheet of carbon, has attracted significant attention due to its electronic properties, including a tunable band gap and high electron mobility for use in semiconducting devices and sufficiently high transparency and low sheet resistance for use as a transparent conductor. For any of these applications, it is desirable to obtain single-crystalline graphene layers with uniform thickness. This is an extremely challenging task that requires a fundamental understanding of the mechanisms controlling the nucleation and growth of graphene. Here, using in situ low-energy microscopy (LEEM), we investigated the growth of graphene via surface segregation of carbon dissolved in the bulk of the substrate. In this process, surface concentration of carbon depends on the substrate temperature T: at T > 920 ^oC the Pd surface is free of carbon; upon cooling to 880 ^oC, we observer monolayer graphene formation on the surface; and, at T = 710 ^oC, we obtain multi-layer graphene. In order the follow the kinetics of graphene growth, we acquired LEEM images as a function of incident electron energy while cooling the sample from 920 ^oC to 880 ^oC. From the LEEM images, electron reflectivity (electron energy dependent variations in image intensities) values, a measure of local surface work function, are extracted. This data is used to follow the changes in surface carbon adatom concentration during nucleation and growth of graphene. For monolayer growth, we find that the electron reflectivity decreases non-linearly with annealing time. This behavior is qualitatively similar to that observed during the growth of graphene on Ru(0001) [1], where the graphene layers grow from carbon adatoms present on the surface. In case of multilayer graphene growth induced by cooling the sample to lower temperatures (< 710 ^oC), we observed spontaneous formation of graphene mounds consisting of multiple layers. Low-energy electron diffraction patterns acquired from the layers reveal that both in-plane and out-of-plane stacking in the graphene layers is random with respect to the substrate. Moreover, we find that the spot intensities are weaker in the subsequent layers compared to the first layer, suggestive of growth of subsequent layers from below the surface at the graphene-substrate interface.

1. K. McCarty, P. Feibelman, E. Loginova, N.C. Bartelt, Kinetics and Thermodynamics of Carbon Segregation and Graphene Growth on Ru (0001). Carbon 2009, 47(7):1806-1813.

Succesfull development of graphene based nano- and macro-elenctronics would require not only high quality graphene but also the design of low power devices and integrated systems. Energy consumption has become among the three top challenges of current, as well as future technologies, as stated in the International Technology Roadmap for Seminconductors. To this end, the introduction of high-capacitance gate dielectrics in field-effect transistors has been considered a vaiable route to decrease the operating voltages and thus the overall power dissipation. In this presentation I will discuss the development of low operating voltage (<|1.5| V) chemical vapour deposition (CVD) graphene transistors based on different solution processable organic self-assembled monolayer (SAM) nanodielectrics. Despite the simple fabrication paradigm adopted, SAM based CVD graphene transistors show excellent characteristics that includes; hysteresis-free operation, low leakage currents, weak doping effect and bias-stress free operation. Importantly, the electronic properties of the graphene-dielectric interface can accurately be tuned hence opening the possibility for fine control of the operating characteristics of graphene transistors.

A microwave plasma chemical vapor deposition MPCVD growth from hydrogen-methane mixture was recently demonstrated by our group as a fast and low temperature method for graphene growth. This paper presents results of surface characterization of graphene films produced by MPCVD on polycrystalline copper foils, (111) oriented single crystal copper, and single crystal SiC substrates. XPS, HRTEM, Raman, AFM, and STM methods were used to characterize chemistry, bonding, and structure of the produced graphene. The results show that films of 3-6 monolayer graphene can be produced over the large areas with no oxygen contamination, nevertheless of the growth without the use of the substrate heating. Films were found to contain some structural defects, leading to disorder peaks in Raman spectra. These defects may result in decoupling vibration resonance in adjusted monolayers, making them to appear as monolayer graphene in characteristic Raman signatures. To overcome this difficulty, an alternative method with XPS thickness measurements was developed, which is using attenuation of escaped photoelectrons while passing through graphene and is shown to work reliably with graphene on both copper and SiC substrates. HRTEM images, as well as STM and thermal AFM analysis are discussed to highlight the effects of graphene structural defects on film electrical and thermal conductivity properties.

We report a unique process for rapid synthesis and in-situ doping of few-layer graphene films on various substrates by microwave plasma chemical vapor deposition (MPCVD). Few-layer graphene film on Cu foil, Cu block, Ni foil and on SiC has been demonstrated. The strong plasma/metal interaction helps to promote growth in a very short time. Particularly on a Cu foil the process can produce films of controllable quality from amorphous to highly crystalline by adjusting plasma conditions during processes of only few minutes and with no supplemental substrate heating. Films have been characterized using Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, scanning tunneling microscopy, transmission electron microscopy and atomic force microscopy. The results elucidate MPCVD deposition of thin carbon films on these substrates using the MPCVD method and also open new pathways for a rapid growth of few-layer graphene films. Further, we also show that the same system can be used for in-situ nitrogen doping of graphene by introducing nitrogen to the gas-phase precursors.

Graphite oxide sheet, now named as graphene oxide (GO), is the product of chemical exfoliation of graphite that has been known for more than a century. Interest in this old material has resurged with the rapid development of graphene since 2004, as GO is considered to be a promising precursor for bulk production of graphene. However, apart from making graphene, GO itself has many intriguing structural features and properties. For example, it can be viewed as an unconventional type of soft material as it has characteristics of polymers, membranes, colloids, liquid crystals and as highlighted here, amphiphiles. In this talk, some new insights into the processing, characterization and properties of this old material will be presented including (1) a high-throughput, high contrast fluorescence quenching microscopy (FQM) technique that can visualize graphene-based sheets on arbitrary substrates and even in solution; (2) the amphiphilicity of GO and the implications of this “world's thinnest bar of soap” in thin film processing; (3) Making graphene aggregation-resistant by mechanical deformation; and (4) GO based interfacial layer and active layer in solution processed photovoltaic devices.

We have studied the effect of the native oxide layer on the growth and performance of aligned carbon nanotubes (CNT) on copper substrates tested as double layer capacitors using a “Swagelok” type arrangement. A sputtered Inconel thin film and iron, delivered in vapor phase from ferrocene decomposition during chemical vapor deposition growth of the CNTs both act as catalysts for CNT growth on the copper surface. We analyzed the effectiveness in using this Inconel-Iron “co-catalyst” combination for dense, aligned CNT growth for potential access to greater surface area and subsequent charge storage. SEM, TEM and Raman measurements were made to study the structure and quality of the CNTs under various growth conditions. The results of the characterization show an improvement in density of the CNTs when the copper native oxide layer was removed before Inconel deposition. In addition, these samples also show larger values of power density and specific capacitance.

Here, Low Energy Electron Microscopy (LEEM) and micro Low Energy Electron Diffraction (µ-LEED) have been employed to study in situ the initial growth of the organic semiconductor para-sexiphenyl (6P) on Ir(111) supported graphene. For low deposition temperatures, indeed layer-by-layer growth of lying molecules is observed as it is desired for OLEDs [1]. After formation of a low density layer, the full first monolayer already shows a bulk like structure. The nucleation of the graphene islands occurs at wrinkles in the metal supported graphene layer. Larger islands composed of flat lying molecules detach from the original nucleation sites and move rapidly as entities across wrinkle free substrate areas [2]. µLEED reveals the surface unit cells in the different growth stages and at various substrate temperatures.

This work has been supported by the FWF project S9707-N20, STW and FOM project 04PR2318.

[1] G. Hlawacek, F. S. Khokhar, R. van Gastel, B. Poelsema, C. Teichert, Nano Lett. 11 (2011) 333.

[2] G. Hlawacek, F. S. Khokar, R. van Gastel, C. Teichert, B. Poelsema, IBM J. Res. Devel. 55 (2011) 15:1.