ICMCTF2013 Session TS4-1: Graphene and 2D Nanostructures

Significant progress has been made during the past decade in preparing nanosheets from a wide range of materials, which are actively pursued for various applications such as energy storage, catalysis, sensing, and membranes. One of the next critical challenges is developing a robust and versatile assembly method which allows the construction of the nanosheets into functional structures tailored for each specific purpose. An interesting characteristic of nanosheets is that they often behave as charged macromolecules, and thus can readily interact with an oppositely charged polyelectrolyte to form a stable complex. In this report, we demonstrate how such complexation process could be utilized for directing the self-assembly of nanosheets. By confining the nanosheet-polyelectrolyte complexation at air-liquid or liquid-liquid interfaces, the nanosheets are successfully assembled into various mesoscale architectures including fibers, capsules, and films. Furthermore, incorporation of additional components such as nanoparticles or small molecules can be easily achieved for further tailoring of material properties. This novel assembly method opens pathway to many useful nanosheets superstructures, and may be further extended to other types of nanomaterials in general.

Transparent conducting films (TCFs) are used in many modern technological devices such as solar cells, displays and touch screens. The current most popular TCF is indium tin oxide (ITO). But due to the limited supply of Indium and high expenses, it is urgent need to look for a substitute material to replace ITO. Generally thin films of carbon based materials are regarded as a suitable alternative for this purpose. Out of all carbon based materials, recently developed graphene is a promising material for flexible TCF due to its high electrical conductivity and high optical transmittance. But, it is a very challenging task to prepare high quality large area graphene films using the existing methods such as mechanical exfoliation of graphite, Silicon carbide sublimation and chemical methods due to the existence of various defects during synthesis.

Recently, advances on chemical vapor deposition (CVD) growth of graphene on Ni and Cu polycrystalline films have been achieved, which have stimulated various applications owing to the scalability and transferability. Graphene films on Ni are grown as mixture of various layers and Cu has very low carbon solubility (i.e. <0.001 atom % at 1000 ℃). In this aspect, Cu-Ni alloy is the best choice for substrate to fabricate high-quality uniform graphene layers in compared to pure Ni and Cu because it exhibits moderate as well as controllable carbon solubility by tuning atomic fraction of Ni in Cu.

In the present work, we report the synthesis of mono-to few-layer graphene films on commercial Cu-Ni alloy foils (70:30 wt %) by varying various experimental parameters such as growth temperature, growth time and cooling rate during chemical vapor deposition. Such films were transferred to glass substrates after etching the metal substrate in an aqueous solution of FeCl₃. The quality and microstructure of graphene film were characterized by Raman spectroscopy, optical microscope (OM), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). From the sheet resistance and transmittance measurements on the prepared graphene films, it is observed that such films are potentially useful for transparent thin conducting electrodes. The details analysis of graphene films as transparent conducting electrodes will be presented in the paper.

A recent and rapidly expanding research field is demonstrating, and exploiting, the fact that large-area (~0.03 to 3 cm), thin sheets (~5-500 nm) made up of materials of any class and any structure can be released from their original substrate and bonded to a new host. These new structural elements quickly came to be called nanomembranes (NMs). Especially of interest are crystalline semiconductor sheets. Nanomembranes are mechanically ultra-compliant, they are readily transferable to other hosts and conform and bond easily, they can take on a large range of shapes (including nanoribbons, micro-/nanotubes, and structures with combinable dimensions from 0D to 3D) via appropriate strain engineering and patterning, and, in planar geometry, they are stackable.

I have exploited the unique properties of Group IV semiconductor NMs to fabricate bi- or multi-material composites characterized by a large mismatch between the elastic moduli of an inherently stiff, but quite thin, top layer, such as Si or Ge, and a low-stiffness (i.e., compliant) supporting substrate. Specifically I have developed a high-yield process to transfer and bond NMs onto a soft substrate, and have they form both into planar and wavy geometries.

In my talk, I will present detailed mechanical analysis showing that the exceptional compliance of NMs, and the possibility of mechanically engineering them in their 2D form allow matching the mechanical properties of cellular environments and hence achieving a successful bio-inorganic integration from a mechanical/electronic perspective.

Furthermore I will illustrate how NMs can be engineered to create novel device architectures, i.e., 3D devices integrating multiple functionalities, such as scaffolding for cell culture, electronics, optics, and fluidics. Finally I will present a few examples of 3D devices having potential application [#] [#] [#] in traditional and newly developed biomedical fields, such as electrophysiology, biomechanics, and optogenetics.

Surface contamination of graphene is known to significantly degrade device performance. Although various ways to clean graphene has been reported, little is known about the nature and especially the source of the contamination. We find that that within 1 hour of exposure to ambient air, a clean graphene surface is contaminated with a thin layer of hydrocarbon. We show that contrary to the conventional wisdom that graphene is hydrophobic, a clean graphene is actually hydrophilic.

We have shown recently by in situ Low Energy Electron Microscopy experiments that the organic semiconductor para-sexiphenyl (6P) grows at low deposition temperatures on Ir(111) supported graphene in a layer-by-layer mode with the molecules lying on the substrate [1]. This molecular orientation is indeed desired for applications of the prepared films in organic light emitting diodes or solar cells for which the graphene acts as a transparent and flexible electrode. At substrate temperatures above room temperature, crystalline 6P needles form on a wetting layer composed of lying molecules [2].

Here, we changed our fabrication route to less expensive substrates and deposition techniques. 6P was grown on exfoliated graphene flakes transferred to silicon oxide [3] by hot-wall epitaxy. The substrate temperature ranged between 275 K and 400 K. The resulting film morphology was measured by ex situ atomic force microscopy (AFM). Whereas on silicon oxide terraced 6P mounds have been observed in the entire temperature range studied, there is on graphene a clear change in growth morphology with temperature. At low temperatures, terraced mounds coexist with short needles whereas above 360 K exclusively straight needles grow in selected directions. Such nanostructures offer the possibility to be studied by photoconductive AFM to probe their local response on irradiation with monochromatic light.

This work has been supported by the Austrian Science Fund project S9707-N20, and the Serbian Ministry of Science under project # OI171005.

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

[2] F. S. Khokhar, G. Hlawacek, R. van Gastel, H.J.W. Zandvliet, C. Teichert, B. Poelsema, Surf. Sci. 606 (2012) 475.

[3] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, A. K. Geim, Proc. Natl. Acad. Sci. 102 (2005) 10451.

Since its discovery in 2004, graphene has attracted tremendous interest due to its exceptional and advantageous physical properties [1]. However, to implement graphene as a leader material for future applications not only the challenges posed by the large scale growth need to be tackled but also a crystalline quality as comparable to mechanically exfoliated samples from graphite need to be obtained. Although, chemical vapor deposition (CVD) constitutes the mainstream among the large scale growth techniques [2], new routes still needed to be explored.

In the present study, we proposed to fabricate graphene by pulsed laser deposition (PLD) technique. PLD is a well-known technique for the deposition of amorphous carbon (a-C) such as DLC (Diamond-Like-Carbon) [3]. By ablating a graphite target, thin DLC films has been deposited by PLD under high vacuum condition either on catalytic metal (Ni, Cu) thin film or directly on the substrates (n-doped Si, fused silica) followed by a sputtered nickel metal thin capping layer. The growth process involves vacuum annealing of the ablated a-C layer and a natural cooling way. The dependence of graphene synthesis on process conditions, including the a-C thickness, temperature, setting time, and gas flow (Ar/H₂) were investigated. The quality of graphene is examined through Raman analysis, UV-VIS spectroscopy and microscopic studies (AFM, SEM…). SERS applications of the PLD-based graphene layer have also been demonstrated. Our method provides a new route to produce large scale graphene with good crystalline quality and good electrical properties, which is an important step for future applications.

[1] C. Yan, J. H. Cho and J. –H. Ahn, Nanoscale, 4 (2012) 4870.

[2] D. R. Cooper et al., ISRN Condensed Matter Physics, D. R. Cooper, B. D’Anjou, N. Ghattamaneni, B. Harack,M. Hilke, A. Horth, N. Majlis, M. Massicotte, L. Vands-burger, E. Whiteway, et al., Volume 2012, ID 501686.(doi:10.5402/2012/501686)

[3] F. Garrelie, N. Benchikh, C. Donnet, R. Y. Fillit, J. N. Rouzaud, J. Y. Laval, and A. Pailleret, Appl. Phys. A 90 (2008) 211-217.