Graphene is a two-dimensional material that has unique chemical and electrical properties. It is studied widely for applications in catalysis, optics and electronics. One area of current research is centred on creating and isolating graphene nanostructures with well-defined shapes along specific crystallographic orientations.
Nanopatterned few layer graphene (FLG) flakes and single layer graphene nanoribbons are of particular interest. This is because they allow for new, useful devices and applications. However, in order to create these structures, a well-controlled fabrication method is needed.
Graphene nanoribbons exhibit distinctly different electronic behavior depending on size and exposed edge type. For example, zigzag-edged nanoribbons, shown in Figure 1A, are considered to be “half-metal”. Depending on the electron spin polarization they serve either as an insulator or metal.
This behavior is useful in devices including spin valves in computer memory and is important for spintronic applications. Armchair-edged nanoribbons may be either metallic or semiconducting, depending on their width. They are shown in Figure 1B and are useful in semiconductor applications like field effect transistors.
Figure 1. A) graphene nanoribbon with an exposed zigzag edge as indicated by the green dotted line and B) Graphene nanoribbon with an exposed armchair edge.
Pristine armchair-edged and zigzag nanoribbons are important for the exploitation of the unique properties of graphene.
During gas-phase reactions, the activity of the graphene and FLG catalyst depends highly on the available surface area. Upon drying, van der Waals forces may cause FLG and graphene to restack. This reduces the specific surface area and surface accessibility significantly.
Consequently, these systems are ill-adapted for catalyst applications, especially gas-phase reactions. This limitation can be resolved using catalytic nanopatterning of FLG. This uses metal nanoparticles to create nanochannels on the FLG surface which improves accessibility.
These nanoparticles enable the development of new anchorage sites on the FLG surface. Metallic and/or oxide active phases can adhere on these sites.
Creating nanoribbons out of SLG and FLG with a highly porous structure or with pristine armchair or zigzag edges is difficult. Patterning techniques that use scanning tunneling microscopy tip or electron beam lithography lead to ribbons of low-quality. Other methods of chemical etching also result in ribbons that have imprecise edges.
When exposed to oxygen, hydrogen, CO2 or water vapor at high temperatures, metal nanoparticles like Ni, Ag, Pt, Co and Fe can etch channels on graphite and graphene surfaces with pristine edges. This is the only method currently used to pattern graphene along specific crystallographic directions in order to create edges that consist of only zigzag or armchair chirality.
Scientists have recently shown that nanoparticle size and reaction time can result in different channel properties if the reaction temperature is varied. Higher temperatures and/or smaller nanoparticles and longer reaction times can result in differences in channel length and width. Therefore, the size and type of the nanoparticle, as well as the temperature all exhibit some amount of control over the etch process.
Often, graphene etching results are characterized in the transmission electron microscope (TEM). This provides the resolving power needed to view materials at the atomic scale and to identify key properties, like crystal orientation.
Etching experiments need to be exposed to a controlled gas and high temperature environment. This is not possible in a typical TEM, which requires high vacuum to operate. Therefore, etching is carried out in a separate reactor and the results are then characterized in the TEM.
The process needs to be observed in real time so as to better understand the behavior of the etching process. So that the reaction and the characterization can take place at the same time, a controlled gas and temperature source needs to be introduced into the microscope itself.
The Protochips Atmosphere 2000 Gas E-cell system integrates the reaction chamber and the sample analysis tool. This means that, within the TEM, samples can be exposed to temperatures up to 1000 oC in a highly controlled gas environment of up to 1 atm.
The holder-based cell design features a software-controlled gas handling system that is fully automated and can convert almost any TEM into an environmental TEM, without modifying the microscope. Atmosphere uses a patented thin film ceramic heating for closed loop temperature control and for ultra-stable high-resolution imaging. This provides accurate heating, regardless of the gas environment.
Dr. S. Moldovan, G. Melinte, and Prof. O. Ersen, at DSI-IPCMS-CNRS/University of Strasbourg, France, used Atmosphere to view the FLG etching process under relevant reaction conditions. Firstly, the team reduced the Fe3O4 nanoparticles to metallic Fe by exposing the sample to 150 Torr of H2 at high temperatures within the TEM. The etching catalyst was metallic Fe.
Figure 2 shows the reduction of nanoparticle from Fe3O4 to metallic Fe. The related EEL spectrum (background subtracted) of the Fe3O4 phase at 400 oC, and the graphene EEL spectrum (raw) are illustrated in Figure 3. After the reduction, the environment conditions were adjusted to 900 oC and 600 Torr H2, which are the conditions needed to start the etching process. The experiment was performed in a JEOL 2100F operating at 200 kV using bright field TEM mode.
Previously, the IPCMS researchers demonstrated the FLG etching process using ex situ techniques with metallic Fe nanoparticles. In situ experiments using the same materials were performed using Atmosphere to better understand the etching process. In this instance, the Fe nanoparticles catalyze the reaction of hydrogen with carbon from graphene edges. This forms methane (CH4) and continues thus:
C(FLG) + 2H2CH4 (on the nanoparticle surface)
The reaction can be understood as the reverse of catalytic carbon nanotube growth. The reaction is the main force for particle movement and the removal of carbon atoms at the interface between the catalyst and graphene.
Figure 2. In situ reduction of iron oxide to metallic iron. The left BFTEM image show a Fe3O4 nanoparticle. The crystal structure of this nanoparticle was determined by the FFT, show in the inset. The center image is a partially reduced FeO nanoparticle after exposure to 150 Torr of H2 at 700 °C. The right image is a fully reduced, metallic Fe nanoparticle after exposure to 150 Torr of H2 and 800 °C. The scale bar is 2 nm in each image.
Figure 3. EELS analysis of iron oxide nanoparticles and FLG material. The top image shows the carbon edge from the FLG material. The bottom image shows the oxygen and iron edges from the iron oxide nanoparticles present in the sample.
Figure 4. Schematic of the etch reaction. The iron nanoparticle catalyzes a reaction between H2 and carbon from the FLG material to create methane, CH4. This reaction removes carbon from the lattice and the iron nanoparticle moves forward.
Figure 4 shows a schematic that visually describes how the reaction takes place. In order to initiate the nanopatterning, the nanoparticles have to be in contact with a FLG edge. This is because the C-H reaction can only begin if the C atoms from the edge are not part of a closed hexagon.
A movie showing the reaction taking place in situ is shown in Figure 5. Generally, it is accepted that the H2 molecules dissociate on the surface of the metal nanoparticle before being brought into contact with the C atoms. The nanoparticle moves to regain contact with the edge when one row of carbon atoms is removed. This leaves an etched track behind.
Figure 5. BF-TEM image of a FLG flake after a reaction has occurred.Trenches along specific crystallographic directions are readily apparent.
In order to obtain a complete understanding of the etching process, material dynamics should be considered. The most critical issues that should be considered are particle facets, changes in support defects, and structural and morphological evolution upon etching.
Using Atmosphere, the scientists successfully reproduced the etching process and visualized the dynamic process at high resolution. More research is required to investigate the actual conditions for the process initiation and control the speed of etching.
Graphene and FLG are materials that can potentially be used for a number of applications such as catalysis, composites, optics and nanoelectronics. Before their application, it is vital to understand the fabrication process and large-scale synthesis.
The results discussed above demonstrate that the process can be viewed and that conditions can be adjusted so that the behavior can be better understood. These results also show a subset of potential application areas that the Atmosphere system can analyze.
Under different conditions, the surface and bulk evolution of different types of nanoparticles can be visualized in real-time, just like the example in which iron oxide is reduced to metallic iron. Often, nanoparticles interact strongly with their support.
In this example, the interaction may promote different coalescence and diffusion behavior. Under certain gas environments, this can be visualized directly at the atomic scale. EELS analysis can be used to probe the materials’ composition and electronic structure.
Atmosphere allows researchers to study material behavior under real-world reaction conditions, in real time, and without affecting the resolving power of TEM.
- D.A. Areshkin, C.T. White, Nano Lett., 7, pp 3253, 2007
- G. Melinte, S. Moldovan, O. Ersen, et al., Nat. Comm., 5, pp 4109, 2014
- F. Schäffel, M.H. Rümmeli, et al., Nano Res., 2, pp 695, 2009
- L.C. Campos, P. Jarillo-Herrero, et al., Nano Lett., 9, pp 2600, 2009
- M. Lukas, R. Krupke, et al., Nat. Comm., 4, pp 1379, 2013
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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