Graphene is a two-dimensional material that has unique electrical and chemical properties. It is widely studied for applications in optics, electronics, and catalysis. One area of current research focuses on creating and isolating graphene nanostructures with well- defined shapes along specific crystallographic orientations.
An area of particular interest in nanoelectronics and catalysis are single layer graphene nanoribbons and nanopatterned few layer graphene (FLG) flakes, because they allow for new, useful devices and applications. However, a well-controlled fabrication method is required to create these structures.
Depending on size and exposed edge type, graphene nanoribbons exhibit distinctly different electronic behavior. For instance, zigzag-edged nanoribbons shown in Figure 1A are considered a “half-metal”, and serve as an insulator or metal depending on the electron spin polarization.
This behavior is important for spintronic applications, and useful in devices including spin valves in computer memory. Armchair-edged nanoribbons, shown in Figure 1B, may either be semiconducting or metallic depending on their width, and are useful in semiconductor applications such as field effect transistors.
Pristine armchair-edged and zigzag nanoribbons are important for exploiting the unique properties of graphene.
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.
In gas-phase reactions, graphene and FLG catalyst activity is highly dependent on the available surface area. However, van der Waals forces may cause FLG and graphene to restack upon drying, considerably reducing the specific surface area and surface accessibility.
As a result, these systems are not well-adapted for catalyst applications, especially for gas-phase reactions. Catalytic nanopatterning of FLG using metal nanoparticles can resolve this limitation by creating nanochannels on the FLG surface with improved accessibility.
These nanochannels support the development of new anchorage sites on the FLG surface, where metallic and/or oxide active phases can adhere.
It is difficult to create nanoribbons out of FLG and SLG with pristine armchair or zigzag edges, or with a highly porous structure. Patterning techniques using electron beam lithography, or a scanning tunneling microscopy tip, lead to low-quality ribbons. Other chemical etching methods also result in ribbons with imprecise edges.
Metal nanoparticles, such as Ag, Ni, Co, Fe and Pt, can etch channels on graphene and graphite surfaces with pristine edges when exposed to hydrogen, oxygen, water vapor or CO2 at increased temperatures. So far, this is the only method used to pattern graphene along specific crystallographic directions in order to leave edges consisting of only armchair or zigzag chirality.
Recently, researchers showed that if the reaction temperature is varied, nanoparticle size and reaction time can result in different channel properties, where higher temperatures and/or smaller nanoparticles and longer reaction times can lead to differences in channel width and length. Hence, the nanoparticle size, type, and temperature provide some amount of control over the etch process.
Graphene etching results are often characterized in the transmission electron microscope (TEM), which provides the required resolving power to view materials at the atomic scale, and to identify key properties such as crystal orientation.
Etching experiments have to be exposed to a controlled gas and high temperature environment, which is not viable in a typical TEM as it needs high vacuum to operate. Instead, etching is carried out in a separate reactor, and the results are subsequently characterized in the TEM.
In order to better understand the behavior of the etching process, the reaction must be observed in real time. A controlled gas and temperature source must be introduced into the microscope itself, so that the reaction and characterization take place at the same time.
The Protochips Atmosphere 200 Gas E-cell system integrates the sample analysis tool and reaction chamber. Samples can now be exposed to a highly controlled gas environment up to 1 atm while applying temperatures to 1000 °C within the TEM.
The holder-based closed cell design features a fully automated, software-controlled gas handling system, and converts almost any TEM into an environmental TEM without modifying the microscope. A patented thin film ceramic heating is used by Atmosphere for closed loop temperature control and ultra-stable high-resolution imaging for accurate heating, regardless of the gas environment.
At DSI-IPCMS-CNRS/University of Strasbourg, France, Dr. S. Moldovan, G. Melinte, and Prof. O. Ersen used Atmosphere to view the FLG etching process under relevant reaction conditions. The research team initially reduced the Fe3O4 nanoparticles to metallic Fe by exposing the sample to 150 Torr of H2 at high temperatures within the TEM. Metallic Fe was used as the etching catalyst.
The reduction of nanoparticle from Fe3O4 to metallic Fe is shown in Figure 2. Figure 3 illustrates the related EEL spectrum (background subtracted) of the Fe3O4 phase at 400 °C, and the graphene EEL spectrum (raw). Following reduction, the environment conditions were adjusted to 900 °C and 600 Torr H2, which match the conditions needed to begin the etching process. The experiment was performed in a JEOL 2100F operating at 200 kV using bright field TEM mode.
Previously, the IPCMS researchers used ex situ techniques to demonstrate the FLG etching process with metallic Fe nanoparticles. In order to better understand the etching process, in situ experiments using the same materials were performed using Atmosphere. In this case, the Fe nanoparticles catalyzes the reaction of hydrogen with carbon from graphene edges, forming methane (CH4), and continues as follows:
C(FLG) + 2H2 CH4 (on the nanoparticle surface)
The reaction can be understood as the reverse of catalytic carbon nanotube growth. The reaction is the driving force for the particle movement and removal of carbon atoms at the catalyst-graphene interface.
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.
The shematic shown in Figure 4 visually describes how the reaction takes place. The nanoparticles have to be in contact with a FLG edge to initiate the nanopatterning, as the C-H reaction can only begin if the C atoms from the edge are not part of a closed hexagon.
Figure 5 displays a movie, showing the reaction taking place in situ. It is generally accepted that the H2 molecules dissociate first on the surface of the metal nanoparticle before being brought into contact with the C atoms. When one row of carbon atoms is removed, the nanoparticle moves to regain contact with the edge, leaving an etch track behind.
Figure 5. BF-TEM image of a FLG flake after a reaction has occurred.Trenches along specific crystallographic directions are readily apparent.
Material dynamics should be considered to obtain a complete understanding of the etching process. The most critical issues that need to be considered are changes in support defects, particle facets, and morphological and structural evolution upon etching.
The researchers successfully reproduced the etching process and visualized the dynamic process at high resolution using Atmosphere. However, more research is needed to investigate the actual conditions for the process initiation and to control the etching speed.
FLG and graphene are potential materials that can be used for a range of applications such as composites, catalysis, nanoelectronics, and optics. It is essential to understand the fabrication process and large-scale synthesis before their widespread adoption.
The above results demonstrate that the process can be directly viewed, and conditions can be adjusted to better understand the behavior. These results also represent a subset of potential application areas that can be analyzed with the Atmosphere system.
Real-time visualization of surface and bulk evolution of different types of nanoparticles can be realized under different reaction conditions, just like the iron oxide to metallic iron reduction example. Nanoparticles often strongly interact with their support.
For instance, the interaction may promote different diffusion and coalescence behavior, and this can be directly visualized under certain gas environments at the atomic scale. Materials’ composition and electronic structure can also be probed using EELS analysis, as described here.
Atmosphere provides researchers with the ability to study material behavior at atomic scale under real-world reaction conditions, without affecting the TEM‘s resolving power.
1. D.A. Areshkin, C.T. White, Nano Lett., 7, pp 3253, 2007
2. G. Melinte, S. Moldovan, O. Ersen, et al., Nat. Comm., 5, pp 4109, 2014
3. F. Schäffel, M.H. Rümmeli, et al., Nano Res., 2, pp 695, 2009
4. L.C. Campos, P. Jarillo-Herrero, et al., Nano Lett., 9, pp 2600, 2009
5. 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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