Chemical vapor deposition (CVD) has enabled industries around the globe to manufacture single to multilayer layer sheet graphene on an industrial level.
Image Credit: Production Perig/Shutterstock.com
To provide precise measurement protocols and verified data, a standardized measurement approach to evaluate the graphene's quality is still being developed by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC). This article aims to provide information on the new ISO standard on the structural characterization of CVD-produced graphene.
CVD Graphene: Synthesis and Applications
Graphene was first successfully synthesized using the scotch tape approach by Prof. Konstantin Novoselov and Prof. Andre Geim in 2004. An experimental demonstration of thermal CVD for the synthesis of large area single-layer graphene was made five years after the first reported studies on graphene synthesis.
During CVD, graphene is grown on a metal substrate such as copper by introducing hydrocarbon gas species such as methane or acetylene into the reactor at a high temperature of ~1000 °C. Hydrocarbon precursors break down into carbon radicals at the metal substrate. During cooling, these carbon radicals self-assemble to form the graphene layer. Finally, the graphene is transferred to the desired surface for further applications.
CVD graphene has impacted many different application areas, including biosensors, displays, solar cells, and flexible electronics due to its exceptional properties such as superior flexibility, high transparency (97.4%), high thermal conductivity (3000 Wm/K), high electron mobility (200000 cm2/Vs).
For example, Paragraf reported the first successful demonstration of CVD graphene as a replacement for indium tin oxide (ITO) in the transparent display. Researchers from Graphene Square Inc have recently won the CEC Best Innovation Award for their CVD graphene-based transparent and flexible home heating and cooking appliances.
As the industry adoption of graphene is increasing, reproducing the consistent quality of graphene on a global scale is becoming more challenging. To that purpose, in 2016, a datasheet for international standards was proposed for ‘Graphene and Related 2D Materials’ through Versailles Project on Advanced Materials and Standards (VAMAS).
Explained: Chemical Vapor Deposition (CVD)
Explained: Chemical Vapor Deposition (CVD). Video Credit: Massachusetts Institute of Technology (MIT)/YouTube.com
The ISO and the Global Graphene Economy
The International Organization for Standardization (ISO) is regarded as one of the innovative supports to bridge the gap between research and the market, facilitating the transfer of reliable, reproducible and accurate measurements to the global market consistently and accurately. Therefore, international standardization is crucial to facilitating commercialization as industry adoption of graphene is increasing.
Many graphene characteristics, in a particular number of layers, lateral thickness, and flake size, might vary based on the manufacturing method and related processing parameters. Given the variety of production methods and suppliers for graphene, international standards must be developed to provide similar measurements for all graphene products manufactured globally regardless of producer or manufacturing process. This will aid in ensuring material quality across the supply chain, as well as allowing for effective judgments on application and health safety.
Standardization is essential with regard to the terminology, performance, and validation of the results. In addition, standards are utilized as valuable tools to promote communication and build credibility between suppliers and buyers. Several standards are being developed by the ISO TC229 'Nanotechnologies,' in collaboration with the IEC, on 'Structural characterization of graphene,' which emphasizes on determining the physical characteristics of graphene flakes and CVD graphene.
The Significance of ISO/TS 21356-2 Nanotechnologies Standards
ISO/TS 21356-2 Nanotechnologies —Structural characterization of graphene— Part 2: CVD grown graphene, deals with graphene synthesized from CVD. It defines methods for structural characterization of monolayer, bilayer, few-layer, or multilayer CVD-grown graphene utilizing optical microscopy, transmission electron microscopy (TEM), and Raman spectroscopy.
This standard, for example, is the measurement protocol of Raman spectra of single-layer graphene or graphene with increasing layers. Along with the number of layers, the other properties determined are the level of disorder, stacking sequence of layers and percentage of substrate coverage of graphene. Sample preparation routines, data analysis for the characterization of CVD-grown graphene and measurement protocols are also provided.
For example, Scientists from the National Physical Laboratory (NPL) in the United Kingdom (UK), published data on comparing Raman spectroscopy measurements of CVD-grown graphene using a similar measurement protocol across various laboratories and institutes. It was found that the relative difference between crucial measured quantities, such as the Raman peak intensity ratios used to measure the number of layers, was found to be up to 200%.
However, users can dramatically reduce this inconsistency by executing an intensity calibration using a verified reference material. Similarly, utilizing a consistent manner of data processing could reduce the variation in measured Raman peak full-width half maxima (FWHM) by orders of magnitude. These findings and method improvements will be incorporated into ISO/AWI TS 21356-2.
The ISO/PWI TS 21356-2 standard is currently being developed. Dr. Denis Koltsov, Chairman of ISO TC229 (Nanotechnologies), and experts from the National Physical Laboratory (NPL), UK, recently discussed a crucial improvement in ISO/TS 21356-2 document. A new draft on Structural characterization of graphene — Part 2: CVD grown graphene is expected soon with the goal of providing a trusted source of data and more precise measurement standards for the worldwide graphene market.
Continue reading: Following Graphene Along the Industrial Supply Chain
References and Further Reading
Turner, Piers, et al. (2022) International interlaboratory comparison of Raman spectroscopic analysis of CVD-grown graphene. 2D Materials. Available at: https://doi.org/10.1088/2053-1583/ac6cf3
National Physical Laboratory, UK (2022) The accuracy for International Standard. Available at https://www.npl.co.uk/news/accurate-international-graphene-standards
National Physical Laboratory, UK (2019) VAMAS TWA 41 - Graphene and Related 2D Materials Project 1 - Structural characterization of CVD-grown graphene. Available at https://eprintspublications.npl.co.uk/8383/
Ntziouni, Afroditi, et al. (2022) "Review of Existing Standards, Guides, and Practices for Raman Spectroscopy." Applied Spectroscopy. Available at https://journals.sagepub.com/doi/full/10.1177/00037028221090988
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