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How to stack graphene up to four layers

Posted By Graphene Council, Wednesday, July 29, 2020
Graphene, an atomically thin hexagonal structure of carbon atoms is a potential candidate for electronic and optoelectrical applications such as transparent electrodes and interconnect for integrated circuits. Yet, it is one thing to possess such useful properties and to induce an intended characteristic from this "wonder material" is another. In the face of the end of the "Moore's Law", chip makers have set their sight on multi-layered graphene for its scaling ability of integrated circuits to smaller physical dimensions and the electric-field induced bandgap, which is not affordable in monolayer graphene. Furthermore, owning to exotic physical properties controlled by its stacking orders (the arrangement of graphene layer along vertical direction) such as superconductivity and quantum Hall effect to name a few, multi-layer graphene is an interesting material for condensed matter physicists. Still, the unknown growth method for uniform single-crystalline multilayer graphene growth in a wafer scale presents a challenge.

Led by professor LEE Young Hee at the Center for Integrated Nanostructure Physics, the Institute for Basic Science (IBS) in Sungkyunkwan University, South Korea, an IBS research team reports a novel method to grow multi-layered, single-crystalline graphene with a selected stacking order in a wafer scale. They obtained four-layered graphene using chemical vapor deposition (CVD) via Cu-Si alloy formation.

There have been several approaches to control the number of graphene layers. Conventionally, the monolayer graphene, which is easily grown on Cu-substrate, can be detached from the Cu-substrate and transferred onto insulator substrates such as SiO2/Si. Therefore, the simplest method to make multilayer graphene is to stack them layer-by-layer via the transfer process. However, this transfer process may cause tearing, wrinkles, and/or polymer residues. Though such issues can be avoided via a direct method, i.e. CVD on Cu substrate, the low solubility of carbon (C) in copper (Cu) hampers the controlling of the number of graphene layers with high uniformity in a large area. By depositing Ni or Co to form Cu-Ni/ Cu-Co alloys or employing oxygen-rich Cu substrate, C solubility in Cu is boosted and thus stacks more layers of graphene. Nevertheless, a small portion of inhomogeneous multilayers occurs. Controlling the crystallographic stacking sequence of graphene films thicker than two layers with high uniformity has not been demonstrated to date.

Dr. Van Luan Nguyen, the first author of the study (now at Samsung Advanced Institute Technology) proposed to use silicon carbide (SiC) on the surface of Cu substrate alloy, via the sublimation of Si atoms at a high temperature. They controlled the C solubility in the Cu film by introducing Si content on Cu surface by heat treatment of Cu substrate with a constant H2 gas flow inside the quartz tube of CVD chamber. "The formation of a homogeneous Cu-Si alloy, as a role of catalyst, was critical to control the number layers of graphene film in a wafer scale with methane gas. With the presence of Cu-Si alloy, SiC can be formed when methane gas is injected and the following sublimation process of Si atoms leaves C atoms behind to form multilayer graphene. Si amount is fixed at 28.7 % for uniform multilayer graphene film. Depending the concentrations of argon (Ar)-diluted methane gas, the number of graphene layers varies," says Dr. Van.

Growing in a large scale of this much-hyped graphene has seen much progress over a decade, but building multi-layered graphene is just in its early stages. Our study offers a novel approach to upgrade the conventional CVD method by introducing an intermediate process of in-situ formation of SiC film," notes Dr. LEE Sang Hyub, coauthor of the study. Importantly, this study provides a new platform to synthesize graphene multilayer towards the uniform large-area single-crystalline layer-tunable multilayer graphene as well as graphite thin film. This is an initial step to incorporate multilayer graphene to display panels and integrated circuits such as via-holes and replacement of Cu electrodes as well as photoelectronic and photovoltaic devices.

"Deposition of Si by conventional methods at low temperatures such as thermal evaporation or sputtering does not work for uniform multilayer graphene growth. The key in our new approach is to form uniform Cu-Si alloy on quartz tube chamber in which Si is sublimated at high temperature of 900 ? with H2 gas flow in a controllable manner," explains Director LEE Young Hee, the corresponding author of the study. Although the substantial achievement has been demonstrated in our current work, Director Lee cautions that the method to deposit Si at high temperature during the growth process is not practical and can be harmful for the quart tube for long-term use. They are searching for a solution to replace the current one for mass product.

Tags:  chemical vapor deposition  CVD  Graphene  LEE Young Hee  monolayer graphene  Samsung Advanced Institute Technology  Sungkyunkwan University  Van Luan Nguyen 

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Graphene’s ultimate stretchability and ‘realistic’ strength discovered

Posted By Graphene Council, Wednesday, March 11, 2020
The realistic mechanical properties of monolayer graphene have been successfully studied by a new method developed by a research team led by Dr Lu Yang, Associate Professor of Department of Mechanical Engineering at City University of Hong Kong (CityU). The groundbreaking discovery will promote the application of graphene in different areas, such as the touch monitor on flexible mobile phones.

Dr Lu’s research achievement has been published in the prestigious international journal Nature Communications, titled “Elastic straining of free-standing monolayer graphene.” This paper was also highlighted in “Editors' Choice” in the 21 February 2020 issue of Science.

A two-dimensional carbon substance, graphene is the strongest material known with excellent electrical and thermal conductivity. Hence, it is deemed a "super material", ideal for many fields, for example, transistors, biosensors and batteries.

Graphene’s structure as a single layer of atoms has made it extremely difficult for scientists to test its actual mechanical properties such as elasticity and tensile strength. The studies in this area so far have covered only its ideal limits by local indentation experiments and theoretical calculations.

“No one has really stretched a large-area, free-standing monolayer graphene and tested its elastic tensile properties,” Dr Lu said.

Over the years, Dr Lu has researched the mechanical properties of various nanomaterials. His research team has successfully developed a new method for transferring large-area graphene onto his unique nanomechanical testing platform, performing in situ tensile tests in a scanning electron microscope to study changes in stretching and shaping.

“One major challenge in our study is how to transfer and lay an extremely light and thin monolayer graphene sample onto a testing platform without damage, and apply the strain evenly when stretching it,” Dr Lu said.

The experiment showed that the tensile strength of chemical vapour deposition (CVD)-grown monolayer graphene can reach 50 to 60 GPa (gigapascal), with elastic strain up to 6%, and the measured Young’s modulus (or the “elastic modulus”) is 920 GPa, which is very close to the theoretical value of ~1,000 GPa. Pascals are units of measurement for stress.

“It took us nearly four years to overcome a lot of difficulties for the experiment, but our work has revealed the realistic mechanical properties of graphene for engineering relevance,” Dr Lu said.

Its strength and stretchability make graphene a suitable material for manufacturing flexible electronic devices, such as transistors with better robustness, organic light-emitting diodes, and other mechanical components.

It can also be used for the production of composite materials and in the areas of biomedical research, aviation and national defence.

Dr Lu said he was grateful to CityU for providing top-notch facilities for his team to conduct their research, such as the Nano-Manufacturing Laboratory at the CityU Shenzhen Research Institute, the Centre for Super-Diamond and Advanced Films, and the Centre for Advanced Structural Materials.

In addition, CityU’s emphasis on interdisciplinary collaboration helped his research. “Our experiment required experts from the disciplines of mechanics, materials science, chemistry and physics to work together, and the outstanding talents in these fields can be readily found at CityU,” Dr Lu said.

Members of the research team include PhD students Cao Ke and Han Ying in the Department of Mechanical Engineering and Dr Ly Thuc-hue, Assistant Professor in the Department of Chemistry, at CityU, as well as experts from Tsinghua University and Xidian University.

Tags:  chemical vapour deposition  City University of Hong Kong  Electronics  Graphene  Lu Yang  monolayer graphene 

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