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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 from sugar, a sweet protocol

Posted By Graphene Council, Saturday, February 29, 2020

Scientists from the Centre for Nano and Soft Matter Sciences, Bengaluru, an autonomous institution under the Department of Science & Technology, Government of India have synthesised reduce graphene oxide (rGO) by the combustion of table-sugar.

The group led by Prof. C. N. R. Rao consisting of Dr. P. Chithaiah from CeNS and Prof. G. U. Kulkarni from JNCASR, Bengaluru has developed a rapid and simple route for the synthesis of rGO by the combustion of table-sugar. This method being single-step and reproducible is advantageous compared to the reported protocols used presently. Further, the synthesis doesn’t involve any metal catalysts, expensive reagents, solvents, hazardous chemicals, and, most importantly, it has the ability to produce graphene oxide in large quantities at rapid rates.

Graphene, a one-atom-thick, two-dimensional sheet of sp2 hybridized carbon atoms is known as a wonder material, as it is stronger than diamond, conducts better than copper along with many other interesting properties. However, the production of graphene in large scale has many challenges to address. 

Till date, methods like chemical vapor deposition, arc discharge, aerosol pyrolysis, mechanical exfoliation, solvothermal, hydrothermal synthesis, laser reduction of graphite oxide have been developed to prepare graphene (reduce graphene oxide, rGO).

All these methods either involve hazardous chemicals, high temperatures, and inert atmosphere making them expensive and thus becoming irrelevant for bulk scale applications.

The team believes that the process developed may have a significant impact on various products, including batteries. Their work has been published in the ‘Beilstein Journal of Nanotechnology.’

Tags:  2D materials  C. N. R. Rao  Centre for Nano and Soft Matter Sciences  chemical vapor deposition  G. U. Kulkarni  Graphene  graphene oxide  nanotechnology  P. Chithaiah 

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Converting graphene into diamond film without high pressure

Posted By Graphene Council, Wednesday, December 11, 2019
Can two layers of graphene be linked and converted to the thinnest diamond-like material? Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) have reported in Nature Nanotechnology ("Chemically Induced Transformation of CVD-Grown Bilayer Graphene into Fluorinated Single Layer Diamond") the first experimental observation of a chemically induced conversion of large-area bilayer graphene to the thinnest possible diamond-like material, under moderate pressure and temperature conditions.

This flexible, strong material is a wide-band gap semiconductor, and thus has potential for industrial applications in nano-optics, nanoelectronics, and can serve as a promising platform for micro- and nano-electromechanical systems.

Diamond, pencil lead, and graphene are made by the same building blocks: carbon atoms (C). Yet, it is the bonds’ configuration between these atoms that makes all the difference. In a diamond, the carbon atoms are strongly bonded in all directions and create an extremely hard material with extraordinary electrical, thermal, optical and chemical properties. In pencil lead, carbon atoms are arranged as a pile of sheets and each sheet is graphene. Strong carbon-carbon (C-C) bonds make up graphene, but weak bonds between the sheets are easily broken and in part explain why the pencil lead is soft. Creating interlayer bonding between graphene layers forms a 2D material, similar to thin diamond films, known as diamane, with many superior characteristics.

Previous attempts to transform bilayer or multilayer graphene into diamane relied on the addition of hydrogen atoms, or high pressure. In the former, the chemical structure and bonds’ configuration are difficult to control and characterize. In the latter, the release of the pressure makes the sample revert back to graphene. Natural diamonds are also forged at high temperature and pressure, deep inside the Earth. However, IBS-CMCM scientists tried a different winning approach.

The team devised a new strategy to promote the formation of diamane, by exposing bilayer graphene to fluorine (F), instead of hydrogen. They used vapors of xenon difluoride (XeF2) as the source of F, and no high pressure was needed. The result is an ultra-thin diamond-like material, namely fluorinated diamond monolayer: F-diamane, with interlayer bonds and F outside.

For a more detailed description; the F-diamane synthesis was achieved by fluorinating large area bilayer graphene on single crystal metal (CuNi(111) alloy) foil, on which the needed type of bilayer graphene was grown via chemical vapor deposition (CVD).

Conveniently, C-F bonds can be easily characterized and distinguished from C-C bonds. The team analyzed the sample after 12, 6, and 2-3 hours of fluorination. Based on the extensive spectroscopic studies and also transmission electron microscopy, the researchers were able to unequivocally show that the addition of fluorine on bilayer graphene under certain well-defined and reproducible conditions results in the formation of F-diamane. For example, the interlayer space between two graphene sheets is 3.34 angstroms, but is reduced to 1.93-2.18 angstroms when the interlayer bonds are formed, as also predicted by the theoretical studies.

“This simple fluorination method works at near-room temperature and under low pressure without the use of plasma or any gas activation mechanisms, hence reduces the possibility of creating defects,” points out Pavel V. Bakharev, the first author and co-corresponding author.

Moreover, the F-diamane film could be freely suspended. “We found that we could obtain a free-standing monolayer diamond by transferring F-diamane from the CuNi(111) substrate to a transmission electron microscope grid, followed by another round of mild fluorination,” says Ming Huang, one of the first authors.

Rodney S. Ruoff, CMCM director and professor at the Ulsan National Institute of Science and Technology (UNIST) notes that this work might spawn worldwide interest in diamanes, the thinnest diamond-like films, whose electronic and mechanical properties can be tuned by altering the surface termination using nanopatterning and/or substitution reaction techniques. He further notes that such diamane films might also eventually provide a route to very large area single crystal diamond films.

Tags:  2D material  bilayer graphene  Center for Multidimensional Carbon Materials  chemical vapor deposition  Graphene  Institute for Basic Science  Ming Huang  nanoelectronics  Nature Nanotechnology  Pavel V. Bakharev  Rodney S. Ruoff  semiconductor  Ulsan National Institute of Science and Technology 

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GrollTex Tackles Sensor Markets With High Quality Graphene

Posted By Dexter Johnson, The Graphene Council, Thursday, March 1, 2018

  Jeffrey Draa, CEO, GROLLTEX

 

Last month, The Graphene Council's Executive Director, Terrance Barkan, and its Editor-in-Chief, Dexter Johnson, had the opportunity to have a talk with the CEO of California-based Grolltex Inc., Jeffrey Draa, about the company's business strategies in bringing graphene products to market and his views on graphene's future. Here is that conversation.

Could you tell us a little bit about the background of GrollTex. How did the company get started and how did you get involved with graphene? In particular, could you provide the history of Grolltex as a company?

Sure, so the name Grolltex is short for graphene rolling technologies and the brief history of the company is that my partner and co-founder and really the inventor, Dr. Alexander Zaretski, was a researcher at University of California San Diego.

He was involved with graphene growth and really got deep into graphene manufacturing techniques while he was at the University of California San Diego. One of the issues with this specific kind of graphene, as generated by chemical vapor deposition (CVD), of course, is the ‘transfer’ issue: How does one get single-layer graphene synthesized from copper off of the copper growth substrate and onto a substrate of interest without destroying the copper growth substrate? Of course, the current state-of-the art is to either acid etch the copper off of CVD graphene, or to use an electrolytic solution to sort of bubble the graphene off of the copper and have it rise to the top after a long period of time.

So both of these two processes, which had been state-of-the art, impact the copper in a very negative way so it's very expensive and not manufacturable. And my partner, Alexander, decided if graphene is going to go forward, there has to be a way to manufacture graphene and not destroy or impact that copper.  So he came up with a process to do that, a process that has a rolling schema where we reuse that growth copper over and over again. So that's kind of the background of the company. Alex had decided that he wanted, and felt so passionately about, this transfer technology and bringing it to graphene manufacturing that after completing his work as a researcher at UCSD, he broke out on his own and he asked me if I would be the business side of the company and he had the technical side. So that's kind of a brief background of Grolltex and how we came to be.

I understand you’re privately held company, correct?

We are, yes. We were funded roughly a year and a half ago with our seed funding and we've since about six months ago taken another round.

In terms of your graphene manufacturing that you just laid out, as you said you focused on producing single-layer graphene of the highest quality, so what are the markets that this product offering opens up to you? And what do you see as your strongest market now and do you see that market changing five years from now?

Well, as anybody that has knowledge of the graphene markets knows, single-layer high purity graphene like that synthesized via CVD has many theoretical use cases. We see on the short-term horizon three particular applications that are really kind of starting to command our attention. Those three are number one: sensing. So graphene given its electrical and mass properties makes an excellent sensor at a very, very small level. So sensing is number one.

We also are doing some work in the advanced solar cell arena and we have a grant from the California Energy Commission where we're working on a two-sided solar cell where graphene not only plays the part of barrier material but it's also the electrode material. So that's really exciting.

And for number three we’re also starting to get some inquiries for an application that actually Dr. Andre Geim at the University of Manchester, who, of course, was the discoverer of graphene was very passionate about. This is one of the very first applications that he thought futuristically would really make the world a better place, and that third application that we're starting to see on the horizon is graphene as a proton exchange membrane in a hydrogen fuel cell.

So those are kind of the three leading candidates we see right now. We’re judging that by some initial business that we’re getting in those areas.

You were discussing a number of applications you are pursing, including sensors. On your website you talk about enabling sensors that could be used for the Internet of Things. Can you explain why you see graphene playing such an important role in the development Internet of Things?

I’ll speak a little about graphene as a sensor material. When you combine the electrical conductivity properties along with the fact that graphene is one atom thick, you've got the potential for a sensor that could take us into the future for the next hundred years. We have patents around some designs of graphene-based sensing materials that are so sensitive that, for example, in the biotech world we had some bioengineering folks at Stanford use our sensor to sense the ability of individual heart cells to contract. Currently there only exists a different kind of test that can only count the number of contractions, but our sensor is so sensitive that it picks up the strength of contraction of the individual heart cell when it beats and it's a very robust signal; there's no mistaking it. So that's just one example of the potential of graphene as a sensor and we're seeing good activity there.

What is consistently your biggest challenge when you're talking to potential customers and convincing them how to use your product? Are they worried about pricing of graphene, the quality of product, a consistent supply chain? What stands out as one of the key issues that keeps coming up when you're speaking to these people?

So, I think the first consistent theme would surprise no one, and it's price. Almost any inquiry goes down along the lines of price, especially for a field like solar. If solar is implemented it’s going to need miles and miles of cheap graphene. Now the case of a sensor is not quite as price sensitive, but with regards to the big kind of large applications people think about like flexible displays and some of the other big idea changes for graphene those are really price sensitive. So price is the first one.

We don't get too many concerns with regards to the supply chain. Quality of product is sometimes discussed and that's partly because graphene is such a new field. But a lot of folks have what they are calling graphene and maybe debatably it is not. We don't necessarily have that problem because no one argues that single-layer graphene made by CVD is not graphene, so we don't have any discussion of the quality, but that sometimes can be an issue. So to kind of summarize, and get back to your main question, really price is the first thing that people want and that's the first hurdle you have to get over with almost everyone.

In addition to your graphene product, you're also producing hexagonal boron nitride (sometimes called white graphene). How do you see this material filling out your portfolio and what are the applications for this material that you're currently targeting and do you expect to develop other two-dimensional materials?

Hexagonal boron nitride is something we’re very excited about for several reasons. For the folks that aren't familiar with hexagonal boron nitride, you need to understand how it works with graphene. Graphene is, of course, the most conductive substance known at room temperature; it's on the order of seven times more conductive than copper depending on who you talk to. So as a conductor, graphene is really unparalleled. Now if you're going to design an electronic device of any type, of course, you worry about a conducting material because you can make the wire, the battery, and the switch with the conductive material. But the other thing you have to worry about is the insulating material. What are you going to use for the insulation for graphene? You have to separate the layers of the devices and hexagonal boron nitride is as good an electrical insulator as graphene is a conductor. And hexagonal boron nitride has a hexagonal pattern when it is synthesized in the proper way and that pattern lines up perfectly with the hexagonal lattice pattern of graphene so it also provides the strength benefit too. So it is really the ideal cousin of graphene. If you're an electronic designer, you're going to want both a conductor and an insulator and now we're going to be delivering both.

So that answers your first question and your second question, which was “are we going to develop other two dimensional materials?” As far as basic building blocks, we are going to rest on graphene and hexagonal boron nitride for a time because again those are your two basic building blocks: you need the insulation and the conducting but we also are developing other materials that go into specific devices. So, an example of this is the sensors I talked about that require some precious metal in small quantity—atomic quantity.  There are other materials involved when you go to make a specific device, but as far as the basic building blocks we're going to stand pat on graphene and hBN probably for a while.

What is your perspective on hybrid graphene materials? I am referring to this combination of a conductor and an insulator, or even a conductor with a semiconductor, and based on that will you look to develop those hybrids yourself or have your client make the next step in the value chain?

At the moment our clients are doing that work. Now I'm not going to say that we won't get into it, but we're going to be opportunistic with regard to that. With regard to opportunistic roads that we can go down today, our plate is pretty full, but there are several routes we can take. We are seeing folks in the semiconductor world, which is my background, starting to use other materials and creating devices out of some of those second and third level hybrids as you described and that's really exciting work. So we may get into some of that, but again we have a lot on our plate right now just based on what I described already.

I’d like to get your view of the overall industry over the short, middle and long term—five, ten, fifteen years expectations of graphene and the industry. And what is your strategy for best placing your company in the environment that you see developing?

With regard to our company, just saying the word “graphene” to a lot of people opens up so many thought patterns, channels and ideas that one of the things that's going to have to happen is the standards are going to need to be put in place fairly soon so that people can know what they're saying when they say “graphene”.

There's a lot of graphitic solutions out there and hybrids and powders and all kinds of things that debatably aren't graphene (of course, I would say that because of my company is involved in the area that there is no argument that it is pure graphene). But the point of that is we will need some standards and some nomenclature put in place to help take this whole field to the next level.

There's all kinds of great use cases for graphitic solutions that aren't graphene—great use cases, don't get me wrong—but let's make sure that we can assign proper nomenclature so people know what they are talking about and looking at. With regard to my company specifically, one of our challenges is picking our targets because again there are so many kinds of different opportunities. And when we first started out we decided we were going to have a two-phased approach to our business and we're doing the phase one part of that now.

Phase one for us is to make and sell graphene material as research material. So our core customer for our phase one is the university lab and commercial lab. So we sell graphene on copper substrates, on wafers, we sell graphene on customer specific substrates. You send us your substrate of interest and we’ll put our graphene on it and send it back to you. That phase one of our business that I just described to you is allowing us to pursue all kinds of exciting applications and some of them are helping us go in new directions. So, our challenge in the first five years I think is; number one stay on that phase-one path, get to profitability just as a business and number two really pick our paths carefully with regard to what are going to be the first real big market businesses out there in graphene—the ones that have paying customers.

So from a commercialization perspective, I think what you mention is that the majority, or is it basically all, of your customers are they in the testing or R&D category right now?

Yes, that’s fair to say. There's a population of big players in the industry that have their own graphene “skunkworks” that they're just not talking about. For example, I'm just going to throw some names around freely about big companies that we happen to know that do have graphene labs internally that it's just really very hush-hush. The reason we know this is because we know some of the people that have been hired out of other graphene places into these big companies. For example, Apple is one of them. They don't talk about it but they have a big graphene effort. Hewlett Packard is of one of those. Samsung is not bashful about their graphene efforts. So there are a lot of big companies where there is a lot of activity going on but nobody is talking about it so I think there's a lot more happening in graphene than people are even aware of because it’s not being leaked.

One of the things we’re very interested in doing as the Graphene Council is helping to act as a catalyst and accelerate commercialization.

One of the biggest obstacles to commercialization we’ve seen is simply the education of potential end users and consumers.  Can you talk a little bit about that? I mean as a company trying to educate potential clients one by one is a time consuming and expensive proposition.

What are the some of the other vertical markets or specific application areas—you mentioned sensors, of course? Are there some other specific areas where you think there's good commercial opportunity where we can help educate those populations?

The first one that comes to mind is the display market. So the display folks, of course, have been using indium tin oxide (ITO) as their core material for decades. ITO is really not a great material for them; it's expensive, it involves dirty mining and it is very prone to pollution when getting it out of the earth. It's also brittle which is why everyone’s display on their phone, their laptop, their television, all displays are brittle; they're like glass.

Graphene is actually a plug and play replacement for ITO and graphene enables flexible displays. So, the first big use case I can think of and that would be the most exciting and the most impactful for the most people is ITO replacement for making flexible displays.

But also I think it’s a use case that's pretty far down the road. It’s very price sensitive for one reason and number two there is a huge infrastructure with multiple large multinational corporations already in place and has been in place for decades with a big manufacturing schema, billions of dollars all lined up to process ITO, etc.. That's not going to shut off overnight and just accept a graphene replacement, right? So, from a price perspective and from an implementation perspective there are some challenges with ITO replacement, but I think that it strikes me as the kind of area where we can start hammering away at some of the existing thinking.

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The Graphene Council thanks Jeffrey Draa, CEO of GROLLTEX for his time and unique insights into the developing market for graphene. 

 

Tags:  chemical vapor deposition  Hexagonal boron nitride  ITO  photovoltaics  sensors 

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