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Waterproof graphene electronic circuits

Posted By Graphene Council, The Graphene Council, Thursday, February 14, 2019
Updated: Thursday, February 14, 2019
Water molecules distort the electrical resistance of graphene, but a team of European researchers has discovered that when this two-dimensional material is integrated with the metal of a circuit, contact resistance is not impaired by humidity. This finding will help to develop new sensors –the interface between circuits and the real world– with a significant cost reduction.

The many applications of graphene, an atomically-thin sheet of carbon atoms with extraordinary conductivity and mechanical properties, include the manufacture of sensors. These transform environmental parameters into electrical signals that can be processed and measured with a computer.

Due to their two-dimensional structure, graphene-based sensors are extremely sensitive and promise good performance at low manufacturing cost in the next years.
To achieve this, graphene needs to make efficient electrical contacts when integrated with a conventional electronic circuit. Such proper contacts are crucial in any sensor and significantly affect its performance.

But a problem arises: graphene is sensitive to humidity, to the water molecules in the surrounding air that are adsorbed onto its surface. H2O molecules change the electrical resistance of this carbon material, which introduces a false signal into the sensor.

However, Swedish scientists have found that when graphene binds to the metal of electronic circuits, the contact resistance (the part of a material's total resistance due to imperfect contact at the interface) is not affected by moisture.

“This will make life easier for sensor designers, since they won't have to worry about humidity influencing the contacts, just the influence on the graphene itself,” explains Arne Quellmalz, a PhD student at KTH Royal Institute of Technology (Sweden) and the main researcher of the research.

The study, published in the journal ACS Applied Materials & Interfaces, has been carried out experimentally using graphene together with gold metallization and silica substrates in transmission line model test structures, as well as computer simulations.

“By combining graphene with conventional electronics, you can take advantage of both the unique properties of graphene and the low cost of conventional integrated circuits.” says Quellmalz, “One way of combining these two technologies is to place the graphene on top of finished electronics, rather than depositing the metal on top the graphene sheet.”

As part of the European CO2-DETECT project, the authors are applying this new approach to create the first prototypes of graphene-based sensors. More specifically, the purpose is to measure carbon dioxide (CO2), the main greenhouse gas, by means of optical detection of mid-infrared light and at lower costs than with other technologies.

Tags:  2D materials  Arne Quellmalz  Electronics  Graphene  KTH Royal Institute of Technology 

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Scientists Probe into the Effect of Graphene on Light-wave Interaction

Posted By Graphene Council, The Graphene Council, Wednesday, February 13, 2019
Updated: Wednesday, February 13, 2019
Two-dimensional (2D) nanomaterials are helping facilitate nanostructure science. Their outstanding nonlinear optical properties like enhanced two-photon absorption and absorption saturation make new applications possible in laser technologies, optical computing and telecommunications. 

Nowadays, ongoing wave mixing studies in 2D materials mainly focus on harmonic generation. Four-wave mixing in near infrared was recently carried out on a graphene monolayer, and revealed a third-order nonlinear susceptibility χ(3) value, which is about 7 orders of magnitude larger than in bulk insulators like silica and BK7 glass, 3-5 orders larger than in bulk semiconductors like silicon, germanium, cadmium and zinc chalcogenides, metal oxides, and 10 times larger than in thin plasmonic gold films and nanoparticles.

Most recently, an even larger value was obtained in graphene nanoribbons at mid-infrared frequencies close to the transverse plasmon resonance. 

Despite these not yet abundant but impressive advances of phase conjugation in graphene, the effect of 2D materials on Stimulated Brillouin scattering (SBS) remains overlooked. Due to its fundamental importance in laser and fiber telecommunications, the effect currently attracts theoretical considerations concerning bulk and composite semiconductor materials, including practical designs. 

Recently, a collaborative study led by Prof. Dr. WANG Jun at Laboratory of Micro-Nano Optoelectronic Materials and Devices, Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, investigated the character of SBS of low-concentration graphene nanoparticle suspensions in N-methyl-2-pyrrolidone (NMP) and water. 

They found a strong SBS quenching effect which was attributed to the interference of density gratings formed in the liquid by electrostriction and thermal expansion forces (see Fig. 1).

Established linear dependences of SBS threshold on graphene absorption coefficient (i.e., concentration) can be used for the detection of small nanomaterial quantities in liquid media down to 5×10-8g·cm-3. 

Computer simulations of the Brillouin gain factor show the efficiency of different thermodynamic, electrooptic and photoacoustic parameters in the SBS quenching. The role of density and compressibility, which change as a result of carbon vapor bubble formation, is found to be decisive in leading to dramatic changes of refractive index, electrostrictive and acoustic damping coefficients. 

The effect can give tools to bubble nanosecond dynamics studies and a method of SBS suppression in optical composites applicable in laser technologies and optical telecommunication networks. 

This study, entitled "Stimulated Brillouin scattering in dispersed graphene" has been published online in Optics Express on Dec. 18, 2018. 

This work was supported by the Chinese National Natural Science Foundation, the Strategic Priority Research Program of CAS, the Key Research Program of Frontier Science of CAS, the Program of Shanghai Academic Research Leader and President’s International Fellowship Initiative of CAS.  

Tags:  2D materials  Graphene 

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Engineers develop novel strategy for designing semiconductor nanoparticles for wide-ranging applications

Posted By Graphene Council, The Graphene Council, Tuesday, February 5, 2019
Updated: Tuesday, February 5, 2019
Two-dimensional (2D) transition metal dichalcogenides (TMDs) nanomaterials such as molybdenite (MoS2), which possess a similar structure as graphene, have been donned the materials of the future for their wide range of potential applications in biomedicine, sensors, catalysts, photodetectors and energy storage devices.

The smaller counterpart of 2D TMDs, also known as TMD quantum dots (QDs) further accentuate the optical and electronic properties of TMDs, and are highly exploitable for catalytic and biomedical applications. However, TMD QDs is hardly used in applications as the synthesis of TMD QDs remains challenging.

Now, engineers from the National University of Singapore (NUS) have developed a cost-effective and scalable strategy to synthesise TMD QDs. The new strategy also allows the properties of TMD QDs to be engineered specifically for different applications, thereby making a leap forward in helping to realise the potential of TMD QDs.

Bottom-up strategy to synthesise TMD QDs

Current synthesis of TMD nanomaterials rely on a top-down approach where TMD mineral ores are collected and broken down from millimetre to nanometre scale via physical or chemical means. This method, while effective in synthesising TMD nanomaterials with precision, is low in scalability and costly as separating the fragments of nanomaterials by size requires multiple purification processes. Using the same method to produce TMD QDs of a consistent size is also extremely difficult due to their minute size.

To overcome this challenge, a team of engineers from the Department of Chemical and Biomolecular Engineering at NUS Faculty of Engineering developed a novel bottom-up synthesis strategy that can consistently construct TMD QDs of a specific size, a cheaper and more scalable method than the conventional top-down approach. The TMD QDs are synthesised by reacting transition metal oxides or chlorides with chalogen precursors under mild aqueous and room temperature conditions. Using the bottom-up approach, the team successfully synthesised a small library of seven TMD QDs and were able to alter their electronic and optical properties accordingly.

Associate Professor David Leong from the Department of Chemical and Biomolecular Engineering at NUS Faculty of Engineering led the development of this new synthesis method. He explained, “Using the bottom-up approach to synthesise TMD QDs is like constructing a building from scratch using concrete, steel and glass component; it gives us full control over the design and features of the building. Similarly, this bottom-up approach allows us to vary the ratio of transition metal ions and chalcogen ions in the reaction to synthesise the TMD QDs with the properties we desire. In addition, through our bottom-up approach, we are able to synthesise new TMD QDs that are not found naturally. They may have new properties that can lead to newer applications.”

Applying TMD QDs in cancer therapy and beyond

The team of NUS engineers then synthesised MoS2 QDs to demonstrate proof-of-concept biomedical applications. Through their experiments, the team showed that the defect properties of MoS2 QDs can be engineered with precision using the bottom-up approach to generate varying levels of oxidative stress, and can therefore be used for photodynamic therapy, an emerging cancer therapy.

“Photodynamic therapy currently utilises photosensitive organic compounds that produce oxidative stress to kill cancer cells. These organic compounds can remain in the body for a few days and patients receiving this kind of photodynamic therapy are advised against unnecessary exposure to bright light. TMD QDs such as MoS2 QDs may offer a safer alternative to these organic compounds as some transition metals like Mo are themselves essential minerals and can be quickly metabolised after the photodynamic treatment. We will conduct further tests to verify this.” Assoc Prof Leong added.

The potential of TMD QDs, however, goes far beyond just biomedical applications. Moving forward, the team is working on expanding its library of TMD QDs using the bottom-up strategy, and to optimise them for other applications such as the next generation TV and electronic device screens, advanced electronics components and even solar cells.

Tags:  2D materials  Graphene 

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Promising steps towards large scale production of graphene nanoribbons for electronics

Posted By Graphene Council, The Graphene Council, Thursday, January 31, 2019
Two-dimensional sheets of graphene in the form of ribbons a few tens of nanometers across have unique properties that are highly interesting for use in future electronics. Researchers have now for the first time fully characterised nanoribbons grown in both the two possible configurations on the same wafer with a clear route towards upscaling the production.

Graphene in the form of nanoribbons show so called ballistic transport, which means that the material does not heat up when a current flow through it. This opens up an interesting path towards high speed, low power nanoelectronics. The nanoribbon form may also let graphene behave more like a semiconductor, which is the type of material found in transistors and diodes. The properties of graphene nanoribbons are closely related to the precise structure of the edges of the ribbon. Also, the symmetry of the graphene structure lets the edges take two different configurations, so called zigzag and armchair, depending on the direction of the long respective short edge of the ribbon.

The nanoribbons were grown in two directions along ridges on the substrate. This way both the zigzag- and armchair-edge varieties form and can be studied at the same time. The positions of the atoms in the graphene layer as well as the zig zag edge can be seen from the scanning tunneling microscopy image (Å stands for Ångström, 0.1 nanometers).

The nanoribbons were grown on a template made of silicon carbide under well controlled conditions and thoroughly characterised by a research team from MAX IV Laboratory, Technische Universität Chemnitz, Leibniz Universität Hannover, and Linköping University. The template has ridges running in two different crystallographic directions to let both the armchair and zig-zag varieties of graphene nanoribbons form. The result is a predictable growth of high-quality graphene nanoribbons which have a homogeneity over a millimeter scale and a well-controlled edge structure.

One of the new findings is that the researchers were able to show ballistic transport in the bulk of the nanoribbon. This was possible due to extremely challenging four probe experiments performed at a length scale below 100 nm by the group in Chemnitz, says Alexei Zakharov, one of the authors.

The electrical characterization also shows that the resistance is many times higher in the so called armchair configuration of the ribbon, as opposed to the lower resistance zig-zag form obtained. This hints to a possible band gap opening in the armchair nanoribbons, making them semiconducting. The process used for preparing the template for nanoribbon growth is readily scalable. This means that it would work well for development into the large-scale production of graphene nanoribbons needed to make them a good candidate for a future material in the electronics industry.

So far, we have been looking at nanoribbons which are 30–40 nanometers wide. It’s challenging to make nanoribbons that are 10 nanometers or less, but they would have very interesting electrical properties, and there´s a plan to do that. Then we will also study them at the MAXPEEM beamline, says Zakharov.

The measurements performed at the MAXPEEM beamline was done with a technique not requiring X-rays. The beamline will go into its commissioning phase this spring and will start welcoming users this year.

Tags:  2d materials  Graphene  graphene production 

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Open-source automated chemical vapor deposition system for the production of two-dimensional nanomaterials

Posted By Graphene Council, The Graphene Council, Wednesday, January 30, 2019
Updated: Tuesday, January 29, 2019
A research group at Boise State University led by Assistant Professor David Estrada of the Micron School of Materials Science and Engineering has released the open-source design of a chemical vapor deposition (CVD) system for two-dimensional (2D) materials growth, an advance which could lower the barrier of entry into 2D materials research and expedite 2D materials discovery and translation from the benchtop to the market.

2-dimensional materials are a class of materials that are one to a few atoms thick. The pioneering work of Nobel Laureates Andre Geim and Konstantin Novoselov in isolating and measuring the physical properties of graphene – a 2D form of carbon arranged in a hexagonal crystal structure - ignited the field of 2D materials research

While 2D materials can be obtained from bulk van der Waals crystals (e.g. graphite and MoS2) using a micromechanical cleavage approach enabled by adhesive tapes, the community quickly realized that unlocking the full potential of 2D materials would require advanced manufacturing methods compatible with the semiconductor industry’s infrastructure.

Chemical vapor deposition is a promising approach for scalable synthesis of 2D materials – but automated commercial systems can be cost prohibitive for some research groups and startup companies. In such situations students are often tasked with building custom furnaces, which can be burdensome and time consuming. While there is value in such endeavors, this can limit productivity and increase time to degree completion.

A recent trend in the scientific community has been to develop open-source hardware and software to reduce equipment cost and expedite scientific discovery. Advances in open-source 3-dimensional printing and microcontrollers have resulted in freely available designs of scientific equipment ranging from test tube holders, potentiostats, syringe pumps and microscopes. Estrada and his colleagues have now added a variable pressure chemical vapor deposition system to the inventory of open-source scientific equipment.

“As a graduate student I was fortunate enough that my advisor was able to purchase a commercial chemical vapor deposition system which greatly impacted our ability to quickly grow carbon nanotubes and graphene. This was critical to advancing our scientific investigations,” said Estrada. “When I read scientific articles I am intrigued with the use of the phrase “custom-built furnace” as I now realize how much time and effort graduate students invest in such endeavors.”

The design and qualification of the furnace was accomplished by lead authors Dale Brown, a former Micron School of Materials Science and Engineering graduate student, and Clinical Assistant faculty member Lizandra Godwin, with assistance from the other co-authors. The results of their variable pressure CVD system have been published in PLoS One ("Open-source automated chemical vapor deposition system for the production of two- dimensional nanomaterials") and include the parts list, software drivers, assembly instructions and programs for automated control of synthesis procedures. Using this furnace, the team has demonstrated the growth of graphene, graphene foam, tungsten disulfide and tungsten disulfide – graphene heterostructures.

“Our goal in publishing this design is to alleviate the burden of designing and constructing CVD systems for the early stage graduate student,” said Godwin. “If we can save even a semester of time for a graduate student this can have a significant impact on their time to graduation and their ability to focus on research and advancing the field.”

“We hope others in the community can improve on our design by incorporating open-source software for automated control of 2D materials synthesis,” said Estrada. “Such an improvement could further reduce the barrier to entry for 2D materials research.”

Tags:  2D materials  Boise State University  CVD  Graphene 

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Creating a roadmap for 2-D materials

Posted By Graphene Council, The Graphene Council, Monday, January 28, 2019
Updated: Monday, January 28, 2019

The rapid growth of research on 2-D materials – materials such as graphene and others that are a single or few atoms thick – is fueled by the hope of developing better performing sensors for health and environment, more economical solar energy, and higher performing and more energy efficient electronics than is possible with current silicon electronics.

Technical roadmaps, such as the International Technology Roadmap for Semiconductors (ITRS), first published in 1998, serve as guides for future advances in a particular field and provide a means for organizations to plan for investments in new technology.

An invited article in the December online edition of the journal 2-D Materials provides a roadmap for the synthesis of electronic-grade two-dimensional materials for future electronic and sensing applications. Led by Penn State, with contributions from five additional universities and national laboratories, the roadmap addresses the grand challenges in 2-D materials with useful electronic or photonic properties, and the outlook for U.S. advances in the field.

"This article is a review of where we currently are in regard to the synthesis of 2-D materials and our thoughts on the top research priorities that need to be addressed to achieve electronic grade 2-D materials," said Joshua Robinson, associate professor of materials science and engineering, whose Ph.D. students Natalie Briggs and Shruti Subramanian are co-lead authors on the report titled "A Roadmap for Electronic Grade 2-Dimensional Materials," published online today, Jan. 17.

The authors divided the paper into four parts: Grand Challenges, which are the technology drivers, such as the internet of things; Synthesis, the techniques and theories required to grow close to perfect 2-D materials; Materials Engineering, which is fine tuning the properties of 2-D and composite materials; and finally, Outlook, which is the future of electronic devices when silicon technology reaches an inevitable roadblock.

"To put our roadmap together, we reached out to experts in various subfields, such as different synthesis approaches, defect engineering and computational theory," said Briggs of the two-year project. "We asked them to talk about the key fundamental challenges and the steps required to address these challenges in their area of expertise."

Robinson added, "This is the first roadmap focused on 2-D synthesis for electronic applications and there are still a lot of open questions. We want to bring some of those topics into the light."

A list of the twenty authors and their affiliations can be found online in the open access article in 2-D Materials.

Tags:  2D materials  Graphene  International Technology Roadmap for Semiconductor 

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How Quantum Dots and Graphene Combined to Change the Landscape for Optoelectronics

Posted By Dexter Johnson, IEEE Spectrum, Wednesday, November 1, 2017

Last June, we covered research that brought graphene, quantum dots and CMOS all together into one to change the future of both optoelectronics and electronics. 

That research was conducted at the Institute of Photonics (ICFO) located just outside of Barcelona, Spain. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics.

Now, in a series of in-person interviews with several researchers at ICFO (the first of which you can find here),  we are gaining better insight into how these technologies came to be and where they ultimately may lead.

Gerasimos Konstantatos - group leader at ICFO

The combination of graphene with quantum dots for use in optoelectronics stems in large part from the contributions of Gerasimos Konstantatos, a group leader at ICFO, who worked with Ted Sargent at the University of Toronto, whose research group has been at the forefront of exploiting colloidal quantum dots for use in a range of applications, most notably high-efficiency photovoltaics.

“Our initial expertise and focus was on actually exploiting the properties of solution-process materials particularly colloidal quantum dots as optoelectronic materials for solar cells and photodetectors,” explained Konstantatos. “The uniqueness of these materials is that they give us access to a spectrum that is very rarely reached in the shortwave and infrared and they can do it at a much lower cost than any other technology.”

Konstantatos and his group were able to bring their work with quantum dots to the point of the near-infrared wavelength spectrum, which falls in the wavelength size range of one to five microns. Konstantos is now developing these solution-based quantum dot materials to produce even more sensitive materials capable of getting to 10 microns, putting them squarely in the mid-infrared range.

“My group is now working with Frank Koppens to sensitize graphene and other 2D materials in order to make very sensitive photodetectors at a very low cost that are capable of accessing the entire spectrum, and this cannot be done with any other technology,” said Konstantatos.

What Konstantatos and Koppens have been able to do is to basically eliminate the junction between graphene and the quantum dots and in so doing have developed a way to control the charge transfer in a very efficient way so that they can exploit the very high mobility and transport conductance of graphene.

“We can re-circulate the charges through the materials so that with a single photon we have several billion charges re-circulating through the material and this constitutes the baseline of this material combination,” adds Konstantatos.

With that as their baseline technology, Konstantatos and his colleagues have engineered the quantum dot layer so instead of just having a passive quantum dot layer they have converted it into an electro-diode. In this way they can make much more complex detectors. In the combination of the graphene-based transistor with the quantum dots, it’s not just a collection of quantum dots but is a photodiode made from quantum dots.

“In this way, we kind of get the benefit of both kinds of detectors,” explains Konstantatos. “You have a phototransistor that has a very high sensitivity and a very high gain, but you also get the high quantum efficiency you get in photodiodes. It’s basically a quantum photodiode that activates a transistor.”

In addition to the use of graphene, the ICFO researchers are looking at other 2D materials in this combination, specifically the semiconductor molybdenum disulfide. While this material is a semiconductor and sacrifices somewhat on the electron mobility of graphene, it does make it possible to switch off the material to control the current. As a result, Konstantatos notes that you can have much lower noise in the detector with much lower power consumption.

In continuing research, Konstantatos hinted at yet to be published work on how all of this combination of quantum dots and graphene could be used in solar cell applications.

In the meantime, the work they have been doing with graphene and quantum dots is much further advanced than what they have yet been able to achieve with molybdenum disulfide, mainly because work has advanced much further in making large scale amounts of graphene. But as the processes for producing other 2D materials improves, there will be a real competition between all of the 2D materials to see which provides the best possible performance as well as manufacturability properties.

In any event, Konstantatos sees that the way forward with both quantum dots and 2D materials is using them together.

He adds: “I think we can explore the synergies in between different material platforms. There's no such thing as a perfect material that can do everything right. But there is definitely a group of materials with some unique properties. And if you can actually combine them in a smart way and make hybrid structures, then I think you can have significant added value.”

Tags:  2D materials  graphene  optoelectronics  photodetectors  photonics  photovoltaics  quantum dots 

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