Graphene - from science fundamentals to low-cost production

Keynote presentations on the third day of Graphene Week 2015 offered an eclectic mix of fundamental science and practical chemical engineering. Here we report briefly on each of the talks, beginning with an introduction to optoelectronics in 2d semiconductors and heterostructures, and concluding with an outline of a highly promising ‘kitchen sink’ approach to graphene production. Xiaodong Xu of the University of Washington in Seattle set the ball rolling with a look at the optoelectronics and valleytronics of 2d semiconductors and heterostructures, such as those based on the sulphides and selenides of molybdenum and tungsten. Xu began with a recap of the electronic properties of MoS2, including the change in band gap with number of layers. Of particular interest to Xu are valley-specific interlayer excitons in monolayer WSe2-MoSe2 vertical heterostructures. Optical pumping leads to coupled spin-valley polarisation of interlayer excitons, with measured lifetimes of more than 30 nanoseconds. Such long-lived polarisation allows for the visualisation of both spin and valley diffusion over length scales of several microns. In practical terms, the effect is important in laser technologies with a tuneable carrier density. Marco Polini of the Scuola Normale Superiore in Pisa discussed plasmon damping mechanisms, with a focus on graphene sheets encapsulated in boron nitride. With 2d materials, plasmon lifetimes are two orders of magnitude longer than with bulk materials such as silicon and silver. With graphene and boron nitride, lifetimes as long as a picosecond have been observed. Damping mechanisms outlined by Polini included electron-electron, electron-impurity, and electron-phonon collisions. He went on to discuss experimental and theoretical work on hybrid plasmon-phonon polaritons. In the second part of his talk, Polini looked at direct current transport in graphene, with hydrodynamic flow and current whirlpools observed at length scales of half a micron. Amir Yacoby from Harvard University in Cambridge, Massachusetts discussed observations of edge currents using Josephson interferometry. The idea here is to use superconductivity to study the intrinsic physical properties of graphene. As for the origin of the observed edge currents, Yacoby suggested that certain edge shapes may guide the current. An alternative explanation is guided electron fibre-optic states at the Dirac point. This guided mode theory can explain edge current observations in bilayer as well as in single layer graphene.
Relevant applications include imaging of topological currents at domain boundaries in bilayer graphene, and induced superconductivity in the quantum spin Hall regime.

Graphene is a two-dimensional polymer, noted Klaus Müllen of the Max-Planck Institute in Mainz, and this makes it something of a challenge for materials synthesis. Müllen looked at both bottom-up and top-down production protocols, including the flattening of 3d, propeller-like molecules. The most promising approach to graphene synthesis is electrochemical exfoliation.

Applications of electrochemically exfoliated graphene identified by Müllen include organic photodetectors and transparent conductive electrodes, with the ability to produce ultrathin and flexible devices. Energy storage is another possibility, using exfoliated graphene and colloidal nanoparticles. Such nanoparticles, wrapped in graphene, offer high reversible charge capacity, retention and Coulomb efficiency.

Müllen concluded his talk with some 3d simulations of carbon networks, and noted, with the illustration of a beehive, that nature sometimes makes mistakes.

Manish Chhowalla of Rutgers University in New Jersey began his talk with an overview of molybdenum and tungsten disulphides. These layered semiconductor materials have a number of interesting properties, but the key problem in using them for electronics applications has been high contact resistance with metals deposited on the semiconducting 2H phase.

Contact resistance in MoS2 can be reduced by inducing a metallic (1T) phase on 2H phase nanosheets. Hybrid field-effect transistors with 2H monolayer MoS2 as the channel, and 1T source and drain contacts, display high electron mobilities, low subthreshold swing values, high on/off ratios and drive currents, and excellent current saturation. Deposition of different metals has a limited influence on transistor performance, suggesting that the 1T-2H interface controls carrier injection into the channel. In practical terms, the MoS2 channel must be locally patterned in order to make such structures. This can be done with a PMMA mask to partially cover certain areas. The result is a contact resistance of 0.2 kiloohms per micrometre. In comparison, 2H phase MoS2 has a contact resistance of 1.12 kiloohms per micron. Jonathan Coleman from Trinity College Dublin spoke of his research group's much-lauded graphene production process known as liquid phase exfoliation, aka kitchen-blender graphene. And not only graphene, as the technique can be used to produce nanoscale flakes of a range of 2d materials. Coleman discussed the fundamentals and practicalities of liquid-phase exfoliation, focusing on such matters as control of flake size. The bulk of Coleman's presentation was given to applications, and here he identified a number of areas. These include the mechanical improvement of composite materials, strain and other motion sensors based on electrical conductivity changes, electrical energy storage and printed electronics. The next challenge for liquid exfoliation is to achieve industrial-scale production of graphene and related 2d materials. To this end, Coleman highlighted a collaboration between his research group and chemical manufacturer Thomas Swan.
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Changing the color of light

Researchers at the University of Delaware have received a $1 million grant from the W.M. Keck Foundation to explore a new idea that could improve solar cells, medical imaging and even cancer treatments. Simply put, they want to change the color of light. They won’t be tinkering with what you see out your window: no purple days or chartreuse nights, no edits to rainbows and blazing sunsets. Their goal is to turn low-energy colors of light, such as red, into higher-energy colors, like blue or green. Changing the color of light would give solar technology a considerable boost. A traditional solar cell can only absorb light with energy above a certain threshold. Infrared light passes right through, its energy untapped. However, if that low-energy light could be transformed into higher-energy light, a solar cell could absorb much more of the sun’s clean, free, abundant energy. The team predicts that their novel approach could increase the efficiency of commercial solar cells by 25 to 30 percent. different colors The UD research team aims to develop new nanostructures that act like a ratchet to combine the energy of two red photons of light into a single blue photon, which has higher energy. Such an advance could improve solar cell efficiency to chemotherapy treatments. The research team, based in UD’s College of Engineering, is led by Matthew Doty, associate professor of materials science and engineering and associate director of UD’s Nanofabrication Facility. Doty’s co-investigators include Joshua Zide, Diane Sellers and Chris Kloxin, all in the Department of Materials Science and Engineering; and Emily Day and John Slater, both in the Department of Biomedical Engineering. “This prestigious $1 million grant from the Keck Foundation underscores the excellence and innovation of our University of Delaware faculty,” says Nancy Targett, acting president of the University. “Clearly, the University of Delaware is pursuing big ideas in renewable energy and biomedicine with the potential to benefit the world.” “The University’s Delaware Will Shine strategic plan challenges us to think boldly as we seek solutions to problems facing society,” Domenico Grasso, UD’s provost, adds. “We congratulate the research team in the College of Engineering for this major award, and we look forward to their findings.” Changing the color of light “A ray of light contains millions and millions of individual units of light called photons,” says project leader Matthew Doty. “The energy of each photon is directly related to the color of the light — a photon of red light has less energy than a photon of blue light. You can’t simply turn a red photon into a blue one, but you can combine the energy from two or more red photons to make one blue photon.” This process, called “photon upconversion,” isn’t new, Doty says. However, the UD team’s approach to it is. They want to design a new kind of semiconductor nanostructure that will act like a ratchet. It will absorb two red photons, one after the other, to push an electron into an excited state when it can emit a single high-energy (blue) photon. These nanostructures will be so teeny they can only be viewed when magnified a million times under a high-powered electron microscope. “Think of the electrons in this structure as if they were at a water park,” Doty says. “The first red photon has only enough energy to push an electron half-way up the ladder of the water slide. The second red photon pushes it the rest of the way up. Then the electron goes down the slide, releasing all of that energy in a single process, with the emission of the blue photon. The trick is to make sure the electron doesn’t slip down the ladder before the second photon arrives. The semiconductor ratchet structure is how we trap the electron in the middle of the ladder until the second photon arrives to push it the rest of the way up.” The UD team will develop new semiconductor structures containing multiple layers of different materials, such as aluminum arsenide and gallium bismuth arsenide, each only a few nanometers thick. This “tailored landscape” will control the flow of electrons into states with varying potential energy, turning once-wasted photons into useful energy. The UD team has shown theoretically that their semiconductors could reach an upconversion efficiency of 86 percent, which would be a vast improvement over the 36 percent efficiency demonstrated by today’s best materials. What’s more, Doty says, the amount of light absorbed and energy emitted by the structures could be customized for a variety of applications, from lightbulbs to laser-guided surgery. How do you even begin to make structures so tiny they can only be seen with an electron microscope? In one technique the UD team will use, called molecular beam epitaxy, nanostructures will be built by depositing layers of atoms one at a time. Each structure will be tested to see how well it absorbs and emits light, and the results will be used to tailor the structure to improve performance. The researchers also will develop a milk-like solution filled with millions of identical individual nanoparticles, each one containing multiple layers of different materials. The multiple layers of this structure, like multiple candy shells in an M&M, will implement the photon ratchet idea. Through such work, the team envisions a future upconversion “paint” that could be easily applied to solar cells, windows and other commercial products. Improving medical tests and treatments While the initial focus of the three-year project will be on improving solar energy harvesting, the team also will explore biomedical applications. A number of diagnostic tests and medical treatments, ranging from CT and PET scans to chemotherapy, rely on the release of fluorescent dyes and pharmaceutical drugs. Ideally, such payloads are delivered both at specific disease sites and at specific times, but this is hard to control in practice. The UD team aims to develop an upconversion nanoparticle that can be triggered by light to release its payload. The goal is to achieve the controlled release of drug therapies even deep within diseased human tissue while reducing the peripheral damage to normal tissue by minimizing the laser power required. “This is high-risk, high-reward research,” Doty says. “High-risk because we don’t yet have proof-of-concept data. High-reward because it has such a huge potential impact in renewable energy to medicine. It’s amazing to think that this same technology could be used to harvest more solar energy and to treat cancer. We’re excited to get started!
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A 'nanomachine' for surgery with no incision

A surgical operation has long been considered one of the first options in cancer treatment; however, a number of issues have been recognized: a highly invasive procedure; a decline in the Quality of Life (QOL) after an operation; the possibility of a recurrence due to missed cancer cells; extended hospitalization, sometimes for as long as one month; and the economic costs. Against such a background, recently neutron capture therapy(1) has been drawing attention. By irradiating the affected area with a pinpoint light beam, ultrasonic waves, and thermal neutrons, which can be safely administered to living organisms, specific chemical compounds (neutron sensitizer elements) are activated and kill the cancer cells. This therapy has a lower burden on patients. However, the technological development to deliver the neutron sensitizer molecules to cancer cells has been a great challenge. A research team led by Professor Kazunori Kataoka, Department of Bioengineering, School of Engineering, The University of Tokyo (concurrently serving as the Director of the Innovation Center of NanoMedicine, Kawasaki Institute of Industry Promotion), and Professor Nobuhiro Nishiyama, Chemical Resources Laboratory, Tokyo Institute of Technology, has successfully developed a nano crystal aggregate (nanomachine) technology to deliver a gadolinium complex (Gd-DTPA or magnevist) — broadly used as an MRI contrast agent — to the affected area ("Hybrid Calcium Phosphate-Polymeric Micelles Incorporating Gadolinium Chelates for Imaging-Guided Gadolinium Neutron Capture Tumor Therapy"). More specifically, it is a drug delivery system (DDS) whereby a nano-level contrast agent (Gd)-DTPA is prepared, and introduced into the interior of calcium phosphate, a bone constituent, and is delivered to cancer tissues. The research team has clarified that selective accumulation of the developed nanomachine in a cancer tumor enables contrast imaging of a solid cancer. Moreover, when the Team applied the nanomachine to cancer neutron capture therapy, they confirmed a remarkable curative effect. This nanomachine therapy enables an imaging-guided thermal neutron irradiation treatment; thus it can be expected to lead to a reliable cancer treatment with no missed cancer cells. The realization of surgery with no incision (chemical surgery) by nanomachine allows us to anticipate outpatient treatment with no need of hospitalization.
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