Black phosphorus could replace silicon computer chips

Silicon Valley in Northern California got its nickname from the multitude of computer chip manufacturers that sprung up in the surrounding area in the 1980’s. Despite its ubiquity as a chip building material, silicon may be facing some competition from a new version of an old substance. Researchers working at the Institute for Basic Science (IBS) Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea, led in part by Director Young Hee Lee, have created a high performance transistor using black phosphorus (BP) which has revealed some fascinating results ("High performance n-type black phosphorus transistors with type control via thickness and contact-metal engineering"). Atomic structure of black phosphorus monolayer Figure 1: Atomic structure of black phosphorus monolayer. Transistors are made up of materials with semiconducting properties, which come in two varieties: n-type (excess electrons) and p-type (excess holes). With the BP crystal, researchers have discovered that they can change its thickness and/or the contact metals and that will determine if it is high performance n-type, p-type, or ambipolar (function as both n- or p-type) material. What does this mean? Silicon has to be extrinsically doped (inserting another element into its crystal structure) to make it n-type or p-type in order for it to work in a semiconductor chip. The BP crystals can operate as both n-type and p-type or something in between, but don’t require extrinsic doping. This means that instead of having to fabricate a silicon-arsenic crystal sandwiched between silicon-boron crystals, a transistor can have a single, lightweight, pure black phosphorus logic chip -- no doping required. Additionally, changing the metals used to connect the chip to the circuit has an influence on whether BP will be n- or p-type. Instead of doping to make an n- and p-type material, both n- and p-type BP can be put all together on one chip just by changing its thickness and the contact metal used. Why is this important? Technology manufacturers are in an arms race to make their devices lighter, smaller and more efficient. By using BP that is only several atomic layers thick, transistors can be made smaller and more energy efficient than what exists now. Silicon chips exist in all of our electronic devices, and as manufacturers make devices smaller and more energy efficient, they begin to approach the threshold for just how small components can be. BP may provide a thinner, more efficient alternative to silicon chips in electrical devices. Atomic structure of black phosphorus and n/p-type transistor property of BP transistor Figure 2: Atomic structure of black phosphorus and n/p-type transistor property of BP transistor. (click on image to enlarge) Another example is tiny autonomous data recording and transmitting devices which will make up the Internet of Things (IoT). A major constraint from preventing IoT from taking off immediately is the inability to scale down the component size and the lack of a long-term power solution. 2 dimensional layered materials (such as black phosphorus) are interesting in this aspect, since both the electrical and mechanical properties are often enhanced compared to their bulk (3 dimensional) counterparts. Is BP a good alternative to current semiconductor materials? It is a great material for transistors since it has a high carrier mobility (how quickly an electron can move through it). This gives BP the ability to operate at lower voltages while also increasing performance, which translates to greatly reduced power consumption. With aluminum as a contact, thicker BP flakes (13 nanometer) show ambipolar properties similar to graphene while thin 3 nm flakes are unipolar n-type with switching on/off ratios greater than 105. The thinner they can make the material, the better the switching performance. Perello explains, “The driving force in back phosphorus is the carrier mobility. Everything centers around that. The fact that the band gap changes with thickness also gives us flexibility in circuit design. As a researcher it gives me a lot of things to play with.” Is it ready to compete with silicon? Unlike other industry standard semiconductor materials, there isn’t a good method for making pure BP on a large scale. Currently, thin layers can be made only from scraping bulk crystalline BP samples, as no other manufacturing method exists yet. Tackling the scaling problem is already underway, with chemical vapor deposition (CVD) and other thin film growth techniques being investigated in labs across the world. The lack of a monolayer fabrication technique isn’t necessarily a problem though. SKKU research fellow David Perello explains, “We can probably operate with 3, 5, or 7 layers and that might actually be better in terms of performance.” When asked if BP was ready to compete with silicon today, Perello said, “I don’t think it can compete with silicon at the moment, that’s a dream everybody has. Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I was able to make in these BP devices.” This doesn’t mean that BP isn’t worth exploring further though. According to Perello, “The fact that it was so simple to make such an excellent transistor without having access to state of the art commercial growth, fabrication and lithography facilities means that we could make it significantly better. We expect the upper bound for carrier mobility in black phosphorus to be much higher than silicon.” At present, BP isn’t ready for commercial use and its potential has just started to be recognized. If it continues to perform in further tests, it should be strong a contender as a chip material for future technology.
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A cost-effective solution to tuned graphene production

Mario Hofmann, National Cheng Kung University
Mario Hofmann is holding an example set up of the electrochemical synthesis.
Today (30 July), in the journal ("Controlling the properties of graphene produced by electrochemical exfoliation"), a team of researchers report that they have developed a simple electrochemical approach which allows defects to intentionally be created in the graphene, altering its electrical and mechanical properties and making the material even more useful.

The researchers used a technique called electrochemical synthesis to break graphite flakes into graphene layers. By varying the voltage they could change the resulting graphene's thickness, flake area, and number of defects - all of which alter the properties of graphene.

"Graphene is basically a metal - so it's somewhat boring!" explains Mario Hofmann, a researcher at National Cheng Kung University in Taiwan. "But when you start adding defects you begin to get interesting effects."

First studies on the electronic properties of graphene that brought received a lot of attention and the Physics Nobel prize in 2010 used graphene that was produced using adhesive tape to remove flakes of graphene from graphite. However, its defective counterpart graphene oxide could be first to carve out a significant market share as polymer fillers and battery electrodes.

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Sol-gel capacitor dielectric offers record-high energy storage

Posted: Using a hybrid silica sol-gel material and self-assembled monolayers of a common fatty acid, researchers have developed a new capacitor dielectric material that provides an electrical energy storage capacity rivaling certain batteries, with both a high energy density and high power density. sol-gel material placed on a clear plastic substrate Samples of the new hybrid sol-gel material are shown placed on a clear plastic substrate for testing. If the material can be scaled up from laboratory samples, devices made from it could surpass traditional electrolytic capacitors for applications in electromagnetic propulsion, electric vehicles and defibrillators. Capacitors often complement batteries in these applications because they can provide large amounts of current quickly. The new material is composed of a silica sol-gel thin film containing polar groups linked to the silicon atoms and a nanoscale self-assembled monolayer of an octylphosphonic acid, which provides insulating properties. The bilayer structure blocks the injection of electrons into the sol-gel material, providing low leakage current, high breakdown strength and high energy extraction efficiency. “Sol-gels with organic groups are well known and fatty acids such as phosphonic acids are well known,” noted Joseph Perry, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology. “But to the best of our knowledge, this is the first time these two types of materials have been combined into high-density energy storage devices.” The research, supported by the Office of Naval Research and the Air Force Office of Scientific Research, was reported July 14 in the journal ("Bilayer Structure with Ultrahigh Energy/Power Density Using Hybrid Sol–Gel Dielectric and Charge-Blocking Monolayer"). The need for efficient, high-performance materials for electrical energy storage has been growing along with the ever-increasing demand for electrical energy in mobile applications. Dielectric materials can provide fast charge and discharge response, high energy storage, and power conditioning for defense, medical and commercial applications. But it has been challenging to find a single dielectric material able to maximize permittivity, breakdown strength, energy density and energy extraction efficiency. Perry and colleagues in Georgia Tech’s Center for Organic Photonics and Electronics (COPE) had been working on other capacitor materials to meet these demands, but were not satisfied with the progress. The hybrid sol-gel materials had shown potential for efficient dielectric energy storage because of their high orientational polarization under an electric field, so the group decided to pursue these materials for the new capacitor applications. Using an aluminized mylar film coated with the hybrid sol-gel capacitor material, they showed that the capacitor could be rolled and re-rolled several times while maintaining high energy density, demonstrating its flexibility. But they were still seeing high current leakage. To address that, they deposited a nanoscale self-assembled monolayer of n-octylphosphonic acid on top of the hybrid sol-gel. Less than a nanometer thick, the monolayer serves as an insulating layer. “Our silica sol-gel is a hybrid material because it has polar organic groups attached to the silica framework that gives the sol-gel a high dielectric constant, and in our bilayer dielectric, the n-octylphosphonic acid groups are inserted between the sol-gel layer and the top aluminum layer to block charge injection into the sol-gel,” Perry explained. “It’s really a bilayer hybrid material that takes the best of both reorientation polarization and approaches for reducing injection and improving energy extraction.” In their structures, the researchers demonstrated maximum extractable energy densities up to 40 joules per cubic centimeter, an energy extraction efficiency of 72 percent at a field strength of 830 volts per micron, and a power density of 520 watts per cubic centimeter. The performance exceeds that of conventional electrolytic capacitors and thin-film lithium ion batteries, though it doesn’t match the lithium ion battery formats commonly used in electronic devices and vehicles. “This is the first time I’ve seen a capacitor beat a battery on energy density,” said Perry. “The combination of high energy density and high power density is uncommon in the capacitor world.” Researchers in Perry’s lab have been making arrays of small sol-gel capacitors in the lab to gather information about the material’s performance. The devices are made on small substrates about an inch square. “What we see when we apply an electric field is that the polarization response – which measures how much the polar groups line up in a stable way with the field – behaves in a linear way,” said Perry. “This is what you want to see in a capacitor dielectric material.” The next step will be to scale up the materials to see if the attractive properties transfer to larger devices. If that is successful, Perry expects to commercialize the material through a startup company or SBIR project. “The simplicity of fully solution-based processes for our dielectric material system provides potential for facile scale-up and fabrication on flexible platforms,” the authors wrote in their paper. “This work emphasizes the importance of controlling the electrode-dielectric interface to maximize the performance of dielectric materials for energy storage application.”
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