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One step closer to a single-molecule device

Molecular diode artist’s impression (credit: Columbia Engineering)

Columbia Engineering researchers have created the first single-molecule diode — the ultimate in miniaturization for electronic devices — with potential for real-world applications in electronic systems.

The diode that has a high (>250) rectification and a high “on” current (~ 0.1 microamps), says Latha Venkataraman, associate professor of applied physics. “Constructing a device where the active elements are only a single molecule … which has been the ‘holy grail’ of molecular electronics, represents the ultimate in functional miniaturization that can be achieved for an electronic device,” he said.

With electronic devices becoming smaller every day, the field of molecular electronics has become ever more critical in solving the problem of further miniaturization, and single molecules represent the limit of miniaturization. The idea of creating a single-molecule diode was suggested by Arieh Aviram and Mark Ratner who theorized in 1974 that a molecule could act as a rectifier, a one-way conductor of electric current.

The future of miniaturization

Single-molecule asymmetric molecular structure (alkyl side chains omitted for clarity) using a donor–bridge–acceptor architecture to mimic a semiconductor p–n junction (credit: Brian Capozzi et al./Nature Nanotechnology)

Researchers have since been exploring the charge-transport properties of molecules. They have shown that single-molecules attached to metal electrodes (single-molecule junctions) can be made to act as a variety of circuit elements, including resistors, switches, transistors, and, indeed, diodes.

They have learned that it is possible to see quantum mechanical effects, such as interference, manifest in the conductance properties of molecular junctions.

Since a diode acts as an electricity valve, its structure needs to be asymmetric so that electricity flowing in one direction experiences a different environment than electricity flowing in the other direction. To develop a single-molecule diode, researchers have simply designed molecules that have asymmetric structures.

“While such asymmetric molecules do indeed display some diode-like properties, they are not effective,” explains Brian Capozzi, a PhD student working with Venkataraman and lead author of the paper.

“A well-designed diode should only allow current to flow in one direction …  and it should allow a lot of current to flow in that direction. Asymmetric molecular designs have typically suffered from very low current flow in both ‘on’ and ‘off’ directions, and the ratio of current flow in the two has typically been low. Ideally, the ratio of ‘on’ current to ‘off’ current, the rectification ratio, should be very high.”

To overcome the issues associated with asymmetric molecular design, Venkataraman and her colleagues — Chemistry Assistant Professor Luis Campos’ group at Columbia and Jeffrey Neaton’s group at the Molecular Foundry at UC Berkeley — focused on developing an asymmetry in the environment around the molecular junction. They created an environmental asymmetry through a rather simple method: they surrounded the active molecule with an ionic solution and used gold metal electrodes of different sizes to contact the molecule.

Avoiding quantum-mechanical effects

Their results achieved rectification ratios as high as 250 — 50 times higher than earlier designs. The “on” current flow in their devices can be more than 0.1 microamps, which, Venkataraman notes, is a lot of current to be passing through a single-molecule. And, because this new technique is so easily implemented, it can be applied to all nanoscale devices of all types, including those that are made with graphene electrodes.

“It’s amazing to be able to design a molecular circuit, using concepts from chemistry and physics, and have it do something functional,” Venkataraman says. “The length scale is so small that quantum mechanical effects are absolutely a crucial aspect of the device. So it is truly a triumph to be able to create something that you will never be able to physically see and that behaves as intended.”

She and her team are now working on understanding the fundamental physics behind their discovery, and trying to increase the rectification ratios they observed, using new molecular systems.

The study, described in a paper published today (May 25) in Nature Nanotechnology, was funded by the National Science Foundation, the Department of Energy, and the Packard Foundation.

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Combining light and sound to create nanoscale optical waveguides

Researchers have shown that a DC voltage applied to layers of graphene and boron nitride can be used to control light emission from a nearby atom. Here, graphene is represented by a maroon-colored top layer; boron nitride is represented by yellow-green lattices below the graphene; and the atom is represented by a grey circle. A low concentration of DC voltage (in blue) allows the light to propagate inside the boron nitride, forming a tightly confined waveguide for optical signals. (credit: Anshuman Kumar Srivastava and Jose Luis Olivares/MIT)

In a new discovery that could lead to chips that combine optical and electronic components, researchers at MIT, IBM and two universities have found a way to combine light and sound with far lower losses than when such devices are made separately and then interconnected, they say.

Light’s interaction with graphene produces vibrating electron particles called plasmons, while light interacting with hexagonal boron nitride (hBN) produces phonons (sound “particles”). Fang and his colleagues found that when the materials are combined in a certain way, the plasmons and phonons can couple, producing a strong resonance.

The properties of the graphene allow precise control over light, while hBN provides very strong confinement and guidance of the light. Combining the two makes it possible to create new “metamaterials” that marry the advantages of both, the researchers say.

The work is co-authored by MIT associate professor of mechanical engineering Nicholas Fang and graduate student Anshuman Kumar, and their co-authors at IBM’s T.J. Watson Research Center, Hong Kong Polytechnic University, and the University of Minnesota.

According to Phaedon Avouris, a researcher at IBM and co-author of the paper, “The combination of these two materials provides a unique system that allows the manipulation of optical processes.”

The two materials are structurally similar — both composed of hexagonal arrays of atoms that form two-dimensional sheets — but they each interact with light quite differently. The researchers found that these interactions can be complementary, and can couple in ways that afford a great deal of control over the behavior of light.

The hybrid material blocks light when a particular voltage is applied to the graphene layer. When a different voltage is applied, a special kind of emission and propagation, called “hyperbolicity” occurs. This phenomenon has not been seen before in optical systems, Fang says.

Nanoscale optical waveguides

The result: an extremely thin sheet of material can interact strongly with light, allowing beams to be guided, funneled, and controlled by different voltages applied to the sheet.

The combined materials create a tuned system that can be adjusted to allow light only of certain specific wavelengths or directions to propagate, they say.

These properties should make it possible, Fang says, to create tiny optical waveguides, about 20 nanometers in size —- the same size range as the smallest features that can now be produced in microchips.

“Our work paves the way for using 2-D material heterostructures for engineering new optical properties on demand,” says co-author Tony Low, a researcher at IBM and the University of Minnesota.

Single-molecule optical resolution

Another potential application, Fang says, comes from the ability to switch a light beam on and off at the material’s surface; because the material naturally works at near-infrared wavelengths, this could enable new avenues for infrared spectroscopy, he says. “It could even enable single-molecule resolution,” Fang says, of biomolecules placed on the hybrid material’s surface.

Sheng Shen, an assistant professor of mechanical engineering at Carnegie Mellon University who was not involved in this research, says, “This work represents significant progress on understanding tunable interactions of light in graphene-hBN.” The work is “pretty critical” for providing the understanding needed to develop optoelectronic or photonic devices based on graphene and hBN, he says, and “could provide direct theoretical guidance on designing such types of devices. … I am personally very excited about this novel theoretical work.”

The research team also included Kin Hung Fung of Hong Kong Polytechnic University. The work was supported by the National Science Foundation and the Air Force Office of Scientific Research.

Abstract of Tunable Light–Matter Interaction and the Role of Hyperbolicity in Graphene–hBN System

Hexagonal boron nitride (hBN) is a natural hyperbolic material, which can also accommodate highly dispersive surface phonon-polariton modes. In this paper, we examine theoretically the mid-infrared optical properties of graphene–hBN heterostructures derived from their coupled plasmon–phonon modes. We find that the graphene plasmon couples differently with the phonons of the two Reststrahlen bands, owing to their different hyperbolicity. This also leads to distinctively different interaction between an external quantum emitter and the plasmon–phonon modes in the two bands, leading to substantial modification of its spectrum. The coupling to graphene plasmons allows for additional gate tunability in the Purcell factor and narrow dips in its emission spectra.

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Light-emitting, transparent flexible paper developed in China

Left: optical images of normal filter paper (bottom layer), nanocellulose-quantum dot paper (middle layer), and with acrylic resin coating added (top layer). Right: photo of luminescent nanocellulose-quantum dot paper in operation. (credit: Juan Xue et al./ACS Applied Materials & Interfaces)

The first light-emitting, transparent, flexible paper made from environmentally friendly materialshas been developed by scientists at Sichuan University in China, the scientists report in the journal ACS Applied Materials & Interfaces.

Most current flexible electronics paper designs rely on petroleum-based plastics and toxic materials.

The researchers developed a thin, clear nanocellulose paper made from wood flour and infused it with biocompatible quantum dots — tiny semiconducting crystals — made out of zinc and selenium. The paper glowed at room temperature and could be rolled and unrolled without cracking.

The researchers are currently developing papers that emit other colors than blue.

The authors acknowledge funding from the Research Fund for the Doctoral Program of Higher Education of China and the National Natural Science Foundation of China.

Abstract of Let It Shine: A Transparent and Photoluminescent Foldable Nanocellulose/Quantum Dot Paper

Exploration of environmentally friendly light-emitting devices with extremely low weight has been a trend in recent decades for modern digital technology. Herein, we describe a simple suction filtration method to develop a transparent and photoluminescent nanocellulose (NC) paper, which contains ZnSe quantum dot (QD) with high quantum yield as a functional filler. ZnSe QD can be dispersed uniformly in NC, and a quite low coefficient of thermal expansion is determined for the resultant composite paper, suggesting its good dimensional stability. These results indicate that the meeting of NC with ZnSe QD can bring a brilliant future during the information age.

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Printing low-cost, flexible radio-frequency antennas with graphene ink

These scanning electron microscope images show graphene ink after it was deposited and dried (a) and then compressed (b)k, which makes the graphene nanoflakes more dense, so it improves its electrical conductivity (credit: Xianjun Huang, et al./University of Manchester)

The first low-cost, flexible, environmentally friendly radio-frequency antenna using compressed graphene ink has been printed by researchers from the University of Manchester and BGT Materials Limited. Potential uses of the new process include radio-frequency identification (RFID) tags, wireless sensors, wearable electronics, and printing on materials like paper and plastic.

Commercial RFID tags are currently made from metals like silver (very expensive) or aluminum or copper (both prone to being oxidized).

Graphene conductive ink avoids those problems and can be used to print circuits and other electronic components, but the ink contains one or more polymeric, epoxy, siloxane, and resin binders. These are required to form a continuous (unbroken) conductive film. The problem is that these binders are insulators, so they reduce the conductivity of the connection. Also, applying the binder material requires annealing, a high-heat process (similar to how soldering with a resin binder works), which would destroy materials like paper or plastic.

Printing graphene ink on paper

So the researchers developed a new process:

1. Graphene flakes are mixed with a solvent and the ink it dried and deposited on the desired surface (paper, in the case of the experiment). (This is shown in step a in the illustration above.)

2. The flakes are compressed (step b above) with a roller (similar to using a roller to compress asphalt when making a road). That step increases the graphene’s conductivity by more than 50 times.

Graphene printed on paper (credit: Xianjun Huang et al./Applied Physics Letters)

The researchers tested their compressed graphene laminate by printing a graphene antenna onto a piece of paper. The material radiated radio-frequency power effectively, said Xianjun Huang, the first author of the paper and a PhD candidate in the Microwave and Communications Group in the School of Electrical and Electronic Engineering.

The researchers plan to further develop graphene-enabled RFID tags, as well as sensors and wearable electronics. They present their results in the journal Applied Physics Letters from AIP Publishing.

Abstract of Binder-free highly conductive graphene laminate for low cost printed radio frequency applications

In this paper we demonstrate realization of printable RFID antenna by low temperature processing of graphene ink. The required ultra-low resistance is achieved by rolling compression of binder-free graphene laminate. With compression, the conductivity of graphene laminate is increased by more than 50 times compared to that of as-deposited one. Graphene laminate with conductivity of 4.3×104 S/m and sheet resistance of 3.8.

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Tunable liquid-metal antennas

Antenna, feed, and reservoir of a liquid metal antenna (credit: Jacob Adams)

Using electrochemistry, North Carolina State University (NCSU) researchers have created a reconfigurable, voltage-controlled liquid metal antenna that may play a role in future mobile devices and the coming Internet of Things.

By placing a positive or negative electrical voltage across the interface between the liquid metal and an electrolyte, they found that they could cause the liquid metal to spread (flow into a capillary) or contract, changing its operating frequency and radiation pattern.

“Using a liquid metal — such as eutectic gallium and indium — that can change its shape allows us to modify antenna properties [such as frequency] more dramatically than is possible with a fixed conductor,” explained Jacob Adams, an assistant professor in the Department of Electrical and Computer Engineering at NCSU and a co-author of an open-access paper in the Journal of Applied Physics, from AIP Publishing.

The positive voltage “electrochemically deposits an oxide on the surface of the metal that lowers the surface tension, while a negative [voltage] removes the oxide to increase the surface tension,” Adams said. These differences in surface tension dictate which direction the metal will flow.

This advance makes it possible to “remove or regenerate enough of the ‘oxide skin’ with an applied voltage to make the liquid metal flow into or out of the capillary. We call this ‘electrochemically controlled capillarity,’ which is much like an electrochemical pump for the liquid metal,” Adams noted.

Although antenna properties can be reconfigured to some extent by using solid conductors with electronic switches, the liquid metal approach greatly increases the range over which the antenna’s operating frequency can be tuned. “Our antenna prototype using liquid metal can tune over a range of at least two times greater than systems using electronic switches,” he pointed out.

Previous liquid-metal designs typically required external pumps that can’t be easily integrated into electronic systems.

Extending frequencies for mobile devices

“Mobile device sizes are continuing to shrink and the burgeoning Internet of Things will likely create an enormous demand for small wireless systems,” Adams said. “And as the number of services that a device must be capable of supporting grows, so too will the number of frequency bands over which the antenna and RF front-end must operate. This combination will create a real antenna design challenge for mobile systems because antenna size and operating bandwidth tend to be conflicting tradeoffs.”

This is why tunable antennas are highly desirable: they can be miniaturized and adapted to correct for near-field loading problems such as the iPhone 4′s well-publicized “death grip” issue of dropped calls when by holding it by the bottom. Liquid metal systems “yield a larger range of tuning than conventional reconfigurable antennas, and the same approach can be applied to other components such as tunable filters,” Adams said.

In the long term, Adams and colleagues hope to gain greater control of the shape of the liquid metal in two-dimensional surfaces to obtain nearly any desired antenna shape. “This would enable enormous flexibility in the electromagnetic properties of the antenna and allow a single adaptive antenna to perform many functions,” he added.

Abstract of A reconfigurable liquid metal antenna driven by electrochemically controlled capillarity 

We describe a new electrochemical method for reversible, pump-free control of liquid eutectic gallium and indium (EGaIn) in a capillary. Electrochemical deposition (or removal) of a surfaceoxide on the EGaIn significantly lowers (or increases) its interfacial tension as a means to induce the liquid metal in (or out) of the capillary. A fabricated prototype demonstrates this method in a reconfigurable antenna application in which EGaIn forms the radiating element. By inducing a change in the physical length of the EGaIn, the operating frequency of the antennatunes over a large bandwidth. This purely electrochemical mechanism uses low, DC voltages to tune the antenna continuously and reversibly between 0.66 GHz and 3.4 GHz resulting in a 5:1 tuning range. Gain and radiation pattern measurements agree with electromagnetic simulations of the device, and its measured radiation efficiency varies from 41% to 70% over its tuning range.

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New technology could fundamentally improve future wireless communications

Novel full-duplex transceiver (top device) in an anechoic chamber for testing (credit: Sam Duckerin)

A new electronics technique that could allow a radio device to transmit and receive on the same channel at the same time (“full duplex,” or simultaneous, two-way transmission) has been developed by researchers at the University of Bristol’s Communication Systems and Networks research group. The technique can estimate and cancel out the interference from a device’s own transmission.

Today’s cell phones and other communication devices use twice as much of the radio spectrum as necessary. The new system requires only one channel (set of frequencies) for two-way communication,so it uses only half as much spectrum compared to current technology.

The new technology combines electrical balance isolation and active radio frequency cancellation. Their prototype can suppress interference by a factor of more than 100 million and uses low-cost, small-form-factor technologies, making it well suited to use in mobile devices such as smartphones.

Significant impacts on mobile and WiFi systems

For future cellular systems (such as 5G systems), the new technology would deliver increased capacity and data rates, or alternatively, the network operators could provide the same total network capacity with fewer base-station sites, reducing the cost and environmental impact of running the network.

In today’s mobile devices, a separate filtering component is required for each frequency band, and because of this, today’s mobiles phone do not support all of the frequency channels available internationally. Different devices are manufactured for different regions of the world, so there are currently no 4G phones capable of unrestricted global roaming.

In Wi-Fi systems, the new design would double the capacity of a Wi-Fi access point, allowing for more simultaneous users or higher data rates.

Replacing these filters with the research team’s duplexer circuit would create smaller and cheaper devices, and would allow manufacturers to produce a single model for the entire world. This would enable global roaming on 4G and would further decrease cost through greater economies of scale.

The team had published papers about their research in the IEEE Journal on Selected Areas in Communications special issue on full duplex radio, and in this month’s issue of the IEEE Communications Magazine and has filed patents.

Abstract of Electrical balance duplexing for small form factor realization of in-band full duplex

Transceiver architectures utilizing various self-interference suppression techniques have enabled simultaneous transmission and reception at the same frequency. This full-duplex wireless offers the potential for a doubling of spectral efficiency; however, the requirement for high transmit-to-receive isolation presents formidable challenges for the designers of full duplex transceivers. Electrical balance in hybrid junctions has been shown to provide high transmit- to-receive isolation over significant bandwidths. Electrical balance duplexers require just one antenna, and can be implemented on-chip, making this an attractive technology for small form factor devices. However, the transmit-toreceive isolation is sensitive to antenna impedance variation in both the frequency domain and time domain, limiting the isolation bandwidth and requiring dynamic adaptation. Various contributions concerning the implementation and performance of electrical balance duplexers are reviewed and compared, and novel measurements and simulations are presented. Results demonstrate the degradation in duplexer isolation due to imperfect system adaptation in user interaction scenarios, and requirements for the duplexer adaptation system are discussed.

Abstract of Optimum Single Antenna Full Duplex Using Hybrid Junctions

This paper investigates electrical balance (EB) in hybrid junctions as a method of achieving transmitter-receiver isolation in single antenna full duplex wireless systems. A novel technique for maximizing isolation in EB duplexers is presented, and we show that the maximum achievable isolation is proportional to the variance of the antenna reflection coefficient with respect to frequency. Consequently, antenna characteristics can have a significant detrimental impact on the isolation bandwidth. Simulations that include embedded antenna measurements show a mean isolation of 62 dB over a 20-MHz bandwidth at 1.9 GHz but relatively poor performance at wider bandwidths. Furthermore, the operational environment can have a significant impact on isolation performance. We present a novel method of characterizing radio reflections being returned to a single antenna. Results show as little as 39 dB of attenuation in the radio echo for a highly reflective indoor environment at 1.9 GHz and that the mean isolation of an EB duplexer is reduced by 7 dB in this environment. A full duplex architecture exploiting EB is proposed.

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NASA new CubeSat concept for planetary exploration

Technologist Jaime Esper and his team are planning to test the stability of a prototype entry vehicle — the Micro-Reentry Capsule (MIRCA) — this summer during a high-altitude balloon mission from Ft. Sumner, New Mexico (credits: NASA/Goddard)

Jaime Esper, a technologist at NASA’s Goddard Space Flight Center has developed a CubeSat concept that would allow scientists to use less-expensive cubesat (tiny-satellite) technology to observe physical phenomena beyond the current low-Earth-orbit limit.

The CubeSat Application for Planetary Entry Missions (CAPE) concept involves a service module that would propel the spacecraft to its  target and a separate planetary entry probe that could survive a rapid dive through the atmosphere of an extraterrestrial planet, all while reliably transmitting scientific and engineering data.

CAPE in its deployed configuration (credit: Jaime Esper/NASA Goddard)

Planetary landings

Esper and his team are planning to test the stability of a prototype entry vehicle, the Micro-Reentry Capsule (MIRCA), this summer during a high-altitude balloon mission from Fort Sumner, New Mexico.

The CAPE/MIRCA spacecraft, including the service module and entry probe, would weigh less than 11 pounds (4.9 kilograms) and measure no more than 4 inches (10.1 centimeters) on a side. After being ejected from a canister housed by its mother ship, the tiny spacecraft would unfurl its miniaturized solar panels or operate on internal battery power to begin its journey to another planetary body.

Once it reached its destination, the sensor-loaded entry vehicle would separate from its service module and begin its descent through the target’s atmosphere. It would communicate atmospheric pressure, temperature, and composition data to the mother ship, which then would transmit the information back to Earth.

The beauty of CubeSats is their versatility. Because they are relatively inexpensive to build and deploy, scientists could conceivably launch multiple spacecraft for multi-point sampling — a capability currently not available with single planetary probes that are the NASA norm today. Esper would equip the MIRCA craft with accelerometers, gyros, thermal and pressure sensors, and radiometers, which measure specific gases; however, scientists could tailor the instrument package depending on the targets, Esper said.

A generic CAPE operations concept, from system deployment to probe release and entry into a given planetary atmosphere. Three mission phases are identified: 1. Deployment, 2. Targeting, and 3. Planetary Entry. (credit: Jaime Esper/NASA Goddard)

 

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How to print stronger, bigger, conductive 3-D graphene structures for tissue engineering

3D graphene inks are produced by simple combination and mixing an elastomer solution with graphene powder in a graded solvent, followed by volume reduction and thickening, a process that can be scaled up to many liters at once (credit: Adam E. Jakus et al./ACS Nano)

Northwestern University researchers have developed a way to print large, robust 3-D structures with graphene-based ink.

The new method could allow for using graphene-printed scaffolds for regenerative medicine and other medical and electronic  applications.

“People have tried to print graphene before,” said Ramille Shah, assistant professor of materials science and engineering at the McCormick School of Engineering and of surgery in the Feinberg School of Medicine.  “But it’s been a mostly polymer composite with graphene making up less than 20 percent of the volume.”

Adding higher volumes of graphene flakes to the mix in these ink systems typically results in printed structures too brittle and fragile to manipulate. At 60–70 percent graphene, the new ink preserves the material’s unique properties, including its electrical conductivity. And it’s flexible and robust enough to print robust macroscopic structures.

The secret: graphene nanoflakes are mixed with a biocompatible elastomer and fast-evaporating solvents.

“After the ink is extruded, one of the solvents in the system evaporates right away, causing the structure to solidify nearly instantly,” Shah explained. “The presence of the other solvents and the interaction with the specific polymer binder chosen also has a significant contribution to its resulting flexibility and properties. Because it holds its shape, we are able to build larger, well-defined objects.”

Could allow neurons to grow and communicate

Shah said her team populated one of the scaffolds with stem cells to surprising results. Not only did the cells survive; they divided, proliferated, and morphed into neuron-like cells.

The printed graphene structure is also flexible and strong enough to be easily sutured to existing tissues, so it could be used for biodegradable sensors and medical implants. Shah said the biocompatible elastomer and graphene’s electrical conductivity most likely contributed to the scaffold’s biological success.

“Cells conduct electricity inherently — especially neurons,” Shah said. “So if they’re on a substrate that can help conduct that signal, they’re able to communicate over wider distances.”

Supported by a Google Gift and a McCormick Research Catalyst Award, the research is described in the paper published in the April 2015 issue of ACS Nano.

Abstract of Three-Dimensional Printing of High-Content Graphene Scaffolds for Electronic and Biomedical Applications

The exceptional properties of graphene enable applications in electronics, optoelectronics, energy storage, and structural composites. Here we demonstrate a 3D printable graphene (3DG) composite consisting of majority graphene and minority polylactide-co-glycolide, a biocompatible elastomer, 3D-printed from a liquid ink. This ink can be utilized under ambient conditions via extrusion-based 3D printing to create graphene structures with features as small as 100 μm composed of as few as two layers (<300 μm thick object) or many hundreds of layers (>10 cm thick object). The resulting 3DG material is mechanically robust and flexible while retaining electrical conductivities greater than 800 S/m, an order of magnitude increase over previously reported 3D-printed carbon materials. In vitro experiments in simple growth medium, in the absence of neurogenic stimuli, reveal that 3DG supports human mesenchymal stem cell (hMSC) adhesion, viability, proliferation, and neurogenic differentiation with significant upregulation of glial and neuronal genes. This coincides with hMSCs adopting highly elongated morphologies with features similar to axons and presynaptic terminals. In vivo experiments indicate that 3DG has promising biocompatibility over the course of at least 30 days. Surgical tests using a human cadaver nerve model also illustrate that 3DG has exceptional handling characteristics and can be intraoperatively manipulated and applied to fine surgical procedures. With this unique set of properties, combined with ease of fabrication, 3DG could be applied toward the design and fabrication of a wide range of functional electronic, biological, and bioelectronic medical and nonmedical devices.

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Wearables and electric vehicles may get boost from boron-infused graphene

Rice University scientists made this supercapacitor with interlocked “fingers” using a laser and writing the pattern into a boron-infused sheet of polyimide. The device may be suitable for flexible, wearable electronics. (credit: Tour Group/Rice University)

Infusing the polymer in a laser-induced graphene supercapacitor (used to rapidly store and discharge electricity) with boric acid quadrupled the supercapacitor’s ability to store an electrical charge while greatly boosting its energy density (energy per unit volume), Rice University researchers have found.

The Rice lab of chemist James Tour uses commercial lasers to create thin, flexible supercapacitors by burning patterns into common polymers. The laser burns away everything but the carbon to a depth of 20 microns on the top layer, which becomes a foam-like matrix of interconnected graphene flakes.

Capacitors charge quickly and release their energy in a burst when needed, as in a camera flash. Supercapacitors add the high energy capacity of batteries and have potential for electric vehicles and other heavy-duty applications. But the potential to shrink them into a small, flexible, easily produced package could make them suitable for many more applications, including catalysts, field emission transistors, and components for solar cells and lithium-ion batteries, the researchers said.

In their earlier work, the team led by Rice graduate student Zhiwei Peng tried many polymers and discovered that a commercial polyimide was the best for the process. For the new work, the lab dissolved boric acid into polyamic acid and condensed it into a boron-infused polyimide sheet, which was then exposed to the laser.

Industrial-scale production

The two-step process produces microsupercapacitors with four times the ability to store an electrical charge and five to 10 times the energy density of the earlier, boron-free version.

The new devices proved highly stable over 12,000 charge-discharge cycles, retaining 90 percent of their capacitance. In stress tests, they handled 8,000 bending cycles with no loss of performance, the researchers reported.

Tour said the technique lends itself to industrial-scale, roll-to-roll production of microsupercapacitors. “What we’ve done shows that huge modulations and enhancements can be made by adding other elements and performing other chemistries within the polymer film prior to exposure to the laser,” he said.

Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of materials science and nanoengineering and of computer science and a member of Rice’s Richard E. Smalley Institute for Nanoscale Science and Technology.

The research is detailed in the journal ACS Nano.

The Air Force Office of Scientific Research and its Multidisciplinary University Research Initiative supported the research.

Abstract of Flexible Boron-Doped Laser Induced Graphene Microsupercapacitors

Heteroatom-doped graphene materials have been intensely studied as active electrodes in energy storage devices. Here, we demonstrate that boron-doped porous graphene can be prepared in ambient air using a facile laser induction process from boric acid containing polyimide sheets. At the same time, active electrodes can be patterned for flexible microsupercapacitors. As a result of boron doping, the highest areal capacitance of as-prepared devices reaches 16.5 mF/cm^2, three times higher than non-doped devices, with concomitant energy density increases of 5 to 10 times at various power densities. The superb cyclability and mechanical flexibility of the device is well-maintained, showing great potential for future microelectronics made from this boron-doped laser induced graphene material.

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New graphene-like two-dimensional material could improve energy storage

Porous, layered structure of highly conductive powder Ni3(HITP)2 (credit: Mircea Dinca, MIT)

MIT and Harvard University researchers have created a graphene-like electrically conductive. porous, layered material as possible new tool for storing energy and investigating the physics of unusual materials.

They synthesized the material using an organic molecule called HITP and nickel ions, forming a new compound: Ni3(HITP)2.

The new porous material is a crystalline, structurally tunable electrical conductor with a high surface area — features that are ideal for supercapacitors, which could extend the range of electric vehicles by capturing and storing the energy that would normally be wasted when brakes slow down a vehicle.

The new material is composed of stacks of unlimited numbers of two-dimensional sheets resembling graphite, with a room temperature electrical conductivity of ~40 S/cm (Siemens per centimeter). The conductivity of this material is comparable to that of bulk graphite and among the highest for any conducting Metal-organic frameworks (MOFs)* reported to date.

Also, the temperature-dependence of its conductivity linear at temperatures between 100 K (Kelvin) and 500 K, suggesting an unusual charge transport mechanism that has not been previously observed in any organic semiconductors, and thus remains to be investigated.

In bulk form, the material could be used for electrocatalysis applications (modifying the rate of chemical reactions) similar to how platinum works (but at lower cost). Upon exfoliation (peeling off of successive layers), the material is expected to behave similar to graphene, but with tunable bandgap and electromagnetic properties, suggesting new uses in electronic circuits and new exotic quantum properties in solid-state physics.

* MOFs are hybrid organic-inorganic materials that have traditionally been studied for gas storage or separation applications owing to their high surface area. Making good electrical conductors out of these normally insulating materials has been a long-standing challenge, as highly porous intrinsic conductors could be used for a range of applications, including energy storage.

Abstract of High Electrical Conductivity in Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2, a Semiconducting Metal–Organic Graphene Analogue

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni2+ in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metal–organic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 and 40 S·cm–1, respectively, both records for MOFs and among the best for any coordination polymer.