Advanced Materials - Graphene

Over the past decade, we have heard much about the promise of graphene. Academic research has exploded and exciting new applications have been demonstrated. However, there is little discussion of the business opportunities arising from graphene. In this article, we present an overview of Graphene, discuss its applications, and give an outline of some challenges that need to be overcome before Graphene is widely commercialized.

Background
The theory of graphene was first explored by Philip Russel Wallace in 1947 – he aimed to understand the electronic properties of more complex, three-dimensional graphite. It was thought that free-standing sheet of graphene was impossible; with noting to support it, it was assumed graphene would curl up into a ball. In 2004 there was a landmark development when Geim and Novoselov from the University of Manchester managed to extract single-atom thick flakes of graphene from graphite blocks using a mechanical exfoliation technique called “the scotch tape method,” – adhesive tape was used to exfoliate graphene plates off of graphite. The duo received a Nobel Prize in Physics in 2010 for their effort that kicked off a flurry of excitement and activity.

Graphene Production
The scotch tape method works well in a lab setting, and since then a number of other methods have been demonstrated that have a better chance of scale-up for larger volumes. Liquid-phase exfoliation of graphite, reduction-oxidation of graphite, CVD (chemical vapor deposition) on metal foils, synthesis on silicon carbide, and unzipping carbon nanotubes, Figure 1. Each of these methods has its advantages and suitability, depending on the application. Graphene suitability for an application is dependent on dimension (crystal size, lateral dimension, and thickness), quality (defects, impurities, variability), cost and scale-up complexity.

Graphene Properties
Graphene has many unusual and superior properties. Electronic properties include high electron mobility, electrical conductivity, and high-efficiency spin transport. It has excellent thermal conductivity. Mechanical properties include exceptional strength and rigidity. Other properties include high optical transparency, resistance to acid attack, super-permeability to water, but impermeable to gases.

These properties give it potential for use for a variety of applications, such as touch-screen displays, electronics (integrated circuits, memories, photodetectors), sensors (biosensors, gas sensors), energy storage (battery anodes, super capacitor electrodes), conductive inks (printed electronics), automotive (tires), polymers, and fiber-reinforced composites. We elaborate on some of these applications in further detail in the following sections.

Touch-Screens
Indium-tin-oxide (ITO) is commonly used as the electrode for transparent conductive panels (e.g. touch screens). But ITO is not suitable for applications requiring mechanical flexibility. Graphene can offer a similar performance as ITO, with the potential for a lower cost, and allows for flexing, and can enable a whole new generation of applications, like foldable smart phones.

In the early days of R&D in this application, graphene monolayers were grown on nickel or copper files at high temperatures and transferred to a polymer film substrate – the method was not cost-effective and thus alternative processing methods are being developed including:

 

  • Roll-to-Roll Production: Enables production of 30+ inch graphene films for transparent electrodes. Graphene films grown by CVD on flexible copper substrates.

  • Spray Coating: spray a mixture of graphene oxide (GO)-based dispersions on a preheated substrate to create a thin film.

  • Transfer Printing: A Graphene Oxide (GO) aqueous dispersion is filtered through a membrane; this membrane (film side down) is placed onto a substrate; dissolving the membrane with acetone leaves behind a GO film.

Graphene can also be used as a transparent electrode in dye solar cells – it has advantages in terms of cost, mechanical flexibility, and electrical performance. There are still significant bottlenecks for the commercialization of graphene touch-screen / transparent electrode – challenges include scale-up, uniformity, and conductivity compared to ITO. Even when commercialized, Graphene based touch-screen electrodes would most likely dominate only the flex-display market.

Electronics (Integrated Circuits, Data Storage, and Communications) 
Transistors and integrated circuits (IC) are the building blocks of electronics. Over the past four decades, continued dimensional scaling of transistors has brought us revolutions in computing, telecommunication, and a variety of other fields where electronics adds critical functionality.

 

Transistors however are reaching the physical limits of size and scaling in silicon, with no clear path to continue scaling dimensions beyond a decade or so.

Graphene provides a potential path to continued scaling; the high electron mobility translates to better performance. However graphene will not be a drop-in replacement for silicon – new architectures will have to be devised along with new ways to compute – e.g. using electron spin. Interconnects used in electronics too benefit from graphene; currently, copper is used to connect silicon transistors; however, copper scaled into the nanoscale has increasing resistivity because of grain boundaries. In our previous work at Georgia Tech, we showed that nanoscale graphene can outperform copper in resistivity and thermal conductivity.

Graphene is also a good candidate for memory devices. Lab prototypes demonstrated include concepts based on a spin valve, nanoribbon memory, graphene oxide-based memory, and hybrid silicon-graphene flash memory. To compete with silicon’s IC and memory density will be a huge challenge, and significant development is needed before commercial graphene devices will hit the market. In the absence of a new computing architecture, a band-gap will have to be engineered in graphene – another big challenge, but one that can be overcome by a mix of top-down and bottom-up lithographic techniques. Either of these scenarios is unlikely to play out in consumer devices at least for the next decade.

Graphene has desirable properties for optical communication devices. Graphene can be applied to high-speed photodetectors, where light is converted into electrical signals, and used as receivers in fiber-optic networks, to replace the traditional silicon and gallium arsenide semiconductors. Amplifiers, where signals can be amplified through a network, would be a great fit for graphene because of excellent electrical and heat conductivity, as well as the material’s unique band structure which makes ultra-wide range of operational wavelengths possible. Graphene can also be applied to frequency multipliers and frequency mixers. Some of the communications applications have more chances of being first commercialized in a military setting, and where performance matters more than cost or volumes.

Sensors
A sensor is a device that respond to physical stimuli (e.g. thermal energy, motion, magnetism) or presence of chemical or biological entities by producing a signal (usually electrical signal for ease of detection). Graphene-based sensor technology is currently under development for detecting chemicals, gases, biosensors, and radiation. Graphene’s two-dimensional structure exposes its entire volume to the environment, and affords more sensitivity in a given volume.

Electro-chemical sensors exhibit a change in electrical characteristics (e.g. conductance) when exposed to a particular chemical or gas. Sensor structures being considered include nanopore, foam, flake, and even pristine graphene. Graphene is also a promising electrode material for sensors.

Graphene sensors have potential applications in life sciences (radiation monitoring, cancer diagnosis), manufacturing (process monitoring, leak detection, automotive (emissions control, cabin air quality monitoring), petroleum (gas detection), homeland security / military (radiation detection), and environmental applications (air and water quality monitoring).

While lab-prototypes have been very promising when using graphene for chemical / biomolecule detection, any sensor commercialization will have to deal with the repeatability of the sensing, along with the manufacturing process for the sensor, both of which need significant further development.

Energy Storage
There is always a search for efficient energy storage technologies, and graphene’s use is being explored for next-generation batteries, super capacitors, fuel cells and solar cells.

In Lithium ion batteries. Graphene has good potential to be used in the anode, either in combination with Silicon or with graphite that is the current material of choice. It is unlikely that the current battery structure can support a 100% graphene-anode; however, with emerging battery designs, an increased use of graphene in anodes can be expected. In the near-term, graphene can serve as an additive to battery anodes, added in small quantities to the host anodic material. Cathodes too can benefit from adding graphene – the electrical and thermal performance of graphene is superior to graphite or carbon black (which are frequently used as additives in the cathode).

Graphene’s high intrinsic electrical conductivity, an accessible and defined pore structure, good resistance to oxidative processes, and high temperature stability would make it a great choice for supercapacitor applications. In addition, graphene nanosheets could be used as a support for platinum catalysts for fuel cells. Unlike carbon black (the typical support material for platinum catalysts in fuel cells), graphene decreases Platinum particle size to under a nanometer because of the strong interaction between platinum particles and graphene. This will lead to an increased catalytic activity in direct methanol fuel cells. Early research also indicates the potential for graphene to replace Platinum for fuel cell catalysis, and achieve a significant performance increase as well as a cost reduction.

Conductive Inks
Radio frequency identification (RFID) tags have steadily been gaining market penetration in a variety of applications from supply chain and logistics to asset tracking. Materials of choice for these tags include copper and aluminum, silver flake inks, silver nanoparticle inks, copper / copper oxide nanoparticle inks.

There are advantages in using graphene in conductive inks for RFID tag production. Ability to print graphene allows for fast design changes, low-volume tag production, and quick prototyping. In addition, the printing is expected to be cost-competitive to current techniques, and allows for printing onto flexible substrates such as paper in a roll-to-roll manner. But there are challenges as well – graphene delivers a conductivity higher than carbon paste, but below most metallic alternatives. Significant cost mark-up will likely take place going from raw graphene to ink; and complex chemistries and/or surfactants may be needed, depending on the graphene precursor. There are already commercial varieties of graphene conductive inks being used for RFID tags; however, their market share is still low but increasing compared to conventional tags.

Polymers
Historically the plastics industry has used various fillers for the enhancement of polymer properties, including mechanical reinforcement, UV-resistance, coloring, conductivity improvement, and fire-retardance. Mechanically reinforced polymers consist of a polymeric matrix and a relatively stiff organic filler. Traditional fillers include talc, glass fibers, carbon black, and calcium carbonate particles in micrometer size-ranges. The use of these micron sized fillers causes problems in melt flow and processing, because it requires high loading for modest property enhancement and this in turn results in high viscosity. The high density of traditional fillers also leads to larger density of the end-product, somewhat negating the light-weighting enabled by polymers. Nanofillers such as graphene have a density advantage over conventional fillers; in addition, its increased interfacial area enables high interactions, thus resulting in a higher modulus. Studies have shown that mechanical reinforcement of graphene is superior to carbon black or single wall nanotube in terms of strength.

Graphene-polymer composites can be processed by one or more of the following methods: melt processing, solvent processing, in situ polymerization, electrospinning, and layer by layer assembly. This involves the agitation of graphene in a polymer that is dissolved in a solvent before casting in a mold – afterwards the solvent is evaporated. Electrospinning is attractive for graphene reinforced polymer because of incorporating nanometer-size particulates into the production of fibers. Graphene oxide has been electrospun with thermoplastic PU, polyimide, gelatin, PVA, nylon 6, PANI, PVP, and PVA. Improvement in mechanical properties of graphene-polymer composites is based on the matrix and process (e.g. sonication, melt), Figure 2.

Beyond mechanical property enhancement, advantages of graphene as a filler includes improved optical, thermal and barrier properties, fire retardancy, and increased electrical conductivity. The excellent electrical conductivity of graphene can be exploited to make polymers electrically conductive for various applications like adhesives, antistatic coatings, and films. Graphene reduces thermal expansion of polymers by constraining the movement of a significant volume of polymer chains because of their interaction with graphene. There is also a permeability advantage resulting from the increase polymer linkage, and can possibly be leveraged in packaging applications.

The interfacial interactions between nanofillers and polymers is important for property enhancement of polymers. Dispersion of nanoparticles in the polymer matrix, to the point where individual particles are coated by the polymer, is critical. Graphene can be dispersed with the aid of a surfactant, depending on the exfoliation procedure. Graphene without functionalization interacts with polymer through van der Waals force and hydrophobic interactions – which are weaker.

Fiber-Reinforced Composites
A reinforced composite is made up of two main components – a filler (e.g. carbon fiber) and a matrix (e.g. epoxy resin). Graphene has the potential to improve mechanical properties of the composite when added to the matrix. In addition, the matrix can be made electrically conductive and this would help composites used for aircraft fuselage, which currently uses a copper mesh structure to prevent damage from lightning strikes.

With proper inclusion of graphene, several important mechanical characteristics (tensile strength, Young’s modulus and fracture toughness) significantly improve at expense of some decrease in plasticity, as compared to pure host polymers. Critical factors influencing the mechanical property enhancement include: (i) uniform dispersion of graphene in the host polymer, (ii) surface-area to volume ratio of graphene (i.e. single- vs multi-layered graphene), and (iii) strength of binding between graphene and host polymer. Commercialization efforts in the graphene composites space have been driven by applications that need electrically conductive polymers as well as areas where higher cost polymers can be replaced by commodity polymers that have been enhanced by graphene.

Outlook
Graphene’s superior and unique characteristics enables a number of applications. Some of these applications can be disrupted by the use of graphene. Significant commercialization efforts are underway but we have not seen graphene displace incumbent materials in a big way as yet because of multiple reasons. The major bottlenecks to commercialization vary based on the application area: manufacturing large-area high quality graphene (transparent electrodes, electronics, sensors), showing a repeatable performance with low variability (sensors, electronics), demonstrating a significant performance improvement compared to incumbents (electronics, energy storage, polymers), and producing cost-effective quality bulk graphene (polymers, fiber-reinforced composites).

We estimate a timeline for application commercialization in Figure 3. This timeline is based on inputs from dozens of researchers, entrepreneurs, and industry executives. However, as with any new technology, a number of other variables also need to be considered in addition to graphene’s usefulness in an application: how graphene competes with existing applications (cost vs. value), the level of disruption enabled, the need of the problem being solved, and adoption timelines for new technology. For market leaders, it is important to be on the lookout for progress in graphene and how it impacts their portfolio, and evaluate what is a good time to enter the graphene space to position for long-term growth.

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