Usesof Graphene: From the Present to the FutureIn1947 P.R. Wallace’s paper 1 was published describing the band structure ofgraphene. It was useful then in understanding the properties of graphite(stacked layers of graphene) but nowadays represents the first step inunderstanding a material whose incredible properties have led to its use in arange of technologies reaching from energy storage via supercapacitors to moredurable bicycle tyres.
Furthermore considering the intense research currentlysurrounding the material, it is all but guaranteed to be utilised in thefuture. Consideringapplications of the substance that are current, possibly realised in a few yearand more hypothetical, should underline just how vital graphene is to manyparts of our lives both now and in the future as well as provide a betterunderstanding of the relevant scientific principles. Whatis It? Carbon is present in the universeas a large number of allotropes, graphene being one of them. Its structure isdeceptively simple, being a single large hexagonal lattice, thus making it a 2Dmaterial.Manyof its physical properties reside at the extremes, it being an excellentconductor and possessing a large specific surface area as well as incrediblemechanical strength. These unusual properties go some way to explaining why itis utilised in so many applications as will be shown in more detail later on.
ThePresent-Graphene and SupercapacitorsWithconventional capacitors rarely possessing a capacitance exceeding the ?F range, a supercapacitor can easily be rated up to several kF. With them possessing avery low equivalent series resistance (ESR: if the supercapacitor was modelledas an ideal capacitor in series with a resistor the ESR would be the value ofsaid resistor) and high power output, withstand thousands of charge anddischarge cycles without significant degradation in performance and having anenergy density many times higher than a conventional capacitor 3 makes themideal devices in certain situations. Limitations are of course present. Lowspecific energies compared to that of a battery, low maximum voltage and a highself-discharge rate plague supercapacitors reducing the applicability in otherareas 4.Whilstthe range of applicability of supercapacitors ranges from defibrillators in themedical industry to regenerative braking systems in automobiles, and thus toolarge to go into detail here, it is worthwhile to give one example forillustrative purposes. Supercapacitorscan be utilised as auxiliary backup power supplies. In the case of the primarypower supply failing or being interrupted, a supercapacitor can provide thenecessary power to the system until functionality is restored. Low ESR(especially than batteries), a long lifetime and their high specific energydensities (compared to capacitors) make them ideal to be used for systems wherehigh power input is required and only limited space available.
Backup of SSDcomputer memory would be a specific example of this. Howdo they work?Aconventional capacitor consists of 2 electrodes separated by an insulatingdielectric. Applying a voltage to a capacitor causes opposite charges toaccumulate on the surfaces of each electrode. The electric field thus developedacross the dielectric allows the device to store energy. Thesame principles apply to the supercapacitor. However the way that the charge isstored is fundamentally different and gives rise to the massive increase incapacitance. In fact there are 3 mechanisms for charge storage giving rise to 3general classes: hybrid, pseudo and electrochemical double-layer capacitors.
Electrochemicaldouble-layer capacitorsThesecapacitors rely on the double layer phenomenon for their increased capacitance.Here an ion-permeable membrane separates 2 electrodes with an electrolyte (amixture of negative and positive ions dissolved in a solvent) electricallyconnecting both electrodes. When a voltage is applied to the capacitor, asbefore charge accumulates in bothelectrode surfaces as layers of opposing polarity. Furthermore due to theadhesive forces between solvent and electrode a single layer of solventparticles (called the inner Helmholtz plane, IHP)is attached to the surface ofthe electrode. Coulombicattraction between the charged internal layer of particles and the dissolvedions distributed in the electrolyte then cause a second layer of oppositepolarity to form fitted against the IHP (now acting as a molecular dielectric).The separation of 2 layers of oppositely charged ions by the IHP storeselectrical charges as in a regular capacitor, with an extremely strong staticelectric field forming across the IHP.
Fig. 1 illustrates this. The capacitancearising due to this double layer phenomenon is thus dependent on the area ofthe layers (and thus dependent on the surface area of the electrode) as well asthe inter layer separation (in this case the thickness of a single molecule) ina fashion similar to the usual parallel plate capacitor. Hence due to theextremely thin nature of molecules and use of electrode materials with anextremely large surface areas, incredibly large capacitances arise. Severaldifferent materials exist that are normally utilised for the electrode,including activated carbon (a processed from of carbon designed to have smalllow volume pores), carbon aerogels, and carbon nanotubes6. Each bring theirown advantages and disadvantages.
Activated carbon for example is lessexpensive whilst the aerogels possess a lower ESR thus yielding higher poweroutputs7. PseudocapacitorsApseudocapacitor is also constructed in a similar fashion to the electrochemicaldouble-layer capacitors. Again when a voltage is applied to the device a doublelayer forms at each of the electrodes. However it is possible for some of theelectrolyte ions that are part of the ion layer to penetrate the IHP to becomede-solvated (the electrolyte ion, the solute, no longer interacts with thesolvent) and adsorb onto the electrode’s surface.
Note that no chemicalreaction is occurring currently between the atoms of the electrode and theadsorbed ion i.e no chemical bonds arise. Then a charge transfer process occurs(in these processes a charge-transfer complex is created, which are 2 or moremolecules that are associated by transfer of charge from one to the other)between the 2. The result is a faradaic current (a current that flows acrossthe electrode-solution interface) caused by one of three processes between theadsorbed electrolyte and electrode.
Fig.2 illustrates this. Hence charge is nowstored via the electrolyte and electrode interaction ready to release it byreversing the process. The size of the resulting capacitance is dependent onthe surface area of the electrode, its material and structure of saidelectrodes.Itis worth noting that whilst here the 2 types of capacitance were described as 2different phenomena, in reality pseudocapacitance cannot occur without thestatic double-layer capacitance as the former is inextricably dependent on thelatter’s existence. Hence the total capacitance of a supercapacitor is in facta sum of the contributions from each type.
HybridCapacitorsThesecapacitors are designed to utilise both double-layer capacitance and pseudocapacitancein order to mitigate the disadvantages of one type with the advantages of theother. Specifically EDLCs usually suffer from lower energy and power densitieswhilst possessing greater cycling stability and affordability.Useof GrapheneSo in what aspect ofsupercapacitors could graphene be utilised? The electrodes in all 3 types ofcapacitors require high surface area per unit mass and volume to maximise thepossible capacitance (both pseudo and double layer capacitance increase withincreasing surface area), good conductivity in order to reduce the ESR and beinert to ensure long lifetimes.
Graphene fulfils all of these requirementsexcellently which is why it is such an ideal candidate for the electrodematerial. Furthermore its excellent mechanical strength makes it well suitedfor next generation flexible thin film supercapacitors. As with all cutting edgetechnologies however there are problems that considerably reduce the efficacyof said technology. In this case processing of the graphene material is mademore difficult by the restacking and agglomeration processes (caused by van derWaals forces) that occur when handling of the substance occurs. As a directconsequence of this is a reduction in diffusion of electrolyte ions betweengraphene layers and available surface area. However by crumpling the graphenesheets, using spacers and template assisted growth has allowed this problem tobe resolved.Whilstgraphene supercapacitors may seem a thing of the future they already existcommercially. The company Skeleton Technologies offers their patented “curvedgraphene” products that according to their website is utilised in a large rangeof applications such as in hybrid city buses 9.
Sowe’ve seen that currently graphene is utilised in supercapacitors, which inturn are used in multiple important applications. Yet research is already beingdone on implementing graphene in many more technologies. For example accordingto the International Technology Roadmap for Semiconductors graphene utilised asthe material for transistor construction is only 10-15 years away, resulting ina massive leap forward in computing ability. TheFuture-Graphene, the New Silicon?Thetransistor is one the most important technological advancements of the 20thcentury. It forms the building block of the integrated circuit, devices thatare virtually omnipresent in all modern electronic equipment ,such as oursmartphones, and thus a foundation on which our society rests.
Nearly all ofsaid transistors are made from silicon and have been for over 60 years, and forthat time have been subject to Moore’s Law, predicting a doubling in transistordensity every 2 years. This was achieved by continually decreasing transistorsize down to just 5nm in June 2017 with the technology giant IMB’s latestdesign10. However decreasing transistor size is and has been for years a lessand less efficient way of improving the performance of transistors due to theincreasing cost of firstly designing devices that small that function well atthat size and producing such minute constructs. Furthermore at such scalesquantum mechanical effects such as tunnelling could cause huge problems. Due tothe ever present need for more powerful computers etc. in our society apressure exists on the electronics industry to continue to improve transistors.
Whilst some solutions involve changing the chip and transistor architecture orinstead moving to more specialised chips instead of more powerful all-rounderdesigns, one possibility would be instead to move to a totally differentsubstance, graphene, as stated by the International Technology Roadmap forSemiconductors 11, a document outlining the most likely directions ofresearch and the probable time line for the semiconductor industry.GrapheneTransistorsInJune 2017 assistant professor Ryan M. Gelfand and the research team he was partof developed a graphene based transistor in the University of Central Florida12 that with further study and refinement could lead to computers a hundredtimes as efficient and a thousand times as powerful (in the terahertz operatingspeed range). But how do they work? To answer this we have to considerspintronics.
Spintronicsand GrapheneThestudy of the spin of the electron in solid-state devices is known asspintronics. By manipulating the spin-degree of freedom in a system it ispossible to design switching devices based on this phenomenon. Howevercascading such devices in order to construct logic gates (which are whatcomputers are based upon) has long been a major challenge, yet an all-carbondesign might be feasible.
Theswitching device (transistor) proposed by the team working in Florida consistsof a graphene nanoribbon (GNR, a thin strips of graphene) created by unzippinga carbon nanotube (CNT) with 2 parallel CNT control wires on either side. Thereexists a constant voltage across all of the 3 components. With current flowing in the CNT a magneticfield is thus generated. Fig.3 illustrates this. Theimportant phenomenon here is the negative magnetoresistance of the GNR, that is to say its resistivitydecreases with increasing external magnetic field. This results due to spininteraction in the material with the magnetic field that will not be explainedin detail here.
Hence the magnitude of the GNR current acts as the binaryoutput of the transistor. Specifically binary 1 is represented by the largecurrent when the CNT magnetic fields are present and binary 0 by the muchsmaller current when the magnetic field is not present. The current from theGNR can now act as the binary input to further cascaded GNR gates and thus beused to form the complex set of logic gates that are used to perform thedesired function. The exceptionally highcomputational ability of computers designed on such transistors (a clock speedof 2 THz is proposed) is a product of the low switching delay, which are inturn caused by the low times required to switch the magnetic field on and off. Nowwhilst other materials exhibiting negative magnetoresistance and highconductivity could be utilised instead of graphene no other material currentlyfits the requirements as well as CNTsand GNRs. 14Whilstgraphene is set to be utilised in a number of applications in the relativelynear future (10-15 years), some research shows possibilities exist for grapheneto be utilised in a generation from now, such as the solar sail.TheFar Future-Solar Sails in Interstellar SpacecraftToday’sspacecraft all rely exclusively on chemical rocket engines as their primarymeans of propulsion in order to escape earth’s gravity well and also makeextensive use of them for travel and manoeuvring in space.
Yet for the purposeof a interstellar spacecraft, rocket engines simply do not generate enoughthrust over a long enough period of time in order to achieve speeds that wouldallow manned craft to reach the nearest solar system in an acceptable timeframe i.e less than the thousands of years currently required. The question ofhow exactly space exploration should be tackled is one that has receivedsignificant consideration since the idea of space travel has even existed.Current answers range from slow to fast, manned and unmanned projects each withtheir own advantages and disadvantages.
Propulsion systems utilised on saidprojects are just as varied, with ideas ranging from the more mundane such asion engines, to the very theoretical such as faster than light travel using anAlcubierre drive. Even a cursory description would warrant an entire article onits own yet one propulsion system is worth going into some detail, the solarsail.Whatare they?Thephenomenon of radiation pressure (electromagnetic radiation incident on asurface exerts pressure) can be utilised to design a propulsion system for aspacecraft called a solar sail. Whilst the actual forces exerted on solar sailare minute, they are constant and thus provide a significant acceleration overa large enough time frame. Furthermore beam sailing, the concept of using highintensity laser beams focused on the solar sail to provide a much greater thrust,provide a way of significantly increasing usability. SailMaterial-Graphene Grapheneis a potential candidate for the material solar sails are constructed from. Notonly does graphene possess a very low density, ensuring minimal mass for thespace craft and thus a large as possible acceleration as well as a highmechanical strength that ensures the sail is able to survive the rigours ofinterstellar travel.
However there is another feature that distinguishesgraphene. In May 2016, a Chinese team of researcher was investigating graphenesponges 15 (layers of graphene fused together), noticed how the laser beingused to cut the material was actually propelling the centimetre size sampleforwards. Radiation pressure was soon discounted as a mechanism for the seenpropulsion as it was estimated to be around 10-9 N, so much toosmall to explain the seen motion. Similarly, the laser burning off some of thematerial to provide thrust was similarly non-feasible due to the much too smalllaser intensity. Insteada third mechanism was suggested.
Graphene absorbs all wavelengths of lightwell, and thus under constant illumination by a laser causes its electronspopulation to become excited into a higher energy state. Further study showedthat some electrons obtained enough energy to be ejected out and to become freeelectrons, thus providing extra thrust ,several orders larger than theradiation pressure, in the direction of the laser beam. Emphasis was placed inthe article on the fact that the macroscale propulsion follows due to theunique optoelectronic properties of the graphene sheets.Hencegraphene seems to be ideally suited to be used as solar sail material.BreakthroughStarshot Initiative Whilstsolar sails already have been constructed and tested in orbit around earth,such as the IKAROS spacecraft (a Japanese experiment that demonstrated solarsails functionality in interplanetary space), the major current proponent ofsolar sail technology as a propulsion system for interstellar flight, that aimsto launch within the next generation, is the Breakthrough Starshot intiative.The objective is to develop a solar sail spacecraft capable of reaching AlphaCentauri, our closest neighbouring star system. Currently graphenebased-materials are being considered as the solar sail substance.
Sowhilst the idea of an interstellar spacecraft powered by graphene may seem likesomething out of a sci-fi novel, rest assured that work is already being doneto implement such a futuristic technology within their lifetime. Of course manyproblems still face the program and the technology as a whole, for example howwill the extremely thin sail stand up to extremely high velocity (up topossibly 20% the speed of light) impacts over its 20 year journey? ConclusionTheaim of this article was to illustrate with a few examples just how integral thesubstance graphene is to our modern technology both presently and in thefuture. With supercapacitors based on graphene offering a device ideal for usein a plethora of applications (such as the medical industry or regenerative brakingsystems), graphene nanoribbon transistors potentially revolutionizing computingability in the near future and graphene solar sails providing a possible propulsionsystem that will allow mankind to make the first interstellar journey within outlifetime, shows this more than adequately. Bibliography