Graphene

1. Graphene Team #3 Phillip Keller Krista Melish Micheal Jones James Kancewick http://www.nanotech-now.com/images/Art_Gallery/AS-graphene.jpg Further Applications and Research

2. Overview • Introduction • Graphene’s mechanical properties • Graphene’s electrical properties • Carbon Vs Silicon • Supercapacitors and Graphene • Gas Detection using low-temperature reduced graphene oxide sheets • Ultrafast graphene photodetector • Electronics and Magnetism of Patterned Graphene Nanoroads • General Conclusions

3. Introduction • Advances in electronics have been the result of the continuous miniaturization or ‘scaling’ of electronic devices, particularly of silicon-based transistors, that has led to denser, faster and more power-efficient circuitry. http://www.scc.spokane.edu/_images/elect/circuit.gif • The realization of the approaching limits has inspired a worldwide effort to develop alternative device technologies. Some approaches involve spin-based devices, while others replace a key component of the device, the conducting channel, with carbon nanomaterials, which have superior electrical properties. Charlier, J.-C., Blase, X., & Roche, S. Electronic and transport properties of nanotubes. Rev. Mod. Phys.

4. Introduction • Among these carbon nanomaterials is graphene. Graphene is a two dimensional allotrope of carbon arranged like a honeycomb structure made out of hexagons and plays an important role since it is the basis for the understanding of the electronic properties in other allotropes. http://www.lbl.gov/Science-Articles/Archive/sabl/2007/Nov/assets/img/lrg/graphene_sheet.jpg

5. History of Graphene • Wallace in 1947 • Created 2D structure to help in the understanding of 3D Graphite • Single layers of graphite grown epitaxially on metallic substrates in the 1970s • Tightly bound to substrate, distorted properties • Term “graphene” coined in 1987 • 2004, Geim and Novoselov mechanically exfoliated sheets of graphene from graphite • Transferred to charge neutral silicon substrate • First successful electrical properties measured Geim, A. K. & MacDonald, A. H. (2007). "Graphene: Exploring carbon flatland". Physics Today.

6. A Closer Look at Graphene • 2D hexagonal carbon crystal lattice • Infinite boundaries • Actual 2D structure is debatable • Graphene sandwich • Thermal effects • Naturally occurring • Multilayer in graphite • Nanospecs in soot from exhaust • Currently one of the most researched materials • Unique physical and electrical properties • Wide array of potential uses http://www.nanotechnow.com/images/Art_Gallery/ASgraphene.jpg Ziegler, K., Robust transport properties in graphene. Phys. Rev. Lett.

7. Graphene Mechanical Properties • Breaking strength 200 times greater than steel • Youngs modulus of ~ 1 tPa • Incredible rigidity lends themselves to nanoscale pressure sensors • Nanoscopic graphene flakes bend with increasing pressure which alters their electrical conductivity which can be related to the pressure • Thermal properties exceed those of diamond • Excellent conductor of heat • Phonon dominated although it can be shown that at certain conditions the electrical portion is significant http://www.kinectrics.com/images/CableSpan.JPG John Scott Bunch. Mechanical and Electrical Properties of Graphene. Cornell University 2008.

8. Graphene Electrical Properties • Anomalous Quantum Hall Effect • Quantization of the Hall effect • Dirac fermions • Carriers have zero effective mass • Room temperature electron mobility of 15,000 cm2/V*s • Theoretically higher conductivity at room temp than silver, but unknown forces are limiting • Possible optical phonon scattering from attached substrate • Both P and N-type transistors have been created • Recent announcement by IBM that graphene transistor was operated at a terahertz frequency • Tunable band gap from 0 to 0.25 eV • Excellent conductivity makes graphene ideal for electrical leads in sensors/capacitors or use in touch screens because of its mechanical strength • Graphene ribbons have tunable electrical conductivity depending on the shape http://www.atwillett.com/lighting_pictures/lightningbolt_closeup.jpg Charlier, J.-C., Blase, X., & Roche, S. Electronic and transport properties of nanotubes. Rev. Mod. Phys.

9. Electrical Component: Transistor • A transistor's operation speed depends on the size of the device — smaller devices can run faster — and the speed at which electrons travel in it. This size dependence has been one of the major driving forces for making ever smaller silicon transistors. • The Consortium of International Semiconductor Companies in its 2001 International Technology Roadmap for Semiconductors projected that transistors have to be smaller than 9 nanometers by 2016 in order to continue the performance trend. Charlier, J.-C., Blase, X., & Roche, S. Electronic and transport properties of nanotubes. Rev. Mod. Phys.

10. Carbon vs. Silicon • Figure (a) is Intel’s 45 nm silicon transistor which uses a Hafniunbased dielectric. • Figure (b) is awafer of the 45 nm transistors photographed with a dime. The processors incorporate 410 million transistors for each dual core chip, and 820 million for each quad core chip. http://www.intel.com/pressroom/kits/45nm/photos.htm.

11. Carbon vs. Silicon • Graphene could offer a way forward. As well as being extremely thin and a semiconductor, electrons move through graphene at extremely high speeds. • The cutting edge of silicon-based transistors is at 32 nanometers. http://mayang.com/textures/Manmade/images/Plastics%20and%20Related/electronic_circuit_board_9131073.JPG • Graphene has the potential to fabricate transistors only a few atoms across. British researchers have unveiled the world’s smallest transistor, which measures one atom thick and ten atoms across. This is in the sub-10 nanometer range Geim, A. K. & Novoselov, K. S. The rise of graphene. Nature Mater.

12. Performance Characteristics for Carbon-Based Transistor Ambipolar transfer characteristics [current versus gate voltage]: drain bias increases from –0.1 V to –1.1 V in –0.2 V steps. Red line represents -0.1 V and the pink line is -1.1 V. Step size is -0.2 V Left Inset: Schematic of the band structure of a Schottky barrier semiconducting carbon nanotube in a field effect transistor under negative gate bias. Holes are injected from the source [S]. Right Inset: Schematic of the band structure of a Schottky barrier semiconducting carbon nanotube in a field effect transistor under positive gate bias. Electrons are injected from the drain [D]. Anantram, M. P. & Leonard, F. Physics of carbon nanotube electronic devices. Rep. Prog. Phys.

13. Electrical Component: Supercapacitors • Supercapacitors are energy storage systems that are able to store and deliver energy at relatively high rates. • They are able to store and deliver energy beyond those accessible by batteries. This is because the mechanism of energy storage is the simple charge-separation at the electrochemical interface between the electrode and the electrolyte. > http://www.johnhenryshammer.com/WOW/willWebPics/battery/superCap.jpg http://carolinepond.files.wordpress.com/2008/12/energizer-bunny.jpg Chen, Y. et al.Supercapacitor Devices Based on Graphene Materials. J. Phys. Chem. C 2009

14. Supercapacitors • An advantage for supercapacitors is that they have several orders of magnitude higher energy density than that of conventional dielectric capacitors. • Furthermore, the deficiencies of other power sources, such as batteries and fuel cells, could be complemented by supercapacitors, owning to their long cycle life and rapid charging and discharging at high power densities. + http://www.johnhenryshammer.com/WOW/willWebPics/battery/superCap.jpg http://carolinepond.files.wordpress.com/2008/12/energizer-bunny.jpg Chen, Y. et al.Supercapacitor Devices Based on Graphene Materials. J. Phys. Chem. C 2009

15. Supercapacitors • When the practical use of electrochemical capacitors for the storage of electrical charge was demonstrated and patented by General Electric, supercapacitors have generated great interest for a wide and growing range of applications such as: http://i.thisislondon.co.uk/i/pix/2008/07/general-electric-415x275.jpg • Load cranes • Forklifts • Electric vehicles • Electric utilities • Factory power back up Links to pictures are in notes Kotz, R.; Carlen, M. Electrochim. Acta2000, 45, 2483.

16. Supercapacitors • Different materials such as various carbon materials, mixed metal oxides, and conducting polymers have been used as supercapacitor electrode materials. • Particularly carbon, in its various forms, has been used as electrode materials of supercapacitors, aiming at high specific capacitance together with high power density. http://www.easycalculation.com/chemistry/elements/images/carbon.jpg Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11.

17. Supercapacitors • Although porous carbon materials have high specific surface area, the low conductivity of porous carbon materials is limiting its application in high power density supercapacitors. • Carbon nanotubes (CNTs), with excellent electrical conductivity and high surface areas, have been fabricated for supercapacitors since 1997. https://www.opassoap.com/opasstore/media/catalog.jpg http://www.t12.lanl.gov/home/afv/nanotube.singleframe.begin.gif Diederich, L.; Barborini, E.; Piseri, P.; Podesta, A.; Milani, P. Appl. Phys. Lett. 1999, 75, 2662.

18. Supercapacitors • However, CNT-based supercapacitors have not met the expected performance; one possible reason is probably due to the observed contact resistance between the electrode and current collector. • Hence, many studies have focused on the morphology of the carbon materials to boost the performance of the capacitor, such as growing CNTs directly on bulk metals to eliminate the contact resistance. http://fc05.deviantart.net/fs24/f/2008/018/e/3/Smoke_Stock_002_by_mross5013.jpg http://www.deskpicture.com/DPs/Technology/CircuitBoard_2.jpg Shaijumon, M. M.; Ou, F. S.; Ci, L. J.; Ajayan, P. M. Chem. Commun. 2008, 2373.

19. Supercapacitors • Graphene is emerging as a unique morphology carbon material with potential for electrochemical energy storage device applications due to its superb characteristics of chemical stability, high electrical conductivity, and large surface area. • Recently, it has been proposed that graphene should be a competitive material for supercapacitor application. Graphene with less agglomeration should be expected to exhibit higher effective surface area and thus better supercapacitor performance. Fig (a). Schematic diagram of graphene-based supercapacitor device Chen, Y. et al.Supercapacitor Devices Based on Graphene Materials. J. Phys. Chem. C 2009

20. Gas Detection using low-temperature reduced graphene oxide sheets By Ganhualu Leonidas E. Ocola Junhong Chen

21. Gas Detection using reduced graphene sheets • high-performance gas sensors made of partially reduced graphene oxide sheets • obtained through low-temperature step annealing at 300 °C in argon flow at atmospheric pressure • Was tested with low concentration NO2 http://www.engineerlive.com/media/images/large/large_1112_D12_Image.JPG W. Frank, J. Vac. Sci. Technol. B 25, 2558 2007

22. Background • The 2D structure of graphene makes every carbon atom a surface atom so that electron transport can be highly sensitive to adsorbed molecules. • Mechanically exfoliated graphene has demonstrated a potential ability to detect gases down to the single molecular level • The gas sensing mechanism of graphene is generally attributed to the adsorption/desorption of gaseous molecules which act as donors or acceptors on the graphene surface, leading to changes in the conductance of graphene http://c0378172.cdn.cloudfiles.rackspacecloud.com/graphene-interconnects.jpg S. Novoselov, Y. Zhang, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 2004.

23. Chemically Reduced Graphene Oxide using Hydrazine • Has been used for the detection of acetone, warfare agents, and explosive agents at parts per billion concentrations • Using Hydrazine for sensor fabrication involves toxic chemicals and introduce extra nitrogen functional groups which may slow the response of the sensor. Below is graphene prepared with Hydrazine http://2.bp.blogspot.com/_VyTCyizqrHs/SRoCXZgYZxI/AAAAAAAABoU/64fpjTGIJjw/s1600-h/graphene_sheet.jpg Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R. D. Piner, S. Stankovich, I. Jung, D. A. Field, C. A. Ventrice, Jr., and R. S. Ruoff, Carbon 47, 145 2009.

24. Modified Hummers Method • Was used for the creation of the graphene oxide sheets used for sensors • Graphite is first synthesized by oxidative treatment • Then is exfoliated in water to produce suspension of single graphene oxide sheets • For a detailed explanation of Hummer’s method please visit http://pubs.acs.org/doi/abs/10.1021/ja01539a017 Chemistry http://www.physics.umanitoba.ca/~tapash/nano/molecule-graphene.jpg http://www.air-intakes.net/Hummer.gif Park, J. An, R. D. Piner, I. Jung, D. Yang, and R. S. Ruoff, Chem. Mater. 20, 6592 2008.

25. Fabrication of the Sensing Device • The graphene oxide was suspended with Au interdigitated electrodes. • Both finger width and interfinger spacing of 1 µm • Drops of the graphene oxide suspension where then placed on the wafer. http://www.physics.upenn.edu/yodhlab/images/research_CMP_Solubilization.jpg H. Lu, L. E. Ocola, and J. H. Chen, J. Nanomater. 2006, 60828 2006

26. The Fabrication of the Electrodes • The Au interdigitated electrodes were fabricated using electron-beam lithography on a Silicon wafer. http://www.cnst.nist.gov/nanofab/nanofab_equipment/images/leica.jpg

27. The Sensor Figure obtained from article FIG. 1. Color online SEM image of a GO sheet bridging two neighboring Au fingers of an interdigitated electrode. Gases are detected by measuring the change in the current while applying a constant dc bias to the device. Leonidas E. Ocola, et al, Gas detection using low-temperature reduced graphene oxide sheets 2009 American Institute of Physics, 2009

28. How it works • When different gases absorb to it the electrical conductance of the Graphene Oxide changes and is used to detect certain gases as extremely low concentrations • For this experiment it was tested using NO2 • After the annealing process at 300 °C absorption sites opened allowing for low concentrations of N02 to cause a change in the resistance of the sensor. http://www.durawear.com/images/catalog/live/imageLibrary/35314ECE1517585314A3B395639BAE23M.jpg Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, NanoLett. 7, 3499 2007.

29. The sensor • Can detect a single atom of N02 which is shown in the figure below • When the N02 binds to the graphene the electrical properties change and this can be detected http://www.rsc.org/images/GrapheneNO2-250_tcm18-95542.jpg 24H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, ACS Nano 2, 463 2008.

30. Problems that need further research • Exploring methods to enhance sensor recovery • Currently it takes the sensors longer than 30 minutes to recover under normal conditions • Methods to be explored • Low temperature heating • UV illumination http://dkamhi.com/uv%20600x450.jpg Leenaerts, B. Partoens, and F. M. Peeters, Phys. Rev. B 77, 125416 2008

31. Ultrafast graphene photodetector http://img.directindustry.com/images_di/photo-g/photodetector-27819.jpg By Fengnian Xia Thomas Mueller Yu-ming Lin Alberto Valdas-Garcia PhaedonAvouris

32. Graphene’s photonic abilities • Ability to absorb ~ 2% of incident light over a broad wavelength • Multiple graphene layer absorb additively • The absorption range of a system can be tuned by changing the Fermi energy using an external gate field http://www.nanotech-now.com/images/NANOIDENTPhotodetectorFunction300.jpg Wang, F. et al. Gate-variable optical transition in graphene. Science 320, 206–209 (2008).

33. Field-Effect Transistors (FETs) • Zero bandgap,large-area single or few-layers of graphene as FETs are used in this paper • Internal fields are shown in this paper to produce an ultrafast photocurrent response in graphene http://rocky.digikey.com/weblib/ST%20Micro/Web%20Photos/New%20Photos/POWERSO-10jpg.jpg http://images.iop.org/objects/phw/news/thumb/14/2/10/graph1.jpg Xia, F. et al. Photocurrent imaging and efficient photon detection in a graphene transistor. NanoLett. 9, 1039–1044 (2009).

34. Applications of Photonic applications • High-speed optical communications • Interconnects • Terahertz detection • Imaging • Remote sensing • Surveillance • Spectroscopy http://www.delen.polito.it/var/ http://www.eecs.umich.edu/optics/html/pics/optics-main4.jpg http://www.sflp.co.uk/xhtml-css/images/surveillance1.jpg Geim, A. K. & Novoselov, K. S. The rise of graphene. Nature Mater. 6, 183–191 (2007).

35. SEM and optical images of High-bandwidth graphene photodetector • The graphene shown here has two to three layers • Two types of wirings are shown: ground–signal (G–S) and ground–signal–ground(G–S–G) • Optical bottom left, SEM is the remaining black and white image Figure obtained from article Chuang, S. Physics of Optoelectronic Devices (Wiley, 1995).

36. Schematic of a Photodetector • Device schematics and electrical model in the high-frequency domain • The green symbols, from top to bottom, represent Cp, Cg and Rg, respectively • The purple sheet represents the graphene, and a pair of dark red strips denote the microwave probe tips Image was obtained from paper Ishibashi, T. et al. InP/InGaAsuni-travelling-carrier photodiodes. IEICE Trans. Electron. E83-C, 938–949 (2000).

37. The High speed Impedance • Impedance – is an important relation of the physical set up of the device in order to filter out the low frequency signals from registering on the device • This equation is what sets the gate bias for the photodetector. Ryzhii, V., Mitin, V., Ryzhii, M., Ryabova, N. & Otsuji, T. Device model for graphenenanoribbon phototransistor. Appl. Phys. Exp. 1, 063002 (2008).

38. Photodetectors • Magnitude of the photocurrent is strong function of the location of the optical illumination and also on the gate bias which are calculated with the impedance equation • The figure to the right displays the absolute a.c. photoresponse as a function of light intensity modulation frequency up to 26 GHz with the gate bias varying from -40 to 80V Meric, I. et al. Current saturation in zero-bandgap, top-gated graphenefieldeffect transistors. Nature Nanotech. 3, 654–659 (2008).

39. Future research for Photodetectors • Enhancing the ability of the photodetectors to detect a broader light position. • The incorporation of photodetectors into use • Large scale manufacture of photodetectors • Use of photodetectors as transistors http://research.microsoft.com/en-us/um/people/jpwang/full_images/led_brdf.jpg

40. Electronics and Magnetism of Patterned Graphene Nanoroads http://www.blogcdn.com/www.engadget.com/media/2010/04/graphene-20100402.jpg By Abhishek K. Singh Boris I. Yakobson

41. Nanoroads • Carving Graphene roads out of fully hydrogenated carbon sheets create “Nanoroads” • This is a method in which the individual characteristics depending upon the zigzag and orientation can be studied to their corresponding affects. Graphane is shown to the left and is full hydrogenated graphene ceramics.org/ceramictechtoday/tag/graphene/ Chen, Z.; Lin, Y.; Rooks, M. J.; Avouris, P. Physica E 2007, 40, 228.

42. Graphene Vs. Graphane http://www.afs.enea.it/project/cmast/Documenti/web/foto/Pulci_fig1.jpg

43. Papers Concept • Can hydrogenation be used to form geometrical areas, such as “roads” of pristine graphene with the desired electronic properties? • This would be done to receive the same electrical properties of graphene nanoribbons without having to cut the graphene. http://images.google.com/imgres?imgurl=http://ceramics.org/ceramictechtoday Hod, O.; Barone, V.; Peralta, J. E.; Scuseria, G. E. Nano Lett. 2007, 7, 2295.

44. Armchair and Zigzag orientation Relaxed structure of (a) armchair nanoroad (b) zigzag nanoroad Na and Nz are the width measured by the number of pristine sp^2 carbon dimer-lines or zigzag chains respectively Figure obtained from article Son, Y.; Cohen, M.; Louie, S. Nature 2006, 444, 347.

45. Conclusions of Paper • In order for the nanoroads to be well defined, the sharpness of the interfaces between hydrogenated and pristine graphene is important. • Conducting and semiconducting nanoroads can in principle be patterned on a graphene by hydrogenation Dewar, M. J. S.; Dougherty, R. C. The PMO Theory of Organic Chemistry, 1st ed.; Plenum: New York, 1975

46. Conclusions of Paper Continued • Antiferromagnetic state is semiconducting, the ferromagnetic state is metallic with a width-independent moment of 0.80 μB/unit cell • The possibility of having metallic and semiconducting roads on the same planar geometry can be an advantage in certain applications Dewar, M. J. S.; Dougherty, R. C. The PMO Theory of Organic Chemistry, 1st ed.; Plenum: New York, 1975

47. Further Research • Achieve a rough patterns and directionality by the masked exposure to a reagent transforming sp2 carbon into sp3, while further energy minimization of the interface • The magnetism in wider nanoroads should display better stability and this should be researched • Applications of this technology also is in need for research

48. Potential Issues • An inability to produce graphene of consistent sizes and consistent electronic properties. • Difficulty integrating graphene into electronic devices using processes suitable for volume production. • High electrical resistance that produces heating and energy loss at junctions between graphene and the metal wires connecting them. http://www.thelocal504.com/wpcontent/uploads/2009/05/broken_computer.jpg Anantram, M. P. & Leonard, F. Physics of carbon nanotube electronic devices. Rep. Prog. Phys.

49. Further Research • Incorporating diamond as a semiconductor in high-temperature, high-power applications. • Accurate mass production techniques. • Essentially developing an entire electronic computing system with carbon-based electronics. ← → http://www.absolutediamonds.co.za/images/diamonds.jpg http://www.cnanorhonealpes.org/IMG/gif/BuckyTube_s.gif http://www.deskpicture.com/DPs/Technology/CircuitBoard_2.jpg

50. Conclusion • Bearing excellent material properties, such as high current-carrying capacity and thermal conductivity, graphene and other carbon based allotropes are ideally suited for creating components for semiconductor circuits and computers. Its planar geometry allows the fabrication of electronic devices and the tailoring of a variety of electrical properties. Because it is only one-atom thick, it can potentially be used to make ultra-small devices and further miniaturize electronics.