Assembling van der Waals (vdW) materials, hexa boronitride, layered transition metal chalcogenide and many strongly correlated materials, together with graphene, we can construct novel quantum structures. These quantum heterostrutured materials can be further modified by electrochemical intercalation and electrolyte gating. In this talk, I will discuss to develop the method of transferring graphene to two-dimensional atomic layers of van der Waals solids to build functional heterostacks. We will discuss novel electron transport phenomena can occur across the heterointerfaces of designed quantum stacks to realize exotic charge transport phenomena in atomically controlled quantum heterostructures and their derivatives.

The quantum capacitance effect in graphene can readily be observed experimentally due to the low density of states near the Dirac energy. In particular, in metal-oxide-graphene structures with thin, high-K dielectrics, the quantum capacitance strongly affects the measurable capacitance as a function of gate voltage. In this work, we show how the quantum capacitance can be utilized to understand numerous properties of graphene, the surrounding dielectrics and even absorbed molecules on the graphene surface. Furthermore, we show that the quantum capacitance can be utilized to realize numerous novel graphene-based devices, including wireless sensors and optical modulators. Finally, the prospects for future materials-related investigations and device applications of quantum capacitance in graphene are described.

Long-distance spin transport is a prerequisite for spin-based logic proposals [1,2]. Graphene is attractive for spin transport studies because its low spin-orbit coupling leads to a long spin diffusion length l_sf [3]. A local magnetoresistance (MR) study suggested l_sf is of order 100 µm in epitaxial graphene [4], but non-local spin precession studies consistently measure l_sf of order 1 µm [5,6]. Local MR measurements require careful interpretation, because MR can arise from diverse physics including anisotropic magnetoresistance in the component materials, local Hall effect, magneto-Coulomb effects and tunnelling anisotropic magnetoresistance (TAMR). The latter in particular can mimic MR signals observed in spin transport.
Here we present large local MR in graphene contacted by epitaxial electrodes of the highly spin-polarized ferromagnet La_0.67Sr_0.33MnO₃ (LSMO). We use a deterministic transfer process [7] to combine the two materials whose growth conditions are incompatible. The formation of an intrinsic interfacial tunnel barrier generates large device resistances of order 100 M#8486;, making non-local measurements impractical. The large local resistance changes ΔR > 30 M#8486; imply giant l_sf values exceeding 500 µm when analysed in spin transport theory. To distinguish spin transport from TAMR, we undertake magnetic imaging of LSMO electrodes by magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism in photoemission electron microscopy (XMCD-PEEM). We conclude that micromagnetic studies provide a health check when interpreting local MR.
[1] Dery et al., Nature 447, 573 (2007)
[2] Behin-Aein et al., Nature Nanotechnology 5, 266 (2010)
[3] Huertas-Hernando et al., Phys. Rev. Lett. 103, 146801 (2009)
[4] Dlubak et al., Nature Phys. 8, 557 (2012)
[5] Tombros et al., Nature 448, 571 (2007)
[6] Han et al., Phys. Rev. Lett. 105, 167202 (2010)
[7] Reina et al., J. Phys. Chem. C 112, 17741 (2008)

Magnetism is today the main technology in use for data storage and spintronics is at the heart of the information storage revolution, with ferromagnets acting as fundamental building blocks: spin sources or analysers. However, these ferromagnetic metals (e.g. nickel, cobalthellip;) are easily prone to oxidation, detrimental to their performances. In turn, this limits their processability, e.g. with low cost wet processes. Here, we present graphene passivated ferromagnetic electrodes (GPFE) as novel oxidation-resistant spin sources [1]. Nickel lines are coated with graphene in a direct and scalable chemical vapour deposition step. An in-situ X-ray photoelectron spectroscopy study shows that the resulting GPFE are reduced during the growth process and remain resistant to oxidation upon air exposure. The GPFE spin polarisation properties are then measured through a complete vertical spin valve structure with a standard Al2O3/Co spin probe. It appears that GPFE not only retain a spin polarisation, but also present a particular majority spin filtering effect, which eventually leads to the reversal of their spin polarisation. Our results open new promising pathways for the integration of novel processes (e.g. ALD) and materials (organics) in spintronics devices [2].
[1] B. Dlubak et al, ACS Nano 6, 10996 (2012).
[2] M.-B. Martin et al, submitted

Several new routes to the growth and manipulation of graphene will be discussed. These will include the preparation of large-area (2.3 millimeters) single-domain graphene grown on simple copper foils, preparation of Bernal-stacked bilayer, trilayer and tetralayer graphene, hexagonal graphene onions, 3D seamless graphene-nanotube hybrid material which shows the requisite 7-membered rings at the interface junction, a top-down route to precisely patterned graphene nanoribbons that are sub 10-nm in width and microns long, and reinforced 2D graphene for increased mechanical performance. The use of graphene, graphene nanoribbons, and their derivatives in supercapacitors, interdigitated microsupercapacitors, field emitters and battery applications will be mentioned.

Graphitic carbon nanomaterials, including fullerene, carbon nanotubes and graphene, attract enormous research attention due to their outstanding material properties along with molecular scale dimension. The optimized utilization of those graphitic carbons in various optoelectronic and energy applications inevitably requires the molecular scale structure engineering as well as subtle controllability of material properties. In this regard, the chemical modification and substitutional doping of heteroatoms, such as Boron or Nitrogen, into the graphitic plane offers robust and reliable solution. In this presentation, our recent research achievements associated to tailored molecular scale assembly of chemically modified & doped graphitic carbon nanomaterials and their ultimate applications for various nanomaterials and nanodevices will be presented. In our approach, graphitic carbons can be efficiently processed into various three-dimensional structures via directed molecular assembly [see our invited feature article, Adv. Funct. Mater. 21, 1338 (2011)]. For instance, vertical carbon nanotube forest can be grown from graphene film surface to constitute three-dimensional carbon hybrid materials. Three-dimensional shape-engineered graphene gels can be spontaneously assembled at metal surfaces by electroless reduction of graphene oxide aqueous dispersion. Those molecular scale carbon assembled structures with extremely large surface and high electro-conductivity are potentially useful for catalysis, energy storage and so on. In addition, the substitutional doping of graphitic carbon with B- or N- was achieved via pre- or post-synthetic treatment. The resultant chemically modified graphitic carbons with tunable workfunction and remarkably enhanced surface activity could be employed for organic solar cells, organic light emitting diodes, nanocomposites, liquid crystalline materials, flexible substrate for nonplanar & flexible nanopatterned structures, nanocatalysts for improved functionalities and devices performances [see our recent invited review article, Adv. Mater. DOI: 10.1002/adma.201303265].

Despite the emergence of chemical vapor deposition (CVD) on polycrystalline Cu as the most widely used route to obtain high-quality, large-area graphene, the fundamental growth mechanisms, which are governed by the interaction between graphene and the catalyst during growth, remain largely unexplored.
Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), and environmental scanning electron microscopy (ESEM) are used (at industrially relevant CVD conditions of pressure ~0.001-0.5 mbar and temperature ~900-1000oC for several hydrocarbon sources) to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding.[1,2]
We note that graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. Further, we note that the coupling is also strongly affected by minor gaseous eg: oxygen contaminations, leading to a change in the graphene-Cu interaction during the growth process.Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. [1]
We compare the graphitic carbon growth mechanism for metal catalysts with a low solubility to novel oxide catalyst systems which inherently exhibit extremely low carbon solubility in an effort towards studying graphene growth directly on a dielectric [3] and highlight the importance of our complementary in-situ approach to study the growth of other 2D nanomaterials during CVD.
1. Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
2. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
3. Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).

Graphene&’s high electron mobility, scalability and excellent thermal conductivity make it a promising semiconductor candidate for post-CMOS electronic devices. We performed in situ nanoindentation to induce uniaxial tensile strain in suspended graphene devices and simultaneously measured electronic transport properties. We found Young&’s modulus to be sim;335 N/m, consistent with previous reports and our atomistic simulation results. Electrical measurements indicate the gauge factor of sim;1.9, comparable to the 2.4 predicted by simulations. Our transport measurements reveal that a moderate uniaxial strain is not capable of opening a band gap in graphene and does not affect its carrier mobility. This result matches our first principle-based MD simulations, as well as other theoretical predictions. The implications are that unlike for many other CMOS devices, mechanical strain, which can be easily controlled through device fabrication, is not an effective means to alter electronic transport properties in graphene.

The mechanical properties of materials depend strongly on their precise crystal structure and defect density. In our current work, we measure the yield strength of suspended single crystal and bicrystal graphene membranes prepared from chemical vapor deposition growth. Membranes of interest are characterized structurally by transmission electron microscopy and mechanically tested using atomic force microscopy. A single crystal diamond tip with a large indentation radius is used to measure the intrinsic strength of suspended membranes for mechanical measurements. Single crystal membranes prepared by chemical vapor deposition have strengths that are similar to previous results of single crystal membranes prepared by mechanical exfoliation. Bicrystal grain boundary membranes with large mismatch angles have higher strengths than their low angle counterparts. These large angle grain boundaries show strengths that are comparable to single crystal graphene. To further investigate this enhanced strength, we use aberration corrected high resolution transmission electron microscopy to explicitly map the atomic scale strain fields in suspended graphene. The enhanced strength of bicrystal membranes is attributed to the presence of low atomic-scale strain in the carbon-carbon bonds at the boundary.

Water, which is most abundant in Earth surface and very closely related to all forms of living organisms, has a simple molecular structure but exhibits very unique physical and chemical properties. Even though tremendous effort has been paid to understand this nature&’s core substance, there amazingly still lefts much room for scientist to explore its novel behaviors. Especially, as the scale goes down to nano-regime, water shows extraordinary properties that are not observable in bulk state. One of such interesting features is the formation of nanoscale bubbles showing unusual long-term stability. Nanobubbles can be spontaneously formed in water on hydrophobic surface or by decompression of gas-saturated liquid. In addition, the nanobubbles can be generated during electrochemical reaction at normal hydrogen electrode (NHE), which possibly distorts the standard reduction potential at NHE as the surface nanobubble screens the reaction with electrolyte solution. However, the real-time evolution of these nanobubbles has been hardly studied owing to the lack of proper imaging tools in liquid phase at nanoscale. Here we demonstrate, for the first time, that the behaviors of nanobubbles can be visualized by in-situ transmission electron microscope (TEM), utilizing graphene as liquid cell membrane. The results indicate that there is a critical radius that determines the long-term stability of nanobubbles. In addition, we find two different pathways of nanobubble growth. The distinctively small and large nanobubbles show Ostwald ripening like process that a small bubble disappears near the surface of a growing larger bubble. In this case, its boundary is not destroyed until the merging is completed. It seems that the gas diffusion from one bubble to another occurs at invisible time scale. On the other hand, two similar-sized nanobubbles show a coalescing process, followed by reshaping into dumbbell-like and spherical morphology. It is interesting that such Ostwald ripening-like behaviour can be observed in gaseous particles. We also observed that a small nanoparticle nucleates at the inter-bubble boundary area and rapidly grow into larger nanoparticles, which is possibly due to higher diffusion rate induced by dynamic equilibrium and high flux rate on nanobubble interface. Furthermore, we observed that the nanoparticles can be included inside the nanobubble cavity to minimize hydration energy and grow into larger nanoparticles. The nanoparticles are often included inside drifting nanobubbles. We suppose that it is one of the catalysing or cleaning pathways common in nature. Our finding is expected to provide a deeper insight to understand unusual chemical, biological and environmental phenomena where nanoscale gas-state is involved.

Growing graphene over large areas and the improvement of transfer techniques increase the need to control the shape and geometry of graphene once deposited onto the destination substrate. After transfer (1), graphene membranes always display unwanted ripples that limit its electrical, thermal and mechanical properties (2).
Indeed, these ripples in graphene-based transistor can between other alter the electrical conductivity (2). Nevertheless, it offers interesting ways to locally tune strain in graphene, which strongly influences its electronic, magnetic and vibrational properties (3,4,5). Local bending of graphene is a mean to induce an electrical gap or create high pseudo-magnetic fields (3), so that a locally tailor strain in graphene for controlled design of devices based on "stresstronics". Before reaching such a stage of control, it appears necessary to understand the interaction process of polycrystalline graphene membranes onto the formation of graphene ripples during transfer. For that purpose, we investigate by spatially resolved Raman spectroscopy, the formation process of strain and ripples in CVD graphene layers, which are deposited onto a corrugated substrate formed by an array of SiO2 nano-pillars with varying spacing and apex radius. This ordered corrugated substrate defines strain domains of parallel ripples, which can reach different regimes by varying the pitch of the array and sharpness of the pillars. We explore both limits of low-density arrays where graphene exhibits ripples domains and of very dense arrays for which no ripples are formed, and so the graphene stays fully suspended over a dense nano-pillars array. Spatially resolved Raman spectroscopy reveals uniaxial strain domains in the transferred graphene, which are induced and controlled by the array. For the tightest arrays, this technique shows the possibility to obtain macroscopically suspended graphene membranes with minimal interaction with the substrate. It also offer perspectives for electron transport and nano mechanical applications.
References:
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(2) Ni, G.-X.; Zheng, Y.; Bae, S.; Kim, H. R.; Pachoud, A.; Kim, Y. S.; Tan, C.-L.; Im, D.;
Ahn, J.-H.; Hong, B. H.; Ozyilmaz, B. ACS nano 2012, 6, 1158-64 ; Chen, C.-C.; Bao, W.; Theiss, J.; Dames, C.; Lau, C. N.; Cronin, S. B. Nano letters 2009, 9, 4172-6 ; Bao, W.; Myhro, K.; Zhao, Z.; Chen, Z.; Jang, W.; Jing, L.; Miao, F.; Zhang, H.; Dames, C.; Lau, C. N. Nano letters 2012, 12, 5470-4.
(3) Levy, N.; Burke, S. a.; Meaker, K. L.; Panlasigui, M.; Zettl, A.; Guinea, F.; Castro Neto, a. H.;Crommie, M. F. Science (New York, N.Y.) 2010, 329, 544-7.
(4) Frank, O.; Tsoukleri, G.; Riaz, I.; Papagelis, K.; Parthenios, J.; Ferrari, A. C.; Geim, A. K.;Novoselov, K. S.; Galiotis, C. Nature communications 2011, 2, 255.
(5) A. Reserbat-Plantey et al, Nature Nanotechnology 7, 151-155 (2012)

Strain engineering has been widely used to modify physical properties of materials to achieve better device performance. Recently, advances in the isolation and fabrication of monolayer materials, have led to a wealth of interest in the engineering of strain in these monolayer materials for applications. Monolayer materials exhibit mechanical strengths that enable the use of strains much larger than can be achieved in bulk materials. However, the unusual mechanical properties of monolayers require different approaches than for bulk materials. Here, we utilize REBO-based interatomic potentials to explore the potential for the engineering of strain in monolayer materials in the context of graphene using lithographically or otherwise patterned adatom adsorption. We discover that the resulting monolayer strain is a complex competition between the in-plane elasticity and out-of-plane relaxation deformations. The strain outside the adatom adsorption region vanishes due to out-of-plane relaxation deformations. Under some circumstances, an annular adsorption pattern generates large strains of approximately 2% inside the adsorption region, and here the elastic plane strain model can be used to provide some qualitative guidance. We find that the strains generated are up to 5.8% of the lattice constant change in the adsorbed region. In addition, an elliptical adsorption pattern proves to have the potential to produce uniaxial strains of as large as 4%. Our results elucidate a potential method for strain control at the nanoscale in monolayer devices.

In this talk, I will present our resent progress in understanding and engineering wetting behavior and electronic transport in graphene via modulating molecular interactions at graphene interfaces. We develop a theory to model the van der Waals interactions between liquid and graphene, including quantifying the wetting behavior of a graphene-coated surface. Molecular dynamics simulations and contact angle measurements were also carried out to test the theory. We show that graphene is only partially transparent to wetting, and that the predicted highest attainable contact angle of water on a graphene-coated surface is 96 degrees. Our findings reveal that graphene is essentially “nonlinearly translucent” to wetting, depending on the substrate considered. In addition, via modulating molecular interactions at graphene interfaces, we demonstrate two methods to enhance the on/off current ratio without sacrificing the field-effect mobility. First, We use an electrochemical approach involving phenyl-diazonium salts to systematically probe electronic modification in bilayer graphene (BLG) with increasing functionalization. Heavily functionalized BLG still retains its signature dual-gated band gap opening due to electric-field symmetry breaking. We find a notable asymmetric deflection of the charge neutrality point under positive bias which increases the apparent on/off current ratio by 50% without losing too much carrier mobility. Finally, we show that field-effect transistor devices comprised of a MoS2-graphene heterostructure can combine the advantages of high carrier mobility in graphene with the permanent band gap of MoS2 for digital applications. The interlayer Schottky impedance formed between MoS2 and graphene dominates carrier transport under negative gate bias, significantly depleting hole transport in graphene. We demonstrate a new type of FET device, which enables a controllable transition from NMOS digital to bipolar characteristics. In the NMOS digital regime, we report the highest on/off current ratio (ION/IOFF ~ 100) among all graphene-based FET devices at room temperature, without sacrificing the field-effect electron mobilities in graphene. The device architecture presented here pioneers a new approach to enable semiconducting behavior in graphene for digital and analog electronics.

Raman spectroscopy is an integral part of graphene research[1-3]. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp2-bonded carbon allotropes, because graphene is their fundamental building block. I will review the state of the art, future directions and open questions in Raman spectroscopy of graphene. The essential physical processes will be described, in particular those only recently been recognized, such as the various types of resonance at play, and the role of quantum interference[3-6]. I will update all basic concepts and notations, and propose a terminology that is able to describe any result in literature[3]. Few layer graphene (FLG) with less than 10 layers do each show a distinctive band structure. There is thus an increasing interest in the physics and applications of FLG. I will discuss the interlayer shear mode of FLG, and show that the corresponding Raman peak, named C, measures the interlayer coupling[7]. A variety of layered materials can also be exfoliated to produce a whole range of two dimensional crystals [8,9]. Similar shear and layer breathing modes are present in all these materials, and their detection provides a direct probe of interlayer interactions. A simple chain model can be used to explain the results, with general applicability to any layered material [10]
1. A. C. Ferrari et al. Phys. Rev. Lett 97, 187401 (2006) 2. A. C. Ferrari et al. Solid State Comm. 143, 47 (2007) 3. A. C. Ferrari, D. M. Basko Nature Nanotech. 8, 235 (2013) 4. D. M. Basko, New J. Phys. 11, 095011 (2009) 5. M. Kalbac et al. ACS Nano 10, 6055 (2010) 6. C. F. Chen et al. Nature 471, 617 (2011) 7. P. H. Tan et al. Nature Materials 11, 294 (2012) 8. J. N. Coleman, et al. Science 331, 568 (2011).
9. F. Bonaccorso et al. Materials Today 15, 564 (2012) 10. X. Zhang et al. Phys. Rev. B 87, 115413 (2013).

Two-dimensional materials have recently gathered significant interest due to properties showing promise for future electronic applications. Research in two-dimensional transition metal dichalcogenide transistors has suffered setbacks due to difficulties in finding efficient metal contacts. Due to the surface to volume ratio of two-dimensional materials, substrate and overlayer interactions can cause significant changes in thin film transistor performance. Additionally, zwitterionic polymers have been shown to alter the work functions of metals. Interactions between zwitterionic polymers and two-dimensional materials systems are studied in different transistor configurations. The properties of zwitterionic polymers could provide modulation to enhance the performance of thin film transistor devices. Topics of this presentation will include the possibilities of doping, changing carrier concentration, and lowering the contact resistance of thin film transistors with zwitterionic polymers.

Graphene device fabrication on large-area graphene typically involves electron-beam or optical lithography, followed by graphene etching and metallization of contacts. However, this method introduces compatibility issues, such as the difficulty of removing resist material from the graphene. The resulting resist contamination causes undesired doping, leading to a reduction in the charge carrier mobility in the graphene. In this work we therefore developed a direct-write, lithography-free method for the fabrication of graphene devices.
In the first step large-area graphene is patterned with a focused ion beam. An in situ Raman microscope allowed for direct observation of the graphene before and after the ion beam processing. Our results show that a Ga-ion dose of 10 C/m2 is sufficient for complete graphene removal. Defects formed by re-deposition or secondary electrons in the graphene away from the ion-milled area were successfully repaired by annealing in an inert atmosphere.
In the second step Pt contacts are formed by using our novel resist-free direct-write technique [1, 2]. This approach consists of the patterning of a thin seed layer of less than 0.5 nm Pt-containing material by electron beam induced deposition (EBID), followed by selective thickening of the seed layer by area-selective atomic layer deposition (ALD). This combined approach gives high-quality material (virtually 100% pure Pt, resistivity of 12 mu;Omega;cm), while it allows for patterning of Pt line deposits of only 10 nm in width.
The electrical transport characteristics of graphene field-effect transistors fabricated using our lithography-free approach will be demonstrated.
[1] A.J.M. Mackus, S.A.F. Dielissen, J.J.L. Mulders, W.M.M. Kessels, Nanoscale 4 (2012) 4477
[2] A.J.M. Mackus, N.F.W. Thissen, J.J.L. Mulders, P.H.F. Trompenaars, M.A. Verheijen, A.A. Bol, W.M.M. Kessels, J. Phys. Chem. C 117 (2013), 10788

It has been demonstrated that low frequency noise (LFN) of graphene is relatively small, a potentially advantageous feature for electronic sensor applications. Although encouraging, most reports have originated from devices on flakes that do not have the scale-up potential for practical applications. This work reports the LFN behavior for gated graphene devices formed on SiC substrates using low pressure sublimation (LPS) of Si in an Ar ambient. In general, we found that the LFN characteristics of LPS graphene to be superior to all prior studies.
The LPS graphene was synthesized in a commercial Aixtron reactor on ~2.5 cm2 nominally on-axis 6H(0001) semi-insulating substrates from the same wafer. The synthesis was tuned to produce nominally 1 ML of graphene on the terraces of the samples; the samples should be identical as the process has been demonstrated to be uniform and run-to-run reproducible. Samples were processed using typical photolithographic methods before dielectric deposition; a Ti/Au stack was used for ohmic and gate contacts. High-κ dielectric deposition was accomplished via a two-step process that includes functionalization of graphene by Fluorine followed by atomic layer deposition (ALD) of 15 nm thick Al2O3 and HfO2. Previously, we have shown that F-functionalization results in pinhole-free coverage of dielectrics and the films possess Dirac voltage shifts of 0.5V and 1.5V and dielectric constants of 9 and 18, for Al2O3 and HfO2, respectively. The LFN data was acquired using a fast Fourier transform analyzer coupled with low noise amplifier and was averaged over 5 different samples on the same substrate for each oxide case; VGS was controlled forward and reverse in the range -3 to 2V.
The LFN magnitude, Si/I2, showed an inverse dependence on frequency, similar to measurements made on flakes, for both bare and dielectric covered graphene. This implies the F-functionalization process does not appreciably add noise generation-recombination centers. In addition, the LFN did not show a strong dependence with area, in contrast to prior results found on flakes, and was generally smaller in magnitude to prior reports. For gated measurements, the LFN magnitude were not flat but showed a slight (20%) dependence on VGS. Both oxides showed noise hysteresis (~15%) although it was more pronounced for the HfO2 devices. The LFN increased with increasing carrier concentration but decreased with increasing mobility implying that the Hooge model cannot explain the origin of the noise and points to carrier scattering mechanisms as the limiting factor in these devices.

As a building block for carbon materials of all other dimensionalities (such as 0D buckyball, 1D nanotube, 3D graphite), the two-dimensional (2D) single atomic carbon sheet of graphene has emerged as an attractive candidate for energy applications due to its unique structure and properties. Like other materials, however, a graphene-based material that possesses desirable bulk properties rarely features the surface characteristics required for certain specific applications. Therefore, surface functionalization is essential, and researchers have devised various covalent and non-covalent chemistries for making graphene materials with the bulk and surface properties needed for efficient energy conversion and storage. Judicious application of these site-selective reactions to graphene sheets has opened up a rich field of graphene-based energy materials with enhanced performance in energy conversion and storage.
In this talk, I will provide some rational concepts for the controlled functionalization of graphene for the development of efficient energy conversion and storage devices (e.g., solar cells, fuel cells, supercapacitors, and batteries), along with an overview on recent progresses in this emerging research area.

In this work, we report on an innovative of pillared graphene nanostructures (PGN) directly on conductive substrates by an ambient pressure chemical vapor deposition (APCVD) process. The seamless connection between current collector and activae materials provides a relatively strong electrode integrity, which facilitates charge transfer in the system. Symmetrical supercapacitors (SCs) are fabricated based on this hybrid CNT and graphene nanostructure. SCs based on RGM show superior high-rate handability of up to 100 V sec-1. The electrochemical stability, excellent capacitive performance, and the ease of preparation suggest this PGN system is promising for future energy storage applications.

Well-design of nanostructures has been emerging a promising approach for the lithium ion batteries (LIBs) to achieve higher capacities, better rate capabilities and improved cyclic performances. Metal alloys and sulfides are promising materials for energy storage and conversion devices. Moreover, recent progress of graphene and surface functionalization to make its hybrids with described above materials revolutionize the applications in LIBs. Here, we synthesized different types of graphene (reduced graphene oxide, graphene, nitrogen and phosphorous doped graphene) 1 and their composites with different types of metal alloys and sulfides like SnS2-rGO,2 Co3S4-G,3 Ni3S4-NG, NiS1.03-NG,4,5 Co2SnO4-NG and Co3Sn2@Co-NG6 for their application as anode electrode in LIBs. To overcome the problem of electrode pulverization, two different strategies were utilized. One is the encapsulation of NPs in elastically strong graphene matrix and the second one is sealing out the NPs in the shell of inactive metal and then wrapped by graphene. The high performance of the composites is attributed to the synergistic effect between graphene and NPs. Because of its high surface area, graphene can provide large contact area between the electrolyte and electrode for better performance. In addition, because of the high conductivity and ions transfer mobility, graphene maintains the fast electrical flow of the composites. Further enhancement in electrochemical properties of graphene is carried out by doping of heteroatom in graphitic planes of graphene. All these composites possess extraordinary performances as anode in LIBs. It is worth noting that Ni3S4-NG and Co3Sn2@Co-NG composites displayed 98.87% and 102% capacity retention with a discharge capacity of 1323.2 and 1615 mAh/g after 100th cycle, respectively. These strategies to combine the different property-enhancing factors with engineered structures will bring the realization of graphene-based nanocomposites in the energy storage
References
1. Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y., Adv Mater 2013, 25, 4932-4937.
2. Jiang, J.; Feng,Y.; Mahmood, N.; Liu, F.; Hou, Y., Sci. Adv. Mater., 2013, 5, 1667-1675.
3. Mahmood, N.; Zhang, C.; Jiang, J.; Liu, F.; Hou, Y., Chem. Eur. J. 2013, 19, 5183-5190.
4. Mahmood, N.; Zhang, C.; Yin, H.; Hou, Y., J. Mater. Chem. A 2013. DOI: 10.1039/C3TA13033A.
5. Mahmood, N.; Zhang, C.; Hou, Y., Small 2013, 9, 1321-1328.
6. Mahmood, N.; Zhang, C.; Liu, F.; Jinghan, Z.; Hou, Y., ACS Nano 2013. DOI: 10.1021/nn4047138.

Carbon nanomaterials including fullerenes, carbon nanotubes, and graphenes represent the most important class of materials today; their unique physical and chemical attributes advance their roles across most advanced scientific and technology platforms. In this presentation, I will describe our recent efforts in developing innovative graphene-based hybrid nanomaterials using layer-by-layer (LbL) assembly. LbL assembly has been widely used as a versatile method for fabricating multilayer thin films with controlled structure and composition with a nanometer scale precision. Specifically in this talk, I will illustrate the hierarchical assembly of multilayered graphene oxide nanosheets for potential applications in transparent conducting thin films, tunable transistors, supercapacitors, and biosensors. Moreover, preliminary results of assembling 2-dimensional metal chalcogenides by LbL approach will be discussed.

Directional motion has long been essential to energy harvesting and conversion in the history of mankind. A typical example is the downhill flow of water guided by gravity which enables hydraulic power generation. The emergency of nanotechnology has resulted in an increasing focus on the nanoscale directional motion due to its importance to nanoscale actuation and energy conversion, and has enabled a series of design of nanomotors powered by applied voltage, electric current, or thermal energy. However, a fundamental law of nanoscale directional motion is notably missing, which could seriously limit the potential of nanotechnology. By investigating the sliding of a graphene flake on a graphene strip via molecular dynamics simulations, here we report an interesting phenomenon of nanoscale directional motion that a nanoscale object spontaneously moves toward a hard region, like water flowing toward a lower place. In this phenomenon, termed nanodurotaxis, the stiffness gradient induces a biased van der Waals potential toward the hard region of the substrate, resulting in an intrinsic mechanism for nanoscale directional motion which need not be maintained by an external source of energy. Our finding is expected to open a new way for nanoscale actuation and energy conversion.

The introduction of local defects such as vacancies or doping impurities is a well-documented way to tune the electronic properties of graphene. In such a context, nitrogen is a natural substitute for carbon in the honeycomb structure due to both its ability to form sp2 bonds and its pentavalent character. However, a clear correlation between the atomic configuration of the chemically modified graphene and the electronic properties remains a challenging task.
Scanning tunneling microscopy and spectroscopy (STM/ STS) are unique tools to measure local electronic properties of graphene and correlate them with their atomic structure [1]. The present work, based on both ab initio DFT, semi-empirical tight-binding (TB) electronic structure calculations and analytical calculations (Green function formalism [2]) aims at looking for interference effects generated by different types of defects. As a first step, the case of simple substitution of nitrogen, where long-range Coulomb effects are expected, will be presented. The Coulomb impurity screening problem in graphene which has been the subject of some debates is discussed [3] and elucidated within a local TB formalism based on the recursion method. This approach is extended to other defects such as vacancy, simple- and double-substitution of nitrogen or pyridine configurations. All the results presented here are discussed in the light of recent experimental STM data [4,5].

[1] F. Joucken et al., Y. Tison, J. Lagoute, J. Dumont, D. Cabosart, V. Repain, C. Chacon, Y. Girard, A. Botello-Méndez, S. Rousset, R. Sporken, J.-C. Charlier and L. Henrard., Phys. Rev. B 85, 161408(R) (2012)
[2] F. Ducastelle, Phys. Rev. B 88, 075413 (2013)
[3] V.N. Kotov, V.M. Pereira, and B. Uchoa, Phys. Rev. B 78 075433 (2008)
[4] Ph. Lambin, H. Amara, F. Ducastelle and L. Henrard, Phys. Rev. B 86, 045448 (2012)
[5] T. Mercier, L. Henrard , H. Amara, and F. Ducastelle (in preparation)

A novel transfer method with chemically controlled interfacial adhesion is reported for the fabrication of novel logic devices such as tunnel FETs and bilayer pseudo-spin field effect transistor (BiSFET). Typically, graphene is transferred with a PMMA mechanical supporting layer requiring several wet chemical steps and a relatively high annealing temperature to remove this support. A novel transfer method has been developed which modifies the bonding between the support material and the graphene, allowing direct transfer onto gate stacks with ultra thin oxides. The top graphene layer was grown on a thin copper layer on a SiO2/Si substrate by CVD. 50 nm thick Au electrodes were deposited on top of the graphene by e-beam evaporation. The predeposited Au electrodes prevent shorting of the gate insulators through grain boundaries in post transfer metallization. To transfer the graphene layer with patterned electrodes, a few microns of PIB (Poly-isobutylene) was coated on the top graphene electrode and cured for 3 minutes. The PIB serves to moderate the adhesion between the PDMS(Poly-dimethylsiloxane) and the gold electrodes.. Thermal release tape was attached to the edge of the PIB for mechanical delamination, and a PDMS stamp was placed on top of the PIB and the tape on the top graphene electrode. The PDMS provides mechanical support during the process. Afterwards, the PDMS/Au/graphene/Cu/SiO2/Si stack was immersed in ammonium persulfate solution to dissolve the Cu, releasing the top graphene electrode. The bottom gate stack was HOPG (Highly ordered pyrolytic graphite) with a subnanometer α-Al2O3 film on monolayer TiOPc (Titanyl Phthalocynine) film. The monolayer TiOPc was prepared via MBE (Molecular Beam Epitaxy) at a substrate temperature of 100 C. The α-Al2O3 layer was deposited by 10 cycles of ALD (Atomic Layer Deposition) using TMA (Trimethylaluminum) and water at 100 C. The α-Al2Onot;3 film is sufficiently dense and uniform that it is insulating and does not dissolve nor detache in organic solvents. After rinsing the gate stack, the PDMS/PIB/Au/Graphene stack was placed on the gate stack, and the PDMS was removed by mechanical delamination with the thermal release tape. By applying hexane solution, the remaining PIB layer was dissolved, leaving a clean graphene surface. Characteristics of the transferred sample were examined by STM (Scanning Tunneling Microscopy), AFM (Atomic Force Microscopy), XPS (X-ray Photoelectron Spectroscopy), and C-V measurements.

We have developed a novel, all-electronic biosensor for opioids employing an engineered mu receptor protein with high binding affinity for opioids chemically bonded to a graphene field-effect transistor (GFET) to read out the binding event. The graphene transistor fabrication process is capable of producing an array of hundreds of devices with an average mobility of ~1500 cm2 V-1 s-1 and yield >98%. The mu protein involved in the experiment has been redesigned, informed by computer simulation, to enhance its solubility and stability in an aqueous environment. The biosensor exhibits extremely high sensitivity and selectivity. A concentration-dependent increase in the Dirac voltage is observed, with a detection limit of 10 pg/mL. These devices exhibit excellent selectivity for naltrexone over other molecules and do not exhibit a response when the mu receptor is omitted or replaced with a control protein. The procedures developed here should be applicable to any protein or engineered antibody containing an accessible amine group, enabling methodologies for earlier detection of disease, more precise monitoring of disease progression, as well as testing the efficacy of pharmaceutical compounds on proteins of interest by electronic interrogation.

The preparation of biorecognition layers on the surface of a sensing platform is a very crucial step for the development of sensitive and selective biosensor. The incredibly large specific surface area, the abundant surface functionalities and the high water solubility indicate graphene oxide (GO) sheets as ideal substrate for such application (1). The immobilization of aptamers as the recognition element on GO allows the formation of a nanocomplex, usable as biosensing platform in an aptasensors (2). In this work, we investigate the immobilization of the biotin-thrombin-aptamer on GO, exploiting the high affinity of avidin for biotin. Avidin was directly immobilized on GO without any chemical functionalization, following a simple incubation procedure, performed in PBS. The ability of Avidin-GO substrate to bind biotinylated aptamers could lead an oriented immobilization of the recognition element (i.e. aptamer), avoiding unwanted interaction between aptamer and GO and keeping intact the exposed binding sites for the thrombin assay. The immobilization was observed directly using atomic force microscopy (AFM) and the modification of the GO functional groups was verified by UV-Vis and Fourier Transform Infrared (FT-IR) spectroscopy. Graphene sheets have been visualized by high-resolution SEM images. Thanks to the SERS properties of Graphene we studied the interface between aptamers and graphene oxide.
1. Y.Liu, X. Dong and P. Chen, Chem. Soc. Rev., 2012, 41, 2283-2307
2. Y. Wang, Z. Li. Et al., J. Am. Chem. Soc., 2010, 132, 9274-9276

Interfacing quantum mechanically active nanomaterials with biological structures has revolutionized nanotechnology and nanoscience. Here, we report on electrostatic incorporation of graphene quantum dots (GQDs) on bacterial cell wall to fabricate an electron-tunneling based device operated by bacterial mechanics. To fabricate this biologically actuated electronic device, the GQDs are functionalized with poly-L-Lysine, a molecule with electrostatic affinity to the Gram-positive bacterial cell wall. The bio-graphenic device demonstrates robust response to the pressure mediated (change in external pressure from 300 to ~ 0.3 Torrs) water dynamics through bacterial cell wall, which results in 1.9 nm decrease in the average tunneling distance between the GQDs anchored on bacterial surface and resultant 5 fold increase in tunneling current. While the activation energy for electron transport was 35 meV, the Coulomb blockade threshold voltage was 31 meV. Further, the Raman spectra and FESEM images confirm the intimate contact between bacterial cell wall and the functionalized GQDs. This work provides an avenue for leveraging the unique bio-molecular construct of the bacteria to achieve nanoscale architecture of GQD network on a pseudo microscopic bag of water.

Doping is an essential process to engineer the conductivity and work-function of graphene for higher performance optoelectronic devices, which includes substitutional atomic doping by reactive gases, electrical/electrochemical doping by gate bias, and chemical doping by acids or reducing/oxidizing agents. Among these, the chemical doping has been widely used due to its simple process and high doping strength. However, it also has an instability problem that the molecular dopants tend to gradually evaporate from the surface of graphene, leading to substantial decrease in doping effect with time. In particular, the instability problem is more serious for n-doped graphene because of unexpected reaction between dopants and oxygen or water in air. Here we report a simple method to tune the electrical properties of CVD graphene through n-doping by vaporized molecules at 70 °C, where the dopants in vapor phase are mildly adsorbed on graphene surface without direct contact with solution. To investigate the dependence on functional groups and molecular weights, we selected a series of ethylene amines as a model system, including ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetramine (TETA) with increasing number of amine groups showing different vapor pressures.
The characteristics of n-doped graphene were studied by Raman spectroscopy at room temperature. The G band of graphene was upshifted from 1585.4 cm-1 (pristine) to 1590.4 cm^-1 (EDA), 1593.5 cm^-1 (DETA) and 1596.5 cm^-1 (TETA), and the ratio of I(2D)/I(G) decreased from 4.0 to 1.2 as a function of the G peak position from 1584 cm^-1 to 1597 cm^-1.
To investigate and compare n-doping effects of ethylene amine vapors, the Dirac voltage of the pristine graphene transistor was initially measured, which was observed at 1.4 ± 2.3 V. The Dirac voltages were shifted to -126.6 ± 5.8 V, -166.4 ± 1.8 V and -192.3 ± 5.8 V respectively for EDA, DETA, and TETA doped and the average sheet resistance of pristine graphene was 925 ± 170 Omega;/sq, and EDA, DETA, and TETA modified graphene respectively showed average sheet resistances of 130 ± 12 Omega;/sq, 124 ± 28 Omega;/sq and 98 ± 12 Omega;/sq.
The results of n-doping stability based on the charge neutral points showed similar tendency in three types of devices (EDA, DETA, and TETA doped case) and charge neutral points were observed to start saturating after 36 hours of exposure in the ambient condition. After 20 days of exposure in the air, the charge neutral point of EDA doped graphene field effect transistors were changed from -126.6 ± 5.8 V to -56 ± 7.3 V, from -166.4 ± 1.8 V to -111.8 ± 3.6 V for DETA doped graphene and from -192.3 ± 5.8 V to -151.4 ± 3.5 V for TETA doped graphene compare to initially doped graphene.
We confirmed that the vapor-phase doping provides not only very high carrier concentration but also good long-term stability in air, which is particularly important for practical applications.

The emerging trend of preventive health care is raising an increasing interest towards point-of-care diagnostics and quantitative detection of low concentration biological and chemical species such as disease markers. Traditional quantitative sensing methods are usually based on labelling of the analyte for detection by e.g. optically, requiring laboratory analysis not suitable for frequent preventive health monitoring. The utilization of the capabilities of novel high-performance nanomaterials such as CNTs, graphene and Si-nanowires can provide the necessary electrical quantitative detection signal, together with an increased sensitivity, simplicity in device design and feasible integration to portable devices, such as mobile phones and computers.
We have demonstrated the use of functionalized hydrophobins (HFBIs) as programmable biomolecular receptors in graphene field effect transistors (GFETs). The HFBIs are small proteins, roughly 2 nm in diameter, with hydrophobic and hydrophilic batches at different sides of the molecule. The hydrophilic batch of the HFBI contains an N-terminus that allows fusing the protein with desired bioreceptors or their fragments. The hydrophobic batch of the HFBI attaches non-covalently on graphene and self-aligns to form a continuous molecular monolayer.
The charge sensitivity and operation principles of the HFBI programmable GFET was demonstrated by using a pair of leuzine zippers, ZE and ZR, with -7 e and +7 e charges in pH7, respectively. The attachment of the negatively charged receptor HFBI-ZE induced a large shift of graphene Dirac peak towards positive voltages as measured against Ag/AgCl liquid electrode. This shift was gradually relaxed by the biorecognition of the positive counter zipper ZR. The dynamic range of the detection reached from 10 fM to 10 µM, where the complete neutralization relaxed the shift of the Dirac point with total 45 % change in GFET resistance.
GFET applicability to biosensing was assessed by studying the detection of class G immunoglobulin antibodies. In this case the graphene channel was functionalized with a HFBI-Protein A fusion. Protein A is known to bind different immunoglobulin subclasses, with the highest affinity to the members of the IgG class. Unlike the zipper peptides, both Protein A and IgG are large biomolecules (much larger than the Debye length in electrolyte) with undefined total charges and charge distributions. However, the sensor response for 80 fM IgG analyte was already 5%, reaching 16% at 80 nM.
By utilizing different tailored proteins to achieve selectivity, the GFET sensor is capable of label-free detection of analytes, whenever the specific binding is associated with a change in the charge density of the receptor-analyte complex. The functionalization is a water-based and reversible process and consists solely on biodegradable components. Aspects related to electrolyte effects, charge distributions and utilization in biodetection will be addressed.

Electron microscopy has been widely used for high-resolution biological imaging and analysis. However, the radiation damage by focused electron beams hinders the high-resolution imaging of non-conducting biological samples. Alternatively, gold or platinum coatings are utilized to prevent the radiation damage, but it disables the chemical analysis by energy dispersive spectroscopy (EDS) utilizing secondary electrons scattered from the sample surface. Here we report that the graphene-coating of biological samples enables high-resolution imaging by scanning electron microscopy (SEM) and chemical analysis by EDS, utilizing the transparency to electron beams as well as the outstanding conductivity and flexibility of graphene films. The graphene synthesized by chemical vapour deposition (CVD) were consecutively transferred to complete a 3-layer film floating on water, and an SEM specimen (an ant) was coated by contacting the graphene layer from the bottom to the interface of water, followed by drying inside a desiccator. We confirmed that the 3-layered graphene is strong enough to cover the conformal surface of the ant without much tearing. The graphene coating also shows the outstanding stability against the strongly focused electron beam with acceleration voltage as high as 20 kV. The graphene is highly transparent to electron beams so that the secondary electrons scattered from the sample can be analyzed by EDS. This is not possible in the case of Pt-coated samples because most of secondary electron signals are blocked by Pt layers. Thus, we successfully imaged various protein and cellulose samples coated with graphene and identified the difference in their chemical composition. We believe that the graphene-assisted biological imaging would be extremely useful by enriching the nanoscale information from electron microscopic analyses.

The growth of graphene by chemical vapor deposition (CVD) is a technique that shows promise for producing graphene over a large area, which is an essential requirement for its practical use. Typically, the CVD synthesis of graphene results in a polycrystalline structure. Because the macroscopic properties of graphene are determined by the number and size of grains and nature of their boundaries, it is necessary to reveal the parameters that can lead to the control over of the growth of grains in order to tailor the properties of the overall graphene film.
So far, most studies of the kinetics of graphene growth have been performed using low pressure chemical vapor deposition (LPCVD). Here we perform a kinetic study on the growth of graphene on copper foil by atmospheric pressure chemical vapor deposition (APCVD) using a methane precursor. By varying the methane concentration and the methane-to-hydrogen ratio over three orders of magnitude at various temperatures we were able to observe by SEM both the kinetics of formation of isolated hexagonal grains of graphene and their evolution into a macroscopic layer. The growth mechanism will be discussed by exploiting the time dependence variation of the number density and area of graphene grains on the copper substrate.

Diffusion between metal and semiconductor materials has been studied for decades as well as the solution to mitigate this process. Diffusion of metal into semiconductor would cause serious problem especially in thermoelectric, where the device would work under high temperature. At high temperature, the diffusion rate of metal into semiconductor increases dramatically and reduces the efficiency of the device performance even damages it. Graphene, a sp2 hybridized carbon sheet, only has a geometric pore radius of 0.064 nm, which would impede metal atoms diffusing through the barrier, thus to protect the semiconductor material from alloying process. Here we demonstrate that a single layer graphene as a diffusion barrier to effectively mitigate the diffusion between metal/PbTe at high temperature up to 600 °C. The usage of graphene as the diffusion barrier increases the life of PbTe thermoelectric devices and only has little influence on the contact owing to its ultrathin nature. We also investigated the graphene as a diffusion barrier between different metals. Our research shows that graphene may be used as a general diffusion barrier in many material and device systems.

It is well known that contact resistance is becoming the main performance killer for building high performance graphene field-effect transistors (G-FETs), especially when the gate length is scaled down to sub-100 nm. Although various kinds of methods have been claimed and demonstrated to decline the contact resistance in G-FETs, contact resistance smaller than180 Omega;*um at room temperature has never been reliably realized.
In this work, we explored the contact resistance between graphene and palladium, and obtained extremely low contact resistance, which is due to the high quality of graphene, heavily doping effect of palladium and clean junction interface. The contact resistance of G-FETs is extracted by a modified transfer length method, in which the parasitic series resistances originated from metal lines and measuring probes are eliminated. According to statistic contact resistance values extracted from many groups of G-FETs, the contact resistance below 100 Omega;*um has been substantially realized at room temperature. The total resistance is as low as 154 Omega; in an as-fabricated G-FET with 2 um channel width, which means the single junction resistance is far below 150Omega;*um, if taking the sheet resistance of graphene channel into account. And more importantly, the contact resistance presents no obvious dependence on the temperature from room temperature (300 K) down to liquid nitrogen temperature (77 K), which is completely different to the published results from IBM.

The goal of this work is the development of advanced thermal interface materials (TIM) for satellite applications. We are focusing on graphene materials and composites to take advantage of the large intrinsic thermal conductivity of graphene (~5,000 W/mK). If even a small fraction of graphene&’s thermal conductivity be realized in new composite TIMs for spacecraft, overall thermal conductivity pathways could be greatly increased impacting many parts of spacecraft.
We are developing synthetic methods with the goal of creating novel materials and composites with high thermal conductivities out of the plane with good coupling to interfaces. Approaches to create three dimensional graphene materials and composites are being pursued, specifically the formation of graphene foams by chemical vapor deposition on three dimensional metal networks. Porous three dimensional graphene, with high intrinsic thermal conductivity can be made by chemical vapor deposition on porous metal frameworks and shows great promise for electrical and thermal applications. Methods to enhance phonon coupling rely on better contact and bonding at the interfaces through the creation of chemical bonds and the formation of compliant composites with high thermal conductivity. This report will describe the chemical vapor deposition of graphene foams, the formation of composites materials for TIM applications and their characterization.

Carbon nanotubes (CNTs) and graphene have been widely applied in various applications due to their extraordinary flexibility, high strength and superior electrical and thermal properties. In order to extend these unique properties into three dimensions, many efforts have been done to combine CNTs and graphene into three dimensional hybrid materials. Graphene aerogel, a three dimensional graphene nanostructure material, has demonstrated high surface area, excellent electrochemical property and extremely low density, which make them very promising materials for many energy related applications. Graphene aerogels dispersed with carbon nanotubes show significant enhancement of electrical and electrochemical properties which benefit from large surface area and porous structure of aerogels as well as superior electrical property of CNTs and graphene. In this work, we develop a new method to fabricate CNT/graphene aerogel hybrid material. Graphene aerogels are synthesized by a mild chemical reduction method. CNTs are grown directly on as-prepared graphene aerogels with iron catalysts by chemical vapor deposition process. The SEM and Raman characterization indicates the coexistence of the nanostructure of CNTs and graphene aerogels in the hybrid materials. Owing to the synergistic effects between CNTs and graphene nanosheets, the developed hybrid CNT/graphene aerogel show significant enhancement of thermal and electrical properties compared with the initial graphene aerogels, which can provide much better performance as electrodes for energy storage devices.

Capacitively transduced silicon nanoelectromechanical systems (NEMS) have been widely explored for applications in RF front-end filters due to their high quality factors, and potential for integration with existing CMOS processes [1]. However, the RF performance of this technology has been limited by poor actuation and sensing of mechanical resonance. To compensate for this, the electrode-to-resonator gap spacing can be decreased and the gap area can be increased to improve the electromechanical coupling, but at the cost of creating larger parasitic feedthrough currents that can easily overwhelm the mechanical resonance signal.
Recently, graphene NEMS (GNEMS) resonators have demonstrated improved RF transduction, and a large frequency tuning range, while maintaining low signal feedthrough [2]. This is achieved by operating the device as a resonant channel transistor, which actively senses and amplifies the mechanical motion of the resonator using the transconductance of the graphene. This allows the resonator to generate an output signal with a magnitude similar to that of Si-NEMS, but with much lower capacitive parasitics.
The fact that GNEMS resonators are biased like regular field-effect transistors and have a transfer function similar to other two-port acoustic resonators [3] allows them to be combined with traditional active devices to design tuned, hybrid NEMS circuits. Circuits using a mix of GNEMS resonators and regular transistors can leverage the properties of both types of devices to create a new class of active frequency-selective circuits.
In this work, we fabricate a resonant RF amplifier by cascoding a GNEMS resonator with a bipolar junction transistor (BJT). The BJT serves to boost the output resistance of the graphene device and improve the overall voltage gain of the hybrid amplifier, while preserving the mechanical resonance and tunability of the resonator. The hybrid amplifier shows an improvement in voltage gain of ~ 10 dB compared with the resonator by itself, and maintains a frequency tuning range of ~25% per volt. A circuit simulation model for the amplifier is presented, as well as several examples of other possible hybrid circuits.
[1] “An MSI micromechanical differential disk-array filter,” S. Li, et al., 14th Intl. Conf. on Solid-State Sensors, Actuators and Microsystems, 2007.
[2] “Electrically integrated SU-8 clamped graphene drum resonators for strain engineering,” S. Lee, et al., Applied Physics Letters, 2013.
[3] "Super-high-frequency two-port AlN contour-mode resonators for RF applications," Rinaldi, M, et al., Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on, 2010.

Graphene is a one-atom-thick material with unusual and highly promising for applications electrical [1] and thermal [2] properties. First obtained by mechanical exfoliation from graphite, graphene is now efficiently grown by chemical vapor deposition (CVD) on copper (Cu) films. It was reported that layered graphene - metal composites have enhanced mechanical strength. However, it was not known how deposition of graphene on Cu films affects their thermal properties. In this talk we will report of our investigation of thermal properties of graphene coated Cu films. The measurements were performed using the modified “laser flash” technique, which allowed for investigation of the in-plane heat conduction properties. It was found hat CVD of graphene enhances the thermal diffusivity and thermal conductivity of graphene coated Cu films. Deposition of graphene increased the thermal conductivity K of 9-mu;m (25-mu;m) thick Cu films by up to 24% (16%) near the room temperature. Interestingly, the increase of thermal conductivity of graphene coated Cu films is primarily due to changes in Cu morphology during graphene deposition and associated with it temperature treatment. Graphene&’s action as an additional heat conducting channel was small due to its small thickness as compared to that of Cu films. Enhancement of thermal properties of metal films via graphene coating may lead to major changes in metallurgy and graphene applications in hybrid graphene - Cu interconnects in Si complementary metal-oxide-semiconductor (CMOS) technology. [1] K.S. Novoselov et al. “Electric field effect in atomically thin carbon films,” Science, 306, 666 (2004); [2] A.A. Balandin, et al., “Superior thermal conductivity of single-layer graphene," Nano Letters, 8, 902 (2008). The work at UC Riverside was supported, in part, by the National Science Foundation (NSF) project ECCS-1307671 on engineering thermal properties of graphene, by DARPA Defense Microelectronics Activity (DMEA) under agreement number H94003-10-2-1003, and by STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) - Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.

Graphene, the atomic thin carbon film with honeycomb lattice, holds great promise in a wide range of applications, arising from its unique band structure and excellent electronic, optical, mechanical and thermal properties. The research of this star material is being stimulated by the development of various emerging preparation techniques, among which chemical vapor deposition (CVD) has received the fastest advances in the last few years. This talk focuses on our recent progresses towards the controlled surface growth of graphene and its two-dimensional (2D) hybrids via CVD process engineering. The general strategy is to design and control the elementary steps of catalytic CVD process for achieving a precise control of layer thickness, stacking order, domain size, doping and energy band structure. A particular emphasis is laid on the design of growth catalysts, including bimetal alloys and groups IVB-VIB transition metal carbides.
The sp2 carbon network of graphene is chemically very stable and hence it is a great challenge for its chemical doping and tailoring. We are working with a photochemical approach for graphene chemistry, where the chemical scissors are the highly reactive radicals generated from photochemical processes. A number of examples are given, including photochlorination, photomethylation, photocatalytic oxidation and Janus chemistry.
References
1)LM Zhang, JW Yu, MM Yang, Q Xie, HL Peng, ZF Liu, Janus graphene from asymmetric two-dimensional chemistry, Nature Comm., 4(2013) 1443-1449.
2)W Yan, WY He, ZD Chu, MX Liu, L Meng, RF Dou, YF Zhang, ZF Liu, JC Nie, L He, Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer, Nature Comm., 4(2013) 2159-2165.
3)K Yan, D Wu, HL Peng, L Jin, Q Fu, XH Bao, ZF Liu, Modulation-doped growth of mosaic graphene with single-crystalline p-n junctions for efficient photocurrent generation, Nature Comm., 3(2012) 1280-1286.
4) BY Dai, L Fu, ZY Zou, M Wang, HT Xu, S Wang, ZF Liu,Rational design of a binary metal alloy for chemical vapour deposition growth of uniform single-layer graphene, Nature Comm., 2(2011) 522-527.
5)K Yan, L Fu, HL Peng, ZF Liu, Designed CVD Growth of Graphene via Process Engineering, Acc. Chem. Res., 10(2013) 2263-2274.

Graphene, the two-dimensional material with honeycomb structure, possesses superior physical properties and thus is a promising candidate for a variety of electronic devices. Actually, there are some issues which should be studied to accomplish the fabrication of devices with high performance. The area of graphene films obtained from mechanically exfoliation is localized while the graphene films synthesized by chemical vapor deposition need to be transferred in spite of the much more complete area. In our investigation, the plasma treatment followed by annealing process is utilized in order to obtain large-scale graphene films from bulk SiC via Nitridation-Induced Carbon Condensation. After exposure of N2 plasma, the N2 annealing process, which promotes nitrogen ions to react with Si and simultaneously condense C around the surface of the SiC, is implemented. A thin silicon nitride layer formed during nitridation may squeeze carbon atoms. Eventually, a uniform large-scale graphene film on SiC wafer will be achieved. The Raman analysis shows typical spectra of graphene and the XPS results indicate that the formation of silicon nitride layer. Furthermore, the TEM images verify that there are several layers of graphene on the top of SiC. In this research, we create a simple method for synthesizing large-scale graphene.

Commercial exploitation of graphene's unique properties depends critically on the development of adequate graphene growth and integration technology. Catalytic growth techniques are widely seen as the most promising for achieving this goal. However, the underlying mechanisms have yet to be fully understood, and control over the material structure and quality remains rudimentary.
Here, we apply complementary in situ techniques to develop a detailed, multi-scale understanding of catalytic graphene growth on polycrystalline Ni catalysts. We focus on technologically relevant low temperature (le;600°C) exposures to gaseous hydrocarbons,^1,2 and vacuum annealing (le;600°C) of Ni/solid-carbon stacks.³ We combine time- and depth- resolved X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction/Reflectivity (XRD/XRR), and Scanning Tunnelling Microscopy (STM) to probe the structural and chemical properties of graphene and the catalyst during growth.^1,2,3,4
We show that graphene forms directly during isothermal hydrocarbon exposures, whilst the contribution of precipitation upon cooling is minor. Similarly, for Ni/tetrahedral amorphous carbon (ta-C) stacks, graphene growth is found to occur predominantly during ramping up and annealing by C dissolution and diffusion through the catalyst. A detailed growth rationale is established from these measurements, and we relate this to the understanding of the growth kinetics we have previously developed.⁵ On the basis of this, we introduce a number of approaches to control the structural properties of the graphene produced.
Catalyst alloying is demonstrated as a means of improving graphene growth by tuning reactivity and selectivity. We show that alloying Au with Ni results in an order of magnitude increase in graphene domain sizes.^1,5 We thus demonstrate the growth of monolayer graphene at 600°C, with a uniformity and quality that has hitherto only been reported for Cu-based CVD at >900°C.
For the catalytic transformation of solid C sources, we introduce C diffusion barriers as a general and simple method to prevent premature C dissolution during temperature ramping and thereby to significantly improve the graphene produced. A thin Al₂O₃(1-3 nm) barrier inserted into a Ni/ta-C stack is shown to enable growth of uniform monolayer graphene at 600°C, with domain sizes >100 mu;m, and a Raman D/G ratio of <0.07.³
The understanding of the growth process we develop is broadly relevant to catalytic graphene growth on a wide range of catalysts, as well as to the growth of other two dimensional materials. Furthermore, the rational engineering of the growth process on the basis of this understanding highlights the importance of our approach.
1) Weatherup et al. Nano Lett. 2011, 11, 4154
2) Weatherup et al. ChemPhysChem 2012, 13, 2544
3) Weatherup et al. Nano Lett. 2013, 13, 4624
4) Patera et al. ACS Nano 2013, 7, 7901
5) Weatherup et al. ACS Nano 2012, 6, 9996

One of the most promising methods to date for graphene large scale synthesis is by chemical vapor deposition (CVD) of carbon sources on catalytical transition metal substrates. Copper as a metal catalyst is ideal due to its low diffusion of carbon. which self-limits the growth of graphene to single layer; however, although CVD is suited for large scale growth of graphene it leaves much to be desired in terms of graphene quality resulting in poor device performance when compared to exfoliated graphene. One responsible entity for the low graphene quality is the contamination which occurs during the graphene synthesis on the surface of the copper catalyst foil. The phase transition of quartz at 573o C allows copper and hydrocarbon to diffuse into the quartz tube, which causes silicate to precipitate onto the copper foil during the CVD process. Here the contamination from quartz tubes commonly used for the CVD furnaces is revealed and the effects on the graphene quality are studied. Scanning Electron Microscopy is used to image the contamination on the Cu surface and Energy Dispersive X-ray Spectroscopy is utilized to identify and trace the potential source of the contamination. Raman Spectroscopy and Mapping is used to demonstrate the effect of the contamination due to the quartz tube on the graphene quality by showing an increase in the D peak and the development of the D&’ peak. A simple method of avoiding growth substrate exposure to turbulent flow during the growth is reported and verified to be an effective way of eliminating contamination from the quartz tube.

The creation and exploration of artificial graphene structures has recently become the focus of great interest. In particular, controlling the interlayer rotation (or twist) angles in multilayer graphene stacks allows one to modulate the overall band structure. However, producing such a structure remains difficult due to the random distribution of twist angles in as-grown samples. Here we report a novel way for creating large area graphene stacks with a pre-determined twist angle. We first grow single layer graphene whose orientation is aligned over a few cm length scale on copper foil using chemical vapor deposition (CVD) method. The overall and local angle alignment of this graphene sample is confirmed using low energy electron microscopy (LEED), dark-field transmission electron microscopy (DF-TEM) and selected area electron diffraction (SAED) techniques. Since the graphene is well aligned over a few centimeters, we can create large area graphene stacks with known twist angle by transferring these graphene layers while controlling the orientation of each layer during transfer. We confirm that the layers are coupled with the designed twist angle, by probing the resulting band structure using angle resolved photoemission spectroscopy (ARPES) and examining their interlayer optical resonance features using spatially resolved hyperspectral (DUV-Vis-NIR wavelengths) measurements. This new method is scalable, controllable, and uses commercially available copper foil, and thus paves the way to explore and exploit the novel properties of two-dimensional crystals in artificial stacks with controlled interlayer structures.

Graphene has been shown to provide a mechanism for the enhancement of vibrational signals of some nano materials and molecules, but not for others. Examples are given for both types of behavior, showing similarities and differences in the nano-structures in each of these categories.

The phase transformation from a non-terminated diamond (111) surface to graphene, has in the present study been simulated using ab initio Molecular Dynamic calculations at different temperatures and under various reaction conditions. For vacuum conditions, the graphitization process was found to start at about 800 K, with a final graphene-like adlayer obtained at 2500 K. The C-C bonds across the interface were found to be broken gradually when increasing the temperature. The resulting graphene-like adlayer at 2500 K was observed to chemisorb to the underlying diamond surface with 33% of the initial C-C bonds, and with a C-C covalent energy value of 3.4 eV. The corresponding density of states spectra show a p-doped character, as compared with graphene.
When introducing H radicals during the annealing process, a graphene-like adlayer started to be formed at a much lower temperature; 500K. The completeness of the diamond-to-graphene process was found to depend on the concentration of H radicals in the lattice. When the number of H radicals reached 34 within a super cell, a final free-standing graphene monolayer was formed at 1000 K. When introducing a larger concentration of H radicals into the lattice in the initial part of the annealing process, the formation of a free-standing graphene layer was found at an even lower H concentration and lower temperature (17 H within the supercell, and at 1000 K).

Due to its exceptional electrical and mechanical properties, graphene is currently being investigated as a transparent conducting electrode material, a resonator material, and as a channel or interconnect material for nanoscale electronics, among many other engineering applications. Graphene synthesis by chemical vapor deposition (CVD) has been studied extensively. However, synthesis of bi-layer graphene over large area on copper foil with good reproducibility is still a problem. It is therefore pertinent to develop better understanding of the growth mechanics that dictate the formation of single or bilayer graphene growth, and to achieve uniform bilayer graphene films over large area on an economically viable substrate with minimal processing time. It was speculated that slower cooling rates encourage the growth of bilayer graphene, and recent CVD studies at atmospheric pressure have shown that cooling rate and cooling environment affect the structure of graphene grown. We have carried out systematic studies to investigate the effect of flowing different gases during the cooling phase on CVD graphene growth. We found that uniform (film uniformity better than 95%) single or bilayer graphene can be a grown on large area copper foils by regulating the gases introduced during the cooling phase. We demonstrate that vacuum cooling enhances the growth of single layer graphene while the introduction of hydrogen gas during the cooling phase encourages the growth of bilayer graphene. Extensive statistical analysis of the Raman spectroscopy data before and after transferring the CVD grown graphene on SiO2 substrate confirmed the formation of single or bilayer graphene with excellent thickness uniformity. Apart from Raman analysis, the formation of single or bilayer graphene over large area (30cm x 30cm) have been confirmed by fabrication of field effect transistors and observing a clear difference in the electrical characteristics of the devices. In addition, optical properties and selected area electron diffraction (SAED) measurements confirmed the single and bilayer graphene growth. Our results demonstrate that hydrogen plays a crucial role during the cooling phase to encourage bilayer graphene growth.
This work was supported by the LDRD Director's Competitive Grants of ANL (Proposal Number #2013-096-N0).
The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory ("Argonne"). Argonne, a U.S. Department of Energy Office of Science Laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Graphene is expected to play a role as an essential building block of 2D integrated circuit due to the simple modification of its electrical properties by doping and patterning. However, in order for 2D integrated circuit to be well worked, it is necessary to find another building block as insulator and combine these two building blocks into one circuit. Recently, although the lateral hetero-structures with graphene and hexagonal boron nitride can be grown using two-step CVD method, it is complicated and not applicable to the system with more than two components. Here, we report that one step growth of graphene/amorphous carbon (G/AC) hetero-structures from solid source as polystyrene (PS) via UV irradiation. The chemical pattern of neat/cross-linked PS via UV irradiation on copper foil converted to the pattern of G/AC in CVD. Because the resistance of amorphous carbon is 100 times higher than that of graphene, amorphous carbon is worked as insulator. Also quantum Hall effect (QHC) is measured in the resulting G/AC lateral hetero-structure, that means the interface of G/AC is well defined and proves good quality of graphene. Our approach can suggest versatile method for growth of hetero-structure with various 2D materials.

Copper (Cu) is a metal catalyst preferred for the controlled growth of graphene. While the atomic structures of Cu surfaces have been extensively studied by surface scientists in the past few decades, it received surprisingly little attention in the graphene community. It has been known that stepped Cu surfaces with the low-index atomic terraces, such as (111) and (100), exhibit multilayer relaxation, which extends to the several layers beneath the surface with alternating contraction and expansion of the interlayer spacing. Because of the surface relaxation, the step-terrace structure of Cu (111) is quite different from a bulk-truncated configuration. Moreover, the surface steps are quite dynamic at elevated temperatures with exhibiting constant fluctuation/migration of the surface steps. Therefore, in order to understand how graphene adapts the highly dynamic stepped surface of Cu and how this affects the growth behavior and crystallinity of graphene, it requires real-time observations of graphene/Cu interface with an atomic resolution during the growth. Here we show in-situ atomic scale observations of the atomic structure of Cu-graphene interface and its dynamic evolution during the growth.
The transmission electron microscopy (TEM) specimen was composed of two pieces of cross-sectioned Cu (111)/α-Al2O3 (0001) samples, graphite bond and Mo support grid. After curing the assembled TEM specimen at 150 oC for 5 hrs, it was polished and then Ar+ ion-milled for electron transparency. The Stuttgart high voltage electron microscope operated at 1250 kV was used for in-situ heating experiments.
At elevated temperatures carbon atoms diffuse out from the solid source towards thin edges of Cu film where they started forming a few-layer graphene (FLG) or graphite. The graphitization initiated already at moderate temperatures (~300 oC) under the electron beam (1250 kV) irradiation. In-situ atomic-scale observations of the Cu/FLG interface revealed that the underlying Cu surface consists of flat (111) terraces separated by like-oriented atomic steps. The mean width of terraces and the vicinal angle are measured to ~2.0 nm and ~5.4 degrees, respectively. This vicinal surface is indexed as (28 28 35).
In order to quantitatively measure the atomic displacement field, the intensity maxima of atomic columns in high-resolution TEM image were determined precisely through peak picking analysis. The displacement field clearly visualizes a relaxation pattern; the step edge atom relaxes towards the bulk while the corner atom relaxes towards the surface. The multilayer surface relaxation resulted in the reduced step height of a stepped Cu (111) surface, over which a few layer graphene bend easily over these step edges like a blanket. This step-terrace structure of Cu surface is highly dynamic due to the temporal step migration/fluctuation, which is accommodated through lattice bending and/or various dislocation motions of graphene layers.

Nanostructures open opportunities for tuning the phonon spectrum and related properties of materials for specific applications at room temperature [1]. Emergence of graphene and 2D materials increased the possibilities for controlled modification of phonon spectrum and pho-non transport - referred to as phononics or phonon engineering. Acoustic phonons are the main heat carriers in graphene while optical phonons are used for determining the number of atomic planes in the few-layer graphene via Raman spectroscopy. Uniqueness of phonon transport in 2D translates to unusual thermal properties, which can be altered more drastically than in corresponding bulk. In this talk I outline the concept of phonon engineering in nanostructures, and describe methods of tuning the phonon spectrum and transport in gra-phene and 2D materials [2-3]. Control of thermal properties in graphene via ribbon edges, isotope composition, grain boundary and defect engineering will be discussed [3-5]. The fo-cus of the talk will be on phonon dispersion in AA-stacked, AB-stacked and twisted bilayer graphene with various rotation angles. It was found that the stacking order affects the out-of-plane acoustic phonon modes the most [6]. Twisting bilayer graphene leads to emergence of new phonon branches - termed hybrid folded phonons - which originate from mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle. By twisting atomic planed in graphene multilayers one can tune the specific heat and thermal conductivity.
This work was supported, in part, by the National Science Foundation (NSF) project ECCS-1307671 on engineering thermal properties of graphene, by DARPA Defense Microelectron-ics Activity (DMEA) under agreement number H94003-10-2-1003, and by STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) - Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.
[1] A.A. Balandin, Nanophononics: Phonon engineering in nanostructures and nanodevices, J. Nanosc. Nanotech., 5, 7 (2005)
[2] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Na-ture Mat., 10, 569 (2011)
[3] A.A. Balandin and D.L. Nika Phonons in low-dimensions: Engineering phonons in nanostructures and graphene, Materials Today, 15, 266 (2012)
[4] S. Chen, Q. Wu, C. Mishra, J. Kang, H. Zhang, K. Cho, W. Cai, A.A. Balandin and R.S. Ruoff, Thermal conductivity of isotopically modified graphene, Nature Mat., 11, 203 (2012)
[5] D.L. Nika, A.S. Askerov and A.A. Balandin, Anomalous size dependence of thermal con-ductivity of graphene ribbons, Nano Lett., 12, 3238 (2012)
[6] A.I. Cocemasov, D.L. Nika and A.A. Balandin, Phonons in twisted bilayer graphene, Phys. Rev. B, 88, 035428 (2013)

This presentation discusses our recent results on scaling predictions associated with thermal and electronic conduction in 2D graphene nanostructures. First part focuses on investigation of interface thermal conductance across graphene nanoribbons (GNRs). Here, we have performed detailed classical non-equilibrium molecular dynamics (MD) simulations of interfacial thermal transport across physically interacting (van der Waals interactions) GNRs of different widths and lengths. The predicted values differ significantly (~factor of 5) based on investigated parameters and show increase in a universal manner with respect to aspect ratio of interacting GNRs in a power-law like behavior. We attribute this scaling to degree of flexibility/conformability which increases with the aspect ratio and lead in better thermal energy exchange between the GNRs. In the second part, we discuss the scaling of energy band-gap of two dimensional semiconducting graphene nano-meshes (GNM) using extensive tight binding (TB) calculations. Based on our simulations and simple geometric arguments, we report that Pedersen scaling (Eg ~ radic;N_removed/N_total) governs not only the energy band-gap but also the effective mass of the Bloch electron of the semiconducting GNMs regardless of its chirality or the crystallography of the mesh holes at low areal fraction of holes (porosity). This study coherently puts together different scaling laws that are reported in the literature alongside their regions of applicability.

We have investigated thermal properties of graphene laminate (GL) films deposited on polyethylene terephthalate polyester (PET) substrates. Graphene laminate films consisted of overlapping graphene and few-layer graphene flakes. Graphene coating of PET substrates was performed using roll compression resulting in high-density films with the thickness of coating in the range from 3 mu;m to 10 mu;m. The measurements of the thermal conductivity of graphene laminate on PET substrate were performed using a non-contact Raman optothermal technique [1]. Thus is a direct steady-state technique, which provides directly the thermal conductivity. For these measurements the GL-PET samples were suspended in the sample holder. The metal clips of the holder played a role of the heat sinks. The samples were illuminated with laser light that provided local heating. Since the films were thick all light was absorbed by GL-PET. The temperature rise was determined from the temperature shift of the Raman G peak of graphene laminate determined independently. Micro-Raman spectroscopy was performed with 488 nm excitation laser and power ranging from 0.1 mW to 10 mW. From the temperature rise and dissipated power data we calculated the thermal conductivity by solving the heat diffusion equation with the help of COLSOL package. The room temperature values of the thermal conductivity of compressed graphene laminate on PET were in the range from 75 W/mK to 135 W/mK. The thermal conductivity of graphene laminate is substantially higher than of PET (0.15 W/mK - 0.4 W/mK). Our results suggest that coating PET and other plastic materials with graphene laminate can substantially improve their heat spreading ability.
The work at UC Riverside was supported, in part, by the National Science Foundation (NSF) project ECCS-1307671 on engineering thermal properties of graphene, by DARPA Defense Microelectronics Activity (DMEA) under agreement number H94003-10-2-1003.
[1] A.A. Balandin, "Thermal properties of graphene and nanostructured carbon materials," Nature Materials, 10, 569 - 581 (2011).

Kinetic theory estimates of the phonon mean free path in graphene imply that significant ballistic effects should be present in graphene devices with physical length scales on the order of microns or smaller. Unfortunately, the quadratic dispersion of the out-of-plane branch in graphene is not consistent with the linear branch approximations used to arrive at these estimates. In addition, the "classical" single mode relaxation time model is incapable of capturing one of the most important transport physics in graphene, namely the coupling between out of equilibrium phonon modes. State of the art phonon transport calculations capture this coupling and include detailed descriptions of the dispersion relation by solving a linearized form of the Boltzmann transport equation based on the ab-initio collision operator. The complexity associated with this operator, however, has limited present algorithms to steady, spatially homogeneous problems.
We present a new computational method which enables the simulation of steady and time-dependent phonon-transport problems in arbitrarily complex geometries using the ab-initio collision operator. The proposed formulation results in an energy-based stochastic particle simulation method in which the particles represent and solve for the deviation from a nearby equilibrium. This control variate formulation retains all advantages of Monte Carlo methods, such as simplicity, efficiency via importance sampling and exact treatment of the advection operator, while alleviating the most important disadvantage of Monte Carlo methods, namely high computational cost for small deviations from equilibrium. Efficient simulation of the ab-initio collision operator including detailed momentum and energy conservation is achieved by leveraging the principles of continuous-time Markov chains. Dispersion relations and transition rates are obtained from density functional perturbation theory calculations of second and third order force constants. The resulting simulation procedure is able to efficiently and accurately describe size dependent effects on thermal transport in simple graphene devices.

Graphene and related materials have been intensively studied for the last few years due to their fascinating electrical, mechanical and chemical properties, and recent advances in large-area graphene synthesis have enabled the potential applications to various electronic devices. There have been many efforts to utilize these fascinating properties of graphene for macroscopic applications such as transparent conducting films useful for flexible electronics. The implementation of graphene requires a production worthy process, and it has been successfully demon-strated that a thermal chemical vapor deposition (T-CVD) process is capable of growing high-quality graphene on Cu substrates. This approach was found to be easily scalable up to meter sizes by employing roll-to-roll (R2R) methods. However, a T-CVD process with an 8-inch tube reactor, for example, typically takes a few hours from heating to cooling, which has been a critical obstacle for the high-throughput production of large-area graphene films. In addition, the synthesis temperature as high as ~1,000 °C often results in the contamination by evaporated Cu, leading to the formation of defective graphene structures. Thus, efforts have been devoted to perfect the CVD synthesis at lower temperature not only by adopting more efficient heating methods but by alloying, annealing or polishing catalytic substrates, leading to larger single-crystalline domains and less structural defects such as grain boundaries and ripples. Nevertheless, the previously proposed high-throughput graphene synthesis methods based on Joule-heating and microwave plasma are found to yield either multilayered or defective graphene films whose sheet resistances are not suitable for capacitive touch screen applications. In addition, the strong chemical doping to enhance conductivity is not desirable because it doesn&’t persist long enough for practical applications without proper encapsulation. Thus, the new equipment based on rapid thermal (RT)-CVD has been designed and manufactured for the following purposes: i) to maintain the high quality of graphene at lower synthesis temperature; ii) to minimize growth time for high-throughput production; and iii) to achieve size, uniformity, reliability, durability and flexibility needed for industrial applications including foldable capacitive touch screen sensors in particular. Here we introduce the RT-CVD growth, etching, and transfer systems that are fully automatized and optimized for massive production of uniform high-quality graphene films satisfying the industrial requirements for touch screen applications. The resulting graphene-based capacitive multi-touch screen devices are fully functional in the most sophisticated mobile phone. The extreme flexibility of graphene further allows the fabrication of foldable touch screen sensors operating at the bending radius of ~3 mm, which is expected to bring the advent of flexible mobile devices forward.

We developed the large-area microwave plasma CVD of graphene for transparent conductive film applications. This technique has been successfully combined with the roll-to-roll process to synthesize graphene on Cu substrates. Fast growth of graphene is one of advantages of plasma CVD which is suitable for the mass production.
Current issue of the plasma CVD of graphene is that the electrical conductivity is limited. This is primarily due to the fact that the crystal size of the plasma CVD graphene is as small as a few nanometers. This could be attributed to very high growth rate with large nucleation density of plasma CVD which suppresses graphene growth in the two-dimensional direction and enhances the stacking of the flakes in multiple layers. To solve this problem in this study, we tried to expand the crystal size of graphene by reducing the carbon source concentration which inhibits the nucleation density and growth rate of graphene. We used a very small amount of carbon contained in the copper foil substrate as a source of graphene synthesis and tried to improve the crystalline quality and electrical conductivity.
We carried out graphene synthesis by plasma technique using the small amount of carbon atoms contained in the Cu foil as the carbon source. The carbon atoms precipitate on the Cu surface by the heat treatment of the Cu foil about 800degC. Then the Cu foil was exposed to hydrogen plasma in several ten seconds to synthesize graphene. Raman spectroscopy and Hall effect measurements revealed that the crystalline quality and the electrical conductivity have dramatically improved compared with the conventional plasma CVD using methane as a carbon source. This new synthesis method of graphene using carbon atoms in Cu foil is suitable for mass production of graphene by roll-to-roll.

Transfer of graphene from metal substrates has typically involved a polymer coating as a support, e.g. polymethyl methacrylate (PMMA), during etching of the metal to limit folding or tearing of graphene; the polymer support is then removed by a chemical or thermal treatment. PMMA residue and metal contaminants on graphene then typically limit its performance by lowering the carrier mobility and increasing the charge impurity density and the Dirac voltage.
To overcome this issue, graphene grown on Cu foils was directly transferred to target polyimide (PI) substrates via electrochemical delamination. The Cu substrate was not etched in this process, reducing metal contaminants on the graphene surface. Meanwhile no sacrificial support (PMMA) layer was used so the graphene is free from polymer residue. As shown by SEM and AFM, such ‘direct delamination&’ also led to a better contact between graphene and the polyimide substrate; complete graphene films with reduced line disruptions (such as ripples and wrinkles) were obtained. Better electrical performance was achieved in the graphene/PI films, such that they may be suitable for transparent conducting films (TCFs) in ‘plastic electronics&’ and potentially for graphene-based electronic devices on PI or other suitable plastic films.
We appreciate support from the Office of Naval Research, under grant N00014-11-1-0190; NSF-NASCENT Engineering Research Center (Cooperative Agreement No. EEC-1160494); and the W. M. Keck Foundation.

Pristine graphene has shown promise in applications to THz technology, its non-equilibrium carrier dynamics making possible the achievement of population-inversion under certain conditions despite the lack of a band gap. However, an optical band gap is essential for photonics applications [1].
It has been shown that a finite band gap in graphene can be opened through its functionalization with H, F or O atoms. Interesting optical properties have been observed in experiments with graphene oxide: Strong absorption in the UV range in aqueous solution, with absorption peaks shifting towards the visible range as oxidization is reduced, and photoluminescence in the visible and IR range from nanoscale flakes. We have investigated the electronic and optical properties of GO by a combination of density functional methods. For structural optimization we have used the generalized gradient approximation, while for estimation of electronic and optical properties we have employed a hybrid Hartree-Fock/DFT approach [2].
The most stable configuration of fully oxidized graphene has a C:O ratio of 2:1, with O atoms occupying positions alternating above and below bridge sites [3]. The formation of sp3 hybridized bonds between C and O 2pz orbitals and minimization of Coulomb repulsion between O atoms renders this structure highly stable with an O binding energy of 3.25 eV per atom. The bond rehybridization results in vertical displacement of C atoms, as well as changes to the electronic structure. We estimate the direct gap at 6.50 eV.
Reduced GO structures were modeled by removing O atoms from the initial structure to form graphene quantum dots. The smallest GQD, with one adatom removed, has a vacancy formation energy of 4.11 eV and an estimated band gap of 4.90 eV. The reduction in the gap is attributed to the appearance of new electronic states localized on C atoms at the desorption sites caused by reversion to sp2 bonding. The removal of 4 O atoms further decreases the gap to 2.33 eV. The electronic structure around the Fermi level is qualitatively similar to that for a single vacancy.
Our results fit a simple relationship Eg=E0exp[-0.25nvac], where E0 is the gap of fully oxidized graphene and nvac the number of O vacancies. This indicates that 8 O vacancies would result in a gap of ~ 1 eV, around the visible range. Predicted trends agree with experimental observations of graphene nanoflakes, which show the band gap to be inversely proportional to the flake diameter. It is evident from our results that tuning the band gap by controlling oxidization is possible, thereby allowing control of the electronic and optical properties.
[1] F. Bonaccorso, Z. Sun, T. Hasan, A. Ferrari, Nature Photon. 4, 611 (2010).
[2] S. Tomic, B. Montanari, N. Harrison, Physica E 40, 2125 (2008).
[3] Z. Sljivancanin, A. Milosevic, Z. Popovic, F. Vukajlovic, Carbon 54, 482 (2013).

The key characteristics of graphene relevant to energy applications - defects, nucleation density, crystallinity, and number of layers - depend critically on the growth mechanisms and kinetics. A combination of real-time Raman spectroscopy, optical imaging, and optical reflectivity were used to characterize the growth kinetics of graphene on Ni films at different growth temperatures by pulsed chemical vapor (CVD) and pulsed laser deposition (PLD). Fundamental synthesis questions are addressed including the timescales for rapid nucleation and growth and whether growth occurs at high temperature or upon cooling. Pulsed CVD is used to provide discrete doses of acetylene with well-defined temporal pulse for kinetics studies. In situ Raman spectroscopy reveals that graphene grows both isothermally, and upon cooling, with the fractional precipitation upon cooling depending upon the flux and temperature - parameters that are crucial for understanding the growth mechanism and controlling the number of layers. Temperature-dependent growth kinetics clearly show two completely different nucleation and growth mechanisms, i.e., surface nucleation and growth at low temperatures (<680 °C) and nucleation and growth due to segregation from the bulk of the Ni film at higher temperatures (>680°C), with an unexpectedly sharp transition temperature between these two growth modes. The described approach combining deposition by CVD or PLD and real-time optical diagnostics opens new opportunities to understand and control graphene growth on various substrates. A simple kinetic growth model which describes our in situ measurements of the growth kinetics will be presented.
Research sponsored by the U.S. Dept. of Energy, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facilities Div. (characterization science).

The effect of gas transport inside a micrometer-scale jig gap on the growth of graphene on Cu foil located in the gap is reported. Due to the small size of the gap, a boundary layer is fully developed inside the gap, and the gas molecule transport is controlled by the molecular flow. Moreover, the conductance of the gas molecules can be tuned by decreasing the gap spacing from 1 mm to 100 mu;m. First, the Cu surface is protected from the sublimation and re-deposition of Cu during pre-annealing, which results from the relatively static gas environment of the molecular gas flow. Second, suppression of the gas conductance resulted in strongly reduced overall graphene coverage with a smaller average grain size but with almost the same density as that of the graphene nuclei. Furthermore, the suppression of gas conductance leads to the formation of well-bounded graphene morphology instead of a dendritic morphology. We will describe how these results contribute to the overall understanding of the mechanism of graphene growth on Cu foil.

In recent years great interest has arisen towards applying graphene and graphene oxide (GO) in sensing devices. Single layer graphene is however an expensive, difficult to produce material. On the other hand, graphene oxide is low cost and may easily be produced using chemical exfoliation of graphite through oxidation followed by dispersion in a solvent or water [1]. In this work a chemical sensor based on partially reduced GO is presented.
For the realization of the sensors, Laser Induced Forward Transfer (LIFT) was used to print GO over 100 mu;m long gold electrodes with spacing between 5 and 40 mu;m. First, a donor quartz substrate was coated with a small amount (sim; 10mu;l) of GO solution (Graphenea, 4 mg/ml in water) with a doctor blade in order to obtain a uniform thin film. Graphene oxide was then laser printed using the 4th harmonic (266 nm) of a pulsed Nd:YAG laser (tau; = 4 ns, 2 Hz) at 200 mJ/cm² first on bare Si/SiO₂ in order to calibrate the printing parameters and then on the gold electrodes of the sensors, while the distance between the donor and the receiver substrates was kept below 200 mu;m for all experiments.
Following the successful printing on the bare Si/SiO₂, printing on the actual sensors was performed, followed by a thermal reduction process. This involved heating [2] of the laser printed graphene oxide at 300 ^oC for 1 h. The efficiency of the reduction was investigated using Raman and electrical characterization. Electrical conductance of the GO, drop casted on the Au electrodes (as received), after printing using LIFT, and after thermal treatment was measured by taking the I-V response. As expected, drop casted, as received, graphene oxide is an insulator with extremely high resistance (in the GOhm range) and this is reflected with an almost flat I-V response. Furthermore, drop casted GO and laser printed GO result in similar resistance values and thus it can be deduced that the LIFT process does not affect GO. On the other hand, the resistance of GO after the thermal treatment is approximately 30 KOhm clearly indicating the partial reduction of the original deposited graphene.
The evaluation of the response of the sensors was conducted using a special measurement setup where the sensors, packaged in Dual in Line (DIL) packages, are placed in a small volume chamber (7 mm³) and in which controlled concentrations of analytes may be introduced.
The devices were, then, tested by exposing them to various concentrations of ethanol, p-xylene and water vapours. The sensor exhibits ΔR/Ro of ~4% and ~0.5% when exposed to 10,000 ppm of water and ethanol vapours respectively, whilst it exhibits considerably higher response upon exposure to p-xylene vapours, which is attributed to p-xylene being aromatic and tends to physisorb to the aromatic rings of the basal plane of reduced graphene oxide.
[1] Y.Zhu et.al, Adv. Mater. 22 (2010) 3906-3924
[2] G.Lu et.al, Nanotechnology 20 (2009) 445502

Recent scanning transmission electron microscopy (STEM) observations of metal-doped graphene have shown that the metal atoms bind exclusively to edge sites and contaminated regions, but not to the pristine regions of graphene. It was hypothesised from this that metal adatoms are very mobile on graphene at room temperature and therefore quickly migrate randomly across the lattice until they bind to more energetically-favourable edge sites by the time the samples reach the microscope. To test this hypothesis, we used density functional theory to optimise the structures of Al, Au and Cr atoms on the adsorption and edge sites of monolayer, bilayer and trilayer graphene and compared their energies and bonding characters. Then we calculated the migration energy barriers between the adsorption sites. It was found that Al, Au and Cr atoms form very weak bonds at the adsorption sites but form strong chemical bonds at the edge sites, and the migration activation barriers were all found to be very small: within an order of magnitude of k_B T at T = 300 K. These theoretical predictions are in striking agreement with the STEM observations. The implications of this are very broad. Much of nanotechnology relies on the passive manipulation physical matter on the microscopic level by means of harnessing naturally occurring processes under controlled conditions. Preferential edge-site binding is one such process which could be exploited in contexts such as patterned nanoscale devices, systematic edge-decoration of 2D nanoribbons and other nanoscale constructions where site-dependent bonding tendencies are an important ingredient in the fabrication process.

Manipulation of various physical fields, including optics, electromagnetics, acoustics, thermotics, etc. through different materials has been a long-standing dream for many researchers over the decades. Analogous to invisible cloak and wave-dynamic illusion, thermal cloak and thermal illusion can potentially transform an actual perception into a pre-controlled perception, thus empowering unprecedented applications in thermal cloaking and camouflage. Here we report our recently two works about thermal cloak and thermal illusion based on simulation and experimental validation. We demonstrate a bilayer thermal cloak made of bulk isotropic materials. Our simulation work derived directly from thermal conduction equation, has been validated as an exact cloak, and we experimentally verified its ability to maintain the heat front and its heat protection capabilities. Also we propose and realize a functional thermal illusion device, which is capable of creating multiple expected images off the original object&’s position in heat conduction. The thermal scattering signature of the object is thus meta-morphosed and perceived as multiple ghost targets with different geometries and compositions. The thermal illusion effect is experimentally confirmed in both time-dependent and temperature-dependent cases, demonstrating excellent thermotics performance.

References:
[1] S. Narayana and Y. Sato, “Heat flux manipulation with engineered thermal materials,” Phys. Rev. Lett. 108, 214303 (2012).
[2] R. Schittny, M. Kadic, S. Guenneau, and M. Wegener, “Experiments on transformation thermodynamics: Molding the flow of heat,” Phys. Rev. Lett. 110, 195901 (2013).
[3] Tiancheng Han, Tao Yuan, Baowen Li and Cheng-Wei Qiu, “Homogeneous thermal cloak with constant conductivity and tunable heat localization”, Sci. Rep. 3, 1593 (2013)
[4] Tiancheng Han, Xue Bai, John T.L. Thong, Baowen Li, and Cheng-Wei Qiu, “Full Control and Manipulation of Heat Sig-natures: Cloaking, Camouflage and Thermal Metamaterials”, Adv. Mater., accepted, 2013.

Wrinkle-formation via thin-sheet compression is a phenomenon readily exhibited by two-dimensional (2D) nanomaterials (like graphene), separating them from their 0D and 1D counterparts (nanoparticles, nanowires and nanotubes). On graphene, these wrinkles can modify the band structure and local electronic states; however, wrinkles with controlled attributes (wavelength, amplitude) on graphene have not been achieved. Here, we show that bacterial cells can be employed as sacrificial scaffolds to induced controlled wrinkles on attached graphene. The wrinkles orient in the longitudinal direction of the cylindrical bacterial cells and the wrinkle-attributes (wavelength and amplitude) were controlled with high precision (in nanometer scale). Further, a modified Herringbone model was used to fit the results with high regression. According to circumferential Young&’s modulus (39MPa) for bacterial cells from bacterial literature, wrinkles wavelength was calculated to be 27.3 nm, which is consistent with the measured wavelength of 30 nm (SEM images). The modulation in the electrical properties with wrinkles will also be presented. The method paves a route to design restrained semiconducting graphene systems and devices.

In this work, we study the self-assembly process of nanowires that are aligned to molecular directions of graphene substrates and apply the process to fabricating graphene nanoribbons with controlled crystallographic orientations.
In MRS 2013 Fall Meeting, we presented the discovery of a liquid-based technique that can synthesize epitaxially aligned nanowires on graphene. The technique was unique and advanced, since previously formed inorganic nanostructures are randomly oriented on graphene derivatives or are only partly aligned on thick graphite or multi-layered graphene. However, our previous study couldn&’t present any details of the process, even including what the nanowire material was. In this work, we analyze chemical details of the process by combining the atomic-resolution TEM imaging and the crystal structure estimation. In addition, we fabricate graphene nanoribbons with zigzag edges using the nanowires as an etching mask. The zigzag-edged graphene nanoribbons are predicted as an important material for spintronics, but current fabrication processes have lack of scalability. Since the synthesized nanowires are naturally aligned to the graphene crystal directions, all of the fabricated nanoribbons have an identical crystal direction - zigzag-edged direction.
The previously reported technique forms the nanowires on pristine graphene and longitudinal directions of the synthesized nanowires are aligned to zigzag-edged directions of graphene. In this work, we claim that these nanowires are made out of one of hydrated forms of gold(III) oxide, 5(Au2O3)H2O. Since classic techniques for the material analysis such as XRD and XPS cannot be applied due to the tiny amounts of samples, we use an indirect way, which is to compare captured atomic-resolution TEM images to the crystal structures of candidate materials. From the EDX analysis and the reaction chemistry, we can expect that the nanowires are made out of one of hydrated forms of gold(III) oxide. The atomic-resolution TEM image of the nanowire shows 2D positions of gold atoms. Among the candidate materials, only 5(Au2O3)H2O has the identical 2D arrangements of gold atoms, and its simulated TEM image from the crystal structure coincides well with the captured TEM image.
The epitaxially aligned nanowires can be used as a mask material of graphene etching, thus enabling us to fabricate graphene nanoribbons with controlled crystallographic orientations (zigzag-edged graphene nanoribbons). Graphene is patterned by O2 plasma, and then the nanowires are selectively etched out in a NaOH solution. The SEM images confirm the successful fabrication of graphene nanoribbons. We believe that these crystallographically-aligned nanostructures are not only useful materials for the specific application (spintronics) but also a good example of nanofabrication techniques combining bottom-up and top-down approaches.

We demonstrate a simple method for preparing hybrid MS2/3DGN serves as template for the deposition of MS2 and also provides good electrical contact between the current collector and deposited MS2. The MS2/3DGN composite are examined by Raman spectroscopy and X-ray diffraction (XRD). Raman spectra results reveal that the composite has two distinct peaks at 384 (353) cmminus;1 and 406 (419) cmminus;1, corresponding to the E12g and A1g Raman modes of MoS2 (WS2), respectively. The characteristic G and 2D peaks of graphene are also observed at sim; 1581 cm minus;1 and sim; 2716 cmminus;1, respectively. The MS2/3DGN show excellent electrochemical performance as an anode material for lithium-ion batteries. The hybrid anode material exhibits reversible capacities of 910 and 750 mAhgminus;1 during the 50 th cycle at current densities of 110 and 522 mA gminus;1, respectively, indicating its good cycling performance. Furthermore, the MS2/3DGN also shows excellent high-current-density performance.

The formation of graphene brushes by grafting polymers onto graphene represents a very attractive route to functionalized graphene, which is readily dispersed in a variety of organic media including polymer melts. This leads to novel families of molecular carbon nanocomposites exhibiting matrix reinforcement combined with markedly improved electrical and thermal conductivity.[1] In most conventional approaches, graphite oxide or reduced graphite oxide, respectively, are functionalized with initiator and chain transfer agents, thus enabling styrene grafting in a subsequent step. In our one-step synthesis, thermal styrene graft copolymerization onto organophilic stearylamine-modified graphite oxide (Stearyl-GO) is achieved without requiring the immobilization of initiators. From the on-line monitoring of this grafting reaction, it is apparent that the concentration of the graphene-centered radicals increases during the initial stage of the graft polymerization. Most likely, this is attributed to the addition of polystyrene radicals to Stearyl-GO and also hydrogen transfer reactions. Following this “grafting-to” reaction, the subsequent “grafting-from” reaction accounts for additional covalent attachment of polystyrene chains. In contrast to the PS/Stearyl-GO blend dissolved in xylene, which is readily separated by filtration, the PS-g-Stearyl-GO forms very stable graphene dispersions which are not separated by filtration. Moreover, Stearyl-GO is a very effective stabilizer improving the thermal stability of polystyrene during prolonged heating at 250°C.[2] The thermal, electrical, rheological and morphological properties of PS-g-Stearyl-GO were compared with PS/Stearyl-GO blends.
[1] H. J, Salavagione et al. Macromol. Rapid Commun. 2011, 32, (22), 1771-1789.
[2] F. Beckert et al., Macromolecules 2013, 46, (14), 5488-5496.

Here, we demonstrate for the first time a highly expendable doping of large area graphene by using chemically versatile monolayer graphene oxide (GO) nanosheets. The charge transfer interaction between graphene and the electron-withdrawing groups of GO nanosheets showed efficient transition of the electronic states of the graphene, revealing the feasibility of the GO as a new non-covalent dopant material. Moreover, GO doping method is also beneficial for maintaining the intrinsic roughness (~0.55nm) and transparency (~96.7%) of graphene, which is unique characteristics of the atomically thin GO dopant. The doping level of graphene was finely tuned by simple variation of the coverage, the degree of oxidation and consecutive reduction of GO nanosheets. The photoemission spectroscopy results showed that the Fermi-level of GO doped graphene shifted up to 0.45eV along with the variation of the coverage from 0 to 100%, and consequently the conductivity of the graphene was almost doubled. The degree of functionality via oxidation and reduction of GO also drastically modulated the electrical characteristics of GO-graphene system. Furthermore, we also demonstrated viability of GO-graphene as the transparent flexible electrode in organic electronics. Organic thin film transistors (OTFTs) with graphene/GO as electrodes showed excellent electrical performance and mechanical stability. The GO doping method will also be very useful for optoelectronics such as organic photovoltaic cells and organic light-emitting diodes as well as OTFTs.

We report on an innovative approach to fabricate lithium ion battery anodes based on optimized growth of hybrid carbon nanotube (CNT) and graphene nanostructures directly on copper foil substrates by an ambient pressure chemical vapor deposition process. Seamlessly connected graphene and CNT pillars provide a relatively strong active material-current collector integrity, which facilitates charge transfer in the system. This innovative architecture provides a binder-free technique for preparing electrodes for lithium ion batteries.

Cu etching is one of the key processes to produce large-area graphene from chemical vapor deposition (CVD) on Cu foils to be transferred to target substrates for further applications. However, the Cu etching method has been much less studied compared to doping and transfer processes although they are equally important for high-performance graphene production. The Cu etchant usually includes a strong oxidizing agent that converts metallic Cu to Cu²⁺ in a short period of time. The highly concentrated Cu²⁺ can catalyze a reaction leading to the formation of defects on graphene, which needs to be prevented for higher quality of graphene films. Here we report that addition of metal-chelating agents such as benzimidazole (BI) suppresses the catalytic activity of Cu²⁺ ions by forming a complex compound (Cu(SO₄)(C₇H₆N₂)₄). This also shifts the equilibrium of Cu oxidation reaction, which facilitates the etching process. Generally, BI molecule was utilized as heterocyclic strong electron acceptor and it has chemically, thermally, and thermooxidatively stable properties. In Raman spectrum, the 2D and G band of BI-graphene were slightly up-shifted and the I(2D)/I(G) ratio was significantly decreased with BI modification, which indicates that stronger p-doping was induced, while the simple etching with ammonium persulfate(APS) solution resulted in mild p-doping. X-ray photoelectron spectroscopy (XPS) of BI-graphene on SiO₂/Si substrate shows that carbon, oxygen and nitrogen bonding states on BI-graphene. XPS peaks indicated a certain degree of oxidation and nitriding, which came from catalyst etching processes. The G and 2D band Raman mapping and atomic force microscopy (AFM) mapping were taken to confirm the surface uniformity of BI-graphene. Tunneling electron microscopy (TEM) was measured to define the crystallinity of BI-graphene and selected area electron diffraction (SAED) confirms single-crystalline structure of BI-graphene. BI-graphene exposed in Cu stabilizing agent has low sheet resistance value (~200 ohm/sq) and high carrier concentration (~1×10¹³cm^-2) without additional doping process. The BI-graphene exposed to 85% humidity and 85 °C temperature condition for 24 hours showed outstanding stability. The sheet resistance of mono-graphene was slightly increased approximately 10%. However, multi stacked graphene exhibited almost no change because of encapsulation effect of additional layers. We also confirmed that the strong doping effect was stably maintained after 4 months at atmospheric condition. Thus, we expect that this simultaneous doping and etching method would be very useful for sustainable high-performance applications of large-area graphene electrodes.

Since graphene was discovered in 2004, there have been many studies on fabrication of graphene-based device.[1-3] Poly (methyl methacrylate) PMMA is commonly used for transferring chemical vapor deposition (CVD) grown graphene. The residue of PMMA after the transfer interrupted graphene&’s intrinsic electric properties.[4] It is problem not to completely remove the residue of PMMA. Annealing in Ar/H2 gas flow has been commonly adopted to remove the residue of PMMA.[5] In this paper, we report on a systematic study of annealing on graphene in the wide temperature range of 350 to 800 °C using Ar/H2 forming gas. According to a temperature change, the effect of annealing on graphene is watched by using Raman spectroscopy and atomic force microscope. The conductivity was increased at moderate temperature, but decreased at excessive temperatures higher than 650 °C. On the other hand, the PMMA residue was not removed effective in all temperature ranges, judging from Raman spectroscopy and atomic force microscopy. By analyzing Raman spectroscopic data, chemisorption of PMMA residue on graphene was confirmed.
Reference
1.Geim, A. K.; Novoselov, K. S. Nat.Mater, 6, 183-191. (2007)
2.Q. Yan, B. Huang, J. Yu, F.Zheng, and W. Dua, Nano letter, 6, 1469-1479 (2007)
3.Geim, A. K. ; Novoselov Science, 5934, 1530-1534 (2009)
4.A. Nourbakhsh, M. Cantoro, J. Phys. Chem. , 114, 6894-6900.(2010)
5.Yung-Chang Lin, Chun-Chieh Lu, and Po-Wen Chiu, Nano letter,12, 414-419.(2012)

Understanding of the physics in metal-graphene interface is important to achieve high spin injection efficiency in graphene-based spintronic devices, which is known to be strongly limited by the conductivity mismatch between ferromagnetic (FM) metal and graphene. Recently, we have tried to alleviate the conductivity mismatch problem by inserting a Cu interfacial layer of different thicknesses ranging from 1.5 to 5 nm between the FM metal and graphene, and successfully increase the spin injection efficiency to 8.4% while keeping the contact resistance in the same order as in transparent contact. [1,2] During the magnetoresistance (MR) measurements, an unusual resistance peak at low in-plane magnetic field was always observed in addition to the anisotropic magnetoresistance (AMR) peaks from the NiFe layer. In order to reveal the origin of this low-field resistance peak, NiFe/Cu/graphene trilayer with different Cu thickness (2.5, 3.5 and 5 nm) and large length-to-width ratio (> 15) were fabricated on SiO2 (300 nm)/p++-Si substrate. Systematic two-probe MR measurements in both in-plane and perpendicular magnetic field were performed at different temperatures (4.2 - 300 K) and at different gate biases (-60 - 60 V). At 4.2 K and with in-plane field, similar MR magnitudes of 0.16% - 0.25% and 0.17% - 0.33% are obtained for the 2.5 nm and 3.5 nm samples, respectively, while the MR signals for the 5 nm Cu samples were much weaker (0.07% - 0.14%). The MR ratio was found to depend on the gate bias with the minimum magnitude occurring at the graphene neutrality point, but insensitive to temperature. Coherence length associated with three different types of scattering mechanisms (i.e. inelastic scattering, elastic intervalley scattering and elastic intravalley scattering) at different temperatures was extracted by fitting the MR curves. Inconsistency between the temperature dependence of coherence length and the results from literature suggests that weak localization is unlikely the origin of the low-field MR peak. The possibility of weak localization was further excluded by the inconsistency in the extracted coherence lengths from the MR measurements with in-plane and perpendicular magnetic fields. On the other hand, it has been reported in literature that intercalation of Au between Ni and graphene can give rise to a giant Rashba splitting in the band structure of graphene, and Ni/Cu/graphene/Ni structure is predicted to exhibit a large MR ratio. The possibility of Rashba induced spin-dependent band splitting in graphene as the origin of low-field MR peak will be discussed with more experiments.
References:
[1] C. Zhang, Y. Wang, B. L. Wu, and Y. H. Wu, Appl. Phys. Lett. 101, 022406 (2012).
[2] C. Zhang, Y. Wang, B. L. Wu, and Y. H. Wu, J. Appl. Phys. 113, 203909 (2013).

Giant magnetoresistance (GMR) effect is observed when the electrical resistance of a thin film changes depending on magnetization alignment of the adjacaent ferromagnetic layers on either side of the thin film. GMR has been observed in graphene nanoribbons1, and other graphene based devices2. We propose an experiment that will investigate the impact of h-BN on the giant magnetoresistance effect observed in graphene. To prepare our heterostructures, we shall employ two methods: (i) we will grow graphene via CVD on h-BN thin film, and (ii) we will transfer already grown graphene layer on to an already grown thin film of h-BN. We will investigate a number of permutations for placement of ferromagnetic layers before presenting our results.
1. Bai, J.; Cheng, R.; Xiu, F.; Liao, L.; Wang, M.; Shailos, A.; Wang, K. L.; Huang, Y.; & Duan, X. (2010). Very large magnetoresistance in graphene nanoribbons. Nature Nanotechnolgoy, 5, 655.
2. Munoz-Rojas, F.; Fernandez-Rossier, J.; & Palacios, J. J. (2009). Giant magnetoresistance in ultrasmall graphene based devices. Phys. Rev. Lett., 102(13), 136810.

Graphene quantum dots (GQDs) exhibit great potential for various optoelectronic applications due to their size-dependent and edge-sensitive photoluminescence properties. Previously, GQDs were mainly synthesized from graphene oxides by chemical exfoliation under strongly acidic environment or by multi-step lithographic methods including masking, patterning and lift-off, which hindered the efficient preparation of high-quality GQDs for practical applications. On the other hand, nitrogen-functionalization or doping is known to be very helpful to tailor the intrinsic properties of GQDs, but it needs further complicated wet-chemical reactions. Here we report a very simple solvent-free method to prepare nitrogen-doped GQDs (N-GQDs) by directly applying nitrogen plasma to as-grown graphene on Cu. The resulting N-GQDs can be transferred as a film-like layer or easily dispersed in an organic solvent. We confirm that the N-GQDs whose sizes are narrowly distributed around ~5 nm show strong photoluminescence when excited at 365 nm. For the further applications, N-GQDs film was used for solar-driven hydrogen evolution reaction on Si-photocathode as a catalyst. The onset potential for photocurrent from the Si was significantly shifted toward the anodic direction without a change in the saturation current density. N-GQDs show excellent catalytic activity for photoelectrochemical hydrogen evolution reaction on the Si photocathode and are the passivation layer that maintains higher onset potential and current density even at neutral pH. Our approach in this study, N-GQDs film would be synthesized by one-step plasma method and be useful for a wide range of applications, such as optoelectronic, electrochemical, and energy storage applications.
Reference
1. U. Sim*, T. Yang*, J. Moon*, J. An, J. Hwang, J. Seo, J. Lee, K. Y. Kim, X. Han, B. H. Hong, K. T. Nam, Ener. Env. Sci. (Published on line) DOI: 10.1039/c3ee42106f.

The opening of a bandgap in graphene has been observed by etching graphene into nanoscale laterally confined sheets or graphene nanoribbons. Although graphene nanoribbons with widths of less than 10 nm have shown substantial ON/OFF ratios, the current and conductance of individual graphene nanoribbons is far too low for practical applications. To obtain higher currents in field effect transistors a network of graphene nanoribbons, or graphene nanomesh, should be utilized. We present a scalable method for the fabrication of graphene nanomesh by the ordering of self-assembled block copolymer templates. Our approach offers excellent control over graphene neck sizes due to the use of well-ordered block copolymer templates. The graphene nanomesh morphology is demonstrated as a highly sensitive gas sensor due to the ability of molecules to bind at the sp3 sites of broken graphene edges and the high surface area to volume ratio of graphene nanomesh.

Graphene based membranes provide ideal materials to implement semi transparent mirrors with tunable position. This very high Young modulus material [1] can be electrostatically actuated [2, 3, 4] with an optical detection of its motion [2,4]. We present fabrication and optical characterization of micro-cavities made of multilayer graphene (MLG) cantilevers clamped by metallic electrodes and suspended over Si/SiO2 substrates. Graphene cantilevers act as semi-transparent mirrors closing air wedge optical cavities. This simple geometry implements a standing-wave optical resonator along with a mechanical one. Equal thickness interference fringes are observed in both Raman and Rayleigh backscattered signals, with interfringe given by their specific wavelength. Chromatic dispersion within the cavity makes possible the spatial modulation of graphene Raman lines and selective rejection of the silicon background signal. Electrostatic actuation of the multilayer graphene cantilever by a gate voltage tunes the cavity length and induces space and time modulation of the backscattered light, including the Raman lines. We demonstrate the potential of these systems for high-sensitivity Raman measurements of generic molecular species grafted on a multilayer graphene surface. The Raman signal of the molecular layer can be modulated both in time and space in a similar fashion and shows enhancement with respect to a collapsed membrane [5].
References
[1] C. Lee et al. Science, 321, 2008.
[2] J. Bunch et al. Science, 15 (5811), 490, 2007.
[3] C. Chen et al. Nature Nanotechnology, 4(12) :861-867, 2009.
[4] A. Reserbat-Plantey et al. Nature Nanotechnology 7, 151-155 (2012)
[5] A. Reserbat-Plantey et al. J. Opt. 15 (2013) 114010

Recently, graphene, the one-atom-thick two-dimensional graphitic carbon system, has attracted tremendous attention due to its extraordinary physical, mechanical and chemical properties. To date, graphene has been formed by physical exfoliation, epitaxial growth, or chemical vapor deposition. Nevertheless, the low yield and expensive equipment involved in these methods prevent potentially scalable production of high-quality graphene. With significant development in past decades, the yield of graphite oxide (GO), the product by oxidizing graphite, has been greatly improved, and the cost has been much decreased. By employing a reduction procedure, GO can be readily converted to graphene.
Herein, we have, for the first time, employed a magnesiothermic reduction to convert microwave-irradiation GO to pure graphene. The magnesiothermic reduction can successfully remove the oxygen by give the graphene product with an ultrahigh C/O ratio of 165.7 from 22.2 in the microwave-irradiation GO. In Raman spectra of the graphene product, a 2D-band peak clearly indicates that the graphene sheets were restored. A postulated reaction mechanism is:
CxOy (s) + yMg (g) → xC (s) + yMgO (s)
The restoration of the graphitic structures may be due to the release of intense heat during the magnesiothermic reaction. Furthermore, the graphene product maintains high porosity even after the reduction. The Brunauer-Emmett-Teller (BET) surface area is calculated to be 249.9 m2/g. This new synthetic methodology may open up new avenues to prepare ultra-pure porous graphene materials that exhibit potential applications in batteries, supercapacitors, and sensors.

Growing graphene on a metal surface is one possible way to obtain high quality graphene, with a controllable number of layers. The synthesis usually relies on a chemical vapor deposition of a carbon bearing gas on the surface of a metal such as Ir, Cu, or Ni. We investigate the case of graphene on Ni that is of particular interest since the role of carbon solubility in subsurface layers is both difficult to investigate experimentally and important to understand the synthesis process.
To study the interaction of carbon with nickel at the atomic level, we have developed a tight binding model [1] implemented in a Grand Canonical Monte Carlo code. It has been used to investigate the nucleation and growth of carbon nanotubes in CVD processes [2, 3]. With the same approach, we investigate the CVD synthesis of graphene on Ni (111) and correlate our results to experimental data. We identify thermodynamic conditions (temperature and carbon chemical potential) to obtain single layer of high quality graphene. Moreover, depending on the growth conditions, we show that variable amounts of carbon atoms can be found in the subsurface layers, while the first subsurface layer shows a tendency for carbon depletion when graphene covers the Ni surface. How this lower stability of carbon close to the surface can be used to control the number of layers formed will be discussed [4].
[1] H. Amara, J.-M. Roussel, C. Bichara, J.-P. Gaspard and F. Ducastelle Phys. Rev. B 79, 014109 (2009)
[2] H. Amara, C. Bichara and F. Ducastelle, Phys. Rev. Lett. 100, 056105 (2008)
[3] M. Diarra, A. Zappelli, H. Amara, F. Ducastelle and C. Bichara Phys. Rev. Lett. 109, 185501 (2012)
[4] M. Bahri, H. Amara, C. Bichara and F. Ducastelle, (in preparation)

Graphene electronics has motivated much of graphene science for the past decade. A primary goal was to develop high mobility semiconducting graphene with a band gap that is large enough for high performance applications. Graphene ribbons were thought to be semiconductors with these properties, however efforts to produce ribbons with useful bandgaps and high mobility has had limited success. We show here that high quality epitaxial graphene nanoribbons 40 nm in width, with annealed edges, grown on sidewall SiC are not semiconductors, but single channel room temperature ballistic conductors for lengths up to at least 16 µm. Sheet resistances below 1Omega; in essentially charge neutral ribbons have been observed, surpassing two dimensional graphene by 3 orders of magnitude and theoretical predictions for perfect graphene by more than a factor of 10. The graphene ribbons behave as electronic waveguides or quantum dots. We show that transport in these ribbons is dominated by two longitudinal components of the ground state transverse waveguide mode, one that is ballistic and temperature independent, and a second thermally activated component that appears to be ballistic at room temperature and insulating at cryogenic temperatures. At room temperature the resistance of both components abruptly increases with increasing length, one at a length of 160 nm and the other at 16 µm. These properties appear to be related to the lowest energy quantum states in the charge neutral ribbons. Since epitaxial graphene nanoribbons are readily produced by the thousands, their room temperature ballistic transport properties can be used in advanced nanoelectronics as well.

The outstanding electronic transport properties of graphene, including its superlative charge carrier mobilities, have been established on pristine samples in idealized conditions. However, for device applications, graphene needs to be interfaced with other materials in a manner that either preserves its intrinsic properties or modifies its properties in a manner that enhances functionality. For example, low power and high speed field-effect transistors require high capacitance, low leakage, and low interface trap density high-k dielectrics. Towards this end, a molecularly thin organic seeding layer, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), has been employed for the growth of atomic layer deposition (ALD) oxides on graphene, leading to significant improvements in dielectric performance limits (e.g., increasing the Weibull scale parameter and shape parameter in the breakdown electric field distribution by more than an order of magnitude). Furthermore, this study shows that vertically heterogeneous oxide stacks provide additional advantages from the perspectives of large-area homogeneity and maximum displacement field. For example, a 2 nm thick interfacial Al2O3 layer is shown to enable robust growth of high-k HfO2, thus achieving a high breakdown field in excess of 7 MV/cm at a capacitance of 785 nF/cm2. In addition to organic seeding layers, this talk will also discuss the chemical activation of the basal plane of graphene with atomic oxygen. Specifically, atomic oxygen creates a chemically homogeneous epoxidation of graphene, which then templates zinc oxide nanoparticle growth using the ALD precursor diethyl zinc. Since this chemistry is nucleated via reversible epoxide functionalization, the resulting nanoparticle growth is achieved without introducing permanent defects in the graphene lattice. Overall, chemical functionalization is shown to be an effective method for forming graphene-based heterostructures without compromising the superlative electronic properties of the underlying graphene substrate.

Reduced graphene oxide (rGO) obtained from graphene oxide (GO) via chemical reduction is a promising material for electronic applications. To produce the high quality rGO, people have already tried various reagents in the reducing process. Hydrazine hydrate and hydroiodic acid are the most popular ones among the reductants in the conventional methods. But it&’s necessary to heat up the reaction solution to a certain temperature to overcome the activation energy barrier. However, until now, researchers are still looking forward to the room temperature reduction and in situ nitrogen doping of rGO for the complete electronic circuit.
Here we introduce a new room temperature reducing method to prepare highly nitrogen doped reduced graphene oxide (rGO) and the in situ n-type rGO field effect transistors (FETs). The ethylenediamine (EDA) is meticulously chosen for the reduction system. It should be liquid state as a solvent under the room temperature and act a strong base with di-amino terminal groups to accept the solvated electrons from the alkali metal, lithium. The Li-EDA has both of reducibility and nucleophilicity which is supposed to bring the competition reactions from the reduction and the substitution. With the help from the Li reductant and the EDA solvent, we could easily access the Li-rGO and the n-type doping rGOFET under the room temperature by just dipping the pre-made GO channel device into the reductant solution for a short while. After the dipping process, both of the Li and the EDA could be easily removed from the surface just by quenching and rinsing with de-ionized (DI) water and ethanol.
This is the first known achievement of the room temperature spontaneous reduction and in situ substitution N-doping of rGO. It&’s a very convenient and important process for the future industrial mass production of rGO and rGO based electronics.

In this work, we report successful synthesis and atomic/electronic study of epitaxially-grown CVD graphene on Cu, which is assisted by self-regulated h-BN etching process in CVD. There are some reports of partial growth of bilayer patches of graphene with limited conditions. Typically one monolayer of graphene, however, can be grown on a Cu surface, because Cu has negligible solubility of carbon atoms. We, therefore, utilized large-area CVD h-BN film as a template for controlling multi-layer growth of graphene. By simply controlling growth parameters of graphene on h-BN thin film/Cu, local etching of h-BN with hydrogen atoms decomposed from carrier gas and graphene precursor makes a gap to supply precursors for second graphene layer underneath of the first graphene layer. CVD growth of second graphene layer, combined with local hydrogen etching of h-BN, repeats the growth of additional layers. Optical microscopy shows that (1) bilayer graphene can be easily grown up to hundreds of micrometer size, (2) most multi-layers share the growth seed, and (3) edges of hexagonal shapes of additional layers are parallel. Thickness-dependent G peak and 2D peak of Raman measurements for multi layer graphene transferred on SiO2 were carefully analyzed. Graphene growth with h-BN etching was confirmed by UV absorption and XPS measurements, which shows no h-BN signal from near one hour, yet keeping carbon peak. Selected area electron diffraction (SAED) study in a transmission electron microscopy (TEM) clearly shows multi layers of graphene have a Bernal stacking, so-called epitaxial growth. If this CVD growth conditions are more optimized such as quality and thickness of h-BN film and graphene growth parameters, then, we expect, we can control the larger area of epitaxial graphene or grow selective number of graphene layers. More details about experimental result, including scanning tunneling microscopy and spectroscopy (STM/STS), will be discussed in the presentation.

The Graphene Flagship is a ten year research program funded by the European Commission, EU member states and project participants. It originates from the science of graphene and related layered materials and targets a disruptive technology shift, bringing these material from academic laboratories to society as new products, employment opportunities and economic growth. Realizing this ambition is only possible by integrating the entire value chain from basic research to applied and industrial research, which requires a large consortium with corresponding resources - the total project cost is about one billion euros (1.4 billion USD).

In this presentation I will cover the origins, motivations and plans for the flagship. I will describe the research program and plans to expand and develop the consortium. The flagship covers a wide range of topics ranging from fundamental research and spintronics to flexible electronics and nanocomposites, and I will not be able to go into the technical details of any of the 16 work packages.

A variety of approaches have been used by our group to create ‘3D graphene&’ and related materials such as ‘activated microwave-expanded graphite oxide&’ that has trivalently bonded carbon in atom thick walls and exceptionally high specific surface areas. I will discuss the approaches used to make such materials, their structure, properties, and potential for application with a particular focus on electrical energy storage.
(We appreciate prior support from the NSF and DOE.) Some references: (a) Zhu, Yanwu; Murali, Shanthi; Stoller, Meryl D.; Ganesh, K. J.; Cai, Weiwei; Ferreira, Paulo J.; Pirkle, Adam; Wallance, Robert M.; Cychosz, Katie A.; Thommes, Matthias; Su, Dong; Stach, Eric A.; Ruoff, Rodney S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science (2011), 332, 1537-1541. (b) Zhang, Li Li; Zhao, Xin; Stoller, Meryl D.; Zhu, Yanwu; Ji, Hengxing; Murali, Shanthi; Wu, Yaping; Perales, Stephen; Clevenger, Brandon; Ruoff, Rodney S. Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors. Nano Letters (2012), 12, 1806-1812. (c) Zhang, Li Li; Zhao, Xin; Ji, Hengxing; Stoller, Meryl D.; Lai, Linfei; Murali, Shanthi; Mcdonnell, Stephen; Cleveger, Brandon; Wallace, Robert M.; Ruoff, Rodney S. Nitrogen doping of graphene and its effect on quantum capacitance, and a new insight on the enhanced capacitance of N-doped carbon. Energy & Environmental Science (2012), 5, 9618-9625 (d) Murali, Shanthi; Quarles, Neil; Zhang, Li Li; Potts, Jeffrey R.; Tan, Ziqi; Lu, Yalin; Ruoff, Rodney S. Volumetric capacitance of compressed activated microwave-expanded graphite oxide (a-MEGO) electrodes. Nano Energy (2013), 2, 764-768 (e) Kim, TaeYoung; Jung, Gyujin; Yoo, Seonmi; Suh, Dwang S.; Ruoff, Rodney S.; Activated Graphene-Based Carbons as Supercapacitor Electrodes with Macro- and Mesopores. ACS Nano (2013), 7 (8), Aug. 2013 , 6899-6905 (f) Ghaffari, Mehdi; Zhou, Yue; Xu, Haiping; Lin, Minren; Kim, TaeYoung; Ruoff, Rodney S.; Zhang, Qiming; High-Volumetric Performance Aligned Nano-Porous Microwave Exfoliated Graphite Oxide-based Electrochemical Capacitors. Advanced Materials (2013), DOI: 10.1002/adma.201301243

Graphene growth on metal substrates produces a wide variety of island shapes, including compact faceted, compact rounded, four-lobed, six-lobed, and dendritic. Graphene on Ir(111) is a unique system for investigating island-shape dynamics because the island shape and growth velocity varies with in-plane orientation relative to Ir. We have investigated the source of these differences by using low-energy electron microscopy (LEEM) to image island growth while simultaneously measuring the concentration of the growth species, carbon adatoms. From these data the velocity of individual graphene edges or the average edge velocity of an entire island can be plotted vs. the concentration. By comparing graphene rotational variants, we propose that the controlling factor for graphene growth kinetics and island shape on Ir(111) is the binding strength of the graphene edge to the metal substrate. Growth occurs by C cluster attachment to kinks. Graphene domains that are rotated with respect to the Ir lattice have facets terminated by the zigzag edge, which is characterized by a low kink density. In contrast, graphene domains aligned with the Ir substrate have a more strongly bound edge than rotated domains, resulting in a rounded edge morphology with a high kink density. We will discuss how the edge binding strength affects the kink density, attachment barrier, and growth kinetics.
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy Contract No. De-Ac04-94AL85000 (SNL). ODD acknowledges support from the NSF (Grant No. DMR-1105541). PCR acknowledges support from a DoD NDSEG fellowship (32 CFR 168a).

Graphene, laterally confined within narrow ribbons, exhibits a bandgap and is envisioned as a next-generation material for high-performance electronics. We report the use of metal salts infused within stretched DNA as catalysts to grow nanoscopic graphitic nanoribbons (GraNRs). The DNA is converted to a GraNR utilizing chemical vapor deposition (CVD) conditions typical for single-layer graphene growth. The resultant GraNRs are micrometers in length and take on the shape of the DNA template. The GraNRs have been characterized using various spectroscopic methods and gave working field-effect transistors. Depending on the growth conditions metallic or semiconducting GraNRs are formed. This discovery opens up a new direction in catalysis of graphene growth via metal salts. In addition to DNA templates, other polymer which has nanometer feature size can also be applied for the GraNRs-synthesis. Improvements in the growth method have potential to lead to bottom-up synthesis of single-layer graphene nanoribbons.
Reference: Zhenan Bao* et al. Nature Communications, 4, 2402, doi:10.1038/ncomms3402

Over the past several years, we have used low-energy electron microscopy to observe the growth of graphene on Cu, Ir, Pd and Ru surfaces in UHV. In this talk, we will overview what we have learned about the processes that control the shape and orientation of the grown graphene crystals. In all systems except Cu(111), we find evidence for a very large barrier for attaching single adsorbed carbon atoms to graphene. The shape of growing crystals is then determined by the factors that can influence the attachment barrier, such as surface step morphology and graphene edge termination. The range of observed graphene orientations is usually determined by the details of the initial nucleation process, but graphene on Ir(111) provides an interesting exception. We find the growth morphology of bilayers is dominated by “growth from below” and so understanding how carbon atoms intercalate between the metal and graphene is important. Finally we will discuss how the UHV growth processes that we have observed can differ from those occurring during CVD. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U. S. Department of Energy under Contract No. DE-AC04-94AL85000.

Over the past few years, graphene has been highlighted as a viable candidate for several applications in electronics and energy storage. Due to its good stability, high transparency and excellent conductivity, graphene has been implemented as electrodes in opto-electronics, batteries and supercapacitors. A novel, high throughput, high resolution method based on fluorescence quenching of fluorescent dyes by graphene, has been developed recently for metrology of graphene-based sheets. In this method, graphene sheets are covered with a thin dye-doped polymer layer. The quenching of the fluorescence intensity provides the contrast required for imaging. The quenching mechanism is reported to be Forster Resonance Energy Transfer (FRET). Herein, we investigate the quenching mechanism of a thiophene-alt-benzothiadiazole-based fluorescent conjugated polymer (PTBTSB) by graphene. Raman spectroscopy, diffuse reflectance spectroscopy, steady-state and time-resolved emission spectroscopy and fluorescence quenching microscopy have been used to investigate the photophysical properties of PTBTSB coated graphene-based sheets. In this case, CVD-grown graphene sheets were immobilized on glass substrates and have been coated with PTBTSB. The average distance between polymers chains and graphene is considerably short and possible charge transfer complexes might form between PTBTSB (donor) and graphene (acceptor). In contrast, the previously reported dye-doped polymer layers are encapsulating the dye molecules and preventing them from having direct electronic interactions with graphene sheets. Therefore, the dominant quenching mechanism is expected to be charge transfer rather than FRET. Our results suggest that PTBTSB is quenched significantly by graphene and provides enhanced contrast for fluorescence imaging of graphene-based sheets.

Graphene is a very promising material for nanotechnology, but structural defects degrade its performance, so the determination of defects in graphene is vital to estimations of graphene quality. Scanning tunneling microscopy and transmission electron microscopy can provide such information about graphene defects on the atomic scale. However, it is difficult to extend this information to large areas. Recently, large scale examinations of graphene grain boundaries with optical microscopy (OM) have been performed. Although these microscopy methods can visualize graphene grain boundaries, they cannot be used for the exact characterization of point defects because such approaches use a humid environment, in which it is easy for water to intercalate graphene and generate vacancies that are almost the same as the pristine vacancy defects of graphene.
We report a simple and accurate method for the characterization of graphene defects by both indicating the density and degree of dislocation of the defects. The key advantage of this method is that it does not induce further functionalization of the graphene layer or water-related damage, so exact information (location, density, etc.) about graphene defects can be obtained. It is realized by mild dry annealing (MDA) graphene/copper film in air and then the oxidation of the Cu film underlying graphene defects is observed with OM. Scanning electron microscopy (SEM), confocal Raman and atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) analysis were performed to demonstrate that our nondestructive approach to characterizing graphene defects with optimized thermal annealing provides rapid and comprehensive determinations of graphene quality.
While no defects are evident in the OM image of CVD-grown pristine graphene on Cu, after optimized MDA process at 200C for 2 hours, defects are visible as lines and points in the OM images with different sizes of the patterns, which indicates that there are different kinds of defects. After dipping the samples in deionized water followed by MDA, the density of point defects increases to a very high level, which indicates that significant defects are generated in humid environments. Similar phenomena are also evident in the SEM images and AFM images combined with corresponding Raman mapping image. After optimized MDA process on pristine graphene, G and 2D peak down shifts are observed in the Raman spectrum, which are caused by the desorption of functional groups that have doping effects on graphene. No significant D-band development and negligible change in sp3 carbon (C-C) bond (functional group) of XPS spectra were observed compared to the pristine sample, which implies that MDA is a nondestructive process.
This approach can be used in the comprehensive characterization of graphene defects and to estimate graphene quality, and furthermore, to tailor the local properties of graphene for further applications.

The huge potential of graphene to disrupt many different application areas of technology has been shown in research laboratories extensively over the last several years and the uptake of this 2-D material is now beginning in industry worldwide. However, the requirement to overcome the practical problems related to 2-D materials, such as quality, reproducibility and contamination, increasingly needs to be met. At the same time, companies in the emerging graphene industry require the ability to accurately, quantitatively and reliably characterise these types of materials to instil market confidence. Raman spectroscopy has been shown to rapidly characterise many different attributes of graphene in a non-destructive manner, attributes such as the number and orientation of layers, strain effects and doping [1]. Raman spectroscopy is arguably also the metrological tool of choice for quantifying the disorder, which is frequently referred to as the ‘quality&’ of graphene.
By creating defects in pristine graphene samples using Argon cluster, Bismuth and Manganese ion guns, and varying both the size and energy of the Ar clusters, we can identify the Raman spectra trends and parameters required to find the size, density and nature of graphene defects. By using a metrological approach with confocal Raman imaging and utilising the nanoscale resolution of tip-enhanced Raman spectroscopy (TERS) mapping, we can achieve the quantification of disorder in graphene layers.
[1] Ferrari and Basko, Nat. Nanotech. 8 (2013) 235

Over the last decade, graphene has gained tremendous attention in the scientific community for its extraordinary electrical and mechanical properties; launching a graphene frenzy. The material has attracted investors, researchers, and corporations alike in the hopes to discover the next novel products and materials. The first step of any organization interested in the development of these novel products and materials is the ability to procure or produce material samples for lab exploration. Samples of graphene can be obtained by a variety of methods including mechanical exfoliation, chemical exfoliation, reduced graphene oxide, Chemical Vapor Deposition (CVD), or epitaxial growth to name a few [1].
In this study, qualitative techniques for the detection of graphene on a Si/SiO2 substrate, without the use of sophisticated equipment, are presented. Once calibrated, this technique can be used to detect Single Layer Graphene (SLG) and Few Layer Graphene (FLG) with the use of an inexpensive optical microscope (OM), OM camera system, and image processing software. This technique could be transferred to graphene deposited on other substrates with minor updates to mathematical theory.
The work first reviewed the optical theory, describing the interaction of light with graphene on top of a Si/SiO2 substrate as described by the Fresnel law [2]. This theory was then used to develop image processing software that transforms the optical image into a clean three dimensional contrast plot. Using this information, the software is able to identify mono-layer and multi-layer graphene, without the use of sophisticated, expensive equipment. Laboratory experiments were also conducted to confirm the predicted theory. The experimental results show that the proposed image processing technique can consistently detect and annunciate single and multi-layer graphene. Suggestions for further researches are also presented that could aid in improving result consistency.
Key words: Graphene, Detection, Image Processing
[1] J. H. Warner, F. Schaffel, A. Bachmatiuk and M. H. Rummeli, Graphene - Fundamentals and Emergent Applications, Kidlington: Elsevier, 2013.
[2] P. Blake, E. W. Hill, A. H. Castro Neto, K. S. Novoselov, D. Jiang, R. Yang, T. J. Booth and A. K. Geim, "Making Graphene Visible," Applied Physics Letters, vol. 91, 2007.

We report electronic structure evolution of graphene oxide (GO) upon thermal reduction with an aim to gain insight into the electronic structure of graphene induced by chemical functionalization. With a focus on the case of oxygen, a degree of functionalization was carefully controlled by thermal annealing and the electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy (UPS). We observed an unexpected increase of density of states (DOS) around the Fermi level upon thermal annealing at 600 oC, in addition to the expected increase of π bond states at ~160 oC. The results indicated that while there is an apparent band gap for GO prior to thermal reduction, the gap closes after the annealing. Simulation results revealed possible ordering of oxygen functional groups for as synthesized GO, which becomes randomly distributed and closes the band gap upon annealing. Our results provide experimental evidence demonstrating a potential importance of functional groups ordering in determination of the electronic structure of graphene.

An ability to induce and control an electrical and/or optical bandgap is an important consideration for graphene material science and devices. Here we report the emergence of an electrical bandgap of up to 150 meV in bilayer graphene through the interaction with physisorbed molecules, such as F2-HCNQ, DMC, DDQ and TTF, on one surface of bilayer graphene. [1] The electrical bandgap is found to scale linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the bandgap varies as 47 meV/10¹³ cm^-2, and p-type dopants where the bandgap varies as 38 meV/10¹³ cm^-2. Optical gaps corresponding to the important 3-5 mu;m region are also found. The emergency of bandgap is explained in terms of the asymmetric charge distributions on the upper graphene layer, which is in contact with the molecules. The high binding energy found upon adsorption of some of these molecules results in an attractive way to a permanent bandgap. Comparison is made with electrical bandgaps induced using dual gate geometries and prospects for graphene based devices are explored.
[1] Alexander J. Samuels and J. David Carey, ACS Nano 7, 2790 (2013).

Indium tin oxide (ITO) which has been widely used as transparent conductive electrodes suffers from poor durability and low flexibility. Nowadays, flexible electronics have attracted a lot of attention due to its utility on various curved platforms. Thus, it is necessary to investigate transparent conductive electrodes with high failure strain. Graphene is considered as one of the replacements of ITO because it has high transparency, exceptional electrical conductivity and great mechanical strength under strained conditions. In this work, 3-layer graphene was employed as a back-side electrode of GaN-based light-emitting diodes (LEDs) on flexible polyethylene terephthalate (PET) and paper substrates.
Firstly, CVD-grown 3-layer graphene was transferred on both PET and paper substrates. Carbon tape was used to attach GaN-based LEDs to the flexible substrates covered with graphene layers. Commercial GaN-based blue LEDs were transferred on carbon tape after laser lift-off (LLO) process to remove the rigid sapphire substrate, where ArF excimer laser (lambda; = 193 nm) was used in LLO process. Final structure of our devices was GaN-based LLO-LED / carbon tape / 3-layer graphene / flexible (PET or paper) substrate. Graphene with high failure strain showed a great potential to demonstrate flexible devices on various substrates (PET and paper). Bright electroluminescence was observed when the graphene-based electrodes on the flexible substrates were strained. The details of our experiments and results will be discussed.

Superb electromechanical properties of graphene, where large mechanical deformation is achievable, provide substantial promise as a candidate material for advanced nano-electromechanical devices. Here, we report a facile, scalable, and selective method of crumpling graphene and graphite into 3-dimensional (3D) textured graphene-graphite hierarchical nanostructures by using soft-matter transformation of shape-memory polymers. We demonstrate that the thermally-induced transformations of graphene/graphite on polymeric substrates create various 3D textured graphene/graphite structures. Quantitative analysis shows that both the periodicity and feature size of textured graphene/graphite are in the order of micrometers for mechanical strains of up to 80%. The resultant 3D morphology (i.e. induced wavelength and height) can be deterministically modulated via simple processing parameters, such as temperature and duration. We further characterize the electrical and mechanical properties of 3D graphene-graphite, and demonstrate the robust electromechanical properties of 3D textured graphene/graphite. Specifically, selective and tunable formation of localized areas of large strain concentrations enable studies into characteristic Raman bands&’ shifts, work function and band-gap modulation, and wavelength-sensitive photo-response. We believe our single-step approach to forming textured 3D nanostructures from graphene and other 2-dimensional nanomaterials via deterministic and selective soft-matter transformation offers a unique avenue for creating advanced nano-electromechanical devices in the future.

Graphene is an appealing material for electron-spin quantum computing (QC) and spintronics, due to the expected weak spin-orbit interaction, and the scarcity of nuclear spin in natural carbon. Due to QC and spintronics, the microwave control and electrical detection of spin have become topics of interest, now in graphene nanostructures, where the small number of spins limits the utility of traditional spin resonance. Here, we report results of an experimental study examining the microwave response of epitaxial graphene.[1] The results suggest the possibility of resistive detection of spin resonance, and they provide a measurement of the g-factor and the spin relaxation time in this novel system.
[1] R. G. Mani, J. Hankinson, C. Berger, and W. de Heer, Nature Comm. 3, 996 (2012).

Conventional means of stacking two-dimensional (2D) crystals inevitably leads to imperfections. To examine the ramifications of these imperfections, rotational disorder and strain are quantified in twisted bilayer graphene (TBG) using a combination of Raman spectroscopic and low-energy electron diffraction imaging. The twist angle between TBG layers varies on the order of 2° within large (50-100 mu;m) single-crystalline grains resulting in changes of the emergent Raman response by over an order of magnitude. Rotational disorder does not evolve continuously across the large grains but rather comes about by variations in the local twist angles between differing contiguous sub-grains , ~1 mu;m in size, that themselves exhibit virtually no twist angle variation (ΔΘ~0.1°). Owing to weak out of plane van der Waals bonding between azimuthally rotated graphene layers, these sub-grains evolve in conjunction with the 0.3% strain variation observed both within and between the atomic layers. Importantly, the emergent Raman response is altered, but not removed, by these extrinsic perturbations. Interlayer interactions are therefore resilient to strain and rotational disorder, a fact that gives promise to the prospect of designer 2D-solid heterostructures created via transfer processes.

Graphene has been under intense exploration as an electronic material in semiconductor industry owing to its excellent electronic, mechanical and thermal properties. However, a number of fundamental limitations to graphene fabrication and integration with CMOS circuits have been preventing application of these materials to electronic devices. Among the major challenges, the most significant one is a large electrical contact resistance between graphene and metal electrodes, which significantly exceeds the graphene channel resistance. A number of theoretical and experimental studies have been performed focusing on the fundamental understanding of the contact resistance and interfacial properties. It was concluded that the intrinsic contact resistance is determined by electronic coupling at the graphene and metal interface, which directly correlates to the nature of interfacial binding properties, i.e., physical/van-der-Waals interactions vs. chemical/ covalent bonding. In order to achieve significant reduction of the contact resistance, one must develop the fabrication protocol for establishing a strong chemical binding between graphene and metal electrodes, which enhances electronic coupling and thus increases carrier transmission efficiency.
Focused Electron Beam Induced Deposition (FEBID) is an emerging chemical vapor deposition (CVD) method, which enables a resist-free “direct-write” additive nanomanufacturing using a variety of materials with a high degree of spatial and time-domain control. FEBID offers a unique potential to engineer graphene-metal interfaces with nanoscale resolution. In this work, we utilized the FEBID technique to improve interfacial properties at the graphene-metal junction by forming graphitic nanojoints using hydrocarbon precursors. Fabrication protocols for i) a buried interfacial metal-graphene coupling interlayer and ii) an electrically conducting overlayer of FEBID graphitic carbon at the mechanically exfoliated graphene and metal junctions will be described along with results of their comprehensive morphological, compositional and electrical characterization. The FEBID graphitic nanojoints are shown to improve thermo-mechanical and electrical properties of graphene-metal interface, resulting in stable Ohmic contact even at high electric bias operation and ~30% reduction in contact resistivity compared to a standard metal contact.
Acknowledgement: The authors would like to thank Dr. Roman Caudillo (Intel) and Dr. An Chen (Global Foundries) for helpful discussions and Ivan Tibavinsky for assistance with fabrication of test substrates. This work was supported Semiconductor Research Corporation (GRC Contract 2011-OJ-2221) and AFOSR BIONIC Center (Award No. FA9550-09-1-0162).

Atomic-resolution low energy scanning transmission electron microscopy and electron energy loss spectroscopy are combined to study the properties of epitaxial graphene grown epitaxially through the controlled graphitisation of a hexagonal SiC(0001) substrate by high temperature annealing. A scanning transmission electron microscopy analysis, carried out at 60KeV of beam energy, below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (11-2n) facets of SiC surface steps, In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp2-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions.A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination.

The unique structure of monolayered graphene composed of a one-atom thick two dimensional (2D) crystal of sp2 carbon atoms arranged in a honeycomb lattice has been attributed to its extraordinary electronic properties such as high intrinsic mobility, and excellent thermal and electrical conductivity. In addition to the superlative electronic properties, recent studies have demonstrated the graphene&’s photoluminescent characteristics, which can expand its application area in optical-related fields. The photoluminescence (PL) property of graphene is derived by controlling the zero-bandgap of graphene. Since the bandgap can be tuned by the size, shape, and fraction of the sp2 domains in the sp3 matrix, a variety of graphene derivatives were explored as PL graphene moieties. Especially, several reports have focused on modulating the size of graphene itself to less than 100 nm, called graphene quantum dots (GQDs), to endow quantum confinement effect that is also a contributing factor for PL emission. Such GQDs display moderate PL signal, non-toxicity, and cell permeability, so their biological applications in the fields of cellular imaging, biosensors, and drug delivery are promising. Despite some successes in GQD synthesis, these methodologies have limitations to overcome. The produced GQDs mainly exist as bi- or multi-layers, and the heterogeneity in size and shape is unavoidable. Moreover, in spite of the term ‘graphene quantum dots&’, the synthesized GQDs, strictly speaking, were graphene oxide quantum dots (GOQDs) or partially reduced GOQDs (rGOQDs).
In this study, we employed graphite nanoparticles (GNPs) whose diameters were 4 nm as a starting material to synthesize both monolayer GQDs and GOQDs that have circular shape and diameter of less than 4 nm. The homogeneity in terms of size and shape is much improved and the blue PL emitting GQD and the green PL emitting GOQD were obtained. By characterizing the optical properties of GQDs and GOQDs, we have revealed that the green luminescence of GOQDs originates from defect states with oxygenous functional groups, whereas the blue luminescence of GQDs is dominated by intrinsic states in the high-crystalline structure. Furthermore, the blue emission from GQDs shows a fast recombination lifetime compared to that of the green emission from GOQDs. In contrast to previously reported GQD fabrication methods, our approach produces stable luminescent pristine GQDs and GOQDs with high yield and reproducibility, which are essential requirements for both biological and electronic applications using nano-sized graphene materials.

Graphene is a one-atom thick material where carbon atoms are arranged into a honeycomb lattice structure. All carbon atoms belong to its surface, and thus electronic properties of graphene are sensitive to surface conditions such as adsorption of foreign atoms/molecules. Especially, adsorption of air molecules such as oxygen is of great importance due to their ubiquitousness. Oxygen adsorption onto bilayer graphene has been found to be controllable by applying gate voltages with a field-effect-transistor structure [1]. In this presentation, we connect the gate-voltage controllability of oxygen adsorption and post-adsorption UV treatment to perform gate-voltage controlled photo-oxidation of graphene. The oxidation reaction is found to occur selectively at edges [2], which might offer a novel methodology to fabricate graphene nanoribbons.
[1] Y. Sato, K. Takai, and T. Enoki, Nano Lett. 11, 3468 (2011).
[2] N. Mitoma and R. Nouchi, Appl. Phys. Lett., in press.

Graphene quantum dots (GQDs) have received an increasing attention in nanoscience and nanotechnology due to properties that include large surface area, low cytotoxicity, excellent solubility, and a tunable band gap. In particular GQDs have utility for various potential applications including optoelectronic devices, biological imaging and labelling, and electroluminescence as a light emitting diodes.
At present, a variety of methods including high-resolution electron-beam, chemical, electrochemical, and hydrothermal methods have been applied to the preparation of GQDs from graphene oxide (GO). However, none of these processes could prepare GQDs directly from graphite with high yield and mass production. Among synthetic methods, microwave irradiation techniques have been used extensively in the synthesis of organic and inorganic materials for industrial applications. Recently, a facile one-pot microwave irradiation reduction method to provide greenish yellow luminescent GQDs from GO under acid conditions has been reported, but not directly from graphite. To make a worse, the isolated yields for these processes were not reported. On the other hand, most other methods to prepare GQDs typically take several days, give low yields, and require expensive raw materials such as GO and carbon fiber (CF).
Even though many strategies have been employed to overcome low yields and mass production with complicate process, until now, none of methods were not successful. Thus, a high yielding with mass production and simple process for the preparation of GQDs is getting an important issue. Herein, we report a one-pot synthesis of GQDs directly from graphite using high-powered microwave irradiation in high yield and mass production. It is strongly suggested that the direct preparation of GQDs from natural resource, graphite powder, by one pot synthesis using high powered microwave irradiation could be one of the most efficient and straightforward for the high yields and mass production methods. It is believed that with high powered microwave irradiation, graphite can be multiply broken by repeated redox reactions during one pot synthesis, leading to give a high yield and mass production.

Graphene is one of the leading candidate materials for high-performance molecular sensors operating at room temperature. Every atom in graphene can be considered as a surface atom, thus electron transport through these materials can be very sensitive to adsorbed molecules. Moreover, the high transparency and considerable stretchability of graphene-based sensors would find use in the flexible electronics where electronic noses, mimicking the mammalian olfactory system, are integrated to detect odorant identity and concentration for further functional convergence. However, graphene-based sensors have a major drawback that the time for a full recovery to the initial state after a sensing event is extremely long, thus making the sensors incapable of producing repeatable sensing signals even upon exposure to the same analyte concentration.
In this study, we present that oxygen-functionalized graphene shows fully reversible, stable and ultrasensitive responses to various molecules including NH3, SO2, CH4, CO and VOCs such as acetone, ethanol, benzene, and toluene, which are distinctly differentiated from those of any other carbon-based nanomaterials such as graphene, reduced graphene oxide (rGO), and carbon nanotubes. From density functional theory calculations, it is revealed that the surface hydroxyl groups make functionalized graphene semiconducting and act as the predominant active sites for the adsorption of gaseous molecules. We also find that the rotation and relaxation of hydroxyl groups during the adsorption and desorption of molecules are responsible for the fast and reversible sensing behavior of oxygen-functionalized graphene.
Another breakthrough of our study is the extremely low power consumption. The power consumption of our sensors under continuous voltage injection is about 31nW. This is three orders of magnitude lower than the power consumption (21.6 µW) of the state-of-the-art sensor based on metal oxide nanostructures which we reported recently. Furthermore, the power consumption goes down to 0.55 nW under pulsed mode operation, suggesting that our oxygen-functionalized graphene sensors can operate for more than 10 years using a typical NiMH rechargeable coin cell.

One-dimensional defects in graphene have strong influence on its physical properties, such as electrical charge transport and mechanical strength. With enhanced chemical reactivity, such defects may allow us to selectively functionalize the material and systematically tune the properties of graphene. By atomic layer deposition (ALD), we demonstrate the selective deposition of Pt nanoparticles at chemical vapor deposition (CVD) graphene&’s line defects, notably grain boundaries. Various characterization methods including transmission electron microscopy (TEM) are utilized to image the structural evolution of Pt deposition as a function of ALD-cycle numbers. The optical transmittance, electrical conductivity, and application towards hydrogen gas sensing of graphene-Pt hybrid structures are also presented.

Graphene aerogels are typically micro- and mesoporous (pores <50 nm), ultra-lighweight, conductive materials that can achieve surface areas in excess of 1000 m2/g. As such, they are used in a wide range of applications ranging including catalysts and catalyst supports, energy storage and conversion, and sorbents for water purification. Aerogels are made via the sol-gel process, in which a reaction solution is gelled and the solvent is extracted in such a way as to leave the porous solid matrix intact. Though their pore sizes can typically be tuned by varying the synthetic parameters of the sol-gel process, limitations do exist. For applications that require faster mass transport through the aerogel, alternative methods for incorporating larger pore networks into the aerogel structure are desired. Herein, we report the development of direct ink writing (DIW) sol-gel materials, which upon drying become DIW aerogels. Characterization of the DIW aerogels will be discussed in comparison to their bulk counterparts.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the DOE Office of Energy Efficiency and Renewable Energy.

As graphene starts to progress from the research laboratory towards industrial applications, the requirement to overcome the practical problems related to 2-D materials, such as quality, reproducibility and contamination, increasingly needs to be met. An emerging global graphene industry requires large-scale production of graphene material that still achieves the exceptional properties demonstrated in smaller scale experiments.
Chemical vapour deposition (CVD) growth methods can produce large-scale graphene sheets utilising roll-to-roll processing, however, the transfer steps required to remove CVD graphene from sacrificial metal substrates and subsequent electronic device manufacturing steps lead to inhomogeneous polymer contamination. This polymer residue from photoresists and transfer polymers cause undesired reductions in conductivity and irreproducibility in the production of graphene devices. Although heating graphene surfaces can improve the consistency of these devices, we show with high-sensitivity secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) that this does not fully remove the contamination present.
However, we reveal that argon cluster ion beam etching, which is commonly used in the semiconductor industry, can be used to remove contamination from graphene layers whilst minimising any damage to the graphene lattice itself. Confocal Raman spectroscopy investigations reveal an impact energy of less than 1 eV per atom in the cluster is required. Optimised conditions for sputter profiling the organic overlayers whilst minimising graphene lattice damage will be presented and the effect on conductivity, as measured using a large-scale and contactless microwave dielectric resonator perturbation technique, is discussed.

In order to make electrocatalytic hydrogen production economically viable, it is necessary to develop efficient, inexpensive and earth-abundant catalysts. In this work, MoSx was grown on crumpled graphene particles and supported on carbon cloth substrates for the hydrogen evolution reaction. By modifying carbon cloth with crumpled graphene, higher loading levels of MoSx could be achieved, thus significantly enhancing the electrocatalytic activity.

The advent of graphene has opened a new era in materials science. More recently, other related graphene-like structures, such as, graphene nanoribbons (GNRs) have attracted a lot o of attention as they can be the basis for new potential technological applications. GNRs are graphene stripes with a high aspect ratio (ultra thin width). In particular, graphene oxide nanorribons (GONRs) have been shown to be a very promising material for biological applications [1]. The structure and composition of GONRs facilitate the chemical incorporation of targeting agents, as well as, other therapeutically relevant agents [1]. However, GONRs have some limitations in some biomedical applications. For instance, they cannot be properly used as a contrast agent in magnetic resonant imaging (MRI), a non-ionizing technique of increasingly importance for medical diagnostics. One possible solution to this problem is through GONR fluorination due to a scanty distribution of fluorine atoms in human body and that fluorine atoms present good contrast in MRI. Recently, we developed a novel method to synthesize large quantities of fluorinated graphene oxide (FGO), and extensive characterizations of the material have been reported [2,3]. Some parameters (such as temperature and nanoribbon sizes) seem to be critical in determining the FGO quality samples. In this work, we investigate through fully atomistic reactive molecular dynamics simulations (MD), the structural and dynamics aspects of the fluorination of graphene oxide nanoribbon (GONR) as a function of some of these parameters. The extensive MD study was carried out using reactive force fields (ReaxFF), as implemented in the Large-scale atomic/Molecular Massively Parallel Simulator (LAMMPS) code. This approach has been successfully applied to the study of hydrogenated [4] and fluorinated graphenes [5]. The process of simulating the fluorination of GONR was carried out considering an isolated graphene sheet immersed into an atmosphere of fluorine, oxygen and hydrogen gases. Our results showed that depending on the relative concentration of the gases and the nanoribbon size, there is a dynamical competition of F, H and O atoms in forming and/or breaking the bonds on GNRs. Another important result is that, in contrast to what was reported to the case of graphene hydrogenation [4], we did not observe the formation of correlated domains (islands of fluorinated carbons) [5]. These results are consistent with the available experimental data.
[1] R. Romero-Aburto et al., Adv. Mater. v25, 5632 (2013).
[2] A. Mathkar et al., Particle & Particle Systems Characterization v30, 266 (2013).
[3] P. Panit et al., J. Phys. Chem. C v116, 25955 (2012).
[4] M. Z. S. Flores, P. A. S. Autreto, S. B. Legoas, and D. S. Galvao, Nanotechnology v20 (2009) 465704.
[5] R. Paupitz, P. A. S. Autreto, S. B. Legoas, S. G. Srinivasan, A. C. T. van Duin, and D. S. Galvao, Nanotechnology v24 (2013) 035706.

The two-dimensional (2D) electron cloud, chemical modifiability, and size- and shape- dependent quantum-confinement makes graphene nanostructures a widely tunable material for electronics. This presentation will discuss the synthesis of graphene quantum dots (GQD), tapered graphene nanoribbons and fabrication of their arrays and electronic devices (sensors and FETs). The GQD arrays are fabricated on both polymeric and biological (bacterial cell) substrates, and are employed to study the modulation of electron tunneling between GQDs. The properties that will be discussed include electron tunneling barrier, height, Coulomb blockade threshold, minimum tunneling distance and gradient in band-gap continuum in tapered ribbons. Other topics which will be presented include synthesis of carbon nanotube quantum dots, graphene-BN composites, functionalization of MoS₂ to modulate electronic states and ultrasensitive graphenic sensors produced via eta;6 functionalization. The work is directed towards understanding the electrical and optical properties of chemically and structurally modified 2D nanomaterials and applying them for sensing, electrical and optical applications.

Recently, the unique electronic and optical properties of graphene have prompted its use in applications ranging from transparent conductive electrodes, to optical modulators. In particular, by changing the optical frequency, changes in the sheet conductivity of graphene can be observed, with an inflection point in conductivity within the terahertz regime. Here, we measure sheet conductivity values of graphene using large-area chemical vapor deposited (CVD) graphene films (~1cm x 1cm) transferred onto high resistivity silicon. We demonstrate, for the first time, the tunable modulation of the conductivity and transparency of monolayer graphene using an external DC gate voltage, in parallel with broadband (GHz - THz) frequency domain measurements.

Graphene functionalization has been an important topic to enable graphene&’s superior electronic properties in nanoionics, nanoelectronics, sensors and other fields [1-3]. In most graphene based devices, it is desirable to modulate the carrier concentration by shifting its Fermi level, which can be realized by noncovalent adsorbates. Adsorption of molecules on graphene is a promising route to achieve an effective doping [4]. However, a high doping level in some cases is not easily obtained due to the small amount of charge transfer between molecules and graphene. Cations such at Li, Na or K may provide effective doping, but they are not stable on graphene surface. The 12 crown-4 ether (CE) molecule has the site-selective binding character with Li [5], and it is expected to be a promising candidate of doping adsorbate.
In this work, we investigated the adsorption energy, geometrical structure, charge transfer, electronic structure and work function of CE and CE-Li adsorbed on graphene by using density functional theory (DFT) calculations. Our results show that the difference of the stabilities among different configurations is small owing to the large distance and weak interaction between the adsorbed molecule or complex and graphene. For CE adsorbed on graphene, the direction of charge transfer depends on the adsorption side of crown ether, which stems from the asymmetric character of crown ether. In contrast, for CE-Li adsorbed on graphene, the amount of charge transfer is much larger, and graphene shows the n-type doping character. Furthermore, the doping effect to graphene in all the adsorbed systems does not influence the linear dispersions of pz bands, which just shifts the Fermi level. Therefore, the carrier concentration and the work function of graphene can be modulated accordingly. These DFT results provide an insight for experimental study of graphene functionalization with crown ether molecules for use in steep-subthreshold-swing transistors and two dimensional crystal memory devices.
This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.
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[3] D. W. Boukhvalov, and M. I. Katsnelson, Nano Lett.,8, 4373 (2008).
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The development of highly specific and easy-to-use sensors is essential for biochemical sensing applications such as blood tests or the detection of harmful substances. A very promising way to fabricate such sensors is the use of field-effect transistors which can be easily integrated in advanced electronic systems. To date, most of these transistors have been fabricated from classical semiconductors such as silicon. Although the technology for such materials is very advanced, they are associated with some major drawbacks, e.g. low chemical stability and high electronic noise. In this respect, the outstanding electronic and electrochemical performance of graphene holds great promise for biosensing applications. Furthermore, graphene can be modified by a variety of methods which were developed successfully for other carbon materials resulting in sophisticated analyte-specific biosensors.
In this contribution, we will present various methods to functionalize graphene surfaces and thus fabricate selective functional transistors. We will discuss the functionalization of graphene first based on polymer brushes grown by self-initiated photografting and photopolymerization of styrene and other methacrylates. With this method, it is possible to obtain transistors with a significantly modified pH response. Furthermore, we will show the grafting of the enzyme acetylcholinesterase by establishing peptide-bonds with the polymer brushes, which enables the detection of small amounts of acetylcholine, an important neurotransmitter in many organisms including humans. A different functionalization technique employs surface grafted monolayers of linker molecules. These molecules are consequently used to attach single-stranded DNA to the transistors. This approach allows the detection of the complementary DNA in the solution.
This work shows the suitability of graphene for the fabrication of biosensors due an extraordinary combination of electronic properties as well as chemical versatility and stability.

The chemical functionalization of graphene is of wide interest; it gives promise of tailoring its physical properties, chemical reactivity and interfacial characteristics when combined into three-dimensional materials. In addition, monolayer sheets can readily be obtained in large quantities through the oxidation and exfoliation of graphite to graphene oxide, making it an attractive route to the production of chemically modified graphene for low cost applications[1]. Significant questions remain over the structure and properties of these chemically modified graphenes. In particular, it is critical to consider changes to the electronic structure - the linear dispersion in the band structure of graphene around the ‘Dirac&’ point is one of its defining features. Such studies require a more controlled system of modification than wet chemical oxidation and functionalization of graphite.
We will introduce the controlled oxygen and nitrogen functionalization of graphene in ultra-high vacuum, allowing systematic study of the effect of functionalization. Starting with the model system of graphene grown by chemical vapour deposition on copper [2,3], we will show how angle-resolved photoemission spectroscopy (ARPES) can reveal the changes in electronic structure induced by functionalization. Complementary insight into the chemical changes is given by X-ray photoemission spectroscopy, whilst Raman spectroscopy and low-voltage aberration-corrected transmission electron microscopy reveal the structural changes.
Combined, the results show that low levels (few %) of functionalization can either disrupt the graphene lattice with irreversible disorder (here for nitrogen), or result in reversible functionalization with no damage to the carbon lattice (here for oxygen), indicating that detailed understanding of the functionalization process is required. Surprisingly, in both cases, even this low level of functionalization is sufficient to destroy graphene&’s band structure such that it is no longer a semi-metal but rather an electronically disordered material. These results have important implications for the properties and application of chemically modified graphene, and for the control of its surface chemistry.
[1] Park, S. & Ruoff, R. S. Chemical methods for the production of graphenes. Nature Nanotechnology 4, 217-224 (2009).
[2] Wilson, N. R. et al. Weak mismatch epitaxy and structural feedback in graphene growth on copper foil. Nano Research 6, 99-112 (2013).
[3] Marsden, A. J. et al. Is graphene on copper doped? Physica status solidi - Rapid Research Letters 7, 643-646 (2013).

Since graphene is an ideal 2-D conductor, deposition of a dielectric layer is required to fabricate graphene devices. Atomic layer deposition (ALD) is a thin film deposition method that employs surface self-saturated reaction of precursors. Because ALD produces dense films without significant damages, it is suitable to deposit dielectric layers on graphene. On ideal graphene surface, it is hard to deposit a film by ALD since there is no chemically active bonding to initiate ALD nucleation. However, several reports on graphene device fabrications using chemical vapor deposited (CVD) graphene showed continuous ALD film growth. In other reports, ALD layer does not nucleate on graphene surface or island growth occurs. This inconsistency may be caused from non-ideal surface properties of graphene which depend on synthesis process, and reaction chemistry of precursors on graphene. Although ALD has been utilized to deposit high-k films to fabricate graphene devices several times, there was no systematic study on surface reactions between ALD precursors and graphene synthesized by various ways.
In this research, we investigated growth characteristics of HfO₂ on exfoliated and CVD graphene by using two Hf precursors, tetrakis(dimethylamino)hafnium (TDMAH) and hafnium tetrachloride (HfCl₄). On exfoliated graphene, no growth was observed on basal planes while growth of HfO₂ was detected only at the edge of graphene when HfCl₄ was used as a precursor. On the contrary, HfO₂ nucleated cross the surface of CVD graphene at the initial growth, then 75 % of surface coverage was obtained at 90 cycles. HfO₂ ALD by using TDMAH on both of exfoliated and CVD graphene showed much more unfavorable nucleation and growth than that by using HfCl₄. Activation energy and surface reaction path were investigated by quantum chemical calculations. The experimental results were discussed with the theoretical calculation results. It should be noted that this comparative research with experimental and theoretical results will be fundamentally and practically useful for fabrication of graphene based electronic devices.

In the last years graphene membranes have been object of intense theoretical and experimental investigations due to its rich electronic [1,2] and mechanical properties [1-3]. They exhibit and combine remarkable properties, such as, high mechanical resistance with low weight [3]. However, in its pristine form graphene is a gapless material, which prevents its applications in some transistor applications [3]. One possible solution to this problem it is to create an electronic gap through chemical functionalizations and/or graphene chemical doping. Effective doping can be achieved for example by replacing C atoms with B or N ones [3].
Recently, Ci et al. [4] synthesised a hybrid material composed of BN domains embedded into graphene membranes. This composition resulted in a new material with properties complementary to those of graphene and hexagonal boron nitride (h-BN), enabling a rich variety of electronic and mechanical properties [4].
It is interesting to determine how the mechanical and electronic properties vary as a function of the number and size of the BN domains. In this work we have used ab initio DFT (Density Functional Theory) and DFTB (Density Functional Tight-Binding) methods to investigate the electronic and mechanical properties of graphene containing different BN domains. Due to the high computational cost ab initio calculations were carried out for small model systems in order to establish a benchmark for the DFTB calculations carried out for larger systems (up to thousands of atoms). Our results showed that the hybrid material stiffness increases with increasing the number of BN domains and when under stress most of mechanical failures (fractures) occur on the interface of the graphene-BN domains. We have also investigated how the electronic structure changes as a function of these BN domains.
[1] A. K. Geim, Science v324, 1530 (2009).
[2] A. K. Geim and K. S. Novoselov, Nature Mater. v6, 183 (2007).
[3] X. Wang et al., Science v324, 768 (2009)
[4] L. Ci et al., Nature Nanotech. v9, 430 (2010).

Graphene has emerged as a promising material for a variety of applications including transistors, photovoltaics, batteries and sensors. In particular graphene holds promise to replace the expensive ITO in photovoltaics and serve as scaffold for engineering electrochemically stable electrodes for batteries. To this end the work function of graphene needs to be tuned, while maintaining its remarkable conductivity and transparency. We recently developed an electrochemical tool [1] which allows the controlled covalent modification of the graphene using a variety of compounds (i.e. donors or acceptors). The graphene functionalization is performed using diaryliodonium salts which are preferred to the well-studied diazonium salts in order to avoid spontaneous functionalization. We show that the grafting density of nitrophenyl groups (-NO2Ph) for instance can be precisely tuned between 4. 1013 up to 3.1014 molecules/cm2.
Local electronic structure and nature of chemical bonding is studied via a combination of Low Temperature Scanning Tunneling Microscopy (LT-STM), Scanning Tunneling Spectroscopy (STS) and Non-Contact Atomic Force Microscopy (NC-AFM) using an Omicron LT-STM/SPM system. We worked with a variety of epitaxial single layer graphene on both insulating and conductive (0001) SiC with and without buffer layer. We show that removal of the buffer layer allows clear observation of the nitrophenyl modifications and are an ideal platform for work function and bandgap engineering studies in graphene. Contrary to suggestions in the literature we do not observe an increased reactivity at defect sites or SiC step edges. Large increase in the density of states is observed at the modification site. STS and XPS data show that the -NO2Ph grafts induce n-type doping in graphene consistent with charge transfer from the grafts to the graphene.
1. C. K. Chan, T. E. Beechem, T. Ohta, M. T. Brumbach, D. R. Wheeler, K. J. Stevenson, J. Phys. Chem. C 2013, 117, 12038.

The performance of optimized graphene devices is ultimately determined by the quality of the graphene itself. Graphene grown on copper foils is often wrinkled, and the orientation of the graphene cannot be controlled. Graphene grown on (0001) SiC via graphitization of the surface has a single orientation, but its thickness cannot be easily limited to one layer. We describe a method in which a graphene film of one or two monolayers grown on SiC is exfoliated via the stress induced with a nickel film and transferred to another substrate. Using the binding energy contrast among graphene and different materials, we selectively removed excess random graphene layers from monolayer graphene by using a gold film. This shows one can “fix” the transferred graphene by removing any excess, leaving only a single layer on the substrate. Two-step exfoliation process applied on a graphitized 4-inch SiC wafer resulted in the formation of a monolayered single-domain graphene in a 4-inch wafer scale. Remarkably, structural and electrical characterizations show that the graphene sheet maintains its high quality during the repeated growth/transfer processes with a single SiC wafer. These results constitute significant progress toward low-cost fabrication of a wafer-scale, oriented graphene on conventional semiconductor wafers for hybrid integrated circuits applications and show a potential for manipulation of two-dimensional materials with a single-atom-thickness precision.

Thermal decomposition of SiC shows a great potential for large-area production of graphene. However, the interaction of the formed monolayer graphene (MLG) with the substrate (via a buffer layer (BL)) reduces its carrier mobility in comparison to free-standing monolayer graphene. By thermally annealing a MLG/BL/SiC(0001) sample in air atmosphere, it has been previously shown that it is possible to decouple the BL from the SiC substrate by O₂ intercalation, which leads to the formation of large area and high quality quasi-free-standing bilayer graphene (Oliveira Jr et al., Carbon 52 (2013) 83). The same result could not be obtained by annealing the samples in a pure O₂ atmosphere. It seems that other species rather than only O₂ play a role in the BL decoupling process. In order to clarify this issue, we investigated the intercalation process in high-quality MLG grown on SiC(0001), which was prepared in an Ar atmosphere. The intercalation experiments consisted of thermal treatments with different proportions of N₂ and O₂, at 600°C for 40 minutes. In addition, the effect of H₂O was probed by interposing a bubbler between the N₂ gas flow and the reaction chamber. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the samples. Raman spectra of samples treated in a N₂+O₂ ambient show that the BL-related features (at ~1300 cm-1) decrease in intensity but do not vanish for all tested O₂ percentages. This indicates that only a partial decoupling of the BL is taking place. Nevertheless, after the introduction of H₂O in the system, the BL-features completely vanish for all annealing conditions. The BL decoupling (and bilayer graphene formation) is further confirmed by the changes observed for the double-resonance peak (2D) at 2700 cm-1, which offers a larger full-width half maximum (FWHM), and exhibits a characteristic bilayer graphene-related shape. Results obtained by XPS corroborate these findings. Following annealings, the binding energy of components assigned to the BL (in Si 2p and C 1s core level spectra) decrease in comparison to that related to pristine MLG. In addition, a component associated to the SiC surface oxidation can be observed. Therefore, the present results points to a synergistic effect of H₂O and O₂, since a complete decoupling of the BL takes place only in the presence of both gases (annealings in N₂+H₂O ambient also promote partial BL decoupling). Considering that a thin silicon oxide layer is formed at BL/SiC interface already in the beginning of the thermal treatment, H₂O molecules will help in stabilizing further interactions between O₂ and the oxidized SiC surface (Ryu et al., Nano Lett. 10 (2010) 4944), resulting in an acceleration of the intercalation process. Finally, we will also show results obtained from magneto transport measurements. They were conducted to investigate possible changes in the electrical properties which might be associated to the observed physico-chemical modifications.

The quest to develop graphene electronics remains hindered by the inability to make a semiconducting form of graphene suitable for room temperature devices. By far the widest band gaps have been generated by chemical functionalization but the intrinsic disorder in the functionalized graphene and the instability of the absorbate at moderate temperatures has proven a challenge. In this talk I will discuss a completely new method to functionalize graphene. Rather than bonding atoms or molecules to the graphene post-growth, we use a predefined nitrogenated SiC surface as a template for graphene growth. Starting from a high temperature-stable sub-monolayer coverage of nitrogen on SiC, the sample is heated to the graphene growth temperature. The nitrogen graphene (NG) that grows from this surface is a wide gap form of graphene that is stable up to at least 1400C. Using angle resolved photoemission, we measure the valance band maximum of the gap to be 0.7eV below EF meaning that the gap is >0.7eV. I will discuss how the nitride surface is prepared and how the semiconducting graphene is formed and characterized. I will also show how scalable bottom-up NG ribbons, seamlessly connected to metallic graphene, can be grown from this material. The NG ribbons offer a direct application as a channel material for high voltage SiC FETs.

Precise stacking of two-dimensional materials provides an exciting new way to tune the electrical and optical properties of atomically thin systems. One key parameter in these materials is the relative rotation angle between adjacent layers (theta;); for instance, it is known that theta; affects the optical properties of twisted bilayer graphene (tBLG). However, even in this simple system, previous measurements have not provided precise enough structure-property relationships for quantitative comparison to theory. Here, we perform comprehensive measurements of the optical conductivity (σ) of tBLG as a function of known theta;. The conductivity is measured between 1.2 and 5.6 eV, more than twice the energy range of previous spectroscopic studies, while theta; between 8° and 30° is determined to within a fraction of a degree by TEM diffraction. There are two peaks in the σ spectrum of tBLG, not found in Bernal stacked bilayer graphene, which are signatures of interlayer coupling. The theta;-dependence of the energies of these peaks is smooth and monotonic, and can be modeled very accurately based on the band structure of single-layer graphene. However, we find that the inclusion of electron-hole interactions in first-principles calculations is essential to reproduce the σ peak shape that we measure, especially for the highest energy features. Finally, we find that the shape of the tBLG absorption peak is tunable as a function of applied vertical electric field, and relate our findings to the band structure of tBLG. Our results provide a framework for understanding interlayer coupling in many similar systems.

Ag thin film is an efficient surface enhanced Raman spectroscopy (SERS) substrate because its quality factor of localized surface plasmon resonance (LSPR) is much larger than other materials, e.g., 10 times larger than that of Au at the wavelength of 500 nm. However, the chemical instability of Ag in ambient environment significantly degrades the SERS properties, and hinders the practical applications. Although conventional protective structures (e.g., silica and alumina barrier) can inhibit the Ag corrosion in ambient, they usually have large light absorption and reduce the plasmonic enhancement. In this work, we transfer monolayer graphene onto Ag SERS substrate, on one hand, to protect Ag from corrosion, and on the other hand, to prevent the photo-induced damages (photocarbonization, photobleaching and metal-catalyzed reaction) on the probed molecules.
Firstly, we comparatively study morphological characteristic of the Ag SERS substrates with and without graphene protective barrier, revealing high corrosion-resistance of monolayer graphene to the oxidizing gas and liquid. We further demonstrate the graphene coated Ag thin films as stable SERS substrate. After 35-day exposure in air, the graphene coated Ag thin film maintains high SERS sensitivity. Secondly, we systematically study the resistance to photo-induced damages with graphene barrier. Our results show that Ag SERS substrate with graphene coating significantly improves the SERS reproducibility, and simultaneously inhibits metal-catalyzed reaction. In addition, the graphene layer can form strong interaction with the probed R6G molecules through π-π bonding stack, reducing the possibility of photo-induced desorption. In summary, our results show that graphene coating on Ag substrate effectively enhances the resistance to the corrosion and photo-induced damages.

The graphene quantum dots (GQDs) and its derivatives have revealed its unique luminescent properties such as pH-dependent and excitation-dependent PL behavior. To date, the PL signal of the GQDs and its derivatives was emitted upon the irradiation of the one-photon excitation by using either UV or blue light source with higher energy than the bandgap of them. The high energy light source inevitably imposes the limitations on the use of the GQDs and graphene oxide quantum dots (GOQDs) as the fluorescent tags for sensing DNA and living cells, since the intensive UV light can cause the photodamage in the biomolecules. To address this issue, the up-conversion or two-photon induced (TPI) PL with near IR light have been normally employed for biomolecular imaging, taking advantages of minor autofluorescence background signal, deep tissue penetration depth, reduced photobleaching, and low photodamage.
Thus, to fully utilize the PL property of the GQD and its derivatives, it is necessary to understand the TPI PL characteristics of them. Although there are a few reports showing that the GQDs display the up-conversion luminescence by a monochromatic light using a xenon lamp as an excitation light source, there are controversies about the up-conversion PL mechanism of the GQDs. Herein, we propose a facile fabrication route for synthesizing uniform GOQDs and clarify its up-conversion green photoluminescence emission by using two-photon excitation via a femto-second Ti:sapphire pulsed laser system. Utilizing the unique TPI PL phenomenon of the GOQDs, the DNAzyme mediated Cu2+ metal ion detection was performed with high sensitivity and selectivity, and the PL quenching of the GOQDs was verified via the electron transfer from the GOQD to the Cu2+ ion. It is expected that the GOQDs with unique TPI PL property, excellent biocompatibility and high water solubility can be broadly applicable in a variety of biosciences and bioengineering as a novel fluorescent probes in vitro and in vivo.

Not to be published.

Reduced Graphene Oxide (RGO) has the advantage of an aqueous and industrial-scalable production route. However, the main problem that prevents the use of RGO in electronics is the high deviation in electrical resistivity between chips. The novel growth of RGO can bridge the gaps in-between existing flakes and thus reduce the electrical resistivity standard deviation from 80.5% to 16.5%. The average resistivity of the treated RGO of ~3.8nm thickness were 200Omega;/square The study uses an atmospheric-pressure chemical vapour deposition (CVD) system with hydrogen and argon gas bubbling through ethanol before entering the furnace. With a treatment of 2 hours, 100% of the silicon dioxide substrate was covered with reduced GO from an initial 65% coverage. This technology could enable RGO to be used in practical electronic devices and molecular sensors.

Graphene nulcei grown on copper foils can exhibit a variety of shapes from dendrites, squares, stars, hexagons, to butterflies and lobes. Understanding the underlying physicochemical mechanisms which dictate the shape of the nuclei and their evolution over time is of fundamental importance for the engineering synthesis of wafer-scale single crystals. Here, we have studied the growth of graphene nuclei onto a variety of Cu facets [high symmetry facets such as (111) and (001) as well as for high-index surfaces such as (221) and (310)] under different growth conditions (CH4 flux, H2 flux, temperature and growth time) [1] with the objective to identify the role of these different parameters. To this aim, we use a phase-field model to provide a unified explanation of the different nuclei shapes. The model can predict the experimentally observed shapes as a function of growth rate for different copper facets and carbon precursor flux and shows that anisotropic diffusion plays a critical role in the determining of the shape of the nuclei.
[1] Esteban Meca, John Lowengrub, Hokwon Kim, Cecilia Mattevi, and Vivek B. Shenoy Nano Lett., Article ASAP, DOI: 10.1021/nl4033928

Recently developed chemical vapor deposition (CVD) methods can produce large-size and uniform polycrystalline graphene on metal substrates (e.g., Ni and Cu), but the electron properties of CVD graphene in such polycrystalline nature are reduced by domain boundaries. Indeed, there have been outstanding advancements in the millimeter-sized growth of Cu-based single-crystal graphene. However, the facile synthesis of graphene with much larger single-crystal domains on commercial Cu foil is more desirable. Here we demonstrate large single crystal graphene formation on commercially available Cu foil using thermal CVD process. In an H2 atmosphere, pre-annealing and the treatment time, particularly H2 flow rate are crucial to tune the most preferred crystal structure of Cu foil. We found that the crystal structure of Cu surface could be engineered to either crystal orientation (100) or (111). Increasing of H2 flow rate of pre-annealing leads to the semi-melting state of Cu foil and simultaneously lowering the melting temperature of the bare polycrystalline Cu surface, as a result, which enables the crystal structure of Cu surface to become a thermodynamically stable state, i.e., Cu (111) single crystal structure, confirmed by electron backscatter diffraction (EBSD). In addition, the kinetics control of graphene deposition contributes to not only single crystal graphene growth but also the reconstruction of Cu crystal structure. Moreover, under low pressure CVD process, the crystal structure of Cu surface can be changed to the most desirable (111) orientation without distinct domain boundaries on the whole area. Eventually, the largest domain boundaries-less single crystal graphene monolayer was ever formed on underlying (111) oriented Cu crystal structure.

Material production is a key challenge for any emerging material. Chemical vapor deposition is the most promising method for the fabrication of high quality large-area graphene. Although graphene manufacturing has evolved tremendously it cannot be considered as mature technology for industrial production. Cost-efficiency is obviously one of the main aspects when considering commercialization. In addition to streamlined fabrication, efficient characterization methods are required in the large-scale fabrication.
We present our findings on efficient and rapid monolayer graphene fabrication and ultrafast characterization of graphene by simultaneous third-harmonic and photoluminescence microscopy. Utilizing photo-induced heating in a cold wall reactor, we are able to produce graphene films covering the whole copper surface only in 20-30 s (~11 mbar). In addition, the total processing time can be significantly reduced due fast ramp rates enabled by minimized thermal mass. Cold wall chamber allows also real-time temperature detection of the substrate using pyrometer. Confocal mu;-Raman mapping revealed a fingerprint of high quality monolayer graphene; for an area of 25×25 mu;m2 average 2D/G > 3 with very low defect density (D/G ratio) while field-effect mobility was 3000 cm2/Vs [1]. These findings can have significant implications while pursuing cost-efficiency in graphene manufacturing. Besides typical global back gate field-effect transistors (FET), we have also manufactured highly tunable local top and bottom gate controlled complementary graphene devices [2]. The complementary operation between two transistors which are individually controlled by electrostatic doping enables switchable performance between inverter (p-n FETs) and voltage controlled resistor (n-p FETs).
In addition to typical graphene analysis, we have also considered alternative methodology. Here, we demonstrate ultrafast characterization capabilities of simultaneous third-harmonic and fluorescence microscopy [3]. Without optimization, imaging is orders of magnitude faster than conventional Raman mapping (an area of hundreds of micrometers squared requires merely a few seconds). High contrast in fluorescence and third harmonic generation was observed for monolayer graphene compared to the substrate, and bilayer was clearly distinguished from monolayer. Moreover, the fluorescence and third harmonic signal increased with the number of layers, until it decreased beyond a certain threshold thickness. Interestingly, some additional features revealed by multiphoton microscopy were practically undetectable by Raman.
References
[1] Riikonen, J.; Kim, W.; Li, C.; Svensk, O.; Arpiainen, S.; Kainlauri, M.; Lipsanen, H., Carbon, 2013, 62, 43-50
[2] Kim, W.; Riikonen, J.; Li, C.; Chen, Y.; Lipsanen, H., Nanotechnology, 2013, 395202-1-5
[3] Saynatjoki, A.; Karvonen, L.; Riikonen, L.; Kim, W.; Mehravar, s.; Norwood, R.A.; Peyghambarian, N.; Lipsanen, H.; Khanh K., ACS Nano, 2013, 8441-8446

Chemical vapour deposition (CVD) is recognized as the most promising technique for the scale-up of higher quality graphene production for a wide range of applications. There is particular interest in the CVD of graphene on copper, where the number of atomic layers can be controlled down to a monolayer.
The quality of the deposited graphene can be quantified in terms of several physical properties, including crystal grain size, percentage of monolayer coverage, defect density, etc. In most cases it is desirable to produce large areas of single-crystal grains of graphene, with a maximized percentage of monolayer and with a low defect density. Many process solutions have been reported for the production of small sized CVD graphene samples. However, very few process solutions exist today to deposit high quality, predominantly monolayer, single-crystal graphene on large size substrates.
We present, for the first time, novel tooling designs for CVD reactors that provide multiple benefits for CVD graphene production. Our designs are scalable to >300 mm substrates, enable manufacturing of higher quality graphene in large quantities, can be beneficially applied to the full range of CVD graphene processes (low pressure, atmospheric pressure and roll-to-roll) and used for a range of substrates (foils or films).
By comparing the quality of CVD graphene obtained using a traditional CVD reactor to the same reactor enhanced by our novel solution, we show substantial improvement in grain size, macroscopic flatness of the substrates, and defect reduction. We also demonstrate that the production throughput for a given reactor can be improved by at least an order of magnitude without loss of quality.

The superior electrical, mechanical, thermal, and optical properties of graphene, a two-dimensional carbon monolayer crystal, suggest that it possesses potential to replace materials currently being used in a wide variety of fields such as electronics, photonics, energy storage and conversion, composite materials, and bioapplications. One of the most promising applications of graphene is chemical sensing. However, semimetallic nature of graphene with zero band gap limits its potential use in chemoresistive sensor applications. Semiconducting conductivity is an essential feature of chemoresistive material to achieve high response, and is closely associated with the number of total carriers in the material and the number of carriers involved in charge transfer between the material and adsorbed molecules. Therefore, opening and tailoring of a band gap is one of the most important challenges for practical application of graphene to chemoresistive sensors.
Fluorination of graphene, in which fluorine (F) atoms are covalently bonded to sp2 carbons accompanied by the structural change of the C-C bonds to sp3 configuration, is a noble and effective method to tune the band gap of graphene. The structural changes of graphene by fluorination induce band gap opening at the K point and cause loss of π-conjugated electron cloud on the graphene plane. Consequently, fully fluorinated graphene behaves like an insulator with a band gap energy of ~3eV.
Here, we demonstrate that fluorinated graphene can be used as an active material for chemoresistive sensors. Fluorinated graphene sensors were fabricated using a solid fluorine source (CYTOP) and laser irradiation. The degree of fluorination, which affects electronic properties including band structure and conductivity, was controlled by changing laser power and irradiation time. The fluorinated graphene sensors show different chemoresponse depending on the degree of fluorination. Especially, controlled sensors shows reversible chemoresponses to various gaseous molecules, which has been never obtained in sensors based on pristine graphene. The experimental results and theoretical calculations reveal the critical role of surface fluorine atoms in chemoresponse to gaseous molecules.

Although chemical modification of graphene creates a band gap, achieving thermal and chemical stability in fluorinated or hydrogenated graphene remains challenging. Band gap engineering through size confinement with graphene nanoribbons suffers from serious impediments to device fabrication [1]. Graphene antidot lattices are an elegant alternative to graphene nanoribbons that have the potential to be easily transferred onto device surfaces. Experimental work on graphene antidot lattices has been limited to top-down fabrication methods which do not reach size scales necessary for significant band gaps [2]. Bottom-up approaches have produced highly ordered polyphenylene networks, but not porous graphene [3]. We examine the formation of graphene antidot lattices through the on-surface polymerization of halogenated aromatic molecules. In addition to thermally mediated self-assembly, tip-induced assembly offers a potential route for directing nanostructure formation. We deposit 1,3,5-tris(2-bromophenyl) benzene (TBB) onto gold. We characterize the surfaces using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Samples prepared with the substrate held at room temperature during deposition appear streaky when imaged with STM, indicating that at room temperature the molecule is highly mobile on gold, and has not polymerized. With the substrate held above 200° C during deposition, TBB nucleates into disordered networks at step edges. When areas imaged as streaky are exposed to high energy tunneling electrons, a disordered network appears underneath the path of the electron beam. Depositing the molecule onto a substrate heated at 400°C and above leads to extended networks that span entire terraces. High index facets show highly ordered regions which we attribute to close-packed bromine adatom islands. A disordered porous network assembles on low index facets. By examining the role of substrate temperature in the deposition of 1,3,5-tris(2-bromophenyl benzene) on gold, we have found conditions that lead to a porous structure. We also show evidence of tip-induced polymerization of halogenated aromatic molecules.
[1] J. Cai, et al., Nature 466, 7305 (2010)
[2] J. Eroms and D. Weiss, New J. Phys. 11(9), 095021 (2009)
[3] M. Bieri, et al., Chem. Commun. 6919-6921 (2009)

Several novel designs for beyond CMOS devices have emerged using two-dimensional semiconductors. These devices require deposition of thin insulators on 2D semiconductors or between two sheets of 2D semiconductors. However, 2D semiconductors are nearly inert surfaces thereby making uniform nucleation of oxide growth challenging preventing scaling of the insulator thickness. A new technique has been developed to employ a monolayer of ordered metal phthalocyanines (MPc) on 2D semiconductors directly as a monolayer low-k dielectric or as a nucleation layer for growth of high-k insulators. This study demonstrates the molecular scale observation of formation of O-TiPc mono and bilayers on graphene with UHV scanning tunneling microscopy (STM). O-TiPc monolayers were deposited on HOPG surfaces by organic molecular beam epitaxy. After deposition, O-TiPc forms a monolayer with only few defects, and the crystal structure of monolayer has four-fold symmetry in a 1.4 x 1.4 nm grid. Observation of bright protrusions on each O-TiPc indicates that each O-TiPc in the monolayer is directed outward to vacuum. STS shows the band gap of the monolayer is 1.7 eV and the band gap of the bilayer is 2.3 eV. The monolayer or bilayer can directly be employed for sub-nanometer insulators on 2D semiconductors at low bias. Multiple cycles of TMA and water were dosed onto O-TiPc/HOPG to investigate nucleation of Al2O3 on the O-TiPc layers. The first cycle of TMA was observed to chemisorb on a 1.4 x 1.4 nm grid on the TiOPc monolayer. After exposure O-TiPc monolayer to 5 cycles ALD pulse (tri-methyl-aluminum (TMA)+H2O), insulating aluminum oxide was deposited uniformly on O-TiPc/HOPG. After formation of Al2O3 on O-TiPc/HOPG, the band gap of surface increases from 1.7 eV to 2.7 eV, while the conductance decreased. As shown in XPS spectra, the quality of Al2O3 can be improved by post annealing, consisting with transition of chemical states in O 1s peak and Al 2p. The chemical shifts of O and Al indicate that post annealing converts remained the Al-OH to Al2O3. Consequently, O-TiPc can not only act as a low-K dielectric but also induce high density ordered nucleation of ALD on central ion of O-TiPc for high-k dielectric growth.

In order to progress from the lab to commercial applications it will be necessary to develop an industrially scalable method to produce large quantities of defect-free graphene. Here we describe a new method for exfoliating graphite in liquids to give large-volume dispersions of graphene flakes. TEM, XPS and Raman spectroscopy show the exfoliated flakes to be thin, unoxidised and defect-free. This procedure can be performed in liquid volumes from 100s of ml up to 100s of litres and beyond and is compatible with industrial processes. We will describe the production of 300 L of graphene dispersion at concentration of ~0.1 mg/ml and production rates of ~5g/hr. We have developed a simple model for the exfoliation mechanism and fully characterized the scaling behaviour of the graphene production rate with processing parameters such as batch volume and processing time. The graphene produced by this method performs well in applications from composites to conductive coatings. We also show this method can be applied to exfoliate BN, MoS2 and a range of other layered crystals.

Among the remarkable variety of semiconducting nanomaterials that have been discovered over the past two decades, single walled carbon nanotubes (SWNTs) remain uniquely well suited for applications in high performance electronics, sensors and other technologies. However, SWNTs are grown as a mixture of both metallic and semiconducting tubes by any available production process, and their practical applications to semiconductor electronics are largely hampered. The most advanced opportunities demand the ability to form perfectly aligned, horizontal arrays of purely semiconducting SWNTs. Techniques based on optical, electrical, or chemical effects involve some combination of drawbacks, including incomplete removal of the metallic SWNTs, partial removal and/or degradation of the semiconducting SWNTs, inability to operate on aligned arrays and/or reliance on uncertain underlying mechanisms. Here, we present strategies, in which nanoscale thermocapillary flows in thin film organic coatings serve as highly efficient means for selectively removing metallic SWNTs from electronically heterogeneous aligned arrays grown on quartz substrates. The low temperatures and unusual physics associated with this process enable robust, scalable operation, with clear potential for practical use. Detailed theoretical and experimental studies reveal all of the essential attributes of the underlying thermophysical phenomena.

One route to realizing graphene as a material for digital-type devices is through the lithographic patterning of graphene nanoribbons (GNRs). GNRs enable band gap engineering that is dependent on nanoribbon width and edge state. We employed two complementary AFM-based lithography techniques to pattern GNRs: (1) thermal dip-pen nanolithography (tDPN)1 and (2) thermochemical nanolithography (TCNL)2. Though inverse in approach, both techniques generate GNRs into a larger sheet of insulating chemically-modified graphene. Both lithographies were performed on CVD-grown single-layered graphene (SLG) on SiO2/Si substrates using heated AFM probes. The first approach, tDPN, used the heated probe to deposit narrow polystyrene (PS) ribbons on pristine graphene. The areas of the graphene not protected by the polymer were then fluorinated, converting them to a highly insulating state, which leaves behind a chemically isolate GNR channel. We show that the PS protected ribbon was the only conductive pathway for active device. Secondly, we use the converse approach by using the heated AFM probe to locally reduce fluorographene back to graphene, leaving behind a conductive GNR channel. Both techniques can generate a wide range of nanoribbon widths while avoiding electron beams which can damage graphene. We discuss the relative merits of each strategy, as well as their impact on electrical properties (e.g., doping).
1. WK Lee, et al., Nano Letters, 11, 5461, 2011
2. WK Lee, et al., ACS Nano, 7, 6219, 2013

We present a study of hybrid graphene nanoribbon-nanopore devices for biomolecule detection[1] and ultimately DNA sequencing. When a graphene nanoribbon(GNR) width is constricted to ~10nm, the variation of the potential created by the different bases of a DNA strand passing through an adjacent nanopore will create a sufficient variation of the ribbon conductivity to enable electrical discrimination between DNA bases. We fabricated back or side gated devices comprised of nanopores with diameters in the range of 2minus;10 nm at the edge or in the center of GNRs with widths between 5nm and 200 nm, on 40 nm thick silicon nitride (SiNx) membranes. We discuss the challenges encountered in the manufacturing of these nanoconstrictions (by lithography or electron beam sculpting) and the irradiation effects of the electron beam during the nanopore formation. GNR conductance is monitored in situ during the nanopore formation process inside a transmission electron microscope (TEM) operating at 200 kV for different doping levels induced by the side or back gates. We identify and study a linear and a superlinear regime for the increase of GNR resistance with the electron dose, and correlate with the decrease by a factor of ten or more in mobility when GNRs are imaged at relatively high magnification prior to the nanopore formation. Bases on our findings we devise a scanning TEM procedure which prevent the GNR electron induced damage, enabling sensitive biosensors. We finally present the operation of this sensor for biomolecule detection and DNA sequencing, correlating the electric signal measured in the GNR to the ionic current measured through the nanopore. The higher current(~mu;A) which can be driven through a GNR compared to the ionic current(~nA)[2] leads to a hundredfold increase in the measuring bandwidth(10-100MHz), possibly enabling DNA sequencing without slowing the molecules - for a projected 10 minutes full genome sequencing.
[1] Towards sensitive graphene nanoribbon-nanopore devices by preventing electron beam induced damage. M. Puster*, J. A. Rodríguez- Manzo*, A. Balan*, M. Drndicacute;. ACS Nano, in review (2013). *equal authorship
[2] Differentiation of short, single-stranded DNA homopolymers in solid-state nanopores. K. Venta, G. Shemer, M. Puster, J. A. Rodríguez- Manzo, A. Balan, J. K. Rosenstein, K. Shepard, M. Drndicacute;. ACS Nano, 7: 4629-4636 (2013).

One of the chief difficulties of incorporating graphene into logic devices is its intrinsic lack of a band gap. One solution involves the formation of nanoribbons (GNRs), less than 2 nm in width. GNRs are commonly created through lithographic techniques or the longitudinal opening of carbon nanotubes. However, these methods often result in irregular ribbon edges adversely affecting their electronic properties. In addition, such GNRs are often too wide to have a band gap sufficient for the high on/off ratios required for logic devices. An elegant solution to these problems involves using organic synthesis to create “bottom-up” GNRs through a polymerization reaction. Such a route results in small ribbon widths that give band gaps comparable to classical semiconductors like Si and have pristine edges. However, this solution-based fabrication technique has its own associated problems, particularly aggregation. GNRs are known to have a propensity to coil, potentially creating kinetically trapped states when the GNRs are deposited from solution onto a substrate.
We are using Molecular Dynamics (MD) techniques to address the aggregation issue that is limiting current ability to create GNR devices. Specifically, we will be looking at GNRs synthesized by oxidizing soluble precursor poly(o-phenylene) polymers[1]. Ideally, this polymerization scheme would form flat, ordered arrays. We shall show that while aggregation can be assuaged by judicious chemical modification of the ribbon edges, GNRs formed when the precursor polymers are oxidized in solution will still tend to aggregate and coil. This will result in kinetically or thermodynamically trapped coils that cannot be expected to effectively flatten when placed on a substrate. If, instead, the oxidation step is performed on the substrate itself, our hypothesis is that aggregation and coiling will be much less of a problem. One complicating factor is that we have found poly(o-phenylenes) tend to form helical secondary structures that also may affect the final conformational state of the GNR on a substrate. We will compare the effectiveness of these two depositions methods using MD with different stabilizing chemical groups on the GNRs as well as different choices of substrate. We will also provide as-yet unreported details of the helical secondary structures poly(o-phenylenes) and the aspects of colloidal stabilization specifically fabricated in this manner. These computational studies will contribute to the community&’s understanding of preferential processing routes that would otherwise be difficult to evaluate using experimental methods. It helps bridge the divide between atomic-scale representation of GNRs and a practical route to recommendations for their physical nanomanufacture.
[1] W.R. Dichtel et al., private communication (2013).

Transformative nanomanufacturing routes is used to create single layer crumpled graphene nanoparticles through innovative electrohydromechanical concepts that capitalize on the salient mechanical features, rich surface chemistry and compelling colloidal properties of graphene in a multiscale and synergistic fashion to transcend the boundary for achieving high power density electrochemical capacitive energy storage. The proposed experimental strategies conceptually mimic the nano-emulsions at interfaces to confine the dimensional transitions of 2-D planar graphene into 3-D crumpled nanoparticles and their assembly into unprecedented superstructures, establishing a paradigm shift in synthesis and processing of crumpled graphene structures at nanoscale. Specifically, the work presented here enables the experimental isolation of single-layer crumpled graphene nanoparticles that first and foremost yield access to fully explore of exceptional “intrinsic material properties”, especially those pertinent to energy applications such as specific surface area, packing density, intrinsic capacitance, porosity, and ionic permeability.