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:
(1) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. Nature
2007, 446, 60-63.
(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-io