Graphene a two-dimensional, single atomic layer material with linear electron dispersion has rather unique electrical and properties. There is currently strong interest in taking advantage of these properties for technological applications. In my talk I will review some of the key properties of free graphene, how these are affected by environmental interactions and how they can be utilized in electronics and optoelectronics. Specifically, I will discuss high frequency (>300 GHz) graphene transistors, nanotube array transistors and simple IC circuits, as well as related device physics aspects, such as the role of electrical contacts, scattering effects, energy dissipation, etc. I will then discuss the key optical properties of graphene and how they can be combined with its excellent electrical properties and used in optoelectronics applications. Specific examples involving applications of graphene in the far-IR and THz ranges, ultrafast graphene photodetectors and their use in the detection of optical data streams will be presented.

Graphene is a two dimensional sheet consisting of carbon hexagon network, from which graphite, a typical layered material, is formed. It was successfully isolated by the group of Andre Geim and Kostya Novoselov at Manchester University in 2004 using the so-called scotch-tape method (mechanical exfoliation of graphite), which has attracted a great attention in the field of physics, chemistry, materials science, and device applications. Graphene has now become not only a central issue in basic science but also a challenging target in cutting-edge nanotechnology. The Nobel Prize 2010 in Physics was awarded to this historic contribution to graphene. Various methods are now being investigated for the purpose of fabricating graphene with large area and good quality. What is most interesting in graphene is the unconventional physics behind its electronic properties as well as the possibility of various applications intensively being explored. Indeed, the electron motion in graphene is governed by the Weyl equation for a neutrino or the Dirac equation with vanishing rest mass. This has been shown to give rise to various unprecedented electronic phenomena, such as half-integer quantum Hall effect, absence of backscattering leading to the Klein tunneling, Berry's phase leading to singularity in the diamagnetic susceptibility and the conductivity, crossover between anti-localization and weak-localization behavior in low-temperature transport, etc. This talk will discuss many of such exciting new physics.

Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO2. These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures. In high quality graphene and hBN hybrid devices, we observe many body induced symmetry breaking integer quantum Hall states and fractional quantum Hall states. In bilayer graphene we observe large band gap opening up when we applied a perpendicular electric field. In this presentation, we will discuss various many body effects, including the quantum Hall ferromagnetism (QHFM) and fractional quantum Hall effect (FQHE) in 2-dimensional electron gas with multiple internal degrees of freedom provides a model system to study the interplay between spontaneous symmetry breaking and emergent topological order.

Pristine graphene isolated from graphite by mechanical peeling has served an invaluable role in studies leading to fundamentals. In contrast, defective and functionalized graphene which is produced through splitting and reduction of graphite oxide, to C/O ratios higher than 10, promises to find entry into a myriad of applications more rapidly. This is mainly because (i) for the development of bulk materials such as ultracapacitors, electrodes for batteries, electrochemical sensors, and graphene-based composites in general, there is a need to produce graphene in > tons/year quantities and (ii) the intrinsically defective and functionalized structure of graphene produced by graphite oxide route provides advantages over pristine graphene in various applications. In this presentation, I will focus on the production and the utilization of functionalized graphene sheets (FGSs) by thermal exfoliation and reduction of graphite oxide. In spite of its rich history dating back to 1840, only recently it has been shown that the thermal exfoliation and reduction of graphite oxide method can indeed yield large fractions (> 80%) of single sheets. The challenge of single sheet production, however, leads into another challenge on the control of aggregate structures as single sheets readily collapse back to form multistacks due to van der Waals and capillary forces. Thus, restacking is not only unavoidable but a certain degree of restacking is also beneficial in most applications. The main challenge is the control of the aggregated network structures to attain the desired properties in applications ranging from electrochemical devices to high strength multifunctional nanocomposites. The reduction of the contact resistance of graphene aggregates is another challenge when high electrical conductivity is a target. While the intrinsic electrical conductivity of functionalized graphene has been shown to be in the order 4x105 S/m, in graphene aggregates, the conductivities are at least three orders of magnitude lower mainly due to contact resistance problems. An effective solution for this problem remains to be demonstrated.

Carbon nanotube (CNT) thin-film transistors (TFTs) have attracted growing interest because of their potential for use in the fabrication of high-performance electronic components on flexible and transparent substrates. However, high-speed potential has not been demonstrated in realistic integrated circuits so far because of the difficulty encountered in integrating many high-performance CNT-TFTs with high uniformity; this difficulty is associated with metallic CNT incorporation which compels researchers to use devices with small CNT density and large channel length. In this presentation, we report on the successful implementation of high-speed medium-scale integrated circuits with 108 CNT-TFTs¹ based on CNT growth using a plasma-enhanced chemical vapor deposition (PECVD) technique; this technique allows the preferential growth of CNTs that exhibit semiconducting behavior in the FET current-voltage characteristics.² Owing to the advantage of preferential growth, a normally-off operation with good pinch-off characteristics with an on/off current ratio of 4.2 Ã- 10⁵ was confirmed even for the device having high-density channel with a relatively small channel length of 10 μm. A normally-off operation is indispensable for all driver TFTs in the circuits to realize error-free logic operations. The drain on current was as high as 800 μA/mm owing to the high-density CNTs, which is expected to lead to high-speed operations. High uniformity was confirmed in the electrical properties of the devices with small fluctuations of 12% for the drain current and 0.5 V for the threshold voltage, leading to the successful implementation of medium-scale integrated circuits. A 53-stage ring oscillator consisting of 108 CNT-TFTs demonstrated a switching speed of 0.51 μs/gate. This speed is approximately two orders of magnitude higher than that demonstrated by previous TFT integrated circuits³, and the integration level is the highest among all the nanocarbon-based devices. The switching speed increased with an increase in the drain current and a decrease in the channel length, indicating the importance of using short-channel devices with high-density CNTs; which has become possible by taking advantage of the preferential growth. The successful demonstration of the operation of a ring oscillator with more than 100 transistors confirms the suitability of the present TFT technology for implementing CNT-TFT integrated circuits with various functions. 1. T. Mizutani, Y. Okigawa, Y. Ono, S. Kishimoto, and Y. Ohno: Appl. Phys. Express, 3, 115101, (2010). 2. H. Ohnaka, Y. Kojima, S. Kishimoto, Y. Ohno, and T. Mizutani: Jpn. J. Appl. Phys., 45, 5485, (2006). 3. M. Ha, Y. Xia, A. A. Green, W. Zhang, M. J. Renn, C. H. Kim, M. C. Hersam, and C. D. Frisbie: ACS Nano, 4, 4388, (2010).

Finding the optimal channel material that can simultaneously offer low temperature processing, ambient stability, high mobility, transparency, and flexibility has been recognized as a fundamental challenge in the field of flexible electronics. Here in this talk, we report our recent progress on large-scale fabrication of flexible macroelectronic integrated circuits using solution-processed carbon nanotube thin-film transistors (TFTs). By using semiconducting nanotubes with 99% purity, high-performance nanotube TFTs with mobility close to 100 cm2/Vs, on/off ratio of 10^4, and exceptional device-to-device uniformity have been demonstrated on flexible substrates, which are significantly superior than the state-of-the-art organic TFTs. Utilizing such nanotube TFTs, flexible integrated circuits such as inverter, NAND, NOR and digital-to-analog converter (DAC) have been designed and demonstrated. The flexible inverters offer idea logic behavior in terms of symmetric input/output, large noise margin (0.4 VDD), and high gain (~30 at VDD = 5 V). Such flexible circuits also offer outstanding bending stability and frequency response, which would work well for flexible wireless communication applications. The demonstration above indicates the significant advantages of our platform to be the ideal candidate for future practical and feasible high-performance flexible macroelectronics.

Carbon nanotube networks (CNNs) have become suitable for applications as sensors, interconnects and flexible electronics through recent development of large-scale fabrication, control of nanotube density, chirality and alignment [1-2]. However, due to the complexity of their structure, performance limits and reliability of CNN-based devices are not well understood. In this study, we combine comprehensive experiments and computational modeling to analyze high-field transport, thermal dissipation, and breakdown in CNN devices. We fabricated thin film transistors with various lengths (5-50 μm), widths (5-150 μm) and nanotube densities, on SiO₂/Si substrates with Cr/Au electrodes. Importantly, we examined all-metallic, all-semiconducting and mixed-chirality CNNs obtained by density gradient ultracentrifugation [2]. Devices were electrically characterized, then imaged by AFM, SEM and infrared (IR) thermal microscopy up to electrical breakdown. The computational model randomly generates tubes with a predefined distribution of orientation, position, density and type based on the experiments. Current and power dissipation and temperature profile were calculated by solving electrical transport equations at each network element [3], self-consistently with the thermal transport equations. CNN breakdown in simulations occurs when all â?ohotâ? tubes are burned out and removed from the network, to mimic the experimental breakdown at high voltage [4]. We find that average breakdown voltages scale linearly with device length with a slope of 1.6 V/μm for mixed-networks. However, we found no strong influence of the gate voltage on the CNN breakdown voltage, even for all-semiconducting networks. Simulations offer the insight that CNN breakdown is initiated at nanotube junctions and that a zigzag burning path is formed along the width of the network, consistent with our SEM imaging [4], but too fast to be imaged in real-time by the IR camera. In addition, average breakdown voltage and its standard deviation decrease (unlike maximum power capacity that increases) when nanotube density or network purity increase (metallic or semiconducting tube percentage becomes very high). For 10x5 μm devices, for example, the breakdown voltage reduces from 90 to 45 V as density increases from 2 to 10 tubes/μm². Furthermore, networks that are very sparse or very pure tend to break instantly, instead of degrading gradually. The results of this study offer a comprehensive look at the complex reliability phenomena in CNN devices for practical applications. 1. X. Ho, et al., Nano Lett. 10, 499 (2010). 2. M. Engel, et al., ACS Nano 2, 2445 (2008). 3. A. Behnam, et al., Phys. Rev. B 75, 125432 (2007). 4. D. Estrada and E. Pop, Appl. Phys. Lett. 98, 073102 (2011).

We have succeeded in the fabrication of very stable n-type single-walled carbon nanotubes-thin film transistors (SWNTs-TFTs) with Cs encapsulated SWNTs (Cs@SWNTs). Cs@SWNTs are prepared by a plasma ion irradiation method [1,2]. It is found that the transport property of SWNT-TFTs clearly changes from p- to n-type after the Cs plasma irradiation. Based on the systematic investigation, it is revealed that there is a suitable irradiation energy (E_i) range of Cs⁺ for the encapsulation (20 â?¤ E_i â?¤ 60 eV) and the optimum energy is about 50 eV. Furthermore, the lower and higher energy limits for the Cs encapsulation are found to be related with the carbon-carbon bond breaking energy and displacement energy of carbon atom from SWNTs, respectively. Moreover, the n-type property is very stable under the long time soak in water and after the high temperature (â?¤ 400 °C) annealing. This stable n-type SWNTs-TFTs are very important for the industrial fabrication of high performance electrical circuits with SWNTs-TFTs. [1] T. Izumida, R. Hatakeyama, Y. Neo, H. Mimura, K. Omote, and Y. Kasama: Appl. Phys. Lett. 89 (2006) 093121. [2] T. Kato, R. Hatakeyama, J. Shishido, W. Oohara, and K. Tohji, Appl. Phys. Lett. 95 (2009) 083109.

A carbon nanotubes transistor inverter can be a probe to test the compatibility of CNT transistors with integrated circuits since a logic inverter is the basic building block of logic circuits. We fabricated top gate carbon nanotubes network field effect transistor(CNTFET)s on as grown network of single wall nanotubes(SWCNT). In the next step, inverters were produced with the ambi-polar CNTFETs and Au wire bonding. The electrical properties of CNTFETs and the inverters made of the CNTFETs were investigated from the point of the hysteresis in the transfer characteristics. The CMOS inverter scheme and the simulated polynomial I_D-V_G characteristic were introduced to understand the operation of the devices. In this presentation, the electrical behaviors of top gate CNTFETs and the inverters will be discussed; the traditional CMOS inverter scheme and the hysteresis induced inverter action will be compared.

In the design of high performance graphene field-effect transistors, gate dielectric deposition and source/drain contacts formation are the two key ingredients determining the basic device performance. Due to the chemical inertness of graphene surface, it has been a great challenge to uniformly deposit gate dielectrics on graphene. Various approaches involving a layer of functionalization material between the graphene and gate dielectric have been applied to achieve a complete surface coverage. However, some undesired effects, such as doping and trapped charges, induced by these functionalization layers can degrade the performance of the graphene field-effect devices. I will present a plasma enhanced atomic layer deposition (ALD) method to deposit ultra-thin, high quality gate dielectrics for top-gated graphene device fabrication. A thorough study of ALD conditions for surface coverage, oxide quality and graphene device performance will be presented. Graphene is a zero band gap semiconductor. Different from most materials, owing to the low density of states in graphene, current modulation by means of a gate in graphene field-effect transistors is still possible even without a band gap. Gating of graphene is thus rather special and needs to be understood in more details to properly design novel nano-devices. We provide experimental data to elucidate a unique property in graphene â?" gating also impacts the graphene areas in direct contact with the source and drain electrodes. I will discuss in how far this effect is critical for the scaling behavior of future generations of graphene field-effect transistors. While the Dirac point has been considered to be indicators for impurity or phonon scattering from the substrate and doping level in graphene, evident from experimental data, we suggest that the shift of Dirac point in graphene devices can also be a consequence of gate length scaling. We propose for the first time to use this shift as one of the signatures of short channel effects in graphene.

Semiconductor optoelectronic devices are extremely popular due to their broad spectral responsivity, excellent linearity, high quantum efficiency, and high dynamic range of operation. Photoswitches are key elements of digital optoelectronic circuits that require high ON/OFF ratios at low optical incident power levels. We present a new type of photoresponse mechanism in heterojunction devices of single-wall carbon nanotubes (SWCNTs) and Si. Organized SWCNT network architectures on SiO2/Si were fabricated by using a template guided fluidic assembly process [1], developed in our laboratory. These devices possess a radically unconventional photo-response that enables extremely high photo-induced ON/OFF ratios and Responsivity, one or two order of magnitude higher than others, at low reverse-bias voltages. We investigated the possible underlying mechanism of this unusually large photo-response by performing an array of optical, electronic transport, and optoelectronic measurements in conjunction with theoretical modeling. A possible mechanism that could give rise to such an effect is Multiple Exciton Gerneration (MEG). MEG occurs when the absorption of a single high-energy photon results in the creation of two or more excitons [2]. With ultra-high responsive photo-detection mechanism, we combine it with a novel template-guided fluidic assembly technique for SWCNTs to develop a range of multifunctional optoelectronic switches. This architecture of heterojunction of SWCNTs and Si demonstrates low-cost, low-power, highly sensitive scalable optoelectronic devices for diverse analog, digital, sensing, imaging, and other suitable applications. [1] Kim et al., ACS Nano 3 (2009) 2818-2826 [2] Hinds et al., Science (2009) 324, 1542

Flexible transparent electrodes are crucial for flat panel display and solar cell technologies. While carbon nanotube (CNT) network and graphene electrodes show promise, their good performance is typically contingent upon treatment with dopants such as nitric acid or thionyl chloride, which are harsh and produce unstable results. Development of mild, stable, reliable doping is critical for enabling carbon nanotube transparent electrode technology. We report the fabrication and electrical properties of MoOx-CNT composite materials. The MoOx can be easily deposited either by thermal evaporation or from solution-based precursors, and the process also works well with graphene. Sheet resistances change less than 10% over 20 days in ambient and less than 2% with overnight heating to 300 degrees C in air. Under appropriate circumstances, such composites can form durable thin film electrodes with sheet resistances as much as seven times higher than comparable CNT-network controls and of less than 100 ohms/sq at 84% transmittance plain and 85 ohms/sq at 83% transmittance with a PEDOT:PSS capping layer.

Platelet derived growth factor (PDGF) is a dimeric polypeptide growth factor comprised of AA, AB, and BB isoforms. It is known to control the proliferation of cells including fibroblasts, glial cells, smooth muscle and more importantly directly implicated in the cell transformation process, i.e. tumor growth and progression. Therefore recently, rapid detection of PDGB-BB has gained considerable attention due to its potential role as cancer biomarker. In particular, lot of research efforts have been focussed to develop detection methods involving labeled aptamers because they provide faster real-time information on cancer-related proteins compared to conventional techniques such as electrophoresis, autoradiography, and enzymed-linked immunosorbent assay (ELISA). On the other hand, with the extensive development of pyrolyzed carbon surface chemistry, considerable efforts have been devoted to the development of different biosensors using this versatile material. In this work, we have developed a fluoroscence-labeled aptamer based sensing platform for PDGF-BB detection. The 3D carbon micropillar arrays functionalized by direct amination technique were used as the electrode for binding the PDGF- binding aptamer. The optical detection of PDGF was based on the release of a fluorescent dye intercalated in the stem loop of the aptamer. We were able to achieve detection limits in the subnanomolar range due to the high surface area of the 3D microarrays. The simple labeled PDGF detection technique offers high sensitivity and selectivity. Finally, we demonstrated the reusability of the sensor through a series of hybridization and denature cycles. The detection strategy reported in this work has the potential to be extended for the detection of other proteins.

Coaxial nanowires composed of metallic cores and semiconductor shells are employed as amperometric enzyme biosensor electrodes to improve the sensitivity which is defined by the current change with the analyte concentration change (Î"I/Î"C). In most cases, the sensitivity is proportional to the measured current level. Besides the enzymatic chemical reaction kinetics on the sensor surface, the resistance and the surface area of the sensor electrode are important factors for high sensitivity because they are closely related to the current level. The resistance and the surface area of the coaxial nanowire network electrode are investigated. The simulation and the experimental results confirm that most current flows through the semiconductor shell rather than the metallic core owing to the work function difference between core and shell. The shell thickness should be increased to lower the resistance but the increase of nanowire diameter causes the decrease of the total surface area in view of the unit volume of nanowire network electrode. Considering the above trade-off relation, the optimal coaxial nanowire structure to maximize the sensitivity is proposed.

Graphene, a two-dimensional honeycomb carbon lattice, is considered to be a promising material for future electronics devices due to its excellent properties. We are actually trying to use graphene as a transistor channel for future large scale integrated circuits (LSIs). In this talk, we explain our recent progress toward such an application. In the first part, we demonstrate graphene growth all over a 200-mm wafer by chemical vapor deposition using Cu film as a catalyst [1, 2]. We explain how the grain size and orientation of graphene are affected by various growth parameters. Electrical properties depending on the grain structures are also described. In the second part, we describe self-organizing formation of graphene nanoribbons (GNRs) on a substrate. One of the examples is preferential GNR formation on steps of a Cu surface [3]. In the last part, we explain our recent results on patterning of GNRs by Helium Ion Microscope (HeIM) and the on-off operation of a GNR transistor thus fabricated [4]. A new device concept using a GNR is also proposed and demonstrated. This work was suported by JSP through the â?oFIRST Program,â? initiated by CSTP, Japan. This work was partly conducted at the Nano-Processing Facility supported by ICAN, AIST, Japan. References [1] S. Sato, et al. ECS Trans. 35(3) (2011) 219. [2] S. Sato, et al. ECS Trans. 37(1) (2011) 121. [3] K. Hayashi, et al. Extended Abstracts of 72th JSAP Fall Meeting 2011, Yamagata (2011) 2a-ZF-9. [4] S. Nakaharai, et al., Extended Abstracts of the 2011 SSDM, Nagoya (2011) 1300.

Flexible displays, stretchable and transparent electronics, low-cost radio-frequency devices, and large-surface sensor networks are some examples of macro-electronic devices that would benefit from the use of high mobility graphene as the semiconducting channel material. However in order to enable graphene integration into the current as well as numerous emerging applications, it would be necessary to develop device concepts that exhibit minimal power dissipation so that can be powered by small batteries or by near-field radio-frequency coupling. Here we demonstrate low-voltage chemical vapor deposited (CVD) graphene field-effect transistors fabricated using a solution-based room temperature process. Specifically, we demonstrate high mobility graphene devices operating at voltages below |1.5| V. This is achieved through the use of different solution processable self-assembled organic monolayer dielectrics [1] with different molecular dipole moment and surface energy characteristics. The transistors display hysteresis-free operation in ambient conditions, low leakage currents and exceptional bias stability for stressing times over 12 hours. The doping effect is weak and varies with the SAM/graphene interfacial properties, opening the possibility to accurately tailor the electrical properties of the conducting channel via surface chemical engineering at the nanometer scale. [1] C. Mattevi, F. Colléaux, H. Kim, M. Chhowalla, and T. D. Anthopoulos â?oLow-voltage graphene transistors based on solution processable self-assembled monolayer nanodielectricsâ?o submitted 2011.

There has been much interest in graphene-based electronic devices because graphene provides excellent electrical, optical, and mechanical properties. Here, we introduce several high performance monolayer and bilayer graphene-based field effect transistors (FETs) controlled by tunable non-covalent molecular doping. Firstly, we controlled carrier doping to monolayer graphene using self-assembled monolayers (SAMs) patterned spatially between graphene and silicon oxide substrate. The doping type and position were controlled by patterning SAMs with different functional groups. This tunable graphene doping method using SAMs as buffer layer was successfully utilized to enhance electrical properties of graphene FETs and organic FETs using graphene electrodes. Secondly, we have devised a new method for band-gap opening of bilayer graphene by dual molecular doping. A perfluorinated molecule deposited onto bilayer graphene induces p-doping from the top, while amine-functionalized SAMs constructed on SiO2/Si substrate induces n-doping from the bottom side. This dual doping-driven perpendicular electric field with opposite directions remarkably increases the on/off current ratio of bilayer graphene FETs, while independently controlling the position of charge neutrality point. The band-gap opening was confirmed by infra-red light absorption spectroscopy and ab-initio calculation of bilayer graphene band structure after dual molecular doping.

Low dimensional carbon allotropes presently provide an unparalleled platform to explore novel electronic properties, and with tremendous progress may one day supplant entrenched materials within the semiconductor industry. In order for graphene to continue on its extraordinary scientific and technological trajectories, many hurdles must be overcome such as reliable bandgap engineering, advances in processability, and removal or mitigation of defects. Noncovalent chemical modification of graphene offers a pathway to address many of these concerns and furthermore provides an opportunity to graft new functionality onto this unique substrate. In this work, we study the effects of noncovalent modification of graphene by rubrene. Rubrene is an organic semiconductor which has garnered considerable interest due to its high mobility and large fluorescence quantum efficiency. It frequently finds use in organic field effect transistors and light emitting diodes. By exploiting Ï?-Ï? interactions between the two highly conjugated systems, we have developed a simple approach to functionalize graphene devices. To this end, optical and electron-beam lithography are used to fabricate graphene field effect transistors, which are subsequently modified by rubrene. Further, our process development allows for rubrene to be patterned on graphene devices, thus providing a pathway to modify the electronic and optical properties in a site-specific manner. In order to better understand the resulting graphene/rubrene structure, a suite of analytical tools has been employed. Raman spectroscopy and microscopy confirm the presence of the rubrene and spatially correlate observed electronic changes with surface modification. Polarized Raman spectroscopy is used to investigate any long range order of rubrene on the graphene surface. Photoluminescence measurements show that rubrene emission is not quenched, and spectral analysis offers insight into rubrene film characteristics. Atomic force microscopy provides detailed information as to film thickness, and suggests that rubrene film morphology is largely disordered. Due to the simplicity of this functionalization procedure, a rubrene-based motif could graft new chemical moieties onto graphene and enable new device opportunities.

Electron chirality effects in Graphene give rise to optics like phenomena such as refraction and focusing of carriers¹ in Graphene p-n junctions (GPNJ). These effects reflect the strong angular dependent transport of carriers across the junction, and can be used to construct devices, including transistors and logic elements. This work reports a method to fabricate a single GPNJ by the use of buried split-gates (SG) which provide the bipolar doping, and a study of device geometries to validate electron chirality effects. By orienting Graphene channels at different angles to the SG, the dependence of the GPNJ resistance on the electron incidence angle is obtained which, even at 300K and modest Graphene mobility, is significant. Electron scattering by GPNJ for other device geometries such as point-source contacts is also investigated. The buried SG, in the form of interdigitated fingers, are formed by patterning 100 nm Polysilicon on top of 100 nm thermal SiO₂, followed by PECVD deposition and CMP of SiO₂ to 100 nm thickness to form the top dielectric, yielding atomically smooth top surfaces. On some samples, Hafnium oxide or hexagonal Boron Nitride is deposited as an alternate surface. Graphene is then deposited either by mechanical exfoliation or by wet-transfer of CVD-grown Graphene. Standard e-beam lithography, metal evaporation and RIE processes are used to yield 2-terminal Graphene devices with channels at different angles to the SG junction, with 10/50 nm Cr/Au or Ti/Au contacts. By sweeping the SG biases V_G1 and V_G2 independently, the doping profile in the Graphene channel is modulated to create unipolar (i.e. p-p), or bipolar (i.e. p-n) junctions, and allows us to map the 2-probe device resistance ( R_T) over these doping levels. For any given doping levels ( V_G1, V_G2) , by subtracting the average unipolar resistance R_T( V_G1, V_G1) from the average bipolar resistance R_T( V_G1, V_G2) , the average junction resistance R_JAV( V_G1, V_G2) is obtained, which subtracts out both the contact resistances and the channel resistances from the total resistance R_T, even for channels with asymmetric p and n-resistances. The extracted R_JAV is then compared for Graphene devices of comparable dimensions and channel mobilities (typically about 2000 cm²/Vs), and with different angles. As the doping changes from unipolar to bipolar, a sharper peak in R_JAV is observed for channels at an angle to the junction than one that is normal to the junction, with the peak being sharpest for an angle of 45°, the largest angle probed. This trend is observed for both exfoliated and CVD-grown Graphene, and is more pronounced for higher mobility and lower width devices, consistent with theory. The ratio of the maximal R_JAV obtained for 45° and 0° devices is about 2.5, which is significant considering the modest Graphene mobilities of our devices. ¹V. Cheianov et al., â?oThe Focusing of Electron Flow and a Veselago Lens in Graphene p-n Junctionsâ?, Science, vol. 315, 2007, pp 1252.

Graphene has been considered as one of the potential post Si-materials due to its high mobility. [1] However, since graphene is semi-conductor with zero band gap, it is difficult to achieve high I_on/I_off ratio, one of the most important requirements for commercial devices. There have been many attempts to open its band gap for high I_on/I_off ratio, but most of them end up lowering the mobility. [2-5]

Thus, we proposed and demonstrated a new device structure for graphene transistor based on one of the unique properties of graphene for high I_on/I_off: using this approach, we were able to achieve the ratio over 10⁵.

[6] Our device has several major advantages over previously proposed graphene based electronic devices. Since our device does not alter the given properties of graphene, such as opening the band gap, it has no fundamental issues on mobility degradations. In addition, our device is fully compatible with current Si technology and we were able to fabricate the devices with 6� wafer scale with CVD (Chemical Vapor Deposition) grown graphene.

In this presentation, we will discuss about the details of our graphene device including the device structure and the detailed understanding of working mechanism. We will present device characteristics including I-V curves with 10⁵ on/off ratio. We will also present the performance of an inverter based on our devices. Finally, we will discuss the current issues and their potential solutions.

[1] C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard and J. Hone, Nature Nanotechnology (2010), 5, 722

[2] Y. Zhang, T.-T. Tang, C. Girit, Z. Hao, M. C. Martin, A. Zettl, M. F. Crommie, Y. R. Shen, and F. Wang, Nature (2009), 459, 820

[3] X. Li, X. Wang, L. Zhang, S. Lee, and H. Dai, Science (2008), 319, 1229

[4] J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. Feng, K. Mullen, and R. Fasel, Nature (2010), 466, 470

[5] L. Ci, L. Song, C. Jin, D. Jaiwala, D. Wu, Y. Li, A. Srivastava, Z. F. Wang, K. Storr, L. Balicas, F. Liu, and P. M. Ajayan, Nature Materials (2010), 9, 430

[6] H. Yang, J. Heo, S. Park, H. J. Song, D. H. Seo, K.-E. Byun, P. Kim, I. Yoo, H.-J. Chung, and K. Kim, submitted.

Planar graphene side gates have found extensive use in mesoscopic graphene quantum devices, such as graphene quantum dots and single-electron transistors, and as a better alternative to top gates in epitaxial graphene FET and nanoribbon devices, as it avoids dielectric breakdown and hysteresis. Despite their prevalence, detailed studies of the electrostatics of side-gating and the impact on channel transport remain lacking. In this talk, we will show how we can systematically quantify the side-gate modulation efficiency in a myriad of side-gated devices. We demonstrate significant modulation in both bilayer and monolayer graphene for various gating configurations. Most importantly, we show that the mechanism behind the side-gate modulation on transport is well accounted for by simple electrostatics and Landauerâ?Ts transport formula. Our approach will prove indispensable in designing versatile planar multi-gated graphene devices for numerous quantum and classical applications.

Understanding the contact between graphene and metal is quite important in the graphene FET applications. It is reported that the Fermi level of graphene in contact with a metal is â?opinnedâ?, and that, the carrier density in graphene under the metal cannot be modulated by applying the back-gate bias. This â?opinningâ? has been intuitively accepted since the existence of the p-n junction in the vicinity of the contacts, which explains electron-hole asymmetry in the conductance, is simply explainable. However, the â?opinningâ? has not been proved experimentally. In this study, we focus on electrical properties of graphene in contact with the Ni electrode and present the experimental evidence of very small pinning at the graphene metal interface. Monolayer graphene was transferred onto SiO₂(90nm)/p⁺-Si substrate by the micromechanical cleavage of Kish graphite. The graphene FET including Ni deposited on the center of channel were fabricated using conventional electron-beam lithography technique. Ohmic contacts were confirmed in all the electrodes. The resistance of graphene/Ni double-layered structure (R_G,M) was evaluated by 4-probe method. It was shown that the conductance in the graphene/metal double-layered structure was modulated by the back-gate bias. This fact implies that graphene in contact with Ni behaves like graphene without Ni, because Ni conductance cannot be modulated due to its large density of states. When the length of Ni on the graphene channel was short, R_G,M was nearly the same as the resistance of graphene without Ni. It also showed the clear ambipolar behaviour with decreasing the Ni length. This indicates that the carrier type in graphene can also be changed even in contact with Ni. When we consider that the contact resistance between graphene and Ni prevents the current from flowing through Ni on graphene, the results are quite reasonable. In addition to this floating electrode case, the conductance modulation was also observed in graphene underneath the source electrode which was connected to the ground. Furthermore, the simple resistor network analysis suggests that electric properties of graphene underneath Ni are not altered by contacting Ni. In other words, the graphene/Ni interaction is not likely to be the chemical bonding but the van-der Waals type interaction since the linear dispersion of graphene is maintained. This conflicts with the Ï?-d coupling reported for graphene on Ni experimentally and theoretically. This discrepancy may come from the effect of substrate SiO₂ on the Ni/graphene interaction, since graphene is one-atom-thick material.

We propose a novel device structure which enables unipolar graphene transistors. The unipolar characteristics are required for stable CMOS operation which must be robust against the threshold voltage shift. The proposed device is a field effect transistor based on bilayer graphene with a comb-shaped top gate which has multiple rectangular gates in series on a bilayer graphene channel. The charge polarity in graphene channel can be changed locally by applying top gate bias (V_tg), inducing a series of p-n junctions. In the configuration of the p-n junctions, drain current (I_ds) is suppressed by the formation of a series of high resistance regions at the p-n interfaces. As the number of comb fingers (N) increases, the number of p-n junctions also increases, and thus I_ds is more effectively suppressed. Consequently, unipolar FET operation is expected by the non-inverted (p-p or n-n) and inverted (p-n or n-p) configurations for on- and off-states, respectively. Bilayer graphene was exfoliated from Kish graphite and deposited on a Si/SiO₂ substrate. The comb-shaped top gate electrodes were formed by direct deposition of Al on the graphene, where Al₂O₃ layer as a gate dielectric was formed naturally at the Al/graphene interface [1]. Electric field between the top gate and the back gate (substrate) opens a band gap [1-3] which enhances the resistance of the p-n junctions induced in the channel. In I_ds-V_tg measurements, unipolar FET behavior was demonstrated by the suppressed I_ds in the p-n or n-p configurations compared to the p-p or n-n cases. In the case of N=8, for example, I_ds in p-n configuration was 23% of that in p-p configuration compared at the fixed carrier density of 6.0Ã-10¹² cm^-2 in both n and p types. In addition, we found that the off current suppression strongly depended on N, although total gated area was exactly the same for the all devices. In fact, in the case of N=1, I_ds in p-n configuration was 81% of that in p-p configuration, which is much larger than the N=8 case. This indicates that the multiple p-n junctions contribute to the unipolar behavior. Finally, we also found that, in the on-state, I_ds showed strong saturation behavior at high source-drain bias (V_ds) in the proposed device structure. Such behavior is considered to be enhanced by the inverted carrier polarity due to a high V_ds comparable to V_tg underneath the top gate [4] which induces p-n junctions. [1] H. Miyazaki et al., Nano Lett. 10, 3888 (2010). [2] E. McCann, Phys. Rev. B 74, 161403 (2006). [3] J. B. Oostinga et al., Nat. Mater. 7, 151 (2008). [4] I. Meric et al., Nat. Nanotech. 3, 354 (2008).

The high frequency characterization of carbon nanotube materials, especially in the form of a nanocomposite, is in its infancy and opens up the potential for large area applications in both rf and microwave frequency bands. The use of a composite has the advantage of accurately tailoring the concentration and dispersion of the conductive nanotube phase and can be potentially scaled up for manufacture. We have measured [1] the sub-THz electrical response of screen printed carbon nanotube - poly(methyl methacrylate) polymer composites up to 220 GHz. The measured electrical losses using mm long coplanar waveguide geometries were as low as 0.15 dB/mm in the frequency range 40 GHz to 110 GHz and showed a reduction in signal loss with increasing frequency; a behaviour opposite to that found in conventional metallic conductors. In the frequency range 140 to 220 GHz the electrical losses averaged 0.28 dB/mm. We show that the low electrical losses are associated with the capacitive coupling between the nanotubes and discuss potential high frequency large area applications.

[1] Ali H. Alshehri, Malgorzata Jakubowska, Marcin Sloma, Michal Horaczek, Diana Rudka, Charles Free and J. David Carey, Appl. Phys. Lett. 99, 153109 (2011).

Recent success in isolating single-walled carbon nanotubes (SWNTs) of narrow chirality distributions enabled making pure metallic (m-) and semiconducting (s-) SWNT films, which are promising components of flexible electronics. Such films are expected to reflect clearly the nature of individual SWNTs in their transport properties. One of the most remarkable characteristics of SWNTs is their one dimensional subband structure and resulting stepwise change of the conductance by subband filling. Therefore, it is important from both scientific and practical viewpoint to investigate one dimensional characteristics of electronic transport in m- and s-SWNT films by controlling their Fermi level. Chemical doping or field-effect transistor is unsuitable for the purpose because of the lack of precise and reversible Fermi level controllability, and the narrow controllable Fermi level range, respectively. The problems are solved by our electric double layer transistor (EDLT) technique, where the gate voltage is applied through an electrolyte. We have realized superconducting transition in SrTiO3 [1] and ZrNCl [2] with EDLT. Therefore, this work was done to investigate conduction properties of s- and m-SWNT films with EDLT. The conductance, optical absorption spectra, and temperature dependence of the resistance of s- and m-SWNT films were measured at various gate voltages. The conductance of the s-SWNT film showed stepwise change against the gate voltage. The absorbance spectra indicate the steps correspond to reaching of the Fermi level to a vHs. The Fermi level shift across a vHs was also observed in the m-SWNT films. The conductance of the m-SWNT film sharply increased at the point and its temperature dependence changed from that of variable range hopping to a metallic behavior. These results demonstrate that the conduction properties strongly depend on the Fermi level, reflecting one dimensionality of SWNTs. [1] K. Ueno, S. Nakamura, H. Shimotani, A. Ohtomo, N. Kimura, T. Nojima, H. Aoki, Y. Iwasa, M. Kawasaki, Nat. Mater. 7, 855 (2008). [2] J. T. Ye, S. Inoue, K. Kobayashi, Y. Kasahara, H. T. Yuan, H. Shimotani, Y. Iwasa, Nat. Mater. 9, 125 (2010).

A low resistance metal-nanocarbon junction is a key for fabrication of high performance nanocarbon devices. Near ballistic transport properties in metallic and semiconducting CNT devices have been demonstrated for device having an ohmic contact[1,2]. The ohmic contact can be obtained by deposition of high work function metals such as Pd. Another important key for fabricating low resistance junction is the application of end-contact geometry[3]. Recent theoretical study has predicted the superiority of end-contact geometry compared to that of the conventional side-contact geometry in various metals[4]. The different chemical bonding of end-contact and side-contact geometries is the major key for the improved junction properties. The fabrication of end-contact geometry, however, is much more difficult since metals should be vertically contacted to open-ended CNTs or grapheneâ?Ts edges so that the contact length is limited to several nanometers. Therefore, the development of a fabrication method of end-contact geometry is a challenge. Here we demonstrate a fabrication method for realization end-contact geometry in CNT and graphene devices. To directly compare the transport properties with those of side-contacted devices, the side-contacted and end-contacted devices have been fabricated using the same CNT (or graphene). A combination of electron beam lithography and photolithography was employed. The CNTs were grown directly by means of thermal chemical vapour deposition on a SiO2-covered Si substrate. Firstly, Au/Ti electrodes of the side contacted devices were deposited. Then, a passivation layer of HfO2 was grown by means of atomic layer deposition. After patterning the electrode areas of end-contacted devices, the CNT of the patterned areas was removed altogether with the HfO2 layer and a part of the SiO2 layer under the CNT by dry etching. In this step, the open-ended CNTs are formed and their air-exposed length is limited to several nanometers due to the presence of HfO2 passivation layer. Then, Au/Ti electrodes were deposited. The transport properties were evaluated using the Si substrate as a backgate. For semiconducting CNTs, we found different transport properties, i.e., the end-contacted device exhibited a p-type field effect transistor characteristic while the side-contacted device showed an n-type characteristic. Since both devices have the same CNT channel and HfO2 passivation layer, the different transport properties should be originated from the junction behaviour. The proposed method is also useful for realization end-contacted graphene devices. [1] W. Liang et al, Nature 411, 665-669 (2001). [2] A. Javey et al, Nature 424, 654-657 (2003). [3] J.J. Palacios et al, Phys. Rev. B 77, 113403 (2008). [4] Y. Matsuda et al, J. Phys. Chem. C 114, 17845 (2010).

The single beam mirage effect, also known as photothermal deflection, is studied using a free-standing, highly-aligned carbon nanotube aerogel sheet as the heat source. The extremely low thermal capacitance and high heat transfer ability of these transparent forest-drawn carbon nanotube sheets enables high frequency modulation of sheet temperature over an enormous temperature range, thereby providing a sharp, rapidly changing gradient of refractive index in surrounding liquid or gas. The advantages of temperature modulation using carbon nanotube sheets are multiple: in inert gases the temperature can reach >2500 K; the obtained frequency range for photothermal modulation is â^¼100 kHz in gases and over 100 Hz in high refractive index liquids; and the heat source is transparent for optical and acoustical waves. Unlike for conventional heat sources for photothermal deflection, the intensity and phase of the thermally modulated beam component linearly depends upon the beam-to-sheet separation over a wide range of distances, which enables convenient measurements of the thermal diffusivity and the temperature dependence of refractive index in both liquids, and gases. The remarkable performance of nanotube sheets suggests possible applications as photo-deflectors and for switchable invisibility cloaks, and provides useful insights into their use as thermoacoustic projectors and sonar. Visibility cloaking is demonstrated in a liquid.

Carbon nanotube (CNT) based X-ray source technology has been investigated for imaging applications because of its attractive characteristics including fast switching, distributed source, and multiplexing [1]. The electron emission from the CNT cathode is then controlled with an electronic circuit. Here we propose a new approach based on the optical control of a CNT photocathode [2]. This photocathode is an array of vertically aligned multi-walled CNTs (electron emitters), each CNT being associated with a p-i-n photodiode. The functions of photon-electron conversion and electron emission are thus separated. This photocathode combines the very good field emission properties of carbon nanotubes and the excellent efficiency provided by semiconductor photodiodes. Schematically, the photodiode acts as an optically driven current source that determines the maximum current that can be emitted by the CNT. We have fabricated a CNT photocathode based on silicon photodiodes. The CNTs have been grown on the p-i-n diodes by plasma enhanced chemical vapor deposition at 700°C using C2H2 and NH3 gases. This photocathode delivers a current ranging from 16 μA to 500 μA which linearly varies with the absorbed optical power delivered by a 532 nm laser. The internal quantum efficiency was 10%. Using a 658 nm wavelength laser source, the cutoff frequency of this photocathode was 50 MHz. We have built the first optically driven X-ray source based on CNT photocathodes. It is a compact dual X-ray source featuring two sources accurately and independently controlled with two laser diodes illuminating the backside of the photocathodes. We used a simple diode configuration with only 1 high voltage supply. As the circuit driving the laser power is insulated from the cathode, the emission current can be easily controlled even if the cathode is biased at high voltage (which was the case). The anode being grounded, its cooling is facilitated. With this dual source, 2 images from two different viewing angles are obtained in parallel. These images exhibit a high quality and a very high stability. This last feature has been attributed to the fact that each emitting CNT is actively controlled with a pin photodiode. Moreover, the fast switching and the frequency modulation of the X ray beams can be implemented by controlling the two laser diodes. This concept can be extended to multiple X-ray sources. Associated with linear or 2D X-ray detectors, low cost, compact/mobile, efficient and stationary (gantry free) tomosynthesis scanners could be realized. Such systems which provide 3D imaging can be used for security applications, non-destructive inspection in the industry and for medical applications. This work has been funded by the French ANR project SPIDERS and the European NMP project TECHNOTUBES. [1] Cao et al., Med. Phys. 37, 5306 (2010). [2] Hudanski et al, Nanotechnology 19, 105201 (2008).

Transparent, elastic conductors are an essential component of electronic and optoelectronic devices that facilitate human interaction and biofeedback, such as interactive electronics, implantable medical devices, and robotic systems with human-like sensing capabilities. The availability of conducting thin films with these properties could lead to the development of skin-like sensors that stretch reversibly, sense pressure (not just touch), bend into hairpin turns, integrate with collapsible, stretchable and mechanically robust displays and solar cells, and also wrap around non-planar and biological surfaces, such as skin and organs, without wrinkling. We report transparent, conducting films of spray-deposited films of single-walled carbon nanotubes that can be rendered stretchable by the applying strain along each axis, and then releasing this strain. This process produces wave-like structures in the nanotubes that accommodate strains of up to 150% and demonstrate conductivities as high as 2,200 S/cm in the stretched state, which make them the most conductive material in the stretched state (> 100%). The nanotube films can be stretched to 30% at least 12,500 times without significant degradation in the electronic properties, and thus, they can be integrated into devices requiring extreme mechanical compliance or resistance to mechanical failure. To demonstrate the utility of these films, we patterned them by spray-deposition through stencil masks and used them as electrodes in arrays of transparent, stretchable parallel-plate capacitors, which behave as pressure and strain sensors. The devices are physically robust, can sense pressures in the range of 50 kPa to >2 MPa, and can sense strains of a few percent.

Nanocarbon materials including carbon nanotubes (CNTs), carbon nanofibers (CNFs), and more recently graphene, enabled by state-of-the-art chemical synthesis processes, have clearly emerged as new classes of promising materials. In the domains of non-planar devices and 3D integration, CNTs and CNFs have promises for a number of important applications ranging from miniaturized field-emission electron and X-ray sources [1,2], to electrochemical probes and scanning probes for microscopes [1], thanks to their attractive electrical, electrochemical, and mechanical properties. Vertically grown and aligned individual CNFs, and arrays of CNFs with controlled spacing and dimensions, offer a unique nanomaterials platform for designing and engineering new types of nanoelectromechanical systems (NEMS). Vertically oriented NEMS, particularly resonant NEMS in vertical CNFs arrays with designed densities, could have strong implications for sensing and communication devices, especially for harsh environment (e.g., for some of NASAâ?Ts extreme environment missions, or generally under high radiation conditions) [3,4]. Almost all of todayâ?Ts NEMS devices and resonators have been based on patterned nanostructures yielded by lithographic planar processes [5,6], or chemically synthesized nanofibers (or nanotubes, nanowires) [7] that are harvested after growth and then painstakingly assembled into planar processes on-chip. In this work, we take this important challenge, to explore very thin vertical CNFs and arrays of vertically aligned CNFs, for NEMS devices and their characterization. The CNF growth is performed using dc plasma-enhanced chemical vapor deposition (PECVD) in an ammonia and acetylene ambient, which yields the designed CNFs and CNF arrays. We perform in-situ mechanical and electrical measurements of the CNFs by using a combination of the scanning electron microscope (SEM) and focused ion beam (FIB). We investigate and quantitatively analyze the adhesion characteristics of the CNFs and the effects of the surface morphology, which we intentionally modulate by using FIB, and characterize by employing energy dispersive spectroscopy (EDS). Based on the measurement and analysis of electromechanical properties of the CNFs, we then devise innovative schemes to engineer the vertical CNFs and arrays into NEMS devices for both quasi-DC and resonant operations. These new vertical NEMS will pave the way for a range of systems and new applications where non-planar processes and three-dimensional integration are desired.

The unique optical and electronic properties of single-walled carbon nanotubes (SWCNTs) make these quasi 1D structures promising building blocks for nanoscale electronic and optoelectronic devices [1]. A powerful method to characterize SWCNT based field-effect transistors is scanning photocurrent microscopy (SPCM), where a diffraction-limited laser spot is raster scanned across a device, recording the photocurrent signal at the same time. Insight into the nature of the band bending at the SWCNT-metal contact and potential modulations along the nanotube was gained by several groups [2-4]. Due to the diffraction limit all SPCM measurements reported in the literature so far have been restricted to a spatial resolution of few hundred of nanometers. Band bending and potential modulations occurring on a smaller length scale could therefore not be resolved or remained undetected up to now. We report on the first photocurrent measurements along single SWCNTs with sub 30 nm spatial resolution. Our approach extends conventional tip-enhanced near-field optical microscopy (TENOM) providing new insight into the optoelectronic properties of nanostructures. In TENOM the diffraction limit is overcome by exploiting the strong localized enhanced fields at the end of a laser-irradiated metal tip that acts as an optical antenna. This allows spatially confined probing of various optical signals, such as Raman scattering and photoluminescence [5]. Combining SPCM and TENOM, we succeeded in simultaneously recording enhanced optical and photocurrent images with nanoscale resolution. At the metal contacts the photocurrent signal is found to decrease on a length scale of few tens of nanometers in the case of metallic nanotubes indicating the extent and the direction of the band bending. Along the nanotubes we observe pronounced photocurrent signal variations in some cases that are correlated with simultaneously recorded Raman spectra. In addition, for semiconducting SWCNTs this technique allows to measure the depletion width of the Schottky barrier as a function of gate voltage. [1] Avouris, P.; Chen, Z.; Perebeinos, V. Nat. Nanotechnol. 2007, 2, 605 [2] Freitag, M.; Tsang, J. C.; Bol, A.; Yuan, D.; Liu, J.; Avouris, P. Nano Lett. 2007, 7, 2037 [3] Balasubramanian, K.; Burghard, M.; Kern, K.; Scolari, M.; Mews, A. Nano Lett. 2005, 5, 507 [4] Freitag, M.; Tsang, J.C.; Bol, A.; Avouris, P. Appl. Phys. Lett 2007, 91, 031101 [5] Hartschuh, A. Angew. Chemie (Int. Edition) 2008, 47, 8178

Nano- and hetero-structures of graphene nanoplatelet (GnP) and carbon nanotube (CNT) can control significantly piezoelectric and optoelectronic properties for acoustic sensors and actuators, i.e., such as flexible transparent speakers and microphones. Optimized preparing conditions of the specimens were obtained for multi-functional applications by obtaining the best dispersion in the solution and interfacial durability for the long-term use. Optical transmittance and electrical resistance were measured for GnP and CNT dip- and spraying coating on piezoelectric poly(vinylidene fluoride) (PVDF) sheets with elapsing time under cyclic loading. Uniform dip-coating was done using Wilhelmy plate machine due to it simple convenience. Specimen was also additionally applied with spraying coating for comparison. The change in the electrical resistance and optical transmittance of coated layer was mainly dependent upon the number of dip-coating, the concentration of GnP and CNT solutions, and the surface treatment condition. Electric properties of coated layers were measured using four-point method, and surface resistance was calculated using a dual configuration method. Optical transmittance of GnP and CNT coated PVDF sheet was also evaluated using UV spectrum. Surface energy and their hydrophilic and hydrophobic properties of GnP and CNT coated PVDF substrates were investigated by wettability test via static and dynamic contact angle measurements. As the elapsing time of cyclic loading passed and heating condition right after coating, the stability of surface resistance and thus comparative interfacial adhesion between coated layer and PVDF sheet was evaluated by comparing with the thermodynamic work of adhesion, Wa. As dip-coating number increased, surface resistance of GnP and CNT coating decreased, whereas the transmittance decreased step-by-step due to the thicker GnP and CNT networking layer. Nano- and hetero-structural effects of GnP and CNT solution on the optical and acoustic effects were studied. The thin film transducers showed good acoustic response over wide frequency ranges.

We have developed a micromanipulation technique that can be used to transfer and assemble suspended graphene into nanoscale mechanical resonators [1]. This new technique enables us to take advantage of a localized gate in close proximity to the sample, which improves the sensitivity of RF-cavity readout schemes by reducing the parasitic contributions. In our experiments on a series of graphene samples, we obtained resonance frequencies up to 178 MHz, a sensitivity of about 300 fm/Hz^{1/2}, and an effective resonator mass down to ~1 femtogram. The achieved combination of high frequency, high sensitivity, and low mass show that our fabrication technique and RF reflection measurement scheme hold promise for building ultrasensitive NEMS structures. By using symmetrized electrical cavities [2], the sensitivity of our scheme can be further increased and the quantum limit is within reach with cavity-coupled graphene mechanical resonators. Furthermore, we are investigating the interplay of electrical and mechanical degrees of freedom, for example, by detecting shot noise in suspended graphene samples. [1] X. Song et al., submitted to Nano Letters. [2] F. Massel, T. Heikkilä, J.-M. Pirkkalainen, S. Cho, H. Saloniemi, P. Hakonen, and M. Sillanpää, Nature, to appear in Dec 2011.

We report the measurement of electrostatically tuned superconducting transition in samples of graphene decorated with tin nanoparticles. The transition towards superconducting state is due to percolation of superconductivity induced by proximity effect of the nanoparticules random array. Depending of the disorder within the graphene layer, superconductivity show different characteristics and significant variations. In case of low disorder exfoliate graphene the superconducting state results from a Berezinski-Kosterlitz-Thouless transition which leads to an homogeneous state [1]. In case of disordered Graphene (CVD-grown) a superconducting to Insulator transition can be tuned by the gate voltage [2]. An intermediate metallic regime is also present at the transition showing sheet resistivity of the order of the resistance quantum h/4e^2. We interpret this transition within the framework of granular superconductivity found in Josephson junction arrays. The intense positive magnetoresistance observed for elds below the critical field of tin nanoparticles is a signature of the localization of Cooper pairs. [1] B. M. Kessler et al., Phys. Rev. Lett. 104,047001 (2010). [2] A. Allain et al., http://arxiv.org/abs/1109.6910

We report that highly crystalline graphene can be obtained from well controlled surface morphology of the metal substrate. Flat copper surface was prepared by using a chemical mechanical polishing method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. Grain boundaries and intentionally formed scratched area of Cu film play an important role for nucleation seeds. Strong correlation between metal grain size and graphene grain size was also observed. We also propose a simple way of observing graphene grain boundary using optical microscope and correlation to the results from atomic force microscopy and confocal Raman spectroscopy will be also fully discussed in this talk.

Carbon nanotube (CNT) thin films are expected to enable the fabrication of high-performance, flexible, and transparent electronic devices with a relatively simple technique [1,2]. In this work, we report all-carbon thin film transistors (TFTs) and integrated circuits (ICs), in which both active channels and passive elements have been formed by CNT thin film. We fabricated top-gate-type TFTs and ICs on a polyethylene naphthalate (PEN) film substrate. The channel was formed by a single-walled CNT thin film with a linear density of about 0.5 tubes/μm. The electrodes and interconnections also consist of thicker single-walled CNT films (transparency: ~85 % @ 550 nm, sheet resistance: ~150 Ω/sq). A polymethyl methacrylate (PMMA) layer with a thickness of 1,000 nm was used as the gate insulator. The CNT thin films were grown by floating-catalyst chemical vapor deposition technique [3], collected by a membrane filter, and then transferred on the PEN substrate. The PMMA layer was spin-coated on the substrate. The patterning of CNT films and PMMA layer were carried out by conventional photolithography and plasma etching processes. The fabricated devices have the transparency of 80% in average. The TFTs simultaneously demonstrated on/off ratios of ~10⁶ and mobility of 240 cm²V^â?"1s^â?"1 evaluated by Caoâ?Ts rigorous model [4]. An inverter circuit with a gain of 5.5 was also demonstrated at an operation voltage of 5 V. A bending test verified the good flexibility of the all-carbon devices. Acknowledgments: This work was partially supported by â?T08 NEDO Grant, R&D promotion scheme funding international joint research promoted by NICT, ALCA-JST, Grant-in-Aid of MEXT, the Aalto University MIDE program via the CNB-E project, and the Academy of Finland (Pr. No. 128445). We acknowledge Canatu Oy for support to produce transparent SWCNT thin films for electrodes. References [1] D.-M. Sun, et al., Nature Nanotech. 6 (2011) 156. [2] A. Kaskela, et al., Nano Lett. (2010) 10 4349. [3] A. Moisala, et al., Chem. Eng. Sci. 61 (2006) 4393. [4] Q. Cao et al., Appl. Phys. Lett. 90 (2007) 023516.

Graphene is a one-atom-thick planar sheet of a sp2-bonded honeycomb carbon crystal. Its gapless and linear energy spectra of electrons and holes lead to nontrivial features such as negative dynamic conductivity in the terahertz (THz) spectral range. In this presentation recent advances in terahertz light amplification by stimulated emission of radiation in optically pumped graphene are reviewed. When graphene is pumped with an infrared photon energy electrons/holes are photogenerated via interband transitions. In case of room temperature environment and/or strong pumping, collective excitations due to the carrier-carrier (CC) scattering, e.g., intraband plasmons perform ultrafast carrier redistribution along the energy. Then optical phonons (OPs) are emitted by carriers on the high-energy tail of the electron and hole distributions. This energy relaxation process accumulates the nonequilibrium carriers around the Dirac points. Due to a fast intraband relaxation (ps or less) and relatively slow interband recombination of photoelectrons/holes, one can obtain the population inversion under a sufficient pumping intensity. The authors have first analytically found the possibility of THz gain in such a system under a cryogenic condition and have recently numerically verified the occurrence of the THz negative dynamic conductivity even at high temperatures. We measured the carrier relaxation and recombination dynamics in optically pumped graphene using THz time-domain spectroscopy based on an optical pump/THz-and-optical-probe technique. An exfoliated fine monolayer-graphene/SiO2/Si sample having a Dirac voltage close to zero and a surface potential distribution of 4.1 meV in variance was placed on the stage. A 0.12-mm-thick CdTe crystal was placed on the sample, acting as a THz probe pulse emitter as well as an electrooptic sensor. A 80-fs, 1550-nm fiber laser beam having 4-mW average power and 20-MHz repetition was split into two: one for optical pumping and generating the THz probe beam in the CdTe crystal, and one for optical probing. The pumping laser, which was linearly polarized, was simultaneously focused at close to the normal incidence from the back surface on graphene sample and the CdTe. The THz probe beam, being generated in the CdTe and reflecting back in part at the CdTe top surface, stimulates the THz emission in graphene, which is electrooptically detected as a THz photon echo signal. The comparison of the THz photon echo signal measured on SiO2/Si substrate and that one with graphene clearly indicates that graphene sheet act like an amplifying medium. The dependence of the gain spectra on pumping intensity shows a threshold like behavior, testifying the THz light amplification by stimulated emission of radiation in graphene. In conclusion, we have successfully observed coherent amplified stimulated THz emissions in an impulsively pumped exfoliated graphene, which may lead to the development of a new type of THz lasers.

Data communications have been growing at a speed even faster than Mooreâ?Ts Law, with a 44-fold increase expected within the next 10 years. Data Transfer on such scale would have to recruit optical communication technology and inspire new designs of light sources, modulators, and photodetectors. An ideal optical modulator will require high modulation speed, small device footprint and large operating bandwidth. Silicon modulators based on free carrier plasma dispersion effect and compound semiconductors utilizing direct bandgap transition have seen rapid improvement over the past decade. One of the key limitations for using silicon as modulator material is its weak refractive index change, which limits the footprint of silicon Machâ?"Zehnder interferometer modulators to millimeters. Other approaches such as silicon microring modulators reduce the operation wavelength range to around 100 pm and are highly sensitive to typical fabrication tolerances and temperature fluctuations. Growing large, high quality wafers of compound semiconductors, and integrating them on silicon or other substrates is expensive, which also restricts their commercialization. In this work, we demonstrate that graphene can be used as the active media for electroabsorption modulators. By tuning the Fermi energy level of the graphene layer, we induced changes in the absorption coefficient of graphene at communication wavelength and achieve a modulation depth above 3 dB. This integrated device also has the potential of working at high speed.

Low-cost, large-size visible light-emitting diode (LED) arrays with high efficiency and low power consumption are required for mobile display applications. Inorganic LEDs based on nitride semiconductors have many advantages over organic LEDs, showing higher device efficiency and long-term stability. However, problems associated with high-quality nitride film growth on a large-size and cheap substrate such as amorphous silica represent one of the major obstacles to the use of nitride LEDs in display application. In particular, the lack of their epitaxial relationship with a nitride film and an amorphous substrate typically yields a poly-crystalline GaN structure, rough surface morphology and random crystal orientation. To resolve this problem, we developed the method to grow GaN thin films heteroepitaxially on mechanically exfoliated graphene layers [1]. For preparation of large size GaN films, we here employed chemical vapor deposited (CVD) graphene films as a growth template. Since the graphene films can be transferred to any substrate, we here demonstrate growth of GaN films on the graphene films transferred to amorphous fused silica substrates and report the fabrications of visible light-emitting diodes (LEDs) on the substrate. GaN films grown on CVD graphene films exhibit a highly c-axis oriented crystal structure and preferable in-plane orientation. In addition, GaN films show strong stimulated emission even at room temperature. Based on the high quality GaN thin films grown on fused silica with the intermediate graphene film, visible LEDs that emit strong electroluminescence emission under room illumination were also fabricated. [1] K. Chung, C.-H. Lee, G.-C. Yi*, Science 330, 655 (2010).

One of the major roadblocks for incorporating graphene in next generation optoelectronic devices is the large efficiency loss caused by a carrier relaxation time that is fast compared to the electron escape time from the p-n junction. Here we probe the relevant timescales associated with photocurrent generation by optically exciting graphene at a p-n junction defined by electrostatic gates. We resolve the kinetics of both optical absorption and the photocurrent response through simultaneous collection of the transient absorption (Î"A(t,λ)) and the transient photocurrent (Î"I(t)). Time-resolution is obtained by a two-pulse correlation technique using temporally delayed ~180 fs long laser pulses whose wavelength and spatial position can be independently tuned.

In the p-n junction we find transient absorption signal Î"A(t,λ) decays biexponentially at 0.26 and 2.2 ps; timescales similar to those previously established for hot electron and hot optical phonon cooling in graphene sheets. In stark contrast, Î"I(t) decays slowly with tails extending for 100s of ps. We find the simple rate equation (dI/dt=-kI²) fits Î"I(t) consistently well (T < 100K). At 10 K, the extracted rate coefficient (k) is 2.7 ± 0.6 ps^-1nA^-1 and increases monotonically tenfold upon warming to room temperature. We further find the decay rate is independent of both incident laser power and optical excitation wavelength (800 to 1490 nm). Such good agreement with a quadratic rate law is expected if the lifetime of the excited carriers is dominated by an electronic two-body carrier recombination process. Using this rate model we can further extract an effective recombination time (1/kÎ"I_max) which scales inversely with the square root of incident photon flux and ranges from 2 to 14 ps. These recombination timescales are used in conjunction with a measured p-n junction carrier escape time to provide an upper bound estimate for current production efficiency achievable at a given incident laser power.

We can attribute the markedly different transient photocurrent and absorption kinetics to a photocurrent response that is predominately sensitive to recombination processes that are occurring well below optical excitation energies, near the Fermi level. Combining measurements, transient photocurrent alongside the well-studied transient absorption allows us to construct a rough timeline of events associated with photocurrent production at a graphene p-n junction.

Inkjet technology is an extremely effective method for producing next-generation printed and flexible electronics. This technique has the potential to drastically reduce ecological impact, energy consumption during manufacturing, and waste by controlling the quantity and location of ink deposition. In particular, single-walled carbon nanotube thin film transistor (SWCNT-TFT) using inkjet method has been investigated because SWCNT can be applied to inkjet technique and has great potential for applications [1]. Recently, progress in purification techniques enables us to obtain samples of a specific electronic type (metallic, semiconducting) SWCNTs using the density-gradient method, and thin film transistor was reported using high purity s-SWCNT as a channel. The challenge for SWCNT printed electronics, however, is still to achieve high-performance transistor without metallic path. In this work, we found the origin of metallic behavior in separated semiconducting SWCNT thick films and realized very high-performance SWCNT thick-film transistors with mobilities of 30-70 cm^2/Vs and on/off ratios of 10^4-10^5. [1] H.Okimoto, T.Takenobu et al., Advanced Materials 22, 3981 (2010).

Central to many proposed electronic device-based applications of carbon nanotubes (CNTs) is the development of assembly methods that could enable organization of these nanoscale building blocks over large areas with controlled orientation and density. In this talk, we will present a novel solution-based technique that combines spin assembly and molecular patterning for preparing large-scale, ultrathin CNT films with unidirectional alignment and controllable density. We investigated the effects of the hydrodynamic force (spin rate), molecular pattern dimensions, and surface interaction force (CNT-molecules), on the adsorption density and final alignment of CNTs on substrates. And we have achieved the unidirectional assembly of controlled density CNTs over the wafer scale and directly integrated them as thin-film transistors (TFTs) that show promising device characteristics with high drive currents, large on/off current ratio and high mobility. The scalable solution-based process for large-scale assembly of well-aligned CNT thin films can lead to many exciting applications ranging from large-scale, highly sensitive biological/chemical nanosensors to large-area flexible displays.

We report the effect of oligomer sequence length on the gas sensing characteristics of DNA decorated single-walled carbon nanotube (SWNT) sensors. Single-stranded poly-G DNAs with various sequence lengths were decorated on to SWNT sensors and the conductance of the nanosensors in response to methanol and IPA vapors were measured. Single-stranded poly-G DNAs were used since base G, compared with other bases, showed largest affinity to SWNTs. Different lengths of poly-G DNA were shown to enhance the gas sensing response of SWNTs to different extend. We attribute such oligomer sequence length dependent sensing performance to the difference in binding affinities of nucleotides on SWNTs and the conformations that DNA molecules form on SWNTs. We found that devices decorated with 24 bases of single-stranded poly-G DNA exhibit the largest gas sensing response to both methanol and IPA vapors. We also observed that the sensors respond to chemical vapors quickly, in the order of minutes, but takes a longer time to recover to the baseline conductance. This suggests a strong interaction between the chemical analytes and the DNA decorated SWNTs and thus a longer desorption time. The gas sensing responses were reproducible and reversible for repeated air-analyte cycles, suggesting rapid, stable, and long-lasting sensing capability. Our results provide further understanding on the sequence dependence of the DNA decorated SWNT gas sensing responses and will facilitate the development of functionalized SWNT nanosensors for the detection of numerous chemicals and biomolecules.

Aerosol-CVD synthesis of Single Walled Carbon Nanotubes (SWCNTs) allows direct dry processing and utilization of variety deposition methods, while eliminating potentially detrimental liquid cleaning and dispersion steps. Previously we have demonstrated membrane filtration combined with dry transfer to transfer the SWCNT networks to wide range of substrate materials [1] and furthermore fabrication of novel large area freestanding SWCNT networks, which both can be used in wide range of applications [2]. The aerosol-CVD synthesis allows straightforward fabrication of high performance transparent and conductive electrodes with sheet resistance of 84 Ω/sq and transmittance of 90%. However, direct and efficient deposition of highly pure, crystalline and well dispersed SWCNTs to the receiving substrate would be preferred in order to eliminate the additional resource and time consuming steps of membrane filtration and subsequent dry transfer, to achieve ultimate process economy for industrial production of SWCNT based components. We present recent results of two alternative methods for direct deposition of SWCNT materials to variety of substrates, namely thermophoretic and electrostatic deposition methods. The results indicate that by combining either electrically or thermally induced driving force with the drag forces of carrier gas flow in specially designed deposition system, variable degree of bundle alignment can be achieved in direction parallel to the carrier gas flow direction. The deposition systems are studied to determine collection efficiencies and the deposits SWCNT networks are characterized by multimodal approach including statistical analysis of high resolution SEM to study the alignment and film uniformity and high-resolution TEM for bundle diameter analysis, combined with optical spectroscopy and direction sensitive electrical transport characterization to estimate the macroscopic transparent electrode performance. [1] A. Kaskela et al., Nano Lett. 10 (2010) 4349â?"4355. [2] A. G. Nasibulin et al., ACS Nano 5 (2011) 3214â?"3221.

The interactions between lectin and carbohydrates are involved in a wide range of biological processes and are important for bacteria recoginition and detection. Here,we fabricated lectin nanosensors based on single-walled carbon nanotubes (SWNTs). SWNTs and their composites are ideal candidates for chemical and biological sensing application due to their unique electronic, chemical and physical properties. Porphyrin-based glycoconjugates were synthesized through click chemistry and were used to noncovalently functionalize the surface of SWNT-FET. The functionalization with different carbohydrate epitopes enabled the specificity of the devices to different lectins, namely PA-IL, PA-IIL and ConA. Lectins PA-IL and PA-IIL were produced in Pseudomonas aeruginosa, and are specific to galactose and fucose, respectively. ConA was purified from jack-bean, Canavalia ensiformis, and specific to mannose. The changes in FET electrical conductance upon exposure to lectin solutions were measured to probe the interaction between carbohydrates and their specific lectins. The detection limit from SWNT-FET was as low as 2 nM [1]. The dissociation constants Kd were also calculated from the FET data and they were comparable to the literature values. Atomic force microscopy(AFM), Fluorescence microscope and isothermal titration calorimetry(ITC) were performed to prove the detection ability. Recently, graphene also gained more and more attention in bioapplication. In order to explore the difference between SWNT and graphene, we also fabricated sensors based on chemically converted graphene (CCG). The comparison between SWNT-FET and CCG-FET incidated that SWNT-FET is a better sensor for lectin using this noncovalent functionalization method. Pseudo-one-dimensional structure and glycoconjugates surface coverage resulted in the optimal lectin detection from SWNTs. [1]H. Vedala, Y. Chen, S. Cecioni, A. Imberty, S. Vidal, A. Star* Nano Lett. 2011, 11, 170-175

The richness of optical and electronic properties of graphene attracts enormous interest. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential to be in photonics, plasmonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, the absence of a bandgap can be beneficial, and the linear dispersion of the Dirac electrons enables ultra-wide-band tuneability [1]. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Despite being a single atom thick, graphene can be optically visualized [2]. Its transmittance can be expressed in terms of the fine structure constant [3]. The linear dispersion of the Dirac electrons enables broadband applications. Saturable absorption is observed as a consequence of Pauli blocking [4,5]. Chemical and physical treatments enable luminescence [1,6]. Graphene-polymer composites prepared using wet chemistry [4-6] can be integrated in a fiber laser cavity, to generate ultrafast pulses, down to 100fs with up to 1 W average power, and enable broadband tunability [4,5]. Graphene-based mode-locked laser are a near term application for this extraordinary material, and can provide simple, low-cost, and convenient light sources for metrology, sensing, medicine and micromachining. Graphene is an ideal transparent flexible conductor. A flexible electrically switchable smart window will be reported, with over 230 contrast ratio, as well as an electro-tactile screen for mobile phone applications. The optoelectronic properties of graphene can be enhanced by combination with plasmonic nanostructures [7], for example in plasmonic-enhanced photovoltage generation [8] 1. F. Bonaccorso et al. Nature Photonics 4, 611 (2010) 2. C. Casiraghi et al. Nano Lett. 7, 2711 (2007). 3. R. R. Nair et al. Science 320, 1308 (2008). 4. T. Hasan, et al. Adv. Mat. 21,3874 (2009) 5. Z. Sun et al. ACS Nano 4, 803 (2010); Nano Research 3, 653 (2010) 6. T. Gokus et al. ACS nano 3, 3963 (2009) 7. F. Schedin, ACS Nano 4, 5617 (2010). 8. T.J. Echtermeyer et al, Nature Comm 2,458 (2011)

It is of scientific and technological interest to analyze the minimal concentration of carbon nanotubes (CNTs) necessary to form a percolating network. From a practical perspective, CNT networks have been proposed as constituents of thin film transistors for electronics and biosensors, polymer composites for electromagnetic interference shielding, etc., While variability in device characteristics was considered, the widespread unpredictability in the intrinsic geometry, e.g., the length (L) of the CNTs, has not yet been modeled. Such issues with predictability of the geometry are typical of nanostructure synthesis processes and could strongly influence the electrical characteristics and device properties. The prediction of a threshold is also pertinent in the synthesis of CNT based composites, where the cost of the nanostructures is a major factor. We suggest a method for the a priori determination of the electrical percolation threshold in CNT networks, of relevance in electronic devices, polymer composites etc. The variability in the CNT lengths, commonly observed in practical processing and dispersion, was also considered and the resulting probability distribution function determined to be of the Weibull type. Subsequently, the predicted percolation threshold volume fractions for single-walled CNTs, ~ 0.00073, and multi-walled CNTs, ~ 0.0193, were found to be in close correspondence to the experimentally determined values of 0.0011 and 0.0147, respectively.

Metallic carbon nanotubes (CNTs) are potential candidates for an alternative interconnect for Cu because of their larger allowable current density than that of Cu. Also, metallic CNTs are expected to be able to respond to sub-THz signal due to their higher mobility compared to conventional materials. However, a dynamical response of metallic CNTs has hardly been clarified thus far, compared with their static response. Previous theoretical investigations has predicted that the AC response of pristine metallic single-walled carbon nanotubes (SWNTs) changes in sub-THz regime [1], and the sub-THz susceptance behavior correlates with the DC conductance one [2] as predicted in other systems [3,4]. Compared to the AC response of pristine metallic SWNTs, however, our understanding of influence of defects on the AC response is not sufficient in spite of its importance. In this study, we have performed a theoretical analysis of sub-THz AC response of metallic SWNTs with a single atomic vacancy, using the Keldysh nonequilibrium Greenâ?Ts function method with the wide-band limit approximation and the nearest neighbor Ï?-orbital tight binding model [5,6]. We found that the sub-THz susceptance behaves capacitively near the defect level induced by the vacancy when the vacancy is at the center of the tubes. In addition, the inductive-capacitive transition of the susceptance occurs at a certain Fermi level when the Fermi level is varied around the defect level. In the case where the vacancy is off-center, we found that the inductive-capacitive transition appears at a certain Fermi level when a distance between the vacancy and the center of tube is smaller than a certain threshold, while the inductive response does regardless of the Fermi level for the distance larger than the certain threshold. This makes a strong contrast to the vacancy-position-independent DC conductance. The vacancy-position dependence of the susceptance can be understood as a consequence of the breaking of parity symmetry. Furthermore, we examined the diameter dependence of the susceptance in metallic SWNTs with a single atomice vacancy, and found that larger diameter metallic SWNTs behave more capacitively than smaller diameter ones. In the presentation, we will discuss the physical origin of these behaviors in detail. [1] T. Yamamoto et al., Phys. Rev. B 81, 115448 (2010). [2] T. Yamamoto et al., Phys. Rev. B 82, 205404 (2010). [3] M. Buttiker et al., Z. Phys. B 94, 133 (1994). [4] T. Christen et al., Phys. Rev. Lett. 77, 143 (1996). [5] D. Hirai et al., Appl. Phys. Express 4, 075103 (2011). [6] D. Hirai et al., submitted.

Great attention has been paid to phonon transport in single-walled carbon nanotubes (SWNTs) because it exhibits various interesting phenomena originating from the coherency of phonons in quasi-one-dimensional structures. In this presentation, we show simulation results of isotope effects on the coherent phonon transport in disordered SWNTs with 13C or 14C isotopes [1,2]. Our simulation is based on the nonequilibrium Green's function technique [3], which enables us to perform fully quantum-mechanical simulations of the coherent phonon transport in an SWNT in contrast to classical molecular dynamics and semi-classical Boltzmann equation. We found that the phonon transport in the isotope-disordered SWNTs is classified into three regimes depending on the phonon frequency Ï?: the low-Ï? ballistic, the mid-Ï? diffusive, and the high-Ï? localized regimes. In the ballistic regime, the isotope-disordered SWNTs exhibit the quantization of thermal conductance even in the presence of isotopes. In the diffusive regime, the phonon transmission fluctuation is always 0.35 ± 0.02, irrespective of the average phonon transmission, the tube chirality, and the concentrations and masses of isotopes. In other words, the transmission histogram has a Gaussian distribution function with the universal standard deviation. In the localized regime, the phonon transmission probability goes to zero exponentially with increasing the tube length. We also found that the histogram has a lognormal distribution function. It is known that the phonon localization in isotope-disordered SWNTs is difficult to observe in thermal transport experiments [1]. In this presentation, we propose that the phonon localization can be observed by illuminating a femtosecond light pulse on isotope-disordered SWNTs. [1] I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008). [2] T. Yamamoto, K. Sasaoka, S. Watanabe, Phys. Rev. Lett. 106, 215503 (2011). [3] T. Yamamoto and K. Watanabe, Phys. Rev. Lett. 96, 255503 (2006).

The remarkable properties of carbon nanotubes (CNTs) have allowed the proposition, design, and exploitation of nanomechanical devices, such as, CNT-based oscillators [1]. In their simplest form, these oscillators consist of a double-walled CNT where the inner tube can move inside the outer tube in an oscillatory low-friction motion of high frequency [1]. This motion is induced and maintained by van der Waals interactions between the inner and the outer tube. Key issues regarding these devices are how to initialize them and how to sustain the motion in a controllable way. Different strategies have been proposed to start the oscillatory motion, e.g., by applying magnetic and electrical fields, by accelerating encapsulated charged elements located inside the inner tube, and by thermal expansion of encapsulated gases. In this work we propose an alternative way to start, maintain, and control the oscillatory motion, based on thermophoresis, i.e., the use of thermal gradients to induce mass transport. In this case, the thermal gradient is imposed on the outer tube by external agents such as electrical currents. This approach was inspired on the recent findings that demonstrated the possibility of producing nanoscale thermal motors based on CNTs [2] Numerical calculations and classical molecular dynamics simulations were used to investigate the behavior of a nanodevice based upon double-walled carbon nanotube oscillators. The nanodevice is driven by periodically applied thermal gradients (â^¼10 K/nm). Our results indicate that thermophoresis can be effective to initiate the oscillator and that suitable heat pulses may provide an appropriate way to tune its behavior. Sustained regular oscillatory as well as chaotic motions were observed [3]. [1] J. Cumings, A. Zettl, Science v289, 602 (2000). [2] A. Barreiro, R. Rurali, E. R. Hernandez, J. Moser, T. Pichler, L. Forro, A. Bachtold, Science v320, 775 (2008). [3] V. R. Coluci, V. S. Timóteo, D. S. Galvão, Appl. Phys. Lett. v95, 253103. (2009)

Graphene is a promising material for transparent conductive films (TCFs) of next generation solar cells because of their high electrical conductivity and transparency in a wide wavelength range. We have developed an original chemical doping method for chemically derived graphene from conjugated organic conductors. However, deposition method of chemically derived graphene onto substrates is challenging issue because poor stacking of graphene flakes degenerate electrical conductivity of obtained films. In this study, we describe a new deposition method of chemically derived graphene based on layer-by-layer (LBL) assembly technique. LBL assembly of chemically derived graphene was utilized to fabricate uniform and better stacked graphene based thin film in a scalable and highly reproducible way. Synthesis of amine-functionalized graphene, showing positive charge in aqueous solution, was intended for its utilization in LBL assembly method. Negatively charged graphene oxide was aminated by ethylenediamine (EDA). Based on the results of x-ray photoelectron spectroscopy and Fourier-transform infrared absorption spectroscopy analysis, it can be said that EDA was successfully introduced to amide groups attached to the edge of graphene plane. Actually zeta potential measurement revealed that amine-functionalized graphene indicated positive charge in aqueous solution. LBL-assembled graphene films were fabricated by alternate immersion of substrates into a negative graphene oxide solution and a positive amine-functionalized graphene solution. It was found that optical transmittance and sheet resistance of the film decreases with an increase in number of LBL cycles in a reproducible way. The sheet resistance of LBL-assembled graphene films decreases by an order of magnitude at the same optical transparency. The estimated Hall carrier mobilities were 2.2 and 57.5 cm^2/Vs for spray-coated film and LBL-assembled film, respectively. This improvement might be derived from decrease in inter-layer resistance due to the electrostatic attraction among graphene layers, though the sheet carrier concentration was slightly decreased from 13.95 x 10^11 to 3.81 x 10^11 1/cm^2. The shorter inter-layer distances estimated by atomic force microscopic analysis supported the better stacking of LBL-assembled graphene films. The sheet resistance of resulting LBL-assembled graphene film was 24 kΩ/sq. at optical transmittance of around 80% with light wavelength of 550 nm. Moreover, we demonstrated our chemical doping method could be incorporated with this LBL assembly technique due to the negatively charged nature of doped graphene flakes. Although the opto-electrical characteristics of our graphene-based TCFs are not enough for real application to transparent conductive films at present, it was demonstrated that our LBL-assembly technique had a huge potential for graphene-based TCFs of next generation solar cells.

Approaches to effectively process carbon nanotubes and graphene are important for their application. In this paper, we report the gel formation of carbon nanotubes and graphene with non-ionic surfactants and fabrication of carbon nanotube, graphene and carbon nanotube/graphene composite films by solvent-free gel coating. The transparency of these films can be tuned by manipulating their thickness. Transparent thin films are obtained and their application to replace indium tin oxide (ITO) as the transparent electrode of polymer solar cells is demonstrated. These films can also be used as the catalytic electrode in electrochemical devices. They are used as the catalytic counter electrode of high-performance dye-sensitized solar cells.

Recently graphene field effect transistors (GFETs) have been popular for possible sensing applications. Many studies have been reported showing the high sensitivity of sensors resulting from the intrinsic properties of graphene, in applications such as pH, DNA, and vapor sensors, among others. However, the sensitivity of GFETs has in some cases proven to be inferior to that of other sensors, instabilities and unreliability of the device has been seen in other cases, and low electrical responses for intrinsic graphene devices in certain examples. It would be highly desirable to develop GFET sensors with higher sensitivities and stabilities. We were motivated in this work by the fact that the hexagonal structure of BCP could possibly result in heterogeneous patterns with nanoscale spacings that may offer new opportunities for achieving a selective sensing function. A sensor was fabricated with patterned graphene which was synthesized with chemical vapor deposition method. It showed different responses to different organic vapors. Detection limit for ethylenediamine, aniline and benzene varies from sub ppm to hundreds of ppm. The time is on the order of seconds for the detection response and minutes for the recovery. Further functionalization the sensor with block co-polymer improves the selectivity of the sensor.

Graphene with two-dimensional honeycomb structure attracted huge attention due to its high conductivity, high transparency, and flexibility. Also, the adsorption/desorption characteristics of the molecules on graphene showed a great potential for ultra-sensitive chemical sensors based on graphene. In our experiments, we demonstrated flexible graphene NO2 chemical sensors by using graphene devices on flexible PET substrate. Also, we compared the sensitivity between strained and relaxed graphene chemical sensors including the fatigue property. Monolayer Graphene was grown on Cu-foil by chemical vapor deposition method. The thickness and quality was characterized by micro-Raman spectroscopy, which confirmed high quality monolayer graphene according to G/2D-peak ratio and very weak D-peak. Graphene was transferred to PET substrate, which was pre-defined with Au patterns sputtered with shadow mask. When the graphene on PET substrate were strained by 7.9 %, the currents decreased, compared with relaxed state. The concentration of NO2 gas was 200 ppm. The sensing mechanism is the charge transfer between molecules (NO2) and graphene. Electron-acceptor such as NO2 can make graphene p-type. When graphene chemical sensor on PET substrate was exposed to NO2 gas, the currents were increased by 26 % due to increased doping. We observed the good sensitivity from both strained and relaxed graphene gas sensors. The details including the fabrication process, sorption mechanism and the response / recovery time will be presented.

Several promising advances have been made using nanoscale 1-D materials such as nanowires and carbon nanotubes as the gate electrode for biologically-active field effect transistors (BioFETs). Recently, we have demonstrated exciting advantages that 2-D chemically modified graphene (CMG) brings to BioFET construction. Grapheneâ?Ts most significant property is the inherent presence of a highly mobile, two-dimensional layer of electron gas near or at its surface that is extremely sensitive to local charge changes. This characteristic, along with grapheneâ?Ts excellent mechanical properties, has guided us to unique modification strategies that offer the potential for label-free, real-time monitoring of biological interactionsâ?"a longstanding goal in the biosensor community. We will briefly describe how the CMG film is modified to accept biological materials such as DNA and proteins, as well as the implementation of this hybrid material as the gate electrode of a field effect transistor. In operation, any protein binding or DNA hybridization at the modified graphene surface can be detected by the disruption of the flow of electron gas due to the charge changes associated with these binding events. Successful results for the detection of specific DNA hybridization at nM concentrations and insensitivity to non-complementary DNA at µM concentrations have been achieved. Acknowledgements: Support for this project was received from the Office of Naval Research and the Defense Threat Reduction Agency-Joint Science and Technology Office for Chemical and Biological Defense.

The development of the future generation of neuroprosthetic devices will require the advancement of novel solid-state sensors with a further improvement in the signal detection capability, a superior stability in biological environments, and a more suitable compatibility with living tissue. In this respect, the outstanding electronic and electrochemical performance of graphene holds great promise for bioelectronic applications [1]. In this work, we report on arrays of graphene solution-gated field effect transistors (G-SGFETs) which can detect the electrical activity of electrogenic cells. G-SGFETs were fabricated using large area CVD-grown graphene films, which were deposited on Cu and then transferred to insulating substrates where arrays of transistors were processed. The high carrier mobilities in graphene, together with the large interfacial capacitance at the graphene/electrolyte interface, lead to very high transconductive sensitivities [2]. Furthermore, G-SGFETs were found to exhibit very low electronic noise, with an RMS value that is equivalent to gate signals of less than 8 µV [3]. The biocompatibility of graphene was investigated by culturing pure retinal ganglion cells from postnatal rats on bare and modified CVD-grown graphene layers. Retinal neurons on graphene surfaces show a healthy growth comparable to standard glass substrates. In this contribution we will also demonstrate the ability of graphene SGFETs to transduce the electrical activity of electrogenic cells. For instance, the generation and propagation of action potentials in cultures of cardiomyocyte-like HL-1 cells will be reported [3]. Further, single cell-transistor coupling will be shown using Human Embryonic Kidney (HEK) cells, in which the response of the graphene FETs to the electrical activity as well as the cell chemical activity will be discussed. Our results confirm that graphene SGFETs can outperform state-of-the-art devices for bioelectronic applications. [1] M. Dankerl et al., Adv. Funct. Mater. 20, 3117 (2010). [2] L. H. Hess et al., Appl. Phys. Lett. 99, 033503 (2011). [3] L. H. Hess et al., Adv. Mater. DOI: 10.1002/adma.201102990

Here we describe the fabrication of artificial lipid bilayer membranes on graphene oxide (GO) and reduced GO (r-GO), as a fundamental platform for biosensing in plasma membrane models using graphene-based techniques. Graphene and its derivatives including GO exhibit the intriguing property of fluorescence energy transfer from fluorescence probes (e.g. dyes, quantum dots, and polymers), which results in fluorescence quenching independent of the emission wavelength of the probes. Recently, the fluorescence quenching properties of graphene and GO were exploited for the highly sensitive detection of biomolecules. Lipid bilayers are the fundamental structure of plasma membranes, and provide reaction fields for the transportation of small molecules, ions, and signals into and out of cells. The behavior of lipids and proteins in plasma membranes is related to a variety of biological reactions and diseases. Supported lipid bilayers (SLBs), artificial bilayer membranes at solid-liquid interfaces, have been investigated as cell membrane models to study the physicochemical properties of lipid bilayers. GO and r-GO exhibit amphiphilicity because they consist of hydrophilic sp³-carbon regions including â?"OH, â?"Oâ?", and â?"COOH groups, and of hydrophobic sp²-carbon regions. A lipid bilayer is a self-assembled structure of amphiphilic lipid molecules, thus the behavior of lipid bilayers on the amphiphilic GO and r-GO surfaces is an important subject for the reproducible fabrication of the SLBs on graphene-related materials. In our research, GO flakes prepared by chemical exfoliation were deposited on thermally oxidized SiO₂/Si substrates, and the GO/SiO₂/Si substrates were incubated in a vesicle suspension of phospholipid (dioleoylphosphatidylcholine: DOPC). AFM topographs showed that planar membranes formed on the GO flakes as well as the bare SiO₂ regions in the presence of Ca²⁺ in the buffer solution, while the GO surface was mainly covered with adhesive vesicular layers in the absence of Ca²⁺. We found two types of the GO regions to be 1.6 nm and 5.6 nm higher than the SiO₂ regions, which were covered with single SLB, and attributed them to single and double SLBs, respectively. Fluorescence recovery after photobleaching (FRAP) measurement showed the SLBs on GO to retain their fluidity, and the SLBs on the GO and SiO₂ regions to be continuous. The behavior of the vesicle drastically changed on the r-GO surface. The hydrophobic r-GO surface was covered with a DOPC monolayer with its hydrocarbon chains on r-GO, and a DOPC bilayer stacked on the DOPC monolayer. FRAP results showed that the DOPC bilayer on the DOPC monolayer/r-GO was fluid, and continuous with the surrounding DOPC-SLB on the bare SiO₂ surfaces. We expect that the lipids assemble similarly on the graphene sheets prepared by mechanical exfoliation or CVD, because the sp² carbon region is dominant in r-GO.