Lewis Gomez De Arco

Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor Deposition for Organic Photovoltaics

We report the implementation of continuous, highly flexible, and transparent graphene films obtained by chemical vapor deposition (CVD) as transparent conductive electrodes (TCE) in organic photovoltaic cells. Graphene films were synthesized by CVD, transferred to transparent substrates, and evaluated in organic solar cell heterojunctions (TCE/poly-3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper phthalocyanine/fullerene/bathocuproine/aluminum). Key to our success is the continuous nature of the CVD graphene films, which led to minimal surface roughness ( approximately 0.9 nm) and offered sheet resistance down to 230 Omega/sq (at 72% transparency), much lower than stacked graphene flakes at similar transparency. In addition, solar cells with CVD graphene and indium tin oxide (ITO) electrodes were fabricated side-by-side on flexible polyethylene terephthalate (PET) substrates and were confirmed to offer comparable performance, with power conversion efficiencies (eta) of 1.18 and 1.27%, respectively. Furthermore, CVD graphene solar cells demonstrated outstanding capability to operate under bending conditions up to 138 degrees , whereas the ITO-based devices displayed cracks and irreversible failure under bending of 60 degrees . Our work indicates the great potential of CVD graphene films for flexible photovoltaic applications.

Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display Applications

Preseparated, semiconductive enriched carbon nanotubes hold great potential for thin-film transistors and display applications due to their high mobility, high percentage of semiconductive nanotubes, and room-temperature processing compatibility. Here in this paper, we report our progress on wafer-scale processing of separated nanotube thin-film transistors (SN-TFTs) for display applications, including key technology components such as wafer-scale assembly of high-density, uniform separated nanotube networks, high-yield fabrication of devices with superior performance, and demonstration of organic light-emitting diode (OLED) switching controlled by a SN-TFT. On the basis of separated nanotubes with 95% semiconductive nanotubes, we have achieved solution-based assembly of separated nanotube thin films on complete 3 in. Si/SiO(2) wafers, and further carried out wafer-scale fabrication to produce transistors with high yield (>98%), small sheet resistance ( approximately 25 kOmega/sq), high current density ( approximately 10 microA/microm), and superior mobility ( approximately 52 cm(2) V(-1) s(-1)). Moreover, on/off ratios of >10(4) are achieved in devices with channel length L > 20 microm. In addition, OLED control circuit has been demonstrated with the SN-TFT, and the modulation in the output light intensity exceeds 10(4). Our approach can be easily scaled to large areas and could serve as critical foundation for future nanotube-based display electronics.

2,4,6-Trinitrotoluene (TNT) Chemical Sensing Based on Aligned Single-Walled Carbon Nanotubes and ZnO Nanowires

2010 WILEY-VCH Verlag Gmb Chemical sensors based on one-dimensional (1D) nanostructures have attracted a great deal of attention because of their exquisite sensitivity and fast response to the surrounding environment. In addition, both carbon nanotubes and metal oxide nanowires are promising candidates for building an electronic nose (e-nose) system. Among these materials, semiconductor single-walled carbon nanotubes (SWNTs) are molecular-scale wires composed entirely of surface atoms, which should be ideal for the direct electrical detection and are expected to exhibit excellent sensitivity to surrounding chemical and biological species. Kong et al. initially utilized SWNT field-effect transistors (FETs) to detect nitrogen dioxide (NO2) and ammonia (NH3), and demonstrated a detection limit of 2 ppm for NO2 and 0.1% for NH3. [11] Subsequently, such SWNT-based chemical sensors have been applied to detect a wide variety of chemicals and the detection limits have been significantly improved. Qi et al. fabricated large arrays of functionalized SWNTsensors, and the detection limit of NO2 was lowered to 100 ppt. [12] In addition, metal oxide nanowires have been widely studied and demonstrated with great potential for chemical sensing applications. Recently, due to the threat of terrorism and the need for homeland security, significant progress has been achieved in the detection of both explosives and nerve agents, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), hexogen (DRX), and dimethyl methylphosphonate (DMMP). One of the leading candidates is 1D nanostructure-based chemoresistors or FETs. Snow et al. and Wang et al. have reported the detection of DMMP at ppb level by using SWNT and SnO2 nanowire-based chemical sensors, respectively. However, to our knowledge, there were only a few reports on the use of 1D nanostructure-based chemoresistors and FETs for detecting explosives, and the detection mechanism is still unclear. In addition, electronic devices fabricated on mechanically flexible substrates have recently attracted enormous attention, due to the proliferation of handheld applications in portable electronics, aerospace science, and civil engineering. Currently, conventional microfabrication techniques or printing methods can be applied to SWNTs on plastic substrates to form devices, allowing inexpensive mass-production and conformable electronics. In this paper, we report the transfer of aligned semiconductor SWNTs onto cloth fabric and successful fabrication of flexible SWNTchemical sensors, which have great potential for wearable electronics. These SWNT chemical sensors exhibited good sensitivity of trace chemical vapors, including 8 ppb TNT and 40 ppb NO2, at room temperature. Besides, to realize the concept of an electronic nose (e-nose) system for explosives, we also fabricated ZnO nanowire-based chemical sensors, which showed a detection limit of 60 ppb for TNT molecules at room temperature. To our knowledge, this is the first TNT sensor built on the basis of metal oxide nanowires. In addition, the detection limit of our chemical sensors is close to the limit of 1.5 ppb TNT set by the U.S. Occupational Safety and Health Administration. The flexible TNT sensors can find immediate applications in systems that demand mechanical flexibility, light weight, and high sensitivity. The fabrication of flexible SWNTchemical sensors started with the synthesis of SWNTs on quartz substrates using a chemical vapor deposition (CVD) method, which have been reported by us and other groups. After growth, we adapted a facile method to transfer the aligned nanotubes from the original substrate to fabric. In brief, a 100-nm-thick gold film was first deposited on the original substrate with aligned SWNTs, followed by applying a thermal tape to peel off the gold film and nanotubes from the growth substrate. The gold film with SWNTs on the thermal tape were pressed against a piece of textile fabric, which was pre-coated with polyethylene at elevated temperature and then transferred from thermal tape onto textile fabric, which had a 50-nm Ti layer as back-gate electrode and 2-mm-thick SU-8 as gate dielectric layer. The thermal tape was released, and KI/I2 gold etchant was then applied to remove gold films. Finally, Ti (0.5 nm) and Pd (40 nm) were deposited on the transferred SWNTs as source/drain electrodes. A schematic diagram of a flexible SWNT chemical sensor is shown in Figure 1a. Figure 1b shows an optical photograph of flexible aligned SWNT FETs on a textile fabric. It can be clearly seen from the SEM image (right) that the nanotubes bridge the two electrodes. Figure 1c displays the current–gate-voltage (I–Vg) characteristics of a typical flexible transistor on fabric before and after electrical breakdown. The device showed significant improvement for the

Resonant micro-Raman spectroscopy of aligned single-walled carbon nanotubes on a-plane sapphire

Resonant micro-Raman spectroscopy was employed to characterize aligned single-walled carbon nanotubes grown on a-plane sapphire to address the alignment mechanism, the metal-to-semiconductor ratio, and the substrate surface influence in nanotube alignment and straightness. Nanotubes aligned predominantly following the [11¯00] direction on the a-plane instead of the atomic step direction. Detailed analysis of radial breathing mode (RBM) and G bands revealed a metallic to semiconducting nanotube ratio of 1:2.6. Improved straightness of nanotubes grown on annealed substrates was attributed to stronger nanotube-substrate interaction along specific lattice directions during growth and confirmed by G′ band broadening and damping of the RBM band.

Large Scale Graphene by Chemical Vapor Deposition: Synthesis, Characterization and Applications

Faster and smaller computers, smarter medicaments, ultrasensitive sensors, dreams of a new generation of products that are increasingly cleaner, lighter, stronger and more efficient; those are aspects that represent the aspirations of a great part of human kind that ever more strives for better technologies. Interestingly, the concept of nanotechnology is at the center of this discussion. Nanotechnology has become instrumental on finding pathways to arrive to processes and products that are not only needed today but will become essential in the future. Nanotechnology can be defined as the understanding and manipulation of matter with at least one dimension of the order of 1 to 100 nanometers, where unique phenomena enable novel applications. For example, whereas elemental carbon is a poor conductor of electricity and not particularly strong, the two-dimensional carbon is a semimetal that exhibits high charge carrier mobility, obeying the laws of relativistic rather than regular quantum mechanics. Furthermore, one-dimensional carbon has mechanical strength 100 times higher than steel, exhibiting either metallic or semiconducting properties depending on their chiral atomic arrangement. Two principal factors cause nanomaterials properties to differ significantly from bulk materials: increased relative surface area, which can change or enhance chemical reactivity (Arenz, Mayrhofer et al. 2005); and quantum effects that can affect the material optical, magnetic and electrical properties (Yu, Li et al. 2003). It is precisely the collection of new and surprising properties of nanomaterials what has motivated the scientific and engineering community to invest a tremendous share of effort towards a better understanding of their physical and chemical properties; as well as finding controllable synthesis and accurate characterization techniques. Graphene sheets are one-atom thick, 2D layers of sp2-bonded carbon. It is interesting that carbon with sp2 hybridization is able to form the two-dimensional graphene, the planar local structure in the closed polyhedral of the fullerene family and the cylinder-shaped carbon nanotubes, all with different physical properties (see table 1). Thus, keeping the sp2 hybridization, the 2D carbon can be wrapped up into 0D fullerenes, rolled into 1D nanotubes, or stacked into 3D graphite. Carbon has four electrons in its valence level with a configuration of 2s22p2. The hexagonal structure of graphene poses an alternate double bond arrangement that makes it perfectly conjugated in sp2 hybridization. In this case its px and py orbitals contain one electron each, and the remaining pz has only one electron. This pz orbital overlaps with the pz orbital of a