Showing papers on "Thin film published in 2008"

Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material

Abstract: The integration of novel materials such as single-walled carbon nanotubes and nanowires into devices has been challenging, but developments in transfer printing and solution-based methods now allow these materials to be incorporated into large-area electronics1,2,3,4,5,6. Similar efforts are now being devoted to making the integration of graphene into devices technologically feasible7,8,9,10. Here, we report a solution-based method that allows uniform and controllable deposition of reduced graphene oxide thin films with thicknesses ranging from a single monolayer to several layers over large areas. The opto-electronic properties can thus be tuned over several orders of magnitude, making them potentially useful for flexible and transparent semiconductors or semi-metals. The thinnest films exhibit graphene-like ambipolar transistor characteristics, whereas thicker films behave as graphite-like semi-metals. Collectively, our deposition method could represent a route for translating the interesting fundamental properties of graphene into technologically viable devices.

Advanced carbon electrode materials for molecular electrochemistry.

Abstract: 3.6.1. Polishing and Cleaning 2663 3.6.2. Vacuum and Heat Treatments 2664 3.6.3. Carbon Electrode Activation 2665 3.7. Summary and Generalizations 2666 4. Advanced Carbon Electrode Materials 2666 4.1. Microfabricated Carbon Thin Films 2666 4.2. Boron-Doped Diamond for Electrochemistry 2668 4.3. Fibers and Nanotubes 2669 4.4. Carbon Composite Electrodes 2674 5. Carbon Surface Modification 2675 5.1. Diazonium Ion Reduction 2675 5.2. Thermal and Photochemical Modifications 2679 5.3. Amine and Carboxylate Oxidation 2680 5.4. Modification by “Click” Chemistry 2681 6. Synopsis and Outlook 2681 7. Acknowledgments 2682 8. References 2682

Solution-processed metal nanowire mesh transparent electrodes.

Abstract: Transparent conductive electrodes are important components of thin-film solar cells, light-emitting diodes, and many display technologies. Doped metal oxides are commonly used, but their optical transparency is limited for films with a low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates, are costly, and require a high-temperature step for the best performance. We demonstrate solution-processed transparent electrodes consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal-oxide thin films for the same sheet resistance. Organic solar cells deposited on these electrodes show a performance equivalent to that of devices based on a conventional metal-oxide transparent electrode.

Graphene segregated on Ni surfaces and transferred to insulators

Abstract: We report an approach to synthesize high quality graphene by surface segregation and substrate transfer. Graphene was segregated from Ni surface under the ambient pressure by dissolving carbon in Ni at high temperatures followed by cooling down with various rates. Different cooling rates led to different segregation behaviors, strongly affecting the thickness and quality of the graphene films. Electron microscopy and Raman spectroscopy indicated that the graphene films synthesized with medium cooling rates have high quality crystalline structure and well-controlled thicknesses. The graphene films were transferred to insulating substrates by wet etching and found to maintain their high quality.

Thin film transitor substrate and method of manufacturing the same

Abstract: A method of manufacturing a thin film transistor (“TFT”) substrate includes forming a first conductive pattern group including a gate electrode on a substrate, forming a gate insulating layer on the first conductive pattern group, forming a semiconductor layer and an ohmic contact layer on the gate insulating layer by patterning an amorphous silicon layer and an oxide semiconductor layer, forming a second conductive pattern group including a source electrode and a drain electrode on the ohmic contact layer by patterning a data metal layer, forming a protection layer including a contact hole on the second conductive pattern group, and forming a pixel electrode on the contact hole of the protection layer. The TFT substrate including the ohmic contact layer formed of an oxide semiconductor is further provided.

Schottky Solar Cells Based on Colloidal Nanocrystal Films

Abstract: We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm -2 ) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport.

Silicon nanowire radial p-n junction solar cells.

Abstract: We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n−p core−shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.

Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells

Abstract: We demonstrate that subwavelength scatterers can couple sunlight into guided modes in thin film Si and GaAs plasmonic solar cells whose back interface is coated with a corrugated metal film. Using numerical simulations, we find that incoupling of sunlight is remarkably insensitive to incident angle, and that the spectral features of the coupling efficiency originate from several different resonant phenomena. The incoupling cross section can be spectrally tuned and enhanced through modification of the scatterer shape, semiconductor film thickness, and materials choice. We demonstrate that, for example, a single 100 nm wide groove under a 200 nm Si thin film can enhance absorption by a factor of 2.5 over a 10 μm area for the portion of the solar spectrum near the Si band gap. These findings show promise for the design of ultrathin solar cells that exhibit enhanced absorption.

Catalyst-Free Efficient Growth, Orientation and Biosensing Properties of Multilayer Graphene Nanoflake Films with Sharp Edge Planes**

Abstract: We report a novel microwave plasma enhanced chemical vapor deposition strategy for the efficient synthesis of multilayer graphene nanoflake films (MGNFs) on Si substrates The constituent graphene nanoflakes have a highly graphitized knife-edge structure with a 2-3 nm thick sharp edge and show a preferred vertical orientation with respect to the Si substrate as established by near-edge X-ray absorption fine structure spectroscopy The growth rate is approximately 16 mu m min(-1), which is 10 times faster than the previously reported best value The MGNFs are shown to demonstrate fast electron-transfer (ET) kinetics for the Fe(CN)(6)(3-/4-) redox system and excellent electrocatalytic activity for simultaneously determining dopamine (DA), ascorbic acid (AA) and uric acid (UA) Their biosensing DA performance in the presence of common interfering agents AA and UA is superior to other bare solid-state electrodes and is comparable only to that of edge plane pyrolytic graphite Our work here, establishes that the abundance of graphitic edge planes/defects are essentially responsible for the fast ET kinetics, active electrocatalytic and biosensing properties This novel edge-plane-based electrochemical platform with the high surface area and electrocatalytic activity offers great promise for creating a revolutionary new class of nanostructured electrodes for biosensing, biofuel cells and energy-conversion applications

Oxidation of Polycrystalline Copper Thin Films at Ambient Conditions

Abstract: Qualitative and quantitative studies of the oxidation of polycrystalline copper (Cu) thin films upon exposure to ambient air conditions for long periods (on the order of several months) are reported in this work. Thin films of Cu, prepared by thermal evaporation, were analyzed by means of X-ray photoelectron spectroscopy (XPS) to gain an understanding on the growth mechanism of the surface oxide layer. Analysis of high-resolution Cu LMM, Cu2p3/2, and O1s spectra was used to follow the time dependence of individual oxide overlayer thicknesses as well as the overall oxide composite thickness. Transmission electron microscopy (TEM) and spectroscopic ellipsometry (SE) were used to confirm the results obtained from XPS measurements. Three main stages of copper oxide growth were observed: (a) the formation of a Cu2O layer, most likely due to Cu metal ionic transport toward the oxide−oxygen interface, (b) the formation of a Cu(OH)2 metastable overlayer, due to the interactions of Cu ions with hydroxyl groups pr...

Measuring the Surface Dynamics of Glassy Polymers

Abstract: The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk α relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk α relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.

Direct measurement of critical Casimir forces

Abstract: When fluctuating fields are confined between two surfaces, long-range forces arise. A famous example is the quantum-electrodynamical Casimir force that results from zero-point vacuum fluctuations confined between two conducting metal plates1. A thermodynamic analogue is the critical Casimir force: it acts between surfaces immersed in a binary liquid mixture close to its critical point and arises from the confinement of concentration fluctuations within the thin film of fluid separating the surfaces2. So far, all experimental evidence for the existence of this effect has been indirect3,4,5. Here we report the direct measurement of critical Casimir force between a single colloidal sphere and a flat silica surface immersed in a mixture of water and 2,6-lutidine near its critical point. We use total internal reflection microscopy to determine in situ the forces between the sphere and the surface, with femtonewton resolution6. Depending on whether the adsorption preferences of the sphere and the surface for water and 2,6-lutidine are identical or opposite, we measure attractive and repulsive forces, respectively, that agree quantitatively with theoretical predictions and exhibit exquisite dependence on the temperature of the system. We expect that these features of critical Casimir forces may result in novel uses of colloids as model systems.

Flexible, stretchable, transparent carbon nanotube thin film loudspeakers.

Abstract: We found that very thin carbon nanotube films, once fed by sound frequency electric currents, could emit loud sounds. This phenomenon could be attributed to a thermoacoustic effect. The ultra small heat capacity per unit area of carbon nanotube thin films leads to a wide frequency response range and a high sound pressure level. On the basis of this finding, we made practical carbon nanotube thin film loudspeakers, which possess the merits of nanometer thickness and are transparent, flexible, stretchable, and magnet-free. Such a single-element thin film loudspeaker can be tailored into any shape and size, freestanding or on any insulating surfaces, which could open up new applications of and approaches to manufacturing loudspeakers and other acoustic devices.

Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics

Abstract: Plasmon-active silver nanoparticle layers were included in solution-processed bulk-heterojunction solar cells. Nanoparticle layers were fabricated using vapor-phase deposition on indium tin oxide electrodes. Owing to the increase in optical electrical field inside the photoactive layer, the inclusion of such particle films lead to increased optical absorption and consequently increased photoconversion at solar-conversion relevant wavelengths. The resulting solar energy conversion efficiency for a bulk heterojunction photovoltaic device of poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester was found to increase from 1.3%±0.2% to 2.2%±0.1% for devices employing thin plasmon-active layers. Based on six measurements, the improvement factor of 1.7 was demonstrated to be statistically significant.

Oxide ultra-thin films on metals: new materials for the design of supported metal catalysts

Abstract: Ultrathin oxide films on metals offer new opportunities for the design of supported nanoclusters with potential use in catalysis. This requires a characterization at the atomistic level of the structure and composition of the thin film, of its morphology and defect structure. A proper selection of metal/oxide interface, film thickness, lattice mismatch, etc. makes it possible to prepare collections of supported metal particles with novel properties. This critical review describes some illustrative examples, emphasizes the role of the interplay between theory and experiment, and relates some recent findings related to the possibility to control the charge state of a supported nanoparticle on an ultrathin oxide film (211 references).

Pt-Doped α-Fe2O3 Thin Films Active for Photoelectrochemical Water Splitting

Abstract: In an attempt to improve photoelectrochemical (PEC) processes that is important in solar-to-chemical energy conversion, various materials have been proposed that have the potential for hydrogen production. For instance, hematite has many advantages for hydrogen production since it is somehow stable, has a relatively narrow bandgap, inexpensive, abundant and environmentally responsible. However, their use in PEC devices have been limited by several factors like poor conductivity and high electron-hole pair recombination rates. Strategies to overcome this limitation have been produced such as designing the hematite structure to permit more efficient transport and collection of photogenerated charge carriers. Others include the addition of surface electrocatalysts on iron oxide photoelectrodes, and doping the iron oxide with heteroatoms. Dopant species have also been introduced in the literature. However, there have been few reports on the synthesis of hematite films by electrodeposition and virtually no reports of electrodeposition of doped hematite. As such, an electrochemical route to prepare thin film photoanodes of nanocrystalline Pt-doped iron oxide has been proposed. It showed improvements in the performance of PEC when compared to pure iron oxide thin films.

Fabrication of “Roll-off” and “Sticky” Superhydrophobic Cellulose Surfaces via Plasma Processing

Abstract: Most of the artificial superhydrophobic surfaces that have been fabricated to date are not biodegradable, renewable, or mechanically flexible and are often expensive, which limits their potential applications. In contrast, cellulose, a biodegradable, renewable, flexible, inexpensive, biopolymer which is abundantly present in nature, satisfies all the above requirements, but it is not superhydrophobic. Superhydrophobicity on cellulose paper was obtained by domain-selective etching of amorphous portions of the cellulose in an oxygen plasma and subsequently coating the etched surface with a thin fluorocarbon film deposited via plasma-enhanced chemical vapor deposition using pentafluoroethane as a precursor. Variation of plasma treatment yielded two types of superhydrophobicity : "roll-off" (contact angle (CA), 166.7 degrees +/- 0.9 degrees ; CA hysteresis, 3.4 degrees +/- 0.1 degrees ) and "sticky" (CA, 144.8 degrees +/- 5.7 degrees ; CA hysteresis, 79.1 degrees +/- 15.8 degrees ) near superhydrophobicity. The nanometer scale roughness obtained by delineating the internal roughness of each fiber and the micrometer scale roughness which is inherent to a cellulose paper surface are robust when compared to roughened structures created by traditional polymer grafting, nanoparticle deposition, or other artificial means.

A Thin Film Approach to Engineering Functionality into Oxides

Abstract: The broad spectrum of electronic and optical properties exhibited by oxides offers tremendous opportunities for microelectronic devices, especially when a combination of properties in a single device is desired Here we describe the use of reactive molecular-beam epitaxy and pulsed-laser deposition to synthesize functional oxides, including ferroelectrics, ferromagnets, and materials that are both at the same time Owing to the dependence of properties on direction, it is often optimal to grow functional oxides in particular directions to maximize their properties for a specific application But these thin film techniques offer more than orientation control; customization of the film structure down to the atomic-layer level is possible Numerous examples of the controlled epitaxial growth of oxides with perovskite and perovskite-related structures, including superlattices and metastable phases, are shown In addition to integrating functional oxides with conventional semiconductors, standard semiconductor practices involving epitaxial strain, confined thickness, and modulation doping can also be applied to oxide thin films Results of fundamental scientific importance as well as results revealing the tremendous potential of utilizing functional oxide thin films to create devices with enhanced performance are described

Transparent conductive zinc oxide : basics and applications in thin film solar cells

Abstract: ZnO and Its Applications.- Electrical Properties.- Optical Properties of ZnO and Related Compounds.- Surfaces and Interfaces of Sputter-Deposited ZnO Films.- Magnetron Sputtering of ZnO Films.- Zinc Oxide Grown by CVD Process as Transparent Contact for Thin Film Solar Cell Applications.- Pulsed Laser Deposition of ZnO-Based Thin Films.- Texture Etched ZnO:Al for Silicon Thin Film Solar Cells.- Chalcopyrite Solar Cells and Modules.

Unipolar resistive switching characteristics of ZnO thin films for nonvolatile memory applications

Abstract: Highly (002)-oriented and columnar-grained ZnO thin films were prepared by radio frequency magnetron sputtering at room temperature The Pt∕ZnO∕Pt devices exhibit reversible and steady bistable resistance switching behaviors with a narrow dispersion of the resistance states and switching voltage Only a low forming electric field was required to induce the resistive switching characteristics The resistance ratios of high resistance state to low resistance state were in the range of 3–4 orders of magnitude within 100cycles of test It was also found that the conduction mechanisms dominating the low and high resistance states are Ohmic behavior and Poole-Frenkel emission, respectively

Ga-assisted catalyst-free growth mechanism of GaAs nanowires by molecular beam epitaxy

Abstract: The mechanisms of Ga-assisted GaAs nanowires grown by molecular beam epitaxy are addressed. The axial and radial growth rates as a function of the Ga rate and As pressure indicate that on the opposite of what is observed in thin film epitaxy, the growth rate of the nanowires is arsenic limited. As a consequence, the axial growth rate of the wires can be controlled by the ${\mathrm{As}}_{4}$ pressure. Additionally, due to the small ${\mathrm{As}}_{4}$ pressure leading to nanowire growth, the deposition on the facets is very slow, leading to a much lower radial growth rate. Finally, we present a model that is able to accurately describe the presented observations and predicts a maximum length of nontapered nanowires of $40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$.

Self-Organization of Ink-jet-Printed Triisopropylsilylethynyl Pentacene via Evaporation-Induced Flows in a Drying Droplet†

Abstract: We have demonstrated the influence of evaporation-induced flow in a single droplet on the crystalline microstructure and film morphology of an ink-jet-printed organic semiconductor, 6,13-bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring-deposited films. However, self-aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface-tension-driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field-effect transistors fabricated with these self-aligned crystals via ink-jet printing exhibit significantly improved performance with an average effective field-effect mobility of 0.12 cm2 V–1 s–1. These results demonstrate that with the choice of appropriate solvent ink-jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct-write fabrication of high-performance organic electronics.

Transparent and conducting electrodes for organic electronics from reduced graphene oxide

Abstract: The deposition and optoelectronic properties of reduced graphene oxide thin films are described. Thin films with thicknesses ranging from 1–10nm have been deposited by the vacuum filtration method. The conductivity of the thin films can be varied over six orders of magnitude by varying the filtration volume of the graphene oxide aqueous suspension while maintaining the transmittance between 60%–95%. In addition, enhancement in the conductance through Cl doping is demonstrated. The combination of the reduction and Cl treatments make the reduced graphene oxide thin films sufficiently conducting to incorporate them as the hole collecting electrode in proof of concept organic photovoltaic devices.

Heteroepitaxy of Corundum-Structured α-Ga2O3 Thin Films on α-Al2O3 Substrates by Ultrasonic Mist Chemical Vapor Deposition

Abstract: Ga2O3 thin films of the α-phase, that is, the corundum structure (in the trigonal system), have been epitaxially obtained on sapphire (α-Al2O3) substrates, in contrast to the strong tendency of Ga2O3 to assume a heterogeneous crystal structure, that is, the β-gallia structure (in the monoclinic system) on sapphire. This result is advantageous for high-quality films and is due to the growth by mist chemical vapor deposition (CVD) at low temperatures of 430–470 °C. The α-Ga2O3 films have narrow full-widths at half maximum (FWHMs) in their X-ray diffraction rocking curves, for example, about 60 arcsec. The root mean square (RMS) roughness of the surface was as small as 1 nm. The optical band gap energy obtained was 5.3 eV, and the films were almost completely transparent in the near-ultraviolet and visible regions. The epitaxial growth of α-Ga2O3 films on sapphire is beneficial for the fabrication of oxide optical and electronic devices.