Noncatalytic chemical vapor deposition of graphene on high-temperature substrates for transparent electrodes

A noncatalytic chemical vapor deposition mechanism is proposed, where high precursor concentration, long deposition time, high temperature, and flat substrate are needed to grow large-area nanocrystalline graphene using hydrocarbon pyrolysis. The graphene is scalable, uniform, and with controlled thickness. It can be deposited on virtually any nonmetallic substrate that withstands ∼1000 °C. For typical examples, graphene grown directly on quartz and sapphire shows transmittance and conductivity similar to exfoliated or metal-catalyzed graphene, as evidenced by transmission spectroscopy and transport measurements. Raman spectroscopy confirms the sp 2-C structure. The model and results demonstrate a promising transfer-free technique for transparent electrode production. © 2012 American Institute of Physics.

Direct chemical vapor deposition of large-area carbon thin films on gallium nitride for transparent electrodes: A first attempt

Direct formation of large-area carbon thin films on gallium nitride by chemical vapor deposition without metallic catalysts is demonstrated. A high flow of ammonia is used to stabilize the surface of the GaN (0001)/sapphire substrate during the deposition at 950°C. Various characterization methods verify that the synthesized thin films are largely sp 2 bonded, macroscopically uniform, and electrically conducting. The carbon thin films possess optical transparencies comparable to that of exfoliated graphene. This paper offers a viable route toward the use of carbon-based materials for future transparent electrodes in III-nitride optoelectronics, such as GaN-based light emitting diodes and laser diodes. © 1988-2012 IEEE.

Controllable chemical vapor deposition of large area uniform nanocrystalline graphene directly on silicon dioxide

Metal-catalyst-free chemical vapor deposition (CVD) of large area uniform nanocrystalline graphene on oxidized silicon substrates is demonstrated. The material grows slowly, allowing for thickness control down to monolayer graphene. The as-grown thin films are continuous with no observable pinholes, and are smooth and uniform across whole wafers, as inspected by optical-, scanning electron-, and atomic force microscopy. The sp 2 hybridized carbon structure is confirmed by Raman spectroscopy. Room temperature electrical measurements show ohmic behavior (sheet resistance similar to exfoliated graphene) and up to 13 of electric-field effect. The Hall mobility is ∼40 cm 2/Vs, which is an order of magnitude higher than previously reported values for nanocrystalline graphene. Transmission electron microscopy, Raman spectroscopy, and transport measurements indicate a graphene crystalline domain size ∼10 nm. The absence of transfer to another substrate allows avoidance of wrinkles, holes, and etching residues which are usually detrimental to device performance. This work provides a broader perspective of graphene CVD and shows a viable route toward applications involving transparent electrodes. © 2012 American Institute of Physics.

Graphene field-effect transistors as room-temperature terahertz detectors

The unique optoelectronic properties of graphene make it an ideal platform for a variety of photonic applications, including fast photodetectors, transparent electrodes in displays and photovoltaic modules, optical modulators, plasmonic devices, microcavities, and ultra-fast lasers. Owing to its high carrier mobility, gapless spectrum and frequency-independent absorption, graphene is a very promising material for the development of detectors and modulators operating in the terahertz region of the electromagnetic spectrum (wavelengths in the hundreds of micrometres), still severely lacking in terms of solid-state devices. Here we demonstrate terahertz detectors based on antenna-coupled graphene field-effect transistors. These exploit the nonlinear response to the oscillating radiation field at the gate electrode, with contributions of thermoelectric and photoconductive origin. We demonstrate room temperature operation at 0.3 THz, showing that our devices can already be used in realistic settings, enabling large-area, fast imaging of macroscopic samples. © 2012 Macmillan Publishers Limited. All rights reserved.

Pixel-level tunable liquid crystal lenses for auto-stereoscopic display

Mobile video and gaming are now widely used, and delivery of a glass-free 3D experience is of both research and development interest. The key drawbacks of a conventional 3D display based on a static lenticular lenslet array and parallax barriers are low resolution, limited viewing angle and reduced brightness, mainly because of the need of multiple-pixels for each object point. This study describes the concept and performance of pixel-level cylindrical liquid crystal (LC) lenses, which are designed to steer light to the left and right eye sequentially to form stereo parallax. The width of the LC lenses can be as small as 20-30 μm, so that the associated auto-stereoscopic display will have the same resolution as the 2D display panel in use. Such a thin sheet of tunable LC lens array can be applied directly on existing mobile displays, and can deliver 3D viewing experience while maintaining 2D viewing capability. Transparent electrodes were laser patterned to achieve the single pixel lens resolution, and a high birefringent LC material was used to realise a large diffraction angle for a wide field of view. Simulation was carried out to model the intensity profile at the viewing plane and optimise the lens array based on the measured LC phase profile. The measured viewing angle and intensity profile were compared with the simulation results. © 2014 SPIE.

Transparent liquid-crystal-based microlens array using vertically aligned carbon nanofiber electrodes on quartz substrates.

A novel transparent liquid-crystal-based microlens array has been fabricated using an array of vertically aligned multi-wall carbon nanofibers (MWCNFs) on a quartz substrate and its optical characteristics investigated. Electron beam lithography was used for the catalyst patterning on a quartz substrate to grow the MWCNF array of electrodes. The structure of the electrode array was determined through simulation to achieve the best optical performance. Both the patterned catalyst and growth parameters were optimized for optimal MWCNF properties. We report an in-depth optical characterization of these reconfigurable hybrid liquid crystal and nanofiber microlens arrays.

Centrifuge coating for low-waste solution processing of transparent nanostructured electrodes

Centrifuge coating was implemented to fabricate nanostructured conductive layers through solution processing at room temperature. This coating procedure allows fast evaporation, thereby fixing the nanomaterials in their dispersed state onto a substrate by the centrifuge action. Material wastes were minimized by mitigating the effects of particle reaggregation. Using this method, we fabricate single-wall nanotube coatings on different substrates such as polyethylene terephthalate, polydimethylsiloxane, and an acrylic elastomer with no prior surface modification of the substrate. The effects of the choice of solvents on the morphology and subsequent performance of the coating network are studied. © 2002-2012 IEEE.

Electro-optic characteristics of a transparent nanophotonic device based on carbon nanotubes and liquid crystals.

We present electro-optic characteristics of a transparent nanophotonic device fabricated on quartz substrate based on multiwall carbon nanotubes and nematic liquid crystals (LCs). The nanotube electrodes spawn a Gaussian electric field to three dimensionally address the LC molecules. The electro-optic characteristics of the device were investigated to optimize the device performance and it was found that lower driving voltages were suitable for microlens array and phase modulation applications, while higher driving voltages with a holding voltage can be used for display-related applications.

Dry-transfer of aligned multiwalled carbon nanotubes for flexible transparent thin films

Herein we present an inexpensive facile wet-chemistry-free approach to the transfer of chemical vapour-deposited multiwalled carbon nanotubes to flexible transparent polymer substrates in a single-step process. By controlling the nanotube length, we demonstrate accurate control over the electrical conductivity and optical transparency of the transferred thin films. Uniaxial strains of up to 140% induced only minor reductions in sample conductivity, opening up a number of applications in stretchable electronics. Nanotube alignment offers enhanced functionality for applications such as polarisation selective electrodes and flexible supercapacitor substrates. A capacitance of 17F/g was determined for supercapacitors fabricated from the reported dry-transferred MWCNTs with the corresponding cyclic voltagrams showing a clear dependence on nanotube length. © 2012 Matthew Cole et al.

Highly electron transparent Graphene for field emission triode gates

The enhanced emission performance of a graphene/Mo hybrid gate electrode integrated into a nanocarbon field emission micro-triode electron source is presented. Highly electron transparent gate electrodes are fabricated from chemical vapor deposited bilayer graphene transferred to Mo grids with experimental and simulated data, showing that liberated electrons efficiently traverse multi-layer graphene membranes with transparencies in excess of 50-68%. The graphene hybrid gates are shown to reduce the gate driving voltage by 1.1 kV, whilst increasing the electron transmission efficiency of the gate electrode significantly. Integrated intensity maps show that the electron beam angular dispersion is dramatically improved (87.9°) coupled with a 63% reduction in beam diameter. Impressive temporal stability is noted (<1.0%) with surprising negligible long-term damage to the graphene. A 34% increase in triode perveance and an amplification factor 7.6 times that of conventional refractory metal grid gate electrode-based triodes are noted, thus demonstrating the excellent stability and suitability of graphene gates in micro-triode electron sources. A nanocarbon field emission triode with a hybrid gate electrode is developed. The graphene/Mo gate shows a high electron transparency (50-68%) which results in a reduced turn-on potential, increased beam collimation, reduced beam diameter (63%), enhanced stability (<1% variation), a 34% increase in perveance, and an amplification 7.6 times that of equivalent conventional refractory metal gate triodes. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Characterization of a liquid crystal microlens array using multiwalled carbon nanotube electrodes.

Reconfigurable liquid crystal microlenses employing arrays of multiwalled carbon nanotubes (MWNTs) have been designed and fabricated. The cells consist of arrays of 2 microm high MWNTs grown by plasma-enhanced chemical vapor deposition on silicon with a top electrode of indium tin oxide coated glass positioned 20 microm above the silicon and the gap filled with the nematic liquid crystal BLO48. Simulations have found that, while its nematic liquid crystal aligns with MWNTs within a distance of 10nm, this distance is greatly enhanced by the application of an external electric field. Polarized light experiments show that light is focused with focal lengths ranging from approximately 7 microm to 12 microm.

Transparent electrode with a nanostructured coating.

Using single-walled nanotubes as an example, we fabricated transparent conductive coatings and demonstrated a new technique of centrifuge coating as a potential low-waste, solution-based batch process for the fabrication of nanostructured coatings. A theoretical model is developed to account for the sheet resistance exhibited by layered random-network coatings such as nanofilaments and graphene. The model equation is analytical and compact, and allows the correlation of very different scaling regimes reported in the literature to the underlying coating microstructure. Finally, we also show a refined experimental setup to systematically measure the curvature-dependent sheet resistance.

Self-aligned electrodes for suspended carbon nanotube structures

We report on a study into electrode fabrication for the gate control of carbon nanotubes partially suspended above an oxidised silicon substrate. A fabrication technique has been developed that allows self-aligned side-gate electrodes to be placed with respect to an individual nanotube with a spacing of less than 10 nm. The suspended multi-walled carbon nanotube (MWCNT) is used as an evaporation mask during metal deposition. The metal forms an island on the nanotube, with increasing width as the metal is deposited, forming a wedge shape, so that even thick deposited layers yield islands that remain separated from the metal deposited on the substrate due to shadowing of the evaporation. The island can be removed during lift-off to leave a set of self-aligned electrodes on the substrate. Results show that Cr yields self-aligned side gates with around 90% effectiveness. © 2003 Elsevier Science B.V. All rights reserved.