Theodoros Dikonimos

Nitrogen-doped graphene films from chemical vapor deposition of pyridine: Influence of process parameters on the electrical and optical properties

Summary Graphene films were produced by chemical vapor deposition (CVD) of pyridine on copper substrates. Pyridine-CVD is expected to lead to doped graphene by the insertion of nitrogen atoms in the growing sp2 carbon lattice, possibly improving the properties of graphene as a transparent conductive film. We here report on the influence that the CVD parameters (i.e., temperature and gas flow) have on the morphology, transmittance, and electrical conductivity of the graphene films grown with pyridine. A temperature range between 930 and 1070 °C was explored and the results were compared to those of pristine graphene grown by ethanol-CVD under the same process conditions. The films were characterized by atomic force microscopy, Raman and X-ray photoemission spectroscopy. The optical transmittance and electrical conductivity of the films were measured to evaluate their performance as transparent conductive electrodes. Graphene films grown by pyridine reached an electrical conductivity of 14.3 × 105 S/m. Such a high conductivity seems to be associated with the electronic doping induced by substitutional nitrogen atoms. In particular, at 930 °C the nitrogen/carbon ratio of pyridine-grown graphene reaches 3%, and its electrical conductivity is 40% higher than that of pristine graphene grown from ethanol-CVD.

Cyclododecane as support material for clean and facile transfer of large-area few-layer graphene

The transfer of chemical vapor deposited graphene is a crucial process, which can affect the quality of the transferred films and compromise their application in devices. Finding a robust and intrinsically clean material capable of easing the transfer of graphene without interfering with its properties remains a challenge. We here propose the use of an organic compound, cyclododecane, as a transfer material. This material can be easily spin coated on graphene and assist the transfer, leaving no residues and requiring no further removal processes. The effectiveness of this transfer method for few-layer graphene on a large area was evaluated and confirmed by microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, and four-point probe measurements. Schottky-barrier solar cells with few-layer graphene were fabricated on silicon wafers by using the cyclododecane transfer method and outperformed reference cells made by standard methods.

Chemical Vapor Deposited Graphene-Based Derivative As High-Performance Hole Transport Material for Organic Photovoltaics

UNLABELLED The development of efficient charge transport layers is a key requirement for the fabrication of efficient and stable organic solar cells. A graphene-based derivative with planar resistivity exceeding 10(5) Ω/□ and work function of 4.9 eV is here produced by finely tuning the parameters of the chemical vapor deposition process on copper. After the growth, the film is transferred to glass/indium tin oxide and used as hole transport layer in organic solar cells based on a PBDTTT-C-T:[70]PCBM blend. The cells attained a maximum power conversion efficiency of 5%, matching reference cells made with state-of-the-art PEDOT PSS as the hole transport layer. Our results indicate that functionalized graphene could represent an effective alternative to PEDOT PSS as hole transport/electron blocking layer in solution-processed organic photovoltaics.

Fast growth of polycrystalline graphene by chemical vapor deposition of ethanol on copper

High conductive graphene films can be grown on metal foils by chemical vapor deposition (CVD). We here analyzed the use of ethanol, an economic precursor, which results also safer than commonly-used methane. A comprehensive range of process parameters were explored in order to obtain graphene films with optimal characteristics in view of their use in optoelectronics and photovoltaics. Commercially-available and electro-polished copper foils were used as substrates. By finely tuning the CVD conditions, we obtained few-layer (2-4) graphene films with good conductivity (~500 Ohm/sq) and optical transmittance around 92-94% at 550 nm on unpolished copper foils. The growth on electro-polished copper provides instead predominantly mono-layer films with lower conductivity (≥1000 Ohm/sq) and with a transmittance of 97.4% at 550 nm. As for the device properties, graphene with optimal properties as transparent conductive film were produced by CVD on standard copper with specific process conditions.

High frequency performance limits of nanointerconnects based on CVD-grown graphene films transferred on SiO 2 -substrate

Graphene films are grown by chemical vapour deposition on copper layer and then transferred onto a silicon substrate, coated with silicon dioxide. The topological characterization of the produced film is performed by atomic force microscopy, and the sheet resistance is measured by applying the four-probe test method. The equivalent single conductor model is then used in order to analyze the signal propagation along a nanointerconnect made with multilayer graphene over silicon dioxide, in a wide frequency range, up to 100 GHz. The comparison of the radio-frequency performances of the nanointerconnect, modeled by using either the measured value of effective resistivity or a theoretical estimation of the p.u.l. resistance, suggests that graphene films grown by chemical vapor deposition are more suitable for application as low frequency electrical interconnections in flexible electronics, than in high-speed integrated circuits.

Annealing free, high quality CVD graphene growth and transfer

Among the different graphene synthesis methods, chemical vapor deposition of graphene on low cost copper foil shows great promise for large scale applications. Here we report on the growth and transfer of uniform and continuous large-sized thin-films composed of single- and few-layered graphene . The foils were grown by chemical vapor deposition (CVD) on polycrystalline copper (Cu) foils at low pressure using ethanol and were transferred onto the destination substrates using a cyclododecane supporting layer. Structural and optical characterizations indicate that the graphene films are composed of single or few layers depending on the growth conditions and exhibit low defect density. The graphene films can be transferred to arbitrary substrates with the help of a green transfer method based on an organic molecule, cyclododecane.

Electrochemical synthesis of self-organized TiO2 crystalline nanotubes without annealing

This work demonstrates that upon anodic polarization in an aqueous fluoride-containing electrolyte, TiO2 nanotube array films can be formed with a well-defined crystalline phase, rather than an amorphous one. The crystalline phase was obtained avoiding any high temperature annealing. We studied the formation of nanotubes in an HF/H2O medium and the development of crystalline grains on the nanotube wall, and we found a facile way to achieve crystalline TiO2 nanotube arrays through a one-step anodization. The crystallinity of the film was influenced by the synthesis parameters, and the optimization of the electrolyte composition and anodization conditions (applied voltage and time) were carried out. For comparison purposes, crystalline anatase TiO2 nanotubes were also prepared by thermal treatment of amorphous nanotubes grown in an organic bath (ethylene glycol/NH4F/H2O). The morphology and the crystallinity of the nanotubes were studied by field emission gun-scanning electron microscopy (FEG-SEM) and Raman spectroscopy, whereas the electrochemical and semiconducting properties were analyzed by means of linear sweep voltammetry, impedance spectroscopy, and Mott-Schottky plots. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) allowed us to determine the surface composition and the electronic structure of the samples and to correlate them with the electrochemical data. The optimal conditions to achieve a crystalline phase with high donor concentration are defined.

Plasma enhanced hot filament CVD growth of thick carbon nanowall layers

Carbon nanowalls are carbon nanostructures consisting of arrays of graphitic carbon plates which are mainly positioned perpendicularly to the growth surface. Carbon nanowalls have received considerable interest in recent years, since they are closely related to graphene from the structural point of view, while maintaining an open honeycomb lattice on the nanoscale. They are thus believed to be an interesting electrode material for many applications since they offer high chemical resistance, low electrical resistance and high surface area. In this paper we are presenting a method that allows the growth of thick layers of carbon nanowalls onto flat and porous substrates, both carbon and refractory metal based. Such methods are promising for making electrodes for use in electrochemical devices.

Pt Alloys on Carbon Nanostructures as Electrocatalysts for Direct Methanol Fuel Cell

Extensive efforts are focused on the development of Direct Methanol Fuel Cells, due to the intrinsic advantages of this type of devices for mobile power supply system. One of the major drawback of the DMFC resides in the easy poisoning of the anode electrocatalyst (platinum) by COlike reaction intermediates, which implies the need of high platinum load in order to obtain reasonable performances. The development of platinum alloys is considered one of the promising routes for overcoming this problem: the second metal in fact acts as inhibitor of the Pt poisoning. In this work we have combined the use of unconventional methods to deposit the electrocatalyst nanoparticles with unconventional carbon supports. PtAu alloys have been deposited by sputter deposition process on carbon nanofibers with platelet morphology grown by plasma enhanced chemical vapour deposition on carbon paper. Cyclic voltammetry in H2SO4 was used to determine the electrochemical active surface and the electrocatalytic performance for methanol oxidation reaction. Even at lower Pt load, respect to the ones prepared with commercial catalysts supported on carbon black, the innovative electrodes showed higher performance and stability.