Highly conducting Langmuir-Blodgett films of the TMTTF-CnTCNQ system

Highly conducting Langmuir-Blodgett (LB) films of the tetramethyltetrathia-fulvalene- alkyltetracyanoquinodimethane (TMTTF-C_n TCNQ, where C_n represents C_nH_2n+1) system are reported. The electron affinities of the acceptors C_nTCNQ (n=14, 18 or 22) in solution are almost the same and lie between those of TCNQ and dimethylTCNQ. These TCNQ derivatives form solid charge transfer complexes with TMTTF. The limiting areas of the complexes, normalized to the number of TCNQ derivatives at the air-water interface, are almost the same irrespective of the donor-to-acceptor ratio and of the length of the alkyl chain, indicating that the areas are governed by the TCNQ moiety. The monolayers of TMTTF-C_n TCNQ (n=14 or 18) are transferred onto solid substrates as LB films. Polarized UV-visible absorption spectra indicate that the long axis of TCNQ is parallel to the film surface. These LB films exhibit lateral conductivities as high as 0.4 S cm^-1 and 0.1 S cm^-1 when n=14 and n=18 respectively.     

On the degradation of conducting Langmuir-Blodgett films

Direct current through conducting Langmuir-Blodgett films was observed as a function of the applied voltage and alternating current as a function of frequency. In some cases voltage-current characteristics appeared to be quasi-ohmic but in other cases they exhibited tunnelling-like behaviour. This phenomenon was interpreted as being due to the progressive detachment of the film from the micro-electrodes, as confirmed by measurements of the conductivity as a function of time. This interpretation was also confirmed by estimating the tunnelling barrier parameters from voltage-current characteristics, by conductivity measurements before and after thermal treatment of the samples and by conductivity measurements of samples stored in air and under vacuum.     

Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir-Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air-water interface and transferred to silicon substrates by Langmuir-Blodgett technique as closely spaced monolayers of 20-40 μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp² bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8-12 cm^− 1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3 ± 0.3 to 0.8 ± 0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.     

High-performance taste sensor made from Langmuir-Blodgett films of conducting polymers and a ruthenium complex

A sensor array made up of nanostructured Langmuir-Blodgett (LB) films is used as an electronic tongue capable of identifying sucrose, quinine, NaCl, and HCl at the parts-per-billion (ppb) level, being in some cases 3 orders of magnitude below the human threshold. The sensing units comprise LB films from conducting polymers and a ruthenium complex transferred onto gold interdigitated electrodes. Impedance spectroscopy is used as the principle of detection, and the importance of using nanostructured films is confirmed by comparing results from LB films with those obtained from cast films.     

The properties of conducting tetracyanoquinodimethane Langmuir-Blodgett films: A study using acoustoelectric devices

Results are presented for Langmuir-Blodgett films of a pyridinium tetracyanoquinodimethane molecular system suitably engineered for deposition with a Langmuir trough. Piezoelectric substrates have been used to demonstrate the reproducibility of the thin film coating procedure, to establish their electrical conductivity and to evaluate their potential use as sensors. The data obtained using surface acoustic wave devices are particularly interesting. They yield a conductivity surface acoustic wave devices are particularly interesting. They yield a conductivity of 0.5 μ^-1 cm^-1 for the charge transfer complex and demonstrate the potential of the devices as a sensor of NO₂, with a detection limit of 10 ppb.     

Highly conducting and transparent tin-doped CdO thin films for optoelectronic applications

Highly conducting and transparent thin films of tin-doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature on structural, optical and electrical properties was studied. These films are highly transparent (78-89%) in visible region, and transmittance of the films depends on growth temperature. It is observed that resistivity increases with growth temperature after attaining minimum at 150 °C, while carrier concentration continuously decreases with temperature. The lowest resistivity of 1.96 × 10^− 5 Ω cm and carrier concentration of 5.52 × 10²¹ cm³ is observed for the film grown at 150 °C. These highly conducting and transparent tin-doped CdO thin films grown via pulsed laser deposition could be an excellent candidate for future optoelectronic applications.     

Highly transparent and conducting indium-doped zinc oxide films by spray pyrolysis

Spray pyrolysis has been used to deposit highly transparent and conducting films of indium-doped zinc oxide. The roles of various deposition parameters have been investigated and the optimum deposition conditions have been outlined. Without any post-deposition heat treatment, as-deposited films with a resistivity of about $\text{(8 ? 9) × 10}{}^{-4}\text{Ω}\text{cm}$ and an average visible transmittance of about 85% have been obtained. The structural, electrical and optical properties have been studied. The electron transport properties suggest that the films are degenerate and the mobility data can be understood in terms of a grain boundary trapping model.     

In situ thermal preparation of polyimide nanocomposite films containing functionalized graphene sheets

Graphene oxides (GO) were exfoliated in N,N-dimethylformamide by simple sonication treatment of the as-prepared high quality graphite oxides. By high-speed mixing of the pristine poly(amic acid) (PAA) solution with graphene oxide suspension, PAA solutions containing uniformly dispersed GO can be obtained. Polyimide (PI) nanocomposite films with different loadings of functionalized graphene sheets (FGS) can be prepared by in situ partial reduction and imidization of the as-prepared GO/PAA composites. Transmission electron microscopy observations showed that the FGS were well exfoliated and uniformly dispersed in the PI matrix. It is interesting to find that the FGS were highly aligned along the surface direction for the nanocomposite film with 2 wt % FGS. Tensile tests indicated that the mechanical properties of polyimide were significantly enhanced by the incorporation of FGS, due to the fine dispersion of high specific surface area of functionalized graphene nanosheets and the good adhesion and interlocking between the FGS and the matrix.     

Synthesis of ethanol-soluble few-layer graphene nanosheets for flexible and transparent conducting composite films

We report a facile method of preparing few-layer graphene nanosheets (FLGs), which can be soluble in ethanol. Atomic force microscopy and high-resolution transmission electron microscopy studies reveal that FLGs have average thicknesses in the range of 2.6-2.8 nm, corresponding to 8-9 layers. A graphene/nafion composite film has a sheet resistance of 9.70 kΩ/sq at the transmittance of 74.5% (at 550 nm) while the nafion film on polyethylene terephthalate has a sheet resistance of 128 kΩ/sq at transmittance of 90.0%. For the cycling/bending test, almost no change in resistance was exhibited when the film was bent at an angle up to 140°, and no obvious deviation in resistance could be found after 100 bending cycles was applied. In addition, an FLGs-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite layer was demonstrated as the effective hole transporting layer to improve the hole transporting ability in an organic photovoltaic device, with which the power conversion efficiency was enhanced from 3.10% to 3.70%. The results demonstrated the promising applications of FLGs on graphene-based electronics, such as transparent electrode and flexible conducting film.     

Highly efficient electron field emission from graphene oxide sheets supported by nickel nanotip arrays

Electron field emission is a quantum tunneling phenomenon whereby electrons are emitted from a solid surface due to a strong electric field. Graphene and its derivatives are expected to be efficient field emitters due to their unique geometry and electrical properties. So far, electron field emission has only been achieved from the edges of graphene and graphene oxide sheets. We have supported graphene oxide sheets on nickel nanotip arrays to produce a high density of sharp protrusions within the sheets and then applied electric fields perpendicular to the sheets. Highly efficient and stable field emission with low turn-on fields was observed for these graphene oxide sheets, because the protrusions appear to locally enhance the electric field and dramatically increase field emission. Our simple and robust approach provides prospects for the development of practical electron sources and advanced devices based on graphene and graphene oxide field emitters.     

Highly conductive and porous activated reduced graphene oxide films for high-power supercapacitors

We present a novel method to prepare highly conductive, free-standing, and flexible porous carbon thin films by chemical activation of reduced graphene oxide paper. These flexible carbon thin films possess a very high specific surface area of 2400 m(2) g(-1) with a high in-plane electrical conductivity of 5880 S m(-1). This is the highest specific surface area for a free-standing carbon film reported to date. A two-electrode supercapacitor using these carbon films as electrodes demonstrated an excellent high-frequency response, an extremely low equivalent series resistance on the order of 0.1 ohm, and a high-power delivery of about 500 kW kg(-1). While higher frequency and power values for graphene materials have been reported, these are the highest values achieved while simultaneously maintaining excellent specific capacitances and energy densities of 120 F g(-1) and 26 W h kg(-1), respectively. In addition, these free-standing thin films provide a route to simplify the electrode-manufacturing process by eliminating conducting additives and binders. The synthetic process is also compatible with existing industrial level KOH activation processes and roll-to-roll thin-film fabrication technologies.     

Phospholipid-based potassium-selective Langmuir-Blodgett films

The preparation and properties of mixed Langmuir-Blodgett films of the phospholipid L--phosphatidic acid dipalmitoyl and the ionophore valinomycin are described. IR spectroscopy shows clear evidence for the formation of the valinomycin-potassium complex when these layers are immersed in KCl solutions. However, and in contrast to previous studies with fatty acid-valinomycin mixtures, dissociation of the potassium complex is also observed.     

Chemical self-assembly of graphene sheets

Chemically derived and noncovalently functionalized graphene sheets (GS) were found to self-assemble onto patterned gold structures via electrostatic interactions between the functional groups and the gold surfaces. This afforded regular arrays of single graphene sheets on large substrates, which were characterized by scanning electron microscopy (SEM), Auger microscopy imaging, and Raman spectroscopy. This represents the first time that self-assembly has been used to produce on-substrate and fully-suspended graphene electrical devices. Molecular coatings on the GS were removed by high current “electrical annealing”, which restored the high electrical conductance and Dirac point of the GS. Molecular sensors for highly sensitive gas detection using the self-assembled GS devices are demonstrated. Electronic Supplementary Material  Further characterization of GS and self-assembled structures by AFM, Raman, and Auger can be found in the ESM with nine figures and one table which are available in the online version of this article at http://dx.doi.org/ and accessible free of charge.     

Langmuir-Blodgett films as chemical sensors

This review covers the fundamental scheme of chemical sensors, fields of major interest in chemical sensors with Langmuir-Blodgett (LB) films, and new and future trends in LB sensor studies. The topics discussed are the relation between sensor characteristics and LB film structure, the biomimetic approach to sensor developments, proposals for transducer improvements, and the possibility of fabricating molecular filters by LB techniques.     

Langmuir-Blodgett films from dioctadecylamine picrate

Langmuir-Blodgett (LB) films prepared from picrate of dioctadecylamine(PDODA) and dioctadecylamine (DODA) were studied for the high picrate dipole moment, potentially effective in pyroelectricity. Stable condensed monolayers were obtained on pure water at pH from 5 to 8. About 90% of the PDODA in the films was hydrolysed to DODA. Moderate repulsion between ionized polar heads in the films promoted the desirable X- or Z-type deposition on solid supports, but it prevented a successive deposition of more than five monolayers. At pH below 5 the content of CdCl₂ or picric acid in the subphase decreased the PDODA hydrolysis. Strong repulsion under these conditions caused an expansion of the films and prevented the deposition of more than a single monolayer.     

A novel highly conducting phosphonium tetracyanoquinodimethane Langmuir-Blodgett film

We describe the properties of pristine and iodine-doped Langmuir-Blodgett films built up from a new semi-amphiphilic tetracyanoquinodimethane (TCNQ) salt, namely trimethyloctadecylphosphonium TCNQ. The films are transferred from a water subphase to a solid substrate. After exposure to diluted iodine vapour, the films become conducting. This behaviour is checked by the IR absorption spectrum. The conductivity estimated from the energy of the maximum of the charge transfer absorption band is about 20–50 Ω^-1 cm^-1.     

还原热处理对石墨烯薄膜导电性的影响

用改进的Hummers法制备了氧化石墨,通过超声、沉聚和自组装等工序制得氧化石墨烯薄膜,真空热处理后获得石墨烯薄膜.利用X射线晶体衍射(XRD)、扫描电子显微镜(SEM)、傅里叶变换红外(FT-IR)和拉曼(Raman)光谱等研究了石墨烯薄膜制备过程中各阶段产物的微观特征变化.结果表明,自组装方法制备氧化石墨烯薄膜简单易行,厚度尺寸可控,微观层状结构良好.热处理使石墨烯薄膜具有导电性,随温度升高导电率不断提高,在1 100℃时可达到536 S/cm.     

Vibrational analysis of single-layered graphene sheets

A molecular structural mechanics method has been implemented to investigate the vibrational behavior of single-layered graphene sheets. By adopting this approach, mode shapes and natural frequencies are obtained. Vibrational analysis is performed with different chirality and boundary conditions. Numerical results from the atomistic modeling are employed to develop predictive equations via a statistical nonlinear regression model. With the proposed equations, fundamental frequencies of single-layered graphene sheets with considered boundary conditions can be predicted within 3% difference with respect to the atomistic simulation.     

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.