Solution casting of transparent and conductive carbon nanotubes/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) films under a magnetic field

Transparent and conductive composite films of carboxyl functionalized single-walled carbon nanotubes and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) were deposited on various substrates under the influence of a magnetic field. It was demonstrated that the sample dewetting under the magnetic field enhances the conductivity of the dried films. Highly transparent films (∼88%) were obtained with a low sheet resistivity of ∼90 Ω/sq. The magnetic field assisted deposition method proposed here suggests scalable production of flexible and cost-effective transparent electronics.     

Space durable polymer/carbon nanotube films for electrostatic charge mitigation

Low color, flexible, space environmentally durable polymeric materials possessing sufficient surface resistivity (10⁶-10¹⁰ Ω/square) for electrostatic charge (ESC) mitigation are of interest for potential applications on Gossamer spacecraft as thin film membranes on antennas, large lightweight space optics, and second surface mirrors. One method of incorporating intrinsic ESC mitigation while maintaining low color, flexibility, and optical clarity is through the utilization of single-walled carbon nanotubes (SWNTs). However, SWNTs are difficult to uniformly disperse in the polymer matrix. The approach reported herein employed amide acid polymers endcapped with alkoxysilane groups that could condense with oxygen containing functionalities that were present on the ends of SWNTs as a result of the oxidative purification treatment. These SWNTs were combined with the endcapped amide acid polymers in solution and subsequently cast as unoriented thin films. Two examples possessed electrical conductivity (measured as surface resistance and surface resistivity) sufficient for ESC mitigation at loading levels of ≤0.08 wt% SWNT as well as good retention of thermo-optical properties. The percolation threshold was determined to lie between 0.03 and 0.04 wt% SWNT loading. Electrical conductivity of the film remained unaffected even after harsh mechanical manipulation.     

Preparation of carbon-based transparent and conductive thin films by pyrolysis of silylated graphite oxides

Thin films of silylated graphite oxide were obtained from a chloroform/cyclohexane dispersion of n-hexadecylamine-intercalated silylated graphite oxide by a casting method at a low temperature. Carbon-based thin films were obtained from the pyrolysis of the resulting films under a reduced pressure at 500 °C or higher temperatures. The resulting samples were well adhered to the substrate because of the presence of silicon containing species as a “glue”. The resistivity decreased with an increase in the film thickness or a decrease in the transparency. Based on the data obtained for the samples prepared from graphite with different particle sizes and graphite oxide with different oxygen contents, the conduction of the electrons within each carbon sheet seemed important for large film thickness and conduction through the boundary seemed important when the film thickness was small. A low sheet resistance of 3.7 kΩ/sq for 80% of transmittance was achieved, when graphite oxide with a lower oxygen content was prepared from graphite with smaller particle sizes and the precursor film was heated at 500 °C. At 900 °C, it further decreased to a value of 700 Ω/sq.     

Mechanical characterization of ultra-thin fluorocarbon films deposited by R.F. magnetron sputtering

Fluorocarbon films were deposited on silicon substrate by R.F. magnetron sputtering using a polytetrafluoroethylene (PTFE) target. Structure of the deposited films was studied by X-ray photoelectron spectroscopy (XPS). Hardness, elastic modulus and scratch resistance were measured using a nanoindenter with scratch capability. -CF_x (x = 1, 2, 3) and C-C units were found in the deposited fluorocarbon films. The hardness and elastic modulus of the films are strongly dependent on the R.F. power and deposition pressure. The film hardness is in the range from 0.8 GPa to 1.3 GPa while the film elastic modulus is in the range from 8 GPa to 18 GPa. Harder films exhibit higher scratch resistance. Differences in nanoindentation behavior between the deposited fluorocarbon films, diamond-like carbon (DLC) films and PTFE were discussed. The fluorocarbon films should find more applications in the magnetic storage and micro/nanoelectromechanical systems.     

Fabrication of highly conducting and transparent graphene films

Graphene-based transparent conducting films were prepared using the following method. A chemically-reduced graphene dispersion was synthesized and graphene films were prepared from it by transfer printing, followed by thermal treatment. The resulting graphene films possessed an excellent electrical conductivity with a high transparency. A sheet resistance lower than ∼2 KΩ/sq and a transparency well over 80% were achieved at a typical wavelength of 550 nm. These properties are considered quite sufficient for many applications, such as transparent conductor films for touch panels.     

Structural, electrical and magnetic properties of carbon-nickel composite thin films

Carbon-nickel composite thin films (600 nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25-800 °C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni₃C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the aspect ratio as high as 1:60 in the sample prepared at 200 °C). The carbon matrix was amorphous, or graphite-like depending on deposition temperature. At low deposition temperatures T_S (25-400 °C) the Ni₃C nanoparticles were of hcp phase. Samples prepared at T_S ? 600 °C contained ferromagnetic fcc Ni nanoparticles. A correlation was found between the structural, electrical and magnetic properties of the composites. To characterise the films, dependences, such as resistivity vs. temperature, current vs. voltage, differential conductivity vs. bias voltage, and magnetoresistivity, were determined. For example, the tunneling effect was found in samples in which the metallic nanoparticles were separated by 2-3 nm thick amorphous carbon. When the metallic nanoparticles were connected by graphite-like carbon regions (having a metallic conductivity, in contrast to a-C), the temperature coefficient of the resistivity became slightly positive. An anisotropic magnetoresistivity of ∼0.1% was found in the sample that contained ferromagnetic columnar fcc Ni. Zero magnetoresistivity was found in the sample in which the metallic nanoparticles were of non-magnetic hcp phase.     

Sol–gel derived undoped and boron-doped ZnO semiconductor thin films: Preparation and characterization

We report the influence of boron doping concentration on the microstructure, electrical and optical properties of solution-processed zinc oxide (ZnO) thin films. The B doping concentration in the resultant solutions was varied from 0 to 5 at%, and the pH value of each synthetic solution was adjusted to 7.0. XRD measurements, SEM observations, and SPM examinations revealed that boron doping produced ZnO thin films consisting of a fine grain structure with a flat surface morphology. Moreover, ZnO thin films doped with B raised the texture coefficient along the (002) plane. All B-doped ZnO (ZnO:B) thin films exhibited higher transparency than that of the undoped ZnO thin film in the wavelengths between 350 and 650 nm. The optical band gap and Urbach energy of the ZnO:B thin films were higher than those of the undoped thin film. According to electrical transport characteristics, the 1% B-doped ZnO thin film exhibited the highest Hall mobility of 17.9 cm²/V s, the highest electron concentration of 1.2×10¹⁵ cm⁻³, and the lowest electrical resistivity of 2.2×10² Ω cm among all of the ZnO:B thin films.     

Optimization of the sputtering process parameters of GZO films using the Grey–Taguchi method

This paper examines the optimization of the process parameters of GZO films deposited on polyethylene terephthalate substrates by R.F. magnetron sputtering using the Taguchi method, aiming to obtain highly transparent and conductive films. The influences of the various sputtering factors (R.F. power, sputtering pressure, deposition time, substrate temperature and post-annealing temperature) on electrical resistivity and structural, morphological and optical transmittance of GZO films are analyzed. The electrical resistivity and the optical transmittance of GZO films were improved by post-annealing the substrate during the deposition process. Experimental results indicate the optimal process parameters in GZO films deposited on polyethylene terephthalate substrates can be determined effectively. The electrical resistivity of GZO films is decreased from 1.194 × 10⁻³ Ω cm to 8.627 × 10⁻⁴ Ω cm and the optical transmittance is increased from 86.148% to 90%, leading to multiple performance characteristics in deposition qualities through the Grey–Taguchi method.     

Reaction-formed W2B5/C composites with high performance

Highly densified W₂B₅/C composites with W₂B₅ content from 30 to 70 vol% were fabricated by reaction hot pressing of the powder mixture of B₄C, WC and carbon black. The reaction products were identified by XRD analysis to consist of only W₂B₅ and carbon, regardless of carbon content. The reaction formed composites have excellent mechanical properties (the maximum flexural strength and fracture toughness of 786 MPa and 8.9 MPa m^1/2 respectively), electrical conductivity (the highest electrical conductivity of 1.64 × 10⁶ Ω⁻¹ m⁻¹), and resistance to both wear and oxidation because of the presence of the plate-like W₂B₅ grains. In this paper, the preparation, microstructure and properties of this new composite are investigated, and the strengthening, toughening, conduction mechanisms are discussed.     

The impact of solvent characteristics on performance and process stability of printed carbon resistive materials

Carbon conductive pastes deposited by screen printing are used in many commercial applications including sensors, PCB, batteries, and PV, and as such represent an important value-added coating. An experimental investigation was carried out into the role of the solvent on the drying characteristics, conductivity, and process consistency in screen printed carbon pastes. Four materials with solvent boiling points between 166 and 219°C were deposited at film thickness between 6 and 16 μm, and the sheet resistance and film thickness were measured after successive passes through an industrial dryer operating with an air temperature of 155°C. Sheet resistances of 14 Ω/sq. were obtained with the thicker films while thinner films produced a sheet resistance of 46 Ω/sq. Thinner films achieved a stable resistivity within a 2.5-min residence time, while the thicker films required a residence time in excess of 12.5 min to achieve a stable resistivity. As well as prolonging drying times, the higher boiling point increased the resistivity of the cured film. It is postulated that the lower resistance of the faster drying materials is a result of film stressing increasing inter particle contact. Process models indicate that multiple thin layers are a more efficient means of manufacture for the process parameters examined.     

Moderate anisotropy in the electrical conductivity of bulk MWCNT/epoxy composites

Bulk aligned multi-walled carbon nanotube films and their epoxy composites were prepared and their DC and AC conductivity studied. Nanotube films of up to 2 mm thickness were grown by catalytic chemical vapor deposition. Composites of nanotubes were made by infiltrating the films with a commercial epoxy. DC electrical resistivities in the axial direction of as-grown and purified films were found to be ∼1.2 Ωmm and ∼3.4 Ωmm, respectively. For the transverse direction the resistivity values were higher only with a factor of ∼2. In the case of composites, anisotropy is more pronounced showing more than an order of magnitude higher resistivity in the transverse direction (∼71.4 Ωmm) as compared to the axial value (∼4.2 Ωmm). AC behavior of the films investigated between 1 MHz and 3 GHz shows the presence of inductive and capacitive components at frequencies above ∼100 MHz. The moderate anisotropy for both DC and AC electrical properties are explained on the basis of the films’ structure combined with percolation theory and equivalent circuit models.     

Composite graphitic nanolayers prepared by self-assembly between finely dispersed graphite oxide and a cationic polymer

Chemical flaking of graphite has been performed by reacting natural graphite with a strong oxidizing agent, NaClO₃/HNO₃. The formed hydrophilic, negatively charged graphite oxide (GO) colloids can be dispersed in water which allows the deposition of thin GO/cationic polymer (poly(diallyldimethylammoniumchloride, PDDA) multilayer films on a glass substrate by wet-chemical self-assembly. The feasibility of the charge-regulated layer-by-layer deposition is demonstrated by mutual charge titrations of the film-forming species. Visible-light spectroscopy revealed progressive growth of the film thickness with the number of deposition of steps, while XRD and AFM showed that partially exfoliated, highly anisometric (aspect ratio >50) graphite oxide platelet aggregates were deposited with an average thickness of the stacked graphite oxide platelets of 10 carbon layers (7.4 nm). Reduction of multilayer assemblies of GO and PDDA on glass yielded a non-conductive turbostratic carbon nanofilm. The original, conductive graphite-like structure was restored by reduction with N₂H₄ and annealing at 400 °C which, by gradual ordering of the carbon crystallites, caused a significant decrease in the resistivity.     

Characterization of indium tin oxide (ITO) thin films prepared by a sol-gel spin coating process

Indium tin oxide (ITO) thin films were prepared by a sol-gel spin coating method, fired, and then annealed in the temperature range of 450-600°. The XRD patterns of the thin films indicated the main peak of the (2 2 2) plane and showed a higher degree of crystallinity with an increase in the annealing temperature. Upon annealing the films at 500 and 600°, two binding energy levels of Sn⁴⁺ ion of 486.9 eV and 486.6 eV, respectively, were measured in the XPS spectra. The ITO film that was annealed at 600° contained two oxidation states of Sn, Sn²⁺ and Sn⁴⁺, and it had a higher sheet resistance based on a rather low doping concentration of Sn⁴⁺. The film that was annealed at 500° and subsequently treated with 0.1 N HCl solution for 40 s showed a sheet resistance of 225 Ω/square. The surface treatment by the acidic solution diminished the RMS (root mean square) roughness value and the residual carbon content (XPS peak intensity of carbon) of the ITO films. It seems that the acid-cleaning of the ITO thin films led to a decrease of the surface roughness and sheet resistance.     

Comparison of experimental and theoretical transverse elastic modulus of carbon fibers

The transverse elastic modulus of PAN-based carbon fibers as measured by experimental methods, calculated from theoretical equations and analyzed by the finite element method (FEM) is discussed. Raman spectroscopy was the primary method utilized to measure the transverse elastic modulus of carbon fibers in carbon-fiber reinforced plastics (CFRP). A lead oxide (PbO) thin film was deposited on the surface of a CFRP specimen using physical vapor deposition as the pretreatment in order to measure the strains of the carbon fibers and epoxy matrix phases by Raman spectroscopy. Since the relation between the Raman peak wave number of PbO thin films and tensile strain has already been developed, the transverse strain of the carbon fibers could be measured. The transverse strain of the carbon fibers was analyzed using a 2-D FEM model. The transverse modulus of the carbon fibers was determined by fitting the experimental result from Raman spectroscopy to the FEM model. The determined transverse modulus (10.4 GPa) is compared with those experimentally measured by nanoindentation (13.4 GPa), numerically analyzed using 2-D and 3-D FEM models (5.25 GPa and 28.7 GPa, respectively), and theoretically calculated from the Mori-Tanaka, Halpin-Tsai, and Uemura equations (24.8 GPa, 17.4 GPa, and 28.4 GPa, respectively).     

Highly conductive and transparent thin films fabricated with predominantly semiconducting single-walled carbon nanotubes

A predominantly semiconducting single-walled carbon nanotube-based thin conductive film was fabricated on a flexible poly(ethylene terephthalate) substrate. The structural features of the nanotubes and careful experimental scrutinization consistently yielded the films with very low surface resistance (59 Ω sq⁻¹) and high transparency (80%). The morphological studies of these films before and after acid treatment revealed the self orientation of nanotubes clustered at favorable centers.     

Effect of doping concentration on the properties of aluminium doped zinc oxide thin films prepared by spray pyrolysis for transparent electrode applications

Zinc oxide possesses many interesting properties, such as modifiable conductivity, wide band gap, high excitonic binding energy, piezo-electric polarisation and cathodoluminiscence. In this study transparent conducting aluminium doped zinc oxide (ZnO:Al) thin films were deposited on float glass substrates by tailor made spray pyrolysis with adaptation for measuring the actual temperature of the substrate surface during deposition. The films were characterised and the effect of aluminium doping concentration [Al/Zn] on their optical, electrical and structural properties was investigated as a function of aluminium doping between 0 and 10 at.%. There was widening of optical band gap with increasing doping concentration. ZnO:Al films with low resistivity of 2.8 × 10⁻² Ω cm and high transmittance of over 85% at 550 nm which are crucial for opto-electrical applications were obtained at a doping ratio of 2 at.%.     

Polyaniline-carbon composite films as supports of Pt and PtRu particles for methanol electrooxidation

Vulcan XC-72 carbon black particles (average size: ca. 50 nm) was incorporated into polyaniline (PANI) matrix by an electrochemical codeposition technique during the electropolymerization process. The doping by carbon particles leads to a higher polymeric degree and a lower defect density in the PANI structure. Furthermore, the incorporation of carbon particles not only increases the electrochemical accessible surface areas (S_a) and electron conductivity of the PANI film, but also decreases charge transfer resistance at PANI/electrolyte interfaces. Therefore, as expected, a fabricated PANI + C composite film with dispersed Pt and PtRu particles exhibited excellent electrocatalytic activity for methanol oxidation due to better Pt dispersion and utilization. The PANI + C composite film is more promising as a support material in electrocatalysis than a PANI film. Meanwhile, a new application for regular carbon black as a doping material into conducting polymer for electrocatalysis was thus demonstrated.     

Characterisation and corrosion studies of steel electrodes covered by polypyrrole/phosphotungstate using Electrochemical Impedance Spectroscopy

Polypyrrole/PW₁₂O₄₀³⁻ hybrid material was electrosynthesised on carbon steel electrodes in acetonitrile medium. The coatings obtained were characterised by Electrochemical Impedance Spectroscopy (EIS). On free-standing polypyrrole films the electrical response was mainly due to ion–ion charge transfer resistance, with a value of 175 Ω cm². A value of 2 × 10⁻⁵ S/cm was determined for the hybrid material conductivity. A charge transfer resistance about 7000 Ω cm² was obtained due to steel/oxide interface. Corrosion tests showed an important improvement in the protection against corrosion when the carbon steel electrodes were coated by these polymeric films.     

Fast and simple fabrication of a large transparent chemically-converted graphene film by spray-coating

We report the fast, low-cost, simple fabrication of a large chemically-converted graphene (CCG) films by spray deposition of graphene oxide (GO)-hydrazine dispersions. The GO-hydrazine dispersion was prepared by mixing GO dispersion with excess amount of hydrazine monohydrate. By spray deposition on preheated substrate, the creation of the thin film and the reduction of GO to CCG were carried out simultaneously. The prepared CCG films had a low sheet resistance of 2.2 × 10³ Ω □⁻¹ and a high transmittance of 84% at a wavelength of 550 nm. Atomic force microscope images clearly showed continuous films resulting from the overlap of graphene sheets and a uniform surface morphology with root mean square about 1 nm.     

Microstructure and through-film electrical characteristics of vertically aligned amorphous carbon films

The microstructure and electrical properties of in-situ annealed carbon films is studied in this paper. In-situ annealing (150 °C to 600 °C) was done during the deposition of carbon films with −300 V substrate bias. Transmission electron microscopy and two points electrical probing studies were performed and the deduced transition for vertical orientated graphitic planes occurs at temperatures above 400 °C. The microstructure of the films strongly depends on the deposition temperature of the films (room temperature, 400 °C and 600 °C). Electrical conductivity of the film strongly depends on texturing due to the formation of preferred orientation in the vertical direction. The vertically orientated carbon (VOC) sheet provides effective nanochannels for electron transport, thus significantly improves the electrical properties of the annealed film.