Room temperature ionic liquids in electrosynthesis and spectroelectrochemical characterization of poly(para-phenylene)

Room temperature ionic liquids (RTILs) were used in electrochemical polymerization and in doping studies (oxidation and reduction) of poly(para-phenylene) (PPP). Cyclic voltammetry was used simultaneously with Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy. Electropolymerization and doping of PPP were done by potential scanning in acetonitrile (ACN + 0.1 M TBAPF₆), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) and butylmethylpyrrolidinium bis (trifluoromethylsulfonyl) imide ([BMP][Tf₂N]). The cyclic voltammograms recorded during polymerization of the PPP film indicate that the best film growth was achieved in [BMIM][PF₆]. The films made in [BMP][Tf₂N] were more stable than films made in ACN (0.1 M TBAPF₆). Results from p-doping studies show that doping can be made at higher potentials in RTILs than in ACN (0.1 M TBAPF₆). It was also found that n-doping can be performed in RTILs at higher negative potentials (−2.2 V) than in ACN (0.1 M TBAPF₆) (−1.8 V). The best n-doping response was achieved in [BMP][Tf₂N]. Also, n-doping in [BMIM][PF₆] was better than in ACN (0.1 M TBAPF₆). The in situ ATR-FTIR spectroscopy was used to study p- and n-doping of PPP films. During both p- and n-doping the spectra indicated formation of infrared active vibration bands (IRAV) in the wavenumber region 1600-800 cm⁻¹. The obtained IRAV bands correlate to the theoretical modes calculated by Zerbi and co-workers according to the effective conjugation coordinate theory (ECC). All these results indicate that RTILs are good solvents in spectroscopic and electrochemical studies of conducting polymers.     

Combustion synthesis of Ca1−xCuxAl2O4 (x = 0.0, 0.4 and 0.8) copper doped calcium aluminate

We report the effect of Cu²⁺ ion on CaAl₂O₄ with different molar concentrations of 0.0, 0.4 and 0.8 M prepared by simple combustion method. The materials have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) and scanning electron microscopy (SEM). DC electrical conductivity has also been measured to study the electrical behavior of the materials. The XRD patterns confirm the formation of single-phase CaAl₂O₄ along with some impurity phases like CaAl₄O₇, CaAl₁₂O₁₉ and Ca₁₂Al₁₄O₃₃. The FT-IR spectra show the stretching and bending vibrations of the synthesized compounds. DC electrical conductivity of the Ca_1−xCu_xAl₂O₄ is found to vary from 26.46 × 10⁻⁴ to 515.68 × 10⁻⁴ S cm⁻¹ for x = 0.0 to x = 0.8 at the measuring temperature of 1000 °C. SEM images show the morphological features of the compounds.     

Electrosyntheses of high quality free-standing poly(9-fluorenone) films in boron trifluoride diethyl etherate

High quality free-standing poly(9-fluorenone) (PFO) films were synthesized electrochemically by direct anodic oxidation of 9-fluorenone (FO) in pure boron trifluoride diethyl etherate (BFEE). The oxidation potential of FO in this medium was measured to be only 1.48 V versus SCE, which was greatly lower than that determined in CH₂Cl₂ + 0.1 mol l⁻¹ Bu₄NBF₄ (2.21 V versus SCE). PFO films obtained from BFEE showed good electrochemical behavior, good thermal stability with electrical conductivity of 7.8 × 10⁻³ S cm⁻¹, indicating that BFEE was a better medium than CH₂Cl₂ for the electrosyntheses of PFO films. Structural studies showed that the polymerization of FO ring occurred at 2,7-position. As-formed PFO films can be partly dissolved in acetone and tetrahydrofuran (THF). Fluorescent spectral studies indicate that PFO is a good blue light emitter. To the best of our knowledge, this is the first report on the electrosyntheses of free-standing PFO films.     

The spatial uniformity and electromechanical stability of transparent, conductive films of single walled nanotubes

We have prepared thin films of arc discharge single walled nanotubes by vacuum filtration. For film thicknesses greater than 40 nm, the films are of high optical quality; the optical transmission varies by <2% over the film area when measured with a spatial resolution of 4 μm. However, the films become spatially non-uniform for film thickness below 40 nm. The in-plane DC conductivity correlates with the uniformity, increasing from ∼3800 S/m for a 10 nm thick film to ∼2-2.5 × 10⁵ S/m for films of thickness >40 nm. Conductive atomic force microscopy maps show reasonably uniform current flow out of the plane of the film. For all thicknesses, the optical transmittance scales with film thickness as expected for a thin conducting film with optical conductivity of 1.7 × 10⁴ S/m (λ = 550 nm). For films with t > 40 nm the ratio of DC to optical conductivity was σ_DC/σ_Op = 13.0, leading to values of transmittance and sheet resistance such as T = 80% and R_s = 110 Ω/□ for the t = 40 nm film. Electromechanically, these films were very stable showing conductivity changes of <5% and <2% when cycled over 2000 times in compression and tension respectively.     

Hole transport in conducting ultrathin films of PEDOT/PSS prepared by layer-by-layer deposition technique

Multilayered ultrathin films of a conductive polymer, poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) were prepared by layer-by-layer deposition technique. These films were characterized by absorption spectroscopy, atomic force microscopy, cyclic voltammetry and potential step chronoamperometry. The PEDOT/PSS films were layered up with a bilayer thickness of 5 nm and the surface roughness of the films was improved after the ultrasonicated pretreatment of a PEDOT/PSS aqueous dispersion prior to the deposition. The ultrathin films thus obtained kept excellent diffusion constant of hole carriers, 5×10⁻¹⁰ cm² s⁻¹, as high as that of spin-cast films of PEDOT/PSS, indicating that the conducting polymer films are fabricated with nanometer-scale precision and act as a junction layer between the electrode and electrochemically active organic materials.     

Effects of the oxygen pressure on the crystalline orientation and strains of YSZ thin films prepared by E-beam PVD

Yttria−stabilized zirconia, YSZ, thin films were prepared by E-beam physical vapor deposition (PVD) at 200 °C under oxygen pressure of 1 × 10⁻³∼1 × 10⁻⁵ Torr. Observations by Field Emission Scanning Electron Microscope (FESEM) proved that different oxygen pressures influenced the thickness of interfacial SiO_x layer formed between the YSZ thin films and Si(100)-substrate. X-ray diffraction (XRD) patterns were used to determine the crystalline structure and calculate the surface grain size of deposited YSZ thin films. XRD patterns also showed that the peaks corresponding to planes (111), (200), (220), and (311) were found and the YSZ thin films revealed the fluorite structure. At lower oxygen pressure (1 × 10⁻⁵∼1 × 10⁻⁴ Torr) YSZ thin films revealed the (111) preferred orientation and at higher oxygen pressure (5 × 10⁻⁴∼1 × 10⁻³ Torr) YSZ thin films revealed the (200) preferred orientation. The effects of oxygen pressure on the lattice constants and the internal strains of YSZ thin films were also investigated.     

Changes of properties of cured and uncured disiloxane bisbenzocyclobutene thin films under irradiation

The effects induced by α-particles and laser beam irradiation in air atmosphere in uncured and cured bisbenzocyclobutene (BCB) 2 μm thick films, spin-coated on glass/ITO surface have been investigated. α-Particle irradiation was done by means of a thin film ²⁴¹Am source (E_α = 5.486 MeV), up to the total fluence of about 5 × 10¹⁰ particles/cm². Laser irradiation was performed by a Nd³⁺:YAG (λ = 1.06 μm) laser in the free generation and the Q-switch regime, using both focused and unfocused beams. Irradiation induced changes were investigated using Light and Atomic Force Microscopy (AFM), infrared (IR), Ultraviolet/visible (UV/vis) and Raman spectroscopy by inspecting several uncured and cured BCB films before and after irradiation. It has been found that both types of irradiation under investigated conditions have produced a novel phase in the material, which is not present either in the uncured or the cured BCB films. Possible implications of the observed effects on curing and degradation of BCB films have been discussed.     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

Solution-processed semiconducting aluminum-zinc-tin-oxide thin films and their thin-film transistor applications

We prepared aluminum-zinc-tin-oxide (AZTO) thin films by the solution spin-coating method and investigated their physical and electrical properties according to different incorporated amounts of Al. AZTO films annealed at 400 °C were amorphous. Though SnO₂ crystallites were detected in films annealed at temperatures higher than 500 °C, the number of crystallites decreased as the Al content increased. Thin films had a smooth and uniform surface morphology with an optical transmittance value higher than 92% in the visible range. Electrical conductivity and its temperature dependence varied markedly according to the amount of Al incorporated in the film. We therefore systematically investigated activation energies for carrier transport for each film composition. Thin-film transistors (TFTs) were fabricated using solution-processed AZTO as an active channel layer. The effects of the amount of Al incorporated in the thin film on TFT characteristics were also evaluated. The best device performance was observed for a TFT with a 5 mol%-Al-incorporated AZTO channel. Field effect mobility, subthreshold swing, and on/off ratio were approximately 0.24 cm² V⁻¹ s⁻¹, 0.69 V/dec, and 1.03×10⁶, respectively.     

Amperometric glutamate biosensor based on chitosan enzyme film

A simple method for enzyme immobilization in electrochemical biosensors for monosodium glutamate (MSG) was developed. The method relied on the precipitation of complexes of polyanionic enzyme l-glutamate oxidase (GmOx) with polycationic chains of chitosan (CHIT) on the surface of platinum electrode. Such ionotropic gelation allowed the CHIT matrix to retain ∼57% of applied GmOx (0.30-3.0 units). The CHIT + GmOx based biosensor displayed a low detection limit of 1.0 × 10⁻⁷ M MSG (S/N = 3, E = 0.400 V), linear range up to 2 × 10⁻⁴ M (R² = 0.991), sensitivity of 85 mA M⁻¹ cm⁻², and a short response time (t_90% = 2 s). The biosensors maintained ∼80% of the MSG signal even after 11 h of continuous use, which indicated good operational stability. Stability studies revealed that a majority of signal loss was due to a slow transformation of glutamate in a solution into the redox inactive pyroglutamate. After 4 months of storage in water at 4 °C, the CHIT + GmOx films retained ∼80-90% of their original activity toward the MSG. The CHIT + GmOx films are promising candidates for the development of simple and reliable chromatographic detectors for glutamate.     

Li-intercalation behaviour of vanadium oxide thin film prepared by thermal oxidation of vanadium metal

In order to produce thin films of crystalline V₂O₅, vanadium metal was thermally oxidised at 500 °C under oxygen pressures between 250 and 1000 mbar for 1-5 min. The oxide films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). The lithium intercalation performance of the oxide films was investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). It was shown that the composition, the crystallinity and the related lithium intercalation properties of the thin oxide films were critically dependent on the oxidation conditions. The formation of crystalline V₂O₅ films was stimulated by higher oxygen pressure and longer oxidation time. Exposure for 5 min at 750 mbar O₂ at 500 °C resulted in a surface oxide film composed of V₂O_5, and consisting of crystallites up to 200 nm in lateral size. The thickness of the layer was about 100 nm. This V₂O₅ oxide film was found to have good cycling performance in a potential window between 3.8 and 2.8 V, with a stable capacity of 117 ± 10 mAh/g at an applied current density of 3.4 μA/cm². The diffusion coefficients corresponding to the two plateaus at 3.4 and 3.2 V were determined from the impedance measurements to (5.2 and 3.0) × 10⁻¹³ cm² s⁻¹, respectively. Beneath the V₂O₅ layer, lower oxides (mainly VO₂) were found close to the metal. At lower oxygen pressure and shorter exposure times, the oxide films were less crystalline and the amount of V⁴⁺ increased in the surface oxide film, as revealed by XPS. At intermediate oxygen pressures and exposure times a mixture of crystalline V₂O₅ and V₆O₁₃ was found in the oxide film.     

Linear sweep anodic stripping voltammetry of heavy metals from nitrogen doped tetrahedral amorphous carbon thin films

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were deposited on p-Si (1 1 1) substrates (1 × 10⁻³ to 6 × 10⁻³ Ω cm) by a filtered cathodic vacuum arc technique with different nitrogen flow rates (3 and 20 sccm). The ta-C:N film coated samples were used as working electrodes to detect trace heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and mercury (Hg) by using linear sweep anodic stripping voltammetry in 0.1 M KCl solutions (pH 1). The influence of nitrogen flow rate on the sensitivity of the films to the metal ions was investigated. The results showed that the current response of the ta-C:N film electrodes was significant to differentiate all the tested trace metal ions (Zn²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) and the three ions (Pb²⁺ + Cu²⁺ + Hg²⁺) could be simultaneously identified with good stripping peak potential separations.     

Diffusional mass transfer model for the adsorption of food dyes on chitosan films

The adsorption kinetics of erythrosine B and indigo carmine on chitosan films was studied by a diffusional mass transfer model. The experimental curves were obtained in batch system under different conditions of stirring rate (80–200 rpm) and initial dye concentration (20–100 mg L⁻¹). For the model development, external mass transfer and intraparticle diffusion steps were considered and the specific simplifications were based on the system characteristics. The proposed diffusional mass transfer model agreed very well with the experimental curves, indicating that the surface diffusion was the rate limiting step. The external mass transfer coefficient (k_f) was dependent of the operating conditions and ranged from 1.32 × 10⁻⁴ to 2.17 × 10⁻⁴ m s⁻¹. The values of surface diffusion coefficient (D_s) increased with the initial dye concentration and were in the range from 0.41 × 10⁻¹⁴ to 22.90 × 10⁻¹⁴ m² s⁻¹. The Biot number ranged from 17.0 to 478.5, confirming that the intraparticle diffusion due to surface diffusion was the rate limiting step in the adsorption of erythrosine B and indigo carmine on chitosan films.     

Effects of rf-power and working pressure on formation of rutile phase in rf-sputtered TiO2 thin film

Thin TiO₂ films have been deposited on glass substrates by a radio-frequency (rf) magnetron sputtering technique. The films were coated under argon atmosphere at three different rf-powers: 80, 100 and 120 W, and three working pressures: 1.0 × 10⁻², 2.5 × 10⁻³ and 1.0 × 10⁻³ mbar. Film structures were analyzed with XRD. At 100 and 120 W, films coated under low working pressure have developed the rutile phase with the preferred (1 1 0) orientation. However, at 80 W, the films have been observed only in an amorphous phase for all working pressures. This effect could be understood as sputtered TiO₂ molecules were more energetic at high rf-powers and encountered fewer collisions at low pressure before deposited onto the substrates. The films have also been annealed at 773 or 873 K. The post-deposition annealing has significantly improved crystallization of the TiO₂ films. In this contribution, results on optical and wetting properties of these films are also reported.     

Spontaneously ordered TiO2 nanostructures

Titanium dioxide thin films were deposited on quartz substrates kept at different O₂ pressures using pulsed laser deposition technique. The effects of reactive atmosphere and annealing temperature on the structural, morphological, electrical and optical properties of the films are discussed. Growth of films with morphology consisting of spontaneously ordered nanostructures is reported. The films growth under an oxygen partial pressure of 3 × 10⁻⁴ Pa consist in nanoislands with voids in between them whereas the film growth under an oxygen partial pressure of 1 × 10⁻⁴ Pa, after having being subjected to annealing at 500 °C, consists in nanosized elongated grains uniformly distributed all over the surface. The growth of nanocrystallites with the increase in annealing temperature is explained on the basis of the critical nuclei-size model.     

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.     

Structural,electrical, and optical properties of Sb-doped SnO2 transparent conductive oxides fabricated using an electrospray technique

Sb-doped SnO₂ (ATO) thin films, for use as transparent conductive oxides (TCOs), were synthesized using an electrospray technique, and their structural, electrical, and optical properties were investigated. To elucidate the optimum fabrication conditions for the best electrical and optical properties, ATO thin films were calcined using four different temperatures, 450 °C, 550 °C, 650 °C, and 750 °C. When calcined at 650 °C, ATO thin films exhibit excellent resistivity (~8.14×10⁻³ Ω cm), superior transmittance (~91.4% at 550 nm), and good figure of merit (~11.4×10⁻⁴ Ω⁻¹) compared to the other samples. The enhanced properties of ATO thin films are attributed to high densification without formation of cracks, and the increased grain size of ATO nanoparticles.     

Spray deposited titanium oxide thin films as passive counter electrodes

Titanium dioxide (TiO₂) thin films were deposited from methanolic solution onto fluorine doped tin oxide coated conducting glass substrates by spray pyrolysis technique. The electrochemical properties of TiO₂ thin films were investigated using cyclic voltammetry, chronoamperometry, chronocoulometry and iono-optical studies, in 0.1N H₂SO₄ electrolyte. Performance of the films deposited at three different substrate temperatures, viz. 350, 400 and 450 °C is discussed in view of their utilization in electrochromic devices, as counter electrode. The magnitude of charge storage capacity, Q/t (4.75-6.13 × 10⁻³ mC/(cm² nm)) and colouration efficiency (3.2-4.3 cm²/mC) of TiO₂ rank these films among the promising counter electrodes in electrochromic devices.     

Sticking probability of CN radicals

The sticking probability, s, of CN(X²Σ⁺) radicals which were the precursor of the formation of amorphous carbon nitride films with high [N]/([N]+[C]) ratios (≤ 0.5) was re-evaluated. CN(X²Σ⁺) radicals were generated from the decomposition of BrCN with the microwave discharge flow of Ar of the pressure of 0.2-0.4 Torr. The number density of CN(X²Σ⁺), n_CN(X), was evaluated from the intensity of the CN(A²Π_i-X²Σ⁺) laser-induced fluorescence spectrum calibrated against Rayleigh scattering intensity of Ar. The weight of the C and N components of films, w, was evaluated from the compositional analysis for the deposited films using Rutherford back scattering and elastic recoil detection analysis. The [N]/([N]+[C]) ratios of films were 0.4-0.5. Based on n_CN(X), w, and the flow speed measured by a time-resolved emission, s was evaluated both under the desiccated and H₂O-added conditions as (8.5 ± 2.1) × 10^− 2 − (6.1 ± 1.2) × 10^− 2 and (11.4 ± 1.3) × 10^− 2 − (7.4 ± 1.8) × 10^− 2, respectively. The variation of s under various experimental conditions was discussed based on the electron densities in the reaction region.     

Facile electrosynthesis of nitro-group-substituted oligopyrene with bicolored emission

Nitro-group-substituted oligopyrene (ONP) film with fairly high electrical conductivity (1.25 × 10⁻¹ S cm⁻¹) and good thermal stability was electrochemically synthesized by direct anodic oxidation of its monomer 1-nitropyrene (NP) in boron trifluoride diethyl etherate (BFEE). The oxidation potential of NP in this medium was determined to be 1.12 V vs. SCE, which was lower than that in acetonitrile +0.1 mol L⁻¹ Bu₄NBF₄ (1.27 V vs. SCE). ONP films obtained from this medium showed good redox activity and structural stability in both BFEE and concentrated sulfuric acid. Fourier transform infrared spectra and theoretical calculations showed that the electropolymerization of the NP monomer mainly occurred at the C(3), C(6) and C(8) positions. The fluorescence spectra suggested that soluble ONP emits strong blue or green fluorescence when excited at 402 nm or 504 nm, respectively. Scanning electron microscopy showed that highly crystalline nitro-group-substituted oligopyrene was formed on the electrode surface. All these results indicate that as-prepared ONP film has many potential applications in various fields.