Iodine-doped Films Research Articles

Nanomorphology and performance of pure and doped lead selenide for infrared detector

Lead selenide (PbSe) has been studied as a promising material for room temperature midwave infrared detection. We have investigated pure PbSe, as well as tin (Tn) and cadmium (Cd)-doped PbSe, nanocrystalline materials produced using physical vapor transport methods on glass and high-resistivity silicon substrates. The morphologies were investigated by scanning electron microscopy and energy-dispersive x-ray analysis. Pure PbSe layers consisted of nanocrystals that change into cubes and cuboids upon annealing. Cuboids generally grew in [100] orientation and ultimately developed in nanorods. Growth on silicon and glass substrates showed different morphologies of pure PbSe material. Parabolic and elongated morphologies resulted in nanowires on the top of thin layers of PbSe nanofilm, which acted as the substrate. Under low gradient annealing conditions (<20 K/cm), elongated morphologies grew into nanorods. Annealing of these samples resulted in coarse nanomorphologies with higher resistivity. In the case of Tn-doped PbSe, annealing dissolved a Tn-rich phase observed in as-grown films. Cd- and iodine-doped films produced through the addition of Cd selenide and Cd iodide, respectively, showed higher resistivity than similarly treated pure PbSe films. Annealing of as-grown materials in the presence of oxygen or iodine showed increased resistivity and significant changes in optical characteristics.

Effect of iodine doping in the deposition solution and iodine vapor pressure in the sensitization treatment on the properties of PbSe films

Effect of iodine-doping in the deposition solution and iodine vapor pressure during the sensitization process on the morphological, microstructural, electrical, and optical properties of PbSe films was studied. Undoped and iodine-doped PbSe films of polycrystalline particles were coated on thermally oxidized silicon substrates by chemical bath deposition. The PbSe films were oxidized at 380°C for 30 min and then iodinated at different iodine vapor pressures at 380°C for 5 min. When the iodine vapor pressure was below 20 Pa, PbSeO3 was the main phase formed on the surface of PbSe microcrystals for both undoped and iodine-doped films. As the iodine vapor pressure was increased above 20 Pa, Pb3I2O2 and PbI2 phases were formed in both types of films and PbSeO3 disappeared in the undoped film. Only the iodine-doped films showed photo response. The sheet resistance and IR signal-to-noise ratio had maximum values at the iodine vapor pressure of 17.5 Pa in the iodine-doped film. The x-ray diffraction spectra, scanning electron microscopy morphologies, and EDS analyses of the sensitized PbSe films show that the main role of iodine in the sensitization is helping solid-state sintering of PbSe microcrystals which may lead to redistribution of oxygen atoms in the effective atomic sites.

Contradictory ageing behaviour and optical property of iodine doped and H2SO4 doped pulsed DC plasma polymerized aniline thin films

Polymerization of aniline in the presence of H2SO4 and iodine vapour is carried out using pulsed DC glow discharge plasma. The as prepared iodine doped plasma polymerized aniline films are found to be highly crystalline due to some crystal structure formed by the iodine incorporated into the film and the ex-situ H2SO4 doped films are found to be amorphous. However, the structure of the iodine doped films transforms to amorphous one while that of the ex-situ H2SO4 doped films transforms to crystalline one due to the effect of ageing. The optical band gap of the samples decreases depending upon the level of doping and annealing, having minimum optical band gap for iodine doped plasma polymerized aniline. Due to ageing, the optical property of the H2SO4 doped film is found to become superior while for iodine doped films a negligible change of the optical property has been noticed even after six months of ageing.

Gold nanoparticle-templated assembly of oligothiophenes: preparation and film properties

Terthiophene-appended gold nanoparticles were prepared by the reduction of AuCl 4 −(C 8H 17) 4N + with NaBH 4 in the presence of bis[2,5-di(3-hexylthiophen-2-yl)thiophene-3-carboxyloxyhexanyl]disulfide. A hexagonal self-assembly of particles with gold core diameters (1.9±0.1 nm) was detected by high-angle annular dark-field scanning transmission electron microscopy. The electric conductivity of the iodine-doped film was 9.1×10 −6 S cm −1, which was ascribable to the terthiophene-based inter-ligand π–π interactions. The Au/terthiophene hybrid spin-coated film consisted of a highly three-dimensional assembled structure of terthiophenes, as inferred from grazing-incidence small-angle X-ray scattering, indicating that such monodispersed and small-sized gold nanoparticles can serve as a template for this organization. In this study, a gold nanoparticle-templated assembly of oligothiophenes has been fabricated for proposing a method to develop tailor-made organizations of π-conjugated oligomers.

Growth of CuI buffer layer prepared by spraying method

CuI thin films with nano-scale grains of about 35nm were deposited via spraying method with using acetonitrile as solvent. The influence of iodine doping concentration in acetonitrile solution on the structure, topographic and optical properties of CuI thin films was investigated. X-ray diffraction results showed that CuI iodine-doped films doped CuI:I 2 were in γ-phase of zinc blende structure with (111) preferential plane. Scanning electron microscopy revealed that the microstructure of CuI films depended on the relative amount of doping iodine in the solution. When the iodine doping amount in acetonitrile solution was 0.025 g, the film was uniform and compact, the optical transmittance was 75.4% in the part of visible region and the energy band gap was close to 2.96 eV.

Fabrication of iodine-doped pentacene thin films for organic thermoelectric devices

We investigate the relation between the morphology and thermoelectric properties of iodine-doped pentacene thin films grown on the glass substrate. The electrical conductivity and thermoelectric power factor are enhanced with decreasing the film thickness and growth temperature due to the volumetric increase in the thin film phase relative to the bulk phase in the film. The 115 and 190 nm thin films grown at room temperature, whose majority phase is the thin film phase, exhibit high power factor of 1.3×10−5 W m−1 K−2. Also, we demonstrate an improved stability of thermoelectric properties by covering the iodine-doped film with a polyimide tape.

Urbach tail analysis on the defect states of polypyrrole thin films prepared by AC plasma polymerization Urbach tail analysis on the defect states of polypyrrole thin films prepared by AC plasma polymerization

AbstractPolypyrrole is a widely studied conducting polymer with excellent optical and electrical properties. Polypyrrole thin films in the pristine and iodine doped forms have been prepared by ac plasma polymerization. The in situ doping of iodine during the process of plasma polymerization modified the optical absorption behaviour of the thin films due to the incorporation of charged defects in the film structure. Optical band gap and related constants are estimated from optical absorption studies. The Urbach tail analysis of the pristine and doped thin films shows that more defect levels are created, when the polymerization was carried out in the iodine atmosphere. The electrical properties of these thin films were estimated from the conductivity measurements. It is found that in the case of iodine doped films, the optical band gap decreased and electrical conductivity enhanced when compared to the pristine ones. The results are correlated with the structural differences of the pristine and doped thin films with the help of Fourier Transform Infrared Spectroscopy (FTIR) analysis.

Chemoenzymatic synthesis of amylose-grafted poly(vinyl alcohol)

In this study, we synthesized an amylose-grafted poly(vinyl alcohol) (PVA) (4) by a chemoenzymatic method. First, synthesis of an amine-functionalized PVA (2) was carried out by reduction of an azide-functionalized PVA (1) using NaBH4. Then, introduction of maltoheptaose to 2 was performed by reductive amination using NaBH3CN to produce a maltoheptaose-grafted PVA (3). Finally, a phosphorylase-catalyzed enzymatic polymerization of α-D-glucose 1-phosphate from the maltoheptaose-chains in 3 was carried out to obtain 4. A film of 4 was prepared by drying its aqueous solution spread on a flat quartz glass substrate. An iodine-doped film was prepared by soaking the film of 4 in KI/I2ethanol solution. UV-Vis analysis indicated that iodine was retained in this film even after it was left standing for 24 h. This is probably because the iodide ion was within the cavities of the amylose graft chains in this film.

Thermal Diffusivity of Plasma-Polymerized Polyaniline Films by Transverse Probe Beam Deflection Method

Undoped and iodine-doped polyaniline films were prepared by radio frequency plasma polymerization technique. The dependence of the thermal diffusivity of these films on doping and the reactor pressure is investigated by transverse probe beam deflection method in the skimming configuration.

Influence of iodine on the electrical properties of nickel phthalocyanines thin film devices

The electrical properties of pure Nickel Phthalocyanine (Ni Pc) thin films and iodine doped Ni Pc thin films with Gold and Aluminium electrode sandwiched devices have been investigated. The electrical properties and the various electrical parameters of the pure and iodine-doped films have been estimated and compared from the analysis of the current density characteristics. From our study we find that the iodine doping enhanced the electrical conductivity compared to pure Nickel Phthalocyanine and the conduction mechanism is much improved with the iodine doping. Interestingly these films after iodine doping showed remarkably increased electrical conductivity nearly ten times that of pure Phthalocyanine. This may be accounted for by the decrease in the metal-metal bond distance. At low voltages the film shows an ohmic conduction whereas at higher voltage levels the conduction is dominated by space charged limited conduction. Further the reverse conduction mechanisms have also been investigated for this sandwiched device. From the current limitations in the reverse condition a strong rectifying behaviour is evident.

Effect of iodine on morphology changes of spray deposited nanocrystalline CdSe:I, CdTe:I and HgTe:I thin films

AbstractThe low‐temperature preparation of nanofibres and nanotubes in iodine‐doped CdX (X = Se, Te) and HgTe thin films is reported. Iodine was used as an n‐type dopant in CdX and HgTe. The films were deposited on glass substrates at 200 °C by spray deposition of iodine‐doped nanoparticles prepared using a solvothermal method. The surface morphology of iodine‐doped CdX films shows the formation of nanofibers and that of iodine‐doped HgTe films shows the formation of nanotubes ∼4 µm long with an average diameter of ∼45 nm. Selected‐area diffraction patterns show the formation of cubic HgTe nanotubes with lattice parameter a = 0.6467 nm. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synthesis, Self-Assembly, and Characterization of Supramolecular Polymers from Electroactive Dendron Rodcoil Molecules

We report here the synthesis and self-assembly of a series of three molecules with dendron rodcoil architecture that contain conjugated segments of oligo(thiophene), oligo(phenylene-vinylene), and oligo(phenylene). Despite their structural differences, all three molecules yield similar self-assembled structures. Electron and atomic force microscopy reveals the self-assembly of the molecules into high aspect ratio ribbon-like nanostructures which at low concentrations induce gelation in nonpolar solvent. Self-assembly results in a blue-shifted absorption spectrum and a red-shifted, quenched fluorescence spectrum, indicating aggregation of the conjugated segments within the ribbon-like structures. The assembly of these molecules into one-dimensional nanostructures is a route to pi-pi stacked supramolecular polymers for organic electronic functions. In the oligo(thiophene) derivative, self-assembly leads to a 3 orders of magnitude increase in the conductivity of iodine-doped films due to self-assembly. We also found that electric field alignment of these supramolecular assemblies can be used to create arrays of self-assembled nanowires on a device substrate.

Organization and Properties of a Conjugated Poly(heteroarylene methines) at the Air–Water Interface and in the Langmuir–Blodgett Films

A poly(heteroarylene methine) derivative, poly[(2,5-thiophenediyl) (p-n-methyl, n-octylaminobenzylidene) (2,5-thiophenequinodimethaneiyl)] (PTABQ), has been synthesized and spread at the air–water interface. The influences of three kinds of solutions on PTABQ monolayer behavior at the air–water interface have been investigated via the measurements of the π–A isotherm and film stability. The results show that all three kinds of PTABQ solutions are apt to form the stable and transferable monolayer film organized with the plane of its π-system nearly perpendicular to the air–water interface. Moreover, the monolayer-forming ability of PTABQ can be improved by introducing a water-soluble amphiphilie as an extractable spread-aiding component, which is further proved by the AFM images and FTIR spectra of the transferred films. UV-visible absorption spectra indicate that the well-ordered layer-by-layer structure is successfully controlled in the LB films. The optical bandgap of PTABQ is reduced for the ordered arrangement of its molecules in LB films. The intrinsic electrical conductivity of PTABQ LB films is 8.1×10−8 S/cm and the conductivity of iodine-doped films is 5.7×10−7 S/cm.

Semiconducting mixed Langmuir–Blodgett films of new charge-transfer complex II: N-octadecylbenzidine-TCNQ

The charge-transfer complex of N-octadecylbenzidine–TCNQ has been synthesized and characterized by various spectroscopic techniques. Mixed Langmuir–Blodgett films were constructed onto hydrophobic glass substrates using a 1:1 molar mixture of this complex with 1-docosanol. These films were characterized by small angle X-ray diffraction, microstructures, d.c. and a.c. electrical analyses. Analyses have also been done after exposing these LB films to iodine vapor in order to compare the results. The results of d.c. electrical conductivity measurements showed that conductivity of these films increased by two orders of magnitude after iodine oxidation as compared with that before iodine doping and reached a maximum of 1.4×10 −4 S cm −1. The conductivity of these films, both the as-deposited as well as iodine doped increased as a function of temperature indicating their semiconducting nature. The values of activation energy calculated from the Arrhenius plot were found to decrease from E a=0.52 eV for as-deposited LB films to E a=0.19 eV in case of iodine doped films. a.c. impedance measurements revealed that the equivalent circuit for these films is R( RQ) in both the cases. Lower values of resistance ( R) were observed in case of iodine doped films.

Synthesis and characterization of regiorandom and regioregular poly(3-octylfuran).

Poly(3-octylfuran) has been synthesized with three regioregularities: P3OF-95, P3OF-75, and P3OF-50, where the number signifies the percentage HT content. The 95% HT material is highly crystalline with a structure similar to that of HT-poly(3-octylthiophene), P3OT. The lamellar spacing is 22.1 A and the pi-stacking distance is 3.81 A. UV-vis spectroscopy reveals that P3OF-95 is aggregated in CHCl(3) solution, and solid films of P3OF-95, but not P3OF-75 or -50, show Davydov and exciton band splitting due to the interactions of the pi-systems in the stacked morphology. An estimate of the Davydov splitting is 0.15 eV (1200 cm(-1)). P3OF is reversibly oxidized at 0.32 V vs ferrocene/ferrocenium, but increasing the potential to 1.15 V leads to irreversible oxidation. Films of P3OF may be p-doped with iodine vapor. Doped P3OF-95 and -75 films have electrical conductivities of 10(-2) and 10(-7) S/cm, respectively. The UV-vis-NIR spectra of the iodine-doped films are interpreted in terms of molecular-like transitions involving the LUMO, HOMO, HOMO-1, and transitions across a Peierls distortion-induced gap in the intermolecular conduction band that is formed by the overlap of the pi-systems of the stacked partially oxidized chains. The conduction band gap estimated for P3OF-95 is 0.34 eV, and that for P3OF-75 is 0.9 eV. The P3OF samples are thermally stable in N(2) atmosphere to between 275 degrees C (P3OF-50) and 380 degrees C (P3OF-95), but suffer thermal oxidation above 150 degrees C or light-induced oxidation at room temperature.

Hyperfine structure of magnetic-resonance electrical conductivity of doped polydiacetylene

Magnetic resonance with a hyperfine structure which can be detected from the electrical conductivity has been observed in iodine-doped polydiacetylene films. This resonance is attributable to transitions between Zeeman levels in the spin-dependent carrier transfer process in a system of localized paramagnetic centers.

Evaporated thin films of tetrathiafulvalene derivatives and their charge-transfer complexes

Thin films of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), dimethyl- and trimethyl-TTF have been prepared using the thermal evaporation technique. The films were investigated using electron/optical microscopy, optical absorption spectroscopy and d.c. electrical measurements. A maximum in-plane, d.c. room temperature conductivity of 1.6±0.6 S cm −1 was noted for iodine-doped films of BEDT-TTF. The thermal activation energy, over the range 77–300 K, was 0.028±0.02 eV for this material.

Nitrogen and iodine doping in amorphous diamond-like carbon films

Abstract Thin films of amorphous diamondlike carbon (a:DLC) were deposited by plasma decomposition of hydrocarbon gas, such as methane gas (CH 4 ). The plasma was produced using a r.f. generator. These thin films of a:DLC were doped by incorporation of nitrogen (a:N-DLC) and iodine (a:I-DLC) gases during the deposition process. Microhardness tests showed high hardness of about 4700–4900 kg mm −2 for a:DLC films. The microhardness of the films was reduced by the doping process (3400 kg mm −2 for 10% nitrogen and 3200 kg mm −2 for 10% iodine partial pressure). From optical measurements at visible light (400–800 nm), optical energy band gaps of about 1.1 eV for a:DLC film, 1.39 eV for a:N-DLC and 0.78 eV for a:I-DLC were determined. From measurements of d.c. conductivity as a function of temperature, the electrical activation energies were determined and found to be 0.34 eV for undoped a:DLC films, 0.20 eV for nitrogen-doped films and 0.23 eV for iodine-doped films. The electrical resistivity at room temperature was reduced by almost three and two orders of magnitude with the doping processes, from 10 8 W·cm for undoped film to 5.10 5 W·cm for nitrogen- and iodine-doped films.

Effect of Morphology on the Electrical Transport Properties of Polyaniline Films for Electronic Applications

AbstractAlthough it is well known that the oxidation state of polyaniline is an important characteristic, there are few reports of its influence on the development of morphology and electrical properties in fibers and films. In this work, differential scanning calorimentry is used in conjunction with measurements of temperature dependence of conductivity and thermoelectric power to elucidate the intimate relationship between structure and properties. By increasing the amount of chemical reduction of polyaniline solutions, films are prepared which exhibit a thermal transition between 300 and 385 °C, indicative of melting. Increasing the chemical reduction also increases the conductivity of iodine doped films. The most reduced film exhibited a semiconductor transport mechanism, while the other films could be modeled with a quasi-one dimensional variable range hopping mechanism. The temperature dependence of conductivity also showed increasing order for increasing reduction, consistent with the DSC results.

Structural and chemical characterization of vapor-deposited polythiophene films

The chemical structure, including changes in charge distribution upon doping, for vapor-deposited polythiophene films has been investigated using x-ray photoelectron spectroscopy (XPS). Two different methods of doping, coevaporation with FeCl3 and exposure to iodine vapor, are contrasted in this study and several chemical states are observed for the ‘‘dopants’’ associated with these complexes. In general, the FeCl3 codeposited complexes have considerably higher conductivities than the iodine exposed films (10–25 S/cm as compared to 0.01 S/cm). Furthermore, besides being a more effective doping mechanism, the FeCl3 codeposited complexes are much more stable upon exposure to atmosphere and also in the ultrahigh vacuum environment of the XPS system. Atomic force microscopy images of the same films show clear morphological differences between the as-deposited, the FeCl3 codeposited, and the iodine doped films. The as-deposited film shows a fibrillar-type structure while the FeCl3 codeposited film is observed to coalesce into larger bundles. For the iodine exposed film, the fibrillar structure is present but small bundles with dimensions of several hundred Å have formed throughout the network.