Iodine-doped Samples Research Articles

Growth and properties of iodine-doped ZnS films grown by low-pressure MOCVD using ethyliodine as a dopant source

Iodine-doped ZnS films have been grown by low-pressure metalorganic chemical vapor deposition using ethyliodine as a dopant source, and their electrical and optical properties have been investigated for several doping conditions. It is shown that the electron density can be controlled from 5 × 10 16 to 2 × 10 19 cm -3 by changing doping conditions. In photoluminescence properties at 18 K of iodine-doped samples, an excitonic emission bound to neutral shallow donors (I 2) and a self-activated (SA) emission have been observed. It has been found that their intensities increase with electron density in lightly doped films. However, in heavily doped films the intensity of the I 2 emission decreases with electron density and that of SA emission tends to saturate. Furthermore, it has been found that the incorporation rate of iodine into the films is affected by growth temperature and [VI]/[II] source gas molar ratio.

Impurity clustering in doped polyacetylene.

We use an augmented Su-Schrieffer-Heeger model to study the changes that occur in polyacetylene in the low-doping regime as doping concentration is increased. The lowest-energy configurations are found to be those for which the impurities are clustered together. The clustering results from the net interaction between impurity atoms and pinned solitons. Clustering explains the doping-induced shifts observed in the infrared spectrum of iodine-doped samples. Infrared results for widely spaced impurities are in marked contrast with experiment.

Synthesis and properties of highly-conductive linear stacked polymers: [FePcFI x] n

Stacked, fluorine-bridged iron phthalocyanine is first synthesized and doped with iodine to give [FePcFI x ] n compositions, where x = 0.8−1.4. The iodine content of the compounds is determined by elemental analysis and thermogravimetric analysis (TGA). Infrared spectroscopy and X-ray diffraction studies are used to characterize the structure of [FePcF] n . It is verified that [FePcF] n possesses a linear stacked fluorine-bridged structure. Partially oxidized complexes, [FePcFI x ] n x = 0.8 and 1.4), are prepared by the reaction with I 2 in CCl 4. The electric conductivity of the compressed powder of [FePcFI 1.4] n is 5 × 10 −3 S cm −1 at room temperature, with an activation energy of 0.09 eV. Iodine doping results in increases in the conductivity by up to five orders of magnitude. This is probably because of the eclipsed ring structure of [FePcF] n , which persists in the iodine-doped samples and makes up for a long inter-ring separation. These complexes are stable in air and unaffected in vacuo at room temperature. It is also verified that the linear fluorine-bridged structure is not affected by the introduction of iodine up to I/Fe = 1.4.

X-ray diffraction studies of pristine and heavily doped porous polyacenic materials

X-ray diffraction studies of porous polyacenic semiconductive materials have been performed with respect to the pristine and heavily doped states. The incorporated ZnCl 2 in the procedure of the heat treatment almost does not change among all the samples. On the other hand, the values of d c in all the iodine-doped samples decreased from those of the pristine samples, which is similar to the manner of d c in the previous iodine-doped polyacenic semiconductive material.

Polarization properties of the Raman spectra in neutral and iodine doped cis-trans (CH) x

Polarized resonant Raman spectra of oriented cis-trans -(CH) x are presented for undoped and iodine doped samples. For undoped films the depolarization ratios of cis lines are consistent with a fibrils misorientation of about 20°. Similar values of depolarization ratios have been measured in iodine doped films as well as for the cis and trans lines than for I 3 − mode. These results mean that doping does not destroy the orientational ordering of polymer chains and that under doping the iodine takes the orientation of these chains.

Morphology and structure of highly oriented polyacetylene

Highly oriented polyacetylene films have been synthesized under high magnetic field (20 KG; 47 KG); using as solvent of the catalyst the nematic phase of a liquid crystal. We report here on morphology of undoped and iodine doped samples studied by transmission electron microscopy (T.E.M.) and electron diffraction (E.D.). Results about orientation, crystalline order, crystallite size are reduced.

Structure of polyacetylene–iodine complexes

We confirm the existence of a 15 Å period in iodine-doped polyacetylene and provide a new interpretation for this key feature as part of a general model for structural changes during iodine doping. The observed diffraction intensities for different samples suggest the existence of structures with two different types of dopant-containing layers: layers obtained by complete replacement of polyacetylene chains by iodine columns (F layers) and layers obtained by replacement of every other polyacetylene chain by an iodine column (P layers). The F layers in the heavily doped complex alternate with dopant-free layers of polyacetylene chains (U layers), corresponding to a (UF)n stacking sequence. The phase obtained at a lower dopant concentration, which provides the 15 Å spacing, is attributed to a (UPUF)n stacking sequence. At still lower dopant concentrations, one obtains a (UP)n stacking sequence. This model, along with published Raman, Mössbauer, and photoelectron spectroscopy data, suggests that the ratio of I−5 to I−3 increases in going from P layers to F layers. Intense and monotonically decreasing, diffuse x-ray scattering suggests that vacancies of size ∼3 Å are present, probably in iodine columns. A diffuse reflection at 3.1 Å, observed in all iodine-doped samples, is due to an average iodine–iodine distance in disordered columnar arrays. On the other hand, ordered arrays of iodine columns in oriented samples give rise to sharp meridional reflections. All ten observed reflections (down to 1.17 Å) in one sample could be indexed based on a 33.8 Å repeat corresponding to (–I−3–I−5–I−3–)n arrays. The observed diffraction pattern was calculated from this model without using any freely adjustable parameters.

Physical properties of highly oriented polyacetylene films

This paper is devoted to the characterization of highly oriented (CH) x films synthesized onto liquid crystal oriented by a strong magnetic field. The morphology of the films is studied by SEM and TEM. The fibril orientation and crystallinity are investigated by X-ray and electron diffraction. The anisotropic characteristics are measured by optical and transport measurements on undoped and iodine-doped samples. A longitudinal d.c. conductivity as high as 20 000 Ω −1 cm −1 was obtained.

Studies on electrical and paramagnetic properties of poly(phenylene sulphide)

AbstractElectrical and paramagnetic properties of poly(phenylene sulphide) synthesized by a direct method from benzene and sulphur were investigated. In darkness dc conductivity of the polymer in its pristine state is of the order 10−10 S/m. Upon doping with iodine it increases by 8 orders of magnitude. For undoped samples the n‐type conductivity and for those doped with iodine the p‐type conductivity was observed. On the basis of ESR experiments the number of spins and g‐value were determined, which for undoped polymer at room temperature are equal to 0.80 · 1017 spins/g and 2.0037, respectively. Next, the changes in spin concentration both for the iodine doped samples and these annealed within the temperature range 513 – 903 K were described. An attempt to discuss the ESR lineshape in a quantitative way was also made.

Studies on some properties of pristine and iodine-doped poly(phenylene sulphide)

Some properties of poly(phenylene sulphide) prepared by direct synthesis from benzene and sulphur are discussed in the light of thermal analysis, X-ray, SEM, and infrared data. Pristine poly(phenylene sulphide) is characterized by great thermal stability; it loses only 22.5% of its mass during dynamic heating to 873 K. Mass loss for iodine-doped samples increases to 41% depending on the iodine content. Doping with iodine also changes crystallinity, morphology and brings about a rise of electrical conductivity by several orders of magnitude. A conductivity of 1.45 × 10−2 S m−1 was obtained in air at room temperature for the poly(phenylene sulphide) sample containing 22.6% iodine.

High electrical conductivity in doped polyacetylene

The electrical conductivity of conducting polymers results from mobile charge carriers introduced into the π-electron system through doping1,2. Because of the large infra-chain transfer integrals, the transport of charge is believed to be principally along the conjugated chains, with inter-chain hopping as a necessary secondary step. In conducting polymers, as in all metals and semiconductors, charge transport is limited by a combination of intrinsic electron–photon scattering and sample imperfection. Although relatively high conductivities (σ ≈ 1,000 S cm−1) have been reported for partially orientated and heavily doped polyacetylene1–3, the absence of a metal-like temperature dependence implies that the observed values are not intrinsic. In doped polyacetylene, (CH)x, electrical transport can be limited both by microscopic defects (leading to scattering and localization) and by the more macroscopic complex fibrillar morphology12 and associated interfibrillar contacts. Thus, with improvements in material quality, one might anticipate corresponding improvements in the electrical conductivity. Here we report the synthesis of polyacetylene with fewer sp3 defects than in material prepared by other methods. The higher-quality material exhibits substantially higher electrical conductivity; maximum values of >20,000 S cm−1 are obtained after doping with iodine. The conductivity has been measured as a function of temperature and pressure: at 0.48 K and 10 kbar, iodine-doped samples remain highly conducting (σ ≈ 9,000 S cm−1).

Analysis of resonant Raman scattering spectra of fully oriented undoped and iodine-doped trans-polyacetylene: Experiments and theory.

Polarized resonant Raman spectra of oriented trans-polyacetylene taken at different excitation wavelengths (${\ensuremath{\lambda}}_{L}$=647.1 and 457.9 nm) are presented for undoped and iodine-doped samples together with theoretical calculations based on distributions of conjugated segments. A detailed analysis of the band shapes and peak positions of the Raman bands in the two different cases reveals important differences, in agreement with a modification upon doping of the morphology of the polymer. The proposed model turns out to be appropriate for interpreting all the experimental data.

The influence of iodine on the electrical properties of lead phthalocyanine

The influence of iodine on the electrical conductivity and thermoelectric power of lead phthalocyanine thin films and pressed samples was investigated. For iodine-doped samples the electrical conductivity rises by seven to nine orders of magnitude. It is very interesting that for these highly conducting materials a compensation rule for the conductivity is also valid. For some samples the mobility values and their temperature dependences are consistent with band model predictions.

Doped poly(thiophene): Electron spin resonance determination of the magnetic susceptibility

E.S.R. studies of poly(thiophene) doped with AsF 5 and iodine are reported. At high AsF 5 concentration (∼ 24 mole % AsF 5), we find a temperature-independent Pauli spin susceptibility indicative of a metallic density of states. Measurements on iodine-doped samples in the intermediate regime (below 10 mole % I 3 −) indicate Curie spin concentration independent of the doping level, but significantly larger than in pristine poly(thiophene). The relaxation of these Curie spins is relatively fast (faster by several orders of magnitude with respect to that of pure poly(thiophene)) and temperature dependent. These features are discussed in terms of charge storage in polarons and bipolarons, and the possibility of formation of a finite density of electronic states in the gap.

X-ray diffraction studies of iodine-doped polyacetylene

The results of an extensive X-ray and synchrotron radiation diffraction study of the structure of cis- and trans-polyacetylene doped with iodine are presented. Studies of rapidly and slowly iodine-doped samples confirmed that undoped regions of (CH) x remain. The undoped regions of the initially cis-(CH) x are shown to isomerize to the trans-(CH) x structure during the doping process. Two new previously unreported X-ray reflections have been recorded for both doped cis- and doped trans-isomers. The data are in agreement with the model of intercalation of iodine between planes of close-packed polyacetylene chains.

The dc conductivity of polyphenylacetylene below room temperature

The temperature dependence of the dc conductivity for doped and undoped cis-polyphenylacetylene (PPA) is analyzed at temperatures of 110 to 300 K. The dc conductivity (σdc) data of arsenic pentafluoride-doped and iodine-doped samples suggest two major current conduction processes separated by a characteristic temperature (Tc) of about 257 K, which is independent of the doping level. At T<Tc, σdc is independent of the temperature within experimental error. At T>Tc, σdc is activated with an activation energy that, for PPA-AsF5, varies from 0.86 eV to 0.51 eV for doping levels of 2×10−3 to 7×10−2 moles dopant/moles monomer, respectively. PPA annealed at 130 °C, which is known to undergo isomerization and cyclization reactions, has a Tc of about 210 K, and σdc does not respond to doping. Possible conduction mechanisms are discussed.

X-RAY DIFFRACTION STUDIES OF IODINE DOPED POLYACETYLENE

X-ray and synchrotron radiation diffraction study of the structure of cis and trans polyacetylene prepared by the Shirakawa method has been carried out. Studies of rapidly and slowly iodine doped samples confirmed that undoped regions of (CH)x remain. The undoped regions of the initially cis (CH)x are shown to isomerize to the trans structure during the doping process.

Morphology of polyacetylene and doped polyacetylene

A detailed study of the morphology of polyacetylene and iodine and arsenic pentafluoride doped polyacetylene has been carried out using scanning electron microscopy and transmission electron microscopy techniques. The results reveal a variety of fibrillar and rod-like morphologies for trans-polyacetylene. The samples retain this morphology upon doping, with, in many cases, a significant increase in the diameter of these structures. Larger increases in diameter were observed for the AsF 5 doped samples than in iodine doped samples. Energy dispersive X-ray analysis and back-scattered electron imaging were used to determine dopant distribution. Both iodine and arsenic distributions were uniform across the film surfaces to a resolution of 5000 Å. In addition, both dull and shiny sides of the films are shown to have approximately the same dopant concentration. This demonstrates that nonuniformity of the doping occurs, if at all, at a much finer scale. Finally, we have determined that small crystals, presumably arsenic trioxide, form on the surface of AsF 5 doped polyacetylene upon even moderate exposure to air.

Investigation on the doping of Polyacetylene Films

We have investigated the oxydation and iodination of (CH)x-films and found that iodine doped samples degrade in oxygen at a lower rate than pure samples. The iodination can be described as a diffusion process, the diffusion constant being D ≈ 10-14 − 10−13 cm2/sec. This value for D is by many orders of magnitude smaller than those normally encountered for diffusion of gases in polymers. The insulator-metal transition, observed at high iodine concentrations, was investigated by optical transmission measurements. The results show that the forbidden gap remains roughly unaffected if the conductivity changes from the semiconducting to the metallic state. Also, below the gap a peak in absorption builds up with increasing iodine concentration but no absorption characteristic for free carriers is observed. These experimental observations are of importance for the selection of models which are to describe the insulator-metal transition and some consequences hereof are discussed.

Thermal conductivity and specific heat of pure and iodine doped polyacetylene (CH)x

A measurement of the thermal conductivity and the specific heat of pure and iodine doped samples of polyacetylene (CH)x are presented here. The thermal conductivity of pure (CH)x is about 75 mW/cm . K at room temperature and decreases monotonically to 1.6 mW/cm . K at 25 K. The specific heat dependence with temperature is roughly linear from about 0.5 J/cm3 . K at 300 K down to 0.04 J/cm3 . K at 25 K. Then the sample was doped in situ and two iodine concentrations were studied : 1.5 % and 4 %. Within experimental error no change in the specific heat behaviour was observed after this operation, but the thermal conductivity increased significantly. The change was ∼ 5 mW/cm . K for the 4 % doped sample at room temperature and could be attributed to a charge carriers contribution. Assuming Wiedeman-Franz law to hold in this case, one finds that the intrinsic electrical conductivity is about 700 times higher than the measured value.