Poly Diphenylene Phthalide Research Articles

ВЛИЯНИЕ ТОЛЩИНЫ ПЛЕНОК ПОЛИДИФЕНИЛЕНФТАЛИДА НА СПЕКТР ТОКОВ ДЕПОЛЯРИЗАЦИИ МНОГОСЛОЙНЫХ СТРУКТУР

The depolarization currents of multilayer polymer films of polydiphenylene phthalide were measured using the thermally stimulated depolarization (TSD) method in the range from 170 to 400 K. The solution concentration was 2.5 and 5 wt.%. Each layer of polymer film was pre-dried at room temperature, then annealed in an oven. The number of layers of polymer film was 1, 2, 3 and 7 layers for 2.5 wt.%. and 1, 2, 3, 5 and 7 layers for 5 wt.%. The polarizing electric field strength was 2.5 × 105 V/cm. The rate of temperature change was 5 K/min. Based on experimental data, the dependence of the temperature of the position of the maximum peak of the depolarization current, the maximum values of the peaks of the depolarization currents, the values of the released charge and the activation energy of relaxation processes on the number of layers of the polymer film is shown. The temperature dependences of depolarization currents revealed two regions for a solution concentration of 2.5 wt.%. and one area for 5 wt.%. The first region is interpreted in the dipole charge approximation. The second region is explained by the presence of a wide distribution of traps occupied by injected charges. Interlayer polarization is observed in inhomogeneous dielectrics and is caused by differences in the electrical conductivity and dielectric constant of individual particles (or microparticles) of the dielectric. In such dielectrics, when an electric field is applied at the interface between its various components, charge will accumulate, which is equivalent to the creation of polarization. The results for the temperature dependences of the TSD currents of the samples showed that the total accumulated charge in the volume increases as the number of polymer layers increases. The reason for such a high concentration of trapped charges is most likely due to the presence of a wide distribution of traps occupied by injected charges. Moreover, the magnitude of the released charge is greater for samples prepared from a polymer solution with 2.5 wt. % This effect depends on the thickness of the polymer films. Namely, the thinner the films, the more charged particles penetrate the interface between the films.

ПОДВИЖНОСТЬ НОСИТЕЛЕЙ ЗАРЯДА ВДОЛЬ ГРАНИЦЫ РАЗДЕЛА ПОЛИМЕР/ПОЛИМЕР

Organic thin-film transistors are increasingly being used in various electronic devices, successfully replacing transistors on silicon and other traditional semiconductors. The advantages of such electronic devices are particularly noticeable in the development of flexible electronic devices. However, one of the intractable problems of organic devices is the low mobility of charge carriers, often not exceeding 10-3 cm2/Vs. Obviously, this significantly limits the scope of their application. In this paper, a new approach to the formation of a transport layer in an organic field-effect transistor is proposed. This approach is based on the use of anomalous transport properties occurring along the interface of two dielectrics. Along such a boundary, a layer of quasi-two-dimensional electron gas with abnormally large values of conductivity and mobility of charge carriers can occur. In this regard, polymer heterostructures representing a field-effect transistor with a transport layer made in the form of an interface between two films of an unconjugated (dielectric) polymer, polydiphenylene phthalide, were investigated in this work. Along such a boundary, a layer of quasi-two-dimensional electron gas with abnormally large values of conductivity and mobility of charge carriers can occur. In this regard, polymer heterostructures representing a field-effect transistor with a transport layer made in the form of an interface between two films of an unconjugated (dielectric) polymer, polydiphenylene phthalide, were investigated in this work. The paper describes the technology of creating a multielectrode device. Polymer films were made by centrifugation from a polymer solution in cyclohexanone. The metal electrodes were manufactured by thermal deposition in vacuum. Measurements of the electrophysical characteristics of an organic field-effect transistor have been carried out. It is established that the main charge carrier along the interface are electrons. Estimates of the mobility of charge carriers were carried out using two different techniques. At zero potential at the gate, an injection model of currents limited by a volumetric charge was used. In the presence of a potential on the gate, the mobility assessment was made within the framework of the field effect model. Comparison of the obtained estimates showed satisfactory agreement of the mobility values. From this fact, it was concluded that it is possible to use an injection model to estimate the mobility of charge carriers when they move along the polymer/polymer interface.

ИССЛЕДОВАНИЕ ПОЛИМЕР-ПОЛИМЕРНЫХ ИНТЕРФЕЙСОВ МЕТОДОМ АТОМНО-СИЛОВОЙ МИКРОСКОПИИ

The results of an experimental study of polymer-polymer interfaces in submicron films of polydiphenylene phthalide are presented. Using atomic force microscopy, it was found that polymer films have an internal structure, and the structural elements have the shape of spheroids. The properties of the polymer and the nature of the observed images made it possible to use a model of brittle polymer fracture to describe the results. A model of the formation of interface layers at the interface between the elements of the supramolecular structure and between sequentially deposited polymer layers is proposed. It was found that a transition layer with a thickness of about 10-20 nm formed by lateral fragments of polymer molecules is formed in the interface region.

НИЗКОТЕМПЕРАТУРНЫЙ ЭЛЕКТРОННЫЙ ТРАНСПОРТ В ГИБРИДНЫХ ТОНКОПЛЕНОЧНЫХ НАНОСТРУКТУРАХ НА БАЗЕ ЭЛЕКТРОПРОВОДЯЩЕГО ПОЛИМЕРА

Most polymers are usually poor conductors of electric current and, accordingly, they can be classified as organic dielectrics. However, there is a special class of polymers, usually characterized by presence of conjugated pbonds, which provide electron delocalization, leading to electrical conductivity in the ground state of the system. In addition to such materials, there have been found polymers that are insulators in the ground state, but under the influence of external factors demonstrate a finite electrical conductivity. Polydiphenylenephthalide (PDP) belongs to the class of carbocyclic organic electroactive polymers that exhibit electrically conductive properties when an external electric field and/or mechanical stress is applied. The effect is explained by the nonzero density of electronic states inside the band gap. The depth of such states increases if the system accepts an additional electron, which indirectly provides electrical conductivity along the polymer chain. Accordingly, the presence of free electrons in contact region with a metal is an important condition for the appearance of final electrical conductivity in PDP. The working hypothesis that stimulates the proposed study is the assumption that superconducting correlations may arise in the electrically conductive state of an electroactive polymer. In the present work, the transport properties of thin-film lead-PDP-lead structures in two configurations are experimentally studied in a wide temperature range: a layered “sandwich” type and a quasi-planar “field-effect transistor” type. At temperatures below ~8 K, in layered samples, features of the PDP electron transport are observed, which can be explained by the effect of induced superconductivity in a thin film of a conducting polymer enclosed between two massive superconductors (lead). In the planar configuration, the electrical conductivity of the polymer in the plane of the structure does not appear. The reason for this observation may be related to the mechanism of the electrically conductive state of the PDP, characterized by formation of quasi-one-dimensional channels of electric current in the direction perpendicular to the plane of contact with the "donor" of electrons: metal in the normal or superconducting state. The effect of appearance of superconductivity in PDP requires further stidies and is potentially of significant interest for various applications in the field of practical microand nanoelectronics.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ УСЛОВИЙ ИЗГОТОВЛЕНИЯ НА ФОТОПРОВОДИМОСТЬ ГРАНИЦЫ РАЗДЕЛА ДВУХ ПОЛИМЕРНЫХ ПЛЕНОК

The study of optical properties in organic materials requires taking into account a wide range of features of these materials in connection with their sensitivity to external influences and aggressive environment. In this work, we studied the features of the photoconductivity of the interface between two dielectrics (polydiphenylene phthalide (PDP)) manufactured under different conditions. Drying was carried out in air and in vacuum. This study is necessary to determine the features of the manufacturing process and its effect on the photoconductive properties of the interface. Currently, the structure of the polymer / polymer interface has been fairly well studied by various methods, in particular, atomic force microscopy. However, there is a certain difficulty in obtaining repeatable results due to the presence of dilution of the lower PDP film when the upper polymer layer is applied. In this work, the photoconductive properties were studied on the basis of the measurements of the current-voltage characteristics (CVC) and the curves of the growth and relaxation of the photocurrent. Based on the results obtained within the framework of this work, it is possible to draw conclusions about the contribution of the drying method of experimental structures to the photoconductivity of the interface. It is known that oxygen makes a significant contribution to the electrical conductivity of thin PDP films. Earlier, it was shown on this polymer material that the presence of oxygen leads to a decrease in the conductivity of PDP films due to the formation of charge carrier traps. Moreover, when oxygen interacts with fragments of a polymer molecule, a chemical bond does not arise, which leads to a reversible change in electrical conductivity with a change in oxygen concentration. Due to this, the PDP films demonstrate a high stability of the electrophysical characteristics for several years when repeated measurements are carried out in an open atmosphere and at room temperature. It can be argued that the effect on the response of photoconductivity in organic materials, as well as on the relaxation time, can be associated with the presence of oxygen traps in the PDP films, increasing the relaxation time. This result is extremely important from the point of view of the practical application of these structures.

Transport properties of layered heterostructures based on conducting polymer

Polydiphenylenephthalide (PDP) belongs to the class of organic dielectrics, which exhibit electric conductive properties when an external electrostatic field and/or mechanical stress are applied. In this work, the transport properties of thin-film layered lead-PDP-lead structures were experimental-ly studied in a wide temperature range. At sufficiently high temperatures, the volt-ampere depend-ences are satisfactorily described in terms of the injection model of currents limited by the space charge. At temperatures below ~7.5 K, a number of samples exhibit features that can be explained by the effect of induced superconductivity in a thin film of a conducting polymer enclosed between two massive superconductors (lead). Keywords: thin films, conducting polymer, superconductivity.

Транспортные свойства слоистых гетероструктур на базе проводящего полимера

Polydiphenylenephthalide (PDP) belongs to the class of organic dielectrics, which exhibit electric conductive properties when an external electrostatic field and/or mechanical stress are applied. In this work, the transport properties of thin-film layered lead–PDP–lead structures were experimental-ly studied in a wide temperature range. At sufficiently high temperatures, the volt–ampere depend-ences are satisfactorily described in terms of the injection model of currents limited by the space charge. At temperatures below ~7.5 K, a number of samples exhibit features that can be explained by the effect of induced superconductivity in a thin film of a conducting polymer enclosed between two massive superconductors (lead).

The Effect of Thickness of Submicron Films of Electroactive Polymers on Thermally Stimulated Depolarization Currents

The results of studying the thermally stimulated depolarization currents in thin films of polydiphenylenephthalide (PDP) with a thickness of 20 nm to 1.4 μm under a constant stress in a structure with an insulating dielectric layer are presented. Nonmonotonic behavior of the dependence of the activation energy of charge carrier trapping sites and the relaxation of the space charge on the thickness of PDP polymer films is found. A correlation between the change in the supramolecular structure of the films and the above parameters is established. The obtained results are interpreted using the theory of thermally stimulated discharge in short-circuit heterogeneous structures.

Photoconductivity along the Polymer/Polymer Interface

The effect of photoexcitation on the electrical conductivity of the interface between two polydiphenylenephthalide (PDP) polymer films is discovered. It is shown that ultraviolet radiation makes the main contribution to the generation of photoconductivity. Long-term relaxation of photoconductivity is observed after extinction of illumination. Based on electrophysical measurements and comparing the results of optical studies, it is concluded that ultraviolet radiation has an effect on the dipole moment magnitude.

Фотопроводимость вдоль границы раздела полимер/полимер

Abstract The effect of photoexcitation on the electrical conductivity of the interface between two polydiphenylenephthalide (PDP) polymer films is discovered. It is shown that ultraviolet radiation makes the main contribution to the generation of photoconductivity. Long-term relaxation of photoconductivity is observed after extinction of illumination. Based on electrophysical measurements and comparing the results of optical studies, it is concluded that ultraviolet radiation has an effect on the dipole moment magnitude.

Влияние толщины субмикронных пленок электроактивных полимеров на токи термостимулированной деполяризации

Abstract The results of studying the thermally stimulated depolarization currents in thin films of polydiphenylenephthalide (PDP) with a thickness of 20 nm to 1.4 μm under a constant stress in a structure with an insulating dielectric layer are presented. Nonmonotonic behavior of the dependence of the activation energy of charge carrier trapping sites and the relaxation of the space charge on the thickness of PDP polymer films is found. A correlation between the change in the supramolecular structure of the films and the above parameters is established. The obtained results are interpreted using the theory of thermally stimulated discharge in short-circuit heterogeneous structures.

Effect of Oxygen on the Conductive Properties of Thin Films of Nonconductive Polymer

The influence of the atmosphere on the electrical conductivity of polydiphenylenephthalide (PDP) films included in the ITO–PDP–Sb sandwich structure is discussed. On the basis of electrophysical measurements and quantum-chemical simulation, a conclusion is drawn that the main contribution to the electrical conductivity of the PDP films comes from oxygen.

Влияние кислорода на электропроводящие свойства тонких пленок полимерного диэлектрика

AbstractThe influence of the atmosphere on the electrical conductivity of polydiphenylenephthalide (PDP) films included in the ITO–PDP–Sb sandwich structure is discussed. On the basis of electrophysical measurements and quantum-chemical simulation, a conclusion is drawn that the main contribution to the electrical conductivity of the PDP films comes from oxygen.

Peculiarities of the magnetic properties of thin-film three-layer Fe/Poly (diphenylene phthalide)/Fe systems

Electrically conductive polymers, created at the end of XX century, attract the attention of researchers due to variety of their mechanical and optical properties, as well as high conductivity. Studies conducted to date show the ability of a polymer to transition from low conductivity to a highly conducting state that is close to metallic and has a strong anisotropy. Due to the above properties, they are widely used as functional elements in devices of microelectronics. At the same time, the magnetic characteristics and behaviour features in magnetic field of polymers, in particular, poly diphenylene phthalide (PDP), have not been studied practically. The present work is devoted to the results of investigation of the magnetic properties of Fe/PDP/Fe thin-film systems.

Atomic-force microscopy of submicron films of electroactive polymer

Atomic-force microscopy is used to study the supramolecular structure of submicron films of electroactive thermally stable polymer (polydiphenylenephthalide (PDP)). It has been demonstrated that PDP films produced using centrifuging are solid homogeneous films with thicknesses down to several nanometers, which correspond to two or three monomolecular layers. The film volume is structurized at thicknesses greater than 100 nm. The study of the rheological properties of solutions used for film production yields a crossover point that separates the domains of strongly diluted and semidiluted solutions. A transition from the globular structure to the associate structure is observed in films that are produced using solutions with a boundary concentration. A model of the formation of polymer film that involves the presence of associates in the original solution is discussed.

Infrared spectra of diphenylphthalide and polydiphenylenephthalide

The splitting of the ν(C=O) absorption band (AB) of about 12 cm−1 is found in the IR spectra of diphenylphthalide (DPP) in the crystalline phase and CCl4 solution. In the crystalline phase, this splitting is likely to be caused by the inequivalence of DPP molecules in the crystallographic cell, while in the solution, by the dimerization of DPP molecules via dipole-dipole and/or hydrogen bonds. A theoretical low-frequency shift of the ν(C=O) AB for a complex of two DPP molecules (in comparison with a single molecule) is 14 cm−1 in the PBE/3ξ approximation, which is close to the experimentally observed splitting. In two quantum chemical approximations (B3LYP/6-311G(d,p) (I) and PBE/3ξ (II)) the optimal structure and vibrational spectrum of DPP are calculated. Approximation I better reproduces the intensities, whereas approximation II better reproduces the IR frequencies of the DPP spectrum. Almost all 48 ABs of the IR spectrum of DPP are assigned to theoretical normal vibrations (modes). Based on the potential energy distribution over natural coordinates and the visualization of vibrations, experimental ABs (and the corresponding modes) are assigned to the stretching and bending vibrations of certain bonds in the DPP molecule. In particular, ABs at 1107 cm−1 and 970 cm−1 are assigned to the ν(-OC-O-) and ν(-C-O-) stretching vibrations, respectively, of the DPP lactonic ring, which differs from the previously accepted assignment. The results of the interpretation of the DPP spectrum are used to assign a number of ABs in the IR spectrum of polydiphenylenephthalide (PDP), for which DPP is a model compound. According to the calculations in approximation II of the vibrational spectrum of a model valence-bonded dimeric molecule, the intense complex AB at 800–870 cm−1 in the IR spectrum of PDP is mainly due to the out-of-plane bending vibrations of C-H bonds in the 1,4-substituted benzene rings of polymer biphenyl moieties and the bending vibrations of the lactonic ring.

The Dynamics of a Free Jet of Solutions of Polysulphone and Polydiphenylene Phthalide in an Electrostatic Field

An investigation was made of the dependences of change in the radius, velocity, velocity gradient, and acceleration of a liquid thread along the longitudinal coordinate on the viscosity of the solution, the volume flow rate of the solution, and the electrostatic field strength. An attempt was made to establish the relationship between the rheological properties of polymer–solvent systems and the real velocities, accelerations, and strains in a Taylor cone.

Electron energy distributions in a metal-polymer-vacuum system

The energy distributions of electrons emitted from a metal coated with a polymer (polydiphenylene phthalide) is studied experimentally using field electron spectroscopy. A considerable decrease in the electron work function for the metal-polymer-vacuum system as compared to pure metal is observed. Analysis of the energy distributions of emitted electrons shows that the distribution in the case with the polymer is broader and displaced towards low energies, and its high-energy edge is slightly extended. The effect of emission voltage on the shape of the energy distribution of emitted electrons is studied. A model is proposed to explain the substantial decrease in the effective electron work function in the case when the metal electrode is coated with a polymer film.

Relative Heat-Stability of Polydiphenylene Phthalide, Brominated Polydiphenylene Phthalide and Polydiphenylene-N-Phenylphthalimidine

Relative Heat-Stability of Polydiphenylene Phthalide, Brominated Polydiphenylene Phthalide and Polydiphenylene-N-Phenylphthalimidine

On the influence of trapping states on electronic instabilities in polydiphenylenephthalide

A phenomenon of switching with memory in polydiphenylenephthalide (PDP) films is studied. The existence of a practically continuous spectrum of the states of polymer film from a dielectric ‘off’ to a metal-like ‘on’ is established. The switching time is less than 0.1 μs. It is shown that highly conducting channels appear as a result of switching. Their diameter is 0.1 μm or less and conductivity ranges from 10 to 10 5 Ω −1 cm −1. This effect is only observed in pristine PDP films less than 3 μm thick. By virtue of the investigations of thermostimulated currents and optical absorption spectra it is established that thin polymer films differ from thick ones in their much higher concentration of trapping states. Simulation of such states in thick PDP films by doping showed that a critical concentration of these states must be achieved in order to realize the switching effect, no matter what the nature of the states. Analysis of the results obtained allows us to make a suggestion about the electronic character of instabilities observed in PDP.