Switching effects in composite films based on the conjugated polymer polyfluorene and ZnO nanoparticles

The S- and N-shaped current—voltage characteristics have been studied for composite films of the conjugated polymer polyfluorene and ZnO nanoparticles deposited onto Al and In₂O₃/SnO₂ electrodes with and without an intermediate sublayer of the conducting polymer PEDT/PSS. The differences in the current— voltage characteristics of the systems (the N- and S-types, respectively) are interpreted using the electro-thermal switching model, which takes into account the structural and electric properties of PEDT/PSS. The switching provides both alignment of polymer molecules and tunneling of charge carriers, which leads to an increase in conductivity. The current flow in this structure causes an increase in temperature of conducting channels; when the temperature reaches certain levels, the conductivity of the channels decreases because the alignment of polymer molecules is upset, which creates an N-shaped form of the current—voltage characteristics.     

Switching and memory effects in composite films of semiconducting polymers with particles of graphene and graphene oxide

The effects of switching were investigated in composite films based on multifunctional polymers. i.e., derivatives of carbazole (PVK) and fluorene (PFD), as well as based on particles of graphene (Gr) and graphene oxide (GO). The concentration of Gr and GO particles in the PVK(PFD) matrix was varied in the range of 2–3 wt %, which corresponded to the percolation threshold in these systems. The atomic composition of the composite films PVK: GO was examined using X-ray photoelectron spectroscopy. It was found that the effect of switching in structures of the form Al/PVK(PFD): GO(Gr)/ITO/PET manifests itself in a sharp change of the electrical resistance of the composite film from a low-conducting state to a relatively high-conducting state when applying a bias to Al-ITO electrodes of ∼0.1–0.3 V (E ∼ 3–5 × 10⁴ V/cm), which is below the threshold switching voltages for similar composites. The mechanism of resistance switching, which is associated with the processes of capture and accumulation of charge carriers by Gr (GO) particles introduced into the matrices of the high-molecular-weight (PVK) and relatively low-molecular-weight (PFD) polymers, was discussed.

     

Influence of the soft phonon mode on charge carrier hopping transfer in bismuth-containing layered ferroelectric ceramic oxides

Formulas and calculations explaining the experimental data on maxima frequency displacement for imaginary dielectric components in a low frequency range with a rise in temperature within the limits of the ferroelectric phase were obtained for the ceramic structure SrBi₂(W _x/2Ti _x/2Nb_{1 − x} )₂O₉. At the heart of our consideration lies our assumption of the interaction of jumping charge carriers with a soft phonon mode.     

Memory effects in field-effect transistor structures based on composite films of polyepoxypropylcarbazole with gold nanoparticles

The memory effects in field-effect transistor structures with an active layer based on composite films of a semiconductor polymer, i.e., the carbazole derivative and gold nanoparticles, manifesting themselves in the hysteresis of the transient characteristics of the transistor have been studied. It has been shown that the observed effects are associated with the features of transport in the polymer-gold nanoparticle structure, where the gold particles serve as a medium of charge carrier collection (accumulation). The data writeerase mechanism based on conductivity modulation of the working channel of the field-effect transistor by the gate voltage have been discussed.     

Transparent conducting hybrid thin films fabricated by layer-by-layer assembly of single-wall carbon nanotubes and conducting polymers

The effect of conducting polymers on the performance of transparent conducting SWNT thin films fabricated by the layer-by-layer (LBL) assembly has been investigated. Transparent conducting SWNT thin films were fabricated by the LBL assembly in two ways, one using conducting polymers, PEDOT-PEG, and the other using non-conducting polymers, PAH, and their electrical and optical properties were compared. The sheet resistance of (PSS-SWNT/PEDOT-PEG) _n films is more than thrice lower than that of (PSS-SWNT/PAH) _n films for the same n while the decrease of optical transmittance due to the absorbance of PEDOT-PEG is fairly small (ca. 2?% at n=30). The conductivity ratio of the (PSS-SWNT/PEDOT-PEG)₃₀ is 3.3 times larger than that of the (PSS-SWNT/PAH)₃₀. These figures indicate that the performance of the transparent conducting SWNT thin films fabricated by the LBL assembly is highly improved by using conducting polymers instead of non-conducting ones.     

Direct visualization of the charge transfer in conjugated polymers

Ion-induced charge-transfer states in conjugated polyelectrolytes were experimentally investigated by Justin M.Hodgkiss and his co-workers [J Am Chem Soc,2009,131(25):8913].In this work,charged and neutral conjugated polyelectrolytes were further studied with quantum chemistry methods.The calculation result shows that the absorption spectra are roughly in visible and ultraviolet light regions,and the two absorption peaks are located in the wavelength span 300-400 nm for charged polyelectrolytes.However,in n...     

Metal-polymer composite films based on polyacrylonitrile and silver nanoparticles. Preparation and properties

The structures consisting of silver nanoparticles and polyacrylonitrile (PAN) are synthesized by the photopolymerization method yielding a homogeneous dispersion and a small size spread of nanoinclusions in PAN matrices, which is observed in images of transmission electron microscopy. The optical properties of the nanocomposites are studied to depend upon conditions of their obtaining. The minimum in the transmission spectra in a wavelength region of 430 nm, which is attributed to the surface plasmon resonance of silver nanoparticles, is found. The absorption peaks in the infrared range are observed at ∼820 and ∼1110 cm⁻¹ for the silver nitrate (AgNO₃) and the photoinitiator, respectively.     

Equilibrium charge of small metal particles and hopping transport in a metal-insulator composite

It is shown that the thermodynamically equilibrium state of a system of small metal particles placed in a dielectric matrix are unavoidably charged. The charge of spherical metal particles with different radii is calculated at low temperatures. Hopping transport in a system of metal particles is studied. It is shown that it is limited by the charging energy, which serves as a typical hopping energy and can be gapless. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 510–515 (25 October 1999)     

Electronic structure of a complex with charge transfer and carrier energy bands in thin films of semiconductors containing carbazole

The electronic structures of hole and electron carrier energy bands and complexes with charge transfer (CCT) in films of semiconductors containing carbazole and sensitized with TNP were determined by the methods of photoelectron and electron spectroscopy as well as by studying the wavelength dependence of the quantum yield of photogeneration at temperatures of 295 and 80 K. The hole carrier bands are determined by the top three filled electronic levels of the carbazole nuclei, while the electron carrier bands are determined by the bottom free electron level of TNP. The first three excited states of CCT are determined, respectively, by electronic transitions from the first three top filled electron levels of the carbazole nuclei to the bottom free electronic level of TNP. Photogeneration of current carriers is observed with excitation of CCT in the region of the second and third electronic transitions.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 7, pp. 47–51, July, 1989.     

Charge carrier transport in conducting polymers on the metal side of the metal-insulator transition: A review

The advances achieved in the field of the synthesis of conducting polymer films and the results of experimental investigations of the mechanism of charge carrier transport in heavily doped conducting polymer films on the metal side of the metal-insulator transition have been considered. The samples studied belong to the last generation of conducting polymers with a low degree of structural disorder, which makes it possible to dope samples to a highly conducting state characterized by an increased stability. Apart from the study of the morphology, structure, and doping conditions, the mechanism of low-temperature transport in these polymers has been investigated in detail. The results of investigations of the transport at low temperatures (T < 1 K) have been described, and a phenomenological model of charge carrier transfer in these systems has been proposed. The polymers under investigation have been widely used as injecting and accumulating layers in light-emitting organic diodes and solar cells.     

Investigations on the mechanism of carrier transport in plasma polymerized pyrrole thin films

Plasma polymerization is found to be an excellent technique for the preparation of good quality, pinhole-free, polymer thin films from different monomer precursors. The present work describes the preparation and characterization of polypyrrole (PPy) thin films by ac plasma polymerization technique in their pristine and in situ iodine doped forms. The electrical conductivity studies of the aluminium-polymer-aluminium (Al-polymer-Al) structures have been carried out and a space charge limited conduction (SCLC) mechanism is identified as the most probable mechanism of carrier transport in these polymer films. The electrical conductivity shows an enhanced value in the iodine doped sample. The reduction of optical band gap by iodine doping is correlated with the observed conductivity results.     

Current instability with an S -shaped current-voltage characteristic in thin films of composites based on polymers and inorganic particles

The switching and memory effects, which manifest themselves in the transition from a high-resistance state to a low-resistance state, are observed in thin films of composites based on polymers (carbazole derivatives) and silicon inorganic particles. It is established that, at small thicknesses of composite films, the resistance (with an initial value of no less than 100 MΩ) upon transition from the high-resistance state to the low-resistance state is changed by three or four orders of magnitude. This transition in the composites under investigation is accompanied by the appearance of an S-shaped current-voltage characteristic and its hysteresis. It is demonstrated that the observed effects are associated with the specific features of the thermomechanical and electrical properties of the polymer in the composite material. Original Russian Text ? é.A. Lebedev, E.L. Alexandrova, A.N. Aleshin, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 195–197.     

Charge carrier accumulation and relaxation effects in the active region of polymer and composite (polymer-gold nanoparticles) field-effect transistor structures

The charge carrier accumulation and relaxation effects in the active region of polymer field-effect transistor structures based on the semiconducting polymer poly(9-epoxypropylcarbazole) have been studied by means of Kelvin probe microscopy. It has been shown that the introduction of gold nanoparticles into the polymer noticeably accelerates the processes considered. The characteristic times of charge accumulation and dissipation upon the application and removal of the potential at the transistor gate in this case are of a few minutes.     

Optical properties of nanocomposites based on polymers and metal nanoparticles

The optical properties of nanocomposites of metal nanoparticles and polymers of two types have been studied. Gold and silver nanoparticles were obtained by laser ablation of corresponding metal targets in acetone and chloroform. The thus formed colloidal solutions were used to prepare nanocomposites of these nanoparticles in polymer matrices of polymethylmethacrylate (PMMA) and fluorine-containing polymer LF-32. The polymer matrix is found to promote aggregation of the metal nanoparticles under study into elongated chains. In turn, metal nanoparticles affect the polymer matrix. In the case of PMMA, suppression of the vibrational peaks of polymer in the low-frequency region of its Raman spectrum occurs. In the case of LF-32, gold and silver nanoparticles amplify the Raman signal of the polymer matrix. In addition, the Raman spectra of nanocomposites indicate aggregation of disordered carbon around the nanoparticles obtained by laser ablation in organic solvents. The possibilities of studying the ultrafast (about 1 ps) optical response of the nanocomposites obtained in order to use it in photonics elements are discussed.     

Electrical and optical properties of light-emitting field-effect transistors based on MEH-PPV polymer composite films with ZnO nanoparticles

The optical and electrical properties of light-emitting field-effect transistor structures with an active layer based on nanocomposite films containing zinc oxide (ZnO) nanoparticles dispersed in the matrix of the soluble conjugated polymer MEH-PPV have been investigated. It has been found that the current-voltage characteristics of the field-effect transistor based on MEH-PPV: ZnO films with a composite component ratio of 2: 1 have an ambipolar character, and the mobilities of electrons and holes in these structures at a temperature of 300 K reach high values up to ～1.2 and ～1.4 cm²/V s, respectively, which are close to the mobilities in fieldeffect transistors based on ZnO films. It has been shown that the ambipolar field-effect transistor based on MEH-PPV: ZnO films emits light at both positive and negative gate bias voltages. The mechanisms of injection, charge carrier transport, and radiative recombination in the studied structures have been discussed.