Egils Fonavs

A Novel Gas Sensor Transducer Based on Phthalocyanine Heterojunction Devices

Experimental data concerning the changes in the current-voltage (I-V) performances of a molecular material-based heterojunction consisting of hexadecafluorinated nickel phthalocyanine (Ni(F16Pc)) and nickel phthalocyanine (NiPc), (Au|Ni(F16Pc)|NiPc|Al) are introduced as an unprecedented principle of transduction for gas sensing performances. The respective n- and p-type doped-insulator behaviors of the respective materials are supported, owing to the observed changes in surface potential (using the Kelvin probe method) after submission to electron donor (ammonia) and electron acceptor gases (ozone). On the other hand, the bilayer device exhibits strong variations in the built-in potential of the junction and in its rectification ratio. Moreover, large increases occur in forward and reverse currents in presence of ammonia vapors. These make possible a multimodal principle of detection controlled by a combined effect between the heterojunction and the NiPc|Al contact. Indeed, this metal/organic junction plays a critical role regarding the steady asymmetry of the I-V profiles during the devices doping even using high ammonia concentrations. This approach offers a more sophisticated alternative to the classically studied, but at times rather operation-limited, resistive gas sensors.

Novel second-order nonlinear optical polymer materials containing indandione derivativatives as a chromophore

Second order non linear optical (NLO) properties of sPMMA based host-guest systems containing eight dimethylaminobenzylidene -1, 3 - indandione (DMABI) related chromophores have been investigated by means of quantum chemical calculations and SHG experimental characterization. Ab initio calculations with basis set 6-31G were used for molecular geometry determination as well as in the calculations of molecular hyperpolarizability by FF approach. Influence of the chromophore concentration on the host-guest film NLO performance was obtained by SHG Maker fringe experiments. The highest value of d53233 =80 pm/V (frequency corrected value d033 =12 pm/V) at chromophore concentration 15 %wt have been recorded for tret-butyl substituted DMABI chromophore.

Structural and electrical characterisation of betaine-type, organic, molecular, thin evaporated films and LB multilayers

Abstract The structure and electrical properties of highly polar indandione-1,3 pyridinium betaine (IPB) derivatives have been studied in vacuum-evaporated thin films and Langmuir-Blodgett (LB) multilayer assemblies. Phase transitions induced by temperature and/or electric field have been observed in LB films of an amphiphilic derivative of IPB. The LB films of IPB, obtained at room temperature, form a Y-like structure which melts at about 50 °C to produce spherical domains, having Z-like structure, which remain stable up to 110 °C. Similar phase transitions can be induced by an electric field with e ≥ 2 × 10 5 V cm −1 at room temperature. In the new Z-like phase of the IPB LB films, the electrical conductivity increases by some five or six orders of magnitude and the activation energy of dark conductivity decreases from 0.18 ± 0.03 eV to practically zero. The vacuum-evaporated IPB films yield low electrical conductivity (σ = 10 −15 –10 −16 S cm −1 ), whereas in the LB multilayers a notable anisotropy of conductivity is observed. In case of coplanar cells the conductivity increases to σ = 10 −8 S cm −1 . In sandwich-type LB samples the conductivity value is similar to that of the vacuum-evaporated polycrystalline thin films.

Optical properties of some novel azobenzenes in thin layers

We have synthesized and investigated a novel electrondonor -- electronacceptor type photoactive amphiphilic azobenzene derivative containing the hydrophobic dicyclohexylamine moiety along with the hydrophylic carboxyl group at the opposite end of the molecule. Reversible trans/cis photoisomerization of this compound in solutions and in Langmuir-Blodgett multilayers has been observed on irradiation with alternating UV (360 nm) and visible (450 nm) light. The reversible changes of absorbance of main absorption band and surface potential of Langmuir-Blodgett multilayers on alternate irradiation is demonstrated.

Incorporated oriented polar molecules in PMMA polymeric films: characterization by Kelvin probe technique and atomic force microscopy

Betaine type dipolar molecules have received considerable attention because of their large dipole moments and optical non-linearities. The host-guest systems are chosen. The high-electric field corona poling was used to generate dipole orientation of betaine molecules in polymer matrices. Atomic force microscopy and surface potential studies yield useful information regarding both structural and electronic properties of casted host-guest polymer films. In this work we present results obtained from surface potential and atom force microscopy studies of poled and un-poled thin films on heating.

Light induced processes in thin films of indandione type organic molecules

The optically induced switching of material properties is important for investigations of opto-electronic effects and optomechanical properties. Investigated organic materials contain chromophore dipole consisting of acceptor and donor groups bridged by a delocalized &pgr;-electron system. Both calculations and experimental data show a reversible highly dipolar photoinduced intra molecular charge transfer in indandione type molecules (DMABI) accompanied by change of the sign and the value of the dipole moment. Investigations of optical properties of thin host-guest polymer films show that the photoinduced process of DMABI is related to the photoinduced switching between two equally stable states of the molecule. In this work first results of formation of the surface relief in polymer films incorporated with DMABI derivatives will be presented. The refractive index gratings of DMABI host-guest films show that red light is less diffracted than blue one. The reversible surface potential changes on irradiation in photoinduced intramolecular electron transfer band in polymer host-guest films is observed. The DMABI molecules in solid state have nonlinear optical properties, which can be used and investigated in host-guest polymer matrix. The influence of concentration of DMABI molecules on photoinduced processes is discussed.

Optically induced switching of dicyclohexylamino substituted azobenzene derivatives in thin ordered films

During the last years, there has been an increasing interest in the photoinduced switching effects. Among the objects of concern azobenzene derivatives, isomerizing under UV and visible light, occupy a prominent position. To understand the photoresponse of these materials in the condensed phase, their spectroscopic and electrical properties are studied. It is shown that the photoresponse depends on the orientation of the molecules and their packing. A number of novel azobenzene derivatives containing a N,N-dicyclohexyl sulfonamide moiety is synthesized. The derivatives differ in the length of alkyl chains between the azobenzene moiety and SH or COOH groups. The morphology and the photoinduced switching of the surface potential of the self-assembled monolayers and Langmuir-Blodgett multilayers are investigated.

Electrical and structural properties of LB multilayers of (methylacylamino) azobenzenes

The present publication is devoted to the studies of the structure of the Langmuir-Blodget films of (methylacylamino)azobenzene derivatives and their electrical properties. Structural studies of monolayer have been carried out by atom force microscopy. The molecules are closely packed in a hexagonal crystalline array. UV irradiation causes a rearrangement of the molecules and a significant structural modification in the monolayer is observed in the frontier region between two crystalline domains of different orientations. The anisotropy of dc conductivity sigma is observed in multilayer assemblies. A low dc conductivity in the bulk sample of the order of sigma(perpendicular) approximately equal to 10-1010-13 S/cm is obtained. In the case of in-plane conductivity the value of conductivity is of the order of sigma(parallel) approximately equals 10-8- 10-10 S/cm. The several sets of hole trapping states are situated at energy depth of Et1 equals 0.55 plus or minus 0.03 eV and Et2 equals 0.48 plus or minus 0.03 eV with the value of the total density of given set of traps of the order of 1012 cm-3. The frequency dependence of capacitance and conductance is nonlinear, with capacitance values decreasing exponentially with higher frequencies while the conductance increases gradually. The behavior is one typical for low mobility materials and the conduction mechanism can be explained in terms of hopping conduction between localized states. The linear dependence of dc in-plane conductivity on relative humidity was observed.

Photoisomerization Processes of Azobenzene Compounds in Thin Poymer Films

Photoisomerization Processes of Azobenzene Compounds in Thin Poymer Films One of the most important properties of the molecular switches is turning of a molecule by external action, e.g. by irradiation with light of definite wavelength. The molecules of promise for the molecular switches are those with azobenzene moiety. When azobenzene is irradiated with light of definite wavelength, it is possible to observe the process of trans/cis isomerisation. In this work, the influence of photoisomerisation processes on the changes in the surface potential of a novel azobenzene compound is investigated. We have studied the surface potential of a PMMA polymer film with polar azobenzene derivative of different concentrations. To orient azobenzene molecules in the film the corona poling method was used, and for studying the surface potential variations - the Kelvin probe method. The photoreaction time constant of the fast response (several seconds) was found to be almost independent of the molecular concentration in the polymer film and of the absorbed light intensity in the bulk of a sample. In contrast, the amplitude of photoinduced changes in the surface potential depends both on the concentration of azobenzene molecules and on the absorbed light intensity in bulk of the host-guest polymer film. Azobenzolu Savienojumu Fotoizomerizācijas Procesi Plānās Kārtiņās Molekulāram slēdzim viena no īpašībām ir molekulas spēja pagriezties citā stāvoklī ārējās iedarbības rezultātā, tādējādi izmainot, piemēram, molekulas dipola momenta vērtību. Kā vienas no tādām molekulām ir azobenzolu savienojumi, kuras apstarojot ar noteiktu viļņu garumu, ir novērojama pāreja starp diviem izomēru trans- un cis-stāvokļiem. Azobenzola molekulu raksturošanai tiek izmantotas tādas vērtības kā fotoierosinātā virsmas potenciāla vērtības un ātruma izmaiņas trans/cis fotoizomerizācijā, kā arī optimālā molekulu koncentrācija polimēra matricā. Darbā izpētītas polimēru plāno kārtiņu ar azobenzola atvasinājumu A-45 fotoierosinātā virsmas potenciāla atkarības no molekulu koncentrācijas, kārtiņu biezuma, kā arī krītošās un absorbētās gaismas intensitātes. Fotoierosinātās virsmas potenciāla izmaiņas ir noteiktas ar Kelvina zondes metodi. Darbā parādīts, ka fotoierosinātās virsmas potenciāla izmaiņu amplitūdas vērtības palielinās, palielinot azobenzola molekulu koncentrāciju no 1 līdz 10 wt%, kā arī palielinot kārtiņā absorbētās gaismas intensitāti. Savukārt, fotoierosinātās virsmas potenciāla izmaiņas ātrums ir nemainīgs

The thermally stimulated surface potential of betaine type host-guest polymer films

In this work betaine-type molecules were investigated. As a result of the asymmetry of charge distribution, molecules in the ground state have a considerable permanent dipole moment. The thermally stimulated surface potential dependences on poling temperature are investigated. The influence of /spl alpha/- and /spl beta/-relaxations of PMMA on the surface potential of the film are shown. The thermal dependence of surface potential is discussed. The transition temperature, where no changes of the surface potential appeared, is related to glass transition temperature of the host-guest system.