Sensitization of the electrooptical effect by a merocyanine dye in polymers of azobenzene derivatives

Absorption spectra of poly[4-metacryloyloxy-(4′-carboxy-3′-oxy)azobenzene] films and their complexes with cobalt, without additives or doped with a merocyanine dye whose absorption spectrum lay in the near IR region, were studied before and after switching on an external electric field. An external electric field effect was found on the light transmittance of films within the dye absorption region, which indicated sensitization of the electrooptical effect. The electrooptical properties of the films resulted from spatial reorientation of dipole moments of azobenzene groups in an external electric field, which initiated turning of dye molecules. A phenomenological model was proposed to explain the sensitization of the electrooptical effect.     

Electrophysical and photophysical properties of films made from an azobenzene-functionalized polymeric cobalt complex

Electric conductivity, photoconductivity, and electro-and magneto-optical properties of films from a novel polymeric complex of 4-methacryloyloxy-(4′-carboxy-3′-hydroxy)azobenzene with cobalt were studied. Electric conductivity and visible photoconductivity are determined by the orientation effects of azobenzene groups and by charge-carrier generation and transport in the polymer films. A change in the spatial orientation of the photoinduced dipole moments of azobenzene groups in an external electric or magnetic field is accounted for by the appearance of forces acting on charged magnetic metal ions that are rigidly bound to these fragments.     

Field-dependent electric conductivities of silicone rubbers deduced from measured currents and surface potential decay characteristics

Measurements of DC electric conductivities of four types of silicone rubber-based polymers for use in high voltage insulation systems of power components are reported. The field dependences of the conductivities obtained by two different techniques, namely by utilizing steady-state currents through materials’ samples placed between metallic electrodes and by employing surface potential decay characteristics in an open circuit configuration, are compared and discussed. It is shown that the surface potential decay technique allows for a wider range of electric field strength and reduces the time span of the measurements.     

Multiresponsive self-assembled liquid crystals with azobenzene groups

An optical and electric field-responsive self-assembled complex containing nitril azobenzene groups and 1,3,5-triazine-2,4-diamine was obtained and characterized. Both the azobenzene precursor and the complex form a liquid-crystalline phase in a certain temperature range. The transition temperature from crystalline phase to liquid-crystalline mesophase was obviously decreased in the complex by the self-assembling. The self-assembled liquid crystals revealed good response to both stimuli of light irradiation and electric field, and the induced molecular orientation could be held even after the removal of the stimuli. The structural and mechanical investigation proved that the formation of hydrogen bonds and assembly-induced molecular dipolar change contributed to the multiresponding action. This kind of self-assembled complex thus has potential applications in imaging and data storage.     

Lifetime of an emitting dipole near various types of interfaces including magnetic and negative refractive materials

A classical electromagnetic calculation of the lifetime of an emitting electric dipole near a material slab is presented. The lifetime is deduced from the imaginary part of the electric field Green's tensor associated with the stratified medium. The method is applied not only to the well known case of metallic reflectors, but also to magnetic reflectors and to negative refractive index slabs. The frequency dependence of the nonradiative decay rate at small distances is analyzed and interpreted in terms of the surface polariton modes of the slab.     

Optically Triggered Control of the Charge Carrier Density in Chemically Functionalized Graphene Field Effect Transistors

Field effect transistors (FETs) based on 2D materials are of great interest for applications in ultrathin electronic and sensing devices. Here we demonstrate the possibility to add optical switchability to graphene FETs (GFET) by functionalizing the graphene channel with optically switchable azobenzene molecules. The azobenzene molecules were incorporated to the GFET channel by building a van der Waals heterostructure with a carbon nanomembrane (CNM), which is used as a molecular interposer to attach the azobenzene molecules. Under exposure with 365 nm and 455 nm light, azobenzene molecules transition between cis and trans molecular conformations, respectively, resulting in a switching of the molecular dipole moment. Thus, the effective electric field acting on the GFET channel is tuned by optical stimulation and the carrier density is modulated.     

Plasmonic Nanorattles as Next‐Generation Catalysts for Surface Plasmon Resonance‐Mediated Oxidations Promoted by Activated Oxygen

Nanorattles, comprised of a nanosphere inside a nanoshell, were employed as the next generation of plasmonic catalysts for oxidations promoted by activated O₂. After investigating how the presence of a nanosphere inside a nanoshell affected the electric‐field enhancements in the nanorattle relative to a nanoshell and a nanosphere, the SPR‐mediated oxidation of p‐aminothiophenol (PATP) functionalized at their surface was investigated to benchmark how these different electric‐field intensities affected the performances of Au@AgAu nanorattles, AgAu nanoshells and Au nanoparticles having similar sizes. The high performance of the nanorattles enabled the visible‐light driven synthesis of azobenzene from aniline under ambient conditions. As the nanorattles allow the formation of electromagnetic hot spots without relying on the uncontrolled aggregation of nanostructures, it enables their application as catalysts in liquid phase under mild conditions using visible light as the main energy input.     

A single-dipole model of surface relief grating formation on azobenzene polymer films

A new model that keeps track of the dynamics of single dipoles is suggested for the photoinduced formation of a surface relief grating on azobenzene polymer films by two optical fields. Interfering optical fields provide a single dipole of an azobenzene molecule with two dynamical resources: rotation of the molecule caused by the torque, leading to an induced dipole, and its subsequent migration because of the electric force. Explicit development of the induced dipole of the molecule and its real-time migration, which depend on the details of the optical fields such as polarization and wavelength, can be understood self-consistently within the model.     

Monolayer alignment on azobenzene surfaces during UV light irradiation: analysis of optical polarized absorption measurement results and theoretical treatment

The influence of the charge separation during the trans-cis conformational change on the surface of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as 4-n-pentyl-4'-cyanobiphenyl(5CB), is investigated. The effective anchoring energy (in the Rapini-Papolar form) is phenomenologically described in the framework of the molecular model, which takes into account the interaction between the surface polarization and surface electric field, for number of conformational states of the boundary surface. It is shown, using the experimental data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique, that the charge separation during the conformational changing, caused by the UV irradiation, may lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the photoisomerization process on the orientational order parameter S2(t) using the optical polarized absorption measurement is also investigated.     

Photo-reorientation of azobenzene side groups of thermotropic ‘hairy rod’ polyglutamate in LB multilayers

A polyglutamate with photochromic azobenzene side groups combines thermotropic properties with the capability to form Langmuir-Blodgett (LB) multilayers. On irradiation with linearly polarised light the azobenzene side groups may be reoriented in spin-coated films and LB multilayers perpendicular to the electric field vector of the incident light. But the photochemically induced reorientation process depends strongly on the supramolecular order and on the irradiation conditions. Thus, the stable nematic-like order of the ‘hairy rod’ backbones restricts the photo-reorientation process of the azobenzene side groups within the LB multilayer, in contrast to the spin-coated film. In this case the degree of the photo-induced anisotropy depends on the primary orientational order of the multilayer, the molecular aggregation of the azobenzene moieties and on the conditions of light exposure. The photo-reorientation process becomes more efficient after weakening the intermolecular interactions by a suitable preparation of the films or by an intermediate photochemical decoupling of the supramolecular order caused by an ultraviolet irradiation step.     

An ab initio study of electrochemical vs. electromechanical properties: the case of CO adsorbed on a Pt(111) surface

We have studied electrochemical vibrational and energy properties of CO/Pt(111) in the framework of periodic density functional theory (DFT) calculations. We have used a modified version of the previously developed Filhol-Neurock method to correct the unphysical contributions arising from homogeneous background countercharge in the case of thick metallic slabs. The stability of different CO adsorption sites on Pt(111) (Top, Bridge, Hcp, Fcc) has been studied at constant electric field. The energies are dominated by the surface dipole interaction with the external electric field: a strong positive electric field favors the surfaces with the lower dipole moment (that correspond to the ones with the lower coordination). The Stark tuning slope of the CO stretching frequency for a Top site was calculated for different surface coverages in very good agreement with both experimental and other theoretical results. Finally, we have performed an analysis of the origin of Stark shifts showing that the total Stark effect can be split into two competing components. The first one corresponds to the direct effect of charging on the C-O chemical bond: it is referred as an electrochemical effect. The second is the consequence of the surface dipole interaction with the applied electric field that modifies the C-O distance, inducing a change of the C-O force constant because of C-O bond anharmonicity: it is referred as the electromechanical effect. In the CO/Pt(111) case, the dominant contribution is electromechanical. The electrochemical contribution is very small because the electronic system involved in the surface charging is mostly non-bonding as analyzed by looking at the surface Fukui function.     

Charge transfer from metallic single-walled carbon nanotube sensor arrays

This paper explores the possibility of using arrays of metallic carbon nanotubes as sensors. Unlike their semiconducting counterparts, single-walled carbon nanotube arrays or networks that are dominated by metallic conduction pathways have not been investigated for their environmental sensitivity. In this work, we demonstrate transduction of molecular adsorption via charge transfer through predominantly metallic single-walled carbon nanotubes. Raman spectroscopy and electric field dependent transport confirm that signal transduction takes place through primarily large diameter metallic nanotubes. This unique signal transduction mechanism might have implications for novel sensors. The scaling of the signal with array impedance is well described using an irreversible binding model developed previously. The arrays have several advantages including a simple, two-electrode fabrication, rapid regeneration, and a responsivity that scales predictably and linearly with the number of adsorption sites. An array-assisted hydrolysis of reactive analytes is found to regenerate the nanotube surface from hydrolyzable species which include important organophosphate nerve agents.     

Preparation of Ag nanoparticles termini‐protected side‐chain liquid crystalline azobenzene polymers by RAFT polymerization

A new synthetic approach to prepare Ag nanoparticles protected side‐chain liquid crystalline (LC) azobenzene polymers was reported. It is based on the reduction of silver ions in presence of a LC polymer polymerized by RAFT. The formation of Ag colloidal nanoparticles was confirmed by TEM and UV analysis. At the same time, according to the results of DSC, XPS, and FTIR spectra, Ag nanoparticles were protected by the side‐chain LC azobenzene polymers through surface attachment interactions between thiol groups and Ag. The out‐plane orientation of side‐chain LC is confirmed by surface‐enhanced Raman spectra analysis and scanning near‐field optical microscope, resulting from the large electromagnetic field arising from the excitation of surface plasmon polariton of Ag nanoparticles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5380–5386, 2007     

Two-layered metallic film-induced surface plasmon polariton for fluorescence emission enhancement in on-chip waveguide

We demonstrate an enhancement of fluorescence emission due to bimetallic silver-gold film-induced surface plasmon wave extension. Rhodamine B (RhB) dyes were excited by the evanescent wave field produced from surface plasmon polaritons excited on metal-deposited sections along an embedded strip waveguide. Various silver-gold combinations were used to quantify for the evanescent field enhancement. The underlying silver yields better evanescent field enhancement, while the overlying gold ensures that the stability of the sensing surface is not compromised. In comparison to the conventional single gold film surface plasmon resonance (SPR) configuration, the two-layered metallic structure is capable of enhancing the surface plasmon polariton (SPP) evanescent field considerably, as verified experimentally by the ca. 4.0 times improvement in the RhB fluorescence emission. The compact waveguide structure and improved electric field probing depth can potentially be exploited for on-chip SPR--fluorescence excitation of less concentrated fluorophore-labelled biological and chemical analytes, with a capability of massively parallel processing for high throughput screening.     

Static electric field effects at the surface of a metallic cluster

A jellium/free-electron model is used to estimate the static electric field at the surface of a metal cluster. The electric field near a cluster with a radius of 10–50 au is found to deviate significantly from the field at the same distance from an infinite metallic plane. This will influence several adsorbate properties, including bonding. At a smooth planar surface the field is sufficiently strong to cause significant contributions to the dipole and transition moments of an admolecule. These contributions become even more important at the surface of a cluster. It is postulated that microscopic roughness may enhance the electron-transfer contribution to surface-enhanced Raman scattering, since the Fermi energy of electrons in a protuberance is increased relative to the bulk. Moreover, for the same reason, the molecule-metal interaction is increased when the molecule is adsorbed at a protuberance. Finally, by means of a WKB formula derived from the model, an accurate prediction can be made of the “magic numbers” which refer to the enhanced abundances of certain clusters when formed in an atomic beam. Therefore these numbers are likely to be determined by the symmetry of the problem and not by the detailed nature of the one-electron potential.     

Degradation of Organic Pollutants Using Atmospheric Pressure Glow Discharge Plasma

A method of plasma treatment in which a glow discharge was generated in the small gas gap between an electrode and a water surface was designed and employed in this study. By using this method, many active species were generated on the wastewater surface to degrade organic pollutants. The electric field distribution of the designed electrode model was simulated using the MAXWELL 3D^® simulation software, and the discharge parameters were measured to investigate the impact of design optimization. In addition, we designed an equipotential multi-electrode configuration to treat a methyl orange solution and an azobenzene solution. The experimental and simulation results indicate that the designed electrodes can realize glow discharge with a relatively low voltage and that the generated plasma covers a large area and is in a stable state. Accordingly, the method helped reduce the cost of the reactor and improved the effectiveness of wastewater treatment.     

pH-敏感的疏水型凝胶在直流电场中的刺激响应

合成了不同疏水链长的甲基丙烯酸酯、甲基丙烯酰胺及丙烯酸与 4 ,4′ 二 (甲基丙烯酰胺基 )偶氮苯交联共聚的聚电解质凝胶 .该凝胶在非接触电场中发生消溶胀 ,但疏水侧链较长的凝胶在发生消溶胀之前经历了一个溶胀过程 .凝胶在电场中首先弯向阴极 ,随着时间的延长或电压的升高弯向阴极 ;并出现弯曲的方向或弯曲的程度的波动 .当撤去电场后 ,凝胶又能恢复到初始的形态 .凝胶的这些性质既与电渗、电扩散及电解有关 ,亦与凝胶的疏水相互作用有关     

Effect of the interaction of azobenzene groups in polymeric composites on their electrooptical characteristics

New azo polymers and a polymeric complex with cobalt based on them were created, and the effect of interaction of the chromophores on the electrooptical effect in films of these materials was investigated. It was established that with increase in the length of the “spacer” and with the possibility of the formation of hydrogen bonds between the substituents in the azobenzene groups the time of the changes in the optical characteristics of the films decreases and their magnitude increases under the influence of the electric field. A phenomenological model of the effect of aggregates of the azobenzene groups attached to the main polymer chain by the spacers on the electrooptical characteristics of the films is proposed.     

Synthesis and photochemical behavior of phosphorus dendrimers containing azobenzene units within the branches and/or on the surface

We describe the synthesis of three series of phosphorus-containing dendrimers having azobenzene derivatives specifically placed at some generations in the interior and/or on the surface. The largest compound obtained possesses 48 azobenzene groups on the surface. Irradiation at 350 nm induces isomerization of the azobenzene groups from the E form to the Z form, whatever their location. The thermal back-isomerization to the E form in the dark at room temperature was observed in all cases. The kinetics of this Z-->E back-isomerization was studied in several cases; the rate is not dependent on the number of azobenzene units or of the generation, when the azobenzene groups are linked to the surface of the dendrimer. A different behavior was observed when the azobenzene groups were located within the framework of the dendrimer.     

Electrical transport characteristics of surface-conductance-controlled, dielectrophoretically separated single-walled carbon nanotubes

Alternating current dielectrophoresis has attracted considerable attention as a possible candidate to separate single-walled carbon nanotubes according to electronic types. Recently, the significant effect of surface charge on the polarizability of semiconducting nanotubes was demonstrated using comparative Raman spectroscopic studies. Here we present electrical transport characteristics of surface-charge-controlled, dielectrophoretically deposited nanotube arrays. The surface charge was controlled using cationic/anionic surfactant mixtures. Complete separation between metallic and semiconducting species was achieved at the electric field frequency of 10 MHz only when the surface charge of nanotubes was neutralized, which is consistent with previous Raman investigation. A theoretical analysis, using zeta potential information as input, further supported the experimental observation.