Aseel Hassan

Modelling of long-period fibre grating response to refractive index higher than that of cladding

In this paper we introduce a new mathematical model, which can be used to investigate the response of a long-period fibre grating to changes in ambient refractive indices of values both greater and less than that of the cladding. A numerical method to calculate the HE core and cladding modes is presented. The results show that even when the ambient refractive index is higher than that of the cladding, the coupling (resonance) wavelengths experience a measurable shift (a few nm). The result obtained suggests that LPGs coated with a material of higher refractive index than the cladding may be used as index sensors.

Study of adsorption of some organic molecules in calix[4]resorcinolarene LB films by surface plasmon resonance

Adsorption of some organic guest molecules in tetra-phosphorylated calix[4]-resorcinolarene LB films was studied by surface plasmon resonance (SPR) techniques. Two methods of registration have been used: measurement of the whole SPR curve at steady state of adsorption and in situ registration of SPR signal at fixed angle during adsorption. Fast and reversible adsorption of toluene and different hydrocarbon vapours was observed. Increasing both film thickness and refractive index implied the swelling behaviour of the LB film during adsorption and can be explained by the capturing and subsequent condensation of guest molecules in the calixarene matrix. Selectivity of SPR response to various hydrocarbons can be attributed to the difference of their condensation temperatures rather than to molecular dimensions.

DC conduction processes and electrical parameters of the organic semiconducting zinc phthalocyanine, ZnPc, thin films

Abstract Electronic conduction in thermally evaporated thin films of organic semiconductor zinc phthalocyanine (ZnPc) has been investigated in a broad temperature range using gold Ohmic contacts. Electronic conduction by charge carrier hopping was dominated at low temperatures and for all applied voltages. At higher temperatures and at voltages just below 2 V conduction was found to obey Ohms law, while at higher voltages space-charge-limited conduction (SCLC) was the dominated mechanism, which was controlled by hole-trapping states distributed exponentially within the band gap. In freshly prepared samples adsorbed oxygen was responsible for lower hole mobility and higher charge carrier concentrations. Prolonged heating of ZnPc films at 425 K resulted in lower defect state density, and thus reduced trap concentration and higher charge carrier mobility.

Condensation of organic vapours within nanoporous calixarene thin films

Adsorption of vapours of benzene, toluene, p-xylene, aniline, hexane and chloroform in LB films of two novel calix[4]resorcinarene derivatives was studied in situ using quartz-crystal microbalance (QCM), ellipsometry and surface plasmon resonance (SPR) techniques. Isotherms of adsorption obtained by both QCM and SPR show that the adsorption ability depends on the condensed vapour pressures of the adsorbates rather than on a structural coincidence between host cavities and guest molecules. The results were interpreted in terms of capillary condensation of organic vapours in the nanoporous matrix of calixarene LB films accompanied by film swelling. Ellipsometric measurements show changes of both the thickness and refractive index of the LB films caused by adsorption, and thus confirm condensation and further accumulation of liquid adsorbate within the film matrix. Unusual adsorption kinetics were observed only when the vapour injection technique was used and their occurrence is believed to be caused by initial vapour condensation on the film surface. This can be eliminated by measurements under constant vapour flow.

Optical dispersion in spun nanocrystalline titania thin films

Sol–gel derived titanium dioxide (TiO2) films are prepared on silicon substrates using the method of spin-coating with different speeds from 1000 to 6000 rpm. Spectroscopic ellipsometric measurements provide dispersion of the refractive index and extinction coefficient within the wavelength range of 300–1000 nm. The behaviour is in agreement with the single oscillator model. Optical absorption is believed to be due to allowed indirect transition over the optical gap of approximately 3.2 eV.

THE EFFECT OF NO2 ON OPTICAL ABSORPTION IN LANGMUIR-BLODGETT FILMS OF OCTA-SUBSTITUTED AMPHIPHILIC COPPER PHTHALOCYANINE MOLECULES

Thin films consisting of 20 layers of 1,4-bis(4-hydroxybutyl)-8,11,15,18,22,25-hexakisdecyl phthalocyaninato copper (CuAmPc) molecules have been deposited onto pre-cleaned glass slides by the Langmuir-Blodgett (LB) technique. The optical absorption of these films were studied in the range 1.4-4.0 eV in air and also in the presence of NO2 gas at 100 ppm level. Prolonged exposures have resulted in dramatic changes in the visible absorption band with the consequence of disappearance of Davydov splitting. Such effects are caused by the domination of local interaction between the central metal atoms and NO2 molecules resulting in a distorted herring-bone structure. This leads to the broadening of the energy levels a(2u) and e(g). Gas adsorption and desorption processes have been described by exponential functions, both characterised by a single time constant

Structural and electrical studies on sol-gel derived spun TiO2 thin films

Titanium dioxide thin films were prepared by spin coating of sol precursor onto microscopic glass slides, silicon and indium tin oxide (ITO) coated glass substrates. Spin speed was varied between 1000 and 6000 rpm. From the morphological analysis, it is found that thin films spun with speed ω≤4000 rpm assume higher ordered structure than those spun at a speed higher than 4000 rpm. Conduction at low voltages is believed to be variable range hopping at temperatures T<220 K while the ionization of donors situated at Ea = 0.46 eV below the conduction band edge becomes dominant at temperatures higher than 220 K. At high field charge transport is due to trap-controlled space charge limited mechanism. Traps with a density Nt≈1×1022 m−3 are thought to be situated at energy level Et = 0.3 eV below the conduction band and are associated with film nonstoichiometry and interface states.

A novel glucose sensor using lutetium phthalocyanine as redox mediator in reduced graphene oxide conducting polymer multifunctional hydrogel

Herein, we report a scalable synthesis of multifunctional conducting polyacrylic acid (PAA) hydrogel (MFH) integrated with reduced grapheme oxide (rGO), vinyl substituted polyaniline (VS-PANI) and lutetium Phthalocyanine (LuPc2) as three dimensional robust matrix for glucose oxidase (GOx) immobilization (PAA-rGO/VS-PANI/LuPc2/GOx-MFH). We have integrated the multicomponents such as PAA with rGO, and VS-PANI through free radical polymerization using methylene bis-acrylamide, and ammonium persulphate as the cross linker and initiator. The LuPc2 was then doped to form multifunctional hydrogel (PAA-rGO/VS-PANI/LuPc2-MFH). Finally, biosensor was fabricated by immobilizing GOx into PAA-rGO/VS-PANI/LuPc2-MFH and subsequently used for electrochemical detection of glucose. The PAA-rGO/VS-PANI/LuPc2/GOx-MFH biosensor exhibited high sensitivity (15.31μAmM-1cm-2) for the detection of glucose over a concentration range of 2-12mM with a low detection limit of 25µm. The PAA-rGO/VS-PANI/LuPc2-MFH biosensor showed a fast response time (1s) to the addition of glucose with high storage stability of 3 months. The real sample analysis reveals that PAA-rGO/VS-PANI/LuPc2/GOx-MFH could be effectively used as an electrochemical biosensor in industrial as well clinical diagnosis.

Resorcinol calixarenes (resorcarenes): Langmuir-Blodgett films and optical properties

Specially synthesized amphiphilic resorcinol calixarene (resorcarene 2) molecules have been deposited as Langmuir-Blodgett (LB) films on a variety of substrates including hydrophobically treated glass slides, silicon and gold-coated glass slides. A value of 1.9 nm(2) is obtained for the area per molecule from measurements of pressure/area isotherms of the floating layer. Optical absorption studies within the ultraviolet-visible frequency region have been performed on these molecules in both LB films and in solution of resorcarene 2 in chloroform, containing 10% ethanol. Molecular aggregation in the form of dimerization is believed to take place during film formation. Further analysis has been carried out for Langmuir-Blodgett films of resorcarene 2 by using Fourier transform infra-red spectroscopy, low-angle X-ray diffraction and surface plasmon resonance (SPR) techniques. The monolayer thickness of 1.6 nm found from SPR measurements is consistent with that from other experimental observations

Electrical characterisation of LB films containing CdS nanoparticles

CdS nanoparticles were formed within LB films of either Cd salts of stearic acid (SA) or calix[8]arene (CA) by exposing the samples to H2S gas. A series of DC and AC electrical measurements were performed for both planar and sandwich structures over a wide range of frequencies. Exponential behaviour of the DC I-V characteristics of the sandwich structures is typical for electron tunnelling through multilayered LB films. The conductivity value decreases after formation of CdS nanoparticles in the film, which is believed to be caused by the reduced layer-by-layer film order. AC measurements show a traditional insulating behaviour for all kind of films studied. Formation of CdS clusters in the LB films resulted in the decrease in both the capacitance and AC conductivity, which can be interpreted by the drop in the polarisability in disordered LB films