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Any crisis (personal or collective) brings along an in-built stress. It cripples people from living a normal life; sometimes it leads people to the extreme condition of permanent damage. The COVID-19 situation has brought in unexpected misery – victimizing millions of people. It has been an equalizer as both the rich and the poor have been affected; both the affluent and the developing countries have become a prey to the pandemic. For nearly one year the world is wondering which way to turn for comfort or solution, as the fear of the second and third waves looms over. Science has not given the timely preventive medication; world leaders are not able to lead the people with effective insight; financial system is on the brink of collapse; and even the faith of people seems to become a question mark. But any problem should have a solution, with often multiple aspects of solutions. Apart from medicine, one could become strengthened to face the situation. Starting with the analysis of the cause and the effect of Coronavirus Pandemic, this talk explores the intellectual and emotional aspects of the situation with the focus on the psychological and spiritual solutions to face the crisis; how to get reconciled with the reality; and how to thrive in his situation. The ultimate goal is to come out of the crisis with a spirit of creativity. If the pandemic is a period of cocoon, the butterfly-joy would be the hope to come out soon. And we need to stay energized to realize the fullness of life with peace of the mind, health of the body, and joy of the heart. Practical suggestions are put forward to face, encounter, and overcome the situation.

Any crisis (personal or collective) brings along an in-built stress. It cripples people from living a normal life; sometimes it leads people to the extreme condition of permanent damage. The COVID-19 situation has brought in unexpected misery - victimizing millions of people. It has been an equalizer as both the rich and the poor have been affected; both the affluent and the developing countries have become a prey to the pandemic. For nearly one year the world is wondering which way to turn for comfort or solution, as the fear of the second and third waves looms over. Science has not given the timely preventive medication; world leaders are not able to lead the people with effective insight; financial system is on the brink of collapse; and even the faith of people seems to become a question mark. But any problem should have a solution, with often multiple aspects of solutions. Apart from medicine, one could become strengthened to face the situation. Starting with the analysis of the cause and the effect of Coronavirus Pandemic, this talk explores the intellectual and emotional aspects of the situation with the focus on the psychological and spiritual solutions to face the crisis; how to get reconciled with the reality; and how to thrive in his situation. The ultimate goal is to come out of the crisis with a spirit of creativity. If the pandemic is a period of cocoon, the butterfly-joy would be the hope to come out soon. And we need to stay energized to realize the fullness of life with peace of the mind, health of the body, and joy of the heart. Practical suggestions are put forward to face, encounter, and overcome the situation.

Pure and Aluminium-doped ZnO (Zn1-xAlxO) x = 0 to 5 wt% thin films were deposited onto glass substrate by sol-gel spin coating method. The influence of various aluminium concentration in ZnO thin films on the structural, surface, optical and photoconducting properties were investigated. The GIXRD studies confirmed the polycrystalline nature with wurtzite structure of pure and Al doped ZnO films. Films with 1.5 % concentration of aluminium showed maximum absorption and transmission in the UV and visible regions respectively. The FESEM images showed crack free films with increasing grain boundaries upon doping. The average grain size is found to decrease due to aluminium doping. AFM images showed doped films with 1.5 % have better smoothness than other films. The photoconductivity measurements reveal that there is increase in the photocurrent compared to dark current for the Al doped ZnO. The photocurrent reaches its maximum value for ZnO: Al-1.5 % and then decreases upon increase in Al concentration. The photoresponse has slightly degraded upon aluminium doping onto ZnO. Temperature dependent conductivity shows that the thermal activation energy for the film decreases up to 1.5 % aluminium concentration and then increases for other concentrations in the temperature range 308 to 375K.

Thin films of undoped and doped ZnO, with different Al concentrations (1–5 wt%) were deposited onto glass substrates, by the sol–gel spin coating method. Grazing incidence X-ray diffraction (GIXRD) studies confirmed the nature of films as poly-crystalline, with typical hexagonal wurtzite structure. The films showed variation in crystallite size and change in relative intensities, upon different Al doping concentrations. The surface morphology of the films examined using FE-SEM, showed the grain size becoming smaller upon Al doping. The influence of Al with different concentrations, onto ZnO on the optical absorption and transmittance was studied using UV–Vis–NIR spectrophotometer in the wavelength range 300–2500 nm. The UV absorption shifted towards shorter wavelength upon Al doping. The average transmittance in the visible region increased for Al doped films up to 1–2 wt% and decreased for other concentration. The dark and photo conductivity measurements of the films indicated increase in the current values upon doping up to 1–2 wt% of Al and decreased for further concentrations. The rise and decay time measured from the photoresponse study, indicate larger values of rise time for the doped films compared to undoped ZnO. However, the film with 1–2 wt% doping of Al showed better response within the doping concentration. The thermal activation energy obtained from temperature-dependant conductivity showed decrease in the value upon Al doping up to 2 wt% and increased beyond this concentration in the temperature range 300–400 K.

Sole components of titania (TiO2), silica (SiO2) nanoparticles, and binary TiO2–SiO2 nanocomposites with various molar ratios of silica contents were prepared by modified sol–gel method. The samples were calcined at 500 °C for 5 h and characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), UV–Vis spectroscopy, Brunauett–Emmett–Teller (BET), and photoconductivity. The crystallite size for TiO2/SiO2 nanocomposites was calculated using Scherrer’s formula and found to be 5 nm for TiO2 nanoparticles. The binary oxide shows the anatase type of TiO2 at the mole ratio up to 80 mol% of TiO2 added. The band gap for as-synthesized nanocomposites was calculated and it was found that the band gap decreases with increase of SiO2 content and then decreases with excess SiO2 content. FTIR confirms that both TiO2 and SiO2 phases have been formed. The BET surface area for TiO2/SiO2 nanocomposite is found to be 303 m2/g, and pore size distribution has an average pore diameter about 10 nm for 40 mol% of TiO2 added. It also confirms the absence of macropores and the presence of micro and mesopores. The field-dependent dark and photoconductivity studies reveal that the dark and photocurrent increase linearly with applied field confirming the ohmic nature of the electric contacts. The dark and photocurrent increase slightly with increase of SiO2 content and decrease with excess amount of SiO2.

Metal-doped (Cu 2 O, CdO and PbO) muscle protein from fish ( Clarious batracus Lin.) was isolated, purified and characterized for its potential as semiconducting biopolymer. The initial characterization of the isolated protein was carried out by circular dichrosim (CD), FTIR and AAS. Protein thin film was prepared using polyvinyl alcohol (PVA) and characterized for its surface morphology by SEM/EDAX and crystallinity by XRD spectrum. The physical properties such as Raman shift, optical coefficient and electron transfer reaction such as electrical conductivity, temperature dependent conductivity and cyclic voltammeter (CV) were studied. The present study concludes that the Cd and Cu-doped fish protein behaves like semiconductor and has the potential application in molecular electronics like protein semiconductors, protein based (proton exchange membrane) fuel cell, drug delivery and nanotechnology.

The addition of ortho-chloranil to the emeraldine base form of polyaniline has been found to increase the dark current and the photocurrent by three orders of magnitude. An electron transfer from polyaniline to ortho-chloranil occurs producing holes in polyaniline. Since ortho-chloranil is an electron acceptor there is a general effect of it acting as an oxidizing agent upon the conductivity properties of polyaniline. The increase in photocurrent upon illumination is due to the transfer of electrons from excited polyaniline molecule to the ortho-chloranil negative ion producing further polyaniline holes and ortho-chloranil ions. The activation energy of polyaniline decreases with increasing doping concentrations. For doping concentrations from 4% (w/w) to 8% (w/w), the activation energy is halved when compared with that of pure polyaniline and it is one fourth of the value of pure polyaniline for 10% doping concentration. The band gap of polyaniline also decreases with increasing doping concentrations. On either side of the band gap, oscillatory structures are observed. On the lower energy level they are ascribed to excitons and on the higher energy level, they indicate structures in band gap system. The dissociation of excitons might take place at the interface at molecular level between polyaniline and ortho-chloranil molecules forming a charge-transfer complex, which explains the increase in conductivity upon the addition of ortho-chloranil. The presence of single transition point in electroreflectance spectra reveals that the mixture of polyaniline and ortho-chloranil behaves as a single entity and not as a simple mixture. The conduction mechanism in this charge-transfer complex is molecular rather than solidstate in nature.

The effects of 125 keV oxygen ion implantation with fluences from 10 14 to 10 16 ions/cm 2 on (1 0 0) cut crystalline silicon (c-Si) have been investigated by Raman scattering and photoluminescence techniques. A systematic decrease in the area and frequency of longitudinal optical (LO) phonon Raman mode and increase in its full-width at half-maximum (FWHM) have been observed with increase of fluence. The temperature at the incident laser spot on the implanted Si increases with fluence. The behavior of peak position and FWHM is due to implantation-induced lattice damage. The decrease in area of the mode is due to the decrease in the volume fraction of c-Si in the implanted layer. The disappearance of the Raman LO mode of c-Si and the appearance of the Raman mode for the amorphous Si (a-Si) have been observed for 10 16 O + /cm 2 fluence, suggesting the amorphization of the implanted layer. The increase in the temperature is due to the poor thermal conductivity of a-Si and increased photon absorption due to implantation induced lattice disorder. Photoluminescence (PL) in the range of 1.8–2.52 eV shows a peak at 2.35 eV with 0.4 eV FWHM for all fluences. Its area increases with fluence and saturates. These results are due to a-Si nanozones created by the O + impact and coalesce of such nanozones at high fluences. The average size of the a-Si nanozone estimated from PL peak position (2.05 nm) and that from the critical fluence for amorphization (1.9 nm) match very well.

It is a difficult process to dope a material using sublimation method when the substances chosen for study have different sublimation temperatures. The two substances used for making the doped crystal are to be maintained at their respective sublimation temperatures before they are allowed to interact at a common temperature gradient zone for the desired crystal growth. Therefore it was decided to construct a furnace which will maintain the two substances in their sublimation temperature simultaneously. The apparatus was constructed to grow metal free phthalocyanine (H2Pc) doped anthracene crystal as they are expected to be active in the entire region (UV-VIS-IR) of the electromagnetic spectrum, for a possible application as a solar cell material. Phthalocyanine (sublimation at 460°C) doped Anthracene (sublimation at 167°C) crystal was successfully grown using this three-zone furnace and was analysed using XRD, IR, TGA and conductivity studies. In general, the furnace will be especially useful for the combined growth of certain substances which do not have a melting point.

The optical response of n-type porous silicon is investigated using spectroscopic ellipsometry in the energy range of 1.5– 5 eV . The close interrelationship between the microstructure and electronic states is examined on (1 0 0) oriented n-type single crystal silicon wafers anodically etched for different durations under illumination with a halogen lamp. Changes occurring in the pseudodielectric response upon etching are analyzed using a multilayer model within the effective medium approximation. Sum rules are used to evaluate changes in the effective number of electrons participating in the optical transition ( N eff ). The process of etching is shown to result in a substantial shift of spectral weights to the visible region.

Structural properties of copper phthalocyanine (CuPc) powder and its thin films coated on glass plate are investigated using X-ray diffraction, Raman scattering and optical absorption techniques. These experiments are carried out at room temperature on CuPc powder and as-coated, 320 °C and 520 °C annealed, CuPc thin films. From the X-ray diffraction (XRD), we find that the CuPc powder exists in monoclinic phase, as-coated film is in orthorhombic α-phase and the film annealed at 320 °C exists in monoclinic β-phase with different lattice parameters as compared to those of the powder phase. The XRD peaks in 520 °C annealed film do not show the presence of CuPc but indicate the presence of copper oxide (CuO) nanoclusters of approximately 15 nm size. The optical Raman phonon frequencies of CuPc powder, as-coated and 320 °C annealed CuPc films show a slight shift in the intramolecular high frequency modes. Whereas, their low frequency Raman spectra from intermolecular modes are characteristically different indicating different phases. The different characteristic behavior of the optical absorption spectra of the as-coated and 320 °C annealed CuPc films corroborate the structural changes in the annealed film.

Anthracene and metal-free phthalocyanine (H 2 Pc) are well-known organic semiconductors, and are reported to be photoconductive in the UV–VIS and IR regions, respectively. Hence H 2 Pc-doped anthracene crystal formed is expected to be active in the entire region UV–VIS–IR, and the compound material could be a viable alternative for a possible solar cell material for the entire range of UV, VIS and IR regions of solar radiation. In the present study, co-crystals of H 2 Pc-doped anthracene are formed using DMSO as the common solvent, and subjected to conductivity studies. The activation energy decreases as the concentration of H 2 Pc increases up to 1% and decreases beyond 1%. In the UV spectrum, the absorption decreases when the concentration of H 2 Pc exceeds 1%. Though the traps play a major role in the electrical and optical conductivity of organic molecular solids, their presence beyond a limit decreases the conductivity. The samples are found to be photoactive and could be a possible alternative material for a semiconductor-based solar cell.

Anthracene is an organic semiconductor extensively investigated over many decades. It finds applications as photoreceptor material, scintillation counter etc. Anthracene is optically active in the UV region of the electromagnetic spectrum whereas H 2 Pc is active from visible through IR regions. Hence phthalocyanine (H 2 Pc) doped anthracene crystal formed is expected to be active in the entire region UV–VIS–IR, and the compound material could be a viable alternative for a possible solar cell material. In the present study doped crystal was grown by two methods namely co-crystallization and vacuum sublimation using a special type three-zone furnace. The co-crystals of anthracene and H 2 Pc were formed using dimethyl sulphoxide as the common solvent, and were subjected to various analysis including electroreflectance method. The electroreflectance method gives the band gap accurately and the calculations show that there is a decrease in the band gap as the concentration of H 2 Pc increases. Also the doped samples were found to be photo active and hence H 2 Pc could be used as a suitable dopant to increase the conductivity of anthracene. A special type three-zone furnace is designed to grow H 2 Pc doped anthracene single crystal. The melting point of anthracene is 217°C while H 2 Pc decomposes above 550°C, but sublimates around 460°C in vacuum. Anthracene and H 2 Pc were placed in two side arms of a main stem and the tube is sealed in vacuum. The sealed tube is heated for few days to enhance the growth of a single crystal. The compound crystal is formed along with the main stem at a convenient temperature gradient.

Photoelectrochemical studies were carried out on [MnMoO2(NCS)(Ox)3(H2O)2] {Ox=8-quinolinol} complex in aqueous dimethylformamide medium in a Honda cell. The system developed a maximum potential of 335 mV when exposed to visible light at 30°C and was found to be reversible. The photogalvanic behaviour has been further investigated by varying the pH, temperature and photosensitizers. When a temperature difference between the illuminated and dark half-cells was maintained, the system generated 410 mV at 60°C. A solid-state galvanic cell, developed using the complex mixed with tetraethylammonium perchlorate (TEAP), showed a maximum voltage of 25 mV. A sandwich galvanic cell, constructed from transparent tin oxide-glass/complex/platinum, developed a maximum photovoltage of 88 mV when irradiated with a tungsten halogen lamp. Copyright © 2000 John Wiley & Sons, Ltd.

Single crystals of potassium penta borate (KB5) exhibit interesting non-linear optical properties such as phasematched second-harmonic generation of the light for a wavelength as short as 216.8 nm. In the present work, single crystals of KB5 with dimensions upto 20]20] 8m m3 are grown by slow evaporation method and subjected to X-ray, TGA, DSC and microhardness studies. The thermogravimetric analysis con"rms the decomposition of KB5 taking place in two stages. The DSC curve of KB5 crystals are recorded in the temperature range 50}5803C. The phase transition temperatures and enthalpies of KB5 were measured and the results are discussed. The grown crystals of KB5 are subjected to microhardness studies using a Vickers diamond pyramid indentor. The microhardness measurements carried out on the (0 1 0), (1 0 0) and (0 0 1) orientations con"rm the anisotropic nature in the microhardness number values. ( 2000 Elsevier Science B.V. All rights reserved.

Lead phthalocyanine thin films (PbPc) were prepared by vacuum sublimation under a pressure of 10(-5) Torr. Photoconductive studies as a function of applied voltage, intensity of incident light, temperature and wavelength were carried out. Photocurrent spectra as a function of wavelength indicate peaks around 1.54, 1.98, 2.4, 3.0 and 3.48 eV. These are interpreted as transitions at critical points. From the temperature dependence of photoconductivity measurements, the activation energies 0.074 and 0.46 eV were obtained and 0.68 and 0.46 eV for dark conductivity. The dependence of the intensity of photocurrent yields a slope of m = 0.6, in the photoconductivity equation I-ph proportional to (In)(m). An energy level diagram is drawn for lead phthalocyanine thin films from the photoresponse studies. (C) 1999 .

Polyaniline (PA) and aniline black (AB) were prepared in powder form by the chemical method of oxidative polymerization, from which free-standing thin films were obtained by solvent evaporation using N-methyl pyrrolidinone ( NMP ). The thin films contained 2, 4, 6 or 8 wt% AB. Electrical measurements showed that the samples containing 4% AB exhibited the highest photoconductivity of the four concentrations. Thus there appears to be a critical ratio of PA to AB for maximum photoconductivity. Annealing up to 570 K has little effect on the conductivity. Upon adding a small quantity of copper phthalocyanine ( CuPc ) to the PA + AB, the electrical conductivity increased considerably and the optical absorption was extended from the UV to the near IR. The electrical conductivity mechanism is a consequence of a redox process, since AB is the oxidized state of PA and, upon illumination, there is an exchange of charge carriers. The extension of the range of optical absorption upon addition of CuPc is interpreted to suggest that CuPc photosensitizes the material and enhances the carrier transport process in the redox couple. The activation energy from the temperature-dependent conductivity and the band gap from the electrolyte electroreflectance method were determined. The activation energy for 0.4% CuPc is lowest (0.52 eV) and the corresponding band gap is determined to be 3.0 eV. This organic compound could be a good candidate for inexpensive, reliable and efficient solar energy-converting devices.

Manganese–molybdenum–diethyldithiocarbamate [MnMoO2(Et2dtc)4(H2O)] {Et2dtc=diethyldithiocarbamate} complex has exhibited reversible photogalvanic behaviour in aqueous dimethylformamide medium when studied in a Honda cell. The photogalvanic behaviour has been further investigated by varying the pH, temperature and photosensitizers. UV, visible and sunlight were used as the radiation sources. A maximum potential of 345 mV was obtained at 80°C in visible light. The system was found to be reversible for several cycles. A photoelectrochemical cell was constructed by coupling a charged nickel electrode with the complex electrode which incorporates the experimental compound in acetylene black on a nickel substrate. A maximum potential of 1·08 V with 80 μA current was obtained when irradiated with tungsten lamp. Copyright © 1999 John Wiley & Sons, Ltd.

Heterobinuclear manganese-molybdenum complex with the Schiff base prepared by the condensation of diethylenetriamine with salicylaldehyde yielded a polymeric mixed metal coordination complex with the composition of [Mn1·5MoO3L]. The infrared spectral data indicated the presence of azomethine (HC=N) and Mo=O groups.1H NMR further confirmed the coordination of azomethine nitrogen and absence of phenolic hydrogen. The EPR studies suggested the presence of Mn(II) in the complex. The solid state conductivity studies with the variation of potential and temperature indicated a specific conductivity of 1·6×10−5 ohm−1 cm−1. The activation energy was found to be 0·019 eV. Hall measurements indicate that the material isp-type which is further confirmed by iodine doping which increases the conductivity by two orders of magnitude.

Many organic semiconductors with conjugate bond structure possess photoconductivity. Conduction mechanism of organic materials exhibits ‘dualism’ since both intramolecular as well as inter-molecular aspects are involved in the excitation, absorption and transport of charge carriers. Modulation spectroscopy promises to be the most accurate method for analysis of organic photoconductors, especially of thin films. In this technique a periodic perturbation is applied to the material under study and the effect of the perturbation is separated from reflection or absorption while scanning through a given wavelength range by use of lock-in phase sensitive detection method. In electromodulation, particularly in electrolyte electromodulation, the applied field on the material produces changes in the dielectric function which corresponds to the change in reflectance. When the applied field is low the line-shape of spectrum is third-derivative like in comparison with the unmodulated reflectance spectrum. Using Aspnes three-point method the transitions corresponding to critical points can be determined. When the field is intermediate Franz-Keldysh oscillations, which are dc bias dependent, appear on the higher energy side of the transition energy from which the role of intra-molecular as well as intermolecular aspects in conduction mechanism can be understood and the carrier concentration could be determined. Though the electroreflectance method has been developed for inorganic semiconductors, it could be effectively applied for organic/molecular semiconductors as well if the constituent molecules are assumed to be the lattices. The study of organic photoconductors is very important since they are more and more promising especially in photocopying, photovoltaic and solar cells.

Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.

Metal-free phthalocyanine (H2Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H2Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H2Pc is in good agreement, thereby indicating that H2Pc is thermally stable even at relatively higher temperature as semiconductor.

Organic semiconductors having conjugate bonds, such as phthalocyanines, are well known photoconductors. Phthalocyanines absorb light on either side of blue-green region in the visible spectrum. And polyaniline which has conjugate bonds is photosensitive. When polyaniline thin films are prepared with copper phthalocyanine powder the magnitude of absorption of the films not only increases but also broadens, indicating a wide absorption region between 1.7 and 2.3 eV and above 3.0 eV of the visible spectrum. Thus copper phthalocyanine is shown to increase the efficiency of solar cells made of conducting polymers such as polyaniline. The use of iodine along with copper phthalocyanine in preparation of thin films is shown to smooth the broadening effect in absorption of solar cells. Attempts are underway to increase the absorption on either end of the visible spectrum, viz., infrared as well as ultraviolet regions, using suitable dopants. This would ensure the overall efficiency of solar cells made of organic photoconductive materials to absorb solar energy from infrared to ultraviolet regions of the optical spectrum, thereby making them more efficient solar energy converters.

Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.

Thin films of CdSe1−x Te x were prepared by using electro deposition with varying doping concentrations of Te(x). Photoconductive studies as a function of applied field, time, wavelength, and intensity of incident light were carried out at room temperature between 1 and 3·5 eV. After establishing that the electrode contacts made using silver paint were ohmic, it was found that as the concentration of Te(x) increases in CdSe1−x Te x , the band gap of the material decreases according to the empirical relationE g = (2 − 1·5x)eV.

An effective method of growing single crystals of organic photoconductors such as phthalocyanine in the presence of doping impurity such as iodine by vacuum sublimation is discussed in this paper. This method is very useful especially when an organic material does not have a melting point but decomposes above a particular temperature. So far, doping has been done by exposing the previously grown organic single crystals to the dopant vapour, but growing the crystal in the presence of dopant vapour makes the doped-crystal more efficient and stable. The photosensitivity as well as the photoconductivity of the doped crystals by this method increases significantly.

Single crystals of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were grown in the presence of iodine vapour. The presence of iodine enhances the spectral dependence of photoconductivity of H2Pc in the visible region but of CuPc in the near-IR region. The dark current is decreased but the photocurrent is increased by one order of magnitude in iodine-doped H2Pc but in the case of iodine-doped CuPc both currents are increased by nearly three orders of magnitude. Introduction of iodine results in about one order of magnitude decrease in response time for both modifications. Thus the introduction of iodine into Pc crystals decreases the energy barrier for conduction and increases the drift mobility of charge carriers thereby enhancing the conductivity of the material.

The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H2Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H2Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H2Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H2Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.

The photocurrent and electrolyte electromodulation (EEM) spectra of thin films of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were investigated. The modulation spectra yielded three distinct features around 1·61, 2·30 and 2·93 eV for H2Pc and around 1·63, 2·04 and 3·20 eV for CuPc. The spectral dependence maxima of photoconductivity correspond to the modulation spectra. These features are interpreted to indicate transitions at critical points, i.e. the existence of transitions between three valence bands, since Pc’s are p-type, and the lowest conduction band in Pc’s.

Microelectronics technology today is dominated exclusively by Silicon (Si). The inefficiency of Si to emit light even at cryogenic temperatures has been overcome with the discovery of porous silicon (PS) and its visible luminescence at room temperature. The present investigation aims at analysing the effect of increasing porosity on the transport properties of porous silicon with reference to field and temperature–dependent dark and photo conductivity and further substantiating the results with modulation techniques. Pure silicon wafer of n–type was made porous by immersion in an appropriate etchant for a few minutes. The conductivity was found to increase as porosity increased and this effect could be attributed to the increase in the trap levels, with increasing porosity. Temperature–dependent studies reveal a decrease in activation energy with increase in porosity indicating an increase in conductivity. Reflectance and electroreflectance measurements were used to calculate the band gap of porous silicon. It was found to lie closer to the direct band gap of silicon. A reduction in the band gap of porous silicon has been observed.

The use of titania–silica in photocatalytic process has been proposed as an alternative to the conventional TiO2 catalysts. Mesoporous materials have been of great interest as catalysts because of their unique textural and structural properties. Mesoporous TiO2, SiO2 nanoparticles and TiO2/SiO2 nanocomposites were successfully synthesized by sol–gel method using titanium (IV) isopropoxide, tetra-ethylorthosilicate as starting materials. The synthesized samples are characterized by X-ray diffraction, UV–Vis spectroscopy, Fourier transform infrared spectroscopy, Brunauett–Emmett–Teller and field-dependent photoconductivity. The UV–Vis spectrum of as-synthesized samples shows similar absorption in the visible range. The crystallite size of the as-synthesized samples was calculated by Scherrer’s formula. The BET surface area for TiO2/SiO2 nanocomposite is found to be 303 m2/g and pore size distribution has average pore diameter about 10 nm. It also confirms the absence of macropores and the presence of micro and mesopores. The field-dependent photoconductivity of TiO2/SiO2 nanocomposite shows nearly 300 folds more than that of TiO2 nanoparticle for a field of 800 V/cm.