Biswanath Mallik

Adsorption-induced unusual changes in the electrical conductivity of ferrocene

Abstract A conductivity peak is observed in the plot of the measured electrical current vs temperature for a sandwich cell of pure ferrocene powders with adsorbed organic vapours at low vapour pressures. The intensity of this peak and the temperature at which it appears depend on the nature of the adsorbed vapours. Intensity as well as the shape of the current vs temperature curves depend drastically on the vapour pressure of (i.e. amounts of) the adsorbed vapours. With some vapours adsorbed at higher vapour pressures multiple conductivity peaks are observed. The position of the conductivity peaks depends significantly on the grain size of the ferrocene powders and also on the rate of heating applied in the experiment. Results show that the adsorption is a physical adsorption. In the adsorbed state the material shows an anomalously high value of the activation energy and pre-exponential factor (in the usual semiconduction equation). The results are discussed in the light of different existing theories. Adsorption-induced reversible structural phase transition in ferrocene seems reasonable for the appearance of such conductivity peaks.

Photoinduced charge-transfer between ferrocene derivatives and chloroform molecules confined in poly(methyl methacrylate) thin films

Abstract The changes in the electronic absorption spectra (UV-Vis) (after photoexcitations) of poly(methyl methacrylate) (PMMA) thin films doped with a ferrocene derivative and containing chloroform molecules as impurities, have been studied as a function of photoexcitation wavelength (210–750 nm), photoexcitation time (duration), amount of ferrocene derivative in the film and amount of chloroform molecules present in the film. Occurrence of photoinduced charge-transfer between some ferrocene derivatives and chloroform molecules confined in the PMMA thin films has been observed. The effects of the substitution group (attached to the ferrocene unit) on the photoinduced changes are discussed.

Anomalous photoconductivity of ferrocene

Abstract Photoconductivity behaviour of ferrocene, a very useful metallo-organic sandwich compound, has been investigated at different constant temperatures using powdery material in a sandwich type of cell configuration and with the exposure of a polychromatic light source (mercury lamp of 125 W). Measurements with a constant d.c. bias voltage (27 V) across the sample cell and a fixed intensity of the exciting light source have shown a drastic change in the photocurrent versus time profile with the increase in temperature. Anomalous changes have been observed in the plot of the photocurrent versus reciprocal of temperature. Such changes are completely absent in the corresponding dark current behaviour. The photoinduced changes have been observed to be almost reversible in the entire temperature range. In a particular temperature range the reversibility of photocurrent is accompanied by fluctuations in equilibrium current obtained after switching off the light source. The observed anomalous changes in photocurrent have been explained by photoinduced phase transition in ferrocene. The possible origin and implications of this photoinduced phase transition are discussed.

Electrical Conductivity of Benzoylferrocene in Presence of Adsorbed Vapors

Enhancement in the dark conductivity of benzoylferrocene upon adsorption of different organic vapors has been studied with the help of sandwich cell technique. Plot of dark conductivity vs temperature shows conductivity peak(s). The position of the peaks as well as the intensity have been found to depend on the amount as well as the nature of adsorbed vapor. In presence of adsorbed vapors, this material shows a very high value of activation energy (E) and the pre-exponential factor (σ0) in the usual semiconduction equation [σ=σ0 exp (-E/2kT)]. Existing theories for adsorption-induced change in the electrical conductivity of materials are not suitable for explaining the observed results. Possible reasons for the adsorption-induced changes in the conductivity behavior have been discussed.

Synthesis of thiol capped CdS nanocrystallites using microwave irradiation and studies on their steady state and time resolved photoluminescence.

Synthesis of cadmium sulfide (CdS) nanocrystallites has been performed through the microwave (MW) assisted reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) in the presence of two capping agents, 1-butanethiol and 2-mercaptoethanol. Attempts were made to control the size and size distribution of the thiol capped CdS nanocrystallites by controlling the number of MW irradiations/exposures for a fixed time (duration). The prepared nanocrystallites have been characterized by UV-vis spectroscopy, FTIR, XRD, FESEM and TEM. The peak position of the absorption band of the 1-butanethiol caped CdS nanocrystals in DMF solution shifted towards longer wavelength with the increasing number of MW exposures indicating the growth of particle size. In contrast, the peak position of absorption band for the 2-mercaptoethanol capped CdS nanocrystals remained nearly at the same wavelength and only the intensity of the absorption band increased with the increasing number of MW exposures. The observed steady state photoluminescence (visible range) of the 1-butanethiol capped CdS nanocrystals in DMF solution shifted towards higher wavelength, showing a decrease in intensity, with the increase in the number of MW exposures. Whereas in the case of 2-mercaptoethanol capped CdS nanocrystals in DMF solution, the photoluminescence peak remained nearly at the same position showing a decrease in intensity with increase in the number of MW exposures. The interesting results on the size-dependent steady state and time resolved photoluminescence (PL) of the CdS nanocrystallites are discussed in the present article. Possible application of such studies in the area of biotechnology has been mentioned.

Effect of adsorption of vapours on the electrical conductivity of a series of some naphthyl polyenes: Adsorption and desorption kinetics

Abstract This paper deals with the change in semiconductivity of a series of naphthyl polyenes of the type R-(CHCH) n - R , where R stands for the naphthyl group and n = 1–6, on adsorption of vapours of some organic solvents at a constant sample temperature. Appreciable enhancement in the conductivity is observed. Such enhancement depends on the chemical nature of the solvent and also on the vapour pressure. The desorption is much slower than that of adsorption. Both the adsorption and desorption kinetics follow the modified Roginsky-Zeldovich relation. Kinetics for the compounds having odd- n is distinctly different from those with even- n . The results show that in all these vapour-semiconductor systems, the adsorption is a two-stage process.

Charge-transfer spectra of ferrocene in halocarbon solvents under photoexcitation

The changes in the electronic absorption spectra of ferrocene in the halocarbon solvents chloroform and carbontetrachloride have been investigated under photoexcitation in nitrogen atmosphere. Photoexcitations have been made with monochromatic light (using an Xe-source and a monochromator), at intervals of a few nanometers in the spectral range 220–750 nm. Analysing the spectra by a modified method the position of the charge-transfer-to-solvent (CTTS) band has been located for both the solvents. The position of the CTTS band in the case of carbontetrachloride solution located (320 nm) by the present study is different from the previously reported value (307 nm), while from the previous studies the position of the CTTS band in the case of the spectra of ferrocene in chloroform was not clear. From the present investigation, the changes in spectra after photoexcitation studied as a function, the concentration of ferrocene in the solution and the time (duration) of photoexcitations, have been observed to be systematic. Using the position of the new band (320 nm) for the CTTS transition in the case of carbontetrachloride, ionization potential of ferrocene has been estimated and the estimated value has shown excellent agreement with the experimental value indicating the exactness of the newly located CTTS band position.

Tuning of electrical conductivity and hysteresis effect in poly(methyl methacrylate)–carbon nanotube composite films

Reproducible current vs. voltage hysteresis has been observed in single-walled carbon nanotubes (SWCNTs) embedded in poly (methyl methacrylate) (PMMA) measured under various experimental conditions. We have observed a stable hysteresis loop along anticlockwise direction in positive bias region and the width of the hysteresis was reduced/quenched when the film was exposed to monochromatic light. By varying the SWCNTs content in PMMA composite thin films, the electrical conductance behavior of the Al/PMMA-SWCNTs/ITO sandwich structure can be tuned in a controlled manner. The conductivity increases with SWCNTs concentration. The hysteresis effects of the composite films have been attributed to the injection of electrons from the electrode to the trap site located in the PMMA and SWCNTs layer. Such nanocomposites are expected to be useful for various nanotechnology-based devices.

Electric field induced tunable bistable conductance switching and the memory effect of thiol capped CdS quantum dots embedded in poly(methyl methacrylate) thin films

Experimental results on conductivity switching and appearance of hysteresis in the current–voltage characteristics of thiol (benzyl mercaptan)-capped CdS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix are presented in this manuscript. The dependence of switching behavior has been studied under different conditions such as sample cell temperature, scan speed of voltage, illumination of light of various intensities, film thickness, size of thiol-capped CdS QDs etc. The voltage at which the conductivity switching occurs i.e. threshold voltage (Vth) appearing in the current–voltage characteristic curve, decreases with the increasing sample cell temperature and also with increasing intensity of photoexcitations but Vth increases with the increasing scan rate of voltage and size of thiol-capped CdS QDs. Between forward and reverse bias sweeps, the switching events exhibit hierarchy of hysteresis loops. The area within the hysteresis loops decreases with increasing sample cell temperature and ultimately disappear at higher temperatures. The switching events can be tuned by changing the external parameter (experimental conditions) such as sample cell temperature, scan speed of bias voltage, irradiation of light, size of thiol-capped CdS QDs etc. The switching mechanism and appearance of hysteresis have been attributed to an electric field-induced charge transfer from the polymer to the thiol-capped CdS QDs. The nanocomposites of PMMA and thiol-capped CdS nanocrystallites may be suitable for polymer based memory devices.

Nanostructured organic–inorganic heterojunction diodes as gas sensors

Nanostructured organic–inorganic heterojunction devices were fabricated by employing n-type zinc sulphide (ZnS) grown on ITO by RF magnetron sputtering and thin film of zinc phthalocyanine (ZnPc) deposited on the top of the ZnS layer by thermal evaporation. On the top of the ZnPc layer, an Al film was coated to form an electrode. Thin films of ZnS and ZnPc were characterized by optical absorption and Field Emission Scanning Electron Microscopy (FESEM). Current–voltage (I–V) characteristics of the fabricated ITO/ZnS/ZnPc/Al hetero-structures were studied at different temperatures (30 to 90 °C) in vacuum and diode-like behaviour was observed. In the case of single layer devices ITO/ZnS/Al and ITO/ZnPc/Al diode-like behavior was not obtained. The effects of donor ammonia and acceptor oxygen gases on the electrical properties of single layer devices ITO/ZnS/Al, ITO/ZnPc/Al and ITO/ZnS/ZnPc/Al heterojunctions were studied in order to investigate the utility of these devices as gas sensors. A drastic increase in forward current of the heterojunction has been noticed in the presence of donor ammonia gas containing water vapor, while a decrease in forward current has been noticed in the presence of acceptor oxygen gas. This change in forward current (diode current) in the presence of gases (under doped conditions) has been explained by the creation of charge carriers inside the ZnPc layer and hence bending of ZnPc bands. The threshold voltage (Vth) of the junction diodes has been found to depend on the temperature and the presence of donor and acceptor gases. Our experimental results suggest that this heterojunction diode could be a novel candidate for gas sensors.