Aloke Kumar Chakraborty

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.

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.

Low-frequency electrical current oscillations in the vapor-adsorbed state of some ferrocene derivatives

Low-frequency oscillations in electrical (dark, i.e., unilluminated) current as a function of time have been observed in a specific temperature range in samples of some ferrocene derivatives, namely ferrocenecarboxylic acid, ferrocenecarbaldehyde and acetylferrocene, with adsorbed ethanol vapor at a pressure of 40 Torr in a sandwich-type cell. The dependence of the oscillatory behavior of current on the applied bias voltage across the electrodes has also been studied at a constant sample cell temperature for each material in the ethanol-vapor-adsorbed state. With increasing bias, increases in both the time interval between two consecutive current peaks (i.e. decrease in frequency of current oscillations) and current peak hight were observed for all of the materials studied. The oscillatory behavior of the current and the temperature at which oscillations occurred depended on the nature of the ferrocene derivative. Existing theories of current oscillations in semiconductors have failed to explain the observed low-frequency current oscillations in these organometallic semiconductors. Such current oscillations are possibly associated with some kind of time-dependent phase changes in the ferrocene derivative–ethanol system at the sample surface layer.

Dark and photoconductive properties of hydroxymethylferrocene

Abstract The dark electrical conduction properties of hydroxymethylferrocene have been studied as a function of voltage at different sample cell temperatures by using sandwich-cell configuration. The analysis of the results has been based on the space charge limited conduction (SCLC) theory. The distribution of traps in this material has been found to be single discrete level type. Photoconductivity in this material has been studied as a function of the intensity of exciting light source (mercury lamp; 125 W) and sample cell temperature. The results have been compared with the results of ferrocene and, in some cases, with other ferrocene derivatives. An unusual temperature-dependence of photocurrent has been observed. The possible origin of this observation has been discussed.

Photoinduced Desorption of Ethanol Vapor from Ferrocene Solid Surface Monitored by Electrical Current Measurements

Photoeffects in ferrocene solid samples with physically adsorbed ethanol vapor have been studied in detail at different sample temperatures by monitoring the change in the electrical current in sandwich cells of the material in powder form. The photoeffects in the ethanol-adsorbed ferrocene sample resulted in the decrease of measured currents in the sample cells. Such experiments have indicated photoinduced desorption of ethanol vapors from the ferrocene solid surface. Previously, desorption of adsorbed vapor in the dark from the ferrocene surface was studied by flushing with dry nitrogen gas, but photoinduced desorption of adsorbed vapors from ferrocene is reported here for the first time. The photoeffects in the vapor-adsorbed ferrocene sample cell were also studied using different vapors. It was found that the photoeffects in the vapor-adsorbed ferrocene sample were very selective, and photoinduced desorption was not observed in all the vapors studied. In some vapors, photoeffects in the vaporadsorbed state resulted in enhanced photocurrents of the material. The above results are discussed.

Observation of low-frequency electrical current oscillations in ferrocene sample with adsorbed 2-propanol vapor

Low-frequency oscillations in electrical current (dark) as a function of time were observed, in the temperature range where unusual/anomalous change in electrical conductivity with temperature was noticed in the samples of ferrocene with physically adsorbed 2-propanol vapor (at 40 torr pressure) molecules as impurities in a sandwich type of cell configuration. With the increasing bias voltage across the electrodes, at a constant sample cell temperature, increase in both the time interval between the two consecutive current peaks (i.e. decrease in frequency of current oscillations) and current peak height was noticed. The current-voltage characteristics behavior of ferrocene with adsorbed 2-propanol vapor has indicated the ohmic nature of conductivity. Existing theories of current oscillations in semiconductors have failed to explain the observed low-frequency current oscillations in this organometallic semiconductor. Such current oscillations are possibly associated with some kind of time dependent phase changes in the ferrocene- (2-propanol) vapor system/surface layer.

Studies on the photoinduced desorption of ethanol vapor from the solid surface of some ferrocene derivatives

The photoinduced changes in solid samples of some ferrocene derivatives namely, ferrocenecarbaldehyde, ferrocenecarboxylic acid, and acetylferrocene with physically adsorbed ethanol vapor were monitored by electrical current measurements in sandwich cells. The decrease in current with time during illumination of light from a mercury lamp (125 W) has indicated the possibility of photoinduced desorption of adsorbed vapors from the solid surface of these materials. Similarly, the increase of current with time in absence of the light source has indicated the possibility of readsorption of vapors from the chamber environment. Possible application of photoinduced desorption has been discussed.

Silver Nanoparticles Dispersed in Poly(methyl methacrylate) Thin Films: Spectroscopic and Electrical Properties

Silver nanoparticles of different sizes were prepared following chemical route and using silver nitrate as starting material. The nanoparticles were dispersed in Poly(methyl methacrylate) (PMMA) matrix and thin films were prepared. The PMMA films with dispersed silver nanoparticles were characterized by UV‐Vis absorption spectroscopy and FESEM. The electrical measurements were carried out by using surface type cells. The absorption spectra of the films have shown plasmon peaks at different wavelengths with the variation in size of the silver nanoparticles. The film with dispersed silver nanoparticles showing plasmon peak at higher wavelength manifested increasing trend in dark current values with time at room temperature. On the contrary, the decreasing dark current values with time were observed in the case of the film showing plasmon peak at lower wavelength under similar measuring conditions. The photocurrent profile of the PMMA film with dispersed silver nanoparticles as a function of photoexcitation ...

Study of Quantum Yield and Photoluminescence of Thiol Capped CdS Nanocrystallites

For the preparation of thiol capped CdS nanocrystallites, the microwave (MW) assisted reaction of cadmium acetate with thiourea in N, N‐dimethylformamide (DMF) was controlled in the presence of two capping agents, 1‐butanethiol and 2‐mercaptoethanol. The peak position of the absorption band of the CdS nanocrystals in DMF solution shifted towards longer wavelength with increasing duration (by repeated exposure for a fixed time) of MW irradiation, for 1‐butanethiol caped CdS nanocrystals indicating growth of particle size. However, the peak position of absorption band remained nearly at the same wavelength and only the intensity of the absorption band increased with increasing duration of MW irradiation for the 2‐mercaptoethanol capped CdS nanocrystals. Photoluminescence (PL) of the CdS nanocrystals in solution with 1‐butanethiol as capping agent was observed to shift towards higher wavelength in the visible range of the spectrum showing a decrease in intensity with increase in the duration of exposure of M...

Photoinduced desorption in ferrocene-ethanol vapor system using polychromatic light source: dependence on light intensity

The photoeffects at 290 K on a ferrocene sample adsorbed with ethanol vapor in a sandwich-type of cell configuration have been studied with various intensities of light from a mercury lamp by monitoring electrical current. Decrease in the measured current under light exposure has been observed at each light intensity. Termination of the light exposure resulted in an enhancement in current value. The analysis of the rates of the photoinduced changes in current under illumination at various intensities indicated the photoinduced desorption of ethanol vapor from the ferrocene surface at each intensity of light used. Similar analysis of the rates of the change of current after the termination of light exposure indicated the possibility of re-adsorption of vapor by the sample. A mathematical analysis of the dependence of photoinduced current on the light intensities has been attempted. Similar studies have been extended for some ferrocene derivatives. The possibility of application of such photoinduced desorption in ethanol vapor sensors has been discussed.