Rakesh Dhar

INVESTIGATION OF TRANSPORT AND OPTICAL PROPERTIES OF MESOPOROUS ANATASE AND RUTILE TiO2 FILMS FOR APPLICATION IN DYE-SENSITIZED SOLAR CELLS

The transport of electrons through TiO2 anatase and rutile thin films synthesized via sol–gel route has been investigated using electronic and optical spectroscopic studies. Mesoporosity of the films is confirmed by poroellipsometry. A smaller electron effective mass in anatase can be accounted for larger effective Bohr radius of trap electrons observed in anatase than in rutile. Further, the smaller effective mass in anatase favors high mobility. A mott transition is observed in anatase with change in temperature but not in rutile. Luminescence of self-trapped excitons is observed in anatase thin films, that implies a strong lattice relaxation.

Effect of capping agents on optical and antibacterial properties of cadmium selenide quantum dots

Cadmium selenide quantum dots (CdSe QDs) were synthesized in aqueous phase by the freezing temperature injection technique using different capping agents (viz. thioglycolic acid, 1-thioglycerol, L-cysteine). Absorption spectra of CdSe QDs exhibited a blue shift as compared to its bulk counterpart, which is an indication of quantum confinement effect. The photoluminescence spectra of CdSe QDs confirmed that the particles are poly-dispersed and possess enhanced luminescent property, depending upon the chemical nature of capping agents. The QDs have been characterized by Fourier-transform infrared spectroscopy, atomic absorption spectroscopy and transmission electron microscopy. Further, antimicrobial activity of as-prepared QDs has also been investigated using the disk diffusion method.

Investigation of the opulent porosity for better performance of dye-sensitized solar cell

The presented research work is an effort to optimize porosity of TiO2 film to get enhanced performance of dye-sensitized solar cell. A novel and economic technique is presented to control the porosity. Poroellipsometry was used to measure porosity of the films. A comparison between microporous (P = 22.3), mesoporous (P = 39.1), and macroporous (63.6) structures has been made by obtaining current density-voltage (JV) characteristics and incident photon to current conversion efficiency curves. It has been found that morphology of mesoporous films with porosity P = 39.1 is best suited for dye-sensitized solar cells. The results are in well agreement with theoretical predictions regarding porosity.

X-ray photoelectron spectroscopy investigations of band offsets in Ga0.02Zn0.98O/ZnO heterojunction for UV photodetectors

Here, we report the valence and conduction band offset measurements in pure ZnO and the Ga0.02Zn0.98O/ZnO heterojunction by X-Ray photoelectron spectroscopy studies for UV photodetector applications. For detailed investigations on the band offsets and UV photodetection behavior of Ga0.02Zn0.98O/ZnO heterostructures, thin films of pristine ZnO, Ga-doped ZnO (Ga0.02Zn0.98O), and heterostructures of Ga-doped ZnO with ZnO (Ga0.02Zn0.98O/ZnO) were deposited using a pulsed laser deposition technique. The deposited thin films were characterized by X-ray diffraction, atomic force microscopy, and UV-Vis spectroscopy. X-ray photoelectron spectroscopy studies were carried out on all the thin films for the investigation of valence and conduction band offsets. The valence band was found to be shifted by 0.28 eV, while the conduction band has a shifting of −0.272 eV in the Ga0.02Zn0.98O/ZnO heterojunction as compared to pristine ZnO thin films. All the three samples were analyzed for photoconduction behavior under UVA ...

Structural optical and electronic properties of Fe and Ga doped ZnO thin films grown using pulsed laser deposition technique

Band gap engineering in ZnO thin films have been subject of intensive studies. The thin films of 2 wt % Fe and 2 wt % Ga doped ZnO and undoped ZnO were deposited on glass substrate by pulse laser deposition technique. Structural, optical and electronic structure properties of these thin films were investigated by X- Ray diffraction (XRD), UV-Vis spectroscopy and X-ray absorption spectroscopy (XAS), respectively. XRD studies show that all the thin films are highly oriented along the c-axis and maintain the wurtzite structure. Out of plane lattice parameter in Ga doped is smaller while in Fe doped is larger, compared to undoped ZnO. The band gaps of doped films have been found to increase due to doping of the Ga and Fe ions. XAS studies across O K edges of doped thin films show that the conduction band edge structure probed via oxygen 1s to 2p transitions have modified significantly in Ga doped sample.

Anti-bacterial biofilm activity of magnesium ferrite thin film

ABSTRACT Thin films of magnesium ferrite (MgFe2O4) are synthesized on the glass substrate by using thermal evaporation technique and studied their anti-bacterial biofilm activity against the gram-negative and gram-positive bacterium. The MgFe2O4 thin films (100 nm) were prepared at a deposition rate of 10 ± 2 Å/S, followed by annealing at 600°C. The morphological and structural study using scanning electron microscope, X-ray diffraction, and transmission electron microscope reveals crystalline structure of the magnesium ferrite. The model organisms for anti-bacterial biofilm activity were tested with E. coli (gram negative) and S. aureus (gram positive). The E. coli and S. aureus biofilms grown on the glass slides and MgFe2O4 coated glass slide were imaged using fluorescent microscope. It is observed that the MgFe2O4 coated glass slides exhibit minimum bacterial growth (both negative and positive bacteria) compared to the uncoated glass slide.

Structural and electrical properties of P doped ZnO films

Structural, electronic and electrical properties of Phosphorus doped ZnO thin films deposited using pulsed laser deposition technique, on P-type Si (111) substrate are studied. X-ray diffraction measurements shows that undoped ZnO and Phosphorus doped ZnO films oriented along c-axis and maintains wurtzite crystalline symmetry. The change in grain size occurs by Phosphorus doping, which can be correlated to stress comes due to presence of Phosphorus. The changes in electronic structure has been observed by Phosphorus doping in ZnO thin films, which is understood from X-ray absorption spectroscopy. Hall measurements shows n-type behavior of Phosphorus doped ZnO samples.Structural, electronic and electrical properties of Phosphorus doped ZnO thin films deposited using pulsed laser deposition technique, on P-type Si (111) substrate are studied. X-ray diffraction measurements shows that undoped ZnO and Phosphorus doped ZnO films oriented along c-axis and maintains wurtzite crystalline symmetry. The change in grain size occurs by Phosphorus doping, which can be correlated to stress comes due to presence of Phosphorus. The changes in electronic structure has been observed by Phosphorus doping in ZnO thin films, which is understood from X-ray absorption spectroscopy. Hall measurements shows n-type behavior of Phosphorus doped ZnO samples.

Study of optical nonlinearity of CdSe and CdSe@ZnO core-shell quantum dots in nanosecond regime

Thioglycolic acid capped cadmium selenide (CdSe) and CdSe@ZnO core–shell quantum dots have been synthesized in aqueous phase. The sample was characterized by UV-vis spectrophotometer, TEM and Z-scan technique. The nonlinear optical parameters viz. nonlinear absorption coefficient (β), nonlinear refractive index (n2) and third-order nonlinear susceptibilities (χ3) of quantum dots have been estimated using second harmonic of Nd:YAG laser. The study predicts that CdSe@ZnO quantum dots exhibits strong nonlinearity as compared to core CdSe quantum dots. The nonlinearity in quantum dots is attributed to the presence of resonant excitation and free optical processes. The presence of RSA in these nanoparticles makes them a potential material for the development of optical limiter.

The growth mechanism of ordered mesoporous electrodes in view of their response in dye-sensitized solar cells

The effect of morphology of photoelectrode films on photoelectrochemical properties of solar cells has been studied. Crack free mesoporous anatase and rutile films of thickness∼16μm have been synthesized and characterized in concern with their use in dye-sensitized solar cells (DSSC). Surface morphologies have been studied with the help of scanning electron microscopy (SEM). The open-circuit photovoltage for mesoporous anatase and rutile based cells is approximately same but short-circuit photocurrent of mesoporous rutile based cell is lower than that of the mesoporous anatase based cell.

Optimized dynamic range cascaded optical power limiter based on nickel complex dye (BDN) encapsulated in PMMA

A novel optical limiter cascaded configuration has been characterized using nanosecond pulsed radiation of a single-mode, Q-switched Nd: YAG laser operating at 532nm. Nickel complex dye bis (4-dimethylaminodithiobenzyl)-nickel (BDN) has been entrapped in PMMA to form polymeric solid-state optical limiter. The optical limiting mechanism arises due to laser induced nonlinear absorption and self-action effects. Optical limiting threshold as well as dynamic range has been reported. The dynamic range optimization has been accomplished using cascaded geometry. A nearly two fold enhancement in dynamic range has been reported.