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Structural, optical and electrical properties study of the cadmium sulphide (CdS) incorporated polyaniline (PANI) thin films with varying layers have been carried out. It is seen that layer variation enhances the particle mean sizes with quenching of photoluminescence and an increase in rectifying nature of current-voltage measurements.
Although semiconductor nanostructures exhibit improved physical properties, their structural instability is be a problem for technological applications. This paper reports a temperature-induced low energy phase transformation that occurs in CdS nanoparticles which is ascribed to thermally generated stacking-faults. The phase transformation is found to change the nature of lattice strain in the samples. The formation of this defect-induced lattice strain and its influence on the physical properties of the samples are investigated. The modified optical absorption and emission properties of the samples are thoroughly studied. Using resonance Raman spectroscopy, the evolution of electron-phonon (e-p) coupling strength with structural changes is studied. An explanation to the contradictory nature of e-p coupling strength observed in the nano-regime is given.
Evolution of the microwave-probed photoconductivity transients and of the proton induced luminescence has simultaneously been examined in polycrystalline CdS layers evaporated in vacuum during exposure to a 1.6 MeV proton beam. The decrease of the intensity of luminescence peaked at 510 and 709 nm wavelengths and of values of the effective carrier lifetime has been correlated in dependence of proton irradiation fluence. The defect introduction rate has been evaluated by the comparative analysis of the laser and proton beam induced luminescence. The difference of a carrier pair generation mechanism inherent for light and for a proton beam has been revealed.
CdS nanoparticles are successively synthesized by chemical precipitation method. The samples prepared at different reaction time and temperature are characterized by X-ray diffraction, Diffuse reflectance spectroscopy, Photoluminescence spectroscopy ans Energy dispersive x-ray analysis. Visible color variation is noted from light yellow to orange, indicates the quantum confinement effect and the results are again got confirmed from the optical studies. A shift in absorption peak is observed towards the lower region of the visible spectra - the “blue shift”- upon decrease in reaction time and temperature. Blue emission observed in the photoluminescence spectrum confirms the grain size induced confinement.
TiO{sub 2} branched nanostructures could be efficient as photoanodes for quantum dot-sensitized solar cells (QDSCs) due to their large surface area for QD deposition. In this study, Mn-doped CdS/Mn-doped CdSe deposited branched TiO{sub 2} nanorods were fabricated to enhance the photovoltaic performance of QDSCs. Mn doping in CdS and CdSe retards the recombination losses of electrons, while branched TiO{sub 2} nanorods facilitate effective electron transport and compensate for the low surface area of the nanorod structure. As a result, the charge-transfer resistance (R{sub CT}), electron lifetime (τ{sub e}), and the amount of QD deposition were significantly improved with branched TiO{sub 2} nanorod based Mn-doped CdS/Mn-doped CdSe quantum dot-sensitized solar cell.
Synthesis of composite photocatalyst is one of the most important strategies to enhance the yield of hydrogen produced by water splitting. However, one photocatalyst usually tends to randomly aggregate on the other's surface, which weakens the electron transport of the heterogeneous interface. Herein, we developed a hydrothermal reaction to synthesize the SiC/CdS composite with a feasible Z-scheme and well-controlled dispersion of CdS on SiC surface. Heterogeneous structure on the catalyst interface is obtained, leading to more light-absorption and effective electron-hole separation between the well-contacted components, which contribute to the doubly enhanced photocatalytic performance of the composite. This work provides a simple and practical route to improve the catalytic activity by optimizing the intrinsic contact of Z-scheme composite semiconductors.
Semiconductor nanowires (NWs) can be applied in gas sensing and cell detection, but the sensing mechanism is not clearly understood. In this study, surface modification effect on the electronic properties of CdS NWs for different diameters with several species (H, F, Cl, Br, and NO{sub 2}) is investigated by first principles calculations. The surface dangling bonds and halogen elements are chosen to represent the environment of the surface. Halogen passivation drastically changes the band gaps due to the strong electronegativity and the energy level of halogen atoms. Density of states analysis indicates that valence band maximum (VBM) of halogen-passivated NWs is formed by the p states of halogen atoms, while VBM of H-passivated NWs is originated from Cd 4d and S 3p orbitals. To illustrate that surface modification can be applied in gas sensing, NO{sub 2}-absorbed NWs with different coverage are calculated. Low coverage of NO{sub 2} introduces a deep p-type dopant-like level, while high coverage introduces a shallow n-type dopant-like level into the band structure. The transformation is due to that at low coverage the adsorption is chemical while at high coverage is physical. These findings might promote the understanding of surface modification effect and the sensing mechanism of NWs as gas sensors.
Quantum dots a few nanometer in size exhibit unique properties in comparison to bulk due to quantum confinement. Their properties can be tuned according to their sizes. Dye sensitized quantum dot (DSQD) solar cells are based on the same principle with surface dangling bonds as a challenge. Researches have shown the existence and stability of nano-cages which are assembled such as to minimize the surface dangling bonds and hence maximize stability. Here, we report a first principles DFT study of optical and electronic properties of CdS-cage (Cd{sub 34}S{sub 34}) sensitized with nkx-2388 dye in three different geometric configurations of dye attachment. A significant distortion is found to occur in the geometric structure of the cage when it interacts strongly with the dye. The relative positioning of dye and cage energy levels is found to be different in different configurations. The absorption spectrum has been analyzed with the help of natural transition orbitals (NTO).
Kinetics of the photo-activated charge carriers has been investigated in Tin substituted Cadmium Sulphide, Cd{sub 1-x}Sn{sub x}S (x=0, 0.05, 0.10 and 0.15), thin films prepared by spray pyrolysis. X-Ray Diffraction shows an increase in strain that resulted in the decreased crystallite size upon Sn substitution. At the first sight, the photo current characteristics show a quenching effect on Sn substitution. However, survival of persistent photocurrents is seen even up to 15% of Sn substitution. Transient photo current decay could be explained with a 2τ relaxation model. CdS normally has an n-type character and the Sn doping expected to inject hole carriers. The two fold increase in τ{sub 1}, increase in activation energy and the decrease in photocurrents upon Sn substitution point towards a band gap cleaning scenario that include compensation and associated carrier injection dynamics. In addition Atomic Force Microscopy shows a drastic change in microstructure that modulates the carrier dynamics as a whole.
Ex-situ spectroscopic ellipsometry measurements are made on radio frequency magnetron sputtered oxygenated cadmium sulfide (CdS:O) thin films. Films are deposited onto glass substrates at room temperature and at 270 °C with varying oxygen to total gas flow ratios in the sputtering ambient. Ellipsometric spectra from 0.74 to 5.89 eV are collected before and after annealing at 607 °C to simulate the thermal processes during close-space sublimation of overlying cadmium telluride in that solar cell configuration. Complex dielectric function (ε = ε{sub 1} + iε{sub 2}) spectra are extracted for films as a function of oxygen gas flow ratio, deposition temperature, and post-deposition annealing using a parametric model accounting for critical point transitions and an Urbach tail for sub-band gap absorption. The results suggest an inverse relationship between degree of crystallinity and oxygen gas flow ratio, whereas annealing is shown to increase crystallinity in all samples. Direct band gap energies are determined from the parametric modeling of ε and linear extrapolations of the square of the absorption coefficient. As-deposited samples feature a range of band gap energies whereas annealing is shown to result in gap energies ranging only from 2.40 to 2.45 eV, which is close to typical band gaps for pure cadmium sulfide.
Metal monochalcogenide quantum dot nanocrystals of ZnS, CdS and SnS were prepared by anaerobic, metal-reducing bacteria using in situ capping by oleic acid or oleylamine. Furthermore, the capping agent preferentially adsorbs on the surface of the nanocrystal, suppressing the growth process in the early stages, thus leading to production of nanocrystals with a diameter of less than 5 nm.
The charge-neutral exciton has been predicted to carry genuine photocurrent due to the geometric Berry phase of the electronic bands, if the inversion symmetry in a crystal is broken. Here we detect such exciton shift current in a prototypical polar semiconductor CdS by using terahertz emission spectroscopy. A distinct peak emerges in the photocurrent spectra at the energy of the exciton resonance, which is demonstrated to result from the distinct displacements of electrons and holes in real space within the excitons to produce a finite transient charge current at subpicosecond time scale. Our findings elucidate the Berry phase physics of the charge-neutral photoexcitations and also shed light on the novel energy harvesting mechanism by exciton generation.
The possibility of using the plasma resonance in semiconductors to excite exoelectron photoemission in the mid-infrared spectral range without special cooling is discussed. Optical reflection spectra in the mid-infrared range of vacuum-deposited radiation-resistant films of limited CdS-PbS solid solutions containing a minimum associated with the plasma resonance are presented. The plasma resonance of secondary-electron emission is compared with the secondary-ion photoeffect and a conclusion is made concerning the possibility of the influence of the plasma resonance in the mid-infrared range on the escape of electrons from the semiconductor photocathode at room temperature.
The photoluminescence kinetics in CdS nanocrystals produced by the Langmuir-Blodgett technique is studied at a temperature of 5 K. The photoluminescence kinetics is described by the sum of two exponential functions, with characteristic times of about 30 and 160 ns. It is found that the fast and slow decay times become longer, as the nanocrystal size increases. Analysis of the data shows that the fast decay time is controlled by trion recombination in nanocrystals with defects, whereas the slow decay time is controlled by the annihilation of optically inactive excitons in nanocrystals without defects. It is established that, as the nanocrystal size is decreased, the fraction of imperfect nanocrystals is reduced because of an increase in the energy of defect formation.
The effect of microwave radiation on the luminescence properties of CdS and CdTe:Cl single crystals is studied. It is established that the exposure of these semiconductors to short-term (≤30 s) microwave radiation substantially modifies their impurity and defect structure. The mechanisms of transformation of the defect subsystem of II–VI single crystals upon microwave treatment are discussed. It is shown that the experimentally observed changes are defined by the nonthermal effects of microwave radiation at a power density of 7.5 W cm{sup –2}; at 90 W cm{sup –2}, nonthermal effects are prevailing.
The specific features of oxygen dissolution in CdS using the example of single crystals grown by the gas-transport method with deviations from stoichiometry at 1100°C are considered. The effect of various types of intrinsic point defects in crystals of different composition on the form in the presence of oxygen is analyzed. It is shown that the most stable composition thermodynamically is that corresponding to nonstoichiometric “self-activated cadmium sulphide” stabilized with oxygen.
The absorption, photoluminescence, and photoluminescence excitation spectra of CdS nanocrystals formed by the Langmuir–Blodgett method are explored. Features of the absorption and photoluminescence excitation spectra defined by optical transitions in the matrix and nanocrystals are identified. The efficiency of electronic excitation transfer from an organic matrix to nanocrystals is studied. It is shown that charge carriers efficiently transfer from the matrix to electron and hole size-quantization levels in nanocrystals and to acceptor defect levels in the band gap of nanocrystals. A large Stokes shift defined by fine exciton structure (bright and dark excitons) is observed. The shift is in the range 140–220 meV for nanocrystals 2.4 and 2.0 nm in radius.
We use a time-resolved technique with three laser pulses (pump, orientation and probe) to study the photocharging dynamics with picosecond resolution on a long timescale ranging from ps to ms in CdS colloidal quantum dots. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the dot core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping on surface states, eventually it evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature.