Undoped CdS Research Articles

Structural, optical and photoconductivity characteristics of manganese doped cadmium sulfide nanoparticles synthesized by co-precipitation method

Mn-doped CdS nanoparticles (NPs) have been synthesized by co-precipitation method and effect of Mn concentration on the structural, photoluminescence and photoconductivity properties have been studied. The X-ray diffraction (XRD) patterns show that the synthesized NPs have a cubical (zinc blende) phase. The clear lattice fringes in the high-resolution transmission electron microscopy (HRTEM) image, and selected area electron diffraction (SAED) patterns with bright circular rings with spots further confirms cubical nature. The estimated average particle size is found to be ∼3.1 nm by TEM image. UV–visible (UV–vis) absorption spectroscopy is used to calculate band-gap of material and blue shift in absorption peak indicates quantum size confinement effect. In photoconductivity study, variation of photocurrent with voltage on ln–ln scale is found to follow power law superlinearly due to injection of additional charge carriers from one of the electrodes. The growth and decay in photocurrent spectra exhibit anomalous behaviors which may be attributed to photo-induced chemisorptions of oxygen molecules on surface of NPs. A significant improvement in photocurrent has been observed in Mn-doped CdS NPs with concentration of Mn as 3 mol% as compared to un-doped CdS NPs. Photoluminescence (PL) spectra of un-doped as well as Mn-doped CdS NPs have one emission peak centered at ∼526 nm whereas Mn-doped CdS NPs show one additional emission peak centered at ∼575 nm which may be assigned to 4T 1 → 6A 1 transitions of Mn 2+.

Structural, optical and magnetic properties of Mn diffusion-doped CdS thin films prepared by vacuum evaporation

The effect of Mn-doping on the vacuum deposited CdS thin films has been investigated by studying the changes in the structural, optical and magnetic properties of the films. A thin Mn layer evaporated on the CdS film surface served as the source layer for the diffusion doping. Doping was accomplished by annealing the CdS/Mn stack layers at the temperature range from 300 °C to 400 °C in step of 50 °C for 30 min under vacuum. The X-ray diffraction results showed that the undoped CdS film had a zinc-blende structure with a strong preferred orientation along the (1 1 1) direction. The incorporation of Mn did not cause any change in the texture but reduced the peak intensity and lead to a smaller crystallite size. Investigation of surface morphology using atomic force microscopy confirmed the decrease in the grain size with the Mn diffusion. In addition, a more uniform grain size distribution was observed in the doped films. X-ray photoelectron spectroscopy analysis showed that Mn atoms on the surface of the films were bounded to either sulphur or oxygen atoms. Auger electron spectroscopy of the diffusion-doped CdS sample at 350 °C indicated that the atomic Mn concentration was higher close to the surface region and Mn was distributed with a steadily decreasing profile through the bulk of the film with an average atomic concentration value around few percent. Band gap energy of the undoped sample decreased from 2.42 to 2.36 eV upon Mn diffusion at 400 °C. The magnetization of films as a function of magnetic field and temperature were measured. Clear ferromagnetic loops were observed for the Mn diffusion-doped CdS films prepared by annealing above 350 °C.

Growth and photoluminescence of CdS and CdS:Mn Nanoribbons

CdS and CdS:Mn 2+ Nanoribbons (NRs) are synthesized via a facile thermal evaporation approach. Their corresponding optical properties are investigated by room-temperature photoluminescence measurements. The spectrum of the undoped CdS NRs features four emission peaks: an intensive sharp one and three wide ones centered at 504 nm, 681 nm, 743 nm and 802 nm, respectively. The origins of them are discussed in detail. After doping with Mn 2+ ions, only the host band-edge emission is retained, therefore, CdS:Mn 2+ NRs present well narrow green band luminescence without any emission peaks from the Mn 2+ ion. This property is quite meaningful for contributing in color light devices.

Structural and optical characterization of Ni-doped CdS quantum dots

Ni-doped CdS quantum dots have been prepared by chemical precipitation technique. The X-diffraction results indicated that the particle size of Ni-doped CdS nanoparticles is smaller than that of undoped CdS and no secondary phase was observed. The average grain size of the nanoparticles is found to lie in the range of 2.7–4 nm. The compositional analysis results show that Cd, Ni, and S are present in the samples. HRTEM studies reveal that the average particle size of undoped and Ni-doped CdS quantum dots is 2 and 3 nm, respectively. Raman spectra shows that 1LO, 2LO, and 3LO peaks of the Ni-doped CdS samples are slightly red shifted when compared to that of undoped CdS. The absorption edge of Ni-doped CdS nanoparticles is found to shift towards the higher-wavelength (red shift) side when compared to that of undoped CdS and the band gap is observed to lie in the range of 3.79–3.95 eV. This band gap is higher than that of the bulk CdS and is due to quantum confinement effect present in CdS nanoparticles.

Upconversion luminescent characteristics and peak shift of CdSeS nanocrystals under different wavelength laser excitation

Upconversion luminescence was obtained from CdSeS nanocrystals (NCs) under 800 nm femtosecond laser excitation. The structural and optical characteristics of the CdSeS NCs were investigated experimentally by use of UV–visible absorption spectroscopy, transmission electron microscopy, X-ray diffractometry, and time-resolved luminescence dynamics. Peak shift of luminescence in CdSeS NCs can be readily observed under different wavelength femtosecond excitation. The pump power dependence of the luminescence intensity and time-resolved decay revealed that one, two, and three-photon absorption occur. It was found that upconversion luminescence is composed of photoinduced trapping and a band-edge excitonic state, and two types of species are involved in the biexponential luminescence decay kinetics. With increasing Se-doped composition, luminescence lifetimes of CdSeS NCs with similar sizes become shorter. This is not consistent with the changes of undoped CdS NCs and is ascribed to impurity level increased doping in the energy gap, which is favorable for trapping luminescence. A simple energy level of doping NCs is used to interpret upconversion luminescence and the peak shift of steady-state emission.

Chlorine-doped n-type CdS nanowires with enhanced photoconductivity

Cl-doped n-type CdS NWs with single-crystal wurtzite structure and growth direction were synthesized by usingCdCl2 as the dopant via a thermal co-evaporation method. By controlling the Cl vapor pressureduring the growth, the conductivity of the CdS:Cl NWs can be tuned in a wide rangeof five orders of magnitude. A nano-photodetector based on the CdS:Cl NWsshows high sensitivity to visible light with excellent stability and reproducibility.Significantly, the photoconductivity of the CdS NWs is greatly enhanced by Cldoping and the responsivity and photoconductive gain of the CdS:Cl NWs havesubstantially increased compared with the undoped CdS NWs. Further study alsodemonstrates the polarization-dependent photoconductivity of the CdS:Cl NWs. Itis expected that the CdS:Cl NWs with tunable optoelectronic properties willhave important applications in high-performance nano-optoelectronic devices.

Studies on optical absorption and structural properties of Fe doped CdS quantum dots

Fe doped CdS quantum dots have been prepared using simple precursors by chemical precipitation technique. Fe doped CdS quantum dots have been synthesized by mixing cadmium nitrate, sodium sulfide and adding Fe under suitable conditions. X-ray diffraction analysis reveals that undoped and Fe doped CdS crystallizes in hexagonal structure. The lattice constants of Fe doped CdS nanoparticles decreased slightly with incorporation of Fe and no secondary phase was observed. The average grain size of the nanoparticles is found to lie in the range of 2.8–4.2 nm. HRTEM results show that undoped and 3.75% Fe doped CdS nanoparticles exhibit a uniform size distribution and average size of the nanoparticles is about 2–3 nm. Raman spectra show that 1LO and 2LO peaks of the Fe doped CdS samples are slightly red shifted compared with those of undoped CdS. Optical absorption spectra of Fe doped CdS nanoparticles exhibited red shift.

Trapping states in CdS:Eu nanobelts studied by excitation-dependent photoluminescence

Pure and Eu-doped CdS nanobelts are synthesized by a thermal evaporation method. For the undoped CdS reference nanobelt, it only exhibits the emission related to free-excitons, very close to the energy of exciton absorption band, and moreover, the excitation power dependent photoluminescence (PL) data show a superlinear increase in integrated intensity with power. For the doped nanobelts, energy cannot transfer effectively between CdS host and incorporated Eu3+ ions. However, incorporated Eu3+ ions can form shallow level trap below the conduction band minimum (CBM), leading to a strong dependence of PL spectra on excitation power and energy. Under the 325 nm He–Cd laser excitation above the band gap energy, free-excitons ionize, and moreover, photogenerated electrons can relax rapidly from CBM to shallow level trap. Thus, the near-band edge (NBE) emission not only redshifts due to the renormalization of band gap but broadens more asymmetrically on its lower-energy side with power. Moreover, the power dependent NBE emission shows a sublinear property. In addition, a trap-related emission appears at higher excitation power, while the two emissions related to free-excitons and deep-levels decrease. Under the 514.5 nm Ar+ laser excitation below the band gap energy, it is just the doped nanobelts that can show the emission related to shallow level trap under the higher excitation condition.

Multicarrier Recombination and Energy Transfer in Mn-Doped CdS Nanocrystals Studied by Femtosecond Pump–Probe Spectroscopy

We studied the photocarrier decay dynamics of Mn-doped CdS nanocrystals (NCs) coated with a ZnS shell layer by femtosecond pump–probe transient absorption spectroscopy. At low excitation intensities, the decay dynamics of photocarriers is determined by energy transfer from electron–hole pairs in CdS NCs to Mn ions. At high excitation intensities, the photocarrier decay curves of Mn-doped CdS NCs are very similar to those of undoped CdS NCs, and they are determined by the fast Auger recombination of photocarriers. These density-dependent photocarrier behaviors in CdS NCs are confirmed by Mn 2+ luminescence spectroscopy.

Features of subthreshold defect formation in CdS and CdS:Cu single crystals subjected to irradiation with X-ray photons

Experimental results of the study of the effect of irradiation with 230-keV electrons and with X-ray photons with energies of 8.06 and 17.5 keV and the effect of quenching on the formation and reconstruction of the centers of slow recombination of nonequilibrium charge carriers (the so-called r centers) in nominally undoped and Cu doped (N Cu ∼ 1018 cm−3) CdS single crystals are reported. It is shown that defects in cadmium sublattice in the crystal (specifically, the V Cd vacancies and the CuCd defects with parameters close to those of the above vacancies) are responsible for the r centers. In the case of the X-ray irradiation of both undoped and Cu-doped CdS single crystals, subthreshold defect formation of cadmium vacancies and CuCd defects takes place; this occurs at sites with distorted and weakened interatomic bonds, i.e., at “weak sites” near large structural imperfections of the lattice, of technological or other origin. Starting with a quenching temperature 170°C, the spectrum of slow-recombination centers is appreciably affected by thermally formed vacancies V Cd and secondary defects CuCd. At quenching temperatures higher than 250°C, a significant contribution to the spectrum of optical quenching of photoconductivity is made by thermally introduced free (away from structural imperfections) r centers, i.e., the V Cd and CuCd defects.

Optical characterization and determination of carrier density of ultrasonically sprayed CdS:Cu films

In this work, CdS and Cu doped CdS films (at the Cu percentages of 1, 3 and 5) have been deposited onto glass substrates at 350 ± 5 °C by ultrasonic spray pyrolysis technique and their application potential for photovoltaic solar cells have been investigated. Optical properties and thicknesses of the films have been investigated by spectroscopic ellipsometry (SE). Ellipsometric angle ψ was used as the source point for optical characterizations. The optical constants ( n and k) and the thicknesses of the films have been fitted according to Cauchy model. Also, optical properties of the produced films have been analyzed by transmittance and reflectance spectra. Refractive index ( n), extinction coefficient ( k) and reflectance ( R) spectra have been taken by spectroscopic ellipsometer, while transmittance spectra have been taken by UV/vis spectrophotometer. The optical method has been used to determine the band gap type and value of the films. Mott-Schottky (M-S) measurements have been made to determine the conductivity type and carrier concentration of the films. Samples showed n-type conductivity and carrier concentration of undoped CdS sample was found to be 1.19 × 10 19 cm −3. Also, it was concluded that Cu doping has an acceptor effect in CdS samples. From the results of these investigations, the application potential of CdS:Cu films for photovoltaic solar cells as window layer was searched.

Effect of indium diffusion on characteristics of CdS films and nCdS/pSi heterojunctions

Diffusion of indium in In/nCdS/glass and In/nCdS/pSi structures at 350–550 °C in vacuum 1.33 × 10 −1 Pa and its effect on structural, electrical, optical and photovoltaic characteristics of nCdS films, fabricated by spray pyrolysis technique and nCdS/pSi heterojunctions have been investigated. The indium-doped and undoped CdS films were characterized by X-ray diffraction (XRD), electrical and optical measurements. The concentration profiles of indium and the components of CdS films (Cd and S) in In/nCdS/pSi structures were studied using the energy dispersive X-ray fluorescence (EDXRF) analysis. The thermal annealing of In/nCdS/pSi structures is accompanied by the diffusion penetration of In through CdS film into Si substrate. At the same time, the Cd and S atoms from CdS film diffuse into Si substrate. The diffusion doping of CdS film by indium did not change the polycrystalline hexagonal structure and the energy band gap of CdS films and increased the electron concentration from 1.4 × 10 13 to 1.3 × 10 16 cm −3. The photovoltaic parameters and spectral responsivity of In-doped nCdS(In)/pSi(In) cells were higher than those for nCdS/pSi cells and reached the maximum values after the thermal annealing at 450 °C. The improvement of the photovoltaic parameters of nCdS(In)/pSi(In) photocells was discussed on the base of the changes introduced in the energy band diagram of nCdS/pSi heterojunction due to the indium diffusion.

Comparison of the Optical Waveguide Behaviors of Se-Doped and Undoped CdS Nanoribbons by Using Near-Field Optical Microscopy

We report the Se-doped and undoped CdS nanoribbons synthesized by a Au-catalyzed thermal evaporation route, have good optical waveguide behaviors. Optical measurements demonstrate light can be guided on sub-wavelength scales along CdS nanoribbons with assembled structures, such as branched and crossed structures. The guided photoluminescence (PL) spectra along Se-doped and undoped CdS nanoribbons at different propagation distance were investigated by using near-field optical microscopy. Both the guided PL spectra of Se-doped and undoped CdS nanoribbons show redshifts because of the band-edge absorption. And the redshift of guided PL spectra of Se-doped nanoribbon is more than undoped CdS nanoribbon at the same PL propagation distance, because the internal disorder of Se-doped nanoribbon is more than that of undoped nanoribbon.

Fabrication of the SnS/ZnO heterojunction for PV applications using electrodeposited ZnO films

In the perspectives of fabricating a nontoxic and low-cost photovoltaic system, SnS has recently received considerable interest as a potential solar cell material. However, doped or undoped CdS has been extensively studied as a window material with p-SnS for the fabrication of a heterojunction solar cell. Because of the carcinogenic character of CdS, there is a prevailing need to search for an alternative window layer and scrutinize the performance of the heterojunction with p-SnS. In this work ZnO thin films were fabricated onto ITO using a galvanostatic electrodeposition approach from the dimethyl sulfoxide bath. The as-deposited ZnO films were subsequently annealed at 400 °C in argon ambiance to reduce the intermediate trap states and effective sintering. Finally, SnS thin films were grown onto the ZnO layer by thermally evaporating high-purity SnS powder. The photovoltaic properties of the SnS/ZnO heterojunction were investigated.

Optical waveguide behavior of Se-doped and undoped CdS one-dimensional nanostructures using near-field optical microscopy

The optical waveguide behaviors of CdS and CdSxSe1−x nanostructures are studied using near-field optical microscopy. Optical measurements demonstrate that light may be guided on sub-wavelength scales along CdS nanoribbons in straight or bent structures. The photoluminescence (PL) spectra from nanoribbon emission using scanning near-field optical microscopy are analyzed under different incident laser intensities. The PL spectra along Se-doped and undoped CdS nanoribbons at different propagation distances are investigated. Both the guided PL spectra of Se-doped and undoped CdS nanoribbons show red-shifts because of the band-edge absorption. Our results are useful for the development of new kinds of functional nano devices.

Specific features of the effect of irradiation with electrons and neutrons on photoelectric properties of CdS single crystals nominally undoped and doped with Cu

Electrical, photoelectric, and magnetic properties of CdS single crystals undoped and doped with copper (N Cu ≈ 1018 cm−3) and irradiated with electrons (E = 1.2 MeV, Φ = 2 × 1017 cm−2) and neutrons (E = 2 MeV, Φ = 1018 cm−2) are studied. It is shown that the donor-acceptor pairs are responsible for extrinsic photoconductivity and paramagnetic properties; in particular, these pairs are represented by Cu Cd − -D + complexes that are destroyed during irradiation and are formed again with time (as secondary radiation defects) in irradiated samples. It is established that the majority of paramagnetic centers and donor-acceptor pairs are located in the near-surface region of the crystal. It is confirmed that large structural defects (defect clusters) formed by irradiation with neutrons are efficient sinks for copper atoms. Specific features of isochronous annealing of paramagnetic centers and donor-acceptor pairs responsible for the variation in magnetic parameters and in the photoconductivity spectra of irradiated undoped and Cu-doped CdS samples are studied.

Structural, optical and magnetic properties of Cd1−xCoxS thin films prepared by spray pyrolysis

Abstract Structural, optical and magnetic properties of CdS thin films with the addition of Co prepared by (i) spray pyrolysis of Cd1−xCoxS (x⩽0.10) thin films (Type 1) and (ii) Co diffusion doped CdS films (Type 2) were investigated. The undoped film has a hexagonal structure with a strong (1 1 2) preferred orientation. As the Co concentration in CdS is increased, the preferred orientation changes from (1 1 2) to (0 0 2) direction. X-ray photoelectron spectroscopy (XPS) analysis shows that Co atoms on the surface of films are found to be bounded either to S atoms or O atoms. Although most of the bindings of Co atoms include Co–O bondings, some of them replace the Cd atoms by making chemical bounds with S atoms. The transmittance spectra indicate the four characteristic absorption maxima at the wavelengths of 680, 685, 729 and 744 nm, which were not observed for the undoped CdS film. Band gap energy Eg decreases from 2.43 to 2.37 eV with increasing Co content from x=0 to 0.10. The Co-doped Cd1−xCoxS films grown by spray pyrolysis (Type 1) didnot show any sign of ferromagnetic behavior. However, the Co diffused CdS films (Type 2) have clear ferromagnetic loops.

Special features of optical and photoelectric properties of nominally undoped and Cu-doped CdS single crystals

Experimental data on the effect of fast reactor neutrons on the absorption spectra, photoconductivity, and luminescence of nominally undoped and Cu-doped CdS single crystals are reported. It is shown that defect clusters formed as a result of neutron irradiation exhibit the properties of getters for easily migrating optically active impurities in the crystal lattice. In neutron-irradiated samples, the defects can be annealed in two stages. The first stage (100–150°C) involves the annealing of point defects, while the second stage (250–420°C) involves mainly the annealing of defect clusters. The degradation of the defect clusters is accompanied with the enrichment of the lattice with Cd and S vacancies.

CdS:Mn Nanorods: Solvothermal Synthesis and Properties

Manganese (0.05-9 mol.%) doped CdS nanorods were synthesized via solvothermal route using ethylenediamine (En) and a mixture of En and water as the solvents. The diameters and the lengths of the doped CdS nanorods varied from 40-100 nm and 600-2500 nm, respectively, with change in the composition of the solvents. The broad photoluminescence (PL) emission from the undoped CdS nanorods centered at approximately 535 nm is found to be blue shifted to 516 nm with the incorporation of Mn in the CdS crystal structure. Also increase in the intensity of the PL was noticed in the Mn doped CdS nanorods for both the solvent systems. Maximum PL intensity was observed for 1 mol.% Mn in case of En system and for 0.5 mol.% Mn in case of En/water system, above which quenching occurred as a result of Mn-Mn clustering. EPR study revealed six-line hyperfine splitting for low Mn concentration in both solvent systems. Increase in the Mn concentration caused EPR signal broadening due to Mn-Mn clustering.

Two-phonon assisted exciton spin relaxation due to exchange interaction in spherical quantum dots

Spin relaxation in undoped quasispherical CdS quantum dots at zero magnetic fields is investigated using time- and polarization-resolved transient absorption measurements. Unlike in previous studies of these systems, the measured signals were corrected for spin-insensitive contributions to the exciton bleaching dynamics, allowing us to determine the pure spin-related exciton dynamics. To explain the observed room-temperature spin-relaxation time of several nanoseconds, we propose a mechanism based on intralevel exciton transitions with the emission of one LO phonon, the absorption of another LO phonon, and an electron spin flip, which is driven by the electron-hole exchange interaction. The transition rates, calculated in the present work for different sizes of quantum dots and temperatures, are in fair agreement with our experimental results.