Surface modification of CdS quantum dots using thiols-structural and photophysical studies

This study is aimed at identifying a suitable organic thiol for CdS by studying its structural, thermal and photophysical characteristics. Quantum dots of the II-VI semiconductor CdS, in the size regime of 2.0-3.3?nm, were prepared in the cubic phase by a wet chemical method. Five organic thiols were used for capping: (i) 1,4-dithiothreitol (DTT), (ii) 2-mercaptoethanol (ME), (iii)?cysteine (Cys), (iv)?methionine (Meth), and (v)?glutathione (GSH). Structural studies were carried out by x-ray diffraction (XRD) and transmission electron microscopy (TEM), which revealed the cubic phase of CdS. Optical properties were studied by FT-IR, UV-visible and fluorescence spectroscopic techniques, and a comparison was made between uncapped and capped CdS. FT-IR studies suggested two different bonding mechanisms of the capping agents with the CdS. GSH and DTT capped CdS showed significant decrease in absorption wavelengths. An increase in band gap was observed in two cases: when (i)?capped and (ii)?decreased in size. The band gap was increased from 2.50?eV for the uncapped to 2.77?eV for the DTT capped CdS. DTT was found to be the best capping agent for CdS among these five organic thiols in two aspects: (i)?yielding lower grain size in cubic phase, and (ii)?good fluorescence properties with efficient quenching of the surface traps.     

Structure and optical properties of capped and uncapped CdS nanoparticles prepared in aqueous medium

We have prepared the hexagonal structure of CdS nanoparticles in an aqueous solution with different sizes and varied surface compositions by using fixed molar ratio of the starting precursors in the presence of capping molecules. In addition, we have prepared uncapped CdS nanoparticles by cadmium chloride and thiourea at low temperature. We showed that the environmental conditions and the type of the aqueous medium are the effective parameters for the exchange of the nanoparticle size. The prepared nanoparticles have sizes in the range from 25 to 100 Å. We have compared the experimentally determined size of CdS nanoparticles with that determined by theoretical calculations. The comparison showed that the size determined by Scherrer’s equation is fitted well with the empirical tight binding calculations, and that the effective mass approximation yields size values is in good agreement with the size estimated by high resolution transmission electron microscopy. Photoluminescence spectroscopy revealed that the nanoparticles with stoichiometries composition S/Cd ~ 1 have a high intensity band edge emission in the blue region for the capped nanoparticles and green emission for the uncapped nanoparticles.     

Comparison of various organic stabilizers as capping agents for CdS nanoparticles synthesis

Results of the studies on the preparation and characterization of CdS nanoparticles capped with various organic stabilizers are presented in this article. Solutions of Cadmium acetate and Sodium sulphide were taken as the precursors. CdS nanoparticles were synthesized in an aqueous medium with Mercaptopropionic acid (MPA) as the stabilizer and non-aqueous methods were used for the synthesis of Polyvinyl Pyrrolidone (PVP) and thiophenol-capped CdS nanoparticles. The synthesized CdS nanoparticles were characterized by the optical absorption and X-ray diffraction (XRD) studies. Particle sizes estimated from the band gap values using Effective Mass Approximation (EMA) agreed fairly well with those calculated from the XRD using Scherrer formula. The quantity and the concentration of the stabilizers needed for effective capping of the CdS nanoparticles were different in the three cases considered. Stability of the synthesized CdS nanoparticles was studied at different intervals of time for 10 days. A change in particle size was observed at lower stabilizer concentrations for the first few days. But at higher stabilizer concentrations there was no change in particle size with time.     

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.     

Thermochemical growth of Mn-doped CdS nanoparticles and study of luminescence evolution

We report a new method of growing Mn-doped CdS (CdS:Mn) nanoparticles in an aqueous solution at boiling temperature. The idea is to use precursors that react only at high temperature, in order to gain crystalline luminescent nanoparticles. CdSO(4), Mn(NO(3))(2) and Na(2)S(2)O(3) were used as the precursors, and thioglycerol was employed as the capping agent and also the reaction catalyst. Na(2)S(2)O(3) is thermally sensitive and it releases S(2-) ions upon heating. The CdS:Mn nanoparticles obtained are about 4?nm in size and show both cubic and hexagonal crystalline phases with a ratio of 35% to 65%. The luminescence of nanoparticles contains a peak at 580?nm, which is related to Mn(2+) ions. Prolonged reaction time results in a decrease of the Mn luminescence peak to about 35% of the maximum value. We discuss the possible causes of the Mn peak reduction and attribute it to preferential dissolution of Mn ions into the solution due to shape reconfiguration of the nanoparticles.     

A novel application of micro-EDM process for the generation of nickel nanoparticles with different shapes

This article explains production of nickel nanoparticles through a micro-electrical discharge machining (EDM) process with a combination of different process parameters. The production of nickel nanoparticles was carried out in a dielectric medium (deionized water) with developed micro-EDM while polyvinyl alcohol worked as the stabilizing agent. The characterization of nickel nanoparticle was done by scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), UV–Vis spectroscopy, and Fourier transform infrared (FTIR) analysis. From this investigation, the mean crystal size of the nickel nanoparticles was found to be in the range of 15–20?mm for a pulse-on time variation of 2–0.3?µs and the crystal size was found to decrease with the decrease of pulse-on time. It was also observed that with this decrease, the shape and size of nickel nanoparticles change from spherical to needle-like. The dispersion stability of nickel nanofluid was determined by viscosity measurements and the dynamic viscosity was noted to decrease by decreasing the pulse duration. From the FTIR spectrum results, it was confirmed that the synthesized nickel nanoparticles in deionized water were pure and monolithic. UV–Vis–NIR spectroscopy depicted that the band gap energy increases with a reduction in the pulse-on time and obtains a higher band gap (5.31?eV) for 0.3?µs pulse-on time.     

Surfactant mediated optical properties of cytosine capped CdSe quantum dots

This letter demonstrates the use of one of the nucleobases, ‘cytosine’ as a new capping agent in controlling the size of the nanoparticles. A size dependent blue shift in optical absorption with enhanced luminescence is observed. Since the calculated density of states do not show any change in the band gap of as-prepared quantum dots after capping, the observed blue shift of the absorption peak can solely be attributed to the so-called size-effect whereas the enhancement in luminescence to surfactant mediated defect passivation. It is expected that the observed properties of the cytosine capped CdSe quantum dots would facilitate a better bio-compatibility of tailor-made nanoparticles for bio-imaging applications.     

Tuning of optical,thermal and antimicrobial capabilities of CdS nanoparticles with incorporated Mn prepared by chemical method

Cadmium sulfide (CdS) nanoparticles with different amounts of incorporated Manganese (Mn: 10, 15 and 20 mol%) has been prepared by chemical method. In optical properties the UV–Vis–NIR absorption spectra of all samples showed blue shift compared with the bulk CdS and decrease in band gap with Mn concentration. The presence of functional groups was identified by fourier transform infrared spectroscopy. It confirmed presence of metal sulfur bonding and weak interaction between anions (S^2?) and cations (Mn²⁺). The Photoluminescence spectra showed two emission peaks at 397 and 541 nm corresponding to the electron-hole recombination of CdS and surface trap induced emission respectively. In thermal properties thermogravimetric curve indicated increase in weight loss with Mn incorporation suggesting that pure CdS nanoparticles are thermally more stable than Mn incorporated CdS nanoparticles. The antimicrobial activities of Mn incorporated CdS were studied against Gram-positive and Gram-negative bacteria as test microorganisms using agar plating-spot inoculation method.     

Ecotoxicological studies of CdS nanoparticles on photosynthetic microorganisms

The potential ecotoxicity of nanosized cadmium sulfide (CdS), synthesized by the polyol process, was investigated using common Anabaena flos-aquae cyanobacteria and Euglena gracilis euglenoid microalgae. The photosynthetic activities of these microorganisms, after addition of free Cd2+ ions and CdS nanoparticles, varied with the presence of tri-n-octylphosphine oxide (TOPO) used to protect surface particle to avoid toxicity and also to control particle size and shape during the synthesis. The nanoparticle concentration was varied from 10(-3) to 5 x 10(-4) M. It was observed that the cadmium concentration, the addition of TOPO protective agent and the particle dissolution process in the culture medium play an important role during the ecotoxicological tests. Viability tests were followed by PAM fluorimetry. Cd2+ ions were very toxic for Anabaena flos aquae. The same behavior was observed after contact with CdS and CdS-TOPO nanoparticles. However, for Euglena gracilis, the photosynthetic activity was stable for more than 1 month in the presence of Cd2+ ions. Moreover, it was observed that the toxicity varies with the concentration of CdS and CdS-TOPO nanoparticles, both kind of nanoparticles are toxic for this microorganism. Transmission electron microscopy (TEM) analyses of microorganisms ultrathin sections showed that polysaccharides produced by Anabaena flos-aquae, after contact with CdS and CdS-TOPO nanoparticles, protect the microalgae against particle internalization. Only some particles were observed inside the cells. Moreover, the nanoparticle internalization was observed after contact with all nanoparticles in the presence of Euglena gracilis by endocytosis. All nanoparticles are inside vesicles formed by the cells.     

ZnO-CdS core-shell quantum dots sensitized solar cell: influence of crystalline and amorphous CdS structures in photovoltaic performance

ZnO nanoparticles (NPs) coated with amorphous and crystalline CdS quantum dots (QDs) were successfully synthesized through chemical bath deposition (CBD) process. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been utilized to characterize the samples morphology and structural properties. The conduction band of CdS QDs is much higher than the ZnO conduction band facilitates electron transfer process through cascade system. The thickness and crystallinity of the CdS QDs coated on ZnO NPs critically controls the electron diffusion length and photovoltaic performance of the solar cell. The red shift from 506 to 524 nm, increased optical absorption in the UV-visible range and electron diffusion length limited by the thickness of the amorphous/crystalline CdS QDs coated on ZnO NPs film, influences the performance of the QDs sensitized solar cell (QDSSCs) under one sun illumination intensity (AM 1.5, 100 mW/cm2). The results discuss the CBD process controlled growth of CdS QDs on ZnO NPs and its influence on the photovoltaic performance of QDSSCs.     

Studies on optical absorption and photoluminescence of thioglycerol-stabilized ZnS nanoparticles

ZnS quantum dots of size 3 nm are prepared at 303 K using ZnSO₄ and Na₂S₂O₃ precursors with thioglycerol as stabilizing agent. Cd²⁺ doped ZnS were prepared by varying doping concentration from 1 to 8 wt.%. ZnS quantum dots were mixed with CdS quantum dots of size 4 nm in the 3:1, 2:1, 1:1, 1:2, 1:3 and 1:4 M ratio. The nanoparticles were characterized by UV–vis, photoluminescence (PL), XRD and high-resolution TEM measurements. The XRD pattern, high-resolution TEM image and SAED pattern reveal that the nanoparticles are in well-crystallized cubic phase. The band gap of ZnS has increased from the bulk value 3.7 to 4.11 eV showing quantum size effect. Excitonic transition is observed at 274 nm in UV absorption and PL emission at 411 nm. Doping with Cd²⁺ red-shifts both UV and PL spectral bands and enhances the PL band of ZnS nanoparticles. Mixing CdS and ZnS quantum dots in different molar ratios shows red-shift of the band edge in the CdS/ZnS hybrid system. In the 1:1 hybrid system of CdS/ZnS nanoparticles, PL band is red-shifted and the intensity is almost doubled with respect to that of CdS nanoparticles.     

核壳结构CdS/ZnS荧光量子点的制备与荧光性能研究

以氧化镉为镉源、硫单质为硫源、油酸为配体、在十八烯体系中合成单分散的CdS纳米颗粒,研究了配体浓度对纳米微粒的生长动力学、颗粒尺寸分布的影响.采用乙基黄原酸锌作为Zn、S源的反应前体,采用逐滴滴加的方法制备了具有核壳结构的CdS/ZnS量子点,吸收光谱和荧光光谱表明CdS/ZnS纳米粒子比单一的CdS纳米粒子具有更优异的发光特性.透射电子显微镜、X射线粉末衍射、X射线光电子能谱、选区电子衍射证明ZnS在CdS表面进行了有效包覆.所制备核壳结构纳米粒子具有较好的尺寸分布,荧光发射峰半高峰宽为18～20nm,荧光量子产率达40%.     

Photoluminescence study of PVP capped CdS nanoparticles embedded in PVA matrix

Photoluminescence properties of polyvinyl pyrrolidone (PVP) capped cadmium sulphide (CdS) nanoparticles embedded in polyvinyl alcohol matrix (PVA) are reported. The PVP-CdS nanoparticles are prepared by non-aqueous method wherein cadmium nitrate is used as the cadmium source and hydrogen sulphide as the sulphur source. The synthesized nanoparticles are dispersed in polyvinyl alcohol (PVA) matrix and cast as self-standing flexible (PVP-CdS)-PVA films. The nanocomposites are characterized by optical absorption spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. XRD and TEM studies show the formation of cubic CdS particles with average size ∼3-5 nm. Thermal studies, carried out to observe the changes in PVA matrix due to the incorporation of PVP-CdS nanoparticles show strong interaction between the polymer matrix and nanoparticles. The photoluminescence emission spectra of the nanocomposites show two peaks, at 502 and 636 nm, which are attributed to the band edge and surface defects respectively, of CdS nanoparticles. Effective surface capping with optimum concentration of polyvinyl pyrrolidone leads to the quenching of surface defect-related emission.     

Synthesis and characterization of CdS nanoparticles

In the present work a chemical reduction method is followed to grow CdS nanoparticles at room temperature with varying the amount of reducing agent sodiumborohydride. The dispersed samples in ethanol are characterized using electron diffraction techniques. Simultaneously optical absorption, photoluminescence and longtime photorelaxation of these samples are studied at room temperature. An increase in band gap is observed in each case as compared to bulk CdS. Also particle size decreases with increased amount of reducing agent. Simultaneously long time relaxation increases with decrease of stoichiometric ratio of reducing agent. An attempt is made to correlate the structural, optical, electrical and opto-electrical properties.     

CdS nanoparticles grown in a polymer matrix by chemical bath deposition

Nanocrystallites of CdS have been grown by chemical bath deposition within the pores of poly(vinyl alcohol) (PVA) on glass and Si substrates. The CdS-PVA composite films are transparent in the visible region. XRD and TEM diffraction patterns confirmed the nanocrystalline CdS phase formation. TEM study of the film revealed the manifestation of nano CdS phase formation and the average particles size was varied in the range 5-12 nm. UV-vis spectrophotometric measurement showed high transparency (nearly 80% in the wavelength range 550-900 nm) of the films with a direct allowed band gap lying in the range 2.64-3.25 eV. Particle sizes have also been calculated from the shift of band gap with respect to that of bulk value and were found to be in the range 3.3-6.44 nm. The high dielectric constant (lies in the range 120-250 at high frequency) of PVA/CdS nanocomposite compared to that of pure PVA (-28) has been observed. The dielectric constant decreases with increase of dispersibility of the CdS nanoparticles within PVA. The nanocrystalline PVA/CdS thin films have also showed field emission properties with a turn-on field of approximately 6.6 V/microm, whereas only PVA thin film and bulk CdS on PVA have shown no field emission.     

Influence of the Cd/S Molar Ratio on the Optical and Structural Properties of Nanocrystalline CdS Thin Films

Nanocrystalline CdS thin films have been deposited using precursors with different thiourea concentrationonto glass substrates by sol-gel spin coating method.The crystalline nature of the films has been observedto be strongly dependent on thiourea concentration and annealing temperature.The CdS films are found tobe nanocrystalline in nature with hexagonal structure.The grain size is found to be in the range of 7.6 to11.5 nm depending on the thiourea concentration and annealing temperature.The high resolution transmissionelectron microscopy (HRTEM) results of the CdS films prepared using cadmium to thiourea molar ratio of0.3:0.3 indicate the formation of nanocrystalline CdS with grain size of 5 nm.Fourier transform infrared (FTIR)analysis shows the absorption bands corresponding to Cd and S.The optical study carried out to determinethe band gap of the nanostructured CdS thin films shows a strong blue shift.The band gap energy has beenobserved to lie in the range of 3.97 to 3.62 eV following closely the quantum confinement dependence ofenergy on crystallite radius.The dependence of band gap of the CdS films on the annealing temperature andthiourea concentration has also been studied.The photoluminescence (PL) spectra display two main emissionpeaks corresponding to the blue and green emissions of CdS.     

Synthesis and Characterization of Fe-Doped CdSe Nanoparticles as Dilute Magnetic Semiconductor

Cd_1?x Fe _x Se (0??x??0.1) nanoparticles were synthesized by hydrothermal method. The solubility limit of Fe in CdSe nanoparticles was found to be less than 6?% as obtained from X-ray diffraction (XRD) study. With the increase in doping concentration at and above it, secondary phase of FeSe₂ starts appearing. The presence of Cd, Se, and Fe has been confirmed by energy dispersive X-ray spectroscopy (EDS). The increase in band gap value has been confirmed by UV-visible spectra and the variation in emission intensity of photoluminescence (PL) measurements further indicates the incorporation of Fe in CdSe nanoparticles. Transmission electron microscopy (TEM) reveals the spherical nature of synthesized nanoparticles, and the particle size decreases with increasing Fe doping concentration. Fourier transform Infrared spectra (FTIR) confirm the capping of sodium dodecyl sulfate (SDS) surfactant on pure and Fe-doped CdSe nanoparticles. The synthesized nanoparticles show room-temperature ferromagnetic behavior, and the saturation magnetization value was found to increase with Fe doping concentration.     

Nanomaterials based on CdS nanoparticles in polyethylene matrix

The synthesis of CdS nanoparticles stabilized in the bulk of a polyethylene matrix is described. The size of synthesized nanoparticles is determined by means of transmission electron microscopy. The composition of nanoparticles is defined by X-ray phase analysis. It is shown that the variation of the process-dependent parameters during synthesis of nanoparticles in a polymer + oil solution melt results in the formation of CdS nanoparticles with average sizes of 4.9, 5.4, and 6.2 nm with a reasonably narrow size distribution and well-formed structure. The optical properties of synthesized nanomaterials are investigated. The investigation of Raman scattering reveals softening of the LO-phonon mode with decreasing CdS nanoparticle size. A broad high-energy band of photoluminescence connected with the exciton annihilation in conditions of size quantization is detected.     

Influence of organic polymers as capping agent on structural and optical properties of ZnS:Mn2+ and CdS nanocrystals

We have synthesized the luminescent ZnS:Mn2+ and bare CdS nanocrystals by employing polyvinyl alcohol (PVA), citric acid (CA) and biotin as organic capping agents by chemical co-precipitation route. The synthesized materials were characterized by X-ray diffraction, small angle X-ray scattering and transmission electron microscopy for the structural analysis, while UV-Visible spectroscopy and photoluminescence (PL) for optical properties. The results show that all the three organic polymers have the same effect and are capable of controlling the growth of the nanoclusters. From UV-Visible absorption spectra, it was observed that different capping agents do not affect the band gap of ZnS:Mn2+ system as well, while capping effect clearly observed on PL properties of ZnS:Mn2+ system. We have observed a novel result that capping with biotin show excellent photoluminescence as compared to capping of PVA and CA on ZnS:Mn2+ -systems. On the other hand, the annealing of these systems leads to degradation of luminescence intensity.     

Photocatalytic performance of CdS nanomaterials for photodegradation of organic azo dyes under artificial visible light and natural solar light irradiation

The CdS semiconductors have been prepared at low temperature via catalyst-free chemical precipitation method without using any surfactant or capping agent. Either water or ethylene glycol, as a solvent, provides spherical CdS nanostructures with a comparable size of 117–121 nm. The molar ratio of Cd/S plays an important role in determining phase structure, morphology and photocatalytic performance of the prepared CdS nanostructures. Increasing molar ratio of S^2? results in not only mixed cubic-hexagonal phases but also low photocatalytic performance. CdS nanoparticles with good dispersibility prepared at Cd/S molar ratio of 1:1 shows high photocatytic efficiency of 95% toward photodegradation of reactive red azo dye (RR141) under visible light irradiation up to 240 min. The degradation efficiency of CdS nanoparticles also reaches 48% under natural solar light irradiation for 80 min. This work demonstrates the promising potential of CdS nanomaterials as photocatalysts for environmental remediation.