A facile route to shape controlled CdTe nanoparticles

Hexadecylamine (HDA) capped CdTe nanoparticles have been synthesized using a facile hybrid solution based/thermolysis route. This method involves the reaction by the addition of an aqueous suspension or solution of a cadmium salt (chloride, acetate, nitrate or carbonate) to a freshly prepared NaHTe solution. The isolated CdTe was then dispersed in tri-octylphosphine (TOP) and injected into pre-heated HDA at temperatures of 190, 230 and 270 °C for 2 h. The particle growth and size distribution of the CdTe particles synthesized using cadmium chloride as the cadmium source were monitored using absorption and photoluminescence spectroscopy. The final morphology of the CdTe nanoparticles synthesized from the various cadmium sources was studied by transmission electron microscopy (TEM) and high resolution TEM. The cadmium source has an influence on the final morphology of the particles.     

Synthesis of size tuneable cadmium sulphide nanoparticles from a single source precursor using ammonia as the solvent

Size tuneable cadmium sulphide nanoparticles of a few nanometres in size were prepared by thermolysis of a single source precursor of cadmium xanthates with variable carbon chain length (Cd(ROCS₂)₂, where R denotes -C₂H₅, -C₄H₉, -C₈H₁₇ and -C₁₂H₂₅, respectively) in an ammonia solution. The particle size, morphology and crystallinity of these nanoparticles were characterized using X-ray powder diffractometry, transmission electron microscopy, and nitrogen adsorption/desorption techniques. The results show that hexagonal CdS nanoparticles can be produced by thermolysis of cadmium alkyl xanthate in an ammonia solution at a temperature as low as 100 °C. The size of CdS particles (between 5.60 nm and 3.71 nm) decreases with increasing length of carbon chain in the precursor, as further confirmed by UV-visible and fluorescence spectrophotometric measurements. The size tuning mechanism of CdS from cadmium alkyl xanthate is also discussed.     

Towards ultra-thin CdTe solar cells using MOCVD

A study was made on very thin CdTe absorber < 1 µm layers to investigate limitations in CdTe collection efficiency. Metal organic chemical vapour deposition (MOCVD) was used to deposit cadmium sulfide (CdS), cadmium zinc sulfide (Cd_0.9Zn_0.1S) and cadmium telluride (CdTe). Improvements in photon collection in the blue, where the absorption length is shorter, have been achieved using a wider band gap Cd_0.9Zn_0.1S ternary alloy to replace CdS as the window layer. Solar cell capacitance simulator (SCAPS) modelling software [M. Burgelman, P. Nollet, S. Degrave, Thin Solid Films, 361-362 (2000) 527-532] has been used to calculate device parameters as a function of the absorber layer thickness (controlled by in situ using laser reflectometry). One feature of the MOCVD grown devices is the apparent absence of pin-holes, demonstrated by growth of an ultra-thin absorber (200 nm) with conversion efficiency of nearly 4%.     

Growth of cubic and hexagonal CdTe thin films by pulsed laser deposition

The paper reports the growth of cadmium telluride (CdTe) thin films by pulsed laser deposition (PLD) using excimer laser (KrF, λ=248 nm, 10 Hz) on corning 7059 glass and SnO₂-coated glass (SnO₂/glass) substrates at different substrate temperatures (T_s) and at different laser energy pulses. Single crystal target CdTe was used for deposition of thin films. With 30 min deposition time, 1.8- to ∼3-μm-thick films were obtained up to 200 °C substrate temperature. However, the film re-evaporates from the substrate surface at temperatures >275 °C. Atomic force microscopy (AFM) shows an average grain size ∼0.3 μm. X-ray diffraction analysis confirms the formation of CdTe cubic phase at all pulse energies except at 200 mJ. At 200 mJ laser energy, the films show hexagonal phase. Optical properties of CdTe were also investigated and the band gap of CdTe films were found as 1.54 eV for hexagonal phase and ∼1.6 eV for cubic phase.     

Syntheses of Mn3O4 and LiMn2O4 nanoparticles by a simple sonochemical method

Mn₃O₄ and LiMn₂O₄ nanoparticles were prepared by a simple sonochemical method which is environmentally benign. First, Mn₃O₄ nanoparticles were prepared by reacting MnCl₂ and NaOH in water at room temperature through a sonochemical method, operated at 20 kHz and 220 W for 20 min. Second, LiOH was coated onto the resulting Mn₃O₄ under the same sonochemical conditions as above. The thickness of coated LiOH on Mn₃O₄ obtained from the reaction ratio of 3:1 between LiOH and Mn₃O₄ was about 4.5–5.5 nm range. Then, by heating those LiOH-coated Mn₃O₄ particles at the relatively low temperature of 300–500 °C for 1 h, they were transformed into phase-pure LiMn₂O₄ nanoparticles of about 50 to 70 nm size in diameter.     

Analysis of electrodeposited CdTe thin films grown using cadmium chloride precursor for applications in solar cells

Deposition of cadmium telluride (CdTe) from cadmium chloride (CdCl₂) and tellurium oxide has been achieved by electroplating technique using two-electrode configuration. Cyclic voltammetry shows that near-stoichiometric CdTe is achievable between 1330 and 1400 mV deposition voltage range. The layers grown were characterised using X-ray diffraction (XRD), UV–Visible spectrophotometry, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), photoelectrochemical (PEC) cell and DC conductivity measurements. The XRD shows that the electrodeposited CdTe layer is polycrystalline in nature. The UV–Visible spectrophotometry shows that the bandgap of both as-deposited and heat-treated CdTe films are in the range of (1.44–1.46) eV. The SEM shows grain growth after CdCl₂ treatment, while, the EDX shows the effect of growth voltage on the atomic composition of CdTe layers. The PEC results show that both p- and n-type CdTe can be electrodeposited and the DC conductivity reveals that the high resistivity is at the inversion growth voltage (V_i) for the as-deposited and CdCl₂ treated layers.     

Syntheses and characterization of CdCO3 and CdO nanoparticles by using a sonochemical method

Nanoparticles of CdCO₃ were synthesized by the reaction of Cd(CH₃COO)₂ and tetramethylammonium hydroxide (TMAH) by a sonochemical method. The CdO nanoparticles were obtained by heating of CdCO₃ nanoparticles at 400 °C. The CdCO₃ and CdO nanoparticles were characterized by scanning electron microscopy, X-ray powder diffraction (XRD), TGA, DTA and FT-IR spectroscopy.     

Effects of CdCl2 in CdTe on the properties of sintered CdS/CdTe solar cells

Sintered CdS films on glass substrates with low electrical resistivity and high optical transmittance have been prepared by a coating and sintering method. All-polycrystalline CdS/CdTe solar cells with different microstructures and properties of the CdTe layer were fabricated by coating a number of CdTe slurries, which consisted of cadmium and tellurium powders, an appropriate amount of propylene glycol and various amounts of CdCl₂, on the sintered CdS films and by sintering the glass-CdS-(Cd + Te) composites at various temperatures. The presence of more than 5 wt% of CdCl₂ in the (Cd + Te) layer enhances the sintering of the CdTe film and the junction formation by a liquid-phase sintering mechanism. A low sintering temperature results in poor densification of the CdTe layer and the CdS-CdTe interface, whereas a high sintering temperature results in a deeply buried homojunction. The optimum temperature for the sintering of the CdTe layer and for junction formation decreases with increasing amount of CdCl₂. All-polycrystalline CdS/CdTe solar cells with an efficiency of 10.2% under solar irradiation have been fabricated by a coating and sintering method using cadmium and tellurium powders for the CdTe layer.     

Linear and non-linear optical properties of capped CdTe nanocrystals prepared by mechanical alloying

CdTe nanocrystals were prepared by mechanical alloying the elemental Cd and Te powders. The formation of CdTe with a single cubic phase after 20 h of ball milling was confirmed by X-ray diffraction (XRD). The surface of as-milled CdTe nanoparticles was then capped with polarization TOP/TOPO or (Na₃PO₄)_n organic ligand, which resulted in colorful dispersion solution with optical absorption peaks located at 573 nm and 525 nm, respectively. The third-order non-linearity, namely, the non-linear refraction and two-photon absorption (TPA) coefficient, of the capped CdTe dispersion samples were evaluated using Z-scan technique. The fitting of Z-scan experimental data with a special equation demonstrated that the capped CdTe nanocrystals possess large third-order susceptibilities at resonant wavelength. The non-linear figure of merit (γ/β) for 20 h as-milled CdTe nanocrystals after capping with TOP/TOPO was determined to be ∼ −2 × 10⁻⁵ m, which is nearly 215 times larger than the value reported for bulk CdTe crystals.     

A simple sonochemical synthesis and characterization of CdWO<Subscript>4</Subscript> nanoparticles and its photocatalytic application

Cadmium tungstate (CdWO₄) nanoparticles were synthesized via a sonochemical method based on the reaction between cadmium(II) nitrate hexahydrate and sodium tungstate dihydrate in an aqueous solution. To the best of authors’ knowledge, it is the first time that cadmium tungstate was synthesized by ultrasonic method. The structural, morphological, and optical properties of as-obtained products were characterized by X-ray diffraction, Scanning electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray microanalysis (EDS), and ultraviolet–visible spectroscopy. The as-synthesized cadmium tungstate indicated a ferromagnetic behavior which evidenced by using vibrating sample magnetometer at room temperature. To evaluate the photocatalysts properties of nanocrystalline cadmium tungstate, the photocatalytic degradation of methyl orange under ultraviolet light irradiation was carried out.     

Preparation of CdTe thin films by laser irradiation of electrodeposits

We describe here the first attempts to electrodeposit cadmium and tellurium layers onto a glass substrate covered with indium tin oxide with the intention of synthesizing CdTe by laser irradiation of the deposit.We successively deposited tellurium and cadmium cathodically in aqueous solutions of H₂SO₄ and TeO₂ or of CdSO₄ respectively. The deposits were then laser irradiated and CdTe was formed.The as-formed CdTe, of cubic crystallographic structure, exhibits a band edge for direct transitions of about 1.5 eV in agreement with the band gap of CdTe.     

Surface characterization of GSH-CdTe quantum dots

The surface characterization of CdTe QDs synthesized by a novel procedure using glutathione (GSH), low temperatures (60–90 °C) and K₂TeO₃ as the –Te precursor is reported. Fluorescence of the produced QDs is stable in the pH range 6–13 and QDs inside eukaryotic cells are highly fluorescent. The surface composition of GSH-CdTe QDs with different spectroscopic properties and particle size distributions was determined by XPS. The XPS analysis indicated that the QDs are essentially CdTe, although all nanoparticles contain 12–24% of CdO (and in one case also TeO₂). GSH decomposes with reaction time releasing small amounts of S⁻² ions that react with Cd(Te) to yield Cd(Te)S in a smaller amount than that of CdTe. Finally, the use of QDs in fluorescence mediated immunodetection of bacterial pathogens has been evaluated.     

Synthesis and kinetics research of ZnS nanoparticles prepared by sonochemical process

Abstract

ZnS nanocrystallites have been successfully prepared by a sonochemical process. The reaction kinetics of the process was also investigated. The as prepared ZnS nanocrystallites were characterised by XRD and TEM. Results show that ZnS nanoparticles can be obtained by sonochemical process using ZnCl₂ and thiacetamide as raw materials. It is found that the as prepared ZnS nanoparticles are hexagonal phase with spherical or spherical-like morphologies. The grain size decreases with increasing ultrasonic irradiation power. Reaction kinetics shows that the weight content of ZnS nanoparticles increases linearly with reaction time at different temperatures. The synthesis activation energy of ZnS nanoparticles is calculated to be 27·80 kJ mol^–1.     

Microstructural features of cadmium telluride photovoltaic thin film devices

In order to study the microstructure of cadmium telluride (CdTe) photovoltaic thin film solar cells, manufactured by an in-line manufacturing process, Scanning Electron Microscopy characterization (SEM) and X-ray diffraction (XRD) characterization were performed. SEM measurement showed that no substantial changes in the grain structure of CdTe layers occurred during the Cadmium Chloride (CdCl₂) treatment. No change in the cubic CdTe lattice parameter “a” was observed for the CdCl₂ treated sample. It is inferred that the primary effect of the CdCl₂ treatment in the devices studied is the passivation of grain boundaries and bulk defects. XRD studies show a loss of preferred orientation (as determined from the peak ratios) of planes during the copper compound treatment indicating recrystallization of the grains due to the Cu treatment. Also the Cu treated sample showed decrease in value of the lattice parameter “a”.     

Rapid sonochemical synthesis of mesoporous MnO2 for supercapacitor applications

Mesoporous MnO₂ samples with average pore-size in the range of 2–20 nm are synthesized in sonochemical method from KMnO₄ by using a tri-block copolymer, namely, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) as a soft template as well as a reducing agent. The MnO₂ samples are found to be poorly crystalline. On increasing the amplitude of sonication, a change in the morphology of MnO₂ from nanoparticles to nanorods and also change in porosity are observed. A high BET surface area of 245 m² g⁻¹ is achieved for MnO₂ sample. The MnO₂ samples are subjected to electrochemical capacitance studies by cyclic voltammetry (CV) and galvanostatic charge–discharge cycling in 0.1 M aqueous Ca(NO₃)₂ electrolyte. A maximum specific capacitance (SC) of 265 F g⁻¹ is obtained for the MnO₂ sample synthesized in sonochemical method using an amplitude of 30 μm. The MnO₂ samples also possess good electrochemical stability due to their favourable porous structure and high surface area.     

Introducing CTAB into CdTe/PVP nanofibers enhances the photoluminescence intensity of CdTe nanoparticles

The major objective of this work is focused on the preparation and characterization of the photoluminescence (PL) property of poly(vinyl pyrrolidone) (PVP) embedding CdTe nanoparticles. The CdTe nanoparticles were generated via the reaction of Cd²⁺ with NaHTe and then stabilized by thiolglycolic acid (TGA). In the process of preparing CdTe/PVP nanofibers by electrospinning, a surfactant, cetyltrimethylammonium bromide (CTAB), was introduced to prevent CdTe nanoparticles from congregating inside the PVP nanofibers. Then the results of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) showed that the average diameter of CdTe/PVP nanofibers was 300 (± 61) nm, in which the CdTe nanoparticles were incorporated into the PVP nanofibers homogeneously. Finally, the PL spectra proved that the photoluminescence intensity of CdTe/PVP nanofibers was enhanced by the addition of CTAB.     

Preparation and characterization of pulsed laser deposited CdTe thin films at higher FTO substrate temperature and in Ar + O2 atmosphere

Pulsed laser deposition (PLD) is one of the promising techniques for depositing cadmium telluride (CdTe) thin films. It has been reported that PLD CdTe thin films were almost deposited at the lower substrate temperatures (<300 °C) under vacuum conditions. However, the poor crystallinity of CdTe films prepared in this way renders them not conducive to the preparation of high-efficiency CdTe solar cells. To obtain high-efficiency solar cell devices, better crystallinity and more suitable grain size are needed, which requires the CdTe layer to be deposited by PLD at high substrate temperatures (>400 °C). In this paper, CdTe layers were deposited by PLD (KrF, λ = 248 nm, 10 Hz) at different higher substrate temperatures (T_s). Excellent performance of CdTe films was achieved at higher substrate temperatures (400 °C, 550 °C) under an atmosphere of Ar mixed with O₂ (1.2 Torr). X-ray diffraction analysis confirmed the formation of CdTe cubic phase with a strong (1 0 0) preferential orientation at all substrates temperatures on 60 mJ laser energy. The optical properties of CdTe were investigated, and the band gaps of CdTe films were 1.51 eV and 1.49 eV at substrate temperatures of 400 °C and 550 °C, respectively. Scanning electron microscopy (SEM) showed an average grain size of 0.3–0.6 μm. Thus, under these conditions of the atmosphere of Ar + O₂ (15 Torr) and at the relatively high T_s (500 °C), an thin-film (FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe: Cu/Ag) solar cell with an efficiency of 6.68% was fabricated.     

Fabrication of glass-capsuled CdTe microcrystals using laser evaporation and plasma chemical vapour deposition method

CdTe microcrystals encapsulated in a silica glass layer were successfully fabricated. Spherical CdTe microcrystals were prepared by laser evaporation of a CdTe target in an argon gas atmosphere. The ensuing microcrystals plus argon gas passed through a tetramethoxysilane (TMOS)+O₂ plasma in which they were encapsulated in an amorphous layer, 2–2.5 nm thick. Characteristic X-rays from the surface layer were measured using an energy dispersive X-ray spectrometer equipped in a high-resolution transmission electron microscope. Measurements indicated that the glass layer consisted of silicon and oxygen, with no cadmium or tellurium included. The CdTe microcrystals fabricated with our laser evaporation system showed two specific kinds of particle: small particles (below 10 nm) and large ones (over 100 nm). Using precise electron-beam diffraction testing, we concluded that the large microcrystal is a single crystal with a hexagonal structure. The deposition rates and infrared transmission of silica glass prepared by TMOS or tetraethoxysilane plasma-enhanced chemical vapour deposition are also discussed. The highest deposition rate, 30 nm s^–1, of silica glass can be achieved in the centre of the plasma when the input r.f. power is 150 W.     

Silver nanoparticles: Ultrasonic wave assisted synthesis, optical characterization and surface area studies

Silver nanoparticles have been successfully synthesized by the sonochemical route using sodium borohydride and sodium citrate as the reducing agents. The effect of the reducing agents on the particle size and morphology has been studied by carrying out the two reactions at the same ultrasound frequency (20 KHz). The strong reducing agent (NaBH₄) produced spherical silver nanoparticles of sizes 10 nm whereas sodium citrate led to much smaller silver nanoparticles of ~ 3 nm diameter. Powder X-ray diffraction studies reveal a high degree of crystallinity and monophasic silver particles. UV-Visible studies show the presence of a surface plasmon band at 405 nm. However the reflectance spectra give a broad band between 340 and 360 nm which is characteristic for the quasi-spherical silver nanoparticles. The specific surface area was found to be 2.6 and 13.1 m²/g and the pore radius was found to be 15.2 and 12.3 Å for silver nanoparticles obtained by the sodium borohydride and sodium citrate reduction respectively.     

Hybrid photovoltaic cells with II–VI quantum dot sensitizers fabricated by layer-by-layer deposition of water-soluble components

We present the fabrication of all solid state heterojunction photovoltaic devices consisting of TiO₂ films sensitized by colloidal CdSe and CdTe quantum dots (QDs) and a hole transport layer of the conjugated polymer poly(9,9-dioctyl-fluorene-co-N-(4-butylenphenyl)diphenylamine). The sensitized films were prepared by alternating the layer-by-layer deposition of TiO₂ nanoparticles, water-soluble semiconductor QDs and polycations. Photovoltaic devices sandwiched between fluorinated tin oxide and gold electrodes showed a high rectification ratio and photovoltages of up to 1.15 V. Effective sensitization was observed for CdSe QDs, while incorporated CdTe QDs apparently had no such effect. These findings are explained by confinement effects in QDs.