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Concept: Cadmium sulfide


Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technology. With laboratory efficiencies approaching 20 per cent, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per watt (ref. 2) and to minimize the environmental impact. A central part of the manufacturing process involves doping the polycrystalline thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface and to passivate the grain boundaries, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per gram) and a water-soluble source of toxic cadmium ions, presenting a risk to both operators and the environment during manufacture. Here we demonstrate that solar cells prepared using MgCl2, which is non-toxic and costs less than a cent per gram, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9 × 10(14) cm(-3)) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concentrations of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of electrically active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell production.

Concepts: Semiconductor, Cadmium, Solar cell, Photovoltaics, Cost, Cadmium telluride, Cadmium sulfide, United States dollar


Over the past years a number of studies have described the instability of the pigment cadmium yellow (CdS). In a previous paper we have shown how cadmium sulfide on paintings by James Ensor oxidizes to CdSO(4)·H(2)O. The degradation process gives rise to the fading of the bright yellow color and the formation of disfiguring white crystals that are present on the paint surface in approximately 50 μm sized globular agglomerations. Here, we study cadmium yellow in the painting “Flowers in a blue vase” by Vincent van Gogh. This painting differs from the Ensor case in the fact that (a) a varnish was superimposed onto the degraded paint surface and (b) the CdS paint area is entirely covered with an opaque crust. The latter obscures the yellow color completely and thus presents a seemingly more advanced state of degradation. Analysis of a cross-sectioned and a crushed sample by combining scanning microscopic X-ray diffraction (μ-XRD), microscopic X-ray absorption near-edge spectroscopy (μ-XANES), microscopic X-ray fluorescence (μ-XRF) based chemical state mapping and scanning microscopic Fourier transform infrared (μ-FT-IR) spectrometry allowed unravelling the complex alteration pathway. Although no crystalline CdSO(4) compounds were identified on the Van Gogh paint samples, we conclude that the observed degradation was initially caused by oxidation of the original CdS pigment, similar as for the previous Ensor case. However, due to the presence of an overlying varnish containing lead-based driers and oxalate ions, secondary reactions took place. In particular, it appears that upon the photoinduced oxidation of its sulfidic counterion, the Cd(2+) ions reprecipitated at the paint/varnish interface after having formed a complex with oxalate ions that themselves are considered to be degradation products of the resin and/or oil in the varnish. The SO(4)(2-) anions, for their part, found a suitable reaction partner in Pb(2+) ions stemming from a dissolved lead-based siccative that was added to the varnish to promote its drying. The resulting opaque anglesite compound in the varnish, in combination with the underlying CdC(2)O(4) layer at the paint/varnish interface, account for the orange-gray crust that is disfiguring the painting on a macroscopic level. In this way, the results presented in this paper demonstrate how, through a judicious combined use of several microanalytical methods with speciation capabilities, many new insights can be obtained from two minute, but highly complex and heterogeneous paint samples.

Concepts: Spectroscopy, Cadmium, Pigment, Cadmium sulfide, Expressionism, Vincent van Gogh, Cadmium pigments, Les XX


Well-aligned CdS nanorod arrays (CdS NRs) with ∼100nm in diameter and ∼700nm in length were fabricated on FTO (fluorine-doped tin oxide) substrate by using glutathione as capping agents. The growth of CdS NRs was studied in details by exploring the roles of each active binding group in glutathione. The thiol group in glutathione plays an important role in forming a compact CdS nanocrystal film, upon which the nanorods grow subsequently via the synergetic effect of thiol and dicarboxyl groups in glutathione. The influence of surface passivation with glutathione on the photoelectrical property of CdS NRs was also tested. The results revealed that glutathione ligands encapsulated in the surfaces of CdS NRs act as insulating barriers between CdS NRs and solution, hindering charge transport. Hybrid photovoltaic cells of FTO/CdS NRs/P3HT (poly(3-hexylthiophene))/Au were then assembled. The performance of the photovoltaic devices was increased with increasing the length of the as-prepared CdS nanorods and further enhanced to the highest efficiency of 0.373% after the thermal sulfuration treatment.

Concepts: Zinc, Disulfide bond, Sulfur, Cadmium, Photovoltaics, Cysteine, Thiol, Cadmium sulfide


A facile electrodeposition technique was utilized to deposit single-walled carbon nanotubes (SWNTs) with cadmium telluride (CdTe) with well-controlled size, density, surface morphology, and composition. By controlling the applied charge, the morphology of these hybrid nanostructures was altered from CdTe nanoparticles on SWNTs to SWNT/CdTe core/shell nanostructures and the composition of the CdTe nanoparticles was altered from Te-rich (29 at% Cd) to Cd-rich (79 at% Cd) CdTe by adjusting the deposition potential. The electrical and optoelectrical properties of these hybrid nanostructures showed that photo-induced current can be tuned by tailoring the conductivity type (n-type or p-type), morphology, and size of the CdTe nanostructures, with a maximum photosensitivity (ΔI/I(0)) of about 30% for SWNT/Cd-rich CdTe (n-type) core/shell nanostructures. This work demonstrates a novel approach for synthesizing metal chalcogenide/SWNT hybrid nanostructures for various electrical and optoelectrical applications.

Concepts: Cadmium, Carbon nanotube, Cadmium telluride, Cadmium sulfide


The optical properties of cadmium sulfide (CdS) nanoparticles in suspension are affected by morphology and suspending solvent. Time dependent stability of these properties is solvent dependent and is affected by illumination conditions under which the suspension is stored. Moreover, minute amounts of dissolved oxygen are sufficient in order to facilitate photodegradation.

Concepts: Oxygen, Colloid, Sulfur, Cadmium, Cadmium telluride, Cadmium sulfide, Cadmium pigments, Greenockite


Nature has evolved several unique biomineralization strategies to direct the synthesis and growth of inorganic materials. These natural systems are complex, involving the interaction of multiple biomolecules to catalyze biomineralization and template growth. Herein we describe the first report to our knowledge of a single enzyme capable of both catalyzing mineralization in otherwise unreactive solution and of templating nanocrystal growth. A recombinant putative cystathionine γ-lyase (smCSE) mineralizes CdS from an aqueous cadmium acetate solution via reactive H2S generation from l-cysteine and controls nanocrystal growth within the quantum confined size range. The role of enzymatic nanocrystal templating is demonstrated by substituting reactive Na2S as the sulfur source. Whereas bulk CdS is formed in the absence of the enzyme or other capping agents, nanocrystal formation is observed when smCSE is present to control the growth. This dual-function, single-enzyme, aerobic, and aqueous route to functional material synthesis demonstrates the powerful potential of engineered functional material biomineralization.

Concepts: Metabolism, Enzyme, Chemistry, Sulfur, Cadmium, Hydrogen sulfide, Nature, Cadmium sulfide


To optimize the optical properties of semiconductor nanoplatelets, simple routes to add high-quality shells are needed. We demonstrate uniform growth of CdS shells on CdSe nanoplatelets at 300 °C, overcoming limitations of previous low-temperature syntheses. We obtain core/shell nanoplatelets with spectrally narrow (20 nm) and efficient emission for shells up to 4 nm thick.

Concepts: Optics, Cadmium sulfide


The kinetics of thiourea synthesis of CdS nanoparticles (NPs) in reverse microemulsions of Tergitol Np-4/n-decane was studied in the temperature range of 40-60°C by spectrophotometry, photon-correlation spectroscopy and transmission electron microscopy. The formation of NPs is described by the kinetic model including two consecutive steps: homogeneous nucleation in a solution as the first step, and autocatalytic growth of NPs due to heterogeneous reaction on a continuously increasing surface as the second step. Effective rate constants of the steps (k1 = 1.52×10(-2) - 1.75×10(-3) s(-1) and k2 = 4.9×10(-1) - 5.1×10(-2) M(-1)s(-1)) and effective activation energies (Ea1 = 156 and Ea2 = 149 kJ/mol) were estimated in the pseudofirst-order reaction with respect to cadmium (CCd = 0.9 mM, CThio = 9 mM). The obtained constants were used to calculate the dependence of nanoparticle diameter on the synthesis time (d(3) ~ t). The calculated values correlate well with experimental data of photon-correlation spectroscopy and transmission electron microscopy.

Concepts: Electron, Chemical reaction, Temperature, Thermodynamics, Nanotechnology, Transmission electron microscopy, Solution, Cadmium sulfide


Novel and sensitive sandwich-type electrochemiluminescence (ECL) immunosensor was fabricated for insulin detection. Au-ZnCd14S combined nitrogen doping mesoporous carbons (Au-ZnCd14S/NH2-NMCs) acted as sensing platform and Au-Cu alloy nanocrystals were employed as labels to quench the ECL of Au-ZnCd14S/NH2-NMCs. Zinc-doping promoted the ECL behavior of CdS nanocrystals, with the best ECL emission obtained when the molar ratio of Zn/Cd was 1:14. Simultaneously, the modification of gold nanoparticles (Au NPs) and combination with NH2-NMC further enhanced the ECL emission of ZnCd14S due to its excellent conductivity and large specific surface area, which is desirable for the immunosensor construction. Au-Cu alloy nanocrystals were employed in the ECL system of ZnCd14S/K2S2O8 triggering ECL quenching effects. The ECL spectra of ZnCd14S, acting as the energy donor, exhibited well overlaps with the absorption band of Au-Cu alloy nanocrystals which acted as the energy acceptor, leading to an effective ECL resonance energy transfer (ECL-RET). On the basis of the ECL quenching effects, a sensitive ECL immunosensor for insulin detection was successfully constructed with a linear response range of insulin concentration from 0.1pg/mL to 30ng/mL and the limit of detection was calculated to be 0.03pg/mL (S/N = 3).

Concepts: Fluorescence, Nanoparticle, Photon, Zinc, Gold, Steel, Specific surface area, Cadmium sulfide


High-density CdTe nanorod arrays are successfully embedded in a uniform and compact CdS layer, forming a novel three-dimensional (3D) CdTe NRs/CdS heterojunction structure. The CdS layer is prepared by homogeneous precipitation (HP) method using decomposition of urea. The effects of temperature and concentration of reactants on the growth and composition of CdS film are investigated in detail. The results demonstrate that the temperature affects the thermal decomposition of urea significantly, and the concentration of CdCl2 and CS (NH2)2 plays an essential role in the compositional ratio of CdS film. Further investigations reveal that, in comparison with the traditional precipitation method, a better coverage of CdS on the surface of CdTe NRs can be obtained by HP method due to the slow and even hydrolysis of urea. Moreover, photovoltaic performance of the novel CdTe NRs/CdS 3D photovoltaic device is also investigated. This study demonstrates that the 3D heterostructure has potential application in thin film solar cells, and the successful deposition of CdS layer on the surface of CdTe NRs by HP method suggests a promising technique for large-scale fabrication of these solar cells.

Concepts: Cadmium, Solar cell, Photovoltaics, Thin film solar cell, Cadmium selenide, Cadmium telluride, Cadmium sulfide, Cadmium oxide