A sulfide/polysulfide-based ionic liquid electrolyte for quantum dot-sensitized solar cells

Further development of quantum dot-sensitized solar cells (QDSCs) will require long-term stability in addition to the continuous increase of photovoltaic (PV) conversion efficiency achieved in the last years. We report a robust S(2-)/S(n)(2-) electrolyte that has been specifically designed for compatibility with CdSe quantum dots in sensitized solar cells. The new pyrrolidinium ionic liquid reaches 1.86% efficiency and a short-circuit current close to 14 mA·cm(-2) under air-mass 1.5 global illumination and improves the device lifetime with good photoanode stability over 240 h. PV characterization showed that the solar cell limitations relate to poor catalysis of regeneration at the counter electrode and high recombination. Further improvement of these factors in the robust electrolyte configuration may thus have a significant impact for advancing the state-of-the-art in QDSCs.     

Chemical bath deposition of thin nanocrystalline tin(II) sulfide films with thioacetamide

Nanocrystalline tin(II) sulfide layers 100–650 nm thick were prepared by hydrochemical deposition on glass–ceramic supports from a citrate system at 323?343 K using thioacetamide as chalcogenizing agent. These films are of interest for the development of thin-film solar radiation converters based on the multicomponent compound Cu₂ZnSnS₄ of kesterite structure, prepared using a cost-saving process. Examination by scanning electron microscopy shows that tin(II) sulfide layers are formed from spherical nanocrystallites of 20–40 nm size. X-ray diffraction analysis shows that they crystallize in the orthorhombic system with the unit cell parameters a = 4.276, b = 11.243, and c = 3.986 Å. Analysis by X-ray photoelectron spectroscopy revealed the presence of up to 44.86 at. % oxygen in the surface layer of the film. This oxygen is mainly present in surface contaminants and is also incorporated in SnO present on the surface.     

Interfacial friction of thin PDMS network films

This study focuses on developing dry, surface‐tethered polymeric lubricant coatings capable of significantly decreasing friction and wear of nano‐ and micrometer scale machines. Vinyl‐terminated polydimethylsiloxane chains are spin‐coated with a crosslinking agent and platinum catalyst onto silicon wafers functionalized with a self‐assembling monolayer containing reactive vinyl groups. Lateral force microscopy (LFM) measurements employing a bead probe are used to quantify the coefficient of friction (COF) and adhesion characteristics of the PDMS‐SAM surface tethered networks. The combined polymer network and SAM layer manifest extremely low friction coefficients, μ = 4 × 10^?3, which is nearly one order of magnitude lower than the friction coefficient of the bare silicon substrate. The lowest friction forces are measured using silicon substrates covered with nanometer thick, peroxide crosslinked PDMS networks; though poorly crosslinked, these networks display COFs as much as ten‐times lower than a solitary SAM coating layer. Micrometer thick end‐linked optimal networks also manifest attractive interfacial friction properties, with COFs approximately three times larger than the thinner, imperfect networks. These observations are discussed in terms of the structure of the polymer networks and the role of adhesion forces on interfacial friction. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1773–1787, 2008     

Study of the structure of thin nanocrystalline films

Structure and phase composition of thin nanocrystalline films of semiconductors and insulators CdS, Cd_xZn_1?xS, SiC_xN_y, and BC_xN_y were studied using registration of weak diffraction intensities with synchrotron radiation. Three methods were developed and applied to obtain diffraction patterns, namely, θ—2-scan (Bragg—Brentano geometry); 2θ-scan (grazing incidence scheme); a scheme with an image plate as detector. The characteristic features of the diffraction patterns, obtained from the aforementioned samples, are discussed. The X-ray diffraction data are compared with the results obtained by HREM, SAED, electron spectroscopy, ellipsometry, IR and Raman spectroscopy. It is stated that: 1) the films are composed of nanocrystals, their size depending on the conditions of film deposition; 2) single crystalline substrate favors formation of oriented crystals, i.e., of the domains comprising uniformly oriented nanocrystals.     

Synthesis of colloidal Mn2+:ZnO quantum dots and high-TC ferromagnetic nanocrystalline thin films

We report the synthesis of colloidal Mn(2+)-doped ZnO (Mn(2+):ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn(2+):ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)(2) was found to strongly inhibit oxidation of Mn(2+) by O(2), allowing the synthesis of Mn(2+):ZnO to be performed aerobically. Mn(2+) ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn(2+):ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn(2+) with very homogeneous speciation, differing from bulk Mn(2+):ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 micro(B)/Mn(2+) and T(C) > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn(2+):ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.     

Interfacial tension and spreading coefficient for thin films

We present a modified mathematical expression for the interfacial tension of very thin films. At large thicknesses the modified expression converges to the classic mathematical expression. We use this modified expression to derive an equation for the spreading coefficient as a function of film thickness. The spreading coefficient equation is then used to calculate the equilibrium thickness of a wetting liquid film for a "pancake drop". Our predictions agree with experimental data. The study is subjected to systems with van der Waals interactions only.     

Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.     

Interfacial effects on moisture absorption in thin polymer films

Moisture absorption in model photoresist films of poly(4-hydroxystryene) (PHOSt) and poly(tert-butoxycarboxystyrene) (PBOCSt) supported on silicon wafers was measured by X-ray and neutron reflectivity. The overall thickness change in the films upon moisture exposure was found to be dependent upon the initial film thickness. As the film becomes thinner, the swelling is enhanced. The enhanced swelling in the thin films is due to the attractive nature of the hydrophilic substrate, leading to an accumulation of water at the silicon/polymer interface and subsequently a gradient in concentration from the enhancement at the interface to the bulk concentration. As films become thinner, this interfacial excess dominates the swelling response of the film. This accumulation was confirmed experimentally using neutron reflectivity. The water rich layer extends 25 +/- 10 A into the film with a maximum water concentration of approximately 30 vol %. The excess layer was found to be polymer independent despite the order of magnitude difference in the water solubility in the bulk of the film. To test if the source of the thickness dependent behavior was the enhanced swelling at the interface, a simple, zero adjustable parameter model consisting of a fixed water rich layer at the interface and bulk swelling through the remainder of the film was developed and found to reasonably correspond to the measured thickness dependent swelling.     

Defect and transport properties of nanocrystalline ceramics and thin films

This paper presents an overview of recent research on the defect and transport properties of nanocrystalline ionic and mixed conducting ceramics and thin films. In the first part, some basic concepts and properties of boundaries are reviewed, including diffusion, segregation, and space charge regions. Experimental data on nanoceramics and thin films made from pure and doped CeO₂, TiO₂, ZrO₂, and CaF₂ are presented and discussed in the second part; opportunities for future work on this topic are outlined. Electronic Publication     

A study of cadmium sulfide nanocrystalline films by grazing incidence X-ray diffraction

Thin cadmium sulfide films were prepared on a monocrystalline-crystal silicon substrate by chemical deposition from aqueous solutions. Grazing incidence X-ray diffraction revealed that the cadmium sulfide films are comprised of nanocrystal particles, with 80% of the particles having a size of 5 ± 1 nm. Some nanocrystals have a wurtzite structure, while others, a sphalerite one. The presence of cubic phase in the films is indicative of a nonequilibrium state of the nanocrystalline films. Thirty minutes after the onset of the formation of cadmium sulfide, the size and crystal structure of the constituent particles of the film become independent of the deposition time—only the film thickness increases. In addition, the initial stage of the formation of the cadmium sulfide film is accompanied by the deposition of cadmium hydroxide Cd(OH)₂.     

Semiconducting thin films of zinc selenide quantum dots

A novel chemical route for deposition of zinc selenide quantum dots in thin film form is developed. The deposited films are characterized with very high purity in crystallographic sense, and behave as typical intrinsic semiconductors. Evolution of the average crystal size, lattice constant, lattice strain and the optical properties of the films upon thermal treatment is followed and discussed. The band gap energy of as-deposited ZnSe films is blue-shifted by ≈0.50 eV with respect to the bulk value, while upon annealing treatment it converges to 2.58 eV. Two discrete electronic states which originate from the bulk valence band are observed in the UV-VIS spectra of ZnSe 3D quantum dots deposited in thin film form via allowed electronic transitions to the 1S electronic state arising from the bulk conduction band—appearing at 3.10 and 3.50 eV. The splitting between these two states is approximately equal to the spin-orbit splitting in the case of bulk ZnSe. The electronic transitions in the case of non-quantized annealed films are discussed in terms of the direct allowed band-to-band transitions with the spin-orbit splitting of the valence band of 0.40 eV. The effective mass approximation model (i.e., the Brus model) with the static relative dielectric constant of bulk ZnSe fails to predict correctly the size dependence of the band gap energy, while only a slight improvement is obtained when the hyperbolic band model is applied. However, when substantially smaller value for ε_r (2.0 instead of 8.1) is used in the Brus model, an excellent agreement with the experimental data is obtained, which supports some earlier indications that the quantum dots ε_r value could be significantly smaller than the bulk material value. The ionization energy of a deep donor impurity level calculated on the basis of the temperature dependence of the film resistivity is 0.82 eV at 0 K.     

Electrodeposition and characterization of copper sulfide (CuS) thin film: towards an understanding of the growth mechanism

Journal of Solid State Electrochemistry - Copper sulfide (CuS) thin film was electrodeposited onto stainless steel (SS 316L) substrate under pulse potential control, from an aqueous acidic solution...     

Interfacial electron transfer in dye sensitised nanocrystalline TiO2 films

Sub-picosecond transient absorption study has been employed to study the electron transfer kinetics in the dye-sensitized TiO₂ films used in commercial photovoltaic devices. The electron injection in these dye sensitized films occurs on an ultrafast time scale with two components, 150 fs and 1·2 ps.     

Bismuth sulfide thin films with low resistivity on self-assembled monolayers

Using self-assembled monolayers (SAMs), highly crystalline bismuth sulfide thin films with low electrical resistivity have been prepared from aqueous solution at low temperature (40-70 degrees C). The nucleation and growth process of Bi2S3 thin films was investigated in detail by XPS, AES, SEM, XRD, SAED, and HRTEM. Solution conditions have marked effects on the microstructure, growth rate, and mechanism of Bi2S3 films. Increased solution temperature resulted in a higher growth rate and a shorter induction time due to a higher supersaturation degree. In the solution of pH 1.12, homogeneous nucleation and the attachment process dominated the formation of Bi2S3 films. In contrast, at pH 0.47 Bi2S3 thin films were formed via heterogeneous nucleation and growth. The c-axial orientation of bismuthinite films was enhanced with the increase of reaction time. By controlling the solution supersaturation and reaction duration, highly crystalline Bi2S3 films composed of closely packed and coalescent crystallites could be realized, whose dark electrical resistivity could reach as low as 0.014 Omega cm without any post-treatment.     

Uniformity analysis in nanocrystalline silver thin films using fuzzy inference system

An automatic and efficient technique to analyze the uniformity of nanoscale images using wavelets, feature similarity index measure (FSIM) and fuzzy inference system is reported. It has been successfully tested on scanning electron micrographs of nanocrystalline silver thin films. Thin films are prepared using on‐axis and off‐axis pulse laser deposition (PLD) technique. It is found that the film prepared using on‐axis PLD is more uniformly distributed and has smoother texture compared with that of the off‐axis technique. In order to analyze the images quantitatively, they are transformed to the wavelet domain to extract the localized frequency variations and a uniformity measure is derived using a fuzzy inference system for quantitatively analyzing the uniformity of each image. The surface plot of the FSIM values of the image is found to be an efficient tool for nanoscientists to evaluate the smoothness of the thin film surfaces. This study is expected to help the nanoscientists to understand these nanostructures in detail. Copyright © 2014 John Wiley & Sons, Ltd.     

Rate constant of free electron-hole recombination in thin cadmium sulfide films

The transient decay kinetics of electrons generated in thin cadmium sulfide films by short laser pulses was studied by the microwave photoconductivity method (9 and 36 GHz) at 295 K. The films were prepared by the pulverization method from thiocarbamide coordination compounds. At the high light intensity I ₀ > 10¹⁴ photon cm^–2 per pulse, the decay kinetics of photoelectrons corresponded to a reaction of the second order. Analysis of the kinetic data made it possible to determine the rate constant of recombination of free electrons and holes: k ₃ 2(±1)·10^–13 cm³ s^–1.     

Structural and optical characterization of thermally evaporated cadmium sulfide thin films

The article reports the structural and optical properties of vacuum‐evaporated cadmium sulfide (CdS) films with different thicknesses at room temperature. The structural investigations performed by means of X‐ray diffraction (XRD) technique have showed that all the films have the zinc‐blende structure, a face‐centered cubic form with lattice constants a = b = c = 5.82 Å and point group F4 3m. Crystallite sizes calculated from Scherrer relation are in the range of 173–345 Å. So far, because the optical parameters of the metastable cubic CdS have not been so well known, we apply spectroscopic ellipsometry to determine the thickness, optical constants and energy band gap of CdS thin film deposited by thermal evaporation onto opaque gold substrate, a perfect reflectivity and inert metal. As shown the measured spectral behavior of the optical constants and the band gap value of CdS thin film are in agreement with those obtained by the reflectance and transmittance methods. The energy band gap of CdS thin film determined from the spectral behavior of the absorption coefficient is about 2.46 eV. Copyright © 2008 John Wiley & Sons, Ltd.     

Solvent dependent growth of fibrous and non-fibrous nanocrystalline thin films of ZnO

Zinc oxide thin films were prepared from three different solvents using the sol–gel technique. Zinc acetate was used as the source of Zn, and the solvents ethanol, 2-methoxy ethanol and ethylene glycol were used to prepare the sols. ZnO thin films were deposited on glass microslides by pre-heating dip coated sol layers, following which they were finally annealed at 450 °C for half an hour. The films were characterized using structural, morphological and optical techniques. XRD studies show that the films grown from all the three solvents were single phase, highly oriented (along the c axis) ZnO having the wurzite structure. Optical transmission and photoluminescence spectra confirm the good quality of the ZnO films. SEM and AFM images show that the surfaces of the ZnO films, obtained using the first two (more volatile) solvents, consist of striations or ridges of height around 100–400 nm and are made up of nanoparticles 20–40 nm in size. The surfaces of the films produced from the less volatile third solvent are however smooth and devoid of striations although they are also covered with nanoparticles.     

Electrochemical behavior of nanocrystalline diamond thin films grown in electrical arc plasma

Nitrogenated nanocrystalline diamond films with controlled electrical conductivity are grown in electrical arc plasma in CH₄/H₂/Ar/N₂ gas mixtures and characterized by scanning electron microscopy and spectroscopic measurements. Their electrochemical properties are studied by electrochemical impedance spectroscopy. Transfer coefficients of reactions in the [Fe(CN)₆]^3−/4− redox system are determined. The electrochemical behavior of the material is controlled by its nitrogenation (3–20% N₂ in the reaction gas mixture). The nitrogenated nanocrystalline diamond has higher differential capacitance in indifferent electrolyte (1 M KCl) solution than not nitrogenated one; the nitrogenation also increases the reversibility of reactions in the [Fe(CN)₆]^3−/4− redox system. By and large, with nitrogenation of diamond, its electrochemical behavior changes from the one characteristic of a “poor conductor” to that characteristic of metallike conductor. In this respect the nanocrystalline diamond electrodes grown in the electrical arc plasma are similar to those grown in microwave plasma.