Surface Review and Letters

Dye sensitized solar cells (DSSCs) provide promisingly, organic–inorganic, clean hybrid, cost effective and efficient molecular solar cell devices. Due to their distinct and multifunctional qualities, zinc oxide (ZnO) nanostructures are promising materials used to create photoanodes for DSSCs due to the availability of larger surface area than bulk sheet substance, effectual light-dispersing centers, and when mixed with titanium dioxide they produce a core–shell formation that diminishes the coalition rate and provide direct charge. Moreover, ZnO thin sheets have been broadly observed due of its potential application in various fields i.e. piezoelectric, photovoltaic, pyroelectric and optoelectronic utilization. This review studies the recent advances in the fabrication of zinc oxide-based photovoltaics; synthesis of ZnO nanostructures with variable morphologies including thin sheets, nanotubes, nanorods, nanoflowers, nanofibers and factors that control the growth and morphologies of these nanospecies and part of crystallographic planes for the fabrication of various zinc oxide nanoshapes. In the next part of this paper, numerous fabrication routes — doped and undoped ZnO thin films — are discussed and different parameters of photovoltaics are investigated, e.g. efficiency pre and post annealing temperatures, fill factors spinning speed and coating time, additives, nature of precursor which impacts on morphological and optical parameters of these sheets. In short, this review is dedicated to the ZnO photoanode, its properties, issues related to ZnO photoanode, various improvement approaches, fabrication methods successfully trialled so far followed by market potential of the DSSC technology, conclusion and recommendations

The chemical etching of the surface of silicon wafers is a critical step in the manufacturing process of all semiconductor devices. In this contribution, we investigate the effect of alkaline etching on minority carrier lifetime and interface-states density ($D_{\mathrm{\text{it}}})$ of silicon wafers intended to be used as solar cell substrates. After alkali treatment, the surface morphology was analyzed using scanning electron microscopy (SEM) and UV-visible-NIR optical spectroscopy. Besides and as electrical characterizations, the minority charge carrier lifetime (${\tau }_{\mathrm{\text{n}}})$ was measured by the Quasi-Steady State Photoconductance technique (QSSPC), and the Electrochemical Impedance Spectroscopy was used to evaluate $D_{\mathrm{\text{it}}}$. These results were correlated with the surface recombination velocity (SRV) calculated by fitting the experimental data to the theory. The results of characterization showed a lower SRV and a higher apparent lifetime (${\tau }_{\mathrm{\text{app}}})$ obtained with 23wt.% KOH etching as compared to those obtained with 30wt.% NaOH; viz. 825cm$\cdot {\mathrm{\text{s}}}^{-1}$ against 1500cm.s${}^{-1}$ and 32$\mu$s against 23$\mu$s, respectively. These findings were corroborated by $D_{\mathrm{\text{it}}}$ measurements which gave $1.55\times 10^{11}$ev${}^{-1}$cm${}^{-2}$ for KOH treatment and $5.67\times 10^{12}$ev${}^{-1}$cm${}^{-2}$ for NaOH treatment.

A theoretical study was carried out of CO₂ adsorption on Cu_mCo_n(2$\le m+n\le$7) clusters using density functional method. Generally CO₂ are located at top or bridge sites, while CO₂ of Cu₂Co₄CO₂, Co₆CO₂ and Cu₂Co₅CO₂ clusters are absorbed at hollow sites. Co₃CO₂ and CuCo₂CO₂ clusters are more stable than their neighbors, while Cu₂CO₂ and Cu₆CO₂ clusters display stronger chemical stability. After adsorption, CO₂ is activated with the elongation of the C–O bond owing to electrons transfer from Cu–Co clusters to 2${\pi }_u$ anti-bonding orbit of CO₂. More charge transfer often corresponds to longer C–O bond and larger adsorption energy, and the chemical activity is stronger correspondingly. Magnetic and electronic properties are also discussed.

The wear behavior of untreated and WC-12% Co laser alloyed nodular cast iron samples were analyzed using a high temperature pin-on-disc tribometer. Response surface methodology (RSM) based Box–Behnken technique was employed to minimize the sum of trials and to develop a mathematical model for the input process parameters namely sliding velocity, temperature and load. The interaction effects of the process parameters were investigated using the developed model. Further, the developed model was analyzed using analysis of variance. Scanning electron microscopy and energy dispersive spectrum were used for characterization. Laser alloyed samples showed a minimum wear rate ($0.3379\times 10^{-3}$ to $0.9537\times 10^{-3}$mm³/m) and friction coefficient (0.10 to 0.39) compared to untreated samples. The results of the validation test showed that the experimental results agreed well with predicted results of the developed mathematical model.

In this research, in situ ultrafine Al–Ni intermetallics were fabricated as reinforcement in the aluminum matrix by Friction Stir Process (FSP). Activated and non-activated powders were used as reinforcement agents. The FSP was performed under the rotating speed of 1250rpm, traverse speed of 24, 32, 44 and 62mm/min to fabricate composite. Scanning Electron Microscopy, Optical Microscopy and XRD patterns were used for microstructural observations and phase analysis of fabricated composite, respectively. In this study, it is shown that by using nickel powder (non-activated), only Al₃Ni compound is formed during FSP. But in the fabricated composite with activated powder because of increasing grain boundaries and dislocation during ball milling, Al₃Ni₂, Al₃Ni and AlNi may be formed. Using activated powder creates ultrafine intermetallic reinforcements and causes more uniform distribution of intermetallics.

Electron microscopy studies were conducted for the fine structure of ion-doped layer on 30ChGSN2A steel obtained by ion implantation of monotectic tin-doped copper–lead alloy Cu64Pb36$+$Sn. Formation of a multi-level hierarchical structure was detected. The features of formation of each layer were analyzed, and it was found that the main mechanism of formation of these structures is diffusion and relaxation resulting in the occurrence of internal stresses both in the surface layer and in the sheet of 30ChGSN2A steel. Relaxation of elastic stress fields results in translational–rotational deformation forming various vortex structures.

Based on density functional theory and nonequilibrium Green’s function method, we study the magnetic order and collinear/noncollinear spin transport of 180${}^{\circ }$ domain wall (DW) made by transition metal (TM)-doped Ni atomic chain. The results show that the TM doping reproduces characteristic features depending on the type of TM elements, the number of dopants and the initial magnetization distribution. In the collinear magnetization, which is obtained from the initial condition of abrupt DW, the two dopants show symmetric features while the single dopant presents a varying magnetic moment on the TM dopant when the number of 3d electrons of dopant increases. As the magnetization is noncollinear (spiral-like), the magnetization becomes complicated. For instance, the rotation sense of magnetization changes from clockwise to counter-clockwise for some TM dopants. In addition, the transmission of doped Ni chain shows two scattering mechanisms, i.e. electronic scattering due to quasi-bound state and due to the spin-flip scattering. Our results reported here provide considerable insights into the doping effect on magnetization and spin transport of atomic-scale DW.

K_0.5Na_0.5Nbo₃ wet films were spin-coated on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method. The microwave irradiation was introduced as the annealing method. The microstructure and electric properties of KNN thin films were tested and analyzed. It was found that the KNN thin film can be well crystallized by microwave irradiation at the temperature as low as 425∘C. The KNN thin film annealed at 425∘C gains the uniform microstructure, grain refinement, better electric properties and low leakage current density.

Wurtzite nanostructures have been recently described using molecular building blocks called wurtzoids. These wurtzoids are utilized in the present work to describe aluminum mononitride (AlN) nanostructures including its surface doping with group IV elements i.e. C Si, Ge and Sn. Calculations are performed for bare, and hydrogen surface passivated wurtzoids. Results show that hydrogen-passivated (HP) AlN-wurtzoids have energy gaps that are very near to the experimental bulk AlN. Longitudinal optical (LO) vibrational frequencies are also very near to bulk experimental value with blue and red frequency shifting for bare, and hydrogen surface passivated wurtzoids. Doped AlN-wurtzoid2c with group IV elements show a reduction of the energy gap. The gap generally decreases as the atomic number of the dopant increases. The electronic and vibrational properties can be interpreted using the charge transfer between atoms. Minimum atomic charge transfer is for carbon atom doping that leads to a maximum reduction of the energy gap of bare and entirely hydrogen surface passivated wurtzoids. The doped carbon atom charge transfer also induces the highest increase in LO vibrational mode.

An experimental study on the tribology behavior and mechanism of NBR-Steel pair has been carried out. Abrasive paper was used to polish the NBR surface. The influences of surface topography on the friction coefficient were investigated based on the block-on-ring tribometer. Results show that polishing with abrasive paper is an effective method to reduce the friction coefficient of NBR on steel. Superlubricity was also found in the test. A new method to explain the superlubricity based on the contact angle and surface molecular structure was put forward in this work. Abrasive paper polishing changes the surface asperities, so affects the contact angle, so as to influence the boundary limiting shearing strength, and then causes the superlubricity. During the friction process the microstructure obviously changed, along with mechanical breaking of the molecular chain.

Layer stacking technique is incorporated in the deposition of copper aluminium oxide (Cu-Al₂O₃) thin films on Al substrate using RF magnetron sputtering. The Cu/Al₂O₃ stack is sputtered using Cu and Al₂O₃ target at ambient temperature and then annealed to yield the resultant Cu-Al₂O₃ films. The structural properties of the films are investigated through X-ray Diffraction (XRD) whereas the chemical structure of the films is studied using Fourier-transform infrared (FTIR). The thermal conductivity analyzer is used to evaluate the thermal properties of coated film on the Al substrate. XRD analysis revealed that the synthesized films are polycrystalline film composed mainly of CuAl₂O₄ phase along with Al₂O₃ and CuO phases. The thermal properties of Cu-Al₂O₃ coated Al substrates showed improvement in terms of thermal conductivity and diffusivity compared to bare Al substrate. The Cu-Al₂O₃ sample annealed at 400∘C exhibited a significant difference in thermal conductivity ($\mathrm{\Delta }k=11$W/mK) compared to bare Al. The difference in thermal conductivity displayed by the annealed sample verified that TIMs did enhance the thermal path of entire substrate by allowing the heat to dissipate to surrounding environment more efficiently, thereby improving the heat dissipation system. From the results observed, it can be concluded that Cu-Al₂O₃ coated Al substrate can be made as alternative TIM in thermal management application.

Nonhydrogen germanium carbide (Ge_xC${}_{1-x}$) films were prepared by KrF pulsed lasers deposition (PLD), and the effects of different germanium contents on the microstructures, optical properties and adhesion properties of the Ge_xC${}_{1-x}$ films were studied. XRD measurements showed that the Ge_xC${}_{1-x}$ films with different germanium contents were of amorphous structure. XPS analysis showed that the C/Ge atomic ratios of Ge_xC${}_{1-x}$ films were close to those of corresponding targets. The intensity of Ge3$d$ peak in the Ge_xC${}_{1-x}$ films increased and the intensity of C1_s peak decreased with the increase of the germanium content. The Ge_xC${}_{1-x}$ films’ $s{p}^3$C bond contents increased from 46.4% to 69.7%. Ellipsometry and infrared transmission spectral measurements showed that the refractive index of Ge_xC${}_{1-x}$ films increased from 2.5 to 3.5 at the wavelength of 1$\mu$m with the increase of germanium content. The extinction coefficient of Ge_xC${}_{1-x}$ film was of the order of 10${}^{-3}$ in the infrared band when the germanium content was less than 10at.%. The adhesion tests showed that the adhesion performances of Ge_xC${}_{1-x}$ films were better when the germanium content increased.

The main purpose of a compact TiO₂ film as a vital part of the electron transport layer in perovskite solar cells (PSCs) is to minimize electron–hole recombination on the surface of F-doped SnO₂ (FTO) and enhance the photoelectric property. By controlling concentrations and using different depressors, an excellent compact TiO₂ film is prepared by sol–gel method. This research utilized four depressors (nitric acid, acetic acid, diethanolamine and acetylacetone) and their volume ratios to ethanol are 0, 1:1 and 1:2 to synthesize the TiO₂ precursor sol. The surface morphology, ingredient and microstructure as well as the optical and electrochemical properties of PSC devices based on them are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV–visible absorption spectroscopy (UV–Vis), respectively. It is found that the device exhibited the most outstanding power conversion efficiency (PCE) of 14.92% via using diethanolamine as the depressor diluted in absolute ethanol in a proportion of 1:1 by volume. Further study showed that the enhanced PCE could be mainly ascribed to the increase of short-current density ($J_{\mathrm{\text{sc}}})$.

The present work is an effort to study the influence of shot peening on the exfoliation corrosion behavior of aluminium alloy (AA) 5083. Surface textural changes induced by shot peening was characterized using microstructural and X-ray diffraction analysis. The surface roughness parameters were measured to study the benefits of peening induced surface topography. Further, the hardness survey was carried out to assess the severe plastic deformation on the peened layers. As a result, excellent resistance against exfoliation corrosion was achieved in the chloride environment. Shot peening plays major role in enhancing the corrosion resistance of AA 5083. In the absence of exfoliation attack, the unpeened sample surfaces such as ground, milled, and as received conditions end up with a significant pitting attack. The findings of this work will be useful for the aluminium alloy fabrications involved in the marine applications.

The friction and wear behavior of the as-cast Mg–6Gd–2Zn–0.4Zr (wt.%, GZ62K) alloy has been studied under dry and simulated body fluid (SBF) conditions for orthopedic application. The results show that the friction coefficient in SBF is much lower than that under dry sliding condition. The mass loss in SBF is lower than that of dry sliding condition before the corrosion products were removed but higher after they were removed. In SBF, wear is significantly alleviated due to lubrication and corroded film, but corrosion is extremely aggravated by wear debris. It is indicated that corrosion contributes much more mass loss than wear with the increase of sliding time.

The microstructure, microhardness and tensile properties of continuously borided and interrupted borided 16MnCr5 steel were compared. In the continuous process, boriding was carried out continuously at 1273K for 16h. In the interrupted process, after 5h of boriding at 1273K, the crucible containing the specimens and the boriding powders was removed from the furnace and cooled to 873K in still air for 30min and this procedure was repeated three times. Both boriding treatments were achieved in a solid medium using the powder pack method with commercial Ekabor-I powders as the boron source. X-ray diffraction studies indicated the presence of FeB and Fe₂B phases in the boride layers for both boriding treatments. The transition zone was more pronounced in the continuously borided steel. The average hardness values of the boride layers were 1759 and 1648HV_0.1 for the continuously borided and interrupted borided steels, respectively. The hardness of the boride layers were considerably higher than the 200HV_0.1 hardness of the untreated steel. No significant difference in tensile properties could be observed between the two different boriding procedures.