Surface Passivation Layer Research Articles

Tribo-corrosion response of additively manufactured high-entropy alloy

High-entropy alloys (HEAs) with multiple principal elements represent a paradigm shift in structural alloy design and show excellent surface degradation resistance in corrosive environment. Here, the tribo-corrosion response of laser-engineered net-shaped CoCrFeMnNi HEA was evaluated in 3.5 wt% NaCl solution at room temperature. The additively manufactured (AM-ed) CoCrFeMnNi showed five times lower wear rate, regenerative passivation, and nobler corrosion potential during tribo-corrosion test compared to its arc-melted counterpart. A significant anisotropy was seen in the tribo-corrosion response with 45° to the build direction showing better performance compared to tests along the build direction and perpendicular to it. The open circuit potential curves were characterized by a sharp drop to more negative values as wear began, followed by continuous change for the active tribo-corrosion duration and finally a jump to nobler value at the end of the test indicating excellent surface re-passivation for the AM-ed alloy. The superior tribo-corrosion resistance of AM-ed CoCrFeMnNi was attributed to the refined microstructure and highly protective surface passivation layer promoted by the sub-grain cellular structure formed during additive manufacturing. These results highlight the potential of utilizing additive manufacturing of HEAs for use in extreme environments that require a combination of tribo-corrosion resistance, mechanical durability, extended service life, and net shaping with low dimensional tolerance.

Fabricating efficient and stable quasi-3D and 3D/2D perovskite solar cells with 2D-sheets connected by inorganic type ionic-bond

In this work, we report a novel two-dimensional (2D) (SrBr)2PbI4 perovskite layered architecture, which were formed by the reaction of strontium bromide (SrBr2), strontium iodide (SrI2) and lead iodide (PbI2). Formation of 2D (SrBr)2PbI4 was verified by small angle XRD peak at 6.4°, which corresponds to the layer distance of 13.78 Å. The best one of solar cells fabricated with Quasi-3D perovskite, (SrBr)2FA59Pb60I181 (n = 60), showed a power conversion efficiency (PCE) of 18.46% and retained 95% of the initial PCE at 1000 h in the dry air. Further, the 2D (SrBr)2PbI4 as the surface passivation layer on the 3D FAPbI3 perovskite greatly reduced the defects at the perovskite/Spiro-OMeTAD interface, and the corresponding solar cells with FAPbI3/(SrBr)2PbI4 3D/2D structure achieved a PCE of 22.14% and over 90% retention of the original PCE at 1000 h. In short, this work provides an example of inorganic complex cations that can form 2D perovskites and achieve perovskite solar cells with high PCE and stabilization at the same time.

Prospects for single-molecule electrostatic detection in molecular motor gliding motility assays

Molecular motor gliding motility assays based on myosin/actin or kinesin/microtubules are of interest for nanotechnology applications ranging from cargo-trafficking in lab-on-a-chip devices to novel biocomputation strategies. Prototype systems are typically monitored by expensive and bulky fluorescence microscopy systems. The development of integrated, direct electric detection of single filaments would strongly benefit applications and scale-up. We present estimates for the viability of such a detector by calculating the electrostatic potential change generated at a carbon nanotube transistor by a motile actin filament or microtubule under realistic gliding assay conditions. We combine this with detection limits based on previous state-of-the-art experiments using carbon nanotube transistors to detect catalysis by a bound lysozyme molecule and melting of a bound short-strand DNA molecule. Our results show that detection should be possible for both actin and microtubules using existing low ionic strength buffers given good device design, e.g., by raising the transistor slightly above the guiding channel floor. We perform studies as a function of buffer ionic strength, height of the transistor above the guiding channel floor, presence/absence of the casein surface passivation layer for microtubule assays and the linear charge density of the actin filaments/microtubules. We show that detection of microtubules is a more likely prospect given their smaller height of travel above the surface, higher negative charge density and the casein passivation, and may possibly be achieved with the nanoscale transistor sitting directly on the guiding channel floor.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

One of the probable reasons behind the limitation of light extraction efficiency (LEE) in III-nitride nanowire (NW) deep ultraviolet (UV) light-emitting diodes (LEDs) is the presence of the high surface density states that significantly contribute to the non-radiative recombination near the surface. Herein, we investigate the LEE of a single AlInN NW UV LED in the entire UV wavelength regime using finite-difference time-domain simulations. It is found that these LEDs favor transverse-magnetic (TM) polarized LEE over transverse-electric (TE) polarized emission. Further, we examine the role of the HfO2 surface passivation layer in the improvement of LEE of AlInN NW LED at ~282 nm wavelength. Our results show that the TM-polarized LEE of such LED without any passivation is only ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 40 nm HfO2 passivation layer. This study provides a promising approach for enhancing the LEE of NW UV LEDs.

Unravelling the essential difference between TiOx and AlOx interface layers on Ta3N5 photoanode for photoelectrochemical water oxidation

Tantalum nitride (Ta3N5) is a very promising photoanode material due to its narrow band gap (2.1 eV) and suitable band alignment for solar water splitting. However, it suffers from severe photocorrosion during water oxidation. In this work, it was found that surface passivation by AlOx and TiOx layers results in dramatically different PEC performance of Ta3N5 photoanode for water oxidation. The mechanism study indicates that the negative charges on AlOx can generate additional field to promote separation of photogenerated charges, while the positive charges on TiOx layer show the opposite effect. As a result, the Ta3N5 based photoanode modified with AlOx layer gives a high photocurrent of 12.5 mA cm−2 for 24 h at 1.23 V versus the reversible hydrogen electrode (RHE). Dynamic analysis implies that the hole extraction and transfer are significantly improved by the modification with the AlOx layer. This work reveals the importance of the charges on surface passivation layer in interface engineering of photoelectrodes.

Quantum dot sensitized solar cells using type-II CdSe-Cu2Se core-shell QDs

Oleylamine capped CdSe-Cu2Se type-II core–shell quantum dots were synthesised by organometallic high temperature route. The nanostructures were characterized using XRD, XPS, HRTEM, optical absorption and EDS and their suitability to serve as an effective sensitizer for TiO2 deposited fluorine doped tin oxide coated glass slides was explored. Employing aqueous sulfide/polysulfide redox couple as the electrolyte, Cu2S as the counter electrode, ZnS as the surface passivation layer and TiO2 as the anode material, quantum dot sensitized solar cells were constructed. A power conversion efficiency of 4.3% was observed for the best performing device. Sintering of the photoanode, before embedding in the cell structure, resulted in an efficiency enhancement to 4.83%.

Theoretical modelling and device structure engineering of kesterite solar cells to boost the conversion efficiency over 20%

Among the several obstacles which impede efficiency enhancement of CZTS solar cells, the sub-optimized Mo back contact and the unfavorable conduction band offset at CZTS/CdS interface are the critical ones, contributing largely to high surface recombination and low open circuit voltage. In this article, a numerical simulation model was used for device structure analysis and performance optimization. A substrate configuration of CZTS solar cell was considered. Benchmarking study of the reference cell gives a conversion efficiency of 8.3% which matches well with the reported experimental value, validating the accuracy of our simulation model. Thereafter, we have implemented several modifications in the experimental device structure including an alternative back contact, a back surface passivation layer, a Cd-free buffer layer with tunable conduction band minimum, and a wide band gap ZnO-based alloy material which enhances the spectral response to shorter wavelength contributing to higher efficiency. A systematic optimization of device structure results in the increase of JSC and VOC due to the increment in diffusion length and optimal free carrier collection which ultimately increases the power efficiency from 8.4 to 20.6%.

Novel cost-effective approach to produce nano-sized contact openings in an aluminum oxide passivation layer up to 30 nm thick for CIGS solar cells

This work presents a novel method of local contact openings formation in an aluminum oxide (Al2O3) rear surface passivation layer by the selenization of the lithium fluoride (LiF) salt on top of the Al2O3 for ultra-thin copper indium gallium (di)selenide (CIGS) solar cells (SCs). This study introduces the potentially cost-effective, fast, industrially viable, and environmentally friendly way to create the nano-sized contact openings with the homogeneous distribution in the thick, i.e. up to 30 nm, Al2O3 passivation layer. The passivation layer is deposited by atomic layer deposition, while the LiF layer is spin-coated. Selenization is done in the H2Se atmosphere and the optimal process parameters are deduced to obtain nano-sized and uniformly allocated openings as confirmed by scanning electron microscopy images. The contact openings were produced in the different thicknesses of the alumina layer from 6 nm to 30 nm. Furthermore, the Al2O3 rear surface passivation layer with the contact openings was implemented into ultra-thin CIGS SC design, and one trial set was produced. We demonstrated that the created openings facilitate the effective current collection through the dielectric Al2O3 layer up to 30 nm thick. However, the upper limit of Al2O3 thickness in which the contact openings can be created by the described method is not established yet. The produced passivated CIGS SCs show increased external quantum efficiency response due to the optical enhancement of the passivated cells. However, the production of SCs on the Al2O3 passivation layer with the openings created by selenization of LiF is not optimized yet.

Intrinsic Amorphous Silicon Bilayers for Effective Surface Passivation in Silicon Heterojunction Solar Cells: A Comparative Study of Interfacial Layers

The impact of intrinsic amorphous silicon bilayers in amorphous silicon/crystalline silicon (a‐Si:H/c‐Si) heterojunction solar cells is investigated. Intrinsic a‐Si:H films with a wide range of film densities and hydrogen contents are prepared via a plasma‐enhanced chemical vapor deposition (PECVD) technique by modifying various process parameters. For silicon heterojunction (SHJ) solar cells with a‐Si:H films applied as single i‐layers, the resulting surface passivation at the a‐Si:H/c‐Si interface is poor. However, surface passivation is significantly improved by applying intrinsic bilayers, which are composed of a porous interfacial layer (≈2 nm) and an overlying dense layer (≈8 nm). The microstructure factor R* of the interfacial a‐Si:H layer, which is related to the SiH bond microstructure and determined by infrared absorption spectroscopy, closely correlates to the surface passivation capability of the bilayers. A variety of PECVD process parameters (temperature, pressure, or precursor gas species) can be utilized to grow an interfacial layer for good surface passivation, provided that its R* is controlled within a suitable range. This indicates that R* is a key universal parameter for optimizing i‐bilayers and realizing high‐efficiency SHJ solar cells.

A more defective substrate leads to a less defective passive layer: Enhancing the mechanical strength, corrosion resistance and anti-inflammatory response of the low-modulus Ti-45Nb alloy by grain refinement

Orthopedic and dental implants made of β-type Ti alloys have low elastic modulus which can better relieve the stress shielding effects after surgical implantation. Nevertheless, clinical application of β-type Ti alloys is hampered by the insufficient mechanical strength and gradual release of pro-inflammatory metallic ions under physiological conditions. In this study, the β-type Ti-45Nb alloy is subjected to high-pressure torsion (HPT) processing to refine the grain size. After HPT processing, the tensile strength increases from 370 MPa to 658 MPa due to grain boundary strengthening and at the same time, the favorable elastic modulus is maintained at a low level of 61-72 GPa because the single β-phase is preserved during grain refinement. More grain boundaries decrease the work function and facilitate the formation of thicker and less defective passive films leading to better corrosion resistance. In addition, more rapid repair of the passive layer mitigates release of metallic ions from the alloy and consequently, the inflammatory response is suppressed. The results reveal a strategy to simultaneously improve the mechanical and biological properties of metallic implant materials for orthopedics and dentistry. Statement of significanceThe low modulus Ti-45Nb alloy is promising in addressing the complication of stress shielding induced by biomedical Ti-based materials with too-high elastic modulus. However, its insufficient strength hampers its clinical application, and traditional strengthening via heat treatments will compromise the low elastic modulus. In the current study, we enhanced the ultimate tensile strength of Ti-45Nb from 370 MPa to 658 MPa through grain-refinement strengthening, while the elastic modulus was maintained at a low value (61-72 GPa). Moreover, substrate grain-refinement has been proved to improve the corrosion resistance of Ti-45Nb with reduced inflammatory response both in vitro and in vivo. A relationship between the substrate microstructure and the surface passive layer has been established to explain the beneficial effects of substrate grain-refinement.

Moisture-Driven Degradation Pathways in Prussian White Cathode Material for Sodium-Ion Batteries.

The high-theoretical-capacity (∼170 mAh/g) Prussian white (PW), NaxFe[Fe(CN)6]y·nH2O, is one of the most promising candidates for Na-ion batteries on the cusp of commercialization. However, it has limitations such as high variability of reported stable practical capacity and cycling stability. A key factor that has been identified to affect the performance of PW is water content in the structure. However, the impact of airborne moisture exposure on the electrochemical performance of PW and the chemical mechanisms leading to performance decay have not yet been explored. Herein, we for the first time systematically studied the influence of humidity on the structural and electrochemical properties of monoclinic hydrated (M-PW) and rhombohedral dehydrated (R-PW) Prussian white. It is identified that moisture-driven capacity fading proceeds via two steps, first by sodium from the bulk material reacting with moisture at the surface to form sodium hydroxide and partial oxidation of Fe2+ to Fe3+. The sodium hydroxide creates a basic environment at the surface of the PW particles, leading to decomposition to Na4[Fe(CN)6] and iron oxides. Although the first process leads to loss of capacity, which can be reversed, the second stage of degradation is irreversible. Over time, both processes lead to the formation of a passivating surface layer, which prevents both reversible and irreversible capacity losses. This study thus presents a significant step toward understanding the large performance variations presented in the literature for PW. From this study, strategies aimed at limiting moisture-driven degradation can be designed and their efficacy assessed.

Nanostructure of Porous Si and Anodic SiO2 Surface Passivation for Improved Efficiency Porous Si Solar Cells.

The photovoltaic effect in the anodic formation of silicon dioxide (SiO2) on porous silicon (PS) surfaces was investigated toward developing a potential passivation technique to achieve high efficiency nanostructured Si solar cells. The PS layers were prepared by electrochemical anodization in hydrofluoric acid (HF) containing electrolyte. An anodic SiO2 layer was formed on the PS surface via a bottom-up anodization mechanism in HCl/H2O solution at room temperature. The thickness of the oxide layer for surface passivation was precisely controlled by adjusting the anodizing current density and the passivation time, for optimal oxidation on the PS layer while maintaining its original nanostructure. HRTEM characterization of the microstructure of the PS layer confirms an atomic lattice matching at the PS/Si interface. The dependence of photovoltaic performance, series resistance, and shunt resistance on passivation time was examined. Due to sufficient passivation on the PS surface, a sample with anodization duration of 30 s achieved the best conversion efficiency of 10.7%. The external quantum efficiency (EQE) and internal quantum efficiency (IQE) indicate a significant decrease in reflectivity due to the PS anti-reflection property and indicate superior performance due to SiO2 surface passivation. In conclusion, the surface of PS solar cells could be successfully passivated by electrochemical anodization.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation.

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm2 with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO2. Here, we applied a TiO2(B) surface passivation layer (SPL) on TiO2/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm2 under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO2 nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO2/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm2 at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

Prevalence of titanium hypersensitivity in patients with titanium implants: A systematic review and meta-analysis

Background:With increase in exposure to titanium in daily life and its use in medical procedures, there is a greater chance for developing titanium hypersensitivity. Our aim and objective is to examine whether patient patients with titanium implants had a greater incidence of titanium hypersensitivity or not. Titanium is widely considered to have good biocompatibility. A passivating surface layer of TiO2 is formed when exposed to air which gives it good corrosion resistance. These properties along with its high strength, lightweight, and improvements in smelting technologies have led to its widespread use as an intraosseous and periosteal implant in dentistry and orthopedics.Materials and Methods:Only case–control studies were used for the meta-analysis. Studies with a sample size of <5 were excluded. Cutaneous and epicutaneous tests were used in the study.Results:Three studies met the inclusion criteria. Titanium hypersensitivity is more prevalent in patients with titanium implants.Conclusions:It is also used in a variety of medical devices such as pacemakers and insulin pumps. Moreover, our external exposure to titanium is increasing. It is used in aerospace, chemical, and automobile industries. It is also used in jewelry, home furnishings, and in making sports and marine equipment. Titanium implants increase the possibility of developing titanium hypersensitivity, and thus, it should be considered as a major factor in unexplained implant failure where other causes have been excluded.

Single-Source Thermal Evaporation Growth and the Tuning Surface Passivation Layer Thickness Effect in Enhanced Amplified Spontaneous Emission Properties of CsPb(Br0.5Cl0.5)3 Perovskite Films.

High-quality inorganic cesium lead halide perovskite CsPb(Br0.5Cl0.5)3 thin films were successfully achieved through evaporation of the precursors and deposition sequentially by a single-source thermal evaporation system. The different melting points of the precursors were enabled us to evaporate precursors one by one in one trip. The resulting films through its fabrication were smooth and pinhole-free. Furthermore, this technique enabled complete surface coverage by high-quality perovskite crystallization and more moisture stability oppositely of that produce by solution-processed. Then the perovskite films were encapsulated by evaporated a polymethyl methacrylate (PMMA) polymer as a specialized surface passivation approach with various thicknesses. The blue emission, high photoluminescence quantum yield (PLQY), stable, and low threshold of amplified spontaneous emission (ASE) properties of CsPb(Br0.5Cl0.5)3 films in the bulk structure at room temperature were achieved. The effects of the surface-passivation layer and its thickness on the optical response were examined. Detailed analysis of the dependence of ASE properties on the surface passivation layer thickness was performed, and it was determined this achieves performance optimization. The ASE characteristics of bare perovskite thin film were influenced by the incorporation of the PMMA with various thicknesses. The improvement to the surface layer of perovskite thin films compared to that of the bare perovskite thin film was attributed to the combination of thermal evaporation deposition and surface encapsulation. The best results were achieved when using a low PMMA thickness up to 100 nm and reducing the ASE threshold by ~11 μJ/cm2 when compared with free-encapsulation and by ~13 μJ/cm2 when encapsulation occurs at 200 nm or thicker. Compared to the bare CsPb(Br0.5Cl0.5)3, ASE reduced 1.1 times when the PMMA thickness was 100 nm.

On high-temperature evolution of passivation layer in Li-10 wt % Mg alloy via in situ SEM-EBSD.

Li-10 wt % Mg alloy (Li-10 Mg) is used as an anode material for a solid-state battery with excellent electrochemical performance and no evidence of dendrite formation during cycling. Thermal treatment of Li metal during manufacturing improves the interfacial contact between a Li metal electrode and solid electrolyte to achieve an all solid-state battery with increased performance. To understand the properties of the alloy passivation layer, this paper presents the first direct observation of its evolution at elevated temperatures (up to 325°C) by in situ scanning electron microscopy. We found that the morphology of the surface passivation layer was unchanged above the alloy melting point, while the bulk of the material below the surface was melted at the expected melting point, as confirmed by in situ electron backscatter diffraction. In situ heat treatment of Li-based materials could be a key method to improve battery performance.

Resolving Heterogeneity in Battery Interphases By Cryogenic Electron Microscopy

The stability of lithium batteries is tied to the physicochemical properties of the solid-electrolyte interphase (SEI), a surface passivation layer on the battery anode. However, owing to the difficulty in characterizing this sensitive interphase with nanoscale resolution, the distribution of SEI components at the nanoscale is poorly understood. Here, we use cryogenic scanning transmission electron microscopy (cryo-STEM) and electron energy loss spectroscopy (EELS) to map the spatial distribution of sensitive SEI components across the metallic Li anode. We reveal that LiF, an inorganic SEI component widely believed to play an important role in battery passivation, is absent within the compact SEI film (~15 nm) in direct contact with the active material. Instead, LiF particles (100-400 nm) are found to precipitate across the electrode surface independently from compact SEI formation. Based on these observations, we conclude that LiF cannot be a dominant contribution to anode passivation nor does it influence Li+ transport across the compact SEI film. Using cryo-STEM and EELS, we refine the traditional view of the SEI structure derived from ensemble-averaged characterizations and nuance the role of SEI components on battery performance.

In Situ Formation of Mixed‐Dimensional Surface Passivation Layers in Perovskite Solar Cells with Dual‐Isomer Alkylammonium Cations

Dimensional engineering of perovskite solar cells has attracted significant research attention recently because of the potential to improve both device performance and stability. Here, a novel 2D passivation scheme for 3D perovskite solar cells is demonstrated using a mixed cation composition of 2D perovskite based on two different isomers of butylammonium iodide. The dual-cation 2D perovskite outperforms its single cation 2D counterparts in surface passivation quality, resulting in devices with an impressive open-circuit voltage of 1.21V for a perovskite composition with an optical bandgap of ≈1.6eV, and a champion efficiency of 23.27%. Using a combination of surface elemental analysis and valence electron spectra decomposition, it is shown that an in situ interaction between the 2D perovskite precursor and the 3D active layer results in surface intermixing of 3D and 2D perovskite phases, providing an effective combination of defect passivation and enhanced charge transfer, despite the semi-insulating nature of the 2D perovskite phase. The demonstration of the synergistic interaction of multiple organic spacer cations in a 2D passivation layer offers new opportunities for further enhancement of device performance with mixed dimensional perovskite solar cells.

Improved Electrical and Deep-UV Sensing Characteristics of Al2O3-Dielectric AlGaN/AlN/SiC MOS-HFETs

Improved electrical and deep-UV sensing characteristics of Al2O3-dielectric Al0.75Ga0.25N/n-AlxGa1−xN/Al0.75Ga0.25N/AlN metal-oxide-semiconductor hetero-structure field-effect transistors (MOS-HFETs), grown on a SiC substrate, with an AlGaN ultra-widegap channel design are investigated. 30 nm thick high-k Al2O3 was deposited as both the gate oxide and surface passivation layer by using a non-vacuum ultrasonic spray pyrolysis deposition (USPD) method. Improved device characteristics, including maximum drain-source current density (IDS,max) of 130.1 mA mm−1, maximum extrinsic transconductance (gm,max) of 11.8 mS mm−1, on/off-current ratio (Ion/Ioff) of 1.4 × 107, gate-voltage swing (GVS) linearity of 5.8 V, two-terminal off-state gate-drain breakdown voltage (BVGD) of −404 V, and three-terminal on-state drain-source breakdown voltage (BVDS) of 364 V at 300 K are obtained for the present MOS-HFET. The device has demonstrated high spectral responsivity (SR) of 737 A W−1 under 250 nm deep-UV radiation.

An Oxidative Chloride Stripping Solution for 14K Gold Alloys

We propose a novel stripping solution containing acids (HCl and HNO3), an oxidant [(NH4)2S2O8], and complexing agents (NaCl and citric acid) to remove surface passivation layers from 14K gold alloys fabricated using an investment casting process. The optimized solution employing only HCl acid is determined by varying molar fractions of HCl and HNO3 on 14K yellow gold samples. Stripping properties are also identified for red and white gold alloy samples under the optimized stripping conditions. The removal of passivation layers, weight loss, and microstructure evolution are characterized using Raman spectroscopy, a precision scale, and optical microscopy. The proposed stripping solution effectively removes passivation layers more rapidly than conventional cyanide stripping. Weight loss increases linearly for up to 5 min for all 14K gold alloys. Red gold exhibits the greatest weight loss, followed by yellow gold and white gold. The results of microstructural analysis reveal that the conformal stripping occurs according to time. These results imply that the proposed oxidative chloride stripping might replace conventional cyanide stripping.