Modification Of Surface States Research Articles

The effect of Laves phases and nano-precipitates on the electrochemical corrosion resistance of Mg-Al-Ca alloys under alkaline conditions

The electrochemical corrosion mechanisms of Mg alloys were extensively studied in previous investigations of different chemical compositions, modified surface states and various electrolyte conditions. However, recent research focused on the active state of Mg dissolution, leading to unresolved effects of secondary phases adjacent to a stable α-solid solution passive layer. The present study investigates the fundamental electrochemical corrosion mechanisms of three different Laves phases with varying phase morphologies and phase fractions in the passive state of Mg-Al-Ca alloys. The microstructure was characterized by (transmission-) electron microscopy and synchrotron-based transmission X-ray microscopy. The electrochemical corrosion resistance was determined with a standard three-electrode setup and advanced in-situ flow cell measurements. A new electrochemical activity sequence (C15>C36>α-Mg>C14) was obtained, as a result of a stable passive layer formation on the α-solid solution. Furthermore, nm-scale Mg-rich precipitates were identified within the Laves phases, which tend to inhibit the corrosion kinetics.

Modification of Topological Surface States in Novel Mn1 – xAxBi2Te4/MnBi2Te4 (A = Si, Ge, Sn, Pb) Topological Systems

Modification of Topological Surface States in Novel Mn1 – xAxBi2Te4/MnBi2Te4 (A = Si, Ge, Sn, Pb) Topological Systems

The effect of charged particle irradiation on the transport properties of bismuth chalcogenide topological insulators: a brief review.

Topological insulators (TIs) offer a novel platform for achieving exciting applications, such as low-power electronics, spintronics, and quantum computation. Hence, the spin-momentum locked and topologically nontrivial surface state of TIs is highly coveted by the research and development industry. Particle irradiation in TIs is a fast-growing field of research owing to the industrial scalability of the particle irradiation technique. Unfortunately, real three-dimensional TI materials, such as bismuth selenide, invariably host a significant population of charged native defects, which cause the ideally insulating bulk to behave like a metal, masking the relatively weak signatures of metallic topological surface states. Particle irradiation has emerged as an effective technique for Fermi energy tuning to achieve an insulating bulk in TI along with other popularly practiced methods, such as substitution doping and electrical gating. Irradiation methods have been used for many years to enhance the thermoelectric properties of bismuth chalcogenides, predominantly by increasing carrier density. In contrast, uncovering the surface states in bismuth-based TI requires the suppression of carrier density via particle irradiation. Hence, the literature on the effect of irradiation on bismuth chalcogenides extends widely to both ends of the spectrum (thermoelectric and topological properties). This review attempts to collate the available literature on particle irradiation-driven Fermi energy tuning and the modification of topological surface states in TI. Recent studies on particle irradiation in TI have focused on precise local modifications in the TI system to induce magnetic topological ordering and surface selective topological superconductivity. Promising proposals for TI-integrated circuits have also been put forth. The eclectic range of irradiation-based studies on TI has been reviewed in this manuscript.

High-Performance Piezophototronic Solar Cells Based on Polarization Modulation Perovskite

Polarization induced by electric field poling or strain can be used for achieving high-performance piezophototronic solar cell by tuning spatial surface potential distribution. Recent experiments presented that performance of perovskite solar cells (PSCs) can be enhanced with poling process due to surface state modification. Here, we characterize the effect of polarization on surface potential distribution of perovskite film, highlighting that piezophototronic effect provides a promising approach for enhancing the performance of PSCs. This method not only paves a new way to develop highly efficient perovskite photovoltaic devices but also designs highly stable PSCs.

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion (GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability. The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline microstructure is more complicated compared to sintered magnets. Here, we investigated the effects of different GBD methods, i.e., intergranular addition (in-situ GBD 1#), in-situ GBD from magnet surface during hot pressing and hot deformation (in-situ GBD 2#), and conventional GBD, on the magnetic properties and microstructure of hot deformed magnets. After the treatment by these three GBD approaches using 2 wt% Pr40Tb30Cu30 diffusion source, the coercivity of the hot-deformed magnet increases from 1281 to 1567, 1412 and 2022 kA/m, respectively. The coercivity enhancement is attributed to the formation of local (Nd,Tb)2Fe14B phase with strong magnetic anisotropy. Reduced grain orientation is found in both in-situ GBD 1# and conventional GBD treated samples mainly due to the local stress state variation and the rotation of platelet grains. Interestingly, the in-situ GBD 2# processed sample has a high orientation at diffusion surface, which may be caused by the modified surface state of the magnet by the diffusion source. Compared with the in-situ GBD processes, the conventional GBD exhibits a higher utilization efficiency of Tb. Since the in-situ GBD is effective to treat thick hot-deformed magnets, further effort should be aimed at enhancing its diffusion efficiency.

Room-temperature gas sensors based on titanium dioxide quantum dots for highly sensitive and selective H2S detection

Room-temperature gas sensors with high performance are crucial in practical application. This work prepared a series of controllable-size titanium dioxide quantum dots (TiO2 QDs) via a rapid microwave-assisted solvothermal method for detecting ppb-level H2S at room temperature. The effect of reaction temperature on the nanostructure and gas sensing properties was investigated systematically. The characterization results showed the samples presented ultra-small size (around 5–6 nm) nanostructure and modified surface state (including Ti3+ and oxygen defects). The S120-TiO2 QDs gas sensor showed the best sensing properties towards ppb-level H2S at room temperature, including high response (25.12 @ 500 ppb H2S), rapid response/recovery, low detecting limit level, good selectivity, and operating stability. The gas sensing mechanism was explained through the synergetic effect of ultra-small size nanostructure, large specific surface area, and surface defects, which are in favor of improving the surface activity and charge transfer efficiency. Moreover, the practical application of TiO2 QDs gas sensor in detecting the volatiles of fish (Pangasius) was also verified. This work developed a room temperature TiO2 quantum dots-based gas sensor, which is promised for practical application because of the rapid synthesis and high performance.

Tunable characteristics of porous silicon optical microcavities by energetic N ion beam interactions

The present study demonstrates the tuning of optical characteristics of porous silicon (PSi)-based microcavities by N ion beam interactions. These optical microcavities are prepared by using electrochemical etching of heavily doped p+-type Si. The PSi microcavities were exposed to N ions of 200 keV and 1 MeV at an optimized ion fluence of 1 × 1015 ions cm−2. A significant red-shifting of 32 ∼ 60 nm in the resonance cavity mode was observed due to ion interaction. The experimental results are in good agreement with the transfer matrix simulations. A substantial modification of the PSi microcavity surface states is visualized through Raman and x-ray photoelectron spectroscopy (XPS) techniques. The Raman spectral results show modifications from crystalline Si to nanostructured Si and subsequently to amorphous Si. The XPS indicates the modification of Si–Si and Si–O bonds and the formation of new Si–N bonds, implying the presence of Si3N4. These experimental observations, along with analytical simulations and transfer-matrix method microcavity modeling, conclusively support the realization of cavity tunability and substantial modification in the optical field intensity and photon confinement within the spacer layer of the microcavity. These results suggest that ion beams are the effective tool to produce wider tunable optical properties in microcavities with highly stable designer optical structures suitable for photonic applications.

Interface-driven electrical magnetochiral anisotropy in Pt/PtMnGa bilayers

Nonreciprocal charge transport, which is frequently termed as electrical magnetochiral anisotropy (EMCA) in chiral conductors, touches the most important elements of modern condensed matter physics. Here, we have investigated the large EMCA in Pt/PtMnGa (PMG) bilayers, which can be attributed to the nonreciprocal response of an interface-driven chiral transport channel. Different from the traditional linear current-dependent EMCA, for Pt/PMG bilayers, higher-order EMCA coefficients should be phenomenally added especially for the small current region. This unusual behavior has been explained based on both quantum transport and semiclassical transport models. Furthermore, a combination of asymmetrical electron scattering and spin-dependent scattering furnish the PMG thickness-dependent chiral transport behaviors in Pt/PMG bilayers. The dramatically enhanced anomalous Hall angle of PMG further demonstrates the modified surface state properties by strong spin–orbit coupling.

Controlling nonreciprocal surface states in magneto-optic photonic crystals by using monolayer graphene

We have theoretically investigated the nonreciprocal surface states created at the interface between the semi-infinite one-dimensional magneto-optic photonic crystal in Voigt configuration and air in the presence of monolayer graphene as a cap layer, using the transfer matrix method. The modification of surface states was studied by changing the Fermi energy of the graphene sheet. The results indicate that the nonreciprocality of surface states could be effectively controlled by Fermi energy. We also analyzed the effect of the termination layer of photonic crystal on surface states and found that the arrangement with termination layer of magnetized semiconductor supports unidirectional surface states.

Concentration dependent thermo-optical properties and nonlinear optical switching behavior of bimetallic Au-Ag nanoparticles synthesized by femtosecond laser ablation

Concentration, morphology and composition-dependent thermal diffusivity changes occurring in bimetallic Au-Ag nanoparticles synthesized by femtosecond laser ablation were investigated using a dual- beam mode matched thermal lens technique. The synthesis utilized the variation in ablation time of silver plate kept immersed in already synthesized gold nanoparticle seeds. Thermal diffusivity values were found to increase both with an increase in concentration of the samples as well as an increase in Ag/Au weight percentage ratio. Au-Ag core–shell and nanoalloy structure show much greater variation in the thermal diffusivity compared to the other samples. A possible explanation could be the modified surface state density of the bimetallic nanoparticles. A single beam z scan technique using an Nd:YAG laser operating at a wavelength of 532 nm was employed for the nonlinear optical characterization of the samples. At a fixed laser input fluence, a switching behavior from saturable absorption (SA) to reverse saturable absorption (RSA) was observed as the ablation time of the silver plate in gold nanoparticle solution increased. The effect of laser input fluence on nonlinear optical properties of core–shell and nanoalloy structures was also studied and the optical limiting threshold of nanoalloy was calculated. The results revealed an increase in the nonlinear absorption coefficient with an increase in input fluence. A linear variation of the limiting threshold was also observed with an increase in input fluence. Thermal diffusivity studies find applications in photo thermal therapies and optical limiting properties find application in optoelectronic switches.

Very high-resolution infrared imagery of misaligned tungsten monoblock edge heating in the WEST tokamak

Fourteen ITER-like plasma-facing units (PFUs) made of bulk tungsten were installed in the WEST tokamak divertor. Five of which were misaligned with their poloidal leading edges (LEs) exposed to the plasma heat flux: two sharp-edge PFUs and three chamfered PFUs (vertical misalignment h = 0.30 ± 0.1 mm). The parallel heat flux is measured on the outer strike point with a very high-resolution (VHR) infrared camera, featuring a sub-millimeter spatial resolution (~0.1 mm/pixel). A Medium Wavelength IR filter (MWIR) was used to lower the temperature detection threshold (Tthreshold,BB ≈ 250 °C) and extend the IR analysis to the full toroidal Monoblock (MB) length on both outer (OSP) and inner (ISP) strike points. The parallel heat flux derived from the VHR data (~70 MW/m2) is consistent with previous analysis and other measurements (Thermocouple and Fiber Bragg Grating) at the maximum heat flux location (OSP). This corresponds to real temperatures of about 1000 °C on the LE. Values obtained on the LE of the ISP are typically lower than of the OSP by a factor of two. However, a significant discrepancy between modeling and measurement is observed along with the toroidal temperature profile, with a temperature drop in the first 5 mm after the LEs. Visual and microscopic analysis performed on the component show clear evidence of surface state modification caused by plasma exposure: the presence of cracks on the LE, polished surface in its vicinity, and rougher surfaces further away from the LE.

Modification of copper and copper oxide surface states due to isopropyl alcohol treatment toward area-selective processes

The reduction of copper oxide by isopropyl alcohol (IPA) gas and its mechanism were investigated toward the selective process of copper (Cu) wiring. Also, the decomposition behavior of IPA gas and surface modification during the IPA treatment on Cu and copper oxide surfaces were studied. Two samples were measured: Cu surface having native oxide film and a metal Cu surface after a hydrogen reduction treatment. The decomposition and reaction behaviors and adsorption characteristics of IPA were investigated using the inline evaluation system equipped with a Cu reactor and Fourier transform infrared spectroscopy. The chemical structures of the Cu and copper oxide surfaces before and after IPA treatment were analyzed by x-ray photoelectron spectroscopy. Based on the experiments, the process condition to induce reduction of copper oxide by IPA gas during the Cu processes temperature range was identified. It was also found that different organic matter derived from IPA was adsorbed on each surface.

Modification of Surface States of Hematite-Based Photoanodes by Submonolayer of TiO2 for Enhanced Solar Water Splitting

Surface states are inherently involved with photoelectrochemical (PEC) solar fuel production; some of them are beneficial and participate in the surface reactions, but some act as recombination cen...

Influence of Cu modified surface states by sol gel Technique on Photocatalytic Activity of Titanium dioxide

In a series of experiments for Cu doping in TiO2 in different precursor ratios and evaluation of MB degradation efficiencies, an interesting finding is observed in which dye degradation efficiency is reduced upon increasing Cu ratios. Cu doping in small ratios firstly increases the degradation efficiency but reduces this efficiency if Cu ratios increase to 2% and more. In the present report, we introduce sol gel technique for preparation and modified surface states of titanium dioxide by copper (Cu) doping in different precursor ratios of 2 w%, 4 w% and 6 w% respectively. The effect of introduced surface states on photo catalytic efficiency of as synthesized products was compared to pure TiO2 nanoparticles prepared with same technique. The crystal structure of the products was studied by XRD and Raman spectroscopy. Anatase and rutile phases of pure TiO2 and Cu doped samples were identified by XRD and the results were confirmed by analyzing Raman spectra of samples. The XRD and Raman spectroscopy results were in agreement. The presence of Cu peak at 43.6° in all the doped samples suggest that surface states were introduced on titanium dioxide nano particle surfaces in the form of un-doped Cu. The morphological characterization for size distribution of samples was performed by FESEM as photo-catalytic activity strongly depends upon the size and shape of the catalyst. The elemental composition of the surfaces was estimated by EDAX which confirmed the presence of Cu and S on the surfaces of the Cu doped samples. The band broadening and band gap calculations were performed by optical characterization using PL and UV–vis spectroscopy results. The modified surface states were analyzed indirectly using results from PL and UV–vis techniques. Finally, the effect of these Cu modified surface states on photo-catalytic activity of as synthesized samples were studied by degradation of MB under visible light irradiation after specific intervals of time. The Langmuir-Hinshelwood (L-H) kinetic model (first order) was successfully applied on MB degradation results under irradiation by xenon lamp.

Effect of different surface functional groups on carbon supports toward methanol electro-oxidation of Pt nanoparticles

With an increased demand for efficient energy conversion system, nanoscale electrocatalysts have been intensively studied for high activity and stability. However, carbon support for nanoparticulate electrocatalyst has been generally thought to be inert for reaction pathway on metal nanocrystals. Therefore, material design for carbon support has mainly focused on development of high surface area with engineered pore structure, while surface state of the support has been largely neglected. Here, we demonstrate that surface functionalization of carbon support can modulate catalytic performance of Pt nanoparticle toward methanol electro-oxidation. Vulcan carbon (VC) surface was successfully modified by either acid oxidation or urea annealing, resulting in oxygen-rich (O-VC) and nitrogen-rich (N-VC) surface states, respectively. Modified surface state of carbon supports were investigated with XPS spectra. With controlled experiments, uniform Pt nanoparticles (d ~ 5 nm) were deposited on each carbon support for the same electrochemical surface area. Through high-resolution XPS analysis, electronic transfer from nitrogen to Pt was monitored in Pt/N-VC. With electron-enriched surface, Pt/N-VC reveals less oxophilic surface property compared to Pt/VC and Pt/O-VC. Notably, methanol electro-oxidation reaction largely depended on types of functional groups. Compared to Pt/VC, methanol oxidation decreased 30% in oxygen-rich Pt/O-VC after surface oxidation, while reaction rate increased double in nitrogen-rich Pt/N-VC. From CO bulk oxidation result, improved MOR activity of Pt/N-VC is attributed to larger number of available Pt active sites from attenuated adsorption of oxygen species. Therefore, this study highlights the importance of surface engineering in carbon supports for electrocatalysis.

Photoelectrochemical Water Splitting: Boosting Photoelectrochemical Water Oxidation of Hematite in Acidic Electrolytes by Surface State Modification (Adv. Energy Mater. 34/2019)

Photoelectrochemical Water Splitting: Boosting Photoelectrochemical Water Oxidation of Hematite in Acidic Electrolytes by Surface State Modification (Adv. Energy Mater. 34/2019)

Boosting Photoelectrochemical Water Oxidation of Hematite in Acidic Electrolytes by Surface State Modification

AbstractState‐of‐the‐art water‐oxidation catalysts (WOCs) in acidic electrolytes usually contain expensive noble metals such as ruthenium and iridium. However, they too expensive to be implemented broadly in semiconductor photoanodes for photoelectrochemical (PEC) water splitting devices. Here, an Earth‐abundant CoFe Prussian blue analogue (CoFe‐PBA) is incorporated with core–shell Fe2O3/Fe2TiO5 type II heterojunction nanowires as composite photoanodes for PEC water splitting. Those deliver a high photocurrent of 1.25 mA cm−2 at 1.23 V versus reversible reference electrode in acidic electrolytes (pH = 1). The enhancement arises from the synergic behavior between the successive decoration of the hematite surface with nanolayers of Fe2TiO5 and then, CoFe‐PBA. The underlying physical mechanism of performance enhancement through formation of the Fe2O3/Fe2TiO5/CoFe‐PBA heterostructure reveals that the surface states’ electronic levels of hematite are modified such that an interfacial charge transfer becomes kinetically favorable. These findings open new pathways for the future design of cheap and efficient hematite‐based photoanodes in acidic electrolytes.

Bloch surface waves engineering in one-dimensional photonic crystals with a chiral cap layer

We investigate the localization properties of surface waves created at the interface between a truncated 1D photonic crystal and homogeneous medium in the presence of a chiral cap layer using the transfer matrix method. The numerical results show that the interface can support surface waves with both transverse electric and transverse magnetic polarizations. We demonstrate that the surface waves can be engineered by varying the chirality parameter of the cap layer, which plays an important role in controlling and localization of surface states. It is shown that the effect of a chirality parameter on surface waves with transverse electric polarization is more remarkable compared with surface waves with transverse magnetic polarization. In addition, the effect of thickness and material type of the cap layer and the termination layer of a photonic crystal on surface waves is investigated. The results show that these parameters have considerable effect on modification of surface states. These findings may open new opportunities in the design of cavity-based devices such as high-Q sensors and perfect absorbers.

A constitutive model of expansive clay considering thermo-hydro-mechanical coupling effect

As a possible buffer material of engineered barriers and seals, expansive clay is often subjected to the thermo-hydro-mechanical (THM) coupling actions. This paper aims to develop a constitutive model which is capable of addressing the THM coupling behavior of expansive clay. Based on the Modified Surface State Approach, two state surfaces are assumed to describe the isotropic compressing behavior of expansive clay with the support of previous works. Subsequently, the yield function and hardening law are derived from the two state surfaces. Furthermore, the yield function and flow rule for triaxial stress states are presented. Through comparing the predicted results to experimental data of the compacted MX 80 bentonite, it is demonstrated that the proposed model has capacity of capturing the typical features of THM coupling behavior of compacted expansive clay. In addition, the proposed model is used to perform triaxial shear compression behaviors of expansive clay specimen. The influences of confining pressure, suction, and temperature are analyzed through the simulated results. The results show that the proposed model is able to address the fundamental characteristics of expansive clay specimen when subjected to triaxial shear compression.

Light-Emitting GaAs Nanowires on a Flexible Substrate.

Semiconductor nanowire-based devices are among the most promising structures used to meet the current challenges of electronics, optics and photonics. Due to their high surface-to-volume ratio and excellent optical and electrical properties, devices with low power, high efficiency and high density can be created. This is of major importance for environmental issues and economic impact. Semiconductor nanowires have been used to fabricate highperformance devices, including detectors, solar cellsand transistors. Here, we demonstrate a technique for transferring large-area nanowire arrays to flexible substrates while retaining their excellent quantum efficiency in emission. Starting with a defect-free self-catalyzed molecular beam epitaxy (MBE) sample grown on a Si substrate, GaAs core-shell nanowires are embedded in a dielectric, removed by reactive ion etching and transferred to a plastic substrate. The original structural and optical properties, including the vertical orientation, of the nanowires are retained in the final plastic substrate structure. Nanowire emission is observed for all stages of the fabrication process, with a higher emission intensity observed for the final transferred structure, consistent with a reduction in nonradiative recombination via the modification of surface states. This transfer process could form the first critical step in the development of flexible nanowire-based light-emitting devices.