ZIFs-Derived Hollow Nanostructures via a Strong/Weak Coetching Strategy for Long-Life Rechargeable Zn-Air Batteries

Recently, zeolitic imidazolate frameworks (ZIFs) composites have emerged as promising precursors for synthesizing hollow-structured N-doped carbon-based noble-metal materials with diverse structures and compositions. Here, a strong/weak competitive coordination strategy is presented for synthesizing high-performance electrocatalysts with hollow features. During the competitive coordination process, the cubic zeolitic-imidazole framework-8 (Cube-8)@ZIF-67 with core-shell structures are transformed into Cube-8@ZIF-67@PF/POM with yolk-shell nanostructures employing phosphomolybdic acid (POM) and potassium ferricyanide (PF) as the strong chelator and the weak chelator, respectively. After calcination, the hollow Mo/Fe/Co@NC catalyst exhibits superior performance in both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Interestingly, the Mo/Fe/Co@NC catalyst exhibits efficient electrocatalytic performance for Zn-air batteries (ZABs), with a high power density (≈150 mW cm-2 ) and superior cycling life (≈500 h) compared to commercial platinum/carbon (Pt/C) and ruthenium dioxide (RuO2 ) mixture benchmarks catalysts. In addition, the density functional theory further proves that after the introduction of Mo and Fe atoms, the adsorption energy with the adsorption intermediates is weakened by adjusting the d-band center, thus weakening the reaction barrier and promoting the reaction kinetics of OER. Undoubtedly, this study presents novel insights into the fabrication of ZIFs-derived hollow structure bifunctional oxygen electrocatalysts for clean-energy diverse applications.

Two-dimensional MXene Nanomaterials: Preparation, Structure Modulation and the Applications in Electrochemical Energy Storage

BACKGROUND

MXenes have attracted intensive attention owing to their unique twodimensional (2D) layered structure, high specific surface area, excellent conductivity, superior surface hydrophilicity, and chemical stability. In recent years, selective etching of the A element layers from MAX phases by fluorine-containing etchants (HF, LiF-HCl, etc) is a common method to prepare multilayered MXene nanomaterials (NMs) with plentiful surface terminations. At present, many studies have been reported on the use of fluorine-free etchants (NaOH, ZnCl2, etc) to etch MAX phases. The properties of MXene NMs are dependent on their structures.

OBJECTIVE

The purpose of this review is to focus on a comprehensive and systematical survey on the preparation, structure modulation, and applications of MXene NMs in electrochemical energy storage devices, including supercapacitors, lithium-ion battery, sodium-ion battery, potassium-ion battery, and aluminum-ion battery.

METHODS

Extensive information related to the preparation and applications of 2D MXene NMs for electrochemical energy storage and their associated patents were collected. This review highlights the recently reported 2D MXene NMs which are used in supercapacitor and various metal ion.

RESULTS

It is found that the preparation methods have great impacts on the layer spacing and surface terminations of MXenes, consequently affecting their performance. Hence, this paper summarizes the research progress of the preparation strategies, layer spacing and surface termination modulation of MXene NMs.

CONCLUSION

The applications of 2D MXene NMs in electrochemical energy storage are outlined. The forward-looking challenges and prospects for the development of MXenes are also proposed.

Engineering Crystallinity Gradients for Tailored CaO2 Nanostructures: Enabling Alkalinity-Reinforced Anticancer Activity with Minimized Ca2+/H2O2 Production

CaO2 nanoparticles (CNPs) can produce toxic Ca2+ and H2O2 under acidic pH, which accounts for their intrinsic anticancer activity but at the same time raises safety concerns upon systemic exposure. Simultaneously realizing minimized Ca2+/H2O2 production and enhanced anticancer activity poses a dilemma. Herein, we introduce a "crystallinity gradient-based selective etching" (CGSE) strategy, which is realized by creating a crystallinity gradient in a CNP formed by self-assembled nanocrystals. The nanocrystals distributed in the outer layer have a higher crystallinity and thus are chemically more robust than those distributed in the inner layer, which can be selectively etched. CGSE not only leads to CNPs with tailored single- and double-shell hollow structures and metal-doped compositions but more surprisingly enables significantly enhanced anticancer activity as well as tumor growth inhibition under limited Ca2+/H2O2 production, which is attributed to an alkalinity-reinforced lysosome-dependent cell death pathway.

Interlayer Incorporation of A-Elements into MXenes Via Selective Etching of A' from Mn+1A'1-xA″xCn MAX Phases

MXenes are a large family of two-dimensional materials with a general formula Mn+1XnTz, where M is a transition metal, X = C and/or N, and Tz represents surface functional groups. MXenes are synthesized by etching A-elements from layered MAX phases with a composition of Mn+1AXn. As over 20 different chemical elements were shown to form A-layers in various MAX phases, we propose that they can provide an abundant source of very diverse MXene-based materials. The general strategy for A-modified MXenes relies on the synthesis of Mn+1A'1-xA″xXn MAX phase, in which the higher reactivity of the A'-element compared to that of A″ enables its selective etching, resulting in A″-modified Mn+1XnTz. In general, the A″-element could modify the interlayer spaces of MXene flakes in a form of metallic or oxide species, depending on its chemical identity and synthetic conditions. We demonstrate this strategy by synthesizing Sn-modified Ti3C2Tz MXene from the Ti3Al0.75Sn0.25C2 MAX phase, which was used as a model system. Although the incorporation of Sn in the A-layer of Ti3AlC2 decreases the MAX phase reactivity, we developed an etching procedure to completely remove Al and produce Sn-modified Ti3C2Tz MXene. The resulting MXene sheets were of very high quality and exhibited improved environmental stability, which we attribute to the effect of a uniform Sn modification. Finally, we demonstrate a peculiar electrostatic expansion of Sn-modified Ti3C2Tz accordions, which may find interesting applications in MXene-based nano-electromechanical systems. Overall, these results demonstrate that in addition to different combinations of M and X elements in MAX phases, an A-layer also provides opportunities for the synthesis of MXene-based materials.

Understanding the Intrinsic Rectification Properties of Nanoporous Anodic Alumina by Selective Chemical Etching

The distribution of oxygen and aluminum vacancies across the hemispherical barrier oxide layer (BOL) of nanoporous anodic alumina (NAA) relies intrinsically on the electric field-driven flow of electrolytic species and the incorporation of electrolyte impurities during the growth of anodic oxide through anodization. This phenomenon provides new opportunities to engineer BOL's inherited ionic current rectification (ICR) fingerprints. NAA's characteristic ICR signals are associated with the space charge density gradient across BOL and electric field-induced ion migration through hopping from vacancy to vacancy. In this study, we engineer the intrinsic space charge density gradient of the BOL of NAA under a range of anodizing potentials in hard and mild anodization regimes. Real-time characterization of the ICR fingerprints of NAA during selective etching of the BOL makes it possible to unravel the distribution pattern of vacancies through rectification signals as a function of etching direction and time. Our analysis demonstrates that the space charge density gradient varies across the BOL of NAA, where the magnitude and distribution of the space charge density gradient are revealed to be critically determined by anodizing the electrolyte, regime, and potential. This study provides a comprehensive understanding of the engineering of ion transport behavior across blind-hole NAA membranes by tuning the distribution of defects across BOL through anodization conditions. This method has the potential to be harnessed for developing nanofluidic devices with tailored ionic rectification properties for energy generation and storage and sensing applications.

Defect-Free Few-Layer M4 C3 Tx (M = V, Nb, Ta) MXene Nanosheets: Synthesis, Characterization, and Physicochemical Properties

High-quality few-layer M4 C3 Tx (M = V, Nb, Ta) MXenes are very important for applications and are necessary for clarifying their physicochemical properties. However, the difficulty in etching for themselves and the existence of MC/MC1-δ and M-Al alloy impurities in their M4 AlC3 precursors seriously hinder the achievement of defect-free few-layer M4 C3 Tx (M = V, Nb, Ta) MXenes nanosheets. Herein, three different defect-free few-layer M4 C3 Tx (M = V, Nb, Ta) nanosheets are obtained by using a universal synthesis strategy of calcination, selective etching, intercalation, and exfoliation. Comprehensive characterizations confirm their defect-free few-layer structure feature, large interlayer spacing (1.702-1.955 nm), types of functional groups (-OH, -F, -O), and abundant valance states (M5+ , M4+ , M3+ , M2+ , M0 ). M4 C3 Tx (M = V, Nb, Ta) free-standing films obtained by vacuum filtration of few-layer M4 C3 Tx inks show good hydrophilia, high thermostability, and conductivity. A roadmap on synthesis of defect-free few-layer M4 C3 Tx (M = V, Nb, Ta) nanosheets are proposed and three key points are summarized. This work provides detailed guidelines for the synthesis of other defect-free few-layer MXenes nanosheets, but also will stimulate extensive functional explorations for M4 C3 Tx (M = V, Nb, Ta) MXenes nanosheets in the future.

Highly selective etching of SiNxover SiO2using ClF3/Cl2remote plasma

Highly selective etching of silicon nitride over silicon oxide is one of the most important processes especially for the fabrication of vertical semiconductor devices including 3D NAND (Not And) devices. In this study, isotropic dry etching characteristics of SiNxand SiO2using ClF3/Cl2remote plasmas have been investigated. The increase of Cl2percent in ClF3/Cl2gas mixture increased etch selectivity of SiNxover SiO2while decreasing SiNxetch rate. By addition of 15% Cl to ClF3/Cl2, the etch selectivity higher than 500 could be obtained with the SiNxetch rate of ∼8 nm min-1, and the increase of Cl percent to 20% further increased the etch selectivity to higher than 1000. It was found that SiNxcan be etched through the reaction from Si-N to Si-F and Si-Cl (also from Si-Cl to Si-F) while SiO2can be etched only through the reaction from Si-O to Si-F, and which is also in extremely low reaction at room temperature. When SiNx/SiO2layer stack was etched using ClF3/Cl2(15%), extremely selective removal of SiNxlayer in the SiNx/SiO2layer stack could be obtained without noticeable etching of SiO2layer in the stack and without etch loading effect.

Evaluation of Self-Adhesive Composite Restorations Bond on Primary Canines: An In Vitro Study

Background and purpose The aesthetic requirements for the restoration of the primary anterior teeth of both the child and his parents increased recently, especially with the presence of materials with good mechanical properties capable of giving a good aesthetic shape. However, the challenge remains in the possibility of applying these materials to primary teeth, where the child's cooperation and ability to keep their mouth open during the completion of the restoration is an important challenge for the dentist. Therefore, this study aimed to evaluate the bonding of self-adhesive composite by studying the microleakage in primary canines. Materials and methods The studied sample consisted of 60 extracted primary canines that were divided into three equal groups (n=20) according to the type of restoration: group 1 (experimental) - self-adhesive composite alone, group 2 (experimental) - self-adhesive composite with selective enamel etching, and group 3 (control) - conventional composite with the self-etching bond and selective enamel etching. A Class V cavity was prepared at the cementoenamel junction, 2 mm deep and 1 mm in diameter. In this way, the gingival wall is dentine, while the incisal wall is enamel-dentine. Restorations were placed according to the group to which the tooth belongs. After that, all teeth were subjected to 500 cycles of thermocycling. Then, a methylene blue dye microleakage test was performed, and longitudinal sections of the teeth were made and studied under x20 magnification using a stereo microscope. Results In the gingival wall, the scores of microleakage in the conventional composite group after selective etching were lower than those of the self-adhesive composite groups, with a statistically significant difference. While in the incisal wall, there were no statistically significant differences in the frequencies of the scores of microleakage between the three groups. Conclusions Within the limitations of this study, the conventional composite bonded better to enamel and dentine than self-adhesive composite resin applied alone or after selective enamel etching. The bonding of the self-adhesive composite was lower than that of the conventional composite, and it is not recommended to use it in Class V restorations of primary anterior teeth.

Monolithically Programmed Stretchable Conductor by Laser-Induced Entanglement of Liquid Metal and Metallic Nanowire Backbone

Owing to its low mechanical compliance, liquid metal is intrinsically suitable for stretchable electronics and future wearable devices. However, its invariable strain-resistance behavior according to the strain-induced geometrical deformation and the difficulty of circuit patterning limit the extensive use of liquid metal, especially for strain-insensitive wiring purposes. To overcome these limitations, herein, novel liquid-metal-based electrodes of fragmented eutectic gallium-indium alloy (EGaIn) and Ag nanowire (NW) backbone of which their entanglement is controlled by the laser-induced photothermal reaction to enable immediate and direct patterning of the stretchable electrode with spatially programmed strain-resistance characteristics are developed. The coexistence of fragmented EGaIn and AgNW backbone, that is, a biphasic metallic composite (BMC), primarily supports the uniform and durable formation of target layers on stretchable substrates. The laser-induced photothermal reaction not only promotes the adhesion between the BMC layer and substrates but also alters the structure of laser-irradiated BMC. By controlling the degree of entanglement between fragmented EGaIn and AgNW, the initial conductivity and local gauge factor are regulated and the electrode becomes effectively insensitive to applied strain. As the configuration developed in this study is compatible with both regimes of electrodes, it can open new routes for the rapid creation of complex stretchable circuitry through a single process.

Digital partition bonding for porcelain veneers guided by sequence 3D printing templates

Obtaining optimal enamel bonding is a common recommendation to ensure the retention, edge closure, and clinical service life of porcelain veneers. Bonding surface is determined after tooth preparation. More or less dentin exposure often occurs due to the uneven thickness of the enamel. Identifying the enamel or dentin on the bonding surface and adopting the "selective etching" technique are useful to obtain the maximal bonding strength. Naked eye recognition in "selective etching" has uncertain results and is experience-based, whereas the proposed technique controls the preparation depth through the first target restoration space template to obtain the accurate enamel and dentin subarea. It is mapped to the second bonding template, depending on the treatment carried out on the enamel and dentin surface, to accurately predict the adhesion of porcelain veneers.

Controlled Synthesis of Perforated Oxide Nanosheets with High Density Nanopores Showing Superior Water Purification Performance

A method for creating genuine nanopores in high area density on monolayer two-dimensional (2D) metallic oxides has been developed. By use of the strong reduction capability of hydroiodic acid, active metal ions, such as Fe^III and Co^III, in 2D oxide nanosheets can be reduced to a divalent charge state (2+). The selective removal of FeO₂ and CoO₂ metal oxide units from the framework can be tuned to produce pores in a range of 1-4 nm. By monitoring of the redox reaction kinetics, the pore area density can be also tuned from ∼0.9 × 10⁴ to ∼3.3 × 10⁵ μm^-2. The universality of this method to produce much smaller pores and higher area density than the previously reported ones has been proven in different oxide nanosheets. To demonstrate their potential applications, ultrasmall metal organic framework particles were grown inside the pores of perforated titania oxide nanosheets. The optimized hybrid film showed ∼100% rejection of methylene blue (MB) from the water. Its water permeance reached 4260 L m^-2 h^-1 bar^-1, which is 1-3 orders of that for reported 2D membranes with good MB rejections.

An Efficient Strategy toward Multichambered Carbon Nanoboxes with Multiple Spatial Confinement for Advanced Sodium-Sulfur Batteries

Intricate hollow carbon structures possess vital function for anchoring polysulfides and enhancing the utilization of sulfur in room-temperature sodium-sulfur batteries. However, their synthesis is extremely challenging due to the complex structure. Here, a facile and efficient strategy is developed for the controllable synthesis of N/O-doped multichambered carbon nanoboxes (MCCBs) by selective etching and stepwise carbonization of ZIF-8 nanocubes. The MCCBs consist of porous carbon shells on the outside and connected carbon grids with a hollow structure on the inside, bringing about a MCCBs structure. As a sulfur host, the multichambered structure has better spatial encapsulation and integrated conductivity via the inner interconnected carbon grids, which combines the characteristics of short charge transfer path and superb physicochemical adsorption along with mechanical strength. As expected, the S@MCCBs cathode realizes decent cycle stability (0.045% capacity decay per cycle over 800 cycles at 5 A g^-1) and enhanced rate performance (328 mA h g^-1 at 10 A g^-1). Furthermore, in situ transmission electron microscopy (TEM) observation confirms the good structural stability of the S@MCCBs during the (de)sodiation process. Our work demonstrates an effective strategy for the rational design and accurate construction of intricate hollow materials for high-performance energy storage systems.

MXenes-A New Class of Two-Dimensional Materials: Structure, Properties and Potential Applications

A new class of two-dimensional nanomaterials, MXenes, which are carbides/nitrides/carbonitrides of transition and refractory metals, has been critically analyzed. Since the synthesis of the first family member in 2011 by Yury Gogotsi and colleagues, MXenes have quickly become attractive for a variety of research fields due to their exceptional properties. Despite the fact that this new family of 2D materials was discovered only about ten years ago, the number of scientific publications related to MXene almost doubles every year. Thus, in 2021 alone, more than 2000 papers are expected to be published, which indicates the relevance and prospects of MXenes. The current paper critically analyzes the structural features, properties, and methods of synthesis of MXenes based on recent available research data. We demonstrate the recent trends of MXene applications in various fields, such as environmental pollution removal and water desalination, energy storage and harvesting, quantum dots, sensors, electrodes, and optical devices. We focus on the most important medical applications: photo-thermal cancer therapy, diagnostics, and antibacterial treatment. The first results on obtaining and studying the structure of high-entropy MXenes are also presented.

Nanoporous B13 C2 towards Highly Efficient Electrochemical Nitrogen Fixation

The electrochemical nitrogen fixation under mild conditions is a promising alternative to the current nitrogen industry with high energy consumption and greenhouse gas emission. Here, a nanoporous boron carbide (np-B₁₃ C₂ ) catalyst is reported for electrochemical nitrogen fixation, which is fabricated by the combination of metallurgical alloy design and chemical etching. The resulting np-B₁₃ C₂ exhibits versatile catalytic activities towards N₂ reduction reactions (NRR) and N₂ oxidation reaction (NOR). A high NH₃ yield of 91.28 µg h^-1 mg_cat.^-1 and Faradaic efficiency (FE) of 35.53% at -0.05 V versus the reversible hydrogen electrode are obtained for NRR, as well as long-term stability of up to 70 h, making them among the most active NRR electrocatalysts. This catalyst can also achieve a NO₃^- yield of 165.8 µg h^-1  mg_cat.^-1 and a FE of 8.4% for NOR. In situ Raman spectroscopy and density functional theory calculations reveal that strong coupling between the BC sites modulates the electronic structures of adjacent B atoms of B₁₃ C₂ , which enables the B sites to effectively adsorb and activate chemical inert N₂ molecules, resulting in lowered energy required by the potential-determining step. Besides, the introduction of carbon can increase the inherent conductivity and reduce the binding energy of the reactants, thus improving N₂ fixation performance.

Hollow Channel Formation inside Sodium Aluminoborate Glass by Femtosecond Laser Writing and Distilled Water Etching

Recently, the effect of nanograting formation was demonstrated for binary sodium borate glass with the possibility of data storage with an enhanced level of security. The obvious disadvantage of such glass is poor chemical stability, which limits real applications. In this paper, we show that the introduction of Al2O3 allows preserving the possibility of nanograting formation with a significant increase of chemical resistance and thus to preserve optical memory applications. On the other hand, the possibility of selective etching of laser-written tracks by means of distilled water is revealed, which was not demonstrated for other types of glasses. The dependence of retardance of nanogratings form birefringence on laser writing parameters is established and discussed. Structural features of laser-modified microdomains are studied via Raman spectroscopy which revealed an increase of three-coordinated boron content. A possible mechanism of selective etching is discussed.

Surface Defect Engineering on Perovskite Oxides as Efficient Bifunctional Electrocatalysts for Water Splitting

The design of high-performance and cost-effective electrocatalysts for water splitting is of prime importance for efficient and sustainable hydrogen production. In this work, a surface defect engineering method is developed for optimizing the electrocatalytic activity of perovskite oxides for water electrolysis. A typical ferrite-based perovskite oxide material La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) is used and regulated by selective acid etching. The optimal parameters for the surface treatment are identified. An efficient bifunctional perovskite oxide, denoted LSCF-30, is prepared by selectively corroding the A-site Sr element in the surface region, which is found to not only increase the exposure and decrease the coordination of B-site metals but also effectively modulate the electronic structure of these metals. The crystal lattice of the perovskite bulk is kept constant during surface engineering, which ensures the structural stability of the perovskite catalyst. The findings demonstrate an effective strategy of surface defect engineering in enhancing the performance of perovskite oxide electrocatalysts for water splitting.

Selective Etching Quaternary MAX Phase toward Single Atom Copper Immobilized MXene (Ti3C2Clx) for Efficient CO2 Electroreduction to Methanol

Single atom catalysts possess attractive electrocatalytic activities for various chemical reactions owing to their favorable geometric and electronic structures compared to the bulk counterparts. Herein, we demonstrate an efficient approach to producing single atom copper immobilized MXene for electrocatalytic CO₂ reduction to methanol via selective etching of hybrid A layers (Al and Cu) in quaternary MAX phases (Ti₃(Al_1-xCu_x)C₂) due to the different saturated vapor pressures of Al- and Cu-containing products. After selective etching of Al in the hybrid A layers, Cu atoms are well-preserved and simultaneously immobilized onto the resultant MXene with dominant surface functional group (Cl_x) on the outmost Ti layers (denoted as Ti₃C₂Cl_x) via Cu-O bonds. Consequently, the as-prepared single atom Cu catalyst exhibits a high Faradaic efficiency value of 59.1% to produce CH₃OH and shows good electrocatalytic stability. On the basis of synchrotron-based X-ray absorption spectroscopy analysis and density functional theory calculations, the single atom Cu with unsaturated electronic structure (Cu^δ+, 0 < δ < 2) delivers a low energy barrier for the rate-determining step (conversion of HCOOH* to absorbed CHO* intermediate), which is responsible for the efficient electrocatalytic CO₂ reduction to CH₃OH.

Selective Digital Etching of Silicon-Germanium Using Nitric and Hydrofluoric Acids

A digital etching method was proposed to achieve excellent control of etching depth. The digital etching characteristics of p⁺-Si and Si_0.7Ge_0.3 using a combination of HNO₃ oxidation and buffered oxide etching oxide removal processes were investigated. Experimental results showed that oxidation saturates as time goes on because of low activation energy and its diffusion-limited characteristic. An oxidation model was developed to describe the wet oxidation process with nitric acid. The model was calibrated with experimental data, and the oxidation saturation time, final oxide thickness, and selectivity between Si_0.7Ge_0.3 and p⁺-Si were obtained. In Si_0.7Ge_0.3/p⁺-Si stacks, the saturated relative etched depth per cycle was 0.5 nm (four monolayers), and variation between experiments was about 4% after saturation. A corrected selectivity calculation formula was also proposed, and the calculated selectivity was 3.7-7.7 for different oxidation times, which was the same as the selectivity obtained from our oxidation model. The proposed model can be used to analyze process variations and repeatability, and it can provide credible guidance for the design of other wet digital etching experiments.

Morphologies, Young's Modulus and Resistivity of High Aspect Ratio Tungsten Nanowires

High aspect ratio tungsten nanowires have been prepared by selective dissolution of Nickel-aluminum-tungsten (NiAl−W) alloys which were directionally solidified at growth rates varying from 2 to 25 μm/s with a temperature gradient of 300 K·cm⁻¹. Young’s modulus and electrical resistivity of tungsten nanowires were measured by metallic mask template method. The results show that the tungsten nanowires with uniform diameter and high aspect ratio are well aligned. The length of tungsten nanowires increases with prolongation of etching time, and their length reaches 300 μm at 14 h. Young’s modulus of tungsten nanowires is estimated by Hertz and Sneddon models. The Sneddon model is proper for estimating the Young’s modulus, and the value of calculating Young’s modulus are 260–460 GPa which approach the value of bulk tungsten. The resistivity of tungsten nanowires is measured and fitted with Fuchs−Sondheimer (FS) + Mayadas−Shatzkes (MS) model. The fitting results show that the specific resistivity of W nanowires is a litter bigger than the bulk W, and its value decreases with decreasing diameter.

The Underlying Chemical Mechanism of Selective Chemical Etching in CsPbBr3 Nanocrystals for Reliably Accessing Near-Unity Emitters

Reliably accessing nanocrystal luminophores with near-unity efficiencies aids in the ability to understand the upper performance limits in optoelectronic applications that require minimal nonradiative losses. Constructing structure-function relationships at the atomic level, while accounting for inevitable defects, allows for the development of robust strategies to achieve near-unity quantum yield luminophores. For CsPbBr₃ perovskite nanocrystals, bromine vacancies leave behind undercoordinated lead atoms that act as traps, limiting the achievable optical performance of the material. We show that selective etching represents a promising path for mitigating the consequences of optical defects in CsPbBr₃ nanocrystals. A mechanistic understanding of the etching reaction is essential for developing strategies to finely control the reaction. We report a study of the selective etching mechanism of CsPbBr₃ nanocrystal cubes by controlling the etchant chemical potential. We observe optical absorption and luminescence trajectories while varying the extent and rate of lead removal, removing in some cases up to 75% of the lead from the original nanocrystal ensemble. At modest etchant chemical potentials, the size and shape uniformity of the nanocrystal ensemble improves in addition to the quantum yield, proceeding through a layer-by-layer etching mechanism. Operating with excessively high etchant chemical potentials is detrimental to the overall optical performance as the etching transitions to nonselective, while too low of a chemical potential results in incomplete etching. Through this general approach, we show how to finely control selective etching to consistently access a steady state or chemical stability zone of near-unity quantum yield CsPbBr₃ nanocrystals postsynthetically, suggesting a practical framework to extend this treatment to other perovskite compositions and sizes.

Two-Dimensional Hydroxyl-Functionalized and Carbon-Deficient Scandium Carbide, ScC xOH, a Direct Band Gap Semiconductor

Two-dimensional (2D) materials have attracted intense attention in nanoscience and nanotechnology due to their outstanding properties. Among these materials, the emerging family of 2D transition metal carbides, carbonitrides, and nitrides (referred to as MXenes) stands out because of the vast available chemical space for tuning materials chemistry and surface termination, offering opportunities for property tailoring. Specifically, semiconducting properties are needed to enable utilization in optoelectronics, but direct band gaps are experimentally challenging to achieve in these 2D carbides. Here, we demonstrate the fabrication of 2D hydroxyl-functionalized and carbon-deficient scandium carbide, namely, ScC _xOH, by selective etching of a layered parent ScAl₃C₃ compound. The 2D configuration is determined as a direct band gap semiconductor, with an experimentally measured band gap approximated at 2.5 eV. Furthermore, this ScC _xOH-based device exhibits excellent photoresponse in the ultraviolet-visible light region (responsivity of 0.125 A/W at 360 nm/10 V, and quantum efficiency of 43%). Thus, this 2D ScC _xOH direct band gap semiconductor may find applications in visible light detectors, photocatalytic chemistry, and optoelectronic devices.

Plasma-Based Nanostructuring of Polymers: A Review

There are various fabrication methods for synthesizing nanostructures, among which plasma-based technology is strongly competitive in terms of its flexibility and friendly uses, economy, and safety. This review systematically discusses plasma techniques and the detailed interactions of charged particles, radicals, and electrons with substrate materials of, in particular, polymers for their nanostructuring. Applications employing a plasma-based nanostructuring process are explored to show the advantages and benefits that plasma treatment brings to many topical and traditional issues, and are specifically related to wettability, healthcare, or energy researches. A short perspective is also presented on strategic plans for overcoming the limitations in dimension from surface to bulk, lifetime of surface functions, and selectivity for interactions.

Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface

Dopants modify the electronic properties of semiconductors, including their susceptibility to etching. In semiconductor nanowires doped during growth by the vapor-liquid-solid (VLS) process, it has been shown that nanofaceting of the liquid-solid growth interface influences strongly the radial distribution of dopants. Hence, the combination of facet-dependent doping and dopant selective etching provides a means to tune simultaneously the electronic properties and morphologies of nanowires. Using atom-probe tomography, we investigated the boron dopant distribution in Au catalyzed VLS grown silicon nanowires, which regularly kink between equivalent ⟨112⟩ directions. Segments alternate between radially uniform and nonuniform doping profiles, which we attribute to switching between a concave and convex faceted liquid-solid interface. Dopant selective etching was used to reveal and correlate the shape of the growth interface with the observed anisotropic doping.

Selective Etching of Silicon in Preference to Germanium and Si0.5Ge0.5

The selective etching characteristics of silicon, germanium, and Si_0.5Ge_0.5 subjected to a downstream H₂/CF₄/Ar plasma have been studied using a pair of in situ quartz crystal microbalances (QCMs) and X-ray photoelectron spectroscopy (XPS). At 50 °C and 760 mTorr, Si can be etched in preference to Ge and Si_0.5Ge_0.5, with an essentially infinite Si/Ge etch-rate ratio (ERR), whereas for Si/Si_0.5Ge_0.5, the ERR is infinite at 22 °C and 760 mTorr. XPS data showed that the selectivity is due to the differential suppression of etching by a ∼2 ML thick C_xH_yF_z layer formed by the H₂/CF₄/Ar plasma on Si, Ge, and Si_0.5Ge_0.5. The data are consistent with the less exothermic reaction of fluorine radicals with Ge or Si_0.5Ge_0.5 being strongly suppressed by the C_xH_yF_z layer, whereas, on Si, the C_xH_yF_z layer is not sufficient to completely suppress etching. Replacing H₂ with D₂ in the feed gas resulted in an inverse kinetic isotope effect (IKIE) where the Si and Si_0.5Ge_0.5 etch rates were increased by ∼30 times with retention of significant etch selectivity. The use of D₂/CF₄/Ar instead of H₂/CF₄/Ar resulted in less total carbon deposition on Si and Si_0.5Ge_0.5 and gave less Ge enrichment of Si_0.5Ge_0.5. These results are consistent with the selectivity being due to the differential suppression of etching by an angstrom-scale carbon layer.

Synthesis and Electrochemical Properties of Two-Dimensional Hafnium Carbide

We demonstrate fabrication of a two-dimensional Hf-containing MXene, Hf₃C₂T_z, by selective etching of a layered parent Hf₃[Al(Si)]₄C₆ compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm^-3 for lithium and sodium ions batteries, respectively, at a current density of 200 mAg^-1 after 200 cycles. Thus, Hf₃C₂T_z MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

Integration of Electrodeposited Ni-Fe in MEMS with Low-Temperature Deposition and Etch Processes

This article presents a set of low-temperature deposition and etching processes for the integration of electrochemically deposited Ni-Fe alloys in complex magnetic microelectromechanical systems, as Ni-Fe is known to suffer from detrimental stress development when subjected to excessive thermal loads. A selective etch process is reported which enables the copper seed layer used for electrodeposition to be removed while preserving the integrity of Ni-Fe. In addition, a low temperature deposition and surface micromachining process is presented in which silicon dioxide and silicon nitride are used, respectively, as sacrificial material and structural dielectric. The sacrificial layer can be patterned and removed by wet buffered oxide etch or vapour HF etching. The reported methods limit the thermal budget and minimise the stress development in Ni-Fe. This combination of techniques represents an advance towards the reliable integration of Ni-Fe components in complex surface micromachined magnetic MEMS.

Well-Ordered Nanoporous ABA Copolymer Thin Films via Solvent Vapor Annealing, Homopolymer Blending, and Selective Etching of ABAC Tetrablock Terpolymers

Solvent vapor annealing treatments are used to control the orientation of nanostructures produced in thin films of a poly(styrene)-block-poly(isoprene)-block-poly(styrene)-block-poly((±)-lactide) (PS-PI-PS-PLA) and its blends with PLA homopolymer. The PS-PI-PS-PLA tetrablock terpolymer, previously determined to adopt a core(PLA)-shell(PS) cylindrical morphology in the bulk, gave perpendicular alignment of PLA cylinders over a limited range of thicknesses using a mixed solvent environment of tetrahydrofuran and acetone. On the other hand, perpendicular alignment was achieved regardless of film thickness by inclusion of 5 wt % homopolymer PLA in the PS-PI-PS-PLA tetrablock. Tapping mode atomic force microscopy (AFM) was used to visualize film surface morphologies. Subsequent reactive ion etching (RIE) and basic hydrolysis of PLA produced 15 nm pores in a PS-PI-PS triblock thin film matrix. Nanoporosity was confirmed by scanning electron microscopy (SEM) images and the vertical continuity of pores was confirmed by cross-sectional SEM analysis.

KCN Chemical Etch for Interface Engineering in Cu2ZnSnSe4 Solar Cells

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

Avoiding Loss of Catalytic Activity of Pd Nanoparticles Partially Embedded in Nanoditches in SiC Nanowires

Nanoditches from selective etching of periodically twinned SiC nanowires were employed to hinder the migration and coalescence of Pd nanoparticles supported on the nanowires, and thus to improve their catalytic stability for total combustion of methane. The results show that the etched Pd/SiC catalyst can keep the methane conversion of almost 100% while the unetched one has an obvious decline in the catalytic activity from 100 to 82% after ten repeated reaction cycles. The excellent catalytic stability originates from the limitation of the nanoditches to the migration and growth of Pd nanoparticles.

Controlling the morphology of side chain liquid crystalline block copolymer thin films through variations in liquid crystalline content

In this paper, we describe methods for manipulating the morphology of side-chain liquid crystalline block copolymers through variations in the liquid crystalline content. By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer (BCP) backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase-segregated BCP microstructures can be precisely manipulated. Increases in LC functionalization lead to stronger preferences for the anchoring of the LC mesophase relative to the substrate and the intermaterial dividing surface. By manipulating the strength of these interactions, the arrangement and ordering of the ultrathin film block copolymer nanostructures can be controlled, yielding a range of morphologies that includes perpendicular and parallel cylinders, as well as both perpendicular and parallel lamellae. Additionally, we demonstrate the utilization of selective etching to create a nanoporous liquid crystalline polymer thin film. The unique control over the orientation and order of the self-assembled morphologies with respect to the substrate will allow for the custom design of thin films for specific nanopatterning applications without manipulation of the surface chemistry or the application of external fields.