Photoluminescence enhancement for blue-green emission in copper-alloyed lead-free cesium halide single crystals.

Recently, metal halides have received extensive attention because of the superior photophysical characteristics. Regardless of the superiority, the limited stability against heat and moisture and the toxicity problem of heavy lead metal are obstacles to the realization of wide range applications. In this case, it is necessary to develop eco-friendly alternatives, which could simultaneously maintain the excellent optoelectronic properties of lead materials. In this paper, the synthesis of lead-free one-dimensional Cs2AgBr3 and Cu(I)-alloyed Cs2AgBr3 single crystals (SCs) has been successfully realized. Experimental results demonstrated that the addition of applicable copper ions could greatly improve their luminescence intensity. A bright blue-green photoluminescence peaking at 510 nm was observed after incorporating Cu+ ions into Cs2AgBr3 SCs under UV irradiation. Theoretical calculation further proved that the incorporation of Cu+ could effectively modulate the materials' electronic band structure; the electronic states limited to the CuBr4 tetrahedron presented a strong localized property, which was beneficial to increase the photoluminescence efficiency. In addition, the SCs displayed favorable structure stability proofing moisture and oxygen under ambient conditions, proving that this material has good prospects for the development of optoelectronic fields.

Bright luminescence from nontoxic all-inorganic low-dimensional cesium halide.

Due to the superiority of low cost, easy manufacture, and tunable light emission owing to the diversity of compositions and dimensionalities, the metal halides have appeared as a promising class of semiconductors. Nevertheless, the toxicity problem along with inherent instability of Pb-based metal halides greatly limits their large-scale applications. Based on this situation, it is necessary to develop eco-friendly materials, which could simultaneously maintain the excellent optoelectronic properties of lead materials. In this Letter, the one-dimensional Cu + -alloyed Cs2AgI3 has been successfully synthesized. An intense blue emission located at 469 nm with a large Stokes shift was observed. Density functional theory calculation indicated that the Cu+ ions could effectively modulate the density of state population, which was the key factor drastically boosting the photoluminescence quantum yield (PLQY). This kind of highly efficient metal halide may overcome the bottlenecks of toxicity and poor efficiency issues of blue emission and will have a promising prospect in the optoelectronic fields.

Well-Ordered Au Nanoarray for Sensitive and Reproducible Detection of Hepatocellular Carcinoma-Associated miRNA via CHA-Assisted SERS/Fluorescence Dual-Mode Sensing.

Ultra-sensitive detection of cancer-related biomarkers in serum is of great significance for early diagnosis, treatment, prognosis, and staging of cancer. In this work, we proposed a surface-enhanced Raman scattering and fluorescence (SERS/FL) dual-mode biosensor for hepatocellular carcinoma (HCC)-related miRNA (miR-224) detection using the composition of well-arranged Au nanoarrays (Au NAs) substrate coupled with the target-catalyzed hairpin assembly (CHA) strategy. The hot spots densely and uniformly distributed on the Au array offers considerably enhanced and reproducible SERS signals, along with their wide and open surface to facilitate miR-224 adsorption. By this sensing strategy, the target miR-224 can be detected in a wide linear range (1 fM to 1 nM) with a limit of detection of 0.34 fM in the SERS mode and 0.39 fM in the FL mode. Meanwhile, this biosensor with exceptional specificity and anti-interference ability can discriminate target miR-224 from other interference miRNAs. Practical analysis of human blood samples also demonstrated considerable reliability and repeatability of our developed strategy. Furthermore, this biosensor can distinguish HCC cancer subjects from normal ones and monitor HCC patients before and after hepatectomy as well as guide the distinct Barcelona clinic liver cancer (BCLC) stages. Overall, benefiting from a well-arranged Au nanoarray, CHA amplification strategy, and SERS/metal enhanced fluorescence effect, this established biosensor opens new avenues for the early prediction, warning, monitoring, and staging of HCC.

Inversion method for soil moisture content based on a distributed fiber optic acoustic sensing system.

The traditional measurement method can't achieve real-time monitoring of soil moisture content (SMC) within a two-dimensional area. To solve the above problems, we propose a rapid SMC monitoring method for two-dimensional area based on distributed acoustic sensing (DAS). DAS demodulates the backward Rayleigh scattering signal containing seismic wave sound velocity information from the active seismic source. The folding ruler approximation is employed to calculate the sound velocity of the soil, which is then inverted to determine the soil moisture content. The experiment measured the soil within a two-dimensional area formed by the seismic source and the acoustic sensing optical cable. The sensing optical cable and the active seismic source are organized into a two-dimensional area and the measurement range is 3 × 10 m with 33 points. The SMC ranges from 15% to 40%. The experiment shows that the absolute error between the measured values obtained by DAS and the water cut meter is 7%. This experiment verifies the feasibility of using the Rayleigh scattering properties to invert SMC and provides a new method for real-time monitoring of SMC in a large area.

Fabrication of a three-dimensional (3D) SERS fiber probe and application of in situ detection.

Surface-enhanced Raman scattering (SERS) fiber probes are useful for remote and online detection of harmful molecules using the SERS effect. In this study, a 3-dimensional (3D) SERS optical fiber probe is proposed. The formation of the 3D optical fiber probe mainly included three steps: construction of monolayer polystyrene (PS) spheres as a mask on the end face of the fiber, reactive ion etching (RIE) for PS spheres and fibers, and metal sputtering deposition. Compared with flat surface fiber probes, these 3D SERS fiber probes are composed of ordered nanocolumn arrays, which have the advantages of a simple manufacturing process, low cost, high sensitivity, and good stability. The structures of the 3D SERS fiber probe can be well controlled by changing the size of the PS sphere and etching time. The formation of the nanocolumn was studied using time evolution experiments. The obtained fiber SERS probe has good stability and high sensitivity for the in situ detection of 4-aminothiophenol (4-ATP) in solution. Therefore, these 3D SERS fiber probes have potential applications in harmful molecules for real-time detection.

COVID-19 lockdowns, stimulus packages, travel bans, and stock returns.

This paper examines the effect of government responses of G7 countries to the coronavirus pandemic (COVID-19) on stock market returns. Using time-series data, we show that lockdowns, travel bans, and economic stimulus packages all had a positive effect on the G7 stock markets. However, lockdowns were most effective in cushioning the effects of COVID-19. Our results are robust to different measures of returns and controls for other factors of returns.

Ordered gold-coated glass nano-sting array with large density tips as highly SERS-active chips for detection of trace organophosphorous toxicant.

A simple and cost-effective route to fabricating gold-coated glass nano-stings' arrays is presented for highly efficient surface enhanced Raman spectroscopy (SERS) chips via reactive ion etching of the glass slide covered with polystyrene (PS) colloidal monolayer and sputtering deposition of gold on it. The as-fabricated SERS chips consist of the nearly-hemispherical particles with submicron-size, which are hexagonally arranged into the ordered arrays. There are many short and long Au-coated glass nano-stings standing vertically on the hemispherical particles, showing high density tips (up to 7.0 × 103 µm-2). The formation of such gold-coated glass hierarchically structured array is attributed to the geometry of PS colloidal monolayer and the nanoscale roughness of glass slide's surface. Further, it has been demonstrated that such gold-coated glass array chip is highly SERS-active and can be utilized to detect trace organophosphorous toxicant with the limit below ppt level and good reproducibility in measurements. The intensity of the Raman peak shows the linear relation, in logarithmic scale, with the organophosphorous concentration from ppt to ppm level, exhibiting the possibility of quantitative detection in a large concentration range. This work has demonstrated the cost-effective method to fabricate the high performance SERS chips for trace detection of toxic molecules.

Engineering of flexible granular Au nanocap ordered array and its surface enhanced Raman spectroscopy effect.

Surface enhanced Raman spectroscopy (SERS) is a new and developing analytical technology in chemical and biological detection. However, traditional hard SERS substrates are struggling to meet the growing demand for flexible devices. In this work, we introduce a simple, cost-effective and large scale preparation route to form a flexible Au nanocap (AuNC) ordered array as SERS substrates via reactive ion etching (RIE) method and then Au deposition. We find RIE is an excellent method for nanoroughening the surface of polystyrene (PS) spheres. Such flexible SERS substrates exhibit high sensitivity and uniformity for detecting organic molecules. The finite-difference time-domain simulation results revealed that a strong electric field coupling effect existed not only in the gap site between the Au nanoparticles (AuNPs), but also in the connection position between the AuNCs and the single AuNP. This study not only offers a novel way for nanoroughening of PS spheres, but also acquires flexible and cheap SERS substrates for quick and sensitive detection of organic molecules.

Fabrication of Ag-nanosheets-built micro/nanostructured arrays via in situ conversion on Cu2O-coated Si nanocone platform and their highly structurally-enhanced SERS effect.

A controllable and flexible route is presented for the fabrication of Ag-nanosheets-built micro/nanostructural ordered arrays via in situ conversion on the Cu2O-coated silicon nanocone (SNC) platform in the AgNO3-contained solution. The platform is pre-prepared by the reactive ion etching of the organic colloidal monolayer-covered silicon wafer, Cu sputtering deposition and in situ oxidation. The obtained Ag micro/nanostructured array consists of nearly spherical and micro-sized particles, which are hexagonally arranged on the substrate. The spherical particles are built of the vertically standing and cross-linked nanosheets with about 30 nm in thickness. This Ag-nanosheets-built array shows high number density of the edges and nanogaps as well as the robust and homogeneous structure. Its formation is attributed to the in situ conversion reaction on the Cu2O-coated SNC platform and the preferentially-oriented connection of Ag nanoparticles. Such Ag array has shown significantly higher surface enhanced Raman scattering (SERS) activity than the Ag nanoparticles' film-covered SNC array, with the enhancement factor up to 107 and the detection limitation down to ∼1 ppt level to the test molecules 4-aminothiophenol, as well as the good reproducibility in measurements. This study not only presents a controllable and flexible fabrication route to the plasmonic micro/nanostructured arrays but also provides the highly efficient and the practical chips for the SERS-based devices.

Controllable corrosion-assisted fabrication of Au-Ag alloyed hollow nanocrystals for highly efficient and environmentally-stable SERS substrates.

Surface enhanced Raman scattering (SERS) substrates with both high activity and long term chemical-stability have been expected in the practical application of the SERS-based detection. In this paper, Au-Ag bimetal nanocrystals are fabricated based on the template-etching reaction in the Ag nanocubes-contained cetylpyridinium chloride (CPC) aqueous solution via adding the HAuCl4 solution. The obtained nanocrystals are Au-Ag alloyed and hollow in structure. Further, it has been found that with the increasing Au/Ag molar ratio, the shape of the alloyed nanocrystals evolve from the truncated nanocubes to the hollow boxes and then nanocages, showing the ever red-shifting surface plasmon resonance from the visible to the infrared region. The formation of the alloyed hollow nanocrystals is attributed to the preferential dissolution of the Ag nanocubes induced by CPC selective adsorption and the three to one galvanic replacement reaction between Ag and Au atoms. Importantly, such Au-Ag alloyed hollow nanocrystals, especially the ones with a low Au/Ag atomic ratio, show both high SERS activity and long term environmental stability compared with pure Ag or Au nanocrystals, and are the ideal candidate for the SERS substrate with practical application value. This work not only demonstrates the nanofabrication route to the alloyed hollow nanocrystals with controllable shapes and tunable optical properties in a large region, but also presents highly active and chemically-stable SERS substrates for the practical SERS-based detection.

One-Step and Surfactant-Free Fabrication of Gold-Nanoparticle-Decorated Bismuth Oxychloride Nanosheets Based on Laser Ablation in Solution and Their Enhanced Visible-Light Plasmonic Photocatalysis.

A simple synthetic route is presented for fabricating gold nanoparticle (NP)-decorated bismuth oxychloride (BiOCl) nanosheets in one step based on laser ablation of a gold target in a hydrochloric acid solution of bismuth nitrate without surfactant. After laser ablation, BiOCl nanosheets with attached Au NPs are obtained. The nanosheets are sub-micron in the planar dimension and around 20 nm thick, and the Au NPs are a mean size of approximately 20 nm. Further experiments reveal that such Au-NP-decorated nanosheets could be formed at a large Cl/Bi molar ratio range (0.01 to 3) in solution. The formation of the BiOCl nanosheets is attributed to the Au plasma plume-induced local fast hydrothermal reaction, which drives the planar growth of BiOCl. Importantly, these Au-NP-decorated BiOCl nanosheets exhibit high photodegradation activity on rhodamine B, a typical organic pollutant, compared with bare nanosheets under visible light irradiation, and show highly stable and recyclable performance. This is attributed to the plasmonic properties of Au NPs, which increase optical absorption and promote separation of electron-hole pairs in the NP-decorated BiOCl nanosheets. This work provides not only a new plasmonic photocatalyst for the oxidative degradation of organic pollutants, but also a general method for fabrication of the metal-NP-decorated nanosheets of other layer-structured oxychlorides.

Nanoscaled Amorphous TiO2 Hollow Spheres: TiCl4 Liquid Droplet-Based Hydrolysis Fabrication and Strong Hollow Structure-Enhanced Surface-Enhanced Raman Scattering Effects.

A very simple route is developed for fast fabrication of nanosized amorphous titanium dioxide (TiO2) hollow spheres (THPs) just via dropping the pure four titanium chloride (TiCl4) liquid droplets into deionized water at around room temperature. The THPs, at around 80 nm in mean diameter, can be formed within a few seconds after dropping TiCl4 droplets into water. The shell layers of the obtained THPs are amorphous and porous in structure with a porosity of 58-80% and show a linear increase in thickness with the size of THPs. Further experiments have revealed that the reaction temperature, initial pH value, and size of the TiCl4 droplet are crucial to the formation, size, productivity, and microstructure of the THPs. A model is proposed on the basis of the fragmentation of liquid droplets, hydrolysis-induced formation, and inward growth of TiO2 shell layers, which can well describe the formation of the THPs. Importantly, such amorphous nanoscaled THPs have exhibited some strong hollow structure-enhanced performances. Typically, the THP-built film shows the highest reflectivity in the visible region compared to the other structured TiO2 films. Especially, if it supports the film of the Au nanoparticle, the surface-enhanced Raman scattering effect is significantly enhanced by more than 1 order of magnitude. This work provides not only a simple and quick fabrication method for the THPs but also a new member for their family.

Ultrathin Oxide Layer-Wrapped Noble Metal Nanoparticles via Colloidal Electrostatic Self-Assembly for Efficient and Reusable Surface Enhanced Raman Scattering Substrates.

Controllable and flexible fabrication of ultrathin and uniform oxide layer-wrapped noble metal nanoparticles (NPs) has been expected. Here a new strategy is presented for them based on colloidal electrostatic attraction and self-assembly on the metal NPs via one-step laser ablation of noble metal targets in the hydrolysis-induced hydroxide sol solutions at room temperature. The Au NPs, with several tens of nanometers in size, are taken as core part and TiO2 as shell-layer to demonstrate the validity of the presented strategy. It has been shown that the TiO2 shell-wrapped Au NPs are obtained after laser ablation of Au target in the hydrolysis-induced Ti(OH)4 sol solution. The Au NPs are about 35 nm in mean size, and the TiO2 shell layers are amorphous in structure and about 2.5 nm in thickness. The shell thickness is nearly independent of the Au NPs' size. Further experiments have shown that the thickness and crystallinity of the shell-layer can be tuned and controlled via changing the temperature or pH value of the Ti(OH)4 sol solution or prolonging the laser ablation duration. The formation of the TiO2 shell-wrapped Au NPs is attributed to attachment and self-assembly of Ti(OH)4 colloids on the laser-induced Au NPs due to the electrostatic attraction between them. Importantly, the presented strategy is universal and suitable for fabrication of many other ultrathin oxide-wrapped noble metal NPs. A series of oxide shell-wrapped noble metal NPs have been successfully fabricated, such as Au@oxides (Fe2O3, Al2O3, CuO, and ZnO) as well as Pt@TiO2 and Pd@TiO2, etc. Further, compared with the pure gold NPs-built film, the TiO2-wrapped Au NPs-built film has exhibited much stronger surface enhanced Raman scattering (SERS) performance to the anions NO3-, which weakly interact with noble metals, and the good reusability for the SERS-based detection of 4-nitrophenol, which could be photodegraded by xenon lamp irradiation. This work provides a flexible and universal route to the ultrathin and uniform oxide layer-wrapped noble metal NPs.

Synthesis of Si/SiO2 core-shell nanowire arrays and broadband anti-reflection effects in diluted Si nanowire arrays by adjusting dielectric shell thickness.

A low filling ratio and enhanced absorption is needed to enable the full potential of Si nanowire (NW) arrays for optoelectronic applications. In this paper, we report a versatile, scalable fabrication technique that uses nanosphere lithography (NSL) patterning for the synthesis of vertically aligned Si and Si/SiO2 NW arrays. The optical reflection of the NW arrays can be substantially suppressed by the addition of the transparent shell. Meanwhile, by the finite-difference time-domain (FDTD) simulation, we find that the absorption enhancement in the core Si NW can be obtained by adding the transparent shell. The special absorption enhancement of the Si NW arrays with a core-shell structure can be theoretically understood by modal analysis. The absorption in such Si NW array structures is very sensitive to the thickness of transparent coating. By the addition of a SiO2 shell layer, the absorption in the inner Si NW array can be substantially enhanced. Furthermore, significant absorption enhancement and broadband anti-reflection effects can be achieved by the diluted Si NWs combined with the single dielectric shell.

Dual-channel extraordinary ultraviolet transmission through an aluminum nanohole array.

Ultraviolet (UV) surface plasmon (SP) has distinct applications in UV filters, high-density optical storage, spectral enhancement, optical detectors, and nanolithography, which are closely related to plasmon-induced extraordinary optical transmission (EOT). However, such EOT in the UV region has not been the subject of detailed research. We report UV transmission based on theoretical research using the finite-difference time-domain method, by modulating the Al thickness, hole size, array periodicity, and SiO2 overlayer thickness. It is notable that we can obtain dual-channel UV transmission peaks with excellent qualities such as high transmissivity, zero cross-talk, narrow bandwidth, and perfect symmetry, by optimizing the parameters. The UV transmission peaks have been discovered to non-monotonously shift with increasing hole size. Although array periodicity has great influence on the transmission peak position, the peak energy in the UV region is much less than the value predicted by the well-known periodicity-related surface plasmon polariton (SPP) wavelength equation; the energy discrepancy in the UV region can reach above 20%, which is much larger than the value (typically 4%) in the visible-infrared region. Furthermore, the SiO2 overlayer may significantly modify the transmission properties. The Al nanohole arrays have also been found to exhibit distinct multi-band UV electric field enhancement properties with special interface effect and size effect. Such extraordinary dual-channel UV transmission with zero cross-talk, based on a very simple Al nanohole array, has promising application in dual-channel UV filters, high-density optical storage, and plasmon-enhanced fluorescence/Raman spectroscopy, which generally involves two wavebands (writing/reading storage or exciting/emission wavelengths). This study is expected to broaden our fundamental understanding of the UV EOT phenomenon, and provide references for experimental research and application of deep-UV and near-UV-related dual-band plasmonic devices.

Effective SERS-active substrates composed of hierarchical micro/nanostructured arrays based on reactive ion etching and colloidal masks.

A facile route has been proposed for the fabrication of morphology-controlled periodic SiO2 hierarchical micro/nanostructured arrays by reactive ion etching (RIE) using monolayer colloidal crystals as masks. By effectively controlling the experimental conditions of RIE, the morphology of a periodic SiO2 hierarchical micro/nanostructured array could be tuned from a dome-shaped one to a circular truncated cone, and finally to a circular cone. After coating a silver thin layer, these periodic micro/nanostructured arrays were used as surface-enhanced Raman scattering (SERS)-active substrates and demonstrated obvious SERS signals of 4-Aminothiophenol (4-ATP). In addition, the circular cone arrays displayed better SERS enhancement than those of the dome-shaped and circular truncated cone arrays due to the rougher surface caused by physical bombardment. After optimization of the circular cone arrays with different periodicities, an array with the periodicity of 350 nm exhibits much stronger SERS enhancement and possesses a low detection limit of 10(-10) M 4-ATP. This offers a practical platform to conveniently prepare SERS-active substrates.

Physical deposition improved SERS stability of morphology controlled periodic micro/nanostructured arrays based on colloidal templates.

An effective and inexpensive method is developed to fabricate periodic arrays by sacrificial colloidal monolayer template route by chemical deposition and further physical deposition. By a colloidal template induced precursor solution dipping strategy, different periodic arrays of semi-hollow sphere array, inverse opal with monolayer pore arrays and hole arrays are obtained under different conditions. After magnetron sputtering deposition, their morphologies are changed to novel micro/nanostructured arrays of honeycomb-shaped arrays, hollow cavity arrays, and regular network arrays due to multiple direction deposition of sputtering deposition and shadow effect. After coating a gold thin layer, these periodic micro/nanostructured arrays are used as SERS active substrates and demonstrate a very stable SERS performance compared with periodic arrays achieved by direct colloidal template-induced chemical deposition. Additionally, a honeycomb-shaped array displays better SERS enhancement than that of a hollow cavity array or a regular network array. After optimization of honeycomb-shaped arrays with different periodicities, an array with periodicity of 350 nm demonstrates much stronger SERS enhancement and possesses a low detection limit of 10(-11) M R6G. Such stable SERS performance is useful for practical application in portable Raman detecting devices to detect organic molecules.

Absorption enhancement of GaInP nanowires by tailoring transparent shell thicknesses and its application in III-V nanowire/Si film two-junction solar cells.

A non-absorbing transparent shell is proposed to be coated on the outer surface of the core photoactive GaInP nanowire array (NWA) of the III-V nanowire (NW)/Si film two-junction solar cell. Interestingly, the diluted (at the filling ratio of 0.25) GaInP NWA with core / transparent shell structure can absorb more light than that in bare denser (at the filling ratio of 0.5) NWA. This allows for less source material consumption during the fabrication of III-V NWA/Si film two-junction cell. Meanwhile, the condition of current matching between the top III-V NWA and Si film sub cell can be easily fulfilled by tailoring the coating thickness of the transparent coating. Beyond the advantages on light absorption, the surface passivation effects introduced by the addition of some transparent dielectric coatings can reduce the surface recombination rate at the top NWA sub cell surface. This facilitates the effective extraction of photo-generated carriers and enhances output stability of the top NWA sub cell. From electrical simulation, a power conversion efficiency of 29.9% can be obtained at the optimized coating geometry.

Gold binary-structured arrays based on monolayer colloidal crystals and their optical properties.

A simple and flexible route is presented to fabricate a gold binary-structured ordered array by one step based on non-shadow deposition on a plasma etching-induced dualistic monolayer colloidal crystal. Such a Au binary-structure array is built of hexagonally arranged nanoshells and nanorings which stand between two adjacent nanoshells. Six gold nanorings surround each nanoshell. The obtained arrays exhibit both the controllable surface-plasmon-resonance (SPR) properties of Au nanoshells and the strong electromagnetic-field-enhancement effects of Au nanorings, with the high structural stability of ordered arrays, and show promising potential as the substrate of surface-enhanced Raman scattering (SERS)-based devices. The method could also be suitable for fabrication of other material binary-structured arrays. This study is important in designing and fabricating basal materials for the next generation of multifunctional nanostructured devices.

CuO-ZnO micro/nanoporous array-film-based chemosensors: new sensing properties to H2S.

CuO-ZnO micro/nanoporous array-films are synthesized by transferring a solution-dipped self-organized colloidal template onto a device substrate and sequent heat treatment. Their morphologies and structures are characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectrum analysis. Based on the sensing measurement, it is found that the CuO-ZnO films prepared with the composition of [Cu(2+)]/[Zn(2+)]=0.005, 0.01, and 0.05 all show a nice sensitivity to 10 ppm H2S. Interestingly, three different zones exist in the patterns of gas responses versus H2S concentrations: a platform zone, a rapidly increasing zone, and a slowly increasing zone. Further experiments show that the hybrid CuO-ZnO porous film sensor exhibits shorter recovery time and better selectivity to H2S gas against other interfering gases at a concentration of 10 ppm. These new sensing properties may be due to a depletion layer induced by p-n junction between p-type CuO and n-type ZnO and high chemical activity of CuO to H2S. This work will provide a new construction route of ZnO-based sensing materials, which can be used as H2S sensors with high performances.