Photonic Fractal Metamaterials: A Metal-Semiconductor Platform with Enhanced Volatile-Compound Sensing Performance

Ab initio investigation of hot electron transfer in CO2 plasmonic photocatalysis in the presence of hydroxyl adsorbate

Photoreduction of carbon dioxide (CO2) on plasmonic structures is of great interest in photocatalysis to aid selectivity. While species commonly found in reaction environments and associated intermediates can steer the reaction down different pathways by altering the potential energy landscape of the system, they are often not addressed when designing efficient plasmonic catalysts. Here, we perform an atomistic study of the effect of the hydroxyl group (OH) on CO2 activation and hot electron generation and transfer using first-principles calculations. We show that the presence of OH is essential in breaking the linear symmetry of CO2, which leads to a charge redistribution and a decrease in the OCO angle to 134°, thereby activating CO2. Analysis of the partial density of states (pDOS) demonstrates that the OH group mediates the orbital hybridization between Au and CO2 resulting in more accessible states, thus facilitating charge transfer. By employing time-dependent density functional theory (TDDFT), we quantify the fraction of hot electrons directly generated into hybridized molecular states at resonance, demonstrating a broader energy distribution and an 11% increase in charge-transfer in the presence of OH groups. We further show that the spectral overlap between excitation energy and plasmon resonance plays a critical role in efficiently modulating electron transfer processes. These findings contribute to the mechanistic understanding of plasmon-mediated reactions and demonstrate the importance of co-adsorbed species in tailoring the electron transfer processes, opening new avenues for enhancing selectivity.

Advances in flame synthesis of nano-scale architectures for chemical, biomolecular, plasmonic, and light sensing

Flame spray pyrolysis (FSP), a key technique under the broader category of flame aerosol synthesis, is being increasingly explored for the design of advanced miniaturized sensor architectures with applications including chemical, biomolecular, plasmonic, and light sensing. This review provides an overview of the advantages of FSP for the fabrication of nanostructured materials for sensing, delving into synthesis strategies and material structures that meet the increasing demands for miniaturized sensor devices. We focus on the fundamentals of FSP, discussing reactor configurations and how process parameters such as precursor compositions, flow rates, and temperature influence nanoparticle characteristics and their sensing performance. A detailed analysis of nanostructures, compositions, and morphologies made by FSP and their applications in chemical, chemiresistive, plasmonic, biosensing, and light sensing is presented. This review identifies the challenges and opportunities of FSP, exploring current limitations and potential improvements for industrial translation. We conclude by highlighting future research directions aiming to establish guidelines for the flame-based design of nano-scale sensing architectures.

Structural Engineering Three-Dimensional Nano-Heterojunction Networks for High-Performance Photochemical Sensing

Nanoscale heterojunction networks are increasingly regarded as promising functional materials for a variety of optoelectronic and photocatalytic devices. Despite their superior charge-carrier separation efficiency, a major challenge remains in the optimization of their surface properties, with surface defects playing a major role in charge trapping and recombination. Here, we report the effective engineering of the photocatalytic properties of nanoscale heterojunction networks via deep ultraviolet photoactivation throughout their cross-section. For the first time, in-depth XPS analysis of very thick (∼10 μm) NixOy-ZnO films reveals localized p-n nanoheterojunctions with tunable oxygen vacancies (Vo) originating from both NixOy and ZnO nanocrystals. Optimizing the amount of oxygen vacancies leads to a 30-fold increase in the photochemoresistive response of these networks, enabling the detection of representative analyte concentrations down to 2 and 20 ppb at an optimal temperature of 150 °C and room temperature, respectively. Density functional theory calculations reveal that this performance enhancement is presumably due to an 80% increase in the analyte adsorption energy. This flexible nanofabrication approach in conjunction with straightforward vacancy control via photoactivation provides an effective strategy for engineering the photocatalytic activity of porous metal oxide semiconductor networks with applications in chemical sensors, photodetectors, and photoelectrochemical cells.

Understanding the Role of (W, Mo, Sb) Dopants in the Catalyst Evolution and Activity Enhancement of Co3 O4 during Water Electrolysis via In Situ Spectroelectrochemical Techniques

Unlocking the potential of the hydrogen economy is dependent on achieving green hydrogen (H₂ ) production at competitive costs. Engineering highly active and durable catalysts for both oxygen and hydrogen evolution reactions (OER and HER) from earth-abundant elements is key to decreasing costs of electrolysis, a carbon-free route for H₂ production. Here, a scalable strategy to prepare doped cobalt oxide (Co₃ O₄ ) electrocatalysts with ultralow loading, disclosing the role of tungsten (W), molybdenum (Mo), and antimony (Sb) dopants in enhancing OER/HER activity in alkaline conditions, is reported. In situ Raman and X-ray absorption spectroscopies, and electrochemical measurements demonstrate that the dopants do not alter the reaction mechanisms but increase the bulk conductivity and density of redox active sites. As a result, the W-doped Co₃ O₄ electrode requires ≈390 and ≈560 mV overpotentials to reach ±10 and ±100 mA cm^-2 for OER and HER, respectively, over long-term electrolysis. Furthermore, optimal Mo-doping leads to the highest OER and HER activities of 8524 and 634 A g^-1 at overpotentials of 0.67 and 0.45 V, respectively. These novel insights provide directions for the effective engineering of Co₃ O₄ as a low-cost material for green hydrogen electrocatalysis at large scales.

Multigram-Scale Production of Hybrid Au-Si Nanomaterial by Laser Ablation in Liquid (LAL) for Temperature-Feedback Optical Nanosensing, Light-to-Heat Conversion, and Anticounterfeit Labeling

Recent progress in hybrid optical nanomaterials composed of dissimilar constituents permitted an improvement in the performance and functionality of novel devices developed for optoelectronics, catalysis, medical diagnostics, and sensing. However, the rational combination of contrasting materials such as noble metals and semiconductors within individual hybrid nanostructures via a ready-to-use and lithography-free fabrication approach is still a challenge. Here, we report on a two-step synthesis of hybrid Au-Si microspheres generated by laser ablation of silicon in isopropanol followed by laser irradiation of the produced Si nanoparticles in the presence of HAuCl₄. Thermal reduction of [AuCl₄]^- species to a metallic gold phase, along with its subsequent mixing with silicon under laser irradiation, creates a nanostructured material with a unique composition and morphology, as revealed by electron microscopy, tomography, and elemental analysis. A combination of basic plasmonic and nanophotonic materials such as gold and silicon within a single microsphere allows for efficient light-to-heat conversion, as well as single-particle SERS sensing with temperature-feedback modality and expanded functionality. Moreover, the characteristic Raman signal and hot-electron-induced nonlinear photoluminescence coexisting within the novel Au-Si hybrids, as well as the commonly criticized randomness of the nanomaterials prepared by laser ablation in liquid, were proved to be useful for the realization of anticounterfeiting labels based on a physically unclonable function approach.

Ag-Decorated Si Microspheres Produced by Laser Ablation in Liquid: All-in-One Temperature-Feedback SERS-Based Platform for Nanosensing

Combination of dissimilar materials such as noble metals and common semiconductors within unified nanomaterials holds promise for optoelectronics, catalysis and optical sensing. Meanwhile, difficulty of obtaining such hybrid nanomaterials using common lithography-based techniques stimulates an active search for advanced, inexpensive, and straightforward fabrication methods. Here, we report one-pot one-step synthesis of Ag-decorated Si microspheres via nanosecond laser ablation of monocrystalline silicon in isopropanol containing AgNO₃. Laser ablation of bulk silicon creates the suspension of the Si microspheres that host further preferential growth of Ag nanoclusters on their surface upon thermal-induced decomposition of AgNO₃ species by subsequently incident laser pulses. The amount of the AgNO₃ in the working solution controls the density, morphology, and arrangement of the Ag nanoclusters allowing them to achieve strong and uniform decoration of the Si microsphere surface. Such unique morphology makes Ag-decorated Si microspheres promising for molecular identification based on the surface-enhanced Raman scattering (SERS) effect. In particular, the designed single-particles sensing platform was shown to offer temperature-feedback modality as well as SERS signal enhancement up to 10⁶, allowing reliable detection of the adsorbed molecules and tracing their plasmon-driven catalytic transformations. Considering the ability to control the decoration degree of Si microspheres by Ag nanoclusters via amount of the AgNO₃, the developed one-pot easy-to-implement PLAL synthesis holds promise for gram-scale production of high-quality hybrid nanomaterial for various nanophotonics and sensing applications.

Surface Functionalization and Texturing of Optical Metasurfaces for Sensing Applications

Optical metasurfaces are planar metamaterials that can mediate highly precise light-matter interactions. Because of their unique optical properties, both plasmonic and dielectric metasurfaces have found common use in sensing applications, enabling label-free, nondestructive, and miniaturized sensors with ultralow limits of detection. However, because bare metasurfaces inherently lack target specificity, their applications have driven the development of surface modification techniques that provide selectivity. Both chemical functionalization and physical texturing methodologies can modify and enhance metasurface properties by selectively capturing analytes at the surface and altering the transduction of light-matter interactions into optical signals. This review summarizes recent advances in material-specific surface functionalization and texturing as applied to representative optical metasurfaces. We also present an overview of the underlying chemistry driving functionalization and texturing processes, including detailed directions for their broad implementation. Overall, this review provides a concise and centralized guide for the modification of metasurfaces with a focus toward sensing applications.

Controlled Deposition of Nanostructured Hierarchical TiO2 Thin Films by Low Pressure Supersonic Plasma Jets

Plasma-assisted supersonic jet deposition (PA-SJD) is a precise technique for the fabrication of thin films with a desired nanostructured morphology. In this work, we used quadrupole mass spectrometry of the neutral species in the jet and the extensive characterization of TiO₂ films to improve our understanding of the relationship between jet chemistry and film properties. To do this, an organo–metallic precursor (titanium tetra–isopropoxide or TTIP) was first dissociated using a reactive argon–oxygen plasma in a vacuum chamber and then delivered into a second, lower pressure chamber through a nozzle. The pressure difference between the two chambers generated a supersonic jet carrying nanoparticles of TiO₂ in the second chamber, and these were deposited onto the surface of a substrate located few centimeters away from the nozzle. The nucleation/aggregation of the jet nanoparticles could be accurately tuned by a suitable choice of control parameters in order to produce the required structures. We demonstrate that high-quality films of up to several µm in thickness and covering a surface area of few cm² can be effectively produced using this PA-SJD technique.

Flexible Plasmonic Biosensors for Healthcare Monitoring: Progress and Prospects

The noble metal nanoparticle has been widely utilized as a plasmonic unit to enhance biosensors, by leveraging its electric and/or optical properties. Integrated with the "flexible" feature, it further enables opportunities in developing healthcare products in a conformal and adaptive fashion, such as wrist pulse tracers, body temperature trackers, blood glucose monitors, etc. In this work, we present a holistic review of the recent advance of flexible plasmonic biosensors for the healthcare sector. The technical spectrum broadly covers the design and selection of a flexible substrate, the process to integrate flexible and plasmonic units, the exploration of different types of flexible plasmonic biosensors to monitor human temperature, blood glucose, ions, gas, and motion indicators, as well as their applications for surface-enhanced Raman scattering (SERS) and colorimetric detections. Their fundamental working principles and structural innovations are scoped and summarized. The challenges and prospects are articulated regarding the critical importance for continued progress of flexible plasmonic biosensors to improve living quality.

Magnetic-Field Directed Vapor-Phase Assembly of Low Fractal Dimension Metal Nanostructures: Experiment and Theory

While gas-phase synthesis techniques offer a scalable approach to production of metal nanoparticles, directed assembly is challenging due to fast particle diffusion rates that lead to random Brownian aggregation. This work explores an electromagnetic-levitation technique to generate metal nanoparticle aggregates with fractal dimension (D_f) below that of diffusion limited assembly. We demonstrate that in addition to levitation and induction heating, the external magnetic field is sufficient to compete with random Brownian forces, which enables the formation of altered fractals. Ferromagnetic metals (Fe, Ni) form chain-like aggregates, while paramagnetic Cu forms compact nanoparticle aggregates with higher D_f values. We have also employed a Monte Carlo simulation to evaluate the necessary field strength to form linear chains in the gas phase.