Photoinduced Enhanced Raman from Lithium Niobate on Insulator Template

Modulation of photo-induced Raman enhancement in Ag nanoparticles deposited on nanometer-thick TiO2 films. An interplay between plasmonic properties and irradiation energy

Photo-induced enhanced Raman spectroscopy (PIERS) is an innovative technology that offers additional enhancement in Raman signal compared to surface-enhanced Raman spectroscopy (SERS). In this study, we fabricated nanohybrids consisting of silver nanoparticles on an ultra-thin anatase film using a photoreduction method. This approach allowed for the controllable synthesis of SERS and PIERS nanoplatforms, characterized by oval-shaped nanoparticles, yet varying in size and surface coverage, leading to distinct plasmonic properties. A mere 15-minute UV pre-treatment with low photon density already initiated significant charge-transfer processes followed by Raman spectra under non-resonant conditions of the molecule and estimated by enhancement factor in the range of 12---17. This phenomenon was observed for a molecular monolayer of a thiol derivative. Not only boosting electron migration appeared. This unique interface of the Ag-anatase composite undoubtedly contributed to extended relaxation times of photo-induced enhancement. Furthermore, we investigated how plasmonic and morphological features of the nanoplatforms, in conjunction with UV and Vis illumination, modulated the migration of photoinduced electrons from the semiconductor to the metal. These findings highlighted the variety of processes contributing to the creation of efficient PIERS materials.

Photo-induced enhanced Raman spectroscopy as a probe for photocatalytic surfaces

Photo-induced enhanced Raman spectroscopy (PIERS) has emerged as a highly sensitive surface-enhanced Raman spectroscopy (SERS) technique for the detection of ultra-low concentrations of organic molecules. The PIERS mechanism has been largely attributed to UV-induced formation of surface oxygen vacancies (Vo) in semiconductor materials, although alternative interpretations have been suggested. Very recently, PIERS has been proposed as a surface probe for photocatalytic materials, following Vo formation and healing kinetics. This work establishes comparison between PIERS and Vo-induced SERS approaches in defected noble-metal-free titanium dioxide (TiO2-x) films to further confirm the role of Vo in PIERS. Upon application of three post-treatment methods (namely UV-induction, vacuum annealing and argon etching), correlation of Vo kinetics and distribution could be established. A proposed mechanism and further discussion on PIERS as a probe to explore photocatalytic materials are also presented. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Dichalcogenide and Metal Oxide Semiconductor-Based Composite to Support Plasmonic Catalysis

Nanocomposites comprising plasmon active metal nanostructures and semiconductors have been used to control the charge states in the metal to support catalytic activity. In this context dichalcogenides when combined with metal oxides offer the potential to control charge states in plasmonic nanomaterials. Using a model plasmonic mediated oxidation reaction p-amino thiophenol ↔ p-nitrophenol, we show that through the introduction of transition metal dichalcogenide nanomaterial, reaction outcomes can be influenced, achieved through controlling the occurrence of the reaction intermediate dimercaptoazobenzene by opening new electron transfer routes in a semiconductor-plasmonic system. This study demonstrates the ability to control plasmonic reactions by carefully controlling the choice of semiconductors.

Effect of Surface Charge Characteristics of Ferroelectric LiNbO3 on Wettability of Ionic Liquids

Electrowetting is a widely used and effective method to tune the wettability of ionic liquids at solid-liquid interfaces, but it usually requires an external electric field. Here, we proposed a strategy for conveniently tuning ionic liquid wettability by adopting ferroelectric LiNbO₃ single crystals as functional substrates. A heating pretreatment process was applied to modulate the surface charge characteristics of LiNbO₃ substrates, leading to an improved wettability of [EMIM][BF₄] and [EMIM][NTf₂] on the LiNbO₃ substrates with both positively poled (+Z) and negatively poled (−Z) surfaces. This work may be of great interest in the field of ferroelectric-based microelectronics.

New Mechanism for Long Photo-Induced Enhanced Raman Spectroscopy in Au Nanoparticles Embedded in TiO2

The photo-induced enhanced Raman spectroscopy (PIERS) effect is a phenomenon taking place when plasmonic nanoparticles deposited on a semiconductor are illuminated by UV light prior to Raman measurement. Results from the literature show that the PIERS effect lasts for about an hour. The proposed mechanism for this effect is the creation of oxygen vacancies in the semiconductor that would create a path for charge transfer between the analyte and the nanoparticles. However, this hypothesis has never been confirmed experimentally. Furthermore, the tested structure of the PIERS substrate has always been composed of plasmonic nanoparticles deposited on top of the semiconductor. Here, gold nanoparticles co-deposited with porous TiO₂ are used as a PIERS substrate. The deposition process confers the nanoparticles a unique position half buried in the nanoporous semiconductor. The resulting PIERS intensity is among the highest measured until now but most importantly the duration of the effect is significantly longer (at least 8 days). Cathodoluminescence measurements on these samples show that two distinct mechanisms are at stake for co-deposited and drop-casted gold nanoparticles. The oxygen vacancies hypothesis tends to be confirmed for the latter, but the narrowing of the depletion zone explains the long PIERS effect.

Enhanced photoluminescence and shortened lifetime of DCJTB by photoinduced metal deposition on a ferroelectric lithography substrate

The photodeposition of metallic nanostructures onto ferroelectric surfaces could enable new applications based on the assembly of molecules and patterning local surface reactivity by enhancing surface field intensity. DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) is an excellent fluorescent dye and dopant material with a high quantum efficiency used for OLED displays on the market. However, how to raise the photoluminescence (PL) and reduce the lifetime of DCJTB in a substrate remain extraordinary challenges for its application. Here, we demonstrate a tunable ferroelectric lithography plasmon-enhanced substrate to generate photo-reduced silver nanoparticles (AgNPs) and achieve enhanced PL with a shortened lifetime depending on the substrate's annealing time. The enhanced PL with shortened lifetimes can attribute to the localized electromagnetic (EM) wave produced by the nanotextured AgNPs layers' surface and gap plasmon resonances. The simulation is based on the three-dimensional finite element method to explain the mechanism of experimental results. Since the absorption increases, the remarkable enhanced PL of DCJTB can attain in the fabricated periodically proton exchanged (PPE) lithium niobate (LiNbO₃) substrate. Furthermore, the proposed fabrication method demonstrates to help tune the surface EM wave distribution in the substrate, which can simultaneously achieve the significantly shortened lifetime and high PL intensity of DCJTB in the substrate. Compared with the un-annealed substrate, the PL intensity of DCJTB in the assembly metallic nanostructures is enhanced 13.70 times, and the PL's lifetime is reduced by 12.50%, respectively. Thus, the fabricated substrate can be a promising candidate, verifying chemically patterned ferroelectrics' satisfaction as a PL-active substrate.

Recent advances and perspectives in photo-induced enhanced Raman spectroscopy

Phototreatment is at the leading edge of a research hot topic as a driving force for structural transformation, spectral and electromagnetism improvements, and the functional performance of nanomaterials. Light irradiation can excite surface plasmons in noble metal nanoparticles, create electron-hole pairs, and produce charge transfer in semiconductor substrates, which have led to it being widely used in surface-enhanced Raman spectroscopy (SERS) for life sciences, environmental protection, and biological analysis. Photo-induced enhanced Raman spectroscopy (PIERS) is a new technology developed on the basis of traditional SERS and has proven to be an efficient way to resolve several critical challenges thanks to its incomparable superiority for incontiguous operation, efficient charge separation and enrichment, and a large signal enhancement for a wide range of biomolecules at the trace level. This makes PIERS a powerful technique with very appealing and promising applications in various branches of analytical science. In this review, the enhancement mechanisms of PIERS are analyzed in comparison with SERS. Afterward, the parameters influencing the enhancement of PIERS, including the substrate, light irradiation, and relaxation are discussed in detail. Finally, some perspectives on further developments of PIERS are exemplified. The PIERS technique will continue to evolve and grow with new developments and its successful application in bioanalysis and life sciences.

Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

Surface enhanced Raman scattering (SERS) is a fingerprint spectral technique whose performance is highly dependent on the physicochemical properties of the substrate materials. In addition to the traditional plasmonic metal substrates that feature prominent electromagnetic enhancements, boosted SERS activities have been reported recently for various categories of non-metal materials, including graphene, MXenes, transition-metal chalcogens/oxides, and conjugated organic molecules. Although the structural compositions of these semiconducting substrates vary, chemical enhancements induced by interfacial charge transfer are often the major contributors to the overall SERS behavior, which is distinct from that of the traditional SERS based on plasmonic metals. Regarding charge-transfer-induced SERS enhancements, this short review introduces the basic concepts underlying the SERS enhancements, the most recent semiconducting substrates that use novel manipulation strategies, and the extended applications of these versatile substrates.

Highly Efficient Photoinduced Enhanced Raman Spectroscopy (PIERS) from Plasmonic Nanoparticles Decorated 3D Semiconductor Arrays for Ultrasensitive, Portable, and Recyclable Detection of Organic Pollutants

Semiconductor materials have become competitive candidates for surface-enhanced Raman scattering (SERS) substrates; however, their limited SERS sensitivity hinders the practical applications of semiconductors. Here, we develop a hybrid substrate by integrating anatase/rutile TiO₂ heterostructure with dense plasmonic hotspots of Ag nanoparticle (AgNPs) for efficient photoinduced enhanced Raman spectroscopy (PIERS). The PIERS mechanism is systematically investigated by means of a portable Raman instrument. When ultraviolet (UV) light irradiates the substrate, the TiO₂-Ag hybrid arrays produce remarkable charge-transfer enhancement, which can be ascribed to the highly efficient charge separation driven by heterojunction and transfer from TiO₂ heterostructure to AgNPs. This platform allows for the rapid detection of multifold organic species, including malachite green (MG), crystal violet (CV), rhodamine 6G (R6G), thiram, and acephate, and as high as 27.8-fold enhancement over the normal SERS is achieved, representing the highest PIERS magnification up to the present time. The intensive PIERS enhancement makes it ultrasensitively detect analyte concentration of an order of magnitude lower than that of SERS method. The improved sensitivity and resolution can be readily realized by simple UV irradiation, which represents a major advantage of our PIERS methodology. Besides, the integration of uniform TiO₂ heterostructure arrays with AgNPs generates superior signal reproducibility with relative standard deviation (RSD) value of less than 14%. In addition, the detected molecules on the substrate can be eliminated by photocatalytic degradation after PIERS measurements by using UV irradiation, which makes the substrate reusable for 15 cycles. The ultrahigh sensitivity, superior reproducibility, and excellent recyclability displayed by our platform may provide new opportunities in field detection analysis coupled with a portable Raman instrument.