Origin of layer-dependent SERS tunability in 2D transition metal dichalcogenides

Recent advances in semiconductor nanostructure-based surface-enhanced Raman scattering sensors

Surface-enhanced Raman scattering (SERS) has become a transformative analytical tool, attracting growing interest for its wide-ranging applications. The development of SERS-active materials is now a central research area, spurring innovation in various types of SERS substrates. While noble metal-based substrates remain extensively studied, semiconductor-based, non-metal substrates are garnering attention due to their unique advantages: excellent chemical stability, high carrier mobility, biocompatibility, and precise fabrication control. However, their generally weaker enhancement effects limit their utility, underscoring the need for strategies to boost their SERS activity. Understanding the complex enhancement mechanisms in semiconductor-based SERS substrates is critical for designing next-generation materials with metal-like enhancement factors (EFs). The interplay of charge transfer, localized surface plasmon resonance, and photonic effects makes the enhancement process inherently challenging to unravel. Therefore, the search for new materials with exciting optoelectronic properties, as well as more innovative solutions to increase their SERS sensitivity, continues to grow. In this review, we explore the latest advancements in semiconductor-based SERS substrates, dissecting the complex enhancement mechanisms and various modification strategies aimed at achieving metal-like high EFs. We present a comprehensive analysis of the methods used to improve the SERS performance of semiconductor substrates and conclude with potential future directions for advancing this dynamic field.

Optical Synaptic Devices with Multiple Encryption Features Based on SERS-Revealed Charge-Transfer Mechanism

2D optical synaptic devices with atomic-scale thickness show potential for building highly integrated tunable artificial visual neural networks. However, their atomic-scale thickness also leads to weak light absorption, limiting device photoresponse. Here, a high-performance optical synaptic device based on a Rhodamine 6G (R6G)/InSe hybrid structure is proposed, achieving a remarkable 328.9% enhancement in photoresponse compared to InSe devices. Using surface-enhanced Raman spectroscopy (SERS) as a nondestructive probing technique, it is demonstrated that light-induced charge transfer between R6G and InSe is the key mechanism enabling the device's high performance. Furthermore, introducing a self-limited oxide layer on the InSe surface provides additional evidence for the charge transfer process. This charge-transfer-based device effectively mimics the neurotransmitter transmission process in biological synapses, showing unique potential in applications such as image preprocessing and decoding within artificial neural networks. In addition, through surface treatment techniques, precise control over the charge transfer process is achieved, enabling the design of a multiple encryption-based anti-counterfeiting array and highlighting their value in on-chip anti-counterfeiting. By employing a spectrally noninvasive method to probe charge transfer, this study elucidates the critical role of charge transfer in optical synaptic devices and opens novel application pathways.

Enhanced Detection of Organic Pollutants Using ReS2/ZnO/Au Ternary Surface-Enhanced Raman Spectroscopy Substrate with Multiple Charge Transfer Channels

Persistent organic pollutants pose significant environmental and health risks, highlighting the urgent need for highly sensitive sensing technologies capable of detecting trace concentrations. In this study, we synthesized a ReS2/ZnO/Au ternary surface-enhanced Raman spectroscopy (SERS) substrate via a simple hydrothermal and reduction method based on a novel strategy that leverages multiple charge transfer channels to significantly enhance SERS performance. The unique bandgap and energy level alignment of ReS2 facilitate both excitonic resonance and charge transfer transitions. Additionally, the semiconductor ZnO acts as an efficient charge transfer mediator by borrowing energy from molecular transitions. As a result of the synergistic combination of electromagnetic and chemical enhancements, the ReS2/ZnO/Au ternary substrate demonstrates excellent versatility and high sensitivity for detecting various pollutants, including rhodamine 6G (R6G), crystal violet, malachite green, and tetracycline (TC). Notably, the detection limit for TC can reach 10-10 M, with an enhancement factor as high as 2.78 × 108. Our strategy provides comprehensive insight into SERS enhancement, offering a pathway for designing sensitive and versatile SERS systems, with significant potential for monitoring and quantitative analysis of organic pollutants.

Utilizing a synergistic strategy that combines electromagnetic and chemical enhancement to analyze the SERS effect of the Fe3O4@GO@Ag on PAHs detection

A comprehensive understanding of the enhancement mechanism of the substrate material is crucial to ensure the repeatability and functionality of SERS detection technology. Therefore, this study introduces a theoretical analysis method that integrates electromagnetic and chemical enhancement to achieve a comprehensive understanding of the SERS effect on the magnetic composite substrate. The visual model is employed in this study to comprehensively analyze and illustrate the electric field enhancement and optical effects of composite substrate materials. The study also elucidated the adsorption and charge transfer between the substrate material and target molecules. Based on this theory, Fe3O4@GO@Ag material was prepared and used to detect hydrophobic organic molecules such as polycyclic aromatic hydrocarbons (PAHs), with a concentration as low as 0.5 nM. This study comprehensively analyzed the SERS enhancement effect of the composite substrate for the first time, and prepared a magnetic composite substrate material for the detection of hydrophobic organic molecules, opening up a new avenue for theoretical guidance and experimental exploration in SERS detection and analysis.

1T'-transition metal dichalcogenide monolayers stabilized on 4H-Au nanowires for ultrasensitive SERS detection

Unconventional 1T'-phase transition metal dichalcogenides (TMDs) have aroused tremendous research interest due to their unique phase-dependent physicochemical properties and applications. However, due to the metastable nature of 1T'-TMDs, the controlled synthesis of 1T'-TMD monolayers (MLs) with high phase purity and stability still remains a challenge. Here we report that 4H-Au nanowires (NWs), when used as templates, can induce the quasi-epitaxial growth of high-phase-purity and stable 1T'-TMD MLs, including WS2, WSe2, MoS2 and MoSe2, via a facile and rapid wet-chemical method. The as-synthesized 4H-Au@1T'-TMD core-shell NWs can be used for ultrasensitive surface-enhanced Raman scattering (SERS) detection. For instance, the 4H-Au@1T'-WS2 NWs have achieved attomole-level SERS detections of Rhodamine 6G and a variety of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike proteins. This work provides insights into the preparation of high-phase-purity and stable 1T'-TMD MLs on metal substrates or templates, showing great potential in various promising applications.

SERS Detection of Trace Carcinogenic Aromatic Amines Based on Amorphous MoO3 Monolayers

Aromatic amines are important commercial chemicals, but their carcinogenicity poses a threat to humans and other organisms, making their rapid quantitative detection increasingly urgent. Here, amorphous MoO3 (a-MoO3) monolayers with localized surface plasmon resonance (LSPR) effect in the visible region are designed for the trace detection of carcinogenic aromatic amine molecules. The hot-electron fast decay component of a-MoO3 decreases from 301 fs to 150 fs after absorption with methyl orange (MO) molecules, indicating the plasmon-induced hot-electron transfer (PIHET) process from a-MoO3 to MO. Therefore, a-MoO3 monolayers present high SERS performance due to the synergistic effect of electromagnetic enhancement (EM) and PIHET, proposing the EM-PIHET synergistic mechanism in a-MoO3. In addition, a-MoO3 possesses higher electron delocalization and electronic state density than crystal MoO3 (c-MoO3), which is conducive to the PIHET. The limit of detection (LOD) for o-aminoazotoluene (o-AAT) is 10-9 M with good uniformity, acid resistance, and thermal stability. In this work, trace detection and identification of various carcinogenic aromatic amines based on a-MoO3 monolayers is realized, which is of great significance for reducing cancer infection rates.

Exploring and Engineering 2D Transition Metal Dichalcogenides toward Ultimate SERS Performance

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive surface analysis technique that is widely used in chemical sensing, bioanalysis, and environmental monitoring. The design of the SERS substrates is crucial for obtaining high-quality SERS signals. Recently, 2D transition metal dichalcogenides (2D TMDs) have emerged as high-performance SERS substrates due to their superior stability, ease of fabrication, biocompatibility, controllable doping, and tunable bandgaps and excitons. In this review, a systematic overview of the latest advancements in 2D TMDs SERS substrates is provided. This review comprehensively summarizes the candidate 2D TMDs SERS materials, elucidates their working principles for SERS, explores the strategies to optimize their SERS performance, and highlights their practical applications. Particularly delved into are the material engineering strategies, including defect engineering, alloy engineering, thickness engineering, and heterojunction engineering. Additionally, the challenges and future prospects associated with the development of 2D TMDs SERS substrates are discussed, outlining potential directions that may lead to significant breakthroughs in practical applications.

Dynamic repulsive interaction enables an asymmetric electron-phonon coupling for improving Raman scattering

Two-dimensional (2D) materials are an excellent platform for surface-enhanced Raman spectroscopy (SERS). For ReS2, the Raman enhancement effect can be highly improved through the dipole-dipole interactions and synergistic resonance effects in the phase-engineering ReS2 films. However, the performance of the substrate can be improved further through regulating the electronic interaction between the ReS2 and probe molecules. Herein, a dynamic coulomb repulsion strategy is proposed to trigger an electronic state redistribution by asymmetric electrostatic interactions. With the phase-engineering ReS2/graphene heterostructure as a prototype, under laser excitation, the generated hot electrons in graphene and ReS2 can repel each other due to Coulomb interaction, which breaks the symmetrical distribution of hot electrons in ReS2, and increases the electronic concentration at the interface between ReS2 and the probe molecule. With R6G as the probe molecule, the asymmetric electron distribution and synergistic resonance effects on their interface improve the limit of detection to 10-12 M with an EF of 2.15 × 108. Meanwhile, the heterostructure also shows good uniformity, stability as well as unique anisotropy. This strategy can be generalized to other 2D heterostructures to obtain the ultrasensitive SERS substrates.

Advancing piezoelectric 2D nanomaterials for applications in drug delivery systems and therapeutic approaches

Precision drug delivery and multimodal synergistic therapy are crucial in treating diverse ailments, such as cancer, tissue damage, and degenerative diseases. Electrodes that emit electric pulses have proven effective in enhancing molecule release and permeability in drug delivery systems. Moreover, the physiological electrical microenvironment plays a vital role in regulating biological functions and triggering action potentials in neural and muscular tissues. Due to their unique noncentrosymmetric structures, many 2D materials exhibit outstanding piezoelectric performance, generating positive and negative charges under mechanical forces. This ability facilitates precise drug targeting and ensures high stimulus responsiveness, thereby controlling cellular destinies. Additionally, the abundant active sites within piezoelectric 2D materials facilitate efficient catalysis through piezochemical coupling, offering multimodal synergistic therapeutic strategies. However, the full potential of piezoelectric 2D nanomaterials in drug delivery system design remains underexplored due to research gaps. In this context, the current applications of piezoelectric 2D materials in disease management are summarized in this review, and the development of drug delivery systems influenced by these materials is forecast.

Porous carbon film/WO3-x nanosheets based SERS substrate combined with deep learning technique for molecule detection

The Surface-enhanced Raman scattering (SERS) is an attractive optical detecting method with high sensitivity and detectivity, however challenges on large-area signal uniformity and complex spectra analysis methods always retards its wide application. Herein, a highly sensitive and uniform SERS detection strategy supported by porous carbon film/WO3-x nanosheets (PorC/WO3-x) based noble-metal-free SERS substrate and deep learning algorithm are reported. Experimentally, the PorC/WO3-x substrate was prepared by high-temperature annealing the PorC/WO3 films under the argon atmosphere. The defect density of the WO3 was controlled by tuning the reducing reaction time during the annealing process. The SERS performance was evaluated by using R6G as the Raman reporter, it showed that the SERS intensity obtained on the substrate with the optimal annealing time of 3 h was about 8 times as high as that obtained on the PorC/WO3 substrate without annealing treatment. And detection limit of 10-7 M and Raman enhancement factor of 106 could be achieved. Moreover, the above optimal SERS substrate was utilized to detect flavonoids of quercetin, 3-hydroxyflavone and flavone, and a deep learning algorithms was incorporated to identify the quercetin. It revealed that quercetin can be accurately detected within the above flavonoids, and lowest detectable concentration of 10-5 M can be achieved.

Crystal-Phase Engineering of Two-Dimensional Transition-Metal Dichalcogenides for Surface-Enhanced Raman Scattering: A Perspective

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising materials for surface-enhanced Raman scattering (SERS) due to their unique electronic, optical, and mechanical properties. In this Perspective, we briefly introduce the fundamental properties, crystal-phase configurations, and phase transition strategies of TMDs materials. We then discuss the importance of the crystal phase in determining the SERS effect of TMDs, highlighting recent advances in phase-engineering approaches to affording remarkable SERS performance. By considering the current challenges and future directions for improving the crystal-phase engineering of TMDs in SERS, we also offer new insights into the design and synthesis of more promising TMD-based SERS substrates.

ReS2 Nanoflowers-Assisted Confined Growth of Gold Nanoparticles for Ultrasensitive and Reliable SERS Sensing

ReS₂, as a new member of transition metal dichalcogenides (TMDCs), has emerged as a promising substrate for semiconductor surface-enhanced Raman spectroscopy (SERS) due to its unique optoelectronic properties. Nevertheless, the sensitivity of the ReS₂ SERS substrate poses a significant challenge to its widespread application in trace detection. In this work, we present a reliable approach for constructing a novel ReS₂/AuNPs SERS composite substrate, enabling ultrasensitive detection of trace amounts of organic pesticides. We demonstrate that the porous structures of ReS₂ nanoflowers can effectively confine the growth of AuNPs. By precisely controlling the size and distribution of AuNPs, numerous efficient and densely packed "hot spots" were created on the surface of ReS₂ nanoflowers. As a result of the synergistic enhancement of the chemical and electromagnetic mechanisms, the ReS₂/AuNPs SERS substrate demonstrates high sensitivity, good reproducibility, and superior stability in detecting typical organic dyes such as rhodamine 6G and crystalline violet. The ReS₂/AuNPs SERS substrate shows an ultralow detection limit of 10^-10 M and a linear detection of organic pesticide molecules within 10^-6-10^-10 M, which is significantly lower than the EU Environmental Protection Agency regulation standards. The strategy of constructing ReS₂/AuNPs composites would contribute to the development of highly sensitive and reliable SERS sensing platforms for food safety monitoring.

Spotting the driving forces for SERS of two-dimensional nanomaterials

Recently, two-dimensional (2D) layered nanomaterials have become promising candidates for surface-enhanced Raman scattering (SERS) substrates due to their unique characteristics of ultrathin layer structure, outstanding optical properties and good biocompatibility, significantly contributing to remarkable SERS sensitivity, stability, and compatibility. Unlike traditional SERS substrates, 2D nanomaterials possess unparalleled layer-dependent, phase transition induced and anisotropic optical properties, which as driving forces significantly promote the SERS performance and development, as well as greatly enrich the SERS substrates and provide versatile resources for SERS research. For a profound understanding of the SERS effect of 2D nanomaterials, a review concentrating on these driving forces for SERS enhancement on 2D nanomaterials is written here for the first time, which strongly emphasizes the importance and influence of these driving forces on the SERS effect of 2D nanomaterials, including their intrinsic physical and chemical properties and external influencing factors. Moreover, the essential mechanisms of these driving forces for the SERS effect are also elaborated systematically. Finally, the challenges and future perspectives of SERS substrates based on 2D nanomaterials are concluded. This review will provide guiding principles and strategies for designing highly sensitive 2D nanomaterial SERS substrates and extending their potential applications based on SERS.

VSe2-x O x @Pd Sensor for Operando Self-Monitoring of Palladium-Catalyzed Reactions

Operando monitoring of catalytic reaction kinetics plays a key role in investigating the reaction pathways and revealing the reaction mechanisms. Surface-enhanced Raman scattering (SERS) has been demonstrated as an innovative tool in tracking molecular dynamics in heterogeneous reactions. However, the SERS performance of most catalytic metals is inadequate. In this work, we propose hybridized VSe2-x O x @Pd sensors to track the molecular dynamics in Pd-catalyzed reactions. Benefiting from metal-support interactions (MSI), the VSe2-x O x @Pd realizes strong charge transfer and enriched density of states near the Fermi level, thereby strongly intensifying the photoinduced charge transfer (PICT) to the adsorbed molecules and consequently enhancing the SERS signals. The excellent SERS performance of the VSe2-x O x @Pd offers the possibility for self-monitoring the Pd-catalyzed reaction. Taking the Suzuki-Miyaura coupling reaction as an example, operando investigations of Pd-catalyzed reactions were demonstrated on the VSe2-x O x @Pd, and the contributions from PICT resonance were illustrated by wavelength-dependent studies. Our work demonstrates the feasibility of improved SERS performance of catalytic metals by modulating the MSI and offers a valid means to investigate the mechanisms of Pd-catalyzed reactions based on VSe2-x O x @Pd sensors.

Significantly increased Raman enhancement enabled by hot-electron-injection-induced synergistic resonances on anisotropic ReS2 films

Two-dimensional (2D) materials are an excellent platform for surface-enhanced Raman spectroscopy (SERS). However, a poor detection sensitivity hinders their practical application. Exciton resonance (μ_ex) can improve SERS significantly by lending intensity to nearby charge-transfer resonance. Coincidentally, for ReS₂, the enhanced μ_ex can be achieved through the injection of excited-state electrons which can adjust the energy band to the SERS detection range. Moreover, ReS₂ has strong anisotropic properties, which adds an additional dimension for SERS. Therefore, ReS₂ is an ideal candidate to realize highly sensitive anisotropic SERS. In this paper, the metallic T phase of ReS₂ is introduced to the semiconducting Td phase by phase engineering. The photoinduced electron tunneling from the T phase to the Td phase can tune exciton emissions to the visible region, which effectively facilitates the photoinduced charge transfer processes. With RhB as the probe molecule, the synergistic resonance effects improve the limit of detection to 10^-9 M with the enhancement factor up to about 10⁸. Meanwhile, the obtained ultrasensitive SERS substrates also show good uniformity, stability as well as unique anisotropy. Our results open a new perspective in the improvement of the SERS performance.

Two-dimensional MBenes with ordered metal vacancies for surface-enhanced Raman scattering

As an emerging class of two-dimensional (2D) materials, MBenes show enormous potential for optoelectronic applications. However, their use in molecular sensing as surface-enhanced Raman scattering (SERS)-active material is unknown. Herein, for the first time, we develop a brand-new high-performance MBene SERS platform. Ordered vacancy-triggered highly sensitive SERS platform with outstanding signal uniformity based on a 2D Mo_4/3B₂ MBene material was designed. The 2D Mo_4/3B₂ MBene presented superior SERS activity to most of the semiconductor SERS substrates, showing a remarkable Raman enhancement factor of 3.88 × 10⁶ and an ultralow detection limit of 1 × 10^-9 M. The underlying SERS mechanism is revealed from systematic experiments and density functional theory calculations that the ultrahigh SERS sensitivity of 2D Mo_4/3B₂ MBene is derived from the efficient photoinduced charge transfer process between MBene substrates and adsorbed molecules. The abundant electronic density of states near the Fermi level of 2D Mo_4/3B₂ MBene enables its Raman enhancement by a factor of 100 000 times higher than that of the bulk MoB. Consequently, the 2D Mo_4/3B₂ MBene could accurately detect various trace chemical analytes. Moreover, with ordered metal vacancies in the 2D Mo_4/3B₂ MBene, uniform charge transfer sites are formed, resulting in an outstanding signal uniformity with a relative standard deviation down to 6.0%. This work opens up a new horizon for the high-performance SERS platform based on MBene materials, which holds great promise in the field of chemical sensing.

Monolayer Iron Oxychloride with a Resonant Band Structure for Ultrasensitive Molecular Sensing

Two-dimensional layered materials (2DLMs) are expected to be next-generation commercial sensors for surface-enhanced Raman scattering (SERS) sensing owing to their unique structural features and physicochemical properties. The low sensitivity and poor universality of 2DLMs are the dominant barriers toward their practical applications. Herein, we report that monolayer iron oxychloride (FeOCl) with a naturally suitable band structure is a promising candidate for ultrasensitive SERS sensing. The generally boosted Raman scattering cross section of different analyte-FeOCl systems benefits from the resonant photoinduced charge transfer processes and strong ground-state interactions. In addition, the strong adsorption ability of monolayer FeOCl is crucial for rapid detection in practical applications, which is proven to be much better than those of conventional SERS sensors. Consequently, monolayer FeOCl enables diverse SERS applications, including multicomponent analysis, chemical reaction monitoring, and indirect ion sensing.

Functionalized MoS2: circular economy SERS substrate for label-free detection of bilirubin in clinical diagnosis

A one-pot hydrothermal synthesis of Fe-doped MoS₂ nanoflowers (Fe-MoS₂ NFs) has been developed as a surface-enhanced Raman spectroscopy (SERS) substrate. The Fe-MoS₂ NFs display high reproducibility, stability, and recyclability, which is beneficial for the development of the sustainable ecological environment. The SERS substrate provides a high enhancement factor of 10⁵, which can be ascribed to the inducing defects by doping Fe that can improve the charge transfer between probe molecules and MoS₂. The Fe-MoS₂ NFs have been used to detect bilirubin in serum. The Fe-MoS₂ NF SERS substrate exhibits a linear detection range from 10^-3 to 10^-9 M with a low limit of detection (LOD) of 10^-8 M. The substrate displays an excellent selectivity to bilirubin in the presence of other potentially interfering molecules (dextrose and phosphate). These results provide a novel concept to synthesize ultra-sensitive SERS substrates and open up a wide range of possibilities for new applications of MoS₂ in clinical diagnosis.

Emerging SERS biosensors for the analysis of cells and extracellular vesicles

Cells and their derived extracellular vesicles (EVs) or exosomes contain unique molecular signatures that could be used as biomarkers for the detection of severe diseases such as cancer, as well as monitoring the treatment response. Revealing these molecular signatures requires developing non-invasive ultrasensitive tools to enable single molecule/cell-level detection using a small volume of sample with low signal-to-noise ratio background and multiplex capability. Surface-enhanced Raman scattering (SERS) can address the current limitations in studying cells and EVs through two main mechanisms: plasmon-enhanced electric field (the so-called electromagnetic mechanism (EM)), and chemical mechanism (CM). In this review, we first highlight these two SERS mechanisms and then discuss the nanomaterials that have been used to develop SERS biosensors based on each of the aforementioned mechanisms as well as the combination of these two mechanisms in order to take advantage of the synergic effect between electromagnetic enhancement and chemical enhancement. Then, we review the recent advances in designing label-aided and label-free SERS biosensors in both colloidal and planar systems to investigate the surface biomarkers on cancer cells and their derived EVs. Finally, we discuss perspectives of emerging SERS biosensors in future biomedical applications. We believe this review article will thus appeal to researchers in the field of nanobiotechnology including material sciences, biosensors, and biomedical fields.

Atomically Thin TaSe2 Film as a High-Performance Substrate for Surface-Enhanced Raman Scattering

An atomically thin TaSe₂ sample, approximately containing two to three layers of TaSe₂ nanosheets with a diameter of 2.5 cm is prepared here for the first time and applied on the detection of various Raman-active molecules. It achieves a limit of detection of 10^-10  m for rhodamine 6G molecules. The excellent surface-enhanced Raman scattering (SERS) performance and underlying mechanism of TaSe₂ are revealed using spectrum analysis and density functional theory. The large adsorption energy and the abundance of filled electrons close to the Fermi level are found to play important roles in the chemical enhancement mechanism. Moreover, the TaSe₂ film enables highly sensitive detection of bilirubin in serum and urine samples, highlighting the potential of using 2D SERS substrates for applications in clinical diagnosis, for example, in the diagnosis of jaundice caused by excess bilirubin in newborn children.

Semiconductor-based surface enhanced Raman scattering (SERS): from active materials to performance improvement

Surface enhanced Raman scattering (SERS) is a powerful spectral analysis technique and has exhibited remarkable application prospects in various fields. The design and fabrication of high-performance SERS substrates is key to promoting the development of SERS technology. Apart from noble metal substrates, non-metal substrates based on semiconductor materials have received increasing attention in recent years owing to their unique physical, chemical, and optical properties. However, compared with noble metal substrates, most semiconductor substrates show weak Raman enhancement ability. Therefore, exploring effective strategies to improve the SERS sensitivity is an urgent task. Numerous reviews have outlined the research progress of semiconductor SERS substrates, which mainly focused on summarizing the material category of semiconductor substrates. However, reviews that systematically summarize the strategies for improving the SERS performance of semiconductor substrates are lacking. In this review, we comprehensively discuss the research on semiconductor SERS from the aspects of mechanism, materials, and modification. Firstly, the Raman enhancement mechanism of semiconductor substrates and the SERS-active materials are discussed. Then, we summarize several effective approaches to boost the SERS performance of semiconductor substrates. In conclusion, we propose some prospects for this field.

Enhanced Raman scattering on two-dimensional palladium diselenide

Two-dimensional (2D) semiconductors with atomic layers, and a flat and active surface provide an attractive platform for the study of surface-enhanced Raman scattering (SERS). Many 2D layered materials, including graphene and transition metal dichalcogenide (TMD), have been exploited as potential Raman enhancers for SERS-based molecule sensing. Herein, atomically-thin palladium diselenide (PdSe₂) used as a SERS substrate for molecule detection was systematically studied. Stable Raman enhancement for molecules such as rhodamine 6G (R6G), crystal violet (CV), and rhodamine B (RhB) on few-layer PdSe₂ has been verified. A detection limit as low as 10^-9 M and an enhancement factor of 10⁵ for the R6G molecule on monolayer PdSe₂ are achieved. With the insertion of a thin Al₂O₃ layer, the Raman spectra confirm the predominant charge transfer mechanism for the large Raman enhancement. Furthermore, the strong thickness-dependent properties, good in-plane anisotropy and excellent air-stability of Raman enhancement are also explored for 2D PdSe₂. Our findings provide not only a promising Raman enhancement platform for sensing applications but also new insights into the chemical mechanism (CM) process of SERS.

A photo-responsive p-Si/TiO2/Ag heterostructure with charge transfer for recyclable surface-enhanced Raman scattering substrates

A Si/TiO2/Ag heterostructure is prepared as a recyclable SERS substrate with EF of 1.23 × 1012 and excellent repeatability, which can boost performance effectively by the synergistic contribution of the EM and CT enhancement effects.

Application of two-dimensional layered materials in surface-enhanced Raman spectroscopy (SERS)

2D materials are promising SERS substrates. Seven feasible strategies to improve the SERS performance of 2D substrate materials are summarized. The prospect of future progress in SERS and possible challenges of 2D layered materials are put forwarded.