Surface-Enhanced Raman Scattering Monitoring of Oxidation States in Defect-Engineered Two-Dimensional 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.

The surface oxidation effect on photocurrent in WSe1.95Te0.05 nanosheets

Surface oxidation effect on photocurrent responsibility was detected in WSe1.95Te0.05 nanosheets, and the photocurrent response depends on the light wavelength. It is enhanced at the wavelength of 405 nm, while showing no change at the wavelength of 532 nm and suppressed at the wavelength of 808 nm. The incident photon-to-current efficiency (IPCE) is expected to increase at 405 nm wavelength, remain unchanged at 532 nm wavelength, and decrease at 808 nm wavelength. Therefore, WO3 contributes to the intrinsic properties. The trend of photocurrent change after half-year exposure corresponds to the absorbance change from pristine WSe1.95Te0.05 to WO3. The wavelength-dependent photocurrent responsibility is understood as the wavelength-dependent IPCE of WO3 that is from the surface-oxidized WSe1.95Te0.05.

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.

Insights into the Semiconductor SERS Activity: The Impact of the Defect-Induced Energy Band Offset and Electron Lifetime Change

The significant boost in surface-enhanced Raman scattering (SERS) by the chemical enhancement of semiconducting oxides is a pivotal finding. It offers a prospective path toward high uniformity and low-cost SERS substrates. However, a detailed understanding of factors that influence the charge transfer process is still insufficient. Herein, we reveal the important role of defect-induced band offset and electron lifetime change in SERS evolution observed in a MoO3 oxide semiconductor. By modulating the density of oxygen vacancy defects using ultraviolet (UV) light irradiation, SERS is found to be improved with irradiation time in the first place, but such improvement later deteriorates for prolonged irradiation even if more defects are generated. Insights into the observed SERS evolution are provided by ultraviolet photoelectron spectroscopy and femtosecond time-resolved transient absorption spectroscopy measurements. Results reveal that (1) a suitable offset between the energy band of the substrate and the orbitals of molecules is facilitated by a certain defect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.

Alloy Engineering Allows On-Demand Design of Ultrasensitive Monolayer Semiconductor SERS Substrates

The chemical mechanism (CM) of surface-enhanced Raman scattering (SERS) has been recognized as a decent approach to mildly amplify Raman scattering. However, the insufficient charge transfer (CT) between the SERS substrate and molecules always results in unsatisfying Raman enhancement, exerting a substantial restriction for CM-based SERS. In principle, CT is dominated by the coupling between the energy levels of a semiconductor-molecule system and the laser wavelength, whereas precise tuning of the energy levels is intrinsically difficult. Herein, two-dimensional transition-metal dichalcogenide alloys, whose energy levels can be precisely and continuously tuned over a wide range by simply adjusting their compositions, are investigated. The alloys enable on-demand construction of the CT resonance channels to cater to the requirements of a specific target molecule in SERS. The SERS signals are highly reproducible, and a clear view of the SERS dependences on the energy levels is revealed for different CT resonance terms.

Ni and O co-modified MoS2 as universal SERS substrate for the detection of different kinds of substances

Surface-enhanced Raman scattering (SERS) has attracted extensive attention as an ultrasensitive detection method. However, the poor biocompatibility and expensive synthesis cost of noble metal SERS substrates have become non-negligible factors that limit the development of SERS technology. Metal chalcogenide semiconductors as an alternative to noble metal SERS substrates can avoid these disadvantages, but the enhancement effect is lower than that of noble metal substrates. Here, we report a method to co-modify MoS₂ by Ni and O, which improves the carrier concentration and mobility of MoS₂. The SERS effect of the modified MoS₂ is comparable to that of noble metals. We found that the improved SERS performance of MoS₂ can be attributed to the following two factors: strong interfacial dipole-dipole interaction and efficient charge transfer effect. During the doping process, the incorporation of Ni and O enhances the polarity and carrier concentration of MoS₂, enhances the interfacial interaction of MoS₂, and provides a basis for charge transfer. During the annealing process, the introduction of O atoms into the S defects reduces the internal defects of doped MoS₂, improves the carrier mobility, and promotes the efficient charge transfer effect of MoS₂. The final modified MoS₂ as a SERS substrate realizes low-concentration detection of bilirubin, cytochrome C, and trichlorfon. This provides promising guidance for the practical inspection of metal chalcogenide semiconductor substrates.

Molybdenum Oxide/Tungsten Oxide Nano-heterojunction with Improved Surface-Enhanced Raman Scattering Performance

By virtue of their high uniformity and stability, metal oxide-based surface-enhanced Raman spectroscopy (SERS) substrates have attracted enormous attention for molecular trace detection. However, strategies for further enhancing the SERS sensitivity are still desired. Herein, MoO_x/WO_x nano-heterojunctions are constructed by mixing MoO_x and WO_x together (MoO_x/WO_x hybrid) with diverse weight ratios. Using a 532 nm laser as the excitation source and R6G as the Raman reporter, it is shown that the Raman signal intensity (for the peak @ 1360 cm^-1) obtained on the optimal MoO_x/WO_x hybrid (MoO_x/WO_x = 1:1/3) is twice that observed on a pure MoO_x or WO_x substrate. Moreover, a limit of detection of 10^-8 M and an enhancement factor of 10⁸ are achieved. In the SERS enhancement mechanism investigation, it is revealed that MoO_x and WO_x form a staggered band structure. During the SERS measurement, electron-hole pairs are generated in the nano-heterojunction using the incident laser. They are then separated by the built-in potential with the electrons moving toward WO_x. The accumulated electrons on WO_x are further transferred to the R6G molecules through the coupling of orbitals. Consequently, the molecular polarizability is amplified, and SERS performance is enhanced. The abovementioned explanation is supported by the evidence that the contribution of the chemical enhancement mechanism in the optimal MoO_x/WO_x hybrid substrate is about 2.5 times or 5.9 times that in the pure WO_x or MoO_x substrate.