Enhancing charge transfer with foreign molecules through femtosecond laser induced MoS2 defect sites for photoluminescence control and SERS enhancement

Ultra-Wide Interlayered WxMo2xSy Alloy Electrode Patterning through High-Precision Controllable Photonic-Synthesis

Ultra-thin 2D materials have great potential as electrodes for micro-supercapacitors (MSCs) because of their facile ion transport channels. Here, a high-precision controllable photonic-synthesis strategy that provided 1 inch wafer-scale ultra-thin film arrays of alloyed WxMo2xSy with sulfur vacancies and expanded interlayer (13.2 Å, twice of 2H MoS2) is reported. This strategy regulates the nucleation and growth of transition metal dichalcogenides (TMDs) on the picosecond or even femtosecond scale, which induces Mo-W alloying, interlayer expansion, and sulfur loss. Therefore, the diffusion barrier of WxMo2xSy is reduced, with charge transfer and ion diffusion enhancing. The as-prepared symmetric MSCs with the size of 100 × 100 µm2 achieve ultrahigh specific capacitance (242.57 mF cm-2 and 242567.83 F cm-3), and energy density (21.56 Wh cm-3 with power density of 485.13 W cm3). The established synthesis strategy fits numerous materials, which provides a universal method for the flexible synthesis of electrodes in microenergy devices.

Ultrafast Laser Processing of 2D Materials: Novel Routes to Advanced Devices

Ultrafast laser processing has emerged as a versatile technique for modifying materials and introducing novel functionalities. Over the past decade, this method has demonstrated remarkable advantages in the manipulation of 2D layered materials, including synthesis, structuring, functionalization, and local patterning. Unlike continuous-wave and long-pulsed optical methods, ultrafast lasers offer a solution for thermal heating issues. Nonlinear interactions between ultrafast laser pulses and the atomic lattice of 2D materials substantially influence their chemical and physical properties. This paper highlights the transformative role of ultrafast laser pulses in maskless green technology, enabling subtractive, and additive processes that unveil ways for advanced devices. Utilizing the synergetic effect between the energy states within the atomic layers and ultrafast laser irradiation, it is feasible to achieve unprecedented resolutions down to several nanometers. Recent advancements are discussed in functionalization, doping, atomic reconstruction, phase transformation, and 2D and 3D micro- and nanopatterning. A forward-looking perspective on a wide array of applications of 2D materials, along with device fabrication featuring novel physical and chemical properties through direct ultrafast laser writing, is also provided.

Direct Z-scheme heterojunction impregnated MoS2-NiO-CuO nanohybrid for efficient photocatalyst and dye-sensitized solar cell

In this present work, the preparation of ternary MoS2-NiO-CuO nanohybrid by a facile hydrothermal process for photocatalytic and photovoltaic performance is presented. The prepared nanomaterials were confirmed by physio-chemical characterization. The nanosphere morphology was confirmed by electron microscopy techniques for the MoS2-NiO-CuO nanohybrid. The MoS2-NiO-CuO nanohybrid demonstrated enhanced crystal violet (CV) dye photodegradation which increased from 50 to 95% at 80 min; The degradation of methyl orange (MO) dye increased from 56 to 93% at 100 min under UV-visible light irradiation. The trapping experiment was carried out using different solvents for active species and the Z-Scheme photocatalytic mechanism was discussed in detail. Additionally, a batch series of stability experiments were carried out to determine the photostability of materials, and the results suggest that the MoS2-NiO-CuO nanohybrid is more stable even after four continuous cycles of photocatalytic activity. The MoS2-NiO-CuO nanohybrid delivers photoconversion efficiency (4.92%) explored efficacy is 3.8 times higher than the bare MoS2 (1.27%). The overall results indicated that the MoS2-NiO-CuO nanohybrid nanostructure could be a potential candidate to be used to improve photocatalytic performance and DSSC solar cell applications as well.

Controllable Fabrication of Hydrophilic Surface Micro/Nanostructures of CFRP by Femtosecond Laser

Carbon fiber reinforced polymer (CFRP), a highly engineered lightweight material with superior properties, is widely used in industrial fields, such as aerospace, automobile, and railway transportation, as well as medical implants and supercapacitor. This work presents an effective surface treatment method for the controllable fabrication of hydrophilic surface micro/nanostructures of CFRP through femtosecond laser processing. Selective removal of the epoxy resin and leaving the carbon fibers exposed are achieved when CFRP is weakly ablated by a femtosecond laser. The diameters and structures of the carbon fibers can be controlled by adjusting the laser processing parameters. Three-dimensional surface micro/nanostructures are processed when CFRP is strongly ablated by a femtosecond laser. Meanwhile, the transformation of the sp2 orbitals to sp3 orbitals of graphitic carbons of carbon fibers is induced by a femtosecond laser. Moreover, the investigation of surface roughness and wettability of femtosecond laser-processed CFRP indicates increased roughness and excellent hydrophilicity (a contact angle of 28.1°). This work reveals the effect of femtosecond laser processing on the regulation of the physicochemical properties of CFRP, which can be applicable to surface treatment and performance control of other fiber-resin composites. The excellent hydrophilicity will be conducive to the combination of CFRP with other materials or to reducing the friction resistance of CFRP used in medical implants.

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.

Design superhydrophobic no-noble metal substrates for highly sensitive and signal stable SERS sensing

Surface-enhanced Raman spectroscopy (SERS) is an analytical technique with a broad range of potential applications in fields such as biomedicine, electrochemistry, and hazardous chemicals. However, it is challenging to develop SERS substrates that are both good sensitive and signal stable. Here we designed a superhydrophobic Nd doped MoS2 uniformly assembled on the activated carbon fiber cloth (CFC) to avoid the coffee ring effect and enrich the analyte, improving the enhancement factor (EF) to 3.9 × 109 and pesticide endosulfan (<10-10) analyte detection. We demonstrate our strategy by density-functional theory (DFT) calculations confirming that both adsorption energy and density of states are enhanced after doping Nd leading to SERS enhancement. Beside DFT calculations, our experiments also provide an effective means to demonstrate that the high SERS sensitivity is based on multiple charge transfer processes combined with the activated carbon cloth. Our results address the limitations of low sensitivity and limit of detection (LOD) of semiconductor SERS substrates for trace analysis and are a step towards practical applications.

Resonance-Assisted Surface-Enhanced Raman Spectroscopy Amplification on Hierarchical Rose-Shaped MoS2/Au Nanocomposites

Surface-enhanced Raman spectroscopy (SERS) has emerged as a highly sensitive trace detection technique in recent decades, yet its exceptional performance remains elusive in semiconductor materials due to the intricate and ambiguous nature of the SERS mechanism. Herein, we have synthesized MoS2 nanoflowers (NFs) decorated with Au nanoparticles (NPs) by hydrothermal and redox methods to explore the size-dependence SERS effect. This strategy enhances the interactions between the substrate and molecules, resulting in exceptional uniformity and reproducibility. Compared to the unadorned Au nanoparticles (NPs), the decoration of Au NPs induces an n-type effect on MoS2, resulting in a significant enhancement of the SERS effect. This augmentation empowers MoS2 to achieve a low limit of detection concentration of 2.1 × 10-9 M for crystal violet (CV) molecules and the enhancement factor (EF) is about 8.52 × 106. The time-stability for a duration of 20 days was carried out, revealing that the Raman intensity of CV on the MoS2/Au-6 substrate only exhibited a reduction of 24.36% after undergoing aging for 20 days. The proposed mechanism for SERS primarily stems from the synergistic interplay among the resonance of CV molecules, local surface plasma resonance (LSPR) of Au NPs, and the dual-step charge transfer enhancement. This research offers comprehensive insights into SERS enhancement and provides guidance for the molecular design of highly sensitive SERS systems.

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.

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.

High performance 1D-2D CuO/MoS2 photodetectors enhanced by femtosecond laser-induced contact engineering

The integration of 2D materials with other dimensional materials opens up rich possibilities for both fundamental physics and exotic nanodevices. However, current mixed-dimensional heterostructures often suffer from interfacial contact issues and environment-induced degradation, which severely limits their performance in electronics/optoelectronics. Herein, we demonstrate a novel BN-encapsulated CuO/MoS₂ 2D-1D van der Waals heterostructure photodetector with an ultrahigh photoresponsivity which is 10-fold higher than its previous 2D-1D counterparts. The interfacial contact state and photodetection capabilities of 2D-1D heterojunctions are significantly improved via femtosecond laser irradiation induced MoS₂ wrapping and contamination removal. These h-BN protected devices show highly sensitive, gate-tunable and robust photoelectronic properties. By controlling the gate and bias voltages, the device can achieve a photoresponsivity as high as 2500 A W^-1 in the forward bias mode, or achieve a high detectivity of 6.5 × 10¹¹ Jones and a typical rise time of 2.5 ms at reverse bias. Moreover, h-BN encapsulation effectively protects the mixed-dimensional photodetector from electrical depletion by gas molecules such as O₂ and H₂O during fs laser treatment or the operation process, thus greatly improving the stability and service life in harsh environments. This work provides a new way for the further development of high performance, low cost, and robust mixed-dimensional heterostructure photodetectors by femtosecond laser contact engineering.

Flexible nano-cloth-like Ag cluster@rGO with ultrahigh SERS sensitivity for capture-optimization-detection due to effective molecule-substrate interactions

Surface-enhanced Raman scattering (SERS) is a rapid and promising detection technique for trace molecules. A central goal of research in this area is to achieve the highly sensitive detection of molecules built on a systematic understanding of enhancement mechanisms. Herein, we develop a Ag cluster@rGO composite nanostructure, which utilizes strong molecular adsorption to achieve ultrahigh SERS sensitivity. Ag clusters are prepared without additional reducing agents, leaving a low carbon footprint in the fabrication process. Finite-difference time-domain (FDTD) simulations show strong electromagnetic field enhancements generated at the edges and interstices of Ag clusters due to the specificity of their structure. Density Functional Theory (DFT) calculations show that the HOMO-LUMO energy gap value is significantly reduced when Ag cluster@rGO forms a composite system with the target molecule, which enables efficient charge transfer between the substrate and molecules, resulting in charge transfer enhancement. A detection limit of 10^-14 M using our substrate can be achieved for the environmental pollutant dye rhodamine 6G (Rh6G). The detection limits of bisphenol A (BPA) and its derivatives reach nanomolar levels with good signal stability. More importantly, we demonstrate the ability to rapidly screen BPA migration in Chinese Baijiu. Our SERS platform can be further developed for environmental pollution control and food safety.

MoS2-Based Substrates for Surface-Enhanced Raman Scattering: Fundamentals, Progress and Perspective

Surface-enhanced Raman scattering (SERS), as an important tool for interface research, occupies a place in the field of molecular detection and analysis due to its extremely high detection sensitivity and fingerprint characteristics. Substantial efforts have been put into the improvement of the enhancement factor (EF) by way of modifying SERS substrates. Recently, MoS2 has emerged as one of the most promising substrates for SERS, which is also exploited as a complementary platform on the conventional metal SERS substrates to optimize the properties. In this minireview, the fundamentals of MoS2-related SERS are first explicated. Then, the synthesis, advances and applications of MoS2-based substrates are illustrated with special emphasis on their practical applications in food safety, biomedical sensing and environmental monitoring, together with the corresponding challenges. This review is expected to arouse broad interest in nonplasmonic MoS2-related materials along with their mechanisms, and to promote the development of SERS studies.

Defect engineering in semiconductor-based SERS

Semiconductor-based surface enhanced Raman spectroscopy (SERS) platforms take advantage of the multifaceted tunability of semiconductor materials to realize specialized sensing demands in a wide range of applications. However, until quite recently, semiconductor-based SERS materials have generally exhibited low activity compared to conventional noble metal substrates, with enhancement factors (EF) typically reaching 103, confining the study of semiconductor-based SERS to purely academic settings. In recent years, defect engineering has been proposed to effectively improve the SERS activity of semiconductor materials. Defective semiconductors can now achieve noble-metal-comparable SERS enhancement and exceedingly low, nano-molar detection concentrations towards certain molecules. The reason for such success is that defect engineering effectively harnesses the complex enhancement mechanisms behind the SERS phenomenon by purposefully tailoring many physicochemical parameters of semiconductors. In this perspective, we introduce the main defect engineering approaches used in SERS-activation, and discuss in depth the electromagnetic and chemical enhancement mechanisms (EM and CM, respectively) that are influenced by these defect engineering methods. We also introduce the applications that have been reported for defective semiconductor-based SERS platforms. With this perspective we aim to meet the imperative demand for a summary on the recent developments of SERS material design based on defect engineering of semiconductors, and highlight the attractive research and application prospects for semiconductor-based SERS.

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.

SERS Selective Enhancement on Monolayer MoS2 Enabled by a Pressure-Induced Shift from Resonance to Charge Transfer

As a newly emerging approach for surface-enhanced Raman spectroscopy (SERS), pressure-induced SERS (PI-SERS) has been attracting increasing interest for its applications in Raman signal enhancement at extreme conditions. However, how to efficiently realize the PI-SERS enhancement and elucidate the corresponding mechanism remain open questions. Herein, we demonstrate the PI-SERS enhancement up to 8.04 GPa using monolayer molybdenum disulfide (ML-MoS₂) as a SERS substrate and three organic molecules with similar energy levels but different symmetries as probes. The combined theory and experiment results show that a pressure-induced increase in the Fermi level of the ML-MoS₂ substrate and a decrease in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap of probe molecules lead to a transition from the multiple resonance-related SERS enhancement to charge transfer (CT)-dominated PI-SERS selective enhancement, depending on the incident laser energy and the pressure applied. Such PI-SERS selective enhancement has been discussed in the framework of CT-induced strengthening of electron-phonon coupling, as well as a possible match of the structural symmetries between probe molecules and the substrate. This study provides deep insights into our understanding of PI-SERS enhancement, and the revealed mechanism can be extended to other molecules for SERS at extreme conditions.

Breast cancer biomarker detection through the photoluminescence of epitaxial monolayer MoS2 flakes

In this work we report on the characterization and biological functionalization of 2D MoS2 flakes, epitaxially grown on sapphire, to develop an optical biosensor for the breast cancer biomarker miRNA21. The MoS2 flakes were modified with a thiolated DNA probe complementary to the target biomarker. Based on the photoluminescence of MoS2, the hybridization events were analyzed for the target (miRNA21c) and the control non-complementary sequence (miRNA21nc). A specific redshift was observed for the hybridization with miRNA21c, but not for the control, demonstrating the biomarker recognition via PL. The homogeneity of these MoS2 platforms was verified with microscopic maps. The detailed spectroscopic analysis of the spectra reveals changes in the trion to excitation ratio, being the redshift after the hybridization ascribed to both peaks. The results demonstrate the benefits of optical biosensors based on MoS2 monolayer for future commercial devices.

Achieving a superlubricating ohmic sliding electrical contact via a 2D heterointerface: a computational investigation

Simultaneously achieving low friction and fine electrical conductance of sliding electrical contacts is a crucial factor but a great challenge for designing high-performance microscale and nanoscale functional devices. Through atomistic simulations, we propose an effective design strategy to obtain both low friction and high conductivity in sliding electrical contacts. By constructing graphene(Gr)/MoS₂ two-dimensional (2D) heterojunctions between sliding Cu surfaces, superlubricity can be achieved with a remarkably lowered sliding energy barrier as compared to that of the homogeneous MoS₂ lubricated Cu contact. Moreover, by introducing vacancy defects into MoS₂ and substituting Cu with active metal Ti, the Schottky and tunneling barriers can be substantially suppressed without losing the superlubricious properties of the tribointerface. Consequently, a high conductivity ohmic contact with low sliding friction could be realized in our proposed Ti-MoS_1.5-Gr-Ti system, which provides a potential strategy for tackling the well-known dilemma for high performance sliding electrical contacts.

Ultrasensitive Plasmon-Free Surface-Enhanced Raman Spectroscopy with Femtomolar Detection Limit from 2D van der Waals Heterostructure

Two-dimensional (2D) materials have been promoted as an ideal platform for surface-enhanced Raman spectroscopy (SERS), as they mitigate the drawbacks of noble metal-based SERS substrates. However, the inferior limit of detection has limited the practical applicability of 2D material-based SERS substrates. Here, we synthesize uniform large-area ReO_xS_y thin films via solution-phase deposition without post-treatments and demonstrate a graphene/ReO_xS_y vertical heterostructure as an ultrasensitive SERS platform. The electronic structure of ReO_xS_y can be modulated by changing the oxygen concentration in the lattice structure, obtaining efficient complementary resonance effects between ReO_xS_y and the probe molecule. In addition, the oxygen atoms in the ReO_xS_y lattice generate a dipole moment on the thin-film surface, which increases the electron transition probability. These synergistic effects outstandingly enhance the Raman effect in the ReO_xS_y thin film. When ReO_xS_y forms a vertical heterostructure on a graphene as the SERS substrate, the enhanced charge-transfer and exciton resonances improve the limit of detection to the femtomolar level, while achieving remarkable flexibility, reproducibility, and operational stability. Our results provide important insights into 2D material-based ultrasensitive SERS based on chemical mechanisms.

Controllable MXene nano-sheet/Au nanostructure architectures for the ultra-sensitive molecule Raman detection

Surface-enhanced Raman scattering (SERS) spectroscopy aims to augment the relatively weak molecular vibrations based on electromagnetic enhancement (EE) and chemical enhancement (CE) mechanisms, and offers a potential way for material identification, even up to the single-molecule level, under atmospheric conditions. We have subtly combined the advantages of EE and CE, and propose new MXene (Ti3C2TX) nano-sheet/Au nanostructure architectures to break through the limitations of the Raman detection with long-time stability. The MXene nanosheets with excellent biocompatibility can effectively prevent structural distortion from the interaction with the Au NSs, and can also guarantee a high enhancement effect owing to the spatially extended electromagnetic field distribution and electron injection into the molecules. The self-assembled Au nanostructures are aggregated based on the Volmer-Weber growth model, and the electromagnetic field distribution radically evolves depending on the morphologies of the resultant Au nanostructures, leading to a drastic compensation for the limited EE of the MXene nano-sheets. Consequently, the intensified Raman vibrational signals of R6G molecules lead to a high enhancement factor of 2.9 × 107, even at an ultra-low concentration of 10-10 M. Similarly, the Raman signals of the methylene blue (MB) and crystal violet (CV) molecules can also be detected at low concentrations below 10-8 M, manifesting universal applications of the MXene/Au architectures for ultra-sensitive molecular detection under atmospheric conditions.