Semiconductor-based Surface-enhanced Raman Scattering Research Articles

Plasmon-Free Surface-Enhanced Raman Spectroscopy Using α-Type MoO3 Semiconductor Nanorods with Strong Light Scattering in the Visible Regime.

Recent developments in semiconductor-based surface-enhanced Raman scattering (SERS) have achieved numerous advancements, primarily centered on the chemical mechanism. However, the role of the electromagnetic (electromagnetic mechanism) contribution in advancing semiconductor SERS substrates is still underexplored. In this study, we developed a SERS substrate based on densely aligned α-type MoO3 (α-MoO3) semiconductor nanorods (NRs) with rectangular parallelepiped ribbon shapes with width measuring several hundred nanometers. These structural attributes strongly affect light transport in the visible range by multiple light scattering generated in narrow gaps between NRs, contributing to the improvement of SERS performance. Engineering the nanostructure and chemical composition of NRs realized high SERS sensitivity with an enhancement factor of 2 × 108 and a low detection limit of 5 × 10-9 M for rhodamine 6G (R6G) molecules, which was achieved by the stoichiometric NR sample with strong light scattering. Furthermore, it was observed that the scattering length becomes significantly shorter compared with the excitation wavelength in the visible regime, which indicates that light transport is strongly modified by mesoscopic interference related to Anderson localization. Additionally, high electric fields were found to be localized on the NR surfaces, depending on the excitation wavelength, similar to the SERS response. These optical phenomena indicate that electromagnetic excitation processes play an important role in plasmon-free SERS platforms based on α-MoO3 NRs. We postulate that our study provides important guidance for designing effective EM-based SERS-active semiconductor substrates.

One-step fabrication of a self-driven point-of-care chip by femtosecond laser direct writing and its application in cancer cell H2O2 detection via semiconductor-based SERS

Self-driven microfluidic systems have attracted significant attention and demonstrated great potential in the field of point-of-care (POC) testing due to their device simplicity, low power consumption, increased portability, and reduced sample consumption. To develop POC detection chips with diverse characteristics that meet different requirements, there is a strong demand for feasible strategies that enable easy operation and reduce processing time. Here, a one-step processing approach using femtosecond laser direct writing technology was proposed to fabricate a capillary-actuated POC microfluidic chip. The driving force of the chip is highly dependent on its surface wettability, which can be easily adjusted by changing the laser processing parameters. This POC microfluidic chip allowed for the detection of intracellular H2O2 through a catalytic reaction system that incorporated 5-aminosalicylic acid -sensitized colloidal TiO2 nanoparticles and horse radish peroxidase, with integrating semiconductor-based surface-enhanced Raman scattering (SERS) quantitative technique. The concentration of H2O2 was determined by the SERS signal of the catalytic products in the microfluidic chip, resulting in rapid detection with minimal sample consumption. Our method provides a simple, feasible, and alternative strategy for POC testing of H2O2, with a linear range of 10−2∼10−6 M and a limit of detection of 0.55 μM. This approach was successfully applied to rapid detection of intracellular H2O2 in MCF-7 breast cancer cells with high sensitivity and minimal sample consumption. Additionally, this study not only demonstrates the exceptional advantages of femtosecond laser processing technology in fabricating diverse microfluidic chips for various applications, but also presents an efficient POC testing strategy for detecting cell signaling molecules.

Construction of composite SERS substrate based on black phosphorus/mesoporous ZIF-67 and its selective monitoring of food additives

Semiconductor-based surface-enhanced Raman scattering (SERS) dependent on chemical mechanism has shown favorable selective enhancement and recognition capabilities. However, the limited Raman enhancement capability of conventional semiconductor-based still remains a great challenge in the effective monitoring of trace molecules in a practical complex environment. In this context, a novel two-dimensional composite semiconductor substrate was developed by in-situ growing reticular ZIF-67 mesoporous film upon black phosphorus (BP) nanosheets. An attractive synergistic chemical enhancement was triggered attributed to the efficient continued charge transfer process in the BP/ZIF-67 system. Particularly, a selective SERS enhancement of food additive molecules with different orbital structures was found owing to the coupling degree of intrinsic molecular resonance and photo-induced charge transfer. A sensitive detection of lycopene in beverage samples with a high recovery rate was finally realized by the BP/ZIF-67 composite substrate, which exhibited great potential in practical molecular selective recognition.

Noble metal-free SERS: mechanisms and applications.

Surface-enhanced Raman scattering (SERS) is a very important tool in vibrational spectroscopy. The coupling of nanomaterials induces local surface plasmon resonance (LSPR), which contributes greatly to SERS. Due to its remarkable sensitivity in trace detection, SERS has gained prominence in the fields of catalysis, biosensors, drug tracking, and optoelectronic devices. SERS activity is believed to be closely related to the LSPR and charge transfer (CT) of the material. Noble metal nanostructures have been commonly used as SERS-active substrates due to their strong local electric fields and relatively mature preparation, application, and enhancement mechanisms. In recent years, SERS research based on semiconductor materials has attracted significant attention because semiconductor materials have advantages such as repeatable preparation, simple pretreatment, stable SERS spectra and superior biocompatibility, stability, and reproducibility. Semiconductor-based SERS has the potential to enrich SERS theory and applications. Thus, the development of semiconductor materials will introduce a new epoch for SERS-based research. In this review, we outline the two main kinds of semiconductor SERS-active substrates: inorganic and organic semiconductor SERS-active substrates. We also provide an overview of the SERS mechanism for different kinds of materials and SERS-based applications.

Dye‐Sensitized Solar Cells Inspired Method to Modulate Photo‐Induced Charge Transfer Efficiency for Enhancing the SERS Activity of Semiconductor

AbstractWhile the Lithium ion modification strategy is widely adopted in tuning the bandgap of semiconductors in dye‐sensitized solar cells (DSSCs), such strategy is yet to be applied to semiconductor‐based surface enhanced Raman scattering (SERS), which is also a photo‐induced charge‐transfer (CT) process. Here, a new DSSCs‐inspired modification method is proposed for TiO2 nanoparticles (NPs): through the adsorption of Li+, the surface states of TiO2 NPs are more abundant, so that the SERS intensity and charge transfer of the adsorbed molecule are significantly enhanced. Similar to DSSCs systems, it is found that the conduction band edge and surface state energy level of TiO2 NPs shift downward with increasing Li+ concentration, which facilitates the CT interaction between the molecules and the SERS substrates. The EF can reach to 104, 1–2 degrees higher than previously reported pure semiconductors. This is the first time to use the DSSCs‐inspired Li+ adsorption strategy on TiO2 NPs as SERS substrate to investigate its SERS enhancement effect, which may provide new ideas for the development of semiconductor nanomaterials in the research of SERS substrates.

Quantum Effects Enter Semiconductor-Based SERS: Multiresonant MoO3·xH2O Quantum Dots Enabling Direct, Sensitive SERS Detection of Small Inorganic Molecules.

There is keen research interest in building highly effective semiconductor-based surface-enhanced Raman scattering (SERS) platforms, due to their selectivity for many probe molecules and suitability for complex scenario applications. However, current tuning approaches have not yet been successful in creating semiconductor-based SERS sensors for small inorganic molecules, due to the challenge of creating sufficient SERS enhancement in semiconductors. Here, we demonstrate the use of MoO3·xH2O quantum dots (QDs), to achieve direct and sensitive fingerprinting of the inorganic species hydrazine, which is a first attempt in semiconductor-based SERS research, as well as various other probe molecules. The resulting SERS platform that uses QDs with average size of 2.2 nm could successfully detect the signal of hydrazine with a limit of detection estimated to be around 4 × 10-5 M, significantly lowering the detectable concentration by at least 1000-fold, in sharp contrast to the weak performance of 10 and 100 nm particles, demonstrating that quantum size effect triggered by small particle size below the Bohr radius is crucially responsible for high SERS activity. The significantly enhanced SERS activity is a result of vibronically coupled multipathway, highly efficient charge-transfer resonances induced by the divergence of energy states in quantum-sized MoO3·xH2O. This is a proof-of-concept demonstration of the exploitation of quantum size effect, toward significantly enhanced intrinsic SERS activity in semiconductor-based SERS materials.

One plus one greater than Two: Ultrasensitive Surface-Enhanced Raman scattering by TiO2/ZnO heterojunctions based on Electron-Hole separation

The separation of electron-hole pairs has been explored to control the performance of photogenerated carriers in several catalytical and optoelectronic systems, yet its effect on the sensitivity of semiconductor-based surface-enhanced Raman scattering (SERS) is still unknown. Herein, we taken TiO2/ZnO heterojunction as an example to study the role of electron-hole separation in semiconductor SERS. The results show that the formation of TiO2/ZnO heterojunction can induce a strong coupling at interface, improving the separation of the photogenerated electron-hole pairs. In SERS spectrum, the selective enhancement of non-totally symmetric modes (b2) of molecule clearly indicated that the charge-transfer efficiency and SERS-activity can be significantly improved using such a semiconductor-heterojunction. Furthermore, the enhancement factor of 105 was achieved for a non-resonance molecule and the lowest detection concentration can be down to 10-8 M. The findings of this work not only provide a fundamental insight into the role of electron-hole separation for semiconductor SERS enhancement, but also open up a novel strategy for design of semiconductor SERS-active substrate with a high charge-transfer contribution.

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 charge-transfer induced by conduction band electrons in aluminum-doped zinc oxide/molecule/Ag sandwich structures observed by surface-enhanced Raman spectroscopy

In the semiconductor/molecule/metal system, enhancing the efficiency of the charge-transfer (CT) plays a pivotal role in improving the sensitivity of semiconductor-based surface-enhanced Raman scattering (SERS). In this work, use of SERS for detection of an enhanced CT in a chemically-etched Al-doped ZnO (AZO), 4-mercaptopyridine (MPy) molecule, and Ag nanoparticles (NPs) (AZO/MPy/Ag) sandwich structure is reported. A series of CT routes are proposed in the energy level diagram of AZO/MPy/Ag assemblies under the excitation line at 633 nm. Very interestingly, for the first of its kind, a significant CT route from the conduction band (CB) of AZO to the lowest unoccupied molecular orbital (LUMO) of MPy molecule is detected. This route can remarkably improve the degree of CT in the AZO/MPy/Ag system by about 48% compared with that of the ZnO/MPy/Ag system. Furthermore, the uniquely enhanced CT route is also further confirmed by alternative probe molecules like p-aminothiophenol (PATP) and 4-mercaptobenzoic acid (MBA). The discovery of this extra CT route will inevitably play an irreplaceable role in SERS enhancement through its participating in the CT enhancement mechanism.

Hollow Multi‐Shelled V2O5 Microstructures Integrating Multiple Synergistic Resonances for Enhanced Semiconductor SERS

AbstractEnhancement of the light‐matter coupling is a promising approach to improve the sensitivity and universality of semiconductor‐based surface‐enhanced Raman scattering (SERS). Herein, it is demonstrated that hollow multi‐shelled V2O5 microstructure can be utilized as an active substrate to integrate multiple synergistic contributions. The experiments and numerical simulations clearly reveal that the excellent SERS activity originates from the unique double‐shelled hollow structures, which allow multiple reflections of electromagnetic waves. This light‐trapping effect provides an efficient resonance absorption for charge‐transfer (CT) and exciton enhancements. More importantly, the coupling of outer and inner shells remarkably generates high electric field contribution for SERS enhancement. Besides, the symmetry analysis of vibrational modes based on density functional theory well explains the origin of the CT coupling of Herzberg–Teller selection rules. The synergy of all factors contributes to a significant increase in the sensitivity and universality of semiconductor SERS. It is anticipated that the strategy demonstrated in this study will widen the scope of currently available methodologies for designing substrates with high semiconductor SERS activity.

Surface Plasmon Resonance from Gallium-Doped Zinc Oxide Nanoparticles and Their Electromagnetic Enhancement Contribution to Surface-Enhanced Raman Scattering

In recent years, semiconductor-based surface-enhanced Raman scattering (SERS) substrates have received considerable attention and led to a forefront in the SERS field. However, the lack of electromagnetic (EM) enhancement contributions highly precludes the development of semiconductive-substrate-based SERS. In this study, Ga-doped ZnO nanoparticles (NPs) were fabricated and employed as novel SERS substrates based on the EM enhancement contribution of surface plasmon resonance (SPR). The obtained Ga-doped ZnO NPs exhibited obvious SPR absorptions in the visible and near- and mid-infrared regions. SPR absorption can be readily tuned by changing the doping ratios of Ga3+ ions. The SERS spectra of Ga-doped ZnO/4-mercaptopyridine (MPy) were investigated at different excitation wavelengths of 488, 532, 633, and 785 nm. The spectral enhancement of Ga-doped ZnO substrates depended on the doping ratios, excitation wavelengths, and nearby SPR absorption. Ga-doped ZnO NPs with the highest free charge carrier density and the doping ratio of 5% showed the strongest SERS spectra. For the fixed doping ratio of 5%, the better is the match between excitation wavelengths and SPR absorption, the higher is the SERS spectral enhancement. This study showed the feasibility of EM contributions to SERS by using semiconductive substrates and can contribute to the development of the semiconductor-based EM mechanism.

Ag-grafted on ZnO nanorod arrays using UV-assisted irradiation for enhanced SERS behavior in CV detection

Introduction: Semiconductor-based surface-enhanced Raman scattering (SERS) substrates with high stability and reproducibility have become one of the essential analytical tools in the analysis of chemical and biological at trace levels. Herein, a growth of the hexagonal-wrapped ZnO nanorod arrays decorating with Ag nanoparticles (AgNPs) at different concentrations of Ag was proposed.
 Methods: The crystallinity, morphology, chemical composition, and optical properties of the prepared samples were investigated by X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Raman system, respectively.
 Results: The results revealed that the SERS performance of ZnO NRs incorporating with AgNPs exhibited higher detection of crystal violet (CV) probe molecules at a low concentration of 10􀀀8 M than that of the pristine ZnO NRs. This effect originates from the localized surface plasmonic resonance of AgNPs that could cause a strong electromagnetic field and synergistic effects of Ag, ZnO, and CV molecules in ZnONRs@Ag/CV SERS system.
 Conclusion: These outcomes reveal that AgNPs play a crucial role in enhanced SERS performance for chemical and biological detection of ZnO substrate.

Significantly increased Raman enhancement on defect-rich O-incorporated 1T-MoS2 nanosheets

Improving charge transfer is the key to the performance of non-noble metal semiconductor-based surface enhanced Raman scattering (SERS) substrates. In this work, the O-incorporated 1T-MoS2 nanosheets with rich sulfur defects (ID-MoS2) are obtained by simple calcination of 1T-MoS2 nanosheets in air atmosphere. Using rhodamine 6G (R6G) as typical probe molecules, ID-MoS2 nanosheets show ultrahigh Raman enhancement effects with an enhancement factor of 1.24 × 107 due to sulfur defects and O incorporation in the 1T-MoS2 lattice. First-principle density functional theory calculations suggest that the existence of sulfur defects and O incorporation significantly increase the Fermi energy level (Ef) and electronic density of states of ID-MoS2. Moreover, O incorporation can enhance the interactions between the substrate and the adsorbed molecules through electrostatic and hydrogen bonding. All these improve the charge transfer resonance and result in the remarkable SERS activity of ID-MoS2 nanosheets. This is the first study on the increasing SERS performance of semiconductor substrates by simultaneously employing defect and dopant incorporation. This study provides an approach to optimize the performance of semiconductor-based SERS substrates.

Hydroxyl Group-Abundant TiO2 Semiconductor SERS Sensor toward Polymerization Inhibitor Sensing

One of the most significant challenges faced by semiconductor-based surface-enhanced Raman scattering (SERS) is achieving high-efficiency SERS detection in complex real-world application scenarios....

Twin-ZnSe nanowires as surface enhanced Raman scattering substrate with significant enhancement factor upon defect.

Semiconductor-based surface enhanced Raman scattering (SERS) substrate design has attracted much interest due to the excellent photoelectronic and biochemical properties. The structural change caused by twin in semiconductor will have an influence on improving the Raman signals enhancement based on the chemical mechanism (CM). Here, we demonstrated the twin in semiconductor ZnSe nanowires as an ultrasensitive CM-based SERS platform. The SERS signals of the rhodamine 6G (R6G) and crystal violet (CV) molecules adsorbed on twin-ZnSe nanowires could be easily detected even with an ultralow concentration of 10-11 M and 10-8 M, respectively, and the corresponding enhancement factor (EF) were up to 6.12 × 107 and 3.02 × 105, respectively. In addition, the charge transfer (CT) between the twin-ZnSe nanowires and R6G molecule has been demonstrated theoretically with first-principles calculations based on density-functional theory (DFT). These results demonstrated the proposed ZnSe nanowires with twin as SERS substrate has a broader application in the field of biochemical sensing.

Increased O 2p State Density Enabling Significant Photoinduced Charge Transfer for Surface-Enhanced Raman Scattering of Amorphous Zn(OH)2.

Enriching the electronic density of states (DOS) of semiconductors is the key to promoting charge transfer (CT) and achieving a large surface-enhanced Raman scattering (SERS) enhancement. Metal hydroxide semiconductors are anticipated to exhibit DOS that are higher than those of metal oxide because of their abundant O atoms; however, their SERS activity has not been verified. Here, combining density functional theory and experiments, we report a SERS sensitivity of amorphous Zn(OH)2 [a-Zn(OH)2] that is much higher than that of amorphous ZnO (a-ZnO), ascribed to the abundant O atoms and hence enriched O 2p state density near the Fermi level in a-Zn(OH)2, which gives rise to higher CT probabilities. Moreover, we find a-Zn(OH)2 exhibits significant advantages in energy-level matching over a-ZnO for efficient photoinduced CT via strong vibronic coupling, ascribed to the upshifted valence band maximum and the narrower band gap of a-Zn(OH)2. Via the synthesis of a-Zn(OH)2 nanocages, an ultrahigh enhancement factor of 1.29 × 106 is obtained in semiconductor-based SERS.

Direct Experimental Observation of Facet-Dependent SERS of Cu2 O Polyhedra.

Semiconductor-based surface enhanced Raman scattering (SERS) has attracted great attention due to its excellent spectral reproducibility, high uniformity, and good anti-interference ability. However, its relatively low SERS sensitivity still hinders its further developments in both performance and applications. Since the SERS is a peculiar surface effect, investigating the facet-dependent SERS activity of semiconductor nanostructures is crucial to boost their SERS signals. Although the semiconductor facet-dependent SERS effect is predicted via numerical calculations, convincing experimental evidence is scarce due to complicated and undefined surface conditions. In this work, three facet-defined ({100}, {110}, and {111} facets) Cu2 O microcrystals (MCs) with clear surface atomic configuration are utilized to investigate the facet-dependent SERS effect. The results from the Kelvin probe force microscopy measurements on single Cu2 O polyhedron, demonstrate that the facet-dependent work function plays a crucial role in the interfacial charge transfer process. Comparing with the {110} and {111} facets, the {100} facet possesses the lowest electronic work function, which enables more efficient interfacial charge transfer. The simulation results further confirm that the {100}-facets can transfer the most electrons from Cu2 O MCs to molecules due to its lowest facet work function, resulting in the largest increment of the molecular polarization.

Remarkable SERS Activity Observed from Amorphous ZnO Nanocages.

Enhancement of the semiconductor-molecule interaction, in particular, promoting the interfacial charge transfer process (ICTP), is key to improving the sensitivity of semiconductor-based surface enhanced Raman scattering (SERS). Herein, by developing amorphous ZnO nanocages (a-ZnO NCs), we successfully obtained an ultrahigh enhancement factor of up to 6.62×105 . This remarkable SERS sensitivity can be attributed to high-efficiency ICTP within a-ZnO NC molecule system, which is caused by metastable electronic states of a-ZnO NCs. First-principles density functional theory (DFT) simulations further confirmed a stronger ICTP in a-ZnO NCs than in their crystalline counterparts. The efficient ICTP can even generate π bonding in Zn-S bonds peculiar to the mercapto molecule adsorbed a-ZnO NCs, which has been verified through the X-ray absorption near-edge structure (XANES) characterization. To the best of our knowledge, this is the first time such remarkable SERS activity has been observed within amorphous semiconductor nanomaterials, which could open a new frontier for developing highly sensitive and stable SERS technology.

Remarkable SERS Activity Observed from Amorphous ZnO Nanocages

AbstractEnhancement of the semiconductor–molecule interaction, in particular, promoting the interfacial charge transfer process (ICTP), is key to improving the sensitivity of semiconductor‐based surface enhanced Raman scattering (SERS). Herein, by developing amorphous ZnO nanocages (a‐ZnO NCs), we successfully obtained an ultrahigh enhancement factor of up to 6.62×105. This remarkable SERS sensitivity can be attributed to high‐efficiency ICTP within a‐ZnO NC molecule system, which is caused by metastable electronic states of a‐ZnO NCs. First‐principles density functional theory (DFT) simulations further confirmed a stronger ICTP in a‐ZnO NCs than in their crystalline counterparts. The efficient ICTP can even generate π bonding in Zn−S bonds peculiar to the mercapto molecule adsorbed a‐ZnO NCs, which has been verified through the X‐ray absorption near‐edge structure (XANES) characterization. To the best of our knowledge, this is the first time such remarkable SERS activity has been observed within amorphous semiconductor nanomaterials, which could open a new frontier for developing highly sensitive and stable SERS technology.