Efficient Photoinduced Charge Transfer Research Articles

Metal-Organic Frameworks/Heterojunction Structures for Surface-Enhanced Raman Scattering with Enhanced Sensitivity and Tailorability.

Metal-organic frameworks (MOFs), which are composed of crystalline microporous materials with metal ions, have gained considerable interest as promising substrate materials for surface-enhanced Raman scattering (SERS) detection via charge transfer. Research on MOF-based SERS substrates has advanced rapidly because of the MOFs' excellent structural tunability, functionalizable pore interiors, and ultrahigh surface-to-volume ratios. Compared with traditional noble metal SERS plasmons, MOFs exhibit better biocompatibility, ease of operation, and tailorability. However, MOFs cannot produce a sufficient limit of detection (LOD) for ultrasensitive detection, and therefore, developing an ultrasensitive MOF-based SERS substrate is imperative. To the best of our knowledge, this is the first study to develop an MOFs/heterojunction structure as an SERS enhancing material. We report an in situ ZIF-67/Co(OH)2 heterojunction-based nanocellulose paper (nanopaper) plate (in situ ZIF-67 nanoplate) as a device with an LOD of 0.98 nmol/L for Rhodamine 6G and a Raman enhancement of 1.43 × 107, which is 100 times better than that of the pure ZIF-67-based SERS substrate. Further, we extend this structure to other types of MOFs and develop an in situ HKUST-1 nanoplate (with HKUST-1/Cu(OH)2). In addition, we demonstrate that the formation of heterojunctions facilitates efficient photoinduced charge transfer for SERS detection by applying the Mx(OH)y-assisted (where M = Co, Cu, or other metals) MOFs/heterojunction structure. Finally, we successfully demonstrate the application of medicine screening on our nanoplates, specifically for omeprazole. The nanoplates we developed still maintain the tailorability of MOFs and perform high anti-interference ability. Our approach provides customizing options for MOF-based SERS detection, catering to diverse possibilities in future research and applications.

Symbiotic coupling of ZnO nanoparticles and coumarin photosensitizer in soft polyurethane matrices for boosting visible-light photocatalytic performance

Photocatalysis provides a practical and efficient solution to eliminate harmful substances from the environment by using light energy to activate the catalysts, and the development of sustainable photocatalysts easily recoverable and highly efficient in visible light remains a significant challenge. To address this issue, herein we present a facile approach to immobilize silane-functionalized ZnO NPs in polyurethanes with coumarin chromophore, resulting in highly active recyclable and reusable hybrid materials as photocatalytic systems for the degradation of organic dyes. Characterization of ZnO NPs revealed that the attachment of silane units to nanoparticles' surface decreased band gap values (up to 2.83 eV) and reduced the rate of charge carrier recombination. The performed investigations showed that incorporation of ZnO NPs into macromolecular chains via quaternization near to coumarin units promoted efficient photoinduced charge transfer between semiconductor nanoparticles and photosensitizer molecules. The band gap energy for hybrid films ranged between 2.73 eV and 2.32 eV, and the best photodegradation results for malachite green (MG) under visible-light irradiation were attained after 17 min (k = 17.7 × 10−2 min−1). The photocatalytic mechanism was clarified using scavenger tests and the photocatalytic efficiency of the best performing sample was tested for the degradation of other dyes (brilliant blue R, sunset yellow, congo red, rhodamine B, s.o.) and for the removal of MG from binary dye mixtures. The stability and integrity evaluation after 7 catalysis cycles reveals that the photocatalytic potential is retained without substantial structural changes.

Structural Conjugation Tuning in Covalent Organic Frameworks Boosts Charge Transfer and Photocatalysis Performances.

Structural conjugation greatly affects the optical and electronic properties of the COF photocatalyst. Herein, we show that 2D hydrazone COFs with either π-extended biphenyl (BPh-COF) or acetylene (AC-COF) frameworks demonstrated distinct charge transfer and photocatalytic performances. The two COFs show good crystallinity and decent porosity as their frameworks are enforced by intra/interlayers hydrogen bonding. However, computational and experimental data reveal that AC-COF managed broader visible-light absorption and narrower optical bandgaps and performed efficient photoinduced charge separation and transfer in comparison with BPh-COF, meaning that the ethynyl skeleton with enhanced planarity better improves the π-conjugation of the whole structure. As a result, AC-COF exhibited an ideal bandgap for rapid oxidative coupling of amines under visible-light irradiation. Furthermore, taking advantage of its better charge transfer properties, AC-COF demonstrated considerable enhanced product conversion and notable functional tolerance for metallaphotocatalytic C-O cross-coupling of a wide range of both aryl bromides and chlorides with alcohols. More importantly, besides being recoverable, AC-COF showcased the previously inaccessible etherification of dihaloarene. This report shows a facile approach for manipulating the structure-activity relationship and paves the way for the development of a COF photocatalyst for solar-to-chemical energy conversion.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes.

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

Rapid and deep photocatalytic degradation of polyvinyl alcohol by black phosphorus quantum dot sensitized g-C3N4

To address the pollution caused by polyvinyl alcohol (PVA) waste, a composite photocatalyst is developed by sensitizing g-C3N4 with black phosphorus quantum dots (BPQDs) using a simple mechanical stirring method. Both g-C3N4 and BPQDs are inorganic nonmetallic semiconductors with well-matched band positions, facilitating efficient photoinduced charge transfer. The periodic table’s adjacent relationship between C, N and P elements allows easy formation of P-N or P-C bonds by replacing C or N atoms with P atoms. The resulting composite shows uniform decoration of two-dimensional layered g-C3N4 with BPQDs with an average size of 2.2 nm. Under solar light simulator irradiation for 20 min, the composite photocatalyst exhibits significantly enhanced photocatalytic activity, with PVA degradation efficiency increasing from 27.1% (pure g-C3N4) to 85.9%. Experimental results and density functional theoretical calculations suggest the formation of a Z-scheme route at the g-C3N4/BPQDs interface. This facilitates photoinduced electron transfer from g-C3N4 to BPQDs, leading to improved carrier production and separation, reduced charge-transfer resistance, and accelerated PVA degradation. The proposed composite photocatalyst holds promise for addressing PVA pollution and improving environmental sustainability.

Fabrication of g-C3N4@N-doped Bi2MoO6 heterostructure for enhanced visible-light-driven photocatalytic degradation of tetracycline pollutant

An effective catalyst for the removal of antibiotic pollutants which severely impact water bodies and the environment is most favorable. In this study, g-C3N4 (gCN) and nitrogen-doped Bi2MoO6 (gCN-NBM) heterostructures are developed using a solvothermal process with enhanced photocatalytic degradation of tetracycline (TC) pollutants under visible-light irradiation. The experimental results confirm that nitrogen-doped Bi2MoO6 (NBM) nanomaterials were dispersed on the gCN surface, and a close combination of NBM and gCN leads to the efficient photocatalytic performance of TC. The photocatalytic efficiency of the heterostructure catalysts is four-fold higher than those of the pristine Bi2MoO6 catalysts owing to the excellent photogenerated charge separation and reduced recombination rate. Photocurrent measurements and electrochemical impedance spectra results disclose that the prepared heterostructure catalysts exhibit efficient photo-induced charge transfer. The electron spin resonance spectra and quenching experiments results reveal that superoxide radicals (.O2−) play a major role in TC degradation. This study presents a promising approach for designing efficient visible-light photocatalysts for environmental remediation applications.

Band gap tuning of Sn1-xCexO2 nanoflower for improved SERS activity of bioassay

Metal oxide-based surface-enhanced Raman scattering (SERS) platforms have attracted considerable interest, which show potentials for biomolecule detection due to their wide bandgap tuning range, stable properties, high electron mobility and good optical properties. However, the types and enhancement factor (EF) of metal oxide SERS substrates are limited. And the band-matched efficient photo-induced charge transfer (PICT) is the important process to further improve the EF. In this study, one efficient SERS substrate Sn1-xCexO2 was synthesized by varying concentrations of Ce ions with band gap tuning and oxygen vacancies introducing. As a result, the charge transfer pathway and strong interaction between the Sn1-xCexO2 nanoflowers (NFs) and methylene blue (MB) molecules are promoted, and the enhancement factor (EF) of this Sn1-xCexO2 (x = 0.03) NF is as high as 1.80 × 106. Then, we constructed a biosensor based on hybridization chain reaction (HCR) strategy combining with Mg2+-dependent DNAzyme cleavage nucleic acid cascade signal amplification to achieve sensitive and quantitative detection of microRNA 21 (miRNA 21). The limit of detection (LOD) was 0.72 fM with a detection linearity ranged from 1 fM to 10 nM. This study gives an approach for preparation of new SERS substrates which can be used in bioanalysis.

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 Mo4/3B2 MBene material was designed. The 2D Mo4/3B2 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 Mo4/3B2 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 Mo4/3B2 MBene enables its Raman enhancement by a factor of 100 000 times higher than that of the bulk MoB. Consequently, the 2D Mo4/3B2 MBene could accurately detect various trace chemical analytes. Moreover, with ordered metal vacancies in the 2D Mo4/3B2 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.

Sulfur Vacancies‐Triggered High SERS Activity of Molybdenum Disulfide for Ultrasensitive Detection of Trace Diclofenac

AbstractDue to the efficient photoinduced charge transfer properties of nonmetals, nonmetallic surface‐enhanced Raman scattering (SERS) substrates have been advanced rapidly these years. And among of these, molybdenum disulfide (MoS2) is perceived as one of the most representative nonmetallic SERS substrates due to its abundant active sites. However, its progress in SERS field is severely hindered because of the lower SERS activity. Herein, one type of vacancies‐triggered highly sensitive SERS substrate based on nanostructured MoS2 with sulfur vacancies (VS) is proposed. The VS‐MoS2 substrates are intended to promote the SERS performance through the VS density regulation. Intrinsically, moderate VS can change the bandgap structure of MoS2, thus promoting the separation of internal carriers and enhancing the interaction with target species. Externally, the presence of VS can bring abundant adsorption sites on MoS2 surface, which greatly improves its ability to enrich target molecules. Both internal and external properties make VS‐MoS2 a high SERS activity substrate. Moreover, with the antibiotic contaminant diclofenac as a model probe molecule, the VS‐MoS2 substrate can accurately perceive its nanomolar concentration level with an enhancement factor up to 5.85 × 103. This work demonstrates exciting potential for analysis in real water environments based on nonmetallic SERS applications.

Construction of Z-scheme Bi12O15Cl6/SnO2−x heterojunction for enhanced photocatalytic degradation of dyes and antibiosis

It is still a great challenge to design efficient and stable photocatalysts to effectively remove organic pollution in water. A new type Bi12O15Cl6/SnO2−x Z-scheme heterojunction was successfully constructed. The best sample 30% Bi12O15Cl6/SnO2−x can degrade 86.7% TC within 90 min and 99.7% RhB within 75 min under LED light. The morphology, structure, optical and electrochemical properties of the photocatalyst were characterized by using different testing techniques. Efficient photoinduced charge transfer and separation efficiency was confirmed by photocurrent and electrochemical impedance spectra (EIS) in Bi12O15Cl6/SnO2−x structure. The active substances, intermediate product and photodegradation pathway in degradation process were revealed by capture test and mass spectrometry (MS) measurement. In photocatalytic process, h+, •O2− and •OH are the main active species, and h+ is the most important active substance. In addition, the photocatalytic mechanism of charge separation is also proposed, which provides a new opportunity for the construction of photocatalysts and the degradation of organic pollutants in the water environment.

In situ self-assembly of mesoporous Zn-Cd-Mo-S quaternary metal sulfides with double heterojunction synergistic charge transfer for boosting photocatalytic hydrogen production

The construction of semiconductor heterojunctions is regarded as an efficient strategy for promoting photogenerated carrier transfer and enhancing photocatalytic performance. [1] Moreover, the in-situ self-assembly method of the main catalyst and the cocatalyst is believed to overcome the shortcomings of the traditional surface loading method and provide a more efficient hydrogen evolution reaction capability. In this work, Zn-Cd-Mo-S quaternary metal sulfide mesoporous nanospheres were generated by in-situ self-assembly via a one-step mild hydrothermal method. A photocatalyst with a double-heterojunction synergistic structure and a large specific surface area was constructed to form a high-speed charge transfer channel as a mechanism to promote photocatalytic hydrogen production. The best quaternary catalyst ZCM5S has a hydrogen evolution yield of 23.32 mmol h−1 g−1, which is 53 times and 11 times that of the corresponding binary (CdS) and ternary (ZnCdS) metal sulfides, respectively, and still has high stability within 20 h. Furthermore, the ZCM5S gets an apparent quantum efficiency (AQE) of 6.9 % at 420 nm. Through further analysis, such as photoluminescence (PL) and photoelectrochemical, etc., the enhanced photocatalytic performance of ZCM5S can be attributed to the synergistic effect of the heterojunction between ZnS and CdS and the heterojunction between ZnCdS and MoS2. In addition, the Pt-like structure of molybdenum sulfide cocatalyst can lead to efficient photoinduced charge separation and transfer. Meanwhile, the mesoporous structure endows the material with abundant reactive sites and increases the catalytic reaction efficiency. The unique in situ construction method of this work also provides a reference for the design and synthesis of heterojunction quaternary catalysts for green energy conversion.

Photoexcited Intramolecular Charge Transfer in Dye Sensitizers: Predictive In Silico Screening for Dye-Sensitized Solar Cell Devices

Efficient photoinduced intramolecular charge transfer (ICT) from donor to acceptor in dye molecules is the functional basis and key property in the working of a dye-sensitized solar cell (DSSC). To understand the ICT process in photoexcited dye molecules, we analyze the electronic properties and structural parameters of a chosen set of experimentally synthesized donor–acceptor (D–A) and donor−π-spacer−acceptor (D−π–A) type dye molecules in their ground, excited, and cationic states. The correlation between structural modification and charge redistribution in different parts of the molecule helps to identify the extent of π-conjugation and spatial rearrangement of electron density localization along the molecular skeleton. We find that prominent twisting of several groups and the resulting molecular bond rearrangements in larger parts of the molecule promote efficient donor to acceptor ICT, such as in D–A type ADEKA1 and C275 dyes. Thus, based on the modest computation of structural and electronic properties of dye molecules in their respective ground, excited, and cationic states, we identify the desired structural changes that facilitate tunable intramolecular charge transfer to highlight a simple and direct prescription to screen out probable efficient dye molecules among many samples. Our approach complements recent experimental evidence of capturing the structural view of the excited-state charge transfer in molecules.

High-Performance Ternary Perovskite-Organic Solar Cells.

Perovskite solar cells in which 2D perovskites are incorporated within a 3D perovskite network exhibit improved stability with respect to purely 3D systems, but lower record power conversion efficiencies (PCEs). Here, a breakthrough is reported in achieving enhanced PCEs, increased stability, and suppressed photocurrent hysteresis by incorporating n-type, low-optical-gap conjugated organic molecules into 2D:3D mixed perovskite composites. The resulting ternary perovskite-organic composites display extended absorption in the near-infrared region, improved film morphology, enlarged crystallinity, balanced charge transport, efficient photoinduced charge transfer, and suppressed counter-ion movement. As a result, the ternary perovskite-organic solar cells exhibit PCEs over 23%, which are among the best PCEs for perovskite solar cells with p-i-n device structure. Moreover, the ternary perovskite-organic solar cells possess dramatically enhanced stability and diminished photocurrent hysteresis. All these results demonstrate that the strategy of exploiting ternary perovskite-organic composite thin films provides a facile way to realize high-performance perovskite solar cells.

Fine fabrication of TiO2/MoOx nano-heterojunctions and investigating on the improved charge transfer for SERS application

Metal-oxide surface-enhanced Raman spectroscopy (SERS) substrates have attracted enormous attentions for trace-level molecule detection, and efficient photo-induced charge transfer is crucial to further enhance the SERS sensitivity. Herein, TiO2@MoOx nanorods are finely prepared to construct the TiO2/MoOx nano-heterojunctions, and then they are implemented as an efficient charge transfer substrate for SERS application. The SERS performance of the substrate is evaluated by using a 532-nm laser as the excitation source and R6G molecules as the Raman reporter. Experimental results show that R6G's Raman signal intensity is gradually improved with the increase of the nano-heterojunction numbers on TiO2@MoOx nanorods. A limit of detection as low as 10−8 M (R6G@4.0 × 10−13 M/cm2) and Raman enhancement factor of 1.445 × 108 (R6G@10−8 M) can be achieved on the most optimal TiO2@MoOx nanorods. Moreover, the mechanism of the SERS enhancement is investigated by analyzing the TiO2 and MoOx band diagrams. It evidences that TiO2@MoOx forms staggered gap heterojunction. Under the laser irradiation, the photo-generate electron-hole pairs are separated in the depletion region and then free electrons move toward the MoOx surface. Thereafter, the electrons are further transferred to the molecules enhancing the SERS performance. Finally, to verify the aforementioned explanation, the surface potential of a single TiO2@MoOx nanorod is measured by Kelvin prober force microscope, it exhibits that the surface potential distribution shifts to a higher level under the light illumination, confirming the accumulation of free electrons on the MoOx surface, and then enhancing charge transfer between the molecules and heterojunctions.

Amorphous Co(OH)2 nanocages achieving efficient photo-induced charge transfer for significant SERS activity

Boosting substrate–molecule interactions, especially the strong vibronic coupling and efficient photo-induced charge transfer (PICT) transitions, is a critical issue to improve the surface-enhanced Raman scattering (SERS) sensitivity of non-metal substrates.

Photoinduced charge transfer in layered 2D PbSe-MoS-=SUB=-2-=/SUB=- nanostructures

Semiconductor 2D nanostructures are a new platform for the creation of modern optoelectronic devices. Layered 2D PbSe-MoS2 nanostructures with efficient photoinduced charge transfer from PbSe nanoplatelets (NPLs) to MoS2 were created. When PbSe NPLs with short organic ligands are deposited onto a thin layer of MoS2 NPLs, a decrease in their photoluminescence intensity and a decrease in the average photoluminescence lifetime are observed. When a layered 2D PbSe-MoS2 nanostructure is illuminated with IR radiation, a photocurrent appears, which indicates the contribution of PbSe NPLs to the electrical response of the system. Ultrathin layers of transition metal dichalcogenides sensitized with nanostructures based on lead chalcogenides can be used in photodetectors with a spectral sensitivity region extended to the near-IR range. Keywords: nanoplatelets, transition metal dichalcogenides, charge transfer, near infrared region.

Фотоиндуцированный перенос заряда в слоистых 2D наноструктурах PbSe-MoS-=SUB=-2-=/SUB=-

Semiconductor 2D nanostructures are a new platform for the creation of modern optoelectronic devices. Layered 2D PbSe-MoS2 nanostructures with efficient photoinduced charge transfer from PbSe nanoplatelets (NPLs) to MoS2 were created. When PbSe NPLs with short organic ligands are deposited onto a thin layer of MoS2 NPLs, a decrease in their photoluminescence intensity and a decrease in the average photoluminescence lifetime are observed. When a layered 2D PbSe-MoS2 nanostructure is illuminated with IR radiation, a photocurrent appears, which indicates the contribution of PbSe NPLs to the electrical response of the system. Ultrathin layers of transition metal dichalcogenides sensitized with nanostructures based on lead chalcogenides can be used in photodetectors with a spectral sensitivity region extended to the near-IR range.

Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution

Tailoring conductive polymers with inorganic photocatalysts, which provide photoinduced electron-hole generation, have significantly enhanced composites leading to excellent photoelectrodes. In this work, MnFe2O4 nanoparticles prepared by a hydrothermal method were combined with polyaniline to prepare mixed (hybrid) slurries, which were cast onto flexible FTO to prepare photoelectrodes. The resulting photoelectrodes were characterized by XRD, FESEM, HRTEM and UV-VIS. The photoelectrochemical performance was investigated by linear sweep voltammetry and chronoamperometry. The photocurrent achieved by MnFe2O4/Polyaniline was 400 μA/cm2 at 0.8 V vs. Ag/AgCl in Na2SO4 (pH = 2) at 100 mW/cm2, while polyaniline alone achieved only 25 μA/cm2 under the same conditions. The best MnFe2O4/Polyaniline displayed an incident photon-to-current conversion efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) of 60% at 405 nm wavelength, and 0.17% at 0.8 V vs. Ag/AgCl, respectively. High and stable photoelectrochemical performance was achieved for more than 900 s in an acidic environment.

Surface-enhanced Raman spectroscopy detection of uranium oxides assisted by Ag2O

The rapid and efficient identification of uranium oxides is of great significance in the field of nuclear security. A feasible SERS strategy assisted by Ag2O for rapid detection of trace amount of uranium oxides (∼1.8 μg/mL for UO2 and ∼ 3.6 μg/mL for U3O8) was proposed and described. The effects of laser power on the Raman and SERS of uranium oxides were investigated. And the SERS mechanism was discussed by constructing several types of SERS substrates. Uranyl ions produced from hydrolysis and oxidation of uranium oxides are the key components for their SERS, where efficient photoinduced charge transfer process from Ag2O to uranyl ions occurs, resulting in a more intense Raman mode. The easily obtained but clearly distinguished SERS signals allow us to selectively and quickly detect the uranium oxides from mixtures, which may be practically applied to nuclear antiterrorism.

Structural engineering of transition-metal nitrides for surface-enhanced Raman scattering chips

Noble-metal-free surface-enhanced Raman scattering (SERS) substrates have attracted great attention for their abundant sources, good signal uniformity, superior biocompatibility, and high chemical stability. However, the lack of controllable synthesis and fabrication of noble-metal-free substrates with high SERS activity impedes their practical applications. Herein,we propose a general strategy to fabricate a series of planar transition-metal nitride (TMN) SERS chips via an ambient temperature sputtering deposition route.These planar TMN (tungsten nitride, tantalum nitride, and molybdenum nitride) chips show remarkable Raman enhancement factors (EFs) with ~105 owing to efficient photoinduced charge transfer process between TMN chips and probe molecules. Further, structural engineering of these TMN chips is used to improve their SERS activity. Benefiting from the synergistic effect of charge transfer process and electric field enhancement by constructing nanocavity structure, the Raman EF of WN nanocavity chips could be greatly improved to 1.29 * 107, which is an order of magnitude higher than that of planar chips. Moreover, we also design the WN/monolayer MoS2 heterostructure chips. With the increase of surface electron density on the upper WN and more exciton resonance transitions in the heterostructure, a 1.94 * 107 level EF and a 5 * 10-10 m level detention limit could be achieved. Our results provide important guidance for the structural design of ultrasensitive noble-metal-free SERS chips.