1. Introduction

Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride and the transition metal dichalcogenides (TMDs), have shown tremendous potential in numerous applications owing to their distinct physical, chemical, electronic and optical properties [1–9]. Among these 2D materials, TMDs (molybdenum disulfide (MoS₂), tungsten disulfide (WS₂) etc.) have aroused the extensive attentions due to the extraordinary properties, such as the indirect-to-direct band-gap transition [2,10,11], valley polarization [12,13], high electrical carrier mobility [5,8,14], tunable excitonic effects [15]. WS₂, as the typical semiconducting TMDs, possesses the van der Waals layered structures where each layer is composed of S-W-S sandwich arrangement [11]. Relying on its extraordinary characteristics, WS₂ have been applied to wide range of applications, such as field-effect transistors [16–18], mode-locked fiber laser [7,19,20], biosensing [21], humidity-sensing [22], cancer theranostics [23], and hydrogen evolution [24–26].

Surface-enhanced Raman scattering (SERS), as a powerful analytical technique with single-molecule detection possible, has already been involved in abundant application fields [27–29]. Generally accepted mechanisms of SERS enhancement are electromagnetic mechanism (EM) which keeps the most dominant status owing to dramatic increase of local electromagnetic field and chemical mechanism (CM) which is related to the charge transfer [29,30]. According to EM, the electrically insulating gap between the vertical metallic nanostructures can achieve the plasmonic hybrids coupling, which helps obtain the tremendously enhanced local electromagnetic field and further produce the extremely amplified Raman signals [31]. Recently, graphene have been designed as the excellent nanogap into the powerful SERS substrate [32–34]. Zhan’ works have developed an Au/Gr/Au sandwiched structure by using graphene as subnanometer gap, the greatly enhanced electromagnetic field have been demonstrated experimentally and theoretically [33]. Zhang and his colleagues have introduced graphene as the tunable nanospacer to design the AgNPs/graphene@AuNPs system, achieving the excellent SERS behaviors by means of 3D hot spots [34]. Even so, the TMDs layers have shown the great possibility in acting as the precise nanospacer between the plasmonic nanostructures [35,36].

Several major aspects could be addressed for introducing WS₂ film into our research. Firstly, there has been rare study regarding the SERS performance of WS₂, compared with other TMDs (especially MoS₂). Wu et al. explored the SERS behavior of monolayer WS₂ while decorated with different distributions of silver nanoparticles (AgNPs) [37]. Sow’s group simply have decorated WS₂ monoflake with gold nanoparticles (AuNPs) to enhance the Raman signal of R6G molecules [38]. Li and associates fabricated the high-crystalline and large-scale WS₂ layers, and further suggested the Raman effects of WS₂ [39]. These work have carried out the preliminary and meaningful study of WS₂ in SERS research field, have not further combined WS₂ with plasmonic nanostructures to realize the greatly improved Raman enhancement effect. Secondly, WS₂ could be utilized as the promising 2D material to form the precise nanospacer between hybrid plasmonic nanostructures, obtaining the highly enhanced electromagnetic field and performing the pleasurable SERS activity. It is a significant attempt to use WS₂ film as nanospacer for surface plasmonics. Thirdly, WS₂ probably cause the promising Raman enhancement due to CM, which is ascribed to the charge transfer between substrate and probe molecules [39]. Lastly, the strong coupling between WS₂ and metallic nanostructures through surface plasmon excitation could be further beneficial to the enhanced Raman signals of probe molecules [38].

What’ more, common methods including mechanical exfoliation [40], chemical exfoliation [20], sulfurization of corresponding metal oxide or metal film [38,41] and thermal decomposition of thiotungstates [24,42] have been widely reported to fabricate the WS₂ layers in various fields. Among these approaches, the thermal decomposition seems to be capable of preparing the promising WS₂ layers, which could achieve tight coupling with plasmonic nanostructures simply. Nonetheless, there is no reported researches concerning the direct fabrication of WS₂ film on the surface of plasmonic nanostructures via the thermal decomposition method up to now.

Inspired by these factors, we designed the AuNPs/WS₂@AuNPs hybrids as the powerful SERS substrate. Bilayer WS₂ film was introduced as the precise nanospacer between the plasmonic nanostructures to generate the highly enhanced local electromagnetic field. Dense three-dimension (3D) hot spots occurring through this hybrid plasmonic nanostructures vastly contributes to SERS enhancement. The thermal decomposition method can realize the fabrication of large-scale and uniform bilayer WS₂ film, and achieve the facile and tight combination of WS₂ film and AuNPs. The desirable SERS behaviors of AuNPs/WS₂@AuNPs hybrids were demonstrated experimentally with R6G and CV as probe molecules and theoretically using COMSOL software. Due to the abundant 3D hot spots, AuNPs/WS₂@AuNPs hybrids could detect the minimum concentration of R6G as low as 10⁻¹¹ M, lower than AuNPs and WS₂@AuNPs substrates. Uniform signals (RSD~5.4%) and reproducible signals (RSD~5.7%) demonstrate the splendid reproducibility of AuNPs/WS₂@AuNPs hybrids with the assistance of uniform hot spots and the continuous bilayer WS₂ film. The synthesized SERS platform likewise perform the excellent anti-oxidation stability under the common air environment. What’s more, the simulation results confirm that the highly enhanced electric field of 3D hot spots is concentrated around the nanospacer introduced by bilayer WS₂. As the outstanding SERS sensor, the AuNPs/WS₂@AuNPs hybrids have exhibited the vast potential in practical applicability.

2. Experimental section

2.1 Fabrication process of the AuNPs /WS₂ @ AuNPs hybrids

The SiO₂ substrate was preprocessed by immersing into the aceton, alcohol, deionized water bath sonication for 15 min in turn, then treated with Ar plasma for 30 min. Then, the Au film was sputtered on the SiO₂ substrate with 5 s, 10 s and 15 s sputtering time by the magnetron sputtering system. The first annealing process was performed in the horizontal quartz tube by introducing 50 sccm Ar. the Au film was transformed into Au NPs when the quartz tube was heated to 800 °C at the rate of 10 °C/min and kept for 60 min. What needs illustration is that AuNPs obtained through first annealing process was further annealed under the similar time and temperature condition of growth process of WS₂ film. Thence, the all involved AuNPs substrates were actually designed through two annealing process under 800 °C. By comparison, AuNPs formed by the Au film with 10 s sputtering time was chosen to fabricate the WS₂@AuNPs and AuNPs/WS₂@AuNPs hybrids. The glycol solution was added into the high purity (NH₄)₂WS₄ powder to form a 1.25 wt% (NH₄)₂WS₄ solution. After the 30 min sonication, the (NH₄)₂WS₄ solution was spin-coated on the surface of AuNPs with 3500 rmp rotating speed to obtain the thin and uniform (NH₄)₂WS₄ film. The WS₂@AuNPs hybrids was fabricated via the thermal decomposition process with the gas flowing (Ar/H₂ = 80 sccm/20 sccm) at 800 °C for the 60 min reaction. Then, another Au film was sputtered on the WS₂@AuNPs hybrids with 5 s sputtering time, the second annealing process was carried out at 500°C for 30 min. Finally, we designed the AuNPs/WS₂@AuNPs hybrids. For comparison, the Au film with 5 s sputtering time was similarly sputtered on the prepared AuNPs without the fabrication of WS₂ film, the AuNPs/AuNPs composites was designed through the annealing process at 500 °C for 30 min.

2.2 Apparatus and characterization

The uniform Au film was sputtered on the substrate via the magnetron sputtering system (Denton Desktop Pro). The thickness of Au film was measured with atomic force microscope (AFM, Park XE-100). The morphologies of AuNPs, WS₂@AuNPs and AuNPs/WS₂@AuNPs substrates were investigated through Scanning electron microscope (SEM, ZEISS Gemini Ultra-55). Transmission electron microscopy system (TEM, JEOL2100F) was carried out to characterize the hybrid plasmonic nanostructures. WS₂@AuNPs composites were also characterized with X-ray Diffraction (XRD, D8-Advance) and X-ray photoelectron spectroscopy (XPS, ESCA Lab-250Xi). The UV–visible spectrophotometer (Hitachi, U-4100) was employed to characterize the localized surface plasmon resonance (LSPR) properties of AuNPs, WS₂@AuNPs and AuNPs/WS₂@AuNPs substrates.

2.3 SERS measurement

Rhodamine 6G (R6G) and crystal violet (CV) were utilized as probe molecules to test the SERS performance of different substrates. R6G and CV were respectively dissolved in the water to prepare the solutions with multiple concentrations. 2 μL of the probe molecules solution was dropped directly on the optional area of corresponding substrate. Then, these samples were naturally dried before performing the SERS experiments. All Raman spectra were collected with the Raman spectrometer system (Horiba HR Evolution 800). (laser wavelength at 532 nm, 4.8 mW laser power, 600 gr/mm diffraction grid, 50 × objective lens, 1 μm laser spot and 8 s integration time). As shown in Fig. 9, optical images of droplet-covered area demonstrate that the measured area we choose for Raman intensity measurement is close to any edge of droplet-covered area. The red dotted lines display the general measured area on different substrates, which is about 60 × 60 µm². All involved Raman spectra are the average of eight Raman spectra that were randomly collected from the regular measured area.

3. Results and discussions

The Schematic illustration of the fabrication of the AuNPs/WS₂ @ AuNPs hybrids is shown in Fig. 1. The detailed illustration could been seen from the description in experimental setup. Before fabricating the AuNPs/WS₂@AuNPs hybrids, we firstly investigated the AuNPs morphology by controlling the sputtering time, and compared the SERS behaviours of different AuNPs distributions. Considering that AuNPs obtained through the first annealing process would subsequently perform another hightemperature process (i.e. growth process of WS₂ film). Thence, all AuNPs substrates were fabricated through two annealing processes under 800 °C.

 

Fig. 1 Schematic illustration of the fabrication of the AuNPs/WS₂ @ AuNPs hybrids.

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As shown in Figs. 10(a)-10(c), the size of AuNPs becomes larger and larger with the sputtering time increasing, and the gaps between the adjacent AuNPs get bigger and bigger. In addition, 3# AuNPs exhibit some irregular sphere shape (marked with red circles) when the sputtering time increases to 15 s, which is not consistent with 2# AuNPs in regular sphere shape. It has been reported that the size of nanoparticles and the distance between nanoparticles play highly crucial roles in SERS enhancement. In contrast with 2# AuNPs, 1# AuNPs with smaller size and 3# AuNPs with bigger space are much likely to perform poor in SERS sensitivity. SERS signals of three different distributions of AuNPs were evaluated subsequently using R6G as the probe molecules, as shown in Fig. 10(d). Obviously, 2# AuNPs annealed by the Au film with 10 s sputtering time exhibit the best SERS signal of R6G, which could be related with the suitable size as well as the dense distribution of AuNPs. Besides, moderate 2# AuNPs are extremely suitable to form the vertical nanometer spacer with other plasmonic nanoparticles under the premise of superior Raman signal. Thence, the detailed analyses were performed for the 2# AuNPs substrate. The thickness of Au film with 10 s sputtering time was firstly measured with AFM. As shown in Fig. 2(a), the boundary between SiO2 substrate (dark area) and Au film (bright area) could be observed clearly, the measured thickness of Au film is around 7 nm. The SEM images of AuNPs and the corresponding size distribution are displayed in Figs. 2(b) and 2(c), respectively. The diameter of AuNPs is approximately 55 nm, which accords with the typical Gaussian distribution. The SERS signals of R6G molecules with concentrations range from 10⁻⁵ M to 10⁻⁹ M on AuNPs are presented in Fig. 2(d). The typical Raman bands of R6G at 612, 773, 1184, 1311, 1362, 1508 and 1649 cm⁻¹ could be observed clearly, consistent with our previous observations [43]. The intensity of Raman bands of R6G decreases with the concentrations dropping, and the AuNPs substrate could detect the minimum concentration of R6G as low as 10⁻⁸ M.

 

Fig. 2 (a) AFM image of Au film with 10 s sputtering time on the SiO₂ substrate. The inset pattern exhibits the corresponding measured thickness. (b) SEM image of the 2# AuNPs annealed by Au film through two annealing process under 800 °C. (c) The size distribution of AuNPs from (b). (d) SERS behaviors of R6G molecules with concentrations range from 10⁻⁵ to 10⁻⁸ M on AuNPs substrate.

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WS₂@AuNPs hybrids were subsequently fabricated through the thermal decomposition approach. The (NH₄)₂WS₄ solution was spin-coated on the surface of the AuNPs substrate, the WS₂ film was fabricated facilely to encapsulate the AuNPs densely. SEM image shown in Fig. 3(a) demonstrates that the size of AuNPs on WS₂@AuNPs hybrids seems to have no obvious difference compared to AuNPs substrate. And the size distribution of AuNPs encapsulated with WS₂ film in Fig. 3(b) exhibits that the diameter of AuNPs is approximately 56.7 nm, slightly larger than that from AuNPs substrate. This tiny difference of size of AuNPs is likely to be caused by the existence of WS₂ film and the minor error in processing data. As shown in Fig. 3(c), Raman spectra of WS₂ film displays the 2LA(M) peak at 351 cm⁻¹ and the A_1g peak at 415 cm⁻¹, Which indicate the second-order mode of longitudinal acoustic mode and the out-of-plane vibrations of the sulfur atoms separately [39,44]. Two stable Raman characteristic bands illustrate that the WS₂ film have successfully synthesized on the WS₂@AuNPs hybrids. In addition, the frequency difference Δ between two characteristic bands is around 64 cm⁻¹, corresponding to the bilayer WS₂ film [45]. TEM was further implemented to exhibit the plasmonic nanostructures of the proposed WS₂@AuNPs composites. As can be seen from TEM and HRTEM patterns in Fig. 3(d), the average diameter of the AuNPs is almost consistent with the SEM result, and AuNPs are densely encapsulated with the WS₂ film. Shape of AuNPs could be approximatively considered as an idealized sphere. The thickness of WS₂ film is measured around 1.6 nm (i.e. bilayer WS₂ film). These results demonstrate that the quasi core-shell structures (WS₂@AuNPs nanostructures) have been successfully synthesized through the thermal decomposition approach. This tight contact between AuNPs and bilayer WS₂ film is beneficial to decrease the loss of electromagnetic enhancement between the vertical plasmonic nanostructures.

 

Fig. 3 (a) SEM image of the WS₂@AuNPs composites. (b) The size distribution of AuNPs encapsulated with the WS₂ film. (c) Raman spectra of WS₂ randomly collected from 50 × 50 µm² area on WS₂@AuNPs substrate. (d) TEM image of WS₂@AuNPs hybrid plasmonic nanostructures. The inset pattern is the HRTEM image of WS₂@AuNPs composites. The green dotted circles label the general outline of AuNPs.

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To further verify the successful fabrication of WS₂@AuNPs composites, we carried out the XRD measurement and the XPS analysis for this hybrid nanostructures. As shown in Fig. 4(a), we can clearly notice the typical diffraction peak at 38.3° corresponding to the (111) crystalline plane of Au, which indicates the existence of AuNPs. However, the (002), (004), (006) and (008) peaks assigned as the characteristic diffraction bands of WS₂ [39] could be observed hardly. This result might be explained that the thickness of synthesized WS₂ layers is too thin. The chemical state of WS₂@AuNPs nanostructures was further analyzed through XPS analysis. The XPS spectrum for WS₂@AuNPs hybrids is exhibited in Fig. 11. The corresponding bands related with Au, W and S elements could be observed obviously, illustrating the existence of three elements on the WS₂@AuNPs hybrids. The atomic radio of W and S elements from inset presents a rational chemical composition (W/S is around 1/2). Moreover, Figs. 4(b)-4(d) display the Au 4f, W 4f, S 2p XPS patterns of WS₂@AuNPs hybrids severally. In Fig. 4(b), the Au-related peaks of Au 4f5/2 and Au 4f7/2 reveal the clear signals at 84.0 and 87.7 eV, respectively. We can easily observe the signals of W 4f at 32.0 and 34.4 eV from Fig. 4(c), which agree with W 4f5/2 and W 4f7/2 separately and indicate the existence of the W⁴⁺ species for WS₂@AuNPs composites. The S 2p XPS spectrum displays that two bands with the binding energy at 163.8 and 162.3 eV are related to S 2p1/2 and S 2p3/2, which suggests the existence of the S^2- species. These measured results further indicate that WS₂@AuNPs composites have been successfully synthesized.

 

Fig. 4 (a) XRD spectrum for WS₂@AuNPs hybrids. (b)-(d) represent the Au 4f, W 4f, S 2p XPS spectra of WS₂@AuNPs composites respectively. (e) Raman spectra of R6G molecules with different concentrations from 10⁻⁵ to 10⁻⁹ M on WS₂@AuNPs hybrids. (f) SERS intensity of R6G molecules at 612 cm⁻¹ as a function of the varying concentrations.

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The SERS intensity of R6G molecules with varying concentrations from WS₂@AuNPs hybrids is displayed in Fig. 4(e). Other than the characteristic bands of R6G, two characteristic peaks of WS₂ are similarly detected, and display a small shift in Raman peak position, which agrees with the previous report [39]. Moreover, as shown in Fig. 9, the droplet-covered area of WS₂@AuNPs substrate exhibit the more even color compared with AuNPs substrate, which is attributed to the existence of WS₂ film. The minimum detectable concentration of R6G on WS₂@AuNPs substrate can reach 10⁻⁹ M, one order of magnitude lower than that on AuNPs substrate. Three major factors could be responsible to the amplified Raman signals from WS₂@AuNPs hybrids. First, WS₂ probably bring the SERS enhancement due to CM, which is attributed to the charge transfer between substrate and probe molecules. Second, the combination of WS₂ and AuNPs could achieve the strong coupling through surface plasmon excitation to enhance the SERS signals. Third, the thermal decomposition method helps ensure that the WS₂ layers tightly wrap the AuNPs, making R6G molecules fall around the hot spots effectively. As shown in Fig. 4(f), we could observe the linear response of the peak intensity of R6G at 612 cm⁻¹ versus the concentrations from 10⁻⁵ to 10⁻⁹ M in log scale. The linear fitted curve (R² = 0.984) with error bars indicates that WS₂@AuNPs hybrids perform the excellent capability of the quantitative detection of target molecules.

Considering that the precise nanospacer which could be introduced through WS₂ layers is likely to trigger the highly enhanced electromagnetic field between the vertical adjacent metallic nanoparticles, and further amplify the SERS signals. Another Au film with 5 s sputtering time was sputtered on WS₂@AuNPs hybrids, so the AuNPs/WS₂@AuNPs substrate was subsequently synthesized through another annealing process at 500 °C. Au film with 5 s sputtering time was sputtered on the blank SiO₂ substrate to characterize its thickness, the AFM image in Fig. 5(a) illustrates that the thickness of Au film with 5s sputtering time is approximately 3 nm. After the annealing process, we can distinctly observe the surface morphology of AuNPs/WS₂@AuNPs hybrids, shown in Fig. 5(b). The AuNPs with small diameter are evenly distributed on the whole AuNPs/WS₂@AuNPs substrate. The space between large AuNPs with 55 nm average diameter are filled with small AuNPs, which have the extremely dense arrangement. What’s more, small AuNPs are tightly decorated on the top surface of large AuNPs (55 nm). The average diameter of small AuNPs is 15 nm with a 5 nm gap. The AuNPs/WS₂@AuNPs composites were further characterized with TEM measurement. The TEM pattern of AuNPs/WS₂@AuNPs composites is exhibited in Fig. 5(c). Two types of AuNPs with significant difference in size are totally observed, and their shape could be approximatively considered as an idealized sphere. HRTEM pattern in Fig. 5(d) demonstrates that the bilayer WS₂ act as the precise nanospacer between small AuNPs and large AuNPs, which indicates that the typical AuNPs/WS₂@AuNPs nanostructures have been successfully synthesized. Furthermore, the Raman spectra of WS₂ were also collected from AuNPs/WS₂@AuNPs composites. As shown in Fig. 5(e), the 2LA(M) peak at 351 cm⁻¹ and the A_1g peak at 415 cm⁻¹ can be still observed and the frequency difference Δ is still 65 cm⁻¹, corresponding to bilayer WS₂ film. However, only the peak intensity of two characteristic Raman peaks have the obvious decrease, which is highly likely been attributed to the slight disorder of the crystalline structure of WS₂ film during the thermal annealing process of Au film [46]. The UV-Vis absorbance spectra were measured to characterize the localized surface plasmon resonance (LSPR) properties of AuNPs, WS₂@AuNPs and AuNPs/WS₂@AuNPs substrates, shown in Fig. 5(f). As the structure of substrates becomes complicated, the absorption band has the obvious red-shift labeled by the black dashed line. WS₂@AuNPs and AuNPs/WS₂@AuNPs substrates respectively displays the strong absorption peaks at ~543 nm and at ~558 nm, which are totally close to the wavelength (532 nm) of laser and beneficial for enhancing the SERS signals of probe molecules.

 

Fig. 5 (a) AFM image of Au film with 5 s sputtering time which is estimated on the SiO₂ substrate. The inset pattern exhibits the corresponding measured thickness (b) SEM image of the AuNPs/WS₂@AuNPs hybrids. The inset pattern shows the size distribution of small AuNPs achieved through annealing process of 3 nm Au film. (c) TEM image of the AuNPs/WS₂@AuNPs composites. The green and red dotted circles label the outline of AuNPs with different average diameters, respectively. (d) HRTEM pattern of the AuNPs/WS₂@AuNPs composites in a high magnification. (e) Raman spectra of WS₂ randomly collected from 50 × 50 µm² area on AuNPs/WS₂@AuNPs substrate. (f) UV-Vis absorbance spectra of AuNPs, WS₂@AuNPs and AuNPs/WS₂@AuNPs substrates. The black dashed line represents the shift of absorption bands.

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The SERS intensity of R6G with varying concentrations was similarly explored on AuNPs/WS₂@AuNPs hybrids. As shown in Fig. 6(a), we could observe the dominating Raman characteristic bands of R6G molecules. The AuNPs/WS₂@AuNPs hybrids can achieve the minimum detectable concentration of R6G molecules as low as 10⁻¹¹ M, three orders of magnitude lower than AuNPs substrate, two orders of magnitude lower than WS₂@AuNPs hybrids. The linear relationship between the peak intensity of R6G at 612 cm⁻¹ and the varying concentrations for AuNPs/WS₂@AuNPs hybrids is demonstrates in Fig. 6(b) in log scale. The linear response curve (R² = 0.993) with error bars indicates that AuNPs/WS₂@AuNPs hybrids perform the brilliant capability of the quantitative detection of probe molecules. To explore the SERS sensitivity of AuNPs/WS₂@AuNPs hybrids for other analyte molecules, crystal violet (CV) molecules were also selected to detect. The characteristic Raman vibrations of CV molecules could be seen from the collected SERS spectra in Fig. 6(c). The bands at 918, 1182, 1375 cm⁻¹ are assigned to ring skeletal vib of radical orientation, ring C-H bend and N-phenyl stretching separately. And the peaks at 1536, 1590, 1622 cm⁻¹ are related to the ring C-C stretching [47]. The linear relationship between the peak intensity of 1622 cm⁻¹ and the varying concentrations for AuNPs/WS₂@AuNPs hybrids is plotted in log scale, shown in Fig. 6(d). The linear response curve (R² = 0.989) with error bars indicates that AuNPs/WS₂@AuNPs hybrids show great potentials for the quantitative analysis of other probe molecules. The thermal decomposition method helps ensure that the bilayer WS₂ could tightly encapsulate large AuNPs, and decrease the mass loss of electromagnetic enhancement from hybrid plasmonic nanostructures. The prominent SERS behaviors of AuNPs/WS₂@AuNPs hybrids might be attributed to the dense 3D hot spots introduced by the lateral nanogap and vertical nanogap between AuNPs, and the CM induced by the bilayer WS₂ film.

 

Fig. 6 (a) SERS spectra of R6G molecules with different concentrations from 10⁻⁵ to 10⁻¹¹ M on AuNPs/WS₂@AuNPs hybrids. (b) SERS intensity of R6G molecules at 612 cm⁻¹ as a function of the varying concentrations. (c) SERS spectra of CV molecules with different concentrations from 10⁻⁵ to 10⁻⁹ M on AuNPs/WS₂@AuNPs hybrids. (d) SERS intensity of CV molecules at 1622 cm⁻¹ as a function of the varying concentrations. (e) SEM image of the AuNPs/AuNPs substrate, the inset was a large-scale SEM pattern. (f) SERS spectra of R6G with the concentration of 10⁻⁵ M collected from AuNPs/WS₂@AuNPs and AuNPs/AuNPs substrates.

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The AuNPs/AuNPs composites were also fabricated to illustrate the benefits from bilayer WS₂ as nanospacer. The 3 nm Au film was deposited on the prepared AuNPs without the growth of bilayer WS₂ film, the AuNPs/AuNPs composites was fabricated through the mentioned annealing process at 500 °C. As shown in Fig. 6(e), SEM image exhibits the morphology of AuNPs/AuNPs composites, the fabricated hybrids nanostructure have widespread and even distribution on substrate. The small AuNPs (15 nm) were evenly distributed between the large AuNPs, identical to the observation from AuNPs/WS₂@AuNPs substrate. However, the top surface of large AuNPs (55 nm) have no decoration of small AuNPs (15 nm), indicating that the vertical plasmonic nanostructures have not been formed without bilayer WS₂ as the nanospacer. Besides, the SERS behaviors of AuNPs/WS₂@AuNPs and AuNPs/AuNPs composites were further compared using 10⁻⁵ M R6G as probe molecules, shown in Fig. 6(f). AuNPs/WS₂@AuNPs hybrids show 1.5 times stronger than AuNPs/AuNPs composites in the peak intensity of 10⁻⁵ M R6G at 612 cm⁻¹. Bilayer WS₂ could not only help achieve the formation of small AuNPs on the top surface of large AuNPs, but also take a crucial role as the precise nanospacer in enhancing the SERS signals of analyte molecules.

What’s more, the peak intensity of R6G at 612 and 773 cm⁻¹ with varying concentrations from three different substrates was summarized to make the quantitative comparisons, as shown in Table 1. Whatever the concentration is, the AuNPs/WS₂@AuNPs composites always show the highest peak intensity of R6G among three substrates. The peak intensity of 612 cm⁻¹ from AuNPs/WS₂@AuNPs composites is around 3.1 times stronger than that from AuNPs substrate, and 1.8 times stronger than that from WS₂@AuNPs composites. In addition, the peak intensity of 773 cm⁻¹ from AuNPs/WS₂@AuNPs hybrids is around 3.7 times stronger than that from AuNPs substrate, and 2.1 times stronger than that from WS₂@AuNPs hybrids. These amplified SERS signals further demonstrate that AuNPs/WS₂@AuNPs hybrids perform the excellent sensitivity for probe molecules relying on dense 3D hot spots provided through hybrid plasmonic nanostructures.

Table 1. The SERS intensity of vibrations of R6G from substrates

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With regard to the remarkable SERS substrate, mass of researchers similarly pay much attention to other crucial parameters apart from the sensitivity, such as the spot-to-spot and substrate-to-substrate reproducibility and the stability (especially the anti-oxidation stability. Thence, the reproducibility of SERS signals for the AuNPs/WS₂@AuNPs hybrids were firstly explored through the detection of R6G molecules with the concentration of 10⁻⁵ M. The SERS spectra of R6G collected from random 20 positions of the same AuNPs/WS₂@AuNPs hybrids are presented in Fig. 7(a). The main vibrational modes of R6G have no significant shift and display the relatively same color without obvious difference. The peak intensity of 612 cm⁻¹ from 20 spectra was quantificationally displayed in Fig. 7(b). The black horizontal line represents the average peak intensity of 612 cm⁻¹, and the blue shaded zone reveals the variation range of the peak intensity. The calculated relative standard deviation (RSD) is 5.4%, confirming that the proposed AuNPs/WS₂@AuNPs hybrids indicate the favourable spot-to-spot reproducibility. Moreover, Figs. 7(c) and 7(d) illustrate SERS spectra of R6G collected from 10 AuNPs/WS₂@AuNPs hybrids in different batches and corresponding band intensity variation of 612 cm⁻¹, respectively. The RSD through the calculation is 5.7%, demonstrating that this synthesized hybrids similarly display the desirable substrate-to-substrate reproducibility. The excellent reproducibility of AuNPs/WS₂@AuNPs hybrids could be attributed to the following factors: the uniform hot spots distribution caused by the homogeneous fabrication of AuNPs and the continuous and large-scale bilayer WS₂ film with the thermal decomposition approach. In general, the SERS substrate with poor anti-oxidation stability exhibits the dissatisfactory performance in SERS sensitivity. Therefore, the antioxidant stability of AuNPs/WS₂@AuNPs hybrids was also investigated by detecting SERS responses of R6G molecules with the concentration of 10⁻⁵ M. In Fig. 7(e), we can observe the SERS responses collected from the same AuNPs/WS₂@AuNPs substrate every three days, which is normally stored in a common air environment. Six spectra plotted in Fig. 7(e) exhibit no obvious shifts for vibrational modes of R6G, simply the SERS intensity of main characteristic peaks decrease with days going on. The changing curve of the peak intensity of R6G at 612 cm⁻¹ versus days was shown in Fig. 7(f). After 15 days, the peak intensity of 612 cm⁻¹ has the little decrease i.e. 6.7%, illustrating the well anti-oxidation stability of AuNPs/WS₂@AuNPs hybrids. The splendid anti-oxidation stability is attributed to several aspects. AuNPs considered as the inert metallic nanostructure under a common air environment could perform the superior anti-oxidation stability during SERS measurement. In addition, the bilayer WS₂, as the protective layers (similar to graphene and MoS₂), might take a dominate role in delaying the long-time oxidation of large AuNPs (55nm) on the AuNPs/WS₂@AuNPs hybrids.

 

Fig. 7 (a) SERS spectra of R6G molecules (10⁻⁵ M) collected from 20 random positions of the same AuNPs/WS₂@AuNPs substrate. (b) Corresponding peak intensity variations of 612 cm⁻¹ for the same AuNPs/WS₂@AuNPs substrate. (c) SERS spectra of R6G molecules (10⁻⁵ M) collected from 10 AuNPs/WS₂@AuNPs substrates in different batches. (d) Corresponding peak intensity variations of 612 cm⁻¹ for different AuNPs/WS₂@AuNPs substrates. (e) SERS responses of R6G molecules (10⁻⁵ M) obtained from the same AuNPs/WS₂@AuNPs substrate every three days. (f) The changing curve of the peak intensity of R6G at 612 cm⁻¹ versus days.

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In order to further analyze the SERS characteristics of WS₂@AuNPs and AuNPs/WS₂@AuNPs hybrids in theory, the COMSOL software was used to simulate the electric field distribution. Based on SEM images of WS₂@AuNPs and AuNPs/WS₂@AuNPs hybrids [insets in Figs. 8 (a) and 8(b)], the 3D models were built up generally to make it easier to understand the simulated plasmonic nanostructures. In view of the reported papers and the simulation aim to study the electric field distribution by using bilayer WS₂ as nanospacer, the modeling structure of AuNPs could be set to be spherical for general distribution of electric field [32–34]. As we can see, Fig. 8(a) presents the simulation set-up of the WS₂@AuNPs hybrids that the diameter of AgNPs is 55 nm, the thickness of WS₂ film is 1.6 nm,and Fig. 8 (b) displays the simulation set-up of AuNPs/WS₂@AuNPs hybrids that the diameter of small AgNPs is 15 nm, the gap is 5 nm. As shown in Fig. 8(c), the y-z view of the electric field distribution for WS₂@AuNPs hybrids demonstrates that the electric field enhancement (E/E₀) of hot spots is around 2.5. The simulated electric field distribution of AuNPs/WS₂@AuNPs hybrids in y-z view is shown in Fig. 8(d). The simulation results can be seen clearly that the prominent electric fields were highly enhanced and concentrated around the nanospacers produced by bilayer WS₂ between large AuNPs (55 nm) and small AuNPs (15 nm), illustrating that 3D dense hot spots have been formed on the AuNPs/WS₂@AuNPs hybrids. The maximum enhancement factor of electric field intensity for AuNPs/WS₂@AuNPs hybrids is 147.7, 59.1-time larger than that for WS₂@AuNPs hybrids. These simulation results illustrate that the precise nanospacer introduced by bilayer WS₂ between plasmonic nanostructures provide the extremely dense 3D hot spots, confirming the sensitive SERS performance theoretically.

 

Fig. 8 (a) Simulation set-up of WS₂@AuNPs hybrids in y–z view. (b) Simulation set-up of AuNPs/WS₂@AuNPs hybrids in x–y view and in y–z view respectively. Insets in (a) and (b) are the SEM images of corresponding nanostructures. (c) COMSOL-simulated y-z view of the electric field distribution for WS₂@AuNPs hybrids that the diameter of AgNPs is 55 nm, the thickness of WS₂ film is 1.6 nm. (d) COMSOL-simulated y-z view of the electric field distribution for AuNPs/WS₂@AuNPs hybrids that the diameter of small AgNPs is 15 nm, the gap is 5 nm.

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4. Conclusions

In summary, we have fabricated the AuNPs/WS₂@AuNPs hybrids by introducing bilayer WS₂ film as the precise nanospacer between the plasmonic metallic nanostructures. Bilayer WS₂ film was prepared directly on the plasmonic nanostructures via the thermal decomposition method, which ensures that AuNPs can be encapsulated with the WS₂ layers facilely and tightly. The tremendously enhanced local electromagnetic field generated through nanospacer contributes vastly to the promising SERS behaviors, which are demonstrated experimentally and theoretically. AuNPs/WS₂@AuNPs hybrids perform the excellent sensitivity with the minimum detectable concentration of R6G as low as 10⁻¹¹ M, the splendid spot-to-spot reproducibility (RSD~5.4%) and substrate-to-substrate reproducibility (RSD~5.7%), as well as the brilliant anti-oxidation stability under the common air environment. The promising results illustrate that the synthesized hybrids perform well in detecting other probe molecules such as CV molecules. The desired SERS behaviors of AuNPs/WS₂@AuNPs hybrids are demonstrated theoretically using the COMSOL software. As the excellent SERS sensor, the AuNPs/WS₂@AuNPs hybrids have shown great potential in practical applicability.

Appendices

 

Fig. 9 (a)-(b) respectively display the optical images of the droplet-covered area of 10⁻⁵ R6G solution on AuNPs, WS₂@AuNPs and AuNPs/WS₂@AuNPs substrates. The red dotted lines exhibit the general measured area on different substrates, which is about 60 × 60 µm².

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Fig. 10 SEM images of AuNPs annealed by the Au film with (a) 5 s, (b) 10 s and (c) 15 s sputtering time through two annealing process under 800 °C, which are respectively labeled as 1#, 2#, and 3# AuNPs. Some AuNPs with irregular sphere shape are marked with red circles in (c). (d) SERS signals of 10⁻⁵ M R6G collected from three different substrates.

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Fig. 11 XPS spectrum for WS₂@AuNPs hybrids.

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Funding

National Natural Science Foundation of China (NSFC) (11674199, 11747072, 11474187, 11774208); Natural Science Foundation of Shandong Province (ZR2017BA004, ZR2017BA018, 2017GGX20120, ZR2013HQ064); Natural Science Foundation of Jiangsu Province (BK20141222); Department of Science and Technology of Shandong Province (J18KZ011).

Acknowledgments

Great thanks to Jing Yu and Wen Yang for their helpful suggestions in our COMSOL analysis. Thanks to Professor Shouzhen Jiang and Zhen Li for great helps in the manuscript writing.

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