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Abstract
Combing the merits of metals and semiconductors,with superior plasmon resonance effects and high charge mobility, 3D-nanocomposite structures consisting of graphene oxide (GO), noble metal nanostructures, and two-dimensional transition metal sulfides (2D-TMDS) are an important topic in surface-enhanced Raman scattering (SERS) research. This paper presents a novel GO/Ag NPs (silver nanoparticles)/WS2 composite SERS substrate, and electric field simulation by COMSOL software. The GO/Ag/WS2 composite substrate shows very high SERS detective sensitivity and stability to probe molecules such as rhodamine 6 g (R6G), crystal violet (CV), methylene blue (MB) and deoxyribonucleic acid (DNA). The SERS sensitivity can reach 10−12 M, the relative standard deviation (RSD) is 8.24%, and the enhancement factor (EF) is approximately 6.60 × 1010 for R6G, which promoted the implementation of the SERS technique in the area of quantitative profiling and testing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As science and technology develop, environmental and biological detection has attracted much attention. As a molecular fingerprint spectroscopy technology, SERS can realize real-time and in-situ detection of target molecules. Because of its high sensitivity and low price, it has been widely used in food safety, natural monitoring, chemical residue detection and other fields [1–4], in the detection processing substrates to convert reaction signals into optical signals for qualitative or quantitative analysis. However, SERS technology, which simultaneously outputs highly sensitive and repeatable signals, is still a challenge to be overcome.
A variety of studies have been used to explore mechanisms and methods to enhance the sensitivity and accuracy of SERS detection. Currently, the SERS enhancement mechanisms mainly include electromagnetic enhancement (EM) and chemical enhancement (CM), where EM is dominant [5,6]. Under the laser, localized surface plasmon resonance (LSPR) generated by metal nanoparticles is considered to be the main reason for the enhancement of electromagnetic formation [4,5,7]. Due to the size effect and quantum effect of precious metal nanoparticles, irradiation by excitation light can cause surface plasmon resonance, which greatly enhances the Raman scattering signal. In addition, the CT (charge transfer) amid the atom with the substrate is the main cause of CM, when the highest and lowest occupied molecular orbital (HOMO and LUMO) of the probe molecule match the valence and conduction band (VB and CB) energy levels of the substrate [8], CT occurs, produces a bigger scrambling cross-section, thus enhancing the Raman spreading.
TMDs, such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have attracted wide attention, due to their specific optical and electronic properties. There is a variety of active states (sulfur defects, vacancies, boundary states, etc.) in WS2, which can adsorb small molecules and polar molecules, which make WS2 have a high application prospects in the field of biological and chemical sensing [9]. Moreover, due to superior electrical conductivity, huge identifiable surface region, admirable biomolecular affinity, tall sensitivity to external stimuli, excellent fluorescence quenching effect, WS2 has been proved to be ideal SERS substrate [10,11]. In 2006, Wu Hanchun obtained high-quality monolayer WS2 films by sulfiding monolayer tungsten trioxide on the sapphire substrate [12]. Li Zhen et al. prepared large-area WS2 films by thermal cleavage and reannealing of ammonium tetrathionate, and performed Raman analysis with R6G acts as a probe molecule with detection limits up to 10−7 M, verifying the Raman enhancement and fluorescence quenching effect of WS2 [13]. However, due to the poor enhancement effect of pure WS2 substrate, its development is limited. Due to controllable morphology and excellent SERS enhancement of silver nanoparticles [14,15], it have been widely used as SERS substrates. It has been demonstrated that Ag NPs can generate a larger number of the “hot spots” [16], which are conducive to the accumulation of molecule Raman signals. But the poor stability of pure Ag NPs substrate limits its development. It is promising to prepare composite substrates combining metals and semiconductors can improve SERS performance. The use of semiconductor materials as substrates or overlays for metal nanostructures increases their specific surface area, this facilitates the molecule absorption and enhances the Raman signal. In addition, GO is rich in oxygen-containing groups, which can be well combined with noble metals and semiconductors, and the intervention of graphene oxide can prevent noble metal nanostructures from being oxidized due to direct exposure to air, improving the stability of the substrate.
In this paper, we use the tube furnace thermal annealing method to grow large-area continuous cerebral reticular WS2, use the water bath method to prepare Ag NPs with uniform particle size and spacing, the van der Waals heterojunction interactions were enhanced by the plasmon resonance effect of Ag and WS2 and CT of the heterojunction was improved. Moreover, GO is added to heighten the stability of the substrate, increase the ability of the substrate to adsorb probe molecules and achieve SERS enhancement. The advantages of this structure are: 1) the strong coupling between WS2 exciton and metal plasma is used to expand and enlarge the amount and intensity of “hot spots”, and increase SERS signal sensitivity; 2) the combined effect of EM and CM of metal nanoparticles and 2D materials, improves the sensitivity of SERS signal and breaks the detection limit of detected molecules; 3) GO is conducive to the adsorption of biomolecules, easy to achieve specific modifications to its surface, and also prevent the oxidation of silver nanoparticles and improve the stability of the substrate.
2. Experimental
2.1 Materials
Ammonium tetrathiotungstate ((NH4)2WS4, ≥99.9% trace metals), dimethyl sulfoxide (C2H6SO, AR, 99%(GC)), silver nitrate (AgNO3, AR, 99.8%), polyvinylpyrrolidone (PVP, Mw = 10000), Sublimed Sulfur (S, AR, ≥99.5%), Acetone (C3H6O, AR, ≥99.5%), Ethylene Glycol (C2H6O2, AR, ≥99.5%), Ethanol (C2H6O, AR, ≥99.5%) were bought from Aladdin Co., Ltd. (Shanghai, China). GO grows through improved Hummers [17] way. The water used in the experiments was deionized.
2.2 Synthesis of WS2 nanofilms and Ag NPs
WS2 is synthesized using a two-step thermal decomposition [18]. Firstly, dissolving 0.015 g (NH4)2WS4 powder in 3 ml C2H6SO by ultrasound for 30 min, to form a yellow-green solution. In the second step, the (NH4)2WS4 solution was dropped on the SiO2 substrate, and gluing machine at 3000 rpm was rotated for 2 minutes to form a thin and uniform (NH4)2WS4 film. The samples were then placed on a heated platform and dried at 120 °C for 20 minutes. After drying, it was put into a tube furnace, evacuated to 10−3 Pa with a molecular pump, and heated to 550 °C. The mixed gas (Ar/H2 = 80/40 sccm) was channeled into the chamber for the first annealing to effectively remove the by-products separated from the precursor (NH4)2WS4. (NH4)2WS4 was thermally degraded to WS2 after 90 min of reaction. In order to grow the WS2 film along the lateral epitaxy direction, the sample was naturally cooled to room temperature. To obtain better crystalline WS2, the quartz tube was then reheated to 850 °C and annealed a second time in an argon/sulfur (Ar/S) vapor atmosphere. Ar was used to avoid sample oxidation and to blow S onto the WS2 film. After annealing for 30 minutes, cool naturally to room temperature.
Ag NPs were made by the reduction of AgNO3 by PVP [20]. First, add PVP and AgNO3 in proportion (10:1) to 20 ml C2H6O2 to make a 2.2 w% solution, heat to 130 °C for 2 hours, when the solution turns milk tea brown, add 50 ml C3H6O to cool quickly. Lastly, the final product is obtained by centrifugation and washing.
2.3 Preparation of GO/Ag NPs/WS2 composite substrates
The preparation process of GO/Ag NPs/WS2 composite substrate is shown in Fig. 1. Firstly, dissolve 0.045 g (NH4)2WS4 in 3 ml C2H6SO to form an (NH4)2WS4 solution. 50 µL of (NH4)2WS4 solution was spin-coated onto a quartz sheet and annealed twice on a tube furnace to form WS2. The WS2 substrate was dipped in Ag colloid, and Ag NPs/WS2 was formed after taking it out. Lastly, GO/Ag NPs/WS2 were gained by spin-coating the graphene dispersion onto Ag NPs/WS2.
2.4 Test apparatus
Scanning electron microscopy (SEM, ZEISS Gemini Sigma 500) was employed in describe the morphology along with elemental composition of GO, WS2 and Ag. The crystal structures of WS2 was profiled by X-ray diffraction (XRD, Rigaku D/MAX-RB). Elementals were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi). An ultraviolet absorption spectrophotometer (UV-Vis, Alpha-1500) was used for calculating the absorption spectrums of Ag NPs and composite substrates. Raman characterizations were obtained from a Raman spectrometer (Horiba HR Evolution 800) with an incident laser wavelength of 532 nm, with a light intensity of 0.48 mW on the substrate, a spot length of 1 µm, integration time 4 s, and 600 g/mm diffraction grating. During SERS experiments, observe the samples with a 50X objective.
3. Results and discussions
3.1 Characterization of composite substrates
SEM features the shape and structure of GO/Ag NPs/WS2 composite substrates. As indicated in Fig. 2(a), large-area continuous cerebral reticular WS2 has been generated. As shown in Fig. 2(b), we can see that the Ag NPs in the Ag NPs/WS2 composite substrate are tightly arranged, and the Ag NPs completely cover WS2 surface. The prepared Ag is in the shape of uniform spherical particles, which is beneficial to improve the plasma coupling efficiency [21]. Likewise, we counted the diameters and spacing of Ag NPs particles in a range of 1.5 × 1.5 µm2, 220 Ag NPs in this region with an average diameter of 106 nm, of which Ag NPs with particle size distribution between 100 - 110 nm accounted for 47% (Fig. 2(d)), and the average spacing between adjacent nanoparticles was 8.8 nm, within these tiny nano-spacers, a robust electromagnetic field will be present, coordinating the Ag NPs to enable the GO/Ag NPs/WS2 composite substrate with high SERS enhancement [22]. Through Fig. 2(c), we can see that Ag NPs/WS2 are covered by satin-like GO, GO/Ag/WS2 composite substrate covered with GO thin film and Ag/WS2 composite substrate without GO film can be clearly distinguished. The elemental composition and distribution of GO/Ag NPs/WS2 can be seen by the EDS (energy-dispersive spectra) in Fig. 2(e), the color distributions of C, O, Si, W, S and Ag are uniform, indicating the uniform distribution of GO, Ag and WS2 on the substrate, moreover, the ratio of tungsten to sulfur is about 1:2 in WS2, which proves that our prepared WS2 has good crystallinity. We analyzed the crystal structure of GO/Ag NPs/WS2 film by XRD, In Fig. 2 (f), six peaks at 2θ = 11.3°, 14.3°, 28.9°, 38.2°, 44.1°, and 60.1° were observed. The peaks at 2θ = 14.3°, 28.9°, 44.1°, and 60.1° correspond to the (002), (004), (006) and (008) planes of WS2 [23], respectively, comparing with the XRD card, it was concluded that the WS2 prepared was 2H crystal phase. The presence of (002) peak at 14.3° indicates that the WS2 is a multilayered nanostructure. Merely the (002) group refraction peaks were observed, indicating that this type of WS2 is highly crystalline. The peaks at 11.3° and 38.2° correspond to the (001) plane of GO and the (111) plane of Ag, respectively, proving that the GO/Ag NPs/WS2 composite substrate has been successfully grown. Figure 2(g) shows the Raman peaks of WS2 and Ag/WS2, and it can be seen that are two distinct peaks at 352 and 419 cm-1, corresponding to the 2LA(M) and A1g(Γ) vibration modes of WS2, respectively, the wavenumber difference between the 2LA(M) and A1g (Γ) is 67 cm-1, which corresponds exactly to 4-layer WS2. Compared to pure WS2 substrate, the position of WS2 Raman peaks in the Ag NPs/WS2 substrate does not change, the peak intensity is stronger and more excitation peaks are generated, for example, the A1g(M)-LA(M) peak at 230 cm-1, the 2LA(M)-E22g(M) peak at 297 cm-1, and the 2LA(M)-2E22g(M) peak at 324 cm-1 [24] were both excited, which can be owing to silver, the existence of silver makes the local electromagnetic field between WS2 and Ag is enhanced, so that more peaks are excited, and the peak intensity increases. The WS2 Raman spectrum also demonstrates that we prepared WS2 as a 2H phase, which is consistent with XRD data. Figure 2(h) is the Raman spectrum of GO/Ag NPs/WS2 composite substrate, we can see the peaks of WS2 and GO (D (1352 cm-1), G (1587 cm-1) and D + G (2940 cm-1)). To prove that GO exists uniformly, the Raman maps of 81 groups of GO measured in the range of 4 × 4 µm2 were counted, and the RSDs of the three peaks were calculated to be 1.05%, 0.44%, and 1.34%, respectively in Fig. 2(i), proving that our GO is uniformly distributed in Ag/WS2. The above results demonstrate that we fabricated GO/Ag NPs/WS2 composite substrates have uniform distribution and high crystallinity.
Fig. 2. (a-c) SEM image of WS2, Ag NPs/WS2, and GO/Ag NPs/WS2 composite substrates. (d) The size distribution of Ag NPs. (e) Energy-dispersive spectra of GO/Ag NPs/WS2. (f) XRD pattern of GO/Ag NPs/WS2. (g) The Raman spectrum of WS2 and Ag NPs/WS2. (h) Raman spectrum of GO/Ag NPs/WS2. (i) RSD distribution of multiple groups of GO Raman peaks.
In addition, the chemical states of elements in WS2, Ag/WS2 and GO/Ag/WS2 were studied and compared, and their XPS results are shown in Fig. 3. C1s (standard value of 284.8 eV) was used to correct the binding energy of the XPS. Figure 3(a, b) show the XPS energy spectrum of W and S in WS2. Three W peaks at 32.7 eV, 34.9 eV, and 38.3 are observed in Fig. 3(a), corresponding to W4f7/2, W4f5/2, and W5p3/2 at the +4 valence [25,26], respectively. The results of XPS proved that the prepared WS2 was 2H phase [27], this result is consistent with XRD. Notably, a slight oxidation peak appears at the high binding energy at 36.4 eV, corresponding to the W4f7/2 of WO3, we consider that CVD-grown 2D-TMDS usually have S-vacancies [28,29], and the presence of W+6 may be due to the oxygen (O2) occupying the surface sulfur vacancies. Another possibility is that O2 molecules adsorbed on WS2. No peak of WO3 was found in XRD, indicating that the W+6 peak is caused by adsorbed oxygen, again proving that our prepared WS2 is highly crystalline. As depicted in Fig. 3(b), two Sp2p peaks were found at 162.2 eV and 163.1 eV, corresponding to the S2p3/2 and Sp1/2 peaks of WS2. Figure 3(c) shows the survey spectrum of Ag/WS2 with O, Ag, C, S and W elements. Figure 3(d, e) shows the detailed XPS spectra XPS of W and S in Ag/WS2: W4f7/2 (33.5 eV), W4f5/2 (352 eV), W5p3/2 (38.3 eV), S2p3/2 (162.2 eV) and S2p1/2 (163.1 eV). The W4f peak of Ag/WS2 moves slightly to higher binding energy compared with WS2, W4f7/2 from 32.7 eV to 33.5 eV and W4f5/2 from 34.9 eV to 35.2 eV, which might be caused by CT between WS2 and Ag NPs. Figure 3(f) shows the peaks at 367.8 eV and 373.8 eV, corresponding to Ag3d5/2 and Ag3d3/2. Figure 3 (i) shows the survey XPS spectrum of GO/Ag NPs/WS2 substrates, for GO, the binding energies of the C1s peaks are 284.8, 286.8 and 288.5 eV, representing C-C, C-O and C = O [30], respectively (Fig. 3(g)), the binding energies of the O1s peaks are 531.50, 532.34, 533.10 and 534.07 eV, representing O-C = O, C = O, C-OH and C-O-C [31], respectively (Fig. 3(h)), the results of XPS also showed that we have successfully prepared GO, Ag and highly crystalline WS2, it lays the foundation for the later research on SERS performance.
Fig. 3. (a) XPS spectroscopy of W4f and W5p in WS2. (b) XPS spectroscopy of S2p in WS2. (c) XPS spectra of W, S, and Ag in Ag NPs/WS2 composite substrate. (d-f) W, S, and Ag XPS spectra in Ag NPs/WS2 composite substrate. (g, h) C1s and O1s XPS spectra in GO/Ag NPs/WS2 composite substrate. (i) XPS spectra of GO/Ag NPs/WS2 composite substrate.
3.2 SERS functioning of GO/Ag NPs/WS2
Preparation of highly sensitive and reproducible substrates is critical for SERS detection [32]. Universal applicability, high sensitivity, uniformity, and stability are important parameters to characterize the sensitivity and accuracy of SERS detection. We selected R6G, CV, and MB dye molecules to test these properties of GO/Ag NPs/WS2 composite substrates. To prepare aqueous R6G solution at a concentration of 10−6 M to 10−12 M, and drop 2 µL liquid droplets on a composite substrate for detection, the obtained spectrums are shown in Fig. 4 (a), the limit of detection (LOD) for R6G on the composite substrate is 10−12 M, the key peaks of R6G (611, 774, 1310, 1362, 1510, 1571, and 1649 cm-1) [33] were observed. To analyze the association between the Raman strength of the substrate and the concentration of the probe molecule, a quantitative analysis was performed. Taking the concentration as the independent variable, the mean and standard deviation of the peak intensity at 15 different locations are used as the dependent variables and errors, and the linear fitting is performed. In Fig. 4(d), the results show that the linear fitting coefficients (R2) at 611 cm-1 and 1362 cm-1 are 0.966 and 0.962, respectively, and the linear response is good. The intensity changes are expressed by linear formulas as 611 cm-1, Log I = 0.308Log C + 5.804; 1362 cm-1, Log I = 0.299Log C + 5.702. We conclude that the Raman spectra of the substrate quantify the R6G dye molecule to some extent, showing a good uniformity and high sensitivity.
Fig. 4. (a) Raman spectra of dissimilar concentrations of R6G (10−7 M - 10−12 M) on GO/Ag NPs/WS2. (b) Raman spectroscopy of CV (10−6 M - 10−10 M) on the GO/Ag NPs/WS2 composite substrates. (c) The Raman spectra of MB (10−5 M-10−10 M) on GO/Ag NPs/WS2. (d) The Log I-Log C diagram of the mean intensity of SERS signals at 611 and 1362 cm−1 versus the concentration of R6G. (e) Log I-Log C diagram of mean intensity versus the concentration of CV at 915 and 1622 cm−1. (f) The Log I-Log C diagram of MB at 449 and 1624 cm−1.
We also used CV and MB for the analysis of composite substrates. Equipped with CV solutions of different concentrations (10−6 M to 10−10 M) for testing, the Raman spectra are shown in Fig. 4(b), where the main Raman peaks of CV clearly seen: 421, 574, 807, 915, 1182, 1372, 1587 and 1622 cm-1 [34]. In Fig. 4(e), fifteen spectra at 915 cm-1 and 1622 cm-1 were selected for quantitative analysis, the intensity and concentration changes are expressed by linear formulas as: Log I = 0.504Log C +6.691; Log I = 0.525Log C + 6.712, the coefficients for a good linear response are: 0.991 (at 915 cm-1), 0.992 (at 1622 cm-1). MB is the other banned bactericide in aquaculture, which has highly toxic, oncogenic, and other species secondary effects, so it is necessary to detect aquatic products quickly and accurately [35]. Similarly, an aqueous MB solution at a concentration of (10−5 M - 10−10 M) is configured, and the solution of 2 $\mathrm{\mu }$L is dripped on the substrate for detection. The respective SERS spectra are presented in Fig. 4(c), the main Raman signature peaks for MB are 449, 502, 678, 774, 806, 1042, 1072, 1153, 1307, 1387, 1398, and 1624 cm-1 [30]. Figure 4(f) shows the average intensity versus concentration at 449 cm-1 and 1624 cm-1. The mean intensity versus concentration can also be expressed by the equation: 1624 cm-1, Log I = 0.414Log C + 5.542;449 cm-1, Log I = 0.448Log C + 5.182. The linear response coefficients R2 at 449 cm-1 and 1624 cm-1 were 0.983 and 0.980, respectively, having a good linear response. The above results show that the GO/Ag NPs/WS2 composite substrate is applicable to the detection of probe molecules, and has good uniformity and reproducibility.
To detect the high sensitivity of the substrate, we took the detection limit of the above probe molecules, respectively, and calculated their EFs for R6G (10−12 M), CV (10−10 M) and MB (10−10 M) using the following equation [36].
${I_{SERS}}$ represents the intensity of SERS signals, ${I_{Si{O_2}}}$ represents Raman signals intensity of SiO2, ${N_{Si{O_2}}}$ and ${N_{SERS}}$ represent the number of molecules in the probe molecules within the laser spot. As displayed in Fig. 5 (a-c), taking the Raman peak at 10−2 M on SiO2 as a control, the EFs of R6G, CV and MB can be calculated to be 6.60× 1010, 4.75 × 109, and 5.75 × 109, respectively. Hence, the above findings indicate that the proposed GO/Ag NPs/WS2 SERS composite substrate is highly sensitive for practical applications and shows that the composite substrate has huge potential for food safety and chemical detection.In parallel to high sensitivity, the strength of the SERS is also closely related to the homogeneity of the substrate, and the parameters characterizing the uniformity are calculated by RSD. 30 sets of R6G (10−12 M) SERS spectra were randomly selected from the GO/Ag NPs/WS2 substrate as seen in Fig. 5(d). The RSD value was computed using the Eigen peak of R6G at 611 cm−1, as follows [37]:
Fig. 5. (a) The spectra of R6G (10−12 M) on GO/Ag NPs/WS2 and R6G (10−2 M) on SiO2. (b) The spectra of CV (10−10 M) on GO/Ag NPs/WS2 and CV (10−2 M) on SiO2. (c) The spectra of MB (10−10 M) on GO/Ag NPs/WS2, MB (10−9 M) on Ag NPs/WS2 and MB (10−2 M) on SiO2. (d) 30 sets of R6G (10−12 M) spectra collected from GO/Ag NPs/WS2 randomly. (e) Raman mapping at 1622 cm−1 of CV (10−10 M). (f) Raman spectrum of MB (10−7 M) on GO/Ag NPs/WS2 at different times. (g) The law of Raman spectral intensity over time on GO/Ag NPs/WS2 and Ag NPs/WS2 substrates. (h) Raman spectrum of PBASE on GO/Ag NPs/WS2. (i) Raman spectrum of DNA on GO/Ag NPs/WS2 composite substrates.
Figure 6 (a) shows the absorption spectrum of GO/Ag NPs/WS2 (green) at 200 - 800 nm, exhibiting broadband absorption at 200 - 300 nm, 300 - 550 nm and 550 - 800 nm, respectively. The UV-Vis spectra of the GO/Ag NPs/WS2 sample show a peak at 250 nm, belonging to GO, originating from the π-π* jump of sp2 hybridized carbon [38]. The absorption spectrum of WS2 (black) shows a small hump at 230 nm, 427 nm and 640 nm, representing the absorption peak of WS2 [39–41], demonstrating the strong UV absorption of WS2. The maximum absorbance of Ag NPs (red) in the visible light section is at 442 nm [19], this SPR peak band is formed due to the resonance of metal free electrons with incident light waves [42], as a result, samples can produce a high signal-to-noise SERS signal at an incident laser at 532 nm. By observation, we can see that the absorption spectrum of Ag NPs/WS2 (blue) is slightly enhanced compared to Ag NPs, probably due the strong LSPR effect of dense Ag NPs particles, which increases the absorption of WS2, resulting in an enhanced absorption peak of the Ag/WS2 composite structure. In the GO/Ag NPs/WS2 composite substrate, the peak strength of the Ag decreases and blue shift occurs, and we believe that the presence of GO leads to a CT between the silver nanoparticles and the GO surface, resulting in a decrease in the peak strength of silver absorption and a blue shift [38].
Fig. 6. (a) The absorption spectra of GO/Ag NPs/WS2, Ag NPs/WS2, Ag NPs, and WS2. (b) The CT process of MB/Ag NPs/WS2.
To explain the CM between MB, WS2 and Ag NPs, the CT process of the SERS structure under the irradiation of visible light at 532 nm, as shown in Fig. 6 (b). The HOMO and LUMO of MB are -3.73 eV and -5.6 eV [43]. The VB and CB of WS2 nanostructures are -5.9 eV and -4.1 eV [44]. Fermi energy (Ef) level of silver is -4.84 eV (vs. vacuum) [45]. There is a CT effect between MB and WS2. Since the resonance energy of MB molecules (1.87 eV) is smaller than the incident laser energy (2.33 eV), photo-CT occurs when the MB are absorbed on WS2 surface, since the band gap between the HOMO level of MB and the CB of WS2 nanoparticles is 1.5 eV, which is a lot smaller than 2.33 eV, electrons can be transferred since the HOMO to the CB. In addition, there are CTs between MB, WS2, and Ag NPs. Electrons can jump from the HOMO energy level of MB to the EF of silver because the energy level difference between them is 0.76 eV. Ag NPS can also be excited to produce “hot electrons”, these hot electrons are transferred from Ag to the LUMO of the MB, since the energy level difference (1.11 eV) between Ef and LUMO is less than the light energy, thereby boosting the Raman signal of the MB molecule. Furthermore, the energy change between the EF and the CB is 0.74 eV, which is less than 2.33 eV, allowing CT to occur [46–48]. In summary, the addition of Ag further increases the number of CT channels. Owing to the possible presence of sulfur vacancies on WS2, the number of silver nanoparticles deposited on the surface of WS2 increases, increasing the chance of charge transfer and leads to further enhancement of the Raman scattering of MB with the composite substrate. Thus, the excellent SERS performance of Ag NPs/WS2 is due to the synergy effects metallic silver particles and semiconducting WS2, the effective CT between them play a considerable role in enhancing SERS signal of MB molecules.
In addition to analysing the CM caused by CT, we have also analysed the electric field distribution of the GO/Ag NPs/WS2 composite substrates using COMSOL software, investigating the EM of GO/Ag NPs/WS2. Based on the results of the experiment, the incident light wavelength was set to 532 nm, Ag diameter was set to 106 nm, the thickness of WS2 solid film was 3 nm, the thickness of GO was 1 nm. Figure 7 (a-d) shows the electric field distributions in the x-z views for WS2, Ag NPs, Ag NPs/WS2, and GO/Ag NPs/WS2, respectively. By comparing Fig. 7(a, b), we can see that the local electric field between adjacent Ag NPs is very strong, up to 1.93 × 105 V/M, which is 8.5 times stronger than that of WS2 alone (2.28 × 104 V/M), this is because the small space between adjacent Ag NPs generate a strong SPR effect, which causes the local electric field between Ag NPs to be enhanced. In Fig. 7(c), the strong electric field on WS2/Ag NPs, comes from the coupling interaction between adjacent Ag NPs and between WS2 and Ag NPs. However, the electric field strength between adjacent silver nanoparticles on the Ag NPs/WS2 and GO/Ag NPs/WS2 composite substrates are slightly less than that between pure silver nanoparticles. It may be due to the addition of WS2, so that the SPR between adjacent silver nanoparticles is weakened, but the overall electric field intensity of GO/Ag NPs/WS2 increases to 2.35×105 V/M, which is 10.3 times that of WS2 alone. By analyze the electric field distribution between GO, Ag NPs and WS2 using COMSOL, we can see that the hot spot regions are concentrated in the nano-spacer layer formed by WS2 and Ag NPs, Ag NPs and GO. In these nano-spacers, there are dense hot spots, which facilitate the collection of SERS signals and improve the sensitivity of the substrate.
Fig. 7. (a)-(d) WS2, Ag NPs, Ag NPs/WS2, and GO/Ag NPs/WS2 simulate the distribution of the electric fields.
4. Conclusions
In summary, we have fabricated GO/Ag NPs/WS2 composite substrates with high sensitivity and uniformity by combining metallic nanostructure Ag with semiconductor WS2. The GO/Ag NPs/WS2 composite substrate has a minimum detection concentration of 10−12 M for R6G in ordinary air environment, with good sensitivity, uniformity (RSD ∼ 5.4%) and good anti-oxidative stability. The experimental results show that the composite also has good SERS performance in detecting other probe molecules such as CV, MB and biomolecular DNA. The GO/Ag NPs/WS2 composite substrate serves as an excellent SERS sensor in ultrasensitive detection and biosensing fields show great potential and practical applications.
Funding
Natural Science Foundation of Shandong Province (ZR2020MA072).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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