Open Access
Add to BibSonomy
Abstract
The simultaneous output of highly sensitive and reproducible signals for surface-enhanced Raman spectroscopy (SERS) technology remains difficult. Here, we propose a two-dimensional (2D) composite structure using the repeated annealing method with MoS2 film as the molecular adsorbent. This method provides enlarged Au nanoparticle (NP) density with much smaller gap spacing, and thus dramatically increases the density and intensity of hot spots. The MoS2 films distribute among the hot spots, which is beneficial for uniform molecular adsorption, and further increases the sensitivity of the SERS substrate. Three kinds of molecules were used to evaluate the SERS substrate. Ultra-sensitive, highly repetitive, and stable SERS signals were obtained, which would promote the application process of SERS technology in quantitative analysis and detection.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The ultra-sensitive and accurate detection of toxic chemicals, food additives, environmental pollutants, chemical reagents, and biological indicators has always been a hot area in scientific and technological development. However, developing sensors with high sensitivity and stability remains challenging, especially for applications that require real-time detection (such as environmental monitoring, explosive detection, and medical diagnostics) [1,2]. Surface-enhanced Raman spectroscopy (SERS) not only possesses the capability for non-destructive finger-print detection, but could also operate in real time and allow in-situ detection, which has attracted widespread attention of scientific researchers [3,4]. Currently, the generally recognized SERS mechanisms mainly include electromagnetic enhancement (EM) and chemical enhancement (CM), and the former has been proven to play a leading role [5]. The origin of EM is that the plasmon resonance on the surface of metal nanostructures would generate strong localized electric fields (hot spots) on the nanometer scale. As already reported, the closer molecules are to the hot spot, the better the enhancement of the Raman signal [6]. Furthermore, the intensity of hot spots generally increase as the gap distance decreases [7]. Thus, the molecular adsorption in the hot spot’s area, and the gap control among metal nanostructures, play an extremely important role in the acquisition of a highly sensitive and uniformSERS signal.
Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (h-BN), and MoS2, possess smooth and uniform surfaces without dangling bonds. This is beneficial for uniform molecular adsorption [8–11]. Furthermore, 2D materials perform excellent SERS effects for the CM mechanism, and could effectively suppress background fluorescence of analytes, benefiting the collection of clear and authentic Raman signal [3,12,13]. Therefore, 2D material films with nanometer thicknesses are widely used as an important elementin the building of SERS substrates. The Bongsoo Kima group report a novel nanowire on graphene-coated Au film structure. This structure exhibits high reproducibility and sensitivity for the uniform nanogap formation caused by graphene, improving the molecular distribution [14]. The Bingqing Wei group directly deposited Cu nanoparticles (NP) on the surface of a graphene-coated Cu film to obtain a composite SERS substrate. The enhancement factor (EF) for copper phthalocyanine (CuPc) molecules reached 1.89×107, which is one of the highest values reported to date in the Cu plasmonic systems [15]. The Wallace C. H. Choy group and the Jie Sun group used the finite-difference time-domain (FDTD) method to explore the electromagnetic field distribution of Ag NP/graphene/Ag films and Au NP/graphene/Au NP composite structures, and found that the strongest EF exist in the graphene spacer area [16,17]. B. Y. Man et al. reported on the Au NP/MoS2/Au NP composite structure [18]. The EF of the composite substrate for rhodamine 6G (R6G) molecules reached 1.98×108, and the RSD of the SERS signal decreased from 25.9% to 7.83% compared with the pure Au NP substrate, proving that the MoS2 film is conducive to the improvement of molecular adsorption uniformity.
These studies confirm that the 2D material films used as a nanospacer exhibit a significant improvement insensitivity and repeatability of SERS signals for the bilayer composite SERS substrates [19,20]. However, there are some obvious disadvantages for these substrates in the practical Raman test: Firstly, the hot spot area lies between the vertical metal NP, and the upper metal NP would hinder the molecules from reaching the hot spot area, which affects the sensitivity of the SERS signal. Secondly, these vertical nanostructures cannot realize the maximum hot spot density. Thirdly, these SERS substrates have not fully exploited the molecular adsorption advantages of 2D materials.
In this paper, we intend to combine the advantages of metal nanostructures and 2D nanomaterials to improve the reproducibility and sensitivity of SERS technology. To achieve this, the development of 2D Au NP/MoS2/Au NP (AuMAu) composite structures with a high density of hotspots is proposed by the simple repeated annealing method, as shown in Fig. 1. Four kinds of AuMAu composite structures were obtained from the repeated annealing process, with different Au film thicknesses. There are several advantages for the 2D AuMAu: Firstly, the repeated annealing provides enlarged AuNP density and much smaller gap spacing, which would dramatically increase the density and intensity of hot spots. Secondly, the MoS2 films wrap around the Au NP and cover the SiO2 substrate, distributing among the hot spots, which is beneficial for molecular adsorption. Thirdly, the 2D mode of the SERS substrate is more favorable for the molecules getting into the hot spots’ area. Three kinds of molecules were selected to evaluate the SERS substrate. The large Raman signal enhancement, high signal reproducibility, and ultra-low detection limit were realized, which would promote the application process of SERS technology in accurate quantitative detection.
2. Experimental
2.1 Preparation of AuMAu SERS substrates
First, 8 nm Au film was deposited on the SiO2 substrate by magnetron sputtering. Before the annealing process, the zone quartz tube was pumped down to 10−3 Pa. Then the Au film/SiO2 substrate was annealed at 700 ℃ for 30 min (Ar: 80 sccm, 10 Pa). The Au film changed into Au NP after annealing treatment. (NH4)2MoS4 (purity of 99.99%; 0.01 g) was added to a dimethylformamide (DMF) solvent to form a 1 wt% solution, and sonicated for 30 min to ensure complete dissolution. The (NH4)2MoS4 solution was spin-coated onto the Au NP/SiO2 at 3000 rpm rotating speed for 1 min, forming a uniform film. The sample was placed in the furnace again, and annealed at 800℃for 90 min in the atmosphere (Ar:H2 = 80:20 sccm, 50 Pa). The quartz tube was cooled naturally down to room temperature, and the MoS2/Au NP hybrid structures were obtained. The MoS2/Au NP substrates were treated by Ar plasma. Ar plasma (13.56 MHz RF source)with power of 10 W and pressure of 10 Pa is used to modify and thin the MoS2 films. On this basis, 3 nm, 6 nm, 9 nm, and 12 nm Au films were deposited on top of separate MoS2/Au NP structures. Four kinds of AuMAu nanostructures were obtained after the annealing process at 500 ℃ for 30 min (Ar: 80 sccm, 10 Pa).
2.2 Apparatus and characterization
The morphologies of the samples were characterized using scanning electron microscopy (SEM, Zeiss Gemini Ultra-55). Images were collected using Transmission Electron Microscope (TEM) and High Resolution TEM (HRTEM, JEM-2100). The elemental composition of the nanostructure was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi). The extinction spectra were obtained by an ultraviolet-visible (UV-vis) photometer (Alpha-1500).
2.3 Assessment of SERS effect
The SERS performance of the AuMAu was investigated using a high-resolution Raman spectrometer (Horiba HR Evolution) with 532 nm laser wavelength, 1 μm spot, 600 gr/mm diffraction grid, 50×objective, and 4 s integration time. During the experiment, R6G (10−6-10−12 M), crystal violet (CV, 10−6-10−10 M), and malachite green (MG, 10−6-10−11 M) solution with different concentrations were prepared as probe molecules. A volume of 2 μl analyte solution was extracted accurately with a pipette gun and dropped on the AuMAu for the SERS test.
3. Results and discussion
The fabrication process of AuMAu is illustrated schematically in Fig. 1. Firstly, Au NP was obtained by a magnetron sputtering and annealing process, and then the MoS2 film was directly grown on the Au NP. After the successful synthesis of MoS2/Au NP, the samples were placed for Ar plasma treatment with the aim to decrease the number of MoS2 layers and modify the structure of the films, which is beneficial for the SERS effect [21]. Then, additional Au films with thicknesses of 3 nm, 6 nm, 9 nm, and 12 nm were deposited on the MoS2/Au NP. After repeated annealing processing, four kinds of AuMAu substrates were obtained, named 3-AuMAu, 6-AuMAu, 9-AuMAu, and 12-AuMAu to correspond with each of these Au film thicknesses, respectively. The morphology of AuMAu was characterized by SEM and TEM. The SEM images of MoS2/Au NP and AuMAu are shown in Figs. 2(a)–2(e), which exhibit completely different morphology. Au films follow a Volmer–Weber growth, and possess a larger binding energy to cohesive energy ratio (Eb/Ec), which results an organized position on MoS2 film [18]. For the 3-AuMAu, the small, ordered spherical Au NP appears to be distributed uniformly aroundtheMoS2/Au NP on the MoS2film surface, as shown in Fig. 2(b). In the high-magnification SEM image (inset image), we can observe that the diameter of the dense Au NP is much smaller than that of the MoS2/Au NP. As the thickness increases, the morphology changes of AuMAu can be clearly observed. When the thickness of the additional Au film increases to 6 nm, the diameter of the Au NP becomes larger, and the density of Au NP decreases after the annealing process, as shown in Fig. 2(c). For 9-AuMAu, the size of MoS2/Au NP increases significantly, and Au NP islands start to appear, as shown in Fig. 2(d). When the thickness of the second Au film increases to 12 nm, the MoS2/Au NP hybrids nearly all changed to the nano-island structure. Fig. 2(f) exhibits the Au NP density line diagram, which illustrates that the nanoparticle density in AuMAu is much larger than that in MoS2/Au NP. In addition, the density of Au NP becomes smaller and smaller as the thickness of the additional Au films increase. Fig. 2(g) shows a statistical analysis of the gap size of MoS2/Au NP and AuMAu. The gap size is distributed in a large range, and the average gap for these five samples is about 44 nm, 6 nm, 14 nm, 20 nm, and 28 nm, respectively. The standard deviations are 11, 2, 4, 8.5, 9, respectively, and the median is 39 nm, 5.5 nm, 17 nm, 21 nm, 33 nm, respectively. Compared with theMoS2/Au NP, the gap size decreases clearly, which is conducive to the generation of much stronger electromagnetic field enhancement.
Fig. 2. SEM images of (a) MoS2, (b) 3-AuMAu, (c) 6-AuMAu, (d) 9-AuMAu, and (e) 12-AuMAu. The inset in each is the corresponding high magnification SEM image. Scale bar: 200 nm. (f) The density line diagrams of AuMAu samples. (g) The gap density frequency histograms of AuMAu samples.
The composition and chemical state of the 6-AuMAu structure were analyzed by XPS. The binding energy was calibrated with C1s=284.8 eV of carbon in the XPS spectra. Two peaks were present, at 232.58 eV and 229.4 eV in Fig. 3(a) of the Mo3d, which correspond to Mo 3d3/2 and Mo 3d5/2, respectively. Meanwhile, the separated S 2p peaks at 163.5 eV (2p1/2) and 162.3 eV (2p3/2) in the AuMAu were also found, as shown in Fig. 3(b). It is also noteworthy that any characteristic bands of metallic Mo or Mo Ox in higher/lower binding energy regions are not found, which demonstrates the high crystallinity of MoS2 film. Moreover, the Au 4f peaks at 84.2 eV and 87.9 eV are shown in Fig. 3(c), corresponding to Au 4f7/2 and Au 4f5/2. While the limited detection depth of X-ray photoelectrons is typically less than 10 nm, the intensity of Mo and S peaks is not weak compared with that of Au. This indicates that MoS2 films are not completely covered by Au, and it should therefore be exposed in the nanogap area among the Au NP, which is beneficial to the molecular adsorption.
Fig. 3. (a) Mo 3d and S 2s in XPS spectra of the 6-AuMAu. (b) S 2p XPS spectra. (c) Au 4f XPS spectra. (d) The Raman spectra of AuMAu. (e)–(g) The TEM and HRTEM images of MoS2.
In this work, we used MoS2 film as the molecular adsorbent among 2D metal nanostructures to improve the reproducibility and sensitivity of SERS signals and inhibit background fluorescence. The uniformity and thickness of the MoS2 films can be identified by the intensity and frequency difference of the characteristic peaks E12g and A1gin the Raman spectra [22]. The E12g mode indicates the in-plane vibration of Mo and S atoms, and the Mo atom moves in the opposite direction to the two S atoms. The A1g mode represents the out-of-plane vibration of S atoms, with two S atoms moving along the opposite direction to both sides of the Mo atoms [23]. One hundred groups of Raman data are collected randomly from AuMAu, as shown in Fig. 3(d).
The small changes of intensity and frequency differences indicate the excellent uniformity of the MoS2 films. The frequency difference between the two vibrational peaks is ∼20. This value is smaller than for the bi-layer MoS2 and slightly larger than that of monolayer MoS2, which means that the MoS2 film should be a single layer [23]. The enlarged frequency difference can be ascribed to the lattice strain effect caused by the curve of the MoS2 shell. To further investigate the 2DAuMAuheterostructures, transmission electron microscopy (TEM) was used. The AuMAu was transferred from the substrate anddissolved in alcohol after 15 minutes ultrasonic treatment. Then the AuMAu solution was drop on a TEM grid for further test. We found that the curving MoS2 films wrap around the Au NP, and typically possess a monolayer structure, which is identified with the Raman analysis. There are also some MoS2 nanosheets that should come from the MoS2 film grown on the surface of the SiO2 substrate, as shown in Fig. 3(e). The HRTEM image in Fig. 3(g) indicates that the continuous and curving single-layer MoS2 film has a thickness of∼0.65 ± 0.2 nm. The lattice strain did not induce any noticeable change on the spacing value, compared with previous reports [24–26].
The extinction spectrum of the prepared 2D nanostructures were investigated by UV-vis spectroscopy, as shown in Fig. 4(a). We note that the extinction bands show a redshift trend with increasing thickness of the additional Au films. The plasmonic coupling for AuMAu is significantly enlarged after the repeated annealing process, which therefore caused much stronger optical absorption compared with the MoS2/Au NP. Besides, the extinction bands are relatively widened as the thickness increases, which is convenient for matching the incident laser to excite the plasmonic resonance. The SERS performance of theMoS2/Au NP, AuAu (obtained via twice annealing process without MoS2) and AuMAu is evaluated using the 10−6 M R6G as the probe molecule, as shown in Fig. 4(b). A volume of 2 μl of R6G solution was dropped onto the substrate and tested after natural drying. We note that the AuMAu exhibits a much better SERS effect than the MoS2/Au NP. The enlarged Au NP density, accompanied by a smaller gap space, apparently increases the density and intensity of plasmonic coupling, and thus enhances the SERS signal. In addition, the 6-AuMAusubstrate shows the best SERS performance. For the 9-AuMAu and 12-AuMAu, the appearance of the Au NP nano-island structures greatly decreases the density of the hot spots, and the extended gap size reduces the electromagnetic field intensity. While the 3-AuMAu possesses the maximal density of hot spots and minimum gap size, the SERS signal is much weaker than that of 6-AuMAu. In general, the enhancement effect of SERS is a synergistic effect of various factors, such as the density and intensity of the hot spots, and the adsorption of analytes. The weak plasmonic coupling among adjacent Au NP and the excessive particle density should be responsible for the weak SERS signal of 3-AuMAu. More interestingly, we found that 6-AuAu exhibit nearly same SERS effect compared with MoS2/Au NP, which indicates the important role of MoS2 film.
Fig. 4. (a) Extinction spectra of the 2D structures. (b) The SERS spectra of R6G (10−6 M) collected from different AuMAu samples. (c)–(f) Cross-sectional view of the electric field distribution for the AuMAu samples. (g) The enhancement factor (|E|2/|E0|2) of these composite structures.
In order to better understand the origin of electromagnetic enhancement for the AuMAu, simulations according to the FDTD method were performed. The AuMAu was modeled as one AuNP covered by 0.7 nm MoS2 shells and surrounded with small AuNP. The optical constants of monolayer MoS2 film (ξ=5.9000+i1.4000 at 532 nm) was obtained from the experimental measurement data by Castellanos-Gomez et al. [27]. The diameter of big particles was set as 55, 55, 66, 70, 70 nm, and the small particles was set as 20, 40, 40, 40 nm, respectively, based on the SEM and TEM images. The polarization of the incident light followed the alignment of the NP. As seen in the results, the plasmonic coupling for 3-AuMAu becomes much stronger than the single MoS2/Au NP, as shown in Fig. 4. The coupling strength was further enhanced by increasing the size of the additional Au NP, as shown in Fig. 4(e). The decline in the coupling strength for the 9-AuMAu and the 12-AuMAu may be ascribed to the enlarged gap size and the formation of nano-islands. The enhancement factors (|E|2/|E0|2) of AuMAu are obtained as shown in Fig. 4(h). We note that the 6-AuMAu exhibits the best enhancement factors, which is in accordance with the experimental results, and the maximum electric field intensity enhancement reached up to≈106. Moreover, the MoS2film could act as an excellent adsorbent for analytes, and makes it easier to adsorb on the hot spot area of AuMAu substrates, which would also benefit the collection of the highly sensitive SERS signal.
Furthermore, the suitability of AuMAu substrates for ultra-low concentration analytes detecting was tested. R6G aqueous solutions with concentrations from10−6 M to 10−12 M were prepared, and 2 μl droplets of aqueous solution were dropped onto the 2D nanostructures for SERS measurement. The SERS spectra of R6G in the concentration range is exhibited in Fig. 5(a). The main characteristic peaks of R6G (611, 774, 1308, 1360, 1506, 1569, and 1645 cm−1) are observed in Raman spectra [26]. Fig. 5(d) shows the average intensity of the R6G band at 611 cm−1 and 1360 cm−1, relative to the concentration from10−6M to10−12M. Error bars indicate standard deviations from at least 20 spectra. The logarithmic scale linear response is obtained between the intensity of the SERS signal and the analytes concentrations. Based on this, quantitative detection would be realized within this concentration range. We use empirical equations to represent the changing curve: Log I = 0.324 Log C + 6.408for611cm−1 and Log I = 0.33 Log C + 6.371 for 1360 cm−1, where I represents the SERS signal intensity of analytes and C represents the concentrations. Moreover, the determination coefficients (R2) are 0.971 for 611 cm−1 and 0.98 for 1360 cm−1, indicating a good linear response.
Fig. 5. (a) SERS spectra of R6G at varying concentrations ranging from 10−6 M to 10−12 M. (b) SERS spectra of CV at varying concentrations ranging from 10−6 M to 10−10 M. (c) SERS spectra of MG at varying concentrations ranging from 10−6 M to 10−11 M. (d) Log−log plot of average intensity of SERS signals at 611 cm−1 and 1360 cm−1 versus the concentration of R6G. (e) Log−log plot of average intensity of SERS signals at 915 cm−1 and 1587 cm−1 versus the concentration of CV. (f) Log−log plot of average intensity of SERS signals at 1182 cm−1 and 1622 cm−1 versus the concentration of R6G.
In addition, the detection of CV and MG was also carried out. CV, more formally known as triphenylmethane disinfectant, is extensively used in aquaculture. However, it has become a banned drug in aquaculture for its dose-related carcinogenicity [28]. Droplets of CV aqueous solution with a volume of 2 μl, and concentrations from 10−6 M to 10−10 M, were dropped onto the AuMAu for SERS measurement. The CVSERS spectra with different concentrations are exhibited in Fig. 5(b). Fig. 5(e) shows the variation of the mean intensity of the CV band at 915 cm−1 and 1587 cm−1 with respect to the concentration range of 10−6 M to 10−10 M, where the quantitative detection could be realized within this range. The intensity change is represented by empirical equations: Log I = 0.47Log C + 7 for 915 cm−1 and Log I = 0.42Log C + 6.68 for 1587 cm−1. The excellent linear response is proved by the determination coefficient (0.987 for 915 cm−1 and 0.984 for 1587 cm−1). MG is another banned fungicide in aquaculture for its high toxicity, carcinogenicity, teratogenicity, amongst other side effects, and therefore needs to be detected accurately and quickly in aquatic products [29]. Similarly, MG aqueous solutions with concentrations from 10−6M to 10−11M were configured, and 2 μl droplets of the solutions were dropped on the AuMAu for further assessment. The SERS spectra of MG with different concentrations are shown in Fig. 5(c). The mean strength of MG characteristic peaks at 1182 cm−1 and 1622 cm−1 with respect to the concentrations is exhibited in Fig. 5(f), where quantitative detection can be realized in this concentration range. The variation of intensity can be also expressed by means of empirical equations: Log I = 0.37Log C + 5.96 for 1182 cm−1 and Log I = 0.32Log C + 5.84 for 1622 cm−1. The R2 values are 0.985 for 1182 cm−1 and 0.983 for 1622 cm−1,which demonstrates a good linear response for MG.
The SERS performance of the prepared composite nanostructures could be evaluated intuitively by calculating the enhancement factor (EF). We calculated the EF of R6G, CV, and MG using the following equation [30]:
where ${I_{SERS}}$ represents the intensity of the SERS signal, ${I_{Si{O_2}}}$ represents the Raman signal intensity collected from the SiO2, ${N_{Si{O_2}}}$ and ${N_{SERS}}$ represent the number of analyte molecules within the laser spot [31]. The R6G, CV, and MG with the concentrations 10−12 M, 10−10 M and 10−11 M, respectively, were selected as the limit concentration for the EF calculation, as presented in Fig. 6(a-c). The analytes with 10−2 M on SiO2 were used as comparison, and the detailed calculation processes were referred to the previous works [32,33]. The experimental EF was calculated to be 1.79×109 for R6G, 1.055×108 for CV and 4.8×108 for MG. The experimental EF of AuMAu have been one of the highest values comparable to the SERS substrates reported in previous studies [34–43].In addition to high sensitivity, the hot spot density of SERS substrates is also closely related with the uniformity of SERS signals [37,44]. The (10−10 M) MG solutions were used as probe molecules. 30 groups of SERS spectra were selected randomly from the AuMAu substrate, as shown in Fig. 6(d). The characteristic peak of MG at 1622 cm−1 was used to calculate the relative standard deviation (RSD) that could reflect the repeatability of the SERS signals. It was calculated using the followed equation [45]:
Fig. 6. (a)–(c) The SERS spectra of R6G (10−12M), CV (10−10 M), and MG (10−11 M) collected from the 6-AuMAu substrate and the Raman spectra of R6G (10−2M), CV (10−2 M), and MG (10−2 M)collected from the SiO2 substrate as comparison.(d) 30 groups of MG (10−10 M) SERS spectra collected from AuMAu randomly. (e) SERS spectra with different aging times.
The stability of the SERS signals is another important parameter affecting SERS performance and practical applications. The AuMAu composite structures are composed of Au and MoS2, which are all stable substances, and hence the composite structures should have good stability. To prove it, the variations of SERS signals were observed under the normal environment at different times (0 days, 2 weeks, four weeks, 6 weeks, and 8 weeks), as shown in Fig. 6(e). As can be seen, the SERS signal is basically unchanged, indicating that the AuMAu2D nanostructures possess long-term stability.
4. Conclusions
In summary, we combined metal nanostructures and 2D nanomaterials to propose a 2DAuMAu composite structure with high hot spot density by repeated annealing. Four kinds of SERS substrates, with different thicknesses of Au films used in the annealing process, were investigated. It was found that the 6-AuMAu shows the strongest SERS effect, which can be attributed to the strong hot spot intensity. Moreover, the MoS2films could act as an excellent adsorbent for the analytes, making it easier to adsorb on the surface of the 2DSERS substrates, which further improves the sensitivity of the substrates. The ultra-low concentration detection of harmful substances (CV and MG) was also achieved. Moreover, the composite substrate possesses superior uniformity and stability. We believe that this substrate should be beneficial for the practical application of SERS substrates in the ultra-sensitive detection of toxic substances.
Funding
National Natural Science Foundation of China (11747072, 11774208, 11804200, 11904214); China post-doctoral foundation (2019M662423).
Disclosures
The authors declare no conflicts of interest.
References
1. P. Karthick Kannan, P. Shankar, C. Blackman, and C. H. Chung, “Recent advances in 2D inorganic nanomaterials for SERS sensing,” Adv. Mater. 31(34), 1803432 (2019). [CrossRef]
2. Z. G. Dai, G. W. Hu, Q. D. Ou, L. Zhang, F. N. Xia, F. J. Garcia-Vidal, C. W. Qiu, and Q. L. Bao, “Artificial Meta photonics Born Naturally in Two Dimensions,” Chem. Rev. 120(13), 6197–6246 (2020). [CrossRef]
3. C. Zhang, S. Z. Jiang, Y. Y. Huo, A. H. Liu, S. C. Xu, X. Y. Liu, Z. C. Sun, Y. Y. Xu, Z. Li, and B. Y. Man, “SERS detection of R6G based on a novel graphene oxide/silver nanoparticles/silicon pyramid arrays structure,” Opt. Express 23(19), 24811–24821 (2015). [CrossRef]
4. J. Yu, Y. Guo, H. Wang, S. Su, C. Zhang, B. Y. Man, and F. C. Lei, “Quasi optical cavity of hierarchical ZnO nanosheets @Ag nanoravines with synergy of near-and far-field effects for in situ Raman detection,” J. Phys. Chem. Lett. 10(13), 3676–3680 (2019). [CrossRef]
5. S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science 275(5303), 1102–1106 (1997). [CrossRef]
6. D. Zheng, Y. Li, W. Chen, T. Fu, J. Sun, S. Zhang, and H. Xu, “The novel plasmonics-transition metal dichalcogenides hybrid nanostructures,” Sci. Sin.-Phys. Mech. Astron. 49(12), 124205 (2019). [CrossRef]
7. W. Zhu and K. B. Crozier, “Quantum mechanical limit to plasmonic enhancement as observed by surface-enhanced Raman scattering,” Nat. Commun. 5(1), 5228 (2014). [CrossRef]
8. Y. Fang, N. H. Seong, and D. D. Dlott, “Measurement of the distribution of site enhancements in surface-enhanced Raman scattering,” Science 321(5887), 388–392 (2008). [CrossRef]
9. Z. G. Dai, X. H. Xiao, W. Wu, Y. P. Zhang, L. Liao, S. H. Guo, J. J. Ying, C. X. Shan, M. T. Sun, and C. Z. Jiang, “Plasmon-driven reaction controlled by the number of graphene layers and localized surface plasmon distribution during optical excitation,” Light: Sci. Appl. 4(10), e342 (2015). [CrossRef]
10. X. L. Zhang, X. H. Xiao, Z. G. Dai, W. Wu, L. Liao, X. G. Zhang, X. H. Xiao, and C. Z. Jiang, “Ultrasensitive SERS Performance in 3D “Sunflower-like” Nanoarrays decorated with Ag Nanoparticles,” Nanoscale 9(9), 3114–3120 (2017). [CrossRef]
11. X. G. Zhang, Z. G. Da, S. Y. Si, X. L. Zhang, W. Wu, H. B. Deng, F. B. Wang, X. H. Xiao, and C. Z. Jiang, “Ultrasensitive SERS Substrate Integrated with Uniform Subnanometer Scale“Hot Spots”created by a Graphene Spacer for the Detection of Mercury Ions,” Small 13(9), 1603347 (2017). [CrossRef]
12. J. Lu, J. H. Lu, H. Liu, B. Liu, L. Gong, E. S. Tok, and C. H. Sow, “Microlandscaping of Au Nanoparticles on Few-Layer MoS2 Films for Chemical Sensing,” Small 11(15), 1792–1800 (2015). [CrossRef]
13. L. Sun, H. Hu, D. Zhan, J. Yan, L. Liu, J. S. Teguh, and Z. Shen, “Plasma modified MoS2 nanoflakes for surface enhanced Raman scattering,” Small 10(6), 1090–1095 (2014). [CrossRef]
14. H. Kim, M. L. Seol, D. I. Lee, J. Lee, I. S. Kang, H. Lee, and B. Kim, “Single nanowire on graphene (SNOG) as an efficient, reproducible, and stable SERS-active platform,” Nanoscale 8(16), 8878–8886 (2016). [CrossRef]
15. X. Li, X. Ren, Y. Zhang, W. C. Choy, and B. Wei, “An all-copper plasmonic sandwich system obtained through directly depositing copper NPs on a CVD grown graphene/copper film and its application in SERS,” Nanoscale 7(26), 11291–11299 (2015). [CrossRef]
16. X. Li, W. C. Choy, X. Ren, D. Zhang, and H. Lu, “Highly intensified surface enhanced Raman scattering by using monolayer graphene as the nanospacer of metal film-metal nanoparticle coupling system,” Adv. Funct. Mater. 24(21), 3114–3122 (2014). [CrossRef]
17. Z. Y. Zhan, L. H. Liu, W. Wang, Z. J. Cao, A. Martinelli, E. Wang, Y. Cao, J. N. Chen, A. Yurgens, and J. Sun, “Ultrahigh Surface-Enhanced Raman Scattering of Graphene from Au/Graphene/Au Sandwiched Structures with Subnanometer Gap,” Adv. Opt. Mater. 4(12), 2021–2027 (2016). [CrossRef]
18. Z. Li, S. Z. Jiang, Y. Y. Huo, A. H. Liu, C. Zhang, J. Yu, M. H. Wang, C. H. Li, Z. Y. Lu, and B. Y. Man, “3D Hybrid Plasmonic Nanostructures with Dense Hot Spots Using Monolayer MoS2 as Sub-Nanometer Spacer,” Adv. Mater. Interfaces 5(19), 1800661 (2018). [CrossRef]
19. Y. Yang, J. Shi, G. Kawamura, and M. Nogami, “Preparation of Au-Ag, Ag-Au core–shell bimetallic nanoparticles for surface-enhanced Raman scattering,” Scr. Mater. 58(10), 862–865 (2008). [CrossRef]
20. Z. Liu, Z. B. Yang, B. Peng, C. O. Cao, C. Zhang, H. J. You, Q. H. Xiong, Z. Y. Li, and J. X. Fang, “Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy from Hollow Au-Ag Alloy Nanourchins,” Adv. Mater. 26(15), 2431–2439 (2014). [CrossRef]
21. D. F. O’kane and K. L. Mittal, “Plasma cleaning of metal surfaces,” J. Mater. Sci. Technol. 11(3), 567–569 (1974). [CrossRef]
22. C. Gong, C. M. Huang, J. Miller, L. X. Cheng, Y. F. Hao, D. Cobden, J. Y. Kim, R. S. Ruoff, R. M. Wallace, K. Cho, X. D. Xu, and Y. J. Chabal, “Metal Contacts on Physical Vapor Deposited Monolayer MoS2,” ACS Nano 7(12), 11350–11357 (2013). [CrossRef]
23. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, “Anomalous lattice vibrations of single- and few-layer MoS2,” ACS Nano 4(5), 2695–2700 (2010). [CrossRef]
24. L. Xiu, X. J. Zhang, T. T. You, N. Yang, G. S. Wang, and P. G. Yin, “Three-dimensional MoS2-NS@ Au-NPs hybrids as SERS sensor for quantitative and ultrasensitive detection of melamine in milk,” J. Raman Spectrosc. 49(2), 245–255 (2018). [CrossRef]
25. Y. Li, J. D. Cain, E. D. Hanson, A. A. Murthy, S. Q. Hao, F. Y. Shi, Q. Q. Li, C. W. X. Q. Chen, and V. P. Dravid, “Au@ MoS2 core-shell heterostructures with strong light-matter interactions,” Nano Lett. 16(12), 7696–7702 (2016). [CrossRef]
26. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, and D. Baillargeat, “From bulk to monolayer MoS2: evolution of Raman scattering,” Adv. Funct. Mater. 22(7), 1385–1390 (2012). [CrossRef]
27. A. Castellanos-Gomez, N. Agraït, and G. Rubio-Bollinger, “Optical identification of atomically thindichalcogenide crystals,” Appl. Phys. Lett. 96(21), 213116 (2010). [CrossRef]
28. S. Chakraborty, S. Chowdhury, and P. D. Saha, “Adsorption of crystal violet from aqueous solution onto NaOH-modified rice husk,” Carbohydr. Polym. 86(4), 1533–1541 (2011). [CrossRef]
29. P. Saha, S. Chowdhury, S. Gupta, and I. Kumar, “Insight into adsorption equilibrium, kinetics and thermodynamics of Malachite Green onto clayey soil of Indian origin,” Chem. Eng. J. 165(3), 874–882 (2010). [CrossRef]
30. Y. K. Kim, S. W. Han, and D. H. Min, “Graphene oxide sheath on Ag nanoparticle/graphene hybrid films as an antioxidative coating and enhancer of surface-enhanced Raman scattering,” ACS Appl. Mater. Interfaces 4(12), 6545–6551 (2012). [CrossRef]
31. S. C. Xu, J. H. Wang, Y. Zou, H. P. Liu, G. Y. Wang, X. M. Zhang, S. Z. Jiang, Z. Li, D. Y. Cao, and R. X. Tang, “HighperformanceSERSactivesubstratesfabricatedbydirectlygrowinggrapheneonAgnanoparticles,” RSC Adv. 5(110), 90457–90465 (2015). [CrossRef]
32. J. W. Jeong, M. M. Arnob, K. M. Baek, S. Y. Lee, W. C. Shih, and Y. S. Jung, “3D cross-point plasmonic nanoarchitectures containing dense and regular hot spots for surface-enhanced Raman spectroscopy analysis,” Adv. Mater. 28(39), 8695–8704 (2016). [CrossRef]
33. Z. Li, M. H. Wang, Y. Jiao, A. H. Liu, S. Wang, C. Zhang, C. Yang, Y. Y. Xu, C. H. Li, and B. Y. Man, “Different number of silver nanoparticles layers for surface enhanced Raman spectroscopy analysis,” Sens. Actuators, B 255, 374–383 (2018). [CrossRef]
34. S. Su, C. Zhang, L. Yuwen, J. Chao, X. Zuo, X. Liu, and L. Wang, “Creating SERS hot spots on MoS2 nanosheets with in situ grown gold nanoparticles,” ACS Appl. Mater. Interfaces 6(21), 18735–18741 (2014). [CrossRef]
35. J. Li, W. Zhang, H. Lei, and B. Li, “Ag nanowire/nanoparticle-decorated MoS2 monolayers for surface-enhanced Raman scattering applications,” Nano Res. 11(4), 2181–2189 (2018). [CrossRef]
36. R. Rani, A. Yoshimura, S. Das, M. R. Sahoo, A. Kundu, K. K. Sahu, and K. S. Hazra, “Sculpting Artificial Edges in Monolayer MoS2 for Controlled Formation of Surface-Enhanced Raman Hotspots,” ACS Nano 14(5), 6258–6268 (2020). [CrossRef]
37. Z. Li, S. Z. Jiang, Y. Y. Huo, T. Y. Ning, A. H. Liu, C. Zhang, Y. He, M. H. Wang, C. H. Li, and B. Y. Man, “3D silver nanoparticles with multilayer graphene oxide as a spacer for surface enhanced Raman spectroscopy analysis,” Nanoscale 10(13), 5897–5905 (2018). [CrossRef]
38. E. Galopin, J. Barbillat, Y. Coffinier, S. Szunerits, G. Patriarche, and R. Boukherroub, “Silicon nanowires coated with silver nanostructures as ultrasensitive interfaces for surface-enhanced Raman spectroscopy,” ACS Appl. Mater. Interfaces 1(7), 1396–1403 (2009). [CrossRef]
39. D. Lin, Z. Wu, S. Li, W. Zhao, C. Ma, J. Wang, and X. Yang, “Large-area Au-nanoparticle-functionalized Si nanorod arrays for spatially uniform surface-enhanced Raman spectroscopy,” ACS Nano 11(2), 1478–1487 (2017). [CrossRef]
40. Y. Kalachyova, M. Erzina, P. Postnikov, V. Svorcik, and O. Lyutakov, “Flexible SERS substrate for portable Raman analysis of biosamples,” Appl. Sur. Sci.458, 95–99 (2018). [CrossRef]
41. O. Guselnikova, P. Postnikov, Y. Kalachyova, Z. Kolska, M. Libansky, J. Zima, and O. Lyutakov, “Large-Scale, Ultrasensitive, Highly Reproducible and Reusable Smart SERS Platform Based on PNIPAm-Grafted Gold Grating,” ChemNanoMat 3(2), 135–144 (2017). [CrossRef]
42. L. Sun, Y. Song, L. Wang, C. Guo, Y. Sun, Z. Liu, and Z. Li, “Ethanol-induced formation of silver nanoparticle aggregates for highly active SERS substrates and application in DNA detection,” J. Phys. Chem. C 112(5), 1415–1422 (2008). [CrossRef]
43. A. M. Michaels, J. Jiang, and L. Brus, “Ag nanocrystal junctions as the site for surface-enhanced Raman scattering of single rhodamine 6G molecules,” J. Phys. Chem. B 104(50), 11965–11971 (2000). [CrossRef]
44. S. G. Park, C. Mun, M. Lee, T. Y. Jeon, H. S. Shim, Y. J. Lee, J. D. Kwon, C. S. Kim, and D. H. Kim, “3D hybrid plasmonic nanomaterials for highly efficient optical absorbers and sensors,” Adv. Mater. 27(29), 4290–4295 (2015). [CrossRef]
45. W. Wu, L. Liu, Z. Dai, J. Liu, S. Yang, L. Zhou, and V. A. L. Roy, “Low-cost, disposable, flexible and highly reproducible screen-printed SERS substrates for the detection of various chemicals,” Sci. Rep. 5(1), 10208 (2015). [CrossRef]
46. S. Han, K. Liu, L. Hu, F. Teng, P. Yu, and Y. Zhu, “Superior adsorption and regenerable dye adsorbent based on flower-like molybdenum disulfide nanostructure,” Sci. Rep. 7(1), 43599 (2017). [CrossRef]
47. Y. Chao, W. Zhu, X. Wu, F. Hou, S. Xun, P. Wu, and H. Li, “Application of graphene-like layered molybdenum disulfide and its excellent adsorption behavior for doxycycline antibiotic,” Chem. Eng. J. 243, 60–67 (2014). [CrossRef]
