Plasmon-Free Surface-Enhanced Raman Spectroscopy Using Metallic

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Plasmon-Free Surface-Enhanced Raman Spectroscopy Using Metallic 2D Materials Xiuju Song,†,‡ Yan Wang,‡,§ Fang Zhao,∥ Qiucheng Li,⊥ Huy Quang Ta,# Mark H. Rümmeli,⊥,#,∇ Christopher G. Tully,∥ Zhenzhu Li,⊥ Wan-Jian Yin,⊥ Letao Yang,⊗ Ki-Bum Lee,⊗ Jieun Yang,‡,§ Ibrahim Bozkurt,‡ Shengwen Liu,†,‡ Wenjing Zhang,*,† and Manish Chhowalla*,†,‡,§ Downloaded via BUFFALO STATE on July 17, 2019 at 08:31:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Shenzhen University, Shenzhen 518060, P.R. China ‡ Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, United States § Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, U.K. ∥ Department of Physics, Princeton University, Jadwin Hall, Princeton, New Jersey 08544, United States ⊥ Soochow Institute for Energy and Materials InnovationS (SIEMIS), Jiangsu Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Soochow University, Suzhou 215006, P.R. China # IFW Dresden, Helmholtz Strasse 20, Dresden 01069, Germany ∇ Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, Zabrze 41-819, Poland ⊗ Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 123 Bevier Road, Piscataway, New Jersey 08854, United States S Supporting Information *

ABSTRACT: Two dimensional (2D) materials-based plasmon-free surface-enhanced Raman scattering (SERS) is an emerging field in nondestructive analysis. However, impeded by the low density of state (DOS), an inferior detection sensitivity is frequently encountered due to the low enhancement factor of most 2D materials. Metallic transition-metal dichalcogenides (TMDs) could be ideal plasmon-free SERS substrates because of their abundant DOS near the Fermi level. However, the absence of controllable synthesis of metallic 2D TMDs has hindered their study as SERS substrates. Here, we realize controllable synthesis of ultrathin metallic 2D niobium disulfide (NbS2) (160 μm). We have explored the SERS performance of as-obtained NbS2, which shows a detection limit down to 10−14 mol·L−1. The enhancement mechanism was studied in depth by density functional theory, which suggested a strong correlation between the SERS performance and DOS near the Fermi level. NbS2 features the most abundant DOS and strongest binding energy with probe molecules as compared with other 2D materials such as graphene, 1T-phase MoS2, and 2H-phase MoS2. The large DOS increases the intermolecular charge transfer probability and thus induces prominent Raman enhancement. To extend the results to practical applications, the resulting NbS2-based plasmon-free SERS substrates were applied for distinguishing different types of red wines. KEYWORDS: chemical vapor deposition, niobium disulfide, surface enhanced Raman scattering, metallic 2D materials, charge transfer

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noble metal substrates such as gold and silver nanostructures have been widely studied due to their high enhancement factors originating from an electromagnetic enhancement mechanism.11−16 Recently, two-dimensional layered (2D)

urface-enhanced Raman spectroscopy (SERS) has been investigated for nondestructive analysis for detection of molecules with high sensitivity and selectivity.1−8 In general, the SERS effect can be attributed to two mechanisms: (i) electric field enhancement due to surface plasmon resonance from metallic nanostructures9 and (ii) chemical enhancement induced by charge transfer between SERS substrates and probe molecules.10 In the past few decades, © XXXX American Chemical Society

Received: May 14, 2019 Accepted: July 8, 2019 Published: July 8, 2019 A

DOI: 10.1021/acsnano.9b03761 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Atomically thin NbS2 growth and its metallic property measurement. (a) Schematic crystal structure of NbS2 on SiO2/Si. (b) Optical microscope image of typical NbS2 flakes grown on SiO2/Si. (c) Statistical evolution of edge length (top) and thickness (bottom) of CVD-grown NbS2 flakes as a function of H2/Ar flow rate. Thin NbS2 flakes with large size can be obtained with the optimized growth conditions of 180 sccm H2/Ar. Inset: optical image of a large NbS2 triangle with 166 μm in edge length (top) and the corresponding AFM height image showing thickness of ∼2.2 nm (bottom). Scale bar: 40 μm (top) and 5 μm (bottom). (d) I−V characteristic curve of NbS2 flake, showing an excellent conductivity values of ∼2000 S/cm. Inset: optical image of as-fabricated device on a 5 nm thick NbS2 flake. Scale bar: 5 μm. (e) Resistance of NbS2 flake measured as a function of temperature in the range of 160 K to 400 K. The positive slope of the resistivity curve indicates metallic behavior of as-grown NbS2.

using various niobium sources, including niobium chloride,31−33 metal amido complexes,34 and niobium metal.35,36 Unlike MoS2 growth using molybdenum oxide, both niobium and niobium oxide precursors have higher melting points (>1500 °C) and low vapor pressure. For this reason, it is challenging to grow NbS2 by the normal CVD where temperatures are limited to less than 1100 °C. Zhou et al.37 reported molten-salt-assisted CVD method to reduce the melting point of the reactants to facilitate growth of various TMDs. Here, we demonstrate that sodium chloride (NaCl) acts as a promoter to decrease the melting point of niobium oxide during the growth, obtaining ultrathin NbS2 flakes with large domain sizes.

materials have emerged as a new class of nanostructure-free substrates for SERS enhancement via a charge-transfer mechanism. Aside from the distinct enhancing mechanism, the flat surface of 2D materials also allows uniform chemisorption of probe molecules, thus facilitating acquirement of stable and repeatable signals essential for practical applications. Graphene is the first explored 2D material for SERS study, where the Raman signal is enhanced via groundstate charge transfer and spectral fluorescence (FL) quenching.17−21 Semiconducting transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2)22−24 and rhenium disulfide (ReS2),25,26 were also demonstrated to have SERS enhancement owing to dipole−dipole coupling and weak charge transfer. To increase their Raman enhancement effect, a branch number of techniques including phase transition,24 plasma treatment,27 and oxygen incorporation28 have been developed. However, the SERS effect on the basis of the above 2D materials is still limited. According to Fermi’s golden rule, electron transition probability during the charge-transfer process is linearly correlated with the density of states (DOS) near the Fermi level.22 Hence, metallic TMDs, which possess abundant energy states near the Fermi level as well as strong interactions with analytes, are well suited as promising enhancing substrates for SERS applications. However, so far it is challenging to fabricate metallic TMDs films with large area and few-layers, discouraging both the enhancing mechanism study and practical applications of SERS. Niobium disulfide (NbS2) is a representative member of the metallic TMDs that has abundant energy near the Fermi level. NbS2 usually exists in the hexagonal (2H phase) or rhombohedral (3R phase) crystal configurations.29 Bulk NbS2 synthesis has been reported by the chemical vapor transport (CVT) technique.30 To achieve two-dimensional thin NbS2 flake growth with precise thickness control, chemical vapor deposition (CVD) of NbS2 has been investigated by

RESULTS AND DISCUSSION Atomically thin NbS2 was grown on SiO2/Si by an ambientpressure CVD method under mixed Ar/H2 gas flow (see further details in Methods, Figure 1a, and Figure S1). Figure 1b shows an optical image of typical NbS2 flakes on SiO2/Si. We found that the edge length of NbS2 flakes and the thickness could be well controlled by changing the flow rate of H2/Ar (see Figure S1). Thin, large NbS2 flakes with edge lengths reaching >160 μm and thickness ∼2 nm were obtained with an optimized flow rate of 180 sccm (Figure 1c). Increasing the flow rate yields a growth of domain size, passing a maximum at 180 sccm and decreasing for higher flow rates, while the thickness trend is the opposite. To investigate the electrical properties, we fabricated devices on 2−5 nm thick NbS2 by evaporating In/Au as electrodes. The I−V curve shown in Figure 1d is linear with a high conductance of ∼2000 S/cm. Temperature-dependent resistance measurements were performed from 400 to 160 K (Figure 1e). The decreasing resistance demonstrates the metallic behavior of NbS2 (Figure 1e and Figure S2). B

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Figure 2. Characterization of mixed phases of NbS2. (a) Raman spectra of as-grown NbS2 on SiO2/Si with various thickness. Inset is the evolution of A1 mode peak with the corresponding thickness. As the thickness decreases, the A1 mode is red-shifted to 379 cm−1. (b, c) XPS spectra and curve fitting of Nb 3d (b) and S 2p (c) core level peak regions for as-transferred NbS2 sample. Both Nb 3d and S 2p spectra are well fitted with two doublets which are assigned to the 2H (purple line) and 3R (blue line) phase of NbS2. (d, e) SAED patterns of thin (d) and thick (e) NbS2 flakes. Both of the SAED patterns exhibit two sets of spots but with different intensity. Insets: corresponding TEM image of the analyzed flakes. Scale bars of insets: (d) 1 μm; (e) 1 μm. (f) High-resolution TEM image of 3R-NbS2 region under focus and the corresponding simulated atomic model of the 3R-NbS2 structure (g, h). The perfect hexagonal atomic structure matches well with the simulated model of 3R-NbS2.

Raman spectra of NbS2 flakes of various thicknesses are shown in Figure 2a. Two characteristic Raman peaks of NbS2 flakes thicker than 6.5 nm are located at 386 and 330 cm−1, which are assigned to A1 and E2 modes of 3R-NbS2, respectively.29,38 We find that as the thickness of NbS2 flakes decreases, the intensity of 303 cm−1 peak increases and the Raman peak of E2 mode is red-shifted to 379 cm−1, suggesting an increase in 2H phase concentration. Only 2H Raman modes are observed on the 1.2 nm thick NbS2. From the Raman spectra, it is revealed that 3R-NbS2 is the predominant phase for thicker flakes, while 2H Raman modes are stronger in thin flakes. This phenomenon is consistent with ref 31. To examine the chemical composition and stoichiometry, X-ray photoelectron spectroscopy (XPS) was performed. Both Nb 3d and S 2p spectra of NbS2 shown in Figure 2b,c can be well fitted by two doublets, indicating the mixed structure of 2H and 3R phases. The chemical composition ratio of S/Nb is calculated to be ∼2.03, close to the stoichiometric ratio of NbS2 (Table S1).35,39 More details about the XPS fitting information and elemental composition results are provided in Figure S3 and Table S1. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectrometry (EDS), and high-resolution TEM were conducted to reveal the atomic structure and extend the chemical composition analysis. Parts d and e of Figure 2 show the SAED patterns performed on thin and thick flakes, respectively. The intensity of the 2H phase (100) diffraction spots is stronger in thin flakes than that in thick flakes, while the 3R (003) diffraction is brighter in thick flakes, suggesting that thick flakes are rich in the 3R phase, in agreement with Raman results (Figure S4). The elemental maps of niobium and sulfur on an NbS2 triangle reveal uniform distributions of Nb and S (Figure S5). High-resolution TEM images performed on an NbS2

triangle display the perfect hexagonal atomic lattice in under and over focus (Figures 2f−h and S5). To investigate the plasmonfree SERS effect of metallic NbS2, we started with methylene blue (MeB) as the probe molecule. The NbS2 sample was first soaked in MeB solution for 30 min with a concentration of 1 × 10−6 mol/L and then washed with ethanol to remove the free dye molecules, followed by blow drying with argon gas. Distinctive Raman signals of MeB were observed on the NbS2 flakes by using a 633 nm laser as the excitation source (Figure 3a). The characteristic peaks of MeB molecules located at 1621, 1472, 1398, 1182 cm−1 and other wavenumbers could be easily observed on NbS2. In contrast, the Raman signal of MeB on SiO2/Si was hindered by strong fluorescence (Figure S6a). To compare SERS effects of NbS2 with other 2D materials, MeB molecules (1 × 10−6 mol/L) were deposited on CVD-grown monolayer graphene, CVDgrown MoS2, chemically exfoliated 1T MoS2, chemically exfoliated 2H MoS2, and partially oxidized NbS2 (Figure S7). The Raman spectra for MeB on different 2D materials are summarized in Figure 3a. Notably, NbS2 exhibits the largest SERS enhancement with MeB as the probe molecule. Metallic 1T MoS2 and semimetal graphene perform better than semiconducting 2H MoS2. Since CVD-grown MoS2 has a large photoluminescence (PL) peak that overlaps with the peaks of MeB molecules in the range of 800−1500 cm−1, MeB signals on CVD-grown MoS2 could not be detected with 633 nm laser as the excitation source (Figure S6b). Table S2 lists all the Raman peaks of MeB on the 2D materials we tested. We performed a detailed study of SERS of MeB on NbS2 at different solution concentrations, NbS2 thickness dependence, and stability in air. NbS2 samples were soaked in MeB solutions with concentrations from 1 × 10−16 to 1 × 10−6 mol/ L. As shown in Figure 3b, the intensities of Raman signals of C

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Figure 3. SERS effect of methylene blue (MeB) on metallic 2D materials. (a) Raman spectra of MeB molecules on blank SiO2/Si substrate (black line), on oxidized NbS2 (green line), on 2H-MoS2 (royal line), on 1T MoS2 (blue line), on graphene (red line), and on 2 nm thick NbS2 (purple line). (b) Raman spectra of MeB on 2 nm thick NbS2 by soaking in the solution with various concentrations from 1 × 10−6 to 1 × 10−16 mol/L. Inset: Raman intensity mapping of 1621 cm−1 peak of MeB (1 × 10−6 mol/L) on a large NbS2 flake, indicating good uniformity. Scale bar of inset: 20 μm. (c) Raman spectra of MeB (1 × 10−6 mol/L) on NbS2 flakes with various thickness. Inset: optical image and the corresponding Raman intensity mapping of the 1621 cm−1 peak of MeB on a layer-varied NbS2 triangle. The intensity of Raman signals of MeB decreased with the increase of the thickness of NbS2. Scale bar of inset: 20 μm. (d) Evolution of Raman spectra of MeB (1 × 10−6 mol/L) on 3 nm thick NbS2 flake after exposure to laboratory atmosphere (22 °C at 60% humidity) for 1 month.

Figure 4. Illustration of charge-transfer mechanism of methylene blue on 2D materials. (a) Enhancement factor of methylene blue (MeB), methyl blue (MB), hemin, and chlorophyll a (Chl a) on NbS2, 1T MoS2, oxidized NbS2, graphene, and 2H MoS2. NbS2 presents the highest enhancement factor with these four probe molecules. (b) Density of state of graphene (red line), 1T MoS2 (blue line), 3R NbS2 (purple line), and 2H NbS2 (black line) near the Fermi level. (c−e) Side views of the electron density difference isosurface (0.003 electron/bohr3) for MeB molecule absorbed on NbS2 (c), 1T MoS2 (d), and graphene (e). Red color represents the region of increased electron density, and blue color shows the region of decreased electron density after formation of the analyte−substrate coupling structure. For NbS2−MeB and 1T MoS2−MeB complexes, the charge transfer is from molecule to 2D material, while for the graphene−MeB complex, the charge transfer is from graphene to molecule.

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Figure 5. SERS effect of 2D materials for red wine detection. (a) Photograph of four types of red wines and one type of white wine analyzed. (b) Raman spectra of Cabernet Sauvignon red wine on blank SiO2/Si substrate (black line), on 2H MoS2 (royal line), on 1T MoS2 (blue line), on graphene (red line), and on 3 nm thick NbS2 (purple line). (c) Raman spectra of Cabernet Sauvignon (green line), Pinot noir (blue line), Syrah/Shiraz (cyan), Merlot (purple line), and white wine (black line) on 3 nm thick NbS2. (d) Percentage of malvidin 3-O-glucoside in the anthocyanin of the analyzed red wine and white wine calculated according to the chromatogram results. The black line is the curve of the Raman intensity of 1530 cm−1 band using NbS2 as the SERS substrates with the five analyzed wines.

chlorophyll a (Chl a), and hemin as probe molecules that are relevant for environmental pollution monitoring and biological sensing (Figure S6). The enhancement factors for MB, Chl a, and hemin molecules along with MeB for comparison on different 2D materials tested here are shown in Figure 4a. NbS2 exhibits the largest enhancement for all the tested molecules. Since the surface-plasmon resonance of NbS2 occurs in midinfrared and terahertz frequency,41,42 the conventional electromagnetic field enhancement mechanism can be excluded in our system. We propose a charge-transfer mechanism to explain the SERS effect on NbS2. Density functional theory was used to study the coupling and charge transfer between MeB and NbS2, graphene and 1T MoS2 (Figures 4b−d). The binding energy of MeB-graphene, MeB-1T MoS2, and MeB-NbS2 was calculated to be −1.64, −3.42, and −3.51 eV, respectively, suggesting the strong coupling between MeB and NbS2. The electron density difference isosurfaces (0.003 electron/bohr3) of NbS2−MeB (Figure 4c) and 1T MoS2−MeB complex (Figure 4d) exhibit high electron perturbation generated by charge transfer from MeB to substrates, thus forming a strong interface dipole. For NbS2−MeB complex, the electron transfer from MeB to NbS2 is 0.7 e/molecule, while there is 0.5 e/ molecule transfer from MeB to 1T MoS2. On the other hand, for graphene−MeB complex, the electron density significantly increases on the graphene surface and slightly increases around the MeB molecule (Figure 4e), suggesting a relatively weak charge transfer from graphene to MeB molecule (0.07 e/ molecule). The Fermi’s golden rule demonstrates that the electron transition probability rate between materials can be 2π expressed as ωlk = ℏ g (Ek )|Hkl′ |2 , where g(Ek) is the density of states and |Hkl′ | is the matrix element for the highest occupied

MeB on NbS2 are still distinguishable even when the concentration is decreased to 1 × 10−14 mol/L. The enhancement factor of the 1621 cm−1 peak is calculated to be 1.07 × 103, which is higher than that of graphene.17 The Raman intensity mapping of 1621 cm−1 band after MeB deposition on a 2.2 nm thickness NbS2 triangle with the edge length of 80 μm demonstrates the uniform Raman enhancement. Figure 3c shows the thickness-dependent Raman enhancement of NbS2 flakes. Notably, the intensity of the Raman signal of MeB on NbS2 flakes becomes weaker as the thickness of NbS2 increases. Similar layer-dependent phenomena have been reported on graphene and 1T′ MoTe2.18,19,40 The variation of the Raman signals with different thicknesses are likely due to changes in electronic structure,18 chemisorption,19 and field intensity of molecules coated on the 2D film40 with thickness. In our case, another reason could be that the composition of 2H and 3R phases changes with the thickness which can cause the DOS to vary near the Fermi level. That is, thin NbS2 is primarily in the 2H phase while thick NbS2 contains more 3R phase. The density functional theory (DFT) calculations demonstrate that the 2H phase presents more abundant DOS near the Fermi level than the 3R phase. Hence, the 2H NbS2 should exhibit better Raman enhancement than 3R NbS2 with the same layers. The stability of SERS measurements was measured by exposure of the MeBdeposited NbS2 sample to typical laboratory atmosphere (22 °C at 60% humidity) for 1 month (Figure 3d). The Raman intensities of MeB peaks exhibit limited degradation after 2 weeks exposure, suggesting reasonably good stability of the NbS2 substrates and strong adsorption of analytes. To demonstrate the universality of plasmon-free SERS with NbS2, we also tested other dyes such as methyl blue (MB), E

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where the detection limit of MeB on NbS2 was achieved to be femtomolar levels with high enhancement factor. The excellent Raman enhancement attributes to the abundant density states near Fermi level, producing high electron transition probability. DFT calculations further verify that the DOS of 2D materials near the Fermi level follows the sequence of NbS2 > 1T MoS2 > graphene, consistent with our experimental SERS performance. As a demonstration, the 2D metallic substrates are used to differentiate red wines by obtaining the direct spectral biomarkers, enabling direct, rapid, and reliable detection in real life.

molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) transition.22,40,43 Hence, the density of states near the Fermi level has a large impact on the electron transition probability. The DOS of graphene, 3R NbS2, 2H NbS2, and 1T MoS2 shown in Figure 4b near Fermi level demonstrates that metallic TMDs possess more electronic states than graphene. Owing to more plentiful electronic states of 2H NbS2 than that of 3R NbS2, 2H NbS2 gives better Raman enhancement, consistent with our experimental results of large SERS from thin flakes. The strong dipole interaction of TMDs also contributes to local symmetry related perturbation, which increases the matrix element |Hkl′ | and the transition probability. To extend the generality of SERS detection with metallic 2D materials, we sought to differentiate commercially available red wines. Four types of red wines (Cabernet Sauvignon, Pinot noir, Syrah/Shiraz Blend, And Merlot) and one white wine (Sauvignon blanc) were tested (Figure 5a). We first conducted the SERS experiments by simply soaking the different 2D materials (CVD-grown graphene, 1T MoS2, 2H MoS2 and NbS2) in Cabernet Sauvignon. Raman spectra from the wine soaked 2D materials exhibit peaks attributed to anthocyanin, an important class of natural compounds responsible for the color of red wine.44,45 Similar to the dye molecules, NbS2 showed a larger Raman enhancement compared to that of graphene and 1T MoS2, while 2H MoS2 presented a broad peak, indicating smaller enhancement (Figure 5b). Raman signals of different wines on NbS2 show similar peak positions but with different degrees of enhancement (Figure 5c and Table S4). The Cabernet Sauvignon soaked NbS2 samples yield the best signals, while the white wine shows no obvious signals. To better understand the origin of different enhancement factors among the wines, we performed high-performance liquid chromatography (HPLC) to analyze their chemical compositions. The HPLC results show that the four kinds of red wines contain peonidin 3-O-glucoside (PEO), malvidin 3O-glucoside (MAL), and delphinidin 3-O-glucoside (DEL) of different concentrations (Figures S8 and S9 and Table S5). Raman measurements were performed on NbS2 samples soaked in these three pure anthocyanin solutions (Figure S10). The Raman spectra of red wines shown in Figure 5c are consistent with the Raman fingerprint of MAL. According to the first layer effect,46 we assume that monolayer anthocyanin molecules are absorbed on NbS2 surface, which is confirmed by AFM results that show only 3 Å height difference between pure NbS2 and NbS2 after soaking in wine (Figure S11). Assuming monolayer deposition of the compound, Figure 5d shows that Raman intensity of five wines obtained experimentally with SERS matches well with the percentage of MAL obtained from HPLC. The higher percentage of MAL of the red wine results in higher percentage absorption of MAL on the NbS2, hence giving a better Raman signal. MAL detection could also be useful for health detection as it has been found that MAL helps in inhibition of pro-inflammatory signaling pathways, which benefits cardiovascular health.47 From our results, it appears that SERS using metallic 2D NbS2 as substrates provides an accessible, prompt, sensitive, and fast technique for red wine discrimination.

MATERIALS AND METHODS CVD Growth of NbS2. The NbS2 growth was performed in a horizontal single zone furnace (Lindberg/Blue M) equipped with a 1in. quartz tube. Prior to the growth, SiO2/Si were cleaned by sonication in acetone and 2-propanol for 15 min, respectively. Nb metal powder (Alfa Aesar, 99.8%) was oxidized in an open quartz tube at 680 °C for 3 min. A 0.7 g portion of partially oxidized NbOx powders mixed with 0.15 g of NaCl powders were loaded in an alumina boat, which was placed in the center of the single-zone furnace. Clean SiO2/Si substrates were put above the powder with the polished side faced down. Another boat containing 1 g of sulfur powder (Alfa Aesar, 99.5%) was placed at the upstream region of the quartz tube. The furnace temperature was ramped to 800 °C in 16 min and kept for another 15 min for NbS2 growth. During the entire process, H2/Ar (10% H2 in Ar) was used as carrier gas. SERS Measurements. An ethanol solution of MeB and water solution of MB solution with different concentrations were obtained via sequential diluting processes. For each SERS measurement, SiO2/ Si substrates were first deposited by graphene, NbS2, 1T MoS2, 2H MoS2, and CVD MoS2 and then were soaked in solutions for 30 min, followed by rinse with ethanol to remove the free molecules. Raman spectra of MeB, MB, chlorophyll a, and hemin molecules on 2D materials were collected using 633 nm laser as the excitation source with a Horiba LabRAM HR Evolution system, while Raman measurement of red wine on 2D materials were conducted with 514 nm laser as the excitation source.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03761. Detailed CVD growth of NbS2, XPS, TEM, SAED, AFM, TEM modulation, Raman bands lists, EF calculations, chromatograms of anthocyanin, UV−vis, and DFT calculation method (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiuju Song: 0000-0001-9777-7016 Mark H. Rümmeli: 0000-0002-4448-1569 Wan-Jian Yin: 0000-0003-0932-2789 Letao Yang: 0000-0002-0572-9787 Wenjing Zhang: 0000-0001-6931-900X

CONCLUSIONS Ultrathin NbS2 flakes (160 μm) were achieved by alkali-assisted APCVD. The metallic NbS2 was demonstrated as an excellent substrate for SERS,

Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (51472164), the Shenzhen Peacock Plan (KQTD2016053112042971), the Educational Commission of Guangdong Province (2015KGJHZ006), the Simons Foundation (377485), and the John Templeton Foundation (58851). REFERENCES (1) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (2) Sharma, B.; Frontiera, R. R.; Henry, A. I.; Ringe, E.; Van Duyne, R. SERS: Materials, Applications, and the Future. Mater. Today 2012, 15, 16−25. (3) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26, 163−166. (4) Cao, Y. C.; Jin, R.; Mirkin, C. A. Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection. Science 2002, 297, 1536−1540. (5) Graham, D.; Thompson, D. G.; Smith, W. E.; Faulds, K. Control of Enhanced Raman Scattering Using a DNA-Based Assembly Process of Dye-Coded Nanoparticles. Nat. Nanotechnol. 2008, 3, 548−551. (6) Qian, X.; Peng, X. H.; Ansari, D. O.; Yin-Goen, Q.; Chen, G. Z.; Shin, D. M.; Yang, L.; Young, A. N.; Wang, M. D.; Nie, S. In Vivo Tumor Targeting and Spectroscopic Detection with SurfaceEnhanced Raman Nanoparticle Tags. Nat. Biotechnol. 2008, 26, 83− 90. (7) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. (8) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (9) Myroshnychenko, V.; Rodríguez-Fernández, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marzán, L. M.; García de Abajo, F. J. Modelling the Optical Response of Gold Nanoparticles. Chem. Soc. Rev. 2008, 37, 1792−1805. (10) Adrian, F. J. Charge Transfer Effects in Surface-Enhanced Raman Scattering. J. Chem. Phys. 1982, 77, 5302−5314. (11) Campion, A.; Kambhampati, P. Surface-Enhanced Raman Scattering. Chem. Soc. Rev. 1998, 27, 241−250. (12) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667− 1670. (13) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. SurfaceEnhanced Raman Scattering. J. Phys.: Condens. Matter 1992, 4, 1143− 1212. (14) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392−385. (15) Jackson, J. B.; Halas, N. J. Surface-Enhanced Raman Scattering on Tunable Plasmonic Nanoparticle Substrates. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 17930−17935. (16) Camden, J. P.; Dieringer, J. A.; Zhao, J.; Van Duyne, R. P. Controlled Plasmonic Nanostructures for Surface-Enhanced Spectroscopy and Sensing. Acc. Chem. Res. 2008, 41, 1653−1661. (17) Ling, X.; Xie, L.; Fang, Y.; Xu, H.; Zhang, H.; Kong, J.; Dresselhaus, M. S.; Zhang, J.; Liu, Z. Can Graphene be used as a Substrate for Raman Enhancement? Nano Lett. 2010, 10, 553−561. (18) Ling, X.; Wu, J.; Xie, L.; Zhang, J. Graphene-ThicknessDependent Graphene-Enhanced Raman Scattering. J. Phys. Chem. C 2013, 117, 2369−2376. G

DOI: 10.1021/acsnano.9b03761 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.9b03761 ACS Nano XXXX, XXX, XXX−XXX