Ultrasensitive and Stable Plasmonic Surface-Enhanced Raman


Jan 21, 2019 - The hybrid platform, an insulating h-BN monolayer covering the Ag NP .... Interface Characteristics of MoS2 Interlayer for Contact Stru...
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Ultrasensitive and stable plasmonic surface-enhanced Raman scattering substrates covered with atomically thin monolayers: effect of the insulating property Na-Yeong Kim, Young-Chul Leem, Sang-Hyun Hong, Jin-Ho Park, and Sang-Youp Yim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17847 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Ultrasensitive and stable plasmonic surfaceenhanced Raman scattering substrates covered with atomically thin monolayers: effect of the insulating property Na-Yeong Kim,§ Young-Chul Leem,§,† Sang-Hyun Hong,§ Jin-Ho Park,‡ and Sang-Youp Yim*,§ §

Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology

(GIST), Gwangju 61005, Republic of Korea ‡ Department of Chemistry, Gwangju Institute of Science and Technology (GIST), Gwangju 61005,

Republic of Korea

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ABSTRACT We demonstrated the effects of monolayer graphene and hexagonal boron nitride (h-BN) on the stability and detection performance of two types of substrates in surface-enhanced Raman scattering (SERS): a two-dimensional (2D) monolayer/Ag nanoparticle (NP) substrate and an Au NP/2D monolayer/Ag NP substrate. Graphene and h-BN, which have different electrical and chemical properties, were introduced in close contact with the metal NPs and had distinctly different effects on the plasmonic near-field interactions between metal NPs in the sub-nanometerscale gap and on the electron transport behavior. A quantitative comparison was possible due to reproducible SERS signals across the entire substrates prepared by simple and inexpensive fabrication methods. The hybrid platform, an insulating h-BN monolayer covering the Ag NP substrate, ensured the long-term oxidative stability for over 80 days, which was superior to the stability achieved using conducting graphene. Additionally, a sandwich structure using h-BN monolayer exhibited excellent SERS sensitivity with a detection limit for rhodamine 6G (R6G) as low as 10-12 M; to the best of our knowledge, this is the best SERS detection limit achieved using monolayer h-BN as a gap-control material. In this study, we suggest an efficient strategy for hybridizing the desired 2D layers with metal nanostructures for SERS applications, where the substrate stability and electromagnetic field enhancement are particularly crucial for the various applications that utilize metal/2D hybrid structures.

KEYWORDS plasmonic hybrid structure, hot-spot, surface-enhanced Raman scattering, hexagonal boron nitride, oxidative stability 2 ACS Paragon Plus Environment

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1. INTRODUCTION Plasmonic hybrid nanostructures fabricated by integrating novel plasmonic metal nanomaterials (e.g., Au, Ag, Cu, and Al) with two-dimensional (2D) materials (e.g., graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs)) have been extensively explored as multifunctional platforms for enhancing the performance for various applications, including plasmonic sensors,1 surface-enhanced Raman scattering (SERS),2,3 photocatalytic reactions,4 water splitting,5 optoelectronic devices,6,7 and biological sensors,8,9 due to their synergetic effects attributed to light-matter interactions and the combination of the inherent structural, optical, and electrical characteristics of each material. Among the above applications, the development of highly sensitive SERS substrates based on hybrid structures with 2D materials has been actively investigated,10-13 in which 2D materials can provide a large reactive surface area, enhanced adsorption capability for the target molecules, protection of the metals from undesired chemical reactions, and hot-spot formation with a sub-nanometer gap between metal nanostructures.14,15 Among the 2D materials available for SERS, graphene and its derivatives are ideal candidates since graphene possesses excellent properties, such as excellent molecular adsorption due to its π-π bond structure, good charge transport with the target molecule, and effective protection of metals.16-20 As a member of a different electrical class than graphene, h-BN has recently emerged as another candidate for SERS. h-BN is known to have excellent thermal stability against high temperatures up to 800 °C, unlike graphene.21 Thus, the removal of target molecules, which requires treatment at high temperatures above 350 °C, can be performed multiple times, thereby improving the reusability of SERS substrates consisting of expensive novel metals.22-24 Similar to graphene, h-BN can be used as an effective passivation layer to improve the stability of metals. However, studies for the direct comparison of the SERS performance of electrically different 2D 3 ACS Paragon Plus Environment

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layers are rare in the literature. Recently, Ling et al. compared Raman enhancement effect on graphene, h-BN, and molybdenum disulfide (MoS2), but exclusively for the effects of chemical interactions depending on the electronic properties of 2D layers without a metal.25 Therefore, for the practical usage of plasmonic hybrid SERS substrates, a comparative study is necessary on the SERS performance, such as the stability and sensitivity, of electrically different 2D layers hybridized with a metal. Meanwhile, it is well established that the SERS enhancement is attributed to an electromagnetic mechanism (EM) and a chemical mechanism (CM), but in general, for SERS substrates, enhancements above 1010 are achieved by the EM, suggesting that the EM is dominant in SERS.26 Recently, however, 2D materials such as graphene have been reported to greatly increase the CM due to their excellent adsorption capacity and charge transfer ability with the target molecules.25 In this regard, the CM enhancement factor, in addition to the unique capabilities mentioned above, must be thoroughly considered when choosing the best 2D materials for plasmonic hybrid SERS substrates. In this study, we fabricated SERS substrates consisting of metal NPs and a graphene or h-BN monolayer to compare how these monolayers with different electrical and chemical properties affect the metal nanoparticles (NPs) and the SERS performances from two perspectives. First, we compared the long-term stability and examined the degradation mechanism of the direct contact structure of 2D monolayer (h-BN or graphene)/Ag NP substrates in which two monolayers with different characteristics tightly covered the curved surface of the Ag NPs, which were immobilized on a substrate by a self-assembly method. We chose Ag NPs as a test element since they have poor stability against oxidation in air, which severely limits their practical applications despite their SERS enhancement being the highest among the noble metals. Surprisingly, many reports have 4 ACS Paragon Plus Environment

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continued to be published until recently on highly sensitive SERS designs based on Ag with no attention paid to oxidative stability.27,28 Here, we present scanning electron microscopy (SEM) observations of the temporal changes of single Ag NPs under different protection conditions and different absorbance and Raman spectral changes. Second, we demonstrated how the different electrical properties of the graphene and h-BN monolayers affected both the strongly amplified electromagnetic field and the SERS performance of Au NP/2D monolayer/Ag NP sandwiched hotspot substrates, in which the 2D monolayer formed a sub-nanometer gap between the stacked layers of Ag NPs and Au NPs. Atomically thin 2D layers are a groundbreaking material for SERS, which can provide uniform sub-nanometer gaps between metal NPs.13,14 However, there are still unexplored issues regarding the effects of 2D layers with different properties, such as graphene and h-BN, on the amplified electromagnetic field and the SERS performance when these layers are in direct contact with metal NPs. We will show below a quantitative comparison of the two substrates prepared by simple and inexpensive fabrication methods, and supporting numerical simulation results.

2. RESULTS AND DISCUSSION 2.1. Long-term structural and optical characteristics of the 2D layer/Ag NP substrates. Figure 1 shows a schematic illustration of the procedure for fabricating the 2D monolayer (h-BN or graphene)/Ag NP substrates and Au NP/2D monolayer/Ag NP sandwiched substrates in which the 2D monolayer of either h-BN or graphene completely covered the self-assembled Ag NP substrate; the latter sandwiched substrates are described below in section 2.3. The 2D monolayer was transferred onto the Ag NPs using a fast and stable dry-transfer method consisting of picking up and pressing the 2D monolayer with an elastomeric stamp of polydimethylsiloxane (PDMS). 5 ACS Paragon Plus Environment

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Rapid dry-transfer methods are crucial for protecting Ag NPs since their oxidation has been reported to begin within a few minutes of exposure to air.29 In addition, chemical and thermal damage to the Ag NPs was minimized by preventing direct contact between the Ag NPs and the solvents, resulting in excellent stability (see the section 4 for details of the experimental procedure).

Figure 1. Schematic illustration of the procedure for fabricating 2D monolayer (h-BN or graphene)/Ag NP substrates and Au NP/2D monolayer/Ag NP sandwiched substrates.

Figure 2a-i shows plan-view SEM images illustrating the morphological evolution of the asprepared Ag NPs, the monolayer graphene/Ag NP substrate, and the monolayer h-BN/Ag NP 6 ACS Paragon Plus Environment

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substrate upon aerobic exposure at room temperature. The Ag NPs shown in Figure 2a have a spherical shape with a smooth surface, and the average diameter is approximately 40 nm. Figures 2b and c show the Ag NPs protected by a monolayer of graphene and a monolayer of h-BN, respectively. The particle size distribution and density of the Ag NPs were nearly same among the three samples because all of the NPs were prepared in the same batch. In addition, we observed no damage to the Ag NPs induced by the dry transfer of the 2D monolayer, as confirmed by the lack of changes in the average size and morphology of the NPs. The SEM images also indicate that the atomically thin monolayers of both graphene and h-BN were much more flexible than a multilayer or a single-crystal structure, enabling sufficient deformation to fully cover each Ag NP, as shown in the inset of Figures 2b and c (scale bar: 50 nm), although some wrinkles were generated by the wrapping of the curved surface of the spherical Ag NPs. After 30 days of exposure to air, no noticeable changes in the Ag NPs covered by graphene and h-BN were observed, as shown in Figures 2e and f, respectively. The smooth and spherical morphology of the Ag NPs remained intact, and the size remained unchanged, revealing the stable protection against oxidation of the Ag NPs by graphene and h-BN. In stark contrast, severe morphological changes were observed for the bare Ag NPs after exposure to ambient conditions for 30 days (Figure 2d). The Ag NPs became larger as a result of aggregation through coalescence, and their morphology was remarkably distorted, forming an irregular and rough surface, possibly due to oxidation under long-term exposure to aerobic conditions. After 80 days of exposure, significant morphological differences among the substrates were observed. The morphological changes of the exposed Ag NPs shown in Figure 2g became more serious and rod-like structures appeared around Ag NPs, considered as newly formed Ag species as described in detail in Figure S1. In addition, a noticeable change was observed in the Ag NPs protected by graphene (Figure 2h). Severe degradation of the graphene 7 ACS Paragon Plus Environment

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was found, particularly at the contact interface with the Ag NPs (inset of Figure 2h), which could be attributed to extensive oxidative degradation of the graphene on the Ag NPs. We speculate that the degradation of graphene could have a significant impact on the stability of the underlying Ag NPs. A further study will address this issue (discussed below). For the Ag NP substrate covered with h-BN, both the morphology of the Ag NPs and the surface morphology of h-BN were maintained, even after 80 days of exposure, unlike the Ag NP substrate covered with graphene (Figure 2i). These results suggest that h-BN performs better than graphene in terms of ensuring the long-term stability of the Ag NPs despite exposure to the atmosphere, even without any heat treatment. The Raman spectra of the monolayer graphene/Ag NP substrate and monolayer hBN/Ag NP substrate with different surface morphologies were measured following prolonged exposure for 80 days. A detailed discussion of the spectra is provided in Figure S2. These results also agree well with the significant morphological differences observed in the SEM images provided in Figures 2h and i, in which the graphene in contact with the Ag NPs degraded, whereas the h-BN monolayer around the Ag NPs remained unchanged.

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Figure 2. (a-i) SEM images showing the morphological changes in Ag NPs with different protective layers at different exposure times under atmospheric conditions. The insets of (b) and (c) show enlarged views of an Ag NP covered by the graphene, and h-BN monolayer, respectively (scale bars: 50 nm). The insets of (g), (h), and (i) show enlarged views of the Ag NP, graphene/Ag NP, and h-BN/Ag NP substrates, respectively, after 80 days of exposure to air (scale bars: 50 nm).

The differences between the protective layers that formed over time also caused differences in the absorption spectra of the Ag NPs, as shown in Figures 3a and b. For reference, note that the bare Ag NP substrate showed drastic changes in spectral shape and intensity within 10 days (Figure S3), indicating that the Ag NPs were very unstable under aerobic conditions. The absorption intensities of the two hybrid substrates gradually decreased over time, but the Ag NPs wrapped 9 ACS Paragon Plus Environment

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with h-BN retained their spectral characteristics better than those wrapped with graphene, even after prolonged exposure. Figure 3c shows the variations in the full width at half maximum (FWHM) and absorption peak intensity of the absorbance spectra over time, from which the effects of graphene and h-BN on the Ag NPs could be clearly compared. The FWHM of the absorbance spectrum of the graphene/Ag NP substrate drastically broadened by approximately 52% after 45 days, while only a 23% broadening was observed for the h-BN sample after 80 days, as shown by the blue plots in Figure 3c. The variations in the absorption peak intensity presented in the red plots of Figure 3c also revealed that the h-BN-protected Ag NPs maintained more than 84% of their initial absorption intensity after 80 days, while only 68% of the initial intensity remained for the graphene-protected Ag NPs. The large differences in both the FWHM and the intensity of the inherent localized surface plasmon resonance (LSPR) spectra of the Ag NPs with different protective layers at different exposure times suggest that the graphene and h-BN monolayers had considerably different electrical, chemical, and optical effects on the protected Ag NPs. A possible mechanism for the degradation of the Ag NPs with different protective layers after long-term exposure to air is illustrated in Figure 3d. First, local fine defects in the 2D layers could be generated near the Ag NPs such as nanometer-scale wrinkles generated by the wrapping of the Ag NPs and local cracks caused by the transfer process, easily observed in various reports on 2D layers-covered nanostructures.30-32 Since these defects are physically weak and highly chemically reactive, some of the Ag NPs located below the defects are likely to react with oxygen species present in the atmosphere, which can cause partial ionization of the Ag NPs to Ag ions and electrons. At that time, large differences in electron diffusion, which depends on the electrical characteristics of the protective layer, may be observed and play a role in the oxidation of the Ag NPs and the degradation of the 2D layer. It is well known that graphene possesses a high electron 10 ACS Paragon Plus Environment

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mobility of approximately 2 × 105 cm2 V-1 s-1;33 thus, electrons can be rapidly transferred to the electrically conductive graphene surrounding the Ag NPs and move laterally along the graphene layer. The rapidly moving electrons in the graphene react with oxygen in the air, leading to the generation of reduced oxygen (O2-), which could react with Ag ions through the defects formed in the graphene monolayer, resulting in the formation of oxidized products, such as Ag2O. Thus, reduced oxygen might be a major factor that promotes the oxidation of the Ag NPs, and this process is assisted by the electrically conductive graphene. Since the size and morphology of the Ag NPs can change with oxidation, it is expected that the graphene monolayer that covers the Ag NPs will be torn or that the defect area will be gradually expanded. These changes were confirmed by the Raman spectra of the monolayer graphene/Ag NPs after 80 days of exposure to air (Figure S2). As a result of degradation, the graphene surrounding the Ag NPs no longer protected the Ag NPs, leaving them exposed to air. In contrast, h-BN is electrically insulating, with a poor electron mobility of approximately 2.5 × 102 cm2 V-1 s-1, unlike graphene;34 thus, the electrons generated by the same process are not easily transferred across the h-BN surface. Therefore, the oxidation rate of the Ag NPs protected by h-BN seems to be very slow relative to that of the Ag NPs protected by graphene because an environment in which reduced oxygen is produced is not present, or the process occurs very slowly. For this reason, h-BN is superior to graphene in terms of ensuring the long-term stability of Ag NPs.

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Figure 3. (a,b) Absorbance spectra of the self-assembled Ag NPs with protective monolayers of (a) graphene or (b) h-BN at different exposure times to air. (c) Variations in the FWHM and absorption peak intensity of the Ag NPs with different protective monolayers at different exposure times under atmospheric conditions. (d) Schematic diagram of the expected degradation mechanism of the Ag NPs after long-term exposure to air for 80 days for the different types of protective layers. 12 ACS Paragon Plus Environment

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In addition, we carried out x-ray photoelectron spectroscopy (XPS) to further determine the change of chemical states on the surface of Ag species by exposing under air for a long time. The XPS data of the as-prepared Ag NPs (Figure 4a) show two peaks observed at 368.18 eV and 374.28 eV, corresponding to the dominant Ag3d5/2 and Ag3d3/2 peak, respectively, and no additional peaks were observed. After 80 days, however, significantly different oxidation behaviors were observed on the three different Ag NPs substrates. For the bare Ag NP substrate, strong additional peaks appeared at 367.5 eV and 373.5 eV corresponding to the silver oxide peaks (e.g., AgO and Ag2O).35 The Ag NP substrate protected by graphene also shows the oxidation peaks, but the relative intensity ratio between Ag2O and Ag are much lower compared to the bare Ag NPs. The results suggest that the graphene acts as a great protective layer to prevent oxidation of Ag NPs only for a short term. In contrast, the Ag NP substrate protected by h-BN retains large amounts of metallic Ag bond, while oxide-related peaks such as AgO and Ag2O are present in much smaller amounts compared to the bare Ag NPs and the Ag NPs covered with graphene. In addition, Figure 4b shows O1s region of the XPS spectra for three different substrates. The O1s peak around at 532.5 eV for all spectra might be attributed to the Si-O bond from the SiO2/Si substrates. The fitting data for the as-prepared Ag NPs show an additional peak with weak intensity at 530.9 eV, which corresponds to the Ag-O bond such as AgO and Ag2O. Obviously, oxidation could start immediately after the Ag NPs are formed on the substrate. For the bare Ag NP substrate after 80 days, a new component peak with weak intensity observed at 529.2 eV is attributed to the Ag2O, which agrees well with the literature for the O1s binding energy of Ag2O.36 In addition, the stronger shoulder peak around at 530.68 eV can be attributed to the formation of Ag2CO3 as well as the further oxidation of Ag NPs into AgO and Ag2O. For the Ag NP substrate covered with graphene and h-BN, the oxidationrelated peak was much weaker after 80 days than that of the bare Ag substrate. These XPS results 13 ACS Paragon Plus Environment

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confirm that h-BN is an excellent material for the long-term protection of Ag NPs from oxidation, which is in good consistent with other experimental results.

Figure 4. XPS spectra of (a) Ag3d and (b) O1s peaks of the Ag NPs covered by different 2D layers after long-term exposure under the aerobic condition.

2.2. Long-term SERS analysis of the 2D layer/Ag NP plasmonic hybrid substrates. It is widely recognized that the oxidation of Ag NPs is primarily responsible for their weakened SERS sensitivity, which can be attributed to damping of the plasmon resonance. Therefore, to further investigate the effect of the interactions between the Ag NPs and the 2D layer on the long-term 14 ACS Paragon Plus Environment

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SERS characteristics, we obtained Raman measurements on three different substrates (bare Ag NPs, monolayer graphene/Ag NPs, and monolayer h-BN/Ag NPs) using rhodamine 6G (R6G), a representative dye molecule, as shown in Figure 5. The R6G dye was diluted in ethanol to a concentration of 10-6 M, and 2 μL of the diluted solution was used on each substrate for the SERS measurement. Representative Raman spectra measured over a period of 80 days were collected, as shown in Figure 5a-c. All the quintessential vibrational modes of R6G, including those at 614, 774, 1088, 1363, 1509, and 1648 cm-1, were clearly observed, and these modes were in good agreement with those reported in the literature.37 In the case of the bare Ag NPs with no protective layer (Figure 5a), the overall Raman intensity decreased rapidly with time, and the observable peak features related to the vibrational modes of R6G were noticeably reduced within a short time. For the Ag NP substrates protected by a monolayer of graphene or h-BN, as shown in Figures 5b and c, respectively, both the Raman intensity and the detectable Raman signal were clearly stable for more than 35 days. In particular, the fine features of the original Raman signals (namely, the intensity and bands) measured on the h-BN/Ag NP substrate were maintained after 80 days, whereas the graphene/Ag NP substrate exhibited a notably reduced Raman intensity after 45 days. To quantitatively and precisely compare the changes in the Raman intensity over time among the three substrates, the intensities of two representative Raman signals, 1363 and 1648 cm-1 corresponding to the aromatic C-C stretching mode of R6G, were extracted and compared, as shown in Figure 5d. The Raman intensity variation was calculated by the equation I/I0(%), where I0 is the Raman intensity of the as-prepared substrate and I is the Raman intensity over time for the selected peak position. The intensity variation of the each two peak for the three samples was averaged and marked with error bars, confirming that the Raman intensity deviation of the two peaks for the each substrate was within approximately 8%. For the bare Ag NPs, the Raman 15 ACS Paragon Plus Environment

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intensity decreased drastically over time, and less than 10% was observed after 35 days, which agrees well with the SEM images and the severe changes that occurred in the absorption peak. For the Ag NPs protected by either graphene or h-BN, however, the Raman intensities of the two peaks remained above 70% for 35 days. The intensities of the two Raman peaks centered at 1363 and 1648 cm-1 remained at 92% for the h-BN/Ag NP substrate, even after 45 days, while the intensities of these peaks on the graphene/Ag NP substrate decrease to 49% and 55%, respectively, after 45 days. Finally, the intensities of the two Raman signals of R6G on the graphene/Ag NP substrate decreased dramatically to 23%, whereas those of the signals of the h-BN/Ag NP substrate remained at approximately 79% (at 1363 cm-1) and 69% (at 1648 cm-1), even after 80 days. Although graphene has been known to contribute to the SERS enhancement by the CM due to its excellent adsorption ability and charge transfer ability with the target molecule, the contribution of the CM to the SERS characteristics is not substantial (Figure S4). In contrast, compared with graphene, hBN has a superior ability to stably protect the structural, electrical and optical properties of the Ag NPs, leading to stable EM characteristics, even though h-BN has a weaker CM enhancement than graphene (however, we also confirmed the adsorption capability of the molecules similar to graphene in h-BN, as described in Figure S5). These results suggest that h-BN is a much more effective protective layer than graphene for ensuring the long-term stability of the underlying Ag NPs against oxidation without any charge transfer phenomenon, which is in contrast to graphene; thus, h-BN results in excellent maintenance of the SERS characteristics for a long period. Nevertheless, slightly decreased Raman intensity for the h-BN/Ag NP substrate after 80 days indicates that oxidative degradation also occurs for the h-BN/Ag NP substrate though the speed is very slow, which can be sufficiently supported by the XPS results presented in Figure 4. We believe that the low electron mobility of the h-BN and the limitation of complete protection of the 16 ACS Paragon Plus Environment

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overall Ag NPs might provide some fine pathways in which the Ag NPs could come into contact with oxygen, which would eventually lead to slight oxidation.

Figure 5. (a) SERS spectra of a 10-6 M R6G solution on bare Ag NPs with increasing exposure time under air. (b,c) SERS spectra of a 10-6 M R6G solution on Ag NPs with a protective layer of (b) graphene or (c) h-BN with increasing exposure time under air. (d) Variations in the SERS intensities at 1363 cm-1 (solid line) and 1648 cm-1 (dotted line) with error bars according to the type of protective layer on the Ag NPs versus the exposure time under aerobic conditions. 17 ACS Paragon Plus Environment

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2.3. Ultrahigh SERS sensitivity of the Au NP/2D layer/Ag NP sandwich structures. The development of an effective method for the stable formation of a hot-spot structure capable of maximizing the strong electromagnetic field induced by the nanogaps between metallic NPs is very important for the use of metal/2D hybrid structure in SERS or LSPR biosensing applications requiring high sensitivity. To examine this issue, we fabricated Au NP/monolayer 2D (graphene or h-BN)/Ag NP sandwich structures by forming colloidal Au NPs on the monolayer 2D/Ag NP substrates using a spray deposition method as depicted in Figure 1. The detailed experimental methods are provided in the section 4. Recently, sandwiched SERS substrates with a graphene monolayer between annealing-formed Au NP layers38 and with a thiolated graphene oxide nanosheet between [email protected] core-shell NP layers11 have been proposed to generate a uniform distribution of hot-spots, since a sandwich structure with 2D materials naturally provide uniform nanometer-scale or sub-nanometer scale gaps between vertically stacked metal NPs, which is extremely challenging for either horizontally deposited NPs39,40 or patterned nanostructures fabricated by electron beam lithography (EBL).41 In these works, NPs were tightly packed to increase the number of hot-spots formed by coincidentally placed metal NPs on the both sides of the 2D layers. However, in a review by Zhao et al., their own works were found to lack reproducibility due to the out-of-order distribution of hot-spots.12 Therefore, in more recent work, they fabricated sandwiched SERS substrates of an Au NP/graphene monolayer/Ag nanohole array, in which the periodic nanohole arrays were fabricated by EBL to avoid random distribution of hot-spots.12 However, we believe that the key issue is not the random distribution of NPs, but a low probability of coincidence of metal NPs. We speculate that this is because highly dense NPs would support the flat 2D layers with a very small contact 18 ACS Paragon Plus Environment

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area on the 2D layers or even no contact if a slight space existed between the NPs and the 2D layers. Therefore, as a paradigm shift, we reduced the density of Ag NPs in the bottom layer by controlling the immobilization condition of Ag NPs (Figures S6 and S7), leading to the complete coverage of the Ag NPs with the 2D monolayers, as noted above. This design has the following three advantages: (i) uniform distribution of hot-spots across entire substrates since almost every Ag NP can contact Au NPs due to a contact area larger than the hemispherical surface of the Ag NPs on the 2D monolayers; (ii) a simple, easy, and inexpensive fabrication procedure that does not require expensive tools such as EBL, which is a critical point for practical applications; and (iii) the feasibility of quantitative comparisons between the graphene and h-BN monolayers because of uniform SERS signals across the entire substrates. In addition, our method does not use annealing treatment to generate hot-spots, which has been applied to h-BN-covered Au NP substrates42 but cannot be applied to graphene-covered metal NP substrates. A resultant SEM image of the hot-spot structure formed between Au NPs and Ag NPs completely covered with 2D monolayers is shown in the inset of Figure 6a. For a quantitative comparison of the two sandwich substrates with different interlayers, the individual SERS intensities in two representative vibration peaks, 1364 cm-1 and 1648 cm-1, of R6G with a concentration of 10-6 M were collected from the SERS spectra measured at five random spots on each sandwich SERS substrate with different types of 2D interlayers, as shown in Figure 6a. The results indicated that the average Raman intensity of the substrate using h-BN was approximately 1.58 times (at 1364 cm-1) and 1.53 times (1648 cm-1) stronger than that of the graphene substrate, and the relative standard deviation in the intensity for the two substrates with graphene and h-BN was approximately 7.96% and 9.14%, respectively. Those values are comparable to the value of 5.2% reported for Zhao et al.’s SERS structure, where the bottom Ag nanohole array was fabricated by EBL.12 Keeping in mind this excellent uniformity 19 ACS Paragon Plus Environment

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as well as the identical preparation of the Ag NP and Au NP layers of both substrates under the same conditions, the difference in the average Raman intensities between the two substrates is expected to be greatly influenced by the 2D monolayer located in the sub-nanometer gap between the Ag NPs and Au NPs. Since the graphene and h-BN monolayers located at the interface between the NP layers have different electrical properties, they could presumably be considered to contribute differently to the amplified electromagnetic field around the gaps between metal NPs. When a graphene layer is inserted between Ag NPs and Au NPs, the electromagnetic field is reduced because graphene is electrically conductive and therefore dissipates the electromagnetic near-field in and around the gap of the metal NPs, thus causing damping of the plasmon resonance. In contrast, the electrically insulating layer of h-BN does not affect the strong electromagnetic field induced around the gaps between the Ag NPs and Au NPs, resulting in the possibility of using this electromagnetic field for molecular detection without any electrical loss. This prediction is consistent with the results shown in Figure 6a, in which the substrate with h-BN showed a larger average SERS enhancement than that with graphene. Figures 6b and c show the results of the Raman measurement using various concentration of R6G deposited on the Au NP/graphene/Ag NP and Au NP/h-BN/Ag NP substrates, respectively, to verify the sensitivity of the plasmonic hotspot substrates mediated by the 2D monolayer. It was confirmed that the Raman intensities for both substrates gradually decreased in proportion to the decreasing number of R6G molecules. The inset of Figures 6b and c show the linear fitting curves for the Raman intensity of R6G located at 1648 cm-1 versus the concentration in an investigation of the capability of quantitative detection of the substrates for R6G molecules. It is clearly shown that reasonable linear responses with a coefficient of determination (R2) of 0.968 and 0.940 for R6G molecules on the plasmonic substrates with graphene and h-BN, respectively, were observed between the SERS intensity and 20 ACS Paragon Plus Environment

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the logarithm of the R6G concentration. As shown in Figures 6b and c, both plasmonic substrates with a graphene and an h-BN monolayer could detect a very low concentration (10-11 M) of R6G molecules, which can be attributed to the formation of a strong electromagnetic field by the monolayer-induced sub-nanometer gap between two metal NPs. The detection limit of the plasmonic structure with h-BN reached as low as 10-12 M, whereas the R6G molecule was no longer detectable on the substrate with graphene at the same concentration. The relatively strong peak observed at 1582 cm-1 corresponds to the G peak of graphene, not a vibration mode of the R6G molecule. These results demonstrate that the ultrasensitive Au NP/monolayer h-BN/Ag NP complex plasmonic structure with detection limit of 10-12 M is much more sensitive than the hBN/Au NP layer previously reported as the most sensitive SERS substrate (10-9 M) using an atomically thin h-BN layer to date.42 The detection limit of our sandwich substrate using monolayer h-BN is comparable to or even much better than that of the graphene-introduced sandwich SERS substrates reported by other groups, such as the Au NP/monolayer graphene/Au NP substrate (10-10 M)38 and the Au NP/monolayer graphene/Ag nanohole array (10-13 M).12 Representative studies on the detection limits of metal and 2D layer (e.g., graphene, graphene derivatives, and h-BN) hybrid SERS substrates are compared in Table S1.

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Figure 6. (a) Comparison of the Raman peak intensities at 1364 and 1648 cm-1 of 10-6 M R6G from five random spots on each sandwich substrate with different types of interlayers (inset: SEM image of hot-spot structure consisting of Au NP/2D monolayer/Ag NP. Scale bar: 1 μm). (b, c) 22 ACS Paragon Plus Environment

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SERS spectra of the Au NP/graphene/Ag NP (b) and Au NP/h-BN/Ag NP (c) substrates with various molecular concentrations of R6G from 10-6 M to 10-12 M. Insets of (b) and (c) show the linear fitting curves for the average Raman peak intensities at 1648 cm-1 according to the R6G concentration.

2.4. Numerical simulation study of the 2D layer effect on the electromagnetic field in plasmonic hybrid substrates. To investigate the effects of each 2D layer on the electromagnetic near-field in the sandwich structures, numerical simulations based on the finite-difference timedomain (FDTD) method were carried out. Figures 7a and b depict the calculated electric field distributions in the xz-plane for the Au NP/graphene/Ag NP and Au NP/h-BN/Ag NP substrates on SiO2/Si, in which the diameter of each metal NP and the thickness of each 2D layer were set to 40 nm and 1 nm, respectively. We clearly observed from the corresponding intensity profiles that the strongly localized electric field between two metal NPs was attributed to the greatly amplified plasmonic near-field interaction in the gaps defined by the 1 nm-thick graphene or h-BN. A comparison of the hot-spot structures with either a graphene or an h-BN layer showed a stronger electromagnetic field distribution at the hot-spot of the substrate with h-BN than at that of the substrate with graphene. Figures 7c and d show the distributions of the electric field intensity at position z = 20 nm in the xy-plane, which corresponds to the contact region between metal NPs with a nanogap defined by the 2D layer. The overall electric field intensity around the nanogap was undoubtedly stronger than that at other positions where no gap was formed. Moreover, the electric field intensity distribution in the nanogap region was remarkably different for the different 2D layers. That is, the strongest electric field was observed at the contact position for the substrate with h-BN, whereas the electric field was severely quenched at the same position for the substrate 23 ACS Paragon Plus Environment

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with graphene. The simulation results indicate that the different electrical characteristics of the 2D layers might induce significantly different near-field interactions between the Ag NPs and the Au NPs. The conductivity at the junction might be relatively high in the case of graphene; thus, electrons creating strong electric field confined in surface plasmonic polaritons can leak through graphene. This phenomenon can be assumed to lead to a reduction in the substantial plasmonic near-field interaction around the gap. Therefore, the local electric field around the gap defined by graphene is weaker than that defined by h-BN, which has less effect on the plasmonic interaction between metal NPs, resulting in a lower SERS sensitivity for the graphene substrate. The SERS enhancement factor contributed by the EM enrichment is approximately proportional to the fourth power of the local electric field enhancement, expressed by the relation |E|4/|E0|4, where |E| and |E0| are the local electric field intensity and the incident electric field intensity, respectively.20,38 Due to dipole-dipole plasmonic near-field coupling, a strong electric field enhancement around the gap between the two metal NPs was clearly observed. The maximum electric field for the Au NP/hBN/Ag NP structure was 106.5, which was approximately 2.7 times larger than that of the same structure with graphene (39.4); therefore, the theoretical SERS enhancement factors are 1.29 × 108 for the hot-spot substrate with h-BN and 2.41 × 106 for that of graphene, respectively. The same factor could be experimentally estimated by the following equation: SERS enhancement factor =

𝐼𝑆𝐸𝑅𝑆 × 𝑁𝑅𝑒𝑓 𝐼𝑅𝑒𝑓 × 𝑁𝑆𝐸𝑅𝑆

In this equation, ISERS and IRef represent the intensity of the SERS signal and the Raman intensity achieved from the reference substrate, respectively. NSERS and NRef correspond to the numbers of R6G molecules within the area of the laser spot on the SERS substrate and on the reference substrate, respectively. The estimated SERS enhancement factor obtained from the experimental data was approximately 9.35 × 107 for the hot-spot substrate with h-BN, which has about 50 times 24 ACS Paragon Plus Environment

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higher value than that for the substrate with graphene (1.92 × 106), well consistent with the theoretical values calculated above. The experimental and simulation results reveal that the sandwiched hot-spot structure defined by a monolayer of h-BN is superior to the graphenemediated structure as a SERS substrate with high sensitivity because h-BN maintains the strong electromagnetic near-field generated around the gap between the Ag NPs and the Au NPs.

Figure 7. (a, b) Normalized electric field intensity distributions in the xz-plane at 514 nm for sandwich structures with (a) graphene and (b) h-BN. (c, d) Simulated electric field distributions at z = 20 nm in the xy-plane for the two metal NPs with interlayers of (c) graphene and (d) h-BN.

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3. CONCLUSION In conclusion, we demonstrated a quantitative comparison of the effects of a monolayer of graphene and of h-BN, which have different electrical and chemical properties, on the stability and SERS performance of SERS substrates (2D monolayer/Ag NP and Au NP/2D monolayer/Ag NP), fabricated by a simple and inexpensive method, but exhibiting high reproducibility. When the graphene and h-BN monolayers with opposite electrical properties were in close contact with the metal NPs, they showed distinct differences in their interactions with the metal NPs depending on the structure of the SERS substrate. The two 2D layers had different effects on the electron transfer behavior, which plays a significant role in the long-term protection of Ag NPs with poor oxidative stability; the hybrid platform using the insulating h-BN monolayer ensured the long-term oxidative stability of the SERS substrate for over 80 days, which is superior to that achieved using conductive graphene. In addition, the 2D layers had different effects on the amplified electromagnetic fields around the hot-spots because they participated in the strong plasmonic nearfield interactions occurring at the two metal NPs separated by a sub-nanometer gap through the 2D monolayer. Some weakening of the near-field interactions between the two metal NPs in contact with graphene resulted in the loss of the amplified electromagnetic field, which was supported by finite element simulations. As a result, the prepared hybrid platform using the insulating h-BN monolayer exhibited excellent SERS sensitivity with a detection limit as low as 10-12 M; to our knowledge, this is the best SERS detection limit achieved using monolayer h-BN as a gap-control material to date. In this study, we suggest an efficient strategy for hybridizing the desired 2D layer with metal nanostructures for SERS applications in which the contribution of the EM is particularly crucial for the various applications that utilize metal/2D hybrid structures. We expect to overcome the limitations of conventional metal nanostructures and the detection limit of 26 ACS Paragon Plus Environment

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plasmonic SERS by further developing a highly efficient metal nanostructure array and designing an effective hybrid structure based on novel metals and 2D materials with fascinating characteristics.

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4. EXPERIMENTAL SECTION 4.1. Materials. Experimental monolayer graphene and h-BN grown by chemical vapor deposition (CVD) on SiO2/Si (p-doped) were purchased from Graphene Supermarket (USA). Hydrogen peroxide solution (30 wt% in H2O), sulfuric acid, acetone (anhydrous), dichloromethane (anhydrous, >99.8%),

poly(L-lactide) (viscosity ~2.0

dL g-1), and (3-aminopropyl)

trimethoxysilane (APTMS) were purchased from Sigma Aldrich (USA). Ethanol (absolute, HPLC grade) was purchased from Duksan (Korea). Silver nanospheres (40 nm) and gold nanospheres (40 nm) dispersed in water were purchased from nanoComposix (USA). Sylgard 184 silicone elastomer base and a curing agent were purchased from Dow Corning (USA). Rhodamine 6G (R590) was purchased from Exciton (USA). 4.2. Preparation of self-assembled Ag NP substrates. A cover slip or Si substrate was cleaned in a piranha solution (H2SO4:H2O2 = 3:1 v/v) at 65 °C for 30 min. The substrates were rinsed repeatedly with deionized (DI) water (18.2 MΩ) and absolute ethanol and then placed into a 2% APTMS solution in ethanol at room temperature for 2 h to modify the surface with amine functional groups. The substrates were subsequently rinsed with ethanol and dried with N2 gas. The APTMS-modified substrates were then heated at 120 °C on a hotplate for 3 h to immobilize the amine groups on the substrates. Finally, self-assembled Ag NPs were formed by immersing the substrates in dilute Ag NP colloids for 4 h and rinsing the substrates with DI water. 4.3. Preparation of 2D monolayer/Ag NP substrates. The transfer of 2D layers using a PDMS stamp was performed according to a published procedure with slight modifications.43 First, a 3 wt% solution of viscose poly(L-lactide) (viscosity ~ 2.0 dL g-1) in dichloromethane was prepared by vigorously stirring the mixture until the viscous polymer was completely dissolved. The solution, which was used as a carrier polymer, was spin-coated onto the monolayer h-BN (or 28 ACS Paragon Plus Environment

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graphene)/SiO2/Si substrate at 3000 rpm for 40 s, and then the sample was heated on a hotplate at 60 °C for 5 min to remove the residual solvent. The polymer at the edge of the substrate was partially removed to expose the SiO2 surface. A PDMS stamp was formed by blending a Sylgard 184 elastomer base and a curing agent at a 10:1 mass ratio and degassing the viscous mixture. The mixture was poured into a petri dish and degassed to form a uniform and flat PDMS stamp, which was subsequently heated in a laboratory oven at 60 °C for 1 h. The resultant 1- to 2- mm-thick PDMS stamp was appropriately cut to completely cover the carrier polymer/monolayer h-BN (or graphene)/SiO2/Si substrate. Next, 20 μL of DI water was dropped on the edge of the substrate to easily separate the polymer film from the SiO2/Si substrate. Then, the PDMS stamp with the carrier polymer/monolayer h-BN (or graphene) was carefully detached from the substrate and brought into contact with the as-prepared Ag NP substrate by vertically pressing the stamp on the substrate. The PDMS stamp with h-BN (or graphene) was prepared in advance before preparation of the selfassembled Ag NP substrate was completed. Then, the PDMS stamp was immediately applied to the Ag NP substrate to minimize the time that the formed Ag NP substrate was exposed to air. Subsequently, the PDMS stamp was slowly peeled off of the polymer/h-BN (or graphene) layer, which completely covered the Ag NPs. Finally, warm (60 °C) dichloromethane was dropped and spin-coated onto the carrier polymer/h-BN (or graphene)/Ag NP substrate at 1000 rpm for 90 s several times until the residual polymer was completely removed by the solvent, resulting in the successful coverage and protection of the self-assembled Ag NP substrate by monolayer h-BN (or graphene). 4.4. Preparation of Au NP/2D monolayer/Ag NP sandwiched substrates. To obtain a uniform distribution of colloidal Au NPs on the as-prepared 2D layer/Ag NP substrate, a spray deposition method using a 0.2 mm diameter spray nozzle (IWATA air-brush HP-SBP, Japan) 29 ACS Paragon Plus Environment

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connected to pressurized inert gas was applied. The 2D layer/Ag NP substrate was placed below the spray apparatus at a distance of approximately 15 cm, which varied depended on the size of the substrate. A 1 mL aliquot of colloidal Au NPs (40 nm in diameter) dispersed in water was injected into a feed cup. Tiny aerosol droplets of the Au NPs were sprayed from the nozzle onto the underside of the 2D layer/Ag NP substrate when pressurized N2 gas (0.05 MPa) was supplied to the nozzle. The increased area of the droplets allowed them to easily vaporize, leading to partial evaporation of the solvent prior to impact with the substrate. Thus, Au NPs could be uniformly distributed over the entire substrate area without causing any chemical or thermal damage to the substrate while stably maintaining the NP size and shape. 4.5. Characterization and instructions. SEM (Hitachi S-4700) was used to characterize the morphology of the Ag NPs, Au NPs, 2D layer/Ag NP structure, and Au NP/2D layer/Ag NP structure. UV-Vis absorption spectra were measured by using a Cary 5000 UV-Vis-NIR spectrophotometer (Varian). Raman spectroscopy and imaging were carried out by using an inVia Raman microscope system from Renishaw and WiRE Raman software. 4.6. Raman measurements. To detect the Raman signals of R6G molecules, R6G solutions with various concentrations ranging from 10-6 M to 10-12 M were prepared by diluting R6G in ethanol. A 2 μL droplet of an R6G solution with a specific concentration was dispersed onto each prepared Ag NP, 2D layer/Ag NP, and Au NP/2D layer/Ag NP substrate and dried under atmospheric conditions. Raman spectra were measured using a 514 nm laser with a 2.5 mW laser power and a 50× objective lens. All Raman spectra were acquired with a grating of 2400 lines per millimeter and an exposure time of 10 s. 4.7. FDTD simulation. In order to obtain the electric field distribution profiles, the simulation configuration consisted of 1 nm-thick graphene or h-BN sandwiched between a spherical Ag NP 30 ACS Paragon Plus Environment

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and an Au NP with a diameter of 40 nm. The 2D layer covered the surface of the Ag NP, and the Au NP was brought into contact with the side of the Ag NP wrapped with the 2D layer. The whole structure was placed on a SiO2 (200 nm)/Si substrate. The dielectric constants of the materials were taken as Johnson and Christy (for Ag and Au) and Palik (for Si and SiO2) from the software database. The optical properties of graphene and h-BN were obtained from the literature.44,45 The boundary conditions of the structures were investigated using a perfectly matched layer (PML) condition along the x, y, and z directions. A total-field scattered-field (TFSF) formulation was illuminated by an incident light source from the top side to the surface of the SERS substrate. The mesh sizes in the domains corresponding to the 2D layer and metal NPs were set to 0.1 nm along the x, y, and z directions. For the regions of the other objects, the mesh sizes were varied from 0.25 to 30 nm with an automatically graded nonuniform mesh. The mesh refinement option employed the Yu-Mittra formulation to ensure the numerical accuracy of the dielectric interface modeling.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. EDS analysis of the bare Ag NP substrate after exposure under air; Raman spectra of monolayer graphene and h-BN depending on the exposure times to air; absorption spectra of bare Ag NPs with different exposure times under atmospheric condition; Raman spectra of R6G molecules on graphene and h-BN monolayer surface; XPS C1s spectra of the Ag NPs covered by different 2D layers after long-term air exposure; SEM images of the immobilized Ag NPs with different density on SiO2/Si substrate; and a table of representative reports on the detection limits of metal and 2D layer hybrid SERS substrates.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (S. Y. Yim) Present Addresses † Department

of Materials Science and Engineering, University of Pennsylvania, Philadelphia,

PA 19104, USA Author Contributions N. Y. Kim, Y. C. Leem, S. H. Hong, J. H. Park, and S. Y. Yim all contributed to conducting the experiments and data analysis. N. Y. Kim and S. Y. Yim wrote the manuscript. Notes 32 ACS Paragon Plus Environment

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The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the GIST Research Institute (GRI) grant funded by the GIST and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03033425).

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