Quasi Optical Cavity of Hierarchical ZnO Nanosheets@Ag

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Cite This: J. Phys. Chem. Lett. 2019, 10, 3676−3680

Quasi Optical Cavity of Hierarchical ZnO Nanosheets@Ag Nanoravines with Synergy of Near- and Far-Field Effects for in Situ Raman Detection Jing Yu,†,‡,¶ Yu Guo,‡,¶ Huijie Wang,§ Shuai Su,∥ Chao Zhang,*,‡ Baoyuan Man,‡ and Fengcai Lei*,†

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†

College of Chemistry, Chemical Engineering and Materials Science, Institute of Biomedical Sciences, Shandong Normal University, Jinan 250014, P.R. China ‡ School of Physics and Electronics, Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, P.R. China § School of Physics and Information Engineering, Shanxi Normal University, Linfen 041004, P.R. China ∥ College of Animal Science and Technology, Shandong Agricultural University, Taian 271018, P.R. China S Supporting Information *

ABSTRACT: The vertically interlaced hierarchical structure (HS) of ZnO nanosheets (NSs)@Ag nanoravines (NRs) as a quasi optical cavity (QOC) for Raman enhancement has been studied experimentally and theoretically in this work. A novel synergism of nearand far-field effects of Ag NRs is facilitated by the multiple oscillation of light inside the ZnO QOC, providing wide distributions of “hot spots” in a large space. The “spatial hot spots” in the HS bring reliable signal collection in in situ Raman detection. Without any specific materials and methods adopted, this HS provides researchers a new way to adjust the light in the fields of Raman enhancement.

he surface plasmonic “hot spot” has generally been considered as one of the most effective means to achieve enhanced Raman spectroscopy (ERS) over the past decades because of its high sensitivity and nonspecificity.1,2 In a narrow sense, the hot spot originates from the near-field effect of the localized surface plasmon resonance (LSPR) by a nanostructured noble metal,3 which can bind the incident electric field around the metal nanoparticle (NP) ca. 5−10 nm and increase the intensity of this localized electric field (LEF) by tens to hundreds of times.1,3 In addition, when numerous metal NPs are close to each other, resonance coupling will occur between the LEFs, further enhancing the intensity of the coupled electric fields.1−3 Therefore, most of the recent research on ERS focuses on designing and preparing special micro−nano metal configurations to achieve more effective near-field effects,4,5 in other words, to obtain a stronger surface plasmonic hot spot. Far-field scattering is another effect caused by LSPR, which can make the incident light scatter in an angular spread around the metal NPs, subsequently resulting in an increase of the optical path length.6,7 Generally, the intensity of the LEFs caused by far-field scattering is far weaker than that by the near-field effect; thus, the far-field effect has rarely been adopted in ERS. However, benefiting from the wide spatial distribution of the enhanced electric field, the farfield scattering still shows brilliant application prospects in

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© 2019 American Chemical Society

many areas (e.g., solar cells, photocatalysis, light-emitting diodes, and photodetectors).8−12 The low utilization of far-field scattering in Raman detection is closely related to the traditional sample preparation process, in which dropping or immersing methods are most commonly used.13,14 In these processes, signal collection is generally operated after the volatilization of the solution, while the probe molecules have completely covered the surface of the substrate. In such cases, to achieve stronger Raman signals, one only needs to concentrate the electric field near the substrate surface as much as possible. Undoubtedly, in contrast with the far-field scattering, the near-field effect accompanied by the high-intensitive LEFs is a better choice. However, not all the Raman detections are based on such process, e.g., in situ Raman detection.15−17 Compared to those in traditional Raman tests, the substrates in in situ Raman detection are usually attached to solid surfaces or placed in liquids and gases.15−17 Under such circumstances, a large part of the probe molecules are dispersed in the entire space instead of covering the surface of the substrate. Thus, to further improve the Raman performance, it is imperative to enhance the signals of Received: May 15, 2019 Accepted: June 15, 2019 Published: June 16, 2019 3676

DOI: 10.1021/acs.jpclett.9b01390 J. Phys. Chem. Lett. 2019, 10, 3676−3680

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The Journal of Physical Chemistry Letters

Ag NRs can further utilize the advantages of far-field scattering without changing any other characteristics of the high-intensity LEFs by the near-field effect. Enhanced hot spots are obtained in a large space, including the surface (localized hot spots, LHSs) and upper space (spatial hot spots, SHSs) near the heterostructure, leading to improvement of the in situ Raman detectability of the substrate. Although some similar works about SHSs have been reported recently, e.g., broadband single-molecule Raman detection by warped optical spaces18 and Raman detection of a virus molecule of adenovirus type 5 by hollow nanocones at the bottom of microbowls,19 the novel and effective combination of the far-field scattering and the near-field effect in ERS is demonstrated for first time in this work. This combination has great importance as a method to further utilize the far-field scattering as an optical modulation method in Raman enhancement. To obtain the HS, vertical ZnO NSs were first prepared on a flexible polydimethylsiloxane (PDMS) film by a facile diluent hydrolytic method at room temperature.20 Subsequently, Ag NRs were covered on these ZnO NSs through a thermal evaporation method (preparation details can be found in the Supporting Information). In the optical image in Figure 1d1, the substrate is shown to be khaki, translucent, and highly flexible (ZnO NSs@Ag NRs, ZNS@ANR). The absorption spectra (Figure S1) show that the LSPR peak of the substrate is located at 506 nm with the full width at half-maximum of 118 nm (from 446 to 564 nm), corresponding well to the incident light (532 nm) of the Raman spectrometer used in subsequent experiments. The X-ray diffraction (XRD) test proves that the ZnO NSs@Ag NRs possess high purity and crystallinity, with clear diffraction peaks corresponding well with the hexagonal phase of ZnO (JCPDS No. 80-0075) and the cubic phase of Ag (JCPDS No. 87-0717), respectively (Figure 1c). The vertical ZnO NSs surround each other and form a structure similar to the whispering wall (as marked in Figure 1b1), also termed QOC in this Letter. As shown in Figure 1b, the average size of them is between 500 and 700

the probe molecules in a large space instead of near the surface of the substrate. Inspired by this idea, a Raman substrate based on a hierarchical structure (HS) of three-dimensional ZnO nanosheets (NSs)@Ag nanoravines (NRs) is put forward in this work. As shown in Figure 1a, quasi optical cavities (QOCs) are

Figure 1. (a) Schematic of the synergy of surface- and spatialenhanced Raman scattering by ZnO NSs@Ag NRs; (b and b1) SEM images of the ZnO NSs@Ag NRs under different scales; (c) XRD patterns of the ZnO seed layer, ZnO NSs, and ZnO NSs@Ag NRs; and (d) TEM image and (d1) photo of the ZnO NSs@Ag NRs.

formed by the vertically interlaced ZnO NSs. With the assistance of multiple oscillations of light between these NSs,

Figure 2. (a) In situ detection of R6G (10−8 to 10−12 M) by vertical ZnO NSs@Ag NRs; (b) in situ detection of R6G by different substrates; and electric field distributions in (c) ZnO seed layer, (d) ZnO NSs, and (e) ZnO NSs@Ag NRs; (e1 and e2) detailed views as indicated (1 and 2) in panel e. 3677

DOI: 10.1021/acs.jpclett.9b01390 J. Phys. Chem. Lett. 2019, 10, 3676−3680

Letter

The Journal of Physical Chemistry Letters

Figure 3. (a) In situ detection of R6G (10−8 to 10−12 M) by exfoliated ZnO NSs@Ag NRs; (b) intensity comparisons of peak 613 cm−1 (R6G) detected by vertical and exfoliated ZnO NSs@Ag NRs; and (c and d) electric field distributions in vertical and exfoliated ZnO NSs@Ag NRs, respectively.

concentration (10−2 M) in comparison with the ZnO seed layer. Figure 2e shows the electric field distribution in ZnO NSs@ Ag NRs, in which the Ag NRs are composed of a number of adjacent nanoblocks (NBs) of 5 nm thickness, 30 nm length and width, spaced 2 nm apart. In this structure, the enhanced electric field can be clearly observed in the whole intra cavity, and the intensity is also remarkable, with the average value being ca. 5.2-times higher than the incident one. We define this electric field as the spatial hot spot (SHS) in this Letter, as distinguished from the traditional localized hot spot (LHS). It can be also found that the intensities of electric fields in the PDMS and the air upon ZnO NSs@Ag NRs decrease obviously in contrast with those in the same areas of the ZnO seed layer or ZnO NSs. These declined electric fields are precisely the sources of the SHSs, which is consistent with the law of energy conservation as well. In view of the wide space of the electric field distribution, the SHSs can be brought about only by the far-field scattering from Ag NBs. On one hand, the scattering light by far-field effect have longer optical path length, and on the other hand, the metal-wrapped QOC can reflect light more effectively than a ZnO NS QOC because of the high reflectivity of metal. These two factors make the incident light be continuously reflected in the QOC and finally be concentrated inside the cavity. The details of the LEFs in Ag NRs (marked as 1 and 2 in Figure 2e) are further presented as panels e1 and e2 in Figure 2, in which the max intensity of the electric field in these gaps is ca. 11-times higher than the incident one, demonstrating that the LHS is available as well in the ZnO NSs@Ag NRs. The synergy of far-field scattering and near-field effect brings an intensity increase of the electric field in the whole area (SHSs and LHSs), which is the main reason for the improved Raman signals of R6G in ZnO NSs@Ag NRs. To further illustrate this conclusion, R6G (10−8 to 10−12 M) was also in situ detected by the exfoliated ZnO NSs@Ag NRs

nm. Gyrus-like Ag NPs are attached on the surfaces of ZnO NSs with an average size between 20 and 40 nm, which are closely adjacent to each other and form plenty of NRs (Figure 1b1,d). When this substrate is immersed into the solution of rhodamine 6G (R6G), the in situ Raman signal collection can be achieved under a wide concentration range from 10−8 to 10−12 M (Figure 2a). By contrast, the PDMS film and the planar ZnO seed layer barely have any Raman enhancement to R6G (Figure 2b). The vertical ZnO NSs can achieve weak detection of R6G at the concentration of 10−2 M (Figure 2b), which can be ascribed to the multiple reflections of incident light in the QOC, similar to the conclusion in Yoon’s report.21 To expound the reasons behind the above phenomena, the finite-difference time-domain (FDTD) simulations were implemented to explore the electric field distributions of the three structures (planar ZnO seed layer, vertical ZnO NSs, and vertical ZnO NSs@Ag NRs; full details can be found in the Supporting Information). First, as shown in Figure 2c, for the planar ZnO seed layer (thickness of 30 nm), the electric field (λ = 532 nm) distributes uniformly in the air layer and the intensity is not enhanced. Because the incident light (λ = 532 nm) irradiates perpendicularly onto the ZnO surface in this simulation, the uniform electric field distribution is mainly caused by the specular reflection of the ZnO seed layer. In subsequent simulations, the vertical ZnO NSs are built with a thickness of 30 nm, length and width of 600 nm, and angle with the z-axis of 15° on the ZnO seed layer (Figure 2d), and four of them wrap around each other to form a QOC (inset in Figure 2a). In this structure, the incident light will be multi reflected between these NSs, similar to the “whispering-gallery mode” in a waveguide. As shown in Figure 2d, the electric field tends to distribute inside the cavity, and the max intensity is ca. 1.8-times higher than the incident one. Therefore, the vertical ZnO NSs are more sensitive to the R6G with low 3678

DOI: 10.1021/acs.jpclett.9b01390 J. Phys. Chem. Lett. 2019, 10, 3676−3680

Letter

The Journal of Physical Chemistry Letters in this work (Figure 3a), put as a disordered heap (DH) in the solution (inset in Figure 3a). The signals collected in this structure are weaker than those measured by vertical ZnO NSs@Ag NRs (QOC), and as the concentration of R6G decreases gradually, the ratio of IQOC and IDH, where the I is the Raman intensity, first increases and then remains essentially unchanged (as shown in Figures 3b and S2). Figure 3c (same as in Figure 2e) and Figure 3d show the simulated electric distributions in ZnO NSs@Ag NRs (QOC) and ZnO NSs@Ag NRs (DH), respectively. In Figure 3d, only the LHSs can be observed around the ZnO NSs@Ag NRs (DH), and no field enhancement is found in the area upon them. The max intensity of the LHSs is ca. 12.5-times higher than the incident one, which is basically the same as that in Figure 2e1,e2. However, in Figure 3c, except for the LHSs near ZnO NSs@ Ag NRs, the strong SHSs can also be found inside the whole cavity, demonstrating that the existence of SHSs is the biggest difference between the two structures. Considering all the analysis presented above, it is reasonable to believe that the signal enhancement in vertical ZnO NSs@Ag NRs is not only caused by the LHSs but also brought about by the SHSs. The enhancement factor (EF) of this hierarchical structure is ca. 3.83 × 109 (full details can be found in the Supporting Information), which is higher than many reported results achieved by hierarchical ZnO and Ag (as shown in Table 1).

Figure 4. (a) In situ detection of MG (10−5 to 10−8 M) by inverted ZnO NSs@Ag NRs, (b) electric field distribution in inverted ZnO NSs@Ag NRs, (c) environment of the in situ detection of MG, and (d) illustration of QOC1 and QOC2 in inverted ZnO NSs@Ag NRs.

QOC1 is easier to be reflected inside the cavity compared with that in QOC2, leading to the stronger electric field in QOC1. This phenomenon is inconspicuous in Figures 2e and 3c, because the bottom ZnO seed layer@Ag NRs can reflect the light backward in that case, which reduces the escaped probability of light in the QOC. However, in Figure 4b, this layer is at the top of the QOC. Moreover, the electric field in the inverted QOC of ZnO NSs@Ag NRs is weaker than that in the normal one, and this is mainly caused by the backward reflection of the top ZnO. Furthermore, the inverted substrate was used to in situ detect the Raman signals of malachite green (MG, a common abusive germicide in aquaculture). Figure 4a shows that the substrate exhibits high sensitivity for MG with concentration of 10−5 to 10−8 M, indicating the promising application prospects of this substrate. Figure 4c and the inset show the environment of this in situ detection. In conclusion, QOCs with hierarchical structure of ZnO NSs@Ag NRs as the Raman substrate are studied on the basis of both experiments and numerical simulations in this Letter. A novel synergism of near- and far-field effects by this structure is proved to bring wide electric distributions in the whole space, including the LHSs and SHSs. These results show promising application prospects in in situ detection and also pave a new way for researchers to adjust light in Raman fields.

Table 1. Comparison of Raman Performance between ZnO NSs@Ag NRs and Other Reported Substrates substrate ZnO NSs@Ag NRs (this Letter) ZnO nanotaper@Ag nanoplate22 ZnO nanorod@Ag nanoparticle23 worm-like ZnO@Ag24 urchin-like ZnO@Ag25 Si/ZnO nanocomb@Ag nanoparticle26

LOD for R6G

EF

in situ detection

10−12 M 10−14 M

3.83 × 109 8.30 × 107

good none

10−13 M

3.58 × 107

none

10−10 M 10−10 M 10−12 M

3.03 × 107 1.00 × 108 1.00 × 109

none none none

The conventional Raman detections further prove that this hierarchical structure is highly uniform, stable, and repeatable (Figures S3 and S4), and the in situ detections of crystal violet (CV, 10−7 to 10−11 M) and Sudan I (10−6 to 10−10 M) indicate that the sensitivity of this hierarchical structure is universal for other probe molecules as well (Figure S5), demonstrating the fantastic Raman performance of it. An interesting phenomenon is that, because of the outstanding photocatalytic properties of ZnO and Ag,22,23 this structure can in situ degrade the molecules remaining on the substrate under ultraviolet (UV) light (Figure S6), which endows the substrate with selfcleaning ability, further increasing its practicality (full details can be found in the Supporting Information). In practical applications, it is more convenient to reverse the substrate on the liquid surface (inset in Figure 4a) for molecular signal collection than to immerse the substrate in the liquid (inset in Figure 2a). Figure 4b shows the electric field distribution in inverted ZnO NSs@Ag NRs. It should be noted that there are two kinds of QOCs in this structure, marked as 1 (QOC1) and 2 (QOC2) in Figure 4b. For QOC1, the QOC shrinks gradually along the propagation direction of the wave vector (K), while for QOC2, the opposite is true (exhibited as the right illustration in Figure 4d). Thus, light in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01390. Details regarding experiments and numerical simulations; discussion of conventional Raman performance tests, UV−vis absorption spectra, and cyclic Raman detection (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.L.). *E-mail: [email protected] (C.Z.). ORCID

Jing Yu: 0000-0003-2289-0748 3679

DOI: 10.1021/acs.jpclett.9b01390 J. Phys. Chem. Lett. 2019, 10, 3676−3680

Letter

The Journal of Physical Chemistry Letters

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Chao Zhang: 0000-0002-3295-8980 Fengcai Lei: 0000-0002-6207-7164 Author Contributions ¶

J.Y. and Y.G. contributed equally to this Letter. This work was completed through contributions of all authors: F.L. and J.Y. designed and directed the whole research; J.Y., Y.G., and F.L. wrote and modified the manuscript; J.Y., Y.G., and H.W. prepared the ZnO NSs@Ag NRs; B.M., J.Y., Y.G., F.L., S.S., and C.Z. contributed to the analysis of the results (SEM, Raman, FDTD simulations, etc.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank the Natural Science Foundation of Shandong Province (Nos. ZR2019YQ09 and ZR2017BA018), National Natural Science Foundation of China (Nos. 21701102 and 21802092), Hong Kong Scholars Program (XJ2018025), Shandong Provincial Science and Technology Project (No. J17KZ002), and China Postdoctoral Science Foundation (Nos. 2017M612322, 2017T100511, and 2016M600550) for financial support.

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DOI: 10.1021/acs.jpclett.9b01390 J. Phys. Chem. Lett. 2019, 10, 3676−3680