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Particle-on-Film Gap Plasmons on Antireflective ZnO Nanocone Arrays for Molecular-Level Surface-Enhanced Raman Scattering Sensors Youngoh Lee, Jiwon Lee, Tae Kyung Lee, Jonghwa Park, Minjung Ha, Sang Kyu Kwak, and Hyunhyub Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09947 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

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ACS Applied Materials & Interfaces

Particle-on-Film Gap Plasmons on Antireflective ZnO Nanocone Arrays for Molecular-Level Surface-Enhanced Raman Scattering Sensors Youngoh Lee†,#, Jiwon Lee†,#, Tae Kyung Lee†,#, Jonghwa Park†, Minjung Ha†, Sang Kyu Kwak*†, and Hyunhyub Ko*† †

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan

National Institute of Science and Technology (UNIST), Ulsan Metropolitan City, 689-798, Republic of Korea. *

Corresponding Author, E-mail: [email protected], [email protected]

ABSTRACT

When semiconducting nanostructures are combined with noble metals, the surface plasmons of the noble metals, in addition to the charge transfer interactions between the semiconductors and noble metals, can be utilized to provide strong surface plasmon effects. Here, we suggest a particle-film plasmonic system in conjunction with tapered ZnO nanowire arrays for ultrasensitive SERS chemical sensors. In this design, the gap plasmons between the metal nanoparticles and the metal films provide significantly improved surface-enhanced Raman spectroscopy (SERS) effects compared to those of interparticle surface plasmons. Furthermore, 3D tapered metal nanostructures with particle-film plasmonic systems enable efficient light trapping and waveguiding effects. To study the effects of various morphologies of ZnO nanostructures on the light trapping and thus the SERS enhancements, we compare the performance of three different ZnO morphologies: ZnO nanocones (NCs), nanonails (NNs), and nanorods (NRs). Finally, we demonstrates that our SERS chemical sensors enable 1

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molecular level of detection capability of benzenethiol (100 zeptomole), rhodamine 6G (10 attomole), and adenine (10 attomole) molecules. This work presents a new design platform based on the 3D antireflective metal/semiconductor heterojunction nanostructures, which will play a critical role in the study of plasmonics and SERS chemical sensors. KEYWORDS: Surface-Enhanced Raman Scattering, gap plasmons, tapered waveguide, ZnO nanocones, anti-reflective INTRODUCTION Surface-enhanced Raman scattering (SERS) has been extensively investigated for chemical and biological sensing applications owing to its high sensitivity, rapid response, and fingerprint effect.1-3 It is now well-known that nanometer-scale gaps, tips, and corners in metal nanostructures provide strong electric-field enhancements when exposed to light, which significantly enhances the Raman scattering of molecules adsorbed onto metal nanostructures.4,5 For the fabrication of nanometer-scale gaps, tips, and corners in metal nanostructures, a wide variety of approaches have been suggested, including nanoparticle dimers,6 core-satellite structures,7,8 nanoparticle assemblies,9,10 and nanoparticle over films1113

for the formation of nanoscale gaps, along with metal films over nanospheres,14 nanostars,15

dendritic structures,16 bi-pyramids,17 and flower-like particles18 for the nanoscale tips and corners. However, these two-dimensional (2D) SERS substrates without the microscale photonic structures have limitations regarding the management of light absorption, reflection, and propagation, which can be synergistically combined with plasmonic nanostructures to provide maximal SERS enhancements. Recently, various semiconducting materials, such as ZnO,19 TiO2,20 GaN,21 and CuO22 have 2


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been demonstrated to provide SERS enhancements arising from the charge transfer effects between the analytes and semiconductors. When these semiconductors are combined with noble metals, the surface plasmons of the noble metals, in addition to the charge transfer interactions between the semiconductors and noble metals, can be utilized to provide strong surface plasmon effects for applications in photocatalysts and SERS sensors.23-26 Among the various semiconductors, ZnO with a wide band gap (~3.37 eV) has exhibited great promise in SERS applications.27,28 It has been reported that ZnO nanocrystals exhibited an SERS enhancement factor of ~103 even without any contribution of nearby noble metals.19 In addition to their large SERS effects, the facile synthesis at low cost, easy control of the nanostructure morphology, and uniform growth into three-dimensional (3D) geometries make ZnO nanomaterials a promising candidate for the fabrication of 3D SERS substrates. The 3D heterostructures of ZnO nanostructures and noble metals have several advantages for SERS substrates. First, the controlled growth of 3D ZnO nanostructures provides a large surface area for the formation of SERS active sites, which are critical for the enhancement of SERS effects.22 Secondly, the light absorbance is increased owing to the metal nanostructures, which act as efficient antennas.29,30 Here, the excitation of surface plasmons in the metal nanostructures are expected to enhance the optical absorption of incident photons in the ZnO nanostructures.31 Finally, one-dimensional ZnO nanostructures with smooth sidewalls and a higher refractive index provide efficient optical waveguide effects32 that enable the multiple internal reflection of propagating light and thus enhance the SERS signals.33,34 Previously, various forms of heterostructures, such as ZnO nanowire arrays decorated with Ag films27 and cone-shaped ZnO nanorods (NRs) coated with Ag nanoparticles (NPs),35 have been suggested for 3D SERS sensors. Although previous 3D SERS sensors based on ZnO heterostructures 3



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have utilized some of the advantages mentioned above, there have been few attempts to fabricate 3D ZnO nanostructures with an efficient light trapping ability and low reflectance. Furthermore, only randomly deposited metal nanoparticles have been utilized, although there are several plasmonic systems with controlled light management and maximal SERS enhancements. In this paper, we report the fabrication of vertically aligned and ultrasharp ZnO nanocone arrays hybridized with particle-on-film plasmonic systems for ultrasensitive SERS substrates. In our design of the SERS substrates, the vertically aligned ultrasharp ZnO nanocone arrays enable efficient light trapping and provide the tapered structure for the fabrication of tapered plasmonic waveguide after coatings with Ag film and AuNPs. Particle-on-film plasmonic systems, which have been known to provide large electric field enhancements, are fabricated on top of the ZnO nanocone arrays based on Au NPs on Ag films. To study the effects of various morphologies of ZnO nanostructures on the SERS enhancements, we compare the performance of three different ZnO morphologies: ZnO nanocones (NCs), nanonails (NNs), and nanorods (NRs). Finally, we show that the optimally designed SERS sensors based on ZnO nanocone heterostructures can detect target molecules at even zeptomole levels. EXPERIMENTAL SECTION Vertical ZnO nanostructure arrays. For the fabrication of seed layers for the growth of vertical ZnO nanostructure arrays, silicon substrates were cleaned in a 2-propanol solution by brief sonication and coated with 10-nm thick Au layers by e-beam evaporation (FC-2000 ebeam evaporator). For the growth of the ZnO nanostructures, an alumina boat was loaded with ZnO powders (Alfa Aesar, 99.9%) and placed at the center of a quartz tube (80 mm in diameter) in the chemical vapor deposition (CVD) furnace. Here, the amount of ZnO powder was 1g for 4


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the ZnO NCs and NRs and 2g for the ZnO NNs. The silicon substrates with seed layers were placed next to the alumina boat (2 cm apart) with locations in the downstream of the Ar gas flowing direction. A constant Ar gas flow (40 sccm) was fed for 30 min at a tube pressure of 300 mTorr. The quartz tube was heated up to 550 C at a rate of 70 C/min and maintained at this temperature for 30 min, with 10 sccm of O2 gas flow for the reaction. The pressure during the reaction was maintained at 7000 mTorr for the synthesis of the ZnO NRs and 4000 mTorr for the synthesis of the ZnO NCs and NNs. After the growth, the furnace was naturally cooled down to ambient temperature. The e-beam evaporator (FC-2000, Temescal, USA) was used to deposit 30-nm Ag films on the ZnO nanosrtructures. For the deposition of the AuNPs on the ZnO/Ag nanostructures, the substrates were coated with a poly(diallyldimethylammonium chloride) (PDDA) layer by spin coating with a 0.2% PDDA solution. Subsequently, the substrate was immersed into a AuNP solution (18-nm AuNP diameter). The AuNP solution was prepared using sodium citrate as a reducing agent in an aqueous solution.36,37 Characterization. The morphology and crystal structure of the ZnO nanostructures were characterized using field-emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi) and an X-ray diffractometer (D8 Advance, Bruker). The SPR spectra of the Au nanoparticles were analyzed using UV-vis spectrophotometry (Cary 5000, Agilent, USA). For the SERS analysis, different solution of target molecules are prepared. The SERS substrate was immersed in a ethanol solution of benzenethiol (Aldrich) for 15 min and aqueous solutions of rhodamine 6G (Aldrich) and adenine (Aldrich) for 2 h, and rinsed with its solvent, and dried with nitrogen gas. The Raman spectra from the SERS substrate were collected using confocal Raman spectroscopy (Alpha300, WITec) with a 785-nm (benzenethiol, adnine, laser power: 15 mW) and 532-nm excitation laser (rhodamine 6G, laser power: 1.5 mW). All of the Raman spectra 5



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were collected using a 20 objective lens (numerical aperture = 0.4) and an integration time of 1 s with 5 times accumulations. RESULTS AND DISCUSSION

To investigate the morphological effects of 3D ZnO nanostructures on the SERS enhancements, morphology-controlled ZnO nanostructures are vertically grown on Si substrates using a chemical vapor deposition (CVD) process. Figure 1 shows a schematic illustration of the growth of three different morphologies of ZnO nanostructures: ZnO NCs, NNs, and NRs. The growth mechanism of the ZnO nanostructures can be explained by a combination of vapor-liquid-solid (VLS) and vapor-solid (VS) mechanisms. For the growth of different morphologies of ZnO nanostructures, textured ZnO films are first formed by the VLS mechanism (Figure 1a). In the VLS growth step, alloyed Au-Zn liquids are first formed by the adsoprtion of Zn atoms into Au catalysts, followed by the formation of a ZnO nucleus upon the precipitaion and oxidation of Zn. Then, the ZnO nucleus grows into a textured ZnO seed film by the successive deposition of Zn vapor.38-40 The textured ZnO film provides nucleation sites for the growth of ZnO nanostructures by a VS growth mechanism. Figure 1b shows the VS mechanism for the growth of ZnO NCs, NNs, and NRs. In this VS growth step, the Au catalysts are encapsulated by the growing ZnO crystals and isolated from the Zn vapor and O2 gas.41 Figure S1 shows cross-sectional SEM images of ZnO NCs, NNs, and NRs grown on the textured ZnO seed layers with a layer thickness of 35 μm, which support the VLS and VS mechanisms for the growth of ZnO nanostructures on the textured ZnO films. The morphology-controlled formation of ZnO nanostructures during the VS growth step can be explained by the anisotropic, face-specific growth of the wurtzite crystal.42 The crystal 6


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structure of ZnO is characterized by alternating sub-lattices of O-2 and Zn2+ ions, where each ion is tetrahedrally coordinated. In this structure, oppositely charged planes are stacked along the c-axis, resulting in a noncentrosymmetric structure with polar surfaces.43 In a ZnO crystal, each crystal face has different surface activities, resulting in different growth rates in different directions; the growth rate of the ZnO crystal is on the order of [0001] > [10-1-1] > [10-10] > [10-11] > [000-1] directions.44 For vertical ZnO nanostructures, the relative growth rate between the axial growth along [0001] and the lateral growth along [10-10] critically determine the final nanostructure morphology. When the axial growth rate is far faster than the lateral growth rate, rod-shaped ZnO nanostructures (ZnO NRs) are produced. However, when the axial growth rate is not sufficiently fast compared with the lateral growth rate, the diameter of the bottom region of the ZnO nanostructures keeps increasing, giving rise to cone-shaped ZnO nanostructures (ZnO NCs).45 When sufficient Zn source remains even after the formation of the ZnO NCs, further growth of ZnO occurs on top of the ZnO NCs because the crystal face of the top region of the ZnO NC has a higher surface free energy than the other face of the NCs.46 This further growth leads to the formation of nail-shaped ZnO nanostructures (ZnO NNs). In this work, the growth rate in the axial and radial directions of the vertical ZnO nanostructures is controlled by the partial pressure of the reactants. During the growth step, the partial pressure of the reactants affects the growth rate in the axial direction. Therefore, at a high partial pressure of Zn and oxygen, the axial growth rate is faster than the radial growth rate, resulting in the formation of ZnO NRs. At a low partial pressure of Zn and oxygen, the limited axial growth rate compared with the lateral growth rate results in the formation of coneshaped ZnO NCs.47 The cone-shaped ZnO NCs can be further grown into ZnO NNs with excess amounts of Zn, which can be controlled by increasing the amount of the Zn powder source. It 7



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is known that excess ZnOx vapor can be deposited on the top of the ZnO NCs owing to the high surface free energy.46 The additional growth of ZnO crystals on top of the ZnO NCs leads to the formation of a hexagonal cap of ZnO nanostructures, where the diameter decreases from the top to the bottom of the hexagonal cap.48,49 The growth of the hexagonal cap on top of the ZnO NCs can be confirmed by investigating the change in the ZnO nanostructure morphologies with respect to the growth time. Figure S2 shows that the ZnO NCs grow until the growth time of 20 min, and then the additional growth time (25 min) results in the formation of the hexagonal cap of the ZnO NNs. Figures 2a–c show scanning electron microscopy (SEM) images of three different morphologies (nanocones, nanonails, nanorods) of ZnO nanostructures. As shown in Figures 2a and S1a, the diameter of NCs at the bottom is ~700 nm, which gradually reduces to ~40 nm at the top ends. Similar to the ZnO NCs, the ZnO NNs exhibit a gradual reduction in the diameter from the bottom to the top region, except for the hexagonal cap structure (150–400 nm in diameter) at the top region, resulting in a nail-like morphology (Figures 2b and S1b). In contrast to the ZnO NCs and NNs, Figures 2c and S1c show the ZnO NRs, where the diameters are uniform (~400 nm) from the bottom to the top region. Figures 2d–f show X-ray diffraction (XRD) patterns for the ZnO NCs, NNs, and NRs. In all the XRD patterns, the (002) diffraction peak is dominant, indicating the preferential growth of the ZnO nanostructures in the c-axis direction.50 This c-axis growth orientation is typically observed in the wurtzite hexagonal phase of ZnO nanostructures. In addition, all the XRD patterns contain a Si (211) peak at 33.3° and an Au (111) peak at 38.0°. While the XRD pattern of the ZnO NRs contains characteristic (100), (002), and (101) peaks of ZnO, ZnO NCs and ZnO NNs contain the (002) peak dominantly, indicating different morphologies of NRs. 8


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The tip morphologies of ZnO nanostructures strongly affect the antireflective properties. Figure 3a shows the optical reflectance spectra of ZnO NCs, NRs, and NNs. ZnO NCs exhibit a significantly lower reflectance than ZnO NNs and NRs, which is attributed to the gradual increase in the effective refractive index from the top to the bottom regions of NC arrays due to the tapered geometry (Figure S3).51-53 For ZnO NN arrays, the effective refractive index gradually decreases from the flat cap structures at the top region to the end of the inverted nanocones (Figure S3b), which results in a significantly higher reflectance compared with ZnO NCs and NRs. Previously, the enhanced reflectance of inverted nanocone structures has been reported for inverted silicon nanocone arrays.54 When ZnO nanostructures were coated with Ag films, all the ZnO/Ag nanostructures exhibited reflectance minima spectra within the wavelength range of 400–600 nm, which is attributed to the enhanced absorption of incoming light by the excitation of the surface plasmon resonances of metallic nanostructures (Figure 3b). In particular, ZnO/Ag NCs exhibit a reflectance of 2.3% at 550 nm, which is 3 and 13 times lower than those of NRs (6.7%) and NNs (29.4%), respectively. These different antireflective properties, along with the surface plasmon excitations, strongly affect the SERS enhancements of ZnO/Ag nanostructures. We also investigated the quality of Ag film on the surface of ZnO NCs, NNs, and NRs by using cross-sectional SEM analysis (Figure S4). The Ag films are evenly deposited along the surface of ZnO NCs, NNs and NRs from the top to bottom areas. We also observed that the Ag film on ZnO NCs is smoother than those on NNs and NRs, which may affect the SERS enhancement degree. To investigate the morphology-dependent SERS enhancements of ZnO/Ag nanostructures, benzenethiol molecules were used as Raman markers for the comparison of SERS spectra (Figure 3c). For all the morphologies (NC, NR, NN) of ZnO/Ag nanostructures, the Raman 9



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spectra of benzenethiol are clearly observable at 998, 1021, 1071, and 1572 cm1 for the Raman modes of the in-plain ring-breathing mode (998 cm-1), in-plain C-H bending mode (1021 cm1

), in-plane ring-breathing mode coupled with the C-S stretching mode (1071 cm-1), and C-C

stretching mode (1572 cm-1), respectively.55 In particular, the ZnO/Ag NCs exhibited the strongest Raman intensity compared with the other morphologies (nail, rod) owing to the unique tapered nanostructures of the NCs.56-58 This tapered structure provides tapered plasmonic waveguide modes that cause enhanced E-fields near the tip region. In addition, the tapered nanostructures provide an antireflective property and more spaces for the analyte molecules to be easily adsorbed on the surface of NCs due to the open spaces of NCs. We also note that the nail morphology provides a lower Raman intensity than the rod morphology. This indicates that the higher reflectance of the inverted tapered nanostructure for the ZnO NNs critically affects the SERS enhancements. To further elucidate the morphology-dependent E-field distributions and thus the SERS enhancements, we calculated E-fields of ZnO/Ag nanostructures with various morphologies by the discrete dipole approximation (DDA) method (detailed descriptions in Supporting Information S2). Figure S5a-b represents the simulated E-field distribution normalized to the incident E-field for ZnO NCs, NNs, and NRs with 30 nm Ag films. The DDA simulation result indicates that there are strong E-fields along the metal-air interfaces for all the morphologies. The average E-field intensity profile according to ZnO height (Figure S5c-d) indicates that ZnO NCs exhibit the larger E-field than those of NNs and NRs. In addition, we also observe that the E-field intensity increases from the bottom to top area for the ZnO NC structure. The morphology-dependent SERS enhancements are also clearly visible in the Raman mapping images. As shown in Figures 3d-f, the ZnO NC structures exhibit many bright yellow 10


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regions, indicating the strong SERS enhancements. Compared with the ZnO NC and NR structures, ZnO NNs exhibit few bright areas, indicating the lower SERS enhancements. In addition to the morphology, the length of the ZnO nanostructures also affects the reflectance and SERS performance. Figure S6 shows the reflectance and SERS spectra of ZnO NC arrays with different lengths (3, 10, and 15 μm). The ZnO NCs with 10 m in length exhibit the lowest reflectance (Figure S6a) and the highest SERS intensity (Figure S6b). The higher performance of the 10-m-long ZnO NCs compared with the 15-m ones can be attributed to the more vertical geometry of the shorter ZnO NCs. The vertically aligned ZnO/Ag NC arrays can be hybridized with particle-on-film plasmonic systems for further enhancement of the SERS effects (Figure 4). In this study, the particle-onfilm plasmonic systems are based on an Ag film decorated with Au nanoparticles. Here, Au NPs can function as nanoantennae and thereby enable the coupling of incoming light into surface plasmon polaritons (SPPs) in the Ag film.59-61 We employed Ag film because Ag is known to have a lower optical loss and thus provide a longer SPP propagation length compared with Au,62 which can lead to stronger particle-film and interparticle plasmon couplings, yielding larger SERS effects.11 In addition, the ZnO nanocone coated with Ag film behaves as a tapered plasmonic waveguide, in which excited SPPs propagate toward the tip and induce giant local E-fields there.63,64 Figure 4a indicates that the Au NPs induce the coupling of the incoming light into the SPPs in the Ag film, which propagate toward and accumulate at the tip of the metal-coated NCs, resulting in the generation of localized surface plasmons (LSPs) and thus the giant local E-fields at the tip. In addition, the propagating SPPs in the Ag film couple with the LSPs of the Au NPs, which induce large E-fields (gap plasmons) at the particle-film gap. When the analytes are attached on these hot spots (tip ends, particle-film gaps) with large 11



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E-fields, their Raman spectra are significantly enhanced by the SERS phenomena. Figure S7 shows the fabrication of ZnO nanocone arrays hybridized with particle-on-film plasmonic systems. In the fabrication process, Ag films are first evaporated on the ZnO NC arrays. On top of the Ag films, we coated PDDA adhesion layers for the deposition of Au NPs through electrostatic interactions between the positively charged amine groups in PDDA and negatively charged Au NPs during the dip-coating process. Notably, the PDDA adhesion layer is only ~1 nm thick and the loosely entangled polymer chains of PDDA layer allows the adsorption of analyte molecules near or between the particle-film gaps,11 maintaining strong particle-film plasmon couplings. Figures 4b-d show SEM images of the ZnO NCs (Figure 4b), ZnO NCs with 30-nm Ag film (ZnO/Ag NCs, Figure 4c), and ZnO/Ag NCs decorated with Au NPs (ZnO/Ag/AuNP NCs, Figure 4d). The optical reflectance spectra of the ZnO/Ag NCs exhibit reflectance minima at ~420 nm, which is due to the localized surface plasmon resonance (LSPR) peak of the ZnO/Ag NCs (Figure 4e and S8). The reflectance increased when the PDDA was coated on the ZnO/Ag NCs, owing to the change in the refractive index. The reflectance decreased again when the Au NPs were coated on the ZnO/Ag/PDDA NCs because they acted as nanoantennae, collecting the incident light and converting it into SPP modes in the Ag film.60,61 We observed that the small deep peak at ~545 nm, which is attributed to the LSPR peak of the Au, is generated when the Au NPs are coated (Figure S8). Figure 4f shows the Raman spectra of benzenethiol adsorbed on ZnO/Ag/AuNP and ZnO/Ag NC substrates. In particular, the ZnO/Ag/AuNP NCs exhibit Raman intensities ~2 times higher than those of the ZnO/Ag NCs, indicating the critical role of the AuNPs in the enhancement of the SERS effects by the light coupling into the SPPs and the particle-film plasmon couplings. The significantly higher SERS enhancement of the ZnO/Ag/AuNP NCs compared with that of 12


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the ZnO/Ag NCs is also observable by comparing the Raman mapping images. While the Raman image of the ZnO/Ag NCs exhibits bright yellow regions in some parts of the scanned Raman image (Figure 4g), the ZnO/Ag/AuNP NCs exhibits bright yellow regions in most of the scanned image (Figure 4h), indicating the existence of SERS hot spots in almost every location of the substrates. To theoretically evaluate the light coupling into SPPs via nanoparticle antenna and the resulting enhancement of SERS effect after AuNP coating, DDA method was employed for the calculation of E-fields on ZnO/Ag/PDDA NCs and ZnO/Ag/PDDA/AuNP NCs under 532- and 785-nm excitation lasers (Figures 4i-j and S9a-b). Under excitation of 532- and 785-nm lasers, intense E-fields are present near the AuNPs on the Ag films. In addition, we observed the enhancement of E-fields near the tip area of ZnO/Ag/PDDA/AuNP NCs when AuNPs are located near the tip area (Figures 4k-l and S9cd). The propagating SPPs are affected by the metal film thickness, which influences the reflectance and the tip diameter of the ZnO nanocones. Figure S10a indicates that the Raman peak intensity decreases as the Ag film thickness increases from 30 to 200 nm. We noted that the reflectivity of the ZnO-NC/Ag film increases as the film thickness increases (Figures S10bc), which resulted in a decrease in the SERS spectra owing to the reflection loss of the incoming light. The SEM images in Figures S10d-f indicate that the tip diameter also increases with an increase in the Ag film thickness. The larger tip diameter with the increase in the Ag film thickness leads to a decreased local E-field intensity at the tip area.63 ZnO/Ag/AuNP nanocone arrays with strong E-field enhancements at the sharp tips and particle-film gaps can be utilized for the trace-level detection of analytes. Figure 5a shows the typical Raman spectra of benzenethiol (BT), adenine, and rhodamine 6G (R6G) molecules (10 13



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μM solution) absorbed on ZnO/Ag/AuNP nanocone arrays. Here, we used 785 nm laser excitation for the detection of BT and adenine molecules because the laser with longer excitation wavelength results in the longer penetration depth,65 which can induce an enhanced scattering with analyte molecules attached on 3D SERS substrates. For the detection of rhodamine 6G, 532 nm laser excitation was used because the resonance Raman scattering can induce large SERS enhancements. Rhodamine 6G is known to have a strong absorption peak around 530 nm,66 which can induce resonance Raman scatterings with the 532 nm laser excitation. For these analytes and laser excitations, high SERS enhancement factors with about 1010 – 1011 were achieved (detailed descriptions in Supporting Information S1). Figures 5b-d show the Raman spectra of these molecules when trace-level amounts of molecules are absorbed on the SERS substrates. Here, the individual Raman spectra for each concentration of target molecules were obtained by averaging the Raman spectra from ten different locations on the SERS substrates. For the Raman spectra of BT molecules in Figure 3b, the Raman fingerprint peaks of 998, 1021, and 1071 cm-1 (red arrows) was clearly observable down to 100 zeptomole (zM) of BT solution. The trace-level detection of benzenethiol molecules (1 aM and 100 zM concentrations) is difficult because the probability of finding molecules within the laser spot area decreases with the concentration, which requires a statistical analysis for the tracelevel detection. In our measurements, among ten different locations on the same SERS substrate, the SERS signals were observed for 8 locations for 1 aM benzenethiol and 3 locations for 100 zM benzenethiol (Figure S11). The statistical detection probability depending on the measurement locations can be also observed in Raman mapping images (Figure S12). For 1 aM benzenethiol, many parts of the scanned image show bright yellow regions, indicating the presence of benzenethiol molecules. However, for 100 zM benzenethiol, only several parts of the image show bright yellow regions, indicating the lower detection probability. For R6G 14


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molecules (Figure 5c), the Raman peaks at 611, 773, 1182, 1360, 1509, and 1647 cm-1 agree well with the literature data and can be identifiable at the concentration down to 10 aM.66 For adenine molecule, which is a DNA base with a great importance in clinical diagnosis, the Raman spectra show the characteristic purine-ring breathing mode at 736 cm-1 and stretching modes of CN bonds at around 1337 cm-1 at the concentrations down to 10 aM.67 Figure S13 shows the variation of SERS intensities with the wide range of analyte concentrations. At high concentration above 10-11 M, the SERS intensity vs. concentration shows a linear relationship. However, at low concentrations under 10-11 M, the SERS intensities remain relatively constant. The lowest detectable level of BT (100 zM) is 1 order of magnitude better than the lowest detection limit of BT molecules (1 aM) reported in prior literatures, demonstrating the ultrasensitive detection capability of the SERS sensors based on ZnO/Ag/AuNP nanocones arrays.11 In addition, the lowest detection limit of R6G (10 aM) and adenine (10 aM) is also comparable with detection limit of R6G (10 aM) and adenine (1 fM) reported in prior literatures.67,68 CONCLUSIONS

We demonstrated highly sensitive SERS substrates with molecular-level (~100 zeptomole) detection capabilities based on the 3D design of ultrasharp ZnO/Ag/AuNP nanocone arrays. In this design, we achieved strong SERS effects by simultaneously utilizing the light trapping properties of nanocone arrays with a graded refractive index, tapered plasmonic waveguide effects of Ag-coated nanocone arrays to induce giant local E-field at the nanocone tips, and particle-film gap plasmons between AuNPs and Ag films. The novelty of our design is the combination and simultaneous utilization of all of the three aforementioned light management designs (nanocone, tapered waveguide, particle-film gap plasmons), which have been studied 15



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separately, to realize ultrasensitive SERS substrates. We showed that the tip morphology of the ZnO nanostructures (nanocone, nanonail, nanorod) critically affect the light absorption and surface plasmon excitation, resulting in different SERS effects. Finally, we showed that the multiple field enhancements in ZnO/Ag/AuNP nanocone arrays can be utilized for the molecular-level detection (100 zeptomole) of target molecules. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected], [email protected]

Author Contributions #

These authors contributed equally.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program (2015M3A6A5065314) and by the National Research Foundation of Korea (NRF-2011-0014965, NRF-2012-K1A3A1A20031618) of the Ministry of Science,

16


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ICT & Future Planning, Korea. S.K.K. acknowledges the financial support from NRF2013R1A1A2007491 and computational resources from UNIST-HPC and KISTI-PLSI.

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Figure captions Figure 1. Schematic of growth mechanism of ZnO NC, NN, and NR. (a) Vapor-liquid-solid 24


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(VLS) mechanism for formation of textured ZnO films. (b) Vapor-solid (VS) mechanism for growth of ZnO NCs, NNs, and NRs.

Figure 2. Characteristics of ZnO NCs, NNs, and NRs. (a–c) Tilted-SEM images of ZnO NCs, NNs, and NRs. Inset images are high-resolution SEM images showing shape of tip. (d–f) XRD patterns of ZnO NCs, NNs, and NRs indicating characteristic patterns of ZnO nanostructure.

Figure 3. Comparison of reflectance and SERS performance among different morphologies of ZnO nanostructures (NC, NN, and NR). (a) Reflectance spectra of different ZnO nanostructures. (b) Reflectance spectra of different ZnO nanostructures with 30-nm Ag film. (c) Raman spectra of BT (10 mM) adsorbed on the different ZnO nanostructures with 30-nm Ag films. (d–f) Raman mapping images of BT (10 mM) adsorbed on ZnO/Ag NCs, ZnO/Ag NNs, and ZnO/Ag NRs.

Figure 4. Ultrasharp ZnO nanocone arrays hybridized with particle-on-film plasmonic systems. (a) Schematic showing detection principle of fabricated SERS sensor: localized E-field at the tip and gap plasmons at the particle-film gap. (b–d) SEM images of ZnO NCs, ZnO/Ag/ NCs, and ZnO/Ag/AuNP NCs. (e) Reflectance spectra of different substrate structures (ZnO/Ag NCs, ZnO/Ag/PDDA NCs, and ZnO/Ag/PDDA/AuNP NCs). (f) Raman spectra of BT (10 mM) adsorbed on the different sensor structures (ZnO/Ag/AuNP NCs and ZnO/Ag NCs). (g, h) Raman mapping images of BT (10 mM) adsorbed on ZnO/Ag NCs and ZnO/Ag/AuNP NCs. (i-l) DDA simulation data under 532-nm excitation laser. (i-j) Normalized electric field 25



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distribution for the ZnO/Ag/PDDA NCs and ZnO/Ag/PDDA/AuNP NCs. (k) 3D electric field contour plot for for the ZnO/Ag/PDDA NCs and ZnO/Ag/PDDA/AuNP NCs. (l) Calculated averaged electric field intensity for the tip part (501-800 nm) of ZnO/Ag/PDDA NCs and ZnO/Ag/PDDA/AuNP NCs along the height. Directions of incident light k and polarized electric field E were indicated with white and red arrows, respectively, and two directions of E rotated by 90o were independently applied at a fixed direction of k.

Figure 5. Detection of trace level molecules adsorbed on ZnO/Ag/AuNP nanocone arrays. (a) Raman spectra of 10uM BT, R6G, adenine adsorbed on SERS sensor. High-resolution Raman spectra for different molecules, (b) BT, (c) R6G, (d) adenine.

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Figure 1

(a) Vapor-Liquid-Solid (VLS)

ZnO Nucleation

Au-Zn alloy

Annealing Au film

(b) Vapor-Solid (VS) Slow axial growth

Fast axial growth

ZnO nanorod (NR)

Excess Zn source

ZnO nanonail (NN)

27


Slow axial growth

ZnO nanocone (NC)


Figure 2 (b)

(c)

500 nm

500 nm

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40 50 2(degree)

60

60

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30

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40 50 2(degree)

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(002) 30

40

(102)

(101)

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(100)

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(f) Intensity (a. u)

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ZnO NNs Au (111)

(102)

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40

(002)

(e)

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2 m Si (211)

Si (211)

(d)

(002)

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40 50 2(degree)

(110)

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60

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Figure 3 (b) ZnO

20

Reflectance (%)

25

Cone Nail Rod

15 10 5 0 200

400

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(d)

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(c) Ag/ZnO Cone Nail Rod

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(f)

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50000 CCD cts

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0 CCD cts


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Figure 4

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Figure 5

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