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Synergetic SERS Enhancement in Metal-like/Metal Double Shell Structure for Sensitive and Stable Application Rongcheng Ban, Yingjian Yu, Meng Zhang, Jun Yin, Binbin Xu, DeYin Wu, Min Wu, Zhigang Zhang, Huiling Tai, Jing Li, and Junyong Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15396 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Synergetic SERS Enhancement in Metal-like/Metal Double Shell Structure for Sensitive and Stable Application Rongcheng Ban1, Yingjian Yu1, Meng Zhang2, Jun Yin*1, Binbin Xu2, De-Yin Wu2, Min Wu3, Zhigang Zhang3, Huiling Tai4, Jing Li*1,4, Junyong Kang1 1

Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, PenTung Sah Institute of Micro-Nano Science and Technology/Department of Physics, Xiamen University, Xiamen, Fujian, 361005, China.

2

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. 3

4

Xiamen Entry-Exit Inspection and Quarantine Bureau, Xiamen, 361026, China.

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China.

ACS Paragon Plus Environment

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ABSTRACT Due to either thermal/chemical instability or high optical loss in noble metal nanostructures, searching for alternative plasmonic materials is becoming more and more urgent, considering the practical bio-sensing applications under various extreme conditions. In this work, titanium nitride (TiN), a low-loss metal-like material with both excellent thermal and chemical stability, was proposed to composite with Ag hollow nanosphere (HNS) nanostructures as an effective surface-enhanced Raman scattering (SERS) substrate to realize both high sensitive and stable molecular detecting. Due to the multiple-mode local surface plasmon resonance (LSPR) around the spherical composite nanospheres and the ‘gap-effect’ derived from the ultra-small nanogaps within the precisely controlled plasmonic arrays, an intensively enhanced local field was successfully induced on this SERS substrate. Combined with the unique charge transferring process between Ag and TiN, a synergistically enhanced SERS sensitivity both involving physical and chemical mechanisms was achieved. Especially, with the isolation of TiN, a timedurable Raman detection on this TiN-Ag HNS arrays was accomplished, showing great potential for practical applications.

KEYWORDS SERS stability, TiN, multiple plasmon resonance, gap-effect, charge transfer

INTRODUCTION Surface plasmon resonance (SPR) effect has been intensively researched for the last several decades due to its wide applications in optical and biomedical regimes1, such as plasmonic optical tweezers,2 solar cells,3 thermal therapy,4 DNA detection,5 bio-sensors6 and so on. The SPR refers to the collective oscillating of free electrons around the nucleus in subwavelength when light incident on metallic nanostructures, which would induce either the surface plasmon


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polaritons (SPPs) or localized surface plasmons (LSPs).7-8 Meanwhile, various specific enhanced near-field distributions around these nanostructures would be generated by the oscillation in different modes which are well accepted as the main enhancement mechanism for surfaceenhanced Raman scattering (SERS) in bio-sensing. 9 It is well acknowledged that the plasmonic properties are greatly influenced by the choice of metallic materials.10 Noble metals, such as Ag and Au, are well accepted as the ideal plasmonic materials,11-14 especially Ag, which shows active plasmonic properties with high sensitivity in SERS detection.15 However, the unsatisfied chemical and thermal stability or high optical loss in Ag limited its effective applications in SERS sensing or other fields10. For example, deteriorated SERS performance and repeatability issue were often found with the sulfuration or oxidation on Ag nanostructures. Kinds of strategies have been implemented to improve the stability of Ag, such as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS),16-17 non-contact testing by tip-enhanced Raman spectroscopy (TERS)18 and so on.

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Besides the using of

ultrathin dielectric shell layer for metal isolation, searching for alternative plasmonic materials also has been paid much attention with the aim to realize good bio-compatibility, well chemical/thermal stability and low optical loss for better SERS performance. 10, 20 Titanium nitride (TiN), as one of metal nitrides with good chemical stability, high melting point, super hardness and metal-like optical properties, has been considered as an important alternative low-loss plasmonic material.21 Compared to the conventional noble metamaterials, its negative permittivity in real part and intermediate carrier concentration make TiN to be an attractive plasmonic material with ultra-low optical loss.22 Series of researches have been performed to investigate the unique plasmonic properties in TiN nanostructures, such as the


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hyperbolic metamaterials,23 perfect absorbers,24 light trapping25 and so on. However, the actual application of nitride based plasmonic material has rarely been carried out. In this work, TiN, as a specially functionalized dielectric coating layer, was introduced to be composited with Ag plasmonic nanostructures, which is proposed to synergistically enhance the stability and sensitivity for SERS applications. The TiN-Ag double shell hollow nanospheres (HNS) arrays were technically fabricated by sputtering the ultra-thin (down to ~ 5 nm) TiN film on the prepared Ag HNS arrays. Obviously enhanced SERS signals were accomplished on this TiN-Ag composite structure compared to only Ag-based substrate. Theoretical simulation by finite difference time domain (FDTD) method indicated that the multiple-mode LSPRs around the TiN-Ag shells and the ‘gap-effect’ induced inter-coupling within the precisely controlled plasmonic nanogaps are mainly responsible for the local near-field enhancement towards an applicable SERS detection. Furthermore, the feasible charge transfer from Ag to TiN also ensures the adequate charge density and strong local near-field distribution, which is believed to further veritably improve the SERS sensitivity. Meanwhile, due to the good thermal and chemical stability of TiN, a time-durable SERS performance with the well-maintained Raman signals lasting over 1 month was realized in the ambient air.

EXPERIMENTAL SECTION Fabrication process of TiN-Ag double shell HNS arrays. As illustrated in Figure 1, the TiN-Ag double shell HNS arrays were fabricated by template method using the polystyrene (PS) nanospheres followed by the radio frequency (RF) magnetron sputtering deposition and solution treatment in tetrahydrofuran (THF), as referring to our previous works.26 Briefly, monolayer PS nanospheres in the size of ~ 500 nm were spin-coated on cleaned silicon or sapphire substrates


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and then adjusted in size and separation by O2 plasma etching. Subsequently, Ag film in the thickness of ~ 40 nm was sputtering deposited on the PS template followed by the removal of PS nanospheres with immersing in THF solution, resulting in the Ag HNS arrays.26 Then a thin layer of TiN was coated on the Ag HNSs by the magnetron sputtering (JS3X-100B) system. By employing the power of 240 W and the pressure of 1 Pa with the Ar2/N2 ratio of 40:1, the TiN thin films in the thickness of ~ 5 nm and ~ 10 nm can be well deposited on Ag HNSs within the duration of 1 and 2 minutes, respectively. The samples were correspondingly denoted as TiN5nm-Ag and TiN-10nm-Ag. Characterization. The morphology and structure properties of the composites were studied by a field emission scanning electron microscopy (SEM, Hitachi S-4800) and high resolution transmission electron microscopy (HRTEM, JEM-2100), which both equipped with energy dispersive X-ray (EDX) spectrum analyzer. Extinction spectra were collected by a UV-Vis-NIR spectrophotometer (Varian, UV-Vis-NIR Cary 5000). Simulated extinction spectra and near-field distribution were calculated by a commercial finite difference time domain (FDTD) simulation package (FDTD Solutions, Lumerical Solutions Inc.). The Raman spectra were recorded by XploRA Raman Spectroscopy (HORIBA J Y) using a 532-nm laser excitation source with the power of 1% (0.2 mW, 5 s). Rhodamine 6G (R6G) as an analysis probe in SERS detection with the concentration adjusted from 10-4 to 10-8 M in deionized (DI) water, and 4-aminothiophenol (PATP) was used to verify the charge transfer mechanism with the concentration of 10-4 M. SERS substrates were prepared by directly dropping trace amounts of probe solution on the sample and then dried in air. SERS-substrate stability was evaluated by Raman intensity for the samples stored in ambient environment (25~28 °C, RH: 40~45%) for different days. Each sample was measured using the R6G probe with the concentration of 10-4 M.


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RESULTS AND DISCUSSION As displayed in Figure S1a (Supporting Information, SI), large area of self-assembled monolayer PS template was successfully prepared. To ensure the effective formation of the ideal hollow shell structure, the diameter of PS nanospheres (the original dimeter is 500 nm) was reduced to be about 440 nm with the gaps around 60 nm by O2 plasma etching, as seen in the Figure S1b. After the deposition of Ag film and PS nanospheres removal, Ag HNS arrays were produced with the average diameter of about 470 nm (Figure 2a). The spherical hollow structure in Ag HNSs with the thickness of around 40 nm can be well observed in the inverted image in the inset. The cross-sectional image was displayed in Figure 2b. As demonstrated by the corresponding EDS pattern in the inset (Figure 2c), the ultra-thin TiN film has been successfully deposited on the rough Ag HNSs with the top surface becoming smoother, because of its low surface energies.23 Also, the double shell hollow structure can be well visualized in the inverted structure (Figure 2d and f) with a thin coating layer (TiN) thoroughly covering around the inner Ag shell, and the thickness of TiN can be referred to Figure S2 and S3. Further detailed information about TiN-Ag HNSs was investigated by TEM as shown in Figure 3 with the corresponding XRD patterns illustrated in Figure S4a. As characterized in either a bunch of HNSs or an individual sphere (Figure 3a and b), a thin coating layer can be well resolved around the hollow spheres with the thickness of TiN indicated in Figure S3. Polycrystalline structure was characterized in this kind of Ag and TiN composited sphere as seen in the SAED pattern of Figure 3c, which is further proved by the high-resolution image in Figure 3d with the lattice space of ~ 0.15 nm corresponding to TiN (220) and the XRD patterns in


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Figure S4b. Similar TEM images of TiN-10nm-Ag were displayed in Figure S5. To further visualize the distribution of titanium nitride and sliver, the dark field images and EDX elemental mapping were introduced, as shown in Figure 3e and f. Uniform TiN layer covering around the Ag sphere can be well distinguished, although there is a spreading of N element in the image generally due to its light weight. In order to evaluate the plasmonic properties of this TiN-Ag double shell structure for biosensing, SERS detections were firstly carried out. Different concentrations of R6G were applied as the probe on the TiN-5nm-Ag substrate with their SERS activity shown in Figure 4a (on Ag and TiN-10nm-Ag substrates in Figure S6). The characteristic Raman vibrational peaks of R6G can be well resolved at 611, 770, 1185, 1310, 1359, 1509, and 1648 cm-1.27 The SERS enhancement factors were calculated as around 1.76×105, 3.24×105 and 1.23×105 on the Ag, TiN-5nm-Ag and TiN-10nm-Ag HNS samples, respectively (Figure S7). These results evidently suggest that this TiN-Ag composite structure can be used as an effective SERS substrate, although there is a dielectric coating over the Ag HNSs. It has been well accepted that SERS performance is subject to the characteristics (e.g. size or separation) of metal nanostructures and their surrounding environment.1 Many studies have demonstrated that the coating or isolation of dielectric materials, such as SiO2 and Al2O3, would lower the SERS activity on bare metal nanostructures, commonly due to the field isolation.16 While in this work, with the introduction of ultra-thin TiN layer, an obvious Raman signal enhancement instead of reduction was characterized on these composite HNS arrays, as seen in Figure 4b. In order to reveal the in-depth mechanisms of the SERS enhancement on this TiN-Ag double shell composite structure, the FDTD simulation was carried out. As seen in Figure 4c, although there is an isolation layer of ~ 5 nm TiN around the Ag nanospheres, a strong near-field


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distribution still can be visualized. Especially, as displayed in both Figure 4c and d, the intensified field can be found within the nanogaps between spheres, which are generally regarded as the hot-spot region with most contribution to the SERS enhancement. In this work, periodic plasmonic nanostructure arrays with highly controlled interspaces can be facilely realized by using the closely packed PS nanosphere arrays as template combined with the highly controllable nanofabrication process, such as RIE etching and film deposition. The plasmonic coupling induced by the ‘gap-effect’ within these interspaces makes the nanostructures to be an ideal SERS substrate.28 Moreover, the introduction of smooth TiN coating layer produced much smaller gaps within the composite hollow nanospheres, resulting in an even stronger intercoupling. So, more intensified field can be induced around the TiN-5nm-Ag HNSs than that in only Ag HNS arrays substrate. Obviously, this kind of near-field enhancement should be one of the main reasons for the SERS-activity improvement on this TiN-Ag composite substrate. (The corresponding near-field distribution patterns in TiN-10nm-Ag HNSs can be referred to Figure S8.) The extinction spectra in the TiN coated Ag HNS arrays were collected as displayed in Figure 5a with the comparison to only Ag HNS arrays. As expected, the multiple mode resonances (i.e. dipole, quadrupole, octupole, hexadecapole) were revealed in the only Ag HNS arrays, which is consistent with previous reports.26, 29 When the thin layer of TiN in the thickness of either 5 or 10 nm was introduced around Ag HNSs, a red shift in frequencies and higher intensity of corresponding resonance modes were identified in both extinction spectra.25 Reasonably, these changes can be attributed to the size increment after introducing the TiN layer. The significant red shift in lower order resonance modes and less prominent shift in higher order modes of metal nanostructures were well demonstrated. 30-31 Besides, a resonance peak within the wavelength of


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1000 to 1200 nm (as the marked) was resolved in both TiN-Ag HNS arrays, which might be originated from the mode hybridization between TiN and Ag HNSs (Figure S9).32 The similar trend was also displayed in a single HNS of Ag, TiN-5nm-Ag, and TiN-10nm-Ag, as seen in the simulated results of Figure 5b. The extracted near-field distribution around the single HNS (Figure 5c and Figure S12) further evidenced the multi-mode resonances in either only Ag or TiN-Ag HNSs. The calculated scattering and absorption spectra of Ag, TiN-5nm-Ag and TiN10nm-Ag can be seen in Figure S10 with the corresponding ratios illustrated in Figure S11, indicating that the multiple LSPR based scattering effect dominated the extinction spectra. Since the permittivity of TiN embraces a negative real part and small imaginary part in visible range, it exhibits metallic properties with low optical loss.10,

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Thus, even with the coating of the

additional TiN over the Ag HNS, the near field around Ag nanostructure still presents considerable intensity, as visualized in Figure 5c. Besides the SPR induced physical enhancement in SERS phenomena, chemical enhancement, e. g. charge transfer, also has been considered as another important process for the Raman signals’ enhancing.34-35 Here, the energy band diagram of TiN/Ag interface was illustrated in Figure 6a. Since the work functions of Ag and TiN are different (~ 4.1 eV in Ag vs ~ 4.87 eV in TiN),36-37 the electrons will understandably transfer from Ag to TiN when the outer TiN layer directly contact with Ag HNS. The SERS performance of 4-aminothiophenol (PATP) on TiN-Ag composite structure well evidenced the charges transfer between TiN and Ag. As shown in Figure 6b, the characteristic resonance modes at 1142, 1388, 1433 cm-1 were clearly identified on both substrates.38-39 These resonance peaks have been well accepted to be corresponding to 4, 4’-dimercaptoazobenzene


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(DMAB), which is the oxidized product of PATP with the involvement of charges as illustrated in the process of Figure 6c and following equations:40 Ag – hv Ag*

(1)

TiN + hv TiN (e-)

(2)

O2 + TiN (e-) TiN + O* (Oxygen Activation)

(3)

Obviously, those charges can be well transferred from Ag to TiN as seen in Figure 6d and then involved into the oxidation of PATP to form DMAB. Also, due to the transfer process, charge accumulation and a certain potential would generate within the TiN/Ag interface, which would be much beneficial for the LSPR effect.36 Hence, an enhancement in local near-field around TiN layer still can be accomplished. It should be pointed out that when the thickness of TiN was increased to ~ 10 nm the near-field became less intensified than that in the TiN-5nm-Ag HNS (Figure 6b). Reasonably, the increased layer thickness in this metal-like shell would further retard the local-field enhancement around the outer layer. Therefore, the less Raman enhancement was characterized in the TiN-10nm-Ag HNSs substrate as seen in Figure 6b. Besides the high sensitivity, excellent stability, both thermal or chemical, has also been a challenge issue in SERS substrates, given the unsatisfied thermal resistance in commonly used noble metals e.g. Au, Ag or Cu, or chemical vulnerability in Ag or Cu/Al.41-43 So, in this work, the metal-like TiN, with good thermal and chemical resistance, can facilely be used to protect the Ag inner shell from the attack of surrounding environment. As evidenced by the evolution of Raman intensity on the three kinds SERS substrates with time aging, including only Ag, TiN5nm-Ag and TiN-10nm-Ag, as displayed in Figure 7a-c, the TiN coated Ag double shell nanostructures do exhibit better stability than that in only Ag HNSs substrate. With using the SERS peak of R6G at 611 cm-1 as the reference peak, the normalized Raman intensity I/I0 (I stands for the resonance intensity at different sample store time and I0 represents the intensity in


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the corresponding freshly prepared SERS substrate) vs time is illustrated in Figure 7d. Obviously, with the time going, the Raman intensity declined in TiN coated Ag HNS arrays was much less than that in the only Ag HNS arrays. Also, with increasing the thickness of TiN layer, the Raman intensity experiences less reduction. Of course, as mentioned above, the thicker layer of TiN would weaken the local-field intensity around the composites’ outer surface, resulting in a sacrificed Raman sensitivity. Therefore, the TiN-5nm-Ag HNS arrays is considered as a better SERS substrate with both high sensitivity and satisfied stability.

CONCLUSIONS In summary, in this work, the metal like TiN was proposed to composite with metal Ag HNS arrays with the aim to accomplish both high sensitive and stable SERS detection. The multiple mode LSPR effect and gap-effect induced near field enhancement were experimental and theoretically demonstrated in this double-shell structure. Also, due to the metal-like property in TiN, the charge transfer from Ag to TiN shells was evidenced to maintain the strong near field around the double shell. Thus, both high Raman sensitivity, synergistically induced by the physical and chemical mechanisms within Ag and TiN shells, and excellent stability due to the protection of TiN were realized in this TiN-Ag composite structure. Promisingly, stemmed from the strategy in this work, other noble metal based composite structures might be developed as stable SERS substrates or with other possible applications in photocatalytic, photovoltaic, thermal therapy and so on.

ASSOCIATED CONTENT


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Supporting Information.

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Additional supplementary figures of the morphology and

electrochemical characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (J. L.) [email protected] (J. Y.) ACKNOWLEDGMENT This work is financially supported by the National Basic Research Program of China 2015CB932301), National Natural Science Foundation of China (Grant No. 61675173 and 61505172), Science and Technology Program of Xiamen City of China (3502Z20161223 and 3502Z20144079), and China Postdoctoral Science Foundation Funded Project (No. 2015M582038). REFERENCES 1.

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26. Yin, J.; Zang, Y.; Xu, B.; Li, S.; Kang, J.; Fang, Y.; Wu, Z.; Li, J. Multipole Plasmon Resonances in Self-assembled Metal Hollow Nanospheres. Nanoscale 2014, 6 (8), 39343940. 27. Michaels, A. M.; Nirmal, M.; Brus, L. Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals. J. Am. Chem. Soc. 1999, 121 (43), 9932-9939. 28. Nam, J. M.; Oh, J. W.; Lee, H.; Suh, Y. D. Plasmonic Nanogap-Enhanced Raman Scattering with Nanoparticles. Acc. Chem. Res. 2016, 49 (12), 2746-2755. 29. Yin, J.; Zang, Y.; Yue, C.; He, X.; Yang, H.; Wu, D.-Y.; Wu, M.; Kang, J.; Wu, Z.; Li, J. Multiple Coupling in Plasmonic Metal/Dielectric Hollow Nanocavity Arrays for Highly Sensitive Detection. Nanoscale 2015, 7 (32), 13495-13502. 30. Wang, H.; Brandl, D. W.; Le, F.; Nordlander, P.; Halas, N. J. Nanorice: a Hybrid Plasmonic Nanostructure. Nano Lett. 2006, 6 (4), 827-832. 31. Lin, M.; Nien, L.; Chen, C.; Lee, C.; Chen, M. Surface Enhanced Raman Scattering and Localized Surface Plasmon Resonance of Nanoscale Ultrathin Films Prepared by Atomic Layer Deposition. Appl. Phys. Lett. 2012, 101 (2), 023112. 32. Prodan, E.; Radloff, C.; Halas, N.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302(5644):419-422. 33. Guler, U.; Boltasseva, A.; Shalaev, V. Refractory Plasmonics. Science 2014, 344 (6181), 263-264. 34. Park, W. H.; Kim, Z. H. Charge Transfer Enhancement in the SERS of a Single Molecule. Nano Lett. 2010, 10 (10), 4040-4048. 35. Zhou, Q.; Li, X.; Fan, Q.; Zhang, X.; Zheng, J. Charge Transfer between Metal Nanoparticles Interconnected with a Functionalized Molecule Probed by Surface-enhanced Raman Spectroscopy. Angew. Chem. 2006, 45 (24), 3970-3973. 36. Shan, G.; Xu, L.; Wang, G.; Liu, Y. Enhanced Raman Scattering of ZnO Quantum Dots on Silver Colloids. J. Phys. Chem. C 2007, 111, 3290-3293. 37. Liu, Y.; Hayashida, T.; Matsukawa, T.; Endo, K.; Masahara, M.; O'uchi, S.; Sakamoto, K.; Ishii, K.; Tsukada, J.; Ishikawa, Y. Nitrogen Gas Flow Ratio and Rapid Thermal Annealing Temperature Dependences of Sputtered Titanium Nitride Gate Work Function and Their Effect on Device Characteristics. Jpn. J. Appl. Phys. 2008, 47 (4S), 2433-2437. 38. Huang, Y. F.; Zhu, H. P.; Liu, G. K.; Wu, D. Y.; Ren, B.; Tian, Z. Q. When the Signal is not from the Original Molecule to be Detected: Chemical Transformation of Paraaminothiophenol on Ag during the SERS Measurement. J. Am. Chem. Soc. 2010, 132 (27), 9244-9246. 39. Li, C. Y.; Meng, M.; Huang, S. C.; Li, L. "Smart" Ag Nanostructures for Plasmon-Enhanced Spectroscopies. J. Am. Chem. Soc. 2015, 137 (43), 13784-13787. 40. Zhao, L. B., Zhang, M., Huang, Y. F, Williams, C. T; Wu, D. Y.; Ren, B.; Tian, Z. Q. Theoretical Study of Plasmon-Enhanced Surface Catalytic Coupling Reactions of Aromatic Amines and Nitro Compounds. J. Phys. Chem. Lett. 2014, 5 (7), 1259–1266. 41. Han, Y.; Lupitskyy, R.; Chou, T. M.; Stafford, C. M.; Du, H.; Sukhishvili, S. Effect of Oxidation on Surface-enhanced Raman Scattering Activity of Silver Nanoparticles: a Quantitative Correlation. Anal. Chem. 2011, 83 (15), 5873-5880. 42. Roguska, A.; Kudelski, A.; Pisarek, M.; Opara, M.; Janik-Czachor, M. Surface-enhanced Raman Scattering (SERS) Activity of Ag, Au and Cu Nanoclusters on TiO2-nanotubes/Ti Substrate. Appl. Surf. Sci. 2011, 257 (19), 8182-8189.


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43. Erol, M.; Han, Y.; Stanley, S. K.; Stafford, C. M.; Du, H.; Sukhishvili, S. SERS not to be Taken for Granted in the Presence of Oxygen. J. Am. Chem. Soc. 2009, 131 (22), 74807481.


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Figure 1. Schematic diagram about the fabrication processes of TiN-Ag double shell HNS arrays.


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Figure 2. Ag HNS arrays on a silicon substrate (a) at low magnification with inverted image at high magnification in the inset and (b) the cross-section image. TiN-5nm-Ag HNS arrays on a silicon substrate (c) at low magnification with EDS pattern in the inset and (d) at high magnification. TiN-10nm-Ag HNS arrays on a silicon substrate (e) at low magnification with EDS pattern in the inset and (f) at high magnification.


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Figure 3. TEM images of (a) a bunch of TiN-5nm-Ag nanospheres and (b) A single HNS. (c) The SAED pattern of the TiN-5nm-Ag sphere in (b). And (d) high resolution TEM image in the boundary area as marked in the square of (b). (e) Dark-field image of another individual TiN5nm-Ag sphere (as marked in the red square) and the corresponding elemental mapping of Ag, Ti and N with the line scan analysis in (f).


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Figure 4. (a) SERS activity of R6G with the concentration of 10-4, 10-5, 10-6, 10-7, and 10-8 M on TiN-5nm-Ag HNSs substrate. (b) Comparison of SERS activity on TiN-5nm-Ag HNSs and Ag HNSs substrates using the R6G probe with the concentration of 10-4 M. The laser source of 532 nm was set to the power of 0.2 mW. (c) Near-field distribution around TiN-5nm-Ag HNSs comparing with that around the bare Ag HNSs. (d) The strongest local nearfield profiles of TiN5nm-Ag HNSs and Ag HNSs within the nanogaps (region of white box in (c)).


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Figure 5. Extinction spectra of (a) Ag, TiN-5nm-Ag and TiN-10nm-Ag HNS arrays on the sapphire substrates. (b) Simulated spectra of a single HNS by the FDTD method as the comparison to the experimental data of (a). (c) Near-field distribution around a single HNS of TiN-5nm-Ag at the incident wavelength of i. 471 nm, ii. 556 nm, iii. 790 nm and iv. 1360 nm, respectively. The direction and polarization of incident light are shown in the patterns.


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Figure 6. (a) Energy band of Ag and TiN. (b) SERS spectra of PATP with the concentration of 10-4 M on TiN-5nm-Ag (red line) and TiN-10nm-Ag HNSs (black line). (c) The oxidation activation model of PATP by charge transfer. (d) Charge distribution model of TiN-Ag HNS.


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Figure 7. Time dependent Raman spectra of (a) Ag, (b) TiN-5nm-Ag and (c) TiN-10nm-Ag HNS array substrate with the R6G concentration of 10-4 M. (d) Normalized Raman resonance intensity at the representative frequency of 611 cm-1 in the above three SERS substrates with changing the storing time.


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