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Article

Plasmonic-photonic interference coupling in submicrometer amorphous TiO-Ag nanoarchitectures 2

Rajeshkumar Shankar Hyam, Jihoon Jeon, Songhwa Chae, Yong Tae Park, Sung Jae Kim, Byeongchan Lee, Choongyeop Lee, and Dukhyun Choi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01080 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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Plasmonic-photonic interference coupling in submicrometer amorphous TiO2-Ag nanoarchitectures Rajeshkumar S. Hyama, Jihoon Jeona, b, Songhwa Chaea, b, Yong Tae Parkc, Sung Jae Kimd, Byeongchan Leea, Choongyeop Leea and Dukhyun Choia, b, * a

Department of Mechanical Engineering, Kyung Hee University, 1732, Deogyeong-daero,

Giheung-gu, Yongin-si, Gyeonggi-do, Republic of Korea b

Industrial Liaison Research Institute, Kyung Hee University, 1732, Deogyeong-daero,

Giheung-gu, Yongin-si, Gyeonggi-do, Republic of Korea c

Department of Mechanical Engineering, Myongji University, 241, Geumhak-ro, Cheoin-gu,

Yongin-si, Gyeonggi-do, Republic of Korea d

Department of Electrical and Computer Engineering, Seoul National University, 1

Gwanakro, Gwanak-gu, Seoul, Republic of Korea

*Corresponding author e-mail: [email protected]

Keywords: TiO2 nanotube; Amorphous; Plasmonic; Interference; Surface-enhanced Raman scattering (SERS)


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Abstract

In this study, we report the crystallinity effects of submicrometer titanium dioxide (TiO2) nanotube (TNT) incorporated with silver (Ag) nanoparticles (NPs) on surface-enhanced Raman scattering (SERS) sensitivity. Furthermore, we demonstrate the SERS behaviors dependent to the plasmonic-photonic interference coupling (P-PIC) in the TNT-AgNP nanoarchitectures. Amorphous TNTs (A-TNTs) are synthesized through a two-step anodization on titanium (Ti) substrate and crystalline TNTs (C-TNTs) are then prepared by using thermal annealing process at 500 °C in air. After thermally evaporating 20-nm thick Ag on TNTs, we investigate SERS signals according to the crystallinity and P-PIC on our TNTAgNP nanostructures. (A-TNTs)-AgNP substrates show dramatically enhanced SERS performance compared with (C-TNTs)-AgNP substrates. We attribute the high enhancement on (A-TNTs)-AgNP substrates with electron confinement at the interface between A-TNTs and AgNPs due to the high interfacial barrier resistance caused by band edge positions. Moreover, the TNT length variation in (A-TNTs)-AgNP nanostructures results in different constructive or destructive interference patterns, which in turn affects the P-PIC. Finally, we could understand significant dependency of SERS intensity on P-PIC in (A-TNTs)-AgNP nanostructures. Our results thus might provide the suitable design way for the myriad applications of enhanced EM on plasmonic-integrated devices.


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1. Introduction Nanostructures with periodic space modulation of dielectric function in a period of photon wavelength are often referred to as photonic nanocrystals and can be suitable for a wide variety of applications if coupled/engineered suitably with plasmonic nanomaterials to control different optical phenomena.1-9 In nanotube-based three-dimensional (3D) hybrid structures, both dielectric (photonic crystals) and metal nanostructures (plasmonic nanomaterials) play a pivotal role. Noble metal nanostructures are able to concentrate light into small volumes in order to enhance the local electromagnetic (EM) field near the metal nanostructures. These greatly enhanced EM field areas are known as “hot spots” for surfaceenhanced Raman spectroscopy (SERS), which utilizes the field enhancement properties of metal nanostructures to amplify the weak Raman scattering signals.10,11 This enhancement allows strong modification of the optical properties of photonic crystals by involving the light scattering at electronic excitations in the metal component, resulting in molding of the light flow to the diffraction resonances occurring in the body of the photonic crystals. The coupling of photonic and plasmonic resonance is a direction- and polarization-dependent property because their dispersions depend on the polarization and propagation direction of the light.

Typical excitations in metal nanostructures are localized surface plasmon resonances (LSPR) and propagating surface plasmon polaritons (SPPs) at metal-dielectric interfaces. If the metal film is prepared with holes or periodic corrugation, then diffracted light can be coupled to both LSPR and SPPs. Controlled engineering of the geometric parameters like height and periodicity of corrugation affect the strength and spectral position of LSPR, changing the metal film thickness changes the height of the extraordinary transmission (EOT) peak, and light-SPP coupling can be changed by altering the dimensionality and refractive index


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contrast in the attached photonic layer. Recently, our novel report12 for plasmonic interference coupling (PIC) has shown that the plasmonic EM enhancement is critically dependent to the optical interference in the metal-insulator-metal (MIM) structure, where the top metal has nanopores and the bottom metal plays a role as an optical mirror. By controlling the thickness of the dielectric insulator at the middle, the optical interference patterns (i.e. constructive or deconstructive interference) were alternated at the top nanopore layer, thus resulting in great effects on SERS responses. Therefore, we could clearly understand that such a MIM structure should be well designed by considering PIC characteristics to effectively utilize a plasmonic EM enhancement.

Titanium dioxide (TiO2) nanostructures have been studied for diverse energy and photonicbased applications due to their chemical stability, high electron transport, relatively high dielectric constant (parallel to the c axis; in rutile TiO2 = 173 , anatase TiO2 = 48 and amorphous TiO2 = 12.7), and refractive index (rutile TiO2 = 2.89, anatase TiO2 = 2.48 and amorphous TiO2 = 2.56).13,14 The major disadvantage of this material is its wide bandgap (3 eV for rutile, 3.2 eV for anatase and 3.4 eV for amorphous), resulting in major absorption in the UV range. The current impetus for research is to enhance the absorption of crystalline TiO2 in the visible range of the solar spectrum from 400-800 nm using different methods like doping,15-17 defect engineering,18 and surface sensitization by dye.19-21 Recently, surface plasmon-enhanced absorption of crystalline TiO2 due to attachment of different metal nanoparticles (NPs) like silver (Ag), platinum (Pt), gold (Au), and copper (Cu) is being studied for diverse energy and SERS applications, showing some enhancement in absorption and improvement in molecule detection.22-31 Noble metals often have an advantageous Fermilevel position for accumulation of photo-generated electrons, which can lead to an improved electron–hole lifetime. However, some efficient noble metals such as Pt, Au and Pd are


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generally too expensive to have any practical value in this application, although Ag has been shown to be a good candidate for use at larger industrial scales. Silver usually can act as an electron source/sink and also provides an additional functional improvement for biomedical applications. Still there is ample opportunity to improve its performance and sensitivity.

In this study, we investigated the SERS sensitivity dependent to the crystallinity of submicrometer TiO2 nanotube (TNT) with AgNPs. As-grown TNTs by two-step anodization were amorphous and crystalline TNTs were obtained after annealing process. It was found that amorphous TNT (A-TNTs)-AgNP nanostructures provided highly enhanced SERS signals due to the electron confinement at the interface between A-TNTs and AgNPs, compared with crystalline TNT (C-TNTs)-AgNP nanostructures. We further explored plasmonic-photonic interference coupling (P-PIC) on (A-TNT)-AgNPs by controlling the length of A-TNT. (A-TNT)-AgNP nanostructures with different length of TNT showed totally different absorption spectra, which means the change of P-PIC in our nanostructure. Finally, it was confirmed that SERS intensities were changed by the length of TNT. We analyzed the effects of P-PIC on SERS intensity based on thin film interference theory.

2. Material and Methods 2.1 Fabrication of TiO2 nanotubes Amorphous TiO2 nanotubes (A-TNTs) were synthesized at room temperature using the anodization technique. Platinum foil is used as a counter electrode and titanium foil (Alfa Aesar) with thickness (0.127 mm) as the anode. The electrolyte for anodization was a mixture of ethylene glycol and NH4F (0.5 wt. %) with the addition of 2 wt. % DI water. All other conditions of anodization (applied voltage of 40 V, anodization time of 1 h and electrolyte of


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room temperature) are kept constant. Initially the Ti foil is cleaned in deionized (DI) water, acetone, and ethanol and sonicated for 10 min in the above mentioned solvents to make the surface clean without any grease, debris, or rough sharp edges on the Ti foil surface. After cleaning, the Ti foil surface is immediately dried by N2 gas. A DC power supply was used to apply constant DC 40 V between anode and cathode, which were kept 1.5 cm apart from each other in the electrolyte. After anodization of Ti foil, the sample was immediately washed with DI water, ethanol and dried by CO2 gas to avoid agglomeration and to enhance the uniform drying. Crystalline TiO2 nanotubes (C-TNTs) were then obtained by thermal annealing process at 500 °C in air for 3 hour and natural cooling process at room temperature.

2.2 Formation of a plasmonic layer Ag molecules were deposited on TNT substrates with metal silver (99.99% purity) pellets by thermal evaporation. The substrate holder was connected to an electric motor to rotate the substrate during the deposition to achieve a uniform layer. Electric current for evaporation was at 45 A and the deposition pressure was 2.0 × 10-5 Torr. The deposition rate was 0.5 Å/s. Due to the high agglomeration property of Ag and thin thickness (20 nm) of evaporation, Ag molecules form Ag NPs on the TNT surface as a plasmonic layer.

2.3 Characterizations All the samples were characterized by using different analytical tools such as X-ray diffraction (XRD, D8 Advance Bruker), Raman measurement (The Peak Seeker by Raman Systems) with the laser wavelength of 785 nm, filed-emission scanning electron microscopy (FE-SEM, LEO SUPRA 55, GENESIS 2000), energy dispersive X ray analysis (EDAX, Carl Zeiss), transmission electron microscopy (TEM, JEM-2100F, JEOL), energy dispersive


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spectroscopy (EDS, Oxford INCA), UV-visible absorption (JASCO 660), Keithley meter (model No. 2100), Potentiostat (SP-50, Biologic).

3. Results and Discussion Submicron TiO2 photonic crystals on nanotubes were prepared through a two-step anodization technique (see Figure S1). The details were introduced in Experimental section. We have optimized the length, pore diameter and wall thickness of photonic crystal TNTs. TNTs synthesized by anodization are amorphous in nature and become crystalline after suitable heat treatment at 500 °C in air. However, due to the heat treatment, the majority of crystalline samples experience degradation due to the high strain produced along the sample surface.32 The C-TNT samples showed weakened optical interference properties and became dark brownish-black/grey in color. However, the A-TNT samples exhibited a sharp and comparatively better optical interference phenomenon. After preparing TNT samples, we coated Ag with 20 nm thickness by thermal evaporation, thus finally creating the P-PIC nanoarchitectures consisting of photonic crystal TiO2 and plasmonic AgNPs. Figure S2 shows EDAX results to confirm the presence of Ag on TiO2. Furthermore, TEM and EDS images showed the cross-sectional morphology of TiO2 nanotubes and the distribution of AgNPs on TiO2 nanotubes (see Figure S3).

Figure 1(a) shows the schematic illustration of the mechanism of the SERS sensitivity difference between submicrometer amorphous and crystalline TNT with AgNPs. A-TNT has its distinct structural color depending on its length according to anodization time; the dark blue color in case of the anodization time of 10 seconds was due to optical interference. On the other hand, due to heat treatment of amorphous samples, the TNT samples became dark grey in all cases of C-TNTs (independent of nanotube length), due to relatively weaker


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optical interference. Under solar irradiation, i.e., 1 sun condition, or the light from a UVvisible source (i.e., 200-800 nm), the surface plasmon (collective oscillations of electrons) in AgNPs is excited, and the A-TNT photonic crystal layer generates excitons (i.e., electron hole pairs). The electrons in the conduction band of TiO2 photonic crystals were coupled or in resonance with the collective oscillation of electrons from AgNPs due to P-PIC. Due to the highly resistive nature at the interface of Ag and A-TNT and their band alignments (see Figure 1(a)), the oscillating electrons by surface plasmon in AgNPs are hard to transfer to the conduction band of TNT, resulting in sustaining enhanced EM field on the surface of AgNPs. In order to clearly demonstrate such a high electron confinement between A-TNT and AgNPs, we measured the current flow of a (A-TNT)-AgNP substrate by adjusting light incidence (i.e. on and off). Figure 1(c) shows the dramatically reduced photocurrent in the (A-TNT)-AgNPs sample due to higher resistance at the interface and high decay time (τ[(ATNT)-AgNPs] ) τ[(C-TNT)-AgNPs]). On the other hand, crystalline TNT samples experience relatively weaker optical interference (exhibiting dark gray color), comparatively less resistance at the interface and suitable band edge positions for charge transfer, as shown in Figure 1(b). The charge carriers from AgNPs were able to move through TNT and reach the bottom Ti substrate, decreasing the electron density at the top TiO2 photonic crystal layer. This could be also demonstrated by the large photocurrent in Figure 1(c) due to lower resistance at the interface and low decay time. We further verified the significant difference of interfacial resistance between (A-TNT)-AgNPs and (C-TNT)-AgNPs by the photocurrent spectra of the samples with different growth time of TNTs, as shown in Figure S4. We could find that the interfacial resistance of (A-TNT)-AgNPs was about 100 ~200 times less than that of (C-TNT)-AgNPs.


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Recently reported work has shown that, by introducing crystal defects in TiO2, a new vacancy band can be generated below its conduction band, expanding the photon-absorbance of TiO2 into the visible light region without the recombination effect from doped impurities.33-35 Figure 2(a-d) and (e-h) show the XRD spectra and FE-SEM images of A-TNT, (A-TNT)AgNPs, C-TNT, and (C-TNT)-AgNPs samples. Figure 2(a) clearly illustrates that the sample has no crystalline TiO2 peaks around 2θ = 25º to 30º due to amorphous TiO2 phase and only Ti substrate peaks. When this amorphous TNT sample is coated with Ag thin film (20 nm) using a vacuum/thermal evaporation technique, the peaks due to Ag emerge at 2θ = 38.2º for the (111) plane, as shown in Figure 2(b), which was confirmed by comparing with the standard inorganic crystal structure database (ICSD) file number [01-089-3722]36. The intensities of the Ag peaks are weaker than those of the substrate peaks due to the relatively smaller thickness of the Ag layer (20 nm), which causes the AgNPs to coalescence. After heat treatment of the A-TNT samples, crystalline TiO2 peaks due to anatase phase [A (101) plane at 2θ = 25.3º] and rutile phase [R (110) plane at 2θ = 27.3 º] were observed, as shown in Figure 2(c) and were confirmed by comparison with the standard Joint Committee on Powder Diffraction Standards Card(JCPDs Card) data reference code: [00-021-1272] and [00-021-1276]. When this C-TNT sample was coated with Ag (20 nm film), Ag peaks were produced at different 2θ positions, as shown in Figure 2(d). The amorphous and crystalline TNT phase formations were further supported by Raman data, as shown in Figure S5.

Figure 2(e - f) show the top view of SEM images of the A-TNT and (A-TNT)-AgNPs nanostructure. In A-TNT, a pore diameter of ~100 nm, wall thickness 20 nm and thick top photonic layer are clearly observed. The sample shows a relatively non-uniform, elongated, slightly rough surface of the top photonic layer on the TNT structures. When Ag layer is coated on such samples, AgNPs were observed on the top photonic layer with a small


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interparticle distance, which may play a role of hot spots, thus affecting the SERS signal. Figure 2(g- h) show the SEM images of the C-TNT and (C-TNT)-AgNPs nanostructures. The C-TNT sample has a relatively uniform and smooth photonic layer. When Ag film is coated on such a smooth crystalline TiO2 photonic crystal layer, small interparticle distances are also created, possibly providing effective SERS hot spots.

Figures 3 shows the UV-visible absorption spectra and SERS spectra on photonic amorphous and crystalline TiO2 nanotubes with and w/o AgNPs, where A-TNT and C-TNT were synthesized for 90 seconds. Figure 3(a) clearly shows that the A-TNT sample (bandgap = 3.4 eV) has a sharp optical absorption band gap edge at 360 nm, which was red shifted for the (A-TNT)-AgNPs sample due to the coupling effect of plasmonic and photonic interference. On the other hand, the C-TNT sample (band gap = 3.2 eV for anatase phase) has an optical absorption band gap edge at 380 nm, which was slightly red shifted by Ag coating (i.e., (CTNT)-AgNPs). Figure 3(b) shows the SERS spectra recorded for rhodamine 6G (R6G) using a 785-nm laser at the integration time of 10 seconds. Sharp SERS main peaks were found at 1310, 1362 and 1514 cm-1 on the (A-TNT)-AgNPs sample due to R6G detection. The enhanced EM fields generated by constructive photonic interference were coupled with the electrons from the LSPR of AgNPs, satisfying the resonance condition and resulting in a higher EM field. The detail calculation for P-PIC will be analyzed in Figure 5. On the other hand, there was no SERS signal detection in the uncoated A-TNT sample because there were not plasmonic hot spots to produce a sufficient EM field to detect R6G molecules. Greatly low SERS signals were detected on (C-TNT)-AgNPs due to the high decay time of the oscillating electrons from plasmonic AgNPs into C-TNT, as explained in Figure 1. No SERS signals were also detected for uncoated C-TNT samples. Therefore, we could clearly


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understand that the crystallinity of TNT is critically important to create plasmonic EM enhancement in TNT-AgNP nanostructures, which can provide high SERS signals.

Figures 4(a-d) show UV-visible absorption spectra before and after Ag coating on A-TNT synthesized at different anodization times (10, 30, 60, and 90 seconds). The inset showed cross-sectional SEM images of Ag-coated A-TNT samples synthesized at the different anodization times, where the A-TNT thicknesses were confirmed to be 70, 130, 230, and 320 nm, respectively. Depending on the thickness of A-TNT, its absorption behaviors were sensitively changed due to its unique photonic charter. Based on the band gap of A-TNT, the wavelengths below UV regions (i.e. < 380 nm) were highly absorbed in A-TNT. It could be clearly observed that the wavelengths in the visible ranges (380 ~ 750 nm) were also partially absorbed in A-TNT due to their photonic crystal effect. After Ag coating, the intensities of absorption on the samples were mostly enhanced due to the plasmonic effect. We can expect a P-PIC effect in such an integrated nanostructure of plasmonic NPs and a photonic crystal nanomaterial. However, the UV-visible absorption spectrum is far-field analysis and plasmonic effect is local field event, so that we could not confirm any P-PIC effect only with the UV-visible absorption spectrum.

In order to clearly understand the P-PIC effect in our (A-TNT)-AgNPs nanostructures, we measured SERS signals and examined the behaviors of SERS intensities based on thin film interference theory. We further prepared longer A-TNTs with the anodization time of 3 and 10 minutes to clearly analyze the behaviors of SERS intensities. The detail data for FE-SEM images, UV absorption spectra, and SERS signals of the A-TNT with the anodization time of 10 minutes were provided in Figure S6. Figure 5(a) shows the SERS signals of R6G with respect to our (A-TNT)-AgNP samples with different anodization time (i.e. different


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thickness of A-TNT). We used the laser of 785-nm wavelength and the integration time was 10 seconds. The spectra clearly showed the peaks at 1313, 1362 and 1514 cm-1 due to the detection of R6G on the sample surface. Since SERS signals are obtained by local field events, we could understand P-PIC effects on our samples based on the behaviors of SERS intensity. Figure 5(b) shows the quantitative data for the SERS intensities at the major peaks of R6G. Interestingly, the SERS intensities showed the repeated increase and decrease according to the anodization time, and finally saturated at the anodization time of 10 minutes. Based on the thin film interference theory and our previous report,12 we could determine the positions of construction and deconstruction interference in our nanostructures. The details introduced in the supporting information. By using 785-nm laser, the constructive interference patterns are formed at the positions of around 125 and 375 nm from the bottom Ti substrate, and the deconstructive interference pattern is formed at about 250 nm. By considering the thicknesses of our A-TNT and coated Ag, the plasmonic Ag layers in our samples were located at 70, 130, 230, 320 nm from the bottom Ti substrate. Thus, we could clearly understand that the (A-TNT)-AgNPs nanostructure with the anodization time of 30 seconds, where the A-TNT thickness is 130 nm, provided the highest SERS intensity due to the high P-PIC effect. The (A-TNT)-AgNPs nanostructure with the anodization time of 60 seconds provided the lowest SERS intensity since the position of the plasmonic layer (230 nm) was too close to the deconstructive interference pattern. As the thickness of A-TNT is longer than 1µm, the P-PIC effect is almost disappeared due to the high order of constructive interference patterns, so that the SERS intensity is saturated. Therefore, the SERS behaviors clearly demonstrated the significant effect of P-PIC in our nanostructure on enhanced plasmonic EM field formation.


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4. Conclusion We have demonstrated the crystallinity effects of TNT with AgNPs on SERS performance. Due to the high barrier and band alignment at the interface between photonic crystal A-TNT and plasmonic AgNPs, the oscillating electrons at the plasmonic AgNPs could provide enhanced EM fields for much longer time than on C-TNT, thus providing high SERS performance. Furthermore, we investigated the SERS behaviors dependent to the P-PIC effects in our (A-TNT)-AgNP nanostructures. Depending on the thickness of A-TNT, the plasmonic layer of AgNPs was matched with the constructive or deconstructive interference patterns, thus affecting to P-PIC. When the plasmonic layer is well matched with the constructive interference (in case of A-TNT with the anodization time of 30 seconds in this study), high SERS intensities could be obtained due to reinforced EM fields. Thus, it could be clearly understood that the P-PIC effect in integrated nanostructures of plasmonic AgNPs and photonic crystal nanomaterials is critically important for enhancing SERS performance. Thus, our results might provide the suitable design way for the applications of enhanced EM on plasmonic-integrated devices.

Acknowledgements This research was financially supported by a grant from the Kyung Hee University in 2015 (KHU-20150653)

References (1) F. Pelayo García de Arquer, Agustín Mihi, and Gerasimos Konstantatos Large-Area Plasmonic-Crystal–Hot-Electron-Based Photodetectors. ACS photonics 2015, 2, 950957.


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(2) Romanov, S. G.; Korovin, A. V.; Regensburger, A.; Peschel, U. Hybrid Colloidal Plasmonic-Photonic Crystals. Adv. Mater. 2011, 23, 2515. (3) Yu, X.; Shi, L.; Han, D.; Zi, J.; Braun, P. V. High Quality Factor Metallodielectric Hybrid plasmonic–photonic Crystals. Adv. Funct. Mater. 2010, 20, 1910-1916. (4) Romanova, A.; Korovin, A.; Romanov, S. Effect of Dimensionality on the Spectra of Hybrid Plasmonic-Photonic Crystals. Phys. Solid State 2013, 55, 1725-1732. (5) Valsecchi, C.; Brolo, A. G. Periodic Metallic Nanostructures as Plasmonic Chemical Sensors. Langmuir 2013, 29, 5638-5649. (6) Rubio-Roy, M.; Vlasin, O.; Pascu, O.; Caicedo, J. M.; Schmidt, M.; GoÃ±i, A. R.; Tognalli, N. G.; Fainstein, A.; Roig, A.; Herranz, G. Magneto-Optical Enhancement by Plasmon Excitations in Nanoparticle/Metal Structures. Langmuir 2012, 28, 9010-9020.. (7) Li, S.; Guo, P.; Buchholz, D. B.; Zhou, W.; Hua, Y.; Odom, T. W.; Ketterson, J. B.; Ocola, L. E.; Sakoda, K.; Chang, R. P. H. Plasmonic-Photonic Mode Coupling in Indium-Tin-Oxide Nanorod Arrays. ACS Photonics 2014, 1, 163-172. (8) Kim, S.; Mitropoulos, A. N.; Spitzberg, J. D.; Kaplan, D. L.; Omenetto, F. G. Silk Protein Based Hybrid Photonic-Plasmonic Crystal. Opt. express 2013, 21, 8897-8903. (9) Murray, W. A.; Barnes, W. L. Plasmonic Materials. Adv. Mater. 2007, 19, 3771-3782. (10) Tian, Z.; Ren, B.; Wu, D. Surface-Enhanced Raman Scattering: From Noble to Transition Metals and from Rough Surfaces to Ordered Nanostructures. J. Phys. Chem. B 2002, 106, 9463-9483.


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Author contributions R. S. H. and J. J. contributed equally to this work.


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Figure 1. Schematic illustrations and band alignment for (a) A-TNT with AgNPs and (b) CTNT with AgNPs. Due to the high resistance and band alignment at the interface between ATNT and AgNPs, less electrons from Ag are transported to A-TNT, thus providing high EM field by many oscillating electrons at the AgNPs. (c) Photocurrent flow from (A-TNT)AgNPs and (C-TNT)-AgNPs by Potentiostat (SP-50, Biologic). Due to the high resistance of A-TNT, the current flow was much less than that by C-TNT, which means that the decay time of oscillating electrons from Ag might be longer on A-TNT than on C-TNT (i.e., τ[(ATNT)-AgNPs] > τ[(C-TNT)-AgNPs]).


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Figure 2. XRD data of TNTs synthesized at the second anodization time of 90 seconds. (a) photonic A-TNT, (b) (A-TNT)-AgNPs, (c) C-TNT, (d) (C-TNT)-Ag, (e)-(h) Top views of FE-SEM images for A-TNT, (A-TNT)-AgNPs, C-TNT, and (C-TNT)-AgNP nanostructures. For identifying AgNPs on TNTs, EDAX and Raman spectra were shown in Figures S2 and S4.


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Figure 3. (a) UV-Visible absorption spectra of A-TNT and C-TNT with and w/o AgNPs, (b) SERS spectra of A-TNT and C-TNT with and w/o AgNPs, where TNTs were synthesized at the second anodization time of 90 seconds.

Figure 4. (a) – (d) UV-visible absorption spectra of A-TNT and (A-TNT)-AgNPs nanostructures at the different anodization times (10 seconds – 90 seconds) for A-TNT. The insets are the cross-sectional SEM images of (A-TNT)-AgNPs for examining the thickness and show the photographs of A-TNT samples.


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Figure 5. (a) SERS spectra of (A-TNT)-AgNPs samples according to the anodization time of A-TNT. Clear R6G raman peaks were observed, but the intensities were different according to the anodization time. (b) Quantitative SERS intensity as a functon of the anodization time. Due to the P-PIC effect, the highest SERS intensity occurred at the anodization time of 30 sec, where the constructive interference pattern at the position of around 125 nm from the Ti substrate was well matched with the plasmonic layer on A-TNT with the thickness of 130 nm.

Graphical abstract


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