Surface Plasmon Resonance and Interference Coenhanced SERS

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Surface Plasmon Resonance and Interference Coenhanced SERS Substrate of AAO/Al-Based Ag Nanostructure Arrays Dongzhi Shan,† Liqing Huang,*,† Xin Li,† Weiwei Zhang,† Jun Wang,§ Long Cheng,† Xuehong Feng,† You Liu,† Jingping Zhu,‡ and Yu Zhang† †

Non-equilibrium Condensed Matter and Quantum Engineering Laboratory, The Key Laboratory of Ministry of Education, School of Science, and ‡The School of Electronic and Information Engineering, Xi’an Jiaotong University, 28 Xianning Road,Xi’an, Shaanxi 710049, People’s Republic of China § School of Science, Xi’an Polytechnic University, 19 Jinhua South Road, Xi’an, Shaanxi 710048, People’s Republic of China ABSTRACT: Surface-enhanced Raman scattering (SERS) substrate of Ag nanostructure arrays patterned by porous anodic aluminum oxide (AAO) membrane supported on Al substrate (AAO/ Al) were fabricated by electron beam evaporation technique. By introducing Al substrate, the optical and SERS properties of as-prepared AAO/Al-based Ag nanostructure arrays are much different from those of the AAO-based array. By optimizing the thickness of both deposited Ag and AAO membrane, the SERS enhancement factor (EF) of as-fabricated AAO/Al-based Ag nanostructure arrays reached 9.77 × 107 due to the SPR and destructive interference coenhanced effects, which is higher than that of AAO-based Ag nanostructure arrays. Also, the mechanism of SPR and destructive interference coenhanced local EM fields as well as SERS enhancement were demonstrated by FDTD simulation results.

1. INTRODUCTION Surface-enhanced Raman scattering (SERS) is widely recognized as a powerful spectroscopic tool for detection and characterization of low concentration of molecules in the chemical and biological agents.1−3 The huge SERS enhancement mainly arises from the strong enhancement of local electromagnetic (EM) fields at the surface of metallic nanostructure,4−6 which resulted from the excitation of surface plasmon resonance (SPR), the coherent oscillations of electrons at metal−dielectric interfaces.7−9 Therefore, fabrication of powerful plasmon metallic nanostructures as SERS-active substrate with high Raman enhancement performance as well as good reproducibility has been a major focus in the development of robust and practical SERS techniques. A variety of SPR-based metal nanostructures have been intensively investigated for SERS. Among these, metal nanostructures patterned by porous anodic aluminum oxide (AAO) template have attracted great attention due to their high SERS sensitivity, good reproducibility, and also easy fabrication. The AAO-based plasmon metal nanocap,10−14 nanoparticle,15−17 and nanopore18−20 arrays prepared by evaporating noble metal onto the surface of AAO membranes have been reported, and their optimized plasmon enhanced SERS performance was obtained by controlling the deposited metal thickness and surface morphology of AAO template. However, for nanopore arrays, in addition to the aforementioned optimized approaches, the effect of thickness of AAO membrane on its SERS performance was investigated with equivalent-circuit analysis.20 In this work, silver nanostructure arrays patterned by AAO membrane supported on Al substrate (AAO/Al) were fabricated © 2014 American Chemical Society

by electron beam evaporating Ag onto the porous surface of AAO membranes. By introducing Al substrate to the AAO-based Ag nanostructure arrays, SERS activity can be further improved by taking advantage of the SPR field enhancement in combination with an interference effect to further amplify the local EM. The dependence of SERS activity for AAO/Al-based Ag nanostructure arrays on the thickness of both deposited Ag layer and AAO was experimentally studied, which showed that absorption and SERS properties of the AAO/Al-based Ag nanostructure arrays are strongly enhanced by SPR and interference coenhanced effects. The largest SERS enhancement factor (EF) for the 1363 and 1310 cm−1 Raman line of the R6G molecule was estimated experimentally to be 6.67 × 107 and 6.31 × 107, respectively, which are higher than that of AAO-based Ag nanoparticle arrays.16 Also, the finite-difference time-domain (FDTD) method was used to calculate spatial distribution of local electric field of AAO/Al-based and AAO-based Ag nanostructure arrays, which help us to understand the SPR and interference coenhanced SERS enhancement mechanism of the AAO/Albased Ag nanostructure arrays. As compared to AAO-based Ag nanoparticle arrays,15−17 the AAO/Al-based Ag nanostructure arrays not only possess the much higher SERS enhancement performance but also are more robust, and easy to fabricate, which are very important for the SERS technique to achieve practical application. Received: March 16, 2014 Revised: September 17, 2014 Published: September 18, 2014 23930

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2. EXPERIMENTAL SECTION 2.1. Preparation of AAO Template and AAO/Al-Based Ag Nanostructure Arrays. A highly ordered porous AAO template supported on Al substrate was fabricated at a constant voltage of 40 V in 0.3 mol/L oxalic acid at 3 °C by using the twostep anodization technique,21 and the thickness of the AAO template was determined by the duration of the second anodization. The free-standing AAO templates were obtained by removing residual Al substrate in a CuCl2 and HCl mixture aqueous solution. The AAO/Al templates with pore diameter ∼40 nm, pore period ∼120 nm, and various tAAO values were used in our experiments. The AAO/Al-based Ag nanostructure arrays were fabricated by electron beam evaporating various thicknesses of Ag onto the porous surface of AAO/Al template with different thicknesses of AAO at the rate 0.2 nm/s in a vacuum of approximately 3 × 10−3 Pa. The temperature of AAO/Al template during evaporating was kept at 25 °C. The AAO-based Ag nanostructure arrays were also fabricated by electron beam evaporating various thicknesses of Ag from 10 to 100 nm onto the porous surface of AAO with thickness ∼3 μm under the identical evaporating condition. 2.2. Characterization of AAO/Al-Based Ag Nanostructure Arrays. The surface morphology of fabricated samples was obtained from the field-emission scanning electron microscope (JEOL JSM-6700F). The optical reflection spectra and absorption spectra were measured with the UV−vis spectrophotometer (Lambda 750). Raman and SERS spectral measurements were carried out with a Horiba jobin Yvon HR800 model Raman spectrometer with 633 or 514 nm excitation light. Rhodamine 6G (R6G) was adopted as the probe of SERS signals measurement. SERS measurement samples were prepared by dropping 50 μL of 1 × 10−6 mol/L R6G ethanol solution on the surface of AAO/Al-based Ag nanostructure arrays with area SSample and drying at ambient environment, and 1 × 10−2 mol/L R6G ethanol solution contained in a capillary (∼1 mm in diameter) was used as the normal Raman spectra measurement sample. To verify the reproducibility and homogeneity of AAO/ Al-based Ag nanostructure arrays, for each specification of AAO/ Al-based Ag nanostructure arrays, more than three samples were prepared, and at least five different points on each sample were selected to detect the R6G probes.

Figure 1. SEM images of AAO/Al nanostructure arrays with various deposited Ag thickness: (a) 10 nm; (b) 30 nm; (c) 50 nm; (d) 30 nm. The thickness of the AAO in (a)−(c) is 130 nm and in (d) is 95 nm. The insets in (b) and (d) are corresponding pore histograms.

pore histograms are shown in the insets. It is seen that the pore histograms for the two samples are almost the same. 3.2. Optical Property and SERS Activity of AAO/AlBased Ag Nanostructure Arrays. To obtain SPR and interference coenhanced AAO/Al-based Ag nanostructure arrays SERS-active substrates, the dependence of optical property and SERS activity on the thickness of both deposited Ag and AAO membrane was investigated. Figure 2 shows the

3. RESULTS AND DISCUSSION 3.1. SEM Characterization of AAO/Al Ag Nanostructure Arrays. SEM images of AAO/Al Ag nanostructure arrays with different deposited Ag thickness (tAg) and AAO thickness (tAAO) are shown in Figure 1. As can be seen from Figure 1a−c, the nanoscale surface roughness of AAO/Al-based Ag nanostructure arrays strongly depends on the deposited Ag thickness. With the increase of the deposited Ag thickness, the surface roughness increased first and then decreased. By optimizing the deposited Ag thickness (40 nm in our experiments), the highest surface roughness with the narrowest gap (interparticle separation) was obtained, which dominates the SPR properties as well as SERS activity of the metal nanostructure arrays.13,22 The surface roughness of Figure 1b is nearly the same as that of Figure 1d, indicating that the surface morphology of the AAO/Al-based Ag nanostructure arrays is almost not affected by the thickness of AAO. To further demonstrate this feature, the SEM images of Figure 1b and d are analyzed through precise edge detection by digital image processing technique, and the corresponding

Figure 2. Optical extinction spectra of AAO-based Ag nanostructure arrays with tAAO = 3 μm and different tAg values.

optical extinction spectra of AAO-based Ag nanostructure arrays fabricated by depositing various thicknesses of Ag from 10 to 100 nm onto the porous surface of free-standing AAO with thickness ∼3 μm. As indicated by the red arrows in 23931

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Figure 3. Optical reflection spectra of AAO/Al-based Ag nanostructures arrays. (a) tAg = 30 nm, tAAO = 95, 130, 170, 225, 290, 310, and 410 nm, respectively. (b) tAg = 40 nm, tAAO = 100, 120, 175, 200, 240, and 275 nm, respectively.

Figure 2, the extinction peak that resulted from SPR of Ag nanostructure arrays is red-shifted first and then blue-shifted with the increase of tAg. The strongest and stronger extinction bands near the 514 and 633 nm excitation laser appear when the tAg reached 30 and 40 nm, respectively, which were used as optimized Ag deposited thickness in a later section. The optical reflection spectra of AAO/Al-based Ag nanostructure arrays with tAg 30 and 40 nm but different tAAO values (indicated by legends) are shown in Figure 3a and b, respectively. The oscillation interval dependence on tAAO presents typical interference characters, decreasing with the increase in tAAO, but the oscillation amplitude at SPR range (∼350−750 nm in our experiments) presents the combined feature of SPR and interference, which are marked by green shadow in Figure 3. By adjusting tAAO, the lowest reflectivity occurs near the wavelength range of the excitation (indicated by dash dot lines in Figure 3), such as 610 and 638 nm for tAAO 170 and 290 nm in Figure 3a, and 630 and 678 nm for tAAO 175 and 275 nm in Figure 3b, which are indicated by red arrows and caused by the SPR and destructive interference coenhanced absorption effects. On the contrary, the constructive interference results in the adverse reflection feature, such as 682 and 628 nm, indicated by a blue arrow, for tAAO 130 and 225 nm in Figure 3a. The influence of SPR and interference on the reflection character of AAO/Al-based Ag nanostructure arrays can also be identified from the reflection spectra of AAO/Al-based Ag nanostructure arrays, AAO-based array, and AAO/Al template without Ag23 (shown in Figure 4). The reflection minima of AAO/Al-based Ag nanostructure arrays (pit 1, pit 2, and pit 3, respectively) exhibit different features. Outside of the range of SPR (smaller than 400 nm), pit 1 exhibits shape similar to that of pit 1′ (reflection minimum of AAO/Al template without Ag) in addition to the little red-shift in position and increase in depth, which resulted from reflectivity variation at the top interface (AAO/air → Ag/air). Yet in the range of SPR (larger than 400 nm), two reflection minima (pits 2 and 3) correspond to one

Figure 4. Reflection spectra of AAO/Al-based Ag nanostructure arrays, AAO-based array, and AO/Al template without Ag.

reflection minimum (pit 2′), and the shape, position, and depth of pit 2 and pit 3 are different from each other and that of pit 2′; all of these further indicate that in the range of SPR, the reflection character of AAO/Al-based Ag nanostructure arrays is determined by both interference and SPR. The SERS spectra of R6G (∼10−6 mol/L) molecules absorbed on the AAO/Al-based Ag nanostructure arrays with tAg 30 nm and different tAAO values are shown in Figure 5a; the excitation light is 633 nm laser. Also, the tAAO-dependent EF for Raman lines 612, 1310, and 1363 cm−1 is shown in Figure 5b; the EF were calculated on the basis of the relation EF = (ISERS/ NSERS)/(IRaman/NRaman),24,25where ISERS and IRaman are the intensities of the SERS and the normal Raman spectra, respectively. NSERS and NRaman are the numbers of molecules adsorbed on AAO/Al-based Ag nanostructure arrays and in the capillary within the focused beam spot, respectively. NRaman/ NSERS was estimated from the relation NRaman/NSERS = (SRaman × DCap × 10−2 mol/L)/{[(50 μL × 10−6 × 10−6 mol/L)/SSample] × SSERS},11 where SRaman and SSERS are area of focused beam spot on the SERS sample surface and the axial plane of capillary, respectively. A 50× objective (NA = 0.75) and 10× objective 23932

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Figure 5. SERS spectra and the tAAO-dependent EF for AAO/Al-based Ag nanostructure arrays with tAg 30 nm and different tAAO values. The excitation light is a 633 nm laser.

Figure 6. SERS spectra of AAO/Al-based Ag nanostructure arrays with tAg 40 nm and different tAAO values. The excitation light used in (a) and (b) is 514 and 633 nm laser, respectively.

tAAO (290 nm), and so does the maximum EF, which is 9.77 × 107 and 6.67 × 107 for 612 and 1363 cm−1 Raman lines, respectively (the maximum EF for tAg 30 nm substrate is 1.17 × 107 for 612 cm−1 Raman line). Comparative analysis of the dependence of the intensity of SERS spectra in Figure 6b on the reflectivity of reflective spectra in Figure 3b found that the SERS enhancement not only depends on the reflectivity near excitation wavelength but also its rate of change. For example, although reflectivity of 275 nm sample at excitation wavelength 633 nm is higher than that of 175 nm sample, as shown in Figure 7, the SERS signal for 275 nm sample

(NA = 0.25) were used in SERS and normal Raman measurement, respectively, and the diameter of the focused beam spot can be calculated with 1.22λ/NA. DCap is the diameter of the capillary (∼1 mm). To demonstrate the influence of interference on the EF, the corresponding tAAO-dependent reflectivity is also shown in Figure 5b. As shown in Figure 5a, Raman signals fluctuate with the increase of tAAO, and Figure 5b shows that the fluctuation in EF with tAAO follows the reverse reflectivity variation, for example, high EF corresponding low reflectivity, indicating that the SERS are strongly affected by the interference effect. The maximum EF is 1.17 × 107 for 612 cm−1 Raman line when the tAAO reaches 290 nm. The SERS spectra obtained from AAO/Al-based Ag nanostructure arrays with tAg 40 nm and different tAAO values are shown in Figure 6. The excitation light used for Figure 6a and b is 514 and 633 nm laser, respectively. The Raman signals excited with 514 nm laser are lower than that of 633 nm. The tAAO-dependent SERS feature is different for different excitation lasers. For example, the maximum Raman enhancement is obtained when tAAO is 175 nm for 514 nm excitation laser, then 275 nm for 633 nm, which also arise from interference effects (as shown in Figure 3b, destructive interference for tAAO 175 and 275 nm occurs at about 514 and 633 nm, respectively). Comparing Figure 5a with Figure 6b, we find that under 633 nm excitation, the total SERS signals [blue solid line in Figure 6b] from AAO/Al-based Ag nanostructure arrays with tAg 40 nm and optimal tAAO (275 nm) are higher than those [green solid line in Figure 5a] from tAg 30 nm and optimal

Figure 7. Optical reflection spectra of AAO/Al-based Ag nanostructure arrays with tAg = 30 nm and tAAO = 175 and 275 nm, respectively.

is much higher than that of 175 nm sample due to their difference in rate of change of reflectivity (indicated by green and red solid 23933

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Figure 10. 3D simplified model of AAO/Al-based Ag nanostructure arrays, P = 120 nm, D = 40 nm.

According to the EM enhancement mechanism (EF ≈ EFE4, where EFE = E/E0, E and E0 are the magnitudes of the local and incident electric field, respectively), the local EM distribution of AAO-based and AAO/Al-based Ag nanostructure with different tAAO and tAg values was calculated using FDTD (Finite Difference Time Domain) solutions (version 8.6) software provided by Lumerical Solutions, Inc. for demonstrating the contribution of SPR and interference to the SERS enhancement effects. The Ag nanostructure arrays were described as a rough Ag nanopore array formed on the porous surface of AAO/Al template. A 3D simplified model for AAO/ Al-based Ag nanostructure arrays is shown in Figure 10. The pore diameter (D) and separation (P) of Ag nanopore arrays as well as AAO/Al template are 40 and120 nm, respectively. The thicknesses of Ag layer, AAO, and Al are indicated by tAg, tAAO, and tAl, respectively. The dispersive dielectric constants of silver, AAO, and Al used for simulations were taken from Palik’s data. The incident light is propagating perpendicular to the surface of AAO/Al-based Ag nanostructure arrays and the polarization parallel to its surface. The FDTD simulations were performed using the program FDTD Solutions (version 8.6 Lumerical Solutions, Inc.). Because of the periodicity of AAO/ Al-based Ag nanostructure arrays in the x−y plane, periodic boundariy conditions (PBCs) along the x and y directions were adopted, and then along the z direction the perfectly matched layer (PML) boundary conditions were adopted. Furthermore, we adopted a graded mesh approach to accurately model AAO/Al-based Ag nanostructure arrays in a 3D-FDTD simulation while maintaining tractable simulation times. The fine mesh grid size is 2 nm in simulation volume. In the surrounding air region, the coarse conformal mesh accuracy setting of 3 nm was adopted. The convergence testing was done, and the result indicated that these simulations were computationally feasible. Figure 11 shows the representative EM simulation results. The EFE in Figure 11a is about twice larger than that in Figure 11b, indicating that the existence of Al substrate, which supports the interference of reflected light at the interface of AAO/Al and Ag/air (as shown in Figure 9), further amplifies the local EM fields. As shown in Figure 11a,c, the EFE for AAO/ Al-based substrate strongly depends on the tAAO. Comparing Figure 11a and d shows that EF for AAO/Al-based substrate strongly depends on the tAg. All of the aforementioned results demonstrate that the local EM fields enhancement and the corresponding SERS are determined by both SPR and interference effects.

Figure 8. SERS spectrum average of 5−8 different positions on the AAO/Al-based Ag nanostructure arrays with different tAg and tAAO values.

Figure 9. Schematic diagram of AAO/Al-based Ag nanostructure arrays.

lines, respectively, in Figure 7). Also, the mechanism of this dependence is not explained in this Article. To verify the reproducibility and homogeneity of as-prepared AAO/Al-based Ag nanostructure arrays, SERS spectra were taken from 5−8 different positions on the sample. The average intensity and standard deviation (σ) were calculated. The results shown in Figure 8 indicate that the standard deviation is below 10%. The above experiment results show that by optimizing both tAg and tAAO, the high performance SERS-active substrate was obtained, which resulted from the SPR and interference coenhanced effects. Figure 9 shows the schematic diagram of SPR and interference coenhanced effects in AAO/Al-based Ag nanostructure. Silver nanostructure arrays couple the incident light into SPR mode, while the Ag/AAO and AAO/Al interfaces couple the incident light into interference of reflected lights, for example, R1 and R2 shown in Figure 9. 23934

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Figure 11. EM distribution of AAO-based and AAO/Al-based Ag nanostructure arrays with different tAg and tAAO values. (a and c) AAO/Al-based substrate with tAg 30 nm and tAAO 100 and 300 nm, respectively; (b) AAO-based substrate with tAAO 3 μm and tAg 30 nm; and (d) AAO/Al-based substrate with tAg 32 nm and tAAO 100 nm.

4. CONCLUSION In summary, the AAO/Al-based Ag nanostructure arrays were fabricated by depositing Ag on the surface of the AAO/Al template. Because the SPR and interference of reflected light at the interface of AAO/Al and Ag/air are in coexistence,

by carefully optimizing the thickness of Ag and AAO, optimizing SPR and interference properties, the SPR and interference coenhanced SERS-active substrate was obtained. The optimal thicknesses of Ag and AAO are 40 and 275 nm, respectively. The maximum SERS EF is 9.77 × 107 for 612 cm−1 Raman line, 23935

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which is higher than that of AAO-based Ag nanostructure arrays. Also, the mechanisms of SPR and destructive interference coenhanced local EM as well as SERS enhancement were demonstrated by FDTD simulation results. By comparison with the AAO-based Ag nanostructure arrays SERS substrate, the AAO/Al-based array not only has much higher EF, but also is more robust (supported on Al) and easier to fabricate (without removing residual Al process), which are very important for the SERS technique to be achieved in practical application.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The Industry Key Technologies R&D project in Shaanxi Province of China (Grant no. 2012K07-19). REFERENCES

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