Silver Nanorod Array Substrates Fabricated by Oblique Angle

Apr 15, 2010 - Haley Marks , Monika Schechinger , Javier Garza , Andrea Locke ..... Sasani Jayawardhana , Gorgi Kostovski , Alex P. Mazzolini , Paul R...
0 downloads 0 Views 4MB Size
Download PDF
8176

J. Phys. Chem. C 2010, 114, 8176–8183

Silver Nanorod Array Substrates Fabricated by Oblique Angle Deposition: Morphological, Optical, and SERS Characterizations Y.-J. Liu,* Hsiao Yun Chu, and Y.-P. Zhao Department of Physics and Astronomy, and Nanoscale Science and Engineering Center, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: January 7, 2010; ReVised Manuscript ReceiVed: February 24, 2010

Silver nanorod arrays with different lengths fabricated by oblique angle deposition at various vapor deposition angles have been studied systematically on their morphologies, optical reflections, and surface-enhanced Raman scattering (SERS) responses. The tilting angle β of Ag nanorods increases with the increase of the deposition angle θ, while the diameter D and density n of Ag nanorods grow as power laws of the length L, D ∼ Lpand n ∼ L-γ, where the exponents are p ∼ 0.30-0.34 and γ ∼ 0.23-0.40 for different deposition angles, respectively. The optical reflectance from these substrates depends not only on the length of the Ag nanorods but also on the deposition angle. It is found that the SERS enhancement factor decreases nearly monotonically with the increase of the reflectance at SERS excitation wavelength, and the highest SERS enhancement factor can reach close to 109. We have developed a phenomenological model based on the assumption that the absorbance (reflection) of the nanorod array is directly linked to the local electric field, and it predicts a similar trend to that of the experimental observation. The empirical results can help us to design better Ag nanorod array SERS substrates, and can also be used as a quality control measurement method for SERS substrate production. I. Introduction Surface-enhanced Raman scattering (SERS) is a promising and powerful spectroscopic method in bioagent analysis.1-3 One major enhancement mechanism for SERS is due to the enhancement of localized electric field generated by the specific nanometer scale morphology of metallic structures.4 Therefore, a critical aspect of SERS related research and application is to develop a specific morphology of metallic surface to alter the local electric field and achieve a reproducible and high level of enhancement. Various nanostructures fabricated by different techniques have been used as SERS-active platforms to manifest the SERS effect, such as aggregates of colloidal particles,5,6 nanoparticle-decorated nanocanals,7 Ag and Au nanorods and nanowires fabricated by chemical and electrochemical methods,8,9 regular nanoparticle arrays prepared by nanosphere lithography10 or electron beam lithography (EBL),11,12 or semiconducting nanoparticles (TiO2 hybrid composites).13 Unfortunately, many of these fabrication methods are either expensive or timeconsuming, and fail to produce reproducible substrates to provide maximum SERS enhancements. Recently, Ag nanorod arrays fabricated by oblique angle deposition (OAD) have shown very strong SERS activities.14-16 OAD is a physical vapor deposition technique in which the vapor atoms are deposited on a substrate at a large incident angle θ (>70°) with respect to the surface normal of the substrate. Due to the shadowing effect and surface diffusion, nanocolumnar structures can be formed.19-21 This technique has the following major advantages: the diameter, shape, spacing, and density of the nanostructures can be easily controlled by changing the deposition conditions such as the deposition angle, growth time, growth rate, and substrate temperature. These Ag nanorod array SERS substrates have a good uniformity and To whom correspondence should be addressed. E-mail: leoyjliu@ physast.uga.edu.

reproducibility and have been demonstrated to be able to distinguish and identify viruses and bacteria.17-19 Due to the tilting nature of Ag nanorod arrays, they also have their own unique characteristics. In our previous studies, we have found that the SERS intensity strongly depends on the length of the Ag nanorods,15,18 the incident angle,14 and the polarization of excitation light.16 We have developed a simple qualitative theory to explain some characteristics of SERS from Ag nanorod arrays for different excitation configurations.16,20 Very recently, we have investigated what locations of the Ag nanorods generate the most SERS signal.21 Both the experimental and theoretical results show that SERS signals come mostly from the side surface of Ag nanorod arrays, not from the bottom of Ag nanorods or the junction between Ag nanorods and Ag film (possible “hot spot” locations).21 By combining OAD and EBL techniques, we can also fabricate semiordered Ag nanorod array substrates for SERS detection.22 All of those studies are based on Ag nanorod arrays fabricated only at a specific OAD deposition angle, θ ) 86°. Under this condition, we have obtained optimal Ag nanorod SERS substrates, with a length of Ag nanorods of about 900 nm, diameter of about 100 nm, average separation ∼177 nm, and tilting angle of 71°.18 However, since one could deposit Ag nanorod arrays at different deposition angles with the OAD technique, a systematic morphological and SERS characterization of Ag nanorod arrays deposited at different vapor incident angles with different nanorod lengths is necessary. In this paper, we systematically studied the morphology, optical reflectance, and SERS response of Ag nanorod arrays with different nanorod lengths and deposition angles fabricated by OAD techniques. We have found that the nanorod tilting angles β relative to the substrate normal increase with the increase of the deposition angle θ. The diameter D of Ag nanorods increases as a power law of Ag nanorod length L, D ∼ Lp, and p ∼ 0.30-0.34 for different deposition angles, while

10.1021/jp1001644  2010 American Chemical Society Published on Web 04/15/2010

Silver Nanorod Array Substrates

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8177

Figure 1. (a) Schematic diagram of the Ag nanorod array fabricated by oblique angle deposition. (b) The definition of deposition angle θ and Ag nanorod tilting angle β.

the density n of Ag nanorods decreases as a power law of L, n ∼ L-γ, with γ ∼ 0.23-0.40 for different deposition angles. The reflectance R from the surface of Ag nanorod arrays depends not only on the length of Ag nanorods but also on the deposition angles: the larger the deposition angle, the less the reflectance at the excitation wavelength. It is also found that the SERS enhancement factor (EF) decreases monotonically with the increase of the reflectance of Ag nanorod arrays at the excitation wavelength, regardless of the length of Ag nanorods and the deposition angle. We have developed a phenomenological model based on the assumption that the absorbance (reflection) of the nanorod array is directly linked to the local electric field, and it predicts a similar EF-R trend as the experimental data. II. Experiments The details of the deposition configuration and conditions have been reported elsewhere.15 The Ag nanorod arrays were deposited by a custom-designed electron-beam evaporation system. Figure 1a shows a schematic diagram of a Ag nanorod array fabricated by oblique angle deposition. In Figure 1, the vapor deposition angle θ is the angle between the vapor flux direction and the substrate surface normal. Before the deposition, all Si(100) wafers were cleaned by the RCA-1 method (in 70 °C solution of deionized (DI) water:hydrogen peroxide:ammonium hydroxide ) 5:1:1) for 20 min followed by DI water rinse. A layer of 500 nm of Ag (Alfa Aesar, 99.999%) thin film was first deposited onto the cleaned Si wafers, and then, a layer of Ag nanorod array was prepared by the OAD technique at different deposition angles of θ ) 78, 80, 82, and 84°, respectively. During the evaporation, the thickness of the metal deposited was monitored by a quartz crystal microbalance (QCM) positioned at normal incidence to the vapor source. The base pressure was around 8.6 × 10-7 Torr, and the Ag growth rate was 0.3 nm/s. For each deposition angle, there were eight samples with different lengths deposited. For example, at θ ) 84°, the QCM reading of the Ag nanorods was l ) 393, 787, 1180, 1579, 1968, 2361, 3148, and 3936 nm, respectively. The morphologies of as-deposited samples were characterized by a field-emission scanning electron microscope (SEM) (FEI Inspect F). Due to the shadowing growth mechanism, the OAD deposition results in tilted Ag nanorod arrays on the substrate. The Ag nanorod tilting angle β is defined as the angle between the Ag nanorod tilting direction and substrate surface normal, as shown in Figure 1b, and can be characterized using the crosssectional SEM images. The reflectance spectra of the asdeposited samples were measured by a UV-vis 2450 spectrometer system with an integration sphere (Shimadzu). For SERS characterization, the Raman probe molecule, trans-1,2bis(4-pyridyl) ethylene (BPE, Aldrich, 99.9+%) was used, and a 1 µL droplet of BPE methanol solution with a concentration of 10-5 M was dispensed on the surface of the Ag nanorod

Figure 2. The top-view (up) and cross-section (down) SEM images of Ag nanorod arrays with length L ≈ 1200 nm prepared at θ ) 78, 80, 82, and 84°, respectively. The scale bar is the same for all images.

arrays. After the droplet was dried, the spreading area was observed to be about 1 cm2 for all of the substrates. The Raman spectra were recorded by the HRC-10HT Raman Analyzer from Enwave Optronics Inc., with the excitation wavelength λ0 ) 785 nm, the power 21 mW, the diameter of the laser spot 0.1 mm, and the collection time 10 s. To obtain the SERS enhancement factor, normal Raman spectra were measured from BPE methanol bulk solution with a concentration of 10-2 M contained in the quartz rectangular cuvette with a path length of 1 mm. III. Results and Discussion 1. Morphological Characterization of Ag Nanorod Samples. Figure 2 shows the top-view and cross-section SEM images of four representative Ag nanorod arrays prepared at θ ) 78, 80, 82, and 84°, respectively. The QCM reading length l for all four deposition groups is nearly the same, l ≈ 2310 ( 50 nm, and the measured nanorod length L ) 1190, 1160, 1290, and 1150 nm for θ ) 78, 80, 82, and 84°, respectively. In Figure 2, all four groups show tilted nanocolumar structures with slightly different morphologies: for Ag nanorod samples deposited at smaller incident angles (θ ) 78 and 80°), the Ag nanorods become fused together, forming a porous network structure, as shown in both the top-view and cross-section SEM images, while at large vapor incident angles (θ ) 82 and 84°), the Ag nanorods are well-separated. The nanorod tilting angle β can be directly measured from the cross-sectional SEM images, as shown in Figure 2. In order to obtain a better measurement of tilting angle β, we plot the height h of nanorods, i.e., the thickness of the nanorod layer, versus the measured nanorod length L in Figure 3a for all of the samples deposited at the same deposition angle θ. All of the data plotted in Figure 3a show a linear relationship, and according to the definition of tilting angle β, the slope of the plots is k ) ∆h/∆L ) cos β ) 0.54, 0.49, 0.45, and 0.43 for θ ) 78, 80, 82, and 84°, respectively. Thus, the corresponding tilting angle is β ) 57 ( 2, 61 ( 3, 63 ( 3, and 65 ( 2°, respectively. The column tilting angle β is less than the deposition angle θ. Figure 3b plots the Ag nanorod tilting angle β as a function of deposition angle θ. With an increase of deposition angle θ, the tilting angle β is

8178

J. Phys. Chem. C, Vol. 114, No. 18, 2010

Liu et al.

Figure 3. (a) Plot of Ag nanorod height h versus Ag nanorod length L for samples deposited at θ ) 78, 80, 82, and 84°, respectively. (b) The Ag nanorod tilt angle β as a function of deposition angle θ.

Figure 4. log-log plots of the diameter D (a) and the density n (b) of Ag nanorods versus the length L for various deposition angles θ.

shown to increase. This result is consistent with most of the reports for OAD. In the literature, there are two empirical β-θ relationships proposed, the tangent rule βt ) arctan(1/2 tan(θ))23 or the cosine rule βc ) θ - arcsin[(1 - cos θ)/2].24 The two solid curves in Figure 3b shows the predicted tilting angle at different θ. Unfortunately, the measured tilting angle β for Ag nanorods observes neither the tangent rule nor the cosine rule; rather, it falls in between βt and βc, and is close to the average value β ≈ (βt + βc)/2 (the dashed curve in Figure 3b).25 From the SEM images, we also found that the diameter of the nanorod and the separation between nanorods changed systematically with nanorod length L and deposition angle θ. Figure 4 shows the log-log plots of the diameter D and the density n of Ag nanorods versus nanorod length L for different deposition angle θ. The diameter of the Ag nanorod diameter D was measured at the very growth end (tip) of the nanorod. With the increase of nanorod length L, the nanorod diameter D becomes larger and larger, while the density n becomes smaller and smaller. For example, when the length of Ag nanorods is L ) 1200 nm, the diameters of Ag nanorods are D ) 170 ( 30, 140 ( 20, 130 ( 20, and 120 ( 20 nm for θ ) 78, 80, 82, and 84°, respectively. In fact, both diameter D and density n follow power laws with nanorod length L: D ∼ Lp and n ∼ L-γ,26-28 where p is the growth exponent for nanorod diameter and is determined as p ∼ 0.30-0.34 for different deposition angles θ and γ is the growth exponent for nanorod density and is determined to be γ ∼ 0.23-0.40 for different deposition angles θ. The exponent p is consistent with those reported in the literature. Karabacak et al. reported that p was measured to be ∼0.28-0.34 for different materials, Co, Cu, Si, and W, deposited at θ ) 85°, respectively.26 Buzea et al. reported that

the p value could be changed from 0.3 to 0.6 by varying θ from 75 to 89° for Si deposition.27 However, the exponent γ is very different in the literature. Smith et al. reported that γ ∼ 0.3 for TiO2 deposited at θ ) 86°,29 while Zhou et al. reported that γ ) 1.02 for Al deposited at θ ) 84°. For Ta deposited at θ ) 84°, γ is more complex, with γ ) 2.5 for Ta thicknesses smaller than 250 nm and γ ) 0.5 for thicknesses larger than 250 nm.30 The consistency of our reported p and those in the literature show that p is almost independent of the materials used for deposition, and is mainly determined by the shadowing effect,28 and p is slightly different at different deposition angles θ; for γ, it not only depends on the shadowing effect but also strongly relies on the deposited materials. 2. Optical Reflectance from Ag Nanorod Samples. All SERS related studies have shown that the SERS activity of nanostructured films strongly depends on the optical properties of the structures. We have demonstrated that the absorbance spectra from Ag nanorod arrays deposited on glass substrates are determined by the length of the Ag nanorods.25 Thus, it is very important to characterize the optical properties of our Ag nanorod samples, especially their optical absorbance properties. For our samples, the substrate is Si and there is a 500 nm thick Ag film deposited between Ag nanorods and Si wafer; therefore, one can only characterize the optical reflectance R of the samples. Figure 5a shows reflectance spectra from Ag nanorod substrates with different Ag nanorod lengths L prepared at the vapor incident angle θ ) 84°. For different lengths L, the shapes of reflectance spectra are quite similar: they all have a nearly constant R in the near IR region. Figure 5b plots the reflectance R at λ ) 785 nm as a function of length L: the reflectance R first decreases with the increase of length L, it reaches a

Silver Nanorod Array Substrates

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8179

Figure 5. (a) Reflectance spectra of Ag nanorod arrays with different lengths L deposited at θ ) 84. (b) The reflectance R and absorbance A at λ ) 785 nm as a function of nanorod length L. (c) The spectra of the effective absorbance coefficient R for Ag nanorod arrays with different lengths deposited at θ ) 84°.

Figure 6. Incident and transmission configurations of light interaction with Ag nanorod array substrates, and for SERS signal detection. This sketch illustrates the coordinates to calculate the effective EF based on the phenomenological model.

minimum when L ≈ 1200 nm, and then it increases with further increase of length L. In fact, the reflectance R for the near IR wavelength region can be treated as a transmission through the Ag nanorod array layer after the incident light was absorbed twice by the Ag nanorod array because the reflectance of 500 nm Ag film is ∼1 at λ > 400 nm, which is shown in Figure 6. The reflection can be approximately expressed as

R ) e-2Rh

(1)

where R is the effective absorbance coefficient under the assumption that for the absorbance coefficient R is a constant across the entire thickness of the Ag nanorod film and h is the thickness of the Ag nanorod layer. The absorbance A of the Ag nanorod arrays can be written as

A ) 2Rh ) -ln(R)

(2)

In Figure 5b, the absorbance A at λ0 ) 785 nm is also plotted as a function of nanorod length L: it first increases with L, reaches a maximum when L ≈ 1200 nm, and then decreases with the further increase of L. According to the above formula and using the measured h in Figure 3a, the spectra of the absorbance coefficient R from Ag nanorod arrays can be calculated. Figure 5c shows the spectra of R for nanorods with different length L prepared at the deposition angle θ ) 84°: the R value decreases monotonically with length L within the entire measured spectral region; this is because, with the increase of nanorod length L, the diameter D of the nanorod increases, while the density n of the nanorods decreases in a faster manner so that the overall porosity of the Ag nanorod array decreases with L; i.e., the Ag volume filling ratio increases. According to the effective medium theory, the R value is expected to decrease monotonically. For samples with similar length L (≈1200 nm) deposited at different deposition angles θ, the reflectance spectra R shown in Figure 6a are strongly dependent on the deposition angle, especially for the wavelength region λ > 500 nm; R-θ follows a similar trend. Figure 6b shows the relationship of R at λ0 ) 785 nm and the deposition angle θ for a fixed length of L ≈ 1200 nm: the reflectance R decreases monotonically with deposition angle θ, and an opposite trend of the absorbance A is also shown in Figure 6b. This result is consistent with the morphology of Ag nanorod arrays due to OAD: with the increase of deposition angle θ, the shadowing effect will play a greater role in thin film growth, and the nanorod film becomes more porous; thus, the effective absorbance coefficient R becomes smaller. For nanorod samples with similar thickness (or length) deposited at different deposition angles, the larger the deposition

8180

J. Phys. Chem. C, Vol. 114, No. 18, 2010

Liu et al.

Figure 7. (a) Reflectance spectra of Ag nanorod arrays with a fixed length L ) 1200 nm prepared at different deposition angles. (b) The reflectance R and absorbance A at λ ) 785 nm as a function of deposition angle θ.

Figure 8. Representative Raman and SERS spectra of BPE: (a) normal Raman from bulk BPR solution; (b) two BPE SERS spectra from Ag nanorod arrays deposited at the deposition angle θ ) 78° with lengths L ) 336 and 664 nm, respectively; (c) two BPE SERS spectra from Ag nanorod arrays deposited at θ ) 84° with lengths L ) 152 and 1148 nm, respectively.

angle, the more porous the film, the higher the reflectance, and the less the absorbance of Ag nanorod array substrates. 3. SERS Characterization of Ag Nanorod Samples. Previously, we have demonstrated that the SERS enhancement factor (EF) is strongly dependent on the length of Ag nanorods prepared at θ ) 86°, and the optimum SERS substrate has a length of L ≈ 900 nm.15,18 Using BPE, we also measured the SERS EF for Ag nanorod samples with different length L and deposition angle θ. Figure 8 shows some representative Raman and SERS spectra of BPE obtained in our experiments. Figure 8a shows the normal Raman spectrum obtained from BPE bulk solution; Figure 8b shows two SERS spectra of BPE from Ag nanorod array substrates deposited at the deposition angle θ ) 78° with nanorod length L ) 336 and 664 nm, respectively; Figure 8c shows two SERS spectra of BPE from Ag nanorod array substrates deposited at θ ) 84° with length L ) 152 and 1148 nm, respectively. All of the spectra show the following Raman characteristic peaks of BPE: ∆ν ) 1200 cm-1 (CdC stretching mode), ∆ν ) 1610 cm-1 (aromatic ring stretching mode), and ∆ν ) 1640 cm-1 (in-plane ring mode). The normal Raman signal is significantly smaller than those obtained from Ag nanorod array substrates, and for substrates fabricated at different deposition angles θ, the SERS intensities are also different. In order to quantitatively compare the SERS substrates, the SERS peak at ∆ν ) 1200 cm-1 and the equation SERS EF ) (NRISERS)/(NSERSIR) were used to estimate the SERS enhancement factor (EF), where NR and IR are the molecular number

and Raman intensity in normal Raman measurement and NSERS and ISERS are the SERS molecular number and intensity.31 In our experiment, the molecular number excited in normal Raman measurement was estimated as NR ) 4.89 × 1013, and the normal Raman peak intensity at band ∆ν ) 1200 cm-1 was IR1200 ) 124 counts. The molecular number excited in SERS measurement was about the same for all of the SERS substrates, NSERS ) 4.89 × 108. Figure 9a plots the calculated SERS EF as a function of nanorod length L for Ag nanorod samples prepared at θ ) 78, 80, 82, and 84°, respectively. For different deposition angles, the SERS EF also shows strong length dependence with a similar trend: the EF first increases with the nanorod length L, reaches a maximum at an optimum length L, and then decreases with further increase of length L. More specifically, for θ ) 78°, L < 200 nm, the EF is relatively low (∼104). The EF reaches a maximum value of 8.7 × 105 when L ) 660 nm, and then decreases slightly, and then increases to 2.0 × 106 when L ) 2100 nm. For θ ) 80°, the EF-L relationship follows the same trend of that for θ ) 78°. The EF reaches a maximum value of 7.0 × 106 when L ) 600 nm and 9.8 × 106 when L ) 2100 nm. For θ ) 82°, the EF reaches a maximum value of 2.4 × 107 when L ) 660 nm. For θ ) 84°, the maximum EF value is 7.2 × 108 when L ) 1100 nm. For all of the samples, the SERS EF obtained from samples prepared at θ ) 84° is apparently much larger than those deposited at θ ) 78, 80, and 82°. To further illustrate this point, we have selected four samples with a fixed Ag nanorod length L (≈160 nm) but deposited at different deposition angles θ and plotted the SERS EF as a function of θ in Figure 9b. Figure 9b shows that, for nanorod samples with similar lengths, the larger the deposition angle θ, the larger the SERS EF. It shows the following quantitative trend: as θ increases with a 2° increment, the SERS EF almost increases 1 order of magnitude. There are several possible reasons for such a trend. First, from a morphological point of view, at small deposition angle θ, the density of the Ag nanorod is large. There will be a higher probability for two adjacent nanorods to contact each other, which changes the SERS hot spot distribution; i.e., the two adjacent connected nanostructures could result in low localized electric fields, and thus cause a low SERS enhancement factor for Ag nanorod arrays deposited at smaller deposition angles. As shown in Figure 2, some Ag nanorods are even stuck together at smaller deposition angle. Another possible reason is that the diameters of Ag nanorods deposited at larger angles are relatively small at a fixed length L, and the diameter D of the Ag nanorod is larger than 100 nm for L > 200 nm for most of the samples (Figure 3a). Mo et al.32 and Du et al.33 reported that the optimum

Silver Nanorod Array Substrates

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8181

Figure 9. (a) SERS EF as a function of nanorod length L for samples deposited θ ) 78, 80, 82, and 84°, respectively. (b) SERS EF as a function of deposition angle θ at a fixed nanorod length of L ) 165 nm.

Figure 10. (a) SERS EF versus the reflectance R785 at λ ) 785 nm from the Ag nanorod array samples with different nanorod lengths and prepared at different deposition angles. (b) SERS EF versus the reflectance R785 from samples with a fixed nanorod length of L ) 1200 nm and L ) 164 nm prepared at different deposition angles θ, respectively.

size of the Ag nanoparticles for SERS activity was around 100 nm. When the size was smaller than 100 nm, the SERS signal increased with an increase of the size; otherwise, the SERS intensity decreased with the increase of the size of Ag nanoparticles. At the length L ) 1200 nm, denoting D78°, D80°, D82°, and D84° as the diameter of the Ag nanorod for θ ) 78, 80, 82, and 84°, respectively, we find that all of these diameters are larger than 100 nm: D78° ) 170 nm, D80° ) 140 nm, D82° ) 130 nm, and D84° ) 120 nm. Since D78° > D80° > D82° > D84°, the SERS EF increases with an increase of deposition angle according to the above argument. In fact, we find that the SERS EF closely depends on the reflectance of the SERS samples at the excitation wavelength. Figure 9a shows a semilog plot of the SERS EF versus the reflectance R785 at λ0 ) 785 nm from all of the samples fabricated at different θ and with different length L. Figure 10a shows that, regardless of the deposition angle or the nanorod length, the SERS EF increases with the decrease of reflectance, i.e., log(EF) ∝ -R785, as demonstrated by the straight line in Figure 10a. One extraordinary observation is that, when the reflectance drops from 76 to 48%, i.e., a change of 28%, the SERS enhancement factor changes almost 5 orders of magnitude. Also, in Figure 10a, the data can be categorized into four groups, and each group corresponds to a specific deposition angle θ. When the length L of Ag nanorods is set at a similar value, this relationship can be illustrated even better, as shown in Figure 10b. Figure 10b shows the SERS EF versus R785 for fixed lengths of L ) 1200 nm and L ) 164 nm. For Ag nanorod arrays deposited

at different θ, the SERS EF apparently decreases with the increase of the reflectance at excitation wavelength λ ) 785 nm. Therefore, the SERS EF strongly depends on the reflectance from SERS substrates. As we discussed above, the reflection is directly related to the absorbance by A ) -ln(R), which means for our SERS Ag nanorod array substrates the larger the absorbance, the larger the SERS EF. This generally agrees with predictions from electromagnetic (EM) theory of SERS. For example, Van Duyne’s group has performed a scanned wavelength SERS measurement of the Ag nanoparticle system, and they found that the SERS intensity reached a maximum when the excitation wavelength was close to the localized surface plasma resonant (LSPR) wavelength, i.e., the maximum absorbance.34 The work in ref 34 is different from ours, since it focused on the effect of LSPR, and our Ag nanorod substrates do not show a particular LSPR peak at λ0 ) 785 nm. However, phenomenologically, it is similar to our observation; i.e., when the excitation laser wavelength is moved close to the LSPR wavelength, there will be a higher extinction, and the nanostructure surfaces could have a higher local electric field distribution; thus, the structure will demonstrate the higher SERS EF. Theoretically, the total absorbed power can be derived from the total fields on the surface of a small particle; hence, the absorption directly links to the local electric fields.35 Thus, we can assume that the layer absorbance is closely related to the local electric field of the Ag nanorod substrates, and we propose the following phenomenological model to qualitatively explain our experimental results.

8182

J. Phys. Chem. C, Vol. 114, No. 18, 2010

Liu et al.

We believe that the main SERS mechanism for Ag nanorod array substrates is electromagnetic enhancement. For the low frequency SERS band, the SERS EF is proportional to the fourth power of the local electric field EL, EF ∼ |EL|4. According to the above discussion, assuming that EL of a nanorod array in a specific thickness x is directly proportional to the absorbed intensity, EL2 ∝ RI(x), then the SERS EF, EF(x) ∝ R2I(x)2. Here, we treat the Ag nanorod array with thickness h as having a constant effective absorbance coefficient R, as shown in Figure 6, and I(x) is the excitation intensity at the thickness x. We assume that the SERS probe molecules are uniformly distributed in the Ag nanorod layer and the surface coverage of the probe molecules for different samples with different length L and deposition angle θ is almost the same. According to the incident configuration and coordinate shown in Figure 10, the total SERS EF can be expressed as

EF ∝

∫0h (RI0e-R(h-x))2e-R(h-x) dx

(3)

The additional term e-R(h-x) in the integration is added, since the SERS signal excited at the infinitesimal dx layer should experience absorbance by the Ag nanorods above this dx layer in order to reach the Raman detector. This consideration is due to our recent experimental and theoretical findings regarding the importance of layer absorbance and anisotropic absorbance for Ag nanorod substrates.21,36 After integration, eq 3 give us

EF ∝ 1 - e-3Rh

(4)

EF ∝ 1 - R3/2

(5)

or according to eq 1

Since 0 < R < 1, our model predicts that EF increases with the decrease of reflectance R, which qualitatively agrees with the experimental results. In Figure 10a, the rescaled function EF ) C(1 - R3/2) (red curve) is plotted together with the experimental data, where C is a constant. The predicted data follow the experimental trend. However, since this phenomenological model does not consider the specific local electric field distribution generated by the specific morphology, it cannot predict the exact EF values and the detailed trend. To be more quantitative, one has to consider the contributions from the details of the nanorod arrays, such as the randomness in nanorod distribution, the irregularity, shape, and size distribution of nanorods, the tilting angle and specific length, etc. Only a sophisticated numerical calculation based on classic EM theory could potentially provide such an answer. Nevertheless, our assumption based on the link between the local electric field and absorbance does provide a qualitative view on the potential SERS mechanism of the Ag nanorod array substrates, and further consideration could include this finding. IV. Conclusions In conclusion, we have performed a systematic study on the SERS activity of Ag nanorod array substrates with different nanorod lengths deposited at different angles. We find that both the diameter and density of Ag nanorods follow power laws with the length of Ag nanorods, with the exponents p ) 0.30-0.34 and γ ) 0.23-0.40 for various

deposition angles. For Ag nanorods with similar lengths, a larger deposition angle will yield a smaller diameter. The reflectance from Ag nanorod array substrates is dependent on the length of Ag nanorods and the deposition angle. For Ag nanorods with a similar length deposited at different deposition angles, the larger deposition angle is employed during the deposition, the less reflectance is obtained from the surface of Ag nanorod array substrates. The SERS EF shows a strong dependence on the nanorod length and deposition angle. Most significantly, the SERS EF is found to strongly rely on the reflectance of the Ag nanorod array substrates: the SERS EF decreases almost linearly in a semilog plot with the increase of the reflectance at the excitation wavelength. This EF-R relationship can be qualitatively explained by a phenomenological model by assuming the layer absorbance is directly linked to the local electric field. Regardless of the detailed mechanism, this empirical EF-R relationship can serve two important purposes for SERS based research and development. The result can give us guidance to adjust the reflectance of the Ag nanorod array to perform quality control of the SERS substrate production. Acknowledgment. Y.-J.L., H.Y.C., and Y.-P.Z. thank the National Science Foundation (No. ECS-0701787) for support. The authors thank Mr. Justin Abell for proofreading this manuscript. References and Notes (1) Hudson, S.; Chumanov, G. Anal. Bioanal. Chem. 2009, 394, 679. (2) Jarvis, R. M.; Goodacre, R. Chem. Soc. ReV. 2008, 37, 931. (3) Driskell, J. D.; Shanmukh, S.; Liu, Y.-J.; Hennigan, S.; Jones, L.; Zhao, Y. P.; Dluhy, R. A.; Krause, D. C.; Tripp, R. A. IEEE Sens. J. 2008, 8, 863. (4) Willets, K. A.; Van Duyne, R. P. Annu. ReV. Phys. Chem. 2007, 58, 267. (5) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (6) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667. (7) Hyunhyub, K.; Vladimir, V. T. Small 2008, 4, 1980. (8) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229. (9) Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372. (10) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (11) Liu, Y.-J.; Zhang, Z. Y.; Zhao, Q.; Dluhy, R. A.; Zhao, Y. P. Appl. Phys. Lett. 2009, 94, 033103. (12) Yu, Q.; Guan, P.; Qin, D.; Golden, G.; Wallace, P. M. Nano Lett. 2008, 8, 1923. (13) Musumeci, A.; Gosztola, D.; Schiller, T.; Dimitrijevic, N. M.; Mujica, V.; Martin, D.; Rajh, T. J. Am. Chem. Soc. 2009, 131, 6040. (14) Liu, Y.-J.; Fan, J.-G.; Zhao, Y.-P.; Shanmukh, S.; Dluhy, R. A. Appl. Phys. Lett. 2006, 89, 053117. (15) Chaney, S. B.; Shanmukh, S.; Dluhy, R. A.; Zhao, Y. P. Appl. Phys. Lett. 2005, 87, 031908. (16) Zhao, Y. P.; Chaney, S. B.; Shanmukh, S.; Dluhy, R. A. J. Phys. Chem. B 2006, 110, 3153. (17) Driskell, J. D.; Primera-Pedrozo, O. M.; Dluhy, R. A.; Zhao, Y.; Tripp, R. A. Appl. Spectrosc. 2009, 63, 1107. (18) Driskell, J. D.; Shanmukh, S.; Liu, Y.-J.; Chaney, S. B.; Tang, X. J.; Zhao, Y. P.; Dluhy, R. A. J. Phys. Chem. C 2008, 112, 895. (19) Shanmukh, S.; Jones, L.; Driskell, J.; Zhao, Y.; Dluhy, R.; Tripp, R. A. Nano Lett. 2006, 6, 2630. (20) Liu, Y.-J.; Zhao, Y.-P. Phys. ReV. B 2008, 78, 075436. (21) Liu, Y.-J.; Zhang, Z.-Y.; Zhao, Q.; Dluhy, R. A.; Zhao, Y.-P. J. Phys. Chem. C, 2009, 113, 9664. (22) Liu, Y.-J.; Zhang, Z.-Y.; Dluhy, R. A.; Zhao, Y.-P. J. Raman Spectrosc., 2010. DOI: 10.1002/jrs.2567. (23) Dirks, A. G.; Leamy, H. J. Thin Solid Films 1977, 47, 219. (24) Tait, R. N.; Smy, T.; Brett, M. J. Thin Solid Films 1993, 226, 196. (25) Zhao, Y. P.; Chaney, S. B.; Zhang, Z. Y. J. Appl. Phys. 2006, 100, 063527. (26) Karabacak, T.; Singh, J. P.; Zhao, Y. P.; Wang, G. C.; Lu, T. M. Phys. ReV. B 2003, 68, 125408.

Silver Nanorod Array Substrates (27) Cristina, B.; Gisia, B.; Chelsea, E.; Kevin, R. Nanotechnology 2005, 1986. (28) Yu, J.; Amar, J. G. Phys. ReV. E 2002, 66, 021603. (29) Smith, W.; Ingram, W.; Zhao, Y. Chem. Phys. Lett. 2009, 479, 270. (30) Zhou, C. M.; Gall, D. J. Appl. Phys. 2008, 103, 014307. (31) LeRu, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. J. Phys. Chem. C 2007, 111, 13794. (32) Mo, Y.; Mo¨rke, I.; Wachter, P. Solid State Commun. 1984, 50, 829.

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8183 (33) Du, Y.; Shi, L.; He, T.; Sun, X.; Mo, Y. Appl. Surf. Sci. 2008, 255, 1901. (34) Chan, G. H.; Zhao, J.; Schatz, G. C.; Duyne, R. P. V. J. Phys. Chem. C 2008, 112, 13958. (35) Jackson, J. D. Classical Electrodynamics, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1999; p 474. (36) Zhang, Z. Y.; Liu, Y.-J.; Zhao, Q.; Zhao, Y. P. Appl. Phys. Lett. 2009, 94, 143107.

JP1001644