pubs.acs.org/NanoLett
Au Double Nanopillars with Nanogap for Plasmonic Sensor Wakana Kubo,†,‡ and Shigenori Fujikawa*,†,‡,§ †
Innovative Nanopatterning Research Laboratory, RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan, ‡ JST, CREST, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan, and § Interfacial Nanostructure Research Laboratory, RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan ABSTRACT We propose a simple, precise, and wafer-scale fabrication technique for Au double nanopillar (DNP) arrays with nanogaps of several tens of nanometers. An Au DNP was simply constructed by alternately laminating thin layers of Au and polymer on a template and selectively removing the thin layers. This DNP array was expected to exhibit a specific plasmonic property induced by its narrow gap. When measuring the refractive index sensitivity (RIS), Au DNP arrays with 33 nm gaps exhibited a high RIS of 1075 nm RIU-1 and showed a higher sensor figure of merit than the alternative structures, which did not have a nanogap structure but had almost the same surface area. This indicated that the enhanced plasmon electromagnetic field induced by the nanogap structure improved sensor performance. Our fabrication technique and the optical properties of the nanogap structure will provide useful information for developing new plasmonic applications with nanogap structures. KEYWORDS Au double nanopillar, nanogap, plasmonic sensor
S
double nanopillars (DNPs) with nanogaps and investigated its plasmonic sensor ability. We investigated four types of Au structures. An Au DNP with a hollowed gap is the target structure. For comparison, an inner NP, outer NP, and DNP with a TiO2 spacer were prepared (these four structures are shown in Figure 1i). To examine the effect of nanogap configuration on the plasmonic sensor performance of Au DNP arrays, we investigated the combination of inner and outer NPs as a comparison experiment to the Au DNP with a hollowed gap. The DNP with a TiO2 spacer was also used to investigate the space effect of the gap, i.e., empty gap or not, which will provide information concerning the plasmon coupling and the distribution of the enhanced electromagnetic field between the gaps. A schematic illustration of the fabrication process is shown in Figure 1a-h. The surface and cross-sectional morphologies at each step were observed using a scanning electron microscope (SEM) (HITACHI model S-5200, acceleration voltage 5 kV) and are shown in Figure 2. For the fabrication of a pillar template by nanoimprinting, a silicon mold was prepared by dry etching of the silicon wafer with the patterned masking layer of a resist polymer (Tokyo Ohka Kogyo, TDUR-P015PM). The silicon wafer was subjected to CF4 plasma gas (SAMCO, RIE-10NR; radio frequency, 13.56 MHz; power, 70 W; about 10 Pa; treatment time, 20 min) and successive O2 plasma treatments (SAMCO, Compact Etcher; treatment time, 5 min) to produce a hole array of silicon with a 300 nm width and a 780 nm depth. A mold-releasing agent solution (Daikin; HD-1101Z) was cast on the Si mold surface, and the substrate was kept at room temperature overnight. The Si mold was rinsed with a thinner solvent and dried at room temperature.
ignificant attention has recently been given to the study of plasmonic properties of noble metal nanostructures, owing to their capability of enhancing the electromagnetic field of incident light based on the plasmon resonance. Effective use of the enhanced electromagnetic field energy offers a broad range of applications, including chemical/biological sensors,1-3 single-molecule detections,4,5 and photonic circuits.6 Particularly, the plasmon electromagnetic field can be greatly enhanced when metal nanostructures are neighboring across a nanogap.7-10 Previous reports have pointed out that the large-scale fabrication of few-nanometer-wide nanogap structures are indispensable in using enhanced plasmonic energy.11 Nanogap structures are commonly fabricated using electron beam lithography. However, the fabrication of few-nanometer-wide nanogap structures is still extremely difficult, and their fabrication area is too small for practical applications.12 Therefore, a simple and large-scale fabrication technique for nanogap structures is needed. We previously reported the simple fabrication of noble metal nanofin arrays.13,14 This process is based on the nanocoating of the template surface and the selective removal of the coating layer and the template. The width of the metal nanofin corresponds to the thickness of the metal coating layer. When alternate coating of metal/spacer/metal and a selective removal of the topmost layer are performed, metal nanostructure arrays with nanogaps can be fabricated over a large area. With this approach, we fabricated Au
* Corresponding author,
[email protected]. Received for review: 03/4/2010 Published on Web: 11/29/2010 © 2011 American Chemical Society
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structure between Au NPs, thin layers of a polymer or TiO2 were prepared (Figure 1e) to form a DNP with a hollowed gap or a DNP with a TiO2 spacer, respectively. Prior to coating with these spacer layers, the sample was exposed to mercaptoacetic acid vapor in a Petri dish at 50 °C for 3 h to introduce carboxyl groups on the surface of the Au NP. The polymer layer was formed using the layer-by-layer (LBL) assembly technique. The substrate was immersed in an aqueous solution of poly(dimethyldiallylammonium chloride) (PDDA, 1 mg/mL) for 10 s, rinsed with ion-exchanged water for 10 s, and dried with nitrogen gas flow for 30 s. Then the substrate was immersed in an aqueous solution of poly(styrenesulfonate) (PSS, 1 mg/mL) for 10 s, rinsed with ion-exchanged water for 10 s, and dried with nitrogen for 30 s. This process was repeated 10, 30, and 50 times to form alternating layers of PDDA and PSS on the NP surface (Figure 2d). For the preparation of Au DNPs with TiO2 spacers, the substrate was immersed in a heptane solution of titanium n-butoxide (100 mM, 3 s), rinsed with heptane for 10 s, and dried with nitrogen gas flow for 10 s. Next, the substrate was immersed in water for 3 s and dried again. This process was repeated 80 times to form a TiO2 layer on the surface of the inner Au NP. After the inner Au NP was coated with the polymer or TiO2 layer, the Au layer was coated on the coated-inner Au NP again (Figure 1f), and the topmost Au layer was selectively etched (Figure 1g). Finally, the template and the polymer layer were removed to form the Au DNP with a hollowed gap (Figure 1h and 2e,f). To evaluate the nanogap effect on the plasmonic sensor performance of Au DNP arrays, the inner and outer Au NPs (shown in Figure 1i) were fabricated on different substrates. Since the sensor performance of the Au NP also depends on its Au surface area, it is necessary to measure the optical response of Au NPs with no gap configuration and the same surface area as the Au DNP. For this purpose, the substrates of the inner and outer Au NPs were set in a face-to-face arrangement (Figure S1in the Supporting Information), and this sandwich arrangement was used for the optical investigation of the inner and outer Au NPs. The optical properties of the Au NP arrays were characterized using a UV-visible-NIR spectroscope (Shimadzu, UV-3100). An aluminum plate with a 2 mm hole was put in front of the sample to limit the irradiation area of the incident light (Figure S1in the Supporting Information). Extinction was defined as ε ) (1 - I/I0) and was measured in transmission geometry (I and I0 are the intensities of the incident light and the transmitted light, respectively). Figure 3 shows SEM images of Au DNPs with empty nanogaps prepared using polymer coating with 50, 30, or 10 cycles. From the SEM images, we measured all nanostructure dimensions, including gap widths, film thicknesses, and diameters of NPs, at 10 different points on a single NP structure and measured their average size and standard errors from these
FIGURE 1. (a-h) Schematic representations of fabrication process for Au double nanopillar (DNP) arrays and (i) schematic images of each NP.
Second, cleaned glass plates were coated with a resist polymer (Tokyo Ohka Kogyo, TSMR-8900LB) diluted with a thinner solvent (Tokyo Ohka Kogyo, thinner MG, the original resist solution/thinner MG ) 4/3 v/v) by spin coating (2000 rpm, 20 s). The substrate was baked at 90 °C for 90 s. The resist polymer film on the glass plate was imprinted with the silicon mold under 1000 N at 180 °C for 600 s using a nanoimprinting tool (SCIVAX, X-200) (Figure 1a). The sample was then cooled at 120 °C under pressure, and the mold was separated from it to obtain NP arrays (Figure 1b and 2a). After template fabrication, an Au layer with a thickness of around 50 nm was sputtered on the pillar template (Sanyu Electron Co, Ltd., SVC-700RF) (Figure 1c and 2b). The reactive ion etching (RIE) process is a drying and anisotropic etching technique for treating a substrate surface, which removes the Au layer on the top and bottom surfaces of the template. The sample was subjected to a plasma gas mixture (gas species, Ar and CF4; etching time, 14 min) for leaving an Au layer only on the side of the template (Figure 1d, 2c), followed by exposure of O2 gas plasma for 5 s to make the sample surface hydrophilic. For the formation of the gap © 2011 American Chemical Society
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FIGURE 2. SEM images of the fabrication process. Cross-sectional images of (a) resist nanopillar template, (b) Au sputtering, (c) top layer etching, (d) polymer coating with layer-by-layer (LBL) assembly technique, and (e) Au DNPs with hollowed nanogap after removal of template and polymer layer, and (f); top view of Au DNP arrays with nanogap width of 33 nm.
( 1, and 9 ( 1 nm, respectively (n ) 10). Gap widths were measured at different points on a sample, and almost the same results were obtained, which indicates most structures fit into an average size distribution. One polymer deposition cycle is roughly calculated to be about 0.7 nm, and the gap distance decreased almost linearly as the number of polymer depositions cycles decreased. This result is reasonable since alternate deposition of PDDA and PSS is generally known to produce a uniform polymer layer of which the thickness is precisely defined by the number of deposition cycles.13 Thus, the gap distance is controllable with nanometer-scale precision simply by changing LBL deposition times. As Fujikawa et al. reported,13 we have already succeeded in coating a uniform SiO2 film with 6 nm thickness using the sol-gel technique. Although SiO2 was not investigated in this report, its layer can be also used as a spacer instead of the polymer since SiO2 can be selectively removed using a conventional technique such as dry gas etching and so on. From these facts, we believe the potential size of the gap distance will reach several nanometers, by optimizing the gap depth, coating process, and spacer material.
FIGURE 3. SEM images of Au DNPs with various nanogap widths. Gap width could be controlled with LBL assembly deposition cycles. Relations between number of coating cycles and gap widths are (a) 50 times, 33 ( 1 nm (n ) 10); (b) 30 times, 14 ( 1 nm (n ) 10); and (c) 10 times, 9 ( 1 nm (n ) 10), respectively.
data. In the polymer deposition sample with 30 cycles, the sidewall thicknesses of the inner and outer NPs were 49 ( 1 and 48 ( 1 nm, respectively. The polymer deposition samples with 50, 30, and 10 cycles produced gap widths of 33 ( 1, 14 © 2011 American Chemical Society
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the inner and outer NPs. Currently, such remarkable peak shifts were not observed even in the spectrum of Au DNP with a 9 nm gap (data not shown), which indicate that further optimizations of Au film thicknesses and gap widths will enhance the plasmon hybridization.8 Identifying the hybridized resonance is still quite difficult due to the strong mixing of multiple resonance modes. We will further devote our effort to elucidate the derivations of the resonance by theoretical simulation. In addition, the sharpness of the plasmon peak will depend on the uniformity of structures and the film thickness of NPs, according to Aizpurua.16 Besides, one of the reasons that the spectrum has a broad absorption all over the wavelength is attributed to our experimental setup. As depicted in Figure S1in the Supporting Information, the detector of our spectroscopy cannot detect scattering light. In fact, the effect of scattering light on extinction spectra may not be negligible. Improvement in optical measurements will be required in the future. Each resonance was independent from measurement positions on a sample substrate, which proves the uniformity of our fabrication technique. Moreover, the plasmon peaks under polarized light did not change even after the sample direction was turned around, which indicates that there is no coupling between adjacent NPs. Finally, each extinction spectrum did not change, even though the sample position changed, which shows the uniformity of our fabrication process. Generally, the resonance peak positions of Au nanostructures are sensitive to the surrounding media and are shifted according to the refractive index change of the media near the Au nanostructures. This phenomenon can be used as a refractive index sensor of the surrounding media. For this purpose, extinction spectra were examined in deuterium water instead of air. The following Au NP combinations were investigated: sample 1, inner Au NP (average diameter, Dinner, 477 nm) + outer Au NP (Douter, 639 nm) fabricated on different substrates; sample 2, Au DNP with a 33 nm gap (Dinner, 480 nm, Douter, 650 nm, Figure S2 in the Supporting Information); sample 3, Au DNP with a gap filled with a 31 nm TiO2 spacer (Dinner, 474 nm; Douter, 674 nm). All samples had almost the same NP size because the surface areas strongly affect sensor ability. In sample 1, the inner and outer Au NPs fabricated on different substrates were placed face-to-face, and deuterium water was poured between the two sample plates (Figure S in the Supporting Information). In this experimental setup, the surface area of sample 1 equals that of sample 2, the Au DNP with a 33 nm gap, which enables us to compare each sensor performance under the same surface area. Finally, the gap effect on the sensor performance will be elucidated from their comparison. In the other samples, the Au DNP was set facing a bare glass plate, and deuterium water was poured between the two plates. Since the surface area of the Au nanostructure per unit area was almost equal to those of samples 1 and 2, the effect of the nanogap structure on the refractive index sensitivity (RIS) could be investigated. The extinction spectrum of each sample
FIGURE 4. Extinction spectra in air of (i) Au DNP with nanogap of 33 nm, (ii) inner Au NP, and (iii) outer Au NP and inner SiO2 NP with same size as Au DNP.
We measured the extinction spectrum of Au DNPs in air. Au DNP with a 33 nm gap showed three peaks at 1110, 1780, and 2250 nm (Figure 4i). To identify each peak, we prepared a single Au NP with the same diameter of the inner NP of the Au DNP and measured its extinction spectrum in air. We also prepared an outer Au NP with an inner SiO2 NP (Au/SiO2 NP) on another substrate and measured its extinction spectrum. For the preparation of the Au/SiO2 NP arrays, a SiO2 thin film was first formed using the sol-gel technique on a resist NP template,13 instead of a second Au coating. After that, the same procedure for preparing the gap and the outer Au NP was applied. The diameter of the outer Au NP in the SiO2/Au DNP was 635 ( 5 nm, which was almost identical to that of the outer Au pillar in the Au DNP. The inner and outer Au NPs had one peak at 1780 and 2250 nm, respectively, which corresponds to the dipole peak of each NP (Figure 4, spectra ii and iii). Therefore, the two peaks at 1780 and 2250 nm of the Au DNP were attributed to the dipole peak of the inner and outer Au NPs. The origin of the strong peak at 1110 nm is unclear at present. We thought the extinction spectrum of Au DNP is the result of plasmon-plasmon interaction between the inner and outer NPs. The extinction spectra reported by Radloff and Halas give some hints concerning the origin of the peak.15 They showed an extinction spectrum of a concentric nanoshell particle and plasmon peak shifts due to the plasmon hybridization between the inner and outer metallic shells. Because of the hybridized plasmon resonances, some peaks have two origins, for example, the symmetric dipole mode of the inner nanoshell and antisymmetric quadrupole mode of the outer nanoshell. Considering their report, the peak at 1110 nm might consist of hybridized plasmon resonances such as the symmetric quadrupole and octupole of the inner NP and the antisymmetric octupole of the outer NP. If the Au DNP will show stronger plasmon hybridization in its spectrum, the peak position will shift depending on the coupling strength and the energy between the plasmons of © 2011 American Chemical Society
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As mentioned before, the strong resonance at 1110 nm in air might consist of several resonances such as the symmetric quadrupole and octupole modes of the inner NP and the asymmetric octupole mode of the outer NP. Therefore, it is no wonder that the peak at 1110 nm in air will split in some peaks in deuterium water. A more systematic investigation will be examined in the future. A solution of deuterium water and ethylene glycol was used as the surrounding media to control environmental refractive indices. The ratio of ethylene glycol to deuterium water was changed to 0, 20, 50, 70, and 90% by volume, and each refractive index was 1.328, 1.349, 1.380, 1.400, and 1.421, respectively. As the environmental refractive indices increase, the plasmon resonance shifts to a longwavelength region (Figure 6a). The peak shifted in proportion to the refractive index increase (Figure 6b). Similar peak shift behavior was observed in the Au nanoring,17 Au nanoshell,18 and Ag nanoprism19 samples, and the present findings strongly suggested that an Au DNP can also act as a plasmonic sensor. In addition to the above experiments, other Au DNP samples with different gap sizes were examined, and their RISs are summarized in Table 1. The RIS is calculated with the following equation
RIS ) ∆(plasmon resonance peak shift)/ ∆(refractive index change)
In the Au DNP samples with the hollowed gap structure, the RISs of short-wavelength (b in Figure 5) and longwavelength (1) peaks were in the range of 524-651 nm (refractive index units, RIU) -1 and 712-1056 nm RIU-1, respectively. Note that the RIS of the Au DNP with a 33nm gap showed a considerably higher value of 1056 ( 14 nm RIU-1, which suggests the possibility for application to ultrahigh sensitive sensors. Lazarides et al. pointed out that the RIS depends not on the structural features of metal nanostructures but on the plasmon resonance positions.20 In other words, the RIS of a resonance peak at a longer wavelength tends to be higher than that of a peak at a shorter wavelength, even in the same sample. In sample 2, the RIS of the long-wavelength peak was also higher than that of the short-wavelength peak, and the short-wavelength peak of samples 1 and 2 showed almost the same RIS because these peak positions were the same. Sample 3, the Au DNP with a 31 nm TiO2 spacer, exhibited the lowest RIS, although its size was almost identical to that of the Au DNP with the 33 nm hollowed gap. The reason for the lowest sensitivity may be a consequence of the reduction in its surface area by the filling of the nanogap with TiO2. To elucidate the nanogap effect more clearly, we calculated the sensing figure of merit (FOM) introduced by Van Duyne et al.21 The FOM is the ratio of the RIS (eV·RIU-1) to
FIGURE 5. Extinction spectra in deuterium water of (i) sample 1, inner and outer NPs fabricated separately on different substrates, (ii) sample 2, Au DNP with 33 nm gap, and (iii) sample 3, Au DNP with gap filled by TiO2 spacer of 31 nm.
in deuterium water is shown in Figure 5. Samples 1 and 3 showed a peak at 1355 and 1335 nm, respectively, while sample 2 showed two peaks at 1362 and 1512 nm. As described in Table 1 later, the RISs of sample 2 at short-wavelength and long-wavelength peaks were 642 nm/ refractive index units (RIU) and 1056 nm/RIU, respectively. Since the difference of refractive index between air and deuterium water is 0.328, the short-wavelength and longwavelength peaks in deuterium water will blue shift to 1150 and 1165 nm in air, respectively, which corresponds approximately to the strong resonance at 1110 nm in Figure 4(i). Therefore, the origin of two peaks in Figure 5(ii) may be a strong resonance at 1110 nm in Figure 4(i). © 2011 American Chemical Society
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TABLE 1. Plasmonic Sensor Properties of Inner and Outer Au NPs, Au DNP, and Au DNP with TiO2 Spacer pillar configuration
gap distance
λpeak/nm in D2O
Epeak/eV in D2O
sensitivity nm/RIU
sensitivity eV/RIU
fwhm/eV
FOM
inner + outer of Au NP (sample 1, n ) 3)
-
1355
0.915
646 ( 29
0.439
0.046
9.9 ( 0.8
Au DNP (sample 2, n ) 3)
33 nm
1361 1512
0.911 0.820
642 ( 34 1056 ( 14
0.431 0.537
0.022 0.045
18.1 ( 0.6 12.2 ( 0.2
Au DNP (n ) 3)
13 nm
1368 1404
0.906 0.883
586 ( 71 712 ( 3
0.358 0.428
0.023 0.024
23.0 ( 3.3 18.1 ( 0.8
Au DNP (n ) 2)
9 nm
1368 1486
0.907 0.834
661 ( 20 824 ( 89
0.300 0.449
0.028 0.039
15.9 ( 2.5 11.2 ( 1.3
Au DNP (sample 3, n ) 2)
31 nm (TiO2 gap)
1330
0.932
296 ( 32
0.207
0.054
9.8 ( 0.5
full width at half-maximum (fwhm) of the plasmon resonance peak (eV).
size. This comparison of FOM will clarify the nanogap effect on their sensor performance because the FOM depends not only on the RIS but also on the plasmon electromagnetic field intensity. The results are summarized in Table 1. The FOM of sample 2, the Au DNP with a 33 nm gap, was 18.1 ( 0.6 and 12.2 ( 0.2 at the peaks of 1367 and 1512 nm, respectively. Although sample 1, the combination of the inner and outer NPs fabricated separately, had almost the same surface area as sample 2, its FOM was 9.9 ( 0.8, which was approximately half the value of sample 2. The difference between samples 1 and 2 was the existence of the nanogap structure. Briefly, the FOM increase was due to the strong enhancement of the plasmon electromagnetic field induced by the gap structure. The FOM of sample 3, the Au DNPs with a 31 nm TiO2 gap spacer, was 9.8, and its Au surface area was half that of sample 1 because of the filling of the gap with TiO2. Generally, reduction of a surface area will decrease its FOM value when the shape features are the same. However, the FOM of sample 3 was 9.8, similar to that of sample 1 (9.9). This result suggests that the gap configuration of the Au DNP with the TiO2 spacer may still enhance the plasmon electromagnetic field around the gap. Sonnefraud and Maier have reported that the highest electromagnetic field lies in the nanocavities between a concentric ring and a disk,22 and showed that the gap cavity has the highest sensing capabilities for plasmonic sensor. Similar results were reported in some references, which show the enhanced electromagnetic fields in gap structures.23-25 Their results will support our expectation that the gap between the inner and the outer NPs has the highest electromagnetic field and the gap configuration is a strong candidate for a plasmonic sensor with high performance. In addition, they reported the sensor properties of concentric ring/disk cavities obtained experimentally and theoretically and concluded that if water cannot fill the gap fully where the highest field exists, the sensor performance will decrease. It suggests that the gap between NPs is the most sensitive position for detecting the change in refractive index
FOM ) RIS (eV · RIU-1)/fwhm (eV)
This definition allows nanostructures to be compared with others as sensing platforms regardless of shape and
FIGURE 6. (a) Extinction spectra of Au DNP with 33 nm gap under various refractive indices. Plasmon resonance peaks shifted to longer wavelength region in proportion to refractive index variation. (b) Linear relationship between plasmon resonance shift and environmental refractive indices. © 2011 American Chemical Society
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structure could be designed using infinite combinations of thin film materials such as metals, metal oxides, and organics. Since Au DNP arrays can be formed even on wafer-scale flexible substrates, such as a transparent polymer film,32 it can be applied to flexible functional materials such as a flexible disposable sensor chips, sensitizers for flexible displays, and solar cells. The FOM of the Au DNP with a 33 nm gap was 18.1 and was higher than that of the separated inner and outer NPs, which did not have a nanogap structure. Enhancement of the plasmon electromagnetic field induced by the gap structure was elucidated by comparing plasmon sensor performance, even though the gap distance was comparably wide. This indicated that a few-nanometer-wide nanogap structure is not indispensable for the effective use of the enhanced plasmon electromagnetic field. The proper shape and size for a more sensitive plasmonic sensor will be proposed based on our results. The simple and large-scale fabrication of nanogap structures provides many opportunities for developing functional optical devices and materials. We are currently focusing on more efficient extraction and utilization of the plasmonic energy within nanogap structures.
FIGURE 7. Correlation between gap widths and FOM values. Circle (b), square (9), and triangle (2) plots represent Au DNP with hollowed gap, Au DNP with TiO2 spacer, and combination of inner and outer NPs, respectively. Error bars of x axis and y axis represent standard error of gap width and FOM value.
due to its strong local field. This results support that the decrease in the sensor performance of sample 3, Au DNP with TiO2 gap spacer, might be attributed not only to the reduction of surface area but also to the decrease of electromagnetic field effect. However, the strong electromagnetic field will still exist even on the upper side of DNP with TiO2 spacer,12 which will drive up the sensor ability of the DNP with filled gap. The FOM value, 23.0, of the Au DNP with a 13 nm gap was comparable to those of multiscale Au nanoparticle arrays (FOM, 23.3),26 Au pyramidal gratings (FOM, 24.4 (θ ) 13°)),27 and Ag nanowell surfaces (FOM, 14.5),28 which is surprisingly higher than those of other nanomaterials such as Au nanocrescents (FOM, 2.4, Bukasov et al.),29 Au nanorings (FOM, ∼2, Larsson et al.),17 and Au nanorice particles (FOM, 1.0, Wang, et al.).30 These results suggest that the Au DNP with a nanogap is one of the best solutions for achieving an ultrasensitive plasmonic sensor. Therefore, the design and introduction of a nanogap configuration could enhance the electromagnetic field and improve sensor performance. The correlations between the gap width and the FOM value are plotted in Figure 7. As the gap distance decreased, the FOM of the Au DNP increased. Generally, a narrower gap will produce a stronger electromagnetic field.9,31 These data confirmed that a gap structure narrower than 33 nm could generate an enhanced plasmon electromagnetic field and improve sensor performance. In summary, simple, uniform, and wafer-scale fabrication of Au DNP arrays with nanogaps of several tens of nanometers was achieved by repeating the nanocoating and etching process. The gap distance was accurately controlled by the thickness of the spacer polymer layer deposited using the LBL assembly technique. Moreover, a hollowed or filled-gap © 2011 American Chemical Society
Acknowledgment. We are grateful to Tokyo Ohka Kogyo, Inc., for providing silicon wafers with resist patterns, and to Dr. Takuo Tanaka and Dr. Nobuyuki Takeyasu for providing useful discussions. This work was supported by JST, CREST. Supporting Information Available. Figures showing experimental setup for extinction spectrum measurement of Au NPs under various refractive indicies and top view of Au DNP with TiO2 spacer observed using SEM. This material is available free of charge via the Internet at http://pubs. acs.org. REFERENCES AND NOTES (1)
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DOI: 10.1021/nl100787b | Nano Lett. 2011, 11, 8-–15
