Plasmon-mediated extraordinary optical transmission (EOT) is finding increased interest for biosensing applications. While Ag nanostructures are capable of the highest plasmonic quality factor of all metals, the performance reliability of pure Ag EOT devices is limited by degradation through environmental interactions. Here we show that EOT devices consisting of nanostructured hole arrays in Ag/Co bilayers show comparable transmission with that of identical hole arrays in Ag thin films as well as enhanced reliability measured by the rate of resonance peak redshift and broadening with time. The Ag/Co EOT devices showed 2.6× and 1.9× smaller red shift in short timescales (20 days) and after 100 days, respectively, while they showed a 1.7× steady-state decrease in rate of bandwidth broadening. This improvement is likely due to the Co metal stabilizing the Ag film from morphological changes by reducing its propensity to diffuse or dewet on the underlying substrate. The improved reliability of Ag/Co bilayer EOT devices could enable the use of their superior plasmonic properties for optical detection of trace chemicals.

The resonant interaction of light with metallic nanostructures governed by plasmonic effects has been an intense area of study due to its many potential applications in energy harvesting,1,2 catalysis,3–5 quantum information,6,7 and sensing.8,9 The plasmon-mediated extraordinary optical transmission (EOT) of light through sub-wavelength metallic apertures with orders of magnitude greater transmission than is predicted by diffractive models of light was first shown in 1998.10 EOT has since been demonstrated in isolated holes and periodic hole arrays, mediated by localized surface plasmons and surface plasmon polaritons.11 The combination of highly tunable resonances, strongly confined electric fields, and a compact, easily parallelized form factor has resulted in growing interest in EOT media for biochemical sensing.12,13 Ag offers better confinement and higher quality factors than other commonly used plasmonic metals throughout the visible to near-IR spectrum, enabling enhanced selectivity and sensitivity in EOT based plasmonic sensors.14–17 However, the superior plasmonic performance of Ag is offset by degradation over short time scales by chemisorption of sulfur,18 and over longer periods by oxidation and migration,19–22 making Ag unsuitable for sensing applications except when integrated as a bimetallic structure with inert Au top layers.23 A growing number of intermetallic compounds and transparent conducting oxides have been proposed as alternative materials,24–27 but Ag and Au are still the most commonly used plasmonic materials. The stabilization of Ag against corrosion and diffusion would enable EOT and surface plasmon resonance (SPR) sensing platforms with dramatically better sensitivity and selectivity than is available in Au sensing platforms.

In this work, we report a nanostructured bilayer EOT device based on Ag and highly damped Co metals that demonstrates comparable plasmonic behavior with that of single layer Ag EOT devices while also significantly improving the long-term reliability of the plasmonic signals. Our bilayer design was motivated by the recent discovery that Ag nanoparticles can be kept corrosion free for long times (approximately a year) when in contact with Co metal in the form of bimetallic nanoparticles.28 The overall plasmonic behavior of the bimetals was also found to be comparable with that of pure Ag while the long-term stability was attributed primarily to a galvanic protection of Ag by the sacrificial Co metal.20 However, this phenomenon has not been observed in thin films optimized for use as EOT sensors.12,13,29

Isosceles triangular nanohole arrays with base length 160 nm, leg length 215 nm, and pitch of 425 nm were patterned on indium-tin oxide (ITO) coated glass substrates by electron beam lithography with a JEOL 9300FS 100 kV platform. A two layer resist utilizing 100 nm of PMMA 495 A4 and 200 nm of PMMA 950 A4 was used to control the undercut of the patterned holes as previously reported.6 Ag thin films of 100 nm thickness were deposited at a rate of 1/s on the developed PMMA mask by electron beam evaporation, and Ag/Co hole arrays were fabricated in the same manner using a 0.5/s deposition rate to deposit 1 nm of Co prior to depositing 100 nm of Ag without breaking vacuum. The two devices were aged under identical ambient laboratory environments (room temperature of 22 °C and 40 ± 5% relative humidity) for a period of ∼100 days. The transmission spectra from these devices were periodically measured under a 10× magnification at normal incidence, with polarization parallel to the base of the triangle, over a spectrum covering 200 nm–1700 nm, normalized against the transmission through the bare glass substrate. We also characterized the morphology of the devices using scanning electron microscopy.

Figs. 1(a) and 1(b) show SEM micrographs of the freshly prepared (measured at day 0, corresponding to the same day as fabrication) Ag and Ag/Co EOT devices, respectively. Both devices showed periodically spaced isosceles holes with morphology typical of electron beam evaporated Ag films. The optical transmission (T) from these two freshly prepared devices are shown in Fig. 1(c). The peak transmitted intensity (Ip) was comparable for both (T = 46% at 777 nm for Ag and T = 61% at 786 nm for the Ag/Co case) and confirmed that the presence of Co in the bilayer Ag/Co device enhanced the transmission over that of pure Ag. Figs. 1(d) and 1(e) show SEM micrographs of the devices following aging for 100 days. It was qualitatively evident that both devices showed a change in the overall film morphology in comparison to the freshly prepared devices. Further, in both aged devices, some changes to the dimension of the triangular holes were also evident. Qualitatively, the triangles in the Ag/Co EOT device appeared to better retain their freshly prepared characteristics following the aging as compared to the Ag device. To better qualify the role of aging, Fig. 1(f) compares the Ag and Ag/Co optical transmission for the 100 day aged device. Several important changes are apparent from the freshly prepared devices [Fig. 1(c)]. First, the EOT resonance red shift is nearly halved in the Ag/Co device (shift = 119 nm) as compared to the Ag EOT device (shift = 226 nm). Second, IP of the EOT devices has also dropped and once again the drop is larger for the Ag EOT case (from 46% to 26%) as compared to the Ag/Co case (from 61% to 45%). Finally, the bandwidth ΔλP for the transmission peak, i.e., the full width at half maximum, also increased over time and once again, was larger for the Ag device (from 78 nm on day 0 to 316 nm on day 100), compared to Ag/Co (from 89 nm on day 0 to 209 nm on day 100).

FIG. 1.

Comparison of the morphology and optical transmission from Ag and Ag/Co EOT devices. (a) and (b) show Ag and Ag/Co EOT devices at day 0 while (d) and (e) show the Ag and Ag/Co devices after 100 days of aging. (c) and (f) show the wavelength-dependent optical transmission for the 0 and 100 day devices, respectively. A magnified image of the triangular hole from each device is also shown in (c) and (f).

FIG. 1.

Comparison of the morphology and optical transmission from Ag and Ag/Co EOT devices. (a) and (b) show Ag and Ag/Co EOT devices at day 0 while (d) and (e) show the Ag and Ag/Co devices after 100 days of aging. (c) and (f) show the wavelength-dependent optical transmission for the 0 and 100 day devices, respectively. A magnified image of the triangular hole from each device is also shown in (c) and (f).

Close modal

To better understand the cumulative changes occurring to the resonance wavelength and transmitted intensity at the resonance, we constructed contour plots of the EOT optical transmission as a function of time and this is shown in Fig. 2. Fig. 2(a) is the result of measurements for the Ag EOT device. From this contour plot, it is evident that the peak transmission wavelength λP shows a dramatic red shift and decrease in intensity within the first 10 days of aging but finally stabilizes after long times (40 days) into a significantly red shifted peak of smaller intensity. In distinct contrast, the Ag/Co EOT device [Fig. 2(b)] shows a smaller rate of red shift as well as drop in peak intensity.

FIG. 2.

Spectrally resolved contour plot of the EOT resonance as a function of aging time. (a) Measurements for the Ag EOT device show that the device characteristic changes rapidly within the first 10 days and eventually reaches a stable behavior after 40 days at a significantly red shifted wavelength and lower intensity. (b) Measurements for the Ag/Co EOT device show that the device characteristic changes much more slowly than that of Ag, producing a more stable sensing platform over short and long time scales.

FIG. 2.

Spectrally resolved contour plot of the EOT resonance as a function of aging time. (a) Measurements for the Ag EOT device show that the device characteristic changes rapidly within the first 10 days and eventually reaches a stable behavior after 40 days at a significantly red shifted wavelength and lower intensity. (b) Measurements for the Ag/Co EOT device show that the device characteristic changes much more slowly than that of Ag, producing a more stable sensing platform over short and long time scales.

Close modal

To assess the quantitative differences in optical performance of the two different EOT systems, we plotted the aging dependence of the red shift and bandwidth for the transmission peak. Fig. 3(a) shows the change in λP with respect to its value at day 0 (freshly prepared) for the Ag device (black filled circles) and the Ag/Co device (red open circles). It is evident that the Ag device showed a faster change in λP very early in the aging and then saturated with an overall shift of ∼225 nm after 40 days. This behavior was consistent with the cumulative changes shown by the contour plot in Fig. 2(a). The early stage red shift (i.e., ≤20 days) occurred at a rate of 7.05 nm/day, while the overall shift following 100 days of aging averaged 2.24 nm/day. In contrast, the Ag/Co device (red open circles) showed a slower rate of change during the early stages (≤20 days) of 2.68 nm/day, while its overall shift following 100 days averaged 1.18 nm/day. This quantitative comparison clearly showed that the stability of the device as measured by the rate of red shift was improved for the Ag/Co by 2.63× for the early stage and by 1.89× over the long term after 100 days. Fig. 3(b) shows the normalized inverse bandwidth change measured as ΔλP(t=0)/ΔλP(t), which readily captures the degradation of the resonant peak as measured by peak broadening from its value at day 0. From Fig. 3(b), it was evident that the Ag device (black filled circles) showed a faster rate of degradation as well as larger overall degradation as compared with Ag/Co (red open circles). After initial degradation in both structures over a period of nearly 20 days, the normalized bandwidth of the Ag/Co device had broadened by a factor of 1.7× less than the Ag device in steady-state.

FIG. 3.

Quantitative comparison of the optical transmission versus time for Ag (black filled circles) and Ag/Co (red open circles). (a) Plot of the relative change in peak wavelength with respect to the peak wavelength at day 0. The curves correspond to exponential growth fits for purposes of showing the trends clearly. (b) Plot of the change in bandwidth. The curves correspond to exponential decay fits for Ag (black dashed line) and Ag/Co (red dotted line) and inverse logarithm fit for Ag/Co (solid red line).

FIG. 3.

Quantitative comparison of the optical transmission versus time for Ag (black filled circles) and Ag/Co (red open circles). (a) Plot of the relative change in peak wavelength with respect to the peak wavelength at day 0. The curves correspond to exponential growth fits for purposes of showing the trends clearly. (b) Plot of the change in bandwidth. The curves correspond to exponential decay fits for Ag (black dashed line) and Ag/Co (red dotted line) and inverse logarithm fit for Ag/Co (solid red line).

Close modal

The results of our quantitative investigation of reliability as determined by measuring the rate of change to the optical transmission characteristics in EOT devices upon aging clearly show that the plasmonic performance of Ag-based EOT devices could be improved by the use of an ultrathin layer of Co at the Ag/substrate interface, i.e., a bilayer metal configuration. Notably, depositing a 1 nm Co film on top of the Ag EOT medium and co-depositing a Co film within the Ag film both resulted in degradation comparable with or worse than that experienced by the bare Ag EOT medium. We hypothesize that the origin of the improved long-term reliability in the Ag/Co bilayer must be due to an enhanced morphological stability of the Ag films in the Ag/Co bilayer system leading to better stability of the triangular holes in the array. As stated earlier, the contact of Co with Ag in nanoparticle morphologies has been shown to improve the long-term chemical stability of Ag in air due to a galvanic coupling that causes the Co to degrade at the expense of Ag.28 There is also some evidence based on thermal annealing studies that Co can stabilize the morphology of Ag nanoparticles by preventing it from agglomerating due to the high mobility and diffusivity of Ag.20,30 As evidenced from the magnified SEM images of the triangular holes shown in Figs. 1(c) and 1(f), the better performance of the Ag/Co device could be coming from the fact that the triangular holes of the aged Ag device appear to show much larger contrast variation inside the hole as compared with the Ag/Co, possibly due to the diffusion/migration of Ag and or formation of Ag corrosion products.

To better understand the difference between Ag and Ag/Co, in Fig. 3(b) we have fit the experimental bandwidth decay data for the two devices to trends typically observed when either Ag or Co oxidizes in the environment.28 Ag is known to show an exponential decay in the change in bandwidth as a result of oxidation, and this is evident from the excellent match of the experimental data (closed circles) to the fit (dashed line, y=0.27+0.75×exp(x/8.61),R2=0.975, where y and x denote the normalized inverse bandwidth and time, respectively, and R2 is the regression coefficient of the fit). In the case of the Ag/Co system, the oxidation of Co in the Ag/Co system, which should be preferred if the galvanic effect was in play, is known to occur with an inverse logarithmic fashion in time.31 In Fig. 3(b), the Ag/Co experimental trend (open circles) is compared with the inverse log behavior (solid line, y=1.09(0.147×ln(x+1.54),R2=0.962) as well as with an exponential decay (y=0.45+0.52×exp(x/16.5),R2=0.969). From this it was clear that the galvanic effect of Co in slowing down Ag degradation does not appear to be a dominant process. Therefore, this analysis, combined with our earlier observation of a reduced material build-up within the triangular holes for the Ag/Co device [Fig. 1(f)], led us to infer that the more likely role of Co must be a stabilization of the Ag film morphology, such as by reducing the tendency of Ag to diffuse or dewet on the underlying low energy glass substrate.

A second aspect that requires some clarification is the enhanced day 0 optical transmission shown by the Ag/Co device over the Ag device, as seen in Fig. 1(c). One reason for this behavior could be that our day 0 optical transmission spectra were measured hours after lift-off. While the Ag oxidization that was discussed in this manuscript occurs over longer timescales, the tarnishing of Ag by ambient S occurs over much shorter time scales18 and so it is likely that the Co may have also served to reduce the effects of S tarnishing over shorter timescales than were measured here. A second reason could be that the underlying 1 nm Co layer has a discrete nanostructured island morphology rather than being a continuous film. Recently it has been shown that Ag in contact with Co nanostructures show localized surface plasmons with intensity comparable with that in pure Ag.32 Overall, these results point to the important role of Co in suppressing the plasmonic degradation of Ag in EOT devices. Our future investigations will be aimed at studying the nature of the Ag/Co/substrate interface via techniques like transmission electron microscopy to better understand the role of morphology and chemistry.

In conclusion, the long-term optical behavior of EOT devices fabricated from Ag and Ag/Co bilayer material and exposed to ambient air was investigated. We found that the Ag/Co device exhibited better overall reliability of optical behavior, as evidenced by the smaller decrease in the red shift and bandwidth of the transmission peak over long periods of time of up to 100 days. These observations point to the important role of the buried Co layer in enhancing the reliability of the device. This could lead to the design of multilayered materials with even better short and long time stability, thus enabling the use of EOT devices in a variety of sensing conditions.

This work was performed jointly at The University of Tennessee through a Science Alliance JDRD Grant U013960010 and at Oak Ridge National Laboratory. This manuscript has been authored in part by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains, and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The nanofabrication and SEM imaging were conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. R.C.P., B.L., and R.D. also acknowledge support from the Laboratory Directed Research and Development Program at ORNL.

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