Efficient Plasmonic Gas Sensing Based On Cavity-Coupled Metallic

9. 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Physics, University of. 10. Otago, PO Box 56, Dunedin 9016, New Zea...
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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Efficient Plasmonic Gas Sensing Based on Cavity-Coupled Metallic Nanoparticles Jian Qin,† Yu-Hui Chen,‡ Boyang Ding,*,‡ Richard J. Blaikie,*,‡ and Min Qiu*,† †

State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Physics, University of Otago, PO Box 56, Dunedin 9016, New Zealand S Supporting Information *

ABSTRACT: Here, we demonstrate the gas sensing ability of cavitycoupled metallic nanoparticle systems, comprising gold nanoparticles separated from a gold mirror with a polymer spacer. An increase in relative humidity (RH) causes the spacer to expand, which induces a significant reduction of nanoparticle scattering intensity, as the scattering is highly dependent on the cavity−nanoparticle coupling that closely relates to the nanoparticle−mirror distance. With high structural tolerance, i.e., no requirement for high-precision nanoparticle geometry, this lithography-free system enables a remarkable average sensitivity at 0.12 dB/% RH and 0.25 dB/% RH over a wide RH range (45−75%) and full reversibility with much faster response time than the commercial electrochemical sensors, possessing the characteristics to be used for notable gas sensing.

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INTRODUCTION In metallic nanoparticles (NPs), collective oscillation of conduction electrons can be excited upon light illumination. This is known as the excitation of localized surface plasmons (LSPs), giving rise to the concentration of incident light into deep subwavelength volumes. As a result, the electromagnetic (EM) fields can be significantly enhanced, leading to intensified local light−matter interactions. This characteristics makes metallic NPs promising candidates for the detection of nearby gas or vapor molecules1 with sensitive, remote, and noninvasive optical read-out. For example, when gold (Au) or silver (Ag) NPs are soaked in helium and argon gases,2 their extinction spectra can be modified, as the refractive index change of the surrounding air induces a spectral shift of the nanoparticles’ LSPs. The plasmon resonance shift can be further enhanced if NPs are made of materials that are catalytically active to certain gases.3−7 Owing to their extremely small geometry, metallic NPs have been considered as favorable sensing elements that can be integrated with nanoscale optical or electronic circuitry to construct emerging lab-on-a-chip devices and other future technologies. However, the sensitivity is highly limited by the large LSP line width that is induced by strong radiative damping of metals. To improve this, various methods have been adopted to sharpen the plasmonic resonances, such as the use of dimer NP structures8 or periodic arrays of metallic NPs9−11 lithographically constructed to create EM hot spots or enhance the diffractive coupling of LSPs. Another solution is to make metallic NPs interact with an optical cavity, e.g., placing NPs © XXXX American Chemical Society

above a mirror to form a thin-film cavity-coupled NP system, which can significantly narrow the LSP line width,12 thus greatly improving bio- or chemical sensing.12−16 The distance between NP and the mirror is critical to the cavity−NP coupling state. A proper NP−mirror distance can lead to constructive or destructive interference between the scattered light from NPs and the reflected light from the mirror, significantly modifying the wavelength and strength of plasmon resonances.17,18 As a result, the scattering spectra,19 directions,20 and magnitudes21 of the nanoparticles can be effectively altered by tuning the NP−mirror distance. Here, we report a highly efficient gas sensing regime based on this NP−mirror distance dependent scattering using an example of water vapor sensing. As shown in Figure 1, AuNPs are separated from a Au mirror with a poly(vinyl alcohol) (PVA) film, which can swell/de-swell upon the adsorption/ desorption of ambient water molecules,22 causing the variation of film thickness and hence the NP−mirror distance. The change in NP−mirror distance effectively alters the cavity−NP coupling state, which can be observed from the transformed farfield scattering patterns and near-field distributions. As the result, scattering intensity from individual NPs can be significantly modified; for example, as we will show herein, when relative humidity (RH) changes from 5% to 75%, the scattering intensity of a single NP reduces down to ∼1/8 of its Received: July 3, 2017 Revised: September 1, 2017 Published: October 12, 2017 A

DOI: 10.1021/acs.jpcc.7b06502 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Structure description and optical properties. (a) Measured scattering spectra of individual AuNPs on glass substrate (dashed), and d = 100 nm (red) and d = 150 nm (blue) above a Au substrate; while panels (b)−(d) show far-field images of these AuNPs (marked by red circles) on different substrates, respectively. Note: The white scale bar in the SEM image of the schematic stands for 100 nm, while all the optical measurements were performed at RH = 35%.

original value, resulting in a remarkable average sensitivity of ∼0.12 dB/% RH and an estimated resolution better than 0.5% RH. In our experiment, the cavity-coupled NP system even shows >10 times faster response than the reference commercial electrochemical sensor. This excellent sensing performance is highly improved as compared to most of the recently developed nanoscale humidity sensors. More importantly, the studied cavity-coupled NP system can be prepared using an easy fabrication process with great structural tolerance; e.g., the irregular geometry of metallic nanoparticles does not significantly affect the sensing performance. These results manifest that this NP−cavity system has outstanding potential to be applied in practical gas sensing.

the near-field distributions and optical spectra of the structures, while the refractive index of Au and PVA within in different humidity environments were acquired from refs 25 and 26, respectively.

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RESULTS AND DISCUSSION Turning to details, Figure 1a shows the scattering spectrum of a single AuNP on a glass substrate (black dashed curve), with a single scattering feature that peaks at 554 nm. In contrast, the AuNP placed 100 nm above a Au film (red curve) gains a highly enhanced scattering intensity (up to 20 times compared to those on a glass substrate) and a much wider line width with a maximum at λ = 674 nm, indicating an overlap of multiple resonances.27 If the NP−mirror distance increases up to d = 150 nm (blue curve), the scattering maximum blue shifts back to λ ≈ 550 nm with a large intensity reduction. Far-field scattering patterns of AuNPs also exhibit a high degree of dependence on substrates. For example, the AuNPs on glass (Figure 1b) display dot-shaped green scattering patterns, while NPs with a 100 nm distance (Figure 1c) show much brighter yellow dots. (Scattering spectra of AuNPs on PVA coated glass substrates were demonstrated in Figure S5 of the SI.) We note that, when the NP−mirror distance increases up to 150 nm (Figure 1d), the NP scattering exhibits green doughnut-shaped patterns, highly different from dot-like patterns shown in AuNPs on other substrates. As revealed by ref 20 and our recent study,21 these substrate-induced variations of far-field patterns and scattering intensity are the result of resonance coupling between LSPs and thin-film optical cavity modes. The FDTD simulations (Figure 2) provide more details of such coupling effects. Similar to the experimental results, the modeled scattering of the AuNP with a 100 nm distance (red curve in Figure 2a) acquires much higher magnitude and broader spectrum than the 150 nm distance AuNP (blue curve). The near-field distributions reveal more insights. As shown in Figure 2b,d, the electric fields are concentrated nearby the AuNP with a 100 nm distance, no matter at which spectral maximum (spectral position labeled 1 or 2). In contrast, the

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METHODS In the system described here, an ∼100 nm thick Au film has been thermally evaporated onto a silicon substrate, followed by the deposition of a PVA film using a spin-coater. When the concentration of PVA solution and spinning speed are adjusted, the PVA film thickness can be tuned and was finally confirmed by a surface profilometer (Dektak-XT) at RH = 35% and room temperature, with a precision of ±5 nm. Characterized by a scanning electron microscope (SEM) (inset of the schematic of Figure 1), cube/sphere-like AuNPs with a side length (diameter) of 65 ± 10 nm were chemically prepared23,24 and stored in ethanol suspension. These AuNPs were placed on the top of PVA spacers by drop-casting from the ethanol suspension and ambient-temperature evaporation to form dispersions of separated particles such as shown in Figure 1b−d. A dark-field microscope associated with a gas chamber was used to collect the scattering spectra and far-field images from single NPs. The RH in the gas chamber was adjusted by being ventilated with mixtures of dry and humid nitrogen and was recorded every 1 s with an accuracy of ±2% RH using a commercial electrochemical RH sensor (Thorlabs TSP01). More experimental details can be found in Section 4 of the Supporting Information (SI). Finite-Difference-Time-Domain (FDTD) methods (Lumerical Solutions) were used to simulate B

DOI: 10.1021/acs.jpcc.7b06502 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. (a) Simulated scattering spectra of AuNPs separated 100 (red) and 150 (blue) nm from the Au film. The calculated electric field intensity |E|2 of AuNPs at labeled spectral positions are shown in panels (b, d) and (c, e) for the AuNP placed 100 and 150 nm above the Au mirror, respectively. |E|2 in all panels are displayed from the x− z cross section, while red (blue) areas correspond to positive (negative) maximum.

Figure 3. Optical properties variation upon interaction with water molecules. Scattering spectra of a AuNP separated from the Au mirror with a PVA spacer measured with RH = 5% (black), 45% (blue), 65% (magenta), and 75% (green). The corresponding far-field images are shown above. The thickness of the PVA film is 100 nm measured at RH = 35%. The dashed line indicates the wavelength of λ = 650 nm.

electric fields highly penetrate into the PVA film when the AuNP is placed at 150 nm. These results indicate that varying the NP−film distance can effectively tune the coupling state between AuNPs and the cavity, giving rise to the significant changes of both scattering intensity and far-field patterns observed in Figure 1. We note that there is a spectral difference between the measured and modeled results, which may arise from the irregular shape of our AuNPs as compared to the symmetric cube structure in simulations (see Figure 2); from the SEM images in Figure 1, we can see that the AuNPs acquire various asymmetric cube/sphere-like geometries, which can induce large spectral broadening and resonance red-shifting.28 However, the irregular NP geometry does not significantly affect the scattering dependence on the NP−mirror distance. As shown in Figure S1 in the SI and our recent work,21 the scattering magnitudes of gold nanorods (AuNRs) are also highly dependent on the NP−mirror distance. Next, the humidity sensing capability of the cavity-coupled NP structures was measured and demonstrated. Specifically, here we show the scattering spectra and far-field images of a cavity-coupled AuNP in different humidity environments. In our experiment, the initial thickness of the PVA spacer was confirmed as d = 100 nm at ambient RH = 35%. (This thickness was chosen to give the optimal sensitivity. Please refer to Section 3 in the SI for more details.) As shown in Figure 3, the scattering spectrum at RH = 5% (black curve) shows a broad line width with a maximum at λ = 650 nm. As the humidity increases, the scattering magnitude at λ = 650 nm drastically reduces; meanwhile, the maximum at λ ≈ 550 nm gradually appears. When RH = 75% (green curve), the long wavelength band maximum almost disappears. There is only one maximum at λ = 545 nm, highly resembling the scattering spectrum of the AuNP placed on a d = 150 nm spacer (blue curve in Figure 1a). The far-field observation of the AuNP at RH = 75% displays a green doughnut pattern (green framed image in Figure 3), which is identical to the far-field image of AuNPs 150 nm above the mirror (Figure 1d). These results unambiguously indicate that the humidity induced modification of spectra and far-field patterns is the result of NP−film distance variation together with PVA refractive index change,26 as absorption/desorption of water molecules leads to the expansion/shrinking of the PVA film. (More details of PVA film expansion/shrinking with ambient RH can be found in Table S1 of the SI.)

Figure 4a shows the scattering intensity at λ = 650 nm of the single cavity-coupled NP system with the RH value increasing from 5% to 75% and then decreasing back to 5%. First of all, we note that the RH-dependent scattering magnitudes are almost identical for increase/decrease directions, indicating that the optical response of the cavity-coupled NP system to RH changes is fully reversible and has almost no sensing hysteresis within the measuring error ± 2% RH. In addition, the scattering intensity at RH = 75% is only ∼1/8 of that at RH = 5%. We note that such a large humidity-induced intensity reduction allows for an average RH sensitivity of 0.12 dB/% RH over a wide RH range (5−75%). It is also worth noting that, since the scattering intensity of the cavity-coupled NP system shows a nonlinear change over the whole RH variation, at certain RH ranges, the sensitivity can be even higher. For example, for the RH range from 45% to 75%, the sensitivity of the system achieves as high as ∼0.25 dB/% RH. Given the exceptionally good sensitivity and the photon detection resolution of our spectrometer, we estimate the sensing resolution of the cavitycoupled NP system to be better than 0.5% RH, which is 4−5 times higher than that of most commercially available electrochemical sensors. This RH sensitivity is highly improved as compared to most of the recently developed nanoscale humidity sensors, e.g., 0.05

Figure 4. Sensing performance of the cavity-coupled NP structure. (a) The scattering intensity of the cavity-coupled AuNP (the same one as in Figure 3) measured at λ = 650 nm with increasing and decreasing humidity. (b) The scattering intensity (black dotted-line curve) varies as a function time when humidity cycles between 5% and 70%, as compared to readings (blue curve) from a commercial electrochemical sensor. C

DOI: 10.1021/acs.jpcc.7b06502 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C dB/% RH of the dye-doped ultrathin-film sensor,22,29 0.07 dB/ % RH of the nanofiber-based plasmonic sensor,30 and 0.09 dB/ % RH of the plasmonic gap resonance sensor.31 What is worth noting is that the remarkable RH sensitivity has little relevance to the geometry of NPs, but is highly dependent on the initial NP−mirror distance. As shown in Figures S2 and S3 of the SI, the RH sensitivity of a Au nanorod with a 100 nm NP−mirror distance exhibits similar RH sensitivity with the cube/spherelike nanoparticles, while identical cube/sphere nanoparticles placed at other NP−mirror distances show much lower RH sensitivity. These results indicate that the cavity-coupled NP sensing system possesses high structural tolerance as compared to most of the plasmonic sensing systems, e.g., the plasmonic dimer8 or gap resonators,31 where high precision NP geometry is required to improve sensing performance. Moreover, the studied cavity-coupled NP system exhibits other advantages, e.g., the structural simplicity: our NP−cavity system has a considerably easier fabrication process than other humidity sensing systems requiring nanolithography32 and chemical vapor deposition33 techniques. In addition, constructed using chemically inactive materials, the NP−cavity system will maintain high RH sensitivity over a long period. In contrast, the sensing performance of dye-doped thin-film sensors22 quickly gets worse over time since dye molecules can easily be bleached. Next, we directly compare our cavity-coupled NP system with the electrochemical sensor used in our experiment. Figure 4b shows the scattering intensity of a single AuNP with a 100 nm distance (black dotted-dashed line) changes as a function of time when RH values cycles between 5% and 70%, while the simultaneous readings from the electrochemical sensor (blue curve) are plotted as a reference. We note that the scattering variation mostly overlaps with the sensor readings. However, when the humidity suddenly changes, e.g., at t = 70 s, RH abruptly jumps from 5% to 70%, the scattering intensity of the AuNP shows an immediate response, while the sensor readings change slowly, taking ∼30 s to achieve the accurate values. Similar behaviors can be observed at all time points when RH changes rapidly, and the cavity-coupled NP system’s response time, estimated as 10 times faster response time than the reference electrochemical sensors. In addition, the cavity-coupled NP system can be prepared using a lithography-free fabrication process with large structural tolerance. More importantly, the cavity-coupled NP system provides a versatile platform toward various gas sensing applications, which can be realized by simply replacing the PVA films with other polymer materials that can expand to specific chemicals, e.g., polystyrene to toluene,34 poly(methyl methacrylate) to alcohol,35 or poly(4-vinylpyridine) to nitroaromatic explosives.36

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06502. Reference experiments using NPs with different shapes, NPs placed at different NP−mirror distances and NPs on different substrates; detailed description of experimental setups; thickness measurement for PVA films at different RH environments; brief introduction of cavity−NP coupling (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.D.). *E-mail: [email protected] (R.J.B.). *E-mail: [email protected] (M.Q.). ORCID

Boyang Ding: 0000-0002-6299-5010 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Science Foundation (NSF) (1263236, 0968895, 1102301); The 863 Program (2013AA014402); Priming Partnership Pilot Funding (University of Otago); and New Idea Research Funding 2016 (Dodd-Walls Centre for photonic and quantum technologies).

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CONCLUSIONS In conclusion, we have demonstrated highly efficient humidity sensing based on cavity-coupled metallic nanoparticle systems. Specifically, being separated from a gold film with a PVA spacer, the scattering of AuNPs can be dramatically modified when ambient humidity changes. This is because the shrinking or expanding of the PVA spacer upon interaction with water molecules causes the NP−mirror distance variation, which can alter the cavity−NP coupling state, thus enabling large modification of scattering intensity. This cavity-coupled NP system offers a high average sensitivity of 0.12 dB/% RH and ∼0.25 dB/% RH over the RH range 45−75% with an estimated resolution better than D

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DOI: 10.1021/acs.jpcc.7b06502 J. Phys. Chem. C XXXX, XXX, XXX−XXX