Mechanically Reconfigurable Pd Nanogroove Array: An Ultrasensitive

Feb 9, 2018 - (7, 31, 35-40) To our best knowledge, there has been no reports on the use of elastomer-supported Pd to enhance the performance of optic...
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Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Mechanically Reconfigurable Pd Nanogroove Array: An Ultrasensitive Optical Hydrogen Detector Yang Shen,†,‡ Xiaoyi She,†,§,‡ and Chongjun Jin*,† †

State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China § School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Low-cost hydrogen sensors, designed for ultrasensitive, reliable, and rapid identification of hydrogen gas (H2), are extremely desired in almost all hydrogen-related applications in the forthcoming hydrogen economy, including crude oil refinement, hydrogen-fueled vehicles, and molecular hydrogen therapy. Here, we first report on the experimental realization of an ultrasensitive optical hydrogen sensor based on a new type of flexible palladium (Pd) nanogroove array. Each groove can be driven synchronously by absorbed hydrogen, with the assistance of the underneath elastic substrate, to mechanically reconfigure itself and thus amplify the spectral shift of plasmon resonance for hydrogen sensing. Our experimental results show a plasmon resonance with a narrow line width of 74 nm, which has a wavelength shift of 18 nm after exposed to 4% H2 in nitrogen gas (N2). In addition, the extremely high relative reflectance change of 400% was achieved, giving rise to an ultralow H2 (in N2) detection limit of 0.1% and sensing resolution of 0.013% in the low H2 volume concentration regime. Meantime, exposure to H2 causes a rapid and reversible change in reflectance on a time scale of seconds. This pronounced performance suggests that our flexible Pd nanogroove array provides a promising optical hydrogen detection scheme for practical applications. KEYWORDS: hydrogen sensor, plasmonics, nanogroove array

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plasmon polaritons (SPPs) have attracted enormous interest from the field of hydrogen sensing during the past 10 years.11−29 For plasmonic hydrogen sensing, two schemes were successfully demonstrated. The first one is direct sensing, where the Pd nanostructure serves as both the active material interacting with hydrogen and the plasmonic signal transducer simultaneously.11−16 Nevertheless, the LSPRs of Pd nanostructures suffer from a weak optical response as well as a very broad full width at half-maximum (fwhm, ∼several hundred nanometers) due to interband transitions of Pd over the entire visible region.11 Moreover, the Pd nanoparticles exhibit a poor spectral repeatability arising from the irreversible disintegration on the edges of these particles after each hydrogen cycle.11 To overcome these limits, an indirect approach was introduced recently through using a combination of smaller Pd particles and a chemically inert plasmonic element (i.e., gold).17−29 In such a hybrid resonant cavity system, the Pd element (active material) is placed at the local field of the gold element (plasmonic transducer), leading to a narrower fwhm together with a plasmon-enhanced near-field, which is favorable in gas sensing. According to this hybrid scheme, a variety of welldesigned structures have been proposed, such as heterogeneous

ith the development of hydrogen fuel cell technology and the prospect of a hydrogen economy at hand, it becomes vitally crucial to ensure safety and improve efficiency at all stages of hydrogen production, distribution, storage, and utilization.1−3 As a highly combustible, colorless, and odorless gas, hydrogen gas (H2) has a wide flammability limit in air (4− 75 vol %) in combination with a low ignition energy (0.02 mJ), which leads to a deterrent for the preparation and adoption of hydrogen.4 Thus, an ultrasensitive, fast response, and reliable hydrogen sensor is extremely desirable in industrial applications and hydrogen-based technologies. Palladium (Pd) can absorb/release a substantial quantity of hydrogen, basically without activation barriers, to form a hydride phase in a reversible manner.5 This process is associated with changes in the structural, optical, and electronic properties of Pd, which can be exploited to detect the presence of hydrogen. Conventional Pd-based hydrogen sensors are generally based on electrical readout, namely, monitoring the electrical response (i.e., resistance) changes of Pd structures during hydrogenation.6−10 This method is elegant in their fast and sensitive response; however, there exists an aggravating problem that it may generate sparks. Compared with electrical sensors, optical ones can eliminate the risk of explosion through remote readout. Nanoplasmonic sensors relying on the localized surface plasmon resonances (LSPRs) or surface © XXXX American Chemical Society

Received: November 7, 2017 Published: February 9, 2018 A

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Figure 1. Principle of the FPNA. (a) Schematic of the FPNA. There are mainly two resonances carried by the FPNA: the FP mode, bouncing between the groove bottoms and openings, and the SPP-Bloch mode, propagating along the flat top surface of the Pd film, respectively. (b) Schematic showing a reversible shrinkage and recovery of the groove through H2 absorption and desorption. The white arrows represent the expansion directions of the Pd film in the nanogroove in the x−z plane caused by the absorption of hydrogen atoms. (c) Simulated deformation in the x-direction of the FPNA after hydrogenation. (d) Simulated stress distribution for the FPNA after hydrogenation. (e) Schematic of the fabrication procedure. PMMA refers to poly(methyl methacrylate). IL stands for interference lithography. (f) Top-view SEM image of an FPNA. The inset is the cross sectional view of the SEM image. (g) Simulated reflection spectra under s-polarization at the incidence angle of 8° for the FPNA (lattice constant a = 600 nm, groove depth h = 90 nm, top widths of grooves before/after exposed to hydrogen w0/w0′ = 280/264 nm, respectively, bottom width of grooves w1 = 100 nm, thickness of Pd film t = 35 nm) with the following three states: metallic state (no hydrogen, black solid), fully hydride state without shape change (blue dashed), and fully hydride state with shape change (red solid). (h) Experimental reflection spectra for the FPNA before and after hydrogenation on exposure to 4% H2 (in N2). The scale bar in (f) is 600 nm.

For the first problem, the elastomer substrates provide the compliant interfaces that allow Pd films to expand or contract more easily in response to H2. Actually, such a elastomersupported scheme has been extensively applied in actively tuning optical responses of the nanostructures by stretching32,33 or swelling34 flexible substrates. For the sensing-oriented applications, a polydimethylsiloxane (PDMS)-supported highly mobile Pd thin film was demonstrated recently as a sensitive and low-cost electrical hydrogen sensor, where hydrogenregulated nanogaps on PDMS can function as on−off switches.7,31,35−40 To our best knowledge, there has been no reports on the use of elastomer-supported Pd to enhance the performance of optical hydrogen sensors. In this work, we designed and fabricated a flexible palladium nanogroove array (FPNA) to overcome the aforementioned

antennas,19−21 particle-in-gap structures,22 Au/Pd core/shell structures,23,24 perfect absorbers,25−27 and layered heterogeneous dimers,28,29 providing superior performances compared with the pure Pd nanoparticles and nanofilms. However, these complex plasmonic nanostructures are limited by the following reasons: (i) they were typically fabricated on rigid substrates, which suffered giant mechanical stresses (∼several GPa) at the hard contact interface between the Pd nanofilms/nanostructures and the underneath rigid substrate (e.g., SiO2) due to Pd expansion during hydrogenation.30,31 These stresses will severely limit the volume expansion of Pd and even peel off the Pd structures from the substrates. (ii) They took advantage only of the LSPRs with strong radiative damping and did not exploit the full potential of other surface plasmons with narrow fwhm (e.g., SPPs). B

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Figure 2. Optical tuning of FPNA. (a) Simulated depth-modulated reflection spectra of the FPNAs (a = 600 nm, w0 = 280 nm, w1 = 100 nm, t = 35 nm) for the depth varied from 50 to 1000 nm. (b) Simulated periodicity-modulated reflection spectra of the FPNAs (h = 90 nm, w0 = 280 nm, w1 = 100 nm, t = 35 nm) for the periodicity varying from 300 to 1200 nm. (c) Simulated reflection spectra of the FPNAs (a = 600 nm, h = 90 nm, w1 = 100 nm, t = 35 nm) for the top width of grooves varying from 150 to 400 nm. The white and red dashed lines in (a)−(c) denote the dispersions of FP and SPP modes with different orders, respectively. (d, e) Electric field intensity distributions for the SPP-governed mode marked by point A at λ = 673 nm for the FPNA with the shallower grooves (h = 90 nm) and the FP-governed mode marked by point B at λ = 958 nm for the FPNA with the deeper grooves (h = 200 nm), respectively.

direction (indicated by the white arrows in Figure 1b). Due to the flexibility of PDMS substrates, the strain in the Pd film will be transferred down to and facilitate the expansion in the xdirection and contraction in the z-direction of the PDMS groove, which steepens the sidewalls of the PDMS groove. Finally, the Pd/PDMS grooves will shrink during hydrogenation and recover during dehydrogenation, as schematically shown in Figure 1b. To verify this reshaping of the groove, we performed a 2D simulation on the mechanical deformation in the x-direction of an FPNA (lattice constant a = 600 nm, groove depth h = 90 nm, top width of grooves w0 = 280 nm, bottom width of grooves w1 = 100 nm, thickness of Pd film t = 35 nm) after hydrogenation by the finite element method (FEM, the details on the mechanical simulations can be found in the Methods). As shown in Figure 1c, as the FPNA was exposed to 4% H2 in N2, the edges of the groove opening move closer to each other by ∼16 nm, confirming the shrinkage of the groove. In contrast, the nanogroove array on a glass substrate shows only minuscule deformation of the groove (a shrinkage of ∼4 nm; the simulated results can be found in Figure S1a). Meantime, for the FPNA, a stress of ∼3 GPa (Figure 1d) is located at the bottom of the Pd nanogroove in the equilibrium state, which is lower than that of the nanogrooves on the glass substrate (∼5 GPa) under equal Pd expansion (the simulated result can be found in Figure S1b), indicating that the flexible substrate indeed can relieve the stress in the Pd film. This hydrogen-regulated shrinkage of the groove opening can significantly enhance the frequency shift of the resonance, which is strongly dependent on the shape of the groove. Figure 1g shows the simulated reflection spectra by the finite-difference time-domain (FDTD) method for the FPNA

limits of the traditional plasmonic sensors. First, due to the presence of the elastic substrate, the FPNA exhibited not only a large plasmon resonance shift that is twice the value of that on the rigid substrate but also an enhanced recyclability for its practical use. Second, the interaction of FP and SPP in the nanogroove array created a coupled resonance with a narrow fwhm of 74 nm. These optical and mechanical responses during hydrogenation/dehydrogenation were confirmed and understood with the assistance of electromagnetic and mechanical simulations.

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RESULTS AND DISCUSSION In an FPNA structure, a Pd nanofilm is tightly attached to an array of elastic soft PDMS/hard PDMS (s-PDMS/h-PDMS) composite nanogrooves underneath (Figure 1a). We employed two-beam interference lithography to pattern a periodic array of photoresist nanogrooves, followed by PDMS molding and magnetron sputtering of Pd to produce the FPNA (Figure 1e and f; the detailed fabrication procedures can be found in the Methods). In principle, when light is polarized perpendicular to the grooves impinging on our structure, two types of plasmons are excited: (i) Fabry Pérot (FP) modes originating from the interference of counterpropagating gap surface plasmon (GSP) modes,41 bouncing between the groove bottoms and openings, and (ii) SPP-Bloch modes propagating along the flat top surface of the Pd film. As these two modes spectrally overlap with each other, they will create a coupled resonance whose resonant wavelength is mediated by the geometry of the nanogroove. In our FPNAs, when exposed to H2, the top and bottom of the Pd nanogroove will both expand in the xC

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Figure 3. Lattice-modulated reflection spectra of FPNAs before and after hydrogenation. (a−c) SEM images of the FPNAs with periodicities of 400, 500, and 600 nm, respectively. (d−i) Measured reflection spectra under s-polarized excitation at incident angles of 8° and 20° for FPNAs before and after hydrogenation, respectively. (j−o) Measured reflection spectra under p-polarized excitation at incident angles of 8° and 20° for FPNAs before and after hydrogenation, respectively. The blue and orange curves indicate the reflection spectra when the FPNAs were exposed to pure N2 and 4% H2 in N2, respectively. The other size parameters of these FPNAs are h = 90 nm, w0 = 260 nm, w1 = 90 nm, t = 35 nm, respectively. (a−c) Scale bars, 500 nm.

dashed line. These reflection dip bands can be explained by a simple FP cavity model, which is characterized by a total phase shift of Δφtot = 2πm. In our FPNA, each groove acts as a resonant cavity, where the two counterpropagating GSP modes bounce between the groove bottom and opening (Figure 1a) and the resonance condition reads

with the metallic state (no hydrogen, black solid), the fully hydride state with the change of only permittivity (blue dashed), or changes of both permittivity and groove shape (red solid). The resonance dip at 675 nm results from the coupling between the FP mode and SPPs. Compared to the pure permittivity-induced plasmon resonance shift of 9 nm, the combination of permittivity and reshaping effects leads to a larger shift of 18 nm. In agreement with the simulations, the experimental result (Figure 1h) also exhibits an 18 nm red-shift. Note that the experimental spectra possess much weaker reflectance than those of the simulated ones. This can be ascribed to the scattering effect based on the inherent wrinkles on the surface of the FPNA. In detail, during the Pd sputtering process, argon plasma treatment would inevitably generate surface corrugations on the PDMS substrates and thus Pd nanofilms,31 introducing strong scattering, which will not be captured by the detector. To get further insight into the optical tunability mediated by the geometry of FPNAs, we simulated a series of reflection spectra as functions of the depth of groove h, the periodicity of the array a, and the top width of the groove w0, respectively, as shown in Figure 2a−c. In simulations, the samples are normally illuminated by an incident light polarized perpendicular to the groove. The depth-resolved reflectance map (Figure 2a) presents a set of strong reflection dip bands, which vanish at the SPP(−1) mode at the air/Pd interface, indicated by the red

∫0

2

h

βGSP(z) dz + φr = 2πm

(1)

where βGSP(z) is the GSP propagation constant, z is the depth coordinate, m is the order of FP modes, and φr is the total reflection phase in a round trip, which is expected to be close to π.41 In the moderate-gap approximation, the GSP propagation constant can be approximately given as42 βGSP(z) ≈

2εd εd − εm 2π εd + 2π λ −εm λ w(z)

(2)

where εd and εm are the dielectric constants of the metal and z(w − w )

surrounding medium, respectively. w(z) = w0 − 0h 1 is the width at the corresponding depth z. According to the two equations above, one can obtain the final relation between the groove depth h and the resonant wavelength λ: D

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Figure 4. FPNA as a hydrogen sensor. (a) Schematic showing the setup for hydrogen sensing. (b) Representative reflection spectra of the FPNA exposed to H2 in N2 with varying volume concentrations. (c) Reflection spectra of the FPNA exposed to 4% H2 (orange) and recovered by pure N2 (green) for 10 cycles, respectively. Because of overlapping with each other, these two groups of curves cannot be distinguished clearly. (d) Timeresolved reflectance at λ = 675 nm of the FPNA when exposed to 4% H2 in N2 with different flow rates. (e) Time-resolved reflection spectra at λ = 675 nm of the FPNA when exposed to 400 sccm H2 in N2 with different concentrations. (f) Relationships between the resonance wavelength (orange)/reflectance (green) at λ = 675 nm and the H2 concentration, respectively. The error bars indicate the standard deviations extracted from the three repeated measurements in each concentration of H2. The gray square indicates the detection region where the wavelength and reflectance are both linearly dependent on the H2 concentration in the range of 0−0.9%. By linear fitting, the coefficients of determination for the wavelength and intensity fittings are 0.996 and 0.974, respectively. (g) Time-resolved reflection spectra at λ = 675 nm of the FPNA exposed to 0.4% and 4% H2 in N2 for 10 hydrogenation/dehydrogenation cycles, respectively. (h) Spectral shift of FPNA as a function of duration time, where the sample was stored in air at room temperature. In this figure, all the reflection spectra were recorded at the incident angle of 8° under s-polarization and the size parameters of the FPNA samples are a = 600 nm, h = 90 nm, w0 = 280 nm, w1 = 100 nm, t = 35 nm, respectively. h≈

2πm − φr w0 − w1 × 2 k

⎡ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ −w 1 + ⎢ 1 ⎣

1 A w1

+ w0 1 +

A w0

⎛ A⎜ + 2 ⎜ln ⎜ ⎝

A −1 w1 A 1+ +1 w1

1+

A +1 w0 A 1+ −1 w0

1+

of the reflection dip bands, suggesting that these reflection dips arise from the FP modes. It should be noted that the FP modes with different orders and SPP(−1) (red dashed line in Figure 2a) eventually overlap to create the coupled resonances at certain depths, where the anomalous dispersion behaviors are presented. On the basis of the nature of these two modes, we simulated the intensity distributions of the two orthogonal electric field components (in-plane Ex and out-of plane Ez) for the FPNAs with depths of 90 and 200 nm, respectively. At lower depth (Figure 2a, h = 90 nm, point A), Ez dominates the total electric field (Figure 2d), suggesting that there is an SPPgoverned mode traveling along the top surface. In contrast, at larger depth (Figure 2a, h = 200 nm, point B), a confined Ex

⎤ ⎥ ⎥ ⎥ ⎥ ⎞⎥ ⎟⎥ ⎟⎟ ⎥ ⎠⎦

(3)

where k =

2π εd λ

,A=

2 εd(εd − εm) −εmk

. The analytically calculated

dispersions of the FP modes with different orders (white dashed lines) are shown in Figure 2a, which overlap with a set E

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ACS Photonics inside the groove dominates the total electric field (Figure 2e), confirming it as an FP-governed mode. Limited by the fabrication process, we consider only the lowest order coupled mode. Clearly, for this branch, the coupled mode has a stronger intensity for the SPP-governed region than the FP-governed region. At fixed h = 90 nm, we further examine the effect of periodicity modulation on the FPNA. As shown in Figure 2b, when the periodicity is increased from 350 nm to 1200 nm, the simulated SPP-governed coupled mode red-shifts along with the dispersion of the SPPs (red dashed). For the FPNAs with a > 420 nm, a reduction in fwhm occurs with the increase of the periodicities. Meantime, the lowest order mode of FPNAs yields a ∼0% reflectance when the periodicity is in the range of 600 to 1000 nm. Such an ultralow reflectance is especially suitable for sensing a relative intensity change as a hydrogen sensor. In addition, as the top width of the grooves is increased, a blue-shift occurs (Figure 2c). This can be ascribed to a broader groove leading to a larger GSP propagation constant and thus a shorter resonant wavelength. In this work, benefited from this property, hydrogen-induced shrinkage of the groove opening makes a critical contribution to the spectral shift. To optimize the performance of the FPNA as a hydrogen sensor, we measured the lattice-modulated reflection spectra under s-polarized incident light at different incident angles when exposed to pure N2 and 4% H2 (in N2). It should be noted that we compared the performance of the sensors with different Pd thicknesses in terms of the spectral shifts and response times (the effect of Pd thickness on the spectral shift and response time of the sensor can be found in Figure S2). The experimental results show that the Pd groove with the thickness of 35 nm has the largest spectral shift and the fastest response time in our nanogroove system; therefore the 35 nm thickness of Pd was chosen in our experiments. As shown in Figure 2d−i, for s-polarized excitations at the incident angle of 8°, when the periodicity is increased from 400 nm to 600 nm, the amount of red-shift of the reflection dip stays nearly unchanged at around 18 nm, while the fwhm of this resonance is narrowed down from 139 nm to 74 nm. Furthermore, in accordance with the simulated results (Figure 2b), the measured reflection dips drop down to a lower intensity for increasing periodicities and approach the minimum of 0.3% for a periodicity of 600 nm (Figure 3d−i). We therefore, in terms of sensing capability in either wavelength or intensity manners, choose the FPNA with a periodicity of 600 nm as the optimized one. It should be noted that the simulation shows an even narrower reflection dip for the longer wavelength (Figure 2b). But due to the fabrication limit, we can only scale the periodicity of FPNA to 600 nm if fixing other size parameters in this work. We also measured the reflection spectra of the FPNA under p-polarized excitation (Figure 3j−o). Similar to those under s-polarized excitation, the larger periodicity can lead to a narrower resonance (∼43 nm) under p-polarized excitation. However, the shift Δλ concomitantly reduces to 10 nm, suggesting an inferior performance compared with those under s-polarized excitation. In this study, we therefore only consider the reflection spectra under s-polarized incident light. In addition, a larger incident angle (20°) will give rise to a blueshift and red-shift for s-polarized (Figure 3d−i) and p-polarized (Figure 3j−o) excitations, respectively. These trends agree reasonably well with the simulated angle-resolved reflection spectra (Figure S3).

To demonstrate the sensing capability of the FPNA as a hydrogen sensor, an FPNA (a = 600 nm, h = 90 nm, w0 = 280 nm, w1 = 100 nm, t = 35 nm) was employed. Figure 4a schematically shows the setup for the hydrogen sensing measurements. H2 (4% in N2) and pure N2 were precisely controlled at flow rates by two mass flow controllers, then mixed in a gas mixer, injected into the flow cell, and finally exhausted directly into a gas collector. Meantime, s-polarized probing light was illuminated on the FPNA by passing through the transparent window at the front of the cell. The reflection signals were recorded by a UV−vis−NIR spectrometer. In addition, our measurements were all carried out at room temperature. Figure 4b displays a set of representative reflection spectra when the FPNA was exposed to the H2−N2 mixture gas with varying compositions. A continuous red-shift (Δλmax ≈ 18 nm) was noticeable as the concentrations of H2 increased from 0% to 4% (explosion limit). Also, we carried out the reflection measurement for 10 cycles toggling between the 4% H2 and pure N2 (Figure 4c). It should be noted that the reflection curves perfectly overlap with each other, suggesting an excellent recyclability. Besides the spectral shift of the plasmon resonance, the reflectance at a given wavelength also changes with the H2 volume concentration. Figure 4g presents the time-resolved reflectance at λ = 675 nm (the maximal optical contrast is achieved at this wavelength) for 10 hydrogenation/dehydrogenation cycles. At 4% H2 concentration, we detected a decrease of the reflectance from 3% to 0.6%. Due to the ultralow value of the reflectance at resonance, we obtained a large relative change of reflectanceΔR rel =

R N2 − R H2 R H2

= 400%.

Compared to the conventional hydrogen sensors based on the Pd thin films, we can increase ΔRrel by 1 order of magnitude.43 Moreover, at 0.4% H2, a reproducible reflectance switch from 3% to 1.6% was also distinctly detected for 10 cycles. To determine the response time of FPNA to hydrogen, we measured the corresponding hydrogenation dynamics under varying flow rates and volume concentrations of H2 as shown in Figure 4d,e, respectively. It reveals that the hydrogenation tends to be rapid with increasing H2 flow rates. While for H2 with different concentrations, the case is more complex. When H2 concentration is increased, the hydrogenation response time becomes shorter first, then longer and finally shorter again. The shortest response time of hydrogenation is determined to be 23 s under a flow rate of 400 sccm (standard cubic centimeters per minute) for 4% H2. Figure 4f exhibits the resonance wavelength and reflectance as functions of H2 concentration in the range from 0% to 4%, respectively, according to Figure S4. Clearly, they both show very good linear dependences on the H2 concentrations at low values (gray region, 0−0.9% H2). Moreover, a clear plasmon resonance shift and intensity change can be observed even at H2 volume concentrations as low as 0.1%. By taking into account the linear dependence at low H2 concentrations and if we assume that intensity variation at the 0.04% level can be resolved instrumentally, the detection resolution of our FPNAbased hydrogen sensing platform can be potentially as low as 0.013%. Long-term stability of a plasmonic hydrogen sensor is also a high concern for real applications. Generally, after prolonged use with H2 or exposure to pollutants in the atmosphere, the sensor’s response will deteriorate significantly. Surprisingly, as F

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overcome via alloying with different metals to reduce the effects of the lattice expansion or introducing a buffering or mediating layer to improve the adhesion between Pd and the substrate.44 Here, the results aforementioned further confirm that our FPNA provides two new stress-releasing channels: (i) the PDMS-induced wrinkles can efficiently release the stress along the grooves; (ii) the shrinkage of elastic grooves can further minimize the stress perpendicular to the grooves. The reduction of the elastic stress prevents the cracking and peeling of the film, leading to higher repeatability and long-term stability.

shown in Figure 4h, the spectral shift of the FPNA (caused by the exposure to 4% H2) remains nearly 90% of the original shift after 24 days. It is vastly superior to those Pd nanoparticleinvolved sensors, such as a Pd nanodisk array, whose spectral shift drops down to 50% after 2 days.44 We fabricated an FPNA structure, studied its plasmon modes, and characterized its hydrogen sensing capabilities. FPNA was experimentally demonstrated to be a sensitive sensor with a very high value of Δλ/fwhm up to 0.243, which is superior to other plasmonic hydrogen sensors (Supplementary Table S1), such as Pd/Au antennas,19,21 a Pd nanoparticle in a Au gap,22 a single Au/Pd core/shell nanocrystal,23 and a Pd/ SiO2/Au cavity.28,29 Such a high Δλ/fwhm is mainly caused by the combination of the additional plasmon resonance shift induced by the mechanical reconfiguration on the Pd nanogroove and the narrow fwhm endowed by the coupling of FP and SPP. In addition, the SPP-governed coupled mode also gives rise to an ultralow reflectance down to 0.3%, which pronouncedly improves the sensing capability based on intensity interrogation. Moreover, the presence of the flexible substrate provides a compliant interface, leading to an enhanced adhesion and thus a reduced stress between the Pd film and substrate during hydrogenation, which makes the system more stable and reliable. In addition, the nanogroove structures can also release a large amount of stress perpendicular to the grooves by mechanical shrinkage of the groove opening. This case is very different from a flat film, where wrinkles randomly occur due to isotropic strain.45 To more clearly ascertain the origin of the recyclability of the FPNAs, we fabricated an FPNA (a = 1000 nm, h = 90 nm, w0 = 370 nm, w1 = 240 nm, t = 35 nm) and monitored the evolution of its surface morphology in a hydrogenation/dehydrogenation cycle. Figure 5 illustrates a group of microscopic images of the

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CONCLUSIONS In summary, we have proposed theoretically and demonstrated experimentally a novel approach for achieving sensitive, reliable, and rapid optical hydrogen sensing with a flexible Pd nanogroove array. Each Pd groove on an elastic substrate acts as a mechanically reconfigurable plasmonic cavity. The presence of the flexible substrate not only amplifies the plasmon resonance shift but also enhances the reliability and life of the sensor for practical use. Theoretical analysis and numerical simulations show a broad tunability mediated by the geometry of the FPNA and reveals that an SPP-governed FP− SPP coupled mode will give rise to a larger plasmon resonance shift and a narrower fwhm. We then developed a highthroughput, simple, and cost-effective method to prepare the structure via two-beam interference lithography, PDMS molding, and sputtering of Pd. The experimental spectral shift of the FP−SPP coupled mode is up to 18 nm with the fwhm of 74 nm, accompanied by an ultrahigh relative reflectance change of 400% on exposure to 4% H2. Meantime, a quick response time of 23 s, an excellent recyclability (>10 cycles), and a long-term stability (90% of original spectral shift after 24 days) are achieved simultaneously. The combination of high sensitivity, excellent repeatability, and easy measurements makes our detection method highly promising in developing active plasmonic hydrogen sensors for practical applications.

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METHODS Interference Lithography. The fabrication procedure is schematically shown in Figure 1e. A thin film of poly(methyl methacrylate) (PMMA, molecular weight: 350 000, 1.5 wt % in chlorobenzene) was first spin-coated on a cleaned quartz slide at 4000 rpm for 32 s. The PMMA film functioned as an adherent to ensure the firm attachment of the photoresist grating. The PMMA film was treated on a hot plate at 180 °C for 5 min to evaporate chlorobenzene. Its thickness was ∼50 nm. A positive photoresist (AR-P 3740, Allresist) film was then spun onto the PMMA film and subsequently baked at 95 °C for 90 s. Its thickness was tuned by spin speed. The photoresist film was thereafter subjected to an exposure under an interference pattern of two continuous laser beams (457.9 nm). The incidence angles of the two laser beams on the photoresist film were 34.9°, 27.3°, 22.4°, and 13.2°. In accordance, the lattice constants of the gratings were 400, 500, 600, and 1000 nm, respectively. After the exposure, the samples were immersed in a developer (AR 300-26, Allresist) at 21 °C for 60 s to give photoresist gratings. PDMS Molding. A molding using an h-PDMS/s-PDMS composite was employed.46 Before PDMS molding, the master was placed in a vacuum desiccator, together with a Petri dish containing a few drops of tridecafluoro-1,1,2,2-tetrahydrooctyl-

Figure 5. Evolution of surface morphology of the FPNA in a hydrogenation/dehydrogenation cycle. (a) Initial metal phase before hydrogenation. (b) Intermediate phase during hydrogenation. (c) Fully hydride phase. (d) Intermediate phase during dehydrogenation. (e) Regenerated metal phase after dehydrogenation. The size parameters of the FPNA are a = 1000 nm, h = 90 nm, w0 = 370 nm, w1 = 240 nm, t = 35 nm, respectively. (a−e) Scale bars, 20 μm.

FPNA during a hydrogenation/dehydrogenation cycle. As H2 was loaded, the Pd film began to expand. Meantime, a slight wrinkling of the Pd film appeared in the direction perpendicular to the grooves (Figure 5b) and then became heavier (Figure 5c), which indicates a phase transition from the Pd state to the PdHx state. After removal of H2, the H2-induced stress vanished. Due to the restoring force of the PDMS substrate, the wrinkles became shallow (Figure 5d) and finally flattened out (Figure 5e), suggesting a recovery from the PdHx state to the original Pd state (Figure 5a). Concomitantly, a striking intensity switch between Pd and PdHx was also observed in a cycle as expected. It should be noted that the black line is a crack on the surface of the sample as a reference to focus the Pd film. Generally, the giant stress at the interface between the Pd film and the substrate can be G

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the literature by Silkin et al.47 The used refractive indices for the h-PDMS and s-PDMS were both 1.4. FEM Simulations. FEM simulations were performed to assess the mechanical responses of the sensor in the presence of 4% H2. Each unit in the array was modeled as a 2D structure in the x−z plane due to the nearly infinite length of the groove compared with its width and depth, consisting of three layers: a Pd film with a nanogroove array (thickness of 35 nm), an hPDMS layer with a grooved array (thickness of 30 μm), and a flat s-PDMS layer (thickness of 1 mm), respectively. The linear expansion coefficients in the x- and z-directions of the Pd film when exposed to 4% H2 were both set to 1.04.48 A periodic boundary condition was applied in the x-direction. In addition, the fixed constraint was applied in the bottom of the s-PDMS. The used Young’s modulus and Poisson ratios for the h-PDMS and s-PDMS were Eh‑PDMS = 9 MPa, Es‑PDMS = 2 MPa, σh‑PDMS = 0.49, and σs‑PDMS = 0.49, respectively.46

1-trichlorosilane (TFOCS, Sigma-Aldrich) for 40 min. This process ensured that the entire surface of the photoresist grating was covered by a monolayer of TFOCS molecules through siloxane bonding, which prevents the cured PDMS from sticking to the master. To prepare h-PDMS, a mixture of 3.4 g of (7−8% vinylmethylsiloxane)-(dimethylsiloxane) copolymer (VDT-731, Gelest), 100 mg of 1,3,5,7-tetramethylcyclotetrasiloxane (SIT7900.0, Gelest), and 50 mg of platinum divinyltetramethyldisiloxane (SIP6831.1, Gelest) was stirred and degassed for 5 min. One gram of (25−30% methylhydrosiloxane)-(dimethylsiloxane) copolymer (HMS-301, Gelest) was then added into this mixture and quickly stirred. Immediately (within 3 min), a thin layer of h-PDMS film with a thickness of ∼30 μm was spin-coated onto a TFOCSmodified master at 1500 rpm for 32 s and cured for 20 min at 75 °C. A liquid-state mixture of the base and the curing agent (Sylgard 184, Dow Corning) at a ratio of 10:1 in weight was poured onto the h-PDMS layer, degassed in a vacuum desiccator, and cured at 70 °C for 2 h to generate an hPDMS/s-PDMS composite. Finally, peeled from the surface of the master, an h-PDMS/s-PDMS substrate with periodic grooves was obtained. Pd Deposition. A magnetron sputtering of 35 nm Pd was applied to form the flexible Pd groove array, namely, the FPNA. The base pressure and deposition pressure under Ar flow were 6 × 10−4 and 0.01 mbar, respectively. The sputtering current and time were 15 mA and 240 s, respectively. The thicknesses of palladium films were measured by a step profiler. Characterization and Measurements of the Plasmonic Response to Hydrogen. The plasmonic responses to H2 were monitored by recording the reflection spectra of the FPNAs, which were taken on a UV/vis/NIR spectrophotometer (Lambda 950, PerkinElmer). The schematic of a setup for the hydrogen sensing measurements can be found in Figure 4a. Briefly, 4% H2 (in N2) and pure N2 were precisely controlled at flow rates by two mass flow controllers (CS100, Sevenstar), then mixed in a gas mixer to generate a H2/N2 mixture with varying H2 concentrations from 0% to 4%. The mixture gas was then injected into a homemade flow cell with a volume of ∼16 cm3 and finally exhausted directly into a gas collector. Meantime, s- or p-polarized probing light was illuminated on the FPNA by passing through the transparent window at the front of the cell. The reflection signals were recorded by a UV/ vis/NIR spectrophotometer. For monitoring the evolution of the FPNA’s surface morphology in a hydrogenation/dehydrogenation cycle, an optical microscope system was used. An objective lens (Olympus MPlanFLX100) with a numerical aperture of 0.9 was used to focus the white light from the tungsten-halogen lamp on the samples and collect the backscattered signals. The observed images were finally recorded by a CMOS camera (Zyla 4.2 PLUS, Andor). All the optical measurements were performed at room temperature. FDTD Simulations. The optical simulations were performed using a commercial software (FDTD solutions, Lumerical Solutions) to generate the reflection spectra and electric field distributions of the FPNAs in this work. Each unit in the array was modeled as a 2D structure in the x−z plane. The structure was excited with s- or p-polarized plane-wave light. A Bloch boundary condition was applied in the xdirection. A mesh size of 1 nm for the metal region was utilized. Corresponding to the Pd films, which are free of and saturated by H2, the dielectric functions of Pd and PdH0.67 are taken from

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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/acsphotonics.7b01323. Simulated mechanical response of a Pd nanogroove array on a SiO2 support after hydrogenation; measured reflection spectra of the FPNAs with varying thicknesses of Pd; simulated angle-resolved reflection spectra of the FPNAs; reflection spectra of the FPNA exposed to H2 in N2 with varying volume concentrations from 0% to 4%; summary of the performance of the plasmonic metal nanostructures for hydrogen sensing (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chongjun Jin: 0000-0002-9308-5343 Author Contributions ‡

Y. Shen and X. She contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (11574406, 11504437, 61604179, 11374376), the Key Project of NSF of Guangdong Province (No. 2016A030311049), and the Fundamental Research Funds for the Central Universities (No. 17lgpy07, No. 17lgpy04).

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REFERENCES

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DOI: 10.1021/acsphotonics.7b01323 ACS Photonics XXXX, XXX, XXX−XXX