Bilayer plasmonic nano-lattices for tunable hydrogen sensing platform

Journal Pre-proof Bilayer Plasmonic Nano-lattices for Tunable Hydrogen Sensing Platform Hoang Mai Luong, Minh Thien Pham, Richa Pokharel Madhogaria, Manh-Huong Phan, George Keefe Larsen, Tho Duc Nguyen PII:

S2211-2855(20)30115-4

DOI:

https://doi.org/10.1016/j.nanoen.2020.104558

Reference:

NANOEN 104558

To appear in:

Nano Energy

Received Date: 7 November 2019 Revised Date:

12 January 2020

Accepted Date: 30 January 2020

Please cite this article as: H.M. Luong, M.T. Pham, R.P. Madhogaria, M.-H. Phan, G.K. Larsen, T.D. Nguyen, Bilayer Plasmonic Nano-lattices for Tunable Hydrogen Sensing Platform, Nano Energy, https:// doi.org/10.1016/j.nanoen.2020.104558. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Published by Elsevier Ltd.

Bilayer plasmonic nano-lattices (BPNL) consist of two parallel hexagonal arrays of Pd nanopatchy particles and nano-holes. BPNL shows two-plateau-type behavior and large optical change when exposed to hydrogen due to coupling effect of localized and propagating surface plasmon resonance. This multi-array sensor platform is promising for future designs of hydrogen gas sensor with a large operational range and high detection sensitivity. ToC figure

Bilayer Plasmonic Nano-lattices for Tunable Hydrogen Sensing Platform Hoang Mai Luong*, Minh Thien Pham, Richa Pokharel Madhogaria, Manh-Huong Phan, George Keefe Larsen*, and Tho Duc Nguyen

H. M. Luong, M. T. Pham, Prof. T. D. Nguyen Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, US E-mail: [email protected] R. P. Madhogaria, Prof. M. H. Phan Department of Physics, University of South Florida, Tampa, Florida 33620, USA Dr. G. K. Larsen National Security Directorate, Savannah River National Laboratory, Aiken, South Carolina 29808, United States E-mail: [email protected] Keywords: hydrogen gas sensing, surface plasmon resonance, subwavelength structures, multiple-plateau hydride

Abstract: Gas sensors are critical for facilitating the safety and integrity of systems used in the hydrogen energy and storage systems. It is challenging for any sensing technology to meet all the performance requirements for emerging applications, such as dynamic measuring range, response time, and sensitivity. Here, we propose an optical hydrogen gas sensor platform based on Pd bilayer plasmonic nano-lattices (BPNL), which is comprised of two parallel, subwavelength-separated hexagonal arrays of Pd nano-patchy particles and nano-holes. Optical transmission studies of the BPNL structure and its isolated sub-structures of nano-patches (NPs) and nano-hole arrays (NHs) upon hydrogen sorption show distinct optical isotherms with different spectral changes and response times in the visible-to-near-infrared region. Experiments and finite-difference time-domain (FDTD) calculations show that the strong electromagnetic coupling between localized and propagating plasmonic modes of the NPs and NHs, respectively,

1

leads to the unique optical properties of the BPNL and their subsequent change upon hydrogen sorption. Notably, the BPNL structure possesses an extended optical response range and enhanced sensitivity for hydrogen detection. Additionally, the optical response time of the BPNL structure is an order of magnitude faster than that of the isolated array sensors in the low hydrogen pressure range.

2

1. Introduction Hydrogen (H2) gas has the potential to be a dominant future energy carrier, due to its high gravimetric energy density, sustainability, and lack of carbon emissions upon consumption.[1] As hydrogen generation and hydrogen fuel cell technology continue to develop,[2, 3] the demand for hydrogen sensors for safely handling hydrogen gas in all stages of production, distribution, storage and utilization will also continue to rise.[4] For hydrogen leakage detection and concentration controls, it is essential that hydrogen sensors have good stability, high sensitivity, rapid response time, and most importantly be “spark-free”.[4, 5] In this case, optical-based hydrogen sensors are preferred due to increased safety over electrical-based hydrogen sensors.[5] An interesting optical hydrogen sensor platform that has been intensively studied is based on surface plasmon resonances (SPRs) in nano-structured metal hydrides. In this system, the interactions between hydrogen and metals via the resonantly enhanced light-matter interaction due to SPRs are investigated.[5] Practical application of these phenomena has led to the development of hydrogen sensors with numerous advantages over other technologies, including simple and multiple output readings, fast response time, and high sensitivity.[5, 6] The working principle of an SPR hydrogen sensor relies on the fact that the conductance and optical permittivity of a metal change as it absorbs hydrogen and undergoes a phase transformation into a metal hydride.[7, 8] Due to its strong interactions with hydrogen, palladium or its alloy is typically used.[5] Changes in conductance and optical permittivity are expressed through optical responses that can be resonantly captured and correlated with hydrogen pressure and hydrogen to metal ratio (H/M). Hydrogen-induced changes may be seen in a number of different optical parameters, including spectral extinction/transmission/reflection magnitudes, peak/dip position, and full-width at half maximum (FWHM) of the peak/dip.[7, 9] 3

Several optical hydrogen sensing platforms based on the SPR concepts have been investigated and can be categorized by how the optical responses are induced: (i) through localized surface plasmon resonance (LSPR), such as in Pd nano-sphere,[10, 11] nano-disk,[6, 7, 12] nanotriangle,[13] nano-bipyramid,[14] nano-wire,[15] or other single nano-particle[16, 17] or (ii) by propagating surface plasmon polariton (SPP), such as in Pd nano-hole arrays[18-20] or prism/grating coupling.[21-23] In SPR sensors, the coupling between different modes and structures is important. For example, the coupling of LSPR-enhanced electric fields between different nanoparticles generates “hot-spots” with much stronger field intensities in comparison to that of a single nano-entity, and significantly improves the sensitivity of the sensor. This has been demonstrated in hydrogen sensing using dimer, trimer, and oligomer arrays.[13, 24-26] Another notable phenomenon, which has not been investigated for hydrogen sensing, is the subwavelength interaction of nano-structures that support both SPP and LSPR modes.

These

interactions can alter the field distribution to allow enhanced coupling in the near field along with emission to the far field, yielding new and interesting spectral transmission characteristics.[27-30] The optical responses of Pd nanostructures exhibit absorption-desorption hysteresis and phase transformation behavior that is commonly seen in metal-hydride systems.[31] Specifically, three phases can be seen in the optical-response “isotherm” curve: α phase, mixed phase, and fully formed hydride β phase.[32] The α and β phases coexist in the mix phase region, also known as the plateau region, and the hydrogen detection sensitivity is the highest in this phase. Thus, the plateau region is very useful for SPR sensing applications.[5] However, the tradeoff is that the plateau region only exists over a narrow pressure range. The plateau can be considered the “response region” for SPR hydrogen sensing, since the optical response as a function of 4

hydrogen pressure is typically more non-specific outside of the plateau.[6] In addition, sorption kinetics of pure Pd structures are slow, which leads to large equilibrium response times for Pdbased hydrogen gas sensors, especially when the pressure is in the plateau regime.[33, 34] The use of Pd alloy materials is one strategy that solves the problems of the small response regimes and long response times.[6, 7, 9] However, the sensitivity is reduced considerably, and alloy nanostructures require more complicated fabrication processes. In this paper, we report on the optical properties of a new optical Pd-based sensor platform, bilayer plasmonic nano-lattices (BPNL). The BPNL structure consists of two parallel layers: a hexagonal array of Pd hemispherical patchy particles (NPs) that is adjacent to a hexagonal array of Pd nano-holes (NHs). This heterogeneous structure is designed to harness the specific features of the individual arrays and the unique properties that emerge through their combination. In order to understand the behavior of the BPNL-hydrogen system, the optical properties as a function of hydrogen pressure (

) of BPNL as well as its sub-structures of Pd NPs and Pd NHs are

thoroughly explored experimentally and quantitatively. Finite-difference time domain (FDTD) calculations are also performed to give insight into the experimental results and to better understand the relationship between hydrogen-induced changes to Pd optical properties and subsequent BPNL response. Notably, it is found that the hydrogen detection sensitivity is increased by the coupling between the LSPRs of the NPs with the SPPs of the NHs. Additionally, the response region and response times are also expanded and improved through the differing plateaus of the individual structures. 2. Results and discussion 2.1. Design concept and experimental realization Nano-hole arrays are well-known plasmonic structures that support SPP modes, where the 5

momentum mismatch between a free-space photon and a surface plasmon (SP) wave is compensated by additional quasi-momentum generated from the periodic array/lattice of holes.[35, 36] On the other hand, the NP structure has been shown to support strong LSPR modes.[28-30] To create a coupling mode between these two nano-entities, it is feasible to place these two arrays parallel and in close proximity for interaction.[37] In this case, a non-closepacked polystyrene (PS) nanosphere is a suitable candidate for fabrication template since with a single deposition of Pd, one can create both nano-structures simultaneously in a monolithic structure (Figure 1a). The hole/patch size and period of the nano-entities and the distance between them can be precisely controlled by choosing an appropriate PS bead size and reactive ion etching (RIE) time (see Experimental section). In general, the light and material interaction in the BPNL system can be controlled by many factors, such as film thickness, period, hole/bead size and shape, lattice symmetry, incident angle of probe light, etc. It is also important to note that the NP and NH structures are supported by different substrates, PS and glass, respectively, which may alter their individual hydride phase transformations, as the mechanical properties and adhesive force between the Pd structures and substrates have been shown to affect the pressure range of plateau regions.[38] The proposed BPNL structures are designed to achieve the following unique properties: (1) coexistence of SPP (from NHs) and LSPR (from NPs); (2) strong and tunable local electric field induced by SPR; (3) coexistence of two-plateau regions in hydrogen absorption induced by substrate effects, which might help extend the optical response range of the sensor; and (4) ultrathin thickness for obtaining fast response time. The NHs, NPs and their combined structure of BPNL were realized by a shadowing nanosphere lithography (SNL) method, and the fabrication steps are further described in the Experimental section. The top and bottom panels in Figure S1a 6

in Supporting Information (SI) show the thicknesses of Pd films on NS and glass substrate, respectively, which are predicted by a home-built MATLAB program.[39] Captured top-view and side-view scanning electron microscope (SEM) images of NPs (Figure 1b) show a thickness of ~ 40 nm at the top of nanosphere, and the thickness reduces gradually to zero at the equator, which is consistent with the predicted thickness of NPs. The morphologies of NHs and NPs are also characterized by atomic force microscopy (AFM) as presented in Figure 1c. The measured thickness and hole diameter of NHs are tPd = 41 ± 2 nm and d = 385 ± 11 nm, respectively. 2.2. Optical hydrogen sorption isotherm of NHs. Figure 2a shows the optical transmission spectra T(λ) of NH samples with different . The general profiles of T(λ) spectra are consistent with those of conventional

representative

plasmonic NHs, [35, 36] whose resonance wavelengths can be identified: two transmission dip features at

≈ 430 nm and

≈ 650 nm can be assigned as (1,0) Pd/glass and (1,0) Pd/air

Wood’s anomaly transmission minima, respectively. While the two transmission peak features at ≈ 520 nm and

≈ 900 nm can be attributed to the (1,1) Pd/glass and (1,0) Pd/glass

resonance peaks, respectively (the features are marked in Figure 2a). Details of this are presented in Section S2 of SI. Several changes can be observed in the T(λ) spectra when NH samples are exposed to H2 gas, and the magnitudes of these changes are strongly wavelength-dependent: the transmission increases at the dips

, and

, and

but decreases at

. In terms of transmission peak/dip positions,

red-shift a relatively small amount (< 5 nm), while the peaks

shift to the red much further (~ 25 nm and 150 nm, respectively) with increasing

and . In addition,

T(λ) spectra of NHs behave differently in hydrogen absorption and desorption processes due to

7

hysteresis in the Pd-hydrogen system.[32] For better visualization, we summarize the change of peak/dip positions (∆λ) and corresponding changes of transmission magnitude (∆T) as a function in Figures 2b and 2c. In both sorption processes, general trends of ∆λ and ∆T indicate that

of

the Pd NHs follow the typical phase of hydrogen sorption: α phase at very low-pressure range (

< 45 (20) mbar for absorption (desorption) process), α + β phase for low pressure range

(

≈ 45 to 75 (15 to 20) mbar for absorption (desorption) process), and β phase for high

pressure range (

> 75 (20) mbar for absorption (desorption) process).[40] Note that these

pressure ranges are slightly different from those reported for Pd thin films on α-quartz, sapphire, and Si substrates, due to differences in the mechanical properties and adhesive force between the Pd thin film and substrates.[38] A more detailed discussion of the optical behavior of the Pd NHs is presented in Section S2 of SI, but the main conclusions are summarized below. ∆λ is particularly large in transmission peaks

(~ 25 nm) and

(~ 150 nm), due to the change of optical permittivity of Pd to

PdHx. While small ∆λ observed in the transmission dips is induced mainly by two factors: (i) lattice expansion, which is almost negligible in this case, and (ii) by convolution with adjacent transmission peaks. Regarding ∆T, the absorption of H2 causes the negative dispersion factor /

to decrease, where

and

are the real and imaginary parts of dielectric permittivity of

the Pd/PdHx thin film, respectively,[41, 42] and consequently weakens the plasmonic response of NHs. As a result, the higher transmission is observed at the Wood’s anomalies, while the lower transmission is observed at the resonance peak. Note that the transmission at the peak

does

not follow this trend (the higher transmission at the resonance peak upon hydrogenation), due to this resonance’s proximity to the Wood’s anomalies.

8

2.3. Optical hydrogen sorption isotherm of NPs. Figure 3a illustrates the dynamics of T(λ) spectra of the NPs during the hydrogen sorption processes. The T(λ) spectra show a sharp transmission peak at transmission dip at

≈ 630 nm and a broad

≈ 1000 nm. In general, the transmission increases for λ > 1600 nm, and

the peak and dip red-shift upon the absorption of H2. The optical properties of the NPs are the product of the two main sub-structures: (i) PS nanosphere arrays and (ii) hemispherical Pd caps on nanospheres (FONs). These two sub-structures produce the two features seen in the transmission spectra. Further discussion regarding the optical properties of the NPs, including FDTD simulation results, can be found in SI, Section S3. From these results, the transmission dip at

is identified as the LSPR resonance mode of the Pd caps. The peak

arises from

the photonic band gap of the PS nanosphere template interacting with LSPR mode. The large absorption at the LSPR resonance wavelength

suppresses the transmission below the band

gap and sharpens the peak. It is also worthwhile to note that the NP samples are separate the metal films on PS nanospheres and do not support SPP modes as seen in the NH samples. Similar to the NH samples, the NPs also exhibit hysteresis, which can be noticed in the plots of ∆λ and ∆T versus

(Figure 3b). However, the plateau region of the NPs occurs at a lower

(higher) pressure range during hydrogen absorption (desorption) processes, in comparison with those of NHs (35 mbar <

< 45 mbar and 20 mbar <

< 25 mbar for hydrogen absorption

and desorption, respectively). A plausible explanation is that the curvature of the nanosphere and the substrate material, changed from glass to PS, may induce difference in the mechanical properties and the adhesive force between the Pd thin film and substrates, which eventually results in the pressure range changes of the plateau region.[38] Further systematic experimental and theoretical efforts are needed to clarify this phenomenon. 9

and

In order to further understand the physics of ∆λ, ∆T and

(Figure 3b). The LSPR dip

, we look into the relationship between

shifts exceedingly to the red (~80 nm) when H2

is present, which can be explained by modification of the density states in a hydrogenated system that promotes the interband transitions.[7, 43] This results in a red-shift of the absorption peak spectra as experimentally demonstrated here and also in previous works.[5, 6] Transmission peak re-shifts as a consequence of convolution with

, with a much smaller shifting rate (~ 6.5

nm at 1000 mbar H2). ∆T(λ) spectra (Figure S4 in SI) show a peak at

due to weakening of

the LSPR mode during H2 absorption, which increases the transmittance. The weaker LSPR mode is also seen through decrease of the electric field intensity near the cap edges, as observed in FDTD simulations (Figure S4d in SI). 2.4. The BPNL structure Optical properties of the BPNLs The BPNL structure is a combination of both the NP and NH structures, where they interact at sub-wavelength distances. Therefore, the BPNL possesses optical features inherited directly from its two sub-structures. However, the transmission spectra T(λ)C of the combined structure (BPNL) is not simply a multiplication of T(λ)NP of NPs and T(λ)NH of NHs, it has intertwined optical properties from close-range interactions. Figure 4a shows the T(λ)C spectra of BPNLs along with T(λ) spectra of NP and NH samples in series and T(λ)NP × T(λ)NH spectra for comparison. One can notice several dips/peaks as labelled in Figure 4a: two transmission dips at ≈ 450 nm and Since

=

≈ 650 nm, and two transmission peaks at

≈ 705 nm.

, this implies that the transmission dips of BPNLs come from (1,0) Pd/glass

Wood’s anomaly of NHs. Likewise, the transmission shoulder and

≈ 600 nm and

: strong absorption at LSPR wavelength 10

≈ 705 nm is generated by

suppresses the large transmission at peak

, which eventually pushes the local transmission maximum at wavelength of

≈ 900 nm to a shorter

≈ 705 nm.

The transmission peak

≈ 600 nm, on the other hand, arises from the coupling mode between

the NPs and NHs. We isolate the contribution of the NP in this coupling mode by considering the T(λ)C / T(λ)NH spectra and T(λ)NP spectra, as plotted in Figure 4b. The T(λ)C / T(λ)NH (%) magnitude is about 3 times larger than that of T(λ)NP (%) spectra in the wavelength range of λ = 500 – 1000 nm, and the T(λ)C / T(λ)NH spectral peak at ~ 610 nm is about at the same position and is much broader than the

peak (inset of Figure 4b). This observation can be explained

through the FDTD calculated distribution of electric field of BPNL and its substructures (Figure 4c to 4e), extracted in the yz-plane and at the T(λ)C / T(λ)NH peaks (denoted as green dots in Figure 4b). A large portion of electric field is localized inside the standalone PS nanosphere as seen in Figure 4c.[44] Likewise in the NH structure, one can observe weakly enhanced electric fields at the rim of the hole, which is induced by (1,1) Pd/glass resonance peak (Figure 4d). In the BPNL structure, the partially localized SP mode supported by the Pd cap[44] can couple with the propagating SPP of NHs, relocating the electric field enhancement as seen in Figure 4e. In other words, the NP structure acts as an antenna to localize and enhance the field, and this field enhancement can be released to the far field transmission by the coupling with the propagating SPP mode of NHs. This coupling allows about 3 times more transmission in wavelength range of

λ = 500 – 1000 nm, in comparison with the transmission of separate NP and NH structures in series (Figure 4a). In comparison to the Wood’s anomaly

≈ 430 nm of the NH sample, the transmission dip

≈ 450 nm of the BPNL is slightly red-shifted, and the transmission intensity at

11

of BPNL is

about 2 times lower than that of separate NP and NH structures in series (Figure 4a at

, and

Figure 4b at position denoted by a red dot), which implies that the transmission dip

is not

inherited directly from the NH but is the result of coupling mode between NH and NP. To understand this observation, the normalized electric field intensity |E/E0| maps of BPNL and its substructures at

were extracted and plotted in Figure 4f to 4h. Again here, we find a large

portion of electric field localized inside PS nanosphere (Figure 4f). In the NH map (Figure 4g), strong enhanced electric fields at the rim of the hole are observed, which are induced by the (1,0) Pd/air Wood’s anomaly transmission minima. In the BPNL structure where the PS nanospheres occupy the hole area, the partial localized SPP mode inside is relocated, and a weak dipole resonance at the Pd/air interface of Pd cap can be seen (Figure 4h). Note that the nature of coupling mode at

≈ 450 nm is significantly different from the one at

≈ 600 nm. At

,

the local electric field inside PS nanosphere couples with the (1,1) Pd/glass resonance peak. This allows the near field to be guided and released at the Pd/glass interface to the far field, which promotes the transmission of the BPNL sample. However, at

≈ 450 nm, the enhanced electric

field is concentrated around the hole and cap areas and cannot be guided or emitted to the far field. Therefore, it promotes the absorption of the Pd cap, and a low transmission intensity is seen at

. In addition, the presence of PS nanospheres in the hole area also slightly increases

the effective refractive index explain the red-shift of

of the surrounding medium on the top of NH, which can

≈ 450 nm in comparison to Wood’s anomaly of NH

≈ 430 nm.

Optical hydrogen sorption isotherm of BPNLs The hydrogenation and dehydrogenation of the BPNL structure exhibit an interesting and intricate hysteresis behavior, which can be seen clearly in T(λ) spectra at different

(Figure

5a). The sorption processes of BPNL samples can be considered as a combination of sorption 12

processes of NH and NP samples (further discussion of the BPNL isotherms can be found in Section S4 of SI). When hydrogenating the BPNL sample, one would observe the following phases (1) α phase: both NP and NH sub-structures is in α phase; (2) lower plateau region (a1): α and β phases coexist in NP, while NH is still in α phase; (3) higher plateau region (a2): NP is in β phase, and α and β phase coexist in NH; and (4) β phase: both NP and NH sub-structures is in β phase. Conversely for dehydrogenation, at (5) higher plateau region (d1): α and β phases coexist in NP and NH is still in β phase; while at (6) lower plateau region (d2): NP is in α phase and NH is in α + β phase coexistence region. An illustration of the above processes can be seen in a representative optical isotherm at λ = 705 nm, as shown in Figure 5b. The complex optical behavior of T(λ) spectra of BPNL versus

can be explained qualitatively

by considering the hydride phases of material in NP and NH, as presented in Figure 5c and 5d. The cartoons in Figure 5c portray the hydride phase of NP and NH in phases (1) to (6), and the corresponding T(λ) spectra of the BPNL, NP, and NH structures are presented in three columns of Figure 5d. The T(λ) spectra of stand-alone NP or NH structures only have two hydride states to transition into, that of pure Pd and fully-hydride PdHx, respectively.

However, the

combination of Pd or PdHx NPs with Pd or PdHx NHs can give us four possible optical responsive transitions, which is observed in the BPNL sample and further verified by FDTD calculations (Figure S6 in SI). In addition, the changes in peaks/dips position (∆λ) and transmission magnitude (∆T) versus

(Figure S5a and S5b) can reveal one-plateau-type

behaviors similar to the NP sample (e.g., ∆λ and ∆T at

) and the NH sample (e.g., ∆λ at

),

or two-plateau type-behaviors. This depends on the contributions of NP and NH sub-structure to the optical features of BPNL. Thus, the optical isotherms of the BPNLs depends not only on the

13

electromagnetic coupling of the two different sub-structures, but also on their differing phase transition regimes. Extending response range of a hydrogen plasmonic sensor The response regime of a hydrogen plasmonic sensor can be extended by extending the monotonic optical response over a wider hydrogen pressure range. The optical parameters of the BPNL sample such as ∆λ(

) and ∆T(

) show a strong wavelength dependence, and the

choice of λ determines whether a one- or two-plateau-type behavior is observed. For the BPNL, ∆T(

) at λ = 705 nm exhibits a two-plateau-type behavior, which should thus be useful in

demonstrating an extended response in comparison with that used NP or NH. The normalized ∆T response as a function of

of NP, NH, and BPNL structures are shown in Figure 6a. While the

NP and NH structures show response regimes in pressure ranges of 35 mbar to 45 mbar and 45 mbar to 75 mbar respectively, the BPNL shows a response over the 35 mbar to 75 mbar due to its combined behaviors. Enhanced response time in the low hydrogen pressure range Furthermore, we characterize the response time (t90, the time taken for a sensor to reach 90% of its final equilibrium response)[6] of NP, NH, and BPNL based sensors, and the results are shown in Figure 6b. The response time of NPs are around 1 minute over the pressure range of 3 mbar to 100 mbar, which is much faster than that of NHs due to the larger volume-to-surface area ratio and the reduced pinning effect of the NPs on PS beads.[6] Likewise, the response time of the BPNL sensor is about the average response time of NP and NH samples. When

is lower than

3 mbar, the response time of NP, NH, as well as BPNL sensors at out-of-resonance wavelength (λ = 705 nm) are in order of few hundred seconds, which is also about the average response time

14

of the individual arrays. However, the trend of response time versus time of NH decreases when

are different: response

decreases from 10 mbar to 1 mbar, while response time of NP

increases greatly from t90 ~20 s when

≈ 10 mbar to t90 ~800 s when

≈ 1 mbar.

Interestingly, the response time of BPNL at a coupling wavelength (λ = 460 nm) is significantly lower by up to an order of magnitude (< 100 s) than those of NH, NP, and BPNL at λ = 705 nm. In addition, response time of BPNL decreases when

decreases (from 40 mbar to 1 mbar),

which is similar to the behavior observed in NH samples. Peaks in response time versus plots can also be seen in the plateau regions in all samples, which is consistent with the previously reported Pd-based hydrogen sensors.[33, 34] Note that in this experiment, the response time of NH is long in comparison to other plasmonic hydrogen sensors,[6, 33, 34] which is due to the large thickness of the sample (~40 nm). The response time can be significantly improved by reducing the thickness of the sample to promote the diffusion of hydrogen, or utilizing other materials such as Pd alloy (Details of this can be found in Section S6 of SI).[7] Enhanced sensitivity in BPNL For plasmonic hydrogen sensing, the relative transmission intensity changes ∆T( where ∆T(

) =

2

-



)/



,

, are widely used.[45, 46] Coupling between LSPR and SPP

modes in the BPNL structure significantly modifies the ∆T(λ) in comparison to the spectra of the equivalent structure without coupling effects (i.e. ∆T(λ) spectra of NP and NH in series, as portrayed in Figure S7 in SI). This coupling also enhances the ∆T magnitude at some particular wavelengths (e.g. λ = 460 nm and λ = 700 nm). Based on this effect, we can pick out the wavelength channel that offers the best sensitivity. Figure 6c illustrates the ∆T/T spectrum of 15

BPNL along with that of NP and NH structures. The features of the ∆T/T spectra of these structures all arise from the plasmonic effects. In the ∆T/T spectra of the NH, two peaks are observed for the Wood’s anomalies at

and

, while a dip is observed at the resonance at

). Similarly, a peak is observed at LSPR wavelength

in the ∆T/T spectrum of the NP. The

∆T/T spectrum of the BPNL has a strong peak at λ ≈ 460 nm, which is induced by the coupling mode between (1,0) Wood’s anomaly at Pd-air interface of NH and partial localized SPP mode inside the PS nanosphere, as discussed above. The extracted optical absorption isotherm of ∆T/T spectrum at the peak λ ≈ 460 nm shows a sensitivity of 10.6 %/mbar for pressure range (a1), which is an order of magnitude larger than that of the NP sample (1.0 %/mbar) (Figure 6d). For pressure range (a2), the sensitivities extracted from BPNL and NP are in the same order of magnitude (0.15 and 0.27 %/mbar, respectively). In comparison to the most sensitive response ranges offered by NH and NP (5 and 2.4 %/mbar, respectively, at wavelengths denoted by red and blue circles in Figure 6d), the sensitivity of BPNL shows about 2- and 4-time enhancements, respectively. The t90 response time of the BPNL at λ = 460 nm (Figure 6b) shows reasonable response times of approximately ≤ 100 s when

≥ 40 mbar or

≤ 10 mbar.

Tunable hydrogen sensing platform with BPNL In order to illustrate the effects of the structure on the optical response as well as on the sensing performance of the hydrogen sensor, we attempt to tune the hole size (d) since this parameter can directly affect the coupling effect between two plasmonic layers[28] (separation distance between two layers) as well as the contribution of each component (i.e. NPs and NHs) to the total optical response of the BPNL structure. Three BPNL samples (tPd = 40 nm, D = 500 nm) with different average hole diameters (d = 500, 385, and 290 nm) have been fabricated on PS nanosphere templates with corresponding reduced bead sizes. The diameters of the beads are 16

further confirmed by AFM as seen in Figure 7. In the case the PS nanospheres were not etched, d = 500 nm, we obtained an array of nanotriangles instead of NHs (top figure, Figure 7a). When the PS nanospheres were etched for a longer time, the hole diameter of NHs (d = 290 nm) became smaller (bottom figure, Figure 7a). The optical response of the BPNL sample changes significantly when d changes (Figure 7b). When d = 500 nm, we observe a dominant optical transmission peak at λ ≈ 640 nm and the spectra shape is similar to that of standalone NP (Figure 3). On the other hand, when d = 290 nm, the transmission at the coupling peak (λ ≈ 540 nm) is relatively small, due to the greater material coverage. Upon hydrogenation, all the three samples exhibit different spectral changes; the d = 290 nm sample shows a maximum ∆T/T at the LSPR resonant wavelength of NP (λ ≈ 720 nm, indicated by an orange circle in Figure 7c). In the d = 500 nm sample, the maximum changes of the ∆T/T spectrum can be found at the peaks where the wavelength position matches the diffraction condition of the PS nanosphere template (λ ≈ 440, 490, and 600 nm). The extracted absorption isotherm of the d = 500 and 290 nm samples (wavelength indicated by a colored circle in Figure 7c) shows a larger sensitivity in the response regime than that of the d = 385 nm sample (Figure 7d). In addition, the response time of the d = 500 nm sample is also significantly shorter than those of the d = 385 and 290 nm samples due to the dominant and fast response of the NP contributions to the total optical changes of the sample (Figure 7e). It is important to note that the plateau range also depends on the hole size. These results demonstrate that the optical wavelength, sensitivity, and response time of the BPNL can be optimized by tuning the structural parameters (e.g., hole size). 3. Conclusions

17

We have demonstrated that the Pd plasmonic structures of NPs, NHs, and BPNLs can be realized by a single Pd electron beam deposition on a nanosphere template substrate. The plasmonic properties upon (de)hydrogenation of these structures exhibit a reversible optical change, significant plasmonic peak shifts, and large changes in transmission amplitudes, which are desirable for hydrogen gas sensing applications. Strong electromagnetic coupling between the NPs and NHs induces unusual optical properties upon the hydrogen sorption, which enhances the sensitivity of the hydrogen sensor by factors of 2 and 4 in comparison to those of the isolated substructures of NP and NH at their non-coupling resonance wavelengths. For low hydrogen pressures (< 3 mbar), the time response of BPNL at the coupling resonance wavelength is an order of magnitude faster than that of the isolated array sensors. The NP and NH possess α + β phase mixed-phase regions at different values of

, and combining them together in BPNL can

induce two-plateau-region behaviors and extend its response regime. In order to further improve the performance of the BPNL structure, the plasmonic behavior of the metal layer should be enhanced since Pd is not a strongly plasmonic material. For example, the plasmonic properties as well as coupling resonance effect of Pd nano-entity arrays can be further improved by using Pd/Ag multilayers or Pd-Ag/Au alloys. The use of such composites can also greatly reduce the response times of individual nano-entity and combined structures.[6, 7, 9] In addition, the optical properties of NPs, NHs, and BPNL can be simply controlled by tuning the structural parameters such as nanosphere diameter (D), hole size (d), film thickness, etc., thanks to the flexibility of the fabrication method. This tunability is promising for designs and applications of future hydrogen sensing materials and structures. 4. Experimental Section

18

Materials: Deionized water (18 MΩ.cm) was used for all experiments. PS nanospheres (Polysciences Inc., D = 500 nm) and ethanol (Sigma-Aldrich, 98%) were used to create the nanosphere monolayers. Palladium (99.95%) from Kurt. J Lesker Company were utilized for ebeam depositions. Sample fabrication: Hexagonal non-close-packed nanosphere (period D = 500 nm, diameter d = 290, 380, and 500 nm) monolayer, which was prepared an air/water interface method and reactive ion etching (RIE),[47, 48] was used as a template for electron beam deposition. The substrates were coated with 40 nm of Pd under a constant deposition rate of 0.05 nm/s, and the sample holder rotated azimuthally with a constant rotation rate of 30 rpm during deposition process. The BPNL structure can be furtherly lifted-off by scotch tape to achieve NH and NP structure (Figure 1a). Morphology characterization: The morphologies of fabricated samples were characterized by an atomic force microscope (AFM, Park NX10), and all the AFM images were analyzed by XEI Image Processing and Analysis Software. Scanning electron microscopy (SEM) was performed with a Jeol model JSM-6390LV scanning electron microscope equipped with an INCAEnergy EDS system from Oxford instruments. Optical characterization: The optical transmissions of NH, NP, and BPNL samples with different H2 pressure (

) were characterized by a home-made setup. The samples were loaded into an

in-house-made vacuum chamber with quartz optical windows, and

was controlled by two

pressure transducers (PX409-USBH, Omega). An unpolarized collimated halogen lamp light source (HL-2000, Ocean Optics) was guided to the sample at a normal incident angle. Transmitted light was collected, coupled to an optical fiber, and guided to two spectrometers

19

(USB4000-VIS-NIR-ES, Ocean Optics, λ = 400 – 1000 nm and DWART-STAR, Stellar, λ = 900 – 1650 nm) for capturing the transmission spectra. Before any measurements, the chamber and samples was flushed with >10 times with hydrogen-nitrogen cycles. All the measurements were performed at 25 °C. FDTD calculations: FDTD calculations of Pd hydride samples were carried out using a commercial software (Lumerical FDTD Solutions).[49] The geometric parameters of Pd thickness (tPd), Pd cap thickness, period (D), hole diameter (d) were obtained from MATLAB simulation. A rectangular unit cell for calculation was used, with a 2-dimesional periodic boundary condition in x- and y-axes, and a perfectly matched layer (PML) boundary condition applied on the z-axis of the simulation region (in top panels of Figure S2 and S4, red boxes). The mesh size of 2.5 nm × 2.5 nm × 2.5 nm was chosen to ensure the convergence of the calculations. The refractive index of glass and PS was chosen to be 1.5 and 1.59, respectively, and the optical parameters of Pd and PdHx were extracted from Ref. [42]. Acknowledgements The authors thank Prof. Yiping Zhao for his generosity in sharing his nano-fabrication tools with us. This work was supported by Savannah River National Laboratory’s Laboratory Directed and Development program (SRNL is managed and operated by Savannah River Nuclear Solutions, LLC under contract no. DE-AC09-08SR22470 (G.K.L and T.D.N) and STYLENQUAZA LLC.DBA VICOSTONE USA under Award No. AWD00009492 (T. D. N). Conflict of Interest The authors declare no conflict of interest.

20

References [1] Editorial, Nature Materials, 17 (2018) 565-565. [2] E. Eriksson, E.M. Gray, Applied Energy, 202 (2017) 348-364. [3] I. Dincer, C. Acar, International journal of hydrogen energy, 40 (2015) 11094-11111. [4] W.J. Buttner, M.B. Post, R. Burgess, C. Rivkin, International Journal of Hydrogen Energy, 36 (2011) 2462-2470. [5] C. Wadell, S. Syrenova, C. Langhammer, ACS Nano, 8 (2014) 11925-11940. [6] F.A. Nugroho, I. Darmadi, L. Cusinato, A. Susarrey-Arce, H. Schreuders, L.J. Bannenberg, A.B. da Silva Fanta, S. Kadkhodazadeh, J.B. Wagner, T.J. Antosiewicz, Nature materials, 18 (2019) 489. [7] F.A.A. Nugroho, I. Darmadi, V.P. Zhdanov, C. Langhammer, ACS Nano, 12 (2018) 99039912. [8] K. Sugawa, H. Tahara, A. Yamashita, J. Otsuki, T. Sagara, T. Harumoto, S. Yanagida, ACS Nano, 9 (2015) 1895-1904. [9] I. Darmadi, F.A.A. Nugroho, S. Kadkhodazadeh, J.B. Wagner, C. Langhammer, ACS sensors, 4 (2019) 1424-1432. [10] C. Langhammer, V.P. Zhdanov, I. Zorić, B. Kasemo, Phys. Rev. Lett., 104 (2010) 135502. [11] N. Liu, M.L. Tang, M. Hentschel, H. Giessen, A.P. Alivisatos, Nature materials, 10 (2011) 631. [12] M.A. Poyli, V.M. Silkin, I.P. Chernov, P.M. Echenique, R.D. Muino, J. Aizpurua, J. Phys. Chem. Lett., 3 (2012) 2556-2561. [13] K. Ikeda, S. Uchiyama, M. Takase, K. Murakoshi, ACS Photonics, 2 (2014) 66-72. [14] H.K. Yip, X. Zhu, X. Zhuo, R. Jiang, Z. Yang, J. Wang, Adv. Opt. Mater., 5 (2017) 1700740. [15] F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Science, 293 (2001) 2227-2231. [16] S. Syrenova, C. Wadell, F.A. Nugroho, T.A. Gschneidtner, Y.A.D. Fernandez, G. Nalin, D. Świtlik, F. Westerlund, T.J. Antosiewicz, V.P. Zhdanov, Nature materials, 14 (2015) 1236. [17] A. Baldi, T.C. Narayan, A.L. Koh, J.A. Dionne, Nature materials, 13 (2014) 1143. [18] E. Maeda, S. Mikuriya, K. Endo, I. Yamada, A. Suda, J.-J. Delaunay, Appl. Phys. Lett., 95 (2009) 133504. [19] E. Maeda, T. Matsuki, I. Yamada, J.-J. Delaunay, J. Appl. Phys., 111 (2012) 084502. [20] R. Tsuji, K. Endo, M. Shuzo, I. Yamada, J.-J. Delaunay, Journal of Micro/Nanolithography, MEMS, and MOEMS, 8 (2009) 021140. [21] V.N. Konopsky, D.V. Basmanov, E.V. Alieva, D.I. Dolgy, E.D. Olshansky, S.K. Sekatskii, G. Dietler, New J. Phys., 11 (2009) 063049. [22] M. Morjan, H. Züchner, K. Cammann, Surface Science, 603 (2009) 1353-1359. [23] K. Lin, Y. Lu, J. Chen, R. Zheng, P. Wang, H. Ming, Opt. Express, 16 (2008) 18599-18604. [24] A. Tittl, C. Kremers, J. Dorfmüller, D.N. Chigrin, H. Giessen, Opt. Mater. Express, 2 (2012) 111-118. [25] S. Syrenova, C. Wadell, C. Langhammer, Nano Lett., 14 (2014) 2655-2663. [26] A. Yang, M.D. Huntington, M.F. Cardinal, S.S. Masango, R.P. Van Duyne, T.W. Odom, ACS Nano, 8 (2014) 7639-7647. [27] S.R. Larson, H. Luong, C. Song, Y. Zhao, J. Phys. Chem. C, 123 (2019) 5634-5641. [28] X. Wen, Z. Xi, X. Jiao, W. Yu, G. Xue, D. Zhang, Y. Lu, P. Wang, S. Blair, H. Ming, Plasmonics, 8 (2013) 225-231. [29] T.M. Schmidt, M. Frederiksen, V. Bochenkov, D.S. Sutherland, Beilstein journal of nanotechnology, 6 (2015) 1-10. 21

[30] W. Du, L. Liu, P. Gu, J. Hu, P. Zhan, F. Liu, Z. Wang, Appl. Phys. Lett., 109 (2016) 121108. [31] R. Schwarz, A. Khachaturyan, Acta Materialia, 54 (2006) 313-323. [32] T.B. Flanagan, W. Oates, Annual Review of Materials Science, 21 (1991) 269-304. [33] J. He, N.S. Villa, Z. Luo, S. An, Q. Shen, P. Tao, C. Song, J. Wu, T. Deng, W. Shang, RSC Adv., 8 (2018) 32395-32400. [34] S.-Y. Cho, H. Ahn, K. Park, J. Choi, H. Kang, H.-T. Jung, ACS sensors, 3 (2018) 18761883. [35] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature, 391 (1998) 667-669. [36] L. Martín-Moreno, F.J. García-Vidal, H.J. Lezec, K.M. Pellerin, T. Thio, J.B. Pendry, T.W. Ebbesen, Phys. Rev. Lett., 86 (2001) 1114-1117. [37] T.D. Nguyen, S. Liu, Z.V. Vardeny, A. Nahata, Opt. Express, 19 (2011) 18678-18686. [38] Y. Pivak, R. Gremaud, K. Gross, M. Gonzalez-Silveira, A. Walton, D. Book, H. Schreuders, B. Dam, R. Griessen, Scripta Materialia, 60 (2009) 348-351. [39] W.M. Ingram, C. Han, Q. Zhang, Y. Zhao, J. Phys. Chem. C, 119 (2015) 27639-27648. [40] R. Matelon, J. Avila, U. Volkmann, A. Cabrera, E.H. Morales, D. Lederman, Thin Solid Films, 516 (2008) 7797-7801. [41] Y. Nishijima, A. Balčytis, G. Seniutinas, S. Juodkazis, T. Arakawa, S. Okazaki, R. Petruškevičius, Sensors and Materials, 29 (2017) 1269-1274. [42] K.J. Palm, J.B. Murray, T.C. Narayan, J.N. Munday, ACS Photonics, 5 (2018) 4677-4686. [43] V.M. Silkin, R.D. Muiño, I.P. Chernov, E.V. Chulkov, P.M. Echenique, Journal of Physics: Condensed Matter, 24 (2012) 104021. [44] Y. Li, J. Pan, P. Zhan, S. Zhu, N. Ming, Z. Wang, W. Han, X. Jiang, J. Zi, Opt. Express, 18 (2010) 3546-3555. [45] C. Perrotton, R.J. Westerwaal, N. Javahiraly, M. Slaman, H. Schreuders, B. Dam, P. Meyrueis, Opt. Express, 21 (2013) 382-390. [46] M.E. Nasir, W. Dickson, G.A. Wurtz, W.P. Wardley, A.V. Zayats, Adv. Mater., 26 (2014) 3532-3537. [47] H.M. Luong, M.T. Pham, B. Ai, T.D. Nguyen, Y. Zhao, Phys. Rev. B, 99 (2019) 224413. [48] J. Henzie, J.E. Barton, C.L. Stender, T.W. Odom, Acc. Chem. Res., 39 (2006) 249-257. [49] https://www.lumerical.com/.

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Figure 1. (a) Schematic of the fabrication process. (b) Top-view (top) and size-view (bottom) SEM images of BPNL. (c) AFM images of NH (left) and NP on scotch tape (right).

23

Figure 2. (a) Optical transmission spectra T(λ)NH of NH samples upon hydrogen absorption and desorption processes. (b) ∆λ and (c) ∆T as functions of denote the sorption direction.

24

at

,

,

, and

. Arrows

Figure 3. (a) Optical transmission spectra T(λ)NP of NP samples upon hydrogen absorption and desorption processes. (b) ∆λ (left) and ∆T (right) as a function of denote the sorption direction.

25

at

and

. Arrows

Figure 4. (a) Optical transmission spectra T(λ)C of BPNL samples (

= 0 mbar), along with

T(λ) spectra of NP and NH samples in series and T(λ)NP × T(λ)NH spectra. (b) T(λ)NP spectra in comparison with T(λ)C / T(λ)NH spectra. Inset: normalized T(λ)NP spectra and T(λ)C / T(λ)NH spectra. (c-e) FDTD calculated distribution of electric field of NP, NH, and BPNL at the T(λ)C / T(λ)NH peak (denotes by green dot in Figure 4b), extracted at yz-plane. (f-h) FDTD calculated distribution of electric field of NP, NH, and BPNL at the T(λ)C / T(λ)NH dip (denoted by red dot in Figure 4b), extracted at yz-plane. 26

Figure 5. (a) Optical transmission spectra T(λ)C of BPNL samples upon hydrogen absorption and desorption processes. (b) Representative optical absorption and desorption isotherms of BPNL samples, extracted at λ = 705 nm. Number in the bracket (1)-(6) denotes the material phases of 27

BPNL sample, which are mentioned in Section 2.4. Four plateau regions (a1), (a2), (d1), and (d2) are also highlighted by pale green, grey, blue, and orange boxes, respectively. Arrows denote the sorption direction. (c-d) Phases transformations and corresponding optical responses of BPNL sample, upon hydrogenation/dehydrogenation. (c): a cartoon depicts the transformation from phase (1) to (6) (as described in Figure 5b) of NH and NP sub-structures. (d) shows the corresponding optical transmission spectra of BPNL sample (BPNL column). NH column and NP column show the optical transmission spectra of standalone NH and NP, respectively, with similar hydride phase in BPNL for a comparison purpose (d).

28

Figure 6. (a) Extended response regime covers the pressure range highlight in green (NPs) and grey (NHs). (b) Response times (t90) of NH, NP and BPNL samples (at λ = 460 and 705 nm). (c) ∆T/T spectra of BPNL, NP, and NH samples (where ∆T(

)=

-



and

= 1000

mbar). (d) Extracted absorption isotherm of ∆T/T spectra (NP, NH, and BPNL at λ = 460 nm, NP at λ = 988 nm, and NH at λ = 651 nm; λ = 460, 651, 988 nm are denoted with green, red, and blue dots in Figure 6c, respectively). Inset: zoom in of the gray shadowed area. The sensitivity numbers indicate the change in relative transmission magnitude (%) per 1 mbar increase in

29

.

Figure 7. (a) AFM images of the BPNL sample after the PS nanospheres were lifted-off. d = 385 ± 11 nm (middle) and d = 290 ± 9 (bottom) were estimated. (b) Optical transmission spectra of the BPNL samples with different values of d, when

= 0 and 1000 mbar. (c)

∆T/T spectra of the BPNL samples with different values of d (where ∆T(

and

) =

-

= 1000 mbar). (c) Extracted absorption isotherm of the ∆T/T spectra and (d)

the corresponding response times of the BPNL samples with different values of d (wavelengths indicated by the colored circles in Figure 7b).

30

•

A plasmonic hydrogen gas sensor based on Bilayer Plasmonic Nano-lattices (BPNLs) was investigated.

•

Strong electromagnetic coupling between localized and propagating plasmonic modes induced reversible optical responses upon (de)hydrogenation.

•

Optical hydrogen sensing performances were improved greatly, with expanded response range, higher sensitivity, and shorter response times at low hydrogen pressure range in comparison to these of isolated array sensors.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: