Tamm Plasmon Polaritons Hydrogen Sensors

Tamm plasmon polariton (TPP) resonance can be excited within the stopband of a distributed Bragg reflector (DBR) by combining a thin metal film. When exposured to hydrogen gas, the TPP resonance feature is redshifted for the TPPs structure with palladium (Pd) on the top, due to hydrogen‐induced palladium lattice expansion. By utilizing a DBR‐side TPP structure, near‐zero reflectance can be achieved, leading to more than 3 orders of magnitude changes in reflectance compared to metal‐side TPP structure. The proposed TPP hydrogen structure enables the detection with low H2 concentration from 0.5% to 4%. Comparing to the only Pd film, TPP sensor has increased the sensitivity about 100% at visible wavelengths.


Introduction
Hydrogen (H 2 ) is considered as one of most clean energy carriers in the future, but it is dangerous and highly explosive when the hydrogen concentration is in the range of 4-76 vol% in atmosphere. Hydrogen is difficult to detect by human senses because it is a colorless, odorless and tasteless gases at room temperature and pressure. However, for safe usage of this clean energy, DOI: 10.1002/apxr.202200094 hydrogen sensor would play an important rule of detecting the hydrogen leakage and monitoring the hydrogen concentration. [1] There are several materials can be used for hydrogen sensing, conventional metal oxides have commonly utilized to the semiconductor type sensors such as titanium dioxide (TiO 2 ), [2,3] tin dioxide (SnO 2 ), [4] zinc oxide (ZnO), [5] and tungsten trioxide (WO 3 ) [6,7] with measuring the resistance change caused by the reduction of oxygen ion which releases electrons into the lattice. Moreover, the metal oxide-based hydrogen sensors usually require a heater and catalysis metal to enhance the sensing performance up to the ppm level. Different from the metal oxides, several metals with good absorption for hydrogen can change their optical and structure properties, such as palladium, yttrium, [8] and magnesium. [9] Among those metals, palladium has commonly applied to hydrogen storage, hydrogenation catalysis and hydrogen sensor because of the good selectivity to hydrogen incorporation. The hydrogenation mechanism of palladium undergoes a phase transition from palladium lattice fill up with hydrogen atoms ( -phase) to palladium hydride lattice ( -phase) leads to expanse the lattice, this expansion of lattice causes significant changes in dielectric constant. [10,11] Although yttrium and magnesium have ability to absorb hydrogen and change the dielectric function when exposure to hydrogen, they still require a catalysis metal due to the poor dissociation of hydrogen molecules. [12,13] Plasmonic sensing and strong coupling utilizing metallic nanostructures has attracted a lot of attention in recent years. [14][15][16] Some plasmonic sensors utilized palladium nanostructure as sensing material, when exposure to hydrogen the palladium dielectric function change induces the localized surface plasmon resonance (LSPR) shifting. [17][18][19] The theoretically model of palladium hydride dielectric constant has been proposed by Poyli et al. [20] The others utilize gold nanoantenna as signal transducer to probe the hydride formation of palladium nanoparticles. [21][22][23] Tittl et al. [24] have demonstrated the palladium nanowire with near perfect absorber at visible range could provide the high sensibility about 8.8 times reflectance change when exposure to hydrogen, but the setup requires a polarized light perpendicular to wire. Bagheri et al. [25] proposed the largearea palladium rectangular nanoantennas arrays fabricated by laser lithography method operated at near infrared range with polarization independent. However, a few research has focused on the thin film based plasmonic hydrogen sensors. Tamm plasmon polaritons (TPPs) structures generally combine the distributed Bragg reflector (DBR) with a metal film. [26,27] TPP resonance can be excited in the stopband of DBR in both TE and TM polarization waves. [28] The tunability of TPP resonance wavelength have been realized by modifying the DBR stopband. [29] In recent years, TPP has already been applied in many applications like, lasing, [30] beam steering, [31] and bound states in the continuum. [32,33] Comparison with the nanostructures devices, nanofabrication usually requires the electron beam lithography and increases the difficulty to fabricate large area samples. On the other hand, TPP structures can be simply fabricated in the low-cost and large area thin film deposition such as electron beam evaporator and DC magnetic sputtering. [34] In this work, we first show the design rule of optimizing TPP structure to achieve high hydrogen sensitivity in visible wavelengths. The TPP structures was fabricated by electron beam evaporator composed of dielectric multilayers as DBR and palladium sensing layer with the resonance dip located at 699 nm. Our home-build hydrogen sensing measurement system provides the ability to control various hydrogen concentrations and the real time reflectance spectra measurement. When exposure to the hydrogen concentration from 0.5 to 4 vol%, a red shifted of the resonance dip could be observed and the reflectance will be increased at 699 nm. The change in optical signal could be sensed remotely without the contact electrode.

Simulation and Experiment
The TPP structure with near zero resonance dip was demonstrated. First, the DBR was fabricated on BK7 glass substrate with the 3 pairs of high and low refractive index material Ta 2 O 5 (nH = 2.11 at 792 nm; tH = 93.7 nm) and SiO 2 (nL = 1.44 at 792 nm; tL = 137 nm), respectively. An additional SiO 2 layer (tDBR_last = 54 nm) was placed on the top of DBR for optimizing the resonance wavelength to achieve near zero reflectance. Afterward, a palladium metal layer (tPd = 70 nm) with Ti adhesion layer (tTi = 5 nm) were deposited on the DBR by an electron beam evaporator. The schematic of palladium-based TPP structure in hydrogen environment was shown in Figure 1a. The hydrogen molecules defused into the surface of palladium thin film and break apart to single hydrogen atoms; then the hydrogen atoms fill up the space in the palladium lattice. A scanning electron microscope (SEM) cross section image of the TPP structure was shown in Figure 1b. The reflectance spectra of DBR-side TPP structure showed a well agreement to measurement and simulation result in Figure 1c. The resonance dip is observed at 699 nm. The electrical fields distribution at the resonance wavelength was calculated by the commercial field element method (FEM) simulation, COMSOL. A strong enhancement of electrical fields located at the interface of palladium metal layer and the DBR is observed.
In the literatures, Yang et al. have demonstrated the metal-side TPP structure (light incident form metal layer) compared with DBR-side TPP structure (light incident form DBR) around the resonance wavelength. [35,36] The metal layer thickness and the number of DBR pairs played the important role of designing an optimized TPP structures. To achieve near zero reflectance dip in metal-side TPP structure, the metal layer thickness determined the light transmitted through the metal layer with less losses, so we utilized an ultrathin metal film to reach near zero reflectance dip. However, for DBR-side TPP structure, the number of the DBR pairs determined the influence of performance rather than metal thickness, so the combination of 3 pairs of DBR with 40% in transmission and a thick metal layer behaved as a good reflector to trap the light in the interface between the metal layer and the DBR. In addition, the DBR-side TPP structure decreased the difficulty of the deposition a continuous ultrathin metal film, and also reduced the requirement of number of the DBR pairs. Figure 2 shows the reflectance and sensitivity spectra of both metal-side and DBR-side TPP structure with the calculation with transfer-matrix method. When exposure to 4 vol% hydrogen, the optical property change of palladium was calculated by Bruggeman's effective medium approximation shown in Figure S1 (Support Information). [20,37] For metal-side TPP structure (Figure 2a), the reflectance spectra showed a broad resonance dip at 791 nm with 6.46% reflectance, when exposure in 4 vol% hydrogen, the dip shifted about 17 nm to the longer wavelength. The reflectance decreases to 2.97% in resonance wavelength. However, the spectra of the DBR-side TPP structure (Figure 2b) showed a higher quality factor (Q-factor) and near zero reflectance resonance dip at 699 nm. After exposure in 4 vol% hydrogen in nitrogen, we observed a 3 nm resonance red-shifted. The reflectance increased from 0.02% to 0.77% in the resonance wavelength. Here, the sensitivity of the alternative readout method was calculated by the following Equation (1) in which the R H2 and R 0 were represented the reflectance of exposure to hydrogen and initial state, respectively. Therefore, we optimized the resonance dip to achieve near zero reflectance to observe a high sensitivity.
Comparison of the metal-side and DBR-side TPP structures, we do not only observe the obviously spectral broadening for both cases, but also observed the reflectance change in the initial resonance wavelength. However, the sensitivity of DBR-side structure was about 35 times stronger than the metal-side TPP structure induced by the same hydrogen concentration condition, demonstrate the better sensing performance in the DBR-side TPP structure which was shown in Figure 2c.
To estimate the TPP hydrogen sensor's performance, the 2D reflectance map of two types of TPP structures (Figure 3a,e) with same central wavelength were compared in Figure 3. The DBRside TPP structure required thick metal layer seen in Figure 3b; while the metal-side TPP structure required ultrathin metal film was shown in Figure 3f. To achieve the near zero reflectance, the number of DBR pairs for metal-side TPP structure required more than 8 pairs shown in Figure 3g. However, for the DBR-side TPP structure only required 3 pairs had the advantage to cost down the fabrication shown in Figure 3c. Eventually, the last layer thickness allowed us to control the resonance wavelength in both DBR-side and metal-side TPP structure. When increasing the thickness, the resonance dip can shift to the longer wavelength, which is shown in Figure 3d,h.

Results and Discussions
A TPP hydrogen measurement system could provide the real time measuring for the hydrogen concentration. We used the pure nitrogen cylinder and hydrogen generator as the gas sources. The hydrogen generator utilized the electrolysis pure water to generate 99.99% hydrogen. The mass flow controllers (Alicat-MC) controlled the flow rate of both gas sources. A customize chamber (135 mL in volume) with a window on the top was placed on the stage of optical microscope (Olympus BX51)   with white light source. The reflectance spectrum was detected by spectrometer (Ocean Optics USB2000+UV-vis) in visible range. The schematic of measurement system was shown in Figure 4a.
The measurement result of DBR-side TPP structure with different hydrogen concentrations was shown in Figure 4b. When exposuring to the low hydrogen concentration (2 vol% hydrogen in nitrogen), the resonance wavelength shifted to longer wavelength and the reflectance decreased to the minimum (Figure 4b, red line). As exposure to the higher hydrogen concentration (4 vol% hydrogen), the resonance wavelength further red-shifted and spectral was broaden as well as the reflectance increase were observed in the measurement (Figure 4b, blue line). The sensitivity spectrum is shown in Figure S2 (Support Information).
Here, we compared the palladium thin film with the DBRside TPP structure in same palladium thickness (t Pd = 70 nm with 5 nm Ti adhesion layer). The TPP structure (Figure 5a, red line) and palladium thin film (Figure 5a, black line) were exposed to various hydrogen concentrations and continuously recorded the changes of spectra over time, during the cycle of hydrogen loading and unloading process. Each cycle includes the chamber purging with the pure nitrogen for 1800 s and the loading with different hydrogen concentrations for 1200 s; then purging the hydrogen out of the chamber by exposed to pure nitrogen for 1800 s again.
The time dependent sensitivity for both TPP structure and palladium thin film in various hydrogen concentration at 709 nm was showed in Figure 5a. When applying the 0.5 vol% hydrogen, and the sensitivity increased to 8% and further increased in the higher hydrogen concentration. When applying the hydrogen concentration above 3 vol%, the signal would take longer time to reach the saturation. This phenomenon was considered as the phase change in the process of palladium hydride. The palladium firstly transformed to palladium hydride -phase with no lattice expansion in palladium atoms below 3 vol%. When the loading hydrogen concentration was above 3 vol%, the palladium would start to transform to -phase from -phase, but there was a hys-teresis sate with both -phase and -phase coexistence shown in Figure S3 (Support Information). As the hydrogen concentration increase over 10 vol%, the -phase would finally dominate over the palladium thin film with the maximum sensitivity. The palladium hydride -phase was caused by huge amount hydrogen atoms defused into palladium lattice; in addition to the lattice expansion, it also changes the optical properties. This huge lattice expansion caused irreversible deformation to palladium thin film, when the palladium layer thickness was over 20 nm. Therefore, we added an ultrathin titanium (Ti) adhesion layer to reduce the stress between palladium and DBR interface. [38] For the palladium thin film without DBR, we could only observe a slight change of the reflectance at 2 vol% hydrogen. When the TPP structure is utilized to improve the sensing performance, the obvious sensitivity could increase below the 2 vol% concentration. Moreover, the sensitivity increased about 20 times than pure palladium thin film at 0.5 vol% hydrogen shown in Figure 5b.

Conclusions
In summary, we show the design rule of the optimized TPP structure to achieve high hydrogen sensitivity in visible wavelength. Utilizing the simple home-build hydrogen sensing system, the real time reflectance spectra in various hydrogen concentration is characterized. The TPP structure provides a near zero reflectance at the resonance wavelength. When exposure to hydrogen, the phase change in the palladium lattice is observed in the optical property. In addition, adding the titanium adhesion layer could further reduce the stress between the metal layer and DBR. When increasing the hydrogen concentration, the spectra red-shift and the increase of reflectance showed a good hydrogen sensitivity up to 100% in the 4 vol% hydrogen. The proposed large-area and low-cost plasmonic hydrogen sensor, which can be operated at the room temperature with alternative readout methods, had a strong potential to future green energy and hydrogen applications.

Experimental Section
Statistical Analysis: The data were presented as mean ± SD. The sample size was 20. The software used were COMSOL, Origin, and Matlab.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.