Activation process of reversible Pd thin film hydrogen sensors

Sensors and Actuators B 186 (2013) 258–262

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Activation process of reversible Pd thin film hydrogen sensors Antonin Ollagnier a , Arnaud Fabre b , Thomas Thundat c , Eric Finot a,∗ a

Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS, Université de Bourgogne, 9 Avenue A. Savary, F-21078 Dijon, France DTRI, CEA Valduc, F-21120 Is Sur Tille, France c University of Alberta, Alberta, Canada b

a r t i c l e

i n f o

Article history: Received 6 February 2013 Received in revised form 11 April 2013 Accepted 10 May 2013 Available online 31 May 2013 Keywords: Hydrogen sensor Palladium hydride Film stress Thin film

a b s t r a c t Microcantilever-based thin film palladium hydrogen sensors have high selectivity and sensitivity. Reproducibility and accuracy of the sensor performance depend on the activation process of the polycrystalline palladium film deposited on the cantilever. When the hydrogen is in solid solution (˛-phase), the cantilever bending is mostly governed by the residual film stress induced by the swelling at the grain boundaries in the film. When the palladium hydride (ˇ-phase) starts to be formed, the cantilever undergoes a large deflection due to hydrogen absorption-induced film swelling (10% change in volume). Differences in the phase diagrams of the palladium hydride for two film thicknesses show that the cantilever bending is governed by hydrogen uptake as well as the release of the residual stress of the film through cyclic exposure and cycling number. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Miniature hydrogen sensors with selectivity, sensitivity and reproducibility have immediate relevance in many commercial and technical applications. Hydrogen sensing for practical applications requires detection sensitivities around 1 ppm (part-per-million) [1,2]. This detection limit makes the microcantilever as one of the best strategies for MEMS-based hydrogen sensor. A thin film of palladium (Pd) deposited on one of the sides of the cantilever serves as the recognition layer and the cantilever undergoes bending as a function of hydrogen absorption. The extent of cantilever bending is related to the concentration of hydrogen in the ambient. The selectivity of microcantilever hydrogen sensor is governed by the selectivity of the palladium to the hydrogen. The palladium–hydrogen system is very well documented [3] and the palladium is often used in the industry as a filter or a storage element for hydrogen. We have already demonstrated how a palladium cantilever is sensitive to light gases such as hydrogen, deuterium and tritium [4]. The bending of the cantilever due to hydrogen absorption was explained by the equilibrium internal stress in the palladium film, which is governed by the H2 pressure following Sievert’s law [5]. Indeed, hydrogen atoms can dissolve specifically in palladium after a surface reaction of gaseous dihydrogen (H2 ) and occupy interstitial positions with an atomic ratio governed by the Pd–H phase diagram. The Pd–H system has been extensively studied in the past for bulk systems and more recently

∗ Corresponding author. Tel.: +33 3 80 39 3774; fax: +33 3 80 39 60 24. E-mail address: eric.fi[email protected] (E. Finot). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.041

for nanocolloids and thin films [6]. For sensor applications, thin films provide an interesting alternative to bulk materials [7]. For lower concentrations of hydrogen, the Pd–H forms the ˛-phase and the kinetics is explained using thermodynamics describing the dissociation limited absorption [1,8–10]. Hydrogen absorption in thin Pd film results in a large compressive stress (900 MPa) as reported by Liu et al. [11]. The bending moment of a Pd-coated cantilever beam during hydrogen exposure was first explained by the volume expansion of the metal due the hydrogen uptake. The concept of microcantilever hydrogen sensors based on thin Pd film is well established. However, the reliability of these sensors is not yet fully understood and mastered. For example, the bending response during desorption does not reach equilibrium for a long time. However, cyclic exposure to hydrogen eliminates the absorption–desorption hysteresis to obtain a reproducible cantilever deflection pattern. This reproducibility issue in microcantilever sensors remains as one of the main challenges in their commercialization. The lack of reproducibility of cantilever sensors is primarily due to the lack of understanding of the fundamental mechanisms involved in the signal generation. It is well known that structural characteristics of the deposited thin film depend on the evaporation technique [12] as well as the annealing process [13]. The stresses generated in thin films are always higher than in bulk. The pressure onset of the ˇ-phase of thin Pd films was found to vary depending on the Pd thickness below 70 nm as observed using quartz crystal microbalance and by volumetric technique [14]. Pressure isotherms (hydrogen pressure as a function of swelling) show a hysteresis that was observed in optical transmission and optical microscopy [15,16]. Wafer curvature measurements were employed to determine the mechanical

A. Ollagnier et al. / Sensors and Actuators B 186 (2013) 258–262

stresses generated in the ˇ-phase and the stress was measured to be 5 GPa [17–19]. In all Pd–H systems, investigation of the changes in film stress, the effect of temperature, and material swelling are of fundamental interest. At a microscopic scale, the hydrogen absorption-induced stress results in the deformation of the grain boundaries, the interface stress, film stretching [20] and buckling as observed both by atomic force microscopy [21] and electrical resistivity variation measurements [22]. It is also explained by the hydrogen diffusion leading to a non-uniform hydrogen concentration distribution in the palladium film [23]. Hydrogen absorption in Pd results in ˛ and ˇ phase depending on the concentration of hydrogen in the Pd. The ˇ-phase of the Pd was examined mostly for hydrogen storage for the energy vectorization. It involves the storage of hydrogen or/and its isotopes (H, D, T) in solid form using metal hydrides such as the palladium hydride system. Young’s modulus of the ˇ-phase was measured using different techniques such as resonance response of a palladium cantilever [4,24] and by ultrasound spectroscopy [25]. Identifying and discerning the most influential parameters responsible for the observed changes in the microcantilever response are critical in understanding the basic mechanisms [26]. Basic understanding of the changes in the physical properties induced by the phase transitions in the polycrystalline metals during the early stages of the hydrogen loading is of fundamental interest, especially the role of the surface dissociation of H2 and the effect of surface diffusion barrier. By monitoring the entire phase diagram of the palladium hydride, this work aims to establish the role played by the film thickness and the importance of the hydrogen cycling process for obtaining a reproducible hydrogen absorption-induced stress. Reproducible stresses will enable reproducible sensor response, namely the cantilever deflection. We also present a simple technique that can visualize phase response using a simple optical method.

2. Materials and methods We have used bare silicon microcantilevers with dimensions of 250 ␮m length, 35 ␮m width, and 1 ␮m thickness (MikroMasch, OR). Prior to the metal coating, the cantilevers were thoroughly cleaned in acetone and ethanol. Palladium (Pd) films of 10 and 30 nm in thickness were deposited using thermal evaporation technique. The palladium layers were evaporated directly on the cantilevers without any adhesion layer. The rate of evaporation was 0.05 nm/s for the three thicknesses used in this work. The vacuum pressure was kept between 5.5 × 10−6 mbar and 2 × 10−5 mbar. Atomic Force Microscopy (Digital Instrument, Santa Barbara, CA) was used to measure the thickness and surface roughness of the Pd film deposited on a silicon substrate placed along with the cantilevers in the evaporator. The effect of hydrogen was carried out in a vacuum chamber. One bare and two Pd-coated cantilevers were placed in the vacuum chamber during each experiment. The vacuum was maintained using both conventional mechanical pump (10−3 bar) and turbo molecular pump (10−7 bar). Once a secondary vacuum is reached, the dihydrogen (H2 ) gas produced by a hydrogen generator (Parker DH – UHP-60H, purity >99.999%) is introduced into the chamber. The H2 pressure P is monitored using a vacuum gauge (Infincon AG) at low pressure and a gas sensor (Druke AG) when (PH2 ≥ 10 mbar). The home-built chamber can withstand 2 bar of H2 gas. When the molecular hydrogen is introduced into the chamber, the Pd absorbs the H2 , thereby causing physical changes in the mechanical stress of the film, such as swelling, heat generation during the hydride formation, and the chemical changes. The cantilever deflections were monitored in situ and simultaneously

259

Fig. 1. Reversible H-sorption in the ˛-phase in a cycled 30 nm Pd film coated on a cantilever. (a) Kinetics of the deflection Z induced by the H absorption from t = 0 to 1000 s by changing the H2 pressure from 10−7 to 10−5 bar. After reaching the steady state deflection of Z, the H is desorbed from t = 1000 to 2500 s by vacuum pumping. (b) Sievert’s law is obtained from cumulating the successive steady state deflections by gradual pressure increments both for absorption and desorption. The film stress is obtained from measured z using Eq. (1)

for each cantilever. The reading head was composed of three laser diodes at 670 nm reflecting off each cantilever into three position sensitive detectors (PSD). The PSD signals were then fed into a home-made amplifier circuit. The output voltage from the PSD is directly proportional to the cantilever deflection (1 V/80 nm of cantilever deflection). 3. Results and discussion 3.1. ˛-Phase of Pd hydride In solid solution of ˛-phase of the Pd hydride (PdHx , x being the stoichiometry), the H2 molecules are first adsorbed and dissociated at the Pd surface, before diffusing into the metal, filling the octahedral sites but without any binding to the Pd. From Fig. 1, it is clear that the Pd-coated cantilever is an excellent H2 sensor in ˛-phase in terms of time response and sensitivity in vacuum condition. This result is similar to the results reported by many authors for hydrogen mixed with nitrogen or argon [1,2]. The ˛-phase corresponds to a stoichiometry x < 0.1 namely a dihydrogen pressure P ranging from the ultra high vacuum up to several mbar at a temperature of 25 ◦ C. Fig. 1a illustrates the exponential nature of the kinetics of the exponential H absorption induced by a change in P from 10−7 to 10−5 bar. The pure exponential form, especially the linear slope at the early absorption stage (time <100 s) may not be governed by a limited absorption process, as proposed by Delmelle and Proost [8]. In this range of hydrogen concentration, Sievert’s law governs the H absorption and the hydrogen concentration within the metal varies in proportion to the square root of the H2 pressure. Fig. 1b of the equilibrium cantilever deflection shows the linear variation √ −4 Z as a function of P, in the P range between 10−6 and √ 10 bar (obtained after many cycles). A sensitivity of ∼3.5 mm/ bar is then

260

A. Ollagnier et al. / Sensors and Actuators B 186 (2013) 258–262

√ measured from the slope Z/ P. Therefore, for a deflection threshold of 1 nm, the cantilever sensor will be able to detect at least P = 10−10 bar, which corresponds to a H2 concentration of ppb level. However, the large time required to reach the equilibrium cantilever deflection (Fig. 1a) is the disadvantage of the detector. For example, the time constant was 10 min for P = 10−7 bar and 5 min for P = 10−6 bar and 100 s for P = 10−5 bar. The time response which did not vary between tPd = 10 nm and 30 nm is controlled by the H2 dissociation rather than the H diffusion into the Pd film. The cantilever deflection, Z, induced by the change in the Pd film stress  Pd can be expressed using the Stoney’s formula as: Pd =

2 ESi · tSi Z 6 · tPd L2

(1)

where ESi is the effective Young’s modulus of silicon (200 GPa). L is the cantilever length. tSi and tPd are the thicknesses of the Si cantilever and the Pd film, respectively. The film stress  Pd in the elastic regime is given by: Pd = EPd ·

a a

(2)

At the end of the ˛-phase, the elastic modulus of the Pd film EPd = 127 GPa is almost unchanged and the swelling a/a = 0.1% [4]. Changes in  Pd are only driven by the atomic swelling, a/a, which varies, in a linear proportion with P (0.1%/mbar).  Pd = 250 MPa was estimated using Eq. (1) at the end of the ˛-phase in Fig. 1b. Our experimental value can be compared to the work of Pedersen et al. [17] for (Pd,tPd =10 nm = 800 MPa, PH2 = 1 mbar) and Delmelle et al. [5] for (Pd,tPd =128 nm = 450 MPa, PH2 = 10 mbar) but slightly differs from the theoretical value of 127 MPa expected by Eq. (2). We believe that the differences are due to two effects, namely the Pd film thickness tPd and the hydrogen cycling process. We will address these two effects in the following section. As mentioned earlier the cantilever bending response to hydrogen absorption and desorption show a hysteresis. Initial exposure to hydrogen shows a large hysteresis. It was observed that repeating the absorption–desorption process (cycling) decreases the hysteresis. The hydrogen cycling process is usually thought to clean the surface and remove the pollutants responsible for limiting the surface dissociation of adsorbed hydrogen. We show here that the cycling process acts most likely to release the residual stress of the Pd film. We have repeated the cycling process in the ˛-phase until the hysteresis between the absorption and desorption stages is completely eliminated. The first cycle of absorption and desorption is shown in Fig. 2a for a Pd film thickness of 30 nm. The cycle starts with ultra high vacuum followed by introduction of hydrogen with pressure P = 1 mbar for 850 s, before pumping again down to high vacuum. First, the cantilever bends with a large amplitude of 50 ␮m namely  Pd = 800 MPa, but after desorption the Z is less than 20 ␮m which is comparable to values given in Fig. 1. The kinetics appears to be more complex than in Fig. 1a and does not fit a simple exponential law. The data points plotted in Fig. 2b are obtained from successive steady state sorptions at various pressure levels. The response in Fig. 2b appears to have the characteristics of a hysteresis (red dots represent absorption and blue dots represent desorption). The hysteresis width Zh has been measured as a function of the number of activation cycles, n, for two different film thicknesses and shown in Fig. 2c. The Zh is higher for the thicker film ( Pd,30 nm = 226 MPa) and requires 10 cycles to reduce Zh by 90%, and a least 50 cycles to be fully reversible in terms of  Pd . The value of Zh = 5 ␮m for the 10 nm film appears to be less pronounced ( Pd,10 nm = 136 MPa) than thicker film. However, from Fig. 2c it can be seen that Zh is hardly reduced with increasing n (90% reduction at n = 50). In fact practically there is no difference between the thick film and thin film after about 10 cycles. The

Fig. 2. (a) Irreversibility of the H-sorption in the ˛-phase for the first cycling kinetics using 30 nm Pd film coated on a cantilever. Zh shown by the bending hysteresis represents the release of the residual stress in the cycle 10−7 bar → 10−3 bar → 10−7 bar. (b) First cycling isotherm built by accumulating the steady deflection as a function the increments in pressure P. The absorption and desorption are coloured in red and blue, respectively. (c) Variation of the bending hysteresis Zh are plotted as a function of the number n of pressure cycle for two Pd film thicknesses, 10 nm in green and 30 nm in black. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

hydrogen cycling is therefore acting as an annealing treatment that reduces the residual stress in the Pd thin film on the cantilever. Depending on the evaporation method and the metal thickness, the Pd film can be either in compressive or tensile stress. In this work, thermal evaporation leads to polycrystalline thin film following a Volmer–Weber growth pattern characterized by a compressive residual stress  r , where Pd nanograins press against each other creating a compression (Z < 0 the cantilever is bent down towards the silicon side). The repeated hydrogen diffusion in and out of the film helps releasing the tensile residual stress. Although the high resolution in Atomic Force Microscopy can hardly discern the morphological structure between the 10 nm and 30 nm Pd film, the wetting of an almost closed percolated film at 10 nm must be different from a continuous 30 nm film between the two thicknesses. It is instructive to compare the electrical H sensors with the idea of hydrogen-induced percolation in discontinuous film [27]. The tensile residual stress in the film after hydrogen absorption–desorption may tend to counterbalance the compressive residual stress. The preferential hydrogen diffusion at the grain

A. Ollagnier et al. / Sensors and Actuators B 186 (2013) 258–262

261

boundaries as proposed by [28] in the higher stress sites (Gorski effect) may explain the larger hysteresis observed with thicker continuous film. We also found that thicker films undergo a possible delamination or peeling at the interface between the Pd and the silicon substrate, especially in the ˇ-phase. 3.2. ˇ-Phase of Pd hydride It is well-known that the split photo-diode position-sensitive detector (PSD) is intrinsically non-linear except for very small deflections. When P > 10−3 bar, Z is so large and rapid than it was extremely difficult to rebuilt the phase diagram since the PSD was not in the linear regime. Therefore, the PSD sensor was simply replaced by a ruler of 20 cm long positioned in the vertical longitudinal plane at a distance D = 10 cm from the cantilever. Fig. 3a shows a series of pictures taken at nine equilibrium pressures showing the positions of laser spot, h, reflected from a bare cantilever and a cantilever with 30 nm Pd film. The bare cantilever serves as the reference. While the reference cantilever does not bend, the spot corresponding to the Pd-coated cantilever moves down with an amplitude h = 10 cm. The Z can be obtained from h following geometrical considerations: Z = L tan 

(3)

where the bending angle  is given by:



2 =  − arctan 1 −

h D



(4)

The tilting angle  = 45◦ is the angle of the incident beam to vertical. The size of the beam impinging on the ruler is around 5 mm due to its scattering on the cantilever surface; the center of the optical beam can be measured with 1 mm accuracy. By using Eqs. (3) and (4), the uncertainty in Z can be calculated of less than 1 ␮m. When P > 10 mbar, the H absorption continues to form a new phase more concentrated in which hydrogen is bounded to the palladium. This ˇ-phase coexists in equilibrium with the ˛-phase, which is poor in H concentration. The ˛-phase ends, giving rise to the palladium hydride in which the hydrogen stoichiometry x = H/Pd can reach 0.6. The lattice constant, a, continues to increase linearly with P reaching a = 0.4026 nm in the ˇ-phase. The maximum hydrogen uptake causes a lattice expansion of 10% in volume. Contrary to the ˛-phase, the Young’s modulus EPd starts to decrease from 127 GPa at x = 0.2 down to 116 GPa at x = 0.6 [24]. In addition to this high stress gradient, an exothermic reaction occurs. Knowing the volume and the mass of palladium (VPd = 7.5 × 10−17 m3 ; mPd = 9.02 × 10−13 kg) and silicon (VSi = 7.50 × 10−15 m3 ; mSi = 1.73 × 10−11 kg) estimated from the geometrical parameters as well as the enthalpy of reaction is in the ˇ-phase (H = 36, 400 J/mol at the stoichiometry H/Pd = 0.7) and the heat capacities (Cp = 750 J/kg/K for silicon 244 J/kg/K for palladium), we can deduce a released energy of 2.16 × 10−7 J which is around a transient temperature change of 100 K for the cantilever. The temperature sensitivity of the bimaterial cantilever can be evaluated [29] using the thermal expansion coefficients (˛ = 2 ×10−6 for silicon and 1.25 × 10−5 for palladium). For a change of temperature of 100 K (end of the ˇ-phase), the temperature sensitivity gives a maximum cantilever bending of 1 ␮m for 30 nm coated pd. This deflection is within the uncertainty of the measurement with the ruler. The Pd film is then not thick enough in our case to be sensitive the transient temperature changes. Fig. 3 shows the complete phase diagram of the Pd–H system built from Z. Fig. 3b and c show the values Z and the corresponding  Pd for two cantilever thicknesses. First, we do not observe the vertical ˛ + ˇ plateau of the isothermal diagram expected when the stoichiometry x is plotted versus P [14]. We can conclude that there exists a non linear dependence of  Pd on the H uptake in the ˛ + ˇ phase.

Fig. 3. (a) A series of 9 images showing the reflected beam impinging into the ruler with respect to the H2 pressure. The bare silicon cantilever serving as the reference does not bend. The bending of the cycled 30 nm Pd coated in the ˇ-phase is visible to the naked eye by a beam deflection of more than 10 cm in the ruler. (b) Changes in the cantilever deflection (determined using Eqs. (3) and (4) as a function of the partial pressure of H2 for two cantilever thicknesses 10 nm and 30 nm. The (˛ + ˇ)phase is located by the bending hysteresis, it starts at higher pressure for the thinner film. The ˇ-phase is determined at 2 × 10−2 bar for tPd = 30 nm and at 10−1 bar for tPd = 10 nm. (c) Changes of the film stress  Pd (deduced from b) and Eq. (1) as a function of the H2 partial pressure.

In addition, a large hysteresis loop between the absorption and the desorption is found and explained by the fact that once the H is bound to the Pd lattice, it is then more difficult to extract it from the metal, hence the film stress during the desorption lies above the absorption one. The hydrogen desorption was found to be 4fold slower than the absorption. Note that the maximum  Pd in ˇ phase exceed in both films the yield strength for Pd of 1 GPa. This threshold is located in the ˛ + ˇ-phase where the hysteresis is the most pronounced. The increase in the Pd thickness does not interfere drastically with the  Pd . However, the diffusion coefficients appear to be modified since the hysteresis width appears larger when tPd = 10 nm and the beginning of the ˛ + ˇ phase appears later when P > 3 ×10−3 bar. An isotherm phase diagram can be drawn in 20 min for the absorption and one hour for the desorption part.

262

A. Ollagnier et al. / Sensors and Actuators B 186 (2013) 258–262

This constitutes a huge temporal gain compared to standard volumetric measurements on centimetre bulk sample that might last a day. Taking the Pd thickness into account, the thermal stress generated by the exothermic reaction was estimated to be negligible compared to the changes driven by the loss in the Young’s modulus (−10 GPa) and the swelling (+3%). 4. Conclusion Microcantilevers coated with thin films of Pd are excellent hydrogen sensors. However, the reproducibility of the sensor performance can be poor due to the large hysteresis in the sensor response. Kinetics in the ˛-phase follows a pure exponential form for a cycled cantilever, whereas the phenomenon is more complex when the residual stress is not released. We have shown that increasing the cycling number significantly reduces the hysteresis making the sensor highly reproducible. While the hysteresis depends on the film thickness, the cyclic number is independent of the film thickness. A narrow window of film thickness between 10 nm and 40 nm is available for monitoring the phase changes in Pd. When the thickness of the Pd film is further increased to 60 nm, the cantilever bending was too large to detect the beginning of the ˛ + ˇ phase. A simple optical technique that can provide the phase change-induced cantilever bending is also demonstrated. Acknowledgements The authors acknowledge funding from the Council of Burgundy (EF). This work has been performed in cooperation with the Labex ACTION program (contract ANR-11-LABX-01-01). References [1] Z. Hu, T. Thundat, R. Warmack, Investigation of adsorption and absorptioninduced stresses using microcantilever sensors, Journal of Applied Physics 90 (2001) 427–431. [2] D. Baselt, B. Fruhberger, E. Klaassen, S. Cemalovic, C. Britton, S. Patel, T. Mlsna, D. McCorkle, B. Warmack, Design and performance of a microcantilever-based hydrogen sensor, Sensors and Actuators B: Chemical 88 (2) (2003) 120–131. [3] F.A. Lewis, The Palladium Hydrogen System, Academic Press, New York, 1967, pp. 178. [4] A. Fabre, E. Finot, J. Demoment, S. Contreras, In situ measurement of elastic properties of PdHx, PdDx, and PdTx, Journal of Alloys and Compounds 356 (2003) 372–376, 8th International Symposium on Metal–Hydrogen Systems, Fundamentals and Applications (MH2002), Annecy, France, Sep 02–06, 2002. [5] R. Delmelle, G. Bamba, J. Proost, In-situ monitoring of hydride formation in Pd thin film systems, International Journal of Hydrogen Energy 35 (18, SI) (2010) 9888–9892. [6] L.L. Jewell, B.H. Davis, Review of absorption and adsorption in the hydrogen–palladium system, Applied Catalysis A: General 310 (2006) 1–15. [7] T. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors – a review, Sensors and Actuators B: Chemical 157 (2) (2011) 329–352. [8] R. Delmelle, J. Proost, An in situ study of the hydriding kinetics of Pd thin films, Physical Chemistry Chemical Physics 13 (23) (2011) 11412–11421. [9] J. Prazmowska, T. Piasecki, A. Szyszka, R. Paszkiewicz, M. Tlaczala, Influence of hydrogen absorption on stress changes in thin catalytic metal films dedicated for sensors application, Central European Journal of Physics 9 (2) (2011) 392–397. [10] P. Zoltowski, Effects of self-induced mechanical stress in hydrogen sorption by metals, by EIS, Electrochimica Acta 44 (24) (1999) 4415–4429. [11] S. Liu, Y. Kao, Y. Su, T. Perng, Stresses induced by hydrogen absorption and desorption in Pd nanofilms, Journal of alloys and compounds 316 (1–2) (2001) 280–283. [12] Y.-I. Chou, H.-C. Chiang, C.-C. Wang, Study on Pd functionalization of microcantilever for hydrogen detection promotion, Sensors and Actuators B: Chemical 129 (1) (2008) 72–78. [13] Z. Zhao, M. Carpenter, Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films, Journal of Applied Physics 97 (12) (2005). [14] M.-W. Lee, R. Glosser, Pressure concentration isotherms of thin films of the palladium–hydrogen system as modified by film thickness, hydrogen cycling, and stress, Journal of Applied Physics 57 (1985) 5237–5239.

[15] R. Gremaud, M. Gonzalez-Silveira, Y. Pivak, S. de Man, M. Slaman, H. Schreuders, B. Dam, R. Griessen, Hydrogenography of PdH(x) thin films: influence of H-induced stress relaxation processes, Acta Materialia 57 (4) (2009) 1209–1219. [16] Y. Artemenko, M. Goltsova, V. Zaitsev, Kinetic and morphological peculiarities of ˇ → ˛ phase hydride transformations in the palladium–hydrogen system, International Journal of hydrogen energy 22 (2–3) (1997) 343–345. [17] T. Pedersen, C. Liesch, C. Salinga, T. Eleftheriadis, H. Weis, M. Wuttig, Hydrogeninduced changes of mechanical stress and optical transmission in thin Pd films, Thin Solid Films 458 (1–2) (2004) 299–303. [18] M. Lukaszewski, A. Czerwinski, Electrochemical quartz crystal microbalance study on hydrogen absorption and desorption into/from palladium and palladium–noble metal alloys, Journal of the Electroanalytical Society 589 (1) (2006) 87–95. [19] J.W. Shin, U. Bertocci, G.R. Stafford, In situ stress measurement during hydrogen sorption on ultrathin (1 1 1)-textured Pd films in alkaline electrolyte, Journal of the Electrochemical Society 158 (7) (2011) F127–F134. [20] C. Lemier, J. Weissmueller, Grain boundary segregation, stress and stretch: effects on hydrogen absorption in nanocrystalline palladium, Acta Materialia 55 (4) (2007) 1241–1254. [21] I. Matsumoto, K. Sakaki, Y. Nakamura, E. Akiba, In situ atomic force microscopy observation of hydrogen absorption/desorption by palladium thin film, Applied Surface Science 258 (4) (2011) 1456–1459. [22] S. Wagner, A. Pundt, Electrical resistivity and hydrogen solubility of PdH(c) thin films, Acta Materialia 58 (4) (2010) 1387–1394. [23] F. Yang, J. Li, Diffusion-induced beam bending in hydrogen sensors, Journal of Applied Physics 93 (11) (2003) 9304–9309. [24] A. Fabre, E. Finot, J. Demoment, S. Contreras, Monitoring the chemical changes in Pd induced by hydrogen absorption using microcantilevers, Ultramicroscopy 97 (1–4) (2003) 425–432. [25] R. Schwarz, H. Bach, U. Harms, D. Tuggle, Elastic properties of Pd-hydrogen, Pd–deuterium, and Pd–tritium single crystals, Acta Materialia 53 (3) (2005) 569–580. [26] E. Finot, A. Passian, T. Thundat, Measurement of mechanical properties of cantilever shaped materials, Sensors 8 (5) (2008) 3497–3541. [27] O. Dankert, A. Pundt, Hydrogen-induced percolation in discontinuous films, Applied Physics Letters 81 (9) (2002) 1618–1620. [28] M. Thouless, J. Gupta, J. Harper, Stress development and relaxation in copper films during thermal cycling, Journal of materials research 8 (8) (1993) 1845–1852. [29] J. Mertens, E. Finot, T. Thundat, A. Fabre, M.H. Nadal, Effects of temperature and pressure on microcantilever resonance response, Ultramicroscopy 97 (2003) 1–4.

Biographies Antonin Ollagnier was studied nano-biotechnology at the “Université de Bourgogne” and received his master’s degree in 2009. Since 2010 he follows a Ph.D. at the University of Bourgogne and he is studying the detection of HSP protein by surface plasmon resonance imaging. His research interests are mainly based on studying chemical interactions in proteins using advanced techniques in physics and nanotechnologies. Arnaud Fabre received the M.Sc. degree in Chemical and Physical engineering from the ESIREM of Dijon, France in 1999 and the Ph.D. degree in Physics from University of Dijon France in 2003. His current research interest involves hydrogen-metals interactions and isotopic effects. He is currently working as a research engineer for CEA in Valduc, France, specializing in developing hydrogen processes and participating in various research projects in the fields of hydrogen storage. Thomas Thundat is a Canada Excellence Research Chair professor at the University of Alberta, Edmonton, Canada. Until recently he was a UT-Battelle/ORNL Corporate Fellow and leader of the Nanoscale Science and Devices Group at the Oak Ridge National Laboratory. Thundat holds a Ph.D. in physics from the University at Albany, State University of New York, and a master’s from the Indian Institute of Technology in Madras, India. His research interests include nanomechanics, solid–liquid interface, nanomechanical sensors for physical, chemical, and biological detection, scanning probe microscopy, quantum-confined atoms, and mid infrared spectroscopy of surface adsorbates. His research currently focuses on developing chemical and biological sensors with extreme high sensitivity using micro and nanocantilever arrays. He has authored or coauthored over 305 publications in peer-refereed journals, 52 book chapters, and 38 U.S. patents. Eric Finot is a professor at University of Bourgogne, Dijon since 2005. He holds a Ph.D. in physics from the University of Bourgogne in 1998. He completed his postdoctoral fellowship at the Oak Ridge National Laboratory (USA) in 1999–2000 and at the CEA, Valduc (1999, 2001). His current research interests include scanning probe microscopy, nanomechanics, the development of surface plasmon based sensors in biology, Surface enhanced Raman spectroscopy and microcantilever.