Pd Thin Films Research Articles

Influence of surface condition on the current densities rendering nucleation loop during cyclic voltammetry for electrodeposition of Pd thin films

ABSTRACT Cyclic voltammetry (CV) for Pd electrodeposition on Au from PdCl2+HCl solution between +1.100 and –0.400 V vs Ag/AgCl (3M NaCl) was performed. The CVs exhibit nucleation loop (NL), which can be characterized by higher cathodic current density (j) in the reverse scan rather than that in the forward scan. The reasons for the appearance of NL are investigated by making use of the surface condition of electrode and overpotential (η). Since it is difficult to estimate η during CV, it was obtained by potentiostatic electrodepositions (ED) on the substrates with similar surface conditions as those within NL. These substrates were produced by a potential sweep in (i) forward (FS) and (ii) forward+reverse (RS) scans from open circuit potential up to a representative potential (+0.330 V) within NL. These FS and RS substrates possess partial and significant Pd coverage, respectively. ED at +0.330 V on these substrates shows similar j trends in the initial stages as in NL. η during ED is estimated from the equilibrium potentials and the potential drop due to solution resistance (Rs). Rs is obtained from electrochemical impedance spectroscopy at +0.330 V. The estimated η is higher on RS, supporting higher j during electrodeposition on RS and reverse scan of CV within NL. Electrochemical and morphological analyses suggest that Pd deposition on the FS is nucleation driven whereas that on the RS is growth driven with the surface condition playing an important role in the appearance of NL during CV.

Amphiphilic Poly(vinyl Alcohol) Copolymers Designed for Optical Sensor Applications—Synthesis and Properties

A possible approach for enhancement of Poly(vinyl alcohol) (PVA) humidity-sensing performance using hydrophobically modified PVA copolymers is studied. Series of poly(vinylalcohol-co-vinylacetal)s (PVA–Ac) of acetal content in the range 18%–28% are synthesized by partial acetalization of hydroxyl groups of PVA with acetaldehyde and thin films are deposited by spin-coating using silicon substrates and glass substrates covered with Au–Pd thin film with thickness of 30 nm. Sensing properties are probed through reflectance measurements at relative humidity (RH) in the range 5%–95% RH. The influence of film thickness, post-deposition annealing temperature, and substrate type/configuration on hysteresis, sensitivity, and accuracy/resolution of humidity sensing is studied for partially acetalized PVA copolymer films, and comparison with neat PVA is made. Enhancement of sensing behavior through preparation of polymer–silica hybrids is demonstrated. The possibility of color sensing is discussed.

On the Nucleation Loop in Cyclic Voltammetry for Electrodeposition of Pd Thin Films

Pd is an excellent catalyst layer for Mg-based thin films for hydrogen storage. Cyclic voltammetry (CV) for Pd deposition on Au in PdCl2+HCl solution between +1.100 and –0.400 V vs Ag/AgCl (3M NaCl) exhibits nucleation loop (NL). NL is characterized by higher cathodic current density (j) in reverse scan versus that in forward scan. This work addresses the question “Why does nucleation loop occur during CV?” using substrate conditions and overpotential (η). Since it is difficult to estimate η during CV, the substrate conditions similar to those within NL are reproduced by potential sweep in (i) forward and (ii) complete forward+reverse scans up to a representative potential (+0.330 V) within NL. These scans produce substrates FS and RS with partial and significant Pd coverage, respectively. Eventual potentiostatic electrodepositions (ED) at +0.330 V on these substrates initially show similar j trends as in NL, replicating Pd deposition during CV. η during ED is estimated by equilibrium potentials and potential drop due to solution resistance (R s) from electrochemical impedance spectroscopy at +0.330 V. The estimated η is higher on RS, supporting higher j during electrodeposition on RS and reverse scan of CV within NL. Electrochemical and morphological analyses together suggest that Pd deposition on FS is nucleation driven; that on RS is growth driven, which help in revealing the metal deposition mechanisms.

Electrochemical Activity of Thin-Film Pd Catalysts Modified with Bi for Methanol and Ethanol Oxidation Reaction in Alkaline Solution

Electrochemical Activity of Thin-Film Pd Catalysts Modified with Bi for Methanol and Ethanol Oxidation Reaction in Alkaline Solution

Relationships between electroless plating temperature, Pd film thickness and hydrogen separation performance of Pd-coated Al2O3 hollow fibers

The present work aims to study the formation of Pd thin film at different electroless plating temperatures and optimize the operating conditions to increase the hydrogen permeation performances of Pd-coated Al2O3 hollow fibers. Deposition of Pd thin films on the structured Al2O3 hollow fiber substrate was systematically investigated at different electroless plating temperatures (30 °C, 40 °C, and 50 °C). Herein, we present data about the effects of the electroless plating temperature on the hydrogen separation properties over a wide temperature range from 350 °C to 450 °C. The Pd thin film thickness is an inverse function of the bath temperature, and the hydrogen flux is primarily dependent on the thickness. The hydrogen fluxes increase with increasing electroless plating temperature.

Electrochemistry at Bimetallic Pd/Au Thin Film Surfaces for Selective Detection of Reactive Oxygen Species and Reactive Nitrogen Species.

In this work, we designed and fabricated Pd/Au bimetallic thin film electrodes with isolated Pd nanoparticles via underpotential deposition of copper on a gold substrate followed by in situ redox replace reaction in a Pd salt solution. The Pd/Au electrode was characterized by AFM and XPS as well as multiple electrochemical techniques including CV and electrochemical quartz crystal microbalance (EQCM) in sulfuric acid and phosphate buffer electrolytes. Results show that the reduction reactions of the analytes (i.e., H2O2 and 3-nitrotyrosine (3-NT)) at the Pd/Au thin film surfaces affect the nature and reactivity of Pd/Au surface electrochemistry including the adsorbed/absorbed hydrogen and/or the premonolayer palladium oxide redox processes at Pd. The EQCM experiment supports the arrangement of small size Pd nanoparticles in the Pd thin film in the presence of gold exhibits unusual properties, acting as a new physicochemical dimension between the electrode and target H2O2 and 3-NT molecules. The Pd/Au thin film was demonstrated as an extremely sensitive and selective probe for detection of common ROS and RNS (i.e., H2O2 and 3-NT). The integration of two different metallic species, Pd and Au, into a surface structure on nanoscale by exploiting their unique surface electrochemistry establishes an innovative analytical method for highly sensitive and selective detection of H2O2 and 3-NT simultaneously. This method has a general scope for detecting a broad range of redox active and nonredox active species simultaneously, which opens up new opportunities to develop new electrocatalytic materials and innovative sensing approaches.

Palladium-coated narrow groove plasmonic nanogratings for highly sensitive hydrogen sensing.

In this paper, we propose novel plasmonic hydrogen sensors based on palladium coated narrow-groove plasmonic nanogratings for sensing of hydrogen gas at visible and near-infrared wavelengths. These narrow-groove plasmonic nanogratings allow the incident light to be coupled directly into plasmonic waveguide modes thereby alleviating the need for bulky coupling methods to be employed. We carried out numerical simulations of the palladium coated narrow-groove plasmonic nanogratings using rigorous coupled wave analysis (RCWA). When palladium is exposed to varying concentrations of hydrogen gas, palladium undergoes phase transition to palladium hydride (PdHx), such that there are different atomic ratios ‘x’ (H/Pd) of hydrogen present in the palladium hydride (PdHx) depending on the concentration of the hydrogen gas. RCWA simulations were performed to obtain the reflectance spectral response of the Pd coated nanogratings in both the absence and presence of hydrogen, for various atomic ratios ‘x’ (x ∼ 0.125 to 0.65) in palladium hydride (PdHx). The results of the RCWA simulations showed that as the dielectric permittivity of the palladium (Pd) thin film layers in between the adjacent walls of the plasmonic nanogratings changes upon exposure to hydrogen, significant shifts in the plasmon resonance wavelength (maximum Δλ being ∼80 nm for an increase in the value of the atomic ratio ‘x’ from 0 to 0.65) as well as changes in the differential reflection spectra are observed. The structural parameters of these Pd coated narrow groove nanogratings—such as the nanograting height, gap between the nanograting walls, thickness of the palladium layer, periodicity of the nanogratings—were varied to maximize the shift in the plasmon resonance wavelength as well as the differential reflectance when these nanostructures are exposed to different concentrations of hydrogen (i.e. for different atomic ratios ‘x’ in PdHx).

Electrocatalytic Behavior of Hydrogenated Pd-Metallic Glass Nanofilms: Butler-Volmer, Tafel, and Impedance Analyses

Electrocatalytic activity and sorption behavior of hydrogen in nanosized Pd–Si–(Cu) metallic glass thin film and Pd thin film electrodes sputtered on a Si/SiO2 substrate were investigated by linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. The electrode MG4 (Pd69Si18Cu13) exhibits the best performance with the highest electrocatalytic activity in the hydrogen evolution region with less than half of the Tafel slope of Pd thin film of the same thickness and lowest overpotential at 10 mA cm−2. A new approach has been adopted by a nonlinear fitting of the entire region of the polarization curve (far- and near-equilibrium cathodic and anodic regions) to the Butler-Volmer model. α parameter is lowest for the MG2 electrode (Pd79Si16Cu5), marking that nonequilibrium conditions change the reaction kinetics. Together with MG2, MG4 shows the lowest Bode magnitude values for hydrogen sorption and evolution regions, indicating that the bonding and release of hydrogen atoms to the electrode is easier. MG4 electrode shows a dramatic decrease of the overpotential after 100 cycles, yielding an increase in hydrogen activity. Besides, MG4 exhibits the sharpest current density drop in the HER region in cyclic voltammetry compared with other MG and Pd electrodes, indicating higher electrocatalytic activity towards hydrogen evolution. The findings highlight the influence of the selected metallic glasses for the design and development of metal catalysts with higher sorption kinetics and/or electrocatalytic turnover.

Interdiffusion during heteroepitaxial Au growth on Pd thin films by electroless Au plating (ELGP) at room temperature

Electroless Au plating (ELGP) on Pd thin films is performed. The ELGP time dependence on the surface morphology is investigated by a scanning electron microscope. Continuous nucleation and growth of hemispherical Au islands are observed as the plating time increases. Heteroepitaxial growth of the electroless plated Au on the Pd surface is confirmed by cross-sectional scanning transmission electron microscope images. Cross-sectional energy-dispersive X-ray spectroscopy reveals interdiffusion between the Pd atoms and the plated Au atoms at room temperature, which allows the formation of Au–Pd alloys. The heteroepitaxial Au growth on Pd is significant for reducing contact resistance of Au/Pd structures.

Realizing stable high hydrogen permeation flux through BaCo0.4Fe0.4Zr0.1Y0.1O3-δ membrane using a thin Pd film protection strategy

Hydrogen (H2)-selective membrane is an enabling technology for high purity H2 production from natural gas. Here, we demonstrate the feasibility to apply BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY), a widely studied MPEC composition with high electronic conductivity as H2 permeation membrane by sputtering thin Palladium (Pd) films on its feed and permeate surfaces as a protective layer to prevent BCFZY reduction upon long-term exposure to reducing H2. Pd thin film also acts as a catalytic layer to enhance H2 dissociation in the feed surface and H association in the permeate surface. Pd-sputtered BCFZY (Pd|BCFZY|Pd) membrane displayed exceptionally high H2 flux that varies from 0.101 rise to 0.213 mL min−1 cm−2 (450 °C–650 °C) under the 10% H2-90% N2/Argon (Ar) gradient. The attainment of significant H2 permeability below 500 °C shown here indicates the possibility to lower the operating temperature for H2 permeation using MPEC materials. Pd|BCFZY|Pd membrane also demonstrated stable, high H2 flux that fluctuates more or less 0.138 mL min−1 cm−2 during the continuous operation over 225-h duration.

Preparation of Hydrophobic Porous Silicon by Metal-Assisted Etching with Pd-Catalyst

The process of the porous silicon layer formation by metal-assisted etching of single-crystal Si with the resistivity of ρ = 0.01 Ω cm, coated with thin Pd-film up to 100 nm thick, in HF/H2O2/H2O solution is studied. The porous silicon is studied by scanning electron microscopy and gravimetric analysis. The dependence of the silicon dissolution rate on the HF concentration, in the presence and in the absence of the Pd-film, is investigated. The anodic current facilitating the Si dissolution in the HF solutions of different concentration is studied. By using sessile drop method, the porous silicon formed by the Pd-assisted anodic etching was shown to be superhydrophobic. The porous Si wetting angle reached 172°. The free surface energy for the porous layers and the water surface tension at the porous Si are calculated. The modified Si-surfaces may found their possible use in robotics.

Surface analysis and pumping speed measurements of oxygen-free palladium/titanium nonevaporable getter after heating at 100–450 °C

A novel nonevaporable getter (NEG) consisting of an oxygen-free thin film of Ti covered with a thin film of Pd (oxygen-free Pd/Ti) with an activation temperature of 133 °C has recently been developed [T. Miyazawa et al., J. Vac. Sci. Technol. A 36, 051601 (2018)]. When activated by baking at 150 °C, an NEG pump using oxygen-free Pd/Ti evacuates H2 and CO efficiently at room temperature, and its pumping speeds for H2 and CO do not decrease even after repeated cycles of activation and exposure to the air [T. Kikuchi et al., AIP Conf. Proc. 2054, 060046 (2019)]. However, oxygen-free Pd/Ti does not evacuate other major residual gases, such as H2O or CO2. To expand the range of applications of the NEG, the authors analyzed the surface chemical composition of the oxygen-free Pd/Ti by synchrotron-radiation x-ray photoelectron spectroscopy after heating at various temperatures in the range of 100–450 °C for 10 min. The authors found that Ti segregated to the surface after heating at 300 °C for 10 min. In addition, the pumping speeds for H2 and CO of a vacuum vessel coated with oxygen-free Pd/Ti were measured after baking at various temperatures in the range of 150–400 °C for 12 h. These results suggested that Ti segregates to the Pd surface after baking at 250 °C for 12 h. Therefore, after heating above 250–300 °C, oxygen-free Pd/Ti can be used as an NEG pump for vacuum systems where active residual gases other than H2 and CO must be pumped.A novel nonevaporable getter (NEG) consisting of an oxygen-free thin film of Ti covered with a thin film of Pd (oxygen-free Pd/Ti) with an activation temperature of 133 °C has recently been developed [T. Miyazawa et al., J. Vac. Sci. Technol. A 36, 051601 (2018)]. When activated by baking at 150 °C, an NEG pump using oxygen-free Pd/Ti evacuates H2 and CO efficiently at room temperature, and its pumping speeds for H2 and CO do not decrease even after repeated cycles of activation and exposure to the air [T. Kikuchi et al., AIP Conf. Proc. 2054, 060046 (2019)]. However, oxygen-free Pd/Ti does not evacuate other major residual gases, such as H2O or CO2. To expand the range of applications of the NEG, the authors analyzed the surface chemical composition of the oxygen-free Pd/Ti by synchrotron-radiation x-ray photoelectron spectroscopy after heating at various temperatures in the range of 100–450 °C for 10 min. The authors found that Ti segregated to the surface after heating at 300 °C for 10 min. In addition...

Positron-electron annihilation momentum transfer to trapped deuterons

The probability of positron annihilation momentum transfer to a trapped deuteron is calculated based on neutron measurements from 2H(d,n)3He reactions in deuterated palladium thin films. Deuterium gas loading in Pd thin films creates a high D/Pd fraction at point defects in the metal lattice. Unmoderated positrons from a Na-22 source thermalize and are trapped in the same point defects as deuterium in the Pd thin films. Similar to the Mössbauer effect, an interaction with the surrounding crystal lattice allows for conservation of momentum during electron-positron annihilation energy transfer to the trapped deuteron.

Electrosynthesis and characterization of PdIr using electrochemical atomic layer deposition for ethanol oxidation in alkaline electrolyte

This work reports Pd and PdIr thin films electrodeposition via surface limited redox replacement reaction using electrochemical atomic layer deposition technique. Cyclic voltammetry (CV) showed Pd and PdIr voltammetric signatures confirming the successful utilization of Cu as sacrificial metal for Pd and Ir ions. The resulting deposits were further characterized using XRD, EDS, XPS, SEM and AFM, confirming their structural morphology and surface roughness. The catalytic activity of the thin films towards the oxidation of ethanol was determined using CV. PdIr showed a more negative onset potential and higher current (−0.423 V; 0.664 mA) than Pd (−0.402 V; 0.494 mA) thin film, indicating a more catalytic behavior. Further, PdIr showed a higher if/ib ratio of 1.094 than Pd (if/ib = 0.94) indicating higher tolerance to poisoning by ethanol intermediates than Pd. Moreover, chronoamperometry (CA) confirmed that the addition of Ir to Pd improves the stability of the PdIr film. The resultant behaviour was also confirmed by EI spectroscopy with smaller Rct = 0.23 (kΩ) for PdIr than that of Pd; Rct = 14.8 (kΩ) when.Subjected at a fixed potential of −0.2 V. The higher catalytic activity of PdIr towards EOR makes it a promising candidate as electrocatalyst for alkaline direct ethanol fuel cell (DEFCs) than Pd.

Nitrate protic ionic liquids as electrolytes: Towards hydrogen sorption in Pd

The process of hydrogen electrosorption in the Pd thin film electrode was examined in three nitrate ionic liquids (ILs): ethylammonium nitrate (EAN), propylammonium nitrate (PAN) and 1-ethylimidazolium nitrate (C2IMN). The Pd electrode was obtained by electrodeposition on a golden matrix from palladium chloride solution. Cyclic voltammetry, chronoamperometry and chronopotentiometry were used for the electrochemical analysis. The hydrogen capacity (H/Pd) of Pd electrodes subjected to various pretreatment procedures was determined using cyclic voltammetry. The max. H/Pd equals to 0.6–0.7. In case of C2IMN IL, the noticeable phenomenon of a significant Pd dissolution under the open circuit conditions and during electrochemical hydrogen sorption experiment was observed. The physicochemical properties such as density, ionic conductivity, dynamic viscosity and water content before and after electrochemical measurements were designated. The nitrate ionic liquid stability under the electrochemical measurement conditions was confirmed by the means of Raman and 1H NMR spectroscopies. 1H NMR spectroscopy also served as a tool to compare the acidity of nitrate ILs: C2IMN>PAN>EAN.

Palladium thin films for plasmonic hydrogen gas sensing

In this study, I prepared BK7 glass slides coated by palladium (Pd) layer by PVD technique. These samples have been employed as plasmon active structures in classic Kretschmann-based SPR set-up. The application of H2 sensing structures based on palladium plasmonic active thin films have been tested and investigated. Hydrogen sensing properties of Pd films were investigated at room temperature The reflectances of p-polarized light from Pd thin films as a function of angle of incidence and wavelength were measured in synthetic air (or nitrogen) and in gas mixtures including hydrogen. Variations of the reflectance in the presence of hydrogen gas at room temperature revealed that the samples can sense hydrogen in a wide range of concentration (0–2% vol/vol) without saturation behavior. The dynamic properties with various concentration of H2 at low temperature and dry gas mixtures was investigated and the effects of these factors on the hydrogen sensing properties were analyzed. Full Text: PDF ReferencesG. Korotcenkov, Handbook of Gas Sensor Materials: Properties, Advantages, and Shortcomings for Applications (Springer, New York 2013). CrossRef W. Jakubik, M. Urbanczyk, E. Maciak, "SAW hydrogen gas sensor based on WO3 and Pd nanostructures", Procedia Chemistry 1 (1), 200 (2009). CrossRef W. Jakubik, M. Urbanczyk, E. Maciak, T. Pustelny, "Bilayer Structures of NiOx and Pd in Surface Acoustic Wave and Electrical Gas Sensor Systems", Acta Physica Polonica A 116(3), 315 (2009). CrossRef E. Maciak, Z. Opilski, "Pd/V2O5 fiber optic hydrogen gas sensor", J. Phys. France IV 129, 137 (2005). CrossRef E. Maciak,. "Fiber optic sensor for H2 gas detection in the presence of methane based on Pd/WO3 low-coherence interferometric structure", Proc. SPIE 10455, UNSP 104550W (2017). CrossRef X. Bevenot, A. Truillet, C. Veillas, H. Gagnaire, M. Clement, "Hydrogen leak detection using an optical fibre sensor for aerospace applications", Sens. Actuators B 67, 57 (2000). CrossRef J. Homola, S.S. Yee, G. Gauglitz, "Surface plasmon resonance sensors: review", Sensors and Actuators B 54, 3 (1999). CrossRef H. Raether, Surface plasmons on smooth and rough surfaces and on gratings (Springer-Verlag, Berlin-Heidelberg 1988). CrossRef P. Tobiska, O. Hugon, A. Trouillet, H.Gagnarie, "An integrated optic hydrogen sensor based on SPR on palladium", Sensors and Actuators, B 74, 168 (2001). CrossRef Z. Opilski, E. Maciak, "Optical hydrogen sensor employing the phenomenon of the surface plasmons resonance in the palladium layer", Proc. SPIE 5576, 202 (2004). CrossRef T. Pustelny, E. Maciak, Z. Opilski, A. Piotrowska, E. Papis, K. Golaszewska, "Investigation of the ZnO sensing structure on NH3 action by means of the surface plasmon resonance method", European Physical Journal-Special Topics 154, 165 (2008). CrossRef E. Maciak, M. Procek, K. Kępska, A. Stolarczyk, "Study of optical and electrical properties of thin films of the conducting comb-like graft copolymer of polymethylsiloxane with poly(3-hexyltiophene) and poly(ethylene) glycol side chains for low temperature NO2 sensing", Thin Solid Films 618, 277 (2016). CrossRef

Hydrogen Sensing Characteristics of a Metal–Oxide–Semiconductor Diode With Bimetallic Catalysts and a GaO x Dielectric

A new metal–oxide–semiconductor (MOS) diode with bimetallic catalysts and a GaOx dielectric is employed herein to fabricate a hydrogen sensor. Bimetallic catalysts, including Pt nanoparticles (NPs) and a Pd thin film, are formed by the proper vacuum thermal evaporation (VTE) approach, and a GaOx dielectric is produced by H2O2 treatment on the GaN surface. The presence of this bimetallic structure can effectively increase the surface area-to-volume ratio and provide a “spill-over” effect. This can substantially enhance the dissociation and adsorption of hydrogen molecules and atoms. The use of a GaOx dielectric effectively suppresses the surface leakage current and increases the adsorption sites for hydrogen atoms. Experimentally, excellent hydrogen sensing properties, including a very high sensing response of $1.1 \times 10^{7}$ under 1% H2/air gas at 300 K, an extremely low detection level (≤100 ppb H2/air), a widespread hydrogen concentration sensing range, and a relatively fast sensing speed, were obtained. From a thermodynamic analysis, it is clear that the hydrogen adsorption of the studied device is an exothermic reaction. Therefore, based on the above-mentioned advantages, the studied Pt NP/Pd thin film/GaOx/GaN-based MOS diode shows promise for high-performance hydrogen sensing applications.

Unoccupied surface and interface states in Pd thin films deposited on Fe/Ir(111) surface

We present a systematic first-principles study of the electronic surface states and resonances occuring in thin films of Pd of various thicknesses deposited on a single ferromagnetic monolayer (ML) of Fe on top of Ir(111) substrate. This system is of interest since one Pd layer deposited on Fe/Ir(111) hosts small magnetic skyrmions. The latter are topological magnetic objects with swirling spin-textures with possible implications in the context of spintronic devices since they have the potential to be used as magnetic bits for information technology. The stabilization, detection and manipulation of such non-collinear magnetic entities require a quantitative investigation and a fundamental understanding of their electronic structure. Here we investigate the nature of the unoccupied electronic states in Pd/Fe/Ir(111), which are essential in the large spin-mixing magnetoresistance signature captured using non spin-polarized scanning tunneling microscopy (Crum et al 2015 Nat. Commun. 6 8541, Hanneken et al 2015 Nat. Nanotechnol. 10 1039). To provide a complete analysis, we investigate bare Fe/Ir(111) and Pdn = 2,7/Fe/Ir(111) surfaces. Our results demonstrate the emergence of surface and interface states after deposition of Pd MLs, which are strongly impacted by the large spin–orbit coupling of Ir surface.

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation. In this study, three Fe film samples with thicknesses of 50 monolayers (ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used. The magnetic anisotropy constant obtained from ferromagnetic resonance (FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy (MCA) at the normal deposition angle with larger Fe thickness. However, the in-plane uniaxial magnetic anisotropy (UMA) is induced by a higher oblique deposition angle and a smaller thickness. Its hard axis lies between the [100] and [010] directions. The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA. Due to MCA, when the magnetic field is weaker than the saturated field, the magnetization direction does not always align with the external field. The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07. Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) systems. Thus, a vector network analyzer ferromagnetic resonance (VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.

Palladium thin films on microfiber filtration paper as flexible substrate and its hydrogen gas sensing mechanism

Cost-effective, simple and flexible hydrogen gas sensors operating at room temperature were demonstrated based on DC magnetron sputtered palladium thin films. The main idea is to use commercial microfiber filtration paper as a cheap, fibrous and flexible substrate for palladium. With this objective, Pd thin films with 21, 38 and 54 nm thicknesses were deposited and characterized by grazing incident X-ray diffraction (GIXRD) and scanning electron microscope (SEM). The gas sensing behavior was tested by hydrogen gas in the concentration rang of 0.5–10%. GIXRD showed a PdX/Pd phase and SEM revealed existing of many structural nanocracks. An activation process was needed where sensors were heated up to 110 °C in the presence of 10% H2. Then a desirable hydrogen sensitivity with good stability based on hydrogen induced lattice expansion (HILE) effect in which conductance increases was observed. However, for low Pd thicknesses (21 and 38 nm) and at below 4% H2, an inverse effect was observed in which the conductance decreased instead of increasing. For all samples the response time is incremental in the range of 0.5–4% and declining for 4–10%. Moreover, the deposited Pd sensors operate well when they are bent inward or outward. The governed physical mechanism was expressed and discussed.