Palladium Thin Films Research Articles

Hysteresis behavior of electrical resistance in Pd thin films during the process of absorption and desorption of hydrogen gas

We report the fabrication of a novel hydrogen sensor that utilizes the electrical resistance changes in the palladium thin films with nanometer thicknesses. The sensing mechanism is based on transitory absorption of hydrogen atoms into the palladium layer, which leads to the reversible alteration of the electrical resistance. In concentrated hydrogen ambient, the excess hydrogen absorption process leads to mechanical deformation on the surface of the palladium films, corresponding to the phase transition from α-phase to β-phase. The reversible sensing process results in a hysteresis curve for resistive properties, of which the height (sensitivity) could be controlled by manipulating the thickness of the palladium layers. The peel-off phenomena on the surface of the palladium film were suppressed by decreasing the thickness of the film. At the thickness of 20 nm, a hysteresis curve of resistance was obtained without any structural change in the palladium thin film. These results provide a significant insight to the fundamental understanding of the relationship between the electrical sensitivity of pure Pd thin films and related structural deformation, which is essential to develop robust H-sensors with high sensibility.

A Reduction Process of Palladium Oxide Thin Films and Hydrogen Gas Sensing Properties of Reduced Palladium Thin Films

A Reduction Process of Palladium Oxide Thin Films and Hydrogen Gas Sensing Properties of Reduced Palladium Thin Films

Metallophosphazene Precursor Routes to the Solid-State Deposition of Metallic and Dielectric Microstructures and Nanostructures on Si and SiO2

We present a method for the preparation and deposition of metallic microstructures and nanostructures deposited on silicon and silica surfaces by pyrolysis in air at 800 degrees C of the corresponding metallophosphazene (cyclic or polymer). Atomic force microscopy studies reveal that the morphology is dependent on the polymeric or oligomeric nature of the phosphazene precursor, on the preparation method used, and on the silicon substrate surface (crystalline or amorphous) and its prior inductively couple plasma etching treatment. Microscale and nanoscale structures and high-surface-area thin films of gold, palladium, silver, and tin were successfully deposited from their respective newly synthesized precursors. The characteristic morphology of the deposited nanostructures resulted in varied roughness and increased surface area and was observed to be dependent on the precursor and the metal center. In contrast to island formation from noble metal precursors, we also report a coral of SnP(2)O(7) growth on Si and SiO(2) surfaces from the respective Sn polymer precursor, leaving a self-affine fractal structure with a well-defined roughness exponent that appears to be independent (within experimental error) of the average size of the islands. The nature of the precursor will be shown to influence the degree of surface features, and the mechanism of their formation is presented. The method reported here constitutes a new route to the deposition of single-crystal metallic, oxidic, and phosphate nanostructures and thin films on technologically relevant substrates.

Palladium nanostructures from galvanic displacement as hydrogen peroxide sensor

Electroless plating of palladium on silicon has been developed, resulting in the formation of palladium films and nanoparticles. The deposition process has been monitored by atomic and Kelvin probe force microscopies (AFM/KPFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) in order to understand the deposition mechanism. A fabrication process is developed to yield a high density of Pd particles, while ensuring the formation of a palladium thin film underneath. The formed substrates have been used as amperometric sensors without further modification for the determination of hydrogen peroxide concentrations. The substrates are found to exhibit excellent linear electrocatalytic response down to the micromolar range and under an applied potential of 0.0 V, with the lowest quantifiable concentration of 1 μM. The simplicity in substrate fabrication coupled with the potential for integration in microfabrication processes, as well as the low applied potential needed in the measurements, offers the possibility of using the device in enzymatic biosensors with low interference signals.

Hydrogen gas sensing properties of PdO thin films with nano-sized cracks

We report on a novel method for the fabrication of highly sensitive hydrogen gas sensorsbased on palladium oxide thin films and have investigated their hydrogen sensing propertiesand nanostructures. To our knowledge, this is the first report on the use of palladium oxideand reduced palladium thin films as hydrogen sensors. The palladium oxide thin films weredeposited on thermally oxidized Si substrates using a reactive direct current (DC)magnetron sputtering system. Considerable changes in the resistance of the palladiumoxide thin films were observed when they were initially exposed to hydrogen gas, asa result of the reduction process. After the initial exposure to hydrogen gas ofPdO30%, its sensitivityincreased up to ∼ 4.5 × 103%. The morphology of the PdO surface was analyzed using a scanning electron microscope(SEM), in order to investigate the interactions between palladium oxide and hydrogen. TheSEM images showed a large number of nano-sized cracks on the surface of the palladiumoxide during the reduction process, which acted to increase the effective surface-to-volumeratio. The response behaviors of the reduced Pd films to hydrogen gas were reversible andhad an enhanced sensing property when compared with those of the pure Pd films. Inaddition, their sensitivities and response times were improved due to the nano-sizedcracks on the surfaces. The results demonstrate that palladium oxide and reducedpalladium thin films can be applied for use in highly sensitive hydrogen sensors.

Investigation of transmittance property of Palladium thin films fabricated by pulsed laser deposition with polarization using single layer model

In this paper the structure, morphology and optical properties of Pd thin films deposited on glass substrate by pulse laser deposition technique at two different substrate temperatures have been investigated. The fabricated films were characterized by various methods such as XRD, AFM, and UV–vis-NIR spectroscopy. The influence of surface roughness and angle of incidence with p polarization was investigated experimentally by optical property of palladium (Pd) thin films of two different thicknesses and rms roughness from transmission measurement in the visible spectral range. It has been shown that the experimental transmittance spectra agree well with their theoretical values for absorbing Pd thin film. The transmittance of thin film increases with increase in incident angle for the same sample.

Study of the acoustoelectric effect for SAW sensors

Research has recently begun on the use of ultrathin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., because the basic material parameters may change because of film morphology. As films of various materials are applied to the surface of SAW devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, and which also correspond to the mid-point between the open and short-circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on a YZ LiNbO3 SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, exposure to hydrogen gas, or both. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.

A Finite Element Model of a MEMS-based Surface Acoustic Wave Hydrogen Sensor

Hydrogen plays a significant role in various industrial applications, but careful handling and continuous monitoring are crucial since it is explosive when mixed with air. Surface Acoustic Wave (SAW) sensors provide desirable characteristics for hydrogen detection due to their small size, low fabrication cost, ease of integration and high sensitivity. In this paper a finite element model of a Surface Acoustic Wave sensor is developed using ANSYS12© and tested for hydrogen detection. The sensor consists of a YZ-lithium niobate substrate with interdigital electrodes (IDT) patterned on the surface. A thin palladium (Pd) film is added on the surface of the sensor due to its high affinity for hydrogen. With increased hydrogen absorption the palladium hydride structure undergoes a phase change due to the formation of the β-phase, which deteriorates the crystal structure. Therefore with increasing hydrogen concentration the stiffness and the density are significantly reduced. The values of the modulus of elasticity and the density at different hydrogen concentrations in palladium are utilized in the finite element model to determine the corresponding SAW sensor response. Results indicate that with increasing the hydrogen concentration the wave velocity decreases and the attenuation of the wave is reduced.

Preparation and Hydrogen Absorption/Desorption of Nanoporous Palladium Thin Films

Nanoporous Pd (np-Pd) was prepared by co-sputtering Pd-Ni alloy films onto Si substrates, followed by chemical dealloying with sulfuric acid. X-ray diffractometry and chemical analysis were used to track the extent of dealloying. The np-Pd structure was changed from particle-like to sponge-like by diluting the sulfuric acid etchant. Using suitable precursor alloy composition and dealloying conditions, np-Pd films were prepared with uniform and open sponge-like structures, with interconnected ligaments and no cracks, yielding a large amount of surface area for reactions with hydrogen. Np-Pd films exhibited shorter response time for hydrogen absorption/desorption than dense Pd films, showing promise for hydrogen sensing.

Fixed-angle observation of surface plasmon coupled chemiluminescence from palladium thin films

In this letter, the fixed-angle observation of surface plasmon coupled chemiluminescence (SPCC) from palladium thin films is reported. Fresnel calculations predict that light 492–549 nm spectral range can efficiently induce surface plasmon modes in 15 nm palladium thin films. Free-space emission from blue, green, and chartreuse chemiluminescent solutions on palladium thin films was isotropic, while the SPCC emission was highly directional and predominantly p-polarized in accordance with the predictions of the Fresnel calculations. In addition, the decay rates of the SPCC and free-space emission were similar, which suggests that palladium thin films have no catalytic effect on chemiluminescence emission.

Bilayer Structures of NiOxand Pd in Surface Acoustic Wave and Electrical Gas Sensor Systems

A bilayer sensor structure of nickel oxide NiOx (≈ 60 nm) with a very thin film of palladium (Pd ≈ 18 nm) on the top, has been studied for gas-sensing application at relatively low temperatures of about 30◦C and 60◦C. The bilayer structure was obtained by rf sputtering and by vacuum deposition (first the NiOx and then the Pd film) onto a LiNbO3 Y -cut Z-propagating substrate, making use of the surface acoustic wave method, and additionally (in the same technological processes) onto a glass substrate with a planar microelectrode array for simultaneous monitoring of the planar resistance of the layered structure. Such a bilayer structure was investigated in a low concentration range (from 50 ppm to 400 ppm in air) of nitrogen dioxide (NO2), carbon monoxide (CO) and ammonia (NH3) in a dry and wet air atmosphere and in a medium hydrogen concentration (1–2.5%) in dry air. The NiOx and Pd bilayer structure interact rather weakly with NO2 molecules but with CO and NH3 this interaction is much greater, especially at higher temperature (≈ 60◦C). The hydrogen sensitivity is on the medium level, not exceeding 600 Hz (relative change in the differential frequency of ≈ 2.3%) at interaction temperature of 35◦C.

SAW hydrogen gas sensor based on WO3 and Pd nanostructures

Bi-layer nanostructures of WO3 (∼100 nm and 150 nm) with a very thin film of palladium (Pd∼10 nm) on the top, have been studied for hydrogen gas-sensing application at ∼700C and medium hydrogen concentrations (1 – 4%) in air. The structures were obtained by vacuum deposition (first the WO3 and then the Pd film) onto a LiNbO3 Y- cut Z-propagating substrate and were tested by means of Surface Acoustic Wave method in a three channel delay line configuration. Very repeatable results have been observed for these two nanostructures with changes in frequency on the level of 500 to 1800 Hz for hydrogen concentration from 1 to 4 % in air. The absolute response value depends on the WO3 film thickness and hydrogen gas concentration and is greater for the nanostructure with a thicker WO3 film.

Fractal geometry of the surface potential in electrochemically deposited platinum and palladium films

The surface potentials are distributed very nonuniformly in electrochemically deposited thin films of palladium and platinum, and it reflects the grain structure of their surface profile. The Hausdorf-Bezikovich space dimensions of the surface potentials in these films much exceed the topological dimension of their projections, which is indicative of their fractal geometry. It is found that the surfaces of platinum and palladium films differ both in the spread of magnitudes of the surface-potential nonuniformities and in the shape and character of their distribution. This substantially affects the way of formation (geometry) of their potential profile and the values of their fractal dimensions. In addition, the fractal geometry of the potential profile of these film surfaces leads to the fact that the total electric charge of their surfaces varies much slower-it is proportional to the change in their linear dimensions to power (4 − Df), where 2 < Df < 3, instead of being proportional to the square of the change in the linear dimensions of the areas under investigation as in the two-dimension case. As a result, it is shown that, for the accurate designing of devices with metal components of submicrometer and nanometer sizes on the basis of thin films of platinum and palladium, it is necessary to take into account their fractal geometry.

Palladium Thin-Films on Clean and Hydrogen-Terminated Si(110): The Effect of Hydrogen Termination on Metal Adsorption

A combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED) study of the room-temperature deposition of palladium on wet-chemically-prepared H/Si(110) and vacuum-prepared Si(110) “16×2” was carried out to gauge the ability of a hydrogen barrier layer to restrict reactive adsorption of Pd on Si(110) (e.g. silicide formation) and to determine the effect of the hydrogen barrier layer on the growth mode of the deposited films. For deposition quantities less than 1.5 monolayers (ML), Pd atoms adsorbed in a random fashion on the clean Si(110) surface, while islands formed on the H/Si(110) surface. However, after ∼3 ML of deposition the film morphology took on a disordered hillock motif for both clean and hydrogen-terminated substrates. The size distribution of the hillocks was remarkably narrow, and indistinguishable for clean and hydrogen terminated substrates. Further, the hillock morphology was invariant with Pd deposition between 3 and ∼8 ML. XPS analysis revealed the deposited films to be chemically similar to Pd2Si for both Si(110) “16×2”, and H/Si(110) substrates, indicating that the hydrogen barrier layer does not prevent silicide formation.

Pd Nanoparticles and Thin Films for Room Temperature Hydrogen Sensor

We report the application of palladium nanoparticles and thin films for hydrogen sensor. Electrochemically grown palladium particles with spherical shapes deposited on Si substrate and sputter deposited Pd thin films were used to detect hydrogen at room temperature. Grain size dependence of H2sensing behavior has been discussed for both types of Pd films. The electrochemically grown Pd nanoparticles were observed to show better hydrogen sensing response than the sputtered palladium thin films. The demonstration of size dependent room temperature H2sensing paves the ways to fabricate the room temperature metallic and metal–metal oxide semiconductor sensor by tuning the size of metal catalyst in mixed systems. H2sensing by the Pd nanostructures is attributed to the chemical and electronic sensitization mechanisms.

Real time characterization of hydrogenation mechanism of palladium thin films by in situ spectroscopic ellipsometry

The hydrogenation mechanism of Pd thin films was analyzed in real time by measuring the variation in ellipsometric Ψ and Δ using in situ spectroscopic ellipsometry. In the initial stage, the hydrogenation proceeded from the film surface and a mixture layer of metal and hydride, not a uniform hydride layer, was formed at the surface. With time evolution, the thickness of the mixture layer increased and that of the Pd metal layer decreased rapidly. After the whole Pd metal layer changed to the mixture layer, the concentration of hydride in the mixture layer increased. Finally, the concentration reached one and hydrogenation of Pd was finished.

Fractal properties of gold, palladium and gold–palladium thin films on InP

Thermal interaction of indium phosphide (InP) bulk compound semiconductor with thin gold metal films was investigated in the course of the present work. The interaction of the InP/Au system resulted in a pattern showing fractal dimensions. The temperature dependence of the fractal parameters was investigated in a broad temperature range from 200 to 600 °C. No significant temperature dependence of the fractal dimension was observed. The same calculations will be presented for Au/InP and AuPd/InP systems. Our calculations show that the Pd-based contacts have a different behaviour than AuGe metallization where a strong temperature dependence of the fractal number was observed earlier. Another topology measure, the structural entropy is also calculated for the samples. The structural entropy is usually applied for determining the type of the localization of charge distributions, but it can also be used for generalized charges, such as the lightness of the pixels of an electron microscopy picture.

Polycrystalline Mercuric Iodide Photodetectors for Cesium Iodide Scintillators

We have fabricated thin films of polycrystalline mercuric iodide for use as photodetectors coupled to cesium iodide scintillators. The compound semiconductor mercuric iodide has a large band gap (2.13 eV) causing a peak in its quantum efficiency (570 nm) which is well matched to the output of cesium iodide (550 nm). Single crystal mercuric iodide photodetectors coupled to cesium iodide have demonstrated a full width at half maximum (FWHM) energy resolution of 4.58% at 662 keV. Single crystals providing sufficient surface areas for coupling with scintillators are difficult to grow and are therefore cost prohibitive to use in large field of view applications. However, polycrystalline films can be fabricated quickly and cheaply on a variety of surfaces including curved ones. We have therefore developed a thermal vapor transport method of growing polycrystalline mercuric iodide films onto transparent glass substrates coated with indium tin oxide. A palladium thin film is thermally evaporated on top of the film and the photodetectors are operated at low voltage (up to 0.4 V/mum) demonstrating the first use of polycrystalline mercuric iodide as a photodetector. The polycrystalline mercuric iodide films have a high resistivity producing a very low dark current typically about 1 nA per square cm at 0.4 V/mum. When coupled to cesium iodide a FWHM energy resolution of 9% at 662 keV is obtained. The quantum efficiency at 570 nm is about 40% as compared to single crystal mercuric iodide which is 80%. We are currently optimizing the growth method to improve the quantum efficiency with the ultimate goal of obtaining an energy resolution close to that of single crystal performance.

Towards an integrated ceramic micro-membrane network: Electroless-plated palladium membranes in cordierite supports

In this manuscript the authors detail the fabrication of permselective palladium membranes within cordierite mini-channel networks, and their subsequent evaluation for hydrogen purification. Cordierite extruded honeycomb monoliths (64 cpsi) were first washcoated with micropowder γ-Al 2O 3 to realize cylindrical surfaces for subsequent deposition of palladium films. A second nanopowder γ-Al 2O 3 layer provides a uniform surface for deposition of defect-free palladium films. Electroless plating under kinetic-limited conditions enabled plating of thin (8 μm) defect-free palladium films with crystallite sizes of ∼2 μm. Analysis of resulting two-channel membrane systems for hydrogen separation at 350 °C demonstrated hydrogen flux of 1.0–5.5 × 10 −3 mol m −2 s −1 and hydrogen-to-helium selectivities of 40–360:1. Lastly, the authors detail a novel microreactor design strategy to employ resulting mini-channel membrane networks as part of a thermally integrated portable reformer for high-purity hydrogen production.

Hydrogenation and dehydrogenation processes of palladium thin films measured in situ by spectroscopic ellipsometry

The hydrogenation and dehydrogenation switching processes of Pd thin films were analyzed by measuring in situ in real time the variation in ellipsometric Ψ and Δ during hydrogenation and dehydrogenation using spectroscopic ellipsometry. The hydrogenation proceeded from the film surface and a mixture layer of metal and hydride was formed at the surface. With time elapse, the thickness of the mixture layer increased rapidly. After the whole Pd metal layer transformed to the mixture layer, the concentration of hydride in the mixture layer increased. Finally, the concentration reached 1; the whole Pd in the mixture layer was hydrogenated, and the hydrogenation of Pd was complete. On the other hand, the dehydrogenation proceeded in a way completely opposite to the hydrogenation process; the concentration of hydride in the mixture layer decreased, Pd metal layer was formed at the interface between the film and the substrate, and the whole layer returned to Pd metal. The dehydrogenation proceeds much faster than hydrogenation.