Ultrathin Pd Films Research Articles

Topological Darkness in Optical Heterostructures: Prediction and Confirmation.

Topological darkness is a new phenomenon that guarantees zero reflection/transmission of light from an optical sample and hence provides topologically nontrivial phase singularities. Here we consider topological darkness in an optical heterostructure that consists of an (unknown) layer placed on a composite substrate and suggest an algorithm that can be used to predict and confirm the presence of topological darkness. The algorithm is based on a combination of optical measurements and the Fresnel equations. We apply this algorithm to ultrathin Pd films fabricated on a Si/SiO2/Cr substrate and extract four different points of topological darkness. Our results will be useful for topological photonics and label-free optical biosensing based on phase interrogation.

Hybrid Microstrip Device for Hydrogen Detection at Microwave Frequencies

This article describes the analysis, design, and construction of a microstrip device capable of detecting hydrogen at microwave frequencies. The proposed structure is a hybrid microstrip line, 10-cm-long, having part of the copper (Cu) line replaced by a strip of palladium (Pd) film 10-nm-thick. A simple formulation has been developed to estimate the device’s insertion loss, as well as the spectral dependence of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula> parameter. For a device having a 2-cm-long Pd section, exposed to 1.6% hydrogen at a 0.4-bar pressure in nitrogen gas, detection was accomplished by measuring the changes produced on one of the resonances of the scattering parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula> in the frequency region around 3.2 GHz. The experimental results, corroborated by the theoretical modeling of the device’s response, indicated that, if on one hand, a Pd thickness much smaller than the skin depth, yields a negligible change in attenuation due to hydrogen absorption, on the other, it favors the phase of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula> parameter to become a highly sensitive function of Pd conductivity, in turn facilitating hydrogen detection. This finding opens the possibility of constructing simple hydrogen sensors by incorporating ultrathin Pd films into planar microwave circuits.

Effect of Co, Pd and Pt ultra-thin films on the Ni-silicide formation: investigating the sandwich configuration

The effect of Co, Pd and Pt ultrathin films on the kinetics of the formation of Ni-silicide by reactive diffusion is investigated. 50 nm Ni/1 nm X/ 50 nm Ni (X = Co, Pd, Pt) deposited on Si(100) substrates are studied using in-situ and ex-situ measurements by X-ray diffraction (XRD). The presence of Co, Pd or Pt thin films in between the Ni layers delays the formation of the metal rich phase compared to the pure Ni/Si system and thus these films act as diffusion barriers. A simultaneous silicide formation (δ-Ni2Si and NiSi phases) different from the classic sequential formation is found during the consumption of the top Ni layer for which Ni has to diffuse through the barrier. A model for the simultaneous growth in the presence of a barrier is developed, and simulation of the kinetics measured by XRD is used to determine the permeability of the different barriers. Atom probe tomography (APT) of the Ni/Pd/Ni system shows that the Pd layer is located between the Ni top layer and δ-Ni2Si during the silicide growth, in accordance with a silicide formation controlled by Ni diffusion through the Pd layer. The effect of the barrier on the silicide formation and properties is discussed.

Switching of magnetism via modifying phase shift of quantum-well states by tailoring the interface electronic structure

We demonstrate control of the magnetism of Pd(100) ultrathin films, which show d-electron quantum-well induced ferromagnetism, via modulation of the interface electronic state using density functional calculation. From an analysis based on the phase model, forming the Au/Pd(100) interface induces hybridization of the wave function of d-electron quantum-well states, and modulates the term of the scattering phase shift as a function of the reciprocal lattice point. In contrast, forming the Al interface, which has only s-electrons at the Fermi energy, cannot modify the scattering phase shift. Our finding indicates the possibility of modifying the phase shift by tailoring the interface electronic states using hybridization of the wave function, and this efficiently changes the density of states near the Fermi energy of Pd films, and the switching between paramagnetism and ferromagnetism occurs based on the condition for ferromagnetism (Stoner criterion).

Manipulation of magnetization in Pd(100) ultrathin films with quantum well structure using modification of Schottky barrier potentials

The magnetization of Pd(100) ultrathin films that show ferromagnetism due to quantum well states was manipulated by changing the quantum well state with an applied bias voltage. The voltage dependence of the magnetic moment of Pd/SrTiO$_{3-x}$/Ti/Au intrinsically depends on the Pd film thickness. The induced change in the magnetic moment is due to the modulation of the phase shift at the interface between the Pd thin film and the semiconductor SrTiO$_{3-x}$ substrate.

(Invited) Growth and Catalytic Performance of Pd and Pd-Pt Alloy Ultrathin Films on Au By Surface Limited Redox Replacement of H UPD Layers in One-Cell Configuration

In the ongoing effort of solving the energy problem of humanity the design, fabrication and testing of catalysts has been playing a central role for the last couple of decades. Besides the known application of Pd hydrogen gas storage a growing interest has been seen in the use of this metal and its alloys in catalytic reactions with relevance to fuel cells. While Pt is still considered best catalyst for fuel cell applications its high cost and limited supply calls more and more for intensifying the effort of replacing Pt with Pd. Means of growing Pd on conductive materials include a variety of expensive vacuum deposition approaches that need sophisticated equipment and high temperature. An alternative to those is electrodeposition that besides the standard bulk deposition, offers a variety of ultra-thin film growth approaches that could facilitate the deposition of uniform and flat continuous Pd films on polycrystalline Au. Among those, the electrochemical atomic layer deposition of Pd under optimum conditions was first developed for automated flow cell. Our group has been working recently on developing fine-tuned approaches for the growth of epitaxial Pd on Au using always readily available one-cell configuration instead, of sophisticated automated flow cells. The growth uses surface limited redox replacement (SLRR) reaction of underpotentially deposited (UPD) Cu or H sacrificial layers. After a set of iteratively applied deposition cycles, the as-deposited Pd or Pt-Pd films are tested for surface roughness using characteristic HUPD and CuUPD cyclic voltammetry (CV) curves and the efficiency of the deposition technique is measured by stripping charge determination. Also, XPS characterization was used for determining both, the alloy composition in the case of Pt-Pd deposition and the presence / absence of impurities incorporating in the deposit in the case of pure Pd growth. The quasi-2D growth resulting in smooth and uniform Pd-film morphology has also been confirmed up to 20 SLRR cycles by in-situ STM. Consequently, the accordingly deposited films at minimum thickness are tested for catalytic activity and durability in formic acid oxidation reactions. In this presentation results of this work will be presented in detail and critically discussed. The emphasis is on the illustration of deposition approach that has been developed to take place in one cell and in the specifically in the case of SLRR of HUPD results in a theoretically contamination free deposits. It is also demonstrated that this approach provides for maximum deposition efficiency engaging also adsorbed H in the replacement scenario. The CV results indicate the deposition of smooth Pd films taking place for up to 30 SLRR cycles for HUPD and CuUPD sacrificial layers, respectively. This is followed by a rapid transition to dendritic growth at higher thickness. The underlying kinetic reasons for such abrupt transition are also considered and critically discussed. Finally, the applicability of the proposed facile SLRR approach has been demonstrated through testing for catalytic activity and durability in formic acid oxidation of ultrathin films of Pd, Pt and Pt-Pd alloys with different composition. The critical analysis of the testing outcome suggested that a 1:2 Pt-Pd alloy exhibits activity and durability that are comparable with those of pure Pt and substantially better to those of pure Pd.

Design and Electrocatalytic Properties of Pt - Pd/Au Nanostructures

The key scientific and technological challenge of the future hydrogen economy is to develop new materials with low-content of Pt and Pt-group noble-metals that exhibit high activity, improved selectivity and stability by controlling their structure and composition. Design of Pt-bi and tri-metallic catalysts of different shapes, sizes and configurations (i.e. core-shell nanoparticles, nanoalloys, nanoclusters) are the most promising approaches to resolve challenges of fuel cells commercialisation, particularly in automotive industry and integrated power system technologies. The extensive studies have been conducted on various Pt-Pd bimetallic structures over the past few decades. Epitaxial ultrathin films of Pd (1-10 ML) on single crystal surfaces of Pt(hkl) and Au(hkl) have been shown to exhibit higher electrocatalytic activity for formic acid oxidation (FAO) than the bulk Pd and bulk Pt 1, 2. Pd and PdAu-alloy nanoparticles covered by electrodeposited Pt monolayer shell have been shown to be excellent catalysts for oxygen reduction reaction with much higher activity and stability than the bulk Pt 3, 4 . In a three component system Pt-nanoclusters (Pt-nc) on Au@Pd (core@shell) particles showed higher activity for FAO dependent on the thickness of Pd shell 5. In this work, we explored the effect of sub-ML Pt - nc on the electrocatalytic activity and stability behaviour of Pd thin films electrodeposited on Au. Uniform distribution of Pt-nc was achieved by reduction of the spontaneously adsorbed [PtCl4]2- complex on the surfaces known as spontaneous deposition6 and by the surface limited redox replacement reaction (SLRR) of adsorbed hydrogen 7. The study was focused on the deposition of sub-ML Pt-nc on epitaxial Pd films on Au of different thicknesses between 2ML and 10 ML. It has been shown that 2ML Pd films are pseudomorphic and adsorbs hydrogen, while for higher thicknesses (3-10 ML) Pd films maintain their epitaxial structure with the underlying Au and feature energetically separated H adsorption and absorption processes 8. The comparison of Pd films of different thickness allowed us to explore the effect of sub ML Pt-nc on H-sorption behaviour and FAO activity. The study showed that presence of Pt-nc contributed to the significant changes in the kinetics of H-sorption and increased the stability of Pd toward dissolution during potential cycling in 0.1 M H2SO4 and in particular during FAO. The FAO on Pd films modified with Pt-nc exhibited significant changes compared to the pure Pd/Au surfaces featuring increased COads poisoning. However, the stability of these surface toward dissolution was significantly better than that of pure Pd/Au. For example, 200 potential cycles of FAO with a scan rate of 50 mV/s on 4 ML Pd/Au resulted in a complete dissolution of Pd film. However, deposition of ~ 0.5 ML of Pt via SLRR of H on the 4 ML Pd/Au improved the surface stability such that the same number of potential cycles resulted in a loss of ~ 1 ML of Pd (equivalent thickness measured by the changes of hydrogen absorption).

Spontaneous distortion via the appearance of ferromagnetism in Pd ultrathin films: Observation of an inverse mechanism for the Stoner criterion

We study the crystal structure of Pd(100) ultrathin films, which show ferromagnetism induced by the quantum confinement effect, using in-situ X-ray crystal truncation rod measurement and density functional calculation. The energy gain due to the appearance of ferromagnetism in Pd results in flatter and uniform film growth of ferromagnetic Pd films compared with paramagnetic Pd. In addition, ferromagnetic Pd films expand the lattice constant in order to suppress the increase in kinetic energy of electrons accompanied by the occurrence of exchange splitting. Although the traditional theory of magnetism in metals indicates that the increase in density of states that induces ferromagnetism (Stoner criterion), our present finding reveals a mechanism of modulation in the density of states via the appearance of ferromagnetism, i.e., the inverse mechanism of Stoner's theory.

Change in magnetization of ferromagnetic Pd(001) ultrathin films induced by the strain effect of BaTiO3

We demonstrate changes in the saturation magnetization of Pd(001) ultrathin films exhibiting ferromagnetism in Pd/SrTiO3/BaTiO3 heterostructures due to the strain effect caused by the structural phase transition of BaTiO3 substrates. When a BaTiO3 substrate with a single domain formed by voltage application transitioned from the orthorhombic phase to the tetragonal phase, the saturation magnetization of Pd decreased by about 5%. The first-principles calculations, which assumed the distortion accompanied by the orthorhombic to the tetragonal structural phase transition, indicated a decrease by 3%–4% in the magnetization, which was consistent with the experimental results.

Spin relaxation length for medium energy electrons in Pd and LiF ultrathin films

The development of spintronics, the field of science that examines the behavior of systems, based not only on charge transfer, but on magnetic properties of nano-objects as well, requires studying various magnetic and electrical properties of nanoscale systems. To create a new device, it is often important to know a length of spin relaxation not only for electrons with energies close to the Fermi surface, but also with energies of several hundred eV. In this paper we compare results from the spin relaxation length measurements in Pd and LiF. To measure spin-relaxation length for electrons within energy range 35–550 eV spin-polarized secondary electron spectroscopy was used. Experimental results allow us to estimate the influence of the electron–electron and spin-orbit interactions in the processes of electron polarization loss. Values of spin relaxation lengths were measured at room temperature and lie in the range from 18 to 27 Å for Pd and from 70 to 230 Å for LiF.

Growth of Pd Ultrathin Films on Au in One-Cell Configuration By Surface Limited Redox Replacement of H and Cu UPD Monolayers

In light of dealing with the mitigation of global warming effects and minimization of growing pressure on limited crude oil resources, green renewable energy sources have been the focus of today’s materials science research. Palladium has been used to store hydrogen gas and to, in part, facilitate catalytic reactions in fuel cells (1, 2). However, Platinum is still the major catalytic material that has been used for fuel cells; due to its high cost and limited supply, research is now intensifying to replace Pt with Pd (3, 4). Techniques of growing Pd on conductive materials include a variety of ultrahigh vacuum approaches that very efficient but are also expensive, complicated and operate at high temperatures. A feasible alternative is electrodeposition. Recent research and development has shown that ultra-thin films of Pd can be grown on polycrystalline Au under optimum conditions using automated flow cell (5, 6). Our lab has been working recently on devising optimum conditions to efficiently grow epitaxial Pd on Au using one-cell configuration instead, of expensive automated flow cells (7). This is to embrace nanotechnology that relies on nanoscale atomic layers of Pd that have a large surface area. The growth uses surface limited reduction reaction (SLRR) of Cu or H underpotentially deposited (UPD) sacrificial layers. After the growth, the as-deposited Pd films are tested for surface roughness using HUPD and CuUPD and the efficiency of the deposition technique is measured by stripping charge determination. Consequently, the resulting Pd-Au, deposited via optimum conditions, will be tested for catalytic ability and durability in formic acid oxidation, methanol oxidation and oxygen reduction (8). In this presentation results of this work will be presented in detail and critically discussed. The emphasis is on the illustration of a facile deposition approach that takes place in one cell and in the case of using HUPD mediation requires nothing else than acidified solution of [PdCl4]2- complex. It is demonstrated that this approach provides for maximum efficiency and again in the case of HUPD driven process, eliminates potential contamination thereby rendering “green” the Pd deposition process. The CV results indicate the deposition of smooth Pd films taking place for up to 30 and 20 SLRR cycles for HUPD and CuUPD sacrificial layers, respectively. This is followed by a rapid transition to dendritic growth at higher thickness. The quasi-2D growth resulting in smooth and uniform Pd-film morphology has also been confirmed up to 20 SLRR cycles by in-situ STM. Analysis of results from Pd-film stripping experiments corroborate these findings. The comparison of charges obtained by stripping of Pd films of different thickness with generic growth models suggests that in the case of H not only adsorbed but also absorbed H UPD participates in redox exchange with [PdCl4]2- complex in 2:1 stoichiometric ratio. The applicability of the proposed presented facile SLRR approach has been compared for the use of H and Cu UPD sacrificial layers and the obtained results have been also critically discussed vis-à-vis existing literature results emphasizing the use of flow cell approach.

Electrochemical Atomic Layer Deposition of Pd Ultrathin Films by Surface Limited Redox Replacement of Underpotentially Deposited H in a Single Cell

This work illustrates the application of a surface limited redox replacement (SLRR) protocol for the electrochemical atomic layer deposition (E-ALD) of Pd ultrathin films on Au in a single cell using underpotentially deposited H (HUPD) as the sacrificial mediator. The facile deposition approach presented herein requires neither the use of sophisticated instrumentation nor the presence of metal ions other than the Pd(II) chloride complex. This provides for maximum growth efficiency and eliminates potential contamination of the deposit thereby rendering the Pd deposition a “greener” process. The growth progress is monitored by open circuit chronopotentiometry while the roughness evolution of accordingly deposited Pd films is assessed by cyclic voltammetry (CV) of HUPD and CuUPD. The CV results indicate the deposition of smooth Pd films occurring for up to 30 SLRR cycles (20 equiv monolayers) followed by a rapid transition to dendritic growth at higher thickness. The quasi-2D growth, resulting in smooth and ...

Structural analysis of ultra-thin Pd films on W(110)

We report on a scanning tunneling microscopy (STM) growth study of ultra-thin Pd films on W(110) in the thickness range up to about three atomic layers. At a substrate temperature of (573±20)K during deposition the initial growth at submonolayer coverages takes place in the step-flow mode, whereby the Pd step edges are preferentially oriented along 〈11¯1〉 directions leading to a pronounced saw-tooth–like appearance. It is found that the Pd monolayer is not smooth but instead exhibits a one-dimensional pattern consisting of alternating straight and zigzag lines along the [001] direction with a periodicity of (7.3±0.5)nm. Atomic resolution STM images reveal that these stripes mark transition regions between two different three-fold–coordinated adsorption sites, i.e. left (L) or right (R) triangular. A structural model is presented which explains the straight and zigzag lines as vacancy rows and Pd grown in the Kurdjumov-Sachs configuration, respectively. In a transition regime between 0.93AL–1.03AL the one-dimensional character converts into triangularly shaped L and R domains up to 100nm in size which are separated by short straight segments approximatelly oriented along 〈11¯1〉 and [001] directions. Growth of the second Pd layer occurs via the nucleation of hexagonal islands and at Pd coverages beyond 2AL the growth mode changes from layer-by-layer to island growth, probably due to the structural transition from the bcc to the fcc crystal structure.

Effect of the change in the interface structure of Pd(100)/SrTiO3 for quantum-well induced ferromagnetism

Measurements of temperature dependent magnetization of Pd(100) ultrathin films on SrTiO3(100) substrates which shows quantum-well induced ferromagnetism were performed. We observed the jump in magnetization of Pd(100) due to the structural phase transition of SrTiO3, and then, the disappearance of ferromagnetism after temperature-cycle repetition. X-ray reflectivity measurement revealed that the density of a few layers in the Pd film decreased near the Pd/SrTiO3 interface after temperature cycles. This suggests that the structural change affects the quantum-well induced ferromagnetism, and lowering of the crystallinity of Pd at the interface has a negative effect on quantum-well induced ferromagnetism of Pd(100) ultrathin films.

Surface Chemical Reactivity of Ultrathin Pd(111) Films on Ru(0001): Importance of Orbital Symmetry in the Application of the d-Band Model

The chemical bonding of adsorbate molecules on transition-metal surfaces is strongly influenced by the hybridization between the molecular orbitals and the metal d-band. The strength of this interaction is often correlated with the location of the metal d-band center relative to the Fermi level. Here, we exploit finite size effects in the electronic structure of ultrathin Pd(111) films grown on Ru(0001) to tune their reactivity by changing the film thickness one atom layer at a time, while keeping all other variables unchanged. Interestingly, while bulk Pd(111) is reactive towards oxygen, Pd(111) films below five monolayers are surprisingly inert. This observation is fully in line with the d-band model prediction when applied to the orbitals involved in the bonding. The shift of the d-band center with film thickness is primarily attributed to shifts in the partial density of states associated with the 4dxz and 4dyz orbitals. This study gives an in-depth look into the orbital specific contributions to the surface chemical reactivity, providing new insights that could be useful in surface catalysis.

Electric-field control of magnetic moment in Pd.

Several magnetic properties have recently become tunable with an applied electric field. Particularly, electrically controlled magnetic phase transitions and/or magnetic moments have attracted attention because they are the most fundamental parameters in ferromagnetic materials. In this study, we showed that an electric field can be used to control the magnetic moment in films made of Pd, usually a non-magnetic element. Pd ultra-thin films were deposited on ferromagnetic Pt/Co layers. In the Pd layer, a ferromagnetically ordered magnetic moment was induced by the ferromagnetic proximity effect. By applying an electric field to the ferromagnetic surface of this Pd layer, a clear change was observed in the magnetic moment, which was measured directly using a superconducting quantum interference device magnetometer. The results indicate that magnetic moments extrinsically induced in non-magnetic elements by the proximity effect, as well as an intrinsically induced magnetic moments in ferromagnetic elements, as reported previously, are electrically tunable. The results of this study suggest a new avenue for answering the fundamental question of “can an electric field make naturally non-magnetic materials ferromagnetic?”

Ultrathin Pd Films on PBI-HFA Membranes for Selective Hydrogen Separation

A polymeric gas separation membrane utilizing polybenzimidazole (PBI) based on 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (HFA) with an effective permeating area of 8.3 cm2 and a thickness of 30(± 2) mm was prepared and its gas permeation properties were investigated using H2, CO2, CO, and N2 at various temperatures ranging from 24 to 200 oC. The PBI-HFA membrane not only exhibited excellent H2 permeability, but it also displayed superior gas separation performance particularly for H2/N2 and H2/CO2. The permeation parameters for both permeability and selectivity ( PH2 and α(H2/N2); PH2 and α(H2/CO2)) obtained for the new material were found to surpass those reported by Robeson in 2008. The gas permeation properties of the prepared membrane were found to be dependent on trans-membrane pressure difference as well as temperature. A gas-tight palladium (Pd) membrane was fabricated on a PBI-HFA membrane by a vacuum electroless plating technique (VELP). The thickness and effective permeating area of the Pd/PBI-HFA membrane were estimated in the range of 228–261 nm, and 8.3 cm2, respectively. The Pd membrane prepared using the VELP technique had a more uniform microstructure than those manufactured by a conventional electroless plating method (CELP), with no delamination or agglomerated Pd particles observed. The gas permeation properties of the prepared Pd membranes were evaluated using H2, N2, CO2, and CO as functions of:35 to 200 °C temperature, 0.4 to 0.8 MPa pressure difference, and membrane preparation methods including CELP, VELP, and VELP following a H2O2 surface treatment (H-VELP). The Pd film fabricated by the H-VELP method showed superior performance, and proved to be impermeable to nitrogen as well as carbon monoxide. Additionally, the Pd membranes prepared by H-VELP with a 228 nm thickness also demonstrated an excellent gas tightness with an infinite H2/N2 permselectivity, and its separation performance was above the Robeson’s 2008 upper bound.

Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction

Ultrathin Pd films with one to five atomic layers were decorated on nanoporous gold by underpotential deposition and galvanic displacement.

Ferromagnetism in Pd(100) Ultrathin Films Enhanced by Distortion

A significant distortion of crystal structure was observed in Pd(100) ultrathin film deposited on a SrTiO3 substrate that shows ferromagnetism induced by quantum-well states. An in-situ X-ray crystal truncation rod scattering measurement showed an expansion of the lattice in the ferromagnetic Pd(100) ultrathin film with a thickness of 3.19nm. This result explains the previous observation that the measured ferromagnetic magnetization in Pd(100) is much larger than that predicted by the calculation.

A Discrete Model of the Evanescent Light Emission from Ultra-Thin Layers

A discrete model of the Differential Evanescent Light Intensity (DELI) technique was developed to calculate and map 3D nanolayers thicknesses from the evanescent light intensity captured from optical waveguides. The model was used for ultra-thin Pd nanometric layers sputtered on glass substrates. The layers thickness profiles were displayed in 3D and 1D profiles plots. The total thickness profiles of the ultra-thin Pd films obtained in the range of 1-10 nm were validated using AFM measurements. Based on the model developed the evanescent photon extraction parameter of the material was estimated.