Rh Films Research Articles

EELS study of Rh particle growth on ZrO 2 substrate with different deposition conditions

Electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) have been used to monitor the formation of Rh films on ZrO 2(1 0 0) single crystalline substrates. Rhodium was evaporated step by step at various substrate temperatures. The surface morphology of the deposited material influences EELS spectra, so that the relative metal coverage of the substrate can be calculated. The EELS and AES results were compared and the conclusions concerning the deposit morphology were verified by means of transmission electron microscopy.

Structural Changes of the W(211) Surface Induced by Ultrathin Films of Rh, Pt, and Pd

A W(211) surface covered with a thin film (between 0.5 and 1 physical monolayer) of Rh, Pt, or Pd is found to exhibit an n × 1 superstructure when annealed above a threshold temperature of ∼900 K (500 K for Pd). The superstructure is observed using low energy electron diffraction; phase diagrams are presented to indicate the coverage range and the temperature threshold where the new structure appears. Scanning tunneling microscopy results indicate that in the case of Pd/W(211) the superstructure phase is due to missing overlayer rows. In the case of Rh/W(211), there are two possible interpretations to the observed periodic structure: either the adlayer forms an ordered superstructure or the W surface itself forms a microfaceted structure with {110} faces. On the basis of our STM observations of gas adsorption on the surface, we found the explanation involving microfacets more plausible. A careful calibration of overlayer coverage is performed using a quartz crystal monitor combined with temperature programmed desorption measurements performed at high temperatures (up to 2400 K). Auger electron spectroscopy used in combination with thermal annealing series demonstrates that overlayer coverages above one physical monolayer are thermally unstable, which is explained by the formation of an ultrathin alloy film.

Rhodium and Rhodium Oxide Thin Films Characterized by XPS

RhO2 belongs to the family of conducting platinum group metal oxides, which have attracted attention as new capacitor electrode materials for dynamic random access memories (DRAMs) and nonvolatile ferroelectric random access memories (FeRAMs). Rh, Rh2O3, and RhO2 thin films were prepared by sputtering and their XPS spectra were collected with a monochromatic Al Kα x-ray source. This report includes XPS spectra of Rh 3d and O 1s core regions for these films.

Adsorption Behavior of CO at the Ultrathin Rh and Pt Film Electrodes

Adsorption Behavior of CO at the Ultrathin Rh and Pt Film Electrodes

Morphology and magnetic properties of thin films of Rh on highly oriented pyrolitic graphite

The structure and magnetic properties of ultrathin Rh layers deposited on highly oriented pyrolitic graphite (HOPG) have been investigated by means of core-level photoemission and scanning tunneling microscopy. The Rh growth on HOPG follows the Volmer-Weber mode at 300 K for any coverage, while Rh may form a commensurate $p(1\ifmmode\times\else\texttimes\fi{}1)$ ordered structure when deposited at 150 K for a coverage up to one monolayer. For submonolayer or monolayer Rh films on HOPG the linear magnetic dichroism in the angle distribution of photoelectrons shows no evidence of in-plane long-range magnetic ordering.

Electrochemical Promotion by Sodium of the Rhodium-Catalyzed NO + CO Reaction

XPS shows that the electropumping of Na to a metallic Rh film contacted with a solid electrolyte (Na β‘ ‘ alumina) under potentiostatic conditions results in reversible spillover of the alkali from the electrolyte to the catalytically active surface. As the catalyst potential (VWR) is decreased over a range of 2000 mV, the sodium coverage increases linearly from zero to ∼0.02 monolayers, while the work function (φ) decreases, also linearly, by 0.65 eV. Over this same regime of catalyst potential, under approximately stoichiometric conditions, both the activity and N2 selectivity of the NO + CO reaction are strongly enhanced as Na is pumped to the Rh surface. The measured Na coverage, electrochemical behavior, and catalytic response of the system are fully reversible with catalyst potential. Our data are understandable in terms of the Na-induced dissociation of adsorbed NO, which is thought to be the rate-limiting step. The nature of electrochemical promotion, the reaction mechanism, the mode of promoter a...

Electrochemical promotion of a classically promoted Rh catalyst for the reduction of NO

The reduction of NO by CO in presence of O 2, a reaction of great technological importance, was investigated on porous polycrystalline Rh catalyst-electrodes deposited on YSZ (Y 2O 3-stabilized-ZrO 2) an O 2− conductor. It was found that application of current or potential between the Rh catalyst electrode and a Au counter-electrode enhances the rate of NO reduction and CO 2 formation by up to a factor of 20. These rate increases are strongly non-Faradaic with apparent Faradaic efficiencies, Λ, up to 20, manifesting the effect of Electrochemical Promotion or Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The Rh catalyst electrodes were subsequently promoted in a classical way, via dry impregnation with NaOH, followed by drying and calcination. The thus Na-promoted Rh films were found, as expected, to exhibit much higher catalytic activity than the unpromoted films, with a pronounced decrease in their light-off temperature from 440 to 320°C. The effect of Electrochemical Promotion was then studied on these, already Na-promoted Rh catalyst. Positive (+1 V) potentials were found to further increase the rate of NO reduction by up to a factor of 4 with a Faradaic efficiency up to 20 and concomitant reduction in light-off temperature down to 260°C. This is the first demonstration of electrochemical promotion on an already promoted catalyst surface.

Optical absorption of transition-metal island films: Correlation interaction between conduction electrons of transition metals

Optical plasma-resonance absorption of Pt island films consisting of Pt particles larger than about 25 Å in diameter has been measured in the photon energy range of 0.5–6.5 eV. As in Rh and Pd island films reported previously, the broadening of the optical plasma-resonance absorption reflects a correlation interaction between conduction electrons. Comparison of the broadening for the Pt island films with that for the Rh island films shows that the correlation interaction is strong when the conduction-electron density n is low. In an electron-gas model, the correlation interaction between electrons becomes stronger with lowering electron density, because the magnitude ratio of the Coulomb to kinetic energy increases as the electron density lowers. Thus, the strong correlation-interaction at low n proves that the correlation interaction in transition metals becomes stronger with magnitude ratio. The magnitude ratio in transition metals is pointed out to increase with lowering n and/or with strengthening d character of conduction electrons. Based on the correlation interaction, reflected by the broadening for the Pt, Rh, and Pd island films, and on the strong correlation interaction, found previously for Ir, the order of magnitude ratio is Ir>Pt>Pd>Rh.

Chemical Fluid Deposition: A Hybrid Technique for Low‐Temperature Metallization

Chemical fluid deposition (CFD) is a novel approach to metal deposition that involves the chemical reduction of organometallic compounds in supercritical carbon dioxide to yield high purity films at low temperature. Since supercritical CO2 can exhibit densities that approach those of a liquid solvent while retaining the transport properties of a gas, CFD is essentially a hybrid technique that uniquely blends the advantages of chemical vapor deposition (CVD) and electroless plating. Here, we describe the deposition of high-purity films of Pt, Pd, Au, and Rh onto inorganic and polymer substrates by the reduction of appropriate precursors in CO2 at 60 °C.

Mechanistic Studies of CVD Metallization Processes: Reactions of Rhodium and Platinum β-Diketonate Complexes on Copper Surfaces

The hexafluoroacetylacetonate complexes Rh(hfac)(C2H4)2 and Pt(hfac)2 are known to serve as chemical vapor deposition precursors to Rh and Pt thin films. In the absence of a reducing carrier gas, the depositions are surface-selective and occur preferentially on copper, but under these conditions, the metallization processes are unexpectedly inefficient relative to the rapid deposition of Pd on Cu seen for the palladium analogue Pd(hfac)2. Mechanistic studies of the reactions of Rh(hfac)(C 2H4)2 and Pt(hfac)2 on copper surfaces under ultrahigh vacuum conditions have now been performed in order to elucidate the factors responsible for the differences among these surface-selective metallization processes. The studies demonstrate that adsorption of the rhodium complex Rh(hfac)(C2H4)2 on copper surfaces is accompanied by the loss of the coordinated ethylene groups, even at 100 K. At these low temperatures, the adsorbed Rh(hfac) fragments are oriented in several ways with respect to the surface. Heating the substrate above 150 K causes the hfac ligands to realign to a perpendicular orientation relative to the surface. The platinum precursor Pt(hfac) 2 adsorbs molecularly at 100 K with the molecular planes of the hfac ligands oriented parallel to the copper surface. Heating the substrate to temperatures above 150 K again results in a realignment of the hfac ligands to a perpendicular orientation. This reorientation is accompanied by a partial reduction of the Pt centers (as judged from shifts seen in X-ray photoelectron spectroscopy core level data), a result suggesting that the hfac ligands begin to dissociate from the platinum centers near 150 K. At temperatures above 220 K, the transfer of the hfac ligands from both complexes to the copper surface is complete, as signaled by the reduction of the metal centers to the zero-valent state. The copper-bound hfac ligands are further transformed upon heating, either reacting with copper surface atoms to yield Cu(hfac)2 (which desorbs at temperatures above 250 K) or decomposing (with fragments desorbing above 350 K). The presence of platinum on the copper surface promotes the former reaction as judged by the appearance of a new reaction-limited desorption process for Cu(hfac) 2. The presence of rhodium on the copper surface does not promote the formation of Cu(hfac)2, although autocatalysis is noted in the steady-state reactive scattering data. The inability of the Rh and Pt precursors to engage in a sustained transmetalation reaction with the copper surface is attributed to the slow interdiffusion of copper through the Rh/Cu and Pt/Cu alloys that are produced in the near-surface region.

Chemical waves and adsorbate-induced segregation on a Pt(100) surface microstructured with a thin Rh/Pt film

Reaction fronts formed during the O 2+H 2 and NO+H 2 reactions on a 70 Å thick microstructured Rh film on a Pt(100) substrate have been investigated using photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM). The reactions were carried out in the 10 −7 mbar range at T=670 and 700 K. The photoelectron spectra revealed that the thin Rh film was alloyed with Pt due to vertical intermixing with the underlying Pt(100) substrate. The surface Rh/Pt ratio was found to vary from ∼4 for an oxygen-free film to 4.8 in oxygen ambient. The O 1s spectra indicated the presence of oxide species along with chemisorbed oxygen on the alloyed film. The chemical maps revealed an enhanced tendency for oxide formation at the boundaries of the Rh/Pt film leading, under reducing conditions, to residual oxide and Rh enrichment expanding laterally to ∼10 μm inside the film. This composition inhomogeneity can explain the peculiar ring patterns near Pt/Rh interfaces that were observed by PEEM under the reaction conditions.

In Situ FTIR Spectroscopic Studies of Adsorption of CO, SCN-, and Poly(o-phenylenediamine) on Electrodes of Nanometer Thin Films of Pt, Pd, and Rh: Abnormal Infrared Effects (AIREs)

Pt, Pd, and Rh films of a few nanometers in thickness supported on glassy carbon (GC) and other substrates were prepared by electrochemical voltammetry. STM patterns illustrated that the prepared thin films are composed of crystallites of layer structure and exhibit a low surface roughness. Studies of in situ FTIR spectroscopy on chemisorption of CO and SCN- and formation of a polymer of o-phenylenediamine (POPD) on electrodes of nanometer thin films have been conducted to explore the abnormal infrared effects (AIREs), which consist of two main characteristics: (1) inversion of IR bands; (2) the enhancement of IR absorption of adsorbates. The results demonstrated that the AIREs depend mainly on the structure and the chemical nature of nanometer thin films. In all cases of chemisorption on thin films of platinum-group metals supported on GC or supported on polymer-covered GC, the direction of IR bands of adsorbates is inverted in comparison with the direction of IR bands of the same adsorbates on correspo...

In situ spectroscopic ellipsometry as a surface-sensitive tool to probe thin film growth

Sputtered thin film and multilayer X-ray mirrors are made routinely at the Advanced Photon Source (APS) for APS users. Precise film growth control and characterization are critical in fabricating high-quality X-ray mirrors. Film thickness calibrations were carried out using in situ and ex situ spectroscopic ellipsometry, interferometry, and X-ray scattering. To better understand the growth and optical properties of different thin film systems, we have carried out a systematic study of sputtered thin films of Au, Rh, Pt, Pd, Cu, and Cr grown on Si substrates, using in situ ellipsometry. Multiple data sets were obtained in situ for each film material with incremental thicknesses and were analyzed using both flat-film and rough-film models. We found that the initial growth of these metal films on Si is correlated with their chemical bond strength with oxygen in the native oxide of Si. Metals with higher bond strength (such as Cr) grow smoother at the early stage of growth and have a better adhesion to the Si wafer. The Cr film becomes rougher as its thickness increases. Au and Rh films grow smoothly on a thin (6-nm) Cr-covered Si wafer. But on a thick (100-nm) Cr/Si film, Rh films were initially as rough as the thick Cr film. The roughness decreases as the Rh film thickness increases. When Cr is used as a ‘glue’ layer in metal on oxide systems, it is better to use a thin (<10 nm) Cr film in order to obtain a smooth metal film. We conclude that in situ spectroscopic ellipsometry as a surface-sensitive tool can also be used to probe the growth and morphology of the thin film system.

FIRST PRINCIPLES TOTAL ENERGY STUDY OF THE MORPHOLOGICAL INSTABILITY OF Mo(111) INDUCED BY ULTRATHIN FILMS OF Rh

By using first principles total energy calculations, the surface energies of (100), (110), (111) and (211) surfaces of Mo before and after adsorbing ultrathin films of Rh have been presented. The results show that in the case of clean Mo surfaces Mo(111) is stable against faceting to Mo(211), Mo(110) and Mo(100), while after adsorbing one physical monolayer of Rh, the surface energies of all the four surfaces decrease, and Mo(111) can facet to {211}, but not to {110} and {100} surfaces.

Faceting and steady-state shape of Rh- and Pd-covered W field emitter tips

Field emission microscopy (FEM) has been used to investigate the faceting of tungsten emitter tips of approximately 2000 Å in radius, due to thin overlayer films of Rh or Pd. Both overlayer metals are found to induce the faceting of the nearly spherical emitter tip upon annealing at elevated temperatures, as demonstrated by the increase in size of {211} facets as well as the appearance of new facet orientations, such as {321} or {431}. The faceting transition was observed only above a critical coverage and a threshold temperature of ∼800 K. The question of whether the observed faceted morphology represented a steady-state tip shape has also been investigated by approaching the final end form starting from two different initial geometries: a nearly spherical thermally annealed tip, and a nearly polyhedral built-up tip. When subjected to similar Rh coverages and annealing conditions, both initial geometries converged to the same steady-state tip shape. A similar convergence using Pd was not observable. Similarities and differences between Rh/W and Pd/W results are discussed and compared to the faceting of the metal-overlayer-covered W(111) single crystal surface. Also discussed are the observed faceted steady-state tip shape and its relation to the equilibrium crystal shape in the light of kinematic limitations in surface diffusion.

Growth and thermal stability of ultrathin films of Fe, Ni, Rh and Pd on the Ru(0001) surface

The growth and stability of ultrathin Fe, Ni, Rh and Pd films on the Ru(0001) surface was investigated in the temperature range of 200–2000 K. All films up to a material-dependent thickness agglomerated upon heating to T>600 K. At higher temperatures the films differ: Fe and Rh wet the surface again by forming an interfacial alloy, Ni and Pd do not. This indicates that interfacial mixing is determined mainly by electronic rather than by elastic effects.

Magnetic Properties of Granular Fe–Cr–O and Fe–X–Cr–O (X=Cu, Rh) Films

Soft magnetic properties have been obtained in films cosputtered using dual targets of Fe and Cr2O3. The resultant Fe–Cr–O films are considered to be mainly composed of nanocrystalline Fe and an amorphous intergranular region of Cr and O. The coercivity decreases rapidly with decreasing volume fraction of Fe, resulting in good soft magnetic properties. This rapid decrease is attributed to the grain size reduction of bcc-Fe, where the so-called random anisotropy model for a single-phase nanocrystalline system is quantitatively applicable to the present granular one. The saturation magnetization 4πMs, coercivity Hc, relative permeability µ′ at 500 MHz, and ferromagnetic resonance frequency of the Fe80Cr7O13 film with average Fe grain size of 5.5 nm are 14.0 kG, 1.5 Oe, 1470, and 1.80 GHz, respectively. Furthermore, it is found that the additional elements such as Cu and Rh decrease Hc and increase the in-plane anisotropy field Hk of the Fe–Cr–O film.

Influence of the Deposition Parameters on the Electrical and Mechanical Properties of Physically Vapor-Deposited Iridium and Rhodium Thin Films

AbstractIridium and rhodium thin films have been formed by e-gun physical vapor deposition on thin-chromium-coated, thermally-oxidized, silicon substrates. Cr, Ir and Rh deposition rates and substrate temperature during deposition were measured and controlled. The effects of the latter deposition parameters on the sheet resistance and stress of the Ir and Rh films are presented and it is demonstrated that both stress and sheet resistance can be desirably minimized by proper choice of the process conditions. The resistivity of these Rh and Ir thin films has been measured at room temperature. Rh can be formed in a wider process window than Ir. Rh films with Rsh = 0.1 Ω/square have been obtained at a thickness of 0.6 ¼m.

Electron elastic scattering study of thin film growth mode: Rh on Al 2O 3

Determination of the thin film growth mode during the early stage of the growth, i.e. in the submonolayer region, requires a 1st surface layer sensitive method. Auger electron spectroscopy that gives informations on the composition of few first atomic layers cannot be used in this purpose without ambiguity. In this paper we show that the elastic electron scattering can be used as a morphology sensitive probe in the case of ultra-thin metallic layers deposited on oxide substrates. Rh films were grown on polycrystalline γ-alumina at room temperature. Variation of elastically scattered electron current (elastic reflectivity) during the growth was used for determination of the relative Rh coverage. It was shown that the formation of the three dimensional small cluster structure covering up to 75% of the substrate surface was followed by coalescence and an increase of the free surface area.

Chemically resolved dynamical imaging of catalytic reactions on composite surfaces

The catalytic reduction of NO by hydrogen is investigated at (T = 650 K and (p≈10-6 mbar on a microstructured Rh/Pt(100) surface consisting of Pt(100) domains surrounded by a 600 Athick Rh film. Synchrotron radiation scanning photoemission microscopy (SPEM), using photons focused into a spot of less than 0.2 μm diameter, is employed as a spatially and chemically resolving in situ technique. The chemical waves which arise in the bistable system NO+H2/Rh are imaged with SPEM monitoring the N 1s and O 1s photoelectrons. The reaction fronts initiate transitions from an inactive oxygen-covered surface (ΘO≈0.25 ML) to a reactive nitrogen-covered surface (ΘN≈0.06 ML). At the Pt/Rh interface, synergetic effects can be observed: the chemical waves on the Rh film nucleate preferentially at the Pt/Rh interface. This nucleation is poisoned by carbon contamination on the Pt area but is prevented in the vicinity of the Pt/Rh interface by the adjacent clean Rh film. No segregation of Pt to the surface was observed for the 600 A thick Rh film.