Theoretical studies of ultrathin film-induced faceting on W(111) surfaces

We have used local volume (or EAM) potentials to study the pyramidal faceting (or reconstruction) of a W(111) surface induced by face center cubic (fcc) metals Pd, Pt, Au, and a body center cubic (bcc) metal Mo. We found that the surface-energy differences of (211) and (111) surfaces of bcc W increases as one or few monolayers of Pd, Pt, Au, and Mo films are deposited. We found that the lateral relaxation which is allowed on the (211) surface further increases the surface energy anisotropy as the thickness of the fcc metal film increases. Our calculated results are consistent with the argument that the surface energy anisotropy is the driving force for the faceting, but do not rule out three-dimensional (3D) island growth as another possible mechanism for the (211) faceting. We also found that there is a possible bilayer growth mode in W(211) surfaces with Pt and Pd films.     

Growth of ultrathin Pd films on Al(001) surfaces

High-energy ion backscattering spectroscopy (HEIS) and X-ray photoelectron spectroscopy (XPS) were used to determine the growth mode and the interface structure of ultrathin Pd films deposited on Al(001) surfaces at room temperature. Measured Al and Pd surface peak areas for MeV He⁺ ions incident normal to the surface show that Pd atoms intermix with and displace Al substrate atoms. The mixing continues for Pd coverages from 0–5 monolayers, at which point a Pd metal film begins to grow on the alloy surface. XPS measurements of the Pd 3d photopeaks show a chemical shift that is consistent with the formation of an AlPd-like compound during the mixing phase, and Pd metal thereafter. HEIS results further reveal that the alloyed overlayer as well as the Pd metal film have some degree of axial alignment with respect to the Al substrate. The XPS intensity measurements are consistent with this two-stage growth model.     

A method to pattern Pd over-layers on GdMg films and its application to increase the transmittance of metal hydride optical switches

A process to pattern Pd over-layers on reactive metal films was developed using ion milling through a tungsten trioxide mask patterned by photolithography and wet etching. The WO₃ mask exhibited a low Ar⁺ sputter yield and, unlike conventional mask materials (SiO₂, Si₃N₄), was easily etched in a mild alkaline solution. This procedure was applied to convert a 20-nm Pd cap over a 160-nm GdMg film to a Pd grid with ≈9-μm-diameter openings covering ≈40% of the surface. The Pd grid proved sufficient to catalyze the (de)hydriding reactions required to reversibly switch the GdMg film from reflecting to transparent. The maximum transmittance of the patterned Pd/GdMg hydride bi-layer was twice that of an otherwise identical sample with a continuous Pd cap, with similar hydriding kinetics. Received: 2 May 2000 / Accepted: 9 May 2000 / Published online: 13 September 2000     

Effect of combination of noble metals and metal oxide supports on catalytic reduction of NO by H2

We investigated the effects of combination of noble metals M (Rh, Pd, Ir, Pt) and metal oxide supports S (Al₂O₃, SiO₂, ZrO₂, CeO₂) on the NO + H₂ reaction using planar catalysts with M/S two layered thin films on Si substrate. In this study, NO reduction ability per metal atom were evaluated with a specially designed apparatus employing pulse valves for the injection of reactant molecules onto catalysts and a time-of-flight mass spectrometer to measure multiple transient products: NH₃, N₂ and N₂O simultaneously as well as with an atomic force microscopy to observe the surface area of metal particles. The catalytic performances of Rh and Ir catalysts were hardly affected by a choice of a metal oxide support, while Pd and Pt catalysts showed different catalytic activity and selectivity depending on the metal oxide supports. This assortment is consistent with ability to dissociate NO depending on metals without the effect of any support materials. There, the metals to the left of Rh and Ir on the periodic table favor dissociation of NO and those to the right of Pd and Pt tend to show molecular adsorption of NO. Therefore, the catalytic property of noble metals could be assorted into two groups, i.e. Rh and Ir group whose own property would mainly dominate the catalytic performance, and Pd and Pt group whose interaction with metal oxides supports would clearly contribute to the reaction of NO with H₂. NO reduction activity of Pd and Pt was found to be promoted above that of Rh and Ir, provided that Pd and Pt were supported by CeO₂ and ZrO₂.     

Chemically selective soft X‐ray patterning of polymers

The chemically selective modification of polymer mixtures by monochromated soft X‐rays has been explored using the high‐brightness fine‐focused 50 nm beam of a scanning transmission X‐ray microscope. Four different polymer systems were examined: a polymethylmethacrylate (PMMA) polyacrylonitrile (PAN) bilayer film; a PMMA‐blend‐PAN microphase‐separated film; a poly(MMA‐co‐AN) copolymer film; and a poly(ethyl cyanoacrylate) homopolymer film. A high level of chemically selective modification was achieved for the PMMA/PAN bilayer; in particular, irradiation at 288.45 eV selectively removed the carbonyl group from PMMA while irradiation at 286.80 eV selectively reduced the nitrile group of PAN, even when these irradiations were carried out at the same (x,y) position of the sample. In the last two homogenous polymer systems, similar amounts of damage to the nitrile and carbonyl groups occurred during irradiation at either 286.80 or 288.45 eV. This is attributed to damage transfer between the C[triple‐bond]N and C=O groups mediated by primary electrons, secondary electrons or radical/ionic processes, aided by their close spatial proximity. Although the overall thickness of the bilayer sample at 70 nm is smaller than the lateral line spreading of 100 nm, the interface between the layers appears to effectively block the transport of energy, and hence damage, between the two layers. The origins of the line spreading in homogeneous phases and possible origins of the damage blocking effect of the interface are discussed. To demonstrate chemically selective patterning, high‐resolution multi‐wavelength patterns were created in the PMMA/PAN bilayer system.     

引入纳米氧化层的CoFe/Pd双层膜结构中增强的垂直磁各向异性研究

制备了CoFe/Pd双层结构的界面处或CoFe层 内部引入纳米氧化层后的系列薄膜. 研究结果显示, 引入纳米氧化层后, 可以使薄膜的磁各向异性在退火后从面内转到垂直膜面方向. 并且对于在CoFe层内部引入纳米氧化层的这类样品, 其强烈的垂直磁性可以在相当宽的有效磁性层厚度范围内(1.2-2 nm)维持. 在保持垂直磁性的前提下, 这种特殊的双层膜结构中CoFe磁性层厚度比常规CoFe/Pd 多层膜中的CoFe层厚度至少多出1.4 nm. 本文的研究有助于制备出具有较高热稳定性的垂直磁性器件电极.     

Selective formation of ultrathin PbSe on Ag(111)

Two-dimensional (2D) semiconductors, such as lead selenide (PbSe), locate at the key position of next-generation devices. However, the ultrathin PbSe is still rarely reported experimentally, particularly on metal substrates. Here, we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111). The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag₅Se₂ and Pb, as further evidenced by the theoretical calculation. It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films. The detailed analysis demonstrates the bilayer structure of synthesized PbSe, which could be deemed to achieve the 2D limit. The differential conductance spectrum reveals a metallic feature of the PbSe film, indicating a certain interaction between PbSe and Ag(111). Moreover, the moiré pattern originated from the lattice mismatch between PbSe and Ag(111) is observed, and this moiré system provides the opportunity for studying physics under periodical modulation and for device applications. Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.     

Undercut structures fabricated by microtransfer printing combined with UV exposure and their applications

In this paper, the undercut structures were fabricated by microtransfer printing of metal films on the surface of photoresist combined with UV exposure and photoresist film developing. The patterned metal films were used as mask to realize the selective UV exposure of photoresist firstly. The undercut structures, which consist of the top metal films and the patterned bottom photoresist, formed in the subsequent developing process because of the lateral dissolving of photoresist at the edge of the unexposed regions. The method proposed in this paper has wider tolerance to the changing of the patterning parameters, but without effect on the patterning resolution since the metal film was used as the top layer. The undercut structures were used as separators to pattern passive-matrix display of organic light-emitting diodes (OLEDs). No visible difference of the device performance was observed compared with the OLEDs patterned by the shadow mask.     

A PdO(1 0 1) thin film grown on Pd(1 1 1) in ultrahigh vacuum

We present experimental results demonstrating that a high quality PdO(1 0 1) thin film can be grown on Pd(1 1 1) in ultrahigh vacuum by oxidizing the metal at 500 K using an oxygen atom beam, followed by annealing to 675 K. Low energy electron diffraction (LEED) images show that the [0 1 0] direction of the PdO(1 0 1) thin film aligns with the [−1 1 0] direction of the Pd(1 1 1) substrate, and that the PdO film grows in three degenerate domains, rotated 120° relative to one another. Based on excellent agreement between the experimental and simulated LEED patterns, we conclude that the surface structure of the PdO thin film deviates minimally from bulk-terminated PdO(1 0 1). Recent temperature programmed desorption (TPD) experiments also provide evidence that the PdO(1 0 1) thin film on Pd(1 1 1) is terminated by the stoichiometric surface in which half of the Pd atoms are coordinatively unsaturated (cus), corresponding to a cus-Pd atom density equal to about 35% of the surface density of Pd(1 1 1). The ability to generate a well-defined PdO(1 0 1) surface in ultrahigh vacuum should provide new opportunities for conducting model surface science studies of PdO, particularly studies aimed at elucidating the reactivity of PdO(1 0 1) toward species important in commercial applications of Pd catalysis.     

Visibly Transparent Multifunctional Metascreen for Tunable Infrared Coloration and Microwave Transmission

Metascreens are remarkable optical materials that have attracted significant research interest owing to their ability to manipulate electromagnetic waves by selectively transmitting, reflecting, and absorbing specific wavelengths. The present study introduces a new metascreen concept called visibly transparent multifunctional metascreen (VTMM), which is capable of simultaneously transmitting visible light, selectively transmitting microwaves, and modulating infrared (IR) emissions, all through a single surface. The VTMM is fabricated by patterning a thin metal film in a square-loop pattern on a transparent substrate, resulting in frequency-selective transmission characteristics in the X-band and visible transparency. Before demonstrating independent control of the visible and IR coloration of the VTMM by varying the metal film thickness and using color backgrounds, the visible and IR coloration characteristics of the ultrathin metal film on glass (MFG) are analyzed. Based on the findings, the VTMM preserves optical images behind the metascreen while reducing the average IR signature from 89.0 to 69.3 W m⁻² sr^-1, with up to 23.5% reduction compared to a glass substrate, as the metal film thickness increases from 40 to 100 nm is confirmed. Furthermore, the designed selective transmission characteristics are shown to remain consistent across varying metal film thicknesses.     

Deposition of metal clusters on single-layer graphene/Ru(0001): Factors that govern cluster growth

Fabrication of nanoclusters on a substrate is of great interest in studies of model catalysts. The key factors that govern the growth and distribution of metal on graphene have been studied by scanning tunneling microscopy (STM) based on different behaviors of five transition metals, namely Pt, Rh, Pd, Co, and Au supported on the template of a graphene moiré pattern formed on Ru(0001). Our experimental findings show that Pt and Rh form finely dispersed small clusters located at fcc sites on graphene while Pd and Co form large clusters at similar coverages. These results, coupled with previous findings that Ir forms the best finely dispersed clusters, suggest that both metal–carbon (M–C) bond strength and metal cohesive energies play significant roles in the cluster formation process and that the M–C bond strength is the most important factor that affects the morphology of clusters at the initial stages of growth. Furthermore, experimental results show Au behaves differently and forms a single-layer film on graphene, indicating other factors such as the effect of substrate metals and lattice match should also be considered. In addition, the effect of annealing Rh on graphene has been studied and its high thermal stability is rationalized in terms of a strong interaction between Rh and graphene as well as sintering via Ostwald ripening.     

Surface functionalization of porous ZrO2-TiO2 membranes using γ-aminopropyltriethoxysilane in palladium electroless deposition

A pre-treatment technique was developed to facilitate the electroless deposition of Pd layers onto ZrO₂-TiO₂ ceramic membrane surfaces in the preparation of novel multi-functional porous membranes. Surface functionalization using an aqueous solution of γ-aminopropyltriethoxysilane (γ-APTES) aided the surface immobilization of the Pd activation particles and the subsequent electroless deposition of metal layers onto the hydroxyl-rich membrane surface. The attractiveness of γ-APTES functionalization, in the electroless deposition of metal layers, was thus demonstrated. Characterization techniques employed in the structural study of the surface-modified membranes included SEM, EDS, dynamic analysis in micro-PIXE, and XRD. Special membrane techniques such as electrokinetic analysis and single-gas permeation measurements were also used in the study of surface modification. These membranes were developed for application in tasks associated with the hydrogen economy.     

Quantum-well states in bilayers of Ag and Au on W(1 1 0)

sp-Like quantum-well states (QWS) in thin monocrystalline bilayer films of Ag and Au on W(1 1 0) and of single Ag films were studied by angle-resolved photoemission. We find that the propagation of the electronic states in the bilayer films along [1 1 1] depends on the energy relative to the band edge of Au metal at the L point of the Brillouin zone. In particular, QWS with binding energies less than this band-edge energy (1.1 eV) are strongly confined to the Ag layer, while for higher binding energies the QWS extend across the whole bilayer film. This clearly demonstrates the weakness of the potential barrier at the Ag/Au interface in the context of QWS formation at energies where electronic states exist in both metals.     

Metal nanostructures with complex surface morphology: The case of supported lumpy Pd and Pt nanoparticles produced by laser processing of metal films

In this work we report on the formation of lumpy Pd and Pt nanoparticles on fluorine-doped tin oxide/glass (FTO/glass) substrate by a laser-based approach. In general, complex-surface morphology metal nanoparticles can be used in several technological applications exploiting the peculiarities of their physical properties as modulated by nanoscale morphology. For example plasmonic metal nanoparticles presenting a lumpy morphology (i.e. larger particles coated on the surface by smaller particles) can be used in plasmonic solar cell devices providing broadband scattering enhancement over the smooth nanoparticles leading, so, to the increase of the device efficiency. However, the use of plasmonic lumpy nanoparticles remains largely unexplored due to the lack of simply, versatile, low-cost and high-throughput methods for the controllable production of such nanostructures.Starting from these considerations, we report on the observation that nanoscale-thick Pd and Pt films (17.6 and 27.9 nm, 12.1 and 19.5 nm, respectively) deposited on FTO/glass surface irradiated by nanosecond pulsed laser at fluences E in the 0.5–1.5 J/cm² range, produce Pd and Pt lumpy nanoparticles on the FTO surface. In addition, using scanning electron microscopy analyses, we report on the observation that starting from each metal film of fixed thickness h, the fraction F of lumpy nanoparticles increases with the laser fluence E and saturates at the higher fluences. For each fixed fluence, F was found higher starting from the Pt films (at each starting film thickness h) with respect to the Pd films. For each fixed metal and fluence, F was found to be higher decreasing the starting thickness of the deposited film. To explain the formation of the lumpy Pd and Pt nanoparticles and the behavior of F as a function of E and h both for Pd and Pt, the thermodynamic behavior of the Pd and Pt films and nanoparticles due to the interaction with the nanosecond laser is discussed. In particular, the photothermal vaporization and Coulomb explosion processes of the Pd and Pt nanoparticles are invoked as possible mechanisms for the lumpy nanoparticles formation.     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

On the formation of a phase with tenfold symmetry in amorphous Ni–Zr alloys

The magnetic anisotropy energy (MAE) of 3d transition-metal wires, stripes, and films is calculated self-consistently as a function of stripe width and film thickness. The magnetization-reorientation transitions in stripes are determined along the crossover from the mono-atomic one-dimensional chain to the two-dimensional monolayer. It is shown that the MAE oscillates as a function of stripe width and depends strongly on the considered transition metal. The reorientation transitions in Co films deposited on a highly polarizable substrate such as Pd are discussed. A local analysis of the layer-resolved MAEs provides new insights into the off-plane magnetization observed in Pd-capped Co films on Pd(111). The interfaces responsible for the stability of the off-plane easy axis are characterized microscopically. An unexpected internal magnetic structure of the Co–Pd interfaces is revealed in which the magnetic moments and spin–orbit interactions at Pd atoms play a crucial role. The nature of the reorientation transition from perpendicular to in-plane magnetization with increasing film thickness is studied by means of full-vectorial calculations. The existence of a spin-canted phase at intermediate film thickness is demonstrated.     

Characterization of a Pd-Fe bimetallic model catalyst

Small bimetallic Pd-Fe particles supported on a well ordered alumina film grown on NiAl (1 1 0) were studied focusing on the geometric, electronic, adsorption, as well as magnetic properties. The morphology, growth mode and surface composition were investigated by combining scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and infrared spectroscopy (IRAS) using CO as a probe molecule. Information on the electronic properties of the bimetallic systems was obtained by means of X-ray photoelectron spectroscopy (XPS). These measurements were amended by in situ ferromagnetic resonance spectroscopy to address the magnetic properties of the bimetallic particles. The subsequent deposition of the metals at 300 K varying the order of metal deposition resulted in two distinct bimetallic systems. Pd deposited on existing Fe particles forms a shell, however, FMR and XPS suggest that intermixing of Pd and Fe occurs to some extent. For the reverse order, a larger amount of Fe is required to coat Pd particles, due to the strong tendency of Pd to segregate to the surface of the particles.     

Selective ultrasonic imprinting for micropattern replication on predefined area

Conventional micro/nano patterning processes generally use rectangular patterned molds, so that the pattern replications are usually performed in a corresponding rectangular region. In this study, a selective patterning method based on ultrasonic imprinting is developed to replicate micropatterns on predefined areas with arbitrary profiles. To replicate micropatterns on predefined areas, the conventional ultrasonic imprinting process is modified by placing a profiled metal mask between a target film and an ultrasonic horn. Ultrasonic waves are then selectively transferred to the target film through the mask film, from which micropatterns can be replicated onto the predefined areas. For the implementation of the proposed selective imprinting process, the effects of the mask size and shape are experimentally investigated in terms of the replication characteristics of micropatterns. This selective imprinting process is further applied to micropattern replications on arbitrarily profiled areas. In these applications, the effects of the mask film and imprinting conditions are also discussed in terms of the replication quality in both the masked and the unmasked regions.     

Study of solid-state reactions and order-disorder transitions in Pd/α-Fe(001) thin films

The formation of the hard-magnetic ordered L1₀-FePd phase in thin bilayer Pd/α-Fe(001) films has been experimentally studied. Solid-state reactions initiated by thermal heating in bilayer Pd/α-Fe(001) films with a thickness of 50–60 nm (the atomic ratio Pd: Fe ≈ 50: 50) separated from the substrate have been studied using the in situ electron diffraction methods. It has been shown that the solid-state reaction between the palladium and iron layers in Pd/α-Fe(001) starts at 400°C with the formation of the disordered Fe-Pd phase. At 480°C, the ordered L1₀-FePd phase is formed. The order-disorder phase transition has been studied. It has been established that the transition of the ordered L1₀-FePd phase to the disordered FePd phase starts at 725°C. At 740°C, only the disordered FePd phase is present over the whole volume of the film. The observed temperature of the order-disorder phase transition is shifted from the equilibrium value by 35°C to higher temperatures. This effect is assumingly associated with the higher concentration of palladium atoms at the boundaries of the Fe-Pd crystal grains owing to the grain-boundary adsorption.     

Evolution of a stress-driven pattern in thin bilayer films: spinodal wrinkling

We report the full spectrum of the evolution of the wrinkle pattern formation in a thin bilayer film of an elastic metal on a viscoelastic polymer. Although the origin is different, the transition of an initial islandlike pattern to a labyrinthine structure without any change in the wavelength (q approximately t(0)) and the overall evolutionary process is strikingly similar to that in the spinodal system but the process is robust and takes place on a long time scale (about 10 days). The change into a mountainous topography in the late stages is accompanied by an increase in the length scale from an initial wavelength to another. This change, due to the relaxation of the confined polymer that results in a transition from elastic- to viscouslike behavior, induces wave coarsening (q approximately t(-1.04+/-0.08)) and macroscopic roughening.