Clean Metal Surfaces Research Articles

Nanoporous copper from dual-phase alloy families and its technology application in lithium ion batteries

AbstractNanoporous metals have recently attracted considerable interest in a wide variety of applications, including catalysis, sensors, actuators, fuel cells, microfluidic flow controllers, and so forth. Their inherent special morphology and characteristic is expected to have superior performances, such as the open porosity, high surface area, good electron conductivity, extremely clean metal surface, and high structural stability with no agglomeration in comparison with other nanostructures. Here, we mainly review our group’s systematical studies related to nanoporous copper (NPC) from dual-phase alloy families, aiming at giving a comprehensive summary about our step-by-step progress in its fabrication, microstructure evolution, dealloying behavior, and one-step strategies to special 3-D bicontinuous porous architectures in the last 5 years. Meanwhile, some encouraging test findings on the original application of NPC in lithium ion batteries have also been covered briefly in the last part of this review, which would open a window toward their other technology applications in high-performance electrochemical devices within the green, new energy industry.

Quantum efficiency enhancement in CsI/metal photocathodes

High quantum efficiency enhancement is found for hybrid metal-insulator photocathodes consisting of thin films of CsI deposited on Cu(100), Ag(100), Au(111) and Au films irradiated by 266nm laser pulses. Low work functions (near or below 2eV) are observed following ultraviolet laser activation. Work functions are reduced by roughly 3eV from that of clean metal surfaces. We discuss various mechanisms of quantum efficiency enhancement for alkali halide/metal photocathode systems and conclude that the large change in work function, due to Cs accumulation of Cs metal at the metal–alkali halide interface, is the dominant mechanism for quantum efficiency enhancement.

Vibrational excitation induces double reaction.

Electron-induced reaction at metal surfaces is currently the subject of extensive study. Here, we broaden the range of experimentation to a comparison of vibrational excitation with electronic excitation, for reaction of the same molecule at the same clean metal surface. In a previous study of electron-induced reaction by scanning tunneling microscopy (STM), we examined the dynamics of the concurrent breaking of the two C-I bonds of ortho-diiodobenzene physisorbed on Cu(110). The energy of the incident electron was near the electronic excitation threshold of E0=1.0 eV required to induce this single-electron process. STM has been employed in the present work to study the reaction dynamics at the substantially lower incident electron energies of 0.3 eV, well below the electronic excitation threshold. The observed increase in reaction rate with current was found to be fourth-order, indicative of multistep reagent vibrational excitation, in contrast to the first-order rate dependence found earlier for electronic excitation. The change in mode of excitation was accompanied by altered reaction dynamics, evidenced by a different pattern of binding of the chemisorbed products to the copper surface. We have modeled these altered reaction dynamics by exciting normal modes of vibration that distort the C-I bonds of the physisorbed reagent. Using the same ab initio ground potential-energy surface as in the prior work on electronic excitation, but with only vibrational excitation of the physisorbed reagent in the asymmetric stretch mode of C-I bonds, we obtained the observed alteration in reaction dynamics.

Ignition of flammable hydrogen/air mixtures by mechanical stimuli. Part 1: Ignition with clean metal surfaces sliding under high load conditions

The conditions under which ignition will occur with sliding of clean metals (SS 304 and carbon steel) at high mechanical loading and velocity are investigated in part 1 of this study. A Monte-Carlo analysis has been undertaken to indicate the likely range of variation in the surface temperature with uncertainty in the parameters of most importance. It is concluded that ignition of flammable hydrogen in air atmospheres due to friction between clean metal surfaces will readily occur at high mechanical loading and sliding velocities where surface temperatures can exceed 900 °C. Even with loading of ≈0.9 tonnes, there was no experimental evidence to suggest that ignition could occur with clean surfaces of the materials investigated at velocities less than 1 m/s. However, given the theoretical relationship between generated surface temperature and properties of the sliding metals, it is indicated that ignition would be much more likely to occur at such velocities with harder metals. The study also shows that the raised surface temperature brought about by increasing sliding velocity is of greater significance than the reduction in contact duration between hot spots and individual kernels of partially ignited gas.

Thin-film Deposition of Silicon Nitrides and Oxides from Trihydridosilanes

Trihydridosilanes can provide a route for generating self-assembled monolayers on metal substrates. Under mild conditions, these precursors interact with a variety of clean, hydrogenated and fresh metal and metalloid surfaces, including titanium, silicon and gold. All classes of hydridosilanes have minimal interaction with anhydrous oxide surfaces. After initial deposition, SAMs formed from 2-chloroethyltrihydridosilanes may be converted to silicon nitride by pulsing with ammonia or silicon dioxide by pulsing water or oxygen. A proposed mechanism for the initial steps of deposition involves the dissociative adsorption of the silanes with the formation of hydrogen, followed by topmost atom layer insertion and concomitant surface reconstruction

Thin-film Deposition of Silicon Nitrides and Oxides from Trihydridosilanes

MOCVD of trihydridosilanes provides a route for generating self-assembled monolayers on metal substrates. Under mild conditions, trihydridosilanes interact with a variety of clean, hydrogenated and fresh metal and metalloid surfaces, including titanium, silicon and gold.1 In contrast, monohydridosilanes appear to have minimal interaction. All classes of hydridosilanes have minimal interaction with anhydrous oxide surfaces. A proposed mechanism for the initial steps of deposition involves the dissociative adsorption of the silanes with the formation of hydrogen, followed by topmost atom layer insertion and concomitant surface reconstruction. New alpha, omega bis(trihydrido)silane precursors are also described which offer the ability for dipodal substrate interaction. Alpha, omega trihydridosilane terminated oligopermethylsilanes with the potential to form molecular wires are presented.2 1 B. Arkles, Y.Pan, Y. Kim, E. Eisenbraun, C. Miller, A. Kaloyeros, J. Adhesion Science and Technology, 2014, 2012, 41. 2 B. Arkles, Y. Pan, G. Larson, US Pat. 8,575,381, 2013.

Soft-landing electrospray ion beam deposition of sensitive oligoynes on surfaces in vacuum

Advances in synthetic chemistry permit the synthesis of large, highly functional, organic molecules. Characterizing the complex structure of such molecules with highly resolving, vacuum-based methods like scanning probe microscopy requires their transfer into the gas phase and further onto an atomically clean surface in ultrahigh vacuum without causing additional contamination. Conventionally this is done via sublimation in vacuum. However, similar to biological molecules, large synthetic compounds can be non-volatile and decompose upon heating. Soft-landing ion beam deposition using soft ionization methods represents an alternative approach to vacuum deposition. Using different oligoyne derivatives of the form of R1(CC)nR2, here we demonstrate that even sensitive molecules can be handled by soft-landing electrospray ion beam deposition. We generate intact molecular ions as well as fragment ions with intact hexayne parts and deposit them on clean metal surfaces. Scanning tunneling microscopy shows that the reactive hexayne segments of the molecules of six conjugated triple bonds are intact. The molecules agglomerate into ribbon-like islands, whose internal structure can be steered by the choice of the substituents. Our results suggest the use of ion beam deposition to arrange reactive precursors for subsequent polymerization reactions.

Does local disorder influence secondary ion formation?

The ionization probabilities of particles sputtered from a clean metallic single crystal surface bombarded under self‐sputtering conditions (i.e. with projectiles of the same atomic species as the target material) are calculated using a hybrid computer simulation model based on a combination of molecular dynamics and excitation dynamics. The simulations reveal an apparent correlation between the ionization probability of a sputtered particle and the local lattice disorder at the point in space and time when it is being emitted from the surface. By examining cross correlations between emission time, local order and the local surface electron temperature, however, we find that particles exhibiting the highest ionization probability are being emitted in an early stage of the collision cascade, where the surface is still practically intact. Atoms emitted at later stages of the cascade can in principle benefit from an effective excitation energy confinement induced by the local disorder; the resulting ionization probability, however, is too low to significantly contribute to secondary ion formation. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of plasmon energetics on light emission induced by scanning tunneling microscopy

A theoretical model of plasmon and molecular luminescence induced by scanning tunneling microscopy using a molecule-covered tip on clean metal surfaces is developed. The effects of coupling between molecular exciton and interface plasmon on the luminescence spectra are investigated for variable energy of plasmon modes by means of the nonequilibrium Green's function method. It is found that spectral features arising from interference between the processes of energy absorption by the molecule and interface plasmons appear near the energy of the excitonic mode. For the energy of plasmon above (below) the energy of excitonic mode, an additional peak structure appears in the energy range slightly below (above) the energy of the excitonic mode. Prominent peak and dip structures observed in recent luminescence experiments are interpreted by the developed theory whereby its utility in the fields of plasmonics and nanophotonics is demonstrated.

金属新生面によるグリースの分解と水素発生挙動に及ぼす基油と添加剤の影響（第2報）異なる切削時間で求めた発生量比の比較

金属新生面によるグリースの分解と水素発生挙動に及ぼす基油と添加剤の影響（第2報）異なる切削時間で求めた発生量比の比較

Effects of Interference between Energy Absorption Processes of Molecule and Surface Plasmons on Light Emission Induced by Scanning Tunneling Microscopy

A theoretical analysis of plasmon and molecular luminescence induced by scanning tunneling microscopy is carried out in the case of luminescence from clean metal surfaces using a molecule-covered tip and from molecular layers. The effects of coupling between molecular exciton and surface plasmon (exciton–plasmon coupling) on the luminescence properties of the molecule and surface plasmons are investigated using the nonequilibrium Green's function method. The enhancement by molecular electronic and vibrational modes (molecular modes) and the energy absorption by the creation of molecular excitons lead to peak and dip structures in the luminescence spectra of surface plasmons. The re-emission of energy in surface plasmons by exciton annihilation leads to a dent structure in their luminescence spectra. It is found that the processes of energy absorption by the creation of molecular excitons and the subsequent re-emission by exciton annihilation interfere with each other, resulting in the enhancement and suppression of these energy-transfer processes. Owing to exciton–plasmon coupling, excitation channels of the molecule appear in the energy range lower than the energy of molecular exciton. It is found that electronic transitions via these excitation channels give rise to molecular luminescence and vibrational excitations at a bias voltage lower than the ratio of the energy of molecular exciton to the elementary charge. The results also indicate that vibrational excitations assist in the emission of photons whose energy exceeds the product of the elementary charge and bias voltage (upconverted luminescence). The trends in the luminescence spectra predicted by the developed model are found to be in good agreement with the experimental data reported recently.

Insight into the general rule for the activation of the X–H bonds (X = C, N, O, S) induced by chemisorbed oxygen atoms

Density functional theory calculations are presented for adsorption and dissociation of NH3, H2O, CH3OH, H2S and C2H4 on clean and oxygen atom pre-adsorbed metal surfaces (Cu, Ag, Au, Ni, Pd, Pt, Rh, Ru, Os and Ir). The calculation results indicated that the oxygen-promotion effect depends both on the metallic activity and the character of the X-H bond. On the one hand, for a given reaction on a series metals, a good linear correlation was found between the energy barrier difference of X-H bond breaking on clean and oxygen-covered metals and the binding strength of oxygen on metals, namely an oxygen-promotion effect was favorable to the less active metals but unfavorable to the more active metals. On the other hand, for a series of X-H bond breaking reactions on a given metal, it was found that the promotion effect follows the trend of O-H > N-H > C-H, that is, the O-H bond is most promoted by the oxygen atom. The possible reason is the O-H bond forms the strongest hydrogen bond in the transition state among the X-H bonds investigated in this work. Additionally, it was found that the oxygen coverage has little effect on the X-H bond scission.

Sign-inverted response of aluminum work function to tangential strain

We have investigated the response of the work function, W, of low-index aluminum surfaces to tangential strain by using first-principles calculations based on density functional theory. This response parameter is a central quantity in electrocapillary coupling of metal electrodes relating to the performance of porous metal actuators and surface stress based sensing devices. We find that Al surfaces exhibit a positive response for all orientations considered. By contrast, previous studies reported negative-valued response parameters for clean surfaces of several transition metals. We discuss separately the response of W to different types of strain and the impact of the strain on the Fermi energy and the surface dipole. We argue that the reason for the abnormal positive sign of the Al response parameter lies in its high valence electron density.

The interaction of H2O with the surface of polycrystalline gadolinium at the temperature range of 300–570 K

The interaction of water vapor with polycrystalline gadolinium surface, in the temperature range of 300–570K and water vapor pressure from 2×10−8 and up to 18Torr, was studied by utilizing Direct-Recoil-Spectrometry, X-ray Photoelectron Spectroscopy and Temperature Programmed Desorption. It has been found that a native Gd surface compound as well as one that was formed by an exposure to 18Torr H2O, for 10min, consists of a multilayer hydroxide phase, Gd(OH)3, transforming into oxide at the metal interface by heating, while emitting hydrogen. For initial exposures of water vapor on a clean metallic surface, it has been deduced that initially, full dissociation of the water molecules on the metal surface occurs, for ~1–2L at 300K and up to ~3L at 570K. On top of the formed oxide there is dissociation to H+OH and hydroxyls are adsorbed, forming, with further exposure, a multilayer hydroxide phase. Enhanced inward diffusion of oxygen starts as early as 370K (similar to the case of oxygen exposure).

Minimizing electrostatic charging of an aperture used to produce in-focus phase contrast in the TEM

Microfabricated devices designed to provide phase contrast in the transmission electron microscope must be free of phase distortions caused by unexpected electrostatic effects. We find that such phase distortions occur even when a device is heated to 300 °C during use in order to avoid the formation of polymerized, carbonaceous contamination. Remaining factors that could cause unwanted phase distortions include patchy variations in the work function of a clean metal surface, radiation-induced formation of a localized oxide layer, and creation of a contact potential between an irradiated area and the surround due to radiation-induced structural changes. We show that coating a microfabricated device with evaporated carbon apparently eliminates the problem of patchy variation in the work function. Furthermore, we show that a carbon-coated titanium device is superior to a carbon-coated gold device, with respect to radiation-induced electrostatic effects. A carbon-coated, hybrid double-sideband/single-sideband aperture is used to record in-focus, cryo-EM images of monolayer crystals of streptavidin. Images showing no systematic phase error due to charging are achievable under conditions of low-dose data collection. The contrast in such in-focus images is sufficient that one can readily see individual streptavidin tetramer molecules. Nevertheless, these carbon-coated devices perform well for only a limited length of time, and the cause of failure is not yet understood.

Photodetachment of H− near a Dielectric-Covered Metal Surface

We combine a simple model potential with closed-orbit theory to study the photodetachment of H � near a dielectric-covered metal surface. We calculate photodetachment cross sections to show that the chem- isorption of a dielectric thin layer on the metal surface can significantly affect the photodetachment of negative ions. Compared to the photodetachment of hydrogen negative ions near clean metallic surfaces, our calculated cross sections show stronger oscillations, the amplitude of the oscillation growing with the layer thickness. For fixed thickness, the amplitude depends on the dielectric constant and on the metallic surface. We expect our study to guide future experimental studies of negative- ion photodetachment from dielectric-covered metallic surfaces.

Magnetic properties of Fe and Fe-Si alloys with {100}〈0vw〉 texture

When iron and its alloy sheets with clean metal surfaces undergo the γ to α phase transformation, they develop strong {100}〈0vw〉 texture with grain size being larger than the sheet thickness. For example, when Fe or Fe-1%Si sheets were subjected to the γ to α phase transformation in a reducing gas atmosphere (hydrogen gas having the dew point below −50 °C), strong {100}〈0vw〉 texture developed. Magnetic properties of Fe and Fe-Si alloys show that, by developing the {100}〈0vw〉 texture, the core loss can be reduced by more than 25% and the permeability can be increased by 2–5 times. With 0.35 mm-thick Fe-1%Si with the {100}〈0vw〉 texture, the magnetic properties are W15/50 (core loss at 1.5 T, 50 Hz) = 2.7 W/kg and B50 (magnetic flux density at 5000 A/m) = 1.80 T. The improvement of permeability together with reducing iron loss by texture control will make a significant contribution to improving power density as well as reducing copper losses in induction motors.

A New Approach to Surface Tension According to the Surface Topographical Features in Laser Cutting Technology

Laser - cut quality is mainly characterized by a degree of accuracy in shape, size and also by surface layer conditions after cutting associated with surface roughness. An experimental determination of surface tension (or tensor components) of clean metal surfaces is very difficult and there is no direct method for its measurement. Attention was paid to numerical derivation of surface tensions according to the surface topographical features in laser cutting technology. The surface tensions and temperature dependencies of several metallic materials have been determined and confirmed by data obtained from the literature. It was found to be in very good agreement between our results and data from different sources in the literature.

Easy-axis ferromagnetic chain on a metallic surface

The phases and excitation spectrum of an easy-axis ferromagnetic chain of S = 1/2 magnetic impurities built on the top of a clean metallic surface are studied. As a function of the (Kondo) coupling to the metallic surface and at low temperatures, the spin chain exhibits a quantum phase transition from an Ising ferromagnetic phase with long-range order to a paramagnetic phase where quantum fluctuations destroy the magnetic order. In the paramagnetic phase, the system consists of a chain of Kondo singlets where the impurities are completely screened by the metallic host. In the ferromagnetic phase, the excitations above the Ising gap are damped magnons, with a finite lifetime arising due to the coupling to the substrate. We discuss the experimental consequences of our results to spin-polarized electron energy loss spectroscopy, and we finally analyze possible extensions to spin chains with S > 1/2.

A microscopic view of secondary ion formation

The formation of secondary ions in sputtering is described by combining classical molecular dynamics of the particle kinetics with simple analytical treatments modeling the transfer of kinetic into electronic excitation energy, the transport of excitation away from the point of its generation and the charge transfer between the solid and a sputtered particle. For the simplest case of a metal atom sputtered from a clean metal surface, the predictions of such a model are used to answer a few fundamental questions regarding the ion formation process. The results indicate that the transient local excitation of the bombarded solid plays a dominant role in determining the charge state of a sputtered atom. Moreover, we find that the assumption of a sputtered particle being emitted from an ideal, undisturbed surface with a constant emission velocity – a picture which forms the physical basis of nearly all published secondary ion formation models – is not generally justified.