Bulk Metal Surfaces Research Articles

ChemInform Abstract: Surface Organic Chemistry for Application to Organic Electronics

AbstractReview: [56 refs.

Bulk metallic glassy surface native oxide: Its atomic structure, growth rate and electrical properties

Formation of a native oxide layer on the surface of bulk metallic glasses (BMGs) influences significantly the nanoscale tribological properties and mechanical behavior of the BMGs used in nanodevices. However, our knowledge of the native oxidation process on the BMG surface and structure of the corresponding oxides remains limited because the oxide layer is very thin. Here we conducted a combined state-of-the-art experimental technique study of the atomic structure, oxidations states and electrical conductivity of the native surface oxides on a CuZrAl BMG formed at ambient conditions by aberration-corrected scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (AFM). This allowed shedding light on the atomic structure, metal oxidation state, growth behavior and nanoscale electrical properties of the surface oxide. The conductive AFM measurements reveal that the electrical conductivity of the native oxide layer transits from the initially metallic to a nonlinear one after some air exposure, and finally changes to insulative state. These findings represent a significant step forward in the knowledge of surface oxides and open up the possibility of fabricating nanoscale electrical devices based on BMGs with controllable conductivity.

A Roadmap for Controlled Production of Topological Insulator Nanostructures and Thin Films.

The group V-VI chalcogenide semiconductors (Bi2 Se3 , Bi2 Te3 , and Sb2 Te3 ) have long been known as thermoelectric materials. Recently, they have been once more generating interest because Bi2 Se3 , Bi2 Te3 and Sb2 Te3 have been crowned as 3D topological insulators (TIs), which have insulating bulk gaps and metallic Dirac surface states. One big challenge in the study of TIs is the lack of high-quality materials with few defects and insulating bulk states. To manifest the topological surface states, it is critical to suppress the contribution from the bulk carriers. Controlled production of TI nanostructures that have a large surface-to-volume ratio is an efficient way to reduce the bulk conductance and to significantly enhance the topological surface conduction. In this review article, the recent progress on the preparation of TI nanostructures is highlighted. Basic production methods for TI nanostructures are introduced in detail. Furthermore, several specific production approaches to reduce the residual bulk carriers from defects are summarized. Finally, the progress and the prospects of the production of TI-based heterostructures, which hold promise in both fundamental study and novel applications are discussed.

Tunable Dirac Fermion Dynamics in Topological Insulators

Three-dimensional topological insulators are characterized by insulating bulk state and metallic surface state involving relativistic Dirac fermions which are responsible for exotic quantum phenomena and potential applications in spintronics and quantum computations. It is essential to understand how the Dirac fermions interact with other electrons, phonons and disorders. Here we report super-high resolution angle-resolved photoemission studies on the Dirac fermion dynamics in the prototypical Bi2(Te,Se)3 topological insulators. We have directly revealed signatures of the electron-phonon coupling and found that the electron-disorder interaction dominates the scattering process. The Dirac fermion dynamics in Bi2(Te3−xSex) topological insulators can be tuned by varying the composition, x, or by controlling the charge carriers. Our findings provide crucial information in understanding and engineering the electron dynamics of the Dirac fermions for fundamental studies and potential applications.

Optical response of plasmonic relief meta-surfaces

Bulk metal surfaces patterned with arrays of sub-wavelength surface features can couple incident light into localized plasmon modes, thereby modifying the intensity and phase of reflected light. Beyond previously reported applications to colour control, we report on the functionality of continuously metallic meta-surfaces for optical magnetic reflection, perfect absorption, and active photonic switching/sensing.

Synthesis and characterization of carbon nanotubes over iron carbide nanoparticles coated Al powder using thermal chemical vapor deposition

A simple method is described to synthesize carbon nanotubes (CNTs) by the thermal decomposition of toluene at 750°C over a thin catalyst film deposited on Al powder. This method allows the bulk metal surface to act as both the catalyst and support for CNT growth. The catalyst film on Al was prepared from an ethanol solution of iron nitrate. Under the growth conditions, iron nitrate formed an amorphous iron oxide layer that transform into crystalline Fe2O3, which was further reduced to Fe3O4 and Fe3C. It is believed that the growth of CNTs took place on iron carbide nanoparticles that were formed from FeO. The characterization of CNTs was mainly carried out by powder X-ray diffraction and scanning electron microscopy, X-ray fluorescence and thermogravimatric analysis. The CNTs were found to be highly dispersed in Al powder. This composite powder could be further used for the fabrication of Al matrix composites using powder metallurgy process in which the powder were first cold pressed at 500–550 MPa followed by sintering at 620°C for 2 h under a vacuum of 10−2 torr. The mechanical properties of the sintered composites were measured using a microhardness tester and a Universal testing Instron machine.

Band structure engineering in (Bi1−xSbx)2Te3 ternary topological insulators

Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is to establish the existence of significant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi(1-x)Sb(x))(2)Te(3) ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi(1-x)Sb(x))(2)Te(3), indicating the robustness of bulk Z(2) topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behaves like one-quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on well-established semiconductor technology.

Influence of water on the properties of an Au/Mpy/Pd metal/molecule/metal junction

The geometric and electronic structure of the metal–molecule interface in metal/molecule/metal junctions is of great interest since it affects the functionality of such units in possible nanoelectronic devices. We have investigated the interaction between water and a palladium monolayer of a Au(111)/4-mercaptopyridine/Pd junction by means of DFT calculations. A relatively strong bond between water and the palladium monolayer of the Au/Mpy/Pd complex is observed via a one-fold bond between the oxygen atom of the water molecule and a Pd atom. An isolated H2O molecule adsorbs preferentially in a flat-lying geometry on top of a palladium atom that is at the same time also bound to the nitrogen atom of a Mpy molecule of the underlying self-assembled monolayer. The electronic structure of these Pd atoms is considerably modified which is reflected in a reduced local density of states at the Fermi energy. At higher coverages, water can be arranged in a hexagonal ice-like bilayer structure in analogy to water on bulk metal surfaces, but with a much stronger binding which is dominated by O–Pd bonds.

Application of a liquid crystal spatial light modulator to laser marking

Laser marking is demonstrated using a nanosecond (ns) pulse duration laser in combination with a liquid crystal spatial light modulator to generate two-dimensional patterns directly onto thin films and bulk metal surfaces. Previous demonstrations of laser marking with such devices have been limited to low average power lasers. Application in the ns regime enables more complex, larger scale marks to be generated with more widely available and industrially proven laser systems. The dynamic nature of the device is utilized to improve mark quality by reducing the impact of the inherently speckled intensity distribution across the generated image and reduce thermal effects in the marked surface.

Few-Layer Nanoplates of Bi2Se3 and Bi2Te3 with Highly Tunable Chemical Potential

A topological insulator (TI) represents an unconventional quantum phase of matter with insulating bulk band gap and metallic surface states. Recent theoretical calculations and photoemission spectroscopy measurements show that group V-VI materials Bi(2)Se(3), Bi(2)Te(3), and Sb(2)Te(3) are TIs with a single Dirac cone on the surface. These materials have anisotropic, layered structures, in which five atomic layers are covalently bonded to form a quintuple layer, and quintuple layers interact weakly through van der Waals interaction to form the crystal. A few quintuple layers of these materials are predicted to exhibit interesting surface properties. Different from our previous nanoribbon study, here we report the synthesis and characterizations of ultrathin Bi(2)Te(3) and Bi(2)Se(3) nanoplates with thickness down to 3 nm (3 quintuple layers), via catalyst-free vapor-solid (VS) growth mechanism. Optical images reveal thickness-dependent color and contrast for nanoplates grown on oxidized silicon (300 nm SiO(2)/Si). As a new member of TI nanomaterials, ultrathin TI nanoplates have an extremely large surface-to-volume ratio and can be electrically gated more effectively than the bulk form, potentially enhancing surface state effects in transport measurements. Low-temperature transport measurements of a single nanoplate device, with a high-k dielectric top gate, show decrease in carrier concentration by several times and large tuning of chemical potential.

Low-temperature behavior of graphite–tin composite anodes for Li-ion batteries

The challenge of increasing low-temperature performances of anodes for Li-ion batteries is faced by preparing graphite–tin composite electrodes. The anodes are prepared by mixing partially oxidized graphite with nanometric Sn powder or by coating the oxidized graphite electrode with a thin Sn layer. Long-term cycling stability and intercalation/deintercalation performances of the composite anodes in the temperature range 20 °C to −30 °C are evaluated. Kinetics is investigated by cyclic voltammetry and electrochemical impedance spectroscopy, in the attempt to explain the role of Sn in reducing the overall electrode polarization at low temperature. Two possible mechanisms of action for bulk metal powder and surface metal layer are proposed.

Facile conversion of bulk metal surface to metal oxide single-crystalline nanostructures by microwave irradiation: Formation of pure or Cr-doped hematite nanostructure arrays

We report a method for converting the surfaces of bulk metal substrates (pure iron or stainless steel) to metal oxide (hematite or Cr-doped hematite) nanostructures using microwave irradiation. When microwave radiation (2.45 GHz, single-mode) was applied to a metal substrate under the flow of a gas mixture containing O 2 and Ar, metal oxide nanostructures formed and entirely covered the substrate. The nanostructures were single crystalline, and the atomic ratios of the substrate metals were preserved in the nanostructures. When a pure iron sheet was used as a substrate, hematite nanowires (1000 W microwave radiation) or nanosheets (1800 W microwave radiation) formed on the surface of the substrate. When a SUS410 sheet was used as a substrate, slightly curved rod-like nanostructures were synthesized. The oxidation states of Fe and Cr in these nanorods were Fe 3+ and Cr 3+. Quantitative analyses revealed an average Fe/Cr atomic ratio of 9.2, nearly identical to the ratio of the metals in the SUS410 substrate.

Imaging of Metal Ion Dissolution and Electrodeposition by Anodic Stripping Voltammetry−Scanning Electrochemical Microscopy

We have developed a new imaging method for scanning electrochemical microscopy (SECM) employing fast-scan anodic stripping voltammetry (ASV) to provide sensitive and selective imaging of multiple chemical species at interfaces immersed in solution. A rapid cyclic voltammetry scan (100 V/s) is used along with a short preconcentration time (300-750 ms) to allow images to be acquired in a normal SECM time frame. A Hg-Pt film electrode is developed having an equivalent Hg thickness of 40 nm that has good sensitivity at short preconcentration times and also retains thin-film behavior with high-speed voltammetric stripping. Fast-scan anodic stripping currents are shown to be linear for 1-100 microM of Pb (2+) and Cd (2+) solutions using a preconcentration time of 300 ms. SECM images showing the presence of Pb (2+) and Cd (2+) at concentrations as low as 1 microM are presented. In addition, a single ASV-SECM image is shown to produce unique concentration maps indicating Cd (2+) and Pb (2+), generated in situ from a corroding sample, while simultaneously detecting the depletion of O 2 at this sample. The transient voltammetric response at the film electrode is simulated and shows good agreement with the experimental behavior. We discuss the behavior of images and concentration profiles obtained with different imaging conditions and show that mass-transport limitations in the tip-substrate gap can induce dissolution. ASV-SECM can thus be used to detect and study induced dissolution not only at bulk metal surfaces but also on underpotential deposition layers, in this case Cd and Pb on Pt. In addition, we discuss how surface diffusion phenomena may relate to the observed ASV-SECM behavior.

The entrapment of corrosion products from CoCr implant alloys in the deposits of calcium phosphate: A comparison of serum, synovial fluid, albumin, EDTA, and water

Physical wear of orthopedic implants is inevitable. CoCr alloy samples, typically used in joint reconstruction, corrode rapidly after removal of the protective oxide layer. The behavior of CoCr pellets immersed in human serum, foetal bovine serum (FBS), synovial fluid, albumin in phosphate-buffered saline (PBS), EDTA in PBS, and water were studied using X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). The difference in the corrosive nature of human serum, water, albumin in PBS and synovial fluid after 5 days of immersion was highlighted by the oxide layer, which was respectively 15, 3.5, 1.5, and 1.5 nm thick. The thickness of an additional calcium phosphate deposit from human serum and synovial fluid was 40 and 2 nm, respectively. Co and Cr ions migrated from the bulk metal surface and were trapped in this deposit by the phosphate anion. This may account for the composition of wear debris from CoCr orthopedic implants, which is known to consist predominantly of hydroxy-phosphate compounds. Known components of synovial fluid including proteoglycans, pyrophosphates, phospholipids, lubricin, and superficial zone protein (SZP), have been identified as possible causes for the lack of significant calcium phosphate deposition in this environment. Circulation of these compounds around the whole implant may inhibit calcium phosphate deposition.

The effects of calcium phosphate deposition upon corrosion of CoCr alloys and the potential for implant failure

Physical wear of orthopedic implants is inevitable. CoCr metal samples, typically used in joint reconstruction, corrode rapidly after removal of the protective oxide layer. The behavior of CoCr pellets immersed in human serum, fetal bovine serum (FBS), synovial fluid, and water were studied using time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The differences in the corrosive nature of human serum, FBS, synovial fluid, and water after 5 days immersion were highlighted by the oxide/hydroxide layer, which was, respectively, 25, 10, 1.5, and 3-3.5 nm thick. The thickness of calcium phosphate deposit from human serum, FBS, and synovial fluid was, respectively, 30, 20, and 2 nm. Co and Cr ions migrated from the bulk metal surface and were trapped in the serum deposits, where chromium existed as oxides, hydroxides, and phosphates, whereas the cobalt chemistry was dominated only by phosphates and hydroxides. This may account for the composition of wear debris from CoCr orthopedic implants, which are predominantly hydroxyphosphate compounds. From the literature, proteoglycans, pyrophosphates, phospholipids, lubricin, and superficial zone protein (SZP) have been identified as possible causes for the insignificant deposit of calcium phosphate from synovial fluid. Circulation of these compounds around the whole implant may inhibit calcium phosphate deposition and therefore contribute to osteolysis.

Observation of dendritic growth on Li powder anode using optical cell

Dendritic growth on Li bulk metal and powder electrode surfaces was observed during a charge–discharge cycle. Cell cycling was performed with high current density (1 mA cm −2). Using the ‘optical cell’, it is observed that Li powder electrodes suppress the growth of dendrite at the electrode surface. The potential profile for dendritic growth also well correlates with the optical observations. The use of powder electrodes having an LiF and Li 2CO 3 surface layer seem to cause a further reduction in dendritic growth during charge/discharge cycles. The increased surface area of the powders, as well as the artificially controlled SEI film, which uniformly covered the electrode surface, may reduce the dendritic growth.

Photoionization threshold shapes of metal clusters

We demonstrate the efficacy of the Fowler equation, derived for the near-threshold photoelectron yield of bulk metal surfaces, for fitting the ionization spectra of finite metal clusters. The known photoionization curves of KN=30–101 are matched to a Fowler plot. The fitted ionization potentials agree well with the previously determined values and extrapolate even more accurately to the bulk work function; the fitted temperature closely matches the prediction of evaporation ensemble theory.

Photoinduced charge separation reactions of J-aggregates coated on silver nanoparticles.

The photochemistry of cyanine J-aggregates on the surface of colloidal Ag nanoparticles is reported. The photochemistry is initiated through ultrafast photoexcitation of the plasmon band in Ag nanoparticles, producing an enhanced near-field that interacts with the J-aggregate monolayer. Through transient absorption spectroscopy, we show that photoexcitation of the plasmon in Ag nanoparticles leads to exciton dynamics that differ strongly from J-aggregates alone or for J-aggregate monolayers on bulk metal surfaces. Specifically, charge-separated states with a lifetime of approximately 300 ps between the J-aggregate and Ag colloid are formed. The reduction of the Ag nanoparticles is shown to be a multielectron process.

Temperature Effects in Transition Metal Ion and Cluster Ion Reactions

While reactions on the surface of bulk metal proceed in general under well-defined and experimentally easily measurable temperature and other conditions, this is usually not the case for reactions of metal atoms or clusters and their ions in high vacuum. A collision complex of the metal cluster with a polyatomic molecule (e.g., benzene) may be transiently stabilized by redistribution of the interaction energy, IVR. This energy redistribution results in heating of the collision complex to internal temperatures, which can be estimated to be often very high, but the data for its exact determination are usually not available. Semiquantitatively, the heat capacity of the complex will increase with the cluster size, and the temperature rise upon complex formation will correspondingly decrease. Such size-dependent heating effects are readily observable in FT-ICR studies of anionic and cationic cluster reactions, and when interpreting the data or trying to draw from it conclusions for bulk condensed phase reactions, they have to be taken into consideration. Conversely, the effect of such temperature rise in mass spectrometric studies can be decreased or eliminated, and the reactions of truly cold clusters can be investigated by “soft landing” the reactive molecule on a solvated cluster and exchanging it for inert ligands, e.g., argon atoms.

Cover Picture

The cover picture shows the computed valence electronic structure of the (100) plane of face-centered-cubic metallic lithium in terms of the electron localization function (ELF). Electron localization increases from blue to green to yellow to orange. The geometries of some selected Li clusters are also depicted; counterclockwise from the top these are Li6 (D3h), Li6 (D4h), Li8 (Cs), and Li8 (Td). The evolution of the electronic structure and multicenter bonding of elemental lithium ranging from only a few atoms all the way to the bulk metal and corresponding surfaces is analyzed in detail by R. Rousseau and D. Marx on p. 2982 ff.