Flat Pb Research Articles

Leaded Bronze Foams As Innovative Electrodes for Organic Electrosynthesis

The electrification of organic synthesis represents an outstanding green alternative to prepare valuable and fine chemicals. However, the design of electrode materials for organic electrosynthesis remains as an open field to investigate. Motivated by the superior performance of leaded bronzes as electrodes for reductive transformations,[1, 2, 3] we have developed highly porous binary and ternary leaded metal foams through the dynamic hydrogen bubble template method [4] and tested them as cathodes in the electro-organic synthesis of 2,6-dichlorobenzonitrile from 2,6-dichlorobenzaldoxime.[5]The electrocatalytic performance of the novel leaded foams was compared to the one of the planar CuSn7Pb15 leaded bronze. The novel porous leaded foams outperformed the CuSn7Pb15 electrodes in terms of chemical yield and energy efficiency. Furthermore, due to their highly crystalline surface, these materials are mechanically more stable and less prone to cathodic corrosion than flat Pb and CuSn7Pb15 electrodes.

Domain Boundary Formation Within an Intercalated Pb Monolayer Featuring Charge‐Neutral Epitaxial Graphene

AbstractThe synthesis of new graphene‐based quantum materials by intercalation is an auspicious approach. However, an accompanying proximity coupling depends crucially on the structural details of the new heterostructure. It is studied in detail the Pb monolayer structure after intercalation into the graphene buffer layer on the SiC(0001) interface by means of photoelectron spectroscopy, x‐ray standing waves, and scanning tunneling microscopy. A coherent fraction close to unity proves the formation of a flat Pb monolayer on the SiC surface. An interlayer distance of 3.67 Å to the suspended graphene underlines the formation of a truly van der Waals heterostructure. The 2D Pb layer reveals a quasi ten‐fold periodicity due to the formation of a grain boundary network, ensuring the saturation of the Si surface bonds. Moreover, the densely‐packed Pb layer also efficiently minimizes the doping influence by the SiC substrate, both from the surface dangling bonds and the SiC surface polarization, giving rise to charge‐neutral monolayer graphene. The observation of a long‐ranged () reconstruction on the graphene lattice at tunneling conditions close to Fermi energy is most likely a result of a nesting condition to be perfectly fulfilled.

Epitaxial Pb on InAs nanowires for quantum devices.

Semiconductor-superconductor hybrids are widely used to realize complex quantum phenomena, such as topological superconductivity and spins coupled to Cooper pairs. Accessing new, exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. One challenge is the generation of favourable conditions for heterostructural formation between materials with the desired properties. Here we harness an increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single-crystal, atomically flat Pb films with no axial grain boundaries. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, and therefore they offer a parameter space more than twice as large as those of alternative semiconductor-superconductor hybrids. Additionally, InAs/Pb island devices exhibit magnetic field-driven transitions from a Cooper pair to single-electron charging, a prerequisite for use in topological quantum computation. Semiconductor-Pb hybrids potentially enable access to entirely new regimes for a number of different quantum systems.

Designing xenes with two-dimensional triangular lattice

Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence spin-orbit coupling (SOC). The introduction of an out-of-plane buckling creates a glide mirror, protecting an anisotropic Dirac nodal loop. Both flat and buckled Pb monolayers become topologically trivial after including SOC. A large buckling will make the Pb sheet a 2D semiconductor with symmetry-protected Dirac points below the Fermi level. The electronic structures of other group-IV triangular lattices such as Ge and Sn demonstrate strong similarity to Pb. We further design a quasi-3D crystal PbHfO$_2$ by alternately stacking Pb and 1T-HfO$_2$ monolayers. The new compound PbHfO$_2$ is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to PbHfO$_2$, it is possible to drive an insulator-to-metal transition coupled with an anti-ferroelectric-to-paraelectric phase transition. Our results suggest the potential of the 2D triangular lattice as a complimentary platform to design new type of broadly-defined Xenes.

Quantum-well and modified image-potential states in thin Pb(111) films: an estimate for the local work function

Quantum-confined electronic states such as quantum-well states (QWS) inside thin Pb(111) films and modified image-potential states (IPS) above the Pb(111) films grown on Si(111) 7 × 7 substrate were studied by means of low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS) in the regime of constant current I. By plotting the position of the nth emission resonances Un versus n2/3 and extrapolating the linear fit for the dependence Un(n2/3) in the high-n limit towards n = 0, we estimate the local work function for the Pb(111) film: W ≃ 3.8 ± 0.1 eV. We experimentally demonstrate that modifications of the shape of the STM tip can change the number of the emission peaks associated with the resonant tunnelling via quantized IPS levels for the same Pb terrace; however it does not affect the estimate of the local work function for the flat Pb terraces. We observe that the maxima in the spectra of the differential tunnelling conductance dI/dU related to both the QWS and the modified IPS resonances are less pronounced if the STM tip becomes more blunt.

Dynamical Coulomb Blockade Observed in Nanosized Electrical Contacts

Electrical contacts between nanoengineered systems are expected to constitute the basic building blocks of future nanoscale electronics. However, the accurate characterization and understanding of electrical contacts at the nanoscale is an experimentally challenging task. Here, we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nanocontacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island-substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements.

Semiconductor-superconductor transition and magnetoresistance terraces in an ultrathin superconducting Pb nanobridge

Using focused ion beam etching technique, the authors fabricated a 28 atomic monolayers thick, 500 nm wide, and 10 μm long Pb nanobridge from an atomically flat Pb thin film grown on Si by molecular beam epitaxy. Electric transport measurements show exotic resistance oscillations in the superconducting state far below its critical field HC and cascading terraces near the superconducting transition region. Furthermore, the bridge shows an unusual semiconducting behavior above the superconducting transition temperature TC. The results are in contrast to those observed in its counterpart of the two-dimensional thin film.

A pathway for self-assembly of metallic nanostructures on quantum-modulated flat Pb(111)/Si(111) substrate

A mechanism of self-assembly of metallic nanostructures on a quantum-modulated flat Pb(111) thin film with patterned Si(111) substrate is proposed based on recent experimental observations [S. M. Binz, M. Hupalo, and M. C. Tringides, Phys. Rev. B 78, 193407 (2008)] which indicates that because of quantum size effects (QSE), the buried steps act as real steps on surfaces. This intriguing feature offers a potential pathway for self-assembly of functional metallic nanostructures, e.g., nanowires on flat Pb(111) films with designed patterned Si(111) surface as substrate, where QSE can be controlled artificially. The growth conditions for nanowires on a designed Pb(111)/Si(111) substrate is explored by kinetic Monte Carlo simulations.

Inside Front Cover: A Quantum‐Stabilized Mirror for Atoms (Adv. Mater. 18/2008)

The inside cover shows “the smoothest surface ever made”, a quantum-stabilized, atomically flat Pb film on Si(111), reflecting a He beam. Rodolfo Miranda, Amadeo L. Vazquez de Parga, and co-workers report on p. 3492 that this helium mirror can also be electrostatically bent to focus the He beam into a sub-micrometer spot, implying that current prototype scanning helium atom microscopy/diffractometry instruments could easily reach nanometer resolution.

Negative magnetoresistance in fractal Pb thin films on Si(111)

Using a low temperature method, the authors have prepared atomically flat Pb ultrathin films on Si(111)-7×7 surface. Room temperature annealing of the films results in a percolation morphology with fractal vacancy islands where the Si substrate is exposed. The fractal film with a nominal thickness of 23 ML exhibits enhanced onset superconducting transition temperature of 7.0K and negative magnetoresistance with wide magnetoresistance terrace under perpendicular magnetic field when the film is in superconducting state. They attribute the phenomena to the coexistence of two superconducting phases in this fractal film.

Oscillatory thermal expansion of Pb thin films modulated by quantum size effects

Varied temperature photoemission study is performed to investigate the quantum size effects on the thermal property of atomically flat Pb films grown on Si(111). The binding energies of the quantum well states for the films with thicknesses from 10 to 24 ML exhibit a linear increase with increasing temperature from 75to270K. Under free electron approximation, thermal expansion coefficients of the thin films are determined, which manifest a large enhancement and oscillation behavior. The large enhancement is interpreted by a model based on the quantum confinement along the film normal direction. The oscillation is shown to be closely related to the structural instability and is a result of the formation of the quantized electronic states in thin films.

Quantum oscillations in Pb/Si (111) heterostructure system

This paper summarizes our recent work on the study of quantum size effects (QSE) and novel physical properties of the Pb/Si (111) heterostructure. Two different types of samples were investigated. One is wedge-shaped Pb islands, and the other is atomically flat Pb thin films. With scanning tunneling microscopy (STM) manipulation, we observed an intriguing morphology dynamics of the islands that swings between two extreme energy states, like that in a classical pendulum. We show that the dynamics is a result of the competition between the QSE and the classical step free energy minimizing effect. For the second type of the samples, the QSE is studied in terms of thickness-dependent film stability, electronic structure and physical properties by using STM, angle-resolved photoemission spectroscopy (ARPES) and transport measurement. The results consistently reveal the formation of quantum well states (QWS) due to electron confinement in the films. This size effect could greatly modify the electronic structure near the Fermi level and lead to quantum oscillations in superconductivity, electron-phonon coupling and thermal expansion. The work unambiguously demonstrates the possibility of quantum engineering of physical properties of thin films by exploiting well-controlled and thickness-dependent QSE.

Band Structure and Oscillatory Electron-Phonon Coupling of Pb Thin Films Determined by Atomic-Layer-Resolved Quantum-Well States

Using a low temperature growth method, we have prepared atomically flat Pb thin films over a wide range of film thickness on a Si-(111)-7 x 7 surface. The Pb film morphology and electronic structure are investigated in situ by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Well-defined and atomic-layer-resolved quantum-well states of the Pb films are used to determine the band structure and the electron-phonon coupling constant (lambda) of the films. We found an oscillatory behavior of lambda with an oscillation periodicity of two atomic layers. Almost all essential features in the Pb/Si(111) system, such as the growth mode, the oscillatory film stability, and the 9 monolayer envelope beating pattern, can be explained by our results in terms of the electron confinement in Pb films.

Oscillatory electron phonon coupling in Pb/Si(111) deduced by temperature-dependent quantum well states

Photoemission study of atomically flat Pb films with a thickness from 15 to 24 monolayers (ML) have been performed within a temperature range 75–270K. Well-defined quantum well states (QWSs) are observed, which exhibit interesting temperature-dependent behaviours. The peak position of the QWSs shifts towards higher binding energy with increasing substrate temperature, whereas the peak width broadens linearly due to enhanced electron–phonon coupling strength (λ). An oscillatory λ with a period of 2ML is deduced. Preliminary analysis shows that the oscillation can be explained in terms of the interface induced phase variations and is thus a manifestation of the quantum size effects.

First-principles studies of structures and stabilities of Pb/Si(111)

Various structures of Pb/Si(111) from 1/6 ML to 4/3 ML are investigated using first-principles calculations. Our calculations for the 1/6-ML mosaic phase and 1/3-ML $\ensuremath{\beta}$-phase yield good agreement with the experimental results. The higher coverage range $1/3<\ensuremath{\theta}<4/3\mathrm{ML}$ has been controversial experimentally, especially close to 4/3 ML with several different phases being observed $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{7},$ hexagonal incommensurate, striped incommensurate, etc.). We compare the unit cell energies of different structures to deduce the energetically favorable ones. Structures consisting of very flat Pb overlayers have lower energy than missing-top-layer structures or structures with stacking faults. This is in very good agreement with experiments which measure high Pb mobility on the $\ensuremath{\alpha}$ phase.

Electronic growth of Pb islands on Si(111) at low temperature

The growth of Pb films on the $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface has been investigated at low temperatures using scanning tunneling microscopy. Flat-top Pb islands are formed and at low coverage the thickness of islands is confined in the range of four to nine atomic layers. Among these islands, those of seven-layer height are the most abundant. In low coverage limit, these multilayer islands prefer to grow in size instead of in thickness, showing a quasi-two-dimensional growth property. This growth behavior, different from the conventional growth modes, arises from the quantum size effect. At higher coverage, the growth also reveals layer-by-layer behavior. The Arrhenius plot of the island density versus temperature shows a linear relationship, indicating the formation of islands can be explained by the nucleation and growth theory. We also study the growth of Pb films on the incommensurate Pb/Si(111) surface at low temperatures. Flat Pb islands can be grown as well, but the threshold thickness is reduced to two atomic layers instead of four.

Scanning tunneling microscopy investigations on Pb/Si(111)

We present results on the early stages of room-temperature (RT) deposition of Pb on the Si(111)-7 × 7 surface on the basis of STM characterization coupled with Auger and LEED analysis. These data show that in the submonolayer range of coverage no intermixing of Pb and Si occurs, while the 7 × 7 periodicity remains. Further deposition of Pb in the monolayer range gives rise to the observation of well shaped atomically flat Pb islands whose edges are oriented along the equivalent 〈1 1 0〉 directions.