Pb Adatoms Research Articles

Doping-dependent charge- and spin-density wave orderings in a monolayer of Pb adatoms on Si(111)

In this work we computed the phase diagram as a function of temperature and doping for a system of lead adatoms allocated periodically on a silicon (111) surface. This Si(111):Pb material is characterized by a strong and long-ranged Coulomb interaction, a relatively large value of the spin-orbit coupling, and a structural phase transition that occurs at low temperature. In order to describe the collective electronic behavior in the system, we perform many-body calculations consistently taking all these important features into account. We find that charge- and spin-density wave orderings coexist with each other in several regions of the phase diagram. This result is in agreement with the recent experimental observation of a chiral spin texture in the charge density wave phase in this material. We also find that the geometries of the charge and spin textures strongly depend on the doping level. The formation of such a rich phase diagram in the Si(111):Pb material can be explained by a combined effect of the lattice distortion and electronic correlations.

Electrocatalysis of formic acid oxidation on Pt–Ru alloys modified with Pb adatoms

Abstract Catalytic effect of lead adatoms, deposited at under-potentials, in the oxidation of formic acid on platinum ruthenium alloys was discovered. With increasing the degree of coverage with the lead adatoms, the catalytic activity of Pt–Ru electrodes increases, reaches its maximum and subsequently decreases. No firmly adsorbed intermediates at the Pt–Ru surface were observed at the optimal coverage degree. Then, its catalytic activity is about 200 mV higher than that on pure Pt in the current density range of 10−4 mA cm−2 to 10−1 mA cm−2. Two effects are responsible for the catalysis of formic acid oxidation. The lead adatoms decrease the number of assemblies of few adjacent free atoms of Pt, on which firmly bound intermediates should form. Positive residual charge of lead adatoms improves their interaction with oxy species, placed on the adjacent atoms of ruthenium. The established interaction stabilizes the energetic state of these oxy species causing the increase in their concentration. A larger concentration of the oxy species accelerates oxidation of firmly bound intermediates and thus releases the Pt atoms for the oxidation of subsequent molecules of formic acid.

Isolated Pb adstructures on Ge(1 0 0) slabs with variable thickness

The adsorption of an isolated monomer (1 Pb adatom), dimer (2 Pb adatoms), and infinite Pb 1D chain on the Ge(100) oriented slabs of different thickness are studied using density functional theory. It was found that the energetically most favourable atomic configurations of the Pb monomers are independent of the slab thickness. On the other hand, the alternative monomer structure with Pb adatom located between two Ge-up atoms from adjacent dimer rows depends considerably on the slab thickness – the structural and electronic structures of this monomer obtained with the use of 6 and 12 layers slab are very different. The geometry and electronic structure of the parallel inter-dimer-row configuration of Pb on Ge(100) surface are reliably reproduced by relatively thin asymmetric slabs, however, structural and electronic properties of the perpendicular inter-dimer-row configuration of Pb are quite sensitive to the number of layers in the slab.

Gold Interlayer Promotes Superconductivity in Single and Double Atomic Pb Layers on Si(100).

Introducing an atomic Au monolayer between a Pb film and a Si(100) substrate allows us to fabricate Pb films with single- and double-atom thicknesses. The Pb films have a 2D square-lattice structure with the 1D atomic chains of Pb adatoms on their top, forming Si(100)1 × 7-(Pb, Au) and Si(100)5 × 1-(Pb, Au) superstructures for single and double atomic Pb layers, respectively. Their common characteristic feature is the occurrence of bundles of quasi-1D metallic bands. Transport measurements showed that samples with a Au interlayer demonstrate enhanced superconductor properties, as compared to Pb layers grown on the bare Si(100) surface. Toward improved superconductor properties, the (Pb, Au)/Si(100) system successively avoids risks associated with possible intermixing between adsorbate layers and substrate, as well as with possible Peierls transition into an insulator state, typical for the 1D systems. This finding opens new ways to control low-dimensional superconductivity at the atomic-scale limit.

The Underpotential Deposition of Lead and its Effect on Carbon Monoxide Adsorption on Pt(111) Electrode

In situ scanning tunneling microscopy (STM) was used to examine the spatial structures of lead (Pb) adatom and carbon monoxide (CO) molecule adsorbed onto an ordered Pt(111) electrode in 0.1 M perchloric acid (HClO4) containing 1 mM Pb(ClO4)2 under potential control. Ordered Pb structures were not seen until the potential was made more negative to 0 V (vs Ag/AgCl), where a quasi-square array (√7 × √57) was seen. Shifting the potential more negative substantiated Pb deposition, which triggered restructuring of the Pb adlayer into (√21 × √21)R11° and (3 × 2√3)rect (θ = 0.62 and 0.67) at −0.1 V. Pb adatom reduced the poisoning effect of CO on the Pt(111) electrode by blocking the adsorption of CO and by forcing CO molecule to asymmetric sites. An ordered (√7 × 3)—Pb + CO structure was imaged at 0 V in 0.1 M HClO4 + 1 mM Pb(ClO4)2, from which the surface coverage and binding sites of CO molecule were inferred.

Atomic structure of PbBr2 thin films on Ag (111)

Ultra-thin PbBr2 overlayers on Ag (111) surface have been grown using thermal evaporation (physical vapor deposition) in the coverage range of ∼0.05 monolayers (ML) to ∼ 0.22 ML. The surface structure of these films has been studied by scanning tunneling microscopy, verified by low energy electron diffraction, and simulated using density functional theory. PbBr2 is one of the binary constituents for the growth of CsPbBr3 perovskites and therefore, the growth study of PbBr2 is directly relevant to the growth of the more complex perovskite as segregation of different phases can affect the properties of the material. Atomically resolved images of PbBr2 with a coverage of ∼0.05 ML show randomly scattered areas of Pb adatoms. Increasing PbBr2 to ∼0.16 ML show additional evidence for the dissociation of PbBr2, with separate structures associated with Pb and Br. ∼0.22 ML of PbBr2 results in areas of a well ordered (√3 x √3) R30° reconstruction that has a structure incorporating both elements. This study provides the reference information for the better understanding of CsPbBr3 perovskite thin films grown under similar conditions.

Surface Diffusion of Underpotential‐Deposited Lead Adatoms on Gold Nanoelectrodes

AbstractA simple and readily applicable voltammetric approach is described to characterize and measure the site‐hopping surface diffusion of underpotential‐deposited (UPD) metal adatoms at nanoelectrodes. UPD refers to the deposition of atoms on foreign metal supports at potentials lower than those predicted by Nernst law for a bulk deposition. Despite its importance in several fields of catalysis, advanced nanofabrication or even atomic nanoengineering by atomic layer epitaxy, diffusion of UPD adatoms is difficult to observe at micro‐ and macroelectrodes. In fact, at electrodes of usual dimensions, UPD adatoms surface diffusion is masked by other electrochemical phenomena of greater relative amplitudes. Conversely, at nanowires electrodes sealed in glass, only an extremely small fraction of the surface of the wires is exposed to the electrolyte solution and is rapidly loaded. This allows the spillage of UPD adatoms onto the much larger area of the nanowire rod which is immune to Faradaic reactions due to its isolation from the electrochemical solution by the glass casing. Therefore, surprisingly for a UPD process, voltammetric peaks currents and integral desorption charges primarily reflect these diffusional surface processes so that the integral charges vary linearly with the inverse of the square root of the scan rate. This is easily observable and measurable with usual bench‐level electrochemical instrumentation. In this work, by using nanoelectrodes with diameters between 90 and 260 nm, we were able to establish the major involvement of this site‐hopping surface diffusion of UPD Pb adatoms on polycrystalline gold (Au) and characterize it quantitatively. The equivalent surface diffusion coefficient of UPD Pb adatoms was determined to be ∼4.4×10−11 cm2 s−1 at room temperature, corresponding to a Gibbs free energy activation barrier of ca. 18.72 kJ mol−1 (i. e., 0.19 eV per Pb adatom) for the reaction of inter‐sites exchange of Pb adatoms on a polycrystalline Au surface.

Imaging and manipulation of adsorbed Pb adatom structures on the Si(100) surface by noncontact atomic force microscopy

We performed imaging and manipulation of epitaxially grown Pb adatom structures on the Si(100) surface by noncontact atomic force microscopy (NC-AFM) in ultra-high-vacuum (UHV) and at cryogenic temperatures. We observe several distinct contrast modes during imaging, which we assign to termination of the scanning probe tip by either a single Si, or single Pb, atom, via quantitative comparison of atomic resolution force spectroscopy experiments with ab initio density functional theory (DFT) simulations. We show that the Pb adatom structures can be controllably manipulated via mechanochemical means and identify a novel semideterministic manipulation strategy that arises from the combination of low temperature operation and the asymmetric diffusion barriers present on the Si(100) surface.

Graphene and graphene oxide as adsorbents for cadmium and lead heavy metals: A theoretical investigation

The geometrical and electronic parameters for the interaction of two toxic heavy metals, namely: cadmium (Cd) and lead (Pb), on graphene and graphene oxide (GO) surfaces are investigated by using local Gaussian type basis sets and the hybrid PBE0 functional as implemented in the CRYSTAL code. The role of including long-range dispersion (D3) contribution as well as the basis set superposition error (BSSE) on the adsorption process is found to be crucial in the description of such interactions. Generally, Cd and Pb adatoms are found to be adsorbed more strongly on GO rather than pristine graphene due to the incorporation of oxygen functional groups (epoxy and/or hydroxyl) on the surface. Moreover, Pb has interestingly been found to play as an electron donor and to form covalent bonding with the GO surface. Such findings could have an impact in water treatment applications using graphene-based nanomaterials.

Characterization of (In,Pb)/Si(111): Tuning normal and lateral atom distributions in mixed metal systems

The growth mode, thermal stability and long range order of In on the Si(111)−α(√3 × √3)−Pb reconstructed surface were investigated by Low Energy Diffraction and X−ray Photoelectron Spectroscopy measurements combined with first−principles Density Functional Theory calculations (DFT). It is shown that indium atoms have tendency to exchange their positions with the lead surfactant resulting in the formation of mixed (In,Pb) heterostructure at elevated temperatures (around 470 K). Moreover, with further temperature increase new metastable In–Pb surface alloys are sequentially observed as the fraction of Pb adatoms decreases. DFT simulations confirmed the experimental findings yielding that not only Pb segregation to the topmost layer is energetically favourable, but also its desorption is expected to occur at lower temperature than the desorption of In due to lower binding energy of the former.

Energy barriers for Pb adatom diffusion on stepped ultrathin Pb(111) quantum nanofilms: First-principles calculations

Various properties of Pb(111) nanofilms, prototypical quantum films, have been studied extensively. However, key ab initio-level energy barriers for Pb adatom diffusion on stepped Pb(111) nanofilm surfaces are still not available. Using first-principles density functional theory, we calculate these barriers for films with thicknesses of few monolayers (ML). We find that two-atom exchange is always much more favorable than direct hopping to cross A- or B-type steps. Ehrlich-Schwoebel (ES) barriers for downward transport to a higher-coordination step-edge site depend strongly on the film thickness. For such transport from 2- to 1-ML terraces, or from 4- to 3-ML terraces, there is no an ES barrier, but large ES barriers of more than 100 meV are found from 3- to 2-ML terraces. We also obtain the barriers for diffusion along the step edges and find that these step-edge barriers are significantly larger than terrace diffusion barriers. In addition, we analyze energetics for diffusion on the top flat surface of a nanofilm supported on a vicinal surface, and thus having underlying buried or ghost steps. We quantify the tilted potential energy surface in both ghost A- and B-step regions separating 2- and 3-ML (as well as 3- and 4-ML) terraces. Consequences are discussed for the growth kinetics of supported Pb nanofilms, where the support does not strongly affect electronic states within the film.

Manipulation of epitaxial graphene towards novel properties and applications

Abstract The integration of epitaxial graphene on 4H-SiC with different metals may allow tunability of electronic and optical properties of graphene, enabling novel high-performance devices. Here we present a Raman spectroscopy study on epitaxial graphene decorated with electrodeposited Pb and Li adatoms and with magnetron sputtered 5 nm-thick Ag nano-island films. We find that the presence of metals on the epitaxial graphene surface generates defects and induces n-type doping, which is evidenced by the observation of the defect related Raman modes (namely D, D′ and D + G) and systematic red-shift of the main characteristic modes of graphene. In-depth statistical analysis of the Raman data before and after metal deposition complemented by density functional theory (DFT) calculations allowed to link the interaction strength between the three selected metals and graphene with the metal-induced changes in the vibrational/electronic properties of graphene. Large-area uniform electron doping of epitaxial graphene and surface-enhanced Raman scattering (SERS) effect are reached by room temperature deposition of Ag nano-island films. Very promising results have been obtained from graphene subjected to electrochemical intercalation by Li, which can serve as prerequisites of the construction of Li batteries. The strong interaction between Li or Pb with graphene implies the possibility to exploit the epitaxial graphene as an efficient material for energy storage or for heavy metal sensing, while predominant van der Waals interaction between Ag and graphene favors the formation of extremely thin silver coatings towards two-dimensional metal systems. The present results give better understanding of the nature of epitaxial graphene response to metal deposition and can be useful to design high-performance energy storage devices, optical sensors and heavy metal detection systems.

Tunneling anisotropic magnetoresistance of Pb and Bi adatoms and dimers on Mn/W(110): A first-principles study

We show that Pb and Bi adatoms and dimers have a large tunneling anisotropic magnetoresistance (TAMR) of up to 60% when adsorbed on a magnetic transition-metal surface due to strong spin-orbit coupling and the hybridization of 6p orbitals with 3d states of the magnetic layer. Using density functional theory, we have explored the TAMR effect of Pb and Bi adatoms and dimers adsorbed on a Mn monolayer on W(110). This surface exhibits a noncollinear cycloidal spin spiral ground state with an angle of 173$^\circ$ between neighboring spins which allows to rotate the spin quantization axis of an adatom or dimer quasi-continuously and is ideally suited to explore the angular dependence of TAMR using scanning tunneling microscopy (STM). We find that the induced magnetic moments of Pb and Bi adatoms and dimers are small, however, the spin-polarization of the local density of states (LDOS) is still very large. The TAMR obtained from the anisotropy of the vacuum LDOS is up to 50-60 % for adatoms. For dimers the TAMR depends sensitively on the dimer orientation with respect to the crystallographic directions of the surface due to the formation of bonds between the adatoms with the Mn surface atoms and the symmetry of the spin-orbit coupling induced mixing. Dimers oriented along the spin spiral direction of the Mn monolayer display the largest TAMR of 60 % which is due to hybrid 6p-3d states of the dimers and the Mn layer

Spectroscopic Imaging of Quasiparticle Bound States Induced by Strong Nonmagnetic Scatterings in One-Unit-Cell FeSe/SrTiO_{3}.

The absence of holelike Fermi pockets in the heavily electron-doped iron selenides (HEDISs) challenges the s_{±}-wave pairing originally proposed for iron pnictides, which consists of opposite signs of the gap function on electron and hole pockets. While the HEDIS compounds have been investigated extensively, a consistent description of the superconducting pairing therein is still lacking. Here, by insitu scanning tunneling spectroscopy and theoretical calculations, we study the effects of strong scatterings from nonmagnetic Pb adatoms on the epitaxially grown HEDIS, one-unit-cell FeSe/SrTiO_{3}(001). Systematic tunneling spectra measured on the Pb adatoms show comprehensive signals of quasiparticle bound states, which can be well explained theoretically within the sign-reversing pairing scenarios. The finding implies that, in addition to previously detected phonons, spin fluctuations play an important role in driving the Cooper pairing in FeSe/SrTiO_{3}(001). The sign reversal in the gap function we revealed here is a significant ingredient in a unified understanding of the high-temperature superconductivity in HEDISs.

Suppression of Competing Reaction Channels by Pb Adatom Decoration of Catalytically Active Cu Surfaces During CO2 Electroreduction

The direct electrochemical conversion of carbon dioxide to chemicals and fuels is of fundamental scientific and technological interest. The control of the product selectivity, expressed in terms of the Faradaic efficiency, has remained a great challenge. Herein, we describe a surface-electrochemical synthetic strategy to tune the electrochemical CO2 reduction selectivity and yield by controlled suppression of the hydrogen evolution reaction (HER) reaction channel, resulting in increased Faradaic efficiencies for fuels and chemicals. We demonstrate that bimetallic catalysts consisting of only minute submonolayer amounts of Pb adatoms deposited on Cu surfaces exhibit and maintain unusually high selectivities for formate (HCOO–) over a large range of overpotentials. The bimetallic adatom electrodes were prepared using underpotential electrodeposition (UPD), which is able to precisely control the adatom coverage. While as little as 0.16 ML Pb surface adatoms on a polycrystalline Cu surface boosted the observed Faradaic HCOO– product selectivity 15 times, the 0.78 ML Pb/Cu catalyst showed the most favorable ratio of HCOO–/H2 production rate thanks to the effective suppression of the HER combined with a partial (−1.0 to −1.1 V vs RHE) enhancement of the HCOO– production. We argue that the favorable product efficiency is caused by selective adatom poisoning on the strongest binding hydrogen adsorption sites; in addition, electronic effects of Pb adatoms change the chemisorption of reactive intermediates. Our study reveals synthetic access to tailored selective bimetallic copper catalysts for the electrochemical CO2 reduction and demonstrates the enormous effect of even minute amounts of surface adatoms on the product spectrum.

Interaction of a single acetophenone molecule with group III-IV elements mediated by Si(001)

A theoretical study of an influence of the acetophenone molecule adsorbed on the Si(001) on the local chemical reactivity of silicon surface is presented. The obtained results indicate that the interaction of the molecule with silicon substrate breaks the intra-dimer π bonds in four surface silicon dimers interacting directly with adsorbed molecule. This leads to the formation of two pairs of unpaired dangling bonds at two opposite sides of the molecule. It is demonstrated that these dangling bonds increase considerably the local chemical reactivity of the silicon substrate in the vicinity of the adsorbed molecule. Consequently, it is shown that such molecule bonded with Si(001) can stabilize the position of In and Pb adatoms diffusing on silicon substrate at two sides and initiate the one-dimensional aggregation of the metallic adatoms on the Si(001) substrate anchored at both sides of the adsorbed molecule. This type of aggregation leads to the growth of chain-like atomic structures in opposite directions, pinned to adsorbed molecule and oriented perpendicular to the rows of surface silicon dimers.

Thickness-dependent energetics for Pb adatoms on low-index Pb nanofilm surfaces: First-principles calculations

Adsorption, interaction, and diffusion of adatoms on surfaces control growth and relaxation of epitaxial nanostructures and nanofilms. Previous reports of key diffusion barriers for Pb diffusion on low-index Pb surfaces are limited in scope and accuracy. Thus, we apply density functional theory (DFT) to calculate the adsorption and diffusion energetics for a Pb adatom on Pb(111), Pb(100), and Pb(110) nanofilms with different thicknesses. We find that these quantities exhibit damped oscillatory variation with increasing film thickness. For Pb(111) films, energetics along the minimum energy path for Pb adatom diffusion between adjacent fcc and hcp sites varies significantly with film thickness, its form differing from other metal-on-metal(111) systems. For Pb(111) and Pb(100) nanofilms, diffusion barriers obtained for both adatom hopping and exchange mechanism differ significantly from previous DFT results. Hopping is favored over exchange for Pb(111), and the opposite applies for Pb(100). For Pb(110) nanofilms, Pb adatom hopping over an in-channel bridge is most facile, then in-channel exchange, then cross-channel exchange, with cross-channel hopping least favorable. We also assess lateral Pb adatom interactions, and characterize island nucleation during deposition on Pb(111).

Dynamical and Reversible Control of Topological Spin Textures.

Recent observations of topological spin textures brought spintronics one step closer to new magnetic memories. Nevertheless, the existence of Skyrmions, as well as their stabilization, require very specific intrinsic magnetic properties which are usually fixed in magnets. Here we address the possibility to dynamically control their intrinsic magnetic interactions by varying the strength of a high-frequency laser field. It is shown that drastic changes can be induced in the antiferromagnetic exchange interactions and the latter can even be reversed to become ferromagnetic, provided the direct exchange is already non-negligible in equilibrium as predicted, for example, in Si doped with C, Sn, or Pb adatoms. In the presence of Dzyaloshinskii-Moriya interactions, this enables us to tune features of ferromagnetic Skyrmions such as their radius, making them easier to stabilize. Alternatively, such topological spin textures can occur in frustrated triangular lattices. Then, we demonstrate that a high-frequency laser field can induce dynamical frustration in antiferromagnets, where the degree of frustration can subsequently be tuned suitably to drive the material toward a Skyrmionic phase.

Influence of Pb on an In/Si(111) surface on the phase transition and the surface structure

The influence of Pb atoms adsorbed on an In/Si(111) surface on the phase transition was investigated by using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). LEED data showed that the transition temperature for the 4×1 → 8×2 structural phase transition decreased in proportion to the amount of Pb deposition. The Pb adatoms appeared as bright protrusions in an STM image taken at 90 K, suggesting that they were located high above the In atomic wires. The regions surrounding the Pb adatoms remained in the 4×1 phase while the other areas turned into the 8×2 or 4×2 phase at this temperature below the transition temperature. Near each Pb adatom, a Friedel-oscillation-like modulation with a double (×2) periodicity in the wire direction was developed. This ×2 modulation was found to be different in nature from the low-temperature 8×2 (or 4×2) phase. We suggest that a Pb-adatom-induced deep potential well is responsible for the Tc lowering and the ×2 modulation.

Insights into the origin of the excited transitions in graphene quantum dots interacting with heavy metals in different media.

Exploring graphene quantum dots (GQDs) is an attractive way to design novel optical and electrochemical sensors for fast and reliable detection of toxic heavy metals (HMs), such as Cd, Hg and Pb. There are two main strategies for achieving this: (i) surface modification of an electrochemical working electrode by nanoscale GQDs and (ii) using a GQD solution electrolyte for optical sensing. Further development of these sensing technologies towards reaching or exceeding the WHO permissible limits implies deep understanding of the interaction between GQDs and HMs in different dielectric media. Solvent is expected to be one of the key factors affecting the binding ability of the GQDs to HMs and their electronic and optical properties. Here we show that the solvent-solute interaction changes the geometrical configuration, stability and absorption spectra of zigzag/armchair-edged GQDs after complexation with neutral and charged HM species. We observe physisorption behavior of Cd and Hg adatoms on the sp2 surface with a solvent-mediated enhancement of the binding energy with increasing solvent polarity. For Pb adatoms, an opposite picture is revealed. We find that the solvent effect also manifests itself in weakening of the chemisorption strength in the HM cation-π system with increasing dielectric constant of the solvent. Thus, a solvent engineering strategy based on control of the dielectric permittivity can be a promising approach to reach the desired binding energy in the HM@GQDs and to provide high sensitivity and selectivity of both optical and electrochemical sensors to toxic HMs we are interested in.