Pb Surface Research Articles

Influence of shock pressure and profile on the microjetting from a grooved Pb surface

This work investigates the shock-induced microjetting from a grooved surface (10 nm, 120 degree) of low-melting metal Pb with molecular dynamics simulations. The microjetting processes under surface/release melting conditions are presented in detail, and some properties on the microjet mass and velocity are revealed for different shock pressure and profile cases. It is found that the increase of microjet mass with shock pressure experiences three stages: rapid increase (solid phase), slowdown increase (release melting) and almost no increase (shock melting). For all cases, the ratio of the maximal jetting velocity to the surface velocity approximately keeps a constant (1.5–1.55), but this value undergoes a degree of exponential decay with time for the solid release cases. In addition, the temperature of the microjet is found to be always above the melting point (zero pressure) and keep a continuous increase towards the microjet tip. When introducing slow decaying profiles, the microjet mass begins to increase with the decay rate, which is dominated by the deformation of bubble during pull-back. When the decay rate becomes fast enough, the microspall occurs as expected, meanwhile the microjet appears to reduce because of the shock energy reduction. But that cannot cut off the microjet completely. The velocity distribution along the loading direction shows two linear regions corresponding to the microspall and microjet, and the latter seems to have a greater velocity gradient.

Probing atomic structure and Majorana wavefunctions in mono-atomic Fe chains on superconducting Pb surface

AbstractMotivated by the striking promise of quantum computation, Majorana bound states (MBSs) in solid-state systems have attracted wide attention in recent years. In particular, the wavefunction localisation of MBSs is a key feature and is crucial for their future implementation as qubits. Here we investigate the spatial and electronic characteristics of topological superconducting chains of iron atoms on the surface of Pb(110) by combining scanning tunnelling microscopy and atomic force microscopy. We demonstrate that the Fe chains are mono-atomic, structured in a linear manner and exhibit zero-bias conductance peaks at their ends, which we interpret as signature for a MBS. Spatially resolved conductance maps of the atomic chains reveal that the MBSs are well localised at the chain ends (≲25 nm), with two localisation lengths as predicted by theory. Our observation lends strong support to use MBSs in Fe chains as qubits for quantum-computing devices.

Pb Correction Algorithms for Non-destructive Provenancing of Lead and Tin Glazed Slip Wares

Lead and tin glazed pottery is frequently contaminated with Pb abundances even on the non-glazed surfaces of vessels and fragments. Elevated lead concentrations result in Pb specific matrix absorption and spectral interference effects on several discriminative trace elements - Ga, Rb and Y - which are important for fully non-destructive micro-chemical provenance based on portable x-ray fluorescence (pXRF) analysis. Empirical Lachance-Traill type correction algorithms have been developed by simulating increasing Pb surface abundances on fired clay samples with lead acetate sprayed on 4.0 µm polypropylene film. For paste surface layers with up to approximately 1 % w/w elemental lead concentrations, full compensation could be achieved, as demonstrated for a representative production waster assemblage of early modern lead glazed slip ware (Weser ware). Since the number of discriminative minor and trace elements in portable and stationary XRF of pottery is at the limit, the development of robust empirical correction algorithms for lead based matrix effects and spectral interferences and the validation of the applied quantification software, respectively, is essential.

Kondo screening and spin excitation in few-layer CoPc molecular assembly stacking on Pb(111) surface: A DFT+HEOM study

Transition metal phthalocyanine molecules adsorbed on a metal substrate exhibit rich spin-related phenomena such as magnetic anisotropy, spin excitation, and Kondo effect. In this work, we investigate theoretically few-layer cobalt phthalocyanine (CoPc) molecular assembly stacking on Pb(111) surface with the use of a combined density functional theory (DFT) and hierarchical equations of motion (HEOM) approach. Calculation results indicate that the local spin properties of CoPc/Pb(111) composites depend critically on the number of adsorption layers. The first layer of CoPc on the Pb(111) surface serves as a spin-insulating buffer, while the CoPc molecules in the second layer exhibit spin-1/2 Kondo effect with a Kondo temperature of about 22 K. In a triple-layer CoPc assembly stacking on Pb(111), the antiferromagnetic coupling between the second and third layers leads to local spin-flip excitations under finite bias voltages, which gives rise to characteristic signatures in the differential conductance spectra. The DFT+HEOM approach thus provides a practical means for investigating the local electronic and spin properties of two-dimensional molecular assemblies adsorbed on the metal surface. The insights obtained from the first-principles based simulations could be useful for experimental manipulation or design of magnetic composite systems.

Lateral Electron Confinement with Open Boundaries: Quantum Well States above Nanocavities at Pb(111).

We have studied electron states present at the Pb(111) surface above Ar-filled nanocavities created by ion beam irradiation and annealing. Vertical confinement between the parallel crystal and nanocavity surfaces creates a series of quantum well state subbands. Differential conductance data measured by scanning tunneling spectroscopy contain a characteristic spectroscopic fine structure within the highest occupied subband, revealing additional quantization. Unexpectedly, reflection at the open boundary where the thin Pb film recovers its bulk thickness gives rise to the lateral confinement of electrons.

Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds

Hybridization-related modifications of the first metal layer of a metal-organic interface are difficult to access experimentally and have been largely neglected so far. Here, we study the influence of specific chemical bonds (as formed by the organic molecules CuPc and PTCDA) on a Pb-Ag surface alloy. We find that delocalized van der Waals or weak chemical π-type bonds are not strong enough to alter the alloy, while localized σ-type bonds lead to a vertical displacement of the Pb surface atoms and to changes in the alloy's surface band structure. Our results provide an exciting platform for tuning the Rashba-type spin texture of surface alloys using organic molecules.

Using elemental Pb surface as a precursor to fabricate large area CH3NH3PbI3 perovskite solar cells

The development of new chemical methods to prepare large area perovskite thin film solar cells is desirable for potential future industrial applications. In this paper, a novel fabrication of perovskite CH3NH3PbI3 thin films based on direct metal surface elemental reaction (DMSER) method in an ambient atmosphere is discussed. The as-prepared CH3NH3PbI3 thin films are highly pure and crystalline. Consequent Transient photovoltaic (TPV) tests were conducted and show that these thin films have a long minority carrier lifetime as good as the perovskite thin films obtained via common two-step method from the literature. Although there have been many studies that have developed perovskite solar cells (PSCs) during the past five years, the current study is the first report using elemental Pb as a precursor to fabricate perovskite solar cells, which were found to be relatively stable upon storage without encapsulation in glove box for more than 200 days. This Pb-initiated in-situ reaction allows for the fabrication of large area and uniform perovskite thin films. For example, in our preliminary studies, we have fabricated large area solar cell device samples (1.10±0.05cm2) and have evaluated their photovoltaic performance under standard conditions (AM 1.5, 100mWcm−2).

Majorana fermions detection based on a coupled quantum dot-nanomechanical resonator system

Motivated by a recent experiment providing evidence for Majorana zero modes in iron (Fe) chains on the superconducting Pb surface, in the present work, an optical method for probing Majorana fermions is proposed theoretically, which is very different from the current tunneling spectroscopy experiments with electrical means. The optical detection proposal consists of a hybrid quantum dot-nanomechanical resonator system driven by one strong pump field and one weak probe field. With the optical means, the signal in the coherent optical spectrum indicates a distinct signature of Majorana fermions in the end of iron chains when the quantum dot coupled to Majorana fermions. In addition, an optical scheme for determining the coupling strength between Majorana fermions and the quantum dot is also introduced, which affords a straightforward method to measure such coupling. We further investigate the role of the nanomechanical resonator in the hybrid system. The vibration of the nanomechanical resonator behaving as a phonon cavity will enhance the exciton resonance spectrum, which is robust for the detection of Majorana fermions. This optical scheme affords a potential supplement to detecte Majorana fermions and supports to use Majorana fermions in Fe chains as qubits for potential applications in quantum computing devices.

Influence of shockwave profile on ejecta from shocked Pb surface: Atomistic calculations**Project supported by the National Natural Science Foundation of China (Grant Nos. 11472254 and 11272006).

We conduct molecular dynamics simulations of the ejection process from a grooved Pb surface subjected to supported and unsupported shock waves with various shock-breakout pressures (PSB) inducing a solid–liquid phase transition upon shock or release. It is found that the total ejecta mass changing with PSB under a supported shock reveals a similar trend with that under an unsupported shock and the former is always less than the latter at the same PSB. The origin of such a discrepancy could be unraveled that for an unsupported shock, a larger velocity difference between the jet tip and its bottom at an early stage of jet formation results in more serious damage, and therefore a greater amount of ejected particles are produced. The cumulative areal density distributions also display the discrepancy. In addition, we discuss the difference of these simulated results compared to the experimental findings.

Fast fabrication of superhydrophobic surfaces on Ti–6Al–4V substrates by deposition of lead

In this paper, superhydrophobic Pb surfaces on Ti–6Al–4V substrates were fabricated based on immersion method in just 30–60 s followed by subsequent low surface energy materials modification. Hierarchical micro- and nanoscale structures were generated on titanium alloy substrates when immersed in Pb(CH3COO)2 solution. Then, the as-immersed specimens were chemically modified via fluoroalkylsilane (FAS) so as to reduce the surface energy. The as-prepared superhydrophobic Pb-coated titanium alloy surfaces, including morphologies, chemical compositions, and wettability were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectrophotometry (FTIR) and contact angle measurements. The results showed that the Pb coatings on Ti–6Al–4V substrates displayed good superhydrophobicity with water contact angle of 165.5° ± 2.4° and sliding angle of 4.6° ± 2.1°. Additionally, variations in micro-morphologies, surface roughness, static and dynamic water contact angles of FAS-modified Pb-coated titanium alloy surfaces with immersion time were also investigated, and the prepared superhydrophobic surfaces exhibited long-term stability in air, good self-cleaning and anti-corrosion properties.

Spectroscopic Line Shapes of Vibrational Quanta in the Presence of Molecular Resonances.

Line shapes of molecular vibrational quanta in inelastic electron tunneling spectroscopy may indicate the strength of electron-vibration coupling, the hybridization of the molecule with its environment, and the degree of vibrational damping by electron-hole pair excitation. Bare as well as C60-terminated Pb tips of a scanning tunneling microscope and clean as well as C60-covered Pb(111) surfaces were used in low-temperature experiments. Depending on the overlap of orbital and vibrational spectral ranges different spectroscopic line shapes of molecular vibrational quanta were observed. The energy range covered by the molecular resonance was altered by modifying the adsorption configuration of the molecule terminating the tip apex. Concomitantly, the line shapes of different vibrational modes were affected. The reported observations represent an experimental proof to theoretical predictions on the contribution from resonant processes to inelastic electron tunneling.

Plasticity of single-atom Pb junctions

A low-temperature scanning tunneling microscope was used to fabricate atomic contacts on Pb(111). Conductance characteristics of the junctions were simultaneously recorded with forming and subsequent breaking of the contacts. A pronounced hysteresis effect in conductance traces was observed from junctions comprising the clean Pb(111) surface. The hysteretic behavior was less profound in contacts to single Pb atoms adsorbed to Pb(111). Density-functional calculations reproduced the experimental results by performing a full ab initio modeling of plastic junction deformations. A comprehensive description of the experimental findings was achieved by considering different atomic tip apex geometries.

First-principles study of the structure of water layers on flat and stepped Pb electrodes.

On the basis of perodic density functional theory (DFT) calculations, we have addressed the geometric structures and electronic properties of water layers on flat and stepped Pb surfaces. In contrast to late d-band metals, on Pb(111) the energy minimum structure does not correspond to an ice-like hexagonal arrangement at a coverage of 2/3, but rather to a distorted structure at a coverage of 1 due to the larger lattice constant of Pb. At stepped Pb surfaces, the water layers are pinned at the step edge and form a complex network consisting of rectangles, pentagons and hexagons. The thermal stability of the water layers has been studied by using ab initio molecular dynamics simulations (AIMD) at a temperature of 140 K. Whereas the water layer on Pb(111) is already unstable at this temperature, the water layers on Pb(100), Pb(311), Pb(511) and Pb(711) exhibit a higher stability because of stronger water–water interactions. The vibrational spectra of the water layers at the stepped surfaces show a characteristic splitting into three modes in the O–H stretch region.

Immobilization of lead by phosphate amended Polonite

Polonite is an alkaline material that is used to remove nutrients from domestic wastewater and it has been evaluated as a fertilizer. Stabilization of Pb by Polonite and Polonite amended with orthophosphate, PO4, (Polonite-P) was studied. Octacalcium phosphate (Ca8H2(PO4)6·5H2O) was a primary species of PO4 formed on the surface of Polonite-P. Lead was found to be associated with pozzolanic reaction products in Pb treated Polonite and Pb treated Polonite-P samples. Formation of Pb oxides, as precipitates or surface complexes, were substantial constituents of Pb treated Polonite. Dissolution of Ca8H2(PO4)6·5H2O followed by formation of Pb4O(PO4)2 was a probable mechanism of Pb removal by Polonite-P. Polonite-P could be a suitable replacement for current PO4 sources as a Pb stabilization agent. Finally, LDI-TOF was an effective technique for evaluating forms of Pb on Polonite and Polonite-P.

Synthesis, adsorption kinetics and thermodynamics of ureido-functionalized Pb(ii) surface imprinted polymers for selective removal of Pb(ii) in wastewater

A surface ion imprinting technique was utilized to synthesize a novel ureido-functionalized Pb(ii)-imprinted polymer where the Pb(ii) ion, γ-ureidopropyltrimethoxysilane, AIBN and EGDMA were used as the template ion, functional monomer, initiator and cross-linking agent. ​

Structures and thermal properties of the N2 monolayer on Pb(1 1 1)

The physisorption of N2 molecules has long been a model system of molecular adsorption. We present a low-energy electron diffraction (LEED) study of the adsorption structures and thermodynamics of monolayer N2 on Pb(1 1 1). The results indicate that the monolayer structure has a triangular incommensurate center-of-mass lattice, and that the N2-substrate interaction is weaker than that observed on other metal surfaces. The N2 monolayer undergoes a phase transition between an orientationally ordered phase (low-temperature) and an orientationally disordered phase at a temperature of 20 K. Potential energy and quasiharmonic calculations indicate that the weak N2-Pb(1 1 1) interaction is the main contributing factor for the difference in orientational order of incommensurate N2 monolayers on Pb(1 1 1) and other similar metal surfaces.

End States and Subgap Structure in Proximity-Coupled Chains of Magnetic Adatoms.

A recent experiment [Nadj-Perge etal., Science 346, 602 (2014)] provides evidence for Majorana zero modes in iron (Fe) chains on the superconducting Pb(110) surface. Here, we study this system by scanning tunneling microscopy using superconducting tips. This high-resolution technique resolves a rich subgap structure, including zero-energy excitations in some chains. We compare the symmetry properties of the data under voltage reversal against theoretical expectations and provide evidence that the putative Majorana signature overlaps with a previously unresolved low-energy resonance. Interpreting the data within a Majorana framework suggests that the topological gap is smaller than previously extracted from experiment. Aided by model calculations, we also analyze higher-energy features of the subgap spectrum and their relation to high-bias peaks which we associate with the Fe d bands.

Self-assembly of tetracyanonaphtho-quinodimethane (TNAP) based metal–organic networks on Pb(1 1 1): Structural, electronic, and magnetic properties

We use scanning tunneling microscopy and spectroscopy to investigate structural and electronic properties of tetracyanonaphtho-quinodimethane (TNAP) based metal–organic networks on a superconducting Pb(111) surface. At low temperatures, the TNAP molecules form densely packed islands. When deposited at room temperature, Pb adatoms are incorporated into fourfold bonding nodes with the TNAP molecules leading to long-range ordered porous structures. Co-deposition of NaCl with TNAP yields a Na source for an ionically bonded Na-TNAP structure. Fourfold bonding motifs are also created by Fe atoms with the cyano terminations of TNAP. However, the structures are irregular and do not sustain the formation of long-range ordered networks. Some Fe centers with molecules surrounded in a local C2 symmetry exhibit Shiba states as a fingerprint of a magnetic interaction with the superconducting surface.

Surface complexation modeling calculation of Pb(II) adsorption onto the calcined diatomite

Removal of noxious heavy metal ions (e.g. Pb(II)) by surface adsorption of minerals (e.g. diatomite) is an important means in the environmental aqueous pollution control. Thus, it is very essential to understand the surface adsorptive behavior and mechanism. In this work, the Pb(II) apparent surface complexation reaction equilibrium constants on the calcined diatomite and distributions of Pb(II) surface species were investigated through modeling calculations of Pb(II) based on diffuse double layer model (DLM) with three amphoteric sites. Batch experiments were used to study the adsorption of Pb(II) onto the calcined diatomite as a function of pH (3.0–7.0) and different ionic strengths (0.05 and 0.1molL−1 NaCl) under ambient atmosphere. Adsorption of Pb(II) can be well described by Freundlich isotherm models. The apparent surface complexation equilibrium constants (logK) were obtained by fitting the batch experimental data using the PEST 13.0 together with PHREEQC 3.1.2 codes and there is good agreement between measured and predicted data. Distribution of Pb(II) surface species on the diatomite calculated by PHREEQC 3.1.2 program indicates that the impurity cations (e.g. Al3+, Fe3+, etc.) in the diatomite play a leading role in the Pb(II) adsorption and dominant formation of complexes and additional electrostatic interaction are the main adsorption mechanism of Pb(II) on the diatomite under weak acidic conditions.

Characteristics of biochars derived from fruit tree pruning wastes and their effects on lead adsorption

The aim of this study was to evaluate the biochar characteristics derived from fruit tree pruning wastes (FTPW) and their effects on lead (Pb) adsorption. Based on results from Pb adsorption, surface area, and phosphorus content, the optimum pyrolysis temperature was 600 °C for Pb adsorption capacity. Using the Freundlich isotherm, the Pb adsorption capacity (K) of biochar obtained from various FTPW decreased in the order of pear (3.8001) ≫ persimmon (2.3977) ≥ apple (2.1968). Based on the Langmuir adsorption isotherm, the maximum Pb adsorption capacities (a; mg g−1) of biochar obtained from different FTPW were in the following order: pear (26.2) ≫ persimmon (19.9) ≥ apple (17.7). The maximum Pb adsorption capacity of the pruned pear tree waste biochar was greater than the other FTPW biochars. Pruned apple tree waste biochar had the lowest Pb adsorption capacity among the tested FTPW biochars. The positive correlation between the Langmuir maximum adsorption capacity (L M) values of the biochars and their phosphorus content and surface area indicated difference in adsorption capacity. However, adsorption capacity of the biochar from all FTPW studied could be used for removing Pb and other metal from wastewater.