Bi Overlayer Research Articles

Epitaxial growth of Bi, Sb, and Sn

Graphene analogs composed of Bi, Sb, and Sn, respectively, are predicted to be great candidates to realize the quantum spin Hall effect at high temperatures and have attracted intensive research interest in recent years. However, their structural and electronic properties are greatly affected by substrates. Here, we epitaxially grow Bi, Sb, and Sn overlayers on various substrates. We observed the formation of Au–Bi alloy on Au(111) substrates, while α-Bi was formed on the TaIrTe4, TiSe2and Cr2Ge2Te6 substrates. Large-scale thin films of α-Bi, α-Sb and β-Sn can be prepared on the TiSe2 substrates due to the high quality of the substrates with very few defects. The lattice of the Sb films is slightly compressed on the TiSe2 substrates, due to the interfacial interaction. α-Sn transitions to β-Sn on the TiSe2 substrates with increasing Sn coverages. Our work is very helpful for tuning the structural and electronic properties of epitaxial Bi, Sb, and Sn films via proper substrates.

Complete three-dimensional structure of Bi-adsorbed Si(110) surface: Discovery of heavily reconstructed Si(110) substrate

Silicon is one of the most well-studied crystals in terms of surface structure. In particular, the three low Miller index surfaces---(100), (110), and (111)---which are located at the three apexes of the triangle of the irreducible orientation of the cubic crystal, are the most fundamental and have been extensively studied. However, the surface structure on Si(110) substrates is not as well understood as on other substrates. Here, we have investigated a surface structure induced by submonolayer Bi deposition on Si(110). The complete atomic arrangement from the surface to the fifth layer was directly determined by solving the Patterson function, which was obtained using a very large number of electron diffraction patterns. In contrast to the case of the Si(111) substrate, the Si(110) substrate showed significant reconstructions under a Bi overlayer. The obtained structure shows how dangling bonds can be reduced at the Si(110) substrate. The present result proves the high capability of the surface structure determination of the present method.

Strain-engineered electronic and topological properties of bismuthene on SiC(0001) substrate

By combining density functional theory with low-energy effective Hamiltonian, we demonstrate strain-engineered electronic and topological properties of the recently synthesized two-dimensional (2D) bismuthene on SiC(0001) substrate. As bismuthene on SiC(0001) exhibits an indirect gap of 0.62 eV with nontrivial topology, we show that the band gap size can be further increased by an applied tensile strain, which follows a nearly linear fashion. Especially, with a tensile strain of 7%, the topological gap can be enhanced to an unprecedented value of 0.83 eV, originating from the different deformation potentials of the conduction band minimum principally contributed from p orbitals of Bi and valance band maximum from the hybridized states of Bi overlayer and SiC substrate. Moreover, we discuss the strength of spin–orbit coupling, in additional to the strain effect, in tuning the electronic structures and topological edge states. Our results suggest the promise of strain engineering in manipulating large-gap quantum spin Hall states on conventional semiconductor for practical dissipationless electronic transport and quantum information processing.

Enhanced CO2 Electrochemical Conversion at Bi-Modified Pb Foams

CO2 conversion methods to remediate anthropogenic emissions appear promising, as they offer the potential to produce liquid and gaseous fuels for use either in the aeronautics industry, or as a chemical storage method for intermittent energy produced by windmills and photovoltaic panels. Fuel synthesis may be achieved via either the production of a syngas mixture (H2 and CO) through the water gas shift reaction and subsequent conversion into hydrocarbon fuel, or by direct electrochemical reduction of CO2. Direct electrochemical conversion of CO2 into value-added products is a low-temperature process, with the advantage of requiring relatively simple equipment. Formic acid, or formate salts, are used in a variety of chemical processes such as electrowinning, leather tanning, and aircraft de-icing. Alternatively, formic acid and formate salts may be considered as a hydrogen storage medium. On top of that, direct formic acid, and more recently direct formate, fuel cells, have been investigated as they demonstrate significant benefits over methanol fuel cells, including higher open circuit voltage and lower crossover. Pb and Bi are promising catalysts with good selectivity for formate. In the case of lead, it can be prepared in the form of dendrites with nanometre-sized tips. Local high electric fields can exist at the tip of these dendrites that can induce a concentration of electrolytes which in turn leads to improve electroreduction of CO2 [[1]]. Bismuth is non-toxic and has negligible environmental impact. It has few commercial applications, and its price has been low and relatively stable in recent years. In this work, we studied the activity and stability for CO2 electroreduction onto high surface area metallic Pb films. The latter Pb films were further functionalized by Bi overlayers to enhance CO2 reduction properties. For both materials, the material films were prepared through a potentiostatic method. Both types of films were extensively characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and x-ray diffraction (XRD), while electrochemical activities were characterized by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and potentiostatic measurements. [1] M. Fang, S. Garbarino, G.A. Botton, A.C. Tavares and D. Guay, J. Mater. Chem A 5 (2017) 20747-20756

Sulfur-vacancy-dependent geometric and electronic structure of bismuth adsorbed on MoS2

Through Bi deposition on the single-crystalline $\mathrm{Mo}{\mathrm{S}}_{2}$ surface, we find that the density of the sulfur vacancy is a critical parameter for the growth of the crystalline Bi overlayer or cluster at room temperature. Also, the $\mathrm{Mo}{\mathrm{S}}_{2}$ band structure is significantly modified near \ensuremath{\Gamma} due to the orbital hybridization with an adsorbed Bi monolayer. Our experimental observations and analysis in combination with density functional theory calculation suggest the importance of controlling the sulfur vacancy concentration in realizing an exotic quantum phase based on the van der Waals interface of Bi and $\mathrm{Mo}{\mathrm{S}}_{2}$.

Adsorption structure of Bi on the fivefold surface of i-Ag-In-Yb quasicrystal

The initial stage of the growth of a Bi overlayer on the fivefold surface of icosahedral Ag-In-Yb quasicrystal is studied using the first-principles density functional calculation. Comparing to the previous study about Pb adsorption on the same surface, it turns out that Bi is bound stronger than Pb on the surface, and small differences are found in the relative magnitude of the adsorption energy of some stable sites. Those may lead some differences in the sequence of the growth in the initial stage, it is concluded that the under- and first-layers observed in the growth of Pb are formed also in the Bi case.

Origin of the moiré pattern in thin Bi films deposited on HOPG

Thin Bi(110) films deposited on highly oriented pyrolytic graphite (HOPG) exhibit a pronounced moir\'e pattern; here the origin of the moir\'e pattern is investigated using scanning tunneling microscopy (STM) and spectroscopy (STS), high-resolution transmission electron microscopy (HR-TEM), and density functional theory (DFT). It is shown that the moir\'e pattern forms only on islands which are misoriented by $\ensuremath{\sim}{30}^{\ensuremath{\circ}}$ with respect to the usual substrate symmetry direction. Two models of the moir\'e pattern are presented: (i) a commensurate monolayer construction (CMC) for rectangular overlayer symmetry on hexagonal substrates and (ii) a qualitative model based on simple superposition of a Bi overlayer on graphene. The CMC model has previously been applied only to systems with pure hexagonal symmetry. Both models generate moir\'e patterns with key parameters (period, angles of the pattern measured with respect to the main HOPG and Bi crystal directions) that are consistent with the experimental results, but development of a fully predictive/quantitative model remains an outstanding challenge. The electronic structure of the moir\'e pattern is investigated using STS and DFT, and it is found that the local density of states (LDOS) is modulated by the moir\'e pattern. These results are consistent with a picture in which a small distortion of Bi atomic positions at the film-substrate interface results in periodic modulation of the LDOS, hence allowing observation of the moir\'e pattern in STM images.

Spin structure of Rashba-split electronic states of Bi overlayers on Cu(1 1 1)

We investigate the unoccupied Rashba-type spin-orbit split band structure of the commensurate and incommensurate Bi monolayer on Cu(111) with spin- and angle-resolved two-photon-photoemission spectroscopy. Because of the unique geometrical structure of these Bi monolayers on Cu(111), it can be expected that both in-plane and out-of-plane potential gradients play an important role for the Rashba-type spin-structure in these systems. Our spin-resolved data of spin-split states in Bi/Cu(111) confirm the expected Rashba behavior of the in-plane spin-components that is caused by the out-of-plane potential gradient. But in addition, we indeed find out-of-plane spin components with different magnitudes in both monolayer Bi/Cu(111) systems, which we therefore attribute to the structurally induced in-plane potential gradients.

Temperature-dependent structure, elasticity, and entropic stability of Bi phases on Cu{111}

We have used low energy electron microscopy (LEEM) to characterize the structure and stability of Bi phases on Cu{111}. As a function of temperature we find that the Cu{111}($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)R${30}^{\ensuremath{\circ}}$-Bi surface alloy phase gradually dealloys and is fully depleted from Bi at a temperature of 803 K. The dealloying leads to a defect induced change of its elastic properties. The Bi surface alloy phase coexists with a Bi overlayer phase that exhibits a sharp decrease in density in a narrow temperature interval just below the temperature where the surface alloy phase has fully dealloyed. LEEM is used to directly evaluate the structure as well as the entropic contributions that determine the stability of each of the phases.

Structural and electronic properties of the Bi/Au(110)–1×4 surface

We report on the structural and electronic properties of the Bi/Au(110)-1 x 4 surface, by combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and first-principles calculations. The analysis of the precursor 1 x 8 moire structure shows that the 1 x 4 reconstruction forms at an optimum coverage of one monolayer. A hard-sphere model is proposed for the 1 x 4 structure and further confirmed by calculations. In this model, topmost Bi atoms form rows supported by a Bi overlayer, with no significant alloying with the substrate. This has important consequences regarding the electronic properties and the spin texture. The photoemission measurements evidence typical p Bi-induced states, that can have either quasi-one-or two-dimensional character depending on their binding energy. These states show no Rashba spin splitting, in agreement with the results of first-principles calculations. This finding is discussed by considering the role of hybridization with the substrate in the emergence of the Rashba effect.

Growth of Bi on Cu(111): Alloying and dealloying transitions

We report on the observation of surface alloying and dealloying transitions during the submonolayer growth of Bi on the Cu(111) surface. In-situ scanning tunneling microscopy (STM) analysis reveals that at low coverage, and for quasi thermodynamical equilibrium, embedded Bi atoms are randomly distributed within the first Cu layer. Near the ideal coverage of 1/3, we observe an ordered BiCu2 substitutional surface alloy. Between 1/3 and 1/2 monolayer a dealloying process takes place where the surface alloy coexists with two different domains of dense Bi overlayer. In order to understand the mechanisms behind this process, the ordered alloy has been used as a template for subsequent Bi adsorption. An apparently etched surface develops during deposition and using STM image analysis, we propose that this growth mode is accompanied by a large Bi and Cu atoms redistribution inside the surface alloy.

Phase Transition Driven Discontinuity in Thermodynamic Size Selection

We show how an order-disorder phase transition in a two-dimensional system can discontinuously alter the shape and size of stress-stabilized self-assembled nanostructures. Low energy electron microscopy was used to study the dealloying of the Cu(111)-sqrt[3]×sqrt[3]-R30°-Bi surface alloy. The gradual expulsion of embedded bismuth from the alloy with increasing temperature induces a hard-hexagon-type order-disorder transition in the surface alloy. Our low energy electron microscopy results demonstrate how the loss of long-range order induces enormous changes in the domain patterns that the alloy forms with a Bi overlayer phase. We propose that the occurrence of phase transitions in one of the two surface phases that constitute a self-assembled domain pattern, provides a general, largely unexplored, mechanism that can be used to influence the morphological details of two-dimensional nanostructures.

The evolution of the electronic structure at the Bi/Ag(111) interface studied using photoemission spectroscopy

The growth of Bi on Ag(111) induces different surface structures, including surface alloy, Bi- overlayer and Bi(110) thin film, as a function of increasing Bi coverage. Here we report the study of electronic states of these structures using core level and valence band photoemission spectroscopy at room temperature. The sp-derived Shockley surface state on Ag(111) is rapidly quenched upon deposition of Bi, due to the strong variation of the in-plane surface potential in the Ag2Bi surface alloy. The Bi 4f core levels of the alloy and Bi(110) thin film are shifted to lower binding energy by ∼0.6 eV and ∼0.3 eV compared with the Bi bulk value, respectively. Mechanisms inducing the core level shifts are discussed as due to a complex superposition of several factors. As Bi coverage increases and a Bi(110) overlayer forms on Ag(111), a new state is observed at ∼0.9 ML arising from electronic states localized at the Ag–Bi interface. Finally the change of work function as a function of coverage is discussed on the basis of a charge transfer model.

Observation of a surface alloying-to-dealloying transition during growth of Bi on Ag(111)

The atomic structures that develop as a function of coverage during deposition of Bi on Ag(111) have been studied using low-temperature scanning tunneling microscopy, low-energy electron diffraction, and ab initio calculations. The growth process involves two sequential stages. At low coverage, Bi atoms are incorporated into the topmost layer of Ag(111), resulting in the formation of an Ag${}_{2}$Bi alloy confined to the surface and ordered ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)$R$30\ifmmode^\circ\else\textdegree\fi{} Ag${}_{2}$Bi islands supported on Ag(111). This mode of accommodation of Bi was found to be energetically favorable based on ab initio total-energy calculations. At coverage above a critical value of 0.55 monolayers, the Ag${}_{2}$Bi alloy phase gradually converts into an ordered Bi ($p\ifmmode\times\else\texttimes\fi{}\sqrt{3}$) overlayer structure supported on Ag(111). We postulate that the dealloying transition is likely driven by compressive strain induced by incorporation of large-size Bi atoms into Ag at a high coverage and the subsequent lack of miscibility of Ag and Bi bulk phases. After completion of the dealloying process, Bi(110) thin films can be grown epitaxially on top of Ag(111) with a chemically abrupt interface.

Surface alloying and anomalous diffusion of Bi on Cu(1 1 1)

We have used Low-Energy Electron Microscopy (LEEM) to study the formation and structure of the surface alloy of Bi on the Cu(1 1 1) surface. As a function of Bi coverage we find a progression from a dilute surface alloy to a 3 × 3 – R 30 ° structure, which coexists with a Bi overlayer when the Bi coverage exceeds 1/3 ML. At a temperature of 401 K, an unusual formation of alloy domains is observed. The origin of this effect is traced down to the mobility of Bi adatoms, which is found to strongly depend on the Bi coverage of the surface alloy on which they reside.

Quantum size effects in Bi films grown on GaAs (110)

Photoemission results, obtained with a helium-discharge lamp on the Bi/GaAs(110) interface shows strong modifications in the electronic structure of the low temperature deposited Bi overlayer upon annealing to room temperature, accompanied by developing of a sharp low energy electron-diffraction pattern. This behavior is consistent with the hypothesis of quantum size effects influence on growth morphology.

Structural Kinetics Studies on Phase Transitions of the Bi UPD Layer between the (2 × 2) and (p × √3) Structures Using Surface X-ray Diffraction

The kinetics of the phase transition between the (2 x 2) and (p x square root[3])-Bi structures on Au(111) was investigated using electrochemical methods and time-resolved surface X-ray diffraction. The temporal changes in the current value and the diffracted X-ray intensity that originated from the (2 x 2)-Bi overlayer were monitored during the phase transitions at various over-potentials. The phase transition models and kinetics parameters were deduced from each of the current and X-ray intensity transient curves. We also carried out comparative studies of the phase transition from the structural and electrochemical points of view. For the (p x square root[3]) --> (2 x 2) phase transition, the phase transition models determined by the X-ray and electrochemical measurements were a surface-diffusion controlled instantaneous nucleation-growth process and a Langmuir process, respectively. For the reverse transition, the phase transition models determined by X-ray and electrochemical measurements were a Langmuir adsorption process and a surface diffusion controlled nucleation-growth process, respectively. Our results revealed that the current transient curve does not always reflect the phase transition model in both cases and suggest that a structural analysis is fundamental in the phase transition studies. The disagreements between the phase transition models and their mechanisms are discussed.

UHV and electrochemical transfer studies on Pt(110)-(1 × 2) : the influence of bismuth on hydrogen and oxygen adsorption, and the electro-oxidation of carbon monoxide

We have investigated the anodic stripping of bismuth layers adsorbed on Pt(110)-(1 × 2) by metal vapour deposition, and through under-potential deposition (upd) from solution. We have also studied the effect of the bismuth layers on hydrogen and oxygen adsorption in ultra-high vacuum (UHV), upd hydrogen adsorption-desorption, and the electro-oxidation of CO in dilute sulphuric acid electrolyte. A bismuth monolayer (0.7 ML) undergoes electro-oxidation at 0.77 V and 0.88 V (vs. Pd | H 2) (saturation θ Bi = 0.75), with the lowest potential anodic peak associated with the formation of the bismuth adsorbed at coverages greater than 0.5 ML in the uniaxially compressed structure. An overall charge of two electrons is associated with the oxidation process in the monolayer, whereas three electrons are involved in the anodic stripping of the second layer at 0.59 V, and third and subsequent layers at 0.5 V. Bismuth adsorption results in a linear blocking of the β2 hydrogen temperature programmed desorption (TPD) state without a shift in the desorption temperature, and total blocking of hydrogen at 0.5 ML of bismuth at the completion of the c(2 × 2) overlayer on the unreconstructed surface. The blocking of the upd hydrogen layer by bismuth is non-linear, and is fastest at lowest coverage of bismuth. At 0.5 ML coverage of bismuth, ca. 0.5 ML of upd hydrogen can be adsorbed, and at 0.7 ML of bismuth 0.25 ML of upd hydrogen is adsorbed. The bismuth does not change the potential of the upd hydrogen adsorption-desorption. CO adsorbed from solution on Pt(110)-(2 × 1) at 0.4 V produces on transfer to UHV the TPD characteristic of 1 ML of CO in the Pt(110)-(2 × 1)plgl overlayer. Electro-oxidation of this layer takes place in an anodic peak in the cyclic voltammetry at 0.69 V. CO adsorbed from solution at 0 V saturates at a coverage of 0.8 ML, and undergoes electro-oxidation at lower potentials, in a broader peak centred at 0.66 V and a small peak at 0.38 V. The effect of bismuth adsorption is to decrease the amount of CO adsorbed, and available for electro-oxidation, at the same rate as CO adsorption is blocked in UHV. Bismuth also has the effect of increasing the electrochemical potential for CO oxidation from 0.69 V on the clean surface to 0.78 V when CO is co-adsorbed with bismuth in the Pt(110)-c(2 × 2) Bi overlayer, or at higher coverages. A shift to a higher CO oxidation potential is observed from the smallest coverages studied, although at 0.1 ML the peak is broad. The electro-oxidation of the CO also strongly influences the subsequent oxidation behaviour of the bismuth which takes place in a highly asymmetric anodic peak at 1.0 V. The potential of CO oxidation in the mixed bismuth CO layer on Pt(110)-(1 × 2) is very similar to that on Pt(111), and the results are explained using a model involving substrate-mediated oxygen transfer in CO electro-oxidation.

Potential-Step Chronocoulometric Investigation of the Surface Coverages of Coadsorbed Bi and Hydroxide on Au(111) Electrodes

Bi underpotentially deposited on Au(111) has been studied using potential-step chronocoulometry to determine the actual surface coverage of Bi. In the potential region where this system exhibits catalytic activity for the electroreduction of peroxide to water, the observed coverage is 0.25 monolayer (ML), which agrees well with the coverage of the reported (2 × 2) Bi overlayer observed by scanning probe microscopy in this region. At more cathodic potentials, the coverage increases to 0.67 ML. This coverage agrees with that expected based on the (p × √3) structure proposed from scanning tunneling microscopy, atomic force microscopy, and SXS measurements in this region. The electrosorption valency calculated based on these coverages is 3, indicating that the Bi is fully discharged on the surface. Potential-step chronocoulometry has been used at various pH values to determine the surface coverage of hydroxide anion in the presence of underpotentially deposited (upd) Bi. The coverage is negligible in the absence of upd Bi and at potentials where the Bi adlayer condenses. It rises to a peak of 0.17 ML in the region where the coverage is 0.25 ML, indicating that OH- is coadsorbed with the Bi.

The structural properties of Si(110)1 × 1-Bi surfaces

The structure of Si(110)1 × 1-Bi is studied using tensor LEED. From the features of the LEED patterns, the geometrical model where zigzag chains of Bi atoms are formed along the [1, −1, 0] direction is assumed. However, the experimental I-V curves cannot be explained by this model. Rather, I-V curves calculated from the geometrical model where zigzag chains are composed of Si atoms are in excellent agreement with the experimental ones. This result suggests that the Bi overlayer is short-range-ordered.