Tin Adatoms Research Articles

Evidence for chiral superconductivity on a silicon surface

Tin adatoms on a Si(111) substrate with a one-third monolayer coverage form a two-dimensional triangular lattice with one unpaired electron per site. These electrons order into an antiferromagnetic Mott-insulating state, but doping the Sn layer with holes creates a two-dimensional conductor that becomes superconducting at low temperatures. Although the pairing symmetry of the superconducting state is currently unknown, the combination of repulsive interactions and frustration inherent in the triangular adatom lattice opens up the possibility of a chiral order parameter. Here we study the superconducting state of Sn/Si(111) using scanning tunnelling microscopy, scanning tunnelling spectroscopy and quasiparticle interference imaging. We find evidence for a doping-dependent superconducting critical temperature with a fully gapped order parameter, the presence of time-reversal symmetry breaking and a strong enhancement in zero-bias conductance near the edges of the superconducting domains. Although each individual piece of evidence could have a more mundane interpretation, our combined results suggest the possibility that Sn/Si(111) is an unconventional chiral d-wave superconductor.

Sn-enriched Ge/GeSn nanostructures grown by MBE on (001) GaAs and Si wafers

Elastically stressed metastable GeSn layers with a tin molar fraction as large as 0.185 are grown on (001) Si and GaAs wafers covered with a germanium buffer layer. A set of wafers with a deviation angle in the range 0°–10° is used. It is established that the GeSn crystal undergoes monoclinic deformation with the angle β to 88° in addition to tetragonal deformation. Misorientation of the wafers surface results in increasing efficiency of the incorporation of tin adatoms into the GeSn crystal lattice. Phase separation in the solid solution upon postgrowth annealing of the structures begins long before the termination of plastic relaxation of elastic heteroepitaxial stresses. Tin released as a result of GeSn decomposition predominantly tends to be found on the surface of the sample. Manifestations of the brittle–plastic mechanism of the relaxation of stresses resulting in the occurrence of microcracks in the subsurface region of the structures under investigation are found.

Electrochemical and spectroscopic studies of ethanol oxidation on Pt stepped surfaces modified by tin adatoms

Ethanol oxidation on platinum stepped surfaces vicinal to the (111) pole modified by tin has been studied to determine the role of this adatom in the oxidation mechanism. Tin has been slowly deposited so that the initial stages of the deposition take place on the step, and deposition on the terrace only occurs when the step has been completely decorated. Voltammetric and chronoamperometric experiments demonstrate that tin on the step catalyzes the oxidation. The maximum enhancement is found when the step is completely decorated by tin. FTIR experiments using normal and isotopically labeled ethanol have been used to elucidate the effect of the tin adatoms in the mechanism. The obtained results indicate that the role of tin is double: (i) when the surface has sites capable of breaking the C-C bond of the molecule, that is, when the step sites are not completely covered by tin, it promotes the oxidation of CO formed from the molecular fragments to CO(2) through a bifunctional mechanism and (ii) it catalyzes the oxidation of ethanol to acetic acid.

Perturbation of Ge(1 1 1) and Si(1 1 1)√3α-Sn surfaces by adsorption of dopants

We test the response of the √3 × √3α reconstructions formed by 1/3 monolayer of tin adatoms on silicon and germanium (1 1 1) surfaces upon doping with electrons or holes, using potassium or iodine as probes/perturbers of the initial electronic structures. From detailed synchrotron radiation photoelectron spectroscopy studies we show that doping with either electrons or holes plays a complimentary role on the Si and Ge surfaces and, especially, leads to complete conversion of the Sn 4d two-component spectra into single line shapes. We find that the low binding energy component of the Sn core level for both Si and Ge surfaces corresponds to Sn adatoms with higher electronic charge, than the Sn adatoms that contribute to the core level high binding energy signal. This could be analyzed as Sn adatoms with different valence state.

In situ high-temperature scanning tunneling microscopy studies of two-dimensional TiN island coarsening kinetics on TiN [formula omitted

In situ high-temperature (1023–1223 K) scanning tunneling microscopy was used to study the coarsening/decay kinetics of two-dimensional (2D) TiN adatom islands on TiN(0 0 1) terraces and in single-atom-deep vacancy pits. Island coarsening/decay behavior was found to be non-linear with time and to depend on the local environment (i.e., on adatom concentration gradients on the terrace), the signature of surface-diffusion-limited kinetics. Two simple island geometries––a single 2D adatom island on an atomically smooth terrace and a single 2D adatom island within a vacancy island––were used to extract adatom surface transport parameters. We model diffusion-limited island decay kinetics for these configurations based upon steady-state diffusion equations solved by adaptive finite-element methods with a form of the Gibbs–Thomson equation describing anisotropic islands serving as the boundary condition. Calculated decay rates were compared with the experimental results to obtain an activation energy E a for adatom formation and diffusion on TiN(0 0 1). E a was found to be 2.6±0.6 and 3.1±0.5 eV for adatom islands in vacancy pits and on terraces, respectively. The difference in the two E a values corresponds to the step edge Ehrlich barrier, which for TiN(0 0 1) is less than the experimental uncertainties in the measurements.

Size-dependent detachment-limited decay kinetics of two-dimensional TiN islands on TiN(111).

In situ high-temperature (T(a)=1050-1250 K) scanning tunneling microscopy was used to determine the coarsening and decay kinetics of two-dimensional TiN adatom and vacancy islands on atomically smooth TiN(111) terraces. We report the first observation of an abrupt decrease in decay rates, irrespective of T(a), of adatom islands with areas less than a critical value of 1600 A(2). However, no decay rate transition was observed for vacancy islands. We attribute the size-dependent island decay behavior, which is consistent with detachment-limited kinetics, to anisotropic attachment and detachment barriers.

Absolute orientation-dependent TiN( [formula omitted]) step energies from two-dimensional equilibrium island shape and coarsening measurements on epitaxial TiN( [formula omitted]) layers

In situ high-temperature (1030–1223 K) scanning tunneling microscopy was used to determine the equilibrium shapes of two-dimensional TiN vacancy islands on atomically smooth terraces of epitaxial TiN(0 0 1) layers. Inverse Legendre transformations of the equilibrium island shapes yield relative step energies as a function of step orientation within an orientation-independent scale factor λ, the equilibrium chemical potential of the island per unit TiN molecular area. We then use quantitative TiN(0 0 1) adatom island coarsening measurements to determine λ and, hence, absolute orientation-dependent step energies β and step stiffnesses β ̃ . For 〈1 1 0〉 and 〈1 0 0〉 steps on TiN(0 0 1), we obtain: β 1 1 0 =0.21±0.05 eV/Å, β 1 0 0 =0.25±0.05 eV/Å, β ̃ 1 1 0 =0.9±0.2 eV/Å, and β ̃ 1 0 0 =0.07±0.02 eV/Å. From the β ̃ values, we calculate kink formation energies ε 1 1 0 =0.40±0.2 eV and ε 1 0 0 =0.11±0.1 eV based on the unrestricted terrace-step-kink model.

Influence of charged impurities on the surface phases of Sn/Ge(1 1 1)

The √3α and 3×3 reconstructions on the Ge(1 1 1)–Sn surface have been perturbed by small amounts of iodine and potassium to test the spectral response in the course of a synchrotron radiation photoemission study. We demonstrate the inadequacy of the static model of the √3α phase at room temperature and of the surface charge density wave model of the 3×3 phase at low temperature. Instead, the influence of the charged impurities is consistently explained when considering that dynamical fluctuations in the tin adatom positions are frozen in upon passing the phase transition temperature.

New insights into the influence of upd Sn on the oxidation of formic acid on platinum in acidic solution

The influence of a stabilized submonolayer of upd tin adatoms on the oxidation of adsorbed and bulk formic acid on sputtered platinum electrodes in 0.1 M HClO 4 has been investigated by measuring simultaneously the rate of CO 2 production from both adsorbed and bulk formic acid. The use of on-line MS technique (DEMS) enables us to distinguish the volatile products (CO 2) formed in the oxidation of adsorbed and bulk species, respectively, with the help of isotope-labeled 13C-formic acid as a probe. A considerable promoting effect of upd tin adatoms on the oxidation of the bulk formic acid was observed. This catalytic activity depends strongly on the coverage of the tin. On the other hand, the oxidation of the formic acid adsorbate is accelerated significantly in presence of Sn adatoms on the surface or tin ions in solution. Based on our results, a discrimination between the electrocatalytic models of tin adatoms postulated in literature was given. We suggested that a combination of a bifunctional mechanism and an electronic interaction between the upd Sn adatoms and Pt substrate could account for the pronounced catalytic influence on the oxidation of formic acid.

Electronic instabilities of the two-dimensional Sn/Ge(111) α-phase

The Sn/Ge(111) interface presents at room temperature an ordered 3 × 3 R(30°) surface reconstruction, formed by a submonolayer array of tin adatoms, which reversibly transforms upon cooling into a 3×3 structure. Recently, it has been claimed that the formation of a commensurate charge density wave is responsible for this transition. We present high-resolution synchrotron radiation photoemission results which support, instead, that the driving force of the transition is rehybridization: fluctuations in the tin adatom type of bonding freeze-in at low temperature to yield the static long-range ordering of the 3×3 ground state which is governed by an ideal 1/2 ratio of the sp 3/sp 2 hybridization balance.

Platinum modified by electrochemical deposition of adatoms

Platinized platinum or platinum/graphite catalysts can be modified ‘in situ’ by submonolayer deposition of different additives (sulphur, copper, lead, tin, iron). An electrochemical method is used based on the underpotential deposition of metals on another more noble metal substrate, by control of its potential at values higher than the Nernst potential of bulk deposition of this additive. Linear potential sweep cyclic voltammetry is used for ‘in situ’ characterization of modified platinum catalysts, allowing evaluation of the quantity and the degree of coverage of deposited adatoms. Moreover, their adsorption stoichiometry and their oxidation state can be determined. The study of the adsorption, on modified platinum, of olefinic acids (maleic acids), used as probe molecules, allowed evaluation of the ‘ligand effects’ induced by these additives. Sulphur, copper, lead and tin adatoms behave as ‘poisons’ of the hydrogenation properties of platinum. The deactivation curves obtained ( a = f(θ poison) ) and the initial toxicities of these poisons can be explained by the occurrence of ‘ensemble effects’ (in the case of copper and sulphur), or by the high surface diffusion of adatoms having low melting points (lead, tin). A restructuring effect of iron, deposited on Pt/graphite has been remarked and can account for the obtained promotion of the hydrogenation catalytic activity of platinum.

Structure sensitivity of irreversibly adsorbed tin on gold single-crystal electrodes in acid media

Tin adlayers formed on well defined gold electrodes upon irreversible adsorption from SnII solutions have been characterized by means of cyclic voltammetry in sulfuric and perchloric acid solutions. The surface redox reactions undergone by the tin adatom have been found to be structure sensitive. From the analysis of the voltammetric charges involved in these processes it can be suggested that adsorbed tin is oxidized to oxygenated SnII species, which are stable at the electrode surface as long as the electrode potential is kept below 0.25 V. At higher potentials, soluble SnIV species are formed and the adlayer is removed from the electrode surface. The stability of the adlayer is strongly dependent on the solution pH. On Au(111), adsorbed tin is directly oxidized to SnIV species.

Effect of ad-atoms on the electro-oxidation of ethylene glycol and oxalic acid on platinized platinum

Potential shifts of a pure Pt/Pt electrode and one modified with ad-atoms when in contact with ethylene glycol at open circuit, were examined. Dehydrogenation and hydrogenation processes in the adsorption of ethylene glycol and oxalic acid on the electrode and the formation of two types of chemisorbed molecules (easily and sparingly oxidizable ones) were determined. On the basis of steady-state current-potential curves it was determined that the acceleration of the electro-oxidation of ethylene glycol by tin ad-atoms was 2–2.5 orders of magnitude in acidic systems and 1–1.5 orders of magnitude in alkaline systems, while lead atoms in alkaline systems accelerate the process by 2 orders of magnitude. The electro-oxidation rate of ethylene glycol in acidic systems is not affected by Cd 2+ cations. At higher concentrations Cd 2+ cations inhibit the electro-oxidation of oxalic acid. The latter process is also inhibited by tin ad-atoms. The effect of tin ad-atoms on the oxidation of the chemisorption products of ethylene glycol and the nature of the other effects found, are discussed.

Electrocatalysis by ad-atoms Part XIII. Preparation of ad-electrodes with tin ad-atoms for methanol, formaldehyde and formic acid fuel cells

The practical application of the electrocatalysis by tin ad-atoms to organic fuel cell anodes has been studied. A simplified and versatile preparation method of the electrode having high specific surface area platinum with a well-defined tin coverage has been developed. It consists of underpotential deposition of ad-atoms and subsequent anodic treatment resulting in uniform dispersion of the ad-atoms over all the platinum clusters in the catalyst layer of the electrode. The resulting electrode exhibits enhancement effects in the specific activity over the pure platinum black electrode by a factor of 100 for methanol oxidation, of more than 1000 for formaldehyde oxidation and of 250 for formic acid oxidation, respectively. This method has the advantage that a much higher specific surface area of catalyst is obtained, as well as an optimum composition, compared with the electrochemical co-deposition or immersion methods proposed previously.

Electrocatalysis by AD-atoms: Part XIII. Preparation of ad-electrodes with tin ad-atoms for methanol, formaldehyde and formic acid fuel cells

The practical application of the electrocatalysis by tin ad-atoms to organic fuel cell anodes has been studied. A simplified and versatile preparation method of the electrode having high specific surface area platinum with a well-defined tin coverage has been developed. It consists of underpotential deposition of ad-atoms and subsequent anodic treatment resulting in uniform dispersion of the ad-atoms over all the platinum clusters in the catalyst layer of the electrode. The resulting electrode exhibits enhancement effects in the specific activity over the pure platinum black electrode by a factor of 100 for methanol oxidation, of more than 1000 for formaldehyde oxidation and of 250 for formic acid oxidation, respectively. This method has the advantage that a much higher specific surface area of catalyst is obtained, as well as an optimum composition, compared with the electrochemical co-deposition or immersion methods proposed previously.