Se Adatoms Research Articles

Enhanced electrocatalysis of oxygen reduction on Se-Modified platinum single crystal electrodes

Se-modified platinum single-crystal surfaces in the [011¯] zone have been used as model electrocatalysts for the oxygen reduction reaction (ORR). The results show that: (1) the Pt(hkl)-[n(111)×(100)] electrodes display enhanced ORR electrocatalytic activity with Se modification. (2) Se adatoms have a double effect on the ORR kinetics at model Pt electrodes: they enhance the activity on the neighboring sites and inhibit the reaction on the sites in direct contact with the Se adatoms. (3) The hydrophobicity and the altered electronic properties are probably the reasons for the enhanced electrocatalysis for ORR with Se modification. The positive shift of the potential of zero free charge of Pt(hkl)@Se electrodes compared to that of un-modified Pt(hkl) are in agreement with these interpretations. (4) At very low coverages, the promotion effect of Se modification on ORR activity at (100) sites is stronger than that at (111) sites, which is probably due to the more pronounced hydrophobicity of the former.

Low-energy Se ion implantation in MoS2 monolayers

In this work, we study ultra-low energy implantation into MoS2 monolayers to evaluate the potential of the technique in two-dimensional materials technology. We use 80Se+ ions at the energy of 20 eV and with fluences up to 5.0·1014 cm−2. Raman spectra of the implanted films show that the implanted ions are predominantly incorporated at the sulfur sites and MoS2−2xSe2x alloys are formed, indicating high ion retention rates, in agreement with the predictions of molecular dynamics simulations of Se ion irradiation on MoS2 monolayers. We found that the ion retention rate is improved when implantation is performed at an elevated temperature of the target monolayers. Photoluminescence spectra reveal the presence of defects, which are mostly removed by post-implantation annealing at 200 °C, suggesting that, in addition to the Se atoms in the substitutional positions, weakly bound Se adatoms are the most common defects introduced by implantation at this ion energy.

The Atomic Structure of the Se-Passivated CuInSe2

Through the first-principles calculations, we studied the atomic structure of Se-passivated CuInSe2. It is revealed that, the CuInSe2 can be passivated by selenium, under Se-rich condition, the structures with a full Se layer plus dimerized Se adatoms are favored. The formation energy of Se-terminated surface and the surface with Se overlayer are different, the formation energy of the surface with Se overlayer is more lower than the pure Se-terminated surface, that is to say, the surface with Se overlayer is more stable. On the other hand, the atomic structure of the Se-passivated CuInSe2 are also different from the atomic structure of the pure Se-terminated surface. Studying the structure of the Se-passivated CuInSe2 is very helpful for the high-efficiency solar cells.

Structural and morphological instabilities of the Si(1 1 1)-7 × 7 surface during silicon growth and etching by oxygen and selenium

Using in situ reflection electron microscopy and ex situ atomic force microscopy, we have studied the morphological stability of large-scale (~10–100 μm) Si(111)-7 × 7 terraces during silicon growth and etching by oxygen and selenium. On the large-scale terraces, silicon growth at substrate temperatures T = 600–770 °C and Si deposition rates R = 0.002–0.2 BL/s proceeds in multilayer mode. Based on RMS surface roughness scaling W ∝ Θβ, we have discerned three modes of morphological instability caused by (I) effective adatom diffusion along step edges at low T and R (β ≈ 0.33), (II) effective diffusion and fast step motion along disordered “1 × 1” regions in 7 × 7 domain boundaries at intermediate T and R (β ≈ 0.2), and (III) accumulation of Si adatoms in high-atom-density “1 × 1” regions on the uppermost terraces at high T and R (β ≈ 0.5). The etching of the singular Si(111)-7 × 7 surface by oxygen leads to the slow development of multilayer morphology, while selenium-induced etching preserves flat surface morphology with periodic 2D vacancy island nucleation, growth, and coalesce, which is attributed to Se adatom diffusion. On the step-bunched surface, the Si or Se adatom diffusion to the step bunches leads to the self-organization of pyramidlike or valley-like morphology during Si growth or Se-induced etching, respectively.

Se-Rich MoSe2 Nanosheets and Their Superior Electrocatalytic Performance for Hydrogen Evolution Reaction.

Two-dimensional MoSe2 has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), although its catalytic activity needs to be further improved. Herein, we report Se-rich MoSe2 nanosheets synthesized using a hydrothermal reaction, displaying much enhanced HER performance at the Se/Mo ratio of 2.3. The transition from the 2H to the 1T' phase occurred as Se/Mo exceeded 2. Structural analysis revealed the presence of Se adatoms as well as the formation of Se-Se bonding. Based on first-principles calculations, we propose two equally stable Se-rich structures. In the first one, excess Se atoms bridge two MoSe2 layers via the interlayer Se-Se bonds. In the second one, the Se atoms substitute for the Mo atoms, and extra Se atoms are added closest to the Mo-substituted Se. Calculation of Gibbs free energy along the reaction path indicates that the Se adatoms of the second model are the most active sites for HER.

Signature of multilayer growth of 2D layered Bi2Se3 through heteroatom-assisted step-edge barrier reduction

During growth of two-dimensional (2D) materials, abrupt growth of multilayers is practically unavoidable even in the case of well-controlled growth. In epitaxial growth of a quintuple-layered {{rm{Bi}}}_{2}{{rm{Se}}}_{3} film, we observe that the multilayer growth pattern deduced from in situ x-ray diffraction implies nontrivial interlayer diffusion process. Here we find that an intriguing diffusion process occurs at step edges where a slowly downward-diffusing Se adatom having a high step-edge barrier interacts with a Bi adatom pre-existing at step edges. The Se–Bi interaction lowers the high step-edge barrier of Se adatoms. This drastic reduction of the overall step-edge barrier and hence increased interlayer diffusion modifies the overall growth significantly. Thus, a step-edge barrier reduction mechanism assisted by hetero adatom–adatom interaction could be fairly general in multilayer growth of 2D heteroatomic materials.

Band structure, band offsets, substitutional doping, and Schottky barriers of bulk and monolayer InSe

We present a comprehensive study of the electronic structure of the layered semiconductor InSe using density functional theory. We calculate the band structure of the monolayer and bulk material with the band gap corrected using hybrid functionals. The band gap of the monolayer is 2.4 eV. The band edge states are surprising isotropic. The electron affinities and band offsets are then calculated for heterostructures as would be used in tunnel field effect transistors (TFETs). The ionization potential of InSe is quite large, similar to that of HfSe2 or SnSe2, and so InSe is suitable to act as the drain in the TFET. The intrinsic defects are then calculated. For Se-rich layers, the Se adatom is the lowest energy defect, whereas for In-rich layers, the In adatom is most stable for Fermi energies across most of the gap. Both substitutional donors and acceptors are calculated to be shallow, and not reconstructed. Finally, the Schottky barriers of metals are found to be strongly pinned, with the Fermi level pinned by metal induced gap states about 0.5 eV above the valence band edge.

First principles study of (Cd, Hg, In, Tl, Sn, Pb, As, Sb, Bi, Se) modified Pt(111), Pt(100) and Pt(211) electrodes as CO oxidation catalysts

CO oxidation is a prototype reaction for studying oxidation of small organic molecules. Certain adatom modified Pt electrodes have a large promotional effect on CO oxidation. However, the effect is often coverage dependent, and has a limited effect due to short lifetimes of the adatoms. The coverage dependence as a function of potential for ten different adatom species (Cd, Hg, In, Tl, Sn, Pb, As, Sb, Bi, Se) on bare and CO saturated Pt(111), Pt(100) and Pt(211) surfaces has been established by means of Density Functional Theory calculations. Most of the adatoms are very stable under standard conditions and remain anchored to the surface until high potentials in supporting electrolytes. The stability is contingent on the electrode roughness and reduces in the following order: Pt(211)≈Pt(100)>Pt(111), except for Se adatoms, in which case the trend is reversed. The activity for CO oxidation, given by the OH formation potentials from water, is dependent on the oxophilicity of the adatoms, and is found to scale almost inversely with the adatom stability. In electrolyte solutions saturated with CO, the stability reduces to roughly half of that on bare Pt surfaces. Irrespective of the CO partial pressure, most of the adatoms are found to dissolve before they become oxidized. This has a detrimental impact on the reaction as high activities have been associated with oxidized adatoms moieties. As on Pt(100) is the only adatom electrode stable in a narrow potential range under standard conditions.

Molecular beam epitaxy of single unit-cell FeSe superconducting films on SrTiO3(001)

Based on our previous work, we have systematically investigated the molecular beam epitaxy growth of single unit-cell FeSe films on SrTiO3(001) substrates and studied the surface morphology by scanning tunneling microscopy. We found that there are three key steps to obtain large-scale uniform one unit-cell superconducting FeSe films. First, the STO(001) substrates should be treated by HCl etching and thermal annealing under oxygen flux so that a specific TiO2-terminated STO(001) surface with well-defined step-terrace structure could be obtained. Second, the Fe and Se fluxes and substrate temperature have to be controlled delicately. At last, post-growth annealing is also critical, which can remove extra Se adatoms, and more importantly facilitate the necessary electron transfer for superconductivity transition.

On the description of the coadsorption of cesium and selenium atoms on the silicon surface

Adsorption of cesium and selenium atoms on silicon surface has been considered in terms of the Anderson-Newns-Muskat model with allowance for the dipole-dipole repulsion. The results obtained have been applied to the problem of coadsorption of Cs and Se atoms. It is shown that even a small addition of cesium adatoms to a submonolayer selenium adatom film on silicon strongly increases the charge of selenium adatoms. This effect is initiated by the mutual enhancement of the dipole fields of the Cs and Se adatom sublattices. All the results obtained agree well with experimental observations.

Microgravimetric, rotating ring-disc and voltammetric studies of the underpotential deposition of selenium on polycrystalline platinum electrodes

Selenium underpotential deposition (upd) on platinum electrodes in perchloric acid solutions was analysed by cyclic voltammetry, rotating ring-disc electrode, and electrochemical quartz crystal microbalance techniques. From the results obtained for Se upd on Pt, the presence of two peaks in the positive sweep with a total charge density value of 420 μC cm −2 (found after subtraction of the charge density due to PtO from the total charge density) was observed. These results are attributed to the deposition of one Se monolayer on Pt with the occupation of two active substrate sites by each Se ad-atom. Such results were confirmed by rotating ring-disc electrode experiments. In the electrochemical quartz crystal microbalance experiments the responses for mass variations were divided in three well-defined potential regions: (A) (0.05–0.7 V), (B) (0.7–1.1 V) and (C) (1.1–1.55 V) in which the mass values of: +18, −50 and −28 ng cm −2, respectively, were described by: (i) water adsorption on previously deposited Se, (ii) dissolution of the species (Se + H 2O) and (iii) dissolution of Se and formation of Pt oxide. The electrode process can be represented according to the reaction: SeO 2 + 4H + + 4e − ⇋ Se ads 0 + 2H 2O.

Reconstruction and energetics of the polar (112) and(1¯1¯2¯)versus the nonpolar (220) surfaces ofCuInSe2

First-principles total-energy calculation reveals a number of (112) and $(1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2\ifmmode\bar\else\textasciimacron\fi{})$ surface structures stable at different atomic chemical potentials. All of the stable structures are charge compensated, thus semiconducting, either by cation-on-cation antisites, cation vacancies, Se adatoms, or by Se addimers. This structural richness raises the possibility for engineering ${\mathrm{CuInSe}}_{2}/\mathrm{Cu}({\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$ material properties by surface control during the growth. The experimentally observed puzzling spontaneous decomposition of the (220)/(204) surfaces into (112) and $(1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2\ifmmode\bar\else\textasciimacron\fi{})$ is also confirmed by calculating individual surface energies.

Radioactive ions for solid-state investigations at magnetic surfaces and interfaces

Hyperfine interactions observed at isomeric states of radioactive probe nuclei are used as a tool for solid-state investigations. This method is sensitive to atomic-scale properties. In recent years surface and interface investigations using radioactive probes delivered many results which can hardly be achieved by any other method. Several groups, e.g., from Konstanz, Leuven, Groningen, Aarhus, Uppsala, Tel Aviv, Pennsylvania, contributed to this field. Our group studies magnetic properties at surfaces and interfaces performing perturbed angular correlation (PAC) measurements in the UHV chamber ASPIC (Apparatus for Surface Physics and Interfaces at CERN). We take advantage of the enhanced variety of PAC probes delivered by the on-line mass separator ISOLDE. First, we report on measurements of magnetic hyperfine fields ( B hf) at Se adatoms on a ferromagnetic substrate using 77Se as a PAC probe. The investigation of induced magnetic interactions in nonmagnetic materials is a further subject of our studies. Here the nonmagnetic 4d element Pd is investigated, when it is in contact with ferromagnetic nickel. An outlook will be given on studies to be done in the future. The experiments were performed at the HMI, Berlin, and at CERN, Geneva.

Temperature dependence of electric-field gradients at isolated Se adatoms on (001) and (111) surfaces and derived adatom distances

Increasing electric-field gradients (EFG) with increasing temperature were observed at isolated Se adatoms on Ni(111), Pd(111), and Co(0001) surfaces applying perturbed angular correlation spectroscopy. This is in contrast to EFG's at probes on surfaces w

Adsorption and desorption of Se on Si(100)2×1: surface restoration

This work entails a study of the adsorption of elemental Se on the reconstructed Si(100)2×1 surface. The investigation took place in an ultra high vacuum (UHV) by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements. The adsorption of one monolayer (1 ML) of Se at room temperature (RT) causes the transition of the reconstructed Si(100)2×1 surface to its original bulk terminated Si(100)1×1 configuration, while Se adatoms form a 1×1 structure by breaking the SiSi dimer bonds. The SiSe bond is strong ( E b=2.97 eV/atom), resulting in the formation of a SiSe compound. Above 1 ML, Se forms a SiSe 2 compound with E b=2.67 eV/atom. The heating that follows causes the desorption of Se up until 1000 K, where Θ Se=0.5 ML, and the Si(100)1×1 structure is changed back to the reconstructed Si(100)2×1 with the Se forming a 2×1 structure. The models of Se(1×1)/Si(100)1×1 and of the Se(2×1)/Si(100)2×1 structures are given.

Reflection high energy electron diffraction study on adatom desorption from ZnSe surface under electron beam irradiation and light irradiation

Effects of electron-beam and light irradiation on the desorption and adsorption processes of Zn and Se adatoms on ZnSe(100) surfaces were studied using the intensity variation of the specular spot of RHEED. The desorption rate constants of physisorbed and chemisorbed atoms were obtained and discussed.

Photo-assisted MBE growth of ZnSe on GaAs substrates

ZnSe epilayers have been grown by photo-assisted MBE on GaAs substrates at various substrates temperatures and under various molecular beam intensity ratios of group VI to group III element. A He-Cd laser with 441.6 mm wavelength was used as a light source. From the temperature dependence of the photo-desorption rate constants, the activation energies of photo-desorption of Zn and Se adatoms from the physical adsorption state ( E P Zn, E P Ss) and from the chemical adsorption state ( E C) are obtained, that is, E P Zn =7.8 kcal/ mol, E P Se =6.7 kcal/mol and E C =16.1 kcal/mol. Thus, the photo-desorption observed is photo-enhanced “thermal” desorption. ZnSe epilayers have been successfully grown at a temperature as low as 150°C. A sharp and strong free-excition emission is observed in photoluminescence spectra at 11 K in the low-temperature grown samples, which shows that the crystallinity is almost comparable with that of the unirradiated epilayer grown at 340°C (the optimum growth temperature without irradiation).

The electrochemical behavior of ad-atoms and their effect on hydrogen evolution: Part V. Selenium ad-atoms on gold

Hydrogen evolution on a platinum electrode proceeds through the atom + atom step which needs pairs of two nearest neighbor sites available for hydrogen evolution. The activity for hydrogen evolution decreases with the decrease of the number of pairs of platinum sites with the deposition of foreign ad-atoms. But on a gold electrode it proceeds through the atom + ion step so it does not need such pairs. In this paper, selenium ad-atoms were put on a gold electrode, in order to examine the difference in their effect on hydrogen evolution, which is expected from the difference in hydrogen evolution mechanism mentioned above. The activity decreases not with the number of pairs of gold surface sites but linearly with the number of gold surface sites with the deposition of Se ad-atoms on a gold electrode as expected. This affords experimental evidence for the validity of the electrochemical mechanism of hydrogen evolution on a gold electrode, from molecular level.

Electrocatalysis by ad-atoms Part V. Cu, Hg and Se ad-atoms for ethylene reduction

Electrocatalysis by ad-atoms Part V. Cu, Hg and Se ad-atoms for ethylene reduction

Electrocatalysis by Ad-atoms: Part V. Cu, hg and Se ad-atoms for ethylene reduction

Another type of the enhancement in electrocatalysis by ad-atoms is reported. This is for ethylene reduction by ad-atoms which occupy less than two Pt sites. According to out theory, for the reaction type A plus B going to AB, the enhancement can be obtained on a surface where there are two kinds of sites, one for A and the other for B. However, a Pt surface is nearly completely covered by ethylene molecules, although hydrogen atoms are well adsorbed by Pt sites when there are no ethylene molecules in the solution. What is necessary for the enhancement is to make sites for hydrogen atoms near the adsorbed ehtlene molecules. This problem was solved by utilizing the difference in the number of Pt sites occupied by an ethylene molecule and that by an ad-atom.