Se Coverage Research Articles

Towards Quantitative Interpretation of Normal Incidence Differential Reflectance (NIDR) Measurements for Electrochemical Dynamics Experiments

Normal incidence differential reflectance, or NIDR, is an in-situ technique suitable for high time resolution monitoring of the interfacial structure and composition of electrode-solution interfaces. Recent efforts have been focused in our laboratory on formulating quantitative models for interpreting the potential-, and coverage-dependence of NIDR signals for use in monitoring interfacial dynamics at Au electrodes in various electrolytes. A simple model capturing the aforementioned dependences will be presented. Within the framework of this model, different limiting behaviors are anticipated depending on the optical properties of the adsorbate and the nature of its interactions with the Au surface. In addition, some exemplary data sets on Au electrodes will be discussed, namely cyclic voltammetry and potential steps on single crystals and polycrystalline Au surfaces in aqueous perchloric acid solutions. The NIDR responses to these potential perturbations can be rationalized on the basis of the optical properties of perchlorate and its interaction with the surface. All NIDR data are consistent with adsorbed perchlorate exhibiting optical properties that deviate minimally from that of water, as well as perchlorate physisorbing on Au rather than chemisorbing. In essence, the NIDR signal is proportional to the charge stored on the Au surface, with no change in said proportionality with perchlorate adsorption, rendering this adsorbate essentially ‘invisible’ to NIDR. This has obvious implications when using perchlorate salts as a supporting electrolyte in NIDR experiments, as perchlorate will produce no NIDR response and therefore will not convolute with the NIDR response arising from other adsorbates. To demonstrate this in practice, NIDR experiments for Se covered Au electrodes in aqueous perchloric acid electrolytes will be discussed and the clear dependence of the NIDR response on Se coverage will be demonstrated.

Reversible structural transition of two-dimensional copper selenide on Cu(111)

Structural engineering opens a door to manipulating the structures and thus tuning the properties of two-dimensional materials. Here, we report a reversible structural transition in honeycomb CuSe monolayer on Cu(111) through scanning tunneling microscopy and Auger electron spectroscopy (AES). Direct selenization of Cu(111) gives rise to the formation of honeycomb CuSe monolayers with one-dimensional moiré structures (stripe-CuSe), due to the asymmetric lattice distortions in CuSe induced by the lattice mismatch. Additional deposition of Se combined with post annealing results in the formation of honeycomb CuSe with quasi-ordered arrays of triangular holes (hole-CuSe), namely, the structural transition from stripe-CuSe to hole-CuSe. Further, annealing the hole-CuSe at higher temperature leads to the reverse structural transition, namely from hole-CuSe to stripe-CuSe. AES measurement unravels the Se content change in the reversible structural transition. Therefore, both the Se coverage and annealing temperature play significant roles in the reversible structural transition in CuSe on Cu(111). Our work provides insights in understanding of the structural transitions in two-dimensional materials.

Pristine edge structures of T''-phase transition metal dichalcogenides (ReSe2, ReS2) atomic layers.

Rhenium dichalcogenides (ReX2, X = S, Se), as a representative type of T''-phase transition metal dichalcogenides (TMDs), have a distinct anisotropic crystal structure as compared to the well-known H- and T-phases and show excellent optical, electronic and catalytic properties. While edges are known to have a profound influence on the physical and chemical properties of two-dimensional materials, they have not been systematically investigated in T''-phase TMDs. We investigated the pristine edge configurations of ReX2 atomic layers using atomic-resolution scanning transmission electron microscopy (STEM) low-dose imaging and density functional theory (DFT) calculations. The pristine edges in monolayer and bilayer ReX2 can be atomically flat with a length up to several tens of nanometers, and are preferentially oriented along either the a axis or b axis. The characteristic 4Re diamond clusters are well preserved along the edges, and ordered structures of the outermost dangling Se atoms were observed, with the Se atoms fully retained, 50% retained or all lost. The edges oriented along the a axis with 100% Se coverage show a ferromagnetic ground state, while their counterparts parallel to b present mid-gap states without appreciable spin-polarization. The anisotropic T'' structure also dictates the cracking direction in ReX2, with cracks propagating mainly along the a and b axes. The strain at the crack edges often causes re-orientation of the lattice, which would change the anisotropic behavior of ReX2. Our work provides new insights into the edge configuration in T'' TMD atomic layers, and offers new opportunities to tailor the performance of ReX2 by edge engineering.

Oxygen Reduction on Rh(111) and Se-Modified Rh(111) Surfaces

Oxygen reduction is studied at Rh(111) and Rh(111) modified by a Se adlayer. Controlled mass transport was achieved in a flow through cell. Hydrogen peroxide, detected at a second electrode behind the working electrode, amounted to few percent at Rh(111) and for low Se coverages, at higher coverages it amounted up to 40%. Small coverages of Se lead to a decrease of the over potential by 50 mV. The Tafel slope amounts to 80 mV at potentials positive of the hydroxyl reduction peak and nearly 200 mV at potentials below. These values do not change after modification by Se. Hydrogen adsorption is completely suppressed for Se coverages of 0.5 (Se atoms per Rh atoms). In addition, the point of total zero charge was determined to be 0.2 V using the CO charge displacement method.

Experimental and theoretical evidence for the ferromagnetic edge in WSe2 nanosheets.

Bulk TMDCs are diamagnetic materials; however, two-dimensional TMDCs exhibit spin polarized edge states, which results in a coupling between the unsaturated transition metal and chalcogenide atoms at the edges. The magnetism in two-dimensional TMDCs broadens their applications in spintronic and multi-functional devices. Herein, by combining macro/micro-magnetic experimental measurements and density functional theory (DFT) calculations, we demonstrate that among five possible edge-terminated WSe2 nanosheets only two types have a magnetic ground state, corresponding to the 100% Se edge terminated and 50% Se edge terminated nanosheets, respectively. The calculation results on WSe2 clusters and WSe2 zig-zag nanoribbons with different terminations and Se coverage rate confirmed that the unpaired electrons of the edge atoms play a crucial role in the appearance of ferromagnetism in WSe2 nanosheets. Furthermore, due to the possible quantum confinement effect and surface effect, there exist thickness-dependent magnetic properties, and the magnitude of magnetism at the edge increases as the number of layers decreases.

Formation of Two-Dimensional Copper Selenide on Cu(111) at Very Low Selenium Coverage.

Using scanning tunneling microscopy (STM), we observed that adsorption of Se on Cu(111) produced islands with a (√3×√3)R30° structure at Se coverages far below the structure's ideal coverage of 1/3 monolayer. On the basis of density functional theory (DFT), these islands cannot form due to attractive interactions between chemisorbed Se atoms. DFT showed that incorporating Cu atoms into the √3-Se lattice stabilizes the structure, which provided a plausible explanation for the experimental observations. STM revealed three types of √3 textures. We assigned two of these as two-dimensional layers of strained CuSe, analogous to dense planes of bulk klockmannite (CuSe). Klockmannite has a bulk lattice constant that is 11 % shorter than √3 times the surface lattice constant of Cu(111). This offers a rationale for the differences observed between these textures, for which strain limits the island size or distorts the √3 lattice. STM showed that existing step edges adsorb Se and facet toward ⟨12‾ 1⟩, which is consistent with DFT.

Predicting the stability of surface phases of molybdenum selenides

The selenization of molybdenum might become an important step in the production of nanostructures based on the layered compound MoSe2. It is already technologically relevant for the production of thin film chalcopyrite solar cells. However, the control of the process is still very poor, due to the lack of basic knowledge of the surface thermodynamics of the system. Here, we present a theoretical study on the stability of surface adlayers of Se on the Mo(110) surface, predicting surface patterns and their stability range in terms of temperature and selenium partial pressure. Our results, based on density functional theory, show that the attainable Se coverages range from 1/4 to 3/4 of a monolayer for systems in equilibrium with a gas formed of Se molecules. We provide simulated scanning tunneling microscopy images to help the experimental characterization of adsorbed surface patterns.

Nature of the Selenium Submonolayer Effect on the Oxygen Electroreduction Reaction Activity of Ru(0001)

I present here results of first principles studies of the adsorption energetics of the intermediates of the oxygen electroreduction reaction (ORR) on a Se-modified Ru(0001) surface. The calculations were performed for 1/3 and 2/3 monolayer coverage of Se, as well as for clean Ru(0001) as a reference. The binding energies of O and OH on Ru(0001) are found to decrease significantly with the presence of Se, and this effect increases with the Se coverage. The Se surface modification is found not to change the Ru local density of electronic states noticeably. However, Se atoms accept electronic charge from the surface and thus become negatively charged. As a result, they repeal electrostatically the adsorbed negatively charged O and OH intermediates and in this way reduce their binding energies. This effect provides an alternative way of tuning the reactivity of the catalyst surfaces. Since for the Ru case reduction of the O and OH binding energies makes ORR energetically favorable, Se modification dramatically improves the ORR rate on Ru. Adsorption of O2 and coadsorption of the ORR intermediates and water have also been studied. The effect of coadsorption on the binding energies of O and OH are discussed.

Adsorption of selenium atoms at the Si(1 1 1)-7 × 7 surface: A combination of scanning tunnelling microscopy and density functional theory studies

The adsorption of selenium (Se) atoms at the Si(1 1 1)-7 × 7 surface has been investigated using both scanning tunnelling microscopy (STM) and density functional theory calculations. A single Se atom prefers to adsorb at sites close to a Si adatom and bonds with this Si adatom and one of its backbonding Si atoms. The adsorption sites are referred to as A ∗-type sites in this article. The density of the conduction band (empty states) of the Si adatom increases as a result of the adsorption of a Se atom, which causes the Si adatom to become brighter in the empty state STM images. At the same time, the adsorption of the Se atom weakens the bonding between the Si adatom and its backbonding Si atom due to the charge transfer from them to the Se atom, and consequently destructs the ordered Si(1 1 1)-7 × 7 surface with increasing Se coverage.

First-principles study of formation of Se submonolayer structures on Ru surfaces

The Ru nanoparticles with Se submonolayer coverage (Se/Ru) demonstrate high electrocatalytic activity toward oxygen reduction reaction (ORR) on cathodes of proton exchange membrane fuel cells. To understand the mechanisms of formation of Se structures on Ru surfaces, the geometric and electronic structures and energetics have been calculated in the present work for various distributions of Se atoms on the Ru(0001) surface and in the vicinity of the edge between the (0001) and (1101) facets. The calculations were performed within the density-functional theory with plane-wave expansion for wave functions and the projector augmented wave potentials. It has been found that due to electronic charge transfer from Ru to Se upon selenium adsorption, Se atoms become negatively charged and repel each other. This repulsion makes compact Se islands on Ru(0001) unstable. Se atoms prefer to separate from each other by the distance of $\ensuremath{\sim}5.47\text{ }\text{\AA{}}$ or larger, which is possible for all Se adsorbates if coverage is not exceeding 1/3 ML. Further increase in Se coverage weakens Se-Ru bonding. Three-dimensional Se structure such as 4- and 11-atom pyramids are found to decompose spontaneously with scattering of Se atoms over the Ru(0001) surface. The Se adsorbates are also found to repel in the vicinity of the edge between the Ru facets, and a small increase in Se bonding to undercoordinated Ru atom does not change the trend of Se adsorbates to separate from each other. The obtained most stable configurations of Se on Ru with 1/3 ML coverage or less may also be optimal for ORR because they provide Ru sites available for O and OH adsorption.

Fabrication and sub-band-gap absorption of single-crystal Si supersaturated with Se by pulsed laser mixing

Selenium supersaturated silicon layers were fabricated by pulsed excimer laser induced liquid-phase mixing of thin Se films on Si(001) wafers. Sufficiently low Se coverage avoids destabilization of rapid epitaxial solidification, resulting in supersaturated solid solutions free of extended defects, as shown by transmission electron microscopy. The amount of retained Se depends on the original film thickness, the laser fluence, and the number of laser pulses irradiating the same spot on the surface. Using this method, Se has incorporated into the topmost 300 nm of the silicon with a concentration of 0.1 at.%. Channeling Rutherford backscattering spectrometry measurements show that the substitutional fraction can be as high as 75% of the total retained Se. These alloys exhibit strong sub-band-gap absorption with optical absorption coefficient ranging up to about 104 cm−1, thus making them potential candidates for applications in Si-based optoelectronic devices.

Towards a determination of the active surface area of polycrystalline and nanoparticle electrodes by Cu upd and CO oxidation

Methods for the determination of the surface area for Pt, Ru and Se modified Pt and Ru are compared, in view of their possible application for technical and nanoparticle electrodes. The hydrogen adsorption charge can hardly be used as a reliable measure for the surface area for Ru because it is paralleled by anion adsorption. The charge necessary for the oxidation of adsorbed CO also contains a large contribution due to anion or oxygen adsorption, which amounts to approx. 45% of the charge in the case of Ru. The mass spectrometrically determined amount of CO2 formed gives a more reliable measure for the surface area, provided that the maximum coverages are constant and independent of the particular surface. Values obtained in this way agree to within 20% with surface area values obtained from measuring the charge needed for the desorption of a complete monolayer of Cu upd on Pt(111) and polycrystalline Pt, polycrystalline Ru, submonolayers of Ru on polycrystalline Pt and on Pt(111) and for nanoparticle, carbon supported electrodes. Se modified Ru has recently found attention as a methanol tolerant cathode material for oxygen reduction. CO does not adsorb on Pt or Ru saturated by Se. For surfaces partially covered by Se, a comparison of the charge measured by cyclic voltammetry in the hydrogen region and of the mass spectrometrically determined amount of CO2 suggests that the latter can be used for a determination of the area not covered by Se. Cu upd, on the other hand, also takes place on surfaces completely covered by Se; the Cu desorption charge is independent of the Se coverage on Pt and Ru modified Pt as long as it does not exceed 70% of full coverage. In the presence of multilayers of Se, CuxSe is formed. On Se modified bulk Ru the amount of Cu upd decreases with increasing Se coverage, approaching only 105 μC m−2 for full Se coverage.

Electrochemical Surface Characterization and O2 Reduction Kinetics of Se Surface-Modified Ru Nanoparticle-Based RuSey/C Catalysts

The electrochemical properties of Se surface-modified Ru/C catalysts (RuSey/C with y = 0 to 1) and their O2 reduction characteristics were determined in model studies under well-defined mass transport conditions, combining quantitative differential electrochemical mass spectrometry and double-disk electrode thin-layer flow-cell measurements. Surface characterization of the catalysts including the quantitative evaluation of the active surface area was performed by electrochemical/mass spectrometric (combined H-upd adsorption, preadsorbed CO monolayer oxidation, Cu-upd adsorption/stripping, and RuOx formation) methods. The suitability of these methods for the determination of the active surface area in the high and low Se coverage regime are discussed, and COad stripping is found to be the most relevant method for the present catalysts. The kinetic parameters for the ORR (activity and selectivity) under quasi-steady-state conditions and their variation with Se modification were evaluated in potentiostatic flow-cell measurements. Modification of Ru/C catalyst by Se improves the O2 reduction activity and reduces the tendency for H2O2 formation in the technically relevant potential region of 0.6-0.8 VRHE, but even for the best catalyst compositions a significant ( approximately 0.2 VRHE) overpotential for O2 reduction on the RuSey/C catalysts remains compared to that for the Pt/C catalyst, and we find H2O2 yields of at least 1% at typical cathode operation potentials. Consequences of the relatively high H2O2 yields for membrane/electrode stability in practical applications are discussed.

Selenium adsorption on Cs-covered Si(100) 2×1 surfaces

This report involves the study of Se adsorption on caesiated Si(100) 2×1 surfaces in ultra high vacuum (UHV) using low energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. Selenium atoms on Cs/Si(100) 2×1 surface adsorb initially on uncaesiated portions of Si and subsequently on the Cs overlayer. The presence of Se increases the binding energy of Cs on Si(100). For Cs and Se coverages above 0.5 ml CsSe and Cs x Se y Si z , compound formation was observed. The coadsorption of Se and Cs induces a high degree of surface disorder, while desorption most probably causes surface etching. The presence of Cs on Si(100) 2×1 surfaces prevents the diffusion of Se into the Si substrate and greatly suppresses the formation of SiSe 2 and SiSe 3, detected when Se is adsorbed on clean Si(100) 2×1 surfaces.

Macroscopic studies of the effects of selenate and selenite on cobalt sorption to gamma-Al2O3.

Metal ion sorption can be significantly impacted by the presence of other solutes or complexing species. In this research, macroscopic sorption studies were conducted to evaluate the effect of strongly sorbing Se(IV) and weakly sorbing Se(VI) oxyanions on cobalt(II) sorption to gamma-Al2O3. Se(IV) was found to significantly alter Co(ll) sorption as a function of Co(II) surface coverage, while Se(VI) was found to have no effect on Co(II) sorption. Under low Co(II) surface loadings (<0.1 micromol/m2), Se(IV) increased Co(II) sorption as a function of the Se(IV) coverage. At low Se(IV) surface coverages, no change in Co(II) sorption was detectable, while at high Se(IV) loadings Co(II) sorption was significantly increased. The increase in Co(ll) sorption in the bisorbate systems can be explained by either an electrostatic enhancement mechanism or byternary complex formation. Se(IV) decreased Co(II) sorption at high Co(ll) surface loadings (>0.5 micromol/m2) where coprecipitation of Co(II) and A(III) in the form of layered double hydroxides (LDH) is expected to be the dominant sorption mechanism for the single-sorbate case. The extent of the Co(ll) sorption reduction in Co(III)/Se(IV) bisorbate systems compared to the corresponding single-sorbate systems increased with increasing Co(II) surface coverage. The rate of Co(II) desorption was reduced in the presence of Se(IV) compared to the single-sorbate case, indicating a direct interaction between Co(II) and Se(IV). A reaction between Co(II) and Se(IV) is further supported by an increase in Se(IV) sorption in the same bisorbate samples where Co(II) sorption is decreased. Thus, the macroscopic data indicates Se(IV) may be altering the mechanism of Co(II) sorption, potentially forming a ternary surface complex or different surface precipitate.

A Study of the restoration of Se/Si( [formula omitted])-7×7 reconstructed surfaces: preservation of the bulk-terminated state

In this work we study the role of adsorbed Se on Si(1 1 1)-7×7 surfaces in the heat-induced 7×7→1×1 phase transition of the Si(1 1 1)-7×7 substrate. The experiment took place in an ultra high vacuum utilizing low energy electron diffraction, thermal desorption spectroscopy (TDS) and work function measurements. The maximum deposited Se coverage was ∼1.6 ML (with the exception of 2.5 ML for some TDS measurements). Heating the Se/Si(1 1 1)-7×7 surface at 1025 K causes the desorption of SiSe 2, dominant at θ Se>0.5 ML, with a calculated binding energy of E b=2.7 eV/atom. Increasing the temperature to 1050 K brings about the phase transition: Si(1 1 1)-7×7→Si(1 1 1)-1×1. The resulting restored Si(1 1 1)-1×1 surface is preserved upon cooling to room temperature as long as the initial coverage of Se on the Si(1 1 1)-7×7 surface is at least θ Se=0.5 ML, while Se is strongly bound to the top substrate layer at E b=2.9 eV/atom. The Se which preserves the Si(1 1 1) surface in its bulk terminated form saturates the free Si bonds, forming a non-symmetric adlayer.

Se adlattices formed on Au(100), studies by LEED, AES, STM and electrochemistry

Ordered selenium atomic layers have been formed electrochemically on Au(100) at a series of coverages. Cyclic voltammetry and coulometry were used to study the deposition process, and to determine the corresponding coverages of a number of Se structures. Structures, with Se coverages of 0.25, 0.33, 0.5, and 0.89 monolayers, were identified using ultra high vacuum — electrochemical techniques as well as scanning tunneling microscopy. The corresponding unit cells of those structures were: p(2 × 2), (2 × √10), c(2 × 2), and a mostly (3 × √10), composed of close-packed Se 8 rings. Pit formation, associated with the formation of the densely packed Se 8 ring structure, was observed. They are reminiscent of pits observed in self-assembled monolayers of alkane thiols on Au surfaces. The pits disappeared as the structure, composed of Se rings, was converted to lower coverage structures, such as the 0.25 monolayer p(2 × 2), via anodic stripping. Se atomic layers were formed electrochemically in three ways: direct reduction from a HSeO − 3 solution; anodic stripping of previously formed bulk Se; or cathodic stripping of previously formed bulk Se. All three methods resulted in equivalent atomic layer structures on the Au(100) surface, but with some variation in the homogeneity and distribution of particular structures.

Structural change of selenium-treated GaAs(001) surface observed by STM

We have controlled the surface structures of selenium (Se) treated GaAs(001) using a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of surface reconstructions. The detailed surface structures were observed in-situ using a high-performance scanning tunneling microscope (STM) system. After producing Ga-stabilized GaAs(001) (4 × 6) reconstructions by thermal cleaning, Se was evaporated slowly onto the surfaces. As the Se deposition time was increased, the surface structure could be changed from (4 × 6) to (4 × 3) to (2 × 3) and finally to the (2 × 1) reconstruction. All surfaces were well ordered and STM images were obtained successfully for all of these reconstructions. The (4 × 3) structure was observed to be similar to (2 × 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the [1 1 0] direction. The (2 × 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction. For the (2 × 1) surface, simple dimers row structures were observed. These structural changes were thought to be caused by irregularities in the SeSe or SeGa dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change.

Atomic Level Studies of Selenium Electrodeposition on Gold(111) and Gold(110)

Studies of the electrodeposition of Se atomic layers on Au(111) and Au(110) are presented. Three electrochemical methods of forming Se atomic layers were investigated: reductive deposition, oxidative stripping of bulk Se, and reductive stripping of bulk Se. The resulting Se atomic layers were studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). LEED indicated the formation of Au(111)(√3×√3)R30°-Se and Au(110)(2×3)-Se structures. STM analysis confirmed the presence of those structures along with several others. At low Se coverages on Au(111), a mosaic structure was formed, composed of a large number of small domains of a (√3×√3)R30°-Se structure, separated by areas void of Se. At higher coverages, near 1/3, the (√3×√3)R30° structure covered most of the surface, except for a number of linear phase boundaries. Commensurate with completion of the (√3×√3)R30° structure, some domains of square Se8 rings were usually evident, as well. At still higher coverages, a heterog...

Exchange reactions versus adsorption geometries for Se/GaAs(110).

We present the result of ab initio density-functional calculations of selenium deposited on the GaAs(110) surface. For the coverages FTHETA=1/4,1/2,1, and 3/2 a variety of adsorption and exchange geometries are optimized. For submonolayer coverage a detailed analysis of the total-energy surface is given. For nearly effective monolayer Se coverage, Se-As exchange reactions are compared with adsorption geometries. We observe a tendency toward an exchange of As and Se atoms in the uppermost layers accompanied by a segregation of As at the surface. For the energetically most favored geometries we compare the calculated band structures with recent angle-resolved photoemission spectroscopy results.