arXiv subjects

Jory A. Yarmoff

Publications and source records attributed to Jory A. Yarmoff.

15 recordsLinked to original sources

arXiv2024

Neutralization of low energy Na$^{+}$ scattered from InAs(001)

The neutralization probability of low energy Na$^{+}$ ions scattered from In- and As-rich InAs(001) surfaces is measured by time-of-flight spectroscopy. It is found that the neutralization probability for projectiles scattered from As sites is larger than from In sites for both types of surfaces. A modification of the resonant charge transfer model is proposed in which a freezing contour that follows the atomic structure is combined with molecular dynamics and density functional theory. Together, these approaches show that the neutralization of alkali projectiles scattered from a compound solid material is determined by multiple factors, particularly the surface atomic and electronic structures. This model is applicable to any system in which the surface potential is inhomogeneous, such as compound materials and adsorbate-covered surfaces.

cond-mat.mtrl-sci↗

arXiv2019

Cs adsorption on Bi$_2$Se$_3$

Bi$_2$Se$_3$ is a topological insulator whose unique properties result from topological surface states (TSS) in the band gap. The adsorption of Cs onto a Bi$_2$Se$_3$ surface is investigated by low energy ion scattering and work function measurements. Much of the deposited Cs quickly diffuses to the step edges forming one-dimensional chains of positively charged adatoms, along with some deposition on the terraces. The work function decreases until a coverage of 0.1 ML is reached, beyond which it increases slightly. The minimum in the work function is due to depolarization of the dipoles induced when the concentration of adatoms in the chains reaches a critical value. A slow diffusion of adsorbed Cs from the terraces to the step edges is also marked by changes in the neutralization of scattered Na$^+$ and work function over time. The spatial distribution of the conductive charges in the TSS, which are primarily positioned between the first and second atomic layers, is confirmed by comparison of the neutralization of Na$^+$ scattered from Bi and Se.

cond-mat.mes-hall↗

arXiv2018

Au Nanocluster Growth on Graphene Supported on Ni(111)

Low energy alkali ion scattering is used to investigate the deposition of Au onto a single layer of graphene grown onto Ni(111) by chemical vapor deposition. The yield of 3.0 keV Na$^+$ singly scattered from Au as a function of coverage indicates that it grows in a Volmer-Weber mode forming nanoclusters that increase in size with the amount of deposition. The neutralization probability of the scattered Na$^+$ is high for the smallest clusters and decreases as they increase in size. This is presumably caused by the cluster edge atoms being positively charged combined with the fact that the ratio of edge to center atoms decreases with size, which is similar to the behavior of Au nanoclusters on oxide substrates. In addition, oxygen is intercalated under the graphene film to decouple it from the substrate, but no changes in the growth mode or neutralization probability are observed.

cond-mat.mes-hall↗

arXiv2018

Spatial Distribution of Topological Surface State Electrons in Bi$_{2}$Se$_{3}$ Probed by Na$^{+}$ Low Energy Ion Scattering

Bi$_{2}$Se$_{3}$ is a topological insulator whose unique properties result from topological surface states (TSS) in the band gap. Low energy ion scattering can determine the properties of the outermost few atomic layers of a solid. The deposition of Cs helps to reveal that the neutralization of Na$^{+}$ is larger when scattered from surface Se than from Bi. This is caused by the spatial redistribution of the conductive charges in the TSS, which are primarily positioned between the first and second atomic layers. This provides direct experimental evidence of the spatial distribution of the TSS electrons.

cond-mat.mes-hall↗

arXiv2018

Adsorption of Br$_2$ onto Small Au Nanoclusters

Au nanoclusters grown on SiO$_2$ by physical vapor deposition are exposed to Br$_2$ and then measured with 1.5 keV Na$^+$ low energy ion scattering. It is found that the clusters are able to dissociate the molecules which then adsorb as individual Br atoms, but Br$_2$ does not stick to the bare substrate nor to bulk Au. Adsorption is the first step in any surface chemical reaction, and this result shows how nanoclusters can induce adsorption of species that otherwise do not stick. Results from the literature indicate that catalysis involving nanoclusters occurs at the edges and that the edge atoms are positively charged. This information in conjunction with the ion scattering results lead to the conclusion that the Br adatoms are negatively charged and ionically bonded at the edges of the clusters. Br$_2$ is also a known catalytic poison and this work shows how its adsorption blocks sites that would otherwise be involved in nanocatalysis.

cond-mat.mes-hall↗

arXiv2018

The orientation of CO intercalated between graphene and Ru(0001)

$^{13}$CO molecules are intercalated under a single layer graphene film on Ru(0001) and interrogated with helium low energy ion scattering. Single scattering is used to determine the mass distribution of atomic species visible to the ion beam and detector, and the scattering angle is varied to distinguish adsorbed from intercalated molecules. At room temperature, CO intercalates as molecules that sit upright with the O end on top, as on clean Ru. The intercalated CO tilts, more than it does on clean Ru, when the temperature is raised. This is presumably due to increased vibrational amplitudes combined with the confining effect of the graphene film.

cond-mat.mes-hall↗

arXiv2018

Halogen adsorption and reaction with Bi$_2$(Se,Te)$_3$ and Bi/Bi$_2$(Se,Te)$_3$

Bi$_2$Se$_3$ and Bi$_2$Te$_3$, and these same surfaces covered with Bi films, are exposed to Br$_2$ and Cl$_2$ in ultra-high vacuum. Low energy electron diffraction (LEED) and low energy ion scattering (LEIS) are used to investigate the surface composition before and after halogen exposure. It is found that Br$_2$ weakly chemisorbs to the Se- or Te-terminated clean surfaces and light annealing removes the adsorbates restoring the intact surfaces. In contrast, halogens dissociatively adsorb onto surfaces covered with an additional bilayer of Bi, having a p-doping effect. Annealing these halogen-covered surfaces at 130°C causes Bi atoms to be chemically etched away and the surface reverts to a Se- or Te-termination. This work shows how halogen adsorption and reaction can be used to modify the surface termination of such materials.

cond-mat.mtrl-sci↗

arXiv2018

The growth of bismuth on Bi$_2$Se$_3$ and the stability of the first bilayer

Bi(0001) films with thicknesses up to several bilayers (BLs) are grown on Se-terminated Bi$_2$Se$_3$(0001) surfaces, and low energy electron diffraction (LEED), low energy ion scattering (LEIS) and atomic force microscopy (AFM) are used to investigate the surface composition, topography and atomic structure. For a single deposited Bi BL, the lattice constant matches that of the substrate and the Bi atoms adjacent to the uppermost Se atoms are located at fcc-like sites. When a 2nd Bi bilayer is deposited, it is incommensurate with the substrate. As the thickness of the deposited Bi film increases further, the lattice parameter evolves to that of bulk Bi(0001). After annealing a multiple BL film at 120°C, the first commensurate Bi BL remains intact, but the additional BLs aggregate to form thicker islands of Bi. These results show that a single Bi BL on Bi$_2$Se$_3$ is a particularly stable structure. After annealing to 490°C, all of the excess Bi desorbs and the Se-terminated Bi$_2$Se$_3$ surface is restored.

cond-mat.mtrl-sci↗

arXiv2018

The role of defects in the etching of graphene by intercalated oxygen

Graphene is one of the most promising 2D materials for various applications due to its unique electronic properties and high thermal stability. In previous studies, it was shown that when graphene is deposited onto some transition metal substrates, small molecules, such as O$_2$, intercalate between the graphene and the substrate and react to partially etch the graphene film when heated to desorb the intercalates. Here, carbon vacancy defects are intentionally formed on Gr/Ru(0001) and their effect on the intercalation of oxygen and etching of the graphene layer are investigated. 50 eV Ar$^+$ sputtering with a low fluence is used to create isolated single vacancy defects in the graphene overlayer and helium low energy ion scattering (LEIS) is employed for surface analysis. It is found that the defects both ease the intercalation of the oxygen molecules and improve the etching efficiency of the graphene during annealing.

cond-mat.mtrl-sci↗

arXiv2018

Defect-induced oxygen adsorption on graphene films

Although defects on graphene can degrade electron transport and its ability for use as a protection layer, they can also be helpful to tailor the local properties or activate new sites for particular adsorbates. Here, carbon vacancy defects are formed in graphene films on Ru(0001) using low energy Ar$^+$ bombardment and the materials are then reacted at room temperature with oxygen (O$_2$). Helium low energy ion scattering shows that no oxygen attaches to the intact graphene layer. When isolated single carbon vacancy defects are present, oxygen adsorbs molecularly at the defect sites and intercalates beneath the graphene overlayer after post-annealing at 600 K. When the defects are large enough to consist of open areas of bare substrate, the oxygen dissociatively chemisorbs to the Ru. This work shows that the adsorption depends on the size of the surface vacancies, and that it is important to have defect-free graphene when using it as a protection layer.

cond-mat.mtrl-sci↗

arXiv2017

Intercalation and desorption of oxygen between graphene and Ru(0001) studied with helium ion scattering

Graphene is a fascinating 2D material that is being widely investigated for use in electronic devices due to its unique electronic and materials properties. Also, because of its high thermal stability and inertness, it is considered a promising candidate for use as a protection layer for metal substrates. Here, graphene films grown on Ru(0001) are held at 600 K while reacted with oxygen (O$_2$) and then investigated with helium low energy ion scattering (LEIS). LEIS spectra collected at different scattering angles confirm that oxygen does not adsorb to graphene, but instead intercalates between the graphene and the substrate. The intercalated O$_2$ desorbs when the sample is annealed to 800 K. It is shown that this is a much lower temperature than is needed to remove chemisorbed atomic oxygen from Ru, thus inferring that the intercalated oxygen is molecular. During the desorption process, some of the graphene is etched away via a chemical reaction with the oxygen, with the proportion desorbing as O$_2$ or reacting to etch the graphene being dependent on the amount of intercalated O$_2$.

cond-mat.mes-hall↗

arXiv2017

A detailed analysis of impact collision ion scattering spectroscopy of bismuth selenide

Impact collision ion scattering spectroscopy (ICISS), which is a variation of low energy ion scattering (LEIS) that employs large scattering angles, is performed on Bi2Se3 surfaces prepared by ion bombardment and annealing (IBA). ICISS angular scans are collected experimentally and simulated numerically along the [120] and [-1 -2 0] azimuths, and the match of the positions of the flux peaks shows that the top three atomic layers are bulk-terminated. A newly observed feature is identified as a minimum in the multiple scattering background when the ion beam incidence is along a low index direction. Calculated scans as a function of scattering angle are employed to identify the behavior of flux peaks to show whether they originate from shadowing, blocking or both. This new method for analysis of large-angle LEIS data is shown to be useful for accurately investigating complex surface structures.

cond-mat.mtrl-sci↗

arXiv2016

Preparation of clean surfaces and Se vacancy formation in Bi$_{2}$Se$_{3}$ by ion bombardment and annealing

Bismuth Selenide (Bi$_{2}$Se$_{3}$) is a topological insulator (TI) with a structure consisting of stacked quintuple layers. Single crystal surfaces are commonly prepared by mechanical cleaving. This work explores the use of low energy Ar$^{+}$ ion bombardment and annealing (IBA) as an alternative method to produce reproducible and stable Bi$_{2}$Se$_{3}$ surfaces under ultra-high vacuum (UHV). It is found that a well-ordered surface can be prepared by a single cycle of 1 keV Ar$^{+}$ ion bombardment and 30 min of annealing. Low energy electron diffraction (LEED) and detailed low energy ion scattering (LEIS) measurements show no differences between IBA-prepared surfaces and those prepared by $\textit{in situ}$ cleaving in UHV. Analysis of the LEED patterns shows that the optimal annealing temperature is 450°C. Angular LEIS scans reveal the formation of surface Se vacancies when the annealing temperature exceeds 520°C.

cond-mat.mtrl-sci↗

arXiv2016

Termination of Single Crystal Bi2Se3 Surfaces Prepared by Various Methods

Bismuth Selenide (Bi2Se3) is a topological insulator with a two-dimensional layered structure that enables clean and well-ordered surfaces to be prepared by cleaving. Although some studies have demonstrated that the cleaved surface is terminated with Se, as expected from the bulk crystal structure, other reports have indicated either a Bi- or mixed-termination. Low energy ion scattering (LEIS), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) are used here to compare surfaces prepared by ex situ cleaving, in situ cleaving, and ion bombardment and annealing (IBA) in ultra-high vacuum (UHV). Surfaces prepared by in situ cleaving and IBA are well ordered and Se-terminated. Ex situ cleaved samples could be either Se-terminated or Bi-rich, are less well ordered and have adsorbed contaminants. This suggests that a chemical reaction involving atmospheric contaminants, which may preferentially adsorb at surface defects, could contribute to the non-reproducibility of the termination.

cond-mat.mtrl-sci↗

arXiv2016

Surface Structure of In Situ Cleaved Single Crystal Bi2Se3 Measured by Low Energy Ion Scattering

Bismuth Selenide is a two-dimensional topological insulator material composed of stacked quintuple layers (QL). The layers are held together by a weak van der Waals force that enables surface preparation by cleaving. Low energy ion scattering experiments (LEIS) show that Bi2Se3 cleaved under ultra-high vacuum (UHV) has a Se-terminated structure that is consistent with cleaving between QLs. Comparison of experimental data to molecular dynamics simulations confirms the Se-termination and provides an estimate of the surface relaxation.

cond-mat.mtrl-sci↗