Impact Collision Ion Scattering Spectroscopy Research Articles

Modeling experimental low energy ion scattering multi-angle maps with molecular dynamics FAN

Multi-angle maps obtained with low energy ion scattering (LEIS) are not only a convenient way to visualize the atomic-level surface structure of single crystals, but also provide a comprehensive data set that is useful for quantitative fitting with models. However, modeling these maps is often difficult due to the complexity of the scattering process, as well as the number of simulations needed to accurately reproduce a single experimental map. For example, traditional molecular dynamics (MD) and binary collision approximation (BCA) simulations typically require on the order of one billion simulated ion trajectories to obtain sufficient statistics. This problem is exacerbated when modeling impact-collision ion scattering spectroscopy (ICISS) maps, as backscattering cross sections are much smaller than forward-scattering cross sections. A workaround to this problem was given by Neihus and Spitzl in the form of FAN (Niehus and Spitzl, 1991), a simulation that approximates the angular dependence of the backscattering signal by only simulating ion trajectories that reach and emanate from lattice atoms, using a BCA code. This greatly reduces the number of trajectories that must be calculated, though it can oversimplify some of the more complex collision sequences. In this work, we develop a fully three-dimensional FAN code that uses MD to simulate trajectories rather than the BCA. Our approach exploits the improved speed of FAN, while retaining the more accurate ion trajectory calculations of MD. We apply the MD-FAN simulation to model LEIS measurements on two surfaces. First, we benchmark our MD-FAN simulation in a comparison to a BCA simulation for 3 keV He+ incident on W(111) that has been previously reported in Wong et al. (2020). The MD-FAN map had stronger agreement with the experimental ICISS measurements than did the BCA map, despite containing nearly 100-fold fewer ion trajectories. Second, to obtain experimental data sets for a more complex surface, we perform experimental ICISS measurements using a 3 keV He+ beam on a magnetite single crystal, Fe3O4(001) to generate a multi-angle map. The simulated MD-FAN map for the Fe3O4(001) surface once again agreed well with the experimental map, allowing for quantitative comparisons. The MD-FAN simulation also provided insight into the scattering processes, revealing that some of the observed pattern arises from 3 keV He+ that have backscattered into the detector from Fe atoms more than 2 nm below the surface.

A Low-Energy Ion Scattering Approach for Studying Au Nanoclusters Grown on an H2O Buffer Layer.

A novel form of alkali low-energy ion scattering is used to probe the deposition of nanoclusters onto a solid surface via buffer layer assisted growth (BLAG) in ultrahigh vacuum. A thin amorphous solid water (ASW) buffer layer is grown on a TiO2(110) single crystal cooled to 100 K. Au atoms deposited onto this layer arrange themselves into nanoclusters. The sample is then annealed to 320 K to desorb the ASW and enable the clusters to soft-land onto the substrate. Time-of-flight low-energy ion scattering, using Li+, Na+, and K+ projectiles, probes the materials during each step of the BLAG process to measure the surface composition and reveal the details of how the clusters form. The neutralization probability of Na+ ions singly scattered from the Au nanoclusters indicates that they increase in size after annealing and that the magnitude of the increase is a function of the buffer layer thickness. The adsorption of a thin, incomplete water layer prior to Au deposition forms nanoclusters that are possibly even smaller than those produced by direct deposition onto the clean substrate.

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. 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.

Quantification of deexcitation processes for analyzing liquid surfaces

In the last two decades the mathematical tools for quantitative data evaluation have been developed for several surface spectroscopic techniques like Angular Resolved X-ray Photoelectron Spectroscopy (ARXPS), Neutral Impact Collision Ion Scattering Spectroscopy (NICISS) and Metastable Induced Electron Spectroscopy (MIES). Provided that the experimental data are of good quality, quantitative data processing can add a lot to the information that can be gained from surface spectroscopy. We give a selection of references that contain information on these methods. The emphasis of this contribution aims at providing motivation to apply quantitative data evaluation by presenting a few examples. We try to demonstrate, that careful data evaluation may lead to interesting insight into basic concepts as well as to results that are useful for practical applications.

Evaluation of screening length corrections for interaction potentials in impact-collision ion scattering spectroscopy

Since in impact-collision ion scattering spectroscopy (ICISS) data analysis the interaction potential represented by the screening length as the screening effect is not satisfactorily established up to the present, we introduce commonly the correction factor in the screening length. Previously, Yamamura, Takeuchi and Kawamura (YTK) have suggested the theory taking the shell effect of electron distributions into account for the correction factor to Firsov screening length in the Moliere potential. The application of YTK theory to the evaluation of screening length corrections for the interaction potentials in ICISS manifested that the screening length corrections calculated by the YTK theory agree almost with those determined by simulations or numerical calculations in ICISS and its variants data analyses, being superior to the evaluation of screening length corrections with the O’Connor and Biersack (OB) formula.

Concentration Depth Profiles of Inorganic Ions at Liquid Surfaces

For a decade, it has been discussed whether or not inorganic ions like iodide or bromide adsorb preferentially at liquid surfaces. Thermodynamic relationships cannot be used to derive detailed information about the molecular structure of surfaces. Thus, it is import to investigate the structure of liquid surfaces directly. Neutral Impact Collision Ion Scattering Spectroscopy is one of the very few techniques that can be used to determine directly the molecular structure of liquid surfaces.

Reply to “Comment on `Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface' ”

Reply to “Comment on `Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface' ”

Comment on “Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface”

The clean Pt(111) and Pt(111)-p(2×2)-O surfaces have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the atomic collisions in a crystalline target (ACOCT) program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3 keV He+ ions incident along the [211] azimuth of the clean Pt(111) and Pt(111)-p(2×2)-O surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Pt(111) surface are relatively well reproduced by the ACOCT simulations for no surface relaxation, and that the ACOCT simulations for the Pt(111)-p(2×2)-O surface appear the oxygen occupancy in the fcc-hollow sites over the hcp-hollow sites. The ACOCT results for the clean Pt(111) and Pt(111)-p(2×2)-O surfaces are consistent with the analyzed results of low-energy electron diffraction (LEED).

Surface Characterization of GaN(0001) Grown by Liquid Phase Epitaxy Using Coaxial Impact-Collision Ion Scattering Spectroscopy

We characterized the surface cleanliness, structure, and composition of GaN(0001) grown by liquid phase epitaxy (LPE) using coaxial impact-collision ion scattering spectroscopy (CAICISS). Contamination on the as-received LPE-GaN(0001) was removed by annealing at 900 °C, but annealing was unable to remove the O atoms from the as-grown surface, indicating that the as-grown sample incorporated O atoms. Although the as-grown sample contained O atoms, its structural perfection was maintained. These facts were explained by localization of O atoms near dislocations. The etched surface exhibited high crystalline quality, which was comparable to that of the sample grown by vapor phase epitaxy (VPE).

New method of evaluation for interatomic interaction potential in LEIS with large-angle scattering using the two-atom scattering model

The interaction potential between an incident ion and a target atom in impact-collision ion scattering spectroscopy (ICISS), which is a specialization of low energy ion scattering (LEIS) and its variants, i.e. ICISS with detection of neutrals (NICISS), coaxial ICISS (CAICISS) and impact-collision atom scattering spectroscopy with detection of neutrals (NICASS), has been evaluated by the new method using the dependence of the total scattering angle on the impact parameter for the first collision in the numerical calculations based on the two-atom scattering model (TWASM). From the comparison of determined values of scaling factor for the Firsov screening length by three-dimensional computer simulations with calculated ones by TWASM, it became obviously that the interatomic potentials for the various combinations of an incident ion and a target atom in LEIS are suitably given by the Moliere potential with the reduced Firsov screening length employing the scaling factor obtained in TWASM calculations.

Structural Study of Initial Growth of Nickel on Yttria-Stabilized Zirconia by Coaxial Impact-Collision Ion Scattering Spectroscopy

The atomic structure of yttria-stabilized ZrO2 (YSZ) surfaces was investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS). After a simple treatment to clean the surface, the outermost plane of the clean YSZ (001) surface was found to be a Zr layer. Compared with the clean surface, the initial stage of Ni/YSZ interface formation was studied after the vapor deposition of nickel at room temperature in ultrahigh vacuum (UHV). It was revealed that Ni atoms grow in order even at a thickness of 2 monolayers (MLs) in UHV. The Ni growth conserved the 4-fold symmetry of the YSZ (001) substrate structure, suggesting a certain interaction between the Ni and Zr atoms. However, after air exposure, the ordering structure disappeared and changed into a random phase.

In situ Monitoring of Surface Processes in Plasma by Coaxial Impact-Collision Ion Scattering Spectroscopy

We demonstrated in situ monitoring of surface processes in plasma, using coaxial impact-collision ion scattering spectroscopy (CAICISS) operable in a gas atmosphere, taking the plasma nitridation processes of Si surfaces with (111) and (001) faces as examples. From the results of real-time monitoring of CAICISS spectra, it was found that (1) the top layer of the Si(111) surface is readily nitrided even at room temperature, while the Si(001) surface requires thermal activation for its nitridation and that (2) at a high temperature, the nitridation of Si(001) proceeds to layers deeper than in the case of Si(111).

A structural analysis of Bi/Si(1 0 0) 2 × n surfaces by ICISS

We have investigated the structure of Bi-adsorbed Si(100) 2×n surfaces with various Bi coverages by low-energy electron diffraction (LEED) and impact-collision ion scattering spectroscopy (ICISS). Intensities of He+ scattered from Bi and Si atoms were measured as a function of ion incidence angle. Bi–Bi interatomic distances for the toplayer including Bi-dimers were estimated by the shadow-cone technique. Bi-signal peaks appeared even at considerably large angles of incidence, which are attributed to collisions with Bi atoms in sub-surface layers. Bi adsorption is not completed at a mono-layer level, but continues to form multiple layers, fitted in the Si lattice, in the Frank-van der Merwe growth mode.

Structure Analysis of Monolayer Graphite on Ni(111) Surface by Li+-Impact Collision Ion Scattering Spectroscopy

The surface structure of monolayer graphite (MG) on a Ni(111) surface has been investigated using Li+-impact collision ion scattering spectroscopy (ICISS) and reflection high-energy electron diffraction (RHEED). The RHEED pattern showed a 1 ×1 structure for MG formed on the Ni(111) surface, and the ICISS measurement clearly showed the difference between a clean Ni(111) surface and that with MG. The surface structure was determined via comparison of computer simulations and ICISS measurements. The C atoms locate on top and hollow sites of the Ni(111) surface, to form monolayer graphite, which is consistent with the model proposed by a tensor Low energy electron diffraction study.

A study of reactivity of O 2-gas on the TiO 2(1 1 0) surface: low and heavily reduced surfaces

The reactivity of O 2-gas on the low and heavily reduced TiO 2(1 1 0)-p(1×2) surfaces was investigated using impact collision ion scattering spectroscopy (ICISS). We found that the reactivity of O 2-gas on the low reduced surface which has Ti 2O unit rows (Appl. Surf. Sci. 167 (2000) 169) is low, while that on the heavily reduced surface with added Ti 2O 3 unit rows is high and the p(1×1) phase with bridging oxygen rows grow on the Ti 2O 3 surface.

Atomic structures of TiO 2(1 1 0) surface between p(1×1) and p(1×2) studied by scanning tunneling microscopy

Structures of TiO 2(1 1 0) surfaces between p(1×1) and p(1×2) are observed by scanning tunneling microscopy (STM). We confirm existence of two types of p(1×2) rows by comparison with results of impact collision ion scattering spectroscopy (ICISS), i.e., one has Ti 2O unit rows on low reduced surface, and another has Ti 2O 3 unit rows on the heavily reduced surface. We also found that: (1) the low energy electron diffraction (LEED) on the Ti 2O surface shows the p(1×2) pattern though the every two spots for [ 1 ̄ 1 0] -direction is intense, and (2) the reflection high-energy electron diffraction (RHEED) on the Ti 2O surface shows the only p(1×1) pattern though the surface manifestly has p(1×2) periodicity. The growing process of p(1×2) surface with Ti 2O unit rows from the p(1×1) surface is also elucidated.

Ag/Cu(111) surface structure and metal epitaxy by impact-collision ion-scattering spectroscopy and scanning tunneling microscopy

We have investigated the growth of 3 monolayers (ML) of Ag on Cu(111) for substrate temperatures from 170 through 640 K by using time of flight-impact collision ion scattering spectroscopy (TOF-ICISS). Also, scanning tunneling microscopy (STM) topographs were taken after the deposition of 0.8 ML of Ag atoms at room temperature (RT). We observed that for deposition of Ag at substrate temperatures above 300 K, two different types of epitaxial growth exist: $\mathrm{Ag}[112\ifmmode\bar\else\textasciimacron\fi{}]\ensuremath{\parallel}\mathrm{Cu}[112\ifmmode\bar\else\textasciimacron\fi{}]$ (type-n) and $\mathrm{Ag}[\overline{11}2]\ensuremath{\parallel}\mathrm{Cu}[112\ifmmode\bar\else\textasciimacron\fi{}]$ (type-r). The growth modes of the Ag thin films on Cu(111) surfaces depend strongly on the temperature during deposition with the Ag(111) planes having a preferred orientation of either type-n growth mode or type-r growth mode as a function of the Cu substrate temperature. A part of the first-layer Cu atoms (20% of the surface) is displaced at low Ag coverage, where the stacking changes abruptly from fcc to hcp sites because of the Ag atom deposition at 603 K. The STM image of the Ag coverage of 0.8 ML showed a periodic array of triangular misfit dislocation loops at the deposition temperature of RT. The triangular shape in a localized region where the stacking {Ag---Cu (A)---Cu (B)---Cu (C)} is replaced by {Ag---Cu (C)---Cu (B)---Cu (C)} in the first substrate plane. At 603 K, surface alloying of the Ag--Cu system was confirmed for Ag coverage below 0.15 ML. The experimental results concerning Ag/Cu(111) show many similarities to those in the previous study of Au/Ni(111). This would suggest that observed oscillations in the growth mode, dependent on the substrate temperature during deposition, may be a general phenomenon on solid surfaces, in cases of large misfit since it has now been seen for both Au/Ni(111) and Ag/Cu(111) systems. Furthermore, Cu atoms deposited on the Ag/Cu(111) system form islands with the same orientation of Ag(111) planes. The Cu atoms undergo surface diffusion at room temperature in the direction of type-n domains for both type-r and type-n modes Ag substrates.

Transitional structures of the TiO 2(110) surface from p(1×1) to p(1×2) studied by impact collision ion scattering spectroscopy

The transitional states of surface structure of TiO 2(110) from p(1×1) to p(1×2) on low reduced to heavily reduced surfaces were studied using Impact Collision Ion Scattering Spectroscopy (ICISS). We found that (i) bridging oxygen rows were formed on low reduced surface at anneal temperature of 650°C for 2.5 h, (ii) Ti 2O unit rows were formed between the bridging oxygen rows on low reduced surface at anneal temperature of 930°C for 100 min, (iii) five-fold titanium atom rows on the surface and titanium atom rows of the Ti 2O rows are imperfectly covered with oxygen atoms on heavily reduced surface at anneal temperature of 600°C for 1 h and followed by 670°C annealing for 5 min after ion sputtering, and (iv) Ti 2O 3 unit rows proposed by Onishi and Iwasawa [H. Onishi and Y. Iwasawa, Surf. Sci. Lett., 313, 1994, L783] were completed by covering with oxygen atoms on all of the titanium atom rows on the Ti 2O rows on heavily reduced surface at higher anneal temperature of 1050°C for 40 min after ion sputtering. It is noticeable that the p(1×2) surface structures on low reduced surface is different from that of heavily reduced surface, viz., each has Ti 2O unit rows and Ti 2O 3 unit rows, respectively. The result contradicts “missing unit model” proposed by Bennett et al. [R.A. Bennett, P. Stone, N.J. Price, M. Bowker, Phys. Rev. Lett., 82, 1999, 3831].

Study of Diffusion and Defects by Medium-Energy Coaxial Impact-Collision Ion Scattering Spectroscopy

Study of Diffusion and Defects by Medium-Energy Coaxial Impact-Collision Ion Scattering Spectroscopy

Adsorption of Atomic Hydrogen on Ag-Covered 6H-SiC(0001) Surface

We have investigated atomic hydrogen (H) adsorption on an Ag-covered 6H-SiC(0001) surface, using coaxial impact-collision ion-scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA). It has been revealed that the two-dimensional (2D) layer of Ag on the 6H-SiC(0001) surface is changed to three-dimensional (3D) islands by the adsorption of H on the surface. The 3D Ag islands have an Ag(111) crystalline structure. When this surface is annealed at 550°C, the 3D Ag islands do not revert to a 2D layer but desorb from the surface together with H.