Low Energy Ion Scattering Measurements 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.

Temporally resolved LEIS measurements of Cr segregation after preferential sputtering of WCrY alloy

The dynamic behaviour of thermally driven segregation of Cr to the surface of WCrY smart alloy is studied with low energy ion scattering (LEIS). Sputtering the WCrY sample with 500 eV D2+ ions at room temperature results in preferential removal of the lighter alloy constituents and causes an almost pure W surface layer. At elevated temperatures above 700 K the segregation of Cr atoms towards the surface sets in and prevents the formation of a pure W layer. The simultaneous heating and sputtering of the sample leads to a surface state which reflects the balance between sputter removal and segregation flux, and deviates from the equilibrium due to thermally driven segregation. Stopping the sputter ion beam allows the system to relax and develop toward the segregation equilibrium. The time constants for the temporal changes of W and Cr surface coverage are obtained from a series of LEIS measurements. The segregation enthalpy is determined from the time constants obtained for various sample temperatures.

Experimental characterization of hydrogen adsorption sites for H/W(111) using low energy ion scattering

Low energy ion scattering (LEIS) and direct recoil spectroscopy (DRS) are among the few experimental techniques that allow for the direct detection of hydrogen on a surface. The interpretation of LEIS and DRS measurements, however, is often made difficult by complexities that can arise from complicated scattering processes. Previously, these complexities were successfully navigated to identify the exact binding configurations of hydrogen on a few surfaces using a simple channeling model for the projectile ion along the surface. For the W(111) surface structure, this simple channeling model breaks down due to the large lateral atomic spacing on the surface and small interlayer spacing. Instead, our observed hydrogen recoil signal can only be explained by considering not just channeling along the surface but also scattering from subsurface atoms. Using this more complete model, together with molecular dynamics (MD) simulations, we determine that hydrogen adsorbs to the bond-centered site for the W(111)$+\text{H}$(ads) system. Additional MD simulations were performed to further constrain the adsorption site to a height $h=1.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ and a position ${d}_{\mathrm{BC}}=1.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ along the bond between neighbors in first and second layers. Our determination of the hydrogen adsorption site is consistent with density functional theory simulation results in the literature.

Low energy ion scattering as a depth profiling tool for thin layers - Case of bromine methanol etched CdTe

We have investigated the properties of the Te-rich surface layer formed after a bromine-methanol etch of CdTe single crystal by two methods: Angle Resolved X-Ray Photoelectron Spectroscopy (ARXPS) and Low Energy Ion Scattering (LEIS) in the Dynamic mode. We compare the acquisition time of each method. The results showed similar, exponential decay of the Te/Cd ratio to a depth of 6 nm. At the depths higher than 6 nm, the substrate becomes stoichiometric. Dynamic LEIS provided more detailed information about composition at depths lower than the probing depth of ARXPS. The Dynamic LEIS measurements suggest that the composition of the outermost layer of CdTe after bromine-methanol etching consists of CdTe4.

Low‐Energy Ion‐Scattering Spectroscopy of Modified Silicate Glasses

Low‐Energy Ion‐Scattering (LEIS) spectroscopy is a technique with a unique sensitivity to the elemental composition of the top atomic layer of a solid surface. LEIS measurements of ternary silicate glasses modified with Na2O, Cs2O, CaO, and BaO show that the compositions of the as‐cast (melt) surface and the in‐vacuum fracture surface often differ. While the as‐cast surface is usually depleted of alkali ions (Na+ or Cs+) compared to the nominal (batch) glass composition, there is often strong accumulation of the same mobile cations on the fresh fracture surface. Depth profiles obtained by sputter etching reveal elemental concentration gradients normal to the glass surface. The final concentrations often fail to reach the nominal glass composition, suggesting the likely presence of preferential sputtering effects and thereby the distortion of the measured concentration gradient. At present, the lack of reliable standards and preferential sputtering effects in the LEIS of multicomponent glasses limit somewhat the absolute chemical composition and structural information that can be obtained with this otherwise unique and powerful method of surface analysis.

Low-Energy Ion Scattering spectroscopy of silicate glass surfaces

Low-Energy Ion Scattering (LEIS) spectroscopy is a technique with a unique sensitivity to the elemental composition of the surface top atomic layers of a solid. LEIS measurements of simple binary silicate glasses (with Na2O, Cs2O or BaO) have shown differences in composition between the as-cast glass surface and the vacuum-fracture surface. The as-cast surface is usually depleted of modifier ions compared to the nominal (batch) bulk glass composition, while the fracture surface is typically enriched with the monovalent modifier species, but not the divalent barium. While the melt surface depletion can probably be attributed to evaporation from the hot melt surface, the origin of the strong accumulation on the fracture surfaces is less clear. It is certainly possible that the fracture may have been preferentially guided by regions already alkali-enriched within the bulk of the glass. But considering the extreme surface sensitivity of this method, a surface-monolayer localized enrichment with alkali due to the stress gradient created by the fracture or the electric field created by dangling bonds on the vacuum-fractured surface is also possible. The Si/O ratio shows usually little deviation from the nominal glass batch value, falling within the estimated errors.

Real-space ion scattering maps of the Mg(0001) surface

Low-energy ion scattering (LEIS) is useful for examining the structure of ordered surfaces as well as for identifying surface atoms. However, in some cases the structural information from LEIS measurements is obscured by complex collision processes which contribute to the detected scattering intensity. In this study, we have developed a more precise approach for examining surface structure that includes comparing experimental real-space ion scattering maps with simulations from binary collision codes using reliability factors. This method is demonstrated with the model system 2 keV ${\text{Ne}}^{+}\ensuremath{\rightarrow}\text{Mg}(0001)$. Using an angle-resolved ion energy spectrometer, the intensity of scattered ${\text{Ne}}^{+}$ from the surface was recorded for a complete set of polar and azimuthal angles, which define the orientation of the surface with respect to the incident beam. These angles were then transformed to distances in real space and used to compile an ion scattering map of the Mg(0001) surface. A simulated map was also generated for the same conditions using a modified version of the binary collision code MARLOWE. The maps provide a comprehensive overview of surface scattering and allow the locations of surface atoms to be correlated directly to regions of enhanced scattering intensity. The sensitivity of the LEIS signal to interatomic spacing was simulated using MARLOWE, and methods for comparing with experiments were developed. Because LEIS can distinguish different types of atoms on the surface, the techniques described here could be extended to map compound surfaces and adsorbates.

A novel procedure for elimination of the peak deviations in LEIS spectra influenced by the primary ion beam profile

Quantitative surface composition analysis using low energy ion scattering (LEIS) can be obstructed by the deviations introduced by the primary beam profile and the analyzer optics. A novel procedure is presented for excluding these deviations in LEIS measurements. The LEIS spectra were calculated from the assumed energy distribution, primary beam profile and the analyzer optics. From the calculated energy spectra, the instrumental functions were determined as a deconvolution of the energy spectra and corresponding energy distributions. The obtained instrumental functions can be very well approximated by the Gaussian. It is convincingly shown that the energy distribution evaluation, by means of the deconvolution of the energy spectrum and the calculated instrumental function, significantly suppresses the problems of qualitative (the peak shift) and the quantitative (the peak intensity deviation) surface composition analysis due to the different geometric profiles of the primary ion beam. The calculation method was then successfully applied for the analysis or the real experimental data that are not too noisy.

Influence of compaction and surface roughness on low‐energy ion scattering signals

AbstractInvestigation of the surface composition of powders often requires compaction. To study the effect of compaction on surface analysis, samples have been compacted at various pressures ranging from 0 Pa (i.e. no compaction) up to 2000 MPa (2 × 104 kg cm−2) Low‐energy ion scattering (LEIS) was used to determine the composition of the outermost atomic surface layer. Using scanning electron microscopy, changes in the morphology due to compaction have been detected in the SiO2 test samples. The LEIS yield of a compacted silica powder is found to be independent of the applied pressure during compaction between 2 MPa and 2000 MPa (2 × 104 kg cm−2). Analysis of a submonolayer of Ta2O5 on a silica support shows that the composition of the outermost atomic layer is not changed after compaction up to a pressure of at least 300 MPa. When compaction is applied, the absolute LEIS yield appears to be independent of the specific surface area of silica supports in the range 50–380 m2 g−1. A minor difference in LEIS signals is observed between compacted silica supports and flat quartz samples. In order to determine the surface roughness factor independently, and to study the material dependence of the surface roughness factor, angle‐dependent LEIS measurements have been carried out on oxidized silicon, gallium and gold surfaces. The results on the oxidized silicon confirm the small influence of surface roughness for silica particles, whereas measurements on the more closely packed metallic gallium and gold surfaces indicate a significant surface roughness effect. Copyright © 2004 John Wiley & Sons, Ltd.

The distribution of InCl x compounds in model polymeric LEDs: A combined low and high-energy ion beam analysis study

A combination of low- and high-energy ion beam analysis techniques was used to determine the distribution of indium chloride compounds in model polymeric light-emitting diodes (p-LEDs). Parts of polymeric LEDs (polydialkoxyphenylenevinylene (OC 1C 10-PPV) on indium-tin-oxide (ITO) substrates) were exposed to a HCl/Ar flow to simulate the processes occurring during conversion of precursor PPVs and acid treatment of polymers. Samples with variable exposure times as well as pristine samples were studied with Rutherford backscattering spectrometry (RBS), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS) and particle induced X-ray emission (PIXE). The RBS measurements show that after HCl exposure indium is distributed throughout the OC 1C 10-PPV layer. LEIS and XPS measurements indicate that the indium and chlorine are present at the outermost surface of the OC 1C 10-PPV layer. PIXE measurements in combination with the RBS data demonstrate that the indium chloride in the OC 1C 10-PPV layer is distributed in blisters (∅=50 μm) which are orientated perpendicular to the sample surface. To study the OC 1C 10-PPV/ITO interface, samples were delaminated by peeling the polymeric layer of the ITO using Scotch tape. LEIS measurements on the fracture planes demonstrate the formation of an interfacial layer between the ITO and the PPV. The implications for p-LEDs are discussed.

Intramolecular segregation in polymers and macromolecules studied by low-energy ion scattering

The intramolecular segregation in several generations of poly(propylene imine) dendrimers and oxygen plasma modified HDPE was studied with low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). Although the different generations of poly(propylene imine) dendrimers contain almost the same atomic carbon to nitrogen (C/N) ratio, it is shown that this ratio at the outermost surface depends strongly on the generation of the dendrimer. The conformation of a dendrimer is probably related to the flexibility of the bis(3-aminopropyl)amine endgroups, which becomes lower for higher generations. Furthermore, the formation of a metallodendrimer, by complexation of CuCl 2 with the amine endgroups of a fourth generation dendrimer (D4), was followed with LEIS. Compared to pure CuCl 2, a relatively high copper concentration and a high atomic copper to chlorine ratio was found at the surface of the metallodendrimer. The high copper concentration and the reduced sterical hindrance by chlorine demonstrate the high potential of the metallodendrimers as catalytic material. The hydrophobic surface of HDPE can be made hydrophilic by treating it in an oxygen plasma. During such a treatment, oxygen containing functional groups are introduced at the surface, which improves wetting and thus adhesion. Unfortunately, the effect of an oxygen treatment is only temporary, an effect referred to as ageing or hydrophobic recovery. LEIS has been used to study this ageing process. We were able to follow the surface oxygen concentration as a function of the time between the plasma treatment and the LEIS analysis. Combined information obtained from LEIS and XPS measurements indicates that the ageing is mainly confined to the outermost atomic layers. Moreover, the ageing process depends strongly on the exact experimental conditions.

Composition of the (1 1 0) surface of the Fe–Ni 34 at.% alloy: a study by low-energy ion scattering

The composition of the (1 1 0) surface of the Fe–Ni 34 at.% alloy has been determined by means of low-energy ion scattering (LEIS). Since the masses of the iron and nickel nuclides are rather close, a curve fitting procedure was needed to determine the contribution of the two metals to the LEIS signal. The LEIS measurements have been performed in a temperature range between room temperature and 850 K. Shadow cone effects were exploited, measuring the LEIS spectra with the ion beam incident along appropriate azimuthal direction, in order to determine the composition of the topmost layer only. We found that the surface concentration of nickel increases for increasing temperatures. At 850 K the composition of the topmost layer is 41±2 at.% of Ni.

Low-energy ion scattering (LEIS) study of the ageing of oxygen-plasma-treated high-density polyethylene (HDPE) surfaces

Static low-energy ion scattering (LEIS or ISS) and x-ray photoelectron spectroscopy (XPS) are used to study the surface of high-density polyethylene (HDPE) that has been modified by exposing it to a 'cold' oxygen plasma. The HDPE surface composition is monitored by both techniques as a function of time after the plasma treatment. With LEIS, large differences in ageing are observed between a sample that is exposed to air after the plasma treatment and a sample that is not exposed to air, but rather kept under vacuum. The hydrophobic recovery of the sample exposed to air is very large, whereas no recovery is observed on the other sample. The XPS measurements also show a decrease in oxygen concentration as a function of time for the sample that was exposed to air, but not as steep as in the LEIS measurements. The differences between the air- and vacuum-exposed samples are explained in terms of differences in composition and cross-linking in the surface region. Static LEIS seems to be a promising method to study the ageing of plasma-treated polymers.

Low-Energy Ion Scattering Measurements from an Al-Pd-Mn Quasicrystal

AbstractEnergy-angle distributions of low-energy inert-gas ions scattered from surfaces provide information about surface composition and structure. We have measured energy spectra of He+ scattered from an Al71Pd20Mn9 quasicrystal, which was oriented perpendicular to the 5-fold axis, along various azimuthal directions. Strong scattering signals are seen from Al and Pd, but only a weak Mn signal is observed. From measurements made of He+ at an oblique angle of incidence scattered in the forward direction, we observe a 72° periodicity in the azimuthal dependence of the scattering signal intensity from Al surface atoms. The effect arises from shadowing effects involving neighboring surface atoms and provides direct evidence that Al surface atoms exist in a local environment with 5-fold symmetry. In addition, measuring the variation of the signal intensity with incidence angle provides information about neighboring atom distances, which compare favorably with a model of the quasicrystal surface derived from the bulk structure.

Cu/ZnO and Cu/ZnO/SiO2 Catalysts Studied by Low-Energy Ion Scattering

Low-Energy Ion Scattering (LEIS) was employed to study the surface of unsupported and silica-supported Cu/ZnO catalysts. It was shown that, by applying isotopic enrichment (63Cu and 68Zn), the copper and zinc signals can be separated. LEIS measurements showed a considerable amount of lead on the surface of the 63Cu/68ZnO catalyst. It was concluded that lead originated from the 68ZnO raw material and segregated during catalyst reduction to the surface. In contrast, no impurities were detected on the surface of the 63Cu/68ZnO/SiO2 catalyst. This seemingly contradictory observation was attributed to the higher degree of copper and zinc dispersion in the latter catalyst or interaction of lead with the support. It was shown previously, that catalytic activity of Cu/ZnO/SiO2 catalysts in methanol synthesis and ester hydrogenolysis is proportional with the reduction temperature applied (600−750 K). A depth profile of copper and zinc obtained for the high-temperature reduced silica-supported catalyst revealed that the surface was enriched in zinc, whereas copper was more abundant in layers beneath the surface. In the low-temperature reduced catalyst, the zinc enrichment was less prominent. These results suggest that the reduction at high-temperature results in segregation of zinc oxide to the surface.

COMPOSITIONAL OSCILLATIONS AND STRAIN EFFECTS AT THE ANNEALED Co/Pd(100) INTERFACE: A STUDY BY X-RAY PHOTOELECTRON DIFFRACTION AND LOW ENERGY ION SCATTERING

The properties of the interface obtained by cobalt deposition on the Pd(100) surface and successive annealing have been studied by X-ray photoelectron diffraction (XPD) and low energy ion scattering (LEIS). The "as deposited" cobalt layer shows considerable lattice strain (about 10%) for coverages up to approximately 20-monolayer equivalents. Annealing at temperatures of the order of 700 K leads to intermixing with the formation of a surface alloy and the partial release of the strain. The combination of LEIS and XPD measurements shows that cobalt in the surface phase is preferentially located in the second outermost layer from the surface.

Ultrathin metal films on a metal oxide surface: Growth of Au onTiO2(110)

We have studied the growth of ultrathin Au films on the ${\mathrm{TiO}}_{2}$ (110) surface using low-energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). For substrate temperatures of 160 and 300 K, for fractional monolayer Au coverages, Au evaporated under UHV conditions appears to grow initially on the stoichiometric ${\mathrm{TiO}}_{2}$ (110) surface in a quasi-two-dimensional (2D) form. As the average Au coverage increases to one monolayer (1 ML) and higher, Au forms three-dimensional (3D) islands. The coverage at which 3D islands become apparent in LEIS data decreases as the surface temperature increases. No evidence of significant chemical interactions has been found between the Au and ${\mathrm{TiO}}_{2}$ substrate. By annealing ${\mathrm{A}\mathrm{u}/\mathrm{T}\mathrm{i}\mathrm{O}}_{2}$ to temperatures up to 775 K, the Au islands continue to grow; encapsulation of the Au islands by Ti suboxides is not observed. There is little or no CO adsorption on small Au clusters at 300 K (upper limit of \ensuremath{\sim}0.05 ML), based on sensitive LEIS measurements.

Sequential Saturating Reactions of ZrCl4 and H2O Vapors in the Modification of Silica and γ-Alumina with ZrO2

Zirconia-modified silica and alumina samples were prepared by the atomic layer epitaxy technique using successive saturating reactions of ZrCl4 and H2O vapors. The growth of ZrO2 was followed by X-ray diffraction, diffuse reflectance IR Fourier transform spectroscopy, and 1H magic angle spinning NMR measurements. In addition, coverage of the silica surface was estimated by low-energy ion scattering (LEIS). At the reaction temperature of 300 °C the main reaction of ZrCl4, on both supports, occurred with surface OH groups, leading to amorphous ZrClx species. In addition, part of the molecules reacted to form ZrO2 agglomerates while simultaneously chlorinating OH groups of the supports. Water treatment removed Cl from the ZrClx surface species, leaving behind Zr−OH groups, and released the directly chlorinated OH groups. Both the reappearing OH groups of the support and the Zr−OH groups were then again available during the next ZrCl4 treatment for both main reactions of ZrCl4. As the reaction cycles were repeated the concentration of amorphous Zr in the samples increased together with the amount of crystalline ZrO2 and surface coverage. On silica, after five cycles of repeated reactions when an average surface density of 5 Zr/nm2 had been achieved, 25% of the original accessible OH groups were still present on the surface. At that point, LEIS measurements showed 50% coverage. On the surface of both silica and alumina, ZrOx was present in the form of tetragonal agglomerates and an amorphous network phase. Two types of Zr−OH groups were identified.

Pt 25Ni 75(100) and (110) single crystals: preferential sputtering and segregation reversal

After annealing at 970 K low energy ion scattering (LEIS) reveals the topmost atomic layer of Pt 25Ni 75(100) to be enriched in Pt to a value of 39 at%, whereas for Pt 25Ni 75(110) the first monolayer is found to consist nearly exclusively of Ni atoms and the second layer shows strong Pt enrichment. LEIS measurements performed after ion bombardment at room temperature indicate the superposition of the effects of preferential sputtering of Ni atoms and segregation. Segregation is found to start at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion. Most of the theoretical models fail to reproduce the orientation dependent change in the segregating component of the PtNi system. One of the few successful approaches is a thorough thermodynamic description which is used here to calculate the composition profile and to discuss the segregation behaviour. Good agreement between measurement and calculation is found.

Surface sites and mobilities of In atoms on a stepped Cu( 100) surface studied at low coverage

The various surface sites of In atoms deposited to a coverage of 0.013 monolayer (ML) onto a stepped Cu(100) surface were determined with low-energy ion scattering (LEIS) as a function of deposition temperature. From the fractions of In atoms occupying different sites, observed in the temperature range 63–300 K, the mobilities of In atoms on the terraces and along the steps were deduced. It was found that during the LEIS measurements with 6 keV Ne ions at temperatures below 120 K Cu adatoms were accumulating at the surface. This is explained by diffusion to the surface of interstitial Cu atoms created in the damage cascades. For temperatures above 120 K these Cu adatoms were found to be mobile on the Cu(100) terraces.