Low sputter damage of metal single crystalline surfaces investigated with medium energy ion scattering spectroscopy

It was observed clearly that the sputter damage due to Ar⁺ ion bombardment on metal single crystalline surfaces is extremely low and the local surface atomic structure is preserved, which is totally different from semiconductor single crystalline surfaces. Medium energy ion scattering spectroscopy (MEIS) shows that there is little irradiation damage on the metal single crystalline surfaces such as Pt(111), Pt(100), and Cu(111), in contrast to the semiconductor Si(100) surfaces, for the ion energy of 3–7 keV even above 10¹⁶–10¹⁷ ions/cm² ion doses at room temperature. However, low energy electron diffraction (LEED) spots became blurred after bombardment. Transmission Electron Microscopy (TEM) studies of a Pt polycrystalline thin film showed formation of dislocations after sputtering. Complementary MEIS, LEED and TEM data show that on sputtered single-crystal metal surfaces, metal atoms recrystallize at room temperature after each ion impact. After repeated ion impacts, local defects accumulate to degrade long range orders.     

Resonant charge transfer in low-energy ion scattering: Information depth in the reionization regime

Time-Of-Flight Low-energy ion scattering (TOF-LEIS) experiments were performed for He(+) ions scattered from Cu(100) and Cu(0.5)Au(0.5)(100). Probabilities for resonant neutralization and reionization in close collisions were deduced in a wide energy range. To learn about the information depth in LEIS, in a next step ion spectra were analyzed for polycrystalline Cu samples. The relative yield of backscattered projectiles, which have undergone distinct charge exchange processes, was calculated. Results indicate a strong contribution to the ion yield that origins from particles reionized in a close collision in deeper layers when experiments are performed at energies where reionization is prominent. The surface sensitivity of the ion signal at different energies is quantified. Based on these results, the total ion spectrum was quantitatively modelled by two consistent, but different approaches.     

Shell‐isolated nanoparticle enhanced Raman spectroscopy (SHINERS) investigation of benzotriazole film formation on Cu(100), Cu(111), and Cu(poly)

Benzotriazole (BTAH) is well known as an effective corrosion inhibitor for Cu because of its ability to make a coordination polymer film on the surface that provides a barrier to Cu oxidation. BTA⁻ film formation was investigated on single‐crystal and polycrystalline Cu surfaces with shell‐isolated nanoparticle enhanced Raman spectroscopy (SHINERS) using silica‐encapsulated Au nanoparticles. Potential‐dependent spectra display reversible film formation on polycrystalline Cu and irreversible film formation on single‐crystal Cu. Grain boundaries leading to smaller BTA⁻‐Cu oligomers are proposed to be the reason for cathodic degradation of the BTA⁻ polymeric films on polycrystalline Cu. Copyright © 2011 John Wiley & Sons, Ltd.     

In-beam orientation of low energy mass-separated radioactive beams: Development of the technique and first results

We report on the first results obtained with a system designed to polarize on-line mass-separated radioactive ion beams by scattering the beams from very flat single crystal surfaces at grazing incidence angles. Polarizations of 1 to 2% were already observed for deuterium.     

The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.     

Polarization bremsstrahlung of heavy charged particles in a polycrystal

Polarization bremsstrahlung (PBS) of a fast ion scattered in a polycrystalline medium is calculated and analyzed with allowance for the contributions from the coherent and incoherent channels of the process. It is shown that scattering of a projectile from the crystal lattice of the target is responsible for typical features of the PBS spectrum. For example, PBS is suppressed (as compared to radiation emitted by a single atom) in the low-frequency part of the spectrum, where coherent PBS dominates. In the intermediate spectral region, a step structure is formed as a result of “elimination” of the contribution from the reciprocal lattice vector with a preset magnitude to the coherent component of the process. Finally, incoherent PBS dominates in the high-frequency part of the spectrum and the process occurs as in the case of a single atom. These spectral peculiarities of PBS are determined by the structure of the target and depend on the velocity of the projectile and the emission angle, and can be observed in experiments dealing with radiation emitted by fast charged particles in thin polycrystalline films.     

Surface potentials and their measurement by the diode method

This review deals with the precise meaning of work functions and surface potentials for single crystal and polycrystalline surfaces. The methods of measuring surface potentials using a diode valve are described together with a number of the experimental diode cells used for these measurements. Particular attention is paid to the value of surface potentials measured by the diode method on polycrystalline surfaces.     

Experimental determination of the helium-metal interaction potential by interferometry of nanostructured surfaces

We present a direct experimental comparison of the helium-surface interaction potential for two unreconstructed metal surfaces. We analyze phase shifts in helium atom scattering from a nanostructured bimetallic surface to yield the relative shape and position of an adsorbate's potential with respect to the reference defined by the substrate. In our prototype system, submonolayer growth of Ni on Cu(100), the He-Ni/Cu(100) potential has an attractive well that is 1.6+/-0.4 meV shallower, and a repulsive wall 0.11+/-0.03 A closer to the ion cores, compared to the He-Cu(100) potential. Our observations provide a ready test of state-of-the-art theoretical calculations.     

Velocity scaling of ion neutralization in low energy ion scattering

The ion fraction P+ is measured for He+ ions scattered by 129 degrees from a Cu surface. Both the primary energy and the angles of incidence and of exit are varied. From our results we conclude the following: along the incoming and outgoing trajectories, neutralization is due to Auger processes and depends on the normal velocity component v( perpendicular ) only. At higher energies, additional charge exchange is due to collision induced neutralization and reionization, both depending on the total ion energy only. Also in this regime P+ depends on v( perpendicular ), but via a two-valued function of the scattering geometry at fixed energy.     

Production of high circular polarization by ion surface scattering of ferromagnets

From a theoretical analysis we predict large fractions of circular polarization of light emitted by ions scattered from surfaces of ferromagnetic single crystals. Maximum enhancement of circular polarization as compared to surface ion scattering from non-magnetic materials is again expected at grazing incidence. A circular polarization of 89% was estimated for the case of Ar⁺ ions scattered from the (110) plane of a Ni single crystal.     

Surface segregation in Cu-Ni and Cu-Pt alloys; A comparison of low-energy ion-scattering results with theory

Under equilibrium conditions the surfaces of Cu-Ni and Cu-Pt alloys are enriched in Cu. Since low-energy ion scattering is only sensitive to the outermost atomic layer of the surface, it is the ideal technique to study surface segregation. The results are compared with zeroth-and first-order bond-breaking theories. The influences of parameters such as relaxation, crystal face, atomic size and double-layer formation are discussed.     

Comparison of Low-Energy Neutral Scattering (LENS) with Low-Energy Ion Scattering (LEIS) at clean and adsorbate covered Ni surfaces

Ne and He projectiles were scattered at clean and sulphur or oxygen-covered polycrystalline Ni surfaces. The analysis of backscattered neutrals results in an oxygen to nickel surface atom density of 0.7 for NiO and a sulphur to nickel density of 0.25 for a saturation coverage of sulphur on polycrystalline nickel, which is in good accord with expectation. Only differential scattering cross-sections, which can be calculated with sufficient reliability enter the data reduction. This is a definite advantage over ion scattering which is complicated by the widely unknown neutralization probabilities.     

Optical properties of ion-modified metal surface in the infrared range

The results of study of ion-cleaned and -implanted surfaces of polycrystalline Be, Al, Ti, Fe, Cu, Mo, Zr, and W using optical and infrared (IR) spectroscopy are presented. It is shown that major changes in the optical reflectance spectra are observed in the range of 190–250 nm and are associated with both the formation of scattering centers and features of the chemical state of near-surface layers of materials. It is established using IR spectroscopy that treatment by Ar⁺ ion beam with a broad energy spectrum increases the reflection coefficient of the modified surface and additional resonance lines related to the formation of oxides themselves and hydroxyl groups are revealed in the reflectance spectrum in the case of formation of oxide film on the surface.     

Multiple scattering and neutralization aspects of low energy noble gas ions incident on a Cu (110) crystal

Experiments have been done with 1 keV Ne⁺ ions bombarding a Cu (110) single crystal in a (111) plane. From the measured energy spectra of the scattered ions the minimum and maximum scattering angles for multiple scattering on the surface are determined. These minimum and maximum scattering angles were also calculated using a computer model. The parameters for an interatomic potential function between ion and metal atom are determined by comparison of the experimental and calculated scattering angles. A neutralization model for an energetic ion at an atomic surface is given. The reliability of this model is tested by comparison of the results of computer calculations and the measured peak intensity distributions as a function of the scattering angle.     

Analysis of surface composition with low-energy backscattered ions

The use of low-energy (< 3 keV) ion beam scattering to characterize the surface properties of solids is reviewed. The elemental composition of the first monolayer of surface atoms can be derived from the energy spectrum of backscattered noble gas ions. Positive identification of surface impurity atoms is based on a simple, yet valid collision model involving only the primary ion and a single, isolated surface atom. Backscattered active gas ion spectra, in contrast, yield little information due to pronounced background similar to that observed in the high-energy Rutherford scattering experiments. These differences are ascribed to complicated neutralization phenomena and can be minimized in single crystal targets by utilizing channeling effects. This simple technique is shown to offer several advantages over existing techniques for characterizing thin films.     

Angular dependence of auger electron emission from Cu (111) and (100) surfaces

Experimental angular emission profiles of the M_2,3M_4,5M_4,562 eV Cu Auger electrons from clean copper (100) and (111) surfaces in several azimuths are presented. A simple single scattering theory to account for elastic scattering of the Auger electrons by other ion cores in the solid is presented, and calculations have been performed to assess the importance of this process in contributing to the observed angular dependence. These calculations produce angular structure having a similar magnitude and temperature dependence to that observed experimentally, and some featural similarity in peak positions or shapes. It appears that elastic scattering is an important source of angular dependence, and that studies of adsorbed species on surfaces should provide a very sensitive method of surface structure determination.     

On the energy distribution of sputtered dimers

A theoretical expression is obtained for the energy distribution of sputtered dimers from crystal surfaces. The derivation is based on a model where the atoms which constitute a dimer, are sputtered independently from the crystal according to their respective single particle energy distribution functions. Two neighbouring atoms are then supposed to form a dimer if the sum of their initial relative kinetic energy and potential energy is less than zero. A comparison with experimental results for K₂ and KI sputtered from polycrystalline K and KI surfaces respectively, shows a good agreement.     

Ion-scattering structure studies of UO2 surfaces

Planar (111) and stepped (553) surfaces of single crystalline UO₂ have been studied by low-energy He⁺ ion scattering spectroscopy, (ISS). Angular dependent ISS and comparison data from oxide-coated, polycrystalline U indicate that the outermost layer of UO₂(111) is oxygen occupying, to a first approximation, the lattice position expected from a simple termination of bulk structure. A similar study of UO₂(553) indicates that ledge U is covered with excess oxygen.     

Laser induced single-crystal transition in polycrystalline silicon

Transition to single crystal of polycrystalline Si material underlying a Si crystal substrate of 〈100〉 orientation was obtained via laser irradiation. The changes in the structure were analyzed by reflection high energy electron diffraction and by channeling effect technique using 2.0 MeV He Rutherford scattering. The power density required to induce the transition in a 4500 ? thick polycrystalline layer is about 70 MW/cm² (50ns). The corresponding amorphous to single transition has a threshold of about 45 MW/cm².     

Role of the 2D surface state continuum and projected band gap in charge transfer in front of a Cu(111) surface

Electron capture by Li+ and H projectiles in grazing scattering from Cu(111) and Cu(110) surfaces is studied experimentally and theoretically. Whereas data for Cu(110) can be described by established theoretical methods treating resonant charge transfer with a free-electron metal, data for Cu(111) show pronounced deviations from this approach. We interpret our observations by the effect of the projected L-band gap of the Cu(111) surface. In particular, the quantum states of reduced dimension (2D surface state continuum) play a dominant role in electron transfer.