Low-energy ion scattering is a powerful surface analysis technique for interrogating the structural and chemical makeup of a surface, as well as the electronic structure. We have used this technique to probe the local electrostatic potential of Bi₂Sr₂CaCu₂O₈, and corroborated the findings of density functional theory. Sputtering was conducted on this same material, revealing the formation of a Bi capping layer that prevents further damage, due to the low surface energy of unsputtered Bi. Finally, scattering was used to investigate charge exchange of the Si(111) surface as a function of doping, a fundamental study that revealed the bulk band gap has a strong effect on neutralization of scattered ions, despite the work function being unchanged with added dopants.

Low energy ion scattering (LEIS) is an experimental technique that is of great utility in surface investigations of novel materials. LEIS is uniquely sensitive to the composition of the outermost atomic layer of a solid and can be used to determine the structure of the lattice in the near-surface region of a single crystal. More recently, it has been shown that the neutralization of scattered low energy alkali ions, which occurs via resonant charge transfer (RCT), is sensitive to the localized surface electronic structure.

This thesis further demonstrates that RCT during LEIS reveals correlated-electron behavior at high temperature. The valence electron of a singly charged alkaline-earth ion is a magnetic impurity that interacts with the continuum of many-body excitations in the metal, leading to Kondo and mixed valence resonances near the Fermi energy. The occupation of these resonances is acutely sensitive to the surface temperature, which results in a marked temperature dependence of the ion neutralization. While the neutralization of magnesium (Mg) and gallium (Ga) scattered from polycrystalline gold (Au) shows little dependence on temperature, scattered strontium (Sr) does show an anomalous temperature dependence that provides clear evidence of electron correlations. The difference in neutralization between the alkaline earth Mg and Sr projectiles is discussed in terms of the velocity of the outgoing projectiles.

The Topological Insulator, Bismuth Selenide (Bi₂Se₃), is comprised of Se-Bi-Se-Bi-Se quintuple layers (QLs), so that Se should terminate the stable surface. After cleaving in ultra-high vacuum at room temperature, however, low energy electron diffraction shows order while low energy Na⁺ LEIS reveals a Bi-termination. After cleaving at 80 K, the Se:Bi ratio measured by LEIS is enhanced, but slowly decreases to the room temperature value. Density functional theory suggests that a Bi bilayer positioned atop the nominal Se termination is energetically favorable and consistent with ARPES results from the literature. It is thus concluded that Bi₂Se₃ cleaves between the QLs, but that a thermally activated process leads to the Bi termination. This observation may resolve issues concerning the long-term stability of such materials.

Topological insulator (TI) materials insulate in the bulk while conduct on the surface, which makes them promising for future quantum computing and spintronics. The momentum-spin locked electron states are concentrated in the first few atomic layers, which demands a surface sensitive technique to probe. Meanwhile, the stability of the surfaces under various conditions still needs more study before TIs can be further utilized. Low energy Ion scattering (LEIS) is an extremely surface sensitive tool for investigating both surface structure and electronic properties. In this thesis, the spatial distribution of the filled topological surface states (TSS), halogen and Cs adsorption and reaction on Bi2Se3 and Bi2Te3 TI surfaces and heterostructures of bismuth bilayers on TIs are studied using LEIS and other surface analysis techniques.

Graphene has attracted great interest in many fields due to its outstanding electronic and chemical properties. Among them, its surface inertness and high thermal stability makes graphene a promising candidate as a protective material for transition metal surfaces. Recent studies show, however, that small molecules, such as O2, CO and H2O, intercalate between a graphene film and a metal substrate at particular temperatures.

The intercalation of O2 between graphene and Ru(0001) is studied with 3 keV helium ion scattering and low energy electron diffraction. It is shown that O2 intercalates between the graphene and the Ru(0001) substrate at a temperature of 650 K and does not adsorb onto the graphene surface. Nevertheless, the graphene layer efficiently avoids both intercalation and adsorption of oxygen at room temperature. It is also found that the intercalated oxygen thermally desorbs from the surface after it is heated to 800 K. Such a desorption is not, however, observed for oxygen dissociatively adsorbed on a bare Ru(0001) surface until 1200 K. It is thus inferred that the oxygen intercalated between graphene and Ru(0001) is in a molecular form. In addition, part of the graphene overlayer is etched by a chemical reaction during the thermal desorption of oxygen.

The role of the defects on the graphene layer is also studied. Defects are introduced by 50 eV Ar+ sputtering, which creates single vacancies with a quick sputtering or larger open areas of substrate following a prolonged sputtering. It is found that oxygen molecularly adsorbs at single carbon vacancies even at room temperature, which does not occur on a complete graphene layer. Following post-annealing to 600 K, it is observed that such adsorbed oxygen diffuses to become intercalated between graphene and Ru(0001). Oxygen dissociatively adsorbs in the large open areas of exposed substrate by forming strong oxygen-metal bonds. It is also found that the presence of defects facilitates the intercalation of oxygen and improves the etching efficiency of the graphene during the desorption of oxygen.

Surface science techniques are used to probe Au nanoclusters in an attempt to understand fundamental properties that are related to their unusually high catalytic activity. The main technique used is the neutralization probability of low energy alkali ion scattering. This is performed on Au nanoclusters supported on either TiO2(110) or SiO2.

Au Nanoclusters are deposited onto TiO2(110) via buffer layer assisted growth in ultra-high vacuum, which is a novel process to grow clusters. A thin amorphous solid water buffer layer is condensed onto 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 procedure 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.

The neutralization of low energy Na+ and Li+ ions scattered from Au nanoclusters formed by deposition onto oxide surfaces decreases as the cluster size increases. An explanation for this behavior is provided, which is based on the notion that the atoms in the clusters are not uniformly charged, but that the edge atoms are positively charged while the center atoms are nearly neutral, as reported in the literature. This leads to upward pointing dipoles at the edge atoms that increase the neutralization probability of alkali ions scattered from those atoms. As the clusters increase in size, the number of edge atoms relative to the number of center atoms decreases, so that that the average neutralization also decreases. Calculations employing this model are compared to experimental data and indicate good agreement if the strength of the dipoles at the edge atoms are assumed to decrease with cluster size. This model also explains differences in the neutralization probabilities of scattered Na+ and Li+.

Catalytically active Au nanoclusters on SiO2 are exposed to Br2 and then measured using 1.5 keV Na+ low energy ion scattering. It is found that Br2 adsorbs on the nanoclusters, but not on the substrate nor on bulk Au. These results show that the clusters are able to dissociate the Br2 and then adsorb the individual Br atoms. Results from the literature indicate that the catalytic activity of nanoclusters occurs at the edges and the work from this dissertation confirms that the edge atoms are positively charged. This leads to the conclusion that the outer shells of the electronegative Br atoms become filled so that they ionically bond to the edge atoms of the clusters. Furthermore, Br2 is a known catalytic poison and this work shows how its adsorption blocks sites that would otherwise be involved in a surface chemical reaction.

Topological insulator (TI) materials do not conduct electricity in the bulk, but instead conduct along the surface via topological surface states (TSS). To fully utilize these materials, it is critically important to understand their surface atomic and electronic structures, both of which can be sensitively probed by low energy ion scattering (LEIS).

Bi2Se3 is a TI that consists of stacked quintuple layers ordered as Se-Bi-Se-Bi-Se. The surfaces are expected to show a Se-termination after mechanical cleaving, but instead various surface terminations are found. Time-of-flight (TOF) LEIS spectra show that Bi2Se3 surfaces prepared by in situ cleaving or by Ar+ ion bombardment and annealing (IBA) are always Se-terminated. Impact collision ion scattering spectroscopy (ICISS) finds no differences in the geometric structure of the top three atomic layers of IBA-prepared surfaces and in situ cleaved surfaces. Note that the annealing temperature is critical in preparing surfaces with IBA. If it’s too low, then surface will not be well-ordered. If it’s too high, then surface Se vacancies will form. Ex situ cleaved Bi2Se3 surfaces can be either Se-terminated or Bi-rich, which might be caused by contaminants that adsorb at surface defects and induce a chemical reaction. After exposure to air, IBA-prepared surfaces show less surface contamination than cleaved surfaces and maintain their Se-termination.

Cs is adsorbed on Bi2Se3 as a tool to investigate the surface electronic properties of TIs. Scattered alkali ions exchange charge with surfaces through a non-adiabatic resonant charge transfer (RCT) process that depends on the local electrostatic potential (LEP) above the target atom. It is found that the neutralization probability for Na+ scattered from Se sites is unexpectedly larger than from Bi sites. This can be explained by the spatial distribution of the TSS carriers in which the electron density accumulates below the first layer Se atoms and above the second layer Bi. This spatial distribution leads to an upward pointing dipole on Se sites that reduces the LEP and a downward dipole above Bi that increases the LEP.

Resonant charge transfer during the backscattering of 3.0 keV Li+ from hydrogen- and iodine-covered Ni(100) is probed with time-of-flight spectroscopy. Hydrogen adsorption on Ni(100) induces only a small increase of the surface work function and the neutralization probabilities for backscattered Li are not significantly affected. Iodine adsorbs with some net negative charge, so that a dipole directed into the surface is expected. Such a dipole would increase the work function thereby decreasing the neutralization probability. iodine adsorption decreases the work function of Ni(100), however, and the neutralization probabilities for Li scattered from the iodine sites are always larger than for scattering from nickel sites. These results suggest that the local charge density associated with adsorbed iodine is not uniform. (C) 2003 American Vacuum Society.

Low energy ion scattering (LEIS) is a technique found to be sensitive to the structural and electronic properties of metal on oxide systems. Here, LEIS is used to investigate the effect of sputtering Pt deposited on TiO2 as well as the effect of surface temperature on Au nanoclusters on SiO2. In the case of Pt/TiO2, it was found that sputtering induces a "strong metal-surface interaction" (SMSI) state, where the surface atoms become mobilized and move to cover the Pt. Previously, this state has been observed when heating the sample, but in this case the sputter beam was found to mobilize the substrate atoms instead of an elevated temperature. K+ scattering was used to examine Au/SiO2 at temperatures ranging from 100 to 300 K. It was found that the probability that an ion would be neutralized during scattering is inversely and reversibly proportional to the temperature. Finally, Na+ was used to examine Au/SiO2 after annealing to temperatures ranging from 300 to 1000 K. It was found that the neutralization probability is dependent not only upon the annealing temperature, but also whether the ion undergoes single or multiple scattering. The ratio of single scattering to multiple scattering neutralization probability may provide another tool to analyze surface evolution.

Time-of-flight spectra were collected for 2.5 keV Li-7(+) backscattered from Fe surfaces covered with submonolayers of iodine. Li singly scattered from the adatoms has a consistently larger neutral fraction than for scattering from the substrate, implying a region of positive charge atop the iodine. The neutral fraction decreases for off-normal exit angles, indicating a nonuniform charge distribution around the polarizable adsorbates. This demonstrates that ion scattering can image the internal electronic structure of an adatom and provides an explanation for anomalous work function changes.