Unambiguous interpretation of atomically resolved force microscopy images of an insulator

The (111) surface of CaF 2 was imaged with dynamic mode scanning force microscopy and modeled using atomistic simulation. Both experiment and theory showed a clear triangular contrast pattern in images, and theory demonstrated that the contrast pattern is due to the interaction of a positive electrostatic potential tip with fluorine ions in the two topmost surface layers. We find a good agreement of position and relative height of scan line features between theory and experiment and thus establish for the first time an unambiguous identification of sublattices of an insulator imaged by force microscopy

Role of image forces in non-contact scanning force microscope images of ionic surfaces

AbstractWe consider the effect of the image interaction on the force acting between tip and surface in non-contact scanning force microscope experiments. This interaction is relevant when a conducting tip interacts with either a polar bulk sample or with a thick film grown on a conducting substrate. We compare the atomistic contribution due to the interaction between the microscopic tip apex and the sample with the macroscopic van der Waals and image contributions to the force on the tip for several representative NaCl clusters adsorbed on a metal substrate. We show that the microscopic force dominates above the plain (001) terrace sites and is solely responsible for image contrast. However, the image force becomes comparable to the microscopic force above the surface di-vacancy and dominates the interaction above a charged step

Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the non-relaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio \kappa of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Froehlich coupling constant \alpha and the screened Coulomb interaction strength \beta. Reasonable variation of these two parameters has demonstrated that the magnitude of \kappa remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between O^{2-} ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O$^-$ polarons bound by the cation vacancies (V^-) in MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses: [email protected], [email protected]

Recent Trends in Surface Characterization and Chemistry with High-Resolution Scanning Force Methods

The current status and future prospects of non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM) for studying insulating surfaces and thin insulating films in high resolution are discussed. The rapid development of these techniques and their use in combination with other scanning probe microscopy methods over the last few years has made them increasingly relevant for studying, controlling, and functionalizing the surfaces of many key materials. After introducing the instruments and the basic terminology associated with them, state-of-the-art experimental and theoretical studies of insulating surfaces and thin films are discussed, with specific focus on defects, atomic and molecular adsorbates, doping, and metallic nanoclusters. The latest achievements in atomic site-specific force spectroscopy and the identification of defects by crystal doping, work function, and surface charge imaging are reviewed and recent progress being made in high-resolution imaging in air and liquids is detailed. Finally, some of the key challenges for the future development of the considered fields are identified

Impact of Body-Thickness-Dependent Band Structure on Scaling of Double-Gate MOSFETs: A DFT/NEGF Study

First principles density functional theory has been used to calculate the 2-D band structure of Si slabs with different thicknesses. From the calculated 2-D band structure, electron longitudinal and transverse effective masses have been extracted as a function of the slab thickness. These thickness-dependent electron effective masses have then been used to simulate IID-V-G characteristics of scaled, sub-10 nm double-gate (DG) MOSFETs and to compare them with the results obtained using bulk masses. The channel thickness dependence of the Si band structure starts to affect noticeably DG MOSFET performance at channel lengths below 10-nm, lowering the ON-current by approximately 10, for transistors with a body thickness of 2.6 mn, and by 20, for transistors with a body thickness of 1.3 nm. On the other hand, the subthreshold swing is improved by 10, in the 6-nm-gate length DG MOSFET and by 15, in the 4-nm-gate length device. Finally, the impact of thickness-dependent effective masses has been related to the behavior of the transmission coefficient

Optical absorption and luminescence energies of F centers in CaO from ab initio embedded cluster calculations

We calculated the optical absorption and luminescence energies of electrons trapped at oxygen vacancies in CaO using a consistent embedded cluster method which accounts for the long-range polarization effects and partial covalence of CaO. Optical absorption and luminescence energies of neutral F center and positively charged F+ center vacancies are calculated by means of time dependent density functional theory using the B3LYP exchange-correlation density functional. Our results demonstrate that using large basis sets to describe a diffuse nature of excited states, and properly accounting for long-range polarization induced by charged and excited defect states, is crucial for accurate predictions of optical excitation and luminescence energies of these defects

Oxygen vacancies in amorphous silica: structure and distribution of properties

We used an ab initio embedded cluster method to study and compare three charged states of the Si-Si dimer configurations of oxygen vacancies in alpha-quartz and amorphous silica. The Si-Si bond in the neutral vacancy remains largely the same in both crystalline and amorphous SiO2. In alpha-quartz the positively charged dimer E' centre exists only as a metastable configuration, whereas in amorphous silica stable dimer configuration can be formed at favorable precursor sites. Our results demonstrate that negatively charged dimer oxygen vacancies can be formed in alpha-quartz