AFM tip characterization by Kelvin probe force microscopy

Reliable determination of the surface potential with spatial resolution is key for understanding complex interfaces that range from nanostructured surfaces to molecular systems to biological membranes. In this context, Kelvin probe force microscopy (KPFM) has become the atomic force microscope (AFM) method of choice for mapping the local electrostatic surface potential as it changes laterally due to variations in the surface work function or surface charge distribution. For reliable KPFM measurements, the influence of the tip on the measured electrostatic surface potential has to be understood. We show here that the mean Kelvin voltage can be used for a straightforward characterization of the electrostatic signature of neutral, charged and polar tips, the starting point for quantitative measurements and for tip-charge control for AFM manipulation experiments. This is proven on thin MgO(001) islands supported on Ag(001) and is supported by theoretical modeling, which shows that single ions or dipoles at the tip apex dominate the mean Kelvin voltage.Peer reviewe

Synthesis of nanocomposite glass-like films containing semiconductor nanocrystals and noble bimetallic colloids by sol-gel route and their characterisation

Zwei Arten von Gläsern liegen im Schwerpunkt des aktuellen Interesses und werden zur Zeit intensiv untersucht. Dies sind zum einen dotierte Gläser mit halbleitenden Nanopartikeln und solche mit Edelmetallkolloiden. Zusätzlich zu ihren Anwendungen in der nichtlinearen Optik kann man die mit Halbleiterkolloiden dotierten Gläser dabei als zukünftige optische Filter ansehen. Edelmetallkolloidhaltige Gläser können als farbige Beschichtungen verwendet werden. Gegenstand dieser Arbeit war daher die auf dem Sol-Gel-Prozeß basierende Herstellung von nanokomposithaltigen glasartigen Beschichtungen, welche halbleitende Nanopartikel sowie gemischte und legierte Kolloide zweier Edelmetalle enthalten.Two type of glasses are currently of interest and being studied extensively. One is a glass doped with semiconducting nancrystals while the other type is doped with noble metal colloids. In addition to their applications in non-linear optics, the first type glasses can also be seen as future optical filters and the second type of glasses can be used as coloured glasses. Therefore the present work was carried out with an objective of synthesizing nano-composite glass like thin films containing semiconductor nanocrystals and mixed- and alloy- noble bimetallic colloids using the sol-gel process

Synthesis of nanocomposite glass-like films containing semiconductor nanocrystals and noble bimetallic colloids by sol-gel route and their characterisation

Zwei Arten von Gläsern liegen im Schwerpunkt des aktuellen Interesses und werden zur Zeit intensiv untersucht. Dies sind zum einen dotierte Gläser mit halbleitenden Nanopartikeln und solche mit Edelmetallkolloiden. Zusätzlich zu ihren Anwendungen in der nichtlinearen Optik kann man die mit Halbleiterkolloiden dotierten Gläser dabei als zukünftige optische Filter ansehen. Edelmetallkolloidhaltige Gläser können als farbige Beschichtungen verwendet werden. Gegenstand dieser Arbeit war daher die auf dem Sol-Gel-Prozeß basierende Herstellung von nanokomposithaltigen glasartigen Beschichtungen, welche halbleitende Nanopartikel sowie gemischte und legierte Kolloide zweier Edelmetalle enthalten.Two type of glasses are currently of interest and being studied extensively. One is a glass doped with semiconducting nancrystals while the other type is doped with noble metal colloids. In addition to their applications in non-linear optics, the first type glasses can also be seen as future optical filters and the second type of glasses can be used as coloured glasses. Therefore the present work was carried out with an objective of synthesizing nano-composite glass like thin films containing semiconductor nanocrystals and mixed- and alloy- noble bimetallic colloids using the sol-gel process

Comprehensive studies on the electrical transport of some chalcogenide semiconductors: frequency- and temperature-dependent AC conductivity

The AC conductivity of chalcogenide semiconductors doped with Ag2S was extensively studied, not only for applications in devices but also for academic interests. X-ray diffraction studies reveal the presence of GeS, Ag2S, Se5.1S1.9, Se2.57S5.43, Ag2Se, S3Se5, Se4.7S3.3, and Ag8S nanocrystallites. The characteristic vibration that appeared in the range 500–600 cm−1 is due to the Ag–S bond, and the vibrations at 3,700 and 1600 cm−1 can be assigned as the bending and stretching vibrations of the O–H bond, which may be formed due to the adsorption of H2O molecules on the Ag2S surface. DC electrical conductivity can be increased by optical phonon frequency, which may be involved in the enhancement of structural vibrations. At low temperatures, the “density of states” increases from 3.337 × 1019 to 2.396 × 1021 eV−1 cm−3, and at high temperatures, it enhances from 3.417 × 1028 to 1.1356 × 1031 eV– 1 cm−3. The correlated barrier hopping model explores the maximum barrier height for composition, x = 0.1 as 0.0292 eV. The modified non-overlapping small polaron tunnelling model reveals the polaron transfer activation energy for x = 0.2 as 0.09110 eV. The independence of the electrical relaxation process of the system on temperature and its dependence on composition were exhibited by the scaling of the conductivity spectra

Molecular Crystallization Directed By Nanoparticles And Nanopatterns

This dissertation focuses on nanoparticle/nanopattern-induced molecular self-assembly and crystallization. We propose a universal strategy to synthesize organic/inorganic hybrid nanostructures using inorganic nanoparticles and nanoparticle arrays to induce organic crystalline compound nucleation and crystallization. The nanoconfinement effect will be addressed to study molecular crystallization by changing nanoparticle size, curvature and separation distance. Atomic force microscopy (AFM) is the main characterization tool to investigate the topography of the hybrid nanostructures and the nucleation and crystallization process. This work will contribute to the understanding of seed mediated nucleation and crystallization, molecular ordering under nanoconfinement and hybrid nanomaterials

ICCG-10: Tenth International Conference on Crystal Growth. Oral presentation abstracts

Oral presentation abstracts from the tenth International Conference on Crystal Growth (ICCG) (Aug. 16-21, 1992) are provided. Topics discussed at the conference include superconductors, semiconductors, nucleation, crystal growth mechanisms, and laser materials. Organizing committees, ICCG advisory board and officers, and sponsors of the conference are also included

Development of Self-assembly Templating Methods for Architecture of Porous Core-shell Nanocomposites

Ph.DDOCTOR OF PHILOSOPH

Data-Driven Approaches to Complex Materials: Applications to Amorphous Solids

While conventional approaches to materials modeling made significant contributions and advanced our understanding of materials properties in the past decades, these approaches often cannot be applied to disordered materials (e.g., glasses) for which accurate total-energy functionals or forces are either not available or it is infeasible to employ due to computational complexities associated with modeling disordered solids in the absence of translational symmetry. In this dissertation, a number of information-driven probabilistic methods were developed for the structural determination of a range of materials including disordered solids to transition metal clusters. The ground-state structures of transition-metal clusters of iron, nickel, and copper were determined by a force-biased Monte Carlo method and their structural and electronic properties were studied comparatively via force-biased Monte Carlo and ab initio simulations. The force-biased Monte Carlo approach has shown unambiguously that it can effectively determine the putative ground-state structures of a number of small transition-metal clusters. For complex amorphous materials, an information-driven probabilistic viewpoint was adopted by posing structural determination of disordered solids as an inferential program and the problem of materials design was addressed as an optimization program, jointly supported by experimental data and information. The hallmark of this new approach is that it can produce atomistic configurations of amorphous solids, which are thermodynamically stable and close to a stable local minimum of a quantum-mechanical total-energy functional. The models have structural, topological, electronic, and vibrational properties comparable to experiments. The data-driven approach presented here for amorphous solids not only can produce overall structural and electronic properties but also the microstructural properties of realistic samples from experiments, such as voids and vacancy-type defects, which cannot be addressed directly using currently available computational methods. Ab initio hydrogen dynamics were simulated inside nanometer-size voids in a-Si within the framework of the density-functional theory and the study revealed that the microstructure of the hydrogen distribution and the morphology of the voids were characterized by the presence of a significant number of monohydride Si–H bonds, along with a few dihydride Si–H2 configurations but not any isolated hydrogen. The study also revealed that a considerable number of total H atoms inside voids can appear as H2 molecules. The densities of the bonded and nonbonded hydrogens are observed to be consistent with those from the infrared and Rutherford backscattering spectrometry measurements