En raison de leur compatibilité avec la technologie conventionnelle du silicium, les nanofils de silicium semblent très prometteurs pour être utilisés comme briques de base de composants électroniques à l'échelle. Ce travail de thèse se focalise sur la croissance épitaxiale et la caractérisation de tels nanofil. Les nanofils de silicium sont fabriqués par la méthode Vapeur-Liquide-Solide (VLS) à partir de catalyseurs d'or, en utilisant deux techniques: dépôt chimique en phase vapeur (CVD) et épitaxie par jets moléculaire (MBE). Dans la première partie de cette étude, les catalyseurs d'or sont déposés sur le substrat Si(111) en ultravide pour bénéficier d'interface or-silicium de grande qualité. A partir de ces ilôts d'or, des nanofils orientés <111> sont obtenus par MBE et CVD, lorsque la pression partielle de silane est faible. En profitant de l'orientation contrôlée des fils qui favorise leur intégration dans les composants, plusieurs structures basées sur les nanofils ont donc été développées. Dans la deuxième partie de cette étude, les structures atomiques des surfaces facettées de nanofils orientés <111> ont été étudiées par microscopie à l'effet tunnel (STM) à basse température. En combinant ces observations avec des images de fils identiques en microscopie électronique, nous avons révélé la diffusion d'atomes d'or depuis le catalyseur le long des fils. Cette diffusion a plusieurs conséquences : elle conduit en partie à la forme conique des nanofils et est certainement à l'origine de l'alternance de la taille des parois des nanofils. Une troisième partie a porté sur le dopage des nanofils. Des gaz tels que la phosphine ou le diborane peuvent être utilisés pour incorporer des dopants de type n ou type p dans les nanofils pendant la croissance. La tomographie par sonde atomique (TAP) a été utilisée pour caractériser la distribution des impuretés dans le volume de nanofils de silicium dopés au bore et orientés <111>. Une distribution uniforme de bore à été observée au centre de nanofils et la concentration des impuretés mesurée correspond bien à la valeur estimée par le rapport entre le flux de silane et de diborane. Enfin, ces observations ont été comparés avec des mesures de conductivité dans des nanofils individuels.

The understanding of surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. Therefore, it is important to understand the fundamental surface properties in order to use NCs efficiently in applications. In the work reported in this thesis ZnO NCs were studied. At MAX-lab in Lund, synchrotron radiation based Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) and X-ray Photoelectron Spectroscopy (XPS) were used. At Karlstad University characterization was done using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), Scanning Tunnelling Microscopy (STM), Auger Electron Spectroscopy (AES), and XPS. The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO2/Si surfaces. The conditions for distribution of ZnO NCs were determined to be beneficial when using ethanol as the solvent for ultrasonically treated dispersions. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO2/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs. Flat ZnO NCs with predominantly polar surfaces were grown using a rapid microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms. / Baksidestext The understanding of the surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. In the work in this thesis ZnO NCs were studied. The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO2/Si surfaces. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO2/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs. Flat ZnO NCs with predominantly polar surfaces were grown using a microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms.

Ultrathin graphite films, with thickness from 1-30 atomic layers, are grown on the Si-terminated and C-terminated faces of 6H-SiC and 4H-SiC via thermal desorption of silicon in an ultrahigh vacuum (UHV) chamber or in a high-vacuum RF furnace. Graphite LEED patterns and atom-resolved STM images on graphite films prove that epitaxial growth is achieved on both faces of the SiC substrate. The thickness of graphite films is estimated with modeling the Si:C Auger peak intensities. Through LEED and STM investigations of monolayer graphite grown on the Si-face of SiC(0001) surface, we show the existence of a SiC 6R3*6R3 reconstructed layer between graphite films and the SiC substrate. The complicated LEED patterns can be interpreted partially by the kinematic scattering of the interfacial layer and the 6*6 surface corrugation. Further scanning tunneling spectroscopy (STS) measurements indicate that the graphite films remain continuous over the steps between domains. Carbon nanotubes and carbon nanocaps cover about 40% of the graphitized C-face of SiC. The remaining areas are flat graphite films. Graphite ribbons were made through E-beam lithography. After the lithography process, the graphitic features remain on flat region underneath HSQ residues.

A low temperature scanning tunneling microscope (LTSTM) was built that we could use in an ultra high vacuum (UHV) system. The scanning tunneling microscope (STM) was tested on an existing 3He cryostat and calibrated at room, liquid nitrogen and helium temperatures. We analyzed the operational electronic and vibration noises and made some effective improvements. To demonstrate the capabilities of the STM, we obtained atomically resolved images of the Au (111) and graphite surfaces. In addition, we showed that the stable tunneling junctions can be formed between the Pt/Ir tip and a superconducting thin film PbBi. We observed the atomic corrugation on Au (111) and measured the height of the atomic steps to be approximately2.53Å, which agrees with published values. In our images of the graphite surface, we found both the β atoms triangular structure, as well as the complete α-β hexagonal unit cell, using the same tip and the same bias voltage of 0.2V. The successful observation of the hidden α atoms of graphite is encouraging in regards to the possibility of imaging other materials with atomic resolution using our STM. We also demonstrated that stable tunneling junctions can be formed at various temperatures. To demonstrate this, the superconducting current-voltage and differential conductance-voltage characteristics of a PbBi film were measured from 1.1K to 9K From this data, the temperature dependent energy gap of the superconductor was shown to be consistent with the predictions of the Bardeen, Cooper, and Schrieffer (BCS) theory.

In this dissertation, I have studied four theoretical problems in quantum tunneling, quantum computing, and high-temperature superconductivity. I have developed a generally-useful numerical tool for analyzing impurity-induced resonant-state images observed with scanning tunneling microscope (STM) in high temperature superconductors. The integrated tunneling intensities on all predominant sites have been estimated. The results can be used to test the predictions of any tight-binding model calculation. I have numerically simulated two-dimensional time-dependent tunneling of a Gaussian wave packet through a barrier, which contains charged ions. We have found that a negative ion in the barrier directly below the tunneling tip can deflect the tunneling electrons and drastically reduce the probability for them to reach the point in the target plane directly below the tunneling tip. I have studied an infinite family of sure-success quantum algorithms, which are introduced by C.-R. Hu [Phys. Rev. A {\bf 66}, 042301 (2002)], for solving a generalized Grover search problem. Rigorous proofs are found for several conjectures made by Hu and explicit equations are obtained for finding the values of two phase parameters which make the algorithms sure success. Using self-consistent Hartree-Fock theory, I have studied an extended Hubbard model which includes quasi-long-range Coulomb interaction between the holes (characterized by parameter V). I have found that for sufficiently large V/t, doubly-charged-antiphase-island do become energetically favored localized objects in this system for moderate values of U/t, thus supporting a recent conjecture by C.-R. Hu [Int. J. Mod. Phys. B {\bf 17}, 3284 (2003)].

No description available.

The scanning tunneling microscope (STM) based on quantum tunneling can attain atomic-scale spatial resolution and help elucidate a wealth of phenomena in the microscopic world. However a limitation in scanning tunneling microscopy is the low temporal resolution due to readout circuit frequency rolloff at a few kHz. This limitation can be overcome by using downmixing directly in the tunneling junction. With this technology we measure the high frequency vibrational modes (~ 1 MHz) of MEMS doubly-clamped beams and explore the implication of STM downmixing for nanomechanics.

abstract: This thesis describes several approaches to next generation DNA sequencing via tunneling current method based on a Scanning Tunneling Microscope system. In chapters 5 and 6, preliminary results have shown that DNA bases could be identified by their characteristic tunneling signals. Measurements taken in aqueous buffered solution showed that single base resolution could be achieved with economic setups. In chapter 7, it is illustrated that some ongoing measurements are indicating the sequence readout by making linear scan on a piece of short DNA oligomer. However, to overcome the difficulties of controlling DNA especially ssDNA movement, it is much better to have the tunneling measurement incorporated onto a robust nanopore device to realize sequential reading of the DNA sequence while it is being translocated. / Dissertation/Thesis / Ph.D. Physics 2011

1 Title: Interaction of group III and IV metals with Si(100) surface in temperature range from 20 to 800 K Author: Martin Setvín Department: Departement of Surface and Plasma Science Supervisor of the doctoral thesis: Doc. RNDr. Ivan Ošt'ádal CSc. Abstract: Interaction of group III and IV metals with Si(100) surface was studied by STM (Scanning Tunneling Microscopy) and AFM (Atomic Force Microscopy) in temperature range from 20 to 800 K. Adsorption and hopping of single metal adatoms on Si(100)-c(4×2) reconstruction can be observed by STM at low temperatures. Activation energies and frequency prefactors for hopping of single indium atoms were measured by two meth- ods - direct STM measurement at low temperature and Kinetic Monte Carlo simulations of layer growth at room temperature. Group III and IV atoms self-assemble into single atom wide chains on Si(100) surface at about room temperature. Atomic and electronic structure of the chains was investi- gated by means of STM and dynamic non-contact AFM. Keywords: Si(100), STM, AFM, adsorption, diffusion

abstract: Understanding charge transport in single molecules covalently bonded to electrodes is a fundamental goal in the field of molecular electronics. In the past decade, it has become possible to measure charge transport on the single-molecule level using the STM break junction method. Measurements on the single-molecule level shed light on charge transport phenomena which would otherwise be obfuscated by ensemble measurements of groups of molecules. This thesis will discuss three projects carried out using STM break junction. In the first project, the transition between two different charge transport mechanisms is reported in a set of molecular wires. The shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer wires show weakly length dependent and temperature dependent behavior. This trend is consistent with a model whereby conduction occurs by coherent tunneling in the shortest wires and by incoherent hopping in the longer wires. Measurements are supported with calculations and the evolution of the molecular junction during the pulling process is investigated. The second project reports controlling the formation of single-molecule junctions by means of electrochemically reducing two axial-diazonium terminal groups on a molecule, thereby producing direct Au-C covalent bonds in-situ between the molecule and gold electrodes. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au-C bond compared with the Au-N bond. Finally, the third project investigates the role that molecular conformation plays in the conductance of oligothiophene single-molecule junctions. Ethyl substituted oligothiophenes were measured and found to exhibit temperature dependent conductance and transition voltage for molecules with between two and six repeat units. While the molecule with only one repeat unit shows temperature invariant behavior. Density functional theory calculations show that at higher temperatures the oligomers with multiple repeat units assume a more planar conformation, which increases the conjugation length and decreases the effective energy barrier of the junction. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2013