In the scope of this master thesis, a home-made brass scanning tunneling microscope (STM) was machined, assembled and tested for its functionality. For this microscope, a new approach-technique was used which follows the design suggested by Pan. The difference to Pan's design is the use of piezoplates, instead of piezostacks. Hence, the approach is still based on the stick and slip motion, but it allows the microscope to be more compact. A new and simple electronic circuit, in order to control the approach, is presented and was put together. This circuit is based on mechanical relays, which provide a sufficient long time gap between the single moving steps, due to their mechanical functional principle. Subsequently the approach-technique and the scanning was successfully tested. Finally, first images of HOPG were taken under ambient conditions. / text

Nanomanufacturing is an active research area in academia and industry due to the ever-growing demands for precision surface modifications of thin films or substrates with nanoscale features. Conventional lithographic techniques face many challenges as they approach their fundamental limits. Consequently, new nanomanufacturing tools, fabrication techniques, and precision instruments are being explored and developed to meet these challenges. It has been hypothesized that direct-write nanolithography might be achieved by using a field-emitted electron beam for nanomachining. This dissertation moves this research one step closer by developing a precision instrument that can enable the integration of direct-write nanolithography by a field-emitted electron beam with dimensional metrology by scanning tunneling microscopy. First, field emission from two prospective electron sources, a carbon nanotube field emitter and a sharp tungsten field emitter, is characterized at distances ranging from sub-micrometer to a few micrometers. Also, the design and construction of a low thermal drift piezoelectric linear motor is described for tip-sample approach. Experiments indicate that: the step size is highly repeatable with a standard deviation of less than 1.2 nm and the thermal stability is better than 40 nm/◦C. Finally, the design and construction of the instrument are presented. Experiments indicate that: the instrument is operating properly in scanning tunneling microscope mode with a resolution of less than 2 Å.

Superconductivity is a remarkable quantum phenomenon in which a macroscopic number of electrons form a condensate of Cooper pairs that can be described by a single quantum wave function. According to the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, there is a minimum length scale (the coherence length) below which the condensate has a rigid quantum phase. The fate of superconductivity in a system with spatial dimensions smaller than [the coherence length] has been the subject of intense interest for decades and recent studies of superconductivity in ultra-thin epitaxial metal films have revealed some surprising behaviors in light of BCS theory. Notably, it was found that superconductivity remains robust in thin lead films with thicknesses orders of magnitude smaller than the coherence length (i.e. in the extreme two dimensional limit). Such studies raise the critical question: what happens to superconductivity as all dimensions are reduced toward the zero dimensional limit? By controlling the lateral size of ultra thin 2D islands, we systematically address this fundamental question with a detailed scanning tunneling microscopy/spectroscopy study. We show that as the lateral dimension is reduced, the strength of the superconducting order parameter is also reduced, at first slowly for dimensions larger than the bulk coherence length, and then dramatically at a critical length scale of ~ 40nm. We find this length scale corresponds to the lateral decay length of the order parameter in an island containing regions of different heights and different superconducting strength. Overall, our results suggest that fluctuation corrections to the BCS theory are important in our samples and may need to be systematically addressed by theory. / text

This dissertation provides a brief introduction into scanning tunneling microscopy, and then Chapter III reports on the design and operation of a cryogenic ultra-high vacuum scanning tunneling microscope (STM) coupled to a closed-cycle cryostat (CCC). The STM is thermally linked to the CCC through helium exchange gas confined inside a volume enclosed by highly flexible rubber bellows. The STM is thus mechanically decoupled from the CCC, which results in a significant reduction of the mechanical noise transferred from the CCC to the STM. Noise analysis of the tunneling current shows current fluctuations up to 4% of the total current, which translates into tip-sample distance variations of up to 1.5 picometers. This noise level is sufficiently low for atomic-resolution imaging of a wide variety of surfaces. To demonstrate this, atomic-resolution images of Au(111) and NaCl(100)/Au(111) surfaces, as well as of carbon nanotubes deposited on Au(111), were obtained. Other performance characteristics such as thermal drift analysis and a cool-down analysis are reported. Scanning tunneling spectroscopy (STS) measurements based on the lock-in technique were also carried out and showed no detectable presence of noise from the CCC. These results demonstrate that the constructed CCC-coupled STM is a highly stable instrument capable of highly detailed spectroscopic investigations of materials and surfaces at the atomic-scale. A study of electron transport in single-walled carbon nanotubes (SWCNTs) was also conducted. In Chapter IV, STS is used to study the quantum-confined electronic states in SWCNTs deposited on the Au(111) surface. The STS spectra show the vibrational overtones which suggest rippling distortion and dimerization of carbon atoms on the SWCNT surface. This study experimentally connects the properties of well-defined localized electronic states to the properties of their associated vibronic states. In Chapter V, a study of PbS nanocrystals was conducted to study the effect of localized sub-bandgap states associated with surface imperfections. A correlation between their properties and the atomic-scale structure of chemical imperfections responsible for their appearance was established to understand the nature of such surface states. This dissertation includes both previously published/unpublished and co-authored material.

Submitted by LUIZ HENRIQUE BUGATTI GUESSI null (lhbguessi@gmail.com) on 2016-03-05T22:01:32Z No. of bitstreams: 1 Dissertação_Final_full_Luiz_Henrique_Guessi.pdf: 8393900 bytes, checksum: 733e3b310e036a5d66ec11c8c11f985e (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-03-07T18:18:44Z (GMT) No. of bitstreams: 1 guessi_lhb_me_rcla.pdf: 8393900 bytes, checksum: 733e3b310e036a5d66ec11c8c11f985e (MD5) / Made available in DSpace on 2016-03-07T18:18:44Z (GMT). No. of bitstreams: 1 guessi_lhb_me_rcla.pdf: 8393900 bytes, checksum: 733e3b310e036a5d66ec11c8c11f985e (MD5) Previous issue date: 2016-02-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Foi discutido teoricamente a Densidade Local de Estados (LDOS) de uma folha de grafeno hospedando duas impurezas distantes localizadas no centro da célula hexagonal. Ao acoplar lateralmente a ponta do Microcópio de Varredura por Tunelamento (STM) sobre o átomo de carbono, dois novos notáveis efeitos foram detectados: i) uma estrutura de multiníveis na LDOS e ii) padrões de batimentos na LDOS induzida. Também foram mostrados que ambos os fenômenos ocorrem próximos aos pontos de Dirac e são altamente anisotrópicos. Além disso, foram propostos experimentos de condutância empregando o STM como uma sonda para a observação de tais manifestações exóticas na LDOS do grafeno induzida pela correlação entre as impurezas. / We discuss theoretically the Local Density of States (LDOS) of a graphene sheet hosting two distant adatoms located at the center of the hexagonal cells. By putting laterally a Scanning Tunneling Microscope (STM) tip over a carbon atom, two remarkable novel effects can be detected: i) a multilevel structure in the LDOS and ii) beating patterns in the induced LDOS. We show that both phenomena occur nearby the Dirac points and are highly anisotropic. Furthermore, we propose conductance experiments employing STM as a probe for the observation of such exotic manifestations in the LDOS of graphene induced by inter-adatoms correlations.

The precise control of the resonance frequency of plasmonic nanostructures is critical and depends on the size, composition, shape, and dielectric nature of the environment. The ability to control the shape and size of nanomaterials acutely depends on the fabrication technique and material design. We used a cheap and scalable method known as nanosphere lithography (NSL) to fabricate plasmonic nano-prism (NP) on glass and indium tin oxide substrate (ITO). The methods involve substrate hydrophilicity treatment, polystyrene nanosphere masking, metal deposition, and mask removal. The array and specific morphology of the fabricated NP was established using scanning electron microscope (SEM) and atomic force microscope (AFM). Finally, we used UVVis spectroscopy to determine the plasmonic resonance frequencies of fabricated NP on different substrates. The results reported herein have potential applications in surface-enhanced Raman spectroscopy (SERS), and biosensing. We also used scanning tunneling microscope to obtain high spatial resolution images of supramolecular trigrams.

No description available.

No description available.

No description available.

Graphene, one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has grabbed appreciable attention due to its exceptional electronic, mechanical and optical properties. Chemical functionalization schemes are needed to integrate graphene with the different materials required for potential applications. Molecular self-assembly behavior on graphene is a key method to investigate the mechanism of interaction between molecules and graphene and the promising applications related to molecular devices. In this thesis, we report the molecular self-assembly behavior of phenyl-C61-butyric acid methyl ester (PCBM), C60, perylenetetracarboxylic dianhydride (PTCDA) and Gd3N@C80 on flat and rippled graphene 2D material by the experimental methods of scanning tunneling microscope (STM) and atomic force microscope (AFM) and by the theoretical method of density functional theory (DFT). We found that molecules form ordered structures on flat graphene, while they form disordered structure on rippled graphene. For example, PCBM forms bilayer and monolayer structures, C60 and Gd3N@C80 form hexagonal close packed (hcp) structure on flat graphene and PTCDA forms herringbone structure on flat graphene surface. Although C60 and Gd3N@C80 both form hcp structure, C60 forms a highly ordered hcp structure over large areas with little defects and Gd3N@C80 forms hcp structure only over small areas with many defects. These differences of structure that forms on flat graphene is mainly due to the molecule-molecule interactions and the shape of the molecules. We find that the spherical C60 molecules form a quasi-hexagonal close packed (hcp) structure, while the planar PTCDA molecules form a disordered herringbone structure. From DFT calculations, we found that molecules are more effected by the morphology of rippled graphene than the molecule-molecule interaction, while the molecule-molecule interaction plays a main role during the formation process on flat graphene. The results of this study clearly illustrate significant differences in C60 and PTCDA molecular packing on rippled graphene surfaces. / Doctor of Philosophy / As the first physical isolated two-dimensional (2D) material, graphene has attracted exceptional scientific attention. Due to its impressive properties including high carrier density, flexibility and transparency, graphene has numerous potential applications, such as solar cell, sensors and electronics. 2D molecular self-assembly is an area that focuses on organization and interaction between self-assembly behaviors of molecules on surface. Graphene is an excellent substrate for the study of molecular self-assembly behavior, and study of molecular study is very important for graphene due to potential applications of molecules on graphene. In this thesis, we present investigations of the molecular self-assembly of PCBM, C60, PTCDA and Gd3N@C80 on graphene substrate. First, we report the two types of bilayer PCBM configuration on HOPG with a step height of 1.68 nm and 1.23 nm, as well as two types of monolayer PCBM configuration with a step height of 0.7 nm and 0.88 nm, respectively. On graphene, PCBM forms one type of PCBM bilayer with a step height of 1.37 nm and one type of PCBM monolayer with a step height of 0.87 nm. By building and analyzing the models of PCBM bilayers and monolayers, we believe the main differences between two configurations of PCBM bilayer and monolayer is the tilt angle between PCBM and HOPG, which makes type I configuration the higher molecule density and binding energy. Secondly, we report the investigation of self-assembly behaviors of C60 and PTCDA on flat graphene and rippled graphene by experimental scanning tunneling microscope (STM) and theoretical density functional theory (DFT). On flat graphene, C60 forms hexagon close pack (hcp) structure, while PTCDA forms herringbone structure. On rippled graphene, C60 forms quasi-hcp structure while PTCDA forms disordered herringbone structure. By DFT calculation, we study the effect of graphene curvature on spherical C60 and planar PTCDA. Finally, we report a STM study of a monolayer of Gd3N@C80 on graphene substrate. Gd3N@C80 forms hcp structure in a small domain with a step height of 0.88 nm and lattice constant of 1.15 nm. According to our DFT calculation, for the optimal organization of Gd3N@C80 and graphene, the gap between Gd3N@C80 and graphene is 3.3 Å and the binding energy is 0.95 eV. Besides, the distance between Gd3N@C80 and Gd3N@C80 is 3.5 Å and the binding energy is 0.32 eV.