Tesis doctoral de Paul Dominique Lacharmoise
Nowadays science efforts are strongly focused into the nanoscale world. The huge progress made in so called nano-technology during the last decade brought to biologists, chemists and physicists the proper tools to get insights into phenomena occurring at the nanoscale level. From natural or even living systems to artificially organized structures, most of their significant properties are determined by the physics involved at the nanometer scale. Nano-technology not only provided to the scientific community the possibility of characterizing these systems but also to generate and manipulate them. Nanostructured systems are now comfortably obtained by chemically or physically controlled fabrication techniques. More sophisticated techniques can go further up to atomic resolution in some cases. The diversity of these techniques combined with the exponentially diverging fields of interest explains the fact that nanoscience is practiced in almost every research center in the world. the big family of optical techniques made a huge contribution to science research all along its history. Conventional optical microscopies and spectroscopies are widespreadly used as non-invasive tools to obtain valuable information in almost every physical system. During the last decades, two main requirements were imposed to these techniques in order to be compatible with the downscaling needs of nano-technology: high spatial resolution and high sensitivity. In this context, the classical optical microscope is restricted by diffraction to about half the wavelength, i.E., To about 0.5 micrometers in the visible range. On the other side, the technological progress in detectors, optical elements and light sources improved considerably the sensitivity of the optical spectroscopies in the past years. However, when spectral information at the molecular level is required, the cross sections of conventional optical spectroscopies like fluorescence and raman are often too low due to detection limitations in normal experimental conditions. recent developments in nano-optics have made great contributions to overcome abbe¿s diffraction limit. The advent of the near-field scanning optical microscope (snom) is considered a milestone in this venture. Snom combines the potentials of scanned probed technology with the power of optical microscopy, providing us with «eyes» for the nanoworld. Among the main properties that might be of interest for a nanostructure under investigation are, besides shape and size, its chemical composition, molecular structure and dynamical behavior. Although electron microscopes as well as scanning tunnelling and atomic force microscopes easily achieve 10 nm spatial resolution and beyond, they have practical limitations with respect to spectral and dynamic properties. Instead, snom combines the potential of classical optical microscopy with lateral resolution below the 100nm regime, being sensitive to electromagnetic local fields. Thus, snom has been in the hearth of nano-optics research during the last 20 years. the improvements achieved with the required instrumentation have made the optical spectroscopies much more easily available to a wider community of scientists from various backgrounds: physics, chemistry, biology, engineering. The main interest of these techniques is that they provide structural and chemical information of the system under study in a non-invasive way. In this context, raman spectroscopy is maybe the most popular due to its high specificity. The raman spectrum provides a «fingerprint» of the studied system that can be used for analytical purposes in a huge variety of cases and combinations. The spectroscopic fingerprint of the system is a very valuable information that brings high specificity to the raman technique, making it much more interesting in many cases than other common spectroscopies like fluorescence. However, a more widespread implementation of the technique has been restricted because of its main disadvantage: the weakness of the raman effect. This strong limitation has hampered the use of raman spectroscopy in the cases where the sampled volume combined with its raman cross section and the specific experimental conditions make very difficult to obtain a (proper) spectrum measurable above the noise level. This can be perfectly the case when studying molecules, and is always the case when single molecule probing is desired. the discovery of surface enhanced raman spectroscopy (sers) in the early 70¿s renewed the potential of the raman effect and extended its application to several cases that could not be envisaged with conventional raman due to its poor signal to noise ratio, mainly in molecular systems. The origin of the observed amplifications in the raman signal was in the center of a long debate that is not completely closed even today. Nonetheless, it is now well accepted that these amplifications come from the electromagnetic interaction of the light with metals, which produces large amplifications of the laser field through surface excitations known as plasmon resonances. The molecules must be adsorbed to the metal surface or be very close to it, typically less than 10 nm, to benefit from these amplifications. If we define the sers enhancement factor as the ratio between the signal in sers conditions and the signal in conventional raman of exactly the same system measured at the same experimental conditions, then enhancements up to 10^11 can be achieved, turning the raman effect into an ultra high sensitive and specific tool for molecule analysis. this work is basically devoted to the application of both snom and sers to study different systems in which we profit from their main advantages as listed before. Thus, most of the experimental results presented along the manuscript are mainly analysed and focused in view of these techniques. Although the obtained results for each case are discussed deeply enough to understand the involved physics, it is not the aim of this work to provide a highly rigorous study of a specific system but to enlighten the potential of snom and sers. the manuscript is divided in three main chapters: in the first chapter we introduce the experimental techniques that were used to characterize the studied systems. We put special emphasis and give more details on the experimental features of sers and snom for being the cornerstones of this thesis. A deeper theoretical background concerning the raman effect itself and near-field optics is presented in appendixes a and b. the second chapter is consecrated to the snom experiments. This chapter is subdivided in two sections. In the first section we study the possibility of making nano-spectroscopy and imaging of organic semiconductor nanostructures. The second subsection is devoted to the study of surface plasmon-polaritons in metallic nanovoid arrays. A brief introduction to surface plasmon-polaritons is presented in appendix ef{appc}, which may be useful to understand the plasmonic behavior of the metallic nanovoids. These sections represent (somehow) two of the most popular fields of interest in the snom community cite{kiso07}. in the third chapter we present the results obtained in the sers experiments. Again, this chapter is also subdivided in two main sections based on different studied systems. In the first section, we explore the possibility of performing single molecule (sm) sers. The experimental conditions required to achieve sm sensitivity are discussed around one of the fundamental problems that prevents sm detection in many cases: photobleaching. In the second section we introduce a novel approach to perform sers experiments in a configuration in which the molecules under study are guided to the sers substrates by electrostatic forces. since the thesis is presented in the format of «article compendium», most of the results and discussions in chapters 2 and 3 are contained in the attached articles. Supplementary information and further details are introduced when necessary to ensure, or at least to try to bring up a complete picture of the tackled subjects.
Datos académicos de la tesis doctoral «Optical techniques with high spatial resolution and sensitivity for nanostructure characterization«
- Título de la tesis: Optical techniques with high spatial resolution and sensitivity for nanostructure characterization
- Autor: Paul Dominique Lacharmoise
- Universidad: Autónoma de barcelona
- Fecha de lectura de la tesis: 05/05/2009
Dirección y tribunal
- Director de la tesis
- Alejandro Rodolfo Goñi
- Tribunal
- Presidente del tribunal: clivia Sotomayor torres
- alejandro Pérez rodríguez (vocal)
- (vocal)
- (vocal)