Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others). / <p>QC 20120920</p>

With the use of photoluminescence (PL) spectroscopy one can able to get a great deal of information about electronic structure and optical processes in semiconductors by the aid of optical characterization. Among various compound semiconductors, Cadmium Sulfide (CdS) and Gallium Selenide (GaSe) are interesting materials for their PL emissions. Particularly, due to its strong anisotropy, investigation of GaSe necessitates new experimental approaches to the PL technique. We have designed, fabricated and used new experimental set-up for this purpose. In this thesis, we have investigated the PL spectra of both CdS and GaSe as a function of temperature. We observed interesting features in these samples. These features were analyzed experimentally and described by taking the band structure of the crystals into account. From the excitonic emissions, we determined the bandgap energy of both materials. We studied various peaks that appear in the PL spectra and their origin in the material. We have found that donor acceptor transitions are effective in CdS at low temperatures. A transition giving rise to a red emission was observed and attributed to a donor level which is likely to result form an S vacancy in CdS crystal. The PL peaks with energy close to the bandgap were observed in GaSe. These peak were attributed to the bound excitons connected either to the direct or indirect band edge of GaSe. The striking experimental finding in this work was the PL spectra of GaSe measured in different angular position with respect to the crystal axis. We observed that PL spectra exhibit substantial differences when the angular position of the laser beam and the detector is changed. The optical anisotropy which is responsible for these differences was measured experimentally and discussed by considering the selection rules of the band states of GaSe.

Optical transition energies are widely used for providing experimental insight into the electronic band structure of single-wall carbon nanotubes (SWCNTs). While the first and second optical transitions in semiconducting carbon nanotubes have already been heavily studied, due to experimental difficulties in accessing the relevant excitation energy region, little is known about higher lying transitions. Here, I present measurements of the third and fourth optical transitions of small-diameter (0.7-1.2 nm), semiconducting single-wall carbon nanotubes via resonant Raman spectroscopy in the visible deep blue region (415-465 nm) and photoluminescence excitation spectroscopy in the ultraviolet and visible blue optical regions (280-488 nm). Diameter-dependent Raman radial breathing mode features, as well as resonant energy excitation maxima determined by Raman and photoluminescence measurements, are assigned to specific (n,m) nanotube species. The Raman intensity within a given 2n+m branch is found to increase with decreasing chiral angle, consistent with similar measurements for lower order optical states. Additionally, increased excitation line widths and weaker Raman intensities are observed as higher lying transitions are accessed for a given nanotube, in agreement with previous Raman measurements. Chiefly, a scaling law analysis that removes the chiral-angle-dependent contribution to the optical transition energy indicates that the third and fourth transition energies exhibit a significant deviation from the energy trend line observed for the first and second optical transitions, when the transition energies are plotted as a function of nanotube diameter. This deviation can be understood in the context of a change in the competition between exchange and excitonic correction terms. Furthermore, for semiconducting SWCNTs with diameters less than 0.9 nm, an additional deviation is observed that is interpreted as the first observation of crossing-over of the third and fourth transition energy trend lines for a given 2n+m branch and a chirality dependence in the many-body excitonic effects that becomes significant at high nanotube curvatures.

A series of stereoregular polymers including atactic, syndiotactic and isotactic poly 4(N, N-diphenyl)styrene (PNNDPS) and poly 4(N, N-ethylphenyl)styrene (PNNEPS) were synthesized to exam the tacticity effect on the photoluminescence (PL) behavior. Also, different degrees of the regularity as well as the chemical modification of the fluorophor were explored in the stereoregular polymers. Because of the increase of the steric hindrance among the bulky triphenylamine pendants in the polymer chains, a red shift of the PL emission with an accompanying increase in the emissive intensity was found in contrast to the weakened emission of triphenylamine monomers. In solution state, the PL spectra of these stereoregular polymers reveal multiple PL emissive bands including monomeric and aggregation emissions as evidenced by the time-resolved lifetime measurement. Because of the huge triphenylamine pendants, the triphenylamine pendants attached on the iPNNDPS17 (mmmmmm~50%) might encounter higher steric hindrance than that in the sPNNDPS23 (rrrrrr~59%) due to the stereoregularity evidenced by simulation. Accordingly, the iPNNDPS17 (mmmmmm~50%) exhibits more emissive intensity than the sPNNDPS23 (rrrrrr~59%) due to the effective blockage of the intramolecular rotation of the phenyl blade i.e., the restriction of intramolecular rotation (RIR). Accordingly, the RIR-active PNNDPS is highly sensitive to the temperature variations. The chemical modification of the fluorophor was carried out to examine the effect of the chemical structure. By comparison, the sPNNDPS2 (rrrrrr~70%) with high regularity exhibits much higher emissive intensity than the sPNNDPS23 (rrrrrr~59%) with low regularity due to the less solubility. However, more intense PL emission can be found in the sPNNEPS17 than aPNNEPS8 due to the ethyl substitution of the fluorophor. In aggregation solution, with the increase of the poor water contents, the PL emission decreases significantly in the sPNNDPS23 (rrrrrr~59%) and sPNNDPS2 (rrrrrr~70%) due to the formation of H-aggregate in which extra energetic loss is conducted. By contrast, the PL spectra display that the emissive intensity decreases first as fw=0.1~0.5 and then intensifies later as fw=0.6~0.9. This might be resulted from the competition between the £k-£k interaction and RIR effect. Interestingly, the PL emissive intensity drops down significantly in the sPNNEPS, whereas the PL emissive intensity is almost unchanged in the aPNNEPS8 with the increase of the poor water contents. We suggest that because the aPNNEPS8 might contain both syndiotactic and isotactic configurations, the isotactic configurations having the ethyl group pointing out of the plane may prevent the formation of the £k-stacking between the fluorophors. The PL behavior in thin film is also explored for these stereoregular polymers. After slow cooling from melt, the crystalline sPNNDPS2 (rrrrrr~70%) thin film exhibits very strong emission in comparison with the thin film after quenching from melt, indicating the crystallization-induced emission enhancement. Although the iPNNDPS17 (mmmmmm~50%) is noncrystallizable as evidenced by differential scanning calorimetry (DSC) and polarized light microscope (PLM), the PL emissive intensity of the iPNNDPS17 (mmmmmm~50%) thin film is significantly stronger after slow cooling from melt than that after quenching from melt. We suggest that this might be attributed to the free volume effect varied with the thermal history associated with the steric hindrance. Notably, this enhanced PL emission in the iPNNDPS17 (mmmmmm~50%) thin film is extremely larger than that in the sPNNDPS23 (rrrrrr~59%) and aPNNDPS8, indicating the stereoregularity effect associated with the RIR effect. By contrast, this free volume effect is not significant in the PNNEPS thin film due to the flexible ethyl substitutions. As a result, the stereoregular polymers with different tacticities and regularities indeed exhibit distinct PL behavior in solution and thin film.

Polyfluorene (PF) and its derivates as well-known fluorescent materials are promising materials in optoelectronic applications due to their high quantum yields in the solid state. Nevertheless, the easy chain inter-action in PFs to result in the unfavorable associations (aggregate and excimer) are generally considered to be detrimental to the emission efficiency in the concentrated solid state and/or at high temperatures. In the study, restraining the extent of associations is therefore by embedding fluorene-based alternative and block copolymers in crosslinked network as matrix. Firstly, alternative copolymers with fluorene connected by anthracene (or pyridine or fluorene) ring were prepared through Suzuki coupling. In this way, the steric hindrance between the o-hydrogens in the neighboring aromatic ring causes the twisting of the constructed polymer chain and the resulting twisting chain conformation keeps the polymer chains from the unfavorable inter-chain interactions and reduces the extent of the association. Secondly, the alternative fluorene-anthracene copolymer (a-PFA) from the first approach can be further chemically formulated to obtain a triblock copolymer with the central a-PFA rod block connected by two flexible poly(methyl methacrylate) (PMMA) segments. In this way, the two flexible PMMA chains serve as spatial isolator to keep the central PFA rod from approaching each other and a reduced extent of association is expected for this block polymer of b-PMMA-PFA. Thirdly, all the alternative and block copolymers cited above were immersed in the curable liquid methyl metharcylate (MMA)/ditrimethylolproanetetracrylate (DTTPT) monomer mixtures and photo-irradiated to obtain composites with the fluorescent polymers immersed in the cured crosslinked network. The chain morphology and thus the degree of associations will be successfully frozen by the immobilized crosslinked network. For systems before and after photo-irradiation, the degree of aggregation was evaluated by Uv-vis absorption, photoluminescent (PL) and PL excitation spectroscopy. Polymer concentration was found to be important factor in controlling the degree of aggregation and was discussed in this study. In addition, the cured solid composites after high-temperature annealing were studied and compared with pure PFs to evaluate the effectiveness of this crosslinking strategy in restraining the extent of aggregation. In most cases, quantum yields (£XPLs) also were measured to evaluate the effectiveness of this strategy.

Thesis (M.S.)--West Virginia University, 2001. / Title from document title page. Document formatted into pages; contains x, 84 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 79-83).

Thèse doctorat : Dispositifs de l'Electronique Intégrée : INSA LYON : 2003. / Titre provenant de l'écran-titre. Bibliogr. en fin de chaque chapitre.

Thèse doctorat : Dispositifs de l'Electronique Intégrée : Villeurbanne, INSA : 2005. / Titre provenant de l'écran-titre. Bibliogr. à la fin de chaque chapitre ainsi qu'en fin d'introduction. Bibliogr. de l'auteur p. 164-165.

Thèse doctorat : Génie des Matériaux : Villeurbanne, INSA : 2001. / Titre provenant de l'écran-titre. Bibliogr. à la fin de chaque chapitre.

Thèse de doctorat : Electronique automatique : Ecully, Ecole centrale de Lyon : 2007. / Titre provenant de l'écran-titre. 207 références.