Rozprawy doktorskie na temat „Semoconductor Nanomaterials - Optical Properties”

L'oxyde de zinc (ZnO) est un matériau connu et intensivement étudié pour des applications optoélectroniques dans le domaine de l’ultraviolet en raison de son large gap énergétique (3,34 eV). Cependant, les applications UV basées sur des matériaux nanostructurés représentent un véritable défi : la diminution en taille des particules obtenues généralement par des voie de chimie en solution permet d’accroître la surface spécifique mais en stabilisant des défauts à l’origine d’émissions visibles. Au cours des dernières décennies, des progrès concernant la qualité des particules ont été enregistrés au moyen des techniques physiques basées sur les dépôts en phase gazeuse à haute température. Cependant, la taille et le contrôle de la morphologie des particules restent difficiles. En prenant en compte l'état de l'art portant sur les propriétés optiques des particules de ZnO, c’est la voie supercritique qui a été mise en œuvre dans cette étude. Tout d'abord des réacteurs micro/millifluidiques ont été développés de façon à accroître la quantité de matériaux produits (gramme/jour) tout en conservant des propriétés d’émission dans l’ultraviolet. Puis les caractéristiques physico-chimiques des particules ont été étudiées au regard de l'influence de la dimension des réacteurs et de l'hydrodynamique des systèmes. Les propriétés de luminescence sont reportées à température ambiante et basses températures et comparées expérimentalement à la réponse d’un monocristal et des données de la littérature. Les mécanismes de formation (nucléation et croissance) des nanoparticules ont été élucidés et ont permis de comprendre les réponses optiques uniques de ces particules
Zinc oxide (ZnO) is a well-known and intensively studied material for optoelectronic applications in the ultraviolet (UV) spectrum region due to its wide band gap energy - 3.34 eV. However, the UV applications based on nanostructured ZnO present a big challenge due to the small size of the nanostructures i.e. a large surface-to-volume ratio resulting the appearance of the visible emission originated from the surface defects. In the last decades, the progress concerning the fabrication of UV-emitting ZnO nanostructures was carried out through the high temperature gas phase based approach. However, the size and shape control of ZnO nanostructures obtained with this approach is still difficult. Taking into account the state of the art in the optics based on ZnO nanomaterials, this Ph.D. thesis demonstrates the development of new supercritical fluids based approach for the synthesis of ZnO nanostructures with UV-emitting only PL properties. First of all in this thesis, we have developed continuous supercritical set up from micro- up to millifluidic reactor dimension for the synthesis of a larger quantity of UV-emitting ZnO nanocrystals (a gram scale per day). The influence of reactor dimension associated with hydrodynamics on physico-chemical characteristics was investigated. ZnO nanocrystals formation mechanism was studied as a function of the residence time in our continuous supercritical fluids process for the understanding of the nucleation and growth of the nanocrystals. Moreover, ZnO nanocrystals formation mechanism determines UV-emitting properties of this material. The optical properties at room and low temperature were deeply investigated with comparing to the PL emission of several types of ZnO particles and single crystal for the understanding of the nature of UV emission

In recent years, employing advanced oxidation processes (AOPs) as a means of wastewater remediation has emerged as a promising route towards maintaining a sustainable global water management program. The heterogeneous photocatalytic oxidation process has been of particular interest due to the prospective of utilizing solar radiation as the driving force behind the degradation of pollutants. Of the photocatalyst studied to date, TiO2 remains the most attractive material for environmental applications due to its affordability, stability, biocompatibility and high quantum yield. A key draw back however is roughly only 5% of solar radiation incident on earth can provide the energy required (3.0-3.2 eV) to generate the electron-hole pairs necessary for photo-oxidation. As a means to improve the process under solar irradiance, optical properties such as surface plasmon resonance of metallic nanoparticles and upconversion luminescence of rare earth ions have been exploited for improved light harvesting as well as the generation of more usable UV light from lower energy photons. In order to explore these phenomena and their role in the enhancement of this AOP, the photocatalytic degradation of organic dyes was studied under various conditions employing Degussa P25 TiO2 as the photocatalyst. Ag nanocubes, Ag-Pd core-shell nanoparticles and YAG:Yb+3,Er+3 served as the dopants for the various studies which resulted in enhanced degradation rates, insight into the applicability of utilizing Yb+3 as sensitizing ion under solar radiation and a novel core-shell nanoparticle synthesis.

The production of large area, high quality two-dimensional (2D) materials using chemical vapour deposition (CVD) has been an important and difficult topic in contemporary materials science research, after the discovery of the diverse and extraordinary properties exhibited by these materials. This thesis mainly focuses on the CVD synthesis of two 2D materials; bilayer graphene and monolayer tungsten disulphide (WS2). Various factors influencing the growth of each material were studied in order to understand how they affect the quality, uniformity, and size of the 2D films produced. Following this, these materials were combined to fabricate 2D vertical heterostructures, which were then spectroscopically examined and characterised. By conducting ambient pressure CVD growth with a flat support, it was found that high uniform bilayer graphene could be grown on the centimetre scale. The flat support provides for the consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights not only into the upscaling of CVD methods for growing large area, high quality graphene and but also in how to transfer the product onto flexible substrates for potential applications as a transparent conducting electrode. Monolayer WS2 is of interest for use in optoelectronic devices due to its direct bandgap and high photoluminescence (PL) intensity. This thesis shows how the controlled addition of hydrogen into the CVD growth of WS2 can lead to separately distributed domains or centimetre scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates or low pressure vacuum systems. This CVD reaction is simple and efficient, ideal for mass-production of large area monolayer WS2. Subsequent studies showed that hexagonal domains of monolayer WS2 can have discrete segmentation in their PL emission intensity, forming symmetric patterns with alternating bright and dark regions. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. These results provide important insights into the spatially varying properties of these CVD-grown TMDs materials, which may be important for their effective implementation in fast photo sensors and optical switches. Finally, by introducing a novel non-aqueous transfer method, it was possible to create vertical stacks of mixed 2D layers containing a strained monolayer of WS2, boron nitride, and graphene. Stronger interactions between WS2 on graphene was found when swapping water for IPA, likely resulting from reduced contamination between the layers associated with aqueous impurities. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, a result which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with large area coverage and controllable thickness on the atomic level.

The objective of this study is to examine core-shell type plasmonic metamaterials aimed at the development of materials with unique electromagnetic properties. The building blocks of metamaterials under study consist of gold as a metal component, and silica and precipitated calcium carbonate (PCC) as the dielectric media. The results of this study demonstrate important applications of the core-shells including scattering suppression, airborne obscurants made of fractal gold shells, photomodification of the fractal structure providing windows of transparency, and plasmonics core-shell with a gain shell as an active device. Plasmonic resonances of the metallic shells depend on their nanostructure and geometry of the core, which can be optimized for the broadband extinction. Significant extinction from the visible to mid-infrared makes fractal shells very attractive as bandpass filters and aerosolized obscurants. In contrast to the planar fractal films, where the absorption and reflection equally contribute to the extinction, the shells' extinction is caused mainly by the absorption. This work shows that the Mie scattering resonance of a silica core with 780 nm diameter at 560 nm is suppressed by 75% and only partially substituted by the absorption in the shell so that the total transmission is noticeably increased. Effective medium theory supports our experiments and indicates that light goes mostly through the epsilon-near-zero shell with approximately wavelength independent absorption rate. Broadband extinction in fractal shells allows as well for a laser photoburning of holes in the extinction spectra and consequently windows of transparency in a controlled manner. Au fractal nanostructures grown on PCC flakes provide the highest mass normalized extinction, up to 3 m^2/g, which has been demonstrated in the broad spectral range. In the nanoplasmonic field active devices consist of a Au nanoparticle that acts as a cavity and the dye molecules attached to it via thin silica shell as the active medium. Such kind of devices is considered as a nano-laser or nano-amplifier. The fabricated nanolasers were studied for their photoluminescence kinetic properties. It is shown that the cooperative effects due to the coupling of dye molecules via Au nanoparticle plasmons result in bi-exponential emission decay characteristics in accord with theory predictions. These bi-exponential decays involve a fast superradiant decay, which is followed by a slow subradiant decay. To summarize, this work shows new attractive properties of core-shell nanoparticles. Fractal Au shells on silica cores prove to be a good scattering suppressor and a band pass filter in a broadband spectral range. They can also be used as an obscurant when PCC is used as the core material. Finally, gold nanoparticles coated with silica with dye results in bi-exponential decays.

The popularity of graphene owing to its unique properties has triggered huge interest in other two-dimensional (2D) nanomaterials. Among them, silicene shows considerable promise for electronic devices due to the expected compatibility with silicon electronics. However, the high-end potential application of silicene in electronic devices is limited owing to the lack of an energy band gap. Hence, the principal objective of this research is to tune the electronic and magnetic properties of silicene related nanomaterials through first-principles models. I first explored the impact of edge functionalization and doping on the stabilities, electronic, and magnetic properties of silicene nanoribbons (SiNRs) and revealed that the modified structures indicate remarkable spin gapless semiconductor and half-metal behaviors. In order to open and tune a band gap in silicene, SiNRs were perforated with periodic nanoholes. It was found that the band gap varies based on the nanoribbon’s width, nanohole’s repeat periodicity, and nanohole’s position due to the quantum confinement effect. To continue to take advantage of quantum confinement, I also studied the electronic and magnetic properties of hydrogenated silicene nanoflakes (SiNFs). It was discovered that half-hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flake’s size. Next, I studied the adsorption behavior of various gas molecules on SiNRs. Based on my results, the SiNR could serve as a highly sensitive gas sensor for CO and NH3 detection and a disposable gas sensor for NO, NO2, and SO2. I also considered adsorption behavior of toxic gas molecules on boron carbide (BC3) and found that unlike graphene, BC3 has good sensitivity to the gas molecules due to the presence of active B atoms. My findings divulged the promising potential of BC3 as a highly sensitive molecular sensor for NO and NH3 detection and a catalyst for NO2 dissociation. Finally, I scrutinized the interactions of CO2 with lithium-functionalized germanene. It was discovered that although a single CO2 molecule was weakly physisorbed on pristine germanene, a significant improvement on its adsorption energy was found by utilizing Li-functionalized germanene as the adsorbent. My results suggest that Li-functionalized germanene shows promise for CO2 capture.

Dans cette thèse, la tomographie à sonde atomique assistée par laser est couplée in situ à un banc de photoluminescence (PL), où le rayonnement laser pulsé est utilize pour déclencher l’évaporation ionique des échantillons et, simultanément, pour activer l’émission à partir des centres optiquement actifs présents dans le matériau. Pour ce travail, deux matériaux différents ont été sélectionnés : des nano-aiguilles de diamant avec des défauts optiquement actifs intégrés (centres de couleur) et une hétérostructure multi-quantique (MQW) ZnO / (Mg, Zn)O, qui contient des émetteurs quantiques d’épaisseurs différentes. Grâce à cette configuration originale de photoluminescence, l’influence du champ électrique sur la structure fine de certains centres de couleur, noyés dans les nano-aiguilles de diamant, a été observée. La première étude s’est concentrée sur le centre neutre de vacance d’azote (NV0), qui est l’un des centres de couleur les plus étudiés dans la littérature. L’évolution de la signature optique NV0, en fonction du biais appliqué, a permis d’évaluer la contrainte mécanique (> 1 GPa) et le champ électrique agissant sur les pointes de diamant. Ces résultats démontrent une nouvelle méthode originale pour effectuer la piezo-spectroscopie sans contact des systems nanométriques sous une contrainte de traction uniaxiale, générée par le champ électrique. Cette méthode a également été appliquée à un autre centre de couleur, dont la nature n’est toujours pas claire dans la littérature, émettant à 2,65 eV, et plus sensible que les centres de couleur NV0 au champ de contrainte / déformation. Des nouveaux résultats sur ses propriétés opto-mécaniques ont été obtenus, mais son identité reste à comprendre. Le champ d’évaporation du diamant étant très élevé, les nano-aiguilles de diamant n’ont pas été analysées à l’aide de La-APT. Par conséquent, la technique couplée in situ a été appliquée afin d’étudier l’hétérostructure ZnO / (Mg, Zn) O MQW, en accédant à la structure, à la composition et à la signature optique de l’échantillon sondé dans une seule expérience. Les spectres de photoluminescence acquis par le spécimen au cours de son évaporation en cours représentent une source unique d’informations pour la compréhension du mécanisme de l’interaction lumière-matière et la physique de la photoémission sous champ électrique élevé. La corrélation des informations structurelles et optiques, liées à cette hétérostructure MQW, démontre que la technique couplée in situ peut chevaucher la limite de diffraction du laser PL et que, comme pour les nano-aiguilles de diamant, il est possible d’estimer le stress de traction induit. Les résultats obtenus par couplage in situ de la technique La-APT avec la spectroscopie PL montrent qu’un tel instrument est une technique innovante et puissante pour effectuer des recherches à l’échelle nanométrique. Pour cette raison, ce travail peut ouvrir de nouvelles perspectives pour une compréhension approfondie de la physique liée aux systèmes étudiés en parallèle avec l’amélioration continue de la configuration expérimentale
In this thesis, the Laser-assisted Atom Probe Tomography is coupled in-situ with a photoluminescence (PL) bench, where the pulsed laser radiation is used to trigger the ion evaporation from the specimens and, simultaneously, to activate the emission from optically active centers present into the material. For this work, two different materials were selected: diamond nano-needles with embed- ded optically active defects (color centers) and a ZnO/(Mg,Zn)O multi-quantum-well (MQW) heterostructure, which contains quantum emitters of different thicknesses. Thanks to this original photoluminescence setup, the influence of the electric field on the fine structure of some color centers, embedded into the diamond nanoneedles, was observed. The first study focused on the neutral nitrogen-vacancy center (NV0), which is one among the most studied color centers in literature. The evolution of the NV0 optical signature, as a function of the applied bias, allowed to evaluate the mechanical stress (> 1 GPa) and the electric-field acting on diamond tips. These results demon- strate an original new method to perform contactless piezo-spectroscopy of nanoscale systems under uniaxial tensile stress, generated by the electric field. This method was applied also on another color center, which nature is still not clear in literature, emitting at 2.65 eV, and more sensitive than the NV0 color centers to the stress/strain field. New results on its opto-mechanical properties were obtained, but its identity still needs to be understood. Since the evaporation field of diamond is really high, the diamond nanoneedles were not analyzed using La-APT. Therefore the coupled in-situ technique was applied in order to study the ZnO/(Mg,Zn)O MQW heterostructure, accessing to the structure, composition and optical signature of the probed specimen in only one experiment. The photoluminescence spectra acquired by the specimen during its ongoing evaporation represents a unique source of information for the understanding of the mechanism of light-matter interaction and the physics of photoemission under high electric field. The correlation of the structural and optical information, related to this MQW heterostructure, demonstrates that the coupled in-situ technique can overlap the diffraction limit of the PL laser and that, as done for the diamond nanoneedles, is pos- sible to estimate the induced-tensile-stress. The results achieved by the in-situ coupling of the La-APT technique with the PL spec- troscopy show that such instrument is an innovative and powerful technique to perform research at the nanometric scale. For this reason, this work can open new perspectives for a deeply understanding of the physicics related to the studied systems in parallel with the continuous enhancement of the experimental setup

abstract: The interaction of light with nanoscale structures consisting of metal and two-level quantum emitters is investigated computationally. A method of tilting the incoming electromagnetic wave is used to demonstrate coupling between a sinusoidal grating and two-level quantum emitters. A system consisting of metallic v-grooves and two-level emitters is thoroughly explored in the linear regime, where the spatially uniform fields provide a unique means of characterizing the coupling between the v-grooves and emitters. Furthermore, subwavelength spatial effects in the ground state population of emitters in the v-grooves are observed and analyzed in the non-linear regime. Finally, photon echoes are explored in the case of a one-dimensional ensemble of interacting two-level emitters as well as two-level emitters coupled to metallic slits, demonstrating the influence of collective effects on the echo amplitude in the former and the modifcation of the photon echo due to interaction with surface plasmons on the slits in the latter.
Dissertation/Thesis
Doctoral Dissertation Physics 2016

碩士
中原大學
應用物理研究所
94
The objective of this thesis is to measure the fluorescence, scattering and absorption properties of different organic and inorganic nanomaterials. In organic materials, we measured the spectra of red and green fluorescent proteins during the temperature range between -20℃and 0℃.We found that the spectra of red fluorescent proteins possess the phenomena of blue shifts when the temperature decreases, while the spectral intensity of green fluorescent protein decreases when the temperature decreases. In inorganic materials, we measured the intensities of light scattered by PMMA particles of 200 nm diameter with different angle. We estimated the particle size based on the Mie scattering theory. Furthermore, we also measured the variation of absorbed spectra of gold nanoparticals in real time as well as the pH value of the environment changed.

碩士
國立交通大學
材料科學與工程系所
95
A novel aqueous solution method has been developed for growing well-aligned alumina coated ZnO nanomaterials. The ZnO nanowires and ZnO nanotubes are synthesized by solution method on the Si wafer coated with ZnO film, and the organic structure-directing agents (SDAs), citric acid and diaminopropane (DAP), are found to play different roles in controlling the morphologies through the selective adsorptions on different crystal facets of ZnO. Nano-sized pseudo-boehmite and bayerite mix powder is obtained through precipitation method and characterized using X-ray diffraction, HRTEM, PL and FTIR spectroscopy after thermal annealing. The as-synthesized powder will transfer into γ-phase by annealing process at 400 oC, and then change to α-phase over 1000 oC. Furthermore, it can be observed a strong blue emission from PL result. On the contrary, transparent conductive alumina film could be obtained after peptized growth process. Alumina-coated ZnO nanowires are synthesized by aqueous solution method at low temperature. When the ZnO nanostructure is immersed into peptized alumina solution, the alumina shell would be form on the surface of ZnO nanostructure. The thickness of alumina film could be controlled by modulating the concentration of alumina solution; When the thickness about 8 nm, the alumina-coated nanowires show both the blue emission of alumina and the UV emission of ZnO after thermal annealing. However, ZnO nanotubes show obviously visible emission with increasing annealing temperature, while alumina-coated ZnO nanotubes exhibit a strong UV emission after thermal annealing.

博士
國立中山大學
海洋生物科技博士學位學程
104
Abstract Metal Nanoparticles are excellent candidates for all the biomedical applications due to their size and shape dependent optical and physiological properties. In this study,silver and gold nanoparticles were synthesized using plant extract. It was observed that the size and shape of silver and gold nanoparticles depend strongly on the concentration of chemical solution, type of reducing agent used in the reaction, temperature of the solution and stabilizing agent for reaction. Transmission electron microscopy (TEM) has been used extensively to determine the size and shape of the nanoparticles. Optical properties of the size and shape selected nanoparticles were studied using UV-vis spectrophotometer in absorption mode. Biomimetic synthesized metals nanoparticles were observed to show excellent absorption property which is reflected by the presence of the characteristic surface plasmon resonance (SPR) band peak. The SPR peak was found to be predominantly dependent on the size of nanoparticles. We have observed a strong red shift with increasing the size of silver and gold nanoparticles. The position of the SPR peak was also observed to change with shape of gold and silver nanoparticles. Synthesis and characterization of the nanoparticles have been carried out in this thesis. at the same time we have presents Comprehension of potential biomedical applications of carbon based nanomaterials, especially carbon dots, is a burgeoning interest over past few years. This work proposed a facile method to separate carbon nanoparticles (CNPs) based on their optical properties, size and capacity to produce free radicals. We used sucrose-Cesium chloride density gradient centrifugation to separate CNPs possessing five distinct colored carbon dots called CNPB (deep blue), CNPG (green), CNPY (Yellow), CNPR (Red) and CNPCy (Cyan). In order to prove biological relevance of synthesized CNPs, we used them as a multi-pronged tool for killing of cancer cells as well as biological imaging for the same. Under the influence of green laser (λ=534nm), the CNPs were found to produce reactive oxygen species (ROS) which was measured using fluorescence spectroscope in presence of HMLER shE-cadherins cancer stem cells. This magnificent property was used to induce cells death in cancer cells and analysis was done using FITC-Annexin and Propidium Iodide double staining and quantified by flow Cytometry. It was found that the production of free radicals in the form of reactive oxygen species (ROS) was highly influenced by their optical properties. Key words: Silver NPs,Gold NPs,C -dots,bioimaging, ROS.

Surface passivation of semiconductor quantum dots is essential to preserve their efficient and robust light emitting properties. By using a lattice matched (mismatch = 0.5%) lead halide perovskite matrix, we achieve shell-like passivation of lead sulfide QDs in crystalline films, leading to efficient infrared light emission. These structures are made from a simple one-step spin coating process of an electrostatically stabilized colloidal suspension. Photoluminescence and transient absorption spectroscopy indicate rapid energy transfer between the perovskite matrix and the QDs, suggesting an interface with few trap states. In addition to housing the efficient infrared QD emitters, lead halide perovskites themselves have good carrier mobilities and low trap densities, making these solution-processable heterostructures an attractive option for electrically pumped light emitting devices. The highest performing quantum dots for visible light applications are CdE (E=chalcogenide) core/shell heterostructures. Again, surface passivation plays a huge role in determining the brightness and robustness of visible QD emitters. Multilayer shell passivation is usually used to produce the highest quantum yield particles. Surface trap states are shown to be detrimental to luminescence output, even in thick-shelled particles. Spherical quantum wells allow for thicker shells and with good surface passivation, show promising reduction of biexciton auger recombination, as measured by a time correlated single photon counting (TCSPC) microscope. TCSPC methods were used to diagnose and identify QD architectures for LED applications and explore fundamental recombination dynamics using photon antibunching measurements, and statistical analysis of blinking traces.Introducing new surfaces onto graphitic substrates can be a useful for introducing new electronic properties, patterning device-specific geometries, or appending molecular catalysts. Metal nanoparticles were used to act as a catalyst for the gasification and etching of graphite and graphene. Several methods of controlling the initiation, propagation, and density of these trenches were explored. Patterning defects helped control where initiation occurred, while faceting existing defect sites could also enable more facile initiation and control the direction at the beginning of etching, due to the wetting mechanism of particle movement. Patterning the metal also was shown as a promising avenue to limit unwanted gasification and promote etching in specific, patterned regions. Surface functionalization using reactive gases was performed and characterized with outlook for future experiments.

博士
國立清華大學
材料科學工程學系
97
In this thesis, we present successful growth and characterization (optical, electrical, and thermal) of InN epitaxial films and nanostructures by molecular beam epitaxy. Temperature-dependent photoluminescence (PL) spectroscopy is used as a tool to study the much controversial optical band gap in degenerate InN. Samples with PL peak on the lower and higher energy side of 0.730 eV demonstrate a normal redshift and anomalous blueshift, respectively, with increasing temperature. This can be explained effectively on the basis of a competition between a conventional red shift from lattice dilation and a blue shift of the electron and hole quasi Fermi-level separation. On the electrical characterization part, we report the first observation of negative photoconductivity behavior in InN thin films. Unlike most conventional (non-degenerate) semiconductors, that show increase in conductivity with illumination, InN shows a regular decrease. The results have been qualitatively modeled on the basis of electronic scattering in the conduction band and transitions in degenerate InN with recombination centers. Finally, a systematic thermal diffusivity (related to thermal conductivity) study in the MBE-grown InN thin films on various substrates with different growth temperatures were carried out. A high thermal diffusivity value of 0.55 cm2/s for a combined 1.7 um thick InN film suggests a lower degree of phonon scattering in our sample with fewer structural defects.

碩士
國立臺灣大學
化學研究所
92
In the thesis, we presented some of our progress in size effects on the optical properties and the application toward nanotechnology of gold nanoparticles and organic nanomaterials. This topic was, in fact, composed of many aspects of nanomaterials. They included basic properties and size controlled preparations of gold nanoparticles, fabrication, characterization and photophysical features of organic nanomaterials, photophysical interactions between fluorophores and gold nanoparticles, and growth of semiconductor 1-D nanomaterials. Accordingly, it is obvious that the discussion cannot be merged into a single chapter. Therefore, we divided our discussions into four parts. In Chapter 2, historical introduction of preparation and basic properties of gold nanoparticles is surveyed (Sections 2.1-2.6). This chapter is not the major part of the thesis. They are here to work as a basic knowledge for further discussions. In Section 2.6, we report our fabrication and characterization of size-controlled gold nanoparticles (applied to studies in Chapters 4 and 5). Chapter 3 highlights a simple procedure for the preparation of organic nanomaterials and its application on many organic compounds. Photophysic properties of these nanomaterials were studied. We have proposed an approach to analyze packing order of molecules in nanomaterials. This helps one to correlate photophysic features of organic nanomaterials with the packing order of molecules. Chapter 4 emphasizes the interaction between gold nanoparticles and fluorescent organic nanocrystals. In the system, gold nanoparticles worked as fluorescent quenchers of the organic nanocrystals. The emission of the nanocrystals was gradually reduced as the amount of gold nanoparticles increased. The mechanism of the quenching is analyzed. The quenching rate was found to be size-dependent with the gold nanoparticles. The exciton dynamics in the perylene nanocrystals were altered by interactions between gold nanoparticles and the nanocrystals. In Chapter 5, gold nanoparticles of various sizes were applied to the diameter-selective growth of GaN (gallium nitride) nanowires. The limitation of low solubility of N in Au and the in-equilibrium condition were conquered. By using gold nanoparticles with distinct size, GaN wires with various size distributions were obtained. The growth dynamics were also surveyed. This demonstrates the application of preformed gold nanoparticles in size-controllable preparation of semiconductor materials and envisages promising future of nanotechnology.

博士
國立臺北科技大學
機電科技研究所
97
In this dissertation, a modified plasma arc gas condensation has been successfully developed to fabricate non-stoichiometric tungsten oxide materials and investigate the effect of titanium oxide addition on the morphologies, structures and electro-optical properties of tungsten oxide nanomaterials. All the samples were characterized by field emission scanning electron microscope (FESEM), high-angle annular dark field (HAADF), high-resolution transmission electron microscope (HRTEM), X-ray diffractormeter (XRD), X-ray photoelectron spectrometer (XPS) and Raman spectra for the morphological and structural investigation. Practical field emission (FE) property and photoluminescence (PL) measurement will reveal the electro-optical properties of the as-prepared samples. The growth mechanism for the as-prepared materials has been proposed for vapor-solid (VS) process. Three as-prepared products, namely, W18O49/TiO2 core-shell nanoparticles, W18O49 nanorod bundles and Ti-modified W18O49 nanorods, can be obtained by the experiment and target design. The results show that the diameter of the W18O49/TiO2 core-shell nanoparticles is estimated to be as 43.5 ± 8.0 nm. In Raman spectrum, the characteristic peaks of the core-shell nanoparticles are also shifted; in addition, green emission peak at 483 nm is also observed in the PL spectrum. The possible explanation for these unique phenomena can be attributed to the lattice distortion induced by the defects from the oxygen vacancies or the interface between the core and shell. The stability at high temperature of W18O49 nanoparticles can be also enhanced by TiO2 shell and prevent from the further oxidization. Unique bundle-like structure with crystalline phase can be prepared by directly evaporating a tungsten bulk in an oxygen-deficient environment and a diameter of 25 nm~200 nm via FESEM and TEM observations. Meanwhile, XRD and HRTEM results confirm that the nanorod bundles are in a single crystalline monoclinic W18O49 phase with growth direction along [010] direction. Also, oxygen deficiencies within the nanostructures induce the band-to-band transition emission and blue emission at 350 nm and 420 nm observed in the PL spectrum. The FE measurement shows that the tungsten oxide nanorod bundles exhibits low turn-on and threshold voltages, which are about 3.5 and 4.6 V/μm, respectively. The corresponding field enhancement factor β values at high and low field regions are estimated as 2269 and 2131, which are high enough for various FE applications. To enhance the applications of W18O49 nanorods, the effects of titanium oxide addition on the structures and properties W18O49 nanorods have been investigated. The results show that the average diameter of the as-prepared Ti-modified W18O49 nanorods is ranged from 20 nm to 100 nm. In Raman spectrum, the peaks for Ti-modified W18O49 nanorods are shifted compared those of pure W18O49 nanorods. Meanwhile, Ti modification results in the green emission peak at 497 nm observed in PL spectrum. Better FE performance of W18O49 nanorods can be obtained by introducing titanium element. The turn-on and threshold voltages of Ti-modified W18O49 nanorods can be as low as 2.2 V/μm and 3.4 V/μm, respectively. The corresponding field enhancement factor β value is estimated as 4578. The reasons for the FE enhancement can be attributed to Ti-modified W18O49 nanorods can be attributed to (1) smaller average diameter (2) conductivity and electron mobility enhancement (3) more electron carriers generated by oxygen defects for Ti modification, and (4) reduced work function.

Photophysical, electronic, and compositional properties of single-walled carbon nanotubes (SWCNTs) and bulk nanotube samples were investigated together with graphene oxide photoluminescence. First, we studied the effect of external electric fields on SWCNT photoluminescence. Fields of up to 10 7 V/m caused dramatic, reversible decreases in emission intensity. Quenching efficiency was proportional to the projection of the field on the SWCNT axis, and showed inverse correlation with optical band gap. The magnitude of the effect was experimentally related to exciton binding energy, as consistent with a proposed field-induced exciton dissociation model. Further, the electronic composition of various SWCNT samples was studied. A new method was developed to measure the fraction of semiconducting nanotubes in as- grown or processed samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results provide important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques. The nature of absorption backgrounds in SWCNT samples was also studied. A number of extrinsic perturbations such as extensive ultrasonication, sidewall functionalization, amorphous carbon impurities, and SWCNT aggregation were applied and their background contributions quantified. Spectral congestion backgrounds from overlapping absorption bands were assessed with spectral modeling. Unlike semiconducting nanotubes, metallic SWCNTs gave broad intrinsic absorption backgrounds. The shape of the metallic background component and its absorptivity coefficient were determined. These results can be used to minimize and evaluate SWCNT absorption backgrounds. Length dependence of SWCNT optical properties was investigated. Samples were dispersed by ultrasonication or shear processing, and then length-fractionated by gel electrophoresis or controlled ultrasonication shortening. Fractions from both methods showed no significant absorbance variations with SWCNT length. The photoluminescence intensity increased linearly with length, and the relative quantum yield gradually increased, approaching a limiting value. Finally, a strong pH dependence of graphene oxide photoluminescence was observed. Sharp and structured excitation/emission features resembling the spectra of molecular fluorophores were obtained in basic conditions. Based on the observed pH-dependence and quantum calculations, these spectral features were assigned to quasi-molecular fluorophores formed by the electronic coupling of oxygen-containing addends with nearby graphene carbon atoms.

Nanomaterials are playing an increasing role in novel technologies, and it is important to develop optical methods to characterize them in situ.  To that end, backside absorbing layer microscopy (BALM) has emerged as a powerful tool, being capable to resolve sub-nanometer height profiles, with video-rate acquisition speeds and a suitable geometry to couple live experiments.  In the internship, several techniques involving BALM were developed, and applied to study optical and electrical properties of the transition metal dichalcogenide (TMD) monolayer MoS2, a type of 2-dimensional (2D) crystalline semiconductor.  A simulations toolkit was created in MATLAB to model BALM, a workflow to reliably extract linear intensities from the CMOS detector was realized, and 2D MoS2 was synthesized by chemical vapor deposition followed by transfer to appropriate substrates.  BALM data of the 2D MoS2 was acquired and combined with simulations, giving a preliminary result for its complex refractive index at 5 optical wavelengths.  In addition, the first steps towards coupling BALM with a gate biased 2D MoS2 field-effect transistor were explored.  To complement BALM measurements, the grown samples were also characterized by conventional optical microscopy, scanning electron microscopy, atomic force microscopy, photoluminescence spectroscopy, and Raman spectroscopy.  This work provides new additions to an existing platform of BALM techniques, enabling novel BALM experiments with nanomaterial systems.  In particular, it introduces a new alternative for local extraction of optical parameters and for probing of electrical charging effects, both of which are vital in the research and development of nano-optoelectronics.

Optical metamaterials have triggered extensive studies driven by their fascinating electromagnetic properties that are not observed in natural materials. Aside from the extraordinary progress, challenges remain in scalable processing and material performance which limit the adoption of metamaterial towards practical applications. The goal of this dissertation is to design and fabricate nanocomposite thin films by combining nitrides with a tunable secondary phase to realize controllable multi-functionalities towards potential device applications. Transition metal nitrides are selected for this study due to the inherit material durability and low-loss plasmonic properties that offer stable two-phase hybridization for potential high temperature optical applications. Using a pulsed laser deposition technique, the nitride-metal nanocomposites are self-assembled into various geometries including pillar-in-matrix, embedded nanoinclusions or complex multilayers, that possess large surface coverage, high epitaxial quality, and sharp phase boundary. The nanostructures can be further engineered upon precise control of growth parameters.

This dissertation is composed of a general review of related background and experimental approaches, followed by four chapters of detailed research chapters. The first two research chapters involve hybrid metal (Au, Ag) - titanium nitride (TiN) nanocomposite thin films where the metal phase is self-assembled into sub-20 nm nanopillars and further tailored in terms of packing density and tilting angles. The tuning of plasmonic resonance and dielectric constant have been achieved by changing the concentration of Au nanopillars, or the tuning of optical anisotropy and angular selectivity by changing the tilting angle of Ag nanopillars. Towards applications, the protruded Au nanopillars are demonstrated to be highly functional for chemical bonding detection or surface enhanced sensing, whereas the embedded Ag nanopillars exhibit enhanced thermal and mechanical stabilities that are promising for high temperature plasmonic applications. In the last two chapters, dissimilar materials candidates beyond plasmonics have been incorporated to extend the electromagnetic properties, include coupling metal nanoinclusions into a wide bandgap semiconducting aluminum nitride matrix, as well as inserting a dielectric spacer between the hybrid plasmonic claddings for geometrical tuning and electric field enhancement. As a summary, these studies present approaches in addressing material and fabrication challenges in the field of plasmonic metamaterials from fundamental materials perspective. As demonstrated in the following chapters, these hybrid plasmonic nanocomposites provide multiple advantages towards tunable optical or biomedical sensing, high temperature plasmonics, controllable metadevices or nanophotonic chips.

Functional oxide-based thin films have attracted much attention owing to their broad applications in modern society. The multifunction tuning in oxide thin films is critical for obtaining enhanced properties. In this dissertation, four new nanocomposite thin film systems with highly textured growth have been fabricated by pulsed laser deposition technique. The functionalities including ferromagnetism, ferroelectricity, multiferroism, magnetoelectric coupling, low-field magnetoresistance, transmittance, optical bandgap and dielectric constants have been demonstrated. Besides, the tunability of the functionalities have been studied via different approaches.

First, varies deposition frequencies have been used in vertically aligned nanocomposite BaTiO₃:YMnO₃ (BTO:YMO) and BaTiO₃:La_0.7Sr_0.3Mn₃ (BTO:LSMO) thin films. In both systems, the strain coupling effect between the phases are affected by the density of grain boundaries. Increasing deposition frequency generates thinner columns in BTO:YMO thin films, which enhances the anisotropic ferromagnetic response in the thin films. In contrast, the columns in BTO:LSMO thin films become discontinuous as the deposition frequency increases, leading to the diminished anisotropic ferromagnetic response. Coupling with the ferroelectricity in BTO, the room temperature multiferroic properties have been obtained in these two systems.

Second, the impact of the film composition has been demonstrated in La_0.7Ca_0.3MnO₃ (LCMO):CeO₂ thin film system, which has an insulating CeO₂ in ferromagnetic conducting LCMO matrix structure. As the atomic percentage of the CeO₂ increases, enhanced low-field magnetoresistance and increased metal-to-insulator transition temperature are observed. The thin films also show enhanced anisotropic ferromagnetic response comparing with the pure LCMO film.

Third, the transition metal element in Bi₃MoM_TO₉ (M_T, transition metals of Mn, Fe, Co and Ni) thin films have been varied. The thin films have a multilayered structure with M_T-rich pillar-like domains embedded in Mo-rich matrix structure. The anisotropic magnetic easy axis and optical properties have been demonstrated. By the element variation, the optical bandgaps, dielectric constants as well as anisotropic ferromagnetic properties have been achieved.

The studies in this dissertation demonstrate several examples of tuning the multifunctionalities in oxide-based nanocomposite thin films. These enhanced properties can broaden the applications of functional oxides for advanced nanoscale devices.