Dissertations / Theses on the topic 'Tunable optical properties'

A straightforward, rapid method to create colloidally stable and brightly luminescent core/shell CdSe-based nanoplatelets (NPLs) with fluorescence quantum yields (QYs) up to 50% is demonstrated. A layer-by-layer deposition technique based on a two-phase mixture ‒ consisting of a nonpolar phase which includes the NPLs, and a saturated ionic polar phase ‒ to separate the reagents and hinder the nucleation of the shell material is used. The deposition of the first sulfur layer leads to a significant red-shift (by more than 100 nm) of the optical absorption and emission of the NPLs. Hence, by varying either the sulfur precursor content or the reaction time one can precisely and continuously tune the absorption and emission maxima from 520 to 630 nm. This evolution of the absorption onset during the shell growth is explained quantitatively using density-functional theory and atomistic statistical simulations. The emission can be further enhanced by exposure of the NPL solution to ambient sunlight. Finally, it is demonstrated that the core/shell NPLs can be transferred from the organic solution to aqueous media with no reduction of their QY that opens the door to a broad range of practical applications.

Metal nanostructures exhibit a wide variety of interesting physical and chemical properties, which can be tailored by altering their size, morphology, composition, and environment. Gold and silver nanostructures have received considerable attention for many decades because of their widespread use in applications such as catalysis, photonics, electronics, optoelectronics, information storage, chemical and biological sensing, surface plasmon resonance and surface-enhanced Raman scattering (SERS) detection. This thesis is composed of three main parts about the synthesis, characterization and SERS applications of shape-controlled and surface modified noble metal nanoparticles. The first part is related to a simple synthesis of shape controlled solid gold, hollow gold, silver, gold-silver core-shell, hollow gold-silver double-shell nanoparticles by applying aqueous solution chemistry. Nanoparticles obtained were used for SERS detection of dye molecules like brilliant cresyl blue (BCB) and crystal violet (CV) in aqueous system. v The second part involves the synthesis of surface modified silver nanoparticles for the detection of dopamine (DA) molecules. Determination of a dopamine molecule attached to a iron-nitrilotriaceticacid modified silver (Ag-Fe(NTA)) nanoparticles by using surface-enhanced resonance Raman scattering (SERRS) was achieved. The Ag-Fe (NTA) substrate provided reproducibility and excellent sensitivity. Experimental results showed that DA was detected quickly and accurately without any pretreatment in nM levels with excellent discrimination against ascorbic acid (AA) (which was among the lowest value reported in direct SERS detection of DA). In the third part, a lanthanide series ion (Eu3+) containing silver nanoparticle was prepared for constructing a molecular recognition SERS substrate for the first time. The procedure reported herein, provides a simple way of achieving reproducible and sensitive SERS spectroscopy for organophosphates (OPP) detection. The sensing of the target species was confirmed by the appearance of an intense SERS signal of the methyl phosphonic acid (MPA), a model compound for nonvolatile organophosphate nerve agents, which bound to the surface of the Ag-Eu3+ nanostructure. The simplicity and low cost of the overall process makes this procedure a potential candidate for analytical control processes of nerve agents.

Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS2 films. An understanding of these effects is critical for integrating MoS2 into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS2 films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS2. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS2 for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS2 films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.

PEDOT:PSS is one of the most known conductive polymers, but, despite its great popularity and numerous studies carried on it, the scientific production still shows a lack on this material. This concerns the acidic doping carried by H2SO4 by an in-solution approach. Hence, the aim of this work was to investigate the effects due to the direct acidification of PEDOT:PSS solution. This thesis thus presents a step-by-step study of this novel approach, describing the characteristics of the pristine polymer first, and of its acidified forms then. Results obtained showed a marked improvement of the electrical properties, with also a great versatility in terms of optical and tribological features, until obtaining a plastic, free-standing material with high potentiality. The mechanism behind the proposed method is described, suppling new insights and perspectives. Finally, a novel approach of the block copolymer self-assembly is also described as a use proposal of the material developed.

碩士
國立中央大學
材料科學與工程研究所
105
In recent years, great interest has arisen in flexible substrate and plasmonic. Additionally, gallium has emerged as a promising new material for plasmonics among a growing family of novel materials. This dissertation aims to explore that choose a suitable polymer elastomer substrate and gallium nanoparticles transfer to elastomer substrate by soft lithography, because the elastomer (PDMS) has unique tenability, control spacing between nanoparticles via alter the elastomer strain ratio. We use UV-Visible spectrometer to analyze optical properties. Finally, it can be seen that with the tensile stress of the nanoparticles on the elastomer substrate increasing from 0% to 40%, the reflection characteristic peak shifted from 571 nm to 528 nm, resulting in the optical properties of the blue shift, and the absorption intensity increased from 48% To 62%, and other metals have not the same characteristics, it is speculated that the optical properties of gallium. As the tensile stress increases to 40%, the coupling strength decreases rapidly, resulting in an increase in the spacing of the particles.

碩士
國立中興大學
物理學系所
106
Magnetism is not easily shielded and able to apply an external stimulus in a non-contact method to cause a rapid and reversible response.These two characteristics make the magnetically responsive material unique in its application. In this study, we synthesized Fe3O4 magnetic nanoparticles and modified Fe3O4 with Poly(4-styrenesulfonic acid-comaleic acid) sodium salt (PSSMA) to have a high zeta-potential on the surface. Finally ,we dissolved Fe3O4 magnetic nanoparticles with water to make it become magnetic fluids. The magnetic fluid can form an ordered chain structure by an external magnetic field. The balance between the magnetic attraction between particles and the electrostatic repulsion force of the surface charge determines the particle spacing, resulting in the incident light produces a Bragg scattering phenomenon between the ordered chains of Fe3O4 magnetic nanoparticles. By controlling the size of the applied magnetic field, the spacing of Fe3O4 magnetic nanoparticles in the solution is changed, and the wavelength of the reflected light is then influenced to achieve the effect of discoloration. We studied the size distribution and concentration of Fe3O4 particles to optimize the range of diffraction intensity and magnetron toning. The magnetic fluid composed of Fe3O4 magnetic nanoparticles can be a rapid and reversible magnetic response, so in the future it has potential applications in optoelectronic devices, sensors and color displays.

碩士
國立成功大學
材料科學及工程學系碩博士班
98
The tunable growth of Ge-doped In2O3 nanowires (NWs) and In2Ge2O7 (IGO) NWs was realized in flowing Ar at 550-750?C by modulating the relative weight of Ge/In2O3 powders with the carbothermal reduction method. Ge-doped In2O3 NWs and a mixture of Ge-doped In2O3 and IGO NWs were grown with the weight ratio of Ge/In2O3 powders in the range of 0.1-0.3% and 0.4-10%, respectively, while with that above 10% only IGO NWs were observed. The growth of both Ge-doped In2O3 and IGO NWs followed the self-catalytic vapor-liquid-solid process. The Ge-doping induced a blueshift in the photoluminescence spectrum and optical bandgap of In2O3 NWs with the extent increasing with the Ge content In2O3 NWs. This result can be explained in terms of the compressive stress yielded in the Ge-doped In2O3 NWs and the Burstein –Moss effect.

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.

碩士
國立臺灣大學
物理學研究所
105
Tunable optical properties of semiconductor devices can have useful applications in optoelectronics. The optical transitions of 2D materials are determined by their band structures. Due to the unique physical properties of the 2D materials, there are various methods to modify their band structures, including application of strain, temperature and electric field. In this thesis, we dedicate to the optical properties of field-effect devices based on few-layer InSe and MoS2. In the first part, we study the electric field-effect of bilayer MoS2 on silicon dioxide (SiO2) cover with few-layer boron nitride. We fabricate field-effect device structure to apply vertical electric field on the bilayer MoS2. Based on the theory, electric field can tune the bandgap and optical properties of bilayer MoS2. We perform photoluminescence (PL) spectroscopy of the MoS2 devices and analyze the field-effect of the PL peaks. The peaks energy of the two direct optical transitions of MoS2, known as the A and B excitons, is tunable as the vertical electric field is applied. We observe that PL emission energy can be reduced by increasing external gate voltage at given temperature, which may be due to Stark effect. In the second part, we study the electric field-effect of few-layer InSe on silicon dioxide. Because InSe is easily degraded in ambient condition, we develop a technique to cover the few-layer InSe with polymethylmethacrylate immediately which is very effective. We compare the thickness dependence of photoluminescence of the InSe field-effect devices with different temperature. We then fabricate graphene top gate to create a duel-gate structure for applying vertical electric field on few-layer InSe. For InSe, the bandgap are direct and indirect for bulk and few-layer InSe, respectively. We observe pronounced shift of the peak energy of photoluminescence in few-layer InSe.

碩士
國立成功大學
化學工程學系碩博士班
95
In this research, the water soluble polymer Poly(vinylamine) (PVAm) was employed as a stabilizer to synthesis stable colloidal CdS QDs with good optical properties in the water-methanol co-solvent system. The peculiar feature of this work is that the emitting color of CdS QDs can be tuned from yellow to blue in aqueous solution. The CdS QDs with blue emitting light prepared in water have rarely been mentioned. The conformation of PVAm can vary from extended chains to compact coils as pH value of the solution or methanol volume fraction in the co-solvent increases. The size and emitting color of the CdS QDs can be adjusted by the conformation change of PVAm in the solution. 　　Hydrophobic modified Poly(vinylamine) (HMPVAm) can form micellar-like conformation in the aqueous solution. The micellar-like conformation changes the aggregation behavior of the colloidal CdS QDs but the optical properties are almost the same with the CdS QDs prepared by PVAm. The optical properties of the colloidal CdS QDs are affected by the size and surface properties of the QDs extremely. It’s found that the size and band gap energy of the CdS QDs are controlled by the conformation of PVAm that is controlled by pH value and co-solvent content. However, the intensity of the emitting light of the CdS QDs is not only affected by the pH value and co-solvent but the anime group of the PVAm is considered.

碩士
國立交通大學
材料科學與工程系所
97
This thesis contains three main topics: synthesis, self-assembling dispersions and the optical properties of various metallic nanoparticles. Firstly, we have successfully synthesized pure Ag, Au, and Cu nanoparticles as well as core-shelled Au/Ag, and Au/Cu nanoparticles by the polyol process. According to TEM analysis, the mean diameters of nanoparticles prepared were found to be smaller than 20 nm with a narrow size distribution. In the polyol process, PVP acts as a nucleation promoting agent for nanoparticles, a stabilizer for mono-dispersion, and a protective agent for oxidation. By varying the reaction temperature and PVP concentration, nanoparticles with different structures can be obtained for the further study. 　Nanoparticles usually exhibit distinct structures as well as properties comparing with bulk materials. In order to distinguish these special structures, FCC, Decahedron and Icosahedrons, by nondestructive characterization of X-ray diffraction, a serious of theoretical X-ray diffraction patterns were calculated and compared with experimental data. The results clearly show that X-ray diffraction can effectively distinguish these structures and is in good agreement with the observation of HRTEM. 　For the optical examinations, the nanoparticles prepared were dispersed onto the optical glass by two ways, i.e. typical spin-coating and block-copolymer self-assembling methods. For the prior method, the washed Ag, Au, Cu, Au/Ag and Au/Cu nanoparticles were dissolved in the ethylene-glycol, and then spun with low speed to obtain nanoparticle thin film with various film thicknesses. For the posterior method, PVP-coated Ag nanoparticles were added into the block copolymer PS-P2VP micellar solution, and then spun with high speed to prepare self-assembly Ag nanoparticle thin films. Following with annealing treatments, various periodic patterns of Ag nanoparticle thin films were obtained. The absorption spectra of nanoparticle solutions and the obtained nanoparticle thin films with various thicknesses were then characterized by UV-vis spectrum spectroscopy. In our research, we have successfully simulated the absorption spectra of Ag, Au, Cu, Au/Ag and Au/Cu nanoparticle solutions based on Mie theory. A dramatic change on the absorption spectra was found between aqueous solutions of nanoparticles and nanoparticle thin films. The peak position of the thin film is greatly red-shifted from the general position observed for the nanoparticles in the aqueous solution. With the thickness increases, red-shifts were initially enhanced and then to reach a saturated value.

碩士
國立臺灣大學
化學工程學研究所
103
An innovative intraocular lens (IOL) device fabricated based on chemical vapor deposition encapsulation process by using functionalized poly-p-xylylenes is introduced herein. The advanced device provides non-compromised design parameters for both its optical and biological properties. As an excellent optical device, it provides a high refractive index that stems from intrinsic poly-p-xylylenes and a tunable effective focal length that is realized by manipulating the wetting properties of the encapsulated liquids; the device also offers protection from UV irradiation. As a key medical device, it exhibits excellent biocompatibility and reduced postoperative calcification through the intrinsic properties of poly-p-xylylenes after conducting alizarin red staining, scanning electron microscope, and energy dispersive X-ray spectroscopy examinations. In addition, these synergic functions, cell-adhering RGD peptide and anti-fouling poly(ethylene glycol) molecules, are provided with precise surface chemistry for location to a guided attachment or repellent properties for human lens epithelial cells, fibroblast cells, and human corneal epithelial cells, which is important in preventing device-associated complications.

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.

碩士
國立清華大學
化學系
103
Here we report two surfactant-free methods for synthesis of small Cu2O octahedra and nanocubes with a large range of size tenability in just 10 min. Cu2O octahedra with corner-to-opposite corner distance varying from 52 nm to 157 nm have been obtained by simply mixing aqueous Cu(NO3)2 solution, N2H4 solution, and different volumes of NaOH solution. Cu2O nanocubes with edge lengths from 9 nm to 87 nm can be synthesized by mixing aqueous CuSO4 solution, different volumes of NaOH solution, and ascorbic acid solution at 35 ºC. The particle size can be controlled systematically. This represents the highest level of size and shape control for ultrasmall Cu2O nanocrystals which have been difficult to make. By comparing cubes and octahedra with similar sizes in terms of particle volume for their optical absorption spectra, nanocubes are consistently more red-shifted than octahedra by approximately 15 nm, proving convincingly that Cu2O nanocrystals possess facet-dependent properties, and that cubes bound by the {100} facets have a more red-shifted absorption band similar to observations made in Au‒Cu2O and Pd‒Cu2O nanocrystals.

碩士
國立交通大學
光電工程研究所
101
We have developed an electric tunable-focusing liquid crystal (LC) lens with a built-in polymeric layer. The operating principle of such a LC lens is that a built-in polymeric layer with a spatially distribution of dielectric permittivity helps generating an inhomogeneous electric field to the LC layer and then the LC directors are reoriented by the electric fields to form the lens-like phase profile. The main advantage of such a LC lens is low driving voltage. However, the tunable focusing range is small and the optical properties have not been studied yet. In this study, we analyzed the optical properties of the LC lens. We discussed the reasons why the focusing range is small, and also proposed solutions to improve it. After analyzing and studying, we found out that the main limitation causing small tunable focusing range is that LC the directors in the center of the aperture and at the edge of the aperture moves simultaneously and then results in small difference of refractive indexes when a homogeneous electric field applies to LC layer. To overcome this, we proposed a gradient polymer network liquid crystal (PNLC) with a special design to increase the difference of refractive index, and proved the idea by the experiments. The potential application of the LC lens with a built-in polymeric layer is electrically tunable focusing eye-wears, imaging systems in portable devices and pico-projection systems.

碩士
國立清華大學
化學系所
106
Selenide sources generally cannot be dissolved in aqueous solution, so special preparation of selenium source is necessary for PbSe synthesis in an aqueous solution. Here we have developed a fast and room-temperature method to synthesize lead selenide (PbSe) nanocubes in aqueous solution. The PbSe nanocubes have sizes ranging from 13 nm to 121 nm by adjusting the amounts of acetic acid added. The formation mechanism has been considered to achieve particle size control systematically. Additionally, we conducted the photocatalytic experiment to realize there are absorption peaks in UV-vis region because of the transition into high-energy bands. With increasing PbSe particle size, their absorption bands red-shift progressively. It indicated that there are obviously size-dependent optical properties.

碩士
國立中正大學
物理系
88
Measuring β value（first hyperpolarizabilities）of one dimension organometallic chromophore with heterocycle by hyper-Rayleigh scattering（HRS）technique. Measuring dispersion relation of β value by tunable OPO（optical parametric oscillator） laser. And comparing with theoretic models. We discovered the experiment results much conforming with damped two-level model.

The results of this Ph.D. thesis demonstrate the tunability of photonic platforms by introducing stimuli responsive polymers as constituents of the photonic structure itself or as thermally driven mechanical actuators. In particular, liquid crystalline networks (LCNs) were patterned by lithographic techniques, such as direct laser writing (DLW) and UV polymerization to develop and fabricate tunable photonic crystals for different applications, from tunable telecom filters to tunable structural colors and intelligent sensors, featuring good optical properties that can be controlled and modulated by multiple tuning mechanisms (e.g. temperature and light). In order to optimize the structure design and the tunability of LCN photonic devices, the refractive index and the tunable optical anisotropy (determined by the chemical composition of the material, the fabrication parameters, and the molecular ordering) have been precisely characterized. As first, it has been demonstrated, using a refractometer method, that optical properties of these new photonic materials can be tuned by adjusting mesogenic concentration both in LCN macro- and micro-structures. The tailored chemical formulation allows not only the determination of the shape changing properties of LCNs but also the modulation of the refractive indices and the optical anisotropy of liquid crystalline mixtures, which can be tuned at different temperatures or alternatively by laser light irradiation. Aiming to increase the fabricated structure resolution, a second result demonstrated how refined fabrication resolutions, never yet reached for liquid crystalline networks, can be achieved at low polymerization temperatures (5°C-10°C) using opportune writing parameters. The resulting 3D polymerizable unit, now comparable with the typical voxel of commercial resists, enlarges the application field of photo-responsive elastic materials without degradation of the patterned structure rigidity. Indeed, a spheroidal voxel would be the best polymerization unit to fabricate three-dimensional structures, especially in 3D photonic structures as woodpile photonic crystals for which isotropic voxel dimensions are needed. The best fabrication parameters using the DLW lithographic technique at controlled temperature enabled the fabrication of the first 3D woodpile photonic crystal made by LCN, having a geometric resolution and a light transmission attenuation at the stop band comparable with photonic crystal fabricated with commercial resists. This demonstrates the effectiveness of our previous study. Such photonic crystal has been characterized using temperature as an external stimulus to tune its optical properties in order to demonstrate its potential as a tunable filter at telecom wavelength. Finally, the first proof-of-concept of a smart millimetric optical sensor was developed during a six-month period in collaboration with Prof. Li and Prof. Keller group in Paris, at ChimieParis Tech. A temperature responsive actuator has been combined with the back side of the Morpho Menelaus wing, owing an optimized structural coloration due to its natural photonic crystal structure. Two different strategies have been proposed to control the visual sensor: a macroscopic deformation of the combined system induces an iridescence variation, whereas a nanoscale contraction generates a color shift through the lamellae interspacing variation, parameter that determines the structural coloration. In conclusion, this thesis focused on the material characterization of smart polymers and their nanopatterning for tunable photonic shows as the employ of smart LCNs can be extended from mechanical actuators and microrobotics to micrometric photonic structures for new multifunctional devices.

博士
國立清華大學
化學系
102
Core−shell nanoparticles are highly functional materials with modified properties by changing either the constituting materials or the core to shell ratio. Various core−shell nanostructures have been synthesized such as Au−Cu2O, Au−Ag, Au−Cu, Pt−Pd, and Au−Pd core−shell nanoparticles. Among them, Au−Pd core−shell nanostructures have efficient catalytic properties for a variety of reactions and plasmonic gas sensing upon exposure to H2 as reversible H2 uptake from the Pd shell occurs. Furthermore, synthesis of well-defined Au−Pd core−shell nanocrystals with systematic shape evolution is still challenging by virtue of long reaction time. In Chapter 1, we have developed a facile aqueous solution method to synthesize Au–Pd core–shell nanocrystals with systematic shape evolution from cubic to octahedral structures in just 0.5–2 h at 50 ºC. Octahedral gold nanocrystals were used as cores. Since an important purpose of this work is to systematically examine the plasmonic properties of these particles as a function of particle size, shape, and shell thickness, octahedral Au nanocrystal cores with sizes of 35, 45, 74, and 92 nm have been employed as the templating cores. Au–Pd core–shell cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra with precisely tuned particle morphology and shell thickness have been achieved, allowing a thorough and detailed analysis of the plasmonic band appearance and shifts of these core–shell nanocrystals for the first time. Nanoparticles with uniformly thin shell thicknesses, particularly the core–shell octahedra, exhibit the most pronounced plasmonic band derived from the gold cores. In Chapter 2, we employed Au–Pd core–shell THH particles, octahedra, and nanocubes as hydrogen sensing materials. The nanocrystals were dispersed in an aqueous solution and hydrogen gas was introduced into a 10-mL flask through a syringe-attached balloon. Presence of dissolved hydrogen was detected. All of these nanocrystals were found to be excellent plasmonic hydrogen sensors producing very large spectral red-shifts after hydrogen absorption. THH nanocrystals exposing high-index facets displayed the largest spectral shifts. All these particles are highly selective to hydrogen. The spectral shifts are almost fully reversible with successive hydrogen absorption and desorption cycles. For smaller core–shell octahedra, the spectral changes can be visually observed. Larger particles with thicker Pd shells give the largest spectral red-shift. With all these advantages, these Au–Pd core–shell nanocrystals should find broad applications in which simple detection of hydrogen presence is desirable. In Chapter 3, we utilized gold, gold-palladium, gold-silver, and lead sulfide nanocrystals with cubic and octahedral structures as building blocks to fabricate supercrystals by solvent evaporation and surfactant diffusional methods. The supercrystals prepared were characterized by SEM, TEM, and XRD techniques. The microstructures were studied by small-angle X-ray scattering (SAXS) technique. The growth process was investigated. Shapes and sizes of various supercrystals were also controlled. These supercrystals are considered novel superstructures and may show interesting optical and electrical properties which may be used for the fabrication of metamaterials and photonic devices. In these works, we have developed a facile aqueous solution method to synthesize Au–Pd core–shell nanocrystals with systematic shape evolution from cubic to octahedral structures, allowing a thorough and detailed analysis of the plasmonic band appearance and shifts of these core–shell nanocrystals. We also employed polyhedral Au–Pd core–shell nanocrystals as hydrogen sensing materials. All these particles are highly responsive and reusable hydrogen sensors in aqueous solution. Furthermore, we utilized polyhedral Au–Pd core–shell nanocrystals as building blocks to fabricate 3D supercrystals, which are considered novel superstructures and may have opportunities for the fabrication of metamaterials and photonic devices.

碩士
國立清華大學
化學系
102
Previously we showed that the optical properties of Cu2O crystals are facet-dependent by examining Au‒Cu2O core‒shell nanocubes, cuboctahedra, and octahedra with octahedral gold cores. The substantially red-shifted Au surface plasmon resonance (SPR) absorption band due to the Cu2O shell with a large refractive index remains fixed despite changes in the shell thickness. However, tuning the shell shape, and hence the surface facet, the Au SPR band can differ by as much as 26 nm. The Cu2O shells also display facet-dependent optical properties. Here in the first study, we examined if the same facet-dependent optical properties are observable in Pd‒Cu2O and Au nanorod‒Cu2O core‒shell nanocrystals. Nicely, the same facet-dependent optical properties are observed in these nanostructures. However, the Pd SPR band in Pd‒Cu2O cubes is more blue-shifted than Pd‒Cu2O truncated octahedra, whereas in the Au‒Cu2O case the Au SPR band of cubes is more red-shifted than octahedra. The short Au nanorod cores shift the SPR band directly to 1010‒1235 nm into the second biological window with the Cu2O shells. The small Au‒Cu2O core‒shell nanocrystals were tested for photothermal effect for the first time. Remarkably, giving enough near-IR irradiation energy, the ethanol solvent containing Au‒Cu2O nanoparticles can raise the temperature to 65 ºC after 300 sec from room temperature. This property can be exploited for converting sun light to thermal energy. Au nanoparticles can deform after long illumination time. By coating Cu2O shells outside Au cores, the stability and absorption cross section will increase and the LSPR property is tunable by changing the shell morphology and using short rods. These structures have potential as ultrasmall and highly efficient plasmonic heaters.

The ability to control electromagnetic wavefront is a central key in optics. Conventional optical components rely on the gradual accumulation of the phase of light as it passes through an optical medium. However, since the accumulated phase is limited by the permittivity of naturally existing materials, such a mechanism often results in bulky devices that are much thicker than the operating wavelength.

During the last several years, metasurfaces (quasi-2D nanophotonic structures) have attracted a great deal of attention owing to their promise to manipulate constitutive properties of electromagnetic waves such as amplitude, phase, and polarization. Metasurfaces are ultrathin arrays of subwavelength resonators, called meta-atoms, where each meta-atom imposes a predefined change on the properties of the scattered light. By precisely designing the optical response of these meta-atoms to an incident wave, metasurfaces can introduce abrupt changes to the properties of the transmitted, reflected, or scattered light, and hence, can flexibly shape the out-going wavefront at a subwavelength scale. This enables metasurfaces to replace conventional bulky optical components such as prisms or lenses by their flat, low-profile analogs. Furthermore, a single metasurface can perform optical functions typically attained by using a combination of multiple bulky optical elements, offering tremendous opportunities for flat optics.

The optical response of a metasurface is typically dictated by the geometrical parameters of the subwavelength scatterers. As a result, most of the reported metasurfaces have been passive, namely have functions that are entirely fixed at the time of fabrication. By making the metasurfaces reconfigurable in their phase, amplitude, and polarization response, one can achieve real-time control of optical functions, and indeed, achieve multi-functional characteristics after fabrication. Dynamical control of the properties of the scattered light is possible by using external stimuli such as electrical biasing, optical pumping, heating, or elastic strain that can give rise to changes in the dielectric function or physical dimensions of the metasurface elements.

In this dissertation, we present the opportunities and challenges towards achieving reconfigurable metasurfaces. We introduce a paradigm of active metasurfaces for real-time control of the wavefront of light at a subwavelength scale by investigating different modulation mechanisms and possible metasurface designs and material platforms that let us effectively employ the desired modulation mechanism. We will present multiple electro-optically tunable metasurface platforms. These electronically-tunable schemes are of great interest owing to their robustness, high energy-efficiency, and reproducibility. We will also show the design and experimental demonstration of active metasurfaces for which the tunable optical response can be tailored in a pixel-by-pixel configuration.

The ability to individually control the optical response of metasurface elements has made active optical metasurfaces to be progressively ubiquitous by enabling a wide range of optical functions such as dynamic holography, light fidelity (Li-Fi), focusing, and beam steering. As a result, reconfigurable metasurfaces can hold an extraordinary promise for optical component miniaturization and on-chip photonic integration. Such compact and high-performance devices with reduced size, weight, and power (SWaP) can be used in future free-space optical communications or light detection and ranging (LiDAR) systems.

Bose-Einstein condensates (BECs) have proven to be remarkable systems with which to study some of the foundational models of condensed matter physics. The observation of a critical velocity for the breakdown of superfluidity in a BEC and the superfluid to Mott insulator transition observed in a BEC trapped by an optical lattice are but two examples of the, by now, dozens of exciting results in this field, which combines theoretical tools from condensed matter physics with state-of-the-art experimental techniques from ultra-cold atomic physics. However, any real condensed matter system has to contend with the effects of disorder, a phenomena notably absent in the inherently clean BEC systems. We have developed and implemented a way to add well characterized disorder in a controlled way to the otherwise clean BEC system using the light field from a laser speckle pattern. Using this system, we have investigated the effects of disorder or a single Gaussian defect, on the collective dipole motion of a BEC of 7Li in an optical trap. In addition, we perform transport experiments on a weakly interacting BEC expanding in a disordered one-dimensional atom wave-guide. We have observed that in such a system, the wave nature of matter can lead to spectacular and counterintuitive phenomena. Specifically, we verify that this system exhibits Anderson localization, a phenomena fundamentally resulting from the interference of multiply scattered matter waves. In such a state, the localized gas behaves as an insulator in a regime where it is classically expected to be conducting. We also present results of experiments regarding a repulsive BEC scattering from a semi-permeable, single defect potential. We investigate the transport properties of such a system with special emphasis on the velocity and defect strength dependent dissipation of the collective dipole motion of the BEC. Finally, we present the results of our experiments on the scattering properties of bright matter wave solitons. We have observed fragmentation of the soliton in a disordered potential as well as both splitting and recombination of a soliton after interacting with a single repulsive defect potential.