Mechanical Self-Assembly of a Strain-Engineered Flexible Layer: Wrinkling, Rolling, and Twisting

Self-shaping of curved structures, especially those involving flexible thin layers, has attracted increasing attention because of their broad potential applications in e.g. nanoelectromechanical/micro-electromechanical systems (NEMS/MEMS), sensors, artificial skins, stretchable electronics, robotics, and drug delivery. Here, we provide an overview of recent experimental, theoretical, and computational studies on the mechanical self-assembly of strain-engineered thin layers, with an emphasis on systems in which the competition between bending and stretchingenergy gives rise to a variety ofdeformations,such as wrinkling, rolling, and twisting. We address the principle of mechanical instabilities, which is often manifested in wrinkling or multistability of strain-engineered thin layers. The principles of shape selection and transition in helical ribbons are also systematically examined. We hope that a more comprehensive understanding of the mechanical principles underlying these rich phenomena can foster the development of new techniques for manufacturing functional three- dimensional structures on demand for a broad spectrum of engineering applications.Comment: 91 pages, 35 figures, review articl

Direct Imaging of Ultrafast Charge Carrier Dynamics in Semiconducting Nanowires Using Two-Photon Excitation and Spatially-Separated Pump-Probe Microscopy

The increasing use of nanoscale materials in scientific research and device design places a greater emphasis on characterizing the heterogeneity of nanostructures. When designing electronic components around the use of individual nanoparticles, it is important to understand variability between seemingly identical particles produced in the same synthesis. To do this, we have developed an ultrafast optical microscope capable of studying single nanostructures with spatial resolution of hundreds of nanometers. Emission images of zinc oxide needle-like nanowires show a modulated pattern along the long axis of the wire that are attributed to the coupling of the optical field into structurally dependent resonance modes. Simulations suggest that these are size dependent hybrid modes, containing character of both whispering gallery and Fabry-Perot modes. By incorporating transient absorption pump-probe techniques into the microscope design, we can observe the recombination dynamics of excited carriers on femtosecond timescales following excitation. Due to the high resolution of the instrument, it is possible to observe the dynamics at different locations within a single nanostructure. This technique is used to study the correlation between the decay kinetics of silicon nanowires and doping density for a variety of surface treatments. The motion of excited carriers in silicon nanowires was directly imaged by holding the pump beam in a particular location and scanning the probe beam over the entire structure. The resulting images show free carriers spreading out from the area of excitation, leaving the immobile trapped carriers behind.Doctor of Philosoph

Synthesis, characterization and field emission properties of rare-earth hexaboride nanowires

Rare-earth hexaborides are a family of compounds which have low work function, high melting point, and high mechanical strength. These properties are highly suitable for electron field emission applications. To explore this possibility, we developed a chemical vapor deposition method and produced three different types of 1D nano-structures, which include single crystalline nanowires of LaB6, CeB6, and GdB6; polycrystalline nanowires of YB12, LaB12, MgO, and Mg3N2; core-shell nanocables of MgB2-in-MgO and LaB6-in-CNT. TEM, SEM, EELS and EDX techniques were applied to characterize structural and chemical information about the synthesized nano-structures. Vapor-solid growth, catalyst-assisted vapor-liquid-solid growth and CNT-assisted template growth are proposed to be accounted for the formation mechanisms of these 1D nano-structures and the theoretical predictions match the experimental observations quantitatively. To fabricate a single nanowire field emitter, direct contact, electron beam lithography and focused ion beam welding techniques were used to attach a single LaB6 nanowire to the tip of a tungsten wire. Cold field emission properties were measured from such made single nanowire emitters. Work function values of 2.6 eV and 1.5 eV were obtained from a LaB6 nanowire emitter and GdB6 nanowire emitter respectively. An Emission Current density as high as 5×105 A/cm2 was obtained from a single LaB6 nanowire emitter, under an extraction voltage of 800 V. Emission current stability was also studied for the nanowire emitter and the results indicate surface-contamination induced emission current fluctuations. A home-designed TEM in-situ field emission measurement holder was fabricated for a JEOL 2010F HRTEM. Field electron emission was performed on a single LaB6 nanowire field electron emitter simultaneously with high resolution TEM imaging. Image contrast changes were observed at under-focus imaging conditions when a series of negative biases were applied to the nanowire emitter. Such contrast changes can be used to qualitatively image the charge density distribution on the nanowire emitter tip during field electron emission

Luminescent Nanocrystals: Line broadening and formation mechanisms

Nanomaterials have become an increasingly important class of materials in the past decades due to their size-tunable optical, electronic, and magnetic properties. Nanomaterials are not only of great scientific interest, but their versatility has also resulted in a wide range of applica¬tions. This thesis focuses on two types of luminescent (light-emitting) nanomaterials, cadmium chalcogenide nanocrystals (NCs) and NaYF4 NCs doped with rare earth ions (lanthanides, e.g., erbium and ytterbium). Both the optical properties and nanocrystal growth mechanisms are investigated. Semiconductor NCs, especially CdSe nanoplatelets (NPLs), exhibit narrow emission bands in the visible part of the spectrum, a property needed for more efficient white light LEDs (w-LEDs) and vibrant displays. In these applications, the luminescent materials operate at elevated tem¬peratures, which affects the emission linewidth. Insight into this thermal broadening is important for application in w-LEDs but has so far not been investigated over a temperature range that is relevant for these applications. In this thesis, I report on the temperature-dependent spectral linewidth of cadmium chalcogenide NPLs and QDs. NaYF4 NCs doped with lanthanide ions are efficient upconversion materials that can convert two low-energy infrared photons to one high-energy visible photon. These materials can be used in deep-tissue imaging and to enhance the efficiency of solar cells. The formation mechanism of both NaYF4 NCs and CdSe NPLs is still debated. Control over the NC growth is essential to adjust the NC properties. In this thesis, I report on the mechanisms of their nucleation and growth, monitored using in situ absorption and x-ray scattering techniques

Nanostructures for plasmon enhanced fluorescence sensing: From photophysics to biomedicine

Metallic nanostructures exhibit unique plasmonic properties when optically excited, which includes modification of the spontaneous emission and lifetime of fluorophores in their vicinity. Here we utilize silica (SiO2) core encapsulated in gold (Au) shell nanoshells for emission enhancement of weak near-infrared (NIR) emitting fluorophores, including Indocyanine green (ICG) and IR800. The fluorescence enhancement of ICG molecules as a function of distance from the surface of nanoshells was studied. A maximum enhancement of 50X at a distance of 7 nm from the nanoshells surface, and minimum enhancement of 7X at 42 nm from nanoshells surface was achieved. Additionally, fluorescence enhancement of IR800 molecules induced by nanoshells was compared with that of Au nanorods. The quantum yield of IR800 was enhanced from 7% to 86% in the case of nanoshells and 74 % for nanorods. The native lifetime of IR800 decreased from 564 ps to 121 ps when conjugated to nanorods and 68 ps for nanoshells. We then demonstrated a biomedical application of plasmon enhanced fluorescence sensing by utilizing nanoshell based complexes (nanocomplexes) for simultaneous fluorescence optical imaging as well as magnetic resonance imaging of cancer cells in vitro and in vivo. Nanocomplexes were fabricated by encapsulating nanoshells with a SiO2 epilayer doped with iron oxide nanoparticles and ICG molecules, which resulted in a high T2 relaxivity (390 mM-1sec-1) and 45X fluorescence enhancement of ICG. The nanocomplexes were covalently conjugated with antibodies to enable active targeting in vitro and in vivo. In addition they were utilized for photothermal therapy of cancer cells in vitro. Furthermore, other plasmonic nanostructures relevant for biomedical applications were also synthesized in the sub-100 nm regime including Au/SiO2/Au nanoshells and cuprous oxide core coated with Au shell nanoshells. Excellent agreement between their experimental and theoretical optical properties was achieved. Additionally, physical and chemical properties of mesostructures relevant for photonic devices including sub-micrometer zinc oxide structures and Mesostars composed of a mixture of iron oxides and Au were also studied