Arthur Ashkin's Nobel Prize-winning single-beam gradient force optical tweezers have revolutionized research in many fields of science. The invention has enabled various atomic and single molecular studies, proving to be an essential tool for observing and understanding nature at the nanoscale. This thesis showcases the uniqueness of single-beam gradient force traps and the advances necessary to overcome the limitations inherent in conventional techniques of optical trapping. With decreasing particle sizes, the power required for a stable trap increases and could potentially damage a particle. This is a significant limitation for studying biomolecules using conventional optical traps. Plasmonic nanoaperture optical trapping using double-nanohole apertures is introduced as a solution to overcoming these limitations. Achievements in double-nanohole optical trapping made possible by the pioneering work of Gordon et. al are highlighted as well. This thesis focuses on the advances in nanoaperture fabrication methods and improvements to experimental techniques adopted in single molecular optical trapping studies. The technique of colloidal lithography is discussed as a cost-effective high-throughput alternative method for nanofabrication. The limitation in using this technique for producing double-nanohole apertures with feature sizes essential for optical trapping is analyzed. Improvements to enable tuning of aperture diameter and cusp separation is one of the main achievements of the work detailed in this thesis. Furthermore, the thesis explains the modified fabrication process tailor-made for designing double-nanohole apertures optimized for optical trapping. Transmission characterization of various apertures fabricated using colloidal lithography is carried out experimentally and estimated by computational electrodynamics simulations using the finite-difference time-domain (FDTD) method. Optical trapping with double-nanohole apertures fabricated using colloidal lithography is demonstrated with distinct results revealing trapping of a single polystyrene molecule, a rubisco enzyme and a bovine serum albumin (BSA) protein. / Graduate

Single-walled carbon nanotubes (SWNTs), with their superior combination of electrical and mechanical properties, have drawn attention from many researchers for potential applications in electronics. Many SWNT-based electronic device prototypes have been developed including transistors, interconnects and flexible electronics. In this thesis, a fabrication method for patterned SWNT networks and devices based on colloidal lithography is presented. Patterned SWNT networks are for the first time formed via solution deposition on a heterogeneous surface. This method demonstrates a simple and straight-forward way to fabricate SWNT networks in a controllable manner. Colloidal sphere monolayers were obtained by drop-casting from solution onto clean substrates. The colloidal monolayer was utilized as a mask for the fabrication of patterned SWNT networks. SWNT networks were shown to be patterned either by depositing SWNT solutions on top of a colloidal monolayer or by depositing a mixed SWNT-colloidal sphere aqueous suspension on the substrates. Colloidal monolayers were examined by optical microscopy and it was found that the monolayer quality can be affected by the concentration of colloids in solution. Polystyrene colloidal solution with concentration of 0.02 wt% ~ 0.04 wt % was found optimal for maximum coverage of colloidal monolayers on SiO2 substrates. After removing the colloidal spheres, the topology of the patterned SWNT networks was characterized by atomic force microscopy and scanning electron v microscopy. Two-dimensional ordered arrays of SWNT rings and SWNTs interconnecting the SWNT rings were observed in the resulting network structure. The height of the rings was about 4-10 nm and the diameter was about 400 nm. In some samples, mesh-like patterned SWNT networks are also observed. It is hypothesized that the capillary forces induced by Van der Waals interaction at liquid/air/solid interfaces play an important role during the formation of the patterned SWNT networks. Raman spectroscopy was also employed to identify the chirality and diameter of the SWNTs in the networks. Both metallic and semiconducting SWNTs were found in the networks and the diameter of the SWNTs was about 1 to 2 nm. The electrical properties of SWNT networks, including random SWNT networks, partially patterned SWNT networks and fully patterned SWNT networks were characterized by a probe station and a Keithley 4200 semiconductor measurement system. The random SWNT networks had two-terminal resistance varying between several MΩ to several hundred MΩ. Field effect behavior was observed in some devices with relatively high resistance and nonlinear I-V curves. Those devices had on/off ratio of less than 100. There was significant leakage current in the ―off‖ state likely due to metallic tube pathways in the networks. The partially patterned SWNT networks had resistance that varied from 20 KΩ to 10 MΩ, but did not display field effect behavior in our studies. The resistance of the patterned SWNT networks was about 10 MΩ - 100 MΩ. The electrical characteristics of the patterned SWNT networks as thin film transistors were investigated, and the on/off ratio of the devices varied from 3 to 105. The upper limit of mobility in the devices was about ~ 0.71 – 5 cm2/V·s. The subthreshold slope of patterned SWNT network FETs can be as low as 210 meV/dec. / Graduate / 0544

abstract: Solar energy has become one of the most popular renewable energy in human’s life because of its abundance and environment friendliness. To achieve high solar energy conversion efficiency, it usually requires surfaces to absorb selectivity within one spectral range of interest and reflect strongly over the rest of the spectrum. An economic method is always desired to fabricate spectrally selective surfaces with improved energy conversion efficiency. Colloidal lithography is a recently emerged way of nanofabrication, which has advantages of low-cost and easy operation. In this thesis, aluminum metasurface structures are proposed based on colloidal lithography method. High Frequency Structure Simulator is used to numerically study optical properties and design the aluminum metasurfaces with selective absorption. Simulation results show that proposed aluminum metasurface structure on aluminum oxide thin film and aluminum substrate has a major reflectance dip, whose wavelength is tunable within the near-infrared and visible spectrum with metasurface size. As the metasurface is opaque due to aluminum film, it indicates strong wavelength-selective optical absorption, which is due to the magnetic resonance between the top metasurface and bottom Al film within the aluminum oxide layer. The proposed sample is fabricated based on colloidal lithography method. Monolayer polystyrene particles of 500 nm are successfully prepared and transferred onto silicon substrate. Scanning electron microscope is used to check the surface topography. Aluminum thin film with 20-nm or 50-nm thickness is then deposited on the sample. After monolayer particles are removed, optical properties of samples are measured by micro-scale optical reflectance and transmittance microscope. Measured and simulated reflectance of these samples do not have frequency selective properties and is not sensitive to defects. The next step is to fabricate the Al metasurface on Al_2 O_3 and Al films to experimentally demonstrate the selective absorption predicted from the numerical simulation. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2019

In this thesis, the local field enhancement from multiple plasmonic structures were studied in different experiments. A new approach was applied to enhance the emission from upconverting nanoparticles to harvest energy from photons below the bandgap. A novel nanofabrication method was introduced to make double nanoholes for use in optical trapping, which was implemented to observe the nonlinear response from 2D materials and the enhanced emission from upconverting single nanoparticles. This method makes a large amount of apertures and is inexpensive. Selective plasmon-enhanced emission from erbium-doped nanoparticles using gold nanorods was demonstrated. Upconversion nanoparticles were excited with a dual-wavelength source of 1520~nm and 1210~nm simultaneously. The power dependence of the observed upconversion emission confirmed the contribution of both excitation bands in the upconversion process. Gold nanorods with resonances at 980~nm and 808~nm were implemented to selectively enhance the upconversion emission in order to harvest light with Si and GaAs solar cells, respectively. I also used colloidal lithography to fabricate double nanoholes which were plasmonic structures used for protein and nanoparticle trapping. This bottom-up technique enabled the fabrication of a large number of structures at low cost. Plasma etching of polystyrene nanoparticles using this technique tuned the cusp separation of double nanoholes down to 10~nm. The smaller cups separation enables to have more confined field in the gap which can be used in plasmonic sensing and plasmon enhanced upconversion processes. This technique can be used to fabricate plasmonic structures for nanoparticle trapping, spectroscopy, and sensing. In the next project, hexagonal boron nitride nanoflakes were trapped in a double nanohole fabricated with the colloidal lithography method. A second harmonic signal was detected at 486.5~nm where the particle was trapped and pumped with an ultra-low power laser at 973~nm. The power dependence measurements supported the second order process for second harmonic generation. Finite-difference time-domain (FDTD) simulations showed a 500-fold field intensity enhancement at the fundamental wavelength and a 450-fold enhancement in the Purcell factor at the second harmonic generation wavelength. This scheme is promising for ultra-fast imaging nonlinear optics technologies. In the last project, colloidal lithography double nanoholes were used to trap upconverting nanocrystals. Colloidal lithography double nanoholes with 32~nm cusp separation achieved 50 times larger emission compared to rectangular apertures. FDTD simulations showed the largest field enhancement in the aperture with the largest upconversion enhancement. 1550~nm emission from the trapped nanoparticle can be used as single-photon source. / Graduate

It is of paramount importance to our society that the environment, life style, science and amusement flourish together in a balanced way. Some trends in this direction are the increased utilization of renewable energy, like solar photovoltaics; better health care products, for example advanced biosensors; high definition TV or high resolution cameras; and novel scientific tools for better understanding of scientific observations. Advancement of micro and nanotechnologies has directly and positively impacted our stance in these application domains; one example is that of vertical periodic or aperiodic nano or micro pillar assemblies which have attracted significant research and industrial interest in recent years. In particular, Si pillars are very attractive due to the versatility of silicon. There are many potential applications of Si nanopillar/nanowire assemblies ranging from light emission, solar cells, antireflection, sensing and nonlinear optical effects. Compared to bulk, Si pillars or their assemblies have several unique properties, such as high surface to volume ratios, light localization, efficient light guiding, better light absorption, selective band of light propagation etc.      The focus of the thesis is on the fabrication of Si pillar assemblies and hierarchical ZnO nanowires on Si micro structures in top-down and bottom-up approaches and their optical properties and different applications. Here, we have investigated periodic and aperiodic Si nano and micro structure assemblies and their properties, such as light propagation, localization, and selective guiding and light-matter interaction. These properties are exploited in a few important optoelectronic/photonic applications, such as optical biosensors, broad-band anti-reflection, radial-junction solar cells, second harmonic generation and color filters.         We achieved a low average reflectivity of ~ 2.5 % with the periodic Si micropyramid-ZnO NWs hierarchical arrays. Tenfold enhancement in Raman intensity is also observed in these structures compared to planar Si. These Si microstructure-ZnO NW hierarchical structures can enhance the performance and versatility of photovoltaic devices and optical sensors. A convenient top-down fabrication of radial junction nanopillar solar cell using spin-on doping and rapid thermal annealing process is presented. Broad band suppressed reflection, on average 5%, in 300- 850 nm wavelength range and an un-optimized cell efficiency of 6.2 % are achieved. Our method can lead to a simple and low cost process for high efficiency radial junction nanopillar solar cell fabrication.            Silicon dioxide (SiO2) coated silicon nanopillar (NP) arrays are demonstrated for surface sensitive optical biosensing. Bovine serum albumin (BSA)/anti-BSA model system is used for biosensing trials by photo-spectrometry in reflection mode. Best sensitivity in terms of limit of detection of 5.2 ng/ml is determined for our nanopillar biosensor. These results are promising for surface sensitive biosensors and the technology allows integration in the CMOS platform.         Si pillar arrays used for surface second harmonic generation (SHG) experiments are shown to have a strong dependence of the SHG intensity on the pillar geometry. The surface SHG can be suitable for nonlinear silicon photonics, surface/interface studies and optical sensing.         Aperiodic Si nanopillar assemblies in PDMS matrix are demonstrated for efficient color filtering in transmission mode. These assemblies are designed using the ‘‘molecular dynamics-collision between hard sphere’’ algorithm. The designed structure is modeled in a 3D finite difference time domain (FDTD) simulation tool for optimization of color filtering properties. Transverse localization effect of light in our nanopillar color filter structures is investigated theoretically and the results are very promising to achieve image sensors with high pixel densities (~1 µm) and low crosstalk. The developed color filter is applicable as a stand-alone filter for visible color in its present form and can be adapted for displays, imaging, smart windows and aesthetic applications. / <p>QC 20160407</p>

No description available.

III-V nanostructures have attracted substantial research effort due to their interesting physical properties and their applications in new generation of ultrafast and high efficiency nanoscale electronic and photonic components. The advances in nanofabrication methods including growth/synthesis have opened up new possibilities of realizing one dimensional (1D) nanostructures as building blocks of future nanoscale devices. For processing of semiconductor nanostructure devices, simplicity, cost effectiveness, and device efficiency are key factors. A number of methods are being pursued to fabricate high quality III-V nanopillar/nanowires, quantum dots and nano disks. Further, high optical quality nanostructures in these materials together with precise control of shapes, sizes and array geometries make them attractive for a wide range of optoelectronic/photonic devices. This thesis work is focused on top-down approaches for fabrication of high optical quality nanostructures in III-V materials. Dense and uniform arrays of nanopillars are fabricated by dry etching using self-assembly of colloidal SiO2 particles for masking. The physico-chemistry of etching and the effect of etch-mask parameters are investigated to control the shape, aspect ratios and spatial coverage of the nanopillar arrays. The optimization of etch parameters and the utilization of erosion of etch masks is evaluated to obtain desired pillar shapes from cylindrical to conical. Using this fabrication method, high quality nanopillar arrays were realized in several InP-based and GaAs-based structures, including quantum wells and multilayer heterostructures. Optical properties of these pillars are investigated using different optical spectroscopic techniques. These nanopillars, single and in arrays, show excellent photoluminescence (PL) at room temperature and the measured PL line-widths are comparable to the as-grown wafer, indicating the high quality of the fabricated nanostructures. The substrate-free InP nanopillars have carrier life times similar to reference epitaxial layers, yet an another indicator of high material quality. InGaAs layer, beneath the pillars is shown to provide several useful functions. It effectively blocks the PL from the InP substrate, serves as a sacrificial layer for generation of free pillars, and as a “detector” in cathodoluminescence (CL) measurements. Diffusion lengths independently determined by time resolved photoluminescence (TRPL) and CL measurements are consistent, and carrier feeding to low bandgap InGaAs layer is evidenced by CL data. Total reflectivity measurements show that nanopillar arrays provide broadband antireflection making them good candidates for photovoltaic applications.  A novel post etch, sulfur-oleylamine (S-OA) based chemical process is developed to etch III-V materials with monolayer precision, in an inverse epitaxial manner along with simultaneous surface passivation. The process is applied to push the limits of top-down fabrication and InP-based high optical quality nanowires with aspect ratios more than 50, and nanostructures with new topologies (nanowire meshes and in-plane wires) are demonstrated.  The optimized process technique is used to fabricate nanopillars in InP-based multilayers (InP/InGaAsP/InP and InP/InGaAs/InP). Such multilayer nanopillars are not only attractive for broad-band absorption in solar cells, but are also ideal to generate high optical quality nanodisks of these materials. Finally, the utility of a soft stamping technique to transfer free nanopillars/wires and nanodisks onto Si substrate is demonstrated. These nanostructures transferred onto Si with controlled densities, from low to high, could provide a new route for material integration on Si. / <p>QC 20130205</p>