Browsing by Subject "Dielectrophoresis"
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Item Dielectrophoresis on nanostructured substrates for enhanced plasmonic biosensing(2017-02) Barik, AvijitPerformance of surface-based plasmonic biosensors is often plagued by diffusion-limited transport, which complicates detection from low-concentration analytes. By harnessing gradient forces available from the sharp metallic edges, tips or gaps that are often found in the plasmonic sensors, it is possible to combine a dielectrophoretic concentration approach to overcome the mass transport limitations. A transparent electrode is integrated with the plasmonic substrates that allow dielectrophoresis without interfering with the label-free sensing schemes such as surface plasmon resonance or Raman spectroscopy. Furthermore, by shrinking the gap between gold electrodes to sub-10 nm, we show ultralow-power trapping of nanoparticles and biomolecules. Reducing the operating voltages diminishes Joule heating, bubble formation and electrochemical surface reactions - hurdles associated with traditional electrodes for dielectrophoresis. The ultralow power electronic operation combined with plasmonic detection has potential in high-density on-chip integration and portable biosensing.Item Engineering metallic nanogap apertures for enhanced optical transmission(2016-10) Yoo, DaehanPhysics and technology of metallic nanoapertures have been of great interest in nanophotonics. In particular, enhanced optical transmission mediated by surface plasmon waves in metallic nanoapertures has been widely studied and utilized in biochemical sensing, imaging, optical trapping, nonlinear optics, metamaterials, and optoelectronics. State-of-the-art nanotechnology enables researchers to explore optical physics in complex nanostructures. However, the high cost and tedium of conventional fabrication approaches such as photolithography, electron-beam lithography, or focused-ion-beam milling have limited the utilization of metallic nanoapertures for practical applications. This dissertation explores new approaches to enable high-throughput fabrication of sub-10-nm nanogaps and apertures in metal films. In particular, we focus on a new technique called atomic layer lithography, which turns atomic layer deposition into a lithographic patterning technique and can create ultra-small coaxial nanoapertures. The resulting nanostructures allowed us to observe extraordinary optical transmission in mid-infrared regime that originates from an intriguing physical phenomenon called the epsilon-near-zero (ENZ) condition. Subsequently, we turn this nanogap structure into a high-Q-factor plasmonic resonator, called a trench nanogap resonator, by combining a nanogap and sidewall mirrors. This structure is optimized for electrical trapping of biomolecules and concurrent optical detection, which is demonstrated experimentally via dielectrophoresis-enhanced plasmonic sensing. The fabrication technique and resulting structures demonstrated in this thesis work can facilitate practical engineering of metallic nanoapertures towards harnessing the potential of plasmonics.Item Large-scale engineered metallic nanstructures for high-throughput surface plasmon resonance biosensing and surface-enhanced Raman spectroscopy(2012-07) Lee, Si HoonPrecise measurements of binding kinetics and affinity of receptor-ligand interactions play an important role in pharmaceutical development as well as basic biology. Since a new drug discovery requires tremendous amount of time and cost, the demand for a high-throughput screening as well as precise kinetics measurement has increased dramatically. Although the commercially available BIAcoreTM system has been the gold standard for label-free and real-time biosensing, it is not capable of high-throughput kinetic measurements that are required for large-scale proteomics studies. To address the critical challenges, high-throughput SPR imaging instruments based on plasmonic nanohole arrays is demonstrated in this dissertation. The key advantage of nanohole-based SPR setup is that plasmons can be excited at normal incidence, which enables simple optical alignment and high-resolution imaging. Using template stripping technology, massively parallel and highly homogenous nanohole arrays, which is the prerequisite to perform high-throughput SPR imaging, are obtained over a large area (~cm2). Linewidths of extraordinary transmission (EOT) peaks are optimized by reducing the damping losses of surface plasmon polaritons (SPPs), leading to the improved detection limits of the sensor. By combining the highly parallel microfluidics with periodic nanohole arrays, our SPR imaging spectrometer system enables high-throughput, label-free, real-time SPR biosensing, and its full-spectral imaging capability increases the dynamic range of detection. Additionally, molecular identification via surface-enhanced Raman spectroscopy (SERS) is also presented in the second portion of the dissertation. Two approaches include planar-type nanohole structures aimed for highly reproducible SERS substrates with low-cost and dynamic nanogaps pearlchains via dielectrophoresis (DEP) for the rapid and ultrasensitive molecular detection and identification.Item Recognition and assembly at multiple length-scales.(2010-05) Olmsted, Brian KeithMany molecular materials capable of crystallizing into an ordered solid state may assume multiple packing arrangements. This behavior is called polymorphism and is common among organic molecules such as pharmaceuticals and dyes. Controlling the nucleation of specific polymorphic crystals is not well understood, but is tantamount to the development and manufacture of new industrial products. One phenomena that has been observed to influence crystal orientation, growth rate, and morphology is epitaxy. Epitaxy refers to a condition by which a crystalline substrate presents a similar two-dimensional lattice to a crystalline plane of a nucleating species, resulting in a condition that lowers the energy barrier to nucleation and results in a preferential orientation of crystal growth on the substrate. Therefore, epitaxial nucleation may provide routes to selectively nucleate polymorphs and attain control over otherwise unpredictable crystallization events. The literature provides several examples of epitaxial relationships between a substrate and a crystal overlayer in fields involving inorganic crystals as well as organic crystals, and because epitaxy relies on geometric comparisons between lattice parameters, computational prediction of epitaxy is an active area of research. Our laboratories have developed software; named GRACE, to attempt to predict epitaxial relationships and this software has been used to verify epitaxy reported in the literature. One particularly useful feature of GRACE is its ability to handle a library of substrates and screen them against a corresponding database of crystal structures available as candidate crystal overlayers. In this capacity GRACE allows large libraries of substrates and crystals to be reduced to an experimentally manageable size, whereby combinatorial crystallizations can be tested for selective nucleation arising from epitaxial interfaces. This research also focuses on other aspects of nucleation that are not yet fully understood. Epitaxial interfaces are by definition, abrupt. However, a specialized class of crystals involving a domain that completely overgrows a core crystal by epitaxial mechanisms has revealed a zone of intermixing spanning close to a micron. In situ Atomic Force Microscopy (AFM) reveals the mechanisms for these observations and provides insight into how epitaxial interfaces behave mechanistically. Notably, it was revealed that process conditions between phases of growth in the formation of core-shroud heterocrystals may yield controllable interfacial thicknesses between crystalline domains, It was also discovered that the propensity for abrupt, epitaxial interfaces may be limited by the thermodynamic behavior of specific crystal interfaces under conditions of near-equilibrium. Although the use of in situ AFM is excellent for the study of crystal growth, the mass-transfer limitations at crystallizing interfaces inside an (AFM) fluid cell are not directly discernable and the assumption is typically made that conditions in the bulk solution are the same inside the cell. By implementing computational fluid dynamic (CFD) simulations for flow and mass transport, in situ AFM was studied to determine how the different conditions at the crystal surface are in comparison to the bulk solution outside the cell. The geometry of the internal volume of the AFM fluid cell imparts specific fluid flow and mass transport limitations on the environment directly at the area of investigation for crystal growth and in some cases may have significant ramifications for the appropriate correlation of bulk solution variables to crystal growth variables. The results of the CFD calculations indicate that differences are significant, though usually minor and these results may prove useful for future fluid cell design. Finally, photolithographic techniques were employed to produce millions of micron-sized particles with shapes mimicking molecular contours and other crystallographically significant contours to study how symmetry and packing originates at the micron length-scale. Although much is known about assembly at the molecular level for symmetry and packing, the assembly of anisotropic particles at longer length scales, which involve different interactive forces, has not been studied. This work concludes by performing preliminary work in elucidating the general behavior towards symmetry and packing in two-dimensions of micron-sized particles by using gravitational gradients and dielectrophoresis.Item Understanding and Optimizing Graphene Material and Devices for Sensing Applications(2020-10) Su, QunGraphene is a novel 2D material with extraordinary potentials in many applications including due to its outstanding electrical properties. In particular, its ultra-sensitive doping effect, outstanding carrier mobility, and large surface-area-to-volume ratio have motivated many research efforts in its chemical and biological sensing applications. However, although high-performance graphene-based sensors have been demonstrated toward various inorganic gas, volatile organic compounds (VOCs), and biomolecules, many obstacles still persist for wide application of graphene-based sensing scheme. This dissertation therefore focuses on these topics and demonstrates novel understanding of graphene-based sensors as well as methodologies for improving their sensing performance. The origin of electrical disorder in graphene was first systematically studied by correlating the doping concentration distribution of graphene to its surface topography. Disorder in graphene is attributed to contact with oxide substrate, embedded ripple structure, and contaminations. Both oxide substrate and contaminations contribute to disorder as doping source, whereas the ripples structure causes inhomogeneous doping interaction with the substrate and hence raises the disorder. Thermal annealing effectively “heals” the topographical unevenness from ripples, but also enhances disorder from the substrate. In the second part, a novel technique for transferring CVD (chemical vapor deposition) graphene from its growth substrate onto arbitrary substrates was developed using fluoropolymer (FP) poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] (Teflon AF1600) as sacrificial layer. This transfer method yields cleaner graphene surface with surface RMS of 0.6 – 1.2 nm and can be applied to larger scale. Moreover, self-assembly molecules can be used as passivation layer during AF1600 transfer process which suppresses the formation of clustered residues. Passivated transfer produces ultra-clean graphene surface with roughness of only 0.4 – 0.5 nm, while the remaining passivation layer can be removed or left to add selectivity for graphene-based sensors. Then, using graphene varactor gas sensors, the interaction mechanisms of gas molecules with graphene was comprehensively studied. Gases like H2O and VOCs can be either loosely bonded to graphene which causes immediate, reversible response in the C-Vg characteristic of varactors due to physisorption-like process, or tightly bonded to graphene which generates a strong, drift response in C-Vg characteristic due to chemisorption-like process. A charge redistribution (CR) model was proposed to explain the reversible signal, which suggests that the doping response arises from the displacement of the charge distribution of the adsorbed molecule driven by the electric fringing field when gated. On the other hand, A charge transfer (CT) model was proposed to explain the drift signal, which states that the signal is originated from the net charge flow enabled by the misalignment of the dynamic fermi energy (EF) of graphene and the highest occupied molecular orbit (HOMO) energy of the adsorbed molecule. These two mechanisms are distinguishable in their temperature dependence and Vg sweeping range dependence. Finally, high-density DEP trapping of 10 kbp DNA molecules using graphene varactors was demonstrated at low voltage. Through selective etching, the excessive edges created provide additional trapping sites. In addition, defect in graphene was found to be nanoscale trapping sites for DEP manipulation, which is promising for DEP integrated graphene sensing scheme.