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The two major global problems are to provide health safety and to meet energy demands for ever growing population on a large scale. The study of light interaction with nanostructures has shown a promising solution in improving the fields of bio-sensor and solar energy devices which addresses above mentioned two major global problems. Nanostructures have tunable physicochemical properties such as light absorption, electrical and thermal properties unlike bulk materials, which gives an advantage in applications like bio-sensing and energy harvesting devices. The development of nanofabrication techniques along with the discovery of Surface Enhanced Raman Scattering (SERS) and Plasmon Enhanced Fluorescence (PEF), led to the development of Point of Care (POC) sensing devices. The fundamental understanding of light path in a nanostructured material led to the improvement in solar energy harvesting performance. For both of these applications, engineering nanostructures is the key to improving performance. In this work, different plasmonic nanostructures were designed, fabricated and analyzed for biosensor and light management applications. A new fabrication route, which combines nanosphere lithography with silicon-based clean-room microfabrication processes, has been developed to produce large-area long-range ordered gold nanoring array patterns in a controllable fashion. The developed nanoring structure has SERS enhancement of 2*109 and is used for miRNA detection. A novel pyramid array on gold film 3D plasmonic nanostructure is designed to convert plasmonic light scattering to confined light absorption. This structure generates a cavity mode by hybridization of fundamental modes, which creates a strong electric and magnetic field with a large mode volume. Due to its unique properties pyramids coupled film structure is used for both solar light management device and in Metal Enhanced Fluorescence (MEF). The fabricated structure is used to demonstrate plexiton (plasmon – exciton coupling) generation and is very effective in light trapping in the gap mode. In MEF, the sandwich nanostructure is used for Metal Organic Framework (MOF) fluorescence enhancement and the enhancement factor is around 5*102. With the plasmonic metal nanostructure optimization, the performance of a specific application is improved. However, the metals used for plasmonic applications are noble metals like gold and silver to support strong localized surface plasmon resonance (LSPR), which are expensive. Two-dimensional semiconductor materials have shown plasmon resonance in the visible region, having a lot of applications in sensing and photonics. Heavily doped semiconductors could replace expensive metals without compromising the performance. LSPR in metals is tuned by shape, size and refractive index of surroundings. This restricts plasmon resonance tuning over a narrow wavelength range and need to choose a different metal to exceed the rage of application. In contrast, LSPR in plasmonic semiconductors can be tuned with parameters like carrier density, annealing temperature and doping. This gives an advantage of tuning the plasmon peak over a broad range including visible, Near Infrared (NIR) and Infrared(IR) regions. This is because, for semiconductor materials, the carrier concentration can be varied over a large range. Herein, the molybdenum oxide thin films were directly deposited and nitrogen annealed which showed a tunable localized surface plasmon resonance (LSPR). A chip based 2D semiconductor material is fabricated to study the structural and size dependent plasmon resonance. This work establishes a way to fabricate chip based ordered semiconductor nanostructures, which helps in a systematic study of plasmon properties on nanostructures.

The world’s booming population projected to reach 10 billion by 2050 causes enormous stresses on environmental safety, food supply, and healthcare, which in return threatens human civilizations. One of the most promising solutions lies at innovating point-of-care (POC) sensing technologies to conduct detection of environmental hazards, monitoring of food safety, and early diagnosis of diseases in a timely and accurate manner. The discovery of surface-enhanced spectroscopy in the 1970s has significantly stimulated research on light-matter interaction which gives rise to enhanced optical phenomena such as surface-enhanced Raman scattering (SERS), plasmon-enhanced fluorescence (PEF), and particularly, they have found enormous applications in optical sensing. To fully exploit surface-enhanced spectroscopy to advance sensing technologies, it requires innovations in the sensor design as well as the plasmonic metallic nanostructures, which is exactly the focus of this dissertation. Owing to their strong capabilities of revealing molecular fingerprints and conducting single molecule analysis, both SERS and PEF have received extensive research interests. Since SERS directly correlates with the local electromagnetic (EM) field enhancement, it is featured by the simplicity in signal amplification. However, high SERS spectral resolution cannot be achieved without a tightly focused laser beam, which compromises the design of SERS-based POC sensing platforms. In contrast, the emission nature of fluorescence makes PEF easily coupled with POC readers, but optimal PEF requires a delicate control of the separation distance between the fluorophore and the nanostructure to minimize fluorescence quenching. SERS and PEF are essentially two complementary techniques and both hold great promise for POC sensing technologies. In the dissertation, in the first place, two label-free SERS sensors have been developed aiming to reduce the number of elements used in a sensor, which could potentially minimize interference, reduce the cost, and enhance the performance. In this regard, a label-free SERS sensor for mercury ions (Hg2+) detection has been developed based on functionalized gold nanoparticles, which employs a small molecule 4-mercaptobenzoic acid to capture mercury ions. A coordination bond formed only in the presence of mercury ions produces a new SERS peak at 374 cm-1, allowing unique detection of mercury ions. The other label-free SERS sensor has been developed for nitrite (NO2-) detection following the mechanism of Griess reaction based on the plasmonic coupling between gold nanostars and silver nanopyramid arrays. A newly formed azo compound produces at least three characteristic SERS peaks at 1140 cm-1, 1389 cm-1, and 1434 cm-1, which allow a highly specific detection of nitrite. While label-free SERS sensing has proved effective to enhance the performance, the need for a tightly focused laser beam hinders SERS from being easily coupled with POC readers for rapid signal readout. To address this limitation of SERS, on-chip PEF sensors have been developed, which can be inserted into POC readers for rapid signal readout. Optimizing PEF usually requires a delicate control of the separation distance between the fluorophore and the nanostructure to balance the excitation and emission enhancement which have different distance dependence. In addition to the separation distance, scattering has been found to be strongly correlated with quantum efficiency enhancement, which has been established as another tuning parameter in optimizing PEF. By making PEF work in the near-infrared (NIR) biological transparency window, the strength of PEF is further manifested by its compatibility with biological matrix featured as low background interference and high penetration depth. As a proof of concept, a NIR fluorescent biosensor has been developed for detection of traumatic brain injury biomarker in the blood plasma. The selection of a gold nanopyramid array pattern as the sensing platform not only generates intense localized EM field for the excitation enhancement, but also allows all the tests to be conducted using a POC fluorescence reader. While noble metals such as gold and silver are often used in developing sensing technologies as they support strong localized surface plasmon resonance (LSPR), it remains an open question as whether they could be replaced by alternative inexpensive metals such as copper and aluminum without compromising the performance. The discovery of a strong and sharp LSPR on copper nanoparticles when the shape is made cubic strongly suggests this possibility. By means of a numeric and theoretical study, it is found that the observed LSPR on copper nanocubes originates from the corner mode which survives damping as it is spectrally separated from the interband transitions. Compared to the dipole mode of a gold nanosphere of the same volume, a copper nanocube displays a comparable extinction coefficient but a local EM field enhancement 7.2 times larger. Furthermore, a film-coupled copper nanocube system has been designed for plasmon-enhanced NIR fluorescence. Because of the coupling between the copper nanocube and the underlying film, a plasmonic cavity mode is generated and featured as a spectrally tunable LSPR and an intense local EM field. By tailoring the resonance to the NIR wavelength region, the film-coupled copper nanocube system has been demonstrated to support a large NIR fluorescence enhancement owing to the strong excitation enhancement and the quantum efficiency enhancement.

This dissertation explores the fluid dynamics of nano and microscale liquid metal filaments, with an emphasis on experimentally investigating the influences and causes of filament breakup and metallic nanostructure formation. Understanding and manipulating the liquid state properties of materials, especially metals, have the potential to advance the development of future technology, particularly nanoscale technology. The combination of top-down nanofabrication techniques with bottom-up, intrinsic self-assembly mechanisms are a powerful fusion, because it permits new and unusual nanostructures to be created, whilst revealing interesting nanoscale physics. In fluid dynamics, wetting and dewetting is the spontaneous natural process that occurs when a liquid supported on a substrate seeks to minimize its systems energy. Either by covering the substrate surface, in the case of wetting, or by rupturing and assembling into a collection of smaller liquid fragments; typically droplets, with minimal contact and surface area with the substrate and the surrounding gaseous environment. Due to metal’s unique liquid state properties, like low viscosity and high surface energy, the dewetting phase is the prescient realm to experimentally access and study the governing dynamics, instabilities, and mass transport behind metallic nanostructure formation. The work contained in this dissertation seeks to address some basic scientific questions, such as: How to develop reasonably simple but predictive models to describe the competition between instability mechanisms that result in filament coalescence or fragmentation, as a function of filament extent? How to manipulate the intrinsic material properties of liquid metals, like surface energy, to initiate instabilities, like those similar to the Rayleigh-Plateau instability, to encourage self-assembly at the nanoscale? A focused and collaborative approach is contained herein where experiments will be used to drive theoretical and computational simulations and vice versa.

Surface plasmon is the quantized collective oscillation of the free electron gas in a metallic material. By coupling surface plasmons with photons in different nanostructures, researchers have found surface plasmon polaritons (SPP) and localized surface plasmon resonance (LSPR), which are widely adopted in biosensing, single molecule sensing and detection via surface enhanced raman scattering (SERS), photothermal ablation treatments for cancer, optical tagging and detection, strain sensing, metamaterials, and other applications. The overall objective of this dissertation is to investigate both how mechanics impacts the optical properties, and also how optics impacts the mechanical properties of metal nanostructures reversely. Mechanically engineering individual nanostructures(forward coupling) offers the freedom to alter the optical properties with more flexibility and tunability. It is shown that elastic strain can be applied to gold nanowires to reduce the intrinsic losses for subwavelength optical signal processing, leading to an increase of up to 70% in the surface plasmon polariton propagation lengths at resonance frequencies. Apart from strain engineering, defects are another important aspect of mechanically engineering nanoscale materials, whose impacts on the optical properties of metal nanostructures remain unresolved. An atomic electrodynamic model has been derived to demonstrate that those effects are crucial for ultrasmall nanoparticles with characteristic sizes around 2 nm, and can be safely ignored for those larger than about 5 nm due to the important contribution of nanoscale surface effects. Another key focus of this research project (reverse coupling) is to investigate the currently unknown effects that an external optical field has on the mechanical properties of metal nanostructures. Since each atom in the nanostructure acts as a dipole due to induced electron motions, this optical excitation introduces additional dipolar forces that add to the standard mechanical atomic interactions, which could alter the mechanical properties of the nanostructures. Furthermore, it is shown that when linking mechanics with LSPR, because the metal is dispersive, the mechanical behavior or the strength of the nanostructure should be dependent on the frequency of the electromagnetic excitation. To study this phenomenon, a simpler case with an electrostatic field excitation is considered first, and conclusions are reached on how static fields can be used to tune the elasticity of metallic nanostructures with different sizes and axial orientations and surfaces. Then building upon those understandings, studies were carried out in determining the effects of an optical field, specifically at LSPR frequency, on the mechanical properties of metallic nanostructures. It is found that the initial relaxation strain induced by the static field or optical field is the key factor leading to the variations in the stiffness of the metallic nanostructures that are excited by optical fields at the LSPR frequencies.

Metallic nanostructures have been investigated with various applications especially for integration with light detection devices. The incident light can be manipulated by those nanostructures to enhance light absorption therefor improve device performance. However, previous studies focused on optical design. The electrical properties of these integrated light detection devices have not been fully considered. The photon generated carriers transport and collection are critical for light detection devices as well. An optimized device platform considering from both the optical and electrical aspects to fully utilize these nanostructures is highly desired for future light detection devices. This dissertation targeted on three objectives, beginning with the fabrication process development of various nanostructures on different substrates. High quality nanostructures were achieved with minimum 20nm gap and 45nm line width. The second objective was developing the metallic fishnet nanostructures integrated Schottky contact a-Si solar cell to improve both light absorption and photon generated carrier collection. The fishnet was designed as the light trapping structure and 2D connected top contact to collect carriers. The third objective was developing metallic nanostructures integrated GeSn photodetectors. The H shape nano antennas were integrated on GeSn photodetectors. Multiple resonant absorption peaks at infrared range were observed using spectroscopic ellipsometry. However, there was no obvious photoresponse value improvement of developed solar cells and H shape antennas integrated GeSn photodetectors. For further investigation, interdigitated electrodes integrated GeSn photodetectors were designed. With less carrier transit time, the responsivity value of the integrated Ge0.991Sn0.009 photodetector was 72µA/W at 1.55µm at room temperature which was 6 times higher comparing to device without integration. Meanwhile, with the increased carrier life time by decreasing temperature, the responsivity value of integrated Ge0.93Sn0.07 detectors at 1.55µm at 100K was 8.5mA/W which was 200 times higher than the value at 300K. These results suggest relative large surface recombination rate was the dominant loss mechanism in metallic nanostructures integrated light detection devices, as the ratio of carrier life time and transit time determines the gain of photodetector. The light detection devices integrated with metallic nanostructures can be developed to maximize device performance with light trapping effect and carrier collection efficiency.

Abstract: Interfacing metals with semiconductor surfaces at the nanometer scale has received much attention, as a result of the critical importance of these interfaces for applications such as integrated circuits, optoelectronics, and others. An efficient and versatile approach for the synthesis of metallic nanostructures on a variety of semiconductor surfaces, including GaAs, InP, silicon [Si(111), Si(100) and Si nanowires], and germanium, is galvanic displacement – a spontaneous electrochemical reaction that is a member of the electroless deposition family. These hybrid nanostructures have intriguing properties but have not been elucidated and therefore not understood. To better illuminate the nature of these systems we use a number of different analyses such as X-ray diffraction (XRD), nanobeam (~5 nm) selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES), as well as high resolution transmission electron microscopy (TEM) imaging. In spite of the fact the reaction is carried out in water, the growth of gold on silicon and germanium surfaces is heteroepitaxial. This high degree of alignment (heteroepitaxy) was directly observed by high resolution TEM imaging the interface between gold and single crystal germanium and silicon substrates, revealing a coincident site lattice (CSL) of four gold lattices to three of the semiconductor substrate (low lattice mismatch). In the case of Au/Ge, we were able to tune the texture nature of the gold epilayer by changing the composition of the deposition bath.5 Galvanic displacement of Au nanoparticles (NPs) on Si nanowires (NWs) showed very interesting phenomenon – Au NPs exhibit preferential deposition on the Si(110) faces of Si nanowires, grown along growth direction, than on the Si(111) faces. The direction of elemental diffusion across the metal-semiconductor contacts was investigated. Spectroscopic (AES) investigations suggest little diffusion of the metals into the semiconductor lattice. Finally, the intermetallic nature of metal-semiconductor interfaces was substantiated by depth profile X-ray photoelectron spectroscopy (XPS) and nanobeam diffraction analyses. Hence, galvanic displacement offers a very attractive method for wiring in nanostructures to semiconductor chips, allowing for its use in modern technological applications.

The modern ceria-zirconia based catalysts are used in automobiles to reduce exhaust pollutants. Cobalt germanides have potential applications as electrical contacts in the future Ge-based semiconductor devices. In this thesis, transmission electron microscopy (TEM) techniques were used to study the atomic scale interactions between metallic nanostructures and crystalline substrates in the two material systems mentioned above. The model catalyst samples consisted of precious metal nano-particles (Pd, Rh) supported on the surface of (Ce,Zr)O2 thin films. The response of the microstructure of the metal-oxide interface to the reduction and oxidation treatments was investigated by cross-sectional high resolution TEM. Atomic detail of the metal-oxide interface was obtained. It was found that Pd and Rh showed different sintering and interaction behaviors on the oxide surface. The preferred orientation of Pd particles in this study was Pd(111)//CZO(111). Partial encapsulation of Pd particles by reduced (Ce,Zr)O 2 surface was observed and possible mechanisms of the encapsulation were discussed. The characteristics of the metal-oxide interaction depend on the properties of the oxide, as well as their relative orientation. The results provide experimental evidence for understanding the thermodynamics of the equilibrium morphology of a solid particle supported on a solid surface that is not considered as inert. The reaction of Co with Ge to form epitaxial Co5Ge7 was studied by in situ ultra-high vacuum (UHV) TEM using two methods. One was reactive deposition of Co on Ge, in which the Ge substrate was maintained at 350°C during deposition. The other method was solid state reaction, in which the deposition of Co on Ge was carried out at room temperature followed by annealing to higher temperatures. During reactive deposition, the deposited Co reacted with Ge to form nanosized 3D Co 5Ge7 islands. During solid state reaction, a continuous epitaxial Co5Ge7 film on the (001) Ge substrate was formed at ∼300°C. With further annealing at a higher temperature, the continuous Co5Ge 7 layer broke into 3D islands in order to relieve the strain energy in the epitaxial Co5Ge7 layer. Electron diffraction and X-ray diffraction were used to identify the cobalt germanide phase and epitaxial orientation relationships with respect to the substrate.