Applying nanoparticles to develop specific chemical sensors and optoelectronic devices

In this thesis, we utilized unique physical (optical, photothermal) and chemical properties (chemical affinity, catalytic) of metal nanoparticles (NPs) to develop specific chemical sensors and optoelectronic devices. In chapters 1-3, we briefly introduce the research background, literature reviews and the experimental details. In chapter 4, we suggest two kinds of metal-NP based chemical sensors on Si or flexible substrates. (i) We describe the optical constants of self-assembled hollow gold nanoparticle (HGN) monolayers determined through spectroscopic ellipsometry (SE). The extinction coefficient (k) curves of the HGN monolayers exhibited strong surface plasmon resonance (SPR) peaks located at wavelengths that followed similar trends to those of the SPR positions of the HGNs in solution. The refractive index (n) curves exhibited an abnormal dispersion that was due to the strong SPR extinction. The values of Δn and kmax both correlated linearly with the particle number densities. From a comparison of the optical constant values of HGNs with those of solid Au nanoparticles (SGNs), we used SE measurements to demonstrate a highly sensitive Si-based chemical sensor. HGNs display a slightly lower value of k at the SPR peak but a much higher sensitivity to changes in the surrounding medium than do SGNs. (ii) We fabricated a flexible SPR-based scattering waveguide-sensor by directly imprinting HGNs and SGNs onto flexible polycarbonate (PC) plates—without any surface modification—using a modified reversal nanoimprint lithography (rNIL) technology. Controlling the imprinting conditions, including temperature and pressure, allowed us to finely adjust the depth of embedded metal NPs and their SPR properties. Consistent with the three dimensional FDTD simulations, experimentally We obtained an almost one order of magnitude enhancement in the scattering signal after transferring the metal NPs from a glass mold to a PC substrate. We attribute this enhanced signal to the particles strong scattering of the guiding-mode waves and the evanescent wave simultaneously. In chapter 5, we suggest three kinds of NPs enhanced optoelectronic devices. (i) We demonstrate a new optical data storage method: photomodification of HGN monolayers induced by one-shot of deep-ultraviolet (DUV) KrF laser recording. A single pulse from a KrF laser heated the HGNs and transformed them from hollow structures to smaller solid spheres. This change in morphology for the HGNs was accompanied by a significant blue-shift of the SPR peak. If this spectral recording technique could be applied onto thin flexible tapes, the recorded data density would increase significantly relative to that of current rigid discs. (ii) We describe the preparation of optimal textured structures on Si surfaces through metal-assisted etching using SGNs as catalysts in HF/H2O2 solution. We obtained uniformly textured Si layers containing cylindrical, conical, and bowl-shaped features. A textured surface possessing close-packed pyramidal features with dimensions on the subwavelength scale exhibited the lowest reflectance (< 3%) over the entire visible and NIR spectrum. This low reflectance arose from the refractive index gradient of the Si surface and light trapping phenomena. (iii) We systematically investigated the phenomenon of light trapping in Si solar cells coated with metal and dielectric nanoparticles. Based on the FDTD simulations, we suspected that SGN arrays could be considered as deficient single-layer antireflection coatings: they could reduce the amount of reflected light, scattering it into the Si substrates, while strongly absorbing incident light in the plasmonic resonance wavelength regime. We obtained strong evidence supporting this hypothesis from our observation that the degree of light trapping in Si solar cells was dramatically suppressed when using the SGN arrays under a variety of low reflection conditions. Therefore, we replaced the SGNs with dielectric NPs, which possess lower extinction coefficients and better antireflection ability. Finally, we used a simple, rapid, and cheap solution-based method to prepare close-packed TiO2 NP films on Si solar cells; these devices exhibited a uniform and remarkable increase (ca. 30%) in their photocurrents.