Applications Of Oxide Nanoparticles In Surface-enhanced Vibrational Spectroscopy

Plasmonic nanocavities have the ability to localize light to nanoscale dimensions and have been widely used in ultrasensitive spectroscopy,opto-mechanics,photodetection,and biosensing.In recent years,with the advancement of nano-fabrication technology,surface-enhanced infrared absorption(SEIRA)based on mid-infrared plasmon resonances has been realized in a variety of metallic nanostructures with well-designed sizes,shapes,and geometric arrangements.Meanwhile,the SEIRA signal can also be used to detect the field enhancement effect in mid-infrared plasmonic nanocavities.However,nanofabrication techniques such as electron beam lithography,focused ion beam lithography and template lift-off are difficult to achieve mid-infrared plasmonic nanocavities with nano-or sub-nanometer precision,which limits the field enhancement effect in the mid-infrared plasmonic nanocavity.Different from the top-down nanofabrication,chemically synthesized colloidal nanoparticles can also form mid-infrared plasmonic nanocavities,and the gap size of the nanocavities could reach the sub-nanometer level,which has the opportunity to make a further increased field enhancement in the mid-infrared plasmonic nanocavities.In this thesis,using chemically synthesized colloidal metal oxide nanoparticles,we have probed the plasmonic field strength in mid-infrared plasmonic nanocavities with a sub-nanometer precision,and achieved simultaneous SEIRA and SERS(surface-enhanced Raman scattering)responses with oxide-coated metal nanoparticles,and explored the electrical modulation of plasmonic resonances of metal oxide nanoparticles.The main findings include:1.Using the SEIRA signal of molecular moieties,we have characterized the dependency of field enhancement in the mid-infrared plasmonic nanocavity on the nanocavity gap size with a sub-nanometer scale precision.During the experiments,using gold and chromium oxide(CdO)nanoparticles,we fabricated Au-CdO nanocavities with significant mid-infrared plasmon resonances.These Au-CdO nanocavities exhibit prominent SEIRA enhancement,which is able to detect the self-assembled monolayer of alkanethiol(CH3(CH2)n-1SH,n=3 to 16)molecules.The SEIRA signal of the methyl group-CH3 increases as the number n decreases,indicating that the corresponding plasmonic field strength increases as the gap size of the Au-CdO nanocavity decreases.2.Using tungsten oxide-coated silver nanoparticles,we have realized a nanoparticle substrate with both SEIRA and SERS responses.In our experiments,we synthesized silver nanoparticles coated with tungsten oxide nanolayers(Ag@WO3-x)via the thermal reduction reaction.These Ag@WO3-x nanoparticles show obvious plasmonic response in both mid-infrared and visible frequency range,which leads to both SEIRA and SERS responses.The SEIRA and SERS sensitivities of these Ag@WO3-x nanoparticles are both high enough for successful detection of a molecular monolayer,with good repeatability and time stability.3.Based on the pronouced plasmonic resonance of CdO nanoparticles,we explored the dynamic modulation of mid-infrared plasmons.By applying a DC electric field,the plasmonic resonance of CdO nanoparticles gradually blue shifts as the bias increases,and the maximum modulation reaches 39 cm-1.