Engineering SERS platform in fiber application with a spectroscopic study of bilayer MoS2
Surface Enhanced Raman Spectroscopy (SERS) is a surface-sensitive technique based on plasmon resonance, which can enhance Raman signal intensity by several orders of magnitude, enabling even detection of a single molecule. Material topics involving SERS contain manipulation of specific hot spots through nano-fabrication and enhancement of substrate sensitivity and affinity to interested molecules. The main objective of this research is to gain fundamental understanding of capability limit of SERS optical fiber technology through systematic experimental studies combined with simulations.
SERS fiber probe based on Au nanotriangle has been fabricated and factors affecting Raman signal through fiber are investigated. These factors include numerical aperture of objective lens, slit width of spectrometer, fiber length and SERS array size on fiber facet. Size of SERS array coated on fiber facet is one primary factor accounting for Raman signal loss, which is ultimately due to decrease in laser power density on the fiber end. The most fundamental optimization consideration for SERS fiber probe is a tradeoff between sensitivity and spatial resolution.
A novel Au pyramid/ring SERS platform with enhancement of spatial confinement and electric field is demonstrated for potential SERS fiber probe application through finite-difference time-domain (FDTD) simulation. The FDTD simulation work provides an alternative way to enhance Raman signal intensity while keep high spatial resolution for SERS fiber probe. Excitation of surface plasmon polaritons (SPPs) by circular Au rings and coupling of SPPs with Au pyramids are studied to improve electric field and overall SERS enhancement of the platform. Au pyramids combined with rings can achieve confinement of the electric field within a 700?700 nm region, which could provide higher spatial resolution for SERS fiber probe.
In addition, a practical way of patterning metallic nanostructures for achieving high SERS enhancement factors (EFs) and high hot spot density is demonstrated. By superimposing one layer Au triangle array on another to form multilayer triangle array, SERS signal can be enhanced by two orders of magnitude. The physical understanding of the SERS enhancement of multilayer triangle array is also developed through FDTD simulation. The drastic increased SERS EFs and hot spot density are due to an increase of amount of gaps formed between Au triangles and a decrease of the gap size.
The thesis also presents a serendipitous discovery of bilayer MoS2 spectral features when exploring MoS2 as a potential SERS enhancement substrate. CVD grown AA’ (60? stacking) and AB (0? stacking) stacked bilayer MoS2 have distinct properties in terms of resonance Raman and photoluminescence. Raman and photoluminescence spectra of AA’ and AB stacked bilayer MoS2 are obtained on Au nano-pyramid surfaces under strong plasmon resonance. A Raman peak named as “a” peak appears in AA’ stacked bilayer MoS2 resonance Raman spectra but not in AB stacked bilayer, a feature that can be used as Raman finger print to differentiate the two types of bilayer MoS2.