Laser Processed SERS Substrates And Sensing Performance

As an important sensing technique,surface enhanced Raman scattering(SERS)spectroscopy has multifold advantages,such as high sensitivity,high specificity,high detection speed,and extensive applicability.However,high-performance SERS substrates are not only the first requirements for such sensing technologies,but also the key for realizing the sensing functions.Therefore,simple,practical,and efficient fabrication of metallic micro-and nano-structures is an important aspect of the SERS sensing technology.This thesis focuses its investigations on SERS sensing techniques and put forward two different fabrication techniques for SERS substrates,which are based on the direct laser writing.Furthermore,characterization of the sensor devices and sensing experiments are carried out to verify the success of the fabrications.(1)A fabrication technique is put forth for the SERS substrates consisting of tightly arranged gold nanoparticles with high densities,which was achieved by the combination of direct laser writing and furnace annealing.In a first stage,colloidal gold nanoparticles were spin-coated on a glass substrate,which is annealed in a Muffel furnace to produce a random matrix of gold nanoparticles with a diameter smaller than100 nm and a gap width larger than 100 nm.This substrate is used as a template for further direct laser writing.A second layer of colloidal gold nanoparticles was then spin-coated onto the top surface again,laser annealing was performed by scanning a focused laser beam over the surface of the sample.The newly coated gold nanoparticles would get melted and aggregated in the free space between the original gold nanoparticles,forming new ones within the gaps.The basic principle for this fabrication lies in the difference between laser annealing and furnace annealing.In furnace annealing,the whole sample,including the colloidal gold nanoparticles and the substrate,are heated simultaneously,the molten gold is allowed to aggregate in a long range,allowing formation of larger particles with larger separation.However,in laser annealing,the gold nanoparticles are heated to melt without heating the substrate,so that the molten gold is allowed only for a short-range aggregation.Above process was repeated again to produce a random arrangement of high-density gold nanoparticles,where the size of the gold nanoparticle was increased to larger than 100 nm,while the gap width was shrunk to smaller than 20 nm.This implies high density of SERS hotspots,enhancing significantly the detection sensitivity.In the detection of R6G solution with a concentration of 10^-5mol/L,the fabricated SERS substrate facilitates an enhancement factor of about 1.8×10⁴.(2)A direct laser writing method was investigated for the fabrication of metallic micro-gratings,where the grating grooves supply mechanisms for both plasmonic response and optical feedback,introducing a new approach for the fabrication of SERS substrates.Direct laser writing on a PMMA substrate coated with colloidal gold nanoparticles enables production of grating structures with a period of 250?m and a modulation depth of 10?m.The inner side walls and the bottom of the grating groove are coated with gold nanoparticles,which supply SERS hotspots.Meanwhile,the optical feedback by the inner surfaces enables formation of a microcavity in the grating groove.Direct detection of the R6G solution in water with a concentration of 10^-5mol/L produced a significantly enhanced sensitivity.Compared with the detection using a flat gold film,the substrate consisting of micro-grating structures exhibits an enhancement factor of 50.