The Exploration On Chemical Preparation Of AFM-TERS Probe And The Study Of Their Enhancement Performance

Tip enhanced Raman spectroscopy(TERS)is a new surface analysis technology which combining scanning probe microscopy(SPM)with Raman technology.In recent years,it has attracted more and more attention due to its high spatial resolution and detection sensitivity.In the system,a probe of dozens of nanometers was moved to a close distance to a sample surface by SPM,when the incident light with appropriate wavelength illuminate at the top of the probe,a very strong electromagnetic field will generate because of surface plasmon resonance and the lightning rod effect,which make the Raman signal of the sample below the tip strengthen.Therefore,the material used to make the TERS tip must be gold,silver,copper or their coating in view of their strong surface plasmon resonance in the visible region.According to the feedback mechanisms which keep the tip close to the sample surface,TERS systems are generally divided into three types: atomic force microscopy based TERS(AFM-TERS),scanning tunneling microscopy based TERS(STM-TERS)and shear force microscopy based TERS(SFM-TERS).The simple operation and wide applicability of AFM make AFM-TERS system become a powerful tool for characterizing various materials from single molecules,biological materials to low dimensional nanomaterials.However,as the core of AFM-TERS,probe has higher costs due to the preparation methods,such as vacuum evaporation method,the precision machining method,are expensive and available only in a few laboratories.This largely limits the promotion of AFM-TERS technology.How to prepare AFM-TERS probes with high enhancement,high resolution and high repetition rate by simpler methods is still an important research for TERS.The wet-chemistry procedure which can be performed using commercially available reagents and basic laboratory equipment is a low-cost way.If the wet-chemistry procedure can be used to prepare AFM-TERS probe,the cost of AFM-TERS probe will be greatly reduced.In view of the above analysis,this dissertation makes the following exploration:1.During the preparation of Au nanoparticles by sol gel method,AFM probe was immersed in the solution to make the nanoparticles formed on the surface of the probe.Results showed that the formed nanostructures on the probe surface were flake rather than spherical nanostructures.2.According to the above results,we changed the experimental method.Au nanoparticles of differentsizes were prepared firstly,then the monolayer of Au nanoparticles was formed on the surface of the solution by two-phase method and the nanoparticles were transferred to the probe surface by the membrane extraction method finally.This method can be used to prepare AFM probes modified by nanoparticles.However,the particles were easy to fall off during the TERS test because there was only a simple physical adsorption between the particles and the probe.3.Through the above experimental analysis,we draw a conclusion that in order to prepare stable AFM-TERS probe by chemical method,a layer of molecular glue needs to be grown on the probe firstly,and then AFM probe is connected with Au nanostructure by molecular glue.We modified the surface of AFM probe by hydroxylation followed by silanization.By constantly adjusting the experimental parameters,the probe surface was assembled with a flat thiol-terminal silane layer,which laid a foundation for the subsequent experiments.4.Silanized AFM probes were immersed in the reactant solution by on-surface chemistry.By controlling immersion time or the number of immersion cycles,Au nanofilms of different thickness were grown on the surface of the probes to prepared Au@AFM TERS with different apex size and their enhancement effect were tested by Nile blue molecules.The results showed that the enhancement effect of these probes varied with the diameter of the probe apex.5.It has been reported that crystalline noble metal nanostructures have better enhancement effect than amorphous noble metal nanostructures.Au nanoparticles with different particle sizes were prepared by two-step reduction method.Then the modified probes were immersed in colloidal solution of Au nanoparticles.By controlling immersion time and preparation method,Au nanoparticles with different sizes were grown on the surface of the probes.The experiments proved that these AFM probes attached with nanoparticles had stronger TERS enhancement effect compared with the Au@AFM TERS probes,and compared with SERS,they had a higher resolution.6.The above experimental results show that the nanostructures grown by growing in situ are more stable,while the nanostructures with high crystallinity can bring better enhancement effect.In the presence of surfactants,Au nanostructures were grown on the surface of silanized silicon wafers.The results showed that the Au nanostructures were cubes,which indicated that the presence of surfactant could limit the growth direction of nanostructures,but it did not affect the formation of nanostructures on the silicon wafersurface.The formation of the structure proved the possibility of growing Au nanostructures with high crystallinity on the probe and laid a foundation for the preparation of AFM-TERS probes with high enhancement and stability by chemical methods.The performance of AFM-TERS probe determines the depth and breadth of the sample characterized by the TERS technology.With lower costs,preparing AFM-TERS probe with high enhancement,high stability and high resolution by lower coat is our effort direction in the future.