Hot Spot Generation And Its SERS Studies Based On Colloidal Lithography

Raman spectroscopy is a kind of fingerprint spectroscopic technique,which can reflect the information of chemical and biological molecules at the molecular level.However,the spontaneous signal of Raman spectrum is very weak,which has limited the application of Raman spectrum in trace detection.In 1974,the appearance of surface-enhanced Raman scattering(SERS)solved the problem of poor sensitivity of traditional Raman scattering.With the development of micro and nano processing technology,to fabricate high performance SERS substrates which can effectively enhance electromagnetic fields has attracted a lot of attention.Among others,the construction of,hot spots" plays a key role in the enhancement efficiency of SERS substrates.Since it remains a great challenge to form hot spots between metallic colloidal nanoparticles while preventing random conglomerations,to fabricate ordered metallic structures turns to be a more appealing alternative.Nanostructures based on colloidal lithography have gained in popularity due to their better controllability and uniformity.Up till now,several generations of metallic nanostructures have been developed based on this technique for SERS applications,such as AgFON(silver film over nanosphere).However,the demanding fabrication procedure,the low spatial occupation of the gaps,the broad distribution of SERS enhancement,and the limited reproducibility among different batches of SERS substrates,have hindered these nanostructures from extraordinary SERS performance.Based on colloidal lithography,the work of this paper focuses on the fabrication of metallic nanostructures for excellent SERS performances by constructing plenty of uniformly distributed hot spots,and the research mainly includes:1.A novel tape-imprinting method is employed utilizing the Kapton tape for fabrication of high-performance flexible SERS substrate,which can be activated by incident light even from the back side.A heterogeneous composite structure consisting of silicon pressure sensitive adhesive(SPSA)underlayer and partially embedded polystyrene(PS)nanospheres is created in a highly repeatable way by simply imprinting PS monolayer template using the Kapton tape,thanks to the viscoelasticity and hysteresis of the tape.After reactive ion etching(RIE)and metalization,multi-scale dual morphological lotus seedpod-like silver(LSL-Ag)arrays with three-dimensional full-space-distributed hot spots are obtained.Owing to the presence of high-density high-intensity hot spots,the inherent internal standard which does not compete for the surface adsorption of probe molecules,coupled with the flexible and bidirection-activatable properties,The optimized LSL-Ag arrays exhibit excellent SERS performances with ultrasensitivity(EF as high as 1.06 ×1013,exceeds those of the traditional AgFON arrays by nearly seven orders of magnitudes;LoD as low as 10-13 M),uniformity and reproducibility(absolute intensity RSDs lower than 13.8%versus 60-75%for AgFON,normalized intensity RSDs lower than 1.5%,respectively).The inherent internal standard,flexible and bidirection-activatable properties offer the current structure efficiency in reliable quantitative measurement and practical in-situ detection on arbitrary surfaces,which is not available for the traditional rigid or flexible materials-based SERS substrates.2.Construction of three-dimensional SERS substrate with high performance based on double-layer annular distributed hot spots.In the first part,the outer particle-film nanogaps between the particles on the PS surface and the netlike silver film on the SPSA layer have contributed a lot to the SERS enhancement for the whole substrate,which plays a key role in the excellent SERS performances of LSL-Ag arrays.Inspired by this,and in order to make full use of the three-dimensional active laser confocal volume.of laser,mushroom-like silver arrays(ML-Ag arrays)with double-layer annular hot spots are constructed by a combination of RIE etching of silicon,RIE etching of PS nanospheres and metal evaporation,templated by close-packed PS nanosphere monolayer assembled on silicon wafer.The double-layer annular hot spots are uniformly distributed along the annular nanogaps of the outer layer(lower layer)and the inner layer(upper layer),respectively.The key to the construction of the outer(lower)layer annular hot spots is the RIE etching of silicon.By adjusting the etching conditions of RIE,including gas flow rate,etching duration,chamber pressure and radio frequency power,etc.,the silicon nanoplatform arrays with concave sidewalls are obtained.After the silver evaporation,the outer(lower)hot spots with uniform annular distribution are formed in the nanogaps along the upper surface edge of the silicon nano-platform.The key of inner(upper)hot spot construction is the RIE etching of PS nanospheres.The size of PS nanospheres is adjusted according to the height of the silicon nanoplatform.After the evaporation of silver,the inner layer(upper layer)hot spots with uniform annular distribution are formed along the suface of PS nanospheres at the height of maximum cross-section.Thus three-dimensional double-layer annular hot spots are formed.This substrate has great enhancement efficiency due to the electromagnetic field enhancement thanks to the coupled annular hot spots.