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Fabrication Of Transition-Metal Nanostructures And Ultraviolet Laser Excited Surface-Enhanced Raman Scattering (UV-SERS)

Posted on:2009-12-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:L CuiFull Text:PDF
GTID:1101360272988816Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Surface-enhanced Raman scattering can extremely enhance the Raman signal of species adsorbed on the surface of nanostructures,which makes surface-enhanced Raman spectroscopy(SERS) be a very sensitive tool for the detection of surface species,and thus widely used in many fields like electrochemistry and analytical chemistry.However,the excitation lines used in SERS studies almost always focus on the visible to near infrared region.If it can be extended to the UV region,the use of UV excitation will expand the SERS application and provide new insights and experimental data to discover the complicated SERS mechanisms.SERS enhancements greatly depend on the nanostructure of SERS substrates. Presently,the traditional methods for fabricating SERS substrates are still difficult to obtain UV-SERS active substrates on various metals.In addition,the performance of Raman spectrometer in UV region is much worse than that in visible region,like the throughput,quantum efficiency of CCD and collection efficiency of objectives,which results in a much lower SERS intensity in UV region than that in visible region.The above two facts greatly restrict the development of UV-SERS studies.Therefore,the objective of this dissertation is to develop and optimize UV-SERS active substrates by using several novel nanostructure fabrication methods,and employ UV-SERS to study the enhancement mechanism of some important molecule/metal systems.The main results in this thesis are listed as below:(1) Design and fabrication of various nanostructures with high UV-SERS activity.The methods of electrochemical oxidation reduction cycle(ORC),synthesis of nanoparticles (including core-shell nanoparticles) and electrodeposition of nanovoids through polystyrene sphere templates were employed to prepare diverse metallic nanostructures. By using adenine and SCN- as the probe molecules,for the first time,UV-SERS excited with a 325 nm UV laser was obtained on four metals inâ…§group,i.e.Pt,Pd,Co and Ni.(2) Study of the effect of size,shape and crystallinity of Pt and Pd nanoparticles on the UV-SERS enhancements.It reveals that the nanocrystals with good crystallinity are favorable for the excitation of surface plasmon resonance and the improvement of UV-SERS activity.(3) Study of the wavelength effect on the SERS enhancements of core-shell nanoparticles.By comparing the intensity-shell thickness profile of Au@Pt and Au@Pd core-shell nanoparticles excited with the UV and visible lasers,it was proved that coinage metal(Au) and transition metal(Pt or Pd) had very different enhancements in UV and visible region.In visible region,Au core plays a dominant role in the SERS enhancement, but in UV region,the UV-SERS enhancements are solely contributed by Pt or Pd shell.(4) Fabrication and UV-SERS study on Pd and Pt nanovoids with tunable SPR absorption and uniform morphology.Electrodeposition through a range of close packed arrays of polystyrene sphere templates was employed to produce a series of palladium and platinum nanovoids with different thicknesses.The diameter(D) of spheres used as the template varies from 200 nm to 800 nm and the thickness of nanovoids varies from 0.1 D to 1.2 D.For the first time,the application of these substrates for UV-SERS was demonstrated,using adenine as the probe molecule,under electrochemical potential control. UV-SERS enhancement of palladium and platinum nanocavities was found to be both void diameter and film thickness dependent,and the Pd nanovoids with the thickness of 0.3 D templated by 200 nm diameter sphere show the best UV-SERS activity.(5) Study of the electromagnetic field enhancement mechanism of UV-SERS on Pt and Pd.The surface plasmon resonance(SPR) in UV region was obtained for the first time on Pt and Pd nanoparticles and nanovoids via UV-Vis absorption and normal incidence reflectance measurements.The SPR correlates well with the UV-SERS enhancement, demonstrating that electromagnetic field enhancement plays an important role in the UV-SERS enhancement of Pt and Pd nanostructures.(6) Study of the charge transfer enhancement mechanism of adenine/metal systems. The charge transfer(CT) enhancement mechanism of adenine adsorbed on Rh and Pd was studied by changing either excitation energy(hv) or Fermi level which can be easily tuned by changing the applied potential.The obvious shift of Emax(potential at which intensity reach the maximum) with the excitation energy,and the linear relationship between hv and Emax,convincingly demonstrate the existence of CT enhancement and the charge transfer direction is from the metal to adenine molecules.This result is very important for the understanding of the huge enhancement observed in SM-SERS or TERS spectra of adenine adsorbed on Ag or Au.Finally,it was deduced that there were totally three enhancement mechanisms in adenine/Rh and adenine/Pd systems,i.e.charge transfer,pre-resonance Raman effect and electromagnetic field.(7) Study of the electrochemical coadsorption and adsorption orientation by SERS. The coadsorption behavior of protonated DNA bases with ClO4- and the detection of hydrogen bonds between protonated DNA bases were studied detailedly by SERS;the adsorption orientation of pyrazine on Rh was observed to change both with the applied potentials and the solution concentration,moreover,the coadsorbed O and H were proved to have great influence on its orientation.In summary,the present thesis successfully extended UV-SERS to four important transition metals,i.e.Pt,Pd,Co and Ni,and further studied the important role of EM (electromagnetic field) mechanism in the UV-SERS enhancement of nanostructures. Moreover,charge transfer enhancement of adenine/Rh(Pd) systems was also systematically studied by UV and visible laser excited SERS.This work greatly promotes the UV-SERS studies.Finally,what should be specially mentioned here is that UV-SERS intensity will be stronger than that of Vis-SERS provided that optical elements used in UV region can perform as well as those used in visible region.This predicts the great potential of UV-SERS.With the improvement of the performance of Raman spectrometer and optical elements in UV region,together with the fabrication of nanostructures with higher UV-SERS activity,UV-SERS and UV-SERRS will get extensively applications in the near future.
Keywords/Search Tags:Surface-enhanced Raman spectroscopy, ultraviolet excitation, UV-SERS, electrochemistry, charge transfer enhancement, surface plasmon resonance absorption, core-shell nanoparticle, adenine
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