The Design And Preparation Of Gold Nanostructures And Their Surface-enhanced Raman Scattering Properties

Surface-enhanced Raman scattering(SERS)is an in situ technique with high sensitivity,high resolution and high fingerprint ability.SERS is quite suitable for the areas based on molecular identification and detection.Therefore,SERS could catch researchers eyes on the early age of its discoveries.After the development for four decades,SERS has been widely used in molecular detection,biomedicine,environment protection,architectural conservation and homeland security.The design and fabrication of SERS substrates is still the key issue of SERS studies.Recently,nanoporous gold(NPG)was found to be a new class of SERS substrate with low cost,uniform microstructure,and stable properties.However,there are two significant defects for conventional NPG structure.On the one hand,the SERS enhancement ability of NPG is relatively low,which is adverse for the detection of trace molecules.On the other hand,the microstructure of NPG is disordered,which is adverse for the control of structure and properties.Aiming at solving the mentioned problems above,we fabricated three new types of NPG structures.The major contents are listed follows:(1)Two kinds of numerical methods(DDA and FDTD),that correspond to the simulation of local electric field.during the design and fabrication of SERS substrates,were briefly introduced.The two main issues that we confront in this thesis were theoretical analyzed.Then,three different kinds of NPG structures were designed by the assist of FDTD simulations,which afforded theoretical foundations for the continued fabrication procedures.(2)A novel NPG structure that contains gradient ligament and pore structures along the thickness direction was simply fabricated by combination of the traditional magnetron sputtering and dealloying technique.Using secondary ion mass spectroscopy(SIMS),X-ray photoelectron spectroscopy(XPS)depth profile,and scanning electron microscopy(SEM)to measure the composition and microstructure of samples.It was found that this novel nanoporous gold structure(we called it gradient nanoporous gold,GNPG)brings abundant inner structures into the conventional nanoporous gold,which can produce remarkable properties for GNPG,such as the surface enhanced Raman scattering(SERS).GNPG was demonstrated to have ultrahigh and homogeneous surface enhanced Raman scattering.A simple method was proposed to estimate the relative enhancement factor(EF).The EF of GNPG was estimated to be?8-fold higher than conventional NPG,which means that their EF should over 107.Based on further theoretical analysis,it was found that the SERS properties of GNPG originate from three aspects:the localized surface plasmon resonance(LSPR)of gold nanostructure,the near-field coupling of neighbouring gold ligaments,and the gradient ligament structure of GNPG.(3)Island-like nanoporous gold(INPG)was fabricated by simply controlling the composition of the dealloying precursor.It was found that INPG is constituted with isolated gold islands.INPG has two structural gradations:the external island structure and the internal nanoporous structure.INPG can generate?10 times higher enhancement factor(above 107)with?4 times lower gold consumption than conventional nanoporous gold.The dimensions of the gold islands can be controlled by the composition of the precursor.The enhancement factor can therefore be controlled by the gold island dimensions,which suggests an effective approach to fabricate better Raman enhancement substrates.Using finite-difference time-domain(FDTD)method to simulate the local electric field of INPGs.It was found that the gold island of INPG can effectively improve the local electric field,thus can improve the SERS enhancement factor.Furthermore,INPG exhibits excellent Raman enhancement uniformity and reproducibility with the relative standard deviation of only 2.5%and 6.5%,which originates from the extremely homogeneous structure of INPG at both the microscale and macroscale.The excellent surface-enhanced Raman scattering properties make INPG a potential surface-enhanced Raman scattering substrate.(4)A nanoporous gold film over nanosphere(NPGFON)with hexagonal periodic structure can be created by a simple strategy,which combines NSL with dealloying.Using scanning electron microscopy(SEM)and focus ion beam(FIB)to measure the microstructure of samples.It was found that NPGFON is a kind of periodic hexagonal array structure that constituted with abundant nanoporous half domes.It was demonstrated that NPGFON can generate over 35 times higher Raman enhancement than conventional gold film over nanosphere(AuFON).Further experimental and FDTD simulated results indicate that the large SERS enhancement mainly originates from two aspects.One is the large surface area(?3 times larger than AuFON)introduced by the internal nanoporous structures.The other is the 3D distributed hot spots that localize both near the sharp crevices between adjacent half-domes and in the widely distributed nanopores.The SERS performance of NPGFON can be highly controlled by the pore size and gap size.The capability of NPGFON used for food safety was demonstrated by detecting melamine with the limit of detection as low as 1×10-8 M.Finally,the fabrication methods,SERS properties,and corresponding enhancement mechanisms of the three new types of NPG nanostructures were summarized.The further works were looked forward.