Optimization And Applications Of SERS Substrates Based On ZnO Nanorod Arrays

Surface-enhanced Raman spectroscopy（SERS）has become a powerful tool of detecting trace molecules and exhibits many distinguishing characteristics,which can be exploited in the applications of biomedicine,environmental monitoring,and chemical analysis,thus receiving much attention in the past decades.The SERS of molecules confined in the vicinity of metal nanoparticles（NPs）or on a rough metal surface is due to the enhancement of electromagnetic fields generated by excitation of a localized surface plasmon resonance（LSPR）.Three-dimensional（3D）SERS substrates prepared with high-conductive metal nanomaterials,which can produce extremely strong LSPR in the visible light range,have greatly promoted the development of SERS application due to the rich structures,low cost and good repeatability.In this thesis,high-sensitive and ultra-stable 3D SERS substrates were prepared by depositing a thin layer of Ag film on ZnO nanorod（ZnO-NR）arrays.By a series of experiments and finite difference-time domain（FDTD）simulations,the mechanism of ultra-stability for Ag-ZnO-NR 3D SERS substrates was revealed and the SERS performance was optimized.By soaking the Ag-ZnO-NR arrays in chloride solutions,the 3D SERS substrates were found to be modified and thus the modification mechanisms were explored and the role of chloride ions in enhancing Raman signals was studied.In addition,the 3D SERS substrates were used to detect gas-phase molecules using a specially designed experimental device.The main results in this thesis are summarized as follows:（1）The stabilization mechanism of Ag-ZnO-NR 3D SERS substratesOn the texturized Si wafers,the patterned sapphire single-crystal chips,and the frosted glass slides,ZnO-NR arrays were grown by a solution method and the 3D SERS substrates were prepared by optimizing the thickness of deposited Ag layer.Using Rhodamine-6G（R6G）as the probing molecules,the Ag-ZnO-NR 3D SERS substrates exhibited good performance capable of detecting R6G as low as 10^-14 M.Moreover,the Ag-ZnO-NR 3D SERS substrates are ultrastable,with a shelf-time in air as long as more than 6 years.Using specially prepred samples of ZnO films with controlled conductivity,the Klevin scanning probe microscopy revealed that the electrons in the Ag layer are transferred to the high-resistance ZnO film,which probably plays an important role in stabilizing the 3D SERS substrates.The stabilitzation mechanism was further substantiated by studing the stability of Ag-ZnO-NR arrays with different buffered layers between the Ag films and ZnO-NRs,showing that the Ag-ZnO-NR arrays become unstable as the electron transfer is obstructed by SiO_x layer,whereas the stability of Ag-ZnO-NR substrates with ZnO buffer layer maintains nearly unchanged.As such,the stabilization mechanism of Ag-ZnO-NR 3D SERS substrates can be attributed to the electron transfer from Ag layer to the ZnO-NRs,thus avoiding Ag films from oxidation.In addition,the stability of Ag-ZnO-NR 3D SERS substrates was studied by exploring the crystallinity of Ag layer.（2）FDTD simulation and structure optimization of Ag-ZnO-NR 3D SERS substrates.By constructing an Ag-ZnO-NR model,the extinction spectra and local electromagnetic fields in the vicinity of Ag-ZnO-NR were calculated by an FDTD method.Differing from Ag-NRs,the Ag-ZnO-NRs exhibited a dipole LSPR split into high-and low-frequency modes（H-and L-modes）,in which the L-mode plays a dominant role in enhancing the Raman signals in the range of 400 to 600 nm.At the same time,the radius and length of ZnO-NRs as well as the the thickness of Ag layer were substantiated to play a role in tuning the LSPR frequency and the intensity.Using SiO₂-NR or TiO₂-NR to replace ZnO-NR,the extinction spectra of Ag-SiO₂-NR or Ag-TiO₂-NR were changed as well,indicating that the refractive index of dielectric materials is another parameter of tuning the LSPR frequency.The local electromagnetic fields of Ag-ZnO-NRs with periodic boundary conditions showed that the performance of Ag-ZnO-NR 3D SERS substrates can be optimized at a ZnO-NR denisity of～83/μm²,which is close to the experimental rsults in this work.In addition,the FDTD simulation revealed that the scattering of electromagnetic fields between different Ag-ZnO-NRs plays an crucial role in enhancing the Raman signals.（3）Effects of chloride ions on Ag-ZnO-NR modification and the Raman enhancement.Using chloride solutions,the Ag-ZnO-NR 3D SERS substrates were modified and the morphological evolution and modification mechanism were explored by changing the chloride concentration and the soaking time.Scanning electron microscopy and FDTD simulation showed that the chloride solutions are able to change the Ag-ZnO-NR arrays into Ag nanoparticles decorated ZnO-NR（Ag NP-ZnO-NR）arrays,which become nearly steady as the chloride solution is higher than a critical concentration and the soaking time is longer than10 min.The Ag NP-ZnO-NR arrays are able to effectively depress the thermalization of probe molecules during the signal collection and reduce the background noise of Raman spectra.By loading the R6G molecules onto SERS substrates in the presence and absence of chloride ions addition,the chloride ions were substantiated to be capable of enhancing adsorption of R6G molecules on the surface of Ag nanoparticles,which is the major reason for the chemical enhancement.The Ag NP-ZnO-NR 3D SERS substrates prepared by soaking Ag-ZnO-NR arrays in R6G and chloride mixed solutions are able to obviously enhancing the ratio of signal to noise.（4）Application of Ag-ZnO-NR 3D SERS substrates in detecting gas-phase molecules.On the basis of Ag-ZnO-NR 3D SERS substrates,a device for detecting gas-phase molecules was specially designed and used to detect the methyl blue molecules in ethonal vapor,showing that the Ag-ZnO-NR substrates are capable of on-line detecting the gas-phase molecules as low as 6×10^-8 M.