Fabrication Of Anodic Aluminum Oxide Template For SERS Application

Anodic aluminum oxide （AAO） is a porous material, which can be obtained via theanodization of aluminum. AAO is highly ordered, adjustable in period, pore size, aspect ratio,and can easily be rinsed with acidic or alkali solutions. Owing to these advantages, AAO is atpresent one of the most widely used templates. An ideal AAO template is supposed to be amembrane with close-packed highly ordered hexagonal nanochannels like a honeycomb.Currently a relatively mature two-step anodization method can realize such an organizedstructure in nanoscale. One of the most fascinating properties of AAO is attributed to itsspecial hexagonally arrayed channel structures. But whether the self-organized property ofaluminum oxide can be used for constructing hierarchical periodic structures has not beenstudied in much detail. If utilizing the self-organized property, hierarchical structural AAOmay be obtained via the anodization of pre-patterned aluminum substrate, then the novelstructure will extend the development and application on new field.AAO with the nanochannels structure can be utilized as a template or a mask forfabricating metallic, metal oxide, semiconductor and polymerized nanomaterials, it also canbe applied on optical waveguide sensing and catalysis. One of the application, as we mostconcern, is to fabricate a surface-enhanced Raman scattering （SERS） substrate. SERS spectrahave extensive applications in chemistry and biology analysis because of their excellentperformance in the detection of single molecules and characteristic ‘fingerprint’ signatures ofanalytes. A good SERS substrate requires high sensitivity and good reproducibility. Thehighly-orderd period of AAO can solve the reproducibility of SERS substrate, and thenanopores structure in two-dimension and nanochannels in three-dimension can help construct metal nanomaterials. Therefore AAO can be an ideal template to fabricate SERSsubstrate.In this thesis, a novel structural AAO template was developed, and the propagatingsurface plasmons （PSPs） and localized surface plasmons （LSPs） on hierarchical silversubstrate. These two modes of surface plasmons （SPs） can co-enhance Raman scattering.Moreover, a three-dimensional SERS substrate was fabricated via AAO nanochannels. Themajors of the work are as follows:1. A rapid phase transfer method for nanoparticles using alkylamine stabilizersWe have developed a rapid and simple phase transfer method with alkylamine as thesurfactant, through which gold （106nm in diameter） and silver NPs （118nm in edge length）can be transferred from aqueous to organic phases. Three alkylamines with differentchain-lengths （NH₂–（CH₂）n-1–CH3, n=12,16and18） were compared, and octadecylamine（ODA） was the most efficient. The ODA–NP complex rapidly formed as a result of shakingthe mixture of ODA （in ethanol） and NPs （in water）. After20s, the complex separated fromthe liquid phase because of their hydrophobicity. The undissolved ODA–NP complex couldbe redispersed into organic solvents, such as chloroform. Ultraviolet–visible （UV–vis）spectroscopy and transmission electron microscopy （TEM） results show that the metal NPsare still monodisperse having been transferred into organic solvents. The whole process of thephase transfer of NPs from aqueous to organic media can be made to happen in less than1min.2. A three-dimensional SERS substrate fabricated with AAO templateFirst, a periodic AAO template was prepared via two-step method. Gold nanoparticles of37nm diameter were loaded in the channels through self assembly to form athree-dimensional SERS substrate. Gold nanoparticles were synthesized using Frens methodin aqueous solution. Due to surface tension in aqueous solution, gold nanoparticles can notenter the nanochannels easily. To solve the problem, gold nanoparticles was modified with octadecylamin （ODA）, and transfered from aqueous solution to chloroform phase. Thehydrophobic gold nanoparticles self assembled, aggregated and formed many hot spots in theAAO template through a supramolecular interaction. We chose thiophenol as a probemolecule to evaluate the SERS enhancement ability of this three-dimensional substrate. Theenhancement factor （EF） was calculated to be4.6×10⁶under the radiation of a785-nm laser.By further comparing SERS signals from different points on the same substrate, we confirmedthat this substrate possessed good reproducibility and could be applied for SERS detection.3. Hierarchical Structural Nanopore Arrays Fabricated by Pre-patterningAluminum using Nanosphere LithographyA highly ordered and novel structured anodic aluminum oxide （AAO） template wasprepared by pre-patterning an aluminum substrate surface with a nanoindentation array via thenanosphere lithography （NSL） and following an anodization treatment. We employed aninterface self-assembly method to prepare a polystyrene （PS） sphere （with diameters410,530,680nm） monolayer on a flat aluminum substrate. SEM measurements show that a highlyordered pattern with PS spheres close-packing into a hexagonal structure is obtained. Then,the PS nanosphere monolayer was etched with oxygen reactive ion etching （RIE） in order toreduce the diameter of the PS spheres. An80-nm-thick aluminum film was deposited on theetched PS nanosphere monolayer by way of vacuum evaporation. The aluminum substrateunderwent ultrasonic treatment in an ethanol/water solution to remove PS nanospheres. Afterthese processes, a pre-patterned nanowell structure was obtained. According to this empiricalrelationship, the anodization voltage was supposed to be164,212, and272V for thesubstrates with the periods of410,530, and680nm, respectively. However, in this study, weoptimized the anodization voltage in a certain range and found that anodization under anoptimized voltage only at a half of the supposed voltage （82,106, and136V for the substrateswith the periods of410,530, and680nm, respectively） could realize a hexagonally patternedpore array in one nanoindentation spot. In our study, hierarchical structure means that a smallhexagonal periodic array acts as a “lattice unit” to compose a larger hexagonal periodicstructure. Each level of hierarchical array has a self-similar nature, which is a hexagonal pore structure. The whole process is simple, effective, and low-cost. The structure achieved in ourstudy is new, interesting, and an excellent candidate for use in photonic devices.4. PSPs and LSPs in SERS substrate to co-enhance Raman scatteringSilver film was deposited on the hierarchical AAO surface via vaccum evaporation andahighly ordered silver micro-nano substrate with hierarchical structure was obtained. Thisnovel metallic submicro-nano structure exhibits a honeycomb geometry, which has amultiscale structure involving a submicro hexagonal array structure and a coaxial channelnanostructure in each submicro unit. The submicroarray was designed for the PSPs while thenanostructure in submicroarray was suggested to work for the LSPs. The angle-resolvedreflectivity spectra of this hierarchical structure prove that the PSPs and LSPs co-exist in thishierarchical substrate. We choose4-Mpy as the probe molecule to investigate the SERSactivity of this substrate. These two kinds of SPs can co-enhance SERS and an （EF） of1.5×107has been achieved. In addition, the hierarchical substrate as a SERS substrate can improvethe sensitivity and the reproducibility of SERS signals.