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Various shapes of metal nanostructures fabricated by NSL, such as triangles and spheres, have been characterized for use in localized surface plasmon resonance (LSPR) spectroscopy applications. Localized surface plasmons (LSPs) are charge density oscillations confined to metallic nanoparticles and metallic nanostructures. Excitation of these LSPs by an electric field (light) at an incident wavelength where resonance occurs results in strong light scattering, and the appearance of intense surface plasmon (SP) absorption bands, as well as an enhancement of the local electromagnetic fields. The frequency (i.e absorption maxima or color) and intensity of the SP absorption bands are characteristic of the type of material (typically gold, silver or platinum), and are highly sensitive to the size, size distribution, and shape of the nanostructures as well as to the environment which surround them. These are the precise properties which has prompted the ongoing intense interest of LSPs and fueled the construction of LSP based sensors and devices in ever increasing variety2.
My proposed research encompasses three phases which incorporates a novel NSL technique to fabricate an ordered array of gold nanorings in an innovative way. (1) Use EL gold plating techniques to apply a gold film to the tips of nanoposts in order to fabricate Au nanoparticles on the tips of these nanoposts thereby creating a nanostamp. The nanoposts will be fabricated by a vendor while the gold plating will be done using a novel electroless (EL) Au plating method to fabricate Au thin films exclusively on the tips of the posts with enhanced uniformity so that they can be thermally transformable to Au nanoparticles (NPs). The EL gold plating technique is one that was developed and perfected by Roper's lab but the procedure to exclusively plate on the tips of the nanoposts will be done by a novel technique. (2) Use the nanostamp to fabricate metal nanostructures in a new innovative way that incorporates plasmonic lithography of SU8 negative resist on a glass substrate. My approach will be based on controlled nanoscale photopolymerization triggered by the local enhanced electromagnetic fields that surrounds the gold nanoparticles and (3) characterize these metal nanostructures for use with LSPR & SERS applications.
Because of the diverse microfabrication techniques currently being developed to meet rising demands for the fabrication of highly ordered metal NP arrays, one can see the importance of developing a novel technique that is both effective in creating high resolution nanostructures with controllable size, shape and spacing and does not require expensive equipment like those used for lithography, ion etching, metal deposition and other traditional top down techniques. Although Nanosphere Lithography has attracted much interest due to the above listed advantages over other lithography techniques, expensive and sophisticated metal deposition equipment is still required post lithography to produce ordered arrays of metal nanostructures3. With this novel NSL technique we will eliminate the need for expensive and sophisticated metal deposition equipment and provide a cost effective repeatable process for mass scale production.
Fabrication and characterization of Au nanostructures formed by Nanosphere lithography will be investigated.
Objective 1: Fabrication - Completed projects will be the fabrication of a nanostamp by forming gold nanoparticles on the tips of pre-fabricated nanoposts; this will be done by (1) using a novel EL gold plating technique developed by Roper's lab and (2) using a novel technique to exclusively plate on the tips of the nanoposts.
Objective 2: Characterization - Completed projects will be the spectroscopic analysis of Au NPs. The physical and optical features of these nanoparticles will be characterized by well-known AFM, SEM and UV-vis spectroscopy.
Objective 3: Fabrication - Completed projects will be the fabrication of Au nanostructures (specifically nanorings) by using a novel NSL technique that will incorporate the successful polymerization of SU8-2002 resist via plasmonic lithography of Au nanoparticles.
Objective 4: Characterization - Completed projects will be the spectroscopic analysis of Au nanostructures (specifically nanorings). The physical and optical features of these nanostructures will be characterized by well-known AFM, SEM and UV-vis spectroscopy.
2. Introduction and Background
History of Nanosphere Lithography
Nanosphere lithography is very similar to other types of lithography, but instead, the mask is replaced with a layer of nanospheres4. After exposure and developing, the uncovered resin is washed away leaving behind nanoscale vertical columns. It was developed for SERS by Van Duyne et al and has proven to be reasonably easy to implement and fairly reproducible. The technique basically exploits the regular patterns formed by the self-assembly of dielectric (e.g. polystyrene) nanospheres on a surface upon drying followed by the evaporation of a metal film on top of the array or even by the 'lift-off' of the nano-spheres themselves (in which case an array of interstitial sites is left on the surface)5.
Because of the high number of processing versatility seen with nanosphere lithography a wide range of possibilities for potential applications have arisen. It has been shown, for example, that substrates with LSP resonances that can be tuned across the entire visible range can be obtained with nanosphere lithography.
Nanostamps have been used in the process of nano-imprint lithography to produce nano-scale patterns on a thermoplastic polymeric film covering the surface of a selected substrate. Nano stamps can also be used in novel processes of nanostamping, which can eliminate the use of resist polymers as well as the etching of the substrate covered with the polymer. A novel technique that I will incorporate involves negative resists and the use photopolymerization around nanoparticles that are stamped into the resist to create nanostructures after removing the un-crosslinked resist.
Conventional fabrication of Metallic Nanostructures using soft lithography techniques
Soft lithography is a general term describing a set of non-photolithographic techniques for microfabrication that are based on the printing of SAMs and molding of liquid precursors. Soft lithography techniques include contact printing, micromolding in capillaries, microtransfer molding and replica molding. Soft lithography has been developed as an alternative to photolithography and a replication technology for both micro- and nanofabrication. The techniques of soft lithography were developed at Whitesides' group and have been summarized in many excellent review articles6.
Nanoimprint lithography is a soft lithography technique that was developed in the middle of 1990's and is a conceptually straightforward method in fabrication of patterned nanostructures. Nanoimprint lithography has demonstrated both high resolution and high throughput for making nanometer scale structures. Figure 1 schematically illustrates the principal steps of a typical nanoimprint process. First a stamp with the desired features is fabricated, for example, by optical or electron beam lithography followed by dry etching or reactive ion etching. The material to be printed, typically a thermoplastic polymer, is spun onto a substrate where the nanostructures are to be fabricated, The second step is to press the stamp on the polymer layer with the temperature raised above the glass transition point for a certain period of time to allow the plastic to deform. In the third step, the stamp is separated from the polymer after cooling. The patterned polymer left on the substrate is used for further processing, such as dry etching or lift-off, or for use directly as a device component6.
Advantages of the bottom approach
Although the bottom-up approach is nothing new, it plays an important role in the fabrication and processing of nanostructures and nanomaterials. There are several reasons for this. When structures fall into a nanometer scale, there is little choice for a top-down approach. All the tools we have possessed are too big to deal with such tiny subjects. The bottom-up approach also promises a better chance to obtain nanostructures with less defects, more homogeneous chemical composition, and better short and long range ordering. This is because the bottom-up approach is driven mainly by the reduction of Gibbs free energy, so that nanostructures and nanomaterials such produced are in a state closer to a thermodynamic equilibrium state. On the contrary, top-down approach most likely introduces internal stress, in addition to surface defects and contaminations6.
Fig 1. Principal steps of a typical nanoimprint process. A stamp with the desired features is pressed on the polymer layer with the temperature raised above the glass transition point for a certain period of time to allow the plastic to deform. The stamp is separated from the polymer after cooling and the patterned polymer left on the substrate are used for further processing, such as drying etching or lift off, or for use directly as a device component6.
Microscopic characterization of nanoparticles and nanostructures.
Scanning electron microscopy (SEM) uses a type of electron microscope to image a sample by scanning it with high energy beams of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. The SEM can produce very high-resolution images of a sample surface, revealing details about less than 1 to 5 nm. Atomic force microscopy is also another high resolution type of scanning probe microscopy with demonstrated resolution on the order of fractions of a nanometer. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale. Both of these tools offer the advancements necessary to characterize the nanopartices and nanostructures that we will be fabricating during this project9.
Spectroscopic characterization of Au nanoparticles and nanorings.
UV-vis absorption spectroscopy is the most widely used method for characterizing the optical properties and electronic structures of metallic nanoparticles as the absorption bands are related to diameter and aspect ratio of metallic nanoparticles. At nanometer dimensions the electron cloud can oscillate on the particle surface and absorb electromagnetic radiation at a particular energy. This resonance known as surface plasmon resonance (SPR) or plasmon absorbance of nanoparticles is a consequence of their small size but can be influenced by numerous factors. This dependence on surface defects make the surface plasmon an ideal monitor to adsorption to particle surface which allows nanoparticles assemblies to be used as sensing devices.
Metal nanoparticles are also capable of photoluminescence, which has been shown to correlate strongly with their well-defined plasmon resonances. Photoluminescence from noble metals has been observed as a broad background in SERS. In the case of Gold the optical properties are due to valence and conduction electrons and their transitions to different bands of energy.
Compared to solid gold particles of similar size, nanorings exhibit a red shifted localized surface plasmon that can be tuned over an extended wavelength range by varying the ratio of the ring thickness to its radius. The measured wavelength variation is well reproduced by numerical calculations and interpreted as originating from the coupling of dipole modes at the inner and outer surfaces of the nanoring. The electric field associated with these plasmons exhibits uniform enhancements and polarization in the ring cavity suggesting applications in near-infrared surface enhanced spectroscopy and sensing7.
Theory & Applications
Photopolymerization using Au nanoparticles
Studies have shown that during the photopolymerization of an epoxy based resist using gold nanoparticles and an iodium hexafluoroantimonate (OPPI) as photoinitiator and coinitiator, respectively, polymerization occurred only when the AuNPs, in the presence of the iodonium salt, were irradiated at the particle plasmonic absorption region (Î» > 450 nm). The AuNPs then activate the coinitiator by intermolecular electron transfer since OPPI has no absorption in the visible region. Fourier transform infrared spectroscopy can be used to monitor polymerization and UV-vis spectroscopy and transmission electron microscopy measurements can be used to characterize the NPs12.
Surface Plasmon Resonance (SPR)
At an interface between two transparent media of different refractive index (air and glass), light coming from the side of higher refractive index is partly reflected and partly refracted. Above a certain critical angle of incidence, no light is refracted across the interface, and total internal reflection is observed. While incident light is totally reflected the electromagnetic field component penetrates a short (tens of nanometers) distance into a medium of a lower refractive index creating an exponentially detenuating evanescent wave. If the interface between the media is coated with a thin layer of metal (gold), and light is monochromatic and p-polarized, the intensity of the reflected light is reduced at a specific incident angle producing a sharp shadow (called surface plasmon resonance) due to the resonance energy transfer between evanescent wave and surface plasmons. The resonance conditions are influenced by the material adsorbed onto the thin metal film10.
In summary the optical properties of metal nanostructures in the visible region will be dominated by surface plasmon absorption caused by collective conduction band electron oscillations in response to the electric field of the light radiation8.
Localized Surface Plasmon Resonance (LSPR)
Localized surface plasmon resonance (LSPR) are collective electron charge oscillations in metallic nanoparticles that are excited by light. They exhibit enhanced near-field amplitude at the resonance wavelength. This field is highly localized at the nanoparticle and decays rapidly away from the nanoparticle/dieletric interface into the dielectric background, though far-field scattering by the particle is also enhanced by the resonance. Light intensity enhancement is a very important aspect of LSPRs and localization means the LSPR has very high spatial resolution (subwavelength), limited only by the size of nanoparticles9.
Biomedical applications of LSPR sensors
NSL fabricated nanoparticles demonstrate a great potential for applications in the field of biomedical diagnostics since nanoparticles have tunable optical properties which make them an ideal LSPR-sensing platform. By functionalizing the nanoparticle surface with the appropriate sensor, the LSPR nanosensor can be used to detect specific ligands. Such LSPR sensors can be used as diagnostic tools for a variety of diseases such as cancer and hypothyroidism11.