Fung Ling Yap

Nanoparticle Cluster Arrays for High-Performance SERS through Directed Self-Assembly on Flat Substrates and on Optical Fibers

We demonstrate template-guided self-assembly of gold nanoparticles into ordered arrays of uniform clusters suitable for high-performance SERS on both flat (silicon or glass) chips and an optical fiber faucet. Cluster formation is driven by electrostatic self-assembly of anionic citrate-stabilized gold nanoparticles (~11.6 nm diameter) onto two-dimensionally ordered polyelectrolyte templates realized by self-assembly of polystyrene-block-poly(2-vinylpyridine). A systematic variation is demonstrated for the number of particles (N ≈ 5, 8, 13, or 18) per cluster as well as intercluster separations (S(c) ≈ 37-10 nm). Minimum interparticle separations of <5 nm, intercluster separations of ~10 nm, and nanoparticle densities on surfaces as high as ~7 × 10(11)/in.(2) are demonstrated. Geometric modeling is used to support experimental data toward estimation of interparticle and intercluster separations in cluster arrays. Optical modeling and simulations using the finite difference time domain method are used to establish the influence of cluster size, shape, and intercluster separations on the optical properties of the cluster arrays in relation to their SERS performance. Excellent SERS performance, as evidenced by a high enhancement factor, >10(8) on flat chips and >10(7) for remote sensing, using SERS-enabled optical fibers is demonstrated. The best performing cluster arrays in both cases are achievable without the use of any expensive equipment or clean room processing. The demonstrated approach paves the way to significantly low-cost and high-throughput production of sensor chips or 3D-configured surfaces for remote sensing applications.

Direct growth of aligned graphitic nanoribbons from a DNA template by chemical vapour deposition

Graphene, laterally confined within narrow ribbons, exhibits a bandgap and is envisioned as a next-generation material for high-performance electronics. To take advantage of this phenomenon, there is a critical need to develop methodologies that result in graphene ribbons <10 nm in width. Here we report the use of metal salts infused within stretched DNA as catalysts to grow nanoscopic graphitic nanoribbons. The nanoribbons are termed graphitic as they have been determined to consist of regions of sp(2) and sp(3) character. The nanoscopic graphitic nanoribbons are micrometres in length, <10 nm in width, and take on the shape of the DNA template. The DNA strand is converted to a graphitic nanoribbon by utilizing chemical vapour deposition conditions. Depending on the growth conditions, metallic or semiconducting graphitic nanoribbons are formed. Improvements in the growth method have potential to lead to bottom-up synthesis of pristine single-layer graphene nanoribbons.

Large-Scale Production of Graphene Nanoribbons from Electrospun Polymers

Graphene nanoribbons (GNRs) are promising building blocks for high-performance electronics due to their high electron mobility and dimensionality-induced bandgap. Despite many past efforts, direct synthesis of GNRs with controlled dimensions and scalability remains challenging. Here we report the scalable synthesis of GNRs using electrospun polymer nanofiber templates. Palladium-incorporated poly(4-vinylphenol) nanofibers were prepared by electrospinning with controlled diameter and orientation. Highly graphitized GNRs as narrow as 10 nm were then synthesized from these templates by chemical vapor deposition. A transport gap can be observed in 30 nm-wide GNRs, enabling them to function as field-effect transistors at room temperature. Our results represent the first success on the scalable synthesis of highly graphitized GNRs from polymer templates. Furthermore, the generality of this method allows various polymers to be explored, which will lead to understanding of growth mechanism and rational control over crystallinity, feature size and bandgap to enable a new pathway for graphene electronics.

Large-Area Graphene Nanodot Array for Plasmon-Enhanced Infrared Spectroscopy.

Graphene nanodot arrays (GNDAs) are fabricated by block copolymer lithography in a high-throughput manner. The GNDA shows strong broadband plasmonic resonances in the mid-IR region with high localized field enhancement, thus allowing plasmon-enhanced infrared spectroscopy with reliable sensitivity and selectivity to be performed.

Quantitative Detection with Surface Enhanced Raman Scattering (SERS) Using Self-Assembled Gold Nanoparticle Cluster Arrays

We analysed sensitivity of high-density arrays of self-assembled gold nanoparticle clusters towards trace analyte detection and quantitative determination by surface enhanced Raman spectroscopy (SERS) employing an aromatic thiol as probe molecule. Periodic nanoscale arrays of gold nanoparticle clusters consisting of an average of 18 nanoparticles per cluster, and exhibiting mean inter-particle and inter-cluster separations below 10 nm were prepared using electrostatic self-assembly on block copolymer templates. The concentration dependent scaling of SERS intensities and the lowest detection limits on the cluster arrays on silicon substrate was probed using 1-naphthalenethiol (NT) as test molecule. The substrates show a detection limit of 10 nM along with high sensitivity to changes in NT concentration, which we attribute to high density of hot-spots uniformly organised across the surface. The capability for facile realisation of such arrays without a clean room environment or expensive tools makes the approach suitable for adoption for economic and high-performing SERS sensors.

Fabricating 2D arrays of chemical templates for in situ synthesis of inorganic nanostructures using self-assembly based nanolithography

A versatile approach towards in situ synthesis of inorganic nanostructures by lithographically fabricated chemical templates is presented. Nanolithography is performed using colloidal and copolymer self-assembly based approaches to derive polymeric templates with arbitrary shapes and sizes. Nanodisc, nanohole and nanoparticle arrays of iron oxide and gold synthesized directly on surfaces are shown as proof of concept.

Flexible, transparent and robust SERS tapes through a two-step block copolymer self-assembly process

Surface enhanced Raman spectroscopy (SERS) is an analytical technique that offers the capability of remote sensing, single molecule detection, and detection of trace contaminants (in parts per million) with high sensitivity and accuracy. Here, we demonstrate a simple and economical method for fabricating large area SERS-active tapes that are flexible, transparent and robust using a two-step process. The first is the fabrication of the gold nanoclusters on a flat chip using block copolymer self-assembly followed by directed electrostatic self-organization of gold nanoparticles (AuNPs). The second step involves the transfer of the resulting metal nanoclusters onto a thermal tape by a simple ‘stick and peel’ technique. Such substrates facilitate the detection and quantification of contaminants on irregular surfaces such as fruit skin, fabrics and other non-planar surfaces without the need to extract the analyte. Furthermore, the SERS measurements are highly quantitative, reproducible and the two-step fabrication process is unprecedented and has potential towards realizing large scale manufacturing of low-cost, flexible SERS-tape for on-field applications.

Hierarchically built gold nanoparticle supercluster arrays as charge storage centers for enhancing the performance of flash memory devices

Flash memory devices with high-performance levels exhibiting high charge storage capacity, good charge retention, and high write/erase speeds with lower operating voltages are widely in demand. In this direction, we demonstrate hierarchical self-assembly of gold nanoparticles based on block copolymer templates as a promising route to engineer nanoparticle assemblies with high nanoparticle densities for application in nanocrystal flash memories. The hierarchical self-assembly process allows systematic multiplication of nanoparticle densities with minimal increase in footprint, thereby increasing the charge storage density without an increase in operating voltage. The protocol involves creation of a parent template composed of gold nanoclusters that guides the self-assembly of diblock copolymer reverse micelles which in turn directs electrostatic assembly of gold nanoparticles resulting in a three-level hierarchical system. Capacitance-voltage (C-V) measurements of the hierarchical nanopatterns with a metal-insulator-semiconductor capacitor configuration reveal promising enhancement in memory window as compared to nonhierarchical nanoparticle controls. Capacitance-time (C-t) measurements show that over half the stored charges were retained when extrapolated to 10 years. The fabrication route can be readily extended to programmed density multiplication of features made of other potential charge storage materials such as platinum, palladium, or hybrid metal/metal oxides for next generation, solution-processable flash memory devices.