Fung Suong Ou

Gold Nanofingers for Molecule Trapping and Detection

Here we demonstrate a molecular trap structure that can be formed to capture analyte molecules in solution for detection and identification. The structure is based on gold-coated nanoscale polymer fingers made by nanoimprinting technique. The nanofingers are flexible and their tips can be brought together to trap molecules, while at the same time the gold-coated fingertips form a reliable Raman hot spot for molecule detection and identification based on surface enhanced Raman spectroscopy (SERS). The molecule self-limiting gap size control between fingertips ensures ultimate SERS enhancement for sensitive molecule detection. Furthermore, these type of structures, resulting from top-down meeting self-assembly, can be generalized for other applications, such as plasmonics, meta-materials, and other nanophotonic systems.

Hot-Spot Engineering in Polygonal Nanofinger Assemblies for Surface Enhanced Raman Spectroscopy

Multiparticle assemblies of nanoscale structures are the fundamental building blocks for powerful plasmonic devices. Here we show the controlled formation of polygonal metal nanostructure assemblies, including digon, trigon, tetragon, pentagon, and hexagon arrays, which were formed on top of predefined flexible polymer pillars that undergo self-coalescence, analogous to finger closing, with the aid of microcapillary forces. This hybrid approach of combining top-down fabrication with self-assembly enables the formation of complex nanoplasmonic structures with sub-nanometer gaps between gold nanoparticles. On comparison of the polygon-shaped assemblies, the symmetry dependence of the nanoplasmonic structures was determined for application to surface enhanced Raman spectroscopy (SERS), with the pentagonal assembly having the largest Raman enhancement for the tested molecules. Electromagnetic simulations of the polygonal structures were performed to visualize the field enhancements of the hot spots so as to guide the rational design of optimal SERS structures.

Synthesis of hybrid nanowire arrays and their application as high power supercapacitor electrodes

Arrays of multi-segmented hybrid nanostructures of carbon nanotube and gold nanowires have been synthesized using a combination of chemical vapour deposition and electrodeposition methods and we further demonstrate that ultra-high power electrochemical double layer capacitors can be engineered using these hybrid nanowires, resulting in very high power densities.

Cones fabricated by 3D nanoimprint lithography for highly sensitive surface enhanced Raman spectroscopy

We demonstrated a cost-effective and deterministic method of patterning 3D cone arrays over a large area by using nanoimprint lithography (NIL). Cones with tip radius of less than 10 nm were successfully duplicated onto the UV-curable imprint resist materials from the silicon cone templates. Such cone structures were shown to be a versatile platform for developing reliable, highly sensitive surface enhanced Raman spectroscopy (SERS) substrates. In contrast to the silicon nanocones, the SERS substrates based on the Au coated cones made by the NIL offered significant improvement of the SERS signal. A further improvement of the SERS signal was observed when the polymer cones were imprinted onto a reflective metallic mirror surface. A sub-zeptomole detection sensitivity for a model molecule, trans-1,2-bis(4-pyridyl)-ethylene (BPE), on the Au coated NIL cone surfaces was achieved.

Study of Molecular Trapping Inside Gold Nanofinger Arrays on Surface-Enhanced Raman Substrates

The binding of trans-1,2-bis(4-pyridyl)-ethylene (BPE) molecules on substrates arrayed with flexible gold nanofingers has been studied by surface-enhanced Raman spectroscopy (SERS) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS). On the basis of the SERS and XPS results, BPE molecules are found to interact with the gold nanofingers through the lone pair electrons of pyridyl nitrogens, not through delocalized π electrons. Furthermore, after comparing the AR-XPS spectra of finger arrays preclosed before exposure to BPE with the spectra of arrays that closed after exposure to BPE, we observed in the latter case, at grazing takeoff angles, an increase in the component of the nitrogen photoelectron peak associated with pyridyl nitrogen atoms residing on bridging sites. These results demonstrate that a small percentage of BPE molecules was trapped between the neighboring gold finger tips during the finger closing process. However, because these trapped BPE molecules coincidently resided in the hot spots formed among the touching finger tips, the substantial increase in the observed SERS signal was dominated by the contribution from this small minority of BPE molecules.

Phonon-assisted electroluminescence from metallic carbon nanotubes and graphene.

We report on light emission from biased metallic single-wall carbon nanotube (SWNT), multiwall carbon nanotube (MWNT) and few-layer graphene (FLG) devices. SWNT devices were assembled from tubes with different diameters in the range 0.7-1.5 nm. They emit light in the visible spectrum with peaks at 1.4 and 1.8 eV. Similar peaks are observed for MWNT and FLG devices. We propose that this light emission is due to phonon-assisted radiative decay from populated pi* band states at the M point to the Fermi level at the K point. Since for most carbon nanotubes as well as for graphene the energy of unoccupied states at the M point is close to 1.6 eV, the observation of two emission peaks at approximately 1.6 +/- approximately 0.2 eV could indicate radiative decay under emission or absorption of optical phonons, respectively.

Multisegmented one-dimensional hybrid structures of carbon nanotubes and metal nanowires

Multisegmented one-dimensional hybrid structures of carbon nanotubes and metal nanowires were fabricated using the alumina templates. Metal nanowires are first grown inside part of the nanochannel using electrodeposition technique, which is followed by the growth of carbon nanotubes using chemical vapor deposition. Well-adhered interfaces formed between the carbon nanotubes and the metal nanowires. The hybrid structure reported here results in nanoscale metal contact with carbon nanotube and will provide a solution to problem of using carbon nanotubes in interconnects. Nanotube-nanowire interfaces for different metals have also been examined and are characterized using scanning electron microscopy and transmission electron microscopy.

Controlled Manipulation of Giant Hybrid Inorganic Nanowire Assemblies

The ultimate goal of nanotechnology is the design and fabrication of nanosize building blocks with multiple functionalities and their assembly into large-scale functional structures that can be controllably manipulated. Here we show that hybrid inorganic multisegmented nanowires, with hydrophobic carbon nanotube tails and hydrophilic metal nanowire heads, allow the assembly and manipulation of massive ordered structures in solution, reminiscent of the organic molecular micellar assembly. Further, properly designed assemblies can be manipulated using external stimuli such as magnetic field and light. The hybrid nanowires can have multiple segments including magnetic components, allowing the assembly to be manipulated by external magnetic field. The assembled structures can also be manipulated by modifying the hydrophobicity of the respective components via chemical functionalization and optical irradiation. This approach brings the concept of environment sensitive self-assembling nanomaterials closer to reality.

Metal Coated Si Nanograss as Highly Sensitive SERS Sensors

We created novel SERS substrates by metalizing (Ag or Au) Si nanograss fabricated by a Bosch process on single crystalline silicon. We demonstrated that the fabricated SERS substrates are highly sensitive. The sensitivity of the substrates depends on the target molecules, the excitation laser wavelengths and the metal coating on the silicon nanograss. With the optimal excitation condition at 633 nm, an enhancement factor of 6 × 107 can be achieved for trans- 1,2-bis(4-pyridyl)-ethylene (BPE) molecules on the gold coated silicon nanograss substrate.

Rational engineering of highly sensitive SERS substrate based on nanocone structures

At last meeting, we reported a new type of surface-enhanced Raman Spectroscopy (SERS) substrates based on metal (Au or Ag) coated Si nanocones fabricated by a Bosch etching process. The substrate showed reliable SERS performance with an analytical enhancement factor greater than 6 × 107 for trans-1,2-bis(4-pyridyl)-ethylene (BPE) molecules. However, the process is limited to single crystalline silicon material, also silicon can absorb both incident and scattered light, making it difficult to investigate the SERS enhancement mechanism. To further improve the sensitivity of the SERS substrate, we have recently developed a process to duplicate the Si nanocones by a cross-linked polymer using 3-D nanoimprint lithography (NIL). The SERS substrate made by NIL demonstrated better enhancement factors for both 633 nm excitation and 785 nm excitation with analytical enhancement factors of over 1011 demonstrated. We will report the rational engineering of the nanocone based SERS substrate and the fundamental understanding of the enhancement mechanism.