Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant
attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel
SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and
showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close
proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and
producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the
structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of
the structural-performance relationship.
Nanoimprint lithography is conventionally used to transfer a pattern from a mold to a deformable and curable
resist layer. Here we report a nanoimprinting technique to selectively transfer components of a pre-assembled
nanostructure to a new substrate, while retaining the advantages of nanoimprint lithography such as low cost and high
throughput. We use this technique to study metal particle roughness in Au "nanofinger" substrates, along with the
effects of annealing to reduce roughness, and the impact of annealing on the Surface Enhanced Raman Scattering
(SERS) signal. The nanofinger substrates consist of Au-coated polymer pillars arranged to collapse into a designed
assembly. Upon exposure to a volatile liquid and subsequent drying, microcapillary forces pull the pillars and their
metal caps together into the designed structure. Successful transfer was achieved using the concept of template stripping
via cold welding using a normal nanoimprinting process with no resist layer but under appropriate pressure to ensure
even and complete transfer of all the nanostructures. Particle roughness was not found to be a significant factor in SERS
from naonfinger substrates as annealing did not increase the observed Raman intensity.
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.
We report a numerical study on the frequency property of a system composed of an optical antenna array placed on the
surface of a dielectric grating. Such a periodic structure is designed for Raman spectroscopy application because of its
advantages over the conventional rough-surface based surface enhanced Raman spectroscopy: the position of the high
field intensity and the exact field magnitude is well controlled by the design, and the constructive interference from the
elements placed in periodic array may form a collective resonance and provide a further enhancement to the field
intensity. By integrating and weakly coupling the guided mode resonance (GMR) of a dielectric grating with optical
nano-antennas made of plasmonic materials that are also field-enhancing devices, it provides a further enhanced local
field around the antenna. We specifically studied the behavior of the device under oblique incidence, and show that
multiple resonant peaks are observed in the spectrum. The application of the device in a Raman process is discussed in
details.
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.
An integrated circuit combining imprinted, nanoscale crossbar switches with metal-oxide field effect transistors
(MOSFET) was fabricated and tested. Construction of the circuits began with fabrication of n-channel MOSFET
devices on silicon-on-insulator (SOI) substrates using CMOS compatible process techniques. To protect the FET devices
as well as provide a flat surface for subsequent nanoimprint lithography, passivation and planarization layers were
deposited. Crossbar junctions were then fabricated next to the FETs using imprint lithography to first define arrays of
parallel nanowires over which, a switchable material layer was deposited. This was followed by a second imprint proces
to construct another set of parallel wires on top of, and orthogonal to the first, to complete the nano-crossbar array with a
half pitch (hp) of 50 nm. The switchable crossbar devices were then connected to the gate of the FETs and the resulting
integrated circuit was tested using the FET as the output signal follower. This successful fabrication process serves as a
proof-of-principle demonstration and a platform for advanced CMOS/nanoscale crossbar hybrid logic circuits.
We propose a novel design for a guided-mode resonance (GMR) grating sensor that is optimized for detecting
small average index changes in an extended region of space, retaining sensitivity up to several tens of microns
away from the grating surface at optical detection frequencies. This kind of sensors has high sensitivity in the
half-space above the grating, close to the theoretical limit, together with a controllable - potentially very high - quality factor. It relies on a resonance with a "confined" mode of a sub-wavelength thick grating slab, a mode
that is largely expelled from the grating itself. The small thickness assumption allows us to derive analytical
expressions for many properties of these sensors, expressions that are then tested numerically using a rigorous
coupled-wave analysis (RCWA) method, and in preliminary experiments.
We propose a novel design for a guided-mode resonance (GMR) grating sensor that extends the sensitivity to a
large region of space, possibly several tens of microns away from the grating surface. This type of sensors has
high sensitivity in the half-space above the grating, close to the theoretical limit, together with a controllable -
potentially very high - quality factor. It relies on a resonance caused by a "confined" mode of a sub-wavelength
thick grating slab, a mode that is largely expelled from the grating itself into the grating environment. The small
thickness assumption allows us to derive a simple yet accurate analytical model for the sensor behavior, which
is tested numerically using a rigorous coupled-wave analysis (RCWA) method as well as in preliminary grating
transmission measurements.
Nanoimprinting lithography was initiated as an alternative way to achieve nanoscale structures with high throughput and low cost. We have developed a UV-nanoimprint process to fabricate 34x34 crossbar circuits with a half-pitch of 50 nm (equivalent to a bit density of 10 Gbit/cm2). Our resist was of a single layer, which required fewer processing steps than any bi-layer process, but yielded high quality results. By engineering the surface energy of the substrate, we also eliminated the problem of trapped air during contact with the mold due to non-conformal contact such that it spreads the resist and expels trapped air. Resist adhesion to the gaps between features in the mold during mold separation is a challenge that becomes more severe as the pitch size shrinks. We have improved the resist adhesion to the substrate by applying a monolayer of surface linker molecule on the substrate surface. The surface linker bonded the resist to the substrate surface chemically and produced fine imprinted patterns at 30 nm hp.
Highly sensitive, sequence-specific and label-free DNA sensors were demonstrated by monitoring the electronic conductance of silicon nanowires with chemically bonded single stranded (ss) DNA or peptide nucleic acid (PNA) probe molecules. For a 12-mer oligonucleotide, tens of pM of target ss-DNA in solution was recognized when the complementary DNA oligonucleotide probe was attached to the SiNW surfaces. In contrast, ss-DNA samples of 1000x concentration with single base mismatch only produce a weak signal due to nonspecific binding.
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