We report formation of an optical cavity and observation of Fabry-Perot resonance in GaAs nanowires and nanosheets
grown by metal organic chemical vapor deposition (MOCVD) with selective area growth (SAG). These nanostructures
are grown along the (111)B direction. The formation of an optical cavity in the nanowires and nanosheets are
fundamentally different from each other. In nanowires the optical cavity is formed along the length of the nanowire with
ends of the nanowire behaving as two parallel mirrors. In nanosheets, however, the three non-parallel edges of the GaAs
nanosheets are involved in trapping of the light through total internal reflection, thus forming a 2D cavity. We show that
through surface passivation and local field enhancement, both the photoluminescence intensity and hence Fabry-Perot
peak intensity increases significantly. Transferring the GaAs nanowires and nanosheets to the gold substrate (instead of
Si/SiO2 substrate) leads to substantial enhancement in the photoluminescence intensity by 5X (for nanowires) and 3.7X
(for nanosheets) to infinite enhancement of the FP peaks intensities. In order to reduce the non-radiative recombination
in these nanowires the surface states in the nanowires can be passivated by either an ionic liquid (EMIM-TFSI) or an
AlGaAs surface layer. Both passivations methods lead to an enhancement of the optical response by up to 12X.
Recently nanostructure materials have emerged as a building block for constructing next generation of photovoltaic
devices. Nanowire based semiconductor solar cells, among other candidates, have shown potential to produce high
efficiency. In a radial pn junction light absorption and carrier collection can be decoupled. Also nanowires can increase
choice of materials one can use to fabricate high efficiency tandem solar cells by relaxing the lattice-match constraint.
Here we report synthesis of vertical III-V semiconducting nanowire arrays using Selective-Area Metal Organic
Chemical Vapor Deposition (SA-MOCVD) technique, which can find application in various optoelectronic devices. We
also demonstrate nanosphere lithography (NSL) patterning techniques to obtain ordered pattern for SAMOCVD.
Reflection spectrum of nanowires array made by this technique shows excellent light absorption performance
without additional anti-reflection coating layer. Thus, we show that highly ordered nanowire structure is 'not needed' to
maximize the absorption in vertical nanowire array. Our scalable approach for synthesis of vertical semiconducting
nanowire can have application in high throughput and low cost optoelectronic devices including photovoltaic devices.
We present a novel nanotube-on-insulator (NOI) approach to produce high-yield nanotube devices based on aligned
single-walled carbon nanotubes. First, we managed to grow aligned nanotube arrays with controlled density on
crystalline, insulating sapphire substrates, which bear analogy to industry-adopted silicon-on-insulator substrates. Based
on the nanotube arrays, we demonstrated registration-free fabrication of both top-gated and polymer-electrolyte-gated
field-effect transistors with minimized parasitic capacitance. In addition, we have successfully developed a way to
transfer these aligned nanotube arrays to flexible substrates. Our approach has great potential for high-density, largescale
integrated systems based on carbon nanotubes for both micro- and flexible electronics.
Semiconducting metal oxide nanowires represent a class of novel materials that are of superior
properties to naoparticles currently used in dye sensitized solar cell and polymer hybrid solar cells.
The quasi one-dimensional nanostructure and surface states of nanowires improve carrier mobility
and charge transfer through interface interactions of theses nanocomposite materials. Raman
spectroscopy, especially resonant Raman spectroscopy, is used to correlate nanomaterial synthesis
condition to the structural, optical and electric transport properties that are important to
photocatalysis, exciton transport and recombination and hydrogen storage mechanism. For example,
highly orientated ZnO nanowires studied with Raman and photoluminescence spectroscopy
demonstrated the high efficiency of the phonon and electron coupling. These results are compared
with that of other ZnO forms such as thin film, polycrystalline powder and solid. The Raman
bandwidths and shifts of nanowires revealed the phonon confinement in the quasi one-dimensional
nanostructures, which is further demonstrated with In2O3 nanowires at 5, 10, 20, 30 nm in diameters.
Room temperature photoluminescence results also show band gap shifts with nanowire dimensions.
Nanowire sizes, defects and strains, controlled by synthesis conditions, have shown to determine
band structure and optical phonon properties. We also discuss characterization and synthesis of
carbon nanotube based composite materials including polymer electropolymerization and
infiltration. Combining significantly enhanced mechanical compressive strength and excellent
electric conductivity, these composite materials offer potentials to fuel cell anode materials as
multifunctional hydrogen storage media.
We report complementary detection of prostate-specific antigen (PSA) using n-type In2O3 nanowires and p-type carbon nanotubes. Our innovation involves developing an approach to covalently attach antibodies to In2O3 NW surfaces via the onsite surface synthesis of phosphoric acid-succinylimide ester. Electronic measurements under dry conditions revealed complementary response for In2O3 NW and SWNT devices after the binding of PSA. Real time detection in solution has also been demonstrated for PSA down to 5 ng/mL, a benchmark concentration significant for clinical diagnosis of prostate cancer, which is the most frequently diagnosed cancer.
We demonstrate detection of NO2 down to ppb levels using transistors based on both single and multiple In2O3 nanowires operating at room temperature. This represents orders-of-magnitude improvement over previously reported metal oxide film or nanowire/nanobelt sensors. A comparison between the single and multiple nanowire sensors reveals that the latter have numerous advantages in terms of great reliability, high sensitivity and simplicity in fabrication. Furthermore, selective detection of NO2 can be readily achieved with multiple-nanowire sensors even with other common chemicals such as NH3, O2, CO and H2 around.
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