Transcript Research Progress - University of Guelph

Research Progress
of Prof. Dai
David Ji
Mar. 28 06
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Outline
► Part
one: preparation of nanomaterials
Synthesis of carbon nano tubes
Synthesis of germanium nanowires
► Part
two: application of SWNT on biosensors
► Proposed
research area
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Part one: Preparation of Nanomaterials
 General Synthesis Method we are using
CVD: thermal CVD, Plasma enhanced CVD
During synthesisThe catalyst particles serve as seeds to nucleate the growth
of nanotubes. Including Au, Fe particles etc.
 Research Aim
Site-selective CVD synthesis on catalytic patterned substrates
which means growing nanotube arrays at controllable locations and with
desired orientations on surfaces.
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Synthesis patterned SWNT with
different orientation by various
methods
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Individual Single Walled Carbon Nanotubes on Patterned Silicon Wafers
a. Electron beam lithography was used to fabricate square
holes in a polymethylmethacrylate (PMMA) film on
silicon.
b. Catalyst precursors dispersed on the substrate surface.
c. Lift-off of PMMA leads to the substrate containing
catalyst islands.
d. CVD of methane at 1,000 C produces SWNTs off the
islands
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Kong et al., Nature, 1998 395, 878
Self-Oriented Regular Arrays
Porous silicon with a thin nanoporous layer obtained by
electrochemical etching
Substrate patterned with Fe films (5 nm thick) by
electron beam evaporation through shadow masks
(10 to 250 m)
Ethylene flow CVD
Fan et al. Science 1999, 283, 512-514
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Free-Standing Single-Walled Carbon Nanotubes
a. PDMS (Poly(dimethylsiloxane) stamp spin was
coated with catalyst precursors.
b,c. The catalyst precursors were transferred to the
towers on the silicon substrate, and calcined
d. followed by CVD.
Si pattern: fabricated by photoresist patterning and
anisotropic etching.
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Cassel et al. J. Am. Chem. Soc. 1999, 121, 7975-7976
Synthesis of Vertically Aligned SWNT
Previously, Seldom success of PECVD synthesis of carbon nanostructures
•Water, alcohol assisted and PECVD growth methods, no vertically aligned SWNTs,
•Narrow parameter space and growth window for the synthesis!
Our work found:
Suitable amounts of oxygen to the various types of PECVD systems, and by using
dense and relatively uniform catalyst particles, vertically aligned SWNTs obtained at
high yields. Oxygen assisted PECVD could thus become a powerful and widely used
method for efficient production of vertically aligned SWNTs.
Theoretical stuff: Vertically aligned SWNT has very high pececnt of
C with sp2 hybridization. sp2 needs a C-rich and H-deficient
condition. This is why oxygen helps!
Zhang et al. PNAS vol. 102 16141
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Pay attention to electric property of
SWNT
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Semiconducting SWNT
SWNTs: either metallic or semiconducting depending on their chirality.
Semiconducting nanotubes can exhibit a large conductance change in
response to the electrostatic and chemical gating effects desired for fieldeffect transistors (FETs) important for biosensors.
Our Work on Semiconducting SWNT
In 2002, by employing uniformly distributed Fe2O3 particles as catalyst and using a mixed
methane and ethylene carbon source in CVD, SWNT with 70% semiconducting obtained
Kim et al. Nano Lett., Vol. 2, No. 7, 2002
In 2004, By plasma enhanced CVD method, the growth temperature for SWNT can be lowered
from ~800-900 C down to 600 C with 90% of the nanotubes are semiconductors.
Li et al. Nano Lett., Vol. 4, No. 2, 2004.
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About gemanium nanowires
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First Reported Low Temperature Synthesis of
Single-Crystal Germanium Nanowires by LPCVD
Ge: high carrier mobility and band gap ~ 0.6 eV.
In terms of growing GeNW by CVD method with GeH4 gas as feed stock
We need to think about melting point of alloy of catalyst
which is Au particles and Ge, because the GeNW will grow
following the mechanism of VLS (Vapour, liquid, solid)
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The binary phase diagram for bulk Ge ± Au
Our growth temperature (approximately 275 C) is lower than the eutectic
temperature for bulk materials by about 80 C because the special property
of nano material
Wang et al. Angew. Chem. Int. Ed. 2002 41, 4783
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After we could make GeNW, we further cared
where to grow them and cared more about
their orientation after growth.
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One-to-One Synthesis of GeNW by Individual Gold Nanoseed Patterning
One to one: on one Au dot, grows one Germanium Nanowire
Multiple nanowires grown from one
large particle of Au catalyst
By careful controlling of temperature (very important for size of
gold nanoparticles, feed-stock gas pressure (LPCVD)
Controlling of orientation of the one to one GeNWs
How to make NW parallel to each other
By water gas flow across the substrate surface,
the nanowires are reoriented towards the flow
direction and become quasi-aligned while
maintaining the same spacing between their
ends
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Wang et al. Angew. Chem. Int. Ed. 2005, 44, 2–5
GeNW is not stable in air because GeO2 can be
readily soluble in water.
how to address this problem?
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Oxidation Resistant Germanium
Nanowires: Bulk Synthesis,
Long Chain Alkanethiol Functionalization,
and
Langmuir-Blodgett Assembly
synthesis of bulk quantities of GeNWs that are
uniform in diameter
Sonication and removal of silica by HF
GeNW functionalization by various chain-length
alkanethiols which is essential for its stability in
air and LB assembly.
Wang et al. J. AM. CHEM. SOC. 9 VOL. 127, NO. 33, 2005 11873
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Langmuir-Blodgett Assembly
•The GeNWs functionalized by both
alkanethiols and alkyls form uniform and stable
suspensions in organic solvents such as
chlorobenzene and chloroform
•This is one of the key elements for successful
LB film assembly of GeNWs.
•Adding functionalized GeNW suspensions
dropwise to a subphase of ethanol/
water in an LB trough
Neat assembly can be obtained.
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Part Two: Application of SWNT on Biosensors
Motivation: explore the biological application prospects of solid state nanomaterials
SWNT
First thing we encountered, compatibility between SWNT and bio molecules
In 2001, SWNT was functionalized and attached to protein. in our work “Chen
et al. J. Am. Chem. Soc. 2001, 123, 3838-3839”
Later on we found as-grown SWNT can adsorb bio molecule itself via
hydrophobic interactions.
The interactions between them called nonspecific bondings (NSB)
Another thing comes, how to selectively adsorb biomolecules onto SWNT?
Streptavidin can be spontaneously adsorbed onto SWNT
So it provides an excellent system for investigating nanotube sidewall modification
for resisting NSB of proteins
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Selective? but how?
First thing, SWNT need to be coated with layers resisting protein, Secondly, the layers
need to connect to detector for specific protein.
Among protein resisting polymers, PEG is one of the most effective and widely used
polymers. PEG can be irreversibly adsorbed onto SWNT.
Not working
PEG can’t be directly put to cover every
part of SWNT, SWNT still can find
streptavidin
Working
No spontaneous adsorption
Triton can be adsorbed onto SWNTs as a wetting layer to significantly
enhance PEG adsorption on nanotubes
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Structure of A Key Part of Biosensor Based on SWNT
Biotin detector for streptavidin
Protein resisting layer
Wetting layer
Biotin-Streptavidin dissociation Constant~10-15
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Shim et al. Nano Lett., Vol. 2, No. 4, 2002
SWNT based Biosensors
After we can make SWNT bond to specific bio molecules, it’s ready to
make a device of biosensor
•A nanotubes bridging two metal electrode pads (Ti/Au 20/60 nm thick,
electrode spacing 0.5–1 mm) electrodes.
•The device can work underneath a solution.
•Sensing by monitoring electrical current
Conductance normally decrease after adsorption of proteins.
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Part 3. Proposed Research Area

1. Keep working on nanosized solid state materials. Explore
possible applications for them. Start a project of synthesis organic
nanomaterials

2. Find more applications of SWNT in the biological fields (near
future) and health field (long term work). A thorough understanding
about the mechanisms of delivery of drugs by SWNT into live organisms
needs to be established

3. Our concern will also go to solve the problems in the field of
energy storage based on applications of nano sized materials and/or
devices. Hydrogen storage at relatively higher temperature will be one
our goals.
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Thank you for your attention!
http://www.stanford.edu/dept/chemistry/faculty/dai/group/
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