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Research ProgressResearch Progressof Prof. Daiof Prof. Dai

David JiDavid JiMar. 28 06 Mar. 28 06

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OutlineOutline

► Part one: preparation of nanomaterials Part one: preparation of nanomaterials Synthesis of carbon nano tubes Synthesis of carbon nano tubes

Synthesis of germanium nanowiresSynthesis of germanium nanowires

► Part two: application of SWNT on biosensorsPart two: application of SWNT on biosensors

► Proposed research areaProposed research area

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During synthesisThe catalyst particles serve as seeds to nucleate the growth of nanotubes. Including Au, Fe particles etc.

CVD: thermal CVD, Plasma enhanced CVD

Site-selective CVD synthesis on catalytic patterned substrates

which means growing nanotube arrays at controllable locations and with desired orientations on surfaces.

General Synthesis Method we are using

Research Aim

Part one: Preparation of Nanomaterials Part one: Preparation of Nanomaterials

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Synthesis patterned SWNT with Synthesis patterned SWNT with different orientation by various different orientation by various methodsmethods

5Kong et al., Nature, 1998 395, 878

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

Individual Single Walled Carbon Nanotubes on Patterned Silicon Wafers

6Fan et al. Science 1999, 283, 512-514

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

Self-Oriented Regular Arrays

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Cassel et al. J. Am. Chem. Soc. 1999, 121, 7975-7976

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 calcinedd. followed by CVD.

Si pattern: fabricated by photoresist patterning and anisotropic etching.

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Synthesis of Vertically Aligned SWNTSynthesis of Vertically Aligned SWNT

Our work found:Our work found:

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!

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!

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.

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 field-effect transistors (FETs) important for biosensors.

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.

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 obtainedKim et al. Nano Lett., Vol. 2, No. 7, 2002

Our Work on Semiconducting SWNT Our Work on Semiconducting SWNT

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About gemanium nanowiresAbout 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.

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)

In terms of growing GeNW by CVD method with GeH4 gas as feed stock

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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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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 GeNWsControlling of orientation of the one to one GeNWs

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

One-to-One Synthesis of GeNW by Individual Gold Nanoseed Patterning

Multiple nanowires grown from one large particle of Au catalyst

Wang et al. Angew. Chem. Int. Ed. 2005, 44, 2–5

One to one: on one Au dot, grows one Germanium Nanowire

How to make NW parallel to each other

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GeNW is not stable in air because GeO2 can be readily soluble in water.

how to address this problem?

17Wang et al. J. AM. CHEM. SOC. 9 VOL. 127, NO. 33, 2005 11873

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.

Oxidation Resistant Germanium Nanowires: Bulk Synthesis,

Long Chain Alkanethiol Functionalization, and

Langmuir-Blodgett Assembly

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•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.

Langmuir-Blodgett Assembly

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Part Two: Application of SWNT on BiosensorsPart Two: Application of SWNT on Biosensors

Later on we found as-grown SWNT can adsorb bio molecule itself via hydrophobic interactions.The interactions between them called nonspecific bondings (NSB)

So it provides an excellent system for investigating nanotube sidewall modification for resisting NSB of proteins

Motivation: explore the biological application prospects of solid state nanomaterialsSWNT

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”

Another thing comes, how to selectively adsorb biomolecules onto SWNT?

Streptavidin can be spontaneously adsorbed onto SWNT

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Selective? but how?

Among protein resisting polymers, PEG is one of the most effective and widely used polymers. PEG can be irreversibly adsorbed onto SWNT.

First thing, SWNT need to be coated with layers resisting protein, Secondly, the layers need to connect to detector for specific protein.

Not workingWorkingPEG can’t be directly put to cover

every part of SWNT, SWNT still can find streptavidin

No spontaneous adsorption

Triton can be adsorbed onto SWNTs as a wetting layer to significantly enhance PEG adsorption on nanotubes

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Shim et al. Nano Lett., Vol. 2, No. 4, 2002

Biotin detector for streptavidin

Structure of A Key Part of Biosensor Based on SWNT

Wetting layer

Protein resisting layer

Biotin-Streptavidin dissociation Constant~10-15

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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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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.

Part 3. Proposed Research Area

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Thank you for your attention!

http://www.stanford.edu/dept/chemistry/faculty/dai/group/