Research Progress of Prof. Dai

1. Research Progressof Prof. Dai David Ji Mar. 28 06

2. 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

3. 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.

4. Synthesis patterned SWNT with different orientation by various methods

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

6. 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

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

8. 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, • Narrowparameter 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

9. Pay attention to electric property of SWNT

10. 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. 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 with90% of the nanotubes are semiconductors. Li et al.Nano Lett., Vol. 4, No. 2, 2004.

11. About gemanium nanowires

12. 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)

13. 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

14. After we could make GeNW, we further cared where to grow them and cared more about their orientation after growth.

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

16. GeNW is not stable in air because GeO2 can be readily soluble in water. how to address this problem?

17. 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

18. 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.

19. 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

20. 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 Working PEG 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

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

22. 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.

23. 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.

24. Thank you for your attention! http://www.stanford.edu/dept/chemistry/faculty/dai/group/