Exploration of SWNT Cloning Continues

Amplification of Single-Walled Carbon Nanotubes from Designed Seeds: Separation of Nucleation and Growth

J. Phys. Chem. C, 2007, ASAP Web Release Date: 26-Jun-2007 (as part of upcoming R.Smalley Special Issue).

Scientists at Rice and AFRL continued their exploration on Smalley's ingenius idea of SWNT cloning. The idea was such that known chirality or type of SWNTs (sorted somehow) are cut into small pieces that are then attached with catalyst particles for further SWNT growth. This growth is supposed to follow the same chirality pattern of the original nanotube seed.

The scientists used FeMoC catalysts to couple to the pyridine-functionalized, shortened SWNT. After further growth, the results seem to suggest that around 3 times of length increase was achieved for short nanotubes (< 200 nm) with further growth (and perhaps the lack of growth of longer nanotubes also) limited by SWNT-SOG substrate interactions.

As a short paper, it lacks many details, although it might be deliberate. Some questions are raised immediately:

(1) "Care was taken to ensure that any unreacted FeMoC is removed prior to characterization or growth". HOW??

(2) According to the AFM quality of Figure 1, it seems difficult to have a rather reliable nanotube length counting for "as-synthesized SWNT-cats" and thus the accuracy of the main claim.

(3) Where are the radial breathing modes??

SiNW Needling Through Cells

"Interfacing Silicon Nanowires with Mammalian Cells"
Woong Kim, Jennifer K. Ng, Miki E. Kunitake, Bruce R. Conklin,* and Peidong Yang*

J. Am. Chem. Soc. 2007, ASAP Published on 05/22/07.

A short report from Berkeley scientists showed that SiNWs vertically aligned on substrates - like a panel of needles - can penetrate through mammalian cells, such as mouse embryonic stem (mES) cells and human embryonic kidney (HEK 293T) cells.

The cells were directly cultured on the Si needle panel (CVD on Si-(111) substrates) and found to be able to survive for a certain period of time, depending upon the diameter of the SiNWs. The larger diameter ones (~400 nm) seemed to be more lethal upon penetration (cells died in a day), while the smaller ones (d~30 nm) were much more benign (5-day survival).

The substrate was demonstrated as a substitute (of gelatin-coated tissue culture grade plastic plates) for maintenance of differentiated stem cells. The embroyid bodies grow and beat on the needle panel for more than a month.

When the SiNW panel was coated with plasmid DNA (requiring PEI-coating first to introduce a counter-ion layer), very limited amount (less than one percent) of cells were found to express the gene introduced.

The work is quite preliminary in comparison to the much "maturer" carbon nanotube field, but is certainly a first wave to interface more varieties of inorganic wires with cells to take advantage of some specific characteristics of these NWs. It is anticipated water-soluble NWs would be available and used for much more diverse and in-depth studies.

Gemanium Nanowire Growth with Au Catalyst at Either Solid Or Liquid State

"Germanium Nanowire Growth Below the Eutectic Temperature"
S. Kodambaka,* J. Tersoff, M. C. Reuter, F. M. Ross
Science, 2007, 316, 729-733.

"Perspectives: How Nanowires Grow"
Volker Schmidt and Ulrich Gösele
Sceince, 2007, 316, 698-699.

Nano-Testtube Does Some Tricks

"Tuning of Redox Properties of Iron and Iron Oxides via Encapsulation within Carbon Nanotubes"

Wei Chen, Xiulian Pan, and Xinhe Bao*

J. Am. Chem. Soc. 2007, ASAP published on 05/18/2007.

10.1021/ja0713072

Comment:

It would be more perfect if the i.d. dependence of oxidation could be shown. But a beautifully done piece of work already from Xinhe Bao at Dalian Inst. of Chemical Physics (China)!

Synthesis:

MWNTs of various i.d. (2-5, 4-8, 8-12 nm, from Chengdu Organic Chemicals. Tag 1: It sounds and looks nice. But how accurate was the i.d. control?) were refluxed with conc. HNO3 for 14 h, heat at 60C for 12 h to "open the cap". The solid was thrown into a Fe(NO3)3 pot, sonicated and stirred for 2 h. Solvent was evaporated at ambient conditions, and the mixture was heated to 140C in air for 8h, then heated to 350C in He at 2C/min and held for 3 h. This gives Fe2O3-encap.-MWNT. (80% encap. yield from TEM)

Characterization:

TEM:

• Non-encapsulated Fe/MWNT sample: 5-8 nm
• Encapsulated Fe/MWNT samples: sizes of encap. Fe NPs decrease with tube inner diameter, while those of non-encap. Fe NPs do not change.

Raman:

• Fe-O Eg vibrations (~280 cm-1, ~385 cm-1) shifts to higher frequency when the encap. Fe NPs become smaller; the positions were in the lower end when Fe NPs were not encap.
• Normally, the dependence trend for vibration frequency-Fe NP diameter is toward the other direction, which means the observed shifts could be attributed to the interactions from CNT host.

For the reaction that Fe2O3 reduced by nanotube carbon ("autoreduction"):

• TPR shows the CO evoluted (Fe2O3 reduced) at lower temp. if the nanotube inner diameter was smaller (~600C for 4-nm i.d. MWNTs).
• in situ XRD shows beautifully the conversion from Fe2O3 (2theta = 34.5/42.6) to Fe (2theta = 44.1) (also tracked by Raman peaks).

Proposed Mechanism:

Inner nanotube surface is more electron deficient.

How about oxidation?

• Occurs at a much lower temp (1% O2) than reduction
• Encap. Fe are more stable (fully oxidized at ~400C vs ~290C for extern. Fe), as expected. But is it due to difficulty in diffusion?
• Using an "artificial" channel with similar tubular characteristics ("SBA", a mesoporous silica sample), it was shown that geometrical constriction (which may slow down oxygen diffusion) did not slow down the Fe oxidation (Tag 2: Interesting control. But perhaps need to be more cautious with the generalized conclusion). Thus the interactions of Fe NPs with CNT must have slowed things down. Again, the electron deficiency of CNT inner surface seems to explain it.
• It is interesting that evolution of CO2 (CNT oxidation) is accompanied along with the Fe oxidation, suggesting the Fe2O3 catalyzing the CNT oxidation.

Hydrogen Plasma Does the Conversion

"Transition of Single-Walled Carbon Nanotubes from Metallic to Semiconducting in Field-Effect Transistors by Hydrogen Plasma Treatment"
Gang Zheng, Qunqing Li, Kaili Jiang, Xiaobo Zhang, Jia Chen, Zheng Ren, and Shoushan Fan
Nano Lett. 2007, ASAP published on 05/18/2007.
10.1021/nl070585w

Qunqing Li and coworkers from Tsinghua University in China reported that hydrogen plasma may be used to offer semiconducting properties to the metallic SWNTs in their devices for FET by opening band-gap in the DOS.

The idea seems to be not really novel, since the sidewall chemical functionalization (here by hydrogenation) is known to be able to do such work.

No metallicity selectivity (semiconducting vs metallic) was reported.

Purified SWNTs Become Diamagnetic

"High-Purity Diamagnetic Single-Walled Carbon Nanotube Buckypaper"
Kim, Y.; Torrens, O. N.; Kikkawa, J. M.; Abou-Hamad, E.; Goze-Bac, C.; Luzzi, D. E. Chem. Mater. ASAP published on 05/16/2007.
DOI: 10.1021/cm063006h

Comment:
The magnetic impurities to SWNTs have been like the shadow of things. These leftover from catalysts, such as Fe (for HiPco SWNTs) or Co/Ni (for arc SWNTs) compounds, are encapsulated in stiff, multilayer carbon shells. They are the really bad guys: silencing ESR signals, broadening NMR peaks, misleading phonon/electron mobilities, etc.

Getting rid of them is not nearly as trivial. Chemical methods should be carefully experimented since distroying the carbon layers - to access and dissolve the metals - is shooting the nanotubes in the foot as well. Physical methods are apparently more plausible. In the literature, Kitaygorodskiy et al. (JACS, 2005, 127, 7517) used magnetic separator - often used in biological separation - to separte the magnetic species from a soluble SWNT sample.

Procedure:
Purified SWNTs (PII-SWNTs from Carbon Solutions, Inc.) was heated to 560C in air for 10-30 min (40-60% yield), followed by sonication in HCl (37%) at 60C for 40min and filtering and brief drying. The solids (~200 mg) were suspended in DMF, and filtered through the 1.1T magnetic gradient apparatus containing a "column" of iron granules (for 3 times with fresh iron granules). The suspensions were then filtered through nylon filter and dried (vacuum annealed at 650C for 30min) to obtain the purified buckypaper (yield 10-20%). Speed: 5 mg/h.

Properties:
Magnetically purer:

• Using Quantum Design phyiscal property measurement system (PPMS)
• Magnetic moment reduced from 1.04 to 0.012 emu/g.

• Beautiful 13C-NMR Spectrum: FWHM only 25 ppm (maximum: 116ppm, double T1)
• Estimated SWNT susceptibility: -5x10(-6) emu/g (theoretical -0.64 x 10(-6) emu/g) (Tag: better or worse?)

Generally purer:

• UV/vis/NIR: From absorption peaks (The Haddon Method)
• XRD: SWNT rope peak @5.6 vs MWNT peak @25.6
• C60-Filling (Signature of Luzzi Group): 95% filled
• Ni:Y ratio change: from 4:1 (catalyst ratio) to 20:1 (obtained) to 2:1 (purified).

Mono-dispersed FeCo Alloy NPs Made in Solution

"Synthesis and Stabilization of FeCo Nanoparticles"
Chaubey, G. S.; Barcena, C.; Poudyal, N.; Rong, C.; Gao, J.; Sun, S.; Liu, J. P. J. Am. Chem. Soc. ASAP published on 05/12/2007.
doi: 10.1021/ja0708969

Synthesis:
"Reductive decomposition of Fe(III) acetylacetonate (Fe(acac)3) and Co(II) acetylacetonate(Co(acac)2) in a mixture of surfactants and 1,2-hexadecanediol (HDD) under a gas mixture of 93% Ar + 7% H2 at 300 °C." H2 is believed to be crucial because of the NPs' prone to oxidation.

Properties:
Marginal size control achieved by varying ligands:

• 20 nm using oleic acid/oleyl amine (the latter alone does not work well)
• 10 nm using oleic acid/TOP

Surfactants contribute to more than 20% of the particle weight

Size-dependent magnetic properties:

• Magnetization (Ms): 207 emu/g for 20 nm; 129 emu/g for 10 nm.
• Similar to bulk Fe/Co alloy, maximum magnetization occured at Fe:Co = 1.5.

Overcoming Instability:

• Oxidation in air: 48 h resulting in more than 20 % decrease (20 nm slightly more)
• Annealing (500C, 93%Ar+7%H2, 30min) to stablize particles by forming a carbon shell: Ms improve to 230 emu/g after annealing (compare to 245 emu/g for bulk metal).