Raman spectra of functionalized carbon nanotubes

G. Klupp, F. Borondics, R. Hackl*, K. Kamarás, E. Jakab**, S. Pekker Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest, Hungary,

e-mail: [email protected]

*Walther Meissner Institute, Bavarian Academy of Sciences and Humanities, Garching, Germany **Institute of Materials and Environmental Chemistry, Budapest, Hungary

The samples

Tubes@Rice: Pulsed laser vaporization  Refluxing with HNO Heating to 800 ºC  3  SWCNT + Ni/Co catalyst  SWCNT-COOH SWCNT  Functionalization by modified Birch reduction [1]: Li + n NH 3 e C C C C (NH 3 ) n  + C + BzBr  + BuI  + MeI  + HX  Li +  C + e BuC + I HC + X (NH + nNH BzC + Br MeC + I 3 3 ) n (HX = H 2 O, NH 3 , CH 3 OH) The degree of functionalization (R+H)/100C was determined from TG-MS.

Funding OTKA T 049338 Alexander von Humboldt Foundation

Resonant Raman scattering

S 33 +S 44

468 nm 531 nm

S 3 3

676 nm

M 11

Depth sampling

1400 1500 1300 140 Bz-, H-SWNT 531 nm laser 1600 1700 1300 1400 1500 120 Bu-, H-SWNT 531 nm laser 80 1600 1700 70 40 0 1300 1400 1500 1600 Raman shift (cm -1 ) 1700 1300 1400 1500 1600 Raman shift (cm -1 ) The samples are inhomogeneous  average spectra selected for comparison with TG-MS 1700 [2]

A complete spectrum

300 600 900 1200 60

Bu-,H-SWNT 531 nm laser Lorentzian

1500 40

Gaussian

3000 20 0 300 600 900 1200 1500

Raman shift (cm

-1

)

3000 No functional groups are visible and nanotubes are still in resonance.

 Electronic structure is not collapsed due to functionalization.

The same degree of functionalization leads to smaller changes in the electronic structure in the case of apolar alkyl groups than in the case of polar substituted phenyl groups [3].

Selectivity on tube type

0.05

0.04

0.03

0.02

0.01

0.00

0

Bz

2

Bu

4 6 (R+H)/100C

Me

8 468 nm 531 nm 676 nm 0.15

0.10

0.05

0.00

-0.05

-0.10

0

Bz

2

Bu

4 6 (R+H)/100C 8

Me

468 nm 676 nm 531 nm

I D /I G and I D /I D* increase with the degree of functionalization, as the change of the electronic structure is only minor. The ratio depends on the wavelength of the exciting laser, as in Ref. 3. If we substract the value measured in the pristine sample (arising from the defects of the pristine nanotube) the change is similar for both metallic and semiconducting nanotubes.



The reaction is not selective for tube type

Selectivity on tube diameter

150 200 250 80 60 40

SWNT Me-,H-SWNT 468 nm

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0 150 200

Raman shift (cm

-1

)

250 2 4 6

(R+H)/100C

8 According to the RBM spectrum the

small diameter semiconducting nanotubes react more readily.

This is in accordance with NIR[4, 5] and Raman[6] spectroscopic measurements on alkylated HiPCO tubes. In the case of 531 nm and 676 nm laser excitation the change was obscured by the error.

Explanation of the selectivity

Most of the functionalization reactions are primarily selective to metallic tubes [7], as these tubes have the nonzero DOS at the Fermi level [8].

Birch-type alkylation begins with doping by excess Li, which fills both S 11 , S 22  and M 11 [9].

The selectivity for metallic tubes is masked

The charged nanotubes are dispersed in the liquid NH 3  solution.

The size of the cavity in the bundle does not play a role.

Carbanions having greater s-character are more stable.



Smaller diameter tubes are more reactive

References

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7

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103

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111,

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245

, 1954 (2008) [5]: K. Németh, F. Borondics, E. Jakab, Á. Pekker, K. Kamarás, S. Pekker: Poster #5 on SIWAN 2008 [6]: M. Müller, J. Maultzsch, D. Wunderlich, A. Hirsch, C. Thomsen: Phys. Stat. Sol. B

244

, 4056 (2007) [7]: K. Kamarás, Á. Pekker: Handbook of Nanoscience and Technology, Editors: A. V. Narlikar, Y. Y. Fu, Oxford University Press, 2009 [8]: M. S. Strano: J. Am. Chem. Soc.

125

, 16148 (2003) [9]: S. Kazaoui, N. Minami, R. Jacquemin, H. Kataura, Y. Achiba: Phys. Rev. B

69,

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