Charge transfer between covalently grafted groups and single-walled carbon nanotubes evidenced by Raman and ellipsometric spectroscopies

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Charge transfer between covalently grafted groups and single-walled carbon nanotubes evidenced by Raman

and ellipsometric spectroscopies

M. Dossot, Naoual Allali, Victor Mamane, Yann Battie, Aotmane En Naciri, Laurent Broch, Alexander Soldatov

To cite this version:

M. Dossot, Naoual Allali, Victor Mamane, Yann Battie, Aotmane En Naciri, et al.. Charge trans- fer between covalently grafted groups and single-walled carbon nanotubes evidenced by Raman and ellipsometric spectroscopies. ChemOntubes, 2014, Riva del Garda, Italy. �hal-02900937�

Charge transfer between covalently grafted groups and single-walled

carbon nanotubes evidenced by Raman and ellipsometric spectroscopies.

Manuel DOSSOT ^a , Naoual ALLALI ^a,b , Victor MAMANE ^b , Yann BATTIE ^c , Aotmane EN NACIRI ^c , Laurent BROCH ^c and Alexander V. SOLDATOV ^d

a

LCPME UMR 7564 CNRS-Université de Lorraine, Villers-lès-Nancy, France /

^b

SRSMC UMR 7565 CNRS-Université de Lorraine, Vandoeuvre-lès-Nancy, France

c

LCP-A2MC, Université de Lorraine, Metz, France /

^d

LTU, Department of Engineering Sciences and Mathematics, Lulea, Sweden e-mail: [email protected]

Introduction :

Clean single-walled carbon nanotubes (SWCNTs) synthesized using the HiPCO process and purified (purchased from Nanointegris Inc.) were chemically treated by two different methods:

i) a microwave-assisted acidic oxidation process by a concentrated HNO

₃

solution (ox-CNTs), ii) a covalent grafting of a methoxyaryl groups by a thermal radical functionalization (MeOB- CNTs). Thin films of raw and chemically modified SWCNTs were made and analysed using ellipsometric spectroscopy from 0.6 to 4.96 eV using three incident angles (50°, 60° and 70°) and also in the infrared region from 0.07 eV to 0.6 eV with a unique incident angle (60°). The films were supposed to be thick and dense enough to model them as a homogeneous and isotropic semi-infinite medium. Under this hypothesis, we analytically calculated the complex dielectric function of the films and extracted the real (e

_r

) and imaginary (e

_i

) parts of this function. Raman spectroscopy was also used to asses the number of covalent defects introduced in the graphitic structure of SWCNTs.

Conclusion

Raman spectroscopy showed that ox-CNTs had almost the same D band intensity than raw CNTs, while MeOB-CNTs gave a quantitative increase of this band. The ellipsometric measurements showed very interesting complementary results. The dielectric function of the films was found independent on the incident angle, confirming a posteriori the hypothesis of a homogeneous and isotropic medium. Raw and ox-CNTs showed a negative e

_r

function in the infrared spectrum, which corresponds to a metallic behaviour due to holes as charge carriers. This metallic behaviour was lost with MeOB-CNTs. This can be explained by a partial electron transfer from grafted groups towards CNTs , leading to a decrease of the number of holes and an increase of the CNT Fermi level. The covalent grafting also decreased intensity of the p- plasmon band at 4.5 eV and the peaks due to van Hove singularities. These conclusions agree with and go beyond those obtained from Raman spectroscopy. Our results exemplify the fact that spectroscopic ellipsometry is a powerful method to obtain precious information on the electronic effects resulting from the covalent functionalization of SWCNTs.

0 1 2 3 4 5

20 40 60 80 100 120 140 160 180

HiPCo raw

(°)

Energy (eV)

50°

60°

70°

0 1 2 3 4 5

-5 0 5 10 15 20 25 30 35 40

(°)

Energy (eV)

50°

60°

70°

HiPCo raw

 

0 1 2 3 4 5

-8 -6 -4 -2 0 2 4 6

 r

Energy (eV)

50°

60°

70°

HiPCo raw

0 1 2 3 4 5

0 4 8 12 16

HiPCo raw

 i

Energy (eV)

50°

60°

70°



_r

= Re(  ) 

_i

= Im(  )

 





 





 

 











 -



_i ^j_j^_{ i}

e

e 





²

2

2

tan

tan 1

tan 1 1

sin

Spectroscopic Ellipsometry: some definitions

plane of incidence Ep

Es

Ep

Ep Es

Es

Reflected light Incident light

plane of incidence Ep

Es

Ep

Ep Es

Es

Reflected light Incident light

Fresnel coefficient r

_S

et r

_P



_i



_t

y and  are the "ellipsometric angles"

1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 0.2

0.4 0.6 0.8 1.0

125 150 175 200 225 250 275 300 0.0

0.1 0.2 0.3 0.4 0.5

G+-Nomalized intensity

Raman shift (cm^-1)

Raw HiPCO

HiPCO-MeOB AN213

G+-Nomalized intensity

Raman shift (cm^-1) M

SC

514nm

500 1000 1500 2000 2500 3000 0.0

0.2 0.4 0.6 0.8 1.0 1.2

150 200 250 300 350

0.0 0.1 0.2

G+-Normalized intensity

Raman shift (cm^-1)

raw HiPCO

ox. HNO3 AN276

G+-Normalized intensity

Raman shift (cm^-1)

633nm

SC

Raman spectroscopy

Dielectric functions of raw and modified SWCNTs (  _i = 60°)

0 1 2 3 4 5

0 2 4 6

Imaginary part of the dielctric function 

Energy (eV)

raw HiPCO

AN276 ox. HNO3 AN213 funct. MeOB

0 1 2 3 4 5

-6 -4 -2 0 2 4 6

raw HiPCO

AN276 ox. HNO3 AN213 funct. MeOB

Real part of the dielectric function 

Energy (eV)

- Super-purified SWCNTs synthesized by HiPCO process (SP-HiPCO from Nanointegris)

- AN276 : SP-HiPCO SP oxidized in HNO₃ 65% under

microwave irradiation (20 mn at 400 W), see scheme 1.

- AN213 : SP-HiPCO SP functionalized with methoxy- aryl groups through a radical process (scheme 2).

Scheme 1

Fe

O Cl

Fc-ETGn Toluene/Et₃N

100°C, 24h 70°C, 24h

SOCl₂ O

Conc. HNO₃

Micro-Wave 50°C, 20 min

CNT

O

OH O O

n

Data acquisition and analysis

Samples Film preparation for ellipsometric

measurements

Sonication probe

Sonication in water under ice cooling

Vacuum filtration on a

cellulose paper Homogeneous film ~300 nm thick

on cellulose paper

Scheme 2

VALIDATION OF THE MODEL

p-plasmons van Hove

singularities

SEMI-INFINITE, ISOTROPIC AND HOMOGENEOUS MEDIUM MODEL:

HRTEM micrographs of raw SP-HiPCO sample.



_i

: 50, 60, 70°

Energy of light: from 0.6 to 4.96 eV and also from 0.07 to 0.6 eV for 

_i

= 60°

) cos Re(

cos

cos cos

0

0 i s

s t

t i

i

t t

i i

i s r

s

r e

n n

n n

E

r E

^







 _



 -

 

 



 

) cos Re(

cos

cos cos

0

0 i p

p t

i i

t

t t

i i

i p r

p

r e

n n

n n

E

r E

^







 _



 -

 

 



 

 L’ellipsométrie est une technique optique basée sur la mesure du changement d’état de polarisation de la lumière (Ψ, Δ) suite à son intéraction avec l’échantillon.

Définition de l’Ellipsométrie

Sample fo

Ep

Es Ei

rp

Er rs

  -



 









^{i( ) i}

s p s

p incident

s refl

s

incident p

refl

p

e tan( ) e

r r r

r E

E

E

E

p s

IR data consistent with VIS-UV part

Analytical expression for the complex dielectric function :

Data consistent at three incident

angles.

Few defects introduced by HNO ₃ oxidation, many defects

introduced by radical functionalization with MeOB groups. Functionalization strongly decreases the intensity of van Hove singularities and p -plasmon band (but not our microwave-assisted oxidation process).

Detection of MeOB functions !

(confirmed by XPS analysis, data not shown)

 _r

 _i

S

₁₁

S

₂₂

M

₁₁

S

₃₃

C=C and C-H

vibrations of MeOB groups !

M

(SDS surfactant is used to disperse CNTs and washed after filtration)