Carbon nanotube-functionalized silicon surfaces with efficient redox communication

Article abstract of DOI:10.1039/b610559a

Sidewall functionalized multi-walled carbon nanotubes can be covalently bound parallel to a silicon surface via a self-assembled acid-terminated monolayer used as an organic molecular glue. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610559a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Carbon nanotube-functionalized silicon surfaces with efficient redox
communication{
Fanny Hauquier,a Giorgia Pastorin,b Philippe Hapiot,a Maurizio Prato,*c Alberto Bianco*b and
Bruno Fabre*a
Received (in Cambridge, UK) 24th July 2006, Accepted 29th August 2006
First published as an Advance Article on the web 18th September 2006
DOI: 10.1039/b610559a
Sidewall functionalized multi-walled carbon nanotubes can be
covalently bound parallel to a silicon surface via a self-
assembled acid-terminated monolayer used as an organic
molecular glue.
Owing to their unique electrical, mechanical and thermal proper-
ties, the integration of carbon nanotubes (CNTs) with metallic or
semiconducting substrates is a very promising approach for
developing new nanoscale devices for future applications in
electronics, optoelectronics and chemical/biological detection.1
However, the fabrication of CNT-based devices is severely
hindered by the lack of simple and reliable methods to deposit
this type of nanomaterial in a controlled fashion. Comparatively to
the covalent attachment of CNTs to gold surfaces,2 the anchoring
Fig. 1 Covalent assembly of MWNTs to Si(111) surfaces through a
of CNTs to silicon substrates has been surprisingly much less
surface amidation reaction (see ESI for experimental details{).
developed. Several published procedures consisted in the con-
trolled adsorption of CNTs on non-oxidized and oxidized silicon
surfaces3 but the absence of a covalent link between the CNTs and
the underlying substrate may render the architectures less stable
and less robust. A nice example demonstrating the covalent
attachment of CNTs to silicon has been recently reported by Tour
and co-workers using orthogonally functionalized oligo(phenylene
ethynylene) aryldiazonium salts as linker units.4
Second, activated esters react with amines under very mild
conditions to form the corresponding amides in high yields.
Thus, this surface chemistry is perfectly compatible with different
nanolithography patterning technologies. Also interestingly, the
surface coverage of CNTs can in principle be controlled by either
diluting the acid-terminated chains with chemically inert n-alkyl
chains of the starting monolayer or changing the amount of amino
groups attached to MWNTs. Consequently, the excess of amino
groups remained available on the MWNTs after the silicon
binding can be further functionalized with other specific groups
(e.g. enzymes or other biologically relevant molecules).
Herein, we propose an alternative and versatile method to build
CNT-functionalized silicon surfaces with striking electrochemical
characteristics which could be useful for electroanalytical applica-
tions. This approach is based on a surface amidation reaction
between soluble amino sidewall functionalized multi-walled carbon
nanotubes (MWNTs)5 and the N-hydroxysuccinimide headgroups
of an alkyl monolayer covalently bound to p-type Si(111) (Fig. 1).{
This methodology offers several advantages. First, the molecular
films produced from the reaction of hydrogen-terminated silicon
surfaces (Si–H) with v-substituted 1-alkenes, used in this work, are
usually well-ordered, densely packed and robust monolayers.6
The immobilization of MWNTs was verified using scanning
electron microscopy (SEM) (Fig. 2). The SEM images show the
aMatie`re Condense´e et Syste`mes Electroactifs, Sciences Chimiques de
Rennes, UMR CNRS 6226, Universite´ de Rennes 1, 35042, Rennes
Cedex, France. E-mail: fabre@univ-rennes1.fr; Fax: +33 223 236732;
Tel: +33 223 236550
bInstitut de Biologie Mole´culaire et Cellulaire, UPR CNRS 9021,
Immunologie et Chimie The´rapeutiques, 67084, Strasbourg, France.
E-mail: A.Bianco@ibmc.u-strasbg.fr; Fax: +33 388 610680;
Tel: +33 388 417088
cDipartimento di Scienze Farmaceutiche, Universita` di Trieste, Piazzale
Europa 1, 34127, Trieste, Italy. E-mail: prato@units.it
{ Electronic supplementary information (ESI) available: Experimental
details for the synthesis of amino-substituted MWNTs, the preparation of
the MWNT-modified Si(111) surfaces and fitting procedures of SECM
data. See DOI: 10.1039/b610559a
Fig. 2 SEM images of MWNTs bound to Si(111) surfaces.
This journal is ß The Royal Society of Chemistry 2006
4536 | Chem. Commun., 2006, 4536–4538

View Article Online
silicon surface highly covered by MWNTs with the tubes lying flat
on the surface rather than perpendicular, consistent with the
sidewall functionalization of the MWNTs by the amino groups.
Such an assertion was also supported by atomic force microscopy
(AFM) results. The SEM analysis of different representative areas
enabled the surface coverage of MWNTs to be roughly estimated
at 109–2 6 109 tubes cm22. The attached MWNTs were observed
predominantly as individual nanotubes of 20–60 nm in diameter. It
must be pointed out that no MWNT deposition was observed
using a silicon surface derivatized with the simple nonfunctiona-
lized n-decyl monolayer. Moreover, as a result of the covalent link,
the MWNTs were strongly adherent to the electrode surface as
they were not removed by sonication (e.g. 10 min in CH2Cl2).
The electrochemical properties of the MWNT assemblies have
been characterized by scanning electrochemical microscopy
(SECM).7 SECM is based on the electrochemical interactions of
a redox species produced at a probe ultramicroelectrode (UME)
tip and the substrate under investigation. In the simplest mode
used here (feedback mode), the substrate electrode is not
electrically connected. After diffusion of the electrogenerated
species to the sample, an electrochemical reaction is possible on a
localized spot on the surface where the initial form of the mediator
can be regenerated resulting in an enhancement of the current at
the probe electrode.
conditions, the p-type silicon surface in contact with the electrolytic
solution can be considered in carrier accumulation9 and thus
behaves as a quasi-metallic electrode.10
For L . 5, It is close to one for all the curves as no interaction
occurs between the electrogenerated ferrocenium species and the
substrate for this distance range. For L , 5, It is found to rapidly
diminish for the undecanoic acid monolayer. This negative
feedback confirms the blocking properties of this monolayer which
limits the regeneration of the mediator (the measured apparent
charge-transfer rate constant kapp is 8 6 1025 cm s21). In the case
of the MWNT-modified surface, It enhances dramatically for the
lowest values of L. This positive feedback is caused by the
reduction of the ferrocenium at the modified electrode with a
rate as high as that determined at a p-type Si(111)-H electrode, i.e.
kapp = 1.5 6 1022 cm s21 (see ESI, Fig. S2{).
In order to independently evaluate the redox contribution of the
attached MWNTs, SECM measurements have also been
performed with the cathodic mediator azobenzene. Indeed, the
cathodic reduction of azobenzene to its radical anion occurring at
a potential much lower than Efb (Eu9 = 21.3 V vs. SCE), the
regeneration of this mediator does not take place on p-type silicon
without illumination, in agreement with previous voltammetric
studies.11 As expected, the approach curves monitored for the
undecanoic acid monolayer and Si(111)–H fit the theoretical curve
for an insulating substrate (Fig. 3B). In contrast, the oxidized form
of the mediator can be regenerated at the MWNT-modified
surface with a rate constant kapp of 1.7 6 1023 cm s21 (see ESI,
Fig. S3{). In that case, the electrode behaves as an inert surface
partially covered by randomly distributed redox-active nanotubes.
So, it is expected that the effective rate constant is directly related
to the fraction of active surface.12 Indeed, we observe that the
regeneration rate is dependent on the MWNT surface coverage13
thus demonstrating that the regeneration of the mediator from the
only network of attached MWNTs can occur with a high rate.
From SECM results, three major pathways can be considered to
regenerate the initial form of the mediator: (a) direct hole tunneling
through the monolayer linker, (b) diffusion of the electroactive
species through the pinholes of the monolayer followed by the
injection of a hole into the valence band of silicon and (c) charge
transfer at the attached conducting MWNTs (Scheme 1).
A typical approach curve8 obtained at the MWNT-modified
surface is shown in Fig. 3A using ferrocene as the reduced form of
the redox mediator. For comparison, the curves obtained at
Si(111)–H and undecanoic acid monolayer-modified Si(111) are
also shown. It must be noticed that the formal potential of the
ferrocene/ferrocenium couple Eu9 = 0.45 V vs. SCE is more positive
than the flat-band potential Efb of the MWNT- and alkyl-modified
electrodes (namely 0.00 ¡ 0.02 V) derived from capacitance
measurements using the Mott–Schottky relation. Under these
Fig. 3 SECM approach curves in the dark obtained at the (%) MWNT-,
(e) undecanoic acid monolayer-modified Si(111) and ($) Si(111)–H in
DMF + 0.1 M Bu4NClO4 containing 1022 M (A) ferrocene or (B)
azobenzene as mediator. The solid and dashed lines are theoretical curves
for totally insulating and conducting substrates respectively. a = 10 mm.
Scheme 1 SECM in feedback mode for studying charge-transfer kinetics
at a non-connected MWNT-modified p-type Si(111) surface.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 4536–4538 | 4537

View Article Online
3.5 h with undecylenic acid to provide a COOH-terminated alkyl
monolayer. Then, the terminal COOH groups were activated with NHS
by immersing the modified silicon surface for 3 h in a freshly prepared
mixture of a deaerated DMF solution containing EDC at 0.2 M and NHS
at 0.1 M. The covalent attachment of MWNTs on Si(111) was performed
by immersing overnight the NHS-activated silicon surface in a DMF
solution containing ca. 1 mg mL21 of ammonium-substituted MWNTs
previously neutralized with diisopropylethylamine (amount of amino
groups: 0.50 mmol per g of MWNTs).
1 M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science of
fullerenes and carbon nanotubes, Academic Press, New York, 1996;
R. Saito, M. S. Dresselhaus and G. Dresselhaus, Physical properties of
carbon nanotubes, Imperial College Press, London, 1998; P. M. Ajayan,
Chem. Rev., 1999, 99, 1787.
2 L. Sheeney-Haj-Ichia, B. Basnar and I. Willner, Angew. Chem., Int. Ed.,
2005, 44, 78; J. J. Gooding, R. Wibowo, J. Liu, W. Yang, D. Losic,
S. Orbons, F. J. Mearns, J. G. Shapter and D. B. Hibbert, J. Am. Chem.
Soc., 2003, 125, 9006; P. Diao and Z. Liu, J. Phys. Chem. B, 2005, 109,
20906.
3 K. H. Choi, J. P. Bourgoin, S. Auvray, D. Esteve, G. S. Duesberg,
S. Roth and M. Burghard, Surf. Sci., 2000, 462, 195; T. Hertel,
R. Martel and P. Avouris, J. Phys. Chem. B, 1998, 102, 910;
M. Burghard, G. Duesberg, G. Philipp, J. Muster and S. Roth,
Adv. Mater., 1998, 10, 584; J. Liu, M. J. Casavant, M. Cox,
D. A. Walters, P. Boul, W. Lu, A. J. Rimberg, K. A. Smith,
D. T. Colbert and R. E. Smalley, Chem. Phys. Lett., 1999, 303, 125;
J. Im, J. Kang, M. Lee, B. Kim and S. Hong, J. Phys. Chem. B, 2006,
110, 12839.
Fig. 4 J–V characteristics of the (+)Hg/MWNT/monolayer/p-Si(111)(2)
junction. The rectification ratio is ca. 5 6 104 at ¡2 V.
Although the occurrence of a charge transfer through the
pinhole defects of the monolayer bridge cannot be ruled out, its
contribution in the overall charge-transfer process is expected to be
small because both long-chain alkyl14 and undecanoic acid
monolayers showed good blocking properties in the presence of
the ferrocene mediator. Moreover, the possibility of a direct
electrical connection between MWNT and the silicon surface so
creating a ‘‘short-circuit’’ seems unlikely as a major pathway in the
mechanism of charge transport. To test the occurrence of such a
‘‘short-circuit’’, a mercury contact was formed to the MWNT-
modified silicon surface. A typical current density–applied bias
voltage (J–V) curve of the Hg/MWNT/monolayer/silicon junction
is shown in Fig. 4. A Schottky diode rectifying behaviour is clearly
observed with a transport mechanism governed by a thermionic
emission process as is generally the case for metal/insulator/
semiconductor (MIS) diodes.15 Furthermore, the shape and the
magnitude of the J–V curve are found to be similar to those
obtained for other alkyl monolayer-based silicon devices.16
In conclusion, the functionalized MWNT assemblies prepared
in this study allow an efficient electrical communication between
the semiconductor surface and a redox probe in solution. Further
SECM investigations with a polarized electrode configuration are
currently in progress in order to elucidate the mechanism involved
in the redox communication. We believe that such modified
surfaces can be useful for electrically transducing an biomolecular
recognition event
4 A. K. Flatt, B. Chen and J. M. Tour, J. Am. Chem. Soc., 2005, 127,
8918; J. He, B. Chen, A. K. Flatt, J. J. Stephenson, C. D. Coyle and
J. M. Tour, Nat. Mater., 2006, 5, 63.
5 D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006,
106, 1105; A. Bianco and M. Prato, Adv. Mater., 2003, 15, 1765.
6 J. M. Buriak, Chem. Rev., 2002, 102, 1271; D. D. M. Wayner and
R. A. Wolkow, J. Chem. Soc., Perkin Trans. 2, 2002, 23.
7 A. J. Bard and M. V. Mirkin, Scanning electrochemical microscopy,
Marcel Dekker, New York, 2001; A. J. Bard, F.-R. F. Fan, D. T. Pierce,
P. R. Unwin, D. O. Wipf and F. Zhou, Science, 1991, 254, 68.
8 Plot of the normalized current It = it/iinf vs. the normalized distance
L = d/a where it is the current at the tip electrode localized at a
distance d from the substrate, iinf is the steady-state current when the tip
is at an infinite distance from the substrate and a is the radius of the
UME.
9 X. G. Zhang, Electrochemistry of silicon and its oxide, Kluwer
Academic, New York, 2001.
10 The cyclic voltammogram of ferrocene at a p-type Si(111)–H electrode
gives a fully reversible system. B. Fabre and F. Hauquier, J. Phys. Chem.
B, 2006, 110, 6848.
11 A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. S. Lewis and
M. S. Wrighton, J. Am. Chem. Soc., 1980, 102, 3683.
12 C. Amatore, J. M. Save´ant and D. Tessier, J. Electroanal. Chem., 1983,
146, 37.
13 F. Hauquier, G. Pastorin, P. Hapiot, M. Prato, A. Bianco and B. Fabre,
unpublished work.
14 J. Ghilane, F. Hauquier, B. Fabre and P. Hapiot, Anal. Chem., 2006, 78,
6019.
15 S. M. Sze, Semiconductor Devices: Physics and Technology, Wiley, New
York, 1985.
This work has been supported by CNRS, University of Trieste
and MIUR (PRIN 2004, prot. 2004035502). F. H. and G. P. are
recipients of a fellowship from MNRT. Dr. J. Ghilane (Universite´
de Rennes 1) is fully acknowledged for his help in SECM
experiments.
16 Y.-J. Liu and H.-Z. Yu, J. Phys. Chem. B, 2003, 107, 7803; E. J.
Faber, L. C. P. M. de Smet, W. Olthuis, H. Zuilhof, E. J. R.
Sudho¨lter, P. Bergveld and A. van den Berg, ChemPhysChem, 2005, 6,
2153.
Notes and references
{ Firstly, p-type Si(111)–H, prepared by etching a clean silicon shard in ppb
grade NH4F 40% for 15 min, was reacted photochemically at 300 nm for
4538 | Chem. Commun., 2006, 4536–4538
This journal is ß The Royal Society of Chemistry 2006