Fabrication of carbon nanotube-molecule-silicon junctions

Article abstract of DOI:10.1021/ja051269r

A hybridization strategy for the covalent connection of single wall carbon nanotubes to silicon surfaces via oligo(phenylene ethynylene)s has been demonstrated using an orthogonal bisdiazonium functionalization protocol. Copyright

Full text of DOI:10.1021/ja051269r

Published on Web 06/02/2005
Fabrication of Carbon Nanotube-Molecule-Silicon Junctions
Austen K. Flatt, Bo Chen, and James M. Tour*
Department of Chemistry and Center for Nanoscale Science and Technology, MS 222, Rice UniVersity,
6100 Main Street, Houston, Texas 77005
Received February 28, 2005; E-mail: tour@rice.edu
Scheme 1. Stepwise Attachment of Carbon Nanotubes to Silicon
via 1
Previous work in our laboratory has demonstrated covalent
attachment of arenes via aryldiazonium salts to Si (hydride-
passivated single crystal or poly Si; 111 or 100 , p-doped,
n-doped, or intrinsic), GaAs, and Pd surfaces.1 In the case of Si,
this provides a direct arene-Si bond with no intervening oxide.
We also reported on the use of aryldiazonium salts for the direct
covalent linkage of arenes to single wall carbon nanotubes (SWNTs)
where the nanotubes can exist either as bundles or as individual
structures2,3 (when surfactant-wrapped). Here, we merge these two
concepts for the covalent attachment of individualized (unroped)
SWNTs to Si surfaces via orthogonally functionalized oligo-
(phenylene ethynylene) (OPE) aryldiazonium salts.4-10 To our
knowledge, this is the first report of a procedure to covalently attach
SWNTs to a silicon surface that does not require a CVD growth
process. In addition to functioning as the linker units, OPEs and
related conjugated molecules can serve as electronically active
moieties in sensor and device embodiments.11 Hence, the union of
easily patterned silicon with the often hard-to-affix nanotubes can
provide a critical interface methodology for electronic and sensor
arrays.
Chemical orthogonality provides chemoselection for dual substrate/
nanotube attachment, while OPEs provide a rigid structure to
minimize molecular looping upon surfaces. The target OPE
molecules contain a diazonium salt on one end and an aniline moiety
on the other end (Figure 1). This design allows for selective
an R,ω-bisdiazonium salt OPE were unsuccessful due to the rapid
loss of the terminal diazonium moiety during the silicon assembly.1
In a typical experiment, the diazonium salt (1 or 2) in anhydrous
Figure 1. Oligo(phenylene ethynylene)s 1 and 2 used to covalently attach
SWNTs to silicon.
assembly via the first diazonium salt onto a hydride-passivated
silicon surface followed by diazotization of the aniline using an
alkyl nitrite. Once formed, the new diazonium salt, covalently bound
to the Si surface, will react with an aqueous solution of individual-
ized sodium dodecyl sulfate (SDS)-wrapped SWNTs12 (SWNT/
SDS), resulting in covalent attachment of the SWNTs13 to the silicon
surface using the OPEs (Scheme 1).
The key to developing an orthogonal attachment chemistry was
the formation of a diazonium species with a latent diazonium salt
at the other end. An R-dialkyltriazenyl-ω-aniline proved to be
efficacious upon the demonstration that the triazene could be
converted to a diazo species14 without affecting the aniline. The
synthesis of the mononitro compound 1 is shown in eq 1, and
compound 2 was similarly synthesized (see Supporting Informa-
tion). Indeed, the triazene moiety on 5 was readily converted to
the diazonium salt upon treatment with HBF4, while the aniline
remained intact, the latter thereby serving as a masked diazonium
salt ready for generation after surface attachment. Attempts to use
CH3CN (2.0 mM) was exposed to a hydride passivated silicon 111
surface according to our previous report1 in a nitrogen-filled
glovebox for the desired reaction time (vide infra). Following
monolayer assembly, the substrate was removed from the glovebox
and placed in a 0.3 M solution of isoamylnitrite in CH3CN for 5
min to diazotize the terminal aniline (Scheme 1). The substrate was
then removed and immediately immersed in an aqueous SWNT/
SDS suspension12 (0.7 µM) at pH 10 for 24 h. The SWNT remained
predominantly as individualized SWNTs rather than in bundles.
Following nanotube attachment, the substrate was removed, rinsed
with water and CH3CN, and dried with a stream of nitrogen to
afford the desired structure (Scheme 1).
Molecular attachment to the silicon substrate was analyzed using
ellipsometry (Table 1). Monolayer formation for 1 reached the
theoretical height after 1 h. However, monolayer thicknesses for 2
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C O M M U N I C A T I O N S
Table 1. Calculated and Observed Thicknesses of 1 and 2 on
Si 111
lable by varying the SDS/SWNT reaction time with the terminal
diazonium salt from 3 to 16 h.
Thickness (nm)
Control experiments were performed to ensure both the OPE
and diazotization steps were required for SWNT attachment
(Supporting Information). When the OPE was not employed or
when the second diazotization step was eliminated, there was no
attachment of the SWNTs to the silicon surface. This leads us to
conclude that the SWNTs are indeed covalently attached13 to the
assembled organic molecule via the in situ generated diazonium
salt.
It is important to note that when using NOBF4 to perform the
diazotization (as opposed to the alkylnitrite) on the assembled
monolayer, surface roughening in the form of large peaks and
valleys (10-20 nm) was observed and there were no surface-bound
nanotubes.
The lightly functionalized surface-bound SWNTs are likely to
retain significant degrees of their electronic and optical properties
since it takes functionalization of ca. 1 in 100 carbons along a
SWNT to even cause a loss of the sensitive UV van Hove
singularities.15 Furthermore, when applied only to the end-segments
of SWNTs that straddle patterned gap arrays, the active central
portions of the SWNTs will remain unperturbed by the surface
hybridization moieties. Thus, this Si-nanotube assembly strategy
could provide the basis for directing SWNTs to precise junctions
in electronic, optical, and sensor arrays.
Molecule
founda
calcdb
1
2
2.0c
1.4,c 2.1d
1.9
1.9
a Value measured by ellipsometry with ca. (0.2 nm error. All reported
values are an average of three measurements for reactions of a 2.0 mM
solution of diazonium salt in CH3CN. b The theoretical thickness calculated
by molecular mechanics (not including the arene-silicon bond). c Assembly
performed inside a nitrogen-filled glovebox for 1 and 16d h.
averaged slightly below the theoretical SAM thickness, although
thicknesses close to the theoretical value were attained by reacting
the substrate with a solution of 2 for longer time periods.
Figure 2 shows the XPS spectrum of the N1s region of a
Acknowledgment. This work was supported by DARPA, ONR,
NASA, and the AFOSR. Trimethylsilylacetylene was provided by
Dr. I. Chester of FAR Laboratories and Dr. R. Awartani of Petra
Research Inc. The NSF, CHEM 0075728, provided partial funding
for the 400 MHz NMR.
Figure 2. XPS spectrum of the N1s region of a SAM of compound 1 (45°
take-off angle).
monolayer of 1. The smaller peak at 405.8 eV is due to the nitrogen
atom of the nitro group, and the peak at 399.7 eV is due to the
aniline plus nitrogen gas adsorbed from the atmosphere. Attempts
to measure the nitrogen signal of the surface-tethered diazonium
salt were unsuccessful due to its rapid decomposition during transfer
in air.
Supporting Information Available: Synthetic details and complete
structure preparation and assemblies. This material is available free of
charge via the Internet at http://pubs.acs.org.
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SWNT attachment was further verified using atomic force
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Figure 3. Tapping mode AFM image of SWNTs covalently bound via 1
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