Vol.3, No.3, 208-217 (2011) Natural Science
Copyright © 2011 SciRes. OPEN ACCESS
Fabrication of self-assembled monolayer using carbon
nanotubes conjugated 1-aminoundecanethiol on gold
Mohammed M. Rahman
Laboratory of Interface and Surface Science, Department of Chemistry, Chonbuk National University, Jeonju, Republic of Korea;
Received 7 December 2010; revised 26 January 2011; accepted 29 January 2011.
The carbon nanotube (fundamentally Single-
walled carbon nanotube, SWCNT) based on
1-Amino-undecanethiol (AUT) were extremely
controlled (nano-level) organizing a vertical
self-assembled monolayer (SAM) on gold single
crystal surfaces. The produced nano-surfaces
were explored particularly by Fourier Transform
Infra-red Spectroscopy (FT-IR), Cyclic Voltam-
metry (CV), Raman spectroscopy, Electrochemi-
cal quartz crystal microbalance (EQCM), and
Atomic force microscopy (AFM) techniques. The
SWCNTs were initially cut (chemically) into
short pipes and thiol-derivatized at the open
ends. The vertical aggregation of SWCNT-AUTs
on chemically refined Au(111) substrates was
made-up by their spontaneous chemical bond-
ing among carboxyl derivatized SWCNT-COOH
and AUT SAM on Au(111), via peptide bonds, or
directly by synthesized SWCNT-AUT compos-
ites. Raman spectroscopy and AFM surface
images obviously disclosed that the SWCNT-
AUT (dia. 20~40 nm) has been vertically catego-
rized d on gold (111) substrates, shaping a SAM
with a perpendicular direction.
Keywords: SWCNT-AUT; Self-Assembled Mono-
layer; Surface Coverage; EQCM; Tapping-Mode
AFM; Raman Spectroscopy
Carbon nanotubes (CNTs) have appeared as an at-
tracting new category of electronic materials owing to
their nano-scale dimensions and exceptional properties,
which comprise the capability to conduct a current den-
sity three orders of degree higher than distinctive con-
ductors, such as copper and aluminium and the aptitude
to conduct electrons statically. As unfilled cylindrical
tubes prepared of whole carbon with enormously high
aspect ratios (length/diameter), CNTs have one, two or
few concentric graphite layers capped through fullerenic
hemispheres. SWCNTs have been developed as an in-
teresting candidate for fundamental studies since its pos-
sible applications including miniature chemical, bio-
logical, material as well as electronic devices. SWCNTs
are one dimensional conductor or composer with all
electrons moving in an atomic layer having surface at-
oms. The small dimensions, strength, and the extraordi-
nary physical properties of these structures construct
them an extremely exceptional material with a wide
range of promising applications [1]. Since the innovation
of carbon nanotubes by Iijima in 1991 by transmission
electron microscopy, SWCNTs have been the topic of
numerous investigations in chemical, physical, and ma-
terial areas owing to their novel structural, mechanical,
electronic, and chemical properties [2]. Depending on
their atomic structure, SWCNTs act an electrically as a
metal or as a semiconductor [3]. The functionalization of
SWCNTs has concerned enormous interest in the past
few years. These works are of large significance to sur-
vey the potential application for SWCNTs. The first
work was conceded out by Green and co-workers [4]
when they cut SWCNTs in short pipes using concen-
trated oxidizing agent. Self-assembly of a molecular
monolayer onto a solid surface is an essential and tech-
nical way to build structurally controlled and stable or-
ganic thin films. Many applications of this stable, closely
packed monolayer on solid substrates have been exam-
ined, counting adhesion, lubrication, promotion, corro-
sion inhibition, and microelectronics production. Several
sorts of organic compound on solid substrates are util-
ized in SAMs. SWCNTs are two dimensional nanos-
tructures with distinctive transport, electrical, electronic,
and optical properties. The prospective use of SWCNT
in electronic applications motivates research on their
M. M. Rahman / Natural Science 3 (2011) 208-217
Copyright © 2011 SciRes. OPEN ACCESS
chemical properties and assemblies on the devices or
electrodes. Pristine SWCNT materials are frequently
collected of nanotubes of intently dispersed diameters,
but their self-assembly layer more disseminated, random,
mostly asymmetrical, and haphazard. SWCNTs can be
functionalized with unusual chemical groups using co-
valent and non-covalent methods. Functionalized
SWCNTs are then conjugated to diverse detection or
recognition molecules for various potential applications.
Different objective analytes can be oxidized by
SWCNTs at low potentials with minimum surface foul-
ing, an appealing characteristic for the improvement of
surface modification with high selectivity, stability, and
reusability. Non-covalent functionalization of SWCNTs
is important and fundamental to protect the sp2 nanotube
structure and thus their significant electronic characteris-
tics. Most of the inspected SAM systems use sul-
fur-anchor assemblies (thiol, di-sulfide, or thio-ether)
and gold electrodes [5], since the chemical limpness of
the substrate and its sturdy communication with sulfur
permit a simple preparation of well-defined SAMs on
single crystal gold surfaces. Because of strong intermo-
lecular Vander Waals forces of interaction, such SAMs
are compactly crammed and extremely vertical prear-
ranged [6]. Significant development along these lines
was lately attained by Liu and co-workers [7]. In this
previous exertion, long SWCNT ropes were cut into
short lengths of pen-ended and chemically functional-
ized tubes by oxidation in concentrated sulfuric and ni-
tric acids [8]. Herein, it is reported the primary chemical
congregation of SWCNT fabricated by the wet SAM
techniques. Hence to offer common means of self as-
sembly, we herein confirmed the perpendicular thiol-
functionalized SWCNT-AUT monolayers on gold single
crystal surfaces via peptides chemical bonding directly
or indirectly. For most of these applications, well-ordered
nanotubes with aligned orientations are greatly enviable,
but conventional methods [9] only formed carbon nano-
tubes in an arbitrarily tangled situation. Consequently,
there is a vast confront to unwrap up a new technique to
categorize the SWCNTs into well-ordered arrays. Based
on SAMs of molecular chains, we put frontward a inno-
vative approach to assemble two-dimensional arrays of
SWCNT by a chemical approach for the development of
electrodes or electronic devices.
2.1. Reagents and Materials
Potassium ferricyanide (III) (99%) and Dicyclohexyl-
carbodimide (99%) are obtained from Aldrich Chemical
Company. Sulphuric acid (97%), nitric acid (60%), Tri-
ton X-100 (98%), and hydrogen peroxide (30%) are
purchased from Showa Chemical Company (Japan). All
the reagents are in analytical grade, used as received
without additional purification. Solutions are prepared in
deionized distilled water (resistivity, >18.2 M.cm). The
commercial existing SWCNTs dispersed solution and
1-Amino-undecanethiol (AUT, NH2(CH2)11SH) is used
to derivatize the SWCNT-COOH and SWCNT-AUT
SAM, which is purchased from Tubes@Rice Company.
2.2. Fabrication of SAM on Quartz Crystal
All quartz glasses were originally cleaned by piranha
solution (a mixture of 98% H2SO4 and 30% H2O2 at 2:1
v/v) for ca. 10 minute [Caution! Piranha solution is a
extremely strong oxidizing agent and reacts aggressively
with organic compounds. Storing in a closed container
and revelation to direct contact should be evaded.
Freshly arranged piranha solution should be handled
with excessive concern]. It was then rinsed with deion-
ized water and ethanol and then dried with a stream of
N2 gas. The cleaned gold coated quartz electrode is im-
mersed in ethanolic solution of AUT and SWCNT-AUT.
Then the modified electrode systematically washed with
ethanol and dried with nitrogen gas. All electrochemical
measurements were conceded at room temperature after
the electrolyte solution was de-aerated by purging Ni-
trogen gas for at least 10 minute.
2.3. Electrochemical Technique
Cyclic voltammetry experiments were performed us-
ing an electrochemical analyzer (Shin1000, Electro-
chemical Quartz Crystal Microbalance, EQCM, Korea)
with a conventional three-electrode cell. All of the vol-
tammetric measurements were carried out in a one com-
partment cell having the naked or AUT-SAM, SWCNT-
AUT SAM modified gold as a working electrode, a Pt
wire as a counter electrode and a Ag|AgCl (saturated
KCl) as a reference electrode. The EQCM was executed
using frequency counter (Shin Cor. Model EQCN1000)
with potentiostat and gold coated AT cut quartz crystal
oscillator electrodes supplied by International Crystal
Manufacturer. The resonance frequency of the oscillator
was 10.0 MHz and the frequency change of 1.0 Hz cor-
responds to 4.42 ng/cm2. The perceptible surface area of
gold electrodes was 0.256 cm2 and roughness factor was
about 1.2. Electrochemical reductive desorption of AUT
and SWCNT-AUT SAM was employed in 0.5 M KOH
aqueous solution. The solution in the cell was de-aerated
by bubbling N2 gas for at least 30 minutes before per-
forming the each electrochemical exploration.
M. M. Rahman / Natural Science 3 (2011) 208-217
Copyright © 2011 SciRes. OPEN ACCESS
2.4. Preparation and Purification of
The SWCNTs are high quality and quite pure, al-
though some nanoparticles still exist in the obtained ma-
terial as a by-product. SWCNTs are produced either as
isolated units or as nanotubes prearranged in bundles; no
effort was made to separate the different patterns. The
purification of general SWCNTs is of huge significance
since most carbon nanotube applications require materi-
als of high quality. Nitric acid is a familiar reagent for
refinement of carbon nanotubes and has comprised the
initial step in different purification methods. Nitric acid
treatment is typically developed to eliminate metal cata-
lysts, together with some of the amorphous carbon, but it
can also oxidize carbon atoms at the ends [11]. Sonicat-
ing SWCNTs in nitric acid opens the ends of the nano-
tubes [12] and thereby commences carboxylic acid
groups at the ends or defect sites of SWCNTs [13]. The
preparation of SWCNTs functionalized with carboxylic
acid groups was performed as follows. Firstly, SWCNTs
were soaked in 5.0 M nitric acid, and then ultrasonically
dispersed them for 6 minutes. Secondly, these are diluted
with a great quantity of water and add a little Triton
X-100 surfactant to amplify the solubility, then sonicated
it to be a black solution. Thirdly, the black solution was
filtered with 0.2 µm diameter film and composed the
SWCNTs and then repeated the second and the third
2.5. Preparation of Gold Single Crystal
The single-crystal gold-coated glass plates were pre-
pared according to Arrandee [10], by flame annealing,
Au(111) terraces can be attained. Best is to flame an-
nealing in a dark room, to be capable to examine the
glowing of the gold arrandee by a propane/butane flame.
Hydrogen flame was too high temperature. Gold arran-
dee was put in flame and taken out after heating and
allocates cooling down for about 30 seconds. This proc-
ess was repeated three times, then it was executed to
locate Au(111) terrace of about 500 nm in diameter es-
tranged by irregular boundaries.
2.6. Instrumentations
The measurements were taken at room temperature.
IR spectra were measured by a FT-IR spectrometer
(AVATAR 330, Thermo Nicolet. USA) as a KBr disc.
Non-contact AFM mode (Solver NT-MDT, Nano-Finder,
Japan) is used to confirm the SWCNT-AUT modified
gold single crystal surfaces. Raman spectra (Raman
Spectroscopy, KBSI, Japan) is obtained for SWCNT-
AUT samples concurrently throughout AFM measure-
2.7. Preparation of SWCNT-AUT SAM on
SWCNT-AUT self-assembled is fabricated in two dif-
ferent approaches on gold substrates.
2.7.1. One-Step Method for SWCNT-AUT SAM
The SWCNT-AUT SAM was produced by plunging
cleaned gold(111) electrode in the ethanol solution hav-
ing 1.0 mM of SWCNT-AUT for 48 hours. Subsequently
it was systematically rinsed with ethanol and dried with
N2 gas. The fabricated SWCNT-AUT gold(111) surface
is considered by non-contact AFM images. The elevated
compactness of SWCNT-AUT aggregated monolayers is
employed. The condensation among the carboxylic ter-
mini of the SWCNTs and the amino group of the thiols
(peptide bond) is demonstrated by the manifestation of
an amide band approximately 1600 cm-1 in the FT-IR
2.7.2. Two-Step Method for SWCNT-AUT SAM
The AUT SAM was produced by immersing a cleaned
gold (111) electrode in the ethanol solution containing
1.0 mM AUT for two hours. Subsequently it was com-
prehensively rinsed with ethanol and dried with nitrogen
gas. The AUT SAM-modified electrode was immersed
for 24-72 hours in activated solution of DCC along with
SWCNT-COOH. Occasionally the activated solution
was put in a refrigerator at about 4.0˚C for above a week
for adequate activation. It was afterward rinsed with
ethanol and dried with nitrogen. Covalent attachment of
SWCNT-COOH to the AUT SAM was induced by a
coupling agent, which is DCC solution in ethanol. This
procedure is commonly applied for configuration of am-
ide bonds among amino groups and carboxylic func-
tional groups in the peptide synthesis.
Figure 1 exemplifies the essential schematic method-
ology of to immobilize the SWCNTs on gold via the
surface condensation reaction in existence of condensa-
tion agent. The single crystal gold substrate is first mod-
ified with AUT self-assembled monolayer completed by
amino groups. The AUT molecules were established to
form compactly packed monolayers on gold as confir-
mation by electrochemical extents and high-resolution
scanning tunneling microscopy images. The chemically
shortened SWCNTs having carboxyl groups of AUT
SAM on gold with the help of dicyclohexylcarbodiimide
condensation agent at 4.0˚C. The condensation reaction
among carboxyl and amino groups is well recognized
M. M. Rahman / Natural Science 3 (2011) 208-217
Copyright © 2011 SciRes. OPEN ACCESS
Figure 1. Schematic illustration of the surface condensation reaction (peptide conjugation) method for fabricating highly aligned
SWCNT-AUT SAM on Au(111).
and often used in polypeptide synthesis. We suppose to
immobilize the short SWCNTs on solid gold substrates
in a extremely ordered manner using this attitude.
The carboxylic acid groups exhibit an essential func-
tion in SWCNT chemistry as they can be further reacted
with a multiplicity of organic molecules [7]. In addition,
to take away the metal catalyst commencing SWCNT, it
is obtained that nitric acid demolishes SWCNT to gener-
ate carbonaceous impurities. The exclusion of the acidi-
fied amorphous carbon can be obtained via centrifuga-
tion [14,15]; though, the mechanism of this route and the
function played by centrifugation has not been thor-
oughly premeditated. Liu et al. approach [16], the puri-
fied SWCNTs were cut into short pipes by chemical
oxidation in a mixture of concentrated H2SO4 and HNO3
(3:1, 98% and 70%, respectively) under ultrasonication
for 8 hours [17-19]. The same techniques were em-
ployed to carboxyl derivatized of obtained SWCNTs
sample. The reaction mixture of SWCNTs was then di-
luted with water and permitted to set overnight for pre-
cipitation. The supernant was decanted, and the rest were
diluted with deionized water and filtered with a 1.0 m
(diameter) pore polytetrafluoroethylene membrane (PTFE,
Gelman, commercial name) under vacuum. The solid
shortened SWCNT sample was attained by washing the
remains on the PTFE filter with deionized water until the
filtrate pH became practically neutral. The SWCNT
pipes attained were originated to form stable colloidal
suspensions in water, ethanol, acetone, and dimetylfor-
maldehyde (DMF). Suspensions were equipped by ul-
trasonication without using surfactants, suggesting that
comparatively short nanotubes have been made as com-
pared with Liu’s work [7]. The characteristic stretching
band (C = O) of carboxylic groups at 1710 cm-1 in the
FT-IR spectrum powerfully suggests the development of
open-ended and carboxyl-terminated SWCNTs after
oxidation treatment [20,21].
3.1. Cyclic Voltammetry
Conventional electrochemical method, cyclic volt-
ammetry (CV) is the most versatile electroanalytical
procedure for the investigation of electroactive species,
and is extensively used in industrial applications and
educational or technical research entities. CV is also an
significant method to assess the blocking property of the
monolayer-coated electrodes via diffusion controlled
redox couples. Figure 2 exhibits the cyclic voltammo-
grams of bare Au and SWCNT-AUT SAM-modified Au
electrodes in 1.0 mM K3Fe(CN)6 with 0.1 M KOH as the
supporting electrolyte at a potential scan rate at 100.0
mV/s. It can be seen from the Figure 2, that the nake
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Figure 2. Cyclic voltammogram in 1.0 mM potassium ferro-
cyanide with 0.1 M KOH as supporting electrolyte at a poten-
tial scan rate at 100 mV/s for bare Au electrode (solid line),
dotted lines are SAM of SWCNT-AUT and AUT on Au surface,
gold electrode (solid line) exhibits a reversible voltam-
mogram for the redox couple specifying that the electron
transfer reaction is totally diffusion controlled. In dispar-
ity, the absence of any peak formation in the CVs of the
SWCNT-AUT monolayer-modified electrodes presents
that the redox reaction is slightly inhibited or blocked. It
can also be perceived that in the case of AUT SAM
(Figure 2), the CV shows an ideal blocking manner. So
the CVs for SWCNT-AUT & AUT designate a good
blocking behavior for the electron transfer reaction,
which denotes that a extremely ordered, condensed mo-
nolayer is produced on the Au surface [22].
3.2. EQCM Study
The surface coverage’s of SWCNT-AUT (with con-
sidering roughness factor, 1.2) SAM on Au substrates is
considered according the Eq.1. The charge (Q) recorded
in bulk electrolysis experiments or the areas beneath
slow scan rate cyclic voltammograms can be used in
combination with lower equation to conclude the surface
coverage of electroactive redox centers:
QnFA (1)
where, F is Faraday’s constant and A is the area of the
The desorption peak and mass change are calculated
in 0.5 M KOH electrolyte at 25 mV/s scan rates. The
desorption conduct of the thiol derivatized SWCNT-
AUT and AUT SAM was checked by EQCM method.
After the SWCNT-AUT was accumulated on the Au, the
surface coverage of the SWCNT-AUT compound was
anticipated by reductive desorption. The abrupt ampli-
fied in frequency and cathodic current of AUT samples
was found at around 1.06 V, which was accredited to
the reductive desorption of the SAMs from the gold sur-
face (Figure 3(a)). But for SWCNT-AUT SAM, current
and frequency tainted in two steps during reductive de-
sorption, first for SWCNT-AUT (at elevated potential)
and second one for AUT (at lower potential, non bonded
with SWCNT-COOH) desorption, which is shown in the
Figure 3(b).
The CV’s in the existence of AUT SAM exhibited
desorption peak at 1.06 V and surface full-coverage
4.61 10-10 mole/cm2 in the potential range 0 to 1.3 V
(Figure 3(a)). Adsorbate coverage is expected by inte-
grating the charge under the voltammetric wave for the
adsorbed complex, which shows two reductive desorp-
tion peaks like binary SAM at about 1.08 V (for AUT)
and 0.51 V (for SWCNT-AUT) vs. Ag|AgCl (saturated
KCl) at the scan rate 25.0 mV/s (Figure 3(b)). The ex-
perimental values of surface coverage’s for AUT and
SWCNT-AUT are intended individually 1.73 10-10 and
2.71 1010 mole/cm2 correspondingly. The desorption
potential of AUT peak coincide with previous value at
1.08 V. The new peak at higher desorption potential
region is executed for SWCNT-AUT self-assembled
The calculated total surface coverage for total AUT
(Figure 3(a)) full-coverage (4.61 1010 mole/cm2) was
incredibly similar to that for total desorption (Figure
3(b)) during SWCNT-AUT and AUT SAM (AUT 1.73
1010 + SWCNT-AUT 2.71 1010 mole/cm2) 4.44
1010 mole/cm2 within experimental error boundary. This
signified that the monolayer produced via SWCNT-AUT
had similar surface coverage to that of the AUT for ex-
tended immersion in the solution [23].
3.3. Tapping AFM Images
The chemical accumulated process offers an addi-
tional valuable, efficient, and suitable technique to con-
struct SWCNT investigates for scanning probe micros-
copy studies [24-26], well proscribed aligning of nano-
tubes is of meticulous significance for generating nano-
tube-based nano-electronic and nano-molecular elec-
tronic devices or chips [27]. We have achieved in accu-
mulating carboxylic acid derivatized SWCNTs on an
amino-terminated Au(111) surface via electrostatic in-
teractions. Nanotube assemblies offer broad potential for
nanotube applications in electronic and optoelectronic
devices. We consider that the occupied standing aggre-
gation nanotubes on an electrode surface will signifi-
cantly advanced the field-emission performances of elec-
trons for flat-panel display applications [28], escalating
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Figure 3. Desorption peaks of (a) AUT SAM and (b) SWCNT-
AUT SAM (direct sample’s) in 0.5 M KOH at 25 mV/s scan
rate, immersion time 24 h.
the current density, lowering the operation voltage, and
miniaturizing the device dimensions. It has been studied
non-contact tapping mode atomic force microscopy (AFM)
to exemplify the topograph of the thiol-derivatized car-
bon nanotubes on gold substrates.
Atomic force microscopy was employed to attain the
structural information of the SWCNT assembly prepared
on gold by the surface condensation procedure. Figure 4
represents the distinctive aggregation of SWCNT-AUT
AFM images acquired, where (A) two-dimensional (2D)
and height profiles correspond to the lines drawn in the
2D image, (B) three-dimensional (3D), and (C) statisti-
cal distribution in SWCNT-AUT diameter. Figure 4 is
an tapping mode AFM two and three dimensional repre-
sentations of an attached SWCNT-AUT self-assembled
monolayer on Au(111) surface. Each bright-spot (two-
dimensional) or standing (three-dimensional) character-
istic is accredited to an adsorbed SWCNT-AUT SAMs.
Unlike the predictable organic self assembling molecules
on solid surfaces (e.g., alkanethiols on gold), the carbon
nanotubes immobilized on gold have dissimilar lengths
and form various-sized of aggregates. This examination
is extremely reproducible at different sites of SWCNT
through oxidation of SWCNT samples. The diameter of
the SWCNTs unswervingly computed from tapping
mode AFM images falls in the range of ca. 20-40 nm.
The large diameter resulting from the AFM image may
be owing to the tip broadening consequence. When the
sample is high and steep, the contour line reported by the
AFM can be relatively unusual from the accurate con-
tour of the sample. In the tremendous case, where the
sample is very high and sharp contrast with the tip, the
AFM image is an image of the tip rather than the sample.
Under this condition, the SWCNT-AUT with sharp
geometrical pattern acts as an AFM tip to image the
large innovative AFM tip. The accurate image can be
reconstructed by deconvolution. For the nominal lateral
size examined from the AFM image, the deconvolution
presents a true tube diameter of ca. 20-40 nm.
Aggregations are apprehended by two or more
SWCNT-AUTs simultaneously in dissimilar lengths on
gold surface, its may be owing to the hydrogen bonding
among the neighbor carboxyl-derivatized SWCNT walls
[29] after chemical treatment. The widths of SAM ge-
ometry are considered; the aggregated diameter is broad-
ening by double, triple or more SWCNT-AUT tubes on
gold surface. The largest diameter of nanotubes from
AFM data (ca. 30 nm) gives a cumulatived diameter of
ca. 120 nm. The innovative chemically condensed
SWNT suspension controlled different lengths of nano-
tubes up to 200 nm, which is investigated by AFM re-
sults. Consequently, the above AFM data specify that
only comparatively short tubes can be immobilized on
gold surfaces via a condensation reaction. A related con-
clusion has been drawn Liu et al. [7], in which the thi-
olated nanotubes were deposited or adsorbed on gold via
Au-S chemical bonds. On the other hand, the experi-
mental lateral dimensions of SWCNT descend into a
range of 20 to 120 nm, generally within 20 - 45 nm
(82.1%). Because of the broadening effect of the AFM
tip [28], these data do not straightforwardly reflect the
true lateral sizes of nanotubes. Most of the SWCNTs in
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(a) (b)
Figure 4. AFM images (1000 nm × 1000 nm) of SWCNT-AUT SAM (AUT SAM is immersed in ethanolic SWCNT-COOH solution)
on Au(111) substrates, Immersion time 24 hours, (a) 2D-image & height profiles correspond to the lines drawn in the 2D AFM image,
(b) 3D-image, (c) Statistical distribution in SWCNT-AUT diameter.
the assembly subsist in aggregated shapes, representing
that the carbon nanotubes tend to form aggregates, that is,
bundles during surface condensation. Approximately all
bundles probable have fine saw-tooth structures, signi-
fying the subsistence of dissimilar lengths of nanotubes
within individual bundles. The carbon nanotubes are
carboxylated at both open ends. Both carboxylic termini
may have the same prospect of contributing in the sur-
face condensation reaction. However we never found
nanotubes lying flatly on the surface. In fact, the fine
saw-tooth structures found in the cross section also pro-
pose the standing-up orientation because otherwise we
would observe a flat cross section for the stacked nano-
tubes along the long axis direction. This is logical be-
cause direct contact among the hydrophobic nanotube
side walls and the hydrophilic amino surface is actively
inauspicious. It should be pointed out that the nanotubes
may tilt some degree from the surface normal, depend-
ing on how many carboxylic groups at the tube end
(typically not one) have participated in the condensation
reaction and on the inter-nanotube interactions. The cut-
ting angle of the nanotube at the open end formed during
the oxidative shortening process may also influence its
tilt angle on the surface.
The present studies demonstrate that aggregates
SWCNT-AUT SAM can be chemically assembled on gold
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surfaces using a similar AUT or some other alkanethiols
as amine terminal organic self-assembled species. The
SAM was organized through the covalent attachment of
carboxylic derivatized single-walled carbon nanotubes
(SWCNT-COOH) in an activation solution having DCC
coupling agent to AUT SAMs produced on a gold sub-
strate in two ways at same immersion time interval
(Figure 5). AFM images of SWCNT-AUT were exe-
cuted in two ways. Direct SWCNT-AUT SAM is em-
ployed from the ethanolic solution of SWCNT-AUT
sample in single step method. The elevated density of
SWCNT-AUT aggregated monolayers is formed (Figure
5(a)). The condensation among the carboxylic termini of
the SWCNTs and the amino group of the thiols (peptide
bond) is evidenced by the appearance of an amide bond
about 1600 cm-1 in the FT-IR spectrum. In two steps
method, covalent attachment of SWCNT-COOH on the
AUT SAM (AUT SAM made before immersion in
SWCNT-COOH ethnolic solution) was induced by a
coupling agent (DCC) in ethanol. This method is exten-
sively applied for formation of amide bonds between
amino groups and carboxylic functional groups in the
peptide synthesis. Less number of SWCNT-AUTs SAM
is formed, which is presented in Figure 5(b). This is
may be owing to the DCC covered carboxylic derivat-
ized SWCNT walls, so less activity to make aggregation
and bond formation with AUT SAM. The DCC-aided
condensation reaction between carboxyl and amino
group proceeds in two successive steps: DCC molecules
react first with the carboxyl group, generating an active
ester intermediate, which then reacts with the amino
species. In the present system, DCC is excessive in
magnitude contrasted with the SWCNTs, while the ami-
no groups are bound to solid substrates. Since the ester
intermediates are quite stable in ethanol, we supposed
most of the nanotubes exist in the intermediate form, and
the effective collision of the intermediates to the ami-
no-terminating surface controls the condensation process.
The oxidatively shortened nanotube typically has more
than one carboxyl group at one end. These carboxyl
groups may combine more than one DCC molecule, ge-
nerating an ester intermediate with multiple active sites.
This would facilitate the surface pinning of huge nano-
tubes. It should be noted that the outermost carboxyl
groups of the immobilized SWCNTs on gold may also
be coupled with DCC after the condensation reaction.
However, these ester intermediates will be instantly hy-
drolyzed to the innovative carboxyl groups after rinsing
in water. As noted above, the carbon nanotubes tend to
form bundles during the collision process. This may be
accredited to the strong attractive interactions between
the hydrophobic sidewalls of SWCNTs. It also suggests
that the collision may not directly lead to the condensa-
Figure 5. AFM images of SWCNT-AUT (a) direct SWCNT-
AUT sample’s (b) via peptide bond (AUT SAM immersed in
ethanolic solution of SWCNT-COOH) on Au(111) substrates.
Immersion time 36 hours, non-contact AFM mode, and 1000 ×
1000 nm.
tive pinning of nanotubes, and the following lateral
movement of the intermediates on surface would be pos-
sible. At the initial stage, the collision of nanotube-carry-
ing ester intermediates with the surface amino groups
consequences in reactive immobilization of SWCNT on
gold at few locations. These immobilized individual
nanotubes may have different diameters and serve as
nucleation centers. The following nanotubes then pref-
erentially adsorb surrounding these nuclei, leading to the
M. M. Rahman / Natural Science 3 (2011) 208-217
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formation of various nanotube self-assembly.
3.4. Raman Spectroscopy
Raman spectroscopy is one of the most influential
methods for classification and characterization of nano-
materials, semiconductor materials, and carbon nano-
tubes etc. All allotropic forms of carbon like fullerenes,
carbon nanotubes, amorphous carbon, polycrystalline
carbon, etc. are active in Raman spectroscopy [31]. The
position, width, and relative intensity of bands are modi-
fied according to the carbon forms [32]. This procedure
provides valuable information concerning the structure
of carbon nanotubes. Shortly, there is strong substantia-
tion for a diameter-selective resonant Raman scattering
process. The tangential mode (TM) in the range 1400-
1700 cm-1 gives information on the electronic properties
of the tubes while the analysis of the so called D-band at
around 1360 cm-1 provides information as to the level of
disordered carbon. The size of the D-band relative to the
TM band is a qualitative computed of the formation of
unattractive forms of carbon. In this experiment, it is
utilized 788-nm (semiconductor Sapphire Laser) excita-
tion for checking SWCNTAUT self-assembled
monolayer on gold single crystal substrates. This is the
most direct confirmation of SWCNT- AUT SAM on
Au(111), which is directly detected by Raman spectros-
copy. The Raman spectrum of the SWCNT-AUT modi-
fied gold surface shown in Figure 6, where the G-line at
1598 cm1 originates from the graphitic sheets [33] and
the peak at 1365 cm1 is related to the defects (disorder
mode consistent with sidewall functionalization) in
CNTs [34]. From this, we can also conclude that the
physical structure of the CNTs was not distorted with the
only exception of the opened ends.
A chemical approach was place onward to categorize
and organize the arbitrarily chemisorbed SWCNT-AUT
SAMs on a Au(111) substrate. From all of these experi-
mental scrutinies, it is confirmed that the thiol-derivatized
SWCNT-AUT have been effectively immobilized on
Au(111) via Au-S chemical bonding, with the nanotubes
being vertically aggregated and standing on the gold
surfaces. We established that SAM of SWCNT- AUT is
characterized and developed, and the adsorbed arrays
will offer broad possibilities for different purposes. Us-
ing Raman and AFM, we demonstrated that extremely
stable, vertically aligned, and patterned SWCNT-AUT
assemblies on gold single crystal were executed. It was
also investigated that the massive SWCNT-AUT tends to
form bundles on amino-terminating surfaces. This type
of extremely aligned nanotube assembly may afford wide
Figure 6. Raman spectrum of SWCNT-AUT SAM is shown
the most characteristic features of SWCNTs (D and G band),
Laser focus on SWCNT-AUT aggregated area, Laser exposure
time 30 sec.
possibilities for various applications.
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