Self-lubricating nanoparticles: Self-organization into 3D-superlattices during a fast drying process

Article abstract of DOI:10.1039/c0cc03538f

Fluorinated tetraethylene glycol-stabilized Au nanoparticles (FTEG-AuNPs) were well-dispersed in general polar organic solvents, such as methanol (MeOH) and THF. The cast film of FTEG-AuNPs on a TEM grid and a glass substrate was found to form a highly or

Full text of DOI:10.1039/c0cc03538f

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Self-lubricating nanoparticles: self-organization into 3D-superlattices
during a fast drying processw
Takashi Nishio,a Kenichi Niikura,*b Yasutaka Matsuobc and Kuniharu Ijiro*bc
Received 30th August 2010, Accepted 8th October 2010
DOI: 10.1039/c0cc03538f
Fluorinated tetraethylene glycol-stabilized Au nanoparticles
(FTEG-AuNPs) were well-dispersed in general polar organic
solvents, such as methanol (MeOH) and THF. The cast film of
FTEG-AuNPs on a TEM grid and a glass substrate was found
to form a highly ordered 3D-superlattice assembly, whereas
tetraethylene glycol-stabilized AuNPs (TEG-AuNPs) provide
an amorphous AuNP aggregation. These data indicate that the
fluorine feature on the surface of the FTEG-AuNPs is critical
for the nanostructured assembly.
the particle–particle interactions have yet been developed.
Thus, we focused on the fluorinate modification of nano-
particles since the fluorinated surface is expected to provide
a low-friction property to the nanoparticles. In this study, we
newly synthesized FTEG-C11-SH 1 as a capping reagent for
gold nanoparticles and subsequently prepared FTEG-stabilized
Au nanoparticles (FTEG-AuNPs) as the building blocks of
highly ordered nanostructures. Nanoparticles protected by
perfluorinated alkylthiols have been previously reported;
however, these papers did not focus on their friction property,
and these nanoparticles can only be dispersed in fluorinated
solvents, thus their use and applications are limited due to
their poor solubility.13
The periodic array of metal and semiconductor nanoparticles
(nanoparticle superlattices) has been of great interest to
researchers in a variety of fields.1 These array structures have
applications ranging from catalysis2 to chemical sensors,3
antireflection films and photonic materials.4 Nanoparticle
superlattices have been made by a variety of techniques,
including the cross-linking of nanoparticles using template
molecules,5 the Langmuir–Blodgett technique,6 layer-by-layer
(LbL) deposition7 and the self-organization method.8 The self-
organization method is mainly based on a simple drying
process after nanoparticle solution evaporation. One of the
key factors in creating highly ordered self-assembling super-
lattices is the slow and regulated evaporation rate of the
solvent.9 Therefore, low-volatile solvents, such as toluene or
chlorobenzene, have been often used.10 Furthermore, a large
excess of stabilizer, such as surfactants and polymers, have to
be added to the colloidal cast solution to obtain superlattices.11
However, the drying process under these conditions is time
consuming and not suitable for wet-processes (e.g., inkjet
printing), which would be a powerful technique applicable to
superlattices for electronic, sensing and photonic devices.
Alternatively, previous theoretical analyses and simulations
imply that control of particle–particle interactions by the
chemical modification on the surface of nanoparticles offers
another strategy for particle arrangement.12 Weak interparticle
interactions are important to the formation of a highly
ordered array in that they prevent amorphous aggregation
prior to the formation of a thermodynamically stable, close-
packed structure. However, no chemical approaches to reduce
Perfluoroether derivatives are known lubricant materials
and have been used in many applications, such as the magnetic
hard disk industry, and in MEMS.14 FTEG is a derivative in
which the hydrogen of tetraethyleneglycol is substituted
for fluorine and, because of its good solubility in several
polar organic solvents, FTEG-stabilized nanoparticles were
expected to be well-dispersed in various organic solvents while
maintaining their fluorine feature. We, therefore, investigated
their dispersity in organic solvents and observed their drying-
mediated self-assembly properties on solid substrates using
scanning transmission electron microscopy (STEM) and AFM
for comparison with those of non-fluorinated TEG-stabilized
nanoparticles.
The synthetic procedures for FTEG-C11-SH 1 are described
in ESI.w In order to clarify the effect of surface fluorination,
the hydrocarbon-based TEG compound 2 was also prepared
according to the previous literature.15 An aqueous dispersion
of citricate-protected AuNPs (20 nm in the core diameter) was
concentrated by centrifugation and then added into 10 mM of
compound 1 or 2, followed by vigorous stirring for 2 days. The
surface-modified nanoparticles were then purified by multiple
centrifugations to remove excess thiol compounds. FTEG-AuNPs
(1.0 Â 1013 particles mlÀ1, 0.8 mg mlÀ1) were soluble in general
organic solvents, such as alcohols (e.g., MeOH, EtOH), THF
and acetonitrile, but insoluble in low polarity solvents,
such as toluene and CH2Cl2, and highly polar solvents
such as water (Solubility in solvents is summarized in ESIw).
a Department of Chemistry, Graduate School of Science,
Hokkaido University, N10W8, Sapporo 060-0810, Japan.
b Research Institute for Electronic Science, Hokkaido University,
N21W10, Sapporo 001-0021, Japan.
E-mail: kniikura@poly.es.hokudai.ac.jp; Fax: +81 11-706-9344;
Tel: +81 11-706-9344
c JST, CREST, N21W10, Sapporo 001-0021, Japan.
w Electronic supplementary information (ESI) available: Synthetic
procedure for FTEG-C11-SH 1, solubility, absorption spectra and
DLS analysis of FTEG-AuNPs, AFM image of a cast film of 20 nm
FTEG-AuNPs on the hydrophilic glass substrate. See DOI: 10.1039/
c0cc03538f
c
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Fig. 2 The three-layered region in the FTEG-AuNPs 3D-superlattice
(20 nm in diameter). (A) STEM image of the three-layered region
viewed along the o111> axis of the face-centered cubic (fcc) of the
3D-superlattice, and (B) the corresponding schematic representation.
Scale bar: 20 nm.
Fig. 3 AFM images (phase mode) of a 20 nm FTEG-AuNP cast film
deposited on a hydrophobic glass substrate. (A) Brown regions show
multilayered superlattices formed on the substrate. The bright regions
show the bare glass substrate. (B) The bright regions show nanopar-
ticles. The left inset shows a magnified image (500 Â 500 nm). The
right inset shows the diffraction pattern obtained by Fourier transfor-
mation of the magnified AFM image. Scale bar: 1 mm.
Fig. 1 STEM images of cast-films of AuNPs (core diameter: 20 nm)
on the TEM grid from methanolic solution. (A) FTEG-AuNPs,
(B) TEG-AuNPs. The insets show the diffraction pattern obtained
by Fourier transformation of STEM image. Scale bar: 300 nm.
shorter than the theoretical molecular length of a FTEG
molecule 1 (ca. 3 nm). This means that the capping thiol
molecules apparently either tilt or interdigitate with a molecule
on the opposite side. Fig. 3 shows an AFM image of a cast film
of 20 nm FTEG-AuNPs on a siliconized hydrophobic glass
substrate. We can see the multilayer 3D-superlattice in which
20 nm FTEG-AuNPs are arranged over a large area. On the
other hand, on the hydrophilic substrate (clean glass), we
observed the formation of an unassembled monolayer fraction
of nanoparticles (see ESIw). As the siliconized hydrophobic
glass substrate has a low surface tension and a dewetting
property for MeOH, the height of the droplets can be
maintained, resulting in the successful growth of 3D lattices.
Finally, we tried to construct superlattices composed of
5 nm FTEG-AuNPs by a cast from MeOH; however, no
3D-superlattice was produced, which is contrary to the readily
formed superlattices observed with 20 nm FTEG-AuNPs.
However, lowering the evaporation rate by changing the
solvent from MeOH to isopropanol, whose relative evapora-
tion rate is 1.9 times slower than that of MeOH, gave a
3D-superlattice (Fig. 4). This indicates that FTEG-modification
is effective in the formation of superlattices even for nano-
particles of several nanometres in size when the proper solvent
is used.
The hydrodynamic diameter distribution of FTEG-AuNPs in
alcohol was measured by dynamic light scattering (DLS) to
give an average diameter of 20.1 nm. Moreover, FTEG-AuNPs
showed a plasmon peak at ca. 525 nm in ethanol, which is
similar to that of citrate-stabilized AuNPs in an aqueous
solvent (see ESIw). Therefore, it is apparent that FTEG
molecules 1 effectively stabilized Au nanoparticles in solution
without aggregation.
STEM images were taken for a cast film of FTEG-AuNPs
dispersed in MeOH (5.0 Â 1013 particles mlÀ1, 4 mg mlÀ1) on
carbon-coated TEM grids (Fig. 1). A hexagonal-packed and
layered structure (3D-supperlattice) was observed over a wide
area (B10 mm). In the case of TEG-AuNPs, we were not able
to observe a highly arranged 3D structure in the cast film
under the same evaporation conditions as these used for the
FTEG-AuNPs. This indicates that the fluorination on the
nanoparticle surface must be responsible for the observed
self-assembly. Fig. 2A gives a dark field TEM mode image
of a three-layered region of 20 nm FTEG-AuNPs. The difference
in contrast between the two background layers and the upper
white layer allows us to assign each nanoparticle to its appro-
priate layer and conclude that the FTEG-AuNPs are assembled
in a face-centered cubic (fcc)-type superlattice. Fig. 2B shows a
graphic illustration clarifying the three-layered arrangement of
the nanoparticle layer can be estimated at 1.5 nm, which is
We considered that the well-ordered nature of the
FTEG-AuNPs resulted from the ‘‘self-lubricating’’ property
(low particle–particle friction) of the fluorinated surface. After
the particle is fixed by the capillary force of the liquid thin film
on the substrate, it is necessary to shift from an amorphous
c
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A part of this work was conducted at Hokkaido Innovation
through Nanotechnology Support (HINTS), supported by
"Nanotechnology Network JAPAN" Program of the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT), Japan and the OPEN FACILITY, Hokkaido
University Sousei Hall. This study was supported in part by
Research Fellowships of the Japan Society for the Promotion
of Science for Young Scientists.
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