Formation of low-symmetric 2D superlattices of gold nanoparticles through surface modification by acid-base interaction

Article abstract of DOI:10.1021/ja035187j

In the present work, a novel method was developed for the fabrication of 2D superlattices with different symmetries. Same-surface amino-functionalized Au nanoparticles as building blocks were self-assembled to form different 2D superlattices using surface

Full text of DOI:10.1021/ja035187j

Published on Web 06/24/2003
Formation of Low-Symmetric 2D Superlattices of Gold Nanoparticles through
Surface Modification by Acid-Base Interaction
Masayuki Kanehara,† Yasunori Oumi,† Tsuneji Sano,† and Toshiharu Teranishi*,†,‡
School of Materials Science, Japan AdVanced Institute of Science and Technology and “Organization
and Function”, PRESTO, Japan Science and Technology Corporation, 1-1 Asahidai, Tatsunokuchi,
Nomi, Ishikawa 923-1292, Japan
Received March 16, 2003; E-mail: tosiharu@jaist.ac.jp
Fabrication of ordered metal nanoparticles with well-defined two-
dimensional (2D) configuration would enable us to produce novel
optical and nanoelectronic devices. One of the most straightforward
approaches for construction of 2D superlattices is a self-assembly,
whereby a colloidal solution of chemically stabilized nanoparticles
is placed on a substrate surface and the solvent is allowed to
evaporate in a controlled fashion. The formation of 2D superlattices1-9
and one-dimensional chains10,11 has been vigorously investigated.
Recently, extensive studies have been performed on optical12 and
electronic13-15 properties of 2D superlattices of metal nanoparticles,
and it has been proposed that the structure,14 interparticle spac-
ing,12,15,16 and periodicity17 of 2D superlattices greatly affect their
properties. The symmetry of the 2D superlattice also has a potential
to control its electric conductivity and optical property. For the
production of future Ultra Large Scale Integration (ULSI), elucida-
Figure 1. TEM image of 2D superlattice of TBA-Au nanoparticles. The
insets on the right and left show the chemical structure of the TBA ligand
and a FFT image of superlattice, respectively.
tion of electron transport behaviors in well-ordered 2D superlattices
of metal nanoparticles with different symmetries is of great
importance. The regularly hexagonal close-packed (hcp) 2D su-
with organic acid, 1,3,5-benzenetricarboxylic acid (BTCA) or acetic
perlattices are well established, and several groups have succeeded
acid (AcOH), to endow the Au nanoparticles with good solvility
in ordering metal nanoparticles into 2D superlattice with low-
for water (see Supporting Information for more details). These
symmetric structures, such as square structure.9,18-20 In this work
nanoparticle aqueous solutions were then put on a polyvinyl formal-
we demonstrate, for the first time, that different symmetric structures
coated copper grid, and the solvent was slowly evaporated to obtain
of 2D superlattices can be obtained from the same gold nanopar-
the low-symmetric 2D superlattices. Adding ethylene glycol lowers
ticles protected by amino-derivatized ligands through surface
the evaporation rate and improves the affinity of the solution to
modification with the different kinds of organic acids.
the grid, leading to an expansion of the organized area of the 2D
To obtain a general means for low-symmetric 2D superlattices
superlattice. A similar effect of addition of dodecanethiol to a
of nanoparticles, we have synthesized a newly designed protective
solution of dodecanethiol-protected Au nanoparticles was reported
agent, bis-4,4′-(4,4′-dithiobutylbenzyl)-N,N,N′,N′-tetraethylamine
by Lin et al.5
(TBA, see inset in Figure 1 and Supporting Information for detailed
Figure 1 shows the TEM image of a hcp 2D superlattice of the
synthetic procedure and characterization data), in which a disulfide
TBA-Au nanoparticles, obtained by a slow evaporation of the
group serves to coordinate to the gold surface, benzene rings endow
toluene solution on an amorphous carbon-coated copper grid. The
Au nanoparticles with solvility for organic solvents, and amino
mean interparticle spacing was estimated to be ∼1.4 nm, which
groups form ammonium salts with organic acids to control the
structure of 2D superlattice.
The Au nanoparticles were prepared using TBA as a protective
agent. One milliliter of a 10 mM N,N-dimethylacetamide (DMAc)
solution of HAuCl4‚4H2O (10 µmol) was added to the mixture of
5 µmol of TBA in DMAc (44 mL). Under vigorous stirring, 0.1
mmol of NaBH4 in methanol (5 mL) was added to the solution.
was obtained by a direct measurement of individual spacings within
well-ordered hcp domain. By taking the length of the TBA molecule
as ∼1.3 nm into consideration, this result suggests that each Au
nanoparticle is stabilized by one TBA layer, and presumably the
TBA ligands are interpenetrating each other as a result of a π-π
interaction between benzene rings of adjacent particles (Supporting
Information, Figure S1a).
After additional stirring for 30 min, the color of the solution turned
Compared with this hcp superlattice, similar hcp (Figure 2a) and
brown. The nanoparticles were precipitated with water (∼50 mL),
quasi-honeycomb (Figure 2b) superlattices were obtained from the
filtered, and washed several times with a methanol/water (2:1, vol/
solution of TBA-BTCA salt-protected Au (TBA-BTCA-Au) nano-
vol) mixed solvent. The TBA-protected Au (TBA-Au) nanoparticles
particles. In Figure 2a, the mean interparticle separation in the hcp
were 2.4 ( 0.2 nm in size and showed good solvility, not only for
superlattice was 3.0 nm, well corresponding to the sum of two TBA
layers and one BTCA molecule. Hydrogen bonds between TBA
nonpolar solvents such as toluene but also for polar solvents such
as ethyl acetate and DMAc. After the precipitate was redispersed
molecules and interparticle Coulomb repulsion caused by am-
into 5 mL of ethyl acetate, the Au nanoparticles were neutralized
monium salt formation on the surface of the nanoparticles would
be responsible for the formation of hcp structure (Supporting
Information, Figure S1b).
† Japan Advanced Institute of Science and Technology.
‡ Japan Science and Technology Corporation.
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8708
J. AM. CHEM. SOC. 2003, 125, 8708-8709
10.1021/ja035187j CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. TEM image of TBA-AcOH-Au nanoparticles. (a) Typical
amorphous structure and (b) square structure. The inset in (b) shows a FFT
image.
different symmetric 2D superlattices using some organic acids. The
2D superlattices of quasi-honeycomb and square structures were
obtained by neutralizing amino-functionalized Au nanoparticles with
1,3,5-tribenzenecarboxylic acid and acetic acid, respectively. The
results strongly suggest that different types of 2D or 3D superlattices
can be constructed by the simple addition of the proper acid to
nanoparticles functionalized with amino groups. This method will
allow us to obtain various desired metal superlattices without fully
synthesizing the ligands.
Figure 2. TEM image of TBA-BTCA-Au nanoparticles with (a) hcp and
(b) quasi-honeycomb structures. (c) Schematic illustration of the bilayer
structure of quasi-honeycomb structure. The insets show FFT images.
When another hcp layer was placed on the already-formed hcp
layer, the 3-fold quasi-honeycomb superlattice was obtained (Figure
2b). The formation of quasi-honeycomb structure results from the
occupation of nanoparticles at the second layer onto the 3-fold
hollow sites of the first layer, as shown in Figure 2c. This con-
formation was confirmed by the tilt technique ((15°) in TEM
observation (Supporting Information, Figure S2). The formation
of a similar quasi-honeycomb structure was reported for CoPt3
nanoparticles.8 This fact surprised us because, in general, small
metal nanoparticles are mainly placed on 2-fold saddle sites of
the first layer of superlattices. Actually, in the bilayer system of
TBA-Au nanoparticles, the second layer tends to occupy 2-fold
saddle sites to form line or circle structures. A similar tendency
has been observed in superlattices of larger metal nanoparticles,
such as 3-5-nm tetra-n-octylammonium bromide-protected Au
nanoparticles2 and 4.5-nm dodecanethiol-protected silver nanopar-
ticles.4 For much larger nanoparticles with large numbers of
protective ligands, such as 5.5-nm dodecanethiol-protected Au
nanoparticles, 3-fold hollow sites of the first layer of the hcp
superlattice start to be occupied.5 The formation of the quasi-
honeycomb superlattice might be also due to strong hydrogen bonds
between BTCA molecules. When 2,6-naphthalenedicarboxylic acid
was also used in a similar fashion as an organic acid, well-ordered
hcp superlattices were obtained but quasi-honeycomb structures
were hardly observed, strongly indicating the contribution of
interligand multi-hydrogen bonds to the formation of quasi-
honeycomb structures. A planar honeycomb structure would be
obtained by reverse deionization of the ammonium salts from this
quasi-honeycomb structure, since deionization leads to the formation
of naked TBA-Au nanoparticles without organic acids, followed
by dropping the second layer particles to the first layer.
Acknowledgment. The present work was supported by PRESTO,
Japan Science and Technology Corporation (T.T.), and by a Grant-
in-Aid for Young Scientists (A) (No. 15681009) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan
(T.T.).
Supporting Information Available: Synthetic procedure and
characterization data of TBA, neutralization procedure of TBA-Au, and
Figures S1 and S2 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
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In the present work, a novel method was developed for the
fabrication of 2D superlattices. Same-surface amino-functionalized
Au nanoparticles as building blocks were self-assembled to form
JA035187J
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J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8709