Communications
reproducibility of the results and found that the type of
around the metal centers (Figure 2); however, since the
organic sublayer is much shorter than twice the length of the
mesogenic molecule, we assume that the mesogenic cores
remain orientationally and positionally disordered and prob-
nanoparticle organization is not affected by the number of
mesogenic pendants within the limits given above. The type of
mesogenic molecule in the metal coating had a profound
influence on gold-cluster self-assembly.
The most interesting results were obtained for particles
passivated by rodlike molecules 1, with a long, forklike end
group. The 2D X-ray pattern (Figure 1) of a homogenously
Figure 2. Schematic drawing of the arrangement of nanoparticles in
the columnar phase, as viewed from above (top, left) and from the
side (bottom, left); a, b, and c are the crystal-lattice parameters. A
gold cluster is located at the center of each cylinder, and the cylinder
is filled by orientationally and positionally disordered n-alkyl and
mesogenic thiol molecules redistributed around the gold cluster. The
two colors distinguish particles with their mass center (gold cluster)
positioned at different levels along the c direction. Bottom: The
smectic structure can be obtained from the columnar phase through
movement of the particles from the intermediate layer along the
column axis and decoupling of the layers.
Figure 1. Left: X-ray pattern of an oriented sample of Au@1 in the
smectic phase. The Bragg signals result from the layer structure,
whereas the diffused signal results from the liquidlike in-plane order of
gold particles inside the layers. The plot of intensity against the q
vector was obtained by integration of the pattern along the azimuthal
angle. Right: X-ray pattern of the columnar phase of an oriented
sample of Au@2. A columnar orthorhombic body-centered lattice was
chosen for indexing of the signals (see the Supporting Information).
aligned sample (with the layer wave vector oriented parallel
to the substrate) shows a series of sharp Bragg reflections
along the longitudinal direction and a diffused signal in the
equatorial direction, and is typical for the smectic A phase.[14]
The homogenously aligned sample with monodomains of
approximately millimeter size was obtained readily by shear-
ing a small amount of the material onto a solid (mica)
substrate at a slightly elevated temperature (ca. 808C). This
technique is commonly used for the aligning of liquid crystals
and results in a perpendicular orientation of the layer normal
to the shearing direction. The thickness of the obtained
smectic layer was approximately 8 nm, and the average
interparticle distance inside the layer was approximately
3 nm. Both interlayer and inlayer periodicities were nearly
temperature-independent.
This material can also be made to undergo homeotropic
alignment (with the layer wave vector orientated perpendic-
ular to the substrate) by melting of the structure on the solid
substrate and subsequent fast cooling. For a homeotropic
sample, up to six commensurate signals with apparatus-
limited resolution were detected, which shows that the
electron-density profile across the layer is strongly nonsinu-
soidal, as expected for layers of metal clusters separated by
organic sublayers (see the Supporting Information for an
electron-density profile). An X-ray rocking experiment
showed that the layers can be aligned extremely well, with
the layer normal orientated exactly perpendicular to the
surface; the mosaicity of some samples was less than 0.18.
The much smaller thickness of the organic sublayer (ca.
6 nm) relative to the particle distance inside the layer (ca.
3 nm) implies some redistribution of the mesogenic groups
ably partially interdigitated. The liquidlike character of the
organic layer was further confirmed by the presence of a
diffused signal at approximately 0.45 nm in the X-ray
pattern.[4] The orientational disorder of molecules inside the
organic layer is consistent with nonmeasurable optical
birefringence (Dn < 0.002) of the smectic samples. The lack
of birefringence of the smectic phase described herein should
not be surprising, since the birefringence of liquid crystals
made of rodlike (or disklike) molecules results mainly from
the orientational order of mesogenic cores.[15] The smectic
phase melts reversibly at about 1108C. In the molten state, the
single diffused signal is observed at 54 ꢀ, which corresponds
to the average distance between the particles in the short-
range structure.
The type of nanoparticle lattice is altered by modification
of the organic coating. The nanoparticles with the same metal
centers but with ligand 2, which has a shorter forklike tail, and
with ligand 3, which has a 2-octyl end chain, exhibit a phase in
which the gold particles are arranged into columns. The
structure gives rise to several incommensurate X-ray signals
of a width typical for the long-range order (Figure 1). The
signal positions do not correspond to either fcc or hcp lattices,
which are commonly observed for the packing of hard
spheres. The pattern was indexed by assuming a body-
centered orthorhombic unit cell with dimensions a ꢀ 83, b
ꢀ 48, c ꢀ 67 ꢀ. The ratio between the a and b cell parameters
pffiffiffi
(a=b ꢁ 3) suggests that the arrangement of particles in the
ab crystallographic plane is very close to hexagonal. The
presence of the signal (101), which is positioned neither along
5168
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 5167 –5169