Formation of branched fractal CdS patterns in oligomer LB monolayers: A study using transmission electron microscopy

Article abstract of DOI:10.1021/jp981254y

Transmission electron microscopy images of the treelike fractal aggregates of CdS nanoparticles in amphiphilic oligomer (polymaleic acid with octadecanol ester) LB matrix were observed. The features of these aggregates were developed into the micrometer regime. By changing the ratio between carboxylic groups and hydrocarbon chains, as well as the surface pressure, different fractal CdS patterns could be obtained. These results lead us to propose the mechanism of the formation of branchlike patterns, very much analogous to two-dimensional irreversible fractal-growth models such as diffusion-limited aggregates (DLA). Weak hydrophobic and hydrophilic interaction for interfacial molecular recognition at the organic/inorganic interface had been proposed as providing a possible elucidation for the formation of some aspects of a self-similar fractal-like pattern. At a ratio of 7:1, the fractal dimension was calculated by the box-counting method, and the dominant value was 1.69 ± 0.03, which was in good agreement with two-dimensional DLA.

Full text of DOI:10.1021/jp981254y

5648
J. Phys. Chem. B 1998, 102, 5648-5652
Formation of Branched Fractal CdS Patterns in Oligomer LB Monolayers: A Study Using
Transmission Electron Microscopy
Lin Song Li,† Jian Jin,† San Yu,‡ Yingying Zhao,† Chengxiang Zhang,§ and Tie Jin Li*,†
Center for Intelligent Materials United Research (CIMUR), Department of Chemistry, State Key Laboratory for
Super Hard Materials, and Department of Physical, Jilin UniVersity, Changchun, 130023, P. R. China
ReceiVed: February 24, 1998; In Final Form: May 7, 1998
Transmission electron microscopy images of the treelike fractal aggregates of CdS nanoparticles in amphiphilic
oligomer (polymaleic acid with octadecanol ester) LB matrix were observed. The features of these aggregates
were developed into the micrometer regime. By changing the ratio between carboxylic groups and hydrocarbon
chains, as well as the surface pressure, different fractal CdS patterns could be obtained. These results lead
us to propose the mechanism of the formation of branchlike patterns, very much analogous to two-dimensional
irreversible fractal-growth models such as diffusion-limited aggregates (DLA). Weak hydrophobic and
hydrophilic interaction for interfacial molecular recognition at the organic/inorganic interface had been proposed
as providing a possible elucidation for the formation of some aspects of a self-similar fractal-like pattern. At
a ratio of 7:1, the fractal dimension was calculated by the box-counting method, and the dominant value was
1.69 ( 0.03, which was in good agreement with two-dimensional DLA.
Introduction
SCHEME 1
It has been demonstrated that amphiphilic molecules contain-
ing a hydrophobic chain and a hydrophilic headgroup can form
organized assemblies of various shapes. For example, micelles,
vesicles, bilayers, monolayers, and liquid crystalline structures
have all been observed. It has frequently been used in schemes
to prepare inorganic nanoparticles where the organic structure
serves as either a template or a reaction vessel of limited size.1,2
The Langmuir-Blodgett (LB) technique, as a kind of molecular-
scale technology for the creation of new materials, has been
the object of increasing technological and scientific interesting
over the past decades.3-5 The films deposited by the LB
technique have potential applications in the areas of molecular-
based electronics, photonics, optoelectronics, and bioelectronics.
Most of these applications rely upon the self-organization of
amphiphilic molecules to form very thin and essentially perfect
films. However, the uniform, constant thickness monolayer and
multilayer films are unstable due to bilayer step defects and
quickly spread to cover the entire film.6,7 The molecular
relaxation and reorientation in the uniform film after deposition
exactly happens on a microscopic scale.6,8,9 The driving force
for the reorganization can be attributed to the strength of the
headgroup-headgroup interface relative to the headgroup-
substrate and headgroup-water interfaces. Because the defects
originally appear random and meandering, it will affect the
macroscopic physical properties. So it will be important and
magnificent if the films still keep ordered and reordered after
reorganization.
treelike fractal aggregates. The amphiphilic oligomer’s mo-
lecular structure is shown in Scheme 1. A hydrophobic chain
was induced by chemically linking the amphiphilic molecules
through a hydrophilic space of adjustable length by octadecyl
polymaleicate (PMAO) that we synthesized.10,11 In the pre-
liminary reports, we just described the reaction of cadmium and
lead PMAO LB films with gaseous H2S to form CdS and PbS
nanoparticles within a layered PMAO matrix.11,12 Due to the
sulfide aggregation being restricted within one PMAO molecule,
PbS as well as CdS within PMAO LB films showed a blue
shift of the optical absorption edge that formed within the stearic
acid LB films. It also indicated a larger blue shift of the optical
absorption edge with the decreasing of the ratio between the
carboxylic groups and the hydrocarbon chains. In this paper
we present a complete report of the images of CdS aggregates
by transmission electron microscopy (TEM) and transmission
electron diffraction (TED). We will analyze the structural
properties of this kind of amphiphilic oligomer monolayers and
present an explanation on understanding the formation of fractal
patterns.
Here we introduce a kind of amphiphilic oligomer as a
Langmuir-Blodgett (LB) matrix used for the synthesis of CdS
nanoparticles. CdS nanoparticles are grown and reorganized
in this LB matrix and formed two-dimensional irreversible
Experimental Section
The synthesis of the PMAO has been described elsewhere.10
Stearic acid (SA) was A.R. grade and recrystallized from
ethanol, and chloroform (A.R. grade) was redistilled twice before
use. Cadmium chloride was analytical grade and was recrystal-
lized before use. Water, with a conductivity of 18 MΩ cm,
was used to prepare the subphase. The PMAOs with a
concentration of approximately 5 × 10-4 mol/L (lateral chain
* To whom correspondence should be addressed. Phone 86-431-8922331-
2463; Fax 86-431-8923907; E-mail tjli@mail.jlu.edu.cn.
† CIMUR, Department of Chemistry.
‡ State Key Laboratory for Super Hard Materials.
§ Department of Physical.
S1089-5647(98)01254-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/26/1998

Fractal CdS Patterns in LB Monolayers
J. Phys. Chem. B, Vol. 102, No. 29, 1998 5649
to observe nanostructured materials. In our previous reports,
the oligomer we introduced could be used for the ordered
synthesis and ordered assembly of the inorganic nanopar-
ticles.11,12 But little is known about the growing process of the
inorganic nanoparticles in the LB matrix. A question arise
whether the size and shape of the nanoparticles vary with the
PMAOs chemical ratios between carboxylic groups and hydro-
carbon chains of the monolayers. In particular, transition
pressures can alter molecular orientation, which thereby really
can change the morphological structure of the nanoparticles.
Here we take advantage of transmission electron microscopy
to investigate the morphological structure of nanoparticles that
were produced through the reaction of CdPMAO monolayer
with H2S. The morphologies of CdS nanoparticles are different
from that of the epitaxial growth or oriented growth of inorganic
crystals under Langmuir monolayers or on Langmuir-Blodgett
films.4,13-17 Among these, Fendler et al. reported that rodlike
CdS and equilateral triangular PbS nanocrystallites were grown
under arachidic acid monolayers, respectively.13,14
Figure 1. π-A isotherms of (a) SA, (b) PMAO1, (c) PMAO2, and
(d) PMAO3 on the surface of CdCl2 solution. The isotherms were all
recorded at 20 ( 1 °C, and the compression speed was 3 cm2/min.
Cadmium stearate (CdSt) monolayer is prepared at the dipping
speed of 10 cm/min and the surface pressure of 30 mN/m. After
being exposed to H2S, distinct particles begin to form and appear
as darker regions. It exhibits domains with less uniform shape
(Figure 2a). After being exposed to H2S over a period of 24 h,
the particles have an average size of about 2-5 nm in diameter.
These particles are close to each other and tend to form more
bigger domains (10-20 nm). That is to say, in the CdSt
monolayer matrix, the CdS particles are apt to aggregate into
large domains which are composed of several small particles.
Moreover, the size of domains increases with increasing reaction
time. When the reaction time lasted for 48 h, the bigger domains
with an average size of 30 nm and a relatively close-packed
arrangement can be observed (Figure 2b). Electron diffraction
from a 1.0 mm diameter area of the film shows the formation
of hexagonal CdS arising from the randomly oriented particles
(Figure 2c).
concentration) and stearic acid with a concentration of 1 × 10-3
mol/L dissolved in a chloroform solvent were spread on the
subphase containing CdCl2 (3 × 10-4 mol/L) + NaHCO3 (2 ×
10-4 mol/L). The pH value of the subphase was 5.8. An
appropriate amount of the spreading solutions (50 µL) was
carefully injected onto the cleaned, thermostatic, air/water
interface. After the solvent evaporated, the monolayer was
compressed at a speed of 3 cm2 min-1, and the surface pressure
was recorded versus the area at 20 °C on a LB trough with a
multicompartmental round trough from Mayer-Fein technic
(Germany). The monolayers were built up on the copper grid
substrates (200 mesh) by the vertical dipping method under a
constant pressure. The prepared samples were examined in the
bright-field mode and by electron diffraction using a transmis-
sion electron microscope operating at 200 kV (H-8100IV
transmission electron microscope, Japan). The deposition speed
was well controlled at 10 cm/min. The surface pressure for
transfer was chosen by the detailed experimental condition. The
transfer ratios were nearly 1.0. In a saturated water steam
system (20 °C), monolayers were reacted with H2S, which was
produced from the hydrolysis of thioacetamide. After reacting
with H2S, CdS nanoparticles were grown and reorganized in
the LB matrix and formed a two-dimensional irreversible
morphological multibifurcation. The observed growth patterns
for different PMAO LB templates could be reproduced and kept
for a long time.
The CdPMAO monolayers with a ratio of 3:1 between
carboxylate groups and hydrocarbon chains are transferred in
LC region at the surface pressure of π ) 30 mN/m. After the
CdPMAO LB film was exposed to H2S, its TEM images display
a branchlike morphology up to several micrometers (Figure
3a,b). The morphology of this sample is different from that of
the domain structure of CdS/St LB films. The observed particles
have an average size of about 15 nm with well-defined edges.
Electron diffraction of the film can also be assigned to hexagonal
CdS arising from a randomly oriented particles (Figure 3c). The
intriguing observation is that the morphology depends strongly
on the ratios between carboxylic groups and hydrocarbon chains.
When the CdPMAO with a ratio of 5:1 monolayer was exposed
to H2S, its image is changed. It shows a short branchlike
morphology range with the length from 500 nm up to 2 µm.
The isolated large particles possess featureless morphologies
with a length of 100-250 nm and width of 10-40 nm (image
not shown here). When the surface pressure decreases from
30 to 15 mN/m, the morphology is changed to a less well-
defined shape. The average size of the particles is about tens
of nanometers. The TED pattern is the same as that of CdS/
PAMO with a ratio of 3:1.
Results and Discussion
Monolayers at the Air/Water Interface. Figure 1 shows
surface pressure-surface area (π-A) isotherms for the samples
of PMAOs with three different ratios between carboxylic groups
and hydrocarbon chains (3:1, 5:1, and 7:1) and stearic acid,
respectively. From the isotherm, it can be seen that the area
per molecule increase with increasing ratios between carboxylic
groups and hydrocarbon chains. Monolayers, compressed at a
controlled rate up to the surface pressure with 15 and 30 mN/
m, respectively, are transferred onto electron microscope copper
grids sandwiched between a Formvar film and a glass slide
under two different phase regions. Two different regions
corresponding to different phases can be distinguished in the
π-A curves: (i) a liquid-expanded (LE) phase and (ii) a liquid-
condensed (LC) phase.
As the ratio increased to 7:1, multibifurcate morphology
becomes visible and develops into the scale of micrometers
(Figure 4a, b). The observed particles range in size from 10 to
30 nm. When the surface pressure decreases from 30 to 15
mN/m, we cannot obtain the clear fine bifurcation structures
again. Meanwhile, the isolated particles ranging from 100 to
Morphological Structures Analysis. Transmission electron
microscopy (TEM), as a direct imaging method, is widely used

5650 J. Phys. Chem. B, Vol. 102, No. 29, 1998
Li et al.
Figure 3. TEM of PMAO/CdS LB monolayer (X:Y ) 3): (a) 10 000×
magnification; (b) 80 000× magnification; (c) 200 kV TED pattern
from a 1 µm diameter area in (a). π )30 mN/m, dipping speed ) 10
cm/min, reaction time lasted 48 h, and scale bars ) 100 nm.
300 nm in size can be obviously observed (Figure 5a,b). The
particles are formed with less well-defined edges. This indicates
that the fine bifurcations formed at the higher surface pressure
involve the fractal growth processes; i.e., it is possible to form
bifurcations or branchlike patterns easily at higher surface
pressure. Electron diffraction patterns of selected areas are
Figure 2. TEM image of CdS particles formed in a cadmium stearate
LB monolayer after reaction with H2S: (a) reaction time lasted 24 h;
(b) reaction time lasted 48 h; (c) 200 kV TED pattern from a 1 µm
diameter area in (b). π ) 30 mN/m, dipping speed ) 10 cm/min, and
scale bars ) 100 nm.

Fractal CdS Patterns in LB Monolayers
J. Phys. Chem. B, Vol. 102, No. 29, 1998 5651
Figure 5. TEM of PMAO/CdS LB monolayer (X:Y ) 7): (a) 10 000×
magnification; (b) 80 000× magnification. π )15 mN/m, dipping speed
) 10 cm/min, reaction time lasted 48 h, and scale bars ) 100 nm.
The above results of morphological multibifurcations lead
us to propose a mechanism for the formation of branchlike
patterns, very much analogous to two-dimensional irreversible
fractal-growth models such as diffusion-limited aggregates
(DLA). In this model, the formation of a stable aggregation
nucleus is required, which can then exhibit a one-dimensional
growth, so as to optimize the lateral nucleus-nucleus con-
tacts.18,19 In our work, the fractal dimensions are calculated
by dividing the patterns into a concentric box with various
lengths and counting the occupied and calculated by the box-
counting method. At the ratio of 7:1, the dominant value is
1.69 ( 0.03, which is in good agreement with two-dimensional
DLA.20
Fractal geometry, which repeats on smaller and smaller scales,
can provide a new perspective on nature and allows us to
consider irregularities as intrinsic entities. The property of self-
similarity implies irregularities at all scales.18,19 This model is
applied to elucidate the formation of bifurcations of CdS
nanoparticles in the PMAO matrixes. There are several steps:
(1) These kinds of oligomers interact with each other at the
air/water interface. Because the adjacent -COOH groups
within one oligomer molecules are only separated by no more
than 0.3 nm and the distance of each individual oligomer
molecule separation exceeds 0.5 nm, the hydrophilic part of
Figure 4. TEM of PMAO/CdS LB monolayer (X:Y ) 7): (a) 10 000×
magnification; (b) 80 000× magnification; (c) 200 kV TED pattern
from a 1 µm diameter area in (a). π )30 mN/m, dipping speed ) 10
cm/min, reaction time lasted 48 h, and scale bars ) 100 nm.
taken using a 1 µm aperture of the area as the micrograph for
Figure 4a. The result also shows the formation of a hexagonal
crystal. However, the corresponding electron diffraction pattern
has more dispersed spots to form an electron diffraction ring
(Figure 4c), indicating a disordered nanocrystallite alignment.

5652 J. Phys. Chem. B, Vol. 102, No. 29, 1998
Li et al.
each molecule of oligomer that aggregates by forming the
sideways hydrogen bonding should be limited within itself. The
size of aggregates should be in the nanometer range because
the average degree of the polymerization is approximately 20.12
And it is possible to make every PMAO molecule form a stable
aggregation nucleus at the air/water interface.
understanding the relationship between morphological structures
and the formation of fractal patterns.
These kinds of amphiphilic oligomers could be used to build
different fractal patterns directly through changing the ratio
between carboxylic groups and hydrocarbon chains, the degree
of oligomer’s polymerization, and the surface pressure. The
wide variety of possible patterns presents an opportunity for
future molecular design of synthesis of organized organic/
inorganic heterostructured materials.
(2) The orientation of alkyl side chains at the air/water
interface is a function of the surface pressure, indicating that a
disorder orientation at lower surface pressure changes to an order
orientation at higher surface pressure. The tails of alkyl chains
tilt in the LE state. As the monolayer enters the LC state, the
tilt angle tends to decrease and the alkyl chains are more upright.
This will affect the weak hydrophobic and hydrophilic interac-
tion forces between the aggregates. And it will also affect the
formation of the fractal CdS nanoparticles in the next step.
(3) The molecular relaxation and reorientation in the uniform
monolayer happen during the formation of CdS nanoparticles
in a saturated water steam system. In fact, the homogeneous
and uniformly constant thickness monolayer and multilayer films
are unstable to bilayer step defects and quickly spread to cover
the entire film (a form of “dewetting”).6 Due to the sulfide
aggregate being restricted within one PMAO molecule,11,12 the
formation process of nanoparticulate CdS two-dimensional
domains should be governed by the diffusion of oligomeric
PMAO molecules and their sticking to a cluster. After reacting
with H2S, CdS nanoparticles are produced and reorganized in
the LB matrix and form a two-dimensional irreversible mor-
phological multibifurcation. That is, the uniformly thick LB
film departs from the substrate and reorganizes to form an
orderly structure. So it effectively weakens the CdS crystalline
anisotropy. The total processes lead to an irregular fractal-like
crystal growth of CdS nanoparticles. So the morphologies of
CdS nanoparticles are different from the reports about the rodlike
CdS and equilateral triangular PbS nanocrystallites grown under
arachidic acid Langmuir monolayers.13,14 Moreover, the same
bifurcations or branchlike patterns still can be observed after
half a year. It shows that the films keep relatively stable after
the fractal-like growth structure is reorganized.
Acknowledgment. The authors acknowledge the National
Climbing Program and the National Natural Science Foundation
of China (NNSFC) for providing financial support. Authors also
thank Dr. Yanjing Liu for proofreading the manuscript and Dr.
L. F. Chi for helpful discussions.
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An amphiphilic oligomer as LB matrix used in the synthesis
of CdS nanoparticles was introduced. CdS nanoparticles were
grown and reorganized in this LB matrix and formed a two-
dimensional irreversible morphological multibifurcation. We
analyzed the structural images of CdS nanoparticles in the
oligomer matrixes by TEM and proposed an explanation on