Formation of PbSe Nanocrystals: A Growth toward Nanocubes
Weigang Lu and Jiye Fang*
Department of Chemistry and AdVanced Materials Research Institute, UniVersity of New Orleans,
New Orleans, Louisiana 70148
Yong Ding and Zhong Lin Wang*
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
ReceiVed: May 16, 2005; In Final Form: August 2, 2005
In this paper we report an electron microscopic observation of crystal shape development when PbSe
nanocrystals were synthesized using a dynamic injection technique at different temperatures in the presence
of oleic acid. A two-step evolution mechanism was proposed, indicating that the shape evolution of PbSe
nanocrystals is dependent on the growth time, whereas the crystalline size can be tuned by varying the growth
temperature under the studied conditions. It also implies that a higher growth rate in the 〈111〉 direction
compared to that in the 〈100〉 direction results in the formation of nanocubes.
Lead chalcogenides are inspiring semiconductors with a
narrow band gap. Size- and shape-controlled nanocrystals (NCs)
of this family have demonstrated unique properties1-5and can
potentially be employed in numerous applications, such as near-
IR luminescence6and thermoelectric devices.7,8To produce
monodisperse NCs of lead chalcogenides with high quality and
tunable size and shape, it is significant to understand their NC
formation process. Since the growth of PbS9,10and PbTe NCs11
has been investigated previously, similar exploration on PbSe
NCs should be conducted. Although the relationship between
band gap and PbSe NC shape has been studied by Pietryga et
al.5and the development of PbSe nanowires and nanorings has
been achieved by Cho et al.1as well, efforts on observation of
crystal shape development and understanding the possible
mechanism of PbSe crystal evolution growth are still essential.
In this paper, we concentrate on the time-dependent formation
study of PbSe NCs based on our electron microscopic observa-
tion when a dynamic injection technique is applied to the
PbSe NCs were prepared by rapidly injecting a phenyl ether
solution of lead acetate and trioctylphosphine selenide ((TOP)-
Se) into a vigorously stirred hot phenyl ether at 180 or 230 °C,
in the presence of oleic acid.12In a typical experiment, Pb-
(Ac)2‚3H2O (2.85 mmol), phenyl ether (15 mL), and oleic acid
(3.0 mL) were mixed and heated to 150 °C for 30 min under
an argon stream in a three-neck flask equipped with a condenser.
After the solution was cooled to 40 °C, it was mixed with 4.0
mL of pre-prepared (TOP)Se solution (1 M for Se) in a
glovebox. This mixture (10 mL) as a stock reactant was then
rapidly injected into 15 mL of phenyl ether that was preheated
to a certain temperature (180 or 230 °C) for 5 min with agitation
in a similar device. In both cases, the temperature of the reaction
system was lowered by ∼20-25 °C and then rapidly reached
the desired value. A portion of the hot reaction mixture (2.0
mL) was extracted from the flask after a certain time of growth,
and an equal volume of the premixed stock reactants was
subsequently added into the flask under the argon stream. These
operations of dynamic injections were successively conducted
at an interval of every 5 min at a constant temperature. Each
product was retrieved from the original solvent by centrifugation
with an equal volume of ethanol as a polar solvent, redispersion
in toluene, and evaluation under transmission electron micros-
copy (TEM) observation. Products were identified by TEM
(JEOL 2010 TEM and Hitachi 2000 FE-TEM (FE ) field
emission) instruments) and by field emission scanning electron
microscopy (SEM) (LEO 1530 FE-SEM instrument, operated
at 5 and 10 keV). The phase of the nanocrystals was determined
using an X-ray diffractometer (Philips X’pert system).
Upon injection, small PbSe clusters immediately nucleated
and started to grow with stabilization provided by the capping
ligands (TOP and oleic acid). We monitored the crystalline
growth process by collecting small portions of the reacting
solution from time to time for electron microscopic analyses.
According to the general growth mechanism proposed in the
La Mer model,13many systems exhibit an Ostwald ripening
process. As a result, small clusters tend to grow into larger NCs
to lower their surface energy.14-16Figure 1A,C demonstrates
the images of 5-s-growth clusters based on bright-field and dark-
field TEM determinations, respectively, revealing that small
PbSe NCs (∼3-5 nm in size) tend to aggregate in the shape of
an octahedron at a reaction temperature of 230 °C. The TEM
diffraction pattern (Figure 1B) also indicates that these small
NCs possess a cubic bulk rock-salt crystal structure with high
crystallinity. The high-resolution TEM (HR-TEM) image further
implies that most of the agglomerated particles have been
coupled with each other (Figure 2), showing that the lattice
planes of the depicted particles were mostly aligned. Moreover,
it can be recognized that the lattice planes go straight through
the contact areas, where the particles were epitaxially fused
together, which is the foundation for agglomerating into a single
larger size crystal.17,18On the basis of Murray’s report,8PbSe
NCs are quite easy to nucleate and grow even at a temperature
of lower than 100 °C. The higher reaction temperature we
applied would accelerate the chemical reaction and produce a
burst of nucleation, which arouses the octahedral agglomerates.
According to our TEM observation presented in Figure 3A,B,
most of the agglomerates can be developed into discrete
* To whom correspondence should be addressed. E-mail: email@example.com
(J.F.); firstname.lastname@example.org (Z.L.W.).
J. Phys. Chem. B 2005, 109, 19219-19222
10.1021/jp052573+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/21/2005
hexagonal crystalline stars when the reaction time elapses for
20 s at 230 °C, although transitive morphology can still be
detected. It is noted that the image corner of a single crystalline
star (2D projection) is sharper than that containing small crystal
agglomeration. As shown in Figure 1D, extension of the growth
time to ∼2-5 min at 230 °C results in a further development
Figure 1. (A, C, B) TEM images and diffraction pattern of 5-s-growth PbSe clusters at 230 °C. (D, F, E) TEM images and diffraction pattern of
a selected single PbSe octahedron grown for 2-5 min at 230 °C. (G, H) TEM image and diffraction pattern of PbSe nanocubes after five injections
within a total aging time of ∼25 min at a growth temperature of 230 °C; (I, K, L) TEM images of a size-selected PbSe nanocube assembly and
HR-TEM image of a single PbSe nanocube (the sample was grown through the same procedure at 180 °C for ∼25 min). (J) A scheme illustrating
the formation process of PbSe nanocubes; (A), (D), (G), (K), and (L) are bright-field images, whereas (C), (F), and (I) are dark-field images.
19220 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Lu et al.
of these PbSe stars, with a ∼100 ( 10 nm diagonal. We realize
that this profile star image was actually recorded from a
projected shape of an octahedral crystal if viewing along 
(see Figure 1J).9,19A dark-field TEM image (2D projection) of
a single perfect PbSe star clearly exhibits the structural
information of an octahedral NC along the  direction
(Figure 1F). The electron diffraction pattern (Figure 1E)
recorded on this selected NC verifies the zone axis of 
and displays that the whole NC is a single crystal. The XRD
trace of the PbSe octahedrons is presented in Figure 4 as well,
in which all of the detectable peaks are indexed to almost the
same positions as those from a standard bulk of PbSe (JCPDS-
ICDD card 06-0354). In addition, the observation of the
octahedral PbSe NCs was further supported by a collection of
dark-field TEM images of the NC prepared at 180 °C at different
tilt angles as indicated in Figure 3C,D. The 3D shape and the
six projected corners of the octahedron are clearly presented
by the dark-field images in Figure 3C,D, which also indicate
the single-crystal nature of each octahedron. The HR-TEM
image recorded from a corner of an octahedron apparently shows
its high crystallinity and sharp surfaces (Figure 3E). Further-
more, the low-magnification SEM image (Figure 5A) also
implies that PbSe octahedrons, as intermediate NCs produced
at 180 °C, are uniform and high in yield.
To further develop the NCs into nanocubes, it is necessary
to successively introduce additional reaction precursors for
compensating the consumption of feedstock by the growing
colloidal octahedrons16and to make the system more “open”.
In this work, we conducted a dynamic injection technique that
was employed previously.12,20The advantage of this dynamic
multi-injection is that primary precursors can be replenished
so that enough primary PbSe clusters are provided to keep a
constant rate of the crystal growth. As a result, cubic NCs as
the major component could be detected in the final production
after five injections within a total aging time of ∼25 min at a
growth temperature of 230 °C. Figure 1G is a bright-field TEM
image of these NCs, indicating that a cubic shape dominates
the morphology of the as-produced NCs. The pattern of electron
diffraction (Figure 1H) from restricted nanocubes on a TEM
grid exhibits high crystallinity. Similar to the discussion of PbTe
Figure 2. High-resolution TEM image, showing lattice planes of small
particles restricted in the agglomerates. The sample was grown at 230
°C for 5 s.
Figure 3. (A) TEM image of PbSe aggregates being grown at 230 °C
for 20 s. (B) Diffraction pattern of the sample grown at 230 °C for 20
s. (C, D) Dark-field TEM images of single octahedral PbSe NCs at
different tilt angles. (E) High-resolution TEM image recorded from a
corner of such an octahedron. From (C) to (E), the sample was
processed at 180 °C for 2 min.
Figure 4. XRD traces of self-assembled PbSe NCs on a Si surface:
(A) octahedrons; (B) nanocubes. Both samples were processed at 230
Formation of PbSe Nanocrystals
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19221
NC shape evolution,11the crystal morphology of PbSe NCs is
dependent on the growth ratio between the 〈100〉 and 〈111〉
directions. The driving force of the evolution from octahedrons
(which are presented as stars in a TEM image) to cubes may
lie in the fact that the growth rate in the 〈111〉 direction is higher
than that in the 〈100〉 direction,18although this type of growth
results from various competitive factors such as crystalline size,
growth temperature, the energy contributions from different parts
of a crystal (such as the interfaces, edges, and corners), and the
kinetics.9,21Cheon et al. once studied the evolution of PbS NCs,
concluding a similar insight into growing PbS nanocubes as
well.9In comparison with this observation, we have also
successfully prepared PbSe nanocubes at a low growth tem-
perature, 180 °C, using the same procedure of dynamic injection
and growth time. SEM images of the nanocubes before size-
selection treatment are presented in Figure 5B,C, revealing that
the cubic NCs softly aggregated group-by-group for possibly
minimizing their surface energy. The bright-field and dark-field
TEM images of a size-selected PbSe nanocube assembly,
presented in parts K and I of Figure 1, respectively, demonstrate
perfect orientation order in 2D self-assembly. The HR-TEM
image (Figure 1L) illustrates the cubic lattice structure and its
high crystallinity as well. These observations indicate that a
decrease of the growth temperature from 230 to 180 °C results
in a reduction of the crystalline size by as much as half.
However, a cube is still the dominant shape of these nanocrystals
as long as the total aging time is ∼25 min. We further examined
this sample grown at 180 °C using a powder XRD diffraction
technique, demonstrating that the intensity of diffraction peak
(200) was relatively enhanced very much (Figure 4B). This
observation implies the presence of nanocubes perfectly as-
sembled on the surface of the Si substrate,11,22because only
the (200) facets have a chance to be diffracted in this case.
In summary, we have monitored the crystal shape develop-
ment of PbSe NCs in high-temperature organic solution by using
a growth technique of dynamic injection, and determined a two-
step evolution mechanism: from nucleation to a single-
crystalline octahedron (nanostar) and from a hexagonal nanostar
to a nanocube. Basically, the shape evolution of PbSe NCs is
dependent on the growth time, whereas the crystalline size can
be tuned by varying the growth temperature under the described
conditions. We also realize that the higher growth rate in the
〈111〉 direction compared to that in the 〈100〉 direction results
in the formation of nanocubes. Understanding the crystal-
lographic shape evolution and predicting the final architectures
of lead chalcogenide NCs would favor the controlled synthesis
of nano building blocks as well as the investigation of relevant
unique properties of these NCs.
Acknowledgment. This work was supported by the NSF
CAREER program (Grant DMR-0449580), NSF NIRT program
(Grant ECS-0210332), DARPA program (Grant HR011-05-1-
0031), and BSST LLC.
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Figure 5. (A) SEM image of uniform PbSe NCs produced at 180 °C,
confirming the existence of octahedrons as transitive intermediates. (B)
SEM image of PbSe nanocubes synthesized at 180 °C (the sample was
observed before size-selection treatment).
19222 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Lu et al.