Steric-hindrance-driven shape transition in PbS quantum dots: Understanding size-dependent stability

Article abstract of DOI:10.1021/ja400948t

Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (D) down to 1.5 nm, and found an abrupt transition at D ≈ 4 nm in the a

Full text of DOI:10.1021/ja400948t

Communication
pubs.acs.org/JACS
Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots:
Understanding Size-Dependent Stability
Hyekyoung Choi,†,‡,⊥ Jae-Hyeon Ko, Yong-Hyun Kim,* and Sohee Jeong*
§,⊥
,§
,†,‡
†Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea
‡Department of Nanomechatronics, University of Science and Technology, Daejeon 305-350, Republic of Korea
§Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Republic of Korea
*
S Supporting Information
In this Communcation, we report the synthesis of air-stable,
ultrasmall PbS QDs and propose an oleate-passivated Pb-rich
ABSTRACT: Ambient stability of colloidal nanocrystal
quantum dots (QDs) is imperative for low-cost, high-
efficiency QD photovoltaics. We synthesized air-stable,
ultrasmall PbS QDs with diameter (D) down to 1.5 nm,
and found an abrupt transition at D ≈ 4 nm in the air
stability as the QD size was varied from 1.5 to 7.5 nm. X-
ray photoemission spectroscopy measurements and
density functional theory calculations reveal that the
stability transition is closely associated with the shape
transition of oleate-capped QDs from octahedron to
cuboctahedron, driven by steric hindrance and thus size-
dependent surface energy of oleate-passivated Pb-rich QD
facets. This microscopic understanding of the surface
chemistry on ultrasmall QDs, up to a few nanometers,
should be very useful for precisely and accurately
controlling physicochemical properties of colloidal QDs
such as doping polarity, carrier mobility, air stability, and
hot-carrier dynamics for solar cell applications.
(
111)-only surface chemistry of octahedral PbS QDs on the basis
of X-ray photoemission spectroscopy (XPS) measurements and
density functional theory (DFT) calculations. We have found
that the air stability of QDs undergoes a sharp transition when
the QD size is ∼4 nm. The measured Pb/S ratio and the
calculated surface energy of the oleate-passivated Pb-rich (111)
surface indicate that a shape transition from (111)-only
octahedron to (111)/(100) cuboctahedron should occur as
QD size increases, driven by the increased steric hindrance
among the capping groups for large QDs. The size-dependent air
stability is thus attributed to the (111)-to-(100) transition of QD
facets, i.e., from the air-stable ligand-passivated (111) facets to
the bare self-passivated (100) facets that are prone to surface
oxidation in ambient conditions.
The band gap of PbS QDs can be tuned by changing the
nanocrystal size. A synthetic route for creating size-tuned PbS
QDs in the diameter (D) range of 2.6−7.2 nm, corresponding to
absorption peak (1S ) of 825−1750 nm and band gap of 0.7−
max
6
1
.5 eV, was well established and widely adapted for recent
ead chalcogenide (PbX, where X = S, Se) quantum dots
(QDs) have been extensively investigated lately with a hope
photovoltaic device fabrications. By reducing reactive Pb
L
precursors, one could obtain PbS QDs as small as D = 2.6
for realizing next-generation (low-cost, high-efficiency) photo-
6a
nm. Smaller QDs <2.6 nm could provide larger band gaps,
1
voltaics. In addition to their abundance, the large exciton Bohr
offering improved open-circuit voltage in QD-based Schottky
radii of PbX could provide strong quantum confinement effects
for colloidal QDs. This band gap tunability from infrared to
ultraviolet could enable a multi-junction solar cell absorbing the
entire solar spectrum. Furthermore, successful utilization of high-
energy photons above band gaps could promise a single-junction
solar energy conversion efficiency over the Shockley−Queisser
7
8
devices or QD-based quantum junction devices. Synthesis of
ultrasmall PbS QDs <2.6 nm is not well established and is very
challenging because of the highly reactive surfaces of ultrasmall
QDs, which typically grow fast. In this work, we synthesized PbS
QDs as small as 1.5 nm with an absorption peak of 480 nm, which
2
are the smallest PbS QDs ever reported. Briefly, Pb(oleate) in
2
limit, 33%. Despite these advantages as light-absorbing materials
octadecene was reacted with the S precursor, bis(trimethylsilyl)-
sulfide, at 90 °C and cooled rapidly with hexane and ice-bath to
stop fast crystal growth. Size-controlled QDs were then grown
under relatively low growth temperatures of 5−50 °C (see SI for
details). Unreacted precursors were removed by precipitation,
and PbS QDs were redispersed in hexane or tetrachloroethylene
for further characterization.
Figure 1A shows the absorption and photoluminescence (PL)
spectra of ultrasmall PbS QDs, of which PL peaks at 680 nm with
a Stokes shift of 200 nm. A high-resolution transmission electron
microscopy (TEM) image is shown in Figure 1B. Based on the
for next-generation solar cells, a working QD-based device has
long been a challenge because of uncontrolled surface defects and
thus degradation of QDs during post-processing for device
fabrication and, more seriously, during solar cell operation.
Recent advances in surface passivation engineering resulted in a
3
noticeable photoconversion efficiency of >7%. Urgently
demanded is a breakthrough in improving material and device
stability in atmospheric ambient condition. It is known that, in
4
air, PbSe QDs undergo critical surface oxidation, but PbS QDs
5
are somewhat resistant to surface corrosion. However, even
basic microscopic information about colloidal PbX QDs such as
surface geometry, ligand passivation, and nanocrystal shape is
vastly lacking, and thus the air stability issue is not yet totally
solved.
Received: January 28, 2013
Published: March 15, 2013
©
2013 American Chemical Society
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Journal of the American Chemical Society
Communication
Figure 2. (A) High-resolution Pb 4f XPS spectra of PbS QDs with D =
1
.5−7.5 nm. (B) Diameter-dependent Pb/S atomic ratio of PbS QDs
obtained from ICP-AES and XPS.
When synthesized from PbCl , PbS QDs with an absorption peak
2
at 1590 nm showed remarkable air stability compared to similar-
size oleate-capped PbS QDs, probably because of additional
5
passivation by chlorides. The air stability of ultrasmall, oleate-
capped PbS QDs and the stability transition at ∼4 nm, uniquely
observed in our experiment, can thus be important missing links
for understanding the surface chemistry of PbS QDs.
Figure 1. (A) Absorption and photoluminescence spectra of ultrasmall
PbS QDs dispersed in tetrachloroethylene. (B) TEM image of the
ultrasmall PbS QDs with 1Smax = 480 nm. (Inset) High-resolution TEM
image of individual ultrasmall QDs showing D ≈ 1.5 nm, and a
photograph of a colloidal solution of ultrasmall QDs showing its daylight
color is red. (C) Absorption spectra of ultrasmall and large QDs just
after synthesis and after 12 weeks of being stored in ambient
atmosphere, showing a blue peak shift. (D) Peak shift depending on
QD size, showing a transition at ∼4 nm.
XPS is a viable tool for chemical analysis of nanostructured
materials and has been widely used for chemical state
10
identification of PbX QDs. To understand the size-dependent
ambient stability, we performed XPS studies of air-free PbS QD
films with diameters ranging from 1.5 to 7.5 nm (see SI, Figure 2,
Figures S5 and S6, and Tables S2 and S3 for details). XPS spectra
in the range 130−150 eV are dominated by two features from Pb
4
f
and Pb 4f , as indicated in Figure 2A. As the QD size
/2 7/2
5
image (inset), the QD size is estimated to be 1.5 nm. Because
ultrasmall QDs absorb green and blue light, the QD-dispersed
solution was red (Figure 1B inset). We further tuned the size of
PbS QDs and obtained the absorption spectra with the excitonic
transition (1Smax) varying from 480 to 1640 nm (see SI for
detailed synthesis and Figures S1 and S2).
decreases, the Pb 4f core levels shift toward high binding energy,
representing that Pb atoms in smaller QDs are more oxidized, or
positively ionized on average. In colloidal PbS QDs, there could
exist two kinds of Pb2+ chemical states: one is the bulk Pb
2+
2−
cation surrounded mostly with S anions, and the other is the
surface Pb2+ cation surrounded partly by COO anions from
oleate. By deconvoluting the measured XPS spectra with two
chemical states of Pb−S (bulk) and Pb−O (bulk), we can
−
Size-dependent air stability of PbS QDs was quantified by
measuring the absorption spectra of QD-dispersed solutions
stored in atmospheric ambient conditions for 12 weeks (see
Figure 1C, Figure S3, and Table S1). The absorption edge was
characteristically blue-shifted after 12 weeks. Large PbS QDs
with D = 6.1 nm showed a 94 nm blue shift of 1Smax. At the other
extreme, ultrasmall QDs did not show any noticeable peak shift,
indicating that they are very stable in air. The 1Smax peak shift was
plotted as a function of QD diameter in Figure 1D. From the
analysis, one can clearly notice a transition at ∼4 nm, indicating
that the air stability of PbS QDs is very size-depedent and
critically varied at ∼4 nm. The size-dependent air stability was
also confirmed by monitoring the PL intensity (see Figure S4),
estimate the Pb
/Pb ratios (Table S3) as 4.7 for ultrasmall
surface
bulk
QDs and 0.6 for large-size QDs.
More accurate chemical analysis of PbS QDs can be done by
estimating the Pb/S atomic ratio from integration of Pb 4f and S
2s XPS peaks (Table S2). Estimated atomic ratios confirm that
PbS QDs are all Pb-rich regardless of size. Noticeably, the
ultrasmall QDs have a Pb/S ratio close to 3, and the ratio quickly
drops as QD size increases. We also performed elemental analysis
using inductively coupled plasma atomic emission spectrometry
(ICP-AES), showing a good correlation with the XPS result
(Figure 2B).
9
which is a sensitive measure of surface degradation.
It has been proposed that PbS QDs in the rock-salt structure
can be faceted with the nonpolar (100) and polar (111)
The blue shifts of absorption peaks of PbS QDs can be
attributed to surface oxidation under aerobic conditions. The
poor air stability of PbSe QDs has been discussed in terms of
surface oxidation. It was proposed that PbSe QDs undergo
11
surfaces, depending on QD size, capping ligand, and coverage.
Our XPS elemental analysis revealed that, for the ultrasmall PbS
QDs, Pb2+ is in the most ionized state supported by the highest
XPS binding energy, and the Pb/S ratio approaches 3. The
extreme Pb-rich condition indicates that the ultrasmall PbS QDs
must be mostly covered with the Pb-terminated (111) surface. In
our synthetis conditions, Pb and S were supplied in the forms of
Pb(oleate)2 and bis(trimethylsilyl)sulfide. Because the bis-
(trimethylsilyl) compound is highly volatile, leaving S behind,
oleate-passivated Pb-rich (111) surfaces should dominate during
the synthesis. The Pb atom in the Pb-terminated, unpassivated
(111) surface is half-coordinated (three neighboring S atoms),
compared to the bulk rock-salt form (six neighboring S atoms),
structural transformations in air by forming PbO, SeO , and
2
PbSeO at the surface within a few hours. Surface oxidation leads
3
to uncontrolled size reduction (blue shift) and defect
distribution, thereby deteriorating optical and electrical proper-
4
ties of the QDs. PbS QDs have been known to be more stable in
air than PbSe, because S is more electronegative than Se. Also, it
is known that the size-dependent air stability of PbS is very
10
subjective to synthetic conditions. For example, oleate-capped,
rather large PbS QDs with absorption peaks at 930 and 1500 nm
showed noticeable degradation in air, but with rather high air
5b
+
2+
stability for the smaller size, consistent with our observation.
thus being in the Pb state. To be in the Pb state and thus in
5
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Journal of the American Chemical Society
Communication
observed in the electronic DOS with band gaps, as clearly seen in
Figure S7c−f. Perfect passivation is guaranteed because every
surface Pb in the Pb-rich L(111) surface is coordinated with one
ligand molecule L. The calculated surface energy of L(111)
2
ranges from −0.29 to 1.92 J/m , depending on the size and type
of ligand molecules (Table 1). Remarkably, the F(111) and
A(111) surfaces have much lower surface energies than the (100)
has. This means that formate- and acetate-capped PbS
11c
nanocrystals, according to the Wulff construction, will favor
the octahedral shape consisting of all L(111). On the other hand,
the surface energies of nonanoate- and oleate-capped (111) are
greater than that of (100), even if reactive surface states are
completely passivated (Figure S7). The high surface energy
could thus be attributed to the steric hindrance among ligand
molecules rather tightly jammed on the L(111) surface. The cis-
oleate generates the strongest ligand−ligand repulsion or steric
Figure 3. Ball-and-stick models of Pb-rich cis-O(111)-terminated (A)
surface and (B) ultrasmall QDs (gray, Pb; yellow, S; red, O; cyan, C;
white, H). (C) Calculated electronic DOSs of (100), (111), cis-O(111),
and cis-O QDs near the Fermi energy (E = 0).
2
Table 1. DFT Surface Energy (γ, J/m ) of Various PbS Surface
a
Systems (See Figure S7)
hindrance between molecules, resulting in a surface energy of
trans-
O(111)
cis-
O(111)
2
1
.92 J/m . Therefore, steric hindrance can determine the shape
(100) (111) F(111) A(111) N(111)
of nanocrystal QDs.
γ
0.22 1.04 −0.29 −0.05 0.8
1.12
1.92
The extreme Pb/S ratio (∼3) of ultrasmall PbS QDs can only
be explained with a nanocrystal QD model entirely covered with
the cis-O(111), but this idea, at first glance, contradicts the
highest calculated surface energy of cis-O(111). We postulate
that the surface energy of cis-O(111) is dependent on the size of
the QDs because the steric hindrance can be mitigated at
nanoscale QD geometry. Indeed, our direct DFT calculations of
eq 1 show that formate-, acetate-, and cis-oleate-capped
ultrasmall octahedral QDs, consisting of 19 Pb atoms, 6 S
atoms, and 26 ligand molecules (up to 1403 atoms), have similar
a
(
100) and (111) represent bare stoichiometric surfaces. L(111)
represents L-capped Pb-rich (111) surface passivated with formate
(
(
F), acetate (A), nonanoate (N), trans-oleate (trans-O), or cis-oleate
cis-O).
perfect passivation, one more electron of Pb should be
coordinated with a monovalent anion such as oleate. So, every
exposed Pb ion in the flat (111) surface should be exactly
coordinated with one anionic ligand for perfect passivation, as
named L(111) hereafter.
2
surface energies of −0.19, −0.18, and −0.13 J/m , respectively,
When forming nanocrystal PbS QDs, the surface energy of the
oleate-capped, Pb-rich (111) surface should compete with the
surface energy of the nonpolar, self-passivated (100) surface. To
rather independent of the ligand size. This is clearly in contrast to
the surface energies of the flat surface models. The ultrasmall L-
QD models are all semiconducting, with a band gap of 2.2 eV, as
shown in Figures 3 and S7. DFT-optimized geometries, shown in
Figure 3A,B, clearly indicate that the steric hindrance of the flat
cis-O(111) surface is significantly relaxed on the curved
nanostructured cis-O QD.
Therefore, Pb-rich ultrasmall PbS QDs should be octahedral
and covered with cis-O(111). As QD size increases, the surface
energy of the cis-oleate-capped octahedral QDs increases,
converging to the value of the flat cis-O(111) surface. When
the cis-O(111) surface energy of QDs is comparable to the (100)
surface energy for a certain size of QDs, the (100) facet should
appear, truncating the octahedron and thus leading to a shape
transition to cuboctahedron. The appearance of the stoichio-
metric (100) surface is another source of the Pb/S ratio
reduction for large QDs, in addition to the reduced surface-to-
bulk ratio.
12
see the competition mechanics, we calculated, based on the
DFT formulation, surface energies of (100), (111), and Pb-rich
ligand-capped L(111) passivated with formate (F), acetate (A),
nonanoate (N), trans-oleate (trans-O), and cis-oleate (cis-O). We
also calculated surface energies of formate-, acetate-, and cis-
oleate-capped ultrasmall PbS QDs (see Figures 3 and S7). The
13
theoretical surface energy γ was defined as
γ = [E(nPbS/mPbL ) − nE(PbS) − mE(PbL )]/A
(1)
2
2
where E is DFT total energy, A is surface area, n and m represent
respectively the numbers of PbS and PbL units in the system,
2
and L represents the capping ligand. The total energies of PbS
and PbL were taken respectively from bulk PbS and PbL2
2
precursor molecules. The obtained surface energy is summarized
in Table 1.
The nonpolar (100) surface is self-passivated without any
dangling bond or reactive electron at the Fermi energy, as
represented by semiconducting density of states (DOS) with a
band gap of 1 eV (Figure 3C). In contrast, the polar (111) surface
is not electronically passivated, showing metallic DOS with many
surface states near the Fermi energy. Because of this, the surface
With this in mind, we reconstructed size-dependent shapes of
oleate-capped Pb-rich PbS QDs based on the measured Pb/S
ratio. We first traced all possible Pb/S ratios of octahedral and
cuboctahedral QDs depending on particle size, as marked with
background symbols in Figure 4. Because there is some
uncertainty in estimating the QD size in both experiment and
theory, we used the farthest Pb−Pb distance in theory. For
ultrasmall QDs with D < 3 nm, the measured Pb/S ratios well
correspond to those of cis-O(111)-only octahedral QDs. For
QDs >3 nm in size, the measured Pb/S ratios are noticeably and
consistently smaller than those of cis-O(111)-only QDs. The
reduced Pb/S ratio can thus be explained by the truncation of
octahedral QDs with the appearance of the stoichiometric (100)
surface. Figure 4 inset illustrates the proposed QD models with
the octahedral and cuboctahedral shapes as a function of QD size.
2
energy (0.22 J/m ) of the self-passivated, nonpolar (100) surface
2
is much smaller than that (1.04 J/m ) of the unpassivated, polar
14
(
111) surface, consistent with previously reported results. The
calculated (100) surface energy of PbS is comparable to that of
2
2
NaCl, i.e., 0.14−0.24 J/m from DFT and 0.18−0.38 J/m from
1
5
experiment. So, cubic PbS consisting of all (100) facets is most
likely to form when no passivating ligand presents.
When the (111) surface is passivated with PbL ligand groups
2
to form a Pb-rich L(111) surface, complete surface passivation is
5
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Journal of the American Chemical Society
Communication
ACKNOWLEDGMENTS
■
This work was supported by the Global Frontier R&D Program
by the Center for Multiscale Energy Systems (2011-0031566),
WCU program (R31-2008-000-10071-0), Industrial Core Grant
(
10035274), and NRF grant (2012-046191) funded by the
Korea government (MEST).
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ASSOCIATED CONTENT
Supporting Information
Details of synthesis, characterization, and DFT simulation, and
atomic coordinates of PbS QDs. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
(
*
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AUTHOR INFORMATION
Corresponding Author
sjeong@kimm.re.kr; yong.hyun.kim@kaist.ac.kr
DOI: 10.1021/cm303318f.
■
Author Contributions
⊥
H.C. and J.-H.K. contributed equally.
Notes
The authors declare no competing financial interest.
5
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