On the absence of a phonon bottleneck in strongly confined CsPbBr3 perovskite nanocrystals

Article abstract of DOI:10.1039/c9sc01339c

In traditional solar cells, photogenerated energetic carriers (so-called hot carriers) rapidly relax to band edges via emission of phonons, prohibiting the extraction of their excess energy above the band gap. Quantum confined semiconductor nanocrystals, or quantum dots (QDs), were predicted to have long-lived hot carriers enabled by a phonon bottleneck, i.e., the large inter-level spacings in QDs should result in inefficient phonon emissions. Here we study the effect of quantum confinement on hot carrier/exciton lifetime in lead halide perovskite nanocrystals. We synthesized a series of strongly confined CsPbBr₃ nanocrystals with edge lengths down to 2.6 nm, the smallest reported to date, and studied their hot exciton relaxation using ultrafast spectroscopy. We observed sub-ps hot exciton lifetimes in all the samples with edge lengths within 2.6-6.2 nm and thus the absence of a phonon bottleneck. Their well-resolved excitonic peaks allowed us to quantify hot carrier/exciton energy loss rates which increased with decreasing NC sizes. This behavior can be well reproduced by a nonadiabatic transition mechanism between excitonic states induced by coupling to surface ligands.

Full text of DOI:10.1039/c9sc01339c

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DOI: 10.1039/C9SC01339C
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On the absence of a phonon bottleneck in strongly-confined
CsPbBr3 perovskite nanocrystals
Yulu Li,ab Runchen Lai,a Xiao Luo,a Xue Liu,a Tao Ding, a Xin Lu b and Kaifeng Wu*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In traditional solar cells, photogenerated energetic carriers (so-called hot carriers) rapidly relax to band edges via emission
of phonons, prohibiting the extraction of their excessive energy above the band gap. Quantum confined semiconductor
nanocrystals, or quantum dots (QDs), were predicted to have long-lived hot carriers enabled by a phonon bottleneck, i.e.,
the large inter-level spacings in QDs should result in inefficient photon emissions. Here we study the effect of quantum
confinement on hot carrier/exciton lifetime in lead halide perovskite nanocrystals.We synthesized a series of strongly-
confined CsPbBr3 nanocrystals with edge lengths down to 2.6 nm, the smallest reported to date, and studied their hot
exciton relaxation using ultrafast spectroscopy. We observed sub-ps hot exciton lifetime in all the samples with edge
lengths within 2.6-6.2 nm and thus the absence of a phonon bottleneck. Their well-resolved excitonic peaks allowed us to
quantify hot carrier/exciton energy loss rates which increased with deceasing NC sizes. This behavior can be well
reproduced by a nonadiabatic transition mechanism between excitonic states induced by coupling to surface ligands.
,
16
Introduction
faster than that of hot carrier relaxation.13
On the other
hand, however, recent studies indicated sub-ps hot carrier
cooling in various types of bulk-like lead halide nanocrystals
(NCs) including CsPbX3 NCs,17-20 MAPbBr3 NCs,19, 21 and FAPbBr3
In typical semiconductors, photogenerated carriers with
excessive energy above band edges, the so-called hot carriers,
rapidly relax to band edges mainly via emission of phonons.
The relaxation often occurs on a sub-ps timescale,1-4 making
excessive energy of hot carriers very difficult to be harvested.
As a result, this part of energy is wasted as heat in
conventional optoelectronic devices such as solar cells, which
is a major reason for the Shockley–Queisser limit for single-
NCs19
at low excitation energy densities. Note that slower
, 22
carrier cooling on 10s of ps timescale reported in refs 19-22 is
a result of Auger heating and hot phonon bottleneck that are
significant only at higher excitation densities, which is beyond
the scope of this work.
junction solar cells.5 If hot carriers can be efficiently extracted In light of these previous studies, it is interesting to examine
to selective contacts,6-8 the efficiency of solar cells can be whether strong quantum confinement can enable a phonon
pushed to as high as 66%.6, As such, it is essential to bottleneck and hence slow down hot carrier relaxation for lead
9
understand the mechanisms for hot carrier relaxation and to halide perovskites, particularly because that the Auger-type,
prolong hot carrier lifetime for next-generation solar cells.
electron-to-hole energy transfer mechanism responsible for
sub-ps hot electron relaxation in typical CdSe quantum dots
(QDs)23-25 should be unavailable for these perovskite materials
with similar electron and hole effective masses.This phonon
bottleneck was theoretically predicted in a very recent work
which calculated hot carrier cooling in CsPbBr3 NCs using the
Red-field theory.26 Experimentally, however, the effect of
quantum confinement on hot carrier/exciton relaxation in
perovskite NCs has never been systematically studied. Such a
systematic study was mainly hindered by the lack of methods
to synthesize mono-dispersed and strongly-confined
perovskite NCs due to their rapid growth rates in hot-injection
Lead halide perovskites (APbX3; with A = Cs, MA, FA and X = Cl,
Br, I) have recently proven technologically important for many
light-harvesting and -emitting applications.10-12 Hot carrier
dynamics in these materials have also been intensively studied
because of recent reports of long-lived hot carriers.13-15 It was
proposed that their highly-dynamic perovskite lattices
facilitated formation of large polarons which not only
protected band edge carriers from trapping but also slowed
down hot carrier cooling if the rate of polaron formation was
a. State Key Laboratory of Molecular Reaction Dynamics and Dynamics Research synthesis. Recent progress has enabled controllable synthesis
Center for Energy and Environmental Materials, Dalian Institute of Chemical
of high-quality CsPbBr3 NCs in the quantum confinement
Physics, Chinese Academy of Sciences, Dalian, China, 116023
regime,27-29 which should allow for examination of the effect of
quantum confinement on hot carrier/exciton cooling as have
been done for CdSe and PbSe QDs.23, 24, 30-32
b. Departmental of Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen, Fujian 361005, China
*Corresponding author. E-mail: kwu@dicp.ac.cn
Electronic supplementary information (ESI) available: Figures S1-S13 and
wavefunction calculations. See DOI: 10.1039/x0xx00000x
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Here we study hot exciton relaxation dynamics in mono- band edge excitons.17, 39-41 As a result, in the course of hot exciton
View Article Online
dispersed CsPbBr3 NCs with varying edge lengths (L) in the relaxation to form band edge excitons, DtOhIe: 10g.1ro03w9t/hC9SoCf01B3139Cis
range of 2.6 to 6.2 nm using transient absorption (TA) accompanied by the decay of A1, as observed in Fig. 1c. By
spectroscopy. We found the absence of a phonon bottleneck. simultaneously fitting the kinetics probed at these two features, the
The hot carrier/exciton energy loss rates increased with X2 to X1 relaxation time constant is determined to be τr = 410±30 fs.
deceasing NC sizes. Similar electron and hole densities of This fast relaxation is similar to that previously reported for bulk-
states (near band edge) in CsPbBr3 exclude the Auger type like CsPbBr3 NCs,17-20 indicating that quantum confinement does not
electron-to-hole energy transfer mechanism. Temperature- slow down hot exciton relaxation in these NCs and that there are
independent relaxation rates were inconsistent with relaxation mechanisms bypassing the phonon bottleneck which,
thenonadiabatic multi-phonon emission mechanism. NCs however, are overlooked in the calculations in ref 26. The hot
capped with alkane thiol ligands of low infrared absorbance carrier energy loss rate (dE/dt) can be estimated as ΔE/2τr, with the
showed similar relaxation rates as pristine NCs capped with factor 2 arising from the assumption of symmetric electron and
oleic acid, which also excluded the mechanism of energy hole confinement energies, and is 0.32±0.02 eV/ps for this L = 4.3
transfer to ligands. The hot exciton relaxation mechanism in nm sample. We calculate hot carrier instead of hot exciton energy
CsPbBr3 NCs is most likely associated with nonadiabatic loss rates simply because previous studies on CdSe23 and PbSe30
coupling between excitonic states and surface ligands.
QDs reported hot carrier energy loss rates; see discussions below.
a
L = 4.3 nm
X2 X1
b
Result and discussion
Size-dependent optical properties of CsPbBr3 NCs
2
0
A1
6.2 nm
5.0 nm
4.5 nm
We synthesized mono-dispersed CsPbBr3 NCs with varying sizes by
modifying recent literature methods;29, details can be found in
0.1 ps
0.2 ps
0.3 ps
0.4 ps
0.5 ps
0.8 ps
33
-2
-4
-6
the Methods. Transmission Electron Microscopy (TEM) images of
these NCs are presented in Fig. S1 in the Electronic Supplementary
Information (ESI). The edge lengths (L) of these cube-shaped NCs
are in the range of 2.6 to 6.2 nm (Table 1), which are smaller than
the Bohr exciton diameter of CsPbBr3 (~7 nm28), and hence, all
samples fall in the strong quantum confinement regime.34 Due to
relatively precise control in their sizes, these NCs exhibit well-
defined excitonic peaks on their absorption spectra (Fig. 1a). In
particular, the two lowest energy peaks can be well-resolved, which
are labelled as X1 and X2. According to a recent study,35 X1 and X2
can be assigned to transitions from the first and second hole levels
in the valance band to the first and second electron levels in the
conduction band, respectively. The exact peak positions for X1 and
X2 were determined from the crossing-zero points on the first
derivative curves of absorption spectra (Fig. 1a). Especially
noteworthy is the L = 2.6 nm sample, which is the smallest size
CsPbBr3 NCs reported to date and thus has the strongest quantum
confinement (with X1 at ~439 nm). In previous studies, the smallest
size reported was ~3.0 nm (with X1 at ~451 nm).28 The energy
separations between X1 and X2 (ΔE) in the samples are in the range
of 0.18 eV to 0.32 eV, increasing with decreasing NC sizes (Table 1).
B1
4.3 nm
4.0 nm
440 460 480 500 520 540
Wavelength (nm)
c
0
3.5 nm
3.0 nm
A1
B1
-2
-4
-6
L = 2.6 nm
400 450 500 550 600
Wavelength (nm)
0
1
2
3
Delay time (ps)
Figure 1. (a) Absorption (colored lines) and first derivative of
absorption (gray lines) spectra of NCs of varying sizes. The positions
of X1 and X2 on these spectra are labelled. (b) Transient absorption
(TA) spectra of L = 4.3 nm NCs at indicated delays following the
excitation by a 430 nm pulse. The bleach feature B1 and absorptive
feature A1 are labelled. (c) TA Kinetics probed at the peaks of A1
(wine circles) and B1 (orange diamonds) are their single-exponential
fits (solid lines).
Well-defined excitonic peaks enable studying state-to-state hot
carrier/exciton relaxation dynamics in CsPbBr3 NCs using pump-
probe TA spectroscopy; see Methods for details. Fig. 1b shows the
TA spectra of L =4.3 nm NCs within 1 ps following the excitation by a
430 nm pump pulse which selectively excites X2. The excitation
energy densities were maintained low enough in all TA experiments
such that single exciton dynamics were detected.36 In the examined
time window, TA spectra are dominated by an exciton bleach (B1)
feature at~470 nm and an induced absorption (A1) at ~490 nm. B1
can be assigned to the state-filling effect by band edge excitons
(both electrons and holes),37, 38 whereas A1 is often attributed to
Stark-effect-like signals arising from interactions between hot and
We notice that excitation at 400 nm generates similar hot carrier
relaxation dynamics as the case of 430 nm excitation (Fig. S2),
suggesting that relaxation from Xn (n≥ 3) to X2 occurs with a much
faster rate than that from X2 to X1. Thus, relaxation from X2 to X1 is
the rate determining step (RDS) in the complicated multi-step hot
relaxation process. Similar observations were found for other
samples, such as the L =4.0 nm and L =4.5 nm NCs shown in Fig. S2.
It also justifies previous reports in which the same excitation
wavelength such as 400 nm was used to determine hot exciton
2 | J. Name., 2012, 00, 1-3
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relaxation time in samples with totally different band edge exciton valance band.34 This mechanism should not be applicable for
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energy.17, 18
CsPbBr3 NCs. Because of similar conductDioOnI: 1a0n.1d03v9a/Cle9nScCe01b3a39nCd
structures near their edges and electron and hole effective
masses,12 both electron and hole energy levels in quantum-confined
CsPbBr3 NCs should be discrete. This situation is similar to IV-VI
group QDs (such as PbSe). The absence of the Auger relaxation
mechanism might be responsible for the slower hot carrier energy
loss rates of CsPbBr3 NCs and PbSe QDs than that of CdSe QDs
plotted in Fig. 2b. Still, the energy loss rates of CsPbBr3 NCs and
PbSe QDs are fast,only several folds slower than CdSe, suggesting
alternative hot carrier/exciton cooling mechanisms in CsPbBr3 and
PbSe.
Table 1. Summary of sample information for CsPbBr3 NCs
L (nm)
X1 (eV)
2.54
2.57
2.60
2.62
2.67
2.71
2.75
2.82
X2(eV)
2.72
2.79
2.83
2.88
2.94
3.03
3.08
3.19
∆E (eV)
0.18
0.22
0.23
0.26
0.27
0.32
0.33
0.37
τr(fs)
dE/dt (eV/ps)
0.22 ± 0.01
0.24 ± 0.01
0.30 ± 0.02
0.32 ± 0.02
0.36 ± 0.03
0.44 ± 0.02
0.48 ± 0.02
0.55 ± 0.02
6.2± 0.3
5.0± 0.4
4.5± 0.2
4.3± 0.2
4.0± 0.4
3.5± 0.3
3.0± 0.2
2.6± 0.3
420 ± 20
400 ± 10
390 ± 30
410 ±30
380 ± 30
370 ± 20
350 ± 10
340 ± 30
For PbSe QDs, a multi-phonon emission mechanism was proposed
to account for the ultrafast hot carrier/exciton relaxation.30 This
mechanism is enabled by intrinsic nonadiabatic interactions
induced by electron-lattice coupling and it requires thermal
activation to cross the intersection point between 1P and 1S
potential energy surfaces. Fig. 3a shows the hot exciton relaxation
dynamics (by monitoring B1 formation) for L = 4.3 nm NCs
measured at varying temperatures. The kinetic traces are virtually
the same within the noise level, showing negligible temperature
dependence. Measurements on another sample (L = 4.0 nm) show
similar results (Fig. S10). Thus, we can exclude the nonadiabatic
multi-phonon emission as a major mechanism for hot exciton
relaxation in CsPbBr3 NCs.
a
500
400
300
200
100
0
Hot exciton relaxation dynamics in CsPbBr3 NCs of other sizes are
also measured using TA (see Figs. S3-S9) to extract their hot exciton
relaxation time constants (τr) and hot carrier energy loss rates(dE/dt)
using the procedures described above. The relaxation time
constants are in the range of 340-420 fs and depend very weakly on
NC sizes (Table 1 and Fig. 2a). In contrast, hot carrier energy loss
rates increase with decreasing NC sizes (Table 1 and Fig. 2b). The
hot carrier energy loss rates for CdSe and PbSe QDs adapted from
refs 23 and 30, respectively, are also plotted in the Figure for
comparison. Note that in order to compare cube-like CsPbBr3 NCs
with previous reports on spherical QDs, we define R = L/2 for these
NCs. In general, the hot carrier energy loss rate in CsPbBr3 NCs is
slower than CdSe QDs of comparable sizes. In contrast, there exists
a cross-over between CsPbBr3 NCs and PbSe QDs; the rate is slightly
faster in CsPbBr3 NCs for R < 2 nm whereas it becomes faster in
PbSe QDs for R > 2 nm. Possible reasons for these behaviors are
given below.
2
3
4
5
6
7
L (nm)
b
10
CsPbBr3
CdSe
PbSe
1
0.1
Temperature and ligand independent relaxationdynamics
We examine possible mechanisms for the ultrafast hot exciton
relaxation dynamics observed in CsPbBr3 NCs. As we mentioned in
the introduction, hot electron relaxation in II-VI group QDs (such as
CdSe) has been attributed the Auger-type, electron-to-hole energy
0.3
0.4
0.5 0.6 0.7 0.8 0.9
R-1 (nm-1
)
Figure 2. Hot exciton relaxation time constants (a) and hot carrier
energy loss rates (b) of CsPbBr3 NCs as a function of NC size (orange
circles). Note that in order to compare cube-like CsPbBr3 NCs with
previous reports on spherical QDs in (b), we define R = L/2 for these
25
transfer mechanism,24, because holes in these QDs can quickly
relax via emission of phonons due to high density of states in the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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NCs. The results for CdSe QDs (red diamonds) and PbSe QDs (wine
triangles) are adapted from refs 23 and 30, respectively.
Hot exciton relaxation via nonadiabatic interactions
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In addition to the electron-to-hole energyDtOraI:n1s0fe.1r0,39m/Cul9tSi-Cp0h1o3n39oCn
emission and electron-to-ligand energy transfer mechanisms
mentioned above, a more complete and unified picture for hot
carrier relaxation was established by Kambhampati et al. by
In addition to intrinsic processes inside the NC volume, organic
ligands on NC surfaces also act as a pathway for energy dissipation
via nonradiative, long-range electronic-to-vibrational energy
43
transfer.42,
For example, the 1P to 1S relaxation rate in II-VI
performing more advanced state-resolved, pump-probe
41
spectroscopic studies on QDs.40,
This picture incorporates yet
core/shell QDs was found to be strongly influenced by surface
ligands;44 the relaxation rate can be retarded by more than one
order of magnitude when capping ligands were changed from
stearic acid to 1-dodecanethiol (DDT) because the latter has a much
weaker absorbance in the range of ~0.2-0.3 eV. In order to test this
possibility for CsPbBr3 NCs, we replaced their pristine oleic acid (OA)
ligands by DDT using a literature method (see Methods and Fig. S11
for details). As shown in Fig. 3b, OA and DDT capped NCs (L =
4.3nm) show similar B1 formation dynamics. Measurements on
other two samples (L = 4.0 and 5.0 nm) show similar results (Fig.
S12). Because these three samples have X2 and X1 energy
differences (ΔE) exactly in the range of 0.2-0.3 eV (Table 1), similar
ultrafast relaxation rates observed for OA and DDT capped NCs
suggest that energy transfer to ligands is not the major channel for
hot exciton relaxation either.
another important relaxtion mechanism which is unique for these
quantum-confined nanoscale systems-nonadiabatic transition
between excitonic states induced by surface ligands.31,
mechanism was invoked to explain the absence of a phonon
bottleneck for hot hole relaxation in CdSe QDs.31,
32
This
32
Specifically,
while hot electron relaxation in CdSe QDs occurs via energy transfer
to the hole, hot hole relaxation was expected to display a phonon
bottleneck. Experimentally, however, hot hole relaxation rates are
32
similarly fast for CdSe QDs with various sizes,31, similar to our
observations here for CsPbBr3 NCs. In order to account for this
peculiar behavior, Cooney et al. proposed the nonadiabatic
relaxation channel mediated by surface ligands.31, 32 According to
this mechanism, nonadiabatic hot carrier relaxation rates (1/τr)
scale with the Hellman-Feynman force describing electron-nuclear
interactions inducing the nonadiabatic transition (H) and inversely
with the energy gap (ΔE): 1/τr ∝H/ΔE. As both H and ΔE increase
with decreasing QD sizes, the hot carrier relaxation rates (1/τr) are
relatively size-independent. On the other hand, the energy loss
rates should scale with H: dE/dt = ΔE/2τr∝H.
a
L = 4.3 nm
295 (K)
280
0
260
245
225
-1
165
135
110
0.6
0.4
0.2
0.0
data
F(X1)
-2
85
F(X2)
-3
0
1
2
3
Delay time (ps)
b
L = 4.3 nm
0
-2
-4
-6
DDT
OA
3
4
5
6
L (nm)
Figure 4. Measured hot carrier energy loss rates (orange circles) and
calculated wavefunction surface fraction (F) of X1 (red solid line) and
X2 (wine solid line) for CsPbBr3 NCs of varyingsizes. F(X1)and
F(X2)have been appropriately scaled by constant factors.
For surface ligand induced nonadiabatic relaxation, H should be
proportional to the carrier/exciton wavefunction near NC surfaces,
which increases with decreasing NC sizes. Following the procedures
in ref 31, we calculate wavefunctions for X1 and X2 using effective
mass approximation (EMA) and obtain their surface fraction (F) by
integrating wavefunctions in the outermost unit cells and those
tunneling into ligand shells; see SI for details. It is the surface
fraction that interacts effectively with the ligand vibrational modes,
0
1
2
3
Delay time (ps)
Figure 3. (a) Hot exciton relaxation dynamics (monitored at B1) for L
= 4.3 nm NCs at varying temperatures. (b) Hot exciton relaxation
dynamics (monitored at B1) for L = 4.3 nm pristine NCs (with OA
ligands; orange diamonds) and DDT-capped NCs (wine circles).
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which induces nonadiabatic transition from X2 to X1. The calculated to calculate size-dependent energy loss rates. The energy loss
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surface fractions (F) for X1 and X2 are plotted in Fig. S13. As shown in mechanism for CsPbBr3 NCs is inconsistentDwOitI:h10t.h1a0t39p/rCo9pSoCs0e1d33f9oCr
Fig. 4, experimental energy loss rates (dE/dt) can be well fitted either CdSe (electron-to-hole energy transfer) or PbSe QDs
using: dE/dt = CF, with C being a scaling constant. Because F(X1)and (nonadiabatic multi-phonon emission). Energy transfer to surface
F(X2) show similar size-dependence, both can be used to fit the data. ligands is not the dominant mechanism either because NCs capped
This fit provides strong evidence that hot carrier/exciton relaxation with ligands of low and high infrared absorbances at the X2 to X1
in CsPbBr3 NCs is mainly enabled by nonadiabatic transition induced transition energy showed essentially the same relaxation dynamics.
by surface ligands. Better agreement with experimental data might Rather, hot exciton relaxation in strongly-confined CsPbBr3 NCs is
be achieved by accounting for minor contributions from other likely dominated by a nonadiabatic transition mechanism induced
factors such as phonon emission which becomes more important by surface ligands. Smaller NCs have stronger quantum
for larger-size NCs with smaller ΔE.
confinement and thus higher wavefunction amplitude at NC
surfaces, thus enhancing their interaction with the nuclear
coordinates of surface ligands and bypassing the otherwise
expected phonon bottleneck. This study highlights the difficulty of
achieving long-lived hot carriers even in strongly-confined
perovskite NCs, due to the ligand-induced nonadiabatic transition
mechanism that universally exists for colloidal NCs.
Discussion
In principle, hot carrier/exciton relaxation via nonadiabatic
transition induced by surface ligands should be universal for
quantum-confined colloidal NCs. As such, this channel is also active
for CdSe and PbSe QDs. For CdSe QDs, however, the Auger-type,
electron-to-hole energy transfer adds an extra and dominant
channel for hot electron relaxation, which explains their faster
energy loss rates than PbSe QDs and CsPbBr3 NCs. PbSe QDs have
similar but slightly lower hot carrier energy loss rates than CsPbBr3
NCs for R > 2 nm in which regime both are likely dominated by the
ligand-induced nonadiabatic mechanism. However, PbSe QDs have
yet another lattice-induced nonadiabatic mechanism that is also
strongly enhanced by quantum confinement; as a result, their hot
carrier energy loss rates become faster than that of CsPbBr3 NCs
when R < 2 nm. These explanations qualitatively rationalize the
comparison shown in Fig. 2b. Nonetheless, several unknowns
remain to be elucidated for a more quantitative interpretation of
Fig. 2b. The first one is related to how to quantify the ligand-
induced nonadiabatic relaxation rates for NCs of different
compositions. This would explain the slightly slower hot carrier
energy loss rates in PbSe QDs than in CsPbBr3 NCs for R > 2 nm.
Another unknown is why the lattice-induced nonadiabatic
mechanism is significant in PbSe QDs but not in CdSe QDs and
CsPbBr3 NCs.
Methods
Synthesis of CsPbBr3 NCs
We synthesized CsPbBr3 NCs by modifying previously
reportedprocedures.29 The synthesis started with preparation of Cs
oleate precursors. 0.25g Cs2CO3, 0.8 g oleic acid (OA), and 7 g1-
octadecene (ODE) were loaded into a 50 mL 3-neck flask and
vacuum-dried for 1 h at 120 °Cusing a Schlenk line. The mixture was
heated under Argon atmosphere to 150 ºC until dissolving all the
Cs2CO3. The Cs-oleate precursor solution was kept at 100 ºC to
prevent precipitation of Cs-oleate out of ODE. In another 25 mL 3-
neck flask, the precursor solution of Pb and Br was prepared by
dissolving 75 mg PbBr2 and a varying amount of ZnBr2 (0-600mg) in
a mixture of ODE (5 mL), OA (3 mL), and oleylamine (OAm, 3 mL).
After the precursor solution of Pb and Br was vacuum-dried for 1 h
at 120 °C, it was set to the reaction temperature under Argon
atmosphere. The reaction temperature was varied depending on
the desired NC sizes (88 °C for 2.6 nm, 93 °C for 3.0 nm, 100 °C for
3.5 nm, 110 °C for 4.0 nm, 120 °C for 4.3 nm and 4.5 nm, 130 °C for
5.0 nm and 140 °C for 6.2 nm QDs). When the reaction temperature
was reached, 0.4 mL of Cs precursor solution was swiftly injected to
initiate the reaction. The reaction was quenched after a short
period of time (10-180 s, depending on the temperature in the
range of 190-80 °C) by cooling the flask in an ice bath. After the
crude solution was cooled down to room temperature, the product
was centrifuged at 3500 rpm for 15 min to remove the unreacted
salts as the precipitate, and the NCs dispersed in the supernatant
were collected. For reactions conducted at higher temperatures
(140-190 °C), only small amounts of unreacted salts remained in the
solution, resulting in a higher reaction yield. 8 mL of acetone was
directly added into the supernatant to precipitate the NCs followed
by centrifuging at 3500 rpm for 3 mins. For reactions conducted at
lower temperatures (80-120 °C), larger amounts of unreacted salt
remained in the supernatant after centrifuging. In this case, the
supernatant was left on the benchtop under ambient conditions for
~0.5 hour until the salts precipitated. Then the mixture was
centrifuged at 3500 rpm for another 15 min. After obtaining a clear
supernatant, the NCs were precipitated by dropwise adding
A more general issue associated with the study of hot carrier
dynamics of colloidal NCs is how to disentangle the various hot
carrier relaxation mechanisms and how to determine the absolute
rate for each individual pathway. According to the unified relaxation
picture established by Kambhampati et al.,40, 41 in many cases these
mechanisms co-exist and thus are very difficult to be cleanly
separated. Experimental studies with more extensively tuned
relaxation-related parameters (NC size, electronic structure,
temperature, etc.) combined with high-level theories would be
required for this goal, which is beyond the scope of this work. The
focus here is to unravel the dominant hot carrier relaxation
mechanism responsible for the absence of a phonon bottleneck in
strongly confined perovskite NCs.
Conclusions
In summary, we measured size-dependent hot exciton relaxation
dynamicsin mono-dispersed and strongly-quantum-confined
CsPbBr3 NCs using TA spectroscopy and found the absence of a
phonon bottleneck. Their well-resolved excitonic peaks allowed us
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acetone until the mixture just turned turbid to avoid decomposition
of the NCs. Then the dried NCs were collected and dissolved in
hexane. Note that in order to obtain very small size NCs (2.6 - 3.5
nm), the synthesis was scaled up by 3 times, and the reaction
should be performed for relatively longer time (180 s for 2.6 nm,
120 s for 3.0 nm, and 100 s for 3.5 nm) without any temperature
fluctuations.
View Article Online
DOI: 10.1039/C9SC01339C
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Acknowledgements
We gratefully acknowledge financial supports from the
Strategic Pilot Science and Technology Project of Chinese
Academy of Sciences (XDA21010206), the Ministry of Science
and Technology of China (2018YFA028703) and the National
Natural Science Foundation of China (21773239).
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X2
 sub-ps relaxation
101
100
10-1
X1
 no phonon bottleneck
G
CsPbBr3
CdSe
PbSe
0.3
0.4
0.5 0.6 0.7 0.8
)
R-1 (nm-1
Strongly-confined CsPbBr3 perovskite nanocrystals exhibit sub-ps hot exciton relaxation dynamics and the absence of a phonon
bottleneck.
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