Supporting Information
Electron-hole overlap dictates the hole spin relaxation
rate in nanocrystal heterostructures
Jun He, Haizheng Zhong, and Gregory D. Scholes*
Department of Chemistry, Institute for Optical Sciences, and Centre for Quantum
Information and Quantum Control, University of Toronto, 80 St. George Street, Toronto,
Ontario M5S 3H6, Canada
*Corresponding author：
gscholes@chem.utoronto.ca
Supplementary materials contains:
page:
i. Synthesis of core-shell NC samples.
(2)
ii. TEM images of samples
(4)
iii. Optical characterizations of NCs
(5)
iv. Time-resolved photoluminescence measurement
(6)
v. Experimental methods: CPH-3TG technique
(8)
vi. Determine electron transfer rate of type-II core-shell NCs with VVVV
measurement
(13)
vii. Modeling of strain and band structure
(14)
viii. CPH-3TG measurements on samples with different surface ligands
(15)
1
i. Synthesis of core-shell nanocrystal samples.
Colloidal CdTe NCs were synthesized with a modified method based on the
previous reports [Ref. 27]. Typically, a mixture of 0.0256 g (0.2 mmol) of CdO, 0.116 g
(0.4 mmol) of tetradecylphosphonic acid (TDPA), and 10 mL of octadecene (ODE) was
heated in a three-neck flask to 300 °C to obtain a clear solution. A Te solution, prepared
by dissolving 0.1 mmol of Te powder in 0.5 mL trioctylphosphine (TOP) and 4 mL ODE,
was quickly injected into the hot mixture. The reaction was allowed to cool to 260 °C,
allowing further growth to the desired size. The ODE solution of as-prepared NCs was
combined with an equal-volume mixture of hexane and methanol (1/2, v/v) by vigorous
shaking and subsequent centrifugation. The ODE phase was then extracted. This
procedure was repeated twice, and the final solution was purged with argon flow and
stored in the dark.
The shell growth was accomplished via a successive ion layer adsorption reaction
(SILAR) method by alternating injections of Cd and Se precursors. The solutions of Cd
and Se precursors were prepared at a concentration of 0.04 M in ODE. Cadmium
precursor was prepared by dissolving 0.050 g CdO in 0.90 g oleic acid and 7.08 g ODE
under argon flow at 240 °C. The solution was then cooled to room temperature and
stored under argon in a septum capped conical flask. The Se precursor was prepared by
shaking a mixture of 0.038 g Se powder (100 meshes), 1.55 g tributylphosphine (TBP)
and 7.95 g ODE in a septum capped conical flask. It was then purged with argon flow
and stored for further use. A typical procedure of shell growth is as follows. A fixed
amount of purified CdTe quantum dots were added into a three necked flask containing a
mixture of ODE (3 mL) and oleylamine (3 mL). The reaction flask was attached to a
Schlenk line, and flowed with argon at room temperature to remove the residential
hexane. The solution was then refluxed under vacuum at ~100°C for an additional 40-60
min to ensure complete removal of any low boiling point impurities. The solution was
then purged three times with argon, and the temperature of the solution was increased to
240 °C. At this temperature, the Se precursor was added first, followed by the Cd
precursor 5 min later. The reaction solution was allowed to react for up to 15 min before
another layer of CdSe was applied. Between 1 and 5 monolayers of CdSe were grown for
2
this study. For the preparation of CdTe/0.5 layer CdSe sample, only Se precursor was
added and the reaction lasted for 10 min. These resulting NCs were precipitated by
adding acetone and re-dissolved in toluene for spectroscopic measurements. The asprepared NCs were further purified twice by precipitation with methanol and re-dispersed
in toluene for transmission electron microscopy characterization.
Sample characterization. Both morphology and size distribution of the NCs were
examined using a CTEM Hitachi H-7000 operated at 100kV. Powder X-ray diffraction
(PXRD) measurements were carried out on a Siemens D-5000 diffractometer using a
high-power Cu Kα source operating at 50 kV and 35 mA with a Kevex solid-state
detector. The one-photon absorption spectrum was measured on a CARY 100 Bio UVvisible
spectrophotometer.
The
steady-state
photoluminescence
(PL)
and
photoluminescence excitation (PLE) spectra were collected with a CARY Eclipse
fluorescence spectrophotometer.
3
ii. TEM images of samples
Figure S_1: Typical TEM images of 3.4 nm CdTe cores (top left) and
CdTe/CdSe core-shell NCs with shell thickness of 1.2 (top right), 1.6 (bottom
left) and 2.0 nm (bottom right). Scale bar: 50 nm.
4
iii. Optical characterizations of NCs
2.0-nm shell
Abs/PL/PLE (a.u.)
1.0
1.6-nm shell
0.8
1.2-nm shell
0.6
0.8-nm shell
0.4
0.4-nm shell
0.2-nm shell
0.2
Core
0.0
500
600
700
800
900
Wavelength (nm)
Figure S_2: Absorption (solid lines), PL (dotted lines) and PLE (dashed lines) spectra of
3.4-nm CdTe cores and CdTe/CdSe core-shell NCs with shell thickness of 0.2, 0.4, 0.8,
1.2, 1.6, and 2.0 nm. Consecutive spectra are shifted vertically for clarity.
5
iv. Time-resolved photoluminescence measurement
PL dynamics were measured by time-correlated single photon counting (TCSPC), using
an IBH Datastation Hub system with an IBH 5000M PL monochromator and a R3809U50-cooled MCP PMT detector. The light source was a model 3950 picosecond
Ti:sapphire Tsunami laser (Spectra-Physics), pumped by a Millenium X (Spectra-Physics)
diode laser and frequency doubled using a GWU-23PL multiharmonic generator
(Spectra-Physics). The excitation wavelength was 450 nm and experimental data were
analyzed by least-squares iterative reconvolution of multiexponential decay function with
two experimentally determined instrument response functions. The shortest event that the
TCSPC experiment can resolve is estimated to ~70 ps. More details on the setup and
kinetic data analysis can be found elsewhere [Ref. 31].
Table S_1: Values extracted from two exponent fits to lifetime data of CdTe cores, coreshell NCs with CdSe shell thickness of 0.2−2.0 nm.
Sample
Emission
peak
(nm)
A 1 (%)
τ 1 (ps)
A 2 (%)
τ 2 (ps)
τ avg (ns) a
core
624
12.8
9262
87.2
23330
21.53
0.2-nm shell
641
11.1
8498
88.9
25283
23.42 (23.25)
0.4-nm shell
676
64.2
22911
35.7
40485
29.20 (28.68)
1.2-nm shell
754
54.2
38936
45.8
23013
31.64 (31.27)
1.6-nm shell
774
63.7
70399
36.3
30850
56.06 (54.42)
2.0-nm shell
815
22.1
38951
77.9
101210
87.43 (84.85)
a
P
P
B
B
B
B
B
B
B
B
B
B
P
P
The values in parentheses are the monoexponential fitting results.
6
PL intensity (a.u.)
-1
radiative decay rate (s )
pure core
radiative decay rate
integrated PL intensity
80
60
40
20
0
600
620
640
660
680
Wavelength (nm)
Figure S_2a: Radiative decay rate determined around the steady-state PL emission peak
for pure CdTe core
7
v. Experimental methods: CPH-3TG technique
The experimental setup of cross-polarized heterodyne third-order transient grating
technique (CPH-3TG) is depicted in Figure S3. The details of the setup have been
described elsewhere [Ref. 22]. Each sample, a dilute solution of CdTe/CdSe core-shell
NCs in toluene, was photoexcited in resonance with the CdTe exciton absorption band by
ultrashort laser pulses at a 1 kHz repetition rate. A relatively narrow size distribution of
core-shell NCs were resonantly excited, as determined by the laser spectrum (~30 nm full
width at half maximum) at each excitation wavelength. Depending on the center
wavelength of laser spectrum, pulse durations of 25–30 fs were obtained from 3-TG
autocorrelation measurements using carbon tetrachloride at the sample position, Figure
S3. To prevent any sample degradation, biexciton formation, or thermal grating
contributions to the signal, the pulse energy was kept at less than 5 nJ/pulse. The
intensities of both pump and probe beams were attenuated until the early time signal
shape did not change. The linear absorption spectra were obtained before and after the
pulsed laser irradiation; no measurable difference was observed, showing the high
photostability of the CdTe/CdSe heterostructures in toluene solution. All of the
measurements reported in the present work were performed at room temperature (293 K).
PM1
E1,
E2
E3,
E LO
B
WP P1
E2
E LO , E sig
B
B
B
B
L1
DO M1
E1
B
B
E3
B
B
B
B
B
B
B
P2
CS
C
G
WP
L2
M2
P3 PD1
S
PM2
PD2
Figure S_3: Experimental setup for heterodyne-detected 3-TG measurements:
P, polarizer; L, lens; DO, diffractive optic; PM, parabolic mirror, WP, /2
waveplate; M, mask; CS, cover slip; S, sample; PD, photodiode; G, glass plate;
C, chopper.
8
Intensity (a.u.)
0.03
Experimental
Gauss fit
Pulse width ( tFWHM=28 fs)
0.02
0.01
0.00
-400
-300
-200
-100
0
100
200
300
Time delay, tp (fs)
Figure S_4: Homodyne-detected 3-TG autocorrelation measurements using carbon
tetrachloride at laser wavelength of 633 nm, the dashed line is the Gaussian fitting
curve.
The CPH-3TG techniques use a sequence of three ultrafast laser pulses to initiate and
probe dynamics. The probe step involves interrogation using the third pulse in the
sequence, but the information is obtained in a fourth direction via a third-order
polarization radiated by the sample. We measure that signal using heterodyne detection,
which retrieves the sign of the linearized signal (cf. the comparison of VHVH and VHHV
signals) and also simplifies kinetic analysis of the transients compared to homodynedetected data, where V and H represent the vertical and horizontal polarizations.
Generally speaking, transient grating measures population decay, whereas polarization
grating measures depolarization. A key result is that optical selection rules for quantum
dot excitons, governed by both circularly and linearly polarized light, can be exploited in
nonlinear optical experiments to examine the exciton fine structure of NCs, even though
9
these levels are completely obscured by inhomogeneous line broadening. The CPH-3TG
signal can be written as
I HET t p 


0




dt  Re E LO
t p , 30,t p ,t  C  nC1  nC1 C nF1  n1F  ,
(1)
where E * LO (t p , Δφ) is the electric field of the local oscillator with phase shift relative to
P

PB
B
B
B
the probe field Δφ, and t p is the pump-probe delay time during which the excitons
B
B
equilibrate among the fine structure states, recombine to the ground state, or are
dissociated into surface traps. P (3) (0, t p , t) is the induced third-order polarization, and
P
P
B
B
n C +1 (n C -1 ) and n F +1 (n F -1 ) are the populations of conserved and flipped excitons. The
B
PB
P
B
PB
P
B
PB
P
B
PB
P
coefficients, C and C´, are rotational averaging factors that both take the value of 2/15 in
a population grating experiment, for example when all three pulses and the signal
analyzer are vertically polarized in the laboratory frame (VVVV). Therefore the VVVV
signal decays only with population relaxation like a normal pump-probe experiment. On
the other hand, when the pulses are cross-linearly polarized (producing a polarization
grating), C and C´have values of 2/15 (-2/15) and -2/15 (2/15) for the VHVH (VHHV)
polarization sequence respectively [Refs. 22,26].
The VVVV CPH-3TG data were fit by a sum of three-exponentials,
IVVVV t p  1  exp k1  t p  2  exp k2  t p  3  exp k3  t p .
(2)
All the experimental data were fitted from ~ 60 fs after the zero time delay in order to
 avoid the coherent spike (non-time ordered signals) [Ref. 23]. The initial decays of the
cross-polarized signals with VHVH and VHHV polarization sequences have opposite
signs, which is a prediction of the different rotational averaging captured by each crosspolarized signal [Ref. 22]. The cross-polarized signals were fit by an exponential function
and an offset, multiplied by the VVVV decay function, as dictated by the theory behind
the measurement [Refs. 22,24]:


IVHVH t p  A1  exp 2k s  t p  A2  IVVVV t p ,
(3)
where k s is the rate of exciton spin relaxation and I VVVV (t p ) is the decay profile of the
B
B
B
B
B
B
VVVVsignal as a function of the pump-probe time delay, t p , and A 2 is the amplitude of
B
B
B
B
10
the exciton recombination dynamics represented by the VVVV signal. The fitted negative
amplitudes (A 1 ) for VHHV data indicate the opposite sign of the rotational averaging
B
B
factors for VHHV measurement compared to that for VHVH experiments.
Table S_2: VVVV fitting parameters for the CPH-3TG studies of CdTe/CdSe core-shell
NCs.
(VVVV) a
P
P
A1
B
B
k 1 (ps −1 )
A2
B
B
P
P
B
B
k 2 (ps −1 )
A3
B
B
P
P
B
B
k 3 (ps −1 ) b
B
B
P
P
P
core
0.393
0.746
0.381
0.0715
0.324
0.00005
0.2-nm shell
0.133
1.25
0.112
0.113
0.0303
0.00005
0.4-nm shell
0.0289
1.19
0.0265
0.0944
0.00962
0.00005
0.8-nm shell
0.0745
0.526
0.0572
0.0589
0.0723
0.00005
1.2-nm shell
0.0410
0.326
0.0673
0.0369
0.0731
0.00005
1.6-nm shell
0.0608
0.165
0.180
0.0269
0.0595
0.00005
2.0-nm shell
0.0485
0.629
0.0668
0.0482
0.0360
0.00005
P
a
VVVV data were fitted using Equation (2) in the previous section. b k 3 was determined
by time-resolved PL measurements (Section iv: Table S_1) and fixed during the current
fitting procedure.
P
P
P
P
B
B
11
Table S_3: Spin flip fitting parameters for the CPH-3TG studies of CdTe/CdSe coreshell NCs.
(VHVH)
(VHHV)
P
P
a
a
P
a
P
P
P
Ks (ps −1 )
A2
Ks (ps −1 )
A2
0.499
2.60
−0.0162
−0.445
2.69
−0.0254
0.2-nm shell
0.648
2.34
0.0894
−0.556
2.08
0.0100
0.4-nm shell
0.492
2.16
−0.00884
−0.360
2.22
0.00897
0.8-nm shell
0.120
2.03
-0.0232
-0.137
1.89
0.00296
1.2-nm shell
0.179
0.573
0.00119
−0.129
0.363
−0.0101
1.6-nm shell
0.0582
0.221
0.00323
−0.0675
0.159
−0.0103
2.0-nm shell
0.285
0.202
−0.00657
−0.281
0.0903
−0.0388
2.4-nm shell
0.0426
0.0891
0.0148
−0.0343
0.0786
−0.00273
Sample
A1
core
B
B
P
P
B
A1
B
B
B
P
P
B
B
VHVH and VHHV data were fitted using Equation (3).
12
vi. Determination of the electron transfer rate of type-II core-shell NCs with the
VVVV measurement.
To determine the electron transfer rate from CdTe core to CdSe shell, we choose
excitation and probing of the CdSe shell. The electronic energy transfer from CdSe to
CdTe is too fast for us to resolve (Ref. 26). The electron subsequently transfers back from
CdTe core to CdSe shell, which manifests itself by the fast rise process (~ 600 fs). We
rule out hole transfer directly from photoexcited CdSe because this hole transfer would
not lead to an increased bleach or transient absorption signal at the CdSe resonance
compared to the CdSe exciton. In this case, the VVVV data can be fitted as follows.


IVVVV t p   1  1  exp k1  t p    2  exp k 2  t p 
The decay component of ~ 3040 ps can be assigned to the electron trapping at the CdSe
surface (Ref. 26).

0.7
CdTe/CdSe 2.4-nm shell
VVVV
fitting curve
Intensity (a.u.)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
10
20
30
40
Time delay, tp (ps)
Figure S_5: VVVV measurement of type-II CdTe/CdSe core-shell NCs with
photoexcitation at CdSe exciton absorption band.
Table S_4: Fitting parameters for the VVVV measurement of type-II CdTe/CdSe coreshell NCs with resonant photoexcitation at CdSe exciton absorption band.
CdTe/CdSe NC
A1
B
(2.4-nm shell)
VVVV
B
0.284
k 1 (1/ps)
A2
1.71
0.258
B
B
B
B
k 2 (1/ps)
B
B
0.0271
13
vii. Modeling of strain and band structure.
Simple modeling of the system to show the transition to a type-II core-shell structure as a
function of shell thickness was performed using an effective mass model, where band
offsets were calculated using a combination of the “model-solid theory” and a continuum
elasticity model of concentric spheres [28]. The natural band alignment, the strain effects
and the quantum confinement are all important for the core-shell NCs.
(a)
Ec1
Ue
Ec2
Eg1
Ev1
Eg2
Uh
Ev2
CdTe
CdSe
1.2
1.0
(b)
0.8
Ue,h(eV)
0.6
0.4
0.2
0.0
Ue
Uh
-0.2
-0.4
-0.6
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Shell thickness (nm)
Figure S_6: (a) Illustration of band alignment (b) calculated relative energy
shift for type-II CdTe-CdSe core-shell structures.
14
viii. CPH-3TG measurements on samples with different surface ligands
In order to evaluate the effect of NC surface on spin flip dynamics, we perform the CPH3TG measurements on the same batch of sample with different surface ligand (N-BUTYL
AMINE). No obvious difference was observed within the experimental error.
0
10
20
CdTe/CdSe 2.4-nm shell
VHVH
VHHV
after ligand change
Intensity (a.u.)
Intensity (a.u.)
CdTe/CdSe 2.4-nm shell
VHVH
VHHV
before ligand change
30
40
0
10
Time delay, tp (ps)
20
30
40
Time delay, tp (ps)
Figure S_7: CPH-3TG measurements on type-II CdTe-CdSe core-shell structures
with different NC surface ligands.
Table S_5: Spin flip fitting parameters for CdTe/CdSe core-shell NCs before and after
surface ligand change.
(VHVH)
(VHHV)
P
P
a
a
P
CdTe/CdSe
(2.4-nm shell)
A1
before change
after change
a
P
P
Ks (ps −1 )
A2
0.0426
0.0891
0.0148
0.0235
0.0821
0.0161
B
B
P
P
B
B
Ks (ps −1 )
A2
−0.0343
0.0786
−0.00273
−0.0289
0.0727
0.00113
A1
B
B
P
P
B
B
VHVH and VHHV data were fitted using Equation (3) in Section v.
15