Supporting Information Ultrafast Charge Separation from Highly ...
Supporting Information Ultrafast Charge Separation from Highly Reductive ZnTe/CdSe Type II Quantum Dots Shengye Jin,1,2‡ Jun Zhang,1‡ Richard D. Schaller,1,3 Tijana Rajh,1* Gary P. Wiederrecht1,2 * 1
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL; 2ArgonneNorthwestern Solar Energy Research Center, Northwestern University, Evanston, IL. 3 Department of Chemistry, Northwestern University, Evanston, IL.
Synthesis of ZnTe seeds and ZnTe/CdSe core/shell type II QDs. Chemicals. Trioctylphosphine (TOP, 90%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), dioctyl ether, metal tellurium (Te, 99.999%), selenium (Te, >99.99%), zinc acetate (99.9%) and super-hydride (LiBH(CH2CH3)3) solution in THF (1 M) were all Aldrich products and used as purchased without further purification. Anhydrous ethanol and hexane were purchased from various sources. Superhydride solution in dioctyl ether (1 M) was prepared by exchanging the solvent according to the literature.1 Stock solutions. Tellurium-trioctylphosphine (Te-TOP, 1M for Te) solution was prepared by dissolving metallic Te into TOP in a glovebox. Selenium -trioctylphosphine (Se-TOP, 1M for Se) solution was pre-prepared using a similar method. Cd precursor solution preparation: 0.32 g of CdO, 6.18 g of oleic acid and 18mL of ODE were loaded in a 100mL flask and heated to 260 oC under Ar gas flow and kept at the same temperature until the CdO powder was dissolved completely. The resulting clear solution was then cooled down to room temperature and stored in a glovebox. Synthesis of ZnTe seeds. The synthesis of ZnTe seeds followed previously reported method with some modifications.2 Zinc acetate (1 mmol), benzyl ether (15 mL), and oleic acid (1 mL) were mixed in a flask equipped with Schlenk-line and heated to 150 oC for 30 min under vacuum to remove moisture. The temperature was then raised to 250 oC. A mixture of Te-TOP solution (1 mL) and 1 M superhydride solution in dioctyl ether (1 mL) was then rapidly injected into this hot solution. The colloidal system was kept at 250 o C for 10 min before being cooled down. The products were separated by adding an excessive amount of ethanol followed by centrifugation. The isolated nanoparticles were redispersed in 10 mL of hexane, producing ZnTe colloidal solution. Synthesis of ZnTe/CdSe core/shell QDs. The growth of CdSe shell on ZnTe seeds was according to a reported method.3 5 mL of ZnTe colloidal solution, 10 mL ODE and 0.5 mL of Se-TOP were mixed in a flask and slowly heated to 170 oC under argon gas flow.
1
0.5 mL of Cd precursor solution was added dropwise into the flask during a period of 10 minutes. The reaction system was kept at 170 oC for an additional 10 minutes for CdSe shell growth. The product was separated by using the same procedure as for bare ZnTe nanoparticles and was re-dissolved in hexane or toluene.
Experimental setup for TA measurements. Ultrafast transient absorption measurements were carried out using a Helios spectrometer (Ultrafast Systems). An amplified Ti:Sapphire pulse (800 nm, 120 fs, 1.67 kHz repetition rate utilizing a Spectra-Physics Spitfire Pro) was split into two beams. The first beam, containing 10% of the power, was focused into a sapphire window to generate a white light continuum (440 nm – 750 nm), which serves as the probe. The other beam, containing 90% of the power, was sent into an optical parametric amplifier (SpectraPhysics TOPAS) to generate the pump beam. After the pump beam was sent through a depolarizer, it was focused and overlapped with the probe beam at the sample. The pump power was set at 60 nJ/pulse for the 400 nm pump and 120 nJ/pulse for the 585 nm pump. The kinetics exhibited no pump-power dependent features at these pump energies (60~120 nJ). The QD samples were kept in a 2 mm cuvette and constantly stirred by a magnetic stirrer to avoid thermal lensing.
2
Figure S1. TEM images of (A) ZnTe seeds and (B) ZnTe/CdSe QDs. (C) Distributions of ZnTe seed and ZnTe/CdSe QD diameters. The average diameter of the ZnTe seed and ZnTe/CdSe QD is 4.6±0.4 nm and 5.8±0.4 nm, respectively. TEM images were obtained using a JEOL 7500 FEG SEM operating at 30kV with a transmitted electron detector.
3
Intensity (a.u.)
800 A1 τ1 A2 τ2 61% 0.25 ns 18% 2.4ns
600
A3 τ3 21% 40ns
400 200 0 0
5
10 Delay Time (ns)
15
Figure S2. Fluorescence decay (black circles) of ZnTe/CdSe type II QDs in toluene. Solid red line is the multi-exponential fit of the decay with fitting parameters Ai (amplitude) and τi (lifetime) listed with i=1, 2, 3. The slow component suggests the presence of slow electron and hole recombination dynamics in type II QDs.
The assignment of the positive absorption in the region red of 650 nm in the TA spectra (Figure 3a and Figure 4a) of free ZnTe/CdSe QDs The positive absorption signals in the region red of 650 nm in the TA spectra of free ZnTe/CdSe QDs (Figure 3a and 4a) are attributed to the excitation of conduction band electrons and/or valence band holes to higher energy levels after initial excitation. Because the absorption spectrum of AQ‾ (centered at ~680 nm) has a large overlap with the intrinsic signals of QDs at this region, it is necessary to identify the origin of the positive absorption signals in order to obtain accurate dynamics of AQ‾ for the external charge separation. Therefore, we have conducted the TA measurement on ZnTe/CdSeBQ electron transfer complex. BQ is 1,4-benzoquinone. Like AQ molecules, BQ can also work as an efficient electron acceptor in the complex.4 However, the absorption of its anion radical (BQ‾) generated after charge separation is < 400nm, away from the TA 4
spectra of ZnTe/CdSe QDs. If the positive absorption signal in red of 650 nm is due to the absorption of conduction band electrons, it should be significantly reduced when the external electron transfer occurs in ZnTe/CdSe-BQ complex. The TA spectra of ZnTe/CdSe free QDs and ZnTe/CdSe-BQ complexes are shown in Figure S4. The two samples contained the same amount of QDs and were measured at identical experimental conditions. We observed depletion of bleach signals at 500 and 600 nm bands in ZnTe/CdSe-BQ complexes, suggesting the occurrence of external electron transfer from QDs to BQs. However, the positive absorption signals in the region red of 650 nm were not changed in ZnTe/CdSe-BQ complexes compared with free ZnTe/CdSe QDs. We therefore attributed the positive absorption in this region to the absorption of valence band holes, whose signal amplitude is not affected by the external electron transfer from QDs to BQs or AQs.
∆A (m O.D.)
2
a
ZnTe/CdSe Only
0 Delay Time (ps) 0 1.5 5.5 10 50 2530
-2 -4 -6 500
550 600 650 700 Wavelength (nm)
2
∆A (m O.D.)
b
ZnTe/CdSe-BQ
0 Delay Time (ps) 0 1 3 6.5 50 2530
-2 -4 500
550 600 650 Wavelength (nm)
700
Figure S3. TA spectra of free (a) ZnTe/CdSe QDs and (b) ZnTe/CdSe-BQ complexes at indicated delay times after 400 nm excitation.
5
Figure S4. Radial distribution function of the lowest energy (1s) conduction band electron and valence band hole levels in ZnTe/CdSe core/shell type II QDs. Black dash lines indicate the positions of core/shell and shell/ligand interfaces. The calculation is by treating the electron and hole as particles confined in spherical wells of finite depth. The calculation of the 1se and 1sh energy levels is based on the following parameters. The effective mass of electrons (me*) and holes (mh*) is 0.12 and 0.6 in ZnTe,5,6 0.13 and 0.45 in CdSe.7,8 The bulk conduction and valence band edges are -2.7 and -5 eV (vs. Vacuum) in ZnTe,9,10 and -4 and -5.7 eV in CdSe.11
Fitting of the TA kinetics in Figure 3b and c The TA kinetics of bleach formation and recovery in ZnTe/CdSe QDs with 400 nm pump in Figure 3b and c can be fit by the following equations to obtain internal charge separation and recombination rates: , 600 , 500 ∑ , 500 ! " # ∑
(Eq. S2)
, 500 !
(Eq. S3)
(Eq. S1)
A(t, 600 nm)T2 is the kinetics at 600 nm and associated with T2 transition. A(t, 500 nm)T1+T3 is the kinetics at 500 nm and it contains contributions from both T1 and T3 transitions. A(t, 500 nm)T1 is the kinetics of pure T1 transition band at 500 nm. kIET is the internal electron transfer rate from ZnTe core to CdSe shell and k0 is the 1s exciton bleach formation rate in ZnTe core. ai and kIRi (i=1-4) are the amplitude and time constant of the ith component of multi-exponential function that describes the relaxation process of 6
the 1s electrons in CdSe shell. The kinetics at 600 nm in Figure 3b and c are composed of only transition T2 and can be fit by Eq. S1. The kinetics at 500 nm in Figure 2b and c involve the contributions from both T1 and T3 transitions, and are fit by Eq. S2. The parameter C accounts for the amplitude of the contribution from transition T3. The kinetics at the T1 transition band are described by Eq. S3. As shown in Figure 3b and c, the kinetics are well fit by these equations and the fitting parameters are listed in Table S1. Table S1. Fitting parameters of the TA kinetics in Figure 3b and c according to Eq. S1 to S3. τIR, ave. is the amplitude-weighted average internal charge recombination time. k0 (ps-1)
kIET (ps-1)
a1
kIR1 (ps-1)
a2
kIR2 (ps-1)
a3
kIR3 (ps-1)
a4
kIR4 (ps-1)
τIR,avg
3.5
1.5
0.49
0.96
0.3
0.31
0.12
0.02
0.09
7.6×10-4
115
(ps).
7
2
∆A (m OD)
1
a
0 Delay Time (ps)
-1
0.5 1 5 50 2580
-2 -3 -4 450
500
550 600 650 Wavelength (nm)
0
b
-1
∆A (m OD)
∆A (m OD)
0
700
-2
-1
-3
-3
-4
-4 0.0
0.5 1.0 1.5 Delay Time (ps)
2.0
ZnTe Seed (500 nm) ZnTe/CdSe (600 nm) fit
-2
c
0
20 40 1000 2000 Delay Time (ps)
Figure S5. TA spectra (a) of ZnTe seed QD as indicated delay times after 400 nm excitation. TA kinetics probed at 500 nm (blue circles) in the early (b) and long (c) time windows. The kinetics were fit by a multi-exponential function yielding an amplitudeweighted bleach recovery (1s electron relaxation) time of 23 ps. For comparison, the TA kinetics of ZnTe/CdSe type II QD probed at 600 nm (red squares) was also shown, indicating a slower electron relaxation time due to the separation of electrons (in the CdSe shell) and holes (in the ZnTe core)
8
2.0
Absorption
1.5 QD-AQ
1.0
0.5
Absorption
0.2
0.1
0.0 320 QD
340 360 380 Wavelength (nm)
400
AQ
0.0 400
500 600 Wavelength (nm)
700
Figure S6. UV-Vis absorption spectra of ZnTe/CdSe QD (black solid line) and ZnTe/CdSe-AQ complexes (red dashed line) in toluene, indicating the presence of AQ molecules and the same amount of QDs in two samples. The absorption spectrum of AQ molecules is obtained by subtraction. An extended view is shown in the inset. The ZnTe/CdSe-AQ complex was prepared by adding 10 µl of AQ dissolved in methanol (0.025 M) into 1 ml QD toluene solution. The mixture was first sonicated and then filtered to obtain the ZnTe/CdSe-AQ complexes. The AQ molecules adsorb on the surfaces of QDs since they are not soluble in toluene. The extinction coefficient of AQ is 4405 cm-1M-1.12 The amount of AQ is calculated to be 1.1×10-7 mol in 1 ml QD-AQ complex solution. Because the extinction coefficient of the ZnTe/CdSe type II QD is unknown, the amount of the QD is estimated from the ZnTe seeds. Based on the amount of chemicals (Zn and Te) added in the ZnTe synthesis (assuming a yield of 80%), the size of the ZnTe seed (4.6 nm diameter) and the density of ZnTe bulk (6.34 g/cm3), the amount of the ZnTe/CdSe QDs in 1 ml QD-AQ solution is estimated to be 1.2×10-7 mol. The number of AQ molecules per QD is ~0.9. However, considering the loss of the QDs in the syntheses and purification process, the real number of AQ molecules per QD can be larger than the estimated number. We estimated the number to be 1~2 by assuming the loss of QDs is up to ~50% in the synthesis and purification processes.
Fitting of the TA kinetics in Figure 5c and d. The TA kinetics in Figure 5c and d probed at 680 nm indicate AQ‾ formation and relaxation dynamics. Because the absorption spectrum of AQ‾ has a large overlap with the intrinsic signals of QDs (due to absorption of holes in valence band) at this probed region, the TA kinetics include contributions from the holes in QD valence band. The formation of holes is through direct excitation. The holes and AQ‾ share the same
9
relaxation kinetics through the external charge recombination process from AQ‾ to valence band hole. The kinetics in Figure 5c and d are then fit according to the following equation:
, 680 1 # &'( " # ) " ∑
(Eq. S4)
Where kAQ- is the time constant for the formation of AQ‾; kh is the time constant for the formation of valence band hole; C is a constant indicating the contribution from hole relative to AQ‾ in the signal amplitude; the last term in the equation describes the external charge recombination kinetics from AQ‾ to the hole in valence band. ai and kERi (i=1-4) are the amplitude and time constant of the ith component of the multi-exponential function. The constant C is pre-obtained by comparing the TA amplitude at 680 nm between ZnTe/CdSe free QDs and ZnTe/CdSe-AQ complexes. kh is pre-obtained by fitting the TA kinetics probed at 680 nm of free ZnTe/CdSe QDs in Figure 2A and Figure S5 for 400 and 585 nm excitations, respectively. As shown in Figure 5c and d, the kinetics are well fit by this equation. The fitting parameters are listed in Table S2.
Table S2. Fitting parameters of the TA kinetics in Figure 5c and d according to Eq. S4.
τER, ave. is the amplitude-weighted average external charge recombination time. Excitation
C
kAQ‾ (ps-1)
kh (ps-1)
400 nm
0.5
1.4
2.4
500 nm
0.4
>5.0
>5.1
Excitation
a1
kER1 (ps-1)
a2
kER2 (ps-1)
a3
kER3 (ps-1)
a4
kER4 (ps-1)
τER,avg (ps)
400 nm
0.74
0.95
0.1
0.09
0.05
0.008
0.11 3.9×10-4
314
500 nm
0.5
0.82
0.28
0.085
0.09
0.009
0.13 3.9×10-4
347
With 585 nm excitation, electrons were directly excited to the CdSe shell, and kAQ‾ = kEET = 5.0 ps-1 (0.2 ps), where kEET is the external electron transfer rate from CdSe shell to AQ.
10
With 400 nm excitation, electrons were excited to the ZnTe core, and kAQ‾ = kIET + kEET ≈ kIET ≈ 1.4 ps-1 (0.71 ps).
References: (1) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325-4330. (2) Zhang, J.; Sun, K.; Kumbhar, A.; Fang, J. J. Phys. Chem. C 2008, 112, 5454-5458. (3) Xie, R.; Zhong, X.; Basché, T. Adv. Mater. 2005, 17, 2741-2745. (4) Zhu, H. M.; Song, N. H.; Rodríguez-Córdoba, W.; Lian, T. Q. J. Am. Chem. Soc. 2012, ASAP. (5) Venghaus, H. J. Phys. C: Solid State Phys. 1984, 17, 6229-6243. (6) Bhunia, S.; Bose, D. N. J. App. Phys. 2000, 87, 2931-2935. (7) Berger, L. I. Semiconductor Materials; CRC Press: Boca Raton, FL, 1997. (8) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (9) Pearton, S. J. Wide Bandgap Semiconductors, William Andrew Publishing: New York, 2000, p9. (10) Schrier, J.; Demchenko, D. O.; Wang, L. W.; Alivisatos, A. P. Nano Lett. 2007, 7, 2377-2382. (11) Zhu, H.; Song, N.; Lian, T. J. Am. Chem. Soc. 2011, 133, 8762-8771. (12) Du, H.; Fuh, R. A.; Li, J.; Corkan, A.; Lindsey, J. S. Photochem. Photobiol 1998, 68, 141-142.
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Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL; 2ArgonneNorthwestern Solar Energy Research Center, Northwestern University, Evanston, IL. 3 Department of Chemistry, Northwestern University, Evanston, IL.
Synthesis of ZnTe seeds and ZnTe/CdSe core/shell type II QDs. Chemicals. Trioctylphosphine (TOP, 90%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), dioctyl ether, metal tellurium (Te, 99.999%), selenium (Te, >99.99%), zinc acetate (99.9%) and super-hydride (LiBH(CH2CH3)3) solution in THF (1 M) were all Aldrich products and used as purchased without further purification. Anhydrous ethanol and hexane were purchased from various sources. Superhydride solution in dioctyl ether (1 M) was prepared by exchanging the solvent according to the literature.1 Stock solutions. Tellurium-trioctylphosphine (Te-TOP, 1M for Te) solution was prepared by dissolving metallic Te into TOP in a glovebox. Selenium -trioctylphosphine (Se-TOP, 1M for Se) solution was pre-prepared using a similar method. Cd precursor solution preparation: 0.32 g of CdO, 6.18 g of oleic acid and 18mL of ODE were loaded in a 100mL flask and heated to 260 oC under Ar gas flow and kept at the same temperature until the CdO powder was dissolved completely. The resulting clear solution was then cooled down to room temperature and stored in a glovebox. Synthesis of ZnTe seeds. The synthesis of ZnTe seeds followed previously reported method with some modifications.2 Zinc acetate (1 mmol), benzyl ether (15 mL), and oleic acid (1 mL) were mixed in a flask equipped with Schlenk-line and heated to 150 oC for 30 min under vacuum to remove moisture. The temperature was then raised to 250 oC. A mixture of Te-TOP solution (1 mL) and 1 M superhydride solution in dioctyl ether (1 mL) was then rapidly injected into this hot solution. The colloidal system was kept at 250 o C for 10 min before being cooled down. The products were separated by adding an excessive amount of ethanol followed by centrifugation. The isolated nanoparticles were redispersed in 10 mL of hexane, producing ZnTe colloidal solution. Synthesis of ZnTe/CdSe core/shell QDs. The growth of CdSe shell on ZnTe seeds was according to a reported method.3 5 mL of ZnTe colloidal solution, 10 mL ODE and 0.5 mL of Se-TOP were mixed in a flask and slowly heated to 170 oC under argon gas flow.
1
0.5 mL of Cd precursor solution was added dropwise into the flask during a period of 10 minutes. The reaction system was kept at 170 oC for an additional 10 minutes for CdSe shell growth. The product was separated by using the same procedure as for bare ZnTe nanoparticles and was re-dissolved in hexane or toluene.
Experimental setup for TA measurements. Ultrafast transient absorption measurements were carried out using a Helios spectrometer (Ultrafast Systems). An amplified Ti:Sapphire pulse (800 nm, 120 fs, 1.67 kHz repetition rate utilizing a Spectra-Physics Spitfire Pro) was split into two beams. The first beam, containing 10% of the power, was focused into a sapphire window to generate a white light continuum (440 nm – 750 nm), which serves as the probe. The other beam, containing 90% of the power, was sent into an optical parametric amplifier (SpectraPhysics TOPAS) to generate the pump beam. After the pump beam was sent through a depolarizer, it was focused and overlapped with the probe beam at the sample. The pump power was set at 60 nJ/pulse for the 400 nm pump and 120 nJ/pulse for the 585 nm pump. The kinetics exhibited no pump-power dependent features at these pump energies (60~120 nJ). The QD samples were kept in a 2 mm cuvette and constantly stirred by a magnetic stirrer to avoid thermal lensing.
2
Figure S1. TEM images of (A) ZnTe seeds and (B) ZnTe/CdSe QDs. (C) Distributions of ZnTe seed and ZnTe/CdSe QD diameters. The average diameter of the ZnTe seed and ZnTe/CdSe QD is 4.6±0.4 nm and 5.8±0.4 nm, respectively. TEM images were obtained using a JEOL 7500 FEG SEM operating at 30kV with a transmitted electron detector.
3
Intensity (a.u.)
800 A1 τ1 A2 τ2 61% 0.25 ns 18% 2.4ns
600
A3 τ3 21% 40ns
400 200 0 0
5
10 Delay Time (ns)
15
Figure S2. Fluorescence decay (black circles) of ZnTe/CdSe type II QDs in toluene. Solid red line is the multi-exponential fit of the decay with fitting parameters Ai (amplitude) and τi (lifetime) listed with i=1, 2, 3. The slow component suggests the presence of slow electron and hole recombination dynamics in type II QDs.
The assignment of the positive absorption in the region red of 650 nm in the TA spectra (Figure 3a and Figure 4a) of free ZnTe/CdSe QDs The positive absorption signals in the region red of 650 nm in the TA spectra of free ZnTe/CdSe QDs (Figure 3a and 4a) are attributed to the excitation of conduction band electrons and/or valence band holes to higher energy levels after initial excitation. Because the absorption spectrum of AQ‾ (centered at ~680 nm) has a large overlap with the intrinsic signals of QDs at this region, it is necessary to identify the origin of the positive absorption signals in order to obtain accurate dynamics of AQ‾ for the external charge separation. Therefore, we have conducted the TA measurement on ZnTe/CdSeBQ electron transfer complex. BQ is 1,4-benzoquinone. Like AQ molecules, BQ can also work as an efficient electron acceptor in the complex.4 However, the absorption of its anion radical (BQ‾) generated after charge separation is < 400nm, away from the TA 4
spectra of ZnTe/CdSe QDs. If the positive absorption signal in red of 650 nm is due to the absorption of conduction band electrons, it should be significantly reduced when the external electron transfer occurs in ZnTe/CdSe-BQ complex. The TA spectra of ZnTe/CdSe free QDs and ZnTe/CdSe-BQ complexes are shown in Figure S4. The two samples contained the same amount of QDs and were measured at identical experimental conditions. We observed depletion of bleach signals at 500 and 600 nm bands in ZnTe/CdSe-BQ complexes, suggesting the occurrence of external electron transfer from QDs to BQs. However, the positive absorption signals in the region red of 650 nm were not changed in ZnTe/CdSe-BQ complexes compared with free ZnTe/CdSe QDs. We therefore attributed the positive absorption in this region to the absorption of valence band holes, whose signal amplitude is not affected by the external electron transfer from QDs to BQs or AQs.
∆A (m O.D.)
2
a
ZnTe/CdSe Only
0 Delay Time (ps) 0 1.5 5.5 10 50 2530
-2 -4 -6 500
550 600 650 700 Wavelength (nm)
2
∆A (m O.D.)
b
ZnTe/CdSe-BQ
0 Delay Time (ps) 0 1 3 6.5 50 2530
-2 -4 500
550 600 650 Wavelength (nm)
700
Figure S3. TA spectra of free (a) ZnTe/CdSe QDs and (b) ZnTe/CdSe-BQ complexes at indicated delay times after 400 nm excitation.
5
Figure S4. Radial distribution function of the lowest energy (1s) conduction band electron and valence band hole levels in ZnTe/CdSe core/shell type II QDs. Black dash lines indicate the positions of core/shell and shell/ligand interfaces. The calculation is by treating the electron and hole as particles confined in spherical wells of finite depth. The calculation of the 1se and 1sh energy levels is based on the following parameters. The effective mass of electrons (me*) and holes (mh*) is 0.12 and 0.6 in ZnTe,5,6 0.13 and 0.45 in CdSe.7,8 The bulk conduction and valence band edges are -2.7 and -5 eV (vs. Vacuum) in ZnTe,9,10 and -4 and -5.7 eV in CdSe.11
Fitting of the TA kinetics in Figure 3b and c The TA kinetics of bleach formation and recovery in ZnTe/CdSe QDs with 400 nm pump in Figure 3b and c can be fit by the following equations to obtain internal charge separation and recombination rates: , 600 , 500 ∑ , 500 ! " # ∑
(Eq. S2)
, 500 !
(Eq. S3)
(Eq. S1)
A(t, 600 nm)T2 is the kinetics at 600 nm and associated with T2 transition. A(t, 500 nm)T1+T3 is the kinetics at 500 nm and it contains contributions from both T1 and T3 transitions. A(t, 500 nm)T1 is the kinetics of pure T1 transition band at 500 nm. kIET is the internal electron transfer rate from ZnTe core to CdSe shell and k0 is the 1s exciton bleach formation rate in ZnTe core. ai and kIRi (i=1-4) are the amplitude and time constant of the ith component of multi-exponential function that describes the relaxation process of 6
the 1s electrons in CdSe shell. The kinetics at 600 nm in Figure 3b and c are composed of only transition T2 and can be fit by Eq. S1. The kinetics at 500 nm in Figure 2b and c involve the contributions from both T1 and T3 transitions, and are fit by Eq. S2. The parameter C accounts for the amplitude of the contribution from transition T3. The kinetics at the T1 transition band are described by Eq. S3. As shown in Figure 3b and c, the kinetics are well fit by these equations and the fitting parameters are listed in Table S1. Table S1. Fitting parameters of the TA kinetics in Figure 3b and c according to Eq. S1 to S3. τIR, ave. is the amplitude-weighted average internal charge recombination time. k0 (ps-1)
kIET (ps-1)
a1
kIR1 (ps-1)
a2
kIR2 (ps-1)
a3
kIR3 (ps-1)
a4
kIR4 (ps-1)
τIR,avg
3.5
1.5
0.49
0.96
0.3
0.31
0.12
0.02
0.09
7.6×10-4
115
(ps).
7
2
∆A (m OD)
1
a
0 Delay Time (ps)
-1
0.5 1 5 50 2580
-2 -3 -4 450
500
550 600 650 Wavelength (nm)
0
b
-1
∆A (m OD)
∆A (m OD)
0
700
-2
-1
-3
-3
-4
-4 0.0
0.5 1.0 1.5 Delay Time (ps)
2.0
ZnTe Seed (500 nm) ZnTe/CdSe (600 nm) fit
-2
c
0
20 40 1000 2000 Delay Time (ps)
Figure S5. TA spectra (a) of ZnTe seed QD as indicated delay times after 400 nm excitation. TA kinetics probed at 500 nm (blue circles) in the early (b) and long (c) time windows. The kinetics were fit by a multi-exponential function yielding an amplitudeweighted bleach recovery (1s electron relaxation) time of 23 ps. For comparison, the TA kinetics of ZnTe/CdSe type II QD probed at 600 nm (red squares) was also shown, indicating a slower electron relaxation time due to the separation of electrons (in the CdSe shell) and holes (in the ZnTe core)
8
2.0
Absorption
1.5 QD-AQ
1.0
0.5
Absorption
0.2
0.1
0.0 320 QD
340 360 380 Wavelength (nm)
400
AQ
0.0 400
500 600 Wavelength (nm)
700
Figure S6. UV-Vis absorption spectra of ZnTe/CdSe QD (black solid line) and ZnTe/CdSe-AQ complexes (red dashed line) in toluene, indicating the presence of AQ molecules and the same amount of QDs in two samples. The absorption spectrum of AQ molecules is obtained by subtraction. An extended view is shown in the inset. The ZnTe/CdSe-AQ complex was prepared by adding 10 µl of AQ dissolved in methanol (0.025 M) into 1 ml QD toluene solution. The mixture was first sonicated and then filtered to obtain the ZnTe/CdSe-AQ complexes. The AQ molecules adsorb on the surfaces of QDs since they are not soluble in toluene. The extinction coefficient of AQ is 4405 cm-1M-1.12 The amount of AQ is calculated to be 1.1×10-7 mol in 1 ml QD-AQ complex solution. Because the extinction coefficient of the ZnTe/CdSe type II QD is unknown, the amount of the QD is estimated from the ZnTe seeds. Based on the amount of chemicals (Zn and Te) added in the ZnTe synthesis (assuming a yield of 80%), the size of the ZnTe seed (4.6 nm diameter) and the density of ZnTe bulk (6.34 g/cm3), the amount of the ZnTe/CdSe QDs in 1 ml QD-AQ solution is estimated to be 1.2×10-7 mol. The number of AQ molecules per QD is ~0.9. However, considering the loss of the QDs in the syntheses and purification process, the real number of AQ molecules per QD can be larger than the estimated number. We estimated the number to be 1~2 by assuming the loss of QDs is up to ~50% in the synthesis and purification processes.
Fitting of the TA kinetics in Figure 5c and d. The TA kinetics in Figure 5c and d probed at 680 nm indicate AQ‾ formation and relaxation dynamics. Because the absorption spectrum of AQ‾ has a large overlap with the intrinsic signals of QDs (due to absorption of holes in valence band) at this probed region, the TA kinetics include contributions from the holes in QD valence band. The formation of holes is through direct excitation. The holes and AQ‾ share the same
9
relaxation kinetics through the external charge recombination process from AQ‾ to valence band hole. The kinetics in Figure 5c and d are then fit according to the following equation:
, 680 1 # &'( " # ) " ∑
(Eq. S4)
Where kAQ- is the time constant for the formation of AQ‾; kh is the time constant for the formation of valence band hole; C is a constant indicating the contribution from hole relative to AQ‾ in the signal amplitude; the last term in the equation describes the external charge recombination kinetics from AQ‾ to the hole in valence band. ai and kERi (i=1-4) are the amplitude and time constant of the ith component of the multi-exponential function. The constant C is pre-obtained by comparing the TA amplitude at 680 nm between ZnTe/CdSe free QDs and ZnTe/CdSe-AQ complexes. kh is pre-obtained by fitting the TA kinetics probed at 680 nm of free ZnTe/CdSe QDs in Figure 2A and Figure S5 for 400 and 585 nm excitations, respectively. As shown in Figure 5c and d, the kinetics are well fit by this equation. The fitting parameters are listed in Table S2.
Table S2. Fitting parameters of the TA kinetics in Figure 5c and d according to Eq. S4.
τER, ave. is the amplitude-weighted average external charge recombination time. Excitation
C
kAQ‾ (ps-1)
kh (ps-1)
400 nm
0.5
1.4
2.4
500 nm
0.4
>5.0
>5.1
Excitation
a1
kER1 (ps-1)
a2
kER2 (ps-1)
a3
kER3 (ps-1)
a4
kER4 (ps-1)
τER,avg (ps)
400 nm
0.74
0.95
0.1
0.09
0.05
0.008
0.11 3.9×10-4
314
500 nm
0.5
0.82
0.28
0.085
0.09
0.009
0.13 3.9×10-4
347
With 585 nm excitation, electrons were directly excited to the CdSe shell, and kAQ‾ = kEET = 5.0 ps-1 (0.2 ps), where kEET is the external electron transfer rate from CdSe shell to AQ.
10
With 400 nm excitation, electrons were excited to the ZnTe core, and kAQ‾ = kIET + kEET ≈ kIET ≈ 1.4 ps-1 (0.71 ps).
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