Ultrafast Carrier Dynamics and Efficient Triplet Generation in Black

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Ultrafast Carrier Dynamics and Efficient Triplet Generation in Black Phosphorus Quantum Dots Lan Chen, Chunfeng Zhang, Long Li, Han Wu, Xiaoyong Wang, Shancheng Yan, Yi Shi, and Min Xiao J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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The Journal of Physical Chemistry

Ultrafast Carrier Dynamics and Efficient Triplet Generation in Black Phosphorus Quantum Dots

Lan Chen,† Chunfeng Zhang,†,‡,* Long Li, † Han Wu, § Xiaoyong Wang,† Shancheng Yan,§,* Yi Shi,ǁ and Min Xiao†,‡, #,*

†

National Laboratory of Solid State Microstructures, School of Physics & Collaborative

Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡

Synergetic Innovation Center in Quantum Information and Quantum Physics, University of

Science and Technology of China, Hefei, Anhui 230026, China §

School of Geography and Biological Information, Nanjing University of Posts and

Telecommunications, Nanjing 210023, China ǁ

School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China

#

Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States

1

ACS Paragon Plus Environment


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ABSTRACT The two-dimensional few-layer black phosphorus (BP) has been proposed as a new type of semiconductor for optoelectronic applications. Beyond optoelectronics, the BP nanostructures have recently shown great potential in producing singlet oxygen for photodynamic therapy, but its underlying mechanism remains elusive. Here, we report the observation of efficient triplet formation in BP quantum dots, which might be responsible for the singlet oxygen generation. In addition to the common dynamic behaviors of semiconductors, a long-lived photo-induced bleaching signal with a lifetime of 26 µs has been explicitly identified in BP quantum dots， which can be ascribed to the intersystem crossing from singlet to triplet states as confirmed by oxygen quenching test. The observation of highly efficient intersystem crossing in BPQDs well explains its superior performance in photodynamic therapy and indicates its exotic spintronic and magneto-optical properties.

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1.INTRODUCTION Black phosphorus (BP) has recently emerged as a new two-dimensional (2D) layered material with fascinating electrical, optical, and thermal properties of great technical significance.1-26 The moderate band gaps of BP materials in the range of 0.3-2.0 eV, susceptible to thickness,12,

27-28

, plane strain,29-30 and edge structure31,

bridge the energy gaps between graphene and transition metal dichalcogenides of (Mo,W)(S,Se)2.15 BP-based optoelectronic devices, like field-effect transistors

1-6

and

photodetectors19-22, show intriguing properties with high mobility

7

and

unprecedented anisotropic response.6-7, obtained

in

samples

prepared

by

25-26

3,

While many achievements have been

mechanical

exfoliation,

solution-based

approaches32-40 have recently been introduced to prepare few-layer BP samples with reduced lateral dimensions such as BP nanosheets and quantum dots (QDs), with which the performances in nonvolatile rewritable memory device,33 chemical sensing37, 39, and photo catalyst35 have been improved. More interestingly,

BP nanostructures synthesized with solution-based

approaches have been applied in photodynamic therapy38,

40-41

as near-infrared

photothermal agents38 and singlet oxygen generators40 that can be potentially employed to treat cancers. The production of singlet oxygen has been frequently connected to triplet generation in conventional photo-sensitizers.42-43 Recently, QD structures have been shown to be beneficial for triplet generation.44-46 In conventional semiconductors, QDs of metal chalcogenide compounds (i.e., CdSe, PbS, and PbSe), efficient generation and transfer of triplet energy from/into triplet sensitizers have 3



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been demonstrated, with potential applications in devices of energy harvesting and photon upconversion44-48. In QDs of two dimensional materials like graphene, the intersystem crossing (ISC) between singlet and triplet states has been argued to be susceptible to the energy splitting between the lowest singlet and triplet states ( ∆ ST )49. The long-lived triplet population is responsible for magnetically-sensitive light emission50 and singlet-oxygen generation in graphene QDs.51 In few layer BP materials, theoretical calculations have estimated ∆ ST to be in the order of 10 meV.52 However, the intersystem crossing and its potential involvement in optoelectronic processes as well as singlet-oxygen generation have remained to be experimentally unexplored in BP nanostructures. Here, we report the observation of a highly efficient triplet generation by a systematic study on the dynamics of photo-excited carriers in BPQDs. By combining femtosecond (fs) and nanosecond (ns) – resolved broadband transient absorption (TA) spectroscopy, we have characterized the carrier dynamics of photo-excited BPQDs in the timescales from sub-picosecond (ps) to 100 microsecond (µs). Following the fast thermalization of hot carriers in a time scale of ~ 0.5 ps, the excited carriers undergo biexponential decays with lifetimes of 85 ps and 1.8 ns, respectively. Surprisingly, a long-lived bleaching signal is built up in a time scale < 2 ns, which retains up to 100 µs. This long-lived feature can be assigned to the triplet generation due to intersystem crossing, as supported by control experiments of oxygen quenching, which is probably responsible for the highly efficient singlet oxygen generation in BP nanostructures. 4


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2. RESULTS AND DISCUSSION

Figure 1．． Sample characterization of BPQDs. (a) TEM image of BPQDs. Inset shows a high-resolution TEM image of a single dot (scale bar: 2 nm). (b) Statistical analysis of the lateral sizes of BPQDs. (c) AFM image of BPQDs on a SiO2/Si substrate (3×3 µm2). (d) Height profile along the red line in c). (e) Raman spectrum of the sample of BPQDs. (f) Absorption spectrum of the BPQDs in ethanol solution.

The samples of BPQDs were prepared by a solution-based method modified from the literature procedures as described in Supporting Information (SI).38 The morphology of BPQDs was characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). From TEM images (Figure 1a), the average lateral size of BPQDs is 3.1±0.8 nm (Figure 1b), which is comparable to the literature results and may be further improved.15, 38 The high-resolution TEM image shows the lattice fringe of 0.34 nm, which can be ascribed to the (021) plane of BP crystals as confirmed by X-ray diffraction patterns (Figure S1, SI). The AFM image shows the topographic morphology of BPQDs (Figure 1c). The average height of BPQDs is ~ 4 nm (Figure 1d). Raman spectroscopy was performed to check the crystalline structure of the BPQDs (Figure 1e). Three prominent peaks were observed that can be assigned to A1g (361.5 cm-1), B2g (438.7 cm-1), and A2g (466.5 cm-1) modes, suggesting excellent crystalline quality of the BPQDs. The frequencies of these modes show 5-9 cm-1 blue-shifted with respect to the bulk values, which is likely caused by the size confinement effect as previously observed in BP nanocrystals,38

few-layer BP 5



nanosheets3 and other layered 2D materials.53

Figure 1f shows the absorption

spectrum of the solution of BPQDs. Efficient absorption has been observed with a broadband spectral coverage in visible/near infrared regime, agreeing well with literature results.38 (a)

(b)

3

PIB

τe-ph ~ 0.5 ps

15

2 1

∆T/T (10-3)

∆T/T (10-3)

680 nm

0 -1

10

PIA 620 nm

5 560 nm

-2 0.1 ps 10 ps 1000 ps

-3

0.5 ps 25 ps 3000 ps

500

2 ps 200 ps

600

0

700

500 nm

0 2 4 10

Wavelength (nm)

(d)

1

∆T/T (Normal.)

(c) ∆T/T (Normal.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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680 nm 620 nm 560 nm

0 τ1 ~ 85 ps (18 %) τ2 ~ 1760 ps (82 %)

-1

0

1000

2000

3000

100

1000

Time (ps) 1

0 2.0 mW 1.0 mW 0.5 mW 0.3 mW

-1

0

Time (ps)

1000

2000

3000

Time (ps)

Figure 2．．Ultrafast carrier dynamics. Fs-time resolved TA spectra of BPQDs. (a) The differential transmission spectra (∆T/T) at different time delays. (b) The decay dynamics of differential transmission (∆T/T) probed at different wavelengths. The pump is set at 400 nm with an average power of 0.5 mW. The curves are vertically shifted for clarity. (c) The curves probed at different wavelengths are plotted in the scale normalized to the maximum value of negative signal. (d) The normalized curves probed at 680 nm with different pump powers

The dynamics of photo-excited carriers at different lifetime scales dictates the optoelectronic response in semiconductors.54 We perform broadband TA spectroscopy to uncover different relaxation channels involved in the carrier dynamics of BPQDs. In general, TA signal is consisted of multiple components including ground-state 6


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bleaching (GSB, ∆T/T > 0), excited-state absorption (ESA, ∆T/T < 0) and stimulated emission (SE, ∆T/T > 0) from different excited states with their characteristic lifetime parameters. Figure 2a plots the TA spectra recorded at different time delays with the pulse excitation at 400 nm. The excitation power is ~ 0.5 mW (~ 50 µJ/cm2). A photo-induced bleaching (PIB) signal appears instantaneously with the incidence of pump pulse. Following the recovery of the initial PIB signal in sub-ps time scale, the signal of photo-induced absorption (PIA) emerges in the spectral range > 520 nm. The PIA signal peaks at a time delay of 25 ps and recovers fully in a time scale of 1 ns. Surprisingly, a late-stage PIB signal becomes pronounced after the recovery of the PIA signal at time delay > 1 ns (Figure 2a). We analyze the kinetic curves probed at different wavelengths for the assignments of different TA components. The curve probed at short wavelength (~ 500 nm) exhibits a fast recombination with sub-ps dynamics (Figure 2b). Such a fast recombination is commonly observed during the thermalization of hot carriers whose lifetime is mainly governed by electron-phonon (e-ph) interaction. The characteristic lifetime of the fast recombination shows a slight dependence on the probe wavelength. The kinetics probed at shorter wavelength generally decays faster (Figure S2, SI), which is consistent with the scenario of carrier thermalization in semiconductors (Figure S2, SI). In addition to the thermalization process, the PIA signal is observed with longer probe wavelengths. The temporal evolutions of the PIA signals show a near-identical behavior with different probe wavelengths except some variations in signal amplitude (Figure 2c). The kinetic curves can be reproduced by a 7



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bi-exponential decay function. The lifetime of the fast component is ~ 85 ps, which is comparable to the time parameter of carrier recombination observed in film samples of BPs.14, 55 Due to the relatively broad size distribution, carrier recombination in some BPQDs with large sizes may be characterized with lifetime similar to that in bulk BP samples. In addition, the defect and surface states are possibly rich in BPQDs, which may induce a fast component due to the effect of carrier trapping. However, this fast component (~ 85 ps) plays a much less significant role in the recovery of photo-excited carriers in BPQDs whose amplitude ratio is less than 20% （Figure 2c）. A more primary channel is much slower with a lifetime of ~ 1760 ps whose amplitude ratio is over 80%. Similar parameters were obtained by analyzing the broadband spectra using global fitting algorithm (Figure S3, SI). Other than that, the slow dynamics may be related to the singlet-triplet crossover since the slow component extends to the build-up of a long-lived PIA signal as justified in next section. Auger recombination is frequently observed in semiconductor nanocrystals as Coulomb interaction is strongly enhanced due to quantum size effect,56-58

which may also

affect the carrier dynamics in BPs.55 Here, the weak excitation power (~0.5 mW) kept the carrier dynamics in linear regime with insignificant Auger effect, as confirmed by the pump-power-independent decay dynamics with excitation power below 2 mW (Figure 2d).

8


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(a)

(b) 0.5

4

2

0.0

∆T/T (10-3)

-0.5

∆T/T (10-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

0

Pump wavelength 400 nm 532 nm 800 nm

-1.0

1 0

600

Wavelength (nm)

700

2000

3000

0

Pump wavelength 400 nm 532 nm 800 nm (x 3)

-1

-2

500

1000

Time (ps)

0 2 4 10

100

1000

Time (ps)

Figure 3. Pump-wavelength-dependent decay dynamics. (a) The differential transmission spectra in BPQDs recorded at the time delay of 25 ps with pump at 400, 532, and 800 nm, respectively. The curves are vertically shifted for clarity. The curve pumped at 532 nm is broken to avoid the pump residual. (b) Time-dependent traces probed at 680 nm with pump at different wavelengths. The signal amplitudes are rescaled with the same incident power. Inset shows the normalized curves.

Figure 3 presents the results of TA spectra recorded with pump wavelength at 400 nm, 532 nm, and 800 nm, respectively. The spectral profiles show some differences that the bleaching signal extends slightly to the longer wavelength side with pumping at longer wavelength. The dependence of probe signal on the pump wavelength seems to be a consequence of the size-dependent band structures of BPQDs. It is known that the energy gaps of BPQDs are dependent on their sizes, manifested as size-dependent absorption and emission wavelengths.59 The spectral dispersions of ESA, GSB, and SE signals are very sensitive to the band gap (Figure S4, SI) so that the TA signal of BPQDs should also be size dependent. With a longer pump wavelength, BPQDs with larger bandgaps will not be excited and make less contribution to the TA signal. The small BPQDs may not be excited with pump at

9



longer wavelength, so the TA signal measured with shorter wavelength pump consists of more contributions from BPQDs with larger bandgaps. It is a plausible reason for the slight spectral difference with more bleach signal observed with 800 nm (Figure 3a). Nevertheless, the kinetics for carrier recombination remains nearly unchanged with changing pump wavelength. Figure 3b shows the kinetics probed at 680 nm. While the signal amplitude drops at longer pump wavelength, the temporal dynamics of the ESA recoveries under different wavelength excitations are nearly identical (Inset, Figure 3b). This result suggests that the long-lived PIB feature is a common feature in BPQDs with different sizes, which is probably the ESA feature of the thermalized carriers in BPQDs. ∆T/T (10-3)

(a) -2

-1

0

1

2

-2

-1

0

1

2

700

Wavelength (nm)

Wavelength (nm)

∆T/T (10-3)

(b) 3

700

650

600

550

650

600

550 0

2

4

101

102

Time (ps)

103

0 2 4 6 8101

102

103

104

Time (ns)

105

(d)

(c) 100 ns

∆T/T (a.u.)

2

∆T/T (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

τD~ 26 µs

τR~ 2 ns

-2

100 ps

500

600

Wavelength (nm)

700

0

5

10

15

102

Time (ns)

103

104

105

Figure 4. Triplet generation dynamics of BPQDs. (a) Femtosecond TA spectra of BPQDs in the delay range of 3 ns. The pump pulse is at 532 nm (0.5 µJ) with a pulse duration of ~ 70 fs. (b) Nanosecond-resolved TA spectra of BPQDs in the decay range up to 100 µs. The pump pulse is at 532 nm (5 µJ) with a pulse duration of ~ 800 ps. (c) The spectra of differential reflectivity at 100 ps and 100 ns, respectively. (d) The kinetics probed at 680 nm shows the build-up (~ 2 ns) and recovery (26 µs) of the late-stage bleaching signal. The dashed line shows the resolution of TA spectroscopy. 10


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We now try to understand the PIB feature at the long time scale. The PIB signal observed here persists to a time delay much longer than the maximum temporal window of 3 ns for fs-time-resolved TA spectroscopy. For a full description of the PIB signal, we have conducted ns-time-resolved TA measurements on the carrier dynamics with a temporal window up to 100 µs. Figure 4a & 4b show the counterplots of TA signal as a function of probe wavelength and delay time measured with fs and ns resolutions, respectively. The fs-resolved TA signal is displayed as a PIA feature in sub ns regime. Following the recovery, the PIB signal with broadband coverage (> 500 nm) gradually builds up in a time scale of few ns and persists to an extremely long time up to 100 µs (Figure 4b). It is reasonable to connect the long-lived feature to the triplet population as the recombination of triplet to ground state is spin forbidden. Figure 4c shows the spectra of ∆T/T recorded at the time delays of 100 ps and 100 ns, respectively. Such a difference is a result of different spectra of ESA for singlet and triplet states where the ESA for triplet population is not within the probed wavelength range as schematically shown in Figure S4 (SI). Since the ESA signal at 490 nm for singlet is negligible (Figure 4c), the kinetic curve probed at 490 nm for the ns-resolved TA spectroscopy represents the triplet population with the onset of lifetime parameter to be ~ 2 ns (Figure S5, SI). Figure 4d shows the kinetics probed at 680 nm for comparison with the kinetics shown in Figures 2 & 3. Besides the onset of bleaching (~ 2 ns), the recovery lifetime of the PIB signal is ~ 26 µs. Such a long-lived component can be regarded as a clear evidence for the triplet generation. The TA feature for the long-lived triplets in the visible range is manifested as the PIB, 11



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suggesting the major ESA of triplets is probably in the infrared range. The measured onset lifetime is close to that of the major carrier relaxation channel uncovered in fs-resolved TA spectroscopy, implying a high efficiency of singlet-triplet crossover in BPQDs (Figure 5a). Theoretically, the rate of intersystem crossing between singlet state and triplet state (γ) is sensitive to the energy difference between the singlet and triplet levels ( ∆ ST ).49, 51, 60 The splitting between singlet and triplet states has been calculated to be about 50 meV for a single-layer BP material, and the splitting is dependent on the number of layers (n) as n-2.52 For BP samples studied here, the singlet-triplet splitting in QDs with multiple layers is in the order of meV. As previously studied in graphene QDs,49,

61

the intersystem crossing rate can be approximately expressed by γ ∝

ESOC/EST, where ESOC represents the spin-orbit coupling in the system. The spin-orbit interaction is about one order of magnitude stronger in BP materials than that in graphene;62 and the singlet-triplet splitting in BPQDs is about two orders of magnitude weaker than that in graphene QDs.49 Considering these factors, the high efficiency of triplet formation in BPQDs is reasonable. The assignment of the long-lived component to triplet generation is further confirmed by an oxygen quenching measurement. By introducing oxygen into the solution of BPQDs, lifetime of the long-lived component is significantly shortened (Figure 5b), indicating a strong interaction between oxygen and the excited triplet states in BPQDs. These results strongly support the involvement of triplet formation.63 The average lifetime of the long-lived component is shortened to ~ 6 µs 12


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with exposure to oxygen, suggesting that ~ 80% of triplet excited states may be involved in the generation of singlet oxygen. With the above results, we can safely summarize the dynamics of photo-excited carriers in BPQDs (Figure 5a): Following the sub-picosecond thermalization process, a large portion of carriers undergoes the intersystem crossing to form the triplet carriers while the others undergo interband recombination. The long-lived triplets involve in producing singlet oxygen, which is critical for the biomedical applications of BPQDs.

Figure 5. Schematic model of triplet dynamics in BPQDs. (a) A schematic diagram of the kinetic model describing carrier dynamics in BPQDs. The photo-excitation creates a hot carrier that cools down to the band edge in sub ps. Part of the cooled carriers may recombine to the ground state or be trapped by surface or defect states through a fast channel with lifetime of ~ 85 ps while the rest of carriers undergo the intersystem crossing from singlet to triplet states. The bandgap of BPQDs is in the order of eV while the energy of singlet-triplet splitting is less than 10 meV. Carriers occupying the triplet states recombine to the ground state slowly due to the spin forbidden effect. (b) The kinetic curves probed at 680 nm recorded from BPQDs dissolved in ethanol with and without enriching dissolved oxygen. Inset shows the same data in a double logarithmic scale. After bubbling pure oxygen in ethanol, the relaxation lifetime of the long-lived component is shortened by a factor of ~ 4, indicating the interaction between triplet excited states in BPQDs with dissolved oxygen.

We have also checked the carrier dynamics of BPQDs dissolved in different 13



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solvents. The kinetics of triplet formation of BPQDs in ethanol, N-Methylpyrrolidone, and water are quite similar (Figure S6, SI). In the film of BPQDs, the ultrafast dynamics in nanosecond time scale is similar to that in solution, although the triplet formation is reduced which is possibly caused by the interactions between excitons in different QDs (Figure S7, SI). Notably, the sub-picosecond time response implies that the materials of BPQDs might find potential applications in the fields of high-speed modulators and ultrafast photonics.22, 64 In addition, efficient triplet formation is also observed in the nanosheets of BP (Figure S8, SI). These results indicate that the efficient singlet-triplet crossing is intrinsic in the material of BPQDs. The long lifetime of excited triplet state is beneficial for the interaction with oxygen as confirmed by oxygen quenching experiment (Figure 5), which is probably responsible for the highly efficient singlet oxygen generation.42 in BP nanostructures. With the formation energy of singlet oxygen (~ 95 kJ/mol), only BP materials with the bandgap larger than ~ 1 eV can serve as photosensitizers for singlet oxygen generation, which well explains the improved performance of nanosheets over the bulk BP sample because the few-layer nanosheets have a much larger bandgap. In BPQDs studied here, the efficiency of singlet to triplet conversion can be roughly estimated to be ~ 80%, and ~ 80% of the excited triplets involve in generation of singlet oxygen. In consequence, the efficiency of singlet oxygen generation is above 60 % in BPQDs. 3.CONCLUSIONS In summary, we have experimentally investigated the dynamics of photo-excited carriers in BPQDs with fs- and ns-time resolved broadband TA 14


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spectroscopy. We have observed the intersystem crossing from singlet to triplet states at the nanosecond time scale, which can well explain the excellent performance of singlet oxygen generation for photodynamic therapy using BPQDs. Our finding highlights, for the first time, a pivotal role played by the triplet states in BP materials, which can serve as triplet photosensitizers for various optoelectronic and biophotonic applications. Since triplet states are susceptible to magnetic field, the reported results in this work may stimulate further adventures in exploring various exotic spintronic and magneto-optical properties of this emergent material system.

15



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ASSOCIATED CONTENT Supporting Information. Experimental details, schematic diagrams of mechanism of carrier dynamics, additional experimental data measured from BPQDs dissolved in different solvents, BPQD films and BP nanosheets, XRD results, global fitting results and schematic diagram of experimental setup. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR

INFORMATION

Corresponding authors *

Correspondence to C.Z. ([email protected]), S. Y. ([email protected]), and M.

X. ([email protected]).

Notes The authors declare no competing finance interest.

ACKNOWLEDGEMENTS This work is supported by the National Basic Research Program of China (2013CB932903), the National Science Foundation of China (11574140, and 11621091),

Jiangsu

Provincial

Funds

for

Distinguished

Young

Scientists

(BK20160019), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Fundamental Research Funds for the Central University. The authors acknowledge Dr. Xuewei Wu for his technical assistant.

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Ultrafast Carrier Dynamics and Efficient Triplet Generation in Black

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