2D Transition Metal

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Spectroscopy and Photochemistry; General Theory

Ultrafast Charge Transfer in Perovskite Nanowire/2D Transition Metal Dichalcogenides Heterostructures Qiyi Fang, Qiu yu Shang, Liyun Zhao, Rui Wang, Zhepeng Zhang, Pengfei Yang, Xinyu Sui, Xiaohui Qiu, Xinfeng Liu, Qing Zhang, and Yanfeng Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00260 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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The Journal of Physical Chemistry Letters

Ultrafast Charge Transfer in Perovskite Nanowire/2D Transition Metal Dichalcogenides Heterostructures Qiyi Fang1, 2, #, Qiuyu Shang1, 3, #, Liyun Zhao1, Rui Wang4, Zhepeng Zhang1, 2, Pengfei Yang1, 2, Xinyu Sui4, Xiaohui Qiu4, Xinfeng Liu4, Qing Zhang1, 3, *, Yanfeng Zhang1, 2, * 1

Department of Materials Science and Engineering, College of Engineering, Peking University,

Beijing 100871, P. R. China 2

Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College

of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China 3

Research Center for Wide Gap Semiconductor, Peking University, Beijing 100871, P. R. China

4

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for

Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China

*

Corresponding author: [email protected]; [email protected].

#

These authors contributed equally to this work.

ACS Paragon Plus Environment


Abstract:

Mixed-dimensional van der Waals (vdW) heterostructures between one dimensional (1D) perovskite nanowires and two dimensional (2D) transition metal dichalcogenides (TMDCs) hold great potentials for novel optoelectronics and light harvesting applications. However, the ultrafast carrier dynamics between the 1D perovskite nanowires and 2D TMDCs are currently not well understood, which is critical for related optoelectronic applications. Here we demonstrate vdW heterostructures of CsPbBr3 nanowire/monolayer-MoS2 and CsPbBr3 nanowire/monolayer-WSe2 and further present systematic investigations on their charge transfer dynamics. We show that CsPbBr3/MoS2 and CsPbBr3/WSe2 are type-I and type-II heterostructures, respectively. Both electrons and holes transfer from CsPbBr3 to MoS2 with an efficiency of 71%. As a contrast, holes transfer from CsPbBr3 to WSe2 with a carriers transfer efficiency of 70% and electrons transfer inversely within 7 ps. The ultrafast and efficient charge transfer in the 1D/2D perovskite-TMDC heterostructures suggest great promises in light emission, photodetector and photovoltaic devices.

TOC Graphic (b)

(a)

MoS2

-3 400 405nm nmlaser laser

e h

e

h

Energy (eV)

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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CsPbBr3 -3.80

WSe2 -3.55

e

-4

e

-5 h

-6 -7


-4.15

h -5.95

-6.20

-5.15

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Two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as MoS2, WS2 and WSe2, have been considered as important candidates for developing low energy-consumption and ultra-thin optoelectronic devices including optical detectors, optical transistors, sensors and lasers, etc.1-11 However, it is still challenging for TMDCs to absorb sufficient light because of their atomic-thin structure, which may hinder the practical applications of TMDC-based optoelectronic devices. Building van der Waals (vdW) heterostructures, for example, coating a layer of material with high optical absorption coefficient on the top of the TMDCs, provides an effective solution and has also open new pathways to develop TMDC-based optoelectronic, photovoltaic and spin-valleytronic devices.12-15

Lead halide perovskites are a family of direct bandgap semiconductors with strong optical absorption in eye-sensitive spectra range, which have been considered as great candidates for next generation solar cells.16 Due to the outstanding optoelectronic properties, including bipolar carrier transport, long carrier diffusion length,17 high charge mobility,18 and low trapping-state density,19, 20

perovskites have recently been used as absorption layers for TMDC-based photo-detecting

devices.21, 22 In these perovskite/TMDC based devices, photo-generated carriers in perovskites could transfer to the TMDCs with an efficiency up to 83%, leading to the improvement of device performances.22 The mechanism of the high carrier transfer efficiency among these devices has been studied to a certain degree, especially for the perovskite quantum dots (QDs, 0D)/TMDCs (2D)23 and perovskites films (2D)/TMDCs (2D)21, 22 hybrid systems. However, the fundamental issues of band-alignment and carrier dynamics in the heterostructures of perovskite nanowire (PNW, 1D) and TMDCs (2D) have not been well understood.

Herein, we fabricate a vertical heterostructure of chemical vapor deposition (CVD) grown CsPbBr3 nanowire/monolayer MoS2 (WSe2) by a novel thermal release tape (TRT) transfer method.24 The carrier dynamics are studied by steady-state and time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS). We demonstrate that CsPbBr3 NW and monolayer MoS2 (WSe2) construct type-I (type-II) semiconductor heterostructures respectively. The carrier transfer efficiency and time between CsPbBr3 NW and monolayer TMDCs can reach 70% and 7 ps upon photoexcitation, respectively. The efficient and ultrafast carrier transfer


The Journal of Physical Chemistry Letters 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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suggest the great promise of 1D perovskite/2D TMDC heterostructures in photo-detection, light-harvesting and light source applications.

A schematic illustration of charge transfer pathways at the interface of the PNW/TMDC heterostructures upon photoexcitation is shown in the upper panel of Figure 1a. CsPbBr3 NW is stacked onto monolayer TMDCs to form a vertical 1D/2D heterostructure (bottom panel, Figure 1a). Following the excitation of photons with sufficient energy to generate carriers in both PNW (a)

Cs Pb Mo or W

(b)

(c)

(d)

(e)

Br S or Se C sPb B r3 on m ica

Figure 1. (a) Upper panel: a schematic illustration of charge transfer pathways at the interface of the PNW/TMDC heterostructures upon 405 continuous-wave laser photoexcitation. Bottom panel: side view of the PNW/TMDC heterostructure interface. (b) Raman spectrum of as-grown MoS2 on sapphire. The inset shows the AFM image of MoS2 nanoflakes. (c) Raman spectrum of as-grown WSe2 nanoflakes on sapphire. The inset shows the AFM image of WSe2 nanoflakes. (d) XRD pattern of as-grown PNWs network. The inset shows the SEM characterization of PNWs. (e) Optical microscope image of transferred PNWs network on TMDCs. Scale bars: (b, c) 10 μm, (d) 2 μm, (e) 50 μm.

and TMDCs, i.e. MoS2 and WSe2, the electrons and holes will transfer from wide-bandgap semiconductor PNW to narrower-bandgap TMDCs if type-I heterostructure is formed (circled by


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green dashed line, upper panel) or separate into different sides in type-II heterostructure (circled by blue dashed line, upper panel). A large area of monolayer MoS2 and WSe2 are grown on sapphire by CVD methods, as shown in Figure S1. According to the Raman spectra, the frequency differences between E12g and A1g mode of MoS2 and WSe2 are ~ 20 and ~ 10 cm−1 (Figure 1b and 1c), suggesting that as-synthesized MoS2 and WSe2 are monolayers.25,

26

Further, atomic force

microscopy (AFM) measurements confirm that the thickness of monolayer MoS2 is ~ 0.7 nm (inset of Figure 1b) and monolayer WSe2 is ~ 0.9 nm (inset of Figure 1c), respectively. Figure S2 is the optical microscope image of as-grown PNWs network.27 Figure 1d shows the X-Ray diffraction (XRD) pattern of PNWs network grown on mica substrate. Two newly emerged peaks can be found and match the (011) and (022) faces of cubic-phase CsPbBr3, which supports the formation of cubic perovskite structure.28 The inset of Figure 1d is the scanning electron microscopy (SEM) image of PNWs which exhibit triangle in cross section. Eventually, the PNW/TMDC heterostructures are fabricated by TRT-assisted method (see Methods and Figure S3). To ensure good contact between PNW and TMDCs, necessary acetone clean and thermal treatment processes have been conducted. The optical microscope image of the heterostructure is shown in Figure 1e.

(b)

MoS2

-3

e h h

e

-4.15

CsPbBr3 -3.80

WSe2

760 nm pump 520 nm probe

-3.55 e

-4

2.0E-4

r = 7 ps

h

-7

PNW

e

-5 -6

PNW/WSe2

4.0E-4

ΔA/A

400 405nm nmlaser laser

Energy (eV)

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 a) 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 = 58.7 ps h -5.95

0.0 -6.20

-5.15

0

50 Time Delay (ps)

Figure 2. Band allignments of PNW/MoS2 and PNW/WSe2 heterostructures, which form type I and type II heterostructures, respectively. The arrows represent different charge transfer pathways.

As shown in Figure 2, the position of valence band maximum (VBM) and condution band minimum (CBM) of these materials are detemined by the X-ray photoelectron spectroscopy


100


(Figure S4).29 The interface of PNW/MoS2 heterostructure has a type-I band alignment. Photoexcited holes and electrons will transfer from PNWs to monolayer MoS2. Therefore, the PNWs can serve as absorbing layer to enhance light absorption efficiency for MoS2 based devices. As a constrast, the band alignment of PNW/WSe2 is type II with band offset of ~ 1 eV, which provides a strong driving force to separate the photo-generated electrons and holes into different materials. In the type-II heterostructure, the PNW is functioned not only as absorbing material but also charge transfer layer, which hold great promise for WSe2 based photo-detection and light harvesting devices. The WSe2 in turn works as promising inorganic charge transfer layer for perovskite based devices due to its good structure stability and transparence properties.

Further, steady-state photoluminescence (PL) spectroscopy is conducted to study the carrier transfer mechanism upon optical excitation in the PNW/TMDC heterostructures. In the type-II heterostructure, the PL intensity of both components will quench since photo-induced excitons will dissociate and spatially segregate into different materials. While in the type-I heterostructure, the PL intensity of wide-bandgap material quenches and inversely it may be enhanced for the narrow-bandgap material. Figure 3a and 3b show the PL intensity distributions of the PNW and MoS2 upon 405 nm continuous-wave laser excitation. The bright field optical image of PNW/MoS2 heterostructure can be seen in Figure S5. The PL of PNW significantly quenches in the PNW/MoS2 heterostructure while there is little change in the PL intensity of MoS2. Figure 3c and 3d show the PL spectra collected from PNW, monolayer MoS2 and PNW/MoS2 heterostructure, respectively. It clearly shows that the PL intensity of PNW located above MoS2 is quenched by ~ 85% but the PL intensity of MoS2 in the heterostructure area remains unchanged. Actually, the excitation intensity of MoS2 in the heterostructure area may be weakened due to reflection by PNW comparing to the isolated monolayer MoS2 under same conditions, resulting in actually enhanced PL intensity of MoS2 in PNW/MoS2 heterostructure. The reflectivity of the isolated PNW was measured to support this hypothesis, which resulted in an 85% transmission of light (Figure S6). The steady-state PL spectra of PNW and MoS2 indicate that the energy transfer from PNW to monolayer MoS2, which is consistent with XPS study that PNW and MoS2 construct type-I heterostructure.


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Same measurements are performed on PNW/WSe2 heterostructure (Figure 3e to 3h). Figure 3g and 3h are the PL spectra obtained from PNW, monolayer WSe2 and PNW/WSe2 heterostructure, respectively. Both the PL intensities of PNW and WSe2 are quenched in the heterostructure area, indicating that PNW and WSe2 form type-II heterostructure. Moreover, the PL spectra of the heterostructure has two peaks at 510 and 750 nm that are close to the peaks of the individual PNW (512 nm) and WSe2 (748 nm), respectively. We attribute the small shifts of 1 − 2 nm of these peaks to the different dielectric environments in the heterostructure compared with the individual components.30-32

4 500

@ 509 nm

3 500

-3.0

MoS2

-2.0

3 000

-1.0

2 500

MoS2

MoS22000

0.0

(d)

@ 670 nm

4 000

-4.0

Y (祄 )

(c)

5 000

PNW on MoS2

1.0

PNW

PL intensity (a.u.)

(b)

PL intensity (a.u.)

(a)

-5.0

Intens ity (cnt)

-6.0

MoS2 under PNW MoS2

1 500

(f)

7 500

2

@ 512 nm

X (祄) -1.5

2

7 000 @ 748 nm

-1.0

WSe2

6 500 6 000

-0.5

5 500

Y (祄 )

4

6

8

(g)

4

PL intensity (a.u.)

(e) 0

0

500

0. 4 祄

0.0

5 000

Intens ity (cnt)

-2

8 000

-2.0

WSe2

0.5

4 500

1.0

4 000

WSe2

3 500

1.5 2.0 0

2 X (祄)

9 000

PNW on WSe2 PNW

8 000

7 000

6 000

495 510 525 Wavelength (nm)

2 500

0. 4 祄

10 000

5 000

3 000

10

495 510 525 Wavelength (nm)

(h) PL intensity (a.u.)

1 000

3.0

Intens ity (cnt)

2.0

Y (祄 )

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


600 625 650 675 700 Wavelength (nm) WSe2 under PNW WSe2

700 725 750 775 800 Wavelength (nm)

4 000

4

2.5 0. 2 祄

1.0

2.0

3.0

3 000

4.0

X (祄)

Figure 3. (a, b) PL mapping of PNW, MoS2 and PNW/MoS2 heterostructure taken in the area within the dashed retangular in Figure S5a. (c) Point PL spectra of PNW located above (pink dashed line) and away from (blue solid line) MoS2. (d) Point PL spectra of MoS2 located below (pink dashed line) and away from (blue solid line) PNW. (e, f) PL mapping of PNW, WSe2 and PNW/WSe2 heterostructure taken in the area within the dashed retangular in Figure S5b. (g) Point PL spectra of PNW located above (pink dashed line) and away from (blue solid line) WSe2. (h) Point PL spectra of WSe2 located above (pink dashed line) and away from (blue solid line) PNW. Scale bars: (a, b) 1 μm, (e, f) 1.5 μm.

To further elucidate the carrier dynamics in PNW/TMDC heterostructures, TRPL spectroscopy is conducted to investigate the exciton recombination behaviors in PNW/TMDC heterostructures.



The TRPL spectra are measured by a time-correlated single-photon counting (TCSPC) system. The ultimate temporal resolution of TRPL spectra is ∼ 25 ps (see Methods). In the TRPL measurement, the excitation source is a 400 nm femto-second pulsed laser. A 405 nm long pass filter is used in the signal collection optical path to block the excitation source to probe TRPL signal of bare PNW and TMDCs. Also, the excitation intensity is set as low as possible to reduce the photo-induced degradation.

(a)

(b)

1

MoS2  = 2.4 ns  = 0.7 ns 1 = 20 ps

0.1

2 = 603 ps

PNW PNW/WSe2 Intensity (a.u.)

PNW PNW/MoS2 Intensity (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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WSe2

1

 = 2.9 ns  = 0.8 ns 1 = 20 ps

0.1

2 = 517 ps

0.01 0.2

0.3

0.4 0.5 Delay Time (ns)

0.6

0.0

0.2 0.4 0.6 Delay Time (ns)

0.8

Figure 4. (a) TRPL spectra of isolated PNW, isolated monolayer MoS2, and PNW in heterostructure. The solid curves are fitted results. (b) TRPL spectra of isolated PNW, isolated monolayer WSe2 and PNW in PNW/WSe2 heterostructure along with corresponding fitting curves.

Figure 4a shows the TRPL decay curves of isolated PNW, isolated monolayer MoS2 and PNW in PNW/WSe2 heterostructure, respectively. The TRPL curve of isolated PNW (green hollow) can be well fitted by a single-exponential function (green solid line), which ascribes to exciton radiative recombination process. The lifetime of isolated PNW exciton radiative recombination is 2.4 ns. This lifetime is comparable with the magnitude that reported in previous literatures (~1 − 15 ns),33-36 which indicates a high crystalline quality of the as-grown CsPbBr3 NWs. Meanwhile, the TRPL spectra of PNW show little difference before and after exposure to pulsed laser, sugests good stability of the NWs (Figure S7). For isolated monolayer MoS2, the TRPL decay curve can be well fitted by a bi-exponential function (navy solid line), giving a lifetime of τ1 = 20 ps and τ2 = 603 ps. The fast decay channel is due to many-body effect,37-39 which is further confirmed by the power dependent TRPL spectra for MoS2 excited at 532 nm (Figure S8). The slow decay pathway


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is attributed to exciton radiative recombination of MoS2. Even though optical pass filter (i. e. 633 nm long pass filter) is adopted, the monolayer MoS2 exciton emission are not detectable because the PL intensity of MoS2 is much weaker than that of PNW. Therefore, in PNW/MoS2 heterostructure, the PL signal is mainly collected from PNW. The TRPL curve of PNW in PNW/WSe2 heterostructure can also be fitted by a single-exponential function (red solid line) which is ascribed to exciton radiative recombination of PNW. The lifetime of exciton recombination is 0.7 ns, which is shorter than the lifetime of bare PNW. Similar phenomenon are also observed in PNW/WSe2 heterostructure, as shown in Figure 4b. The TRPL of individual WSe2 monolayer exhibits bi-exponential decay with time constant of 20 (many-body process) and 517 ps (exciton radiative recombination). Meanwhile, the lifetime of PNW decreases from 2.9 to 0.8 ns in the heterostructure area. The PL lifetime decrease of PNW in the both heterostructures areas is identical to PL quench in Figure 3, which suggests that photo-generated carriers in PNW move towards monolayer TMDCS. Combing the studies of XPS, PL and TRPL studies, it can be confirmed that the electrons and holes in PNWs transfer towards MoS2 while only one type of carriers moves from PNWs to WSe2 (holes).

Although the TRPL spectroscopy demonstrates a sort of carriers (holes) transfer from PNW towards TMDCs, it is difficult to probe the inverse carrier transfer process in type-II heterostructure, i. e. from WSe2 to PNW, because the PL signal of TMDC is too weak to be distinguished from the PNW based on present experiment setup. Then, TAS with sub-ps time resolution is carried out using a micro-pump-probe system to probe carrier transfer dynamics from WSe2 to PNW.40 A femto-second pulsed laser (pulse width: 150 fs, repetiton rate: 76 MHz) is used to pump the PNW/WSe2 heterostructure, which induces a population of photo-generated carriers across the bandgap. Meanwhile, a time-delayed probe pulsed laser is applied to interrogate the occupancy of these carriers by monitoring the changes of absorption (see Methods). Here the differential absorption signal is defined as the normalized change of the absorption caused by pump laser, ΔA/A = (A − A0)/A0. A and A0 are the reflection of the probe pulse from the sample with and without the pump pulse, respectively.10 Thus, positive signals indicate depopulations of photo-generated electrons on excited states, which is called as i.e., photobleaching (PB) peaks. Figure 5a shows TAS of PNW/WSe2 heterostructure and isolated PNW. The 760 nm pump pulses



with a fluence of 20 μJ cm−2 is adopted in order to selectively excite monolayer WSe2 only. The 520 nm pulsed laser, in resonance of PNW exciton, is utilized to probe the populations variation of PNW conduction band after the carriers inside WSe2 are generated by pump laser. Significant positive signal is observed in TAS of PNW/WSe2 heterostructure, which confirms that carriers (electrons here) transfer from WSe2 to PNW. The decay curve can be well fitted by a bi-exponential function with a rising time of 7.0 ps and a decay time of 58.7 ps. The sharp rising edge of PB signal reflects the fast build-up statefilling at the PNW exciton band edges. It means that the photo-generated excitons of WSe2 are efficiently dissociated into free carriers and then electrons transfer into nearby PNW within 7 ps, as shown in the inset of Figure 5a. As a comparasion, no PB signal is observed under the below-bandgap excitation in bare PNW (navy hollow data, Figure 5a), strongly suggesting that the PB signal in PNW/WSe2 heterostructure is due to charge transfer process from WSe2.

(a)

(b) xE-4

xE-4

PNW/WSe2


PNW

10 8 6

1

4

r = 7 ps 1 = 58.7 ps

WSe2


2

1

Normlized ΔA/A

10

ΔA/A

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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PNW/WSe2 WSe2 PNW

1 = 2.8 ps 2 = 19.4 ps 1 = 2.0 ps

0.01

2 = 15.2 ps

PNW

0 0.1

1

10 100 Time Delay (ps)

1E-4

5

10 15 Time Delay (ps)

20

Figure 5. (a) Transient absorption signals as a function of the probe delay with 760 nm pump and 520 nm probe. The ultrafast PB signal builds up within 7 ps in PNW/WSe2 heterostructure with a delay time of 58.7 ps of exciton recombination in PNW. The navy data are results for experiments on isolated PNW under same condition. (b) Transient absorption spectra as a function of the probe delay with 520 nm pump and 760 nm probe. The navy dots: isolated WSe2; the red circles: PNW/WSe2 heterostructure; The green dots: isolated PNWs. The curves are fitted by a bi-exponential function (solid lines). The short and long lifetimes are attributed to the many-body process and exciton recombination of WSe2.


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On the other hand, to evaluate the holes transfer behavior in PNW/WSe2 heterostructure, isolated PNW (green hollow), isolated monolayer WSe2 (navy hollow) and PNW/WSe2 (red hollow) heterostructure are excited by using a 520 nm pump pulse with a fluence of 20 μJ cm−2. The carrier dynamics of state filling in WSe2 is probed using 760 nm pulse laser, as shown in Figure 5b. For bare WSe2, two decay channels are observed with lifetimes of 2.0 and 15.2 ps, which are attributed to the many-body effect and exciton radiative recombination, respectively. Moreover, the lifetimes of the two decay channels of WSe2 in the heterostructure region (2.8 and 19.4 ps) are both longer than the respective values of isolated WSe2, which suggests that the WSe2 acquires the holes from PNW. On account of the results of TAS, steady-state PL and XPS, it can be confirmed that PNW and WSe2 construct a type-II heterostructure. It should be noticed that the interlayer exciton is not observed in type-II WSe2/PNW heterostructure, which may be due to the large lattice mismatch between PNW and WSe2, interface allignment, surface bonding states of non-layered PNW and etc.41-44

Furthermore, the carrier transfer efficiency ηCT and rate kCT are introduced to quantify the carrier transfer in PNW/TMDC heterostructures, which can be evaluted by lifetime change according to previous literatures.17 The decay rate can be expressed as k = kr + knr, or 1/ = 1/r + 1/nr, where k ( is the PL decay rate (lifetime) and kr(r, knr(nr are the radiative and nonradiative decay rate (lifetime), respectively. In the PNW/WSe2 heterostructure, the nonradiative decay component of PNW is mainly attributed to the carrier transfer process towards WSe2 in the heterostructure.45 Then, the charge transfer time (CT, hole transfer time here) and efficiency (ηCT = kCT/k) can be estimated to be 1 ns and 70%, respectively. The carrier tranfer rate is given by kCT = τ−1 − τ−1pnw,45 which is calculated to be 9×108 s−1. According to the lifetime change of PNW, the energy transfer efficiency and energy tranfer rate in PNW/MoS2 heterostructure are also caculated to be 71% and 1 × 109 s−1, respectively.

The caculated carrier transfer efficiency and rate in the PNW/TMDC heterostructures are comparable to the magnitudes reported in the TMDCs heterostructures previously. In 2D/2D TMDC heterostructures, such as MoS2/MoSe2, MoS2/WSe2, MoS2/WS2, WS2/graphene, etc., the carrier tranfer efficiency is as high as 99%, which results from reduced dielectric screening in thin



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layer semiconductors having unusually large permittivity and a strong in-plane transition dipole moment.10,

30, 40, 46

The carrier tranfer efficiency is also prominent (> 75%) in QDs/TMDCs

(0D/2D) heterostructures, such as CdSe QDs/WS2, CdSSe QDs/MoS2, CdSe QDs/MoS2, etc., which mainly benefits from the size effect and perfect interface contact.14,

45, 47

In CsPbBr3

QDs/monolayer WS2 heterostructure the carrier transfer efficiency and rate, i. e. ~ 40% and ~ 2 × 108 s−1, are relatively lower in 0D/2D TMDC semiconductor heterostructures.23 The reason may lie in high intensity of trapping states in CsPbBr3 QDs, which restricts the carriers transfer from CsPbBr3 to WS2. Herein, although the width of CsPbBr3 nanowire is above 500 nm, the high carrier transfer efficiency ηCT = 70% is still obtained, which can be ascribed to the long carrier diffusion length, low density of trapping states of PNW and reduced exciton binding energy with the presence of TMDCs.19, 48

In summary, we have studied carrier dynamics in the mixed dimensional heterostructures of CVD grown PNW and monolayer TMDCs (MoS2 and WSe2) using steady-state PL, XPS, TRPL and ultrafast TA spectroscopy. We have confirmed that PNW/MoS2 and PNW/WSe2 heterostructures show type-I and type-II band allignment, respectively. The carrier transfer efficiency and rate of PNW/TMDC heterostructures are ~ 70% and 109 s-1. The ultrafast and efficient charge transfer/separation in CsPbBr3/MoS2 (WSe2) demonstrate that CsPbBr3 could serve as active layer in TMDC-based optoelectronics and TMDC materials in turn can work as inorganic ultrathin carrier transfer layers in perovskite-based optoelectronic devices. These results are significant to construct high performance optoelectronic devices based on TMDCs and perovskite materials.

Experimental Methods

Synthesis of perovskite nanowires. The perovskite nanowires were grown inside a multi-temperature zone tubular furnace (Lindberg/Blue M) equipped with a 1-inch-diameter quartz tube using 30 sccm Ar and 5 sccm H2 as caring gases. 5 mmol of powder cesium bromide (99%, Sigma-Aldric) and powder lead bromide (98%, Sigma-Aldric) were mixed and placed in the center of the furnace as perovskite nanowire’s precursor. Mica substrates were placed at the downstream around ~10 cm away from precursors. The temperature of precursors and substrates were 380 and 350 °C, respectively. The CVD reaction was carried out by 40 minutes and the


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furnace was cooled down naturally.

Synthesis of monolayer TMDCs. The monolayer TMDCs were grown inside a multi-temperature zone tubular furnace (Lindberg/Blue M) equipped with a 1-in-diameter quartz tube. Monolayer MoS2 was synthesized by an air-assisted CVD method. S powder was placed outside the hot zone and mildly sublimated with heating belts at ~ 115 °C. MoO3 powder (purity 99.9%, Alfa Aesar) and as-received sapphire substrates with no pretreatments were successively placed in the hot center of the tube furnace with a MoO3 sublimation temperature of ~ 530 °C, a growth temperature of ~850 °C and a growth time of 20 mins. Monolayer WSe2 was grown by physical vapor deposition. The WSe2 powder (Alfa Aesar, purity 99%) was put in the first zone and as-received sapphire substrates with no pretreatments were successively placed in the second zone. Se powder was put at the end of the heating section. The loading of WSe2 and Se powders were 10 mg, 1 g respectively. WSe2 powder and substrates were heated to 1050 and 750 °C and maintain for 20 minutes. The carrier gases were Ar (80 sccm) and H2 (20 sccm).

Preparation of PNW/TMDC heterostructures. To fabricate PNW/TMDCs vertical heterostructures, TMDCs were transferred onto SiO2/Si substrate by modified PMMA assisted transfer method based on previous reports.49 Because lead halide perovskites can be easily dissolve in water, we applied TRT assisted transfer method, as illustrated in Figure S3. A piece of TRT was placed on the top of mica substrate, and peeled off after 5 minutes. The tape with adhered PNWs was transferred to SiO2/Si substrate with TMDCs and baked for 5 minutes at ~ 95 °C. The tape can then be removed from the substrate easily after this thermal treatment, leaving behind the transferred PNWs network on the top of TMDCs. The residue was removed by annealing in Ar and H2 for 30 mins at 200 °C.

Micro-photoluminescence measurements. For PL mapping, we used a continuous-wave 405 nm laser (photon energy of 3.06 eV) to excite the isolated PNW, isolated monolayer TMDCs and the PNW/TMDC heterostructures. The pump laser was focused to a 1-m-diameter spot by a 100× objective (NA = 0.95) and the PL signals were collected in reflection geometry by the same microscope objective. A monochromator and a liquid-nitrogen-cooled charge-coupled device (CCD) were used to record the PL spectra. PL mapping spectra were carried out by scanning the



computer-controlled piezoelectric stage with a spatial resolution of 50 nm at relevant region.

Time-resolved photoluminescence spectroscopy. For TRPL measurements, the excitation pulse laser (wavelength 400 nm) were doubled frequencies of a Coherent Mira 900 (120 fs, 800 nm, 76 MHz) and filtered by a 655 nm short pass to generate 400 nm laser and the backscattered signal was collected using a time correlated single photon counting (TCSPC, SPC-150) which has an ultimate temporal resolution of ∼25 ps. The output of the frequency doubled laser was focused directly into a microscope to illuminate the samples through a 100× microscope objective (beam spot ~ 1 µm). A 750 nm short pass filter was used in the incident arm to block the original 800 nm laser signal.

Micro-pump-probe measurement. For pump-probe measurements, a diode laser was first used to pump a mode-locked Ti: Sapphire laser to generate 100 fs pulses with a central wavelength of 780 nm and a repetition rate of 76 MHz. A small portion of this beam was reﬂected by a beam splitter and used as the pump laser, which is lately focused to the sample with a spot size of about 2 µm by using a microscope objective lens. The majority of the 780-nm beam was used to pump an optical parametric oscillator (OPO), which generates a signal output with a central wavelength in the range of 1290 to 1500 nm. This pulse was focused to a beta barium borate (BBO) crystal to generate its second harmonic pulse in the range of 645 to 750 nm, which is used as the probe pulse laser. The probe pulse was also focused to the sample with a spot size of about 2.4 µm by using the same objective lens. The pump and probe beams were focused onto the sample at the same spot. A retroreﬂector was used in the pump arm in order to control the time delay between the pump and the probe pulses.

Supporting Information

Optical microscope images of monolayer TMDCs and schematic illustration of TRT transfer method to fabricate PNW/TMDC vertical heterostructures. Valence band XPS spectra of PNWs and monolayer TMDCs. The reflectivity of the isolated PNW. The TRPL spectra of PNW before and after several cycles of transient absorption spectroscopy. Power dependent TRPL decay times for MoS2 excited at 532 nm.


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Notes

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the Ministry of Science and Technology (Nos. 2017YFA0205700; 2017YFA0304600, 2016YFA0200700, and 2017YFA0205004), National Natural Science Foundation of China (Grant Nos. 51290272, 51472008, 61774003, 61521004 and 21673054), the start-up funding from Peking University, one-thousand talent programs from Chinese government and the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (Grant Nos. KF201601 and KF201604).



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(a) Upper panel: a schematic illustration of charge transfer pathways at the interface of the PNW/TMDC heterostructures upon 405 continuous-wave laser photoexcitation. Bottom panel: side view of the PNW/TMDC heterostructure interface. (b) Raman spectrum of as-grown MoS2 on sapphire. The inset shows the AFM image of MoS2 nanoflakes. (c) Raman spectrum of as-grown WSe2 nanoflakes on sapphire. The inset shows the AFM image of WSe2 nanoflakes. (d) XRD pattern of as-grown PNWs network. The inset shows the SEM characterization of PNWs. (e) Optical microscope image of transferred PNWs network on TMDCs. Scale bars: (b, c) 10 µm, (d) 2 µm, (e) 50 µm. 1229x796mm (120 x 120 DPI)


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Band allignments of PNW/MoS2 and PNW/WSe2 heterostructures, which form type I and type II heterostructures, respectively. The arrows represent different charge transfer pathways. 659x542mm (120 x 120 DPI)



(a, b) PL mapping of PNW, MoS2 and PNW/MoS2 heterostructure taken in the area within the dashed retangular in Figure S5a. (c) Point PL spectra of PNW located above (pink dashed line) and away from (blue solid line) MoS2. (d) Point PL spectra of MoS2 located below (pink dashed line) and away from (blue solid line) PNW. (e, f) PL mapping of PNW, WSe2 and PNW/WSe2 heterostructure taken in the area within the dashed retangular in Figure S5b. (g) Point PL spectra of PNW located above (pink dashed line) and away from (blue solid line) WSe2. (h) Point PL spectra of WSe2 located above (pink dashed line) and away from (blue solid line) PNW. Scale bars: (a, b) 1 µm, (e, f) 1.5 µm. 1293x690mm (120 x 120 DPI)


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(a) TRPL spectra of isolated PNW, isolated monolayer MoS2, and PNW in heterostructure. The solid curves are fitted results. (b) TRPL spectra of isolated PNW, isolated monolayer WSe2 and PNW in PNW/WSe2 heterostructure along with corresponding fitting curves. 843x425mm (120 x 120 DPI)



(a) Transient absorption signals as a function of the probe delay with 760 nm pump and 520 nm probe. The ultrafast PB signal builds up within 7 ps in PNW/WSe2 heterostructure with a delay time of 58.7 ps of exciton recombination in PNW. The navy data are results for experiments on isolated PNW under same condition. (b) Transient absorption spectra as a function of the probe delay with 520 nm pump and 760 nm probe. The navy dots: isolated WSe2; the red circles: PNW/WSe2 heterostructure; The green dots: isolated PNWs. The curves are fitted by a bi-exponential function (solid lines). The short and long lifetimes are attributed to the many-body process and exciton recombination of WSe2. 995x506mm (120 x 120 DPI)


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2D Transition Metal

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