Exciton Radiative Recombination Dynamics and Nonradiative Energy

Subscriber access provided by ECU Libraries

C: Physical Processes in Nanomaterials and Nanostructures

Exciton Radiative Recombination Dynamics and Nonradiative Energy Transfer in Two-Dimensional Transition Metal Dichalcogenides Huan Liu, Ting Wang, Chong Wang, Dameng Liu, and Jianbin Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12179 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 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

The Journal of Physical Chemistry

Exciton Radiative Recombination Dynamics and Nonradiative Energy Transfer in Two-dimensional Transition Metal Dichalcogenides Huan Liu, Ting Wang, Chong Wang, Dameng Liu* & Jianbin Luo* State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People’s Republic of China

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT We employ fluorescence lifetime imaging technology to explore exciton radiative recombination dynamics in layered transition metal dichalcogenides (TMDCs) and non-radiative energy transfer from CdSe/ZnS QDs to monolayer TMDCs (MoS2, WS2 and WSe2). Owing to indirect-direct bandgap transition, exciton radiative lifetimes decrease with the TMDCs layer number reducing. The fastest exciton recombination rate is observed in monolayer TMDCs, which is attributed to their reduced dielectric screening. Furthermore, the effect of reduced dielectric screening on non-radiative energy transfer from QDs to monolayer TMDCs is investigated. The fastest energy transfer rate is observed in QDs/WS2 heterostructure owing to weak dielectric screening of monolayer WS2, and the slowest rate in QDs/MoS2 caused by strong dielectric screening of monolayer MoS2. Our experiments provide fundamental insights into exciton recombination dynamics in TMDCs and potentially enable new avenues for controlling motion of excitonic energy conversion at nanoscale.


Page 2 of 24

Page 3 of 24 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


INTRODUCTION Recently, atomically thin transition metal dichalcogenides (TMDCs), including MoS2, WS2, MoSe2 and WSe2, have sparked intensive interest in the community for diverse optoelectronic devices such as transistors, photodetectors, light emitter diodes and photovoltaics.1-6 Owing to reduced dielectric screening effect and enhanced Coulomb interaction in tow-dimensional (2D) TMDCs, exciton has large binding energy (up to 0.7 eV) under room temperature and exciton recombination dynamics dominate the optical properties of these materials.7-12 Exciton radiative recombination can be modulated by bandgap structure, dielectric screening effect, dark exciton state, chemical treatment or disorders.12-15 And these factors give rise to a wide exciton radiative lifetime range from a few picoseconds to tens of nanoseconds.16-19 In addition, excitons in TMDCs can be directly excited by a nearby quantum dot (QD) emitter through non-radiative energy transfer (NRET).20-24 This coupling also can be mediated by the dielectric property of TMDCs.22,

25-31

Reduced dielectric screening leads to

enhanced energy transfer rate in thin layer MoS2, and NRET rate increases with decreasing thickness of MoS2 layers.25,

27

However, how bandgap structure and

dielectric screening affect exciton radiative recombination dynamics and non-radiative energy transfer in 2D TMDCs are not fully addressed in current literatures. Here, we use fluorescence lifetime imaging microscopy (FLIM) technology to investigate exciton radiative recombination dynamics in layered TMDCs and nonradiative energy transfer in CdSe/ZnS QDs to monolayer TMDCs. Our results demonstrate that the exciton radiative lifetime keep a decreasing trend from thick thickness to a monolayer. Then we compare the non-radiative energy transfer rate in QDs/TMDCs heterostructures. The fastest NRET rate appears in QDs/WS2 and the



slowest in QDs/MoS2. These observations are explained by bandgap structure and dielectric screening effect of 2D TMDCs. METHODS Samples preparation. Monolayer and multilayer TMDCs (MoS2, WS2 and WSe2) with different thickness were mechanical exfoliated from commercial bulk TMDCs crystals (2D Semiconductor) on a glass substrate.32 Core-shell CdSe/ZnS QDs (3 mg/mL, Wuhan Jiayuan) are composed of CdSe core and ZnS shell with a layer of oiled acid ligands on the surface. QDs with emission peak at 582 nm were dispersed onto the 2DTMDCs surface and then used spin-coating (3000 rpm) to form the 0D-2D hybrid interface. Time-Resolved Photoluminescence. Time-resolved PL measurement and lifetime image were performed with an inverted fluorescence microscope (IX83, Olympus) equipped with a time-correlated single-photon counting (PicoHarp, PicoQuant). The samples were excited at 485 nm using a pulsed laser triggered at 5 MHz through a 40 × , 0.95 NA objective (UPLSAPO). And PL signals were collected by the same objective, passed a 561 nm long-pass filter (or other) and focused onto a hybrid photomultiplier (Hybrid-PMT). The total time span was 200 ns. The overall system time resolution was ＜120 ps. Spatial resolution of the microscope was at 1 nm and spectrum resolution was 2 nm. The data were analyzed by a software (SymPhoTime 64). Characterization. Raman spectroscopy (Evolution, HORIBA) and atomic force microscopy (AFM, Dimension, Bruker) were used to identify the thickness of TMDCs layers. Raman spectra was collected by 532 nm for excitation. A high-resolution TEM (JEM, JEOL) was employed to characterize the morphology of QDs, and selected area electron diffraction (SAED) confirmed QDs polycrystalline. A High-Angle Annular


Page 4 of 24

Page 5 of 24 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


Dark Field (HAADF) TEM image suggested the high dispersive of QDs. RESULTS AND DISCUSSION Monolayer and few-layer MoS2, WS2 and WSe2 flakes are deposited on glass substrates by dry mechanical exfoliation from bulk materials.32 Figure 1a-1c show the bright-field optical micrograph of three kinds of TMDCs, optical contrasts indicating the different layer number. The thickness of TMDCs are estimated from atomic force microscopy (AFM) in Figure 1d-1f. AFM height profiles in insets indicate a thickness for 1.1~1.3 nm corresponding to the white line in AFM pictures, which are smaller than theoretical value and may be caused by the trapping of adsorbed impurities between TMDCs and substrates.29 Further, Raman spectroscopy is used to confirm the layer number of 2D TMDCs in Figure 1d-1f.33-34 The combination of optical contrast, Raman spectroscopy and AFM is applied to determine the layer number from thick thickness down to a monolayer.

Figure 1. (a) – (c) Optical micrographs of mechanically exfoliated MoS2, WS2 and WSe2 flakes with varying thickness, respectively, supported on a transparent glass substrate. (d) – (f) Raman spectra for MoS2, WS2 and WSe2 with different layer (1-10L)



Page 6 of 24

and thick area. The laser excitation wavelength is 532 nm. The insets are AFM morphology images and the height profiles of the monolayer area. The height profiles are measured along the white line in the insets. The combination of optical contrast, Raman spectroscopy and AFM is used to determine the layer number of TMDCs. FLIM images are shown in Figure 2a-2c, and its laser excitation wavelength is 488 nm. For all TMDCs samples, exciton lifetimes decrease with thickness decreasing from thick layer to monolayer, where blue color represents fast exciton lifetime and red for slow lifetime. Especially, the fastest exciton lifetimes occur in monolayer areas. Timeresolved photoluminescence (TRPL) decay traces are presented in semi-log scale in t  t  Figure 2d-2f and can be well fitted by a biexponential a function of A1e 1  A2 e 2

using an exponential tail-fit method, where

 i are the decay lifetime with its

normalized amplitude component Ai . A short lifetime component lifetime component

 1 and a long

 2 are observed in these TMDCs and Table 1 presents the detailed

fitting parameters. The whole decay lines are shown in Figure S1, and all curves have an instantaneous rise first appears because of photo-excited carriers with high energy band relaxing to lower excitonic level.35 Limited by the time resolved in our experiment, the rise is not discussed in the following. Here, we attribute the short lifetime component TMDCs. The values of

 1 to interband transition in

 1 in multilayer TMDCs are larger than that in monolayer,

which has relation to the transition from the indirect bandgap at the

 -point to direct

bandgap at the K-point of the Brillouin zone as the thickness decreases to a monolayer. This can be confirmed by measuring steady state PL intensity for monnolayer and thick TMDCs, respectively, as shown in Figure S2. In monolayer TMDCs, dramatic enhancements of PL quantum rate are observed owing to direct bandgap transition.


Page 7 of 24 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


Neutral A-exciton recombination is expected to contribute maily to PL spectrum.17 In our experiment, the A-exciton lifetimes are as short as hundreds of picosecond at room temparaure that is in good agreement with other observations.17 As the layer number increasing,

 i continues to increase that results from the

indirect bandgap transition in multilayer TMDCs. The indirect bandgap leads to poor photoluminescence in Figure S2. Although few-layer and thick TMDCs are indirectgap materials, the direct-gap at K still determines the optical absorption and PL spectrum.36 After photoexcitation above the direct-gap at K, the A-exciton becomes delocalized, and subsequently thermalize at K and return to ground state by radiative recombination.17,

36

So the observed radiative lifetmes

 1 in multilayer areas also

mainly result from A-exciton recombination. The slower radiative recombination in few-layer and thick TMDCs than monolayer may stem from the delocalization of Aexciton, which leads to lower dipole matrix elements and longer lifetimes. With the layer number increasing, the increasing delocalization of A-exciton in the layer-normal direction give rise to slower exciton radiative recombination.17 The radiative lifetimes in multiplayer TMDCs are in the range of 0.235–1.663 ns that agrees well with previous results.17, 35 The long lifetime component

 2 is associated with the disorders caused by defects,

substrates, impurities or strains.37-43 These factors from production process or operating conditions can strongly influence the exciton lifetime of TMDCs. Exciton lifetimes as long as 20 ns have been observed in chemical treatment MoS2.13 In order to verify the effect of defects on the exciton lifetime of TMDCs, we intentionally introduce defects in a WSe2 sample by an argon ion bombardment. An mild condition of the defect treatment is used with 5 W plasma at 60 Pa for 10 s by a commercial precision plasma cleaning system (13.56 MHz). The exciton lifetime of defective monolayer sample



Page 8 of 24

obviously becomes longer than in pristine monolayer sample, revealing that defects have a marked effect on exciton dynamics, as shown in Figure S3a and S3b. The PL decay cures of pristine and defective monolayer area are shown in Figure S3c. The value of

 1 are similar, suggesting that defects have slightly impact on the exciton

dynamics of interband transition. However, defects have greatly effect on the long lifetime component

 2 . So the long lifetime component may arise from the effect of

the disorders in our experiment.

Figure 2. (a) – (c) Fluorescence lifetime imaging microscopy (FLIM) images of MoS2, WS2 and WSe2, respectively, its laser wavelength of excitation laser is 488 nm. The bar in upper right corner indicates photon events for collection. The second color bar indicates the fluorescence lifetime of TMDCs. (d) – (f) Time resolved photoluminescence (TRPL) traces for TMDCs extracted from (a) – (c). The solid lines are the fit to a biexponential function by a n-exponential tail-fit method and

 av is the

average lifetime, calculated as:  av  A1 1  A2 2 . Table 1. Fitting Parameters for TRPL Decay Traces of TMDCs with Different Layers


Page 9 of 24 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


t  t  in Figure 2d-2f Using a Biexponential Decay Function of A1e 1  A2 e 2

MoS2

WS2

WSe2

1L

2L

Thick

1L

3L

10 L

Thick

1L

4L

6L

9L

Thick

τ1(ns)

0.289

0.371

0.749

0.169

0.235

0.671

0.764

0.250

0.340

0.460

1.062

1.663

A1(%)

90.91

84.62

61.61

98.78

93.33

56.29

62.09

91.43

84.21

55.17

60.47

57.69

τ2 (ns)

3.257

5.612

6.088

2.297

5.479

4.918

7.318

3.055

3.385

4.744

9.733

18.019

A2(%)

9.09

15.38

38.39

1.22

6.67

43.71

37.91

8.57

15.79

44.83

39.53

42.31

τav(ns)

0.559

1.177

2.798

0.195

0.585

2.527

3.249

0.490

0.821

2.381

4.490

8.583

WSe2

The above thickness-dependent exciton dynamics also can be explained by dielectric screening effect in TMDCs.9-10, 44 In order to evaluate the dielectric screening effect on exciton dynamics, an average lifetime is used that calculated as:  av  A1 1  A2 2 . As the layer number reducing, the average lifetime

 av have a decreasing trend in

Figure 2d-2f. The layer number of TMDCs dominates their dielctric property. The dielectric function of TMDCs has the form of

1 +i 2 , imaginary part  2 reflects the

properties of interband transitions. The  2 decreases as the layer number decreases to the monolayer at about 2.5 eV (488 nm) for exciting.8 Reduced dielectric function leads to weak dielectric screening effect and strong Coulomb interaction.45 So the exciton radiative recombination rate becomes faster and the shortest radiative lifetimes are observed in monolayer TMDCs. Then we compare the radiative lifetimes with a focus on monolayer areas. The radiative lifetime of WS2 is 0.195 ns that is shortest comparing with the other two monolayer TMDCs, and while MoS2 has the longest lifetime (0.558 ns).This difference comes from different dielectric screening effect of monolayer TMDCs, where MoS2 has the largest dielectric functions value corresponding to strong dielectric screening effect and WS2 has the weakest dielectric screening effect.8 In order to implement the dielectric screening effect in energy conversion application, low-dimensional heterostructures based on CdSe/ZnS QDs and monolayer TMDCs are constructed, and the dielectric screening effect on energy



transfer in such heterostructure is discussed in the following. A core–shell CdSe/ZnS QD is used in our experiment with emission peak at 582 nm and its emission spectrum overlaps with TMDCs absorption features to guarantee energy transfer occur.8 In addition, the core-shell structure and surface passivation with long insulating ligands greatly reduce the possibility of charge transfer.27 High-angle annular dark field (HAADF) TEM image suggests high dispersive of QDs with an average diameter of 6.2 nm in Figure 3a. The lifetime distribution of QDs is homogeneous shown in Figure 3b, and its time-resolved PL decay is shown in Figure 3c. The TRPL profile can be well fitted by a tri-exponential function,

B1et 1  B2 et  2  B3et  3 , where  i is decay lifetime with its normalized amplitude components

Bi . And  1 ,  2 and  3 are 1.72 ns, 6.87 ns and 15.76 ns,

respectively. The origin of these lifetime components and more characterizations analysis of QDs are discussed in Figure S4 and S5. Then QDs are dispersed onto TMDCs surface by spin-coating to form 0D-2D hybrid interface. In such heterostructures, excitons in TMDCs can be excited by nearby QD emitters via nonradiative energy transfer (NRET). Steady-state PL intensity pictures of QD/TMDCs heterojunction reveal that PL intensity of QDs decrease with the increasing TMDC layer number in Figure 3d-3f and PL almost all quenching in the thick region. PL quenching can be visually observed by their corresponding confocal fluorescence microscopy images in the insets. Strong PL quenching in QDs/TMDCs hybrids indicates that non-radiative relaxation pathway dominates the exciton dynamics of QDs.25, 27


Page 10 of 24

Page 11 of 24 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


Figure 3. (a) HAADF TEM image of QDs, suggesting QDs have good dispersibility with an average diameter of 6.2 nm. (b) FLIM image of pure QDs indicates the homogeneous lifetime distribution measured by a 561 nm long-pass filter. (c) Timeresolved PL (TRPL) profile of QDs is well fitted by a tri-exponential function. (d)–(f) PL spectra with different layers for QDs/MoS2, QDs/WS2 and QDs/WSe2, respectively. The corresponding confocal microscopy pictures in insets also suggest strong PL quenching in QDs/TMDCs heterostructures. PL intensity decreases as the layer number increasing, indicating TMDCs provides a non-radiative recombination channel for QDs. Figure 4a-4c present PL lifetime images of QDs/TMDCs heterostructures. Compared with the large absorption cross-section and high quantum yield of QDs, the fluorescence intensity of TMDCs is weak and its contribution to the PL decay profile of QDs can be neglected.25 The PL average lifetime of QDs away from TMDCs are closed to their average native lifetime

 av of 9.12 ns. In contrast, the PL decay dynamics of QDs on

TMDCs remarkably become faster than the case of pure QDs. Especially, owing to dielectric screening effect depending on layer number of TMDCs mentioned above, the



Page 12 of 24

shortest PL lifetimes of QDs are seen in monolayer areas. Figure 4d compares PL decay profiles of QDs in the absence and presence of the monolayer MoS2, WS2 and WSe2, which are well fitted by a tri-exponential function and detail parameters are shown in Table S1. The native lifetime are significantly shortened in monolayer regions, which are 5.66 ns, 3.19 ns and 2.58 ns for MoS2, WSe2 and WS2, respectively, where the longest average lifetime  av occurs in QDs/MoS2 heterostructure and the shortest  av is observed in QDs/WS2 heterostructure. This decreasing trend of PL lifetime of QDs in QDs/TMDCs heterostructure is consistent with excitons lifetime in monolayer TMDCs (Figure 2), indicating that the dielectric properties of TMDCs can affect the non-radiative energy transfer rate  NRET , given by the following equation:

 NRET =

1



1

 QD/TMDCs  QD

Where  QD is the PL lifetime of pure QDs on the substrate.  QD/TMDCs is the PL lifetime of QDs on monolayer TMDCs. For monolayer MoS2, the energy transfer rate is 6.7×107 s-1, reaching as high as 2.78×108 s-1 for monolayer WS2. And the energy transfer rate for WSe2 is 2.04×108 s-1 falling in the middle. Now we use a versatile dipole-dipole model to illustrate the increasing trend of nonradiative energy transfer rate (MoS2 ˂ WSe2 ˂ WS2),25,

27-28

and a schematic

representation is presented in Figure 4e. The spherical core-shell structure of QDs and the small size comparing to the emission wavelength make the dipole approximation available. The total exciton decay rate



of the arbitrary oriented electric-dipole

emitter in transparent medium (such as air) with dielectric constant from the top surface of TMDCs can be derived as:


1 at distance d

Page 13 of 24 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


 k dk  1 p s =1+ Re  II II  2 k II2 k12  1 r    r    e 2ik1 z d 0 0 2 k1k1z

(1)

Where  0 is the radiative decay rate of QDs in a uniform medium, d is the distance,

k II is in-plane component of the wave vector with k12z  k12  k II2 , k12  1k02 and k0  2  . Notably, the decay rate in Eq 1 include both radiative and non-radiative

decay channels. The rate of radiative decay  rad can be written as: 2  rad 1 k1 k II dk II   p  2 = + r  r  s   2 Re(  2 k II2 k12  1 r  p   r  s   e 2ik1 z d )    0 2 0 4k1k1z 



k2

0

2 k2 z k II dk II  k II  k1z 2  k12 4k1 k1z 

2

t

 p

2

t

s

2

 2Im k d 1z e 

(2)

The spontaneous decay rate of QDs can be substantially affected in the vicinity of the atomic thin TMDCs, which has highly anisotropic dielectric function and strong inplane excitonic polarizability.21, 27, 46-47 For emission energy of QDs at 2.13 eV, in-plane susceptibility

 II of the monolayer TMDCs dominates the reflection coefficient

amplitudes r ( s ) and r ( p ) for s- and p-polarized light,8, 28 respectively, as following:

r  p   ( 2 k2 z  1 k1z  i  II )( 2 k2 z  1 k1z  i  II ) 1 r  s   (k1z  k2 z  ik02  II )(k1z  k2 z  ik02  II ) 1 Where the  II is associated with

 II  1 t

(3) (4)

for a fixed thickness of monolayer t. Here,

out-of-plane dielectric is neglectful, because it may only contribute to the optical response and it is difficult for exciting the vertical dipole in atomically thin film.44 The  II has a similar effect on the transmission coefficient amplitudes as the following:

t  s   2k1z (k1z  k2 z  ik02  II ) 1

(5)

t  p   2 1 2 k2 z ( 2 k2 z  1 k1z  i  II ) 1

(6)

2 2 2 Where  2 is the dielectric constant of the glass substrate and k2 z  k2  k II ，



Page 14 of 24

k22   2 k02 . So the NRET rate in QDs/TMDCs heterostructures can be described by the in-plane

complex

dielectric

function

 II = i +i j .

As

a

result,

 NRET  0 =   0   rad  0 is obtained and the calculation of absolute rate  NRET requres the QD decay rate  0 in a lossless medium. Here, the average fluorescence lifetime of QDs (9.12 ns) in air and the distance d=3.1×10-9 nm are used. The

1 =1

 2 =2.25 are taken from the literature.25 In this experiment, the emission energy

and

of the QDs is about 2.1 eV, in-plain dielectric functions are  WS  9+6i , 2

 WSe  17+7.5i and  MoS  18+7i 2

2

for the monolayer WS2, WSe2 and MoS2,

respectively. 8, 48 The computed  NRET are 9.14×107 s-1, 1.15×108 s-1 and 2.913×108 s-1 for MoS2 , WSe2 and WS2, respectively. We find the increasing trend is consistent with the experimental results, indicating the dielectric properties of TMDCs have greatly impact on the energy transfer rate in QD/TMDCs heterostructures. The small dielectric function for WS2 implies relatively weak screening effect of in-plane components of donor dipole field and results in the fastest NRET rate.49 However, MoS2 has the relatively strong dielectric screening effect and the slowest NRET rate. The excitation of the monolayer TMDCs by efficient energy transfer from neighboring QDs might be attractive for controlling motion of excitonic energy conversion at nanoscale.


Page 15 of 24 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


Figure 4. (a) – (c) FLIM images of QDs spin-cast on MoS2, WS2 and WSe2, respectively. The fluorescence lifetime of QDs vary as a function of TMDCs layer number and the shortest lifetime are found in the monolayer area. (d) Comparison of TRPL profiles of QDs on monolayer TMDCs. The solid lines are the fit to a triexponential function and the shortest average lifetime  av (2.58 ns) occurs in QDs/WS2 heterostructure and the longest  av (5.66 ns) observed in QDs/MoS2 heterostructure. (e) Schematic representation of energy transfer (ET) from core–shell CdSe/ZnS QDs to a monolayer TMDCs based on a dipole-dipole model. CONCLUSION In summary, we have investigated exciton radiative recombination dynamics in layered TMDCs and non-radiative energy transfer in QDs/TMDCs heterostructures using fluorescence lifetime imaging technology. Exciton decay profiles in TMDCs exhibit biexponential decay behavior, attributed to interband transition and dark exciton state effect. Exciton lifetime decreases as the layer number decreases to a monolayer due to



reduced dielectric screening effect. Furthermore, the effect of dielectric screening on non-radiative energy transfer have been confirmed in QDs/TMDCs heterostructures. The fastest NRET rate (2.78×108 s-1) is observed in QDs/WS2 and the slowest rate (6.7×107 s-1) for QDs/MoS2. A dipole-dipole model has been used to illustrate this phenomenon, which is a result of reduced dielectric screening of in-plain components of the donor dipole field. Our results could provide a deeper understanding of exciton dynamics and energy conversion mechanisms in TMDC based devices. SUPPORTING INFORMATION Supplementary material is available for this manuscript, containing TRPL traces and steady PL spectra of TMDCs; FLIM pictures and TRPL profiles of pristine and defective WSe2; The characterization for QDs using TEM, absorption spectra and emission spectra; Analysis of the origin of QDs’ lifetime components by comparing TRPL traces of QDs using different filters. This information is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (no. 51527901, 51575298, 51705285 and 51705284). REFERENCES 1.

Xie, J.; Huang, K.; Yu, X.; Yang, Z.; Xiao, K.; Qiang, Y.; Zhu, X.; Xu, L.; Wang,


Page 16 of 24

Page 17 of 24 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


P.; Cui, C.; Yang, D., Enhanced Electronic Properties of SnO2 via Electron Transfer from Graphene Quantum Dots for Efficient Perovskite Solar Cells. ACS Nano 2017, 11 (9), 9176-9182. 2.

Shang, J.; Cong, C.; Wang, Z.; Peimyoo, N.; Wu, L.; Zou, C.; Chen, Y.; Chin, X.

Y.; Wang, J.; Soci, C.; Huang, W.; Yu, T., Room-temperature 2D semiconductor activated vertical-cavity surface-emitting lasers. Nature Communications 2017, 8 (1), 543. 3.

Ni, Z.; Ma, L.; Du, S.; Xu, Y.; Yuan, M.; Fang, H.; Wang, Z.; Xu, M.; Li, D.; Yang,

J.; Hu, W.; Pi, X.; Yang, D., Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors. ACS Nano 2017, 11 (10), 9854-9862. 4.

Qian, L.; Zheng, Y.; Xue, J.; Holloway, P. H., Stable and efficient quantum-dot

light-emitting diodes based on solution-processed multilayer structures. Nature Photonics 2011, 5 (9), 543-548. 5.

Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer

MoS2 transistors. Nat Nanotechnol 2011, 6 (3), 147-50. 6.

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S.,

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature Nanotechnology 2012, 7 (11), 699-712. 7.

Chernikov, A.; Berkelbach, T. C.; Hill, H. M.; Rigosi, A.; Li, Y.; Aslan, O. B.;

Reichman, D. R.; Hybertsen, M. S.; Heinz, T. F., Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2. Phys Rev Lett 2014, 113 (7), 076802.



8.

Li, Y.; Chernikov, A.; Zhang, X.; Rigosi, A.; Hill, H. M.; Zande, A. M. V. D.;

Chenet, D. A.; Shih, E. M.; Hone, J.; Heinz, T. F., Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS2 , MoSe2, WS2, and WSe2. Phys.rev.b 2014, 90 (20), 205422. 9.

Lin, Y.; Ling, X.; Yu, L.; Huang, S.; Hsu, A. L.; Lee, Y. H.; Kong, J.; Dresselhaus,

M. S.; Palacios, T., Dielectric screening of excitons and trions in single-layer MoS2. Nano Lett 2014, 14 (10), 5569-76. 10. Hüser, F.; Olsen, T.; Thygesen, K. S., How dielectric screening in two-dimensional crystals affects the convergence of excited-state calculations: Monolayer MoS2. Physical Review B 2013, 88 (24), 245309. 11. Mai, C.; Barrette, A.; Yu, Y.; Semenov, Y. G.; Kim, K. W.; Cao, L.; Gundogdu, K., Many-body effects in valleytronics: direct measurement of valley lifetimes in single-layer MoS2. Nano Lett 2014, 14 (1), 202-6. 12. Yuan, L.; Wang, T.; Zhu, T.; Zhou, M.; Huang, L., Exciton Dynamics, Transport, and Annihilation in Atomically Thin Two-Dimensional Semiconductors. J Phys Chem Lett 2017, 8 (14), 3371-3379. 13. Matin, A.; Der-Hsien, L.; Daisuke, K.; Jun, X.; Angelica, A.; Jiyoung, N.; Madhvapathy, S. R.; Rafik, A.; Santosh, K.; Madan, D., Near-unity photoluminescence quantum yield in MoS2. Science 2015, 350 (6264), 1065. 14. Wu, Z.; Zhao, W.; Jiang, J.; Zheng, T.; You, Y.; Lu, J.; Ni, Z., Defect Activated Photoluminescence in WSe2 Monolayer. The Journal of Physical Chemistry C 2017, 121 (22), 12294-12299.


Page 18 of 24

Page 19 of 24 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


15. Kylänpää, I.; Komsa, H.-P., Binding energies of exciton complexes in transition metal dichalcogenide monolayers and effect of dielectric environment. Physical Review B 2015, 92 (20), 205418. 16. Wang, H.; Zhang, C.; Chan, W.; Manolatou, C.; Tiwari, S.; Rana, F., Radiative lifetimes of excitons and trions in monolayers of the metal dichalcogenide MoS2. Physical Review B 2016, 93 (4), 045407. 17. Palummo, M.; Bernardi, M.; Grossman, J. C., Exciton radiative lifetimes in twodimensional transition metal dichalcogenides. Nano Lett 2015, 15 (5), 2794-800. 18. Jin, C.; Kim, J.; Wu, K.; Chen, B.; Barnard, E. S.; Suh, J.; Shi, Z.; Drapcho, S. G.; Wu, J.; Schuck, P. J.; Tongay, S.; Wang, F., On Optical Dipole Moment and Radiative Recombination Lifetime of Excitons in WSe2. Advanced Functional Materials 2017, 27 (19), 1601741. 19. Wang, G.; Chernikov, A.; Glazov, M. M.; Heinz, T. F.; Marie, X.; Amand, T.; Urbaszek, B., Colloquium: Excitons in atomically thin transition metal dichalcogenides. Reviews of Modern Physics 2018, 90 (2), 021001. 20. Gordon, J. M.; Gartstein, Y. N., Local field effects for spherical quantum dot emitters in the proximity of a planar dielectric interface. Journal of the Optical Society of America B 2014, 31 (9), 2029. 21. Gartstein, Y. N.; Li, X.; Zhang, C., Exciton polaritons in transition-metal dichalcogenides and their direct excitation via energy transfer. Physical Review B 2015, 92 (7), 075445. 22. Prasai, D.; Klots, A. R.; Newaz, A. K.; Niezgoda, J. S.; Orfield, N. J.; Escobar, C.



A.; Wynn, A.; Efimov, A.; Jennings, G. K.; Rosenthal, S. J.; Bolotin, K. I., Electrical Control of near-Field Energy Transfer between Quantum Dots and Two-Dimensional Semiconductors. Nano Lett 2015, 15 (7), 4374-80. 23. Guo, T.; Sampat, S.; Zhang, K.; Robinson, J. A.; Rupich, S. M.; Chabal, Y. J.; Gartstein, Y. N.; Malko, A. V., Order of magnitude enhancement of monolayer MoS2 photoluminescence due to near-field energy influx from nanocrystal films. Sci Rep 2017, 7, 41967. 24. Malko, A. V., Energy transfer interactions between semiconductor nanocrystals and TMDC materials (Conference Presentation). SPIE: 2018; Vol. 10521. 25. Prins, F.; Goodman, A. J.; Tisdale, W. A., Reduced dielectric screening and enhanced energy transfer in single- and few-layer MoS2. Nano Lett 2014, 14 (11), 608791. 26. Zhang, D.; Wang, D. Z.-R.; Creswell, R.; Lu, C.; Liou, J.; Herman, I. P., Passivation of CdSe Quantum Dots by Graphene and MoS2 Monolayer Encapsulation. Chemistry of Materials 2015, 27 (14), 5032-5039. 27. Raja, A.; Montoya Castillo, A.; Zultak, J.; Zhang, X. X.; Ye, Z.; Roquelet, C.; Chenet, D. A.; van der Zande, A. M.; Huang, P.; Jockusch, S.; Hone, J.; Reichman, D. R.; Brus, L. E.; Heinz, T. F., Energy Transfer from Quantum Dots to Graphene and MoS2: The Role of Absorption and Screening in Two-Dimensional Materials. Nano Lett 2016, 16 (4), 2328-33. 28. Sampat, S.; Guo, T.; Zhang, K.; Robinson, J. A.; Ghosh, Y.; Acharya, K. P.; Htoon, H.; Hollingsworth, J. A.; Gartstein, Y. N.; Malko, A. V., Exciton and Trion Energy


Page 20 of 24

Page 21 of 24 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


Transfer from Giant Semiconductor Nanocrystals to MoS2 Monolayers. ACS Photonics 2016, 3 (4), 708-715. 29. Zang, H.; Routh, P. K.; Huang, Y.; Chen, J. S.; Sutter, E.; Sutter, P.; Cotlet, M., Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to SingleLayer and Few-Layer Tin Disulfide. ACS Nano 2016, 10 (4), 4790-6. 30. Mushtaq, A.; Ghosh, S.; Sarkar, A. S.; Pal, S. K., Multiple Exciton Harvesting at Zero-Dimensional/Two-Dimensional Heterostructures. ACS Energy Letters 2017, 2 (8), 1879-1885. 31. Chen, Z.; Berciaud, S.; Nuckolls, C.; Heinz, T. F.; Brus, L. E., Energy transfer from individual semiconductor nanocrystals to graphene. Acs Nano 2010, 4 (5), 2964-8. 32. Castellanos-Gomez, A.; Buscema, M.; Molenaar, R.; Singh, V.; Janssen, L.; van der Zant, H. S. J.; Steele, G. A., Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Materials 2014, 1 (1), 011002. 33. Zhang, X.; Qiao, X. F.; Shi, W.; Wu, J. B.; Jiang, D. S.; Tan, P. H., Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material. Chemical Society reviews 2015, 44 (9), 2757-85. 34. Zhao, Y.; Luo, X.; Li, H.; Zhang, J.; Araujo, P. T.; Gan, C. K.; Wu, J.; Zhang, H.; Quek, S. Y.; Dresselhaus, M. S.; Xiong, Q., Interlayer breathing and shear modes in few-trilayer MoS2 and WSe2. Nano Lett 2013, 13 (3), 1007-15. 35. Shi, H.; Yan, R.; Bertolazzi, S.; Brivio, J.; Gao, B.; Kis, A.; Jena, D.; Xing, H. G.; Huang, L., Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2 2D Crystals. ACS Nano 2013, 7 (2), 1072-1080.



36. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically thin MoS2: a new direct-gap semiconductor. Phys Rev Lett 2010, 105 (13), 136805. 37. Surrente, A.; Dumcenco, D.; Yang, Z.; Kuc, A.; Jing, Y.; Heine, T.; Kung, Y. C.; Maude, D. K.; Kis, A.; Plochocka, P., Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS2/MoSe2/MoS2 Trilayer van der Waals Heterostructures. Nano Lett 2017, 17 (7), 4130-4136. 38. Zandiatashbar, A.; Lee, G. H.; An, S. J.; Lee, S.; Mathew, N.; Terrones, M.; Hayashi, T.; Picu, C. R.; Hone, J.; Koratkar, N., Effect of defects on the intrinsic strength and stiffness of graphene. Nat Commun 2014, 5, 3186. 39. Nan, H.; Wang, Z.; Wang, W.; Liang, Z.; Lu, Y.; Chen, Q.; He, D.; Tan, P.; Miao, F.; Wang, X.; Wang, J.; Ni, Z., Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding. ACS Nano 2014, 8 (6), 5738-5745. 40. Su, W.; Jin, L.; Qu, X.; Huo, D.; Yang, L., Defect passivation induced strong photoluminescence enhancement of rhombic monolayer MoS2. Phys Chem Chem Phys 2016, 18 (20), 14001-6. 41. Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J. C., Intrinsic structural defects in monolayer molybdenum disulfide. Nano Lett 2013, 13 (6), 2615-22. 42. Carozo, V.; Wang, Y.; Fujisawa, K.; Carvalho, B. R.; McCreary, A.; Feng, S.; Lin, Z.; Zhou, C.; Perea-López, N.; Elías, A. L.; Kabius, B.; Crespi, V. H.; Terrones, M., Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide. Science Advances 2017, 3 (4), e1602813.


Page 22 of 24

Page 23 of 24 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


43. Moody, G.; Tran, K.; Lu, X.; Autry, T.; Fraser, J. M.; Mirin, R. P.; Yang, L.; Li, X.; Silverman, K. L., Microsecond Valley Lifetime of Defect-Bound Excitons in Monolayer WSe2. Phys Rev Lett 2018, 121 (5), 057403. 44. Yu, Y.; Yu, Y.; Cai, Y.; Li, W.; Gurarslan, A.; Peelaers, H.; Aspnes, D. E.; Van de Walle, C. G.; Nguyen, N. V.; Zhang, Y. W.; Cao, L., Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films. Sci Rep 2015, 5, 16996. 45. Gordon, J. M.; Gartstein, Y. N., Dielectric polarization, anisotropy and nonradiative energy transfer into nanometre-scale thin semiconducting films. J Phys Condens Matter 2013, 25 (42), 425302. 46. Karanikolas, V. D.; Marocico, C. A.; Eastham, P. R.; Bradley, A. L., Near-field relaxation of a quantum emitter to two-dimensional semiconductors: Surface dissipation and exciton polaritons. Physical Review B 2016, 94 (19), 195418. 47. Schuller, J. A.; Karaveli, S.; Schiros, T.; He, K.; Yang, S.; Kymissis, I.; Shan, J.; Zia, R., Orientation of luminescent excitons in layered nanomaterials. Nat Nanotechnol 2013, 8 (4), 271-6. 48. Rigosi, A. F.; Hill, H. M.; Li, Y.; Chernikov, A.; Heinz, T. F., Probing Interlayer Interactions in Transition Metal Dichalcogenide Heterostructures by Optical Spectroscopy: MoS2/WS2 and MoSe2/WSe2. Nano Lett 2015, 15 (8), 5033-8. 49. Molecular Fluorescence and Energy Transfer Near Interfaces. In Advances in Chemical Physics.



TOC Graphic


Page 24 of 24

Exciton Radiative Recombination Dynamics and Nonradiative Energy

Recommend Documents