Sub-Picosecond Carrier Dynamics Induced by Efficient Charge

Subscriber access provided by GUILFORD COLLEGE

Article

Sub-Picosecond Carrier Dynamics Induced by Efficient Charge Transfer in MoTe2/WTe2 van der Waals Heterostructures Kyusup Lee, Jie Li, Liang Cheng, Junyong Wang, Dushyant Kumar, Qisheng Wang, Mengji Chen, Yang Wu, Goki Eda, Elbert E. M. Chia, Haixin Chang, and Hyunsoo Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04701 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 19, 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 41 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

ACS Nano

Sub-Picosecond Carrier Dynamics Induced by Efficient Charge Transfer in MoTe2/WTe2 van der Waals Heterostructures Kyusup Lee,† Jie Li,‡ Liang Cheng,§ Junyong Wang, Dushyant Kumar,† Qisheng Wang,† Mengji Chen,† Yang Wu,† Goki Eda, Elbert E. M. Chia,§ Haixin Chang,‡ Hyunsoo Yang†,*

†Department

of Electrical and Computer Engineering and NUSNNI, National

University of Singapore, 117576, Singapore

‡Center

for Joining and Electronic Packaging, State Key Laboratory of Material

Processing and Die & Mould Technology, School of Materials Science and

Engineering, Huazhong University of Science and Technology, Wuhan 430074,

China

1

ACS Paragon Plus Environment

ACS Nano 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

§Division

of Physics and Applied Physics, School of Physical and Mathematical

Sciences, Nanyang Technological University, 637371, Singapore

Department of Physics, National University of Singapore, 119077, Singapore

2


Page 2 of 41

Page 3 of 41 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

ACS Nano

ABSTRACT: Demonstration of van der Waals (vdW) semiconductor/metal

heterostructures (SMHs) based on transition metal dichalcogenides has been a central

approach in high-speed electronics by introducing ultrafast carrier dynamics. In this

regard, a Weyl semimetal WTe2 is of great interest due to its vdW layered nature, low work-function, and superior electrical properties. However, little is still known about its

heterostructures and a few-picosecond photocarrier lifetimes hinder its applications in

high-speed electronics. Here, we propose a SMH; Semimetallic Td-phase WTe2 with its sister compound of semiconducting 2H-phase MoTe2. Time-resolved terahertz spectroscopy demonstrated that WTe2 exhibited the significantly shorter carrier lifetimes of sub-ps when forming a junction with MoTe2. We provided explicit characteristic signatures revealing charge transfer across the interface and the

subsequent interlayer-exciton decay. This work not only offers the extension of the

THz detection scope of ultrafast phenomena from atomically-thin materials but also

provides a building block of vertical SMHs for high-speed electronic devices with sub-

ps photocarrier lifetimes.

3


ACS Nano 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

KEYWORDS: van der Waals heterostructure, transition metal dichalcogenide, Weyl

semimetal, charge transfer, time-resolved terahertz spectroscopy

4


Page 4 of 41

Page 5 of 41 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

ACS Nano

One avenue towards the achievement of high-speed devices is the utilization of high-

speed photoconducting materials, because semiconductor electronics in electronic

processors is generally limited in speed by thermal and incoherent delay times to approximately 10 GHz.1 In this regard, van der Waals (vdW) semiconductor/metal

heterostructures (SMHs) based on transition metal dichalcogenides (TMDs) have attracted considerable interest2,3 by introducing sub-picosecond carrier lifetimes. Such

ultrashort lifetimes are attributed to a vdW bonding nature that induces efficient charge transfer (CT) across the interface,4,5 which is limited in conventional semiconductor heterostructures due to the interfacial imperfection caused by lattice mismatch.6 In

addition, a poorly screened Coulomb potential in vdW bonding promotes to form interlayer-excitons,7,8 bound electron-hole (e-h) pairs formed across the interface

following the CT, which primarily govern their electronic properties and carrier dynamics: promoting carrier relaxation,9-11 enhancing light-matter interaction,12 and generating highly-efficient photocurrents.13

Furthermore, vdW SMHs have shown efficient tuning of a Schottky barrier (SB)

height, while it has been generally limited in conventional SMHs due to a Fermi level

5


ACS Nano 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

pinning effect due to finite metal-induced gap states at the junction. The vdW bonding suppresses the gap states and correspondingly weakens the pinning effect,14 playing

an important role in efficient CT as well as ultrashort carrier lifetimes. However,

engineering sub-ps lifetimes still remains a challenge.

Tungsten ditelluride (WTe2), a TMD compound, has been proposed as a type-II Weyl semimetal showing a strongly tilted Weyl cone and adjoined Weyl points between an electron pocket and a hole pocket in the Fermi surface.15 WTe2 hosts topological surface states and the conduction-valence bands touch at the Weyl points in the bulk state,16 providing ample playground for fundamental and practical standpoints. Distinct

from most transition metal sulfide and selenide compounds which are only stable in a

semiconducting 2H-phase, WTe2 forms a stable Td-phase at room temperature, which exhibits semimetallic electronic band structures17,18 as well as the corresponding electronic and transport properties: high mobility,19 quantum anomalous Hall effect,16 and extremely large magnetoresistance.20 However, a few ps photocarrier lifetimes in

WTe2, larger than typical metals and Dirac semimetals, have limited the applications in high-speed electronics.

6


Page 6 of 41

Page 7 of 41 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

ACS Nano

In this report, we propose MoTe2/WTe2 as an example of vdW SMHs to demonstrate ultrashort carrier lifetimes in a sub-ps regime. Semiconducting 2H-phase MoTe2 has a smaller bandgap (Eg ~ 1.0 eV)21 and a lower electron affinity (Eea~3.8 eV)14 among TMD semiconductors. Therefore, it was expected to reduce the SB height when MoTe2 forms a junction with a low work function material of Td-phase WTe2 (W~4.4 eV)22 by diminishing the Fermi level pining effect due to the reduced metal-induced gap states.14 In addition, both elements exhibit the room-temperature stable and uniform

phases at a large size grown by chemical vapor deposition (CVD), which are promising

for practical applications.

We utilized time-resolved terahertz (THz) spectroscopy to directly monitor the photo-

excited carrier dynamics. We revealed CT across the interface and the subsequent

interlayer-exciton decay in MoTe2/WTe2, playing dominant roles in demonstrating subps carrier lifetimes. We noticed that while previous works have shown CT in various

vdW heterostructures, experimental signatures based on time-resolved THz

spectroscopy are still largely unexplored. On the other hand, we present explicit

characteristic signatures of CT and interlayer exciton dynamics. Therefore, this work

7


ACS Nano 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

extends the possibility of THz detection of ultrafast phenomena occurring in atomically-

thin materials. Furthermore, the proposed MoTe2/WTe2 with a sub-ps lifetime is promising for the applications in THz speed electronics.

RESULTS AND DISCUSSION

Material preparation and experimental design. MoTe2, WTe2, and MoTe2/WTe2 thin films were grown by CVD. While mechanically exfoliated WTe2 and MoTe2 materials are typically limited in their size to small flakes,19,23 CVD-grown materials are centimeter-scale with an uniform crystalline phase.24,25 Details on the thin-film growth are addressed in Material section and have been reported elsewhere.25,26

First, we characterized the crystalline phase (Figure 1a). Raman spectra in WTe2 with peaks at 116, 132, 160, and 210 cm-1 corresponding to A13, A14, A17, and A19 vibrational modes, respectively, revealed a semimetallic Td-phase.27,28 The Raman peaks of A1g and E2g1 modes in MoTe2 revealed a semiconducting 2H-phase.29 MoTe2/WTe2 was prepared such that WTe2 was deposited on top of MoTe2 on a glass substrate. The Raman spectra in MoTe2/WTe2 demonstrated the mixture of Td-phase 8


Page 8 of 41

Page 9 of 41 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

ACS Nano

WTe2 and 2H-phase MoTe2. We kept the same thickness for the heterostructures compared to the controls, i.e., participating materials of WTe2 (5, 10, and 30 nm) and MoTe2 (20 nm) (Supporting figure S1). In Raman measurements, the beam shined onto the WTe2 surface, which gives rise to the WTe2 thickness-dependent Raman amplitudes (Supporting figure S2). The Raman spectrum of thin MoTe2 (20 nm)/WTe2 (5 nm), compared to those of thick MoTe2 (20 nm)/WTe2 (10 and 30 nm) showed the reduced amplitudes of Td-phase modes (A13, A14, A17, and A19) but the increased amplitudes of 2H-phase modes (A1g and E2g1). This thickness trend supported that the investigated materials were the vdW SMHs.

X-ray diffraction (XRD) patterns showed that only (0 0 2n) reflections were observed

in the individual WTe2 or MoTe2 film, implying that WTe2 and MoTe2 films with highly oriented in the z-axis were obtained (Supporting figure S3a). The XRD pattern of

MoTe2 (20 nm)/WTe2 (30 nm) was similar to that of individuals. Further fitting analysis, where Td-WTe2 and 2H-MoTe2 contributions were clearly resolved, indicated that the exposed surface of MoTe2/WTe2 was also the c-plane (Supporting figure S3b).

9


ACS Nano 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 1b represents the band diagram of the band-bending at the interface between WTe2 and MoTe2.14,21,22,25 The upward band-bending toward MoTe2 occurs at the interface, and the SB forms in the valence band with a height (MB) of 0.4 eV, given by the equation of MB = Eea + Eg – W.14 The upward-bending feature is expected to induce the flow of electrons (holes) transfer toward WTe2 (MoTe2) under photoexcitation. We utilized a time-resolved THz spectroscopy system, i.e., an optical-pump and

THz-probe (OPTP) method (Figure 2a). Low energies of THz photons enable to

directly monitor photo-excitation and relaxation dynamics of excited carriers without the unwanted interference with other species.30 A temporal resolution of approximately

120 fs was roughly estimated by a pump pulse duration (Supporting figure S4). The

pump wavelength of 800 or 1550 nm (photon energies of 1.5 or 0.8 eV, respectively)

was used for the optical excitation; a 1.5 eV photon excites both MoTe2 and WTe2 layers but a 0.8 eV photon excites the WTe2 layer only due to the larger bandgap of MoTe2 (~1.0 eV) (See Method section for the details). A lifetime of excited mobile carriers can be determined by measuring the photo-

induced change in the peak amplitude of the THz waveform transmitted through a

10


Page 10 of 41

Page 11 of 41 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

ACS Nano

sample, –QT/T0, where QT = T – T0 and, T0 and T are the maximums in the THz waveforms before and after photoexcitation, respectively. Here, the minus sign of – QT/T0 is indicative of pump-induced increase of the THz absorption during propagation through a material, i.e., increase in the imaginary part of the refractive index (nR and

ncomplex = nS + TnR@% This change therefore corresponds to a larger positive real part of the conductivity (US and Ucomplex = US + TUR@ which is mostly arising from mobile charge carriers such as free carriers. The conductivity is proportional to the density of the excited carriers (N) as well as their effective electronic temperature (Teff). Accordingly, a rising trend in –QT/T0 represents photoexcitation and thermalization, through which both N and Teff increase. A peak indicates a thermalized state with the highest Teff. Shortly after, the charge carriers start cooling and in turn recombine to the ground

state, showing a decay trend in the signal and determining the carrier lifetime.

Sub-bandgap excitation. By utilizing two different pump photon energies, we

selectively injected photoexcited carriers into the WTe2 layer only or both MoTe2 and WTe2 layers. First, we successively captured the dynamics of the excited free carriers

11


ACS Nano 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

Page 12 of 41

with a 0.8-eV optical pump as a form of –QT/T0 (Figure 2b). WTe2 (30 nm) clearly showed a rise and decay trend. The rise signal represents the photoexcitation of

charges

and

the

subsequent

thermalization

mainly

via the carrier-carrier

scattering.25,31 The decay signal is attributed to the cooling process mostly via the e-h

recombination. Here, the e-h recombination is assisted by phonons due to the momentum mismatch of the band structure near the Weyl points in WTe2. A mono-exponential fit revealed a lifetime of 1.0 ± 0.01 ps. On the other hand, MoTe2 (20 nm) showed no measurable photoexcitation due to the higher bandgap than the pump photon energy and a tiny

peak signal, pointed by a blue arrow in Figure 2b, was induced by the two-photon

absorption of the pump.

Next, we identically employed the measurement on MoTe2 (20 nm)/WTe2 (30 nm) under the sub-bandgap excitation, similarly showing a clear rise and decay feature

(Figure 2b). A lifetime of 0.6 ± 0.01 ps was obtained by a mono-exponential fit. It was

noted that WTe2 exhibited considerably faster relaxation when forming a junction with MoTe2, although the photoexcitation was only allowed in WTe2.

12


Page 13 of 41 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

ACS Nano

Above-bandgap excitation. We further monitored free carrier dynamics using the

above-bandgap excitation (1.5-eV pump). In contrast to the sub-bandgap excitation,

MoTe2 clearly showed the optical excitation and the subsequent relaxation (Figure 2c). The decay signals of all the three samples were fitted by two distinct exponential decay components, fast and slow components of Vf and Vs, respectively, with a scanning range up to 130 ps. The extracted time constants were Vf1 = 1.46 ± 0.03 ps and Vs1 = 117 ± 12.4 ps in WTe2, as well as Vf2 = 5.42 ± 0.02 ps and Vs2 = 25 ± 2.4 ps in MoTe2 (Figure 2c and Supporting figure S5). In both cases, the fitting weights of Vs were smaller than 1% of those of Vf. Accordingly, Vf and Vs can be assigned to a carrier lifetime and carrier/heat diffusion out of the laser exposure volume, respectively, which has been similarly studied in Dirac semimetals.32 In addition, WTe2, compared to MoTe2, exhibited a shorter Vf and a longer Vs, which were consistent with a previous study.25

We noted that MoTe2/WTe2 similarly exhibited faster relaxation than the controls under the above-bandgap excitation, having a carrier lifetime of 0.77 ± 0.02 ps and

the carrier/heat diffusion for 11 ± 7.4 ps (Figure 2c and Supporting figure S5). If

13


ACS Nano 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

assuming the charges stay only in the excited layer, the overall relaxation should be a

weighted average of the participating materials. Accordingly, the averaged time constant V given by a relation of 1/V = 1/Vf1 + 1/Vf2 is 1.15 ps (Vf1 = 1.46 ps and Vf2 = 5.42 ps), which is significantly longer than the experimentally extracted value (0.77 ps).

It has been recently reported that the electronic states of the participating materials

can be modified by the existence of coupled states induced by strong interlayer

coupling in various vdW semiconductor heterojunctions. They have shown changes in band edge states such as binding energy modification and bandgap renormalization,33-36 which

can subsequently induce significant changes in carrier dynamics. On the other hand, due to

the relatively weak vdW interaction, it insignificantly affects the electronic properties

around the Fermi level, which exhibits that the density of state of SMHs is the

superposition of the individual contributions. However, the strong out-of-plane

hybridization effects have been predicted to induce significant changes in the electronic structure in the higher conduction and deeper valence regions.37 While the

vdW interaction affects the properties close to the Fermi level, its role away from it is

much more pronounced. Nevertheless, the overall carrier relaxation after excitation

14


Page 14 of 41

Page 15 of 41 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

ACS Nano

and thermalization is mainly determined around the Fermi energy, which is not

prominent by the changes in the higher energy states. Furthermore, the density of state near the Fermi level in semimetallic Td-WTe2 is insignificant.17,18 Therefore, the effect of change in the electronic structures of WTe2 with a vdW junction with MoTe2 to a carrier lifetime can be neglected. Consequently, the significantly shorter decay times in

MoTe2/WTe2 measured using both sub- and above-bandgap excitation suggested that interfacial effects play dominant roles in accelerating the relaxation,9,10,14 such as

bandgap renormalization induced by ground-state CT, interlayer exciton formation by

hot-carrier transfer, and/or interfacial defect states.

Photoconductivity. An effect of ground-state CT on a carrier lifetime is considered in

this section. As mentioned above, an energy band alignment after vdW junction can

be modified under a built-in potential induced by ground-state CT, which subsequently

changes a carrier lifetime. In this regard, we quantified a dc conductivity ?Udc) to indirectly elucidate the ground-state CT, without and with photoexcitation based on

THz time-domain spectroscopy (See Method section).

15


ACS Nano 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

Page 16 of 41

Figure 3a showed the extracted Udc from various WTe2-thickness without photoexcitation. The conductivities of MoTe2/WTe2 were almost comparable to the summation of individual contributions ?UMoTe2/WTe2 ~ UMoTe2 + UWTe2). This suggested the negligible ground-state CT, as well as indicated the insignificant change in the

electronic structures of WTe2 after the vdW junction. In contrast, UMoTe2/WTe2 became smaller than the summation of UMoTe2 and UWTe2 for all WTe2-thicknesses with photoexcitation (Figure 3b). This evidenced that the measurable amount of CT occurred after photoexcitation and subsequently they were

quickly recombined, decreasing the carrier density and thus giving rise to the lower

Udc. We attributed both Udc trends to the presence of the SB at the band structure. It was because that thermionic emission without photoexcitation was prohibited due to lower

thermal energy level at room temperature (25 meV)

38,39

than the SB height of

MoTe2/WTe2 (YB ~ 0.4 eV, shown in Fig. 1b). In contrast, photoexcitation energy of 1.5 eV was able to induce the thermionic emission across the SB.40 We indirectly

estimated

the

absence

(presence)

of

charge

transfer

16


without

(with)

the

Page 17 of 41 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

ACS Nano

photoexcitation.

Therefore,

the

effects

of

ground-state

CT

and

bandgap

renormalization to a carrier lifetime were ruled out, while the sub-ps carrier lifetimes in

MoTe2/WTe2 were dominantly induced by hot-carrier transfer.

Zero-crossing point scan. To further elucidate the effect of CT in WTe2/MoTe2, we defined the charge species dominated in relaxation after CT. While an OPTP spectroscopy has been extensively used to investigate the dynamics of mobile charged carriers in various materials, it has been recently noted that low energy of a THz photon (1 THz ~ 4 meV) allows to couple it with the intraexcitonic transition of bound charges, enabling to monitor the photo-induced dynamics of excitons as well. The mechanism of measuring the

exciton dynamics is photo-induced change in the imaginary part of the conductivity (UR@ which can be quantified by the phase shift of the THz transmission through a

material. It is because that the phase-shift is parameterized by the real part of the refractive index (nS@ that also represents the change in the material polarizability.41 The positive phase shift ?Q[) is an increase of the real refractive index (–QnS@ corresponding to a larger negative imaginary photoconductivity (–QUR@ which can also

contain contributions from polarization effects of bound charges such as excitons.

17


ACS Nano 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

Therefore, one can similarly determine an exciton lifetime by monitoring the photoinduced Q[ of the THz transmission. In experiments, the Q[ can be quantified by

measuring the change of the THz transmission at a zero-crossing point of the THz waveform ?QT), as shown in Fig. 4a, through which one is able to exclude the contribution of the photo-induced amplitude change.38,42,43

We separately extracted free carrier and exciton dynamics, plotted by red and blue curves in Fig. 4b–d, respectively. The negative valued signal (–QT/T0 < 0) of the exciton motion is indicative of pump-induced larger positive Q[. Hence, the initial

negative rise and the corresponding decay trends represent the exciton formation and annihilation via recombination, respectively, finally determining an exciton lifetime.

Note that MoTe2/WTe2 and the individuals showed distinct behaviors: 1) An exciton lifetime in semimetallic WTe2 was significantly shorter than that of free carriers, indicating the weak exciton contribution to overall carrier relaxation. 2) In contrast, as

expected in semiconducting MoTe2, the strong exciton contribution was observed by showing comparable lifetimes between free carriers and excitons. In this case, the

excitons formed in the MoTe2 layers (intralayer excitons), which can be efficiently

18


Page 18 of 41

Page 19 of 41 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

ACS Nano

generated in various thin TMD semiconductors due to a weak Coulomb screening. 3)

We similarly observed the strong exciton contribution in MoTe2/WTe2, confirming that the charge specie was dominated by excitons as well. However, the significantly

reduced lifetimes of both free carriers and excitons, compared to MoTe2, indicated that e-h pairs formed not in the individual layers but across the interface, i.e., interlayer-

excitons. Therefore, we assigned the interfacial effect to sub-ps carrier lifetimes to CT

across the interface and the subsequent interlayer-exciton decay.

Figure 5a represents the photoexcitation and the subsequent CT under the sub-

bandgap excitation. Due to the upward band-bending, the CT is mainly attributed to

the hole transfer from WTe2 to MoTe2, whereas the excited electrons remain in WTe2. For the above-bandgap excitation, it is expected that the bidirectional CT dominantly

occurs, wherein holes (electrons) transfer to the MoTe2 (WTe2) layer (Figure 5b). In both cases, the excited carriers are delocalized in the different layers (mainly holes in

MoTe2 and electrons in WTe2). They form weakly-bound e-h pairs across the interface at higher energy states (hot interlayer-excitons), followed by either the dissipation of

19


ACS Nano 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

excess energy into the environment or the formation of tightly-bound interlayer

excitons.

A similar study in MoSe2/graphene heterostructures has shown the CT and the subsequent interlayer-exciton decay.30 The authors showed that the recombination of

tightly-bound interlayer-excitons needs either the momentum conservation or the

cross of the binding barrier, taking extra steps and thus exhibiting longer lifetimes. On

the contrary, we observed the shorter lifetimes in MoTe2/WTe2 than that of the controls. We thus ruled out the effect of tightly-bound interlayer-excitons, but attributed

the shorter lifetimes to the effect of intermediate states, through which ultrafast

dissipation of excess energy of hot interlayer-excitons occurs. The lower-binding

energy and a larger e-h distance in hot interlayer excitons can lead to dissociate the intermediates, reducing the lifetimes of the heterostructures.11 Accordingly, the rise trend of –QT/T0 in MoTe2/WTe2 represents the hot interlayer-exciton formation, followed by the exciton decay with the sub-ps characteristic times.

20


Page 20 of 41

Page 21 of 41 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

ACS Nano

Band-filling effect and hot-phonon effect. In addition, the fluence- and thickness-

dependent studies revealed characteristic signatures of CT and the subsequent exciton decay. First, we systematically varied the optical fluence (Fopt) within 0–10 ]J/cm2 (brown to violet curves, respectively, in Figure 6a-b) using the 1.5-eV pump.

Thin WTe2 (5 nm) showed a weakly-peaking and negligible decay trend within 12 ps (Figure 6a). We assigned this trend to the suppressed photoexcitation induced by the low density of states in a few layers WTe2.17,25 The small phase space in thin WTe2 limits the interband transition and induces a prominent band filling feature; Most of the

excited energy states are quickly filled by photoexcited carriers and Pauli blocking suppresses the interband transition.44 Accordingly, the WTe2 films exhibited significant band-filling effects (Figure 6c). This was further evident by that fact that we obtained a larger band filling effect in the strong excitation regime (up to 80 ]J/cm2) compared to that in the weak excitation regime (< 10 ]J/cm2) (Supporting figure S6). On the other

hand, WTe2 (5 nm)/MoTe2 (20 nm) exhibited a clear photoexcitation and decay trend (Figure 6b). We noticed that the linear behavior to the optical fluence (Figure 6d)

represented a negligible band-filling effect. We assigned the suppressed band-filling

21


ACS Nano 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

effect to efficient CT such that the majority of excited holes in WTe2 efficiently transfer to MoTe2 before filling the states in WTe2. On the other hand, MoTe2/WTe2 with a poor interface-quality showed that the interfacial effect-driven effects on carrier lifetimes

were almost vanishing but bulk effects dominated (See Supporting figure S7 and note

1). Second, we compared the lifetimes of MoTe2/WTe2 and WTe2 with various Fopt. The lifetimes of WTe2 clearly exhibited the fluence-dependence; slower decay at higher fluence (Figure 6e). The Fopt-trend arose from a hot phonon bottleneck effect in which the hot phonons generated during thermalization were re-absorbed by hot carriers and the overall carrier relaxation was delayed.45 We noticed that, on the other hand, MoTe2/WTe2 exhibited insignificant fluence-dependence with the given Fopt range (Figure 3f). In addition, such fluence-independent lifetimes ruled out a defect-mediated

effect on carrier relaxation, which is strongly depending on the fluence (See

Supporting note 2). Therefore, we assigned the absence of a hot-phonon effect to the

suppressed Auger process and the weak phonon contribution during the relaxation.

This further demonstrated that the CT and the interlayer-excitons decay prevail over

22


Page 22 of 41

Page 23 of 41 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

ACS Nano

the intralayer relaxation and play roles as fast relaxation channels. In addition, the

shorter lifetimes at thicker MoTe2/WTe2 (Figure 6f) is attributed to a larger phase space of the thicker WTe2 (10 and 30 nm) 17,25 inducing a higher scattering probability during the relaxation, which provides an extra knob to engineer WTe2-based SMHs.

CONCLUSIONS

We propose Weyl-semimetal based vdW SMHs, of which optoelectronic properties

are of great importance. The photocarrier dynamics on the SMH of semiconducting

2H-phase MoTe2/semimetallic Td-phase WTe2 and its controls were investigated. Time-resolved THz spectroscopy with various conditions revealed explicit signatures

of CT and interlayer exciton decay, demonstrating sub-ps carrier lifetimes. Sub- and

above-bandgap excitation suggested interfacial effects dominated in carrier relaxation.

The dc conductivities demonstrated the presence of CT under photoexcitation and

ruled out the other interfacial effects such as ground-state CT. In addition, exciton

lifetimes were measured by zero-crossing point scanning, revealing the presence of

interlayer excitons. Thickness- and fluence-dependence further provided the 23


ACS Nano 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

characteristic signatures of CT, further supporting the weak phonon contribution in the

relaxation. Therefore, our work extends the possibility of THz detection of ultrafast

phenomena occurring in atomically-thin heterostructures. Furthermore, the proposed

vdW SMH is promising for high-frequency optoelectronics with its ultrashort carrier

lifetimes.

MATERIALS AND METHODS

Material synthesis. The synthesis of MoTe2/WTe2 thin films is based on a one-zone CVD system. First, we grow a MoTe2 thin film on a glass substrate with a modified method in a previous work.26 0.2 g Te powder (99.99%, Alfar Aesar) covered by certain

molecular sieves is placed in a ceramic crucible. Two evaporated MoOx (x 3) films on the substrate, as reaction sources, are covered with each other and placed on top of

the ceramic crucible. We flow a mixture of N2 (3 sccm) and H2 (4 sccm) into a tube at ambient atmosphere. The growth temperature is 700 °C for 30 minutes. At the end of

the growth, the furnace is naturally cooled down to the room temperature. Second, for

the growth of a WTe2 thin film on top of the as-grown MoTe2, the WOx ?'_9@ film is 24


Page 24 of 41

Page 25 of 41 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

ACS Nano

evaporated onto the surface of the MoTe2 film. A ceramic crucible loaded with 0.25 g Te powder (99.99%, Alfar Aesar) is placed in the center of a quartz tube. The prepared

WOx/MoTe2 film placed at the downstream about 3–5 cm away from the Te power is tellurized at 700 °C for 45 minutes. An Al layer (1 nm thick) is capped to protect the

as-grown thin films, which is naturally oxidized to Al2O3 in air.

Time-resolved terahertz spectroscopy: Optical-pump and terahertz-probe. Carrier

dynamics on the thin films is monitored by an all-optical means through which the carriers are excited via interband transition by optical photons (0.8 or 1.55 eV) and

their relaxation motion through intra- and inter-band transitions are probed with the

low-energy THz photons. Ultrashort laser pulses with 120-fs pulse duration based on

a Ti:sapphire regenerative amplified laser system operating at 1 kHz are utilized for

the time-resolved THz time-domain spectroscopy. The laser beam is split into three

for THz emission and detection, as well as the optical pump. When a linearly polarized

optical pulse (300 mW) illuminating a 1-mm thick and 110-cut ZnTe, a THz pulse (0.1– 3 THz) is emitted via the optical rectification. Based on a 2-mm thick and 110-cut ZnTe

25


ACS Nano 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

Page 26 of 41

under a canonical electro-optic sampling technique, we map a THz waveform in the

time domain using a weak optical-probe pulse (T/T0, from the scanning at the peak (zero-crossing) position represents the photo-induced increase in THz absorption (phase-shift) and thus determines a free carrier (exciton) lifetime.

38


Page 39 of 41 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

ACS Nano


ACS Nano

(c)

0.4

1.2

WTe2 5 nm 10 nm 30 nm

3

Tpeak/T0 (%)

- T/T0 (%)

0.6

(e)

4

WTe2 (5 nm)

(ps)

0.8

2

0.9

f

(a)

0.2

1

WTe2

0.6

10 nm 30 nm

0.0 0

4

8

0

12

0

2

4

6

8

10

0

Fopt ( J/cm2)

Time delay (ps)

(d)

(f) MoTe2 (20 nm)/WTe2 (x)

MoTe2 (20 nm)/WTe2 (5 nm)

6

1

5 nm 10 nm 30 nm

4

2

4

6

8

10

Fopt ( J/cm2)

1.2

MoTe2 (20 nm)/WTe2 (x) 5 nm 10 nm 30 nm

0.9

f

- Tpeak/T0 (%)

2

(ps)

(b)

- T/T0 (%)

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

Page 40 of 41

0.6

2 0 0

2

4

2

Time delay (ps)

4

6

8

Fopt ( J/cm

10

2

4

6

8

10

2

)

Fopt ( J/cm )

Figure 6. The differential THz transmission in (a) WTe2 (5 nm) and (b) MoTe2 (20 nm)/WTe2 (5 nm) at variable optical fluences. The brown to violet (bottom to upper) curves correspond to the fluence of 0–10 ?@

2

2-step,

with 1 ?@

respectively. The peak value of the

differential THz transmission, –>Tpeak/T0, as a function of optical fluence at different thicknesses of (c) WTe2 and (d) MoTe2/WTe2. The solid and linear lines are added for eye guidance. The extracted free carrier lifetimes in different thicknesses of (e) WTe2 and (f) MoTe2/WTe2.

40


Page 41 of 41 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

ACS Nano


Sub-Picosecond Carrier Dynamics Induced by Efficient Charge

Recommend Documents