Observation of ultralong valley lifetime in WSe2/MoS2 heterostructures.

TL;DR: In this article, a microsecond-long-lived valley polarization in 2D transition metal dichalcogenides (TMD) was achieved by exploiting the ultrafast charge transfer processes in the heterostructure that efficiently creates resident holes in the WSe2 layer.

Abstract: The valley degree of freedom in two-dimensional (2D) crystals recently emerged as a novel information carrier in addition to spin and charge. The intrinsic valley lifetime in 2D transition metal dichalcogenides (TMD) is expected to be markedly long due to the unique spin-valley locking behavior, where the intervalley scattering of the electron simultaneously requires a large momentum transfer to the opposite valley and a flip of the electron spin. However, the experimentally observed valley lifetime in 2D TMDs has been limited to tens of nanoseconds thus far. We report efficient generation of microsecond-long-lived valley polarization in WSe2/MoS2 heterostructures by exploiting the ultrafast charge transfer processes in the heterostructure that efficiently creates resident holes in the WSe2 layer. These valley-polarized holes exhibit near-unity valley polarization and ultralong valley lifetime: We observe a valley-polarized hole population lifetime of more than 1 μs and a valley depolarization lifetime (that is, intervalley scattering lifetime) of more than 40 μs at 10 K. The near-perfect generation of valley-polarized holes in TMD heterostructures, combined with ultralong valley lifetime, which is orders of magnitude longer than previous results, opens up new opportunities for novel valleytronics and spintronics applications.

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UC Berkeley
UC Berkeley Previously Published Works
Title
Observation of ultralong valley lifetime in WSe
2
/MoS
2
heterostructures.
Permalink
https://escholarship.org/uc/item/4h81w790
Journal
Science advances, 3(7)
ISSN
2375-2548
Authors
Kim, Jonghwan
Jin, Chenhao
Chen, Bin
et al.
Publication Date
2017-07-01
DOI
10.1126/sciadv.1700518
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

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Copyright © 2017
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
Observation of ultralong valley lifetime in
WSe
2
/MoS
2
heterostructures
Jonghwan Kim,
1,2
* Chenhao Jin,
1
* Bin Chen,
3
Hui Cai,
3
Tao Zhao,
1
Puiyee Lee,
1
Salman Kahn,
1
Kenji Watanabe,
4
Takashi Taniguchi,
4
Sefaattin Tongay,
3
Michael F. Crommie,
1,5,6
Feng Wang
1,5,6†
The valley degree of freedom in two-dimensional (2D) crystals recently emerged as a novel information carrier
in addition to spin and charg e. The intrinsic valley lifetime in 2D transition metal dichalcogenides (TMD) is
expected to be markedly long due to the unique spin-valley locking behavior, where the intervalley scattering
of the electron simultaneously requires a large momentum transfer to the opposite valley and a flip of the
electron spin. However, the experimentally observed valley lifetime in 2D TMDs has been limited to tens of
nanoseconds thus far. We report efficient generation of microsecond-long-li ved valley polarization in WSe
2
/
MoS
2
heterostructures by exploiting the ultrafast charge transfer processes in the heterostructure that efficient-
ly creates resident holes in the WSe
2
layer. These valley-polarized holes exhibit near-unity valley polarization
and ultralong valley lifetime: We observe a valley-polarized hole population lifetime of more than 1 ms and a
valley depolarization lifetime (that is, intervalley scattering lifetime) of more than 40 ms at 10 K. The near-perfect
generation of valley-polarized holes in TMD heterostructures, combined with ultralong valley lifetime, which is
orders of magnitude longer than previous results, opens up new opportunities for novel valleytronics and spin-
tronics applications.
INTRODUCTION
Atomically thin layers of semiconducting transition metal dichalcogen-
ides (TMDs) exhibit unique electronic band structure (1, 2)and
fascinating physical properties (3, 4). A pair of degenerate direct bands
are present at the K and K′ points in the momentum space of hexagonal
TMD monolayers, giving rise to a new valley degree of freedom known
as the valley pseudospin (5, 6). The strong spin-orbital coupling present
in TMDs further locks the valley pseudospin to specific electron and
hole spins for electronic states close to the bandgap (5, 6). These coupled
spin and valley degrees of freedom in TMDs can open up new ways to
encode and process information for valleytronics, and they can be
controlled flexibly through optical excitation, electrostatic gating, and
heterostructure stacking. In particular, the spin-valley locking suggests
that the intrinsic valley lifetime can be extremely long because a change
of valley pseudospin requires a rare event with a large momentum trans-
fer (from K to K′ valley) and an electron spin flip at the same time.
Tremendous progress has been made in exploring the valley pseu-
dospin of two dimensional (2D) TMDs, ranging from optical generation
and detection of valley polarization (7–9) to manipulation of valley
pseudospin state with optical and magnetic field (10–14) and obser-
vation of the valley Hall effect (15). However, many challenges still exist
for potential valleytronics applications. Chief among them is the rela-
tively short valley lifetime. It was recently shown both theoretically and
experimentally that the valley lifetime of excitons in TMD monolayers is
severely constrained by the electron-hole exchange interaction through
the Maialle-Silva-Sham mechanism (16–20), which can annihilate an ex-
citoninonevalleyandcreateanotherexcitonintheothervalley(thatis,
depolarize the valley pseudospin) within picoseconds. However, the
valley pseudospin of individual electrons or holes is not affected by this
mechanism and can have a much longer lifetime. Photoinduced valley
polarization of resident carriers in TMD monolayers is reported to have
amuchlongervalleylifetime(21–23). The bright interlayer exciton in
type II van der Waals heterostructure of TMDs provides another way to
achieve a longer valley lifetime, where electrons and holes are separated
into different lay ers and the electron-ho le exchange interaction is
strongly suppressed. However, experimentally observed valley lifetime
for either resident carriers in TMD monolayers or indirect excitons in
TMD heterostructures has been limited to a few tens of nanoseconds
thus far (21–24). Here, we report efficient generation of ultralong-lived
valley polarization in WSe
2
/MoS
2
heterostructures. Using ultrafast
pump-probe spectroscopy that covers the time scale from femtose-
conds to microseconds, we show that perfectly valley-polarized holes
can be generated in the WSe
2
layer within 50 fs because of the ultrafast
charge transfer processes in the WSe
2
/MoS
2
heterostructure (25, 26).
These valley-polarized holes exhibit a population decay lifetime of
more than 1 ms and a depolarization lifetime (that is, intervalley
scattering lifetime) of more than 40 msat10K,whichisordersofmag-
nitude larger than previously reported values. The near-unity valley
polarization and ultralong valley lifetime observed here will enable
new ways to probe and manipulate valley and spin degrees of freedom
in TMDs.
RESULTS
We i nvestigate h igh-quality WSe
2
/MoS
2
heterostructures using
polarization-resolved pump-probe spectroscopy. Figure 1B shows the
optical microscopy image of a representative WSe
2
/MoS
2
heterostruc-
ture. The WSe
2
(encircled by the blue dashed line) and MoS
2
(encircled
by the red dashed line) monolayers are first exfoliated mechanically
from bulk crystals onto SiO
2
/Si substrates and then stacked to form
the heterostructure (denoted by the black dashed line) by a dry transfer
method using a polyethylene terephthalate (PET) stamp (Supplemen-
tary Text). The heterostructure region can be visual ized most strikingly
1
Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.
2
Department of Materials Science and Engineering, Pohang University of Science
and Technology, Pohang 790-784, Korea.
3
School for Engineering of Matter, Transport
and Energy, Arizona State University, Tempe, AZ 85287, USA.
4
National Institute for
Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
5
Materials Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
6
Kavli Energy Na-
noScience Institute, University of California, Berkeley and Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA.
*These authors contributed equally to this work.
†Corresponding author. Email: fengwang76@berkeley.edu
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in the photoluminescence (PL) image (Fig. 1C), where the PL at the
WSe
2
A-exciton resonance (1.65 eV at room temperature) is quenched
by more than four orders of magnitude in the heterostructure region
compared to the WSe
2
-only region. This quenching of PL is a signature
of the type II heterojunction in WSe
2
/MoS
2
heterostructures, where the
conduction band minimum and valence band maximum reside in the
MoS
2
and WSe
2
layers, respectively, and an ultrafast charge transfer
process takes the electron from the WSe
2
to the MoS
2
layer upon photo-
excitation of WSe
2
excitons (Fig. 1A).
The ultrafast electron transfer process in the heterostructure allows
for efficient generation of valley-polarized holes in WSe
2
,asillustrated
in Fig. 2A: Resonant photoexcitation with left circularly polarized (LCP)
light selectively creates electron-hole pairs (that is, excitons) at the K
valley in the WSe
2
layer (encircled with dashed box). After the excita-
tion, the electrons can transfer to the MoS
2
layerwithin50fsandleave
behind resident holes in the K valley of the WSe
2
layer. These valley-
polarized holes can exhibit ultralong lifetime: The population and valley
polarization relaxation processes of resident holes, such as radiative re-
combination and exchange interaction (Maialle-Silva-Sham mecha-
nism), are markedly suppressed because the holes are well-separated
from the electrons in not only real space but also momentum space.
Compared with the previously studied valley polarization of the bright
interlayer exciton (which requires the presence of both electrons and
holes), the lifetime of resident holes is not limited by the decay of the
electrons in the other layer. To achieve the longest valley lifetime, we
choose mechanically exfoliated and stacked WSe
2
/MoS
2
heterostruc-
tures: The exfoliated TMD layers exhibit much higher quality and fewer
defects compared with chemical vapor deposition grown samples, and
the WSe
2
/MoS
2
heterostructure features the largest band offset among
all TMD combinations (27, 28) such that the electrons and holes are well
confined in separate layers.
We investigate the dynamic evolution of valley-polarized holes in
the WSe
2
layer using polarization-resolved pump-probe spectrosco-
py. The LCP pump pulses generate valley-polarized ho les in t he
WSe
2
layer. This valley imbalance leads to a difference in the optical
absorption of the heterostructure for LCP and right circularly polar-
ized (RCP) light close to the WSe
2
A-exciton resonance and can
thereby be probed by pump-induced changes in the reflection con-
trast (RC) spectra of circularly polarized probe pulses. The dynamic
evolution of the polarization-resolved DRC from femtosecond to mi-
crosecond was measured by combining a mechanical delay line
(from femtoseconds to nanoseconds) and an electronic delay (from
nanoseconds to microseconds).
Fig. 1. Ultrafast charge transfer process in the WSe
2
/MoS
2
heterostructure.
(A) Illustration of the ultrafast electron transfer process in the WSe
2
/MoS
2
heterostruc-
ture. The WSe
2
/MoS
2
heterostructure forms a type II heterojunction where the conduc-
tion band minimum and the valence band maximum reside in MoS
2
and WSe
2
,
respectively. Photoexcited electrons transfer rapidly to the MoS
2
layer, whereas holes
remain in the WSe
2
layer. (B) Optical microscope image of a representative WSe
2
/MoS
2
heterostructure. Blue, red, and black dashed lines encircle WSe
2
,MoS
2
, and hetero-
structure (HS) regions, respectively. Scale bar, 10 mm. (C)PLimageoftheWSe
2
/MoS
2
heterostructure at WSe
2
A-exciton resonance (1.65 eV) at room temperature. PL is
quenched by four orders of magnitude in the heterostructure compared to the
WSe
2
-only region due to the ultrafast electron transfer process in the heterostructure.
1.6 1.7 1.8
0.0
0.5
–RC
Energy (eV)
–1.0
–0.5
0.0
0 800 1600 2400
0.01
0.1
1
–Δ(RC
σ+
– RC
σ
–
)
–Δ(RC
σ+
– RC
σ
–
)
Delay (ns)
'
Fig. 2. Photoinduced CD signal of the WSe
2
/MoS
2
heterostructure at 10 K. (A) Schematic of valley-polarized hole generation in the WSe
2
layer within the heterostructure.
Upon photoexcitation with LCP light, excitons are resonantly created in the K valley of the WSe
2
layer (encircled with a dashed box). Ultrafast charge transfer process then
efficiently transfers electrons to the MoS
2
layer and leaves reside nt holes at the K valley in the WSe
2
layer. (B) Top: Photoinduced CD spectrum at 3 ns. Bottom: RC spectrum
of the WSe
2
/MoS
2
heterostructure. RC spectrum is dominated by the optical absorption near the WSe
2
A-exciton resonance at 1.72 eV. The CD spectrum, −D(RC
s+
− RC
s−
), shows
prominent resonant feature near the WSe
2
A-exciton peak under LCP pump light at 1.78 eV (black arrow in the RC spectrum). (C) Decay dynamics of the resonant CD signal at 10 K.
No decay is observed within 3.5 ns (inset). The decay curve over a longer time scale shows a significant slow decay componen t with a lifetime of more than 1 ms.
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The top panel of Fig. 2B displays a photoinduced circular dichroism
(CD) spectrum of the heterostructure probed at 3 ns after the pump
excitation, and the bottom panel of Fig. 2B shows the RC spectrum
of the heterostructure. The RC spectrum is dominated by the optical
absorption near the WSe
2
A-exciton resonance peak at 1.72 eV
(29, 30). In the pump-probe study, we choose an excitation energy at
1.78 eV (black arrow in Fig. 2B) to selectively excite WSe
2
but not MoS
2
(which has an optical bandgap of 1.92 eV). In addition, pump excitation
at this energy can provide full spectr al informati on of the effect of resi-
dent holes on light absorption, as explained later. The CD signal is
measured through −D(RC
s+
− RC
s−
), which is the difference between
the photoinduced changes in the RC of LCP (RC
s+
)andRCP(RC
s−
)
light. The CD spectrum exhibits a prominent resonant feature around
the A-exciton transition, and its magnitude is directly proportional to
thevalley-polarizedholedensity,that is, the difference between the hole
densityintheKvalley(p
+
)andK′ valley (p
−
). The asymmetric derivative-
like shape of the CD spectrum is analogous to the behavior observed in
III-V and II-V semiconductor quantum wells (31, 32), and it originates
from the distinctive spectral response of LCP and RCP light to holes in a
specific valley in WSe
2
, which will be discussed later.
Figure 2C shows the time evolution of the CD signal with 1.71-eV
probe photons at 10 K. The dynamic response from femtoseconds to
nanoseconds is measured using a mechanical delay line, and it shows
an almost constant CD signal from 300 fs to 3.5 ns (black curve in the
inset). To capture the longer-term dynamics, we use an radio frequency
(rf)–coupled diode laser synchronized to the femtosecond pulses to gen-
erate 3-ns-long probe pulses with electronically defined time delay up to
a few microseconds (Supplementary Text). The CD signal from nano-
second to microsecond time scale is shown as red squares. Strikingly,
the CD signal remains significant even after several microseconds, and
the slowest decay component shows a lifetime of more than 1 ms(the
relatively fast initial decay is largely due to interactions between photo-
excited carriers; see pump fluence dependence in Supplementary Text).
This microsecond lifetime of the valley-polarized hole density is orders
of magnitude longer than that reported previously (21–24).
To separate the contributions from the population decay and the
intervalley scattering to the overall lifetime of valley-polarized holes
and quantify the degree of valley polarization upon optical initializa-
tion, we examine in more detail the dynamic behavior and spectrum
dependence of the photoinduced valley polarization in WSe
2
/MoS
2
heterostructures.
One key figure of merit in valley initialization and control is the
degree of valley polarization, as defined by
P ¼
p
þ
 p

p
þ
þ p

where p
+
and p
−
areholedensitiesintheKandK′ valleys, respectively.
Here, we need to establish a quantitative relation between the hole den-
sity in a specific valley and the corresponding circularly polarized DRC
spectra. Toward this goal, we analyze the photoinduced absorption
changes in individual K and K′ valleys by separately examining the
DRC spectra for LCP and RCP light. Figure 3 (A and B) shows the
DRC spectra in the K and K′ valleysat3nsafterresidentholesarecre-
ated in the K valley of WSe
2
through LCP light excitation. Distinctively
different absorpti on changes are obser ved for the K valley (DRC
s+
,black
squares in Fig. 3A) and K′ valley (DRC
s–
, red circles in Fig. 3B): The
holes present in the K valley can modify further absorption at the K
valley through a combination of the phase-space filling and Burstein-
Moss effects, as previously observed for exciton states in quantum wells
(33, 34). It leads to a reduction of the exciton absorption oscillator
strength accompanied by a slight blueshift of the exciton resonance. This
is illustrated in the inset of Fig. 3A, where the original exciton absorption
(black dashed line) is reduced and slightly blueshifted in the presence of
theholeintheKvalley(redsolidline).Consequently,DRC
s+
for the K
valley is dominated by an overall absorption reduction with a small
absorption increase at the higher-energy side. However, the effect of
K-valley holes on the absorption of the K′ valley is completely different
because there is no Pauli blocking from phase-space filling. Instead,
DRC
s–
is characterized by a decrease in the exciton absorption and
an increase in the trion absorption due to the formation of an intervalley
trion state (that is, exciton in the K′ valley and hole in the K valley), with
the total absorption oscillator strength conserved. The inset in Fig. 3B
further illustrates this process, where resident holes lead to oscillator
strength transfer from exciton to intervalley trion absorption (red solid
line) compared to the original spectrum (black dashed line).
0.0
0.5
1.0
K
Hole density
K
Valley polarization
~100%
1.60 1.65 1.70 1.75
0.8–
–0.4
0.0
0.4
σ
+
probe
Probe energy (eV) Probe energy (eV)
0.0
0.2
0.4
0.6
RC
1.60 1.65 1.70 1.75
–0.8
–0.4
0.0
0.4
σ
–
probe
–
ΔRC (10
–3
)
–ΔRC (10
–3
)
0.0
0.2
0.4
0.6
RC
–RC
–RC
'
Fig. 3. An almost perfect valley polarization. (A and B) Resident holes in the K valley induce distinctively different absorption changes for the K (A) and K′ (B) valleys; RC of the
heterostructure (blue solid line) is also shown for comparison. (A) The absorption change in the K valley features an overall reduction of the absorption oscillator strength and a
slight blueshift of the exciton resonance. The inset illustrates the change of absorption spectrum, where the original exciton absorption (black dashed line) is modified in the
presence of holes at the K valley (red solid line). This spectral change can be understood by the phase-space filling and Burstein-Moss effects. (B) The absorption change in the K′
valley shows a transfer of oscillator strength from exciton to trion absorption due to the formation of interval ley trions. However, the total oscillator strength is unaffected because
there is no Pauli blocking effect. The inset illustrates the reduction of exciton absorption and emergence of intervalley trion absorption (red solid line) compared to the original
spectrum (black dashed line). (C) The density of resident holes in the K and K′ valleys obtained by the integrated oscillator strength change of LCP and RCP light, which suggests
near-perfect valley polarization (within 10% experimental uncertainty). The total hole density is normalized to 1.
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The understanding that the holes present in one valley will reduce
the overall optical absorption oscillator strength in the same valley but
not the opposite valley allows for a direct experimental determination of
hole population in each valley: p
+
and p
−
are proportional to the inte-
gration of the DRC signal over frequency for LCP and RCP light, respec-
tively. Following this approach, we find that an almost perfect valley
polarization was created by the LCP excitation pulses: Holes exist only
in the K valley but not in the K′ valley of WSe
2
within the experimental
uncertainty (Fig. 3C). This efficient generation of valley-polarized holes
presumably benefits from the ultrafast electron transfer process in the
heterostructure, which separates the electrons and holes even before the
valley depolarization of excitons produces a noticeable effect. Next, we
quantitatively investigate the time evolution of valley-polarized holes.
The decay of CD signal, which is proportional to the valley-polarized
hole density (that is, p
+
− p
−
), has two different contributions: (i) a pop-
ulation decay of the total hole density (that is, p
+
+ p
−
) and (ii) inter-
valley scattering that reduces the degree of valley polarization P =
(p
+
− p
−
)/ (p
+
+ p
−
). These two contributions can be obtained separately
by examining the time evolution of DRC
s+
+ DRC
s−
and (DRC
s+
−
DRC
s−
)/(DRC
s+
+ DRC
s−
), respectively.
Figure 4 (A and B) shows the photoinduced difference (DRC
s+
−
DRC
s−
)andsum(DRC
s+
+ DRC
s−
) signals at 3-, 80-, and 550-ns
pump-probe delay. All the spectra are normalized to 1. Both the
difference and sum spec tra exhibit a constant profile around the
A-exciton resonance (in the spectral range of 1.69 to 1.78 eV). In
the sum response (Fig. 4B), a weak signal appears at around 1.66 eV
over time, presumably due to some defect states that decay slowly.
However, these defect states do not distinguish the K and K′ valleys
anddonotshowanyeffectinthedifferenceresponse(Fig.4A).Because
both the difference and sum spectra remain constant profiles within
1.69 to 1.78 eV, we can use a single probe photon energy at 1.71 eV
to obtain the time evolution of the difference and sum signals, which
characterize the difference and sum of hole densities in the K and K′
valleys, respect ively.
Figure 4C displays the normalized decay dynamics of the total (p
+
+
p
−
, black dots) and valley-polarized (p
+
− p
−
, red squares) hole densities
in the heterostructure, as well as the degree of valley polarization (P,blue
triangles). We found that the decay of valley-polarized holes (p
+
− p
−
)is
very similar to that of the total hole density (p
+
+ p
−
), indicating that the
1-ms decay lifetime observed in CD signals is dominated by a population
1.60 1.65 1.70 1.75
–0.4
0.0
0.4
0.8
ΔRC
σ+
− ΔRC
σ−
ΔRC
σ+
+ Δ
RC
σ−
3 ns
80 ns
550 ns
1.60 1.65 1.70 1.75
–0.4
0.0
0.4
0.8
3 ns
80 ns
550 ns
0 500 1000 1500 2000 2500
0.01
0.1
1
ΔRC
Delay (ns)Probe energy (eV)Probe energy (eV)
p
+
-
p
–
p
+
+ p
–
P = (p
+
-
p
–
)/(p
+
+ p
–
)
0
200 400 600 800
1000
0.01
0.1
1
10 K
20 K
30 K
40 K
50 K
60 K
77 K
Delay (ns) 1/temperature (1/K)
0.02 0.04 0.06 0.08 0.10
0.01
0.1
1
10
100
Valley depolarization lifetime (μs)
Valley polarization
Fig. 4. Ultralong valley depolarization lifetime. (A and B) Photoinduced difference (A) and sum (B) responses of the two valleys at a pump-probe delay of 3 ns (black),
80 ns (red), and 550 ns (blue). All spectra were measured at 10 K and normalized to 1. Both the difference and sum spectra show constant profile over time, except for a
weak signal at around 1.66 eV in the sum response due to some defect states that decay slowly. (C) Decay dynamics of the total hole population p
+
+ p
−
(black dots),
the valley-polarized hole population p
+
− p
−
(red squares), and the degree of valley polarization P (blue triangles) obtained with a probe energy of 1.71 eV. The decay of
the valley-polarized hole population of ~1 ms is mainly due to the total population decay. However, the valley polarization does not show any apparent decay at 2.5 ms,
corresponding to an ultralong valley depolarization lifetime approaching 40 ms. (D) Temperature-dependent decay dynamics of valley polarization from 10 to 77 K
(symbols). Solid lines are biexponential decay fitting of experimentally measured decay dynamics, with decay lifetime of dominant slow components summarize d in (E). The valley
depolarization lifetime changes strongly with the temperature, suggesting an energy-activated mechanism in the intervalley hole scattering.
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