Angewandte
International Edition
A Journal of the Gesellschaft Deutscher Chemiker
www.angewandte.org
Chemie
Accepted Article
Title:
Ultrafast Formation of Charge Transfer Trions at Molecular-
Functionalized 2D MoS2 Interfaces
Authors:
Yuancheng Jing, Kangkai Liang, Nicole Muir, Hao Zhou,
Zhehao Li, Joseph Palasz, Jonathan Sorbie, Chenglai Wang,
Scott Cushing, Clifford Kubiak, Zdeněk Sofer, Shaowei Li, and
Wei Xiong
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). The VoR will be published online
in Early View as soon as possible and may be different to this Accepted
Article as a result of editing. Readers should obtain the VoR from the
journal website shown below when it is published to ensure accuracy of
information. The authors are responsible for the content of this Accepted
Article.
To be cited as:
Angew. Chem. Int. Ed.
2024
, e202405123
Link to VoR:
https://doi.org/10.1002/anie.202405123
RESEARCH ARTICLE
1
Ultrafast Formation of Charge Transfer Trions at Molecular
-
Functionalized 2D MoS
2
Interfaces
Yuancheng Jing
,
[
a
]
Kangkai Liang
,
[b]
Nicole S. Muir
,
[
a
]
Hao Zhou
,
[b]
Zhehao Li
,
[b]
Joseph M. Palasz
,
[
a
]
Jonathan Sorbie
,
[
a
]
Chenglai Wang
,
[
a
]
Scott K Cushing
,
[
c
]
Clifford P. Kubiak
,
[
a
]
Zdeněk Sofer
,
[
d
]
Shaowei
Li
,
[
a
]
,
[b]
Wei Xiong
*
[a]
,[b]
[a]
Y. Jing, N. S. Muir, J. M. Palasz, J. Sorbie, C. Wang, C. P. Kubiak, S. Li, W. Xiong
Department of Chemistry and Biochemistry
University of
California, San Diego
9500 Gilman Drive, MC 0358, La Jolla, California 92093
-
0358, United States
E
-
mail:
w2xiong@ucsd.edu
[b]
K. Liang, H. Zhou, Z. Li, S. Li, W. Xiong
Material Science and Engineering Program
University of California, San Diego
9500 Gilman Drive, MC 0418, La Jolla, California 92093
-
0418
, United States
[
c
]
S. K. Cushing
Division of Chemistry and Chemical Engineering
California Institute of Technology
1200 E California Blvd, MC 127
-
72,
Pasadena, California 91125, United States
[
d
]
Z.
Sofer
Department of Inorganic Chemistry
University of Chemistry and Technology
,
Prague
Technická 5, 166 28 Prague 6, Czech Republic
Supporting information for this article is given via a link at the end of the document
.
Abstract:
In this work, we
investigate
trion dynamics
occurring
at
the
heterojunction between
organometallic molecule
s and
a
monolayer
transition metal dichalcogenide (TMD)
with
transient electronic sum
frequency generation
(tr
-
ESFG)
spectroscopy.
By
pumping at 2.4
eV
with laser pulses
, we
have
observed an ultrafast hole transfer,
succeeded
by the
emergence
of charge
-
transfer trions.
This
observation is facilitated by
the cancellation
of
ground state bleach
and stimulated emission signal
s due to their opposite
phase
s
, making
tr
-
ESFG especially sensitive to the trion formation dynamics
. The
presence
of charge
-
transfer trion at mol
ecular functionalized TMD
monolayers
suggests the potential for
engineering the local electronic
structures and dynamics of specific locations on TMDs and offers the
potential for transferring unique electronic attributes of TMD to the
molecular layers
.
Introduction
Transition metal dichalcogenides (TMDs) are a group of materials
that are composed of a transition metal atom M from Group III or
IV covalently connecting to two chalcogen atoms X in a hexagonal
lattice.
[1]
The layered structure exhibits weak interlayer van der
Walls interaction
s
, which reduces the electrostatic screening and
thereby gives rise to novel many
-
body interactions between
charge carriers
[2]
. At the monolayer limit, the reduced
dimensionality, combined with the
reduction
of screening for
Coulomb interactions, leads to the emergence of various robust
quasiparticles even at room temperature. These includes
excitons
[3]
, trions
[4]
and bi
-
exciton
[5]
, several of which are
unattainable in other systems
6,7
. Trions, for instance, are one
such charged excitons, i.e. one exciton bounded with an
additional charge carrier. They arise from the interaction between
excitons and residual free charge carriers in monolayer
TMDs.
[4,6,7]
The existence of trions allows electrically guiding and
controlling charged particles more efficiently than excitons.
[8,9]
To harness the unique characteristics of monolayer TMD due
to many
-
body interactions, it is essential to fine
ly
adjust the
materials’ electro
-
optical properties and to transfer these distinct
traits to other materials. One efficient approach is to create
heterojunctions, including TMD
-
TMD
[10
–
12]
and molecule
-
TMD
heterojunctions.
[13
–
17]
For example, it was recently shown that
many
-
body control of electron localization in moiré
heterostructures can greatly improve catalytic performance.
[18]
Similarly, doping monolayer TMD with molecules could control
interfacial band alignment and its chemical and electrostatic
environments.
[19]
In the meantime, due to the relatively weak
screening along the normal direction of the interfaces, the
heterojunction may potentially inherit the electronic properties of
monolayer TMDs. For example, the existence of interfacial
charge transfer (CT) exc
itons in molecule
-
TMD heterojunctions
enable
s
the electronic structure hybridization between TMD and
molecules.
[20,21]
Recently, CT trions, also referred
to
as interlayer
10.1002/anie.202405123
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RESEARCH ARTICLE
2
trions, were also observed in TMD/TMD heterojunctions, offering
another route to control and transfer electronic properties across
the heterojunctions.
[22,23]
Given the dramatic difference between
electronic structures of molecules and TMD
–
molecular
electronic wavefunctions are much more localized, it is a pertinent
question to ask whether the strong many
-
body interactions that
enable CT trion formations in
the TMD/TMD heterojunction can
persist if molecules replace a TMD layer to serve as charge
acceptors. Realizing CT trions in monolayer
-
TMD heterojunctions
may
be able to
transfer specific attributes of trions in TMD to the
surface molecular species, includ
ing spin polarizations to enable
spin
-
polarized chemistry
[24
–
27]
and the in
-
plane delocalization that
facilitates charge hopping among molecular sites. In return, the
surface molecules could dynamically modify these electronic
characteristics in TMD through CT trions, for photonics,
optoelectronic and valleytronic appl
ications.
The lack of studies on CT trions at molecule
-
TMD interfaces
stems from the technical challenges to study charge dynamics.
[28
–
32]
Photoluminescence (PL) is the predominant optical technique
used to investigate monolayer TMD, where the signal can either
be enhanced or quenched by molecular doping
[33]
,
[34]
. However,
the charge transfer dynamics often occurs
on the
femtosecond
time scale, which is beyond the common time resolution of time
-
resolved PL. Transient absorption (TA) has also been applied to
investigate monolayer TMD
[35
–
40]
, interfacial charge transfer
between two TMD layers
[41,42]
, and between TMD and conjugated
molecules.
[43
–
45]
Yet,
it has been not used to
study CT trion
formation at molecule
-
TMD interfaces.
Interfacial sensitive
second harmonic generation (SHG) was also applied on pristine
TMD.
[46]
,
[47]
However, its narrow spectral coverage makes SHG
difficult to differentiate electronic states, making it unsuitable to
identify CT trions.
In this work, we implemented transient electronic sum
frequency generation (tr
-
ESFG) to investigate the ultrafast charge
dynamics at a molecule
-
TMD heterojunction. Tr
-
ESFG is
particular
l
y suitable for this study because of its broad
-
spectral
coverage and surface sensitivity. As a second order nonlinear
optic, SFG only occurs for samples with non
-
centrosymmetric
geometries, such as surfaces and interfaces.
[48
–
54]
Thus, ESFG
has been
used to study
interfacial molecule alignment
[55]
and
charge dynamics at surfaces/interfaces
[56
–
61]
. Using this unique
method, we report ultrafast dynamics of photo
-
induced charge
transfer between the heterojunction of monolayer molybdenum
disulfide (MoS
2
) and ruthenium(ii) polypyridyl complexes
(Ru(DPPZ)
2
L
-
Pro), referred as Ru
-
MoS
2
hereafter. We find that,
in contrast to TA, the ground state bleach (GSB) and stimulated
emission (SE) of tr
-
ESFG are out of phase and thereby cancel.
This phase cancellation removes the larger ground state signals,
promoting the characterization of trion
formation timescale
s.
Furthermore, the spectral signal strongly indicates the formation
of CT trions at the heterojunction. The identification of a CT
-
trion
in molecule
-
TMD heterojunctions offers the potential for
transferring spin or valley degrees of freedom of TMD to
mole
cules for molecular
-
based spintronics,
[62]
spin
-
specific
chemistry,
[63,64]
and engineering of TMD properties through
molecular dopants.
[65]
Results and Discussion
A charge transfer interface was created within a type II
heterojunction between monolayer MoS
2
and Ru(DPPZ)
2
L
-
Pro.
Ruthenium(ii) polypyridyl
complexes are a group of promising
candidates for water oxidation catalyst.
[66
–
68]
The
hole
in
Ruthenium(ii) polypyridyl can enhance the efficiency for
catalyzing water oxidation (oxygen evolution).
[69]
Therefore, the
heterojunctions comprised of MoS
2
and
Ru(DPPZ)
2
L
-
Pro
may be
developed into photocatalyst based on type
II hole transfer,
taking
advantage of the exceptional characteristics of monolayer TMD,
including large absorption cross section and valley degrees of
freedom, and the catalytic nature of the Ruthenium(ii) polypyridyl
complexes.
Based on a combination of UV
-
Vis absorption, PL,
and cyclic voltammetry measurements, (see supplemental Figure
S5, Figure S6), the HOMO of Ru(DPPZ)
2
L
-
Pro lies above
the
MoS
2
valance band maximum (VBM)
[70]
while the LUMO is higher
than the conduction band minimum (CBM)
[70]
of MoS
2
, as
described in Figure1, facilitating the hole transfer from monolayer
MoS
2
to the molecule upon photoexcitation
.
Figure 1.
band alignment of Ru
-
MoS
2
heterojunction
The efficient interfacial charge transfer between monolayer MoS
2
and Ru(DPPZ)
2
L
-
Pro was confirmed by the quenching of Ru
-
MoS
2
PL signals (Figure 2a). Optical microscope shows an image
for a pristine MoS
2
flake containing a large area of monolayer
10.1002/anie.202405123
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RESEARCH ARTICLE
3
Figure
2
.
Photoluminescent
r
esult to confirm interfacial charge transfer. (a) PL spectrum comparison on a typical position of monolayer MoS
2
, 95% quenching
effect after
Ru(DPPZ)
2
L
-
Pro doped; (b) 20
μ
m by 20
μ
m optical image of monolayer MoS
2
flake ; (c) PL microscope mapping on same pristine monolayer MoS
2
,
higher PL intensity at left corner is due to dominated signal from monolayer, while the top area contains both signal from mo
nolayer and multilayer sample (beam
size of PL is
4μ
m as diameter
) leading to a lower intensity;(d) fitting result for PL spectrum of pristine monolayer MoS
2
; (e) PL microscope mapping on Ru
-
MoS
2
;
(f) fitting result of Ru
-
MoS
2
MoS
2
and minor part for bilayer MoS
2
and bulk MoS
2
(Figure 2b).
For pristine monolayer MoS
2
, the PL mapping (Figure 2c) at 690
nm presents a uniform signal intensity, indicating a highly
homogeneous material.
The PL spectrum of the pristine sample
(Figure 2d) can be decomposed into three gaussian peaks: the
radiative
recombination of B
-
exciton, A
-
exciton and negative trion,
at 2.00, 1.87, and 1.
78
eV, respectively.
[71]
Exciton A and B are formed by strong spin orbital coupling
(SOC) that leads to a split in highest valence band of ~130meV.
The trion, with a larger coulomb binding energy, is identified at a
lower energy, in good agreement with the literatures
[27]
. Upon
functionalizing the monolayer MoS
2
by Ru(DPPZ)
2
L
-
Pro, the PL
emission was quenched by 95% uniformly across the entire
monolayer MoS
2
surface, as shown in Figure 2a and e. All three
of the spectra components are reduced in Figure 2f, suggesting
all three quasiparticles are subject to interfacial charge transfer.
We note that the signal reduction is unlikely to be attributed to the
absorbance of the excitation lights by the surface adsorbed
Ru(DPPZ)
2
L
-
Pro layer, as evidenced by the absence of
absorptio
n within the excitation wavelength range from a pure
Ru(DPPZ)
2
L
-
Pro layer (see supplemental Section 10). Thus, an
interfacial charge transfer occurs at the Ru
-
MoS
2
interface.
Although the existence of charge transfer was confirmed, the
mechanism and dynamics of the interfacial charge transfer and
whether CT trion could exist at the interfaces remain unknown. To
elucidate these, we applied Tr
-
ESFG spectroscopy. A 515nm
(2.4eV, 2
00 fs) pump pulse excites electrons to the conduction
band of monolayer MoS
2
(E
g
= 1.9eV). The subsequent charge
dynamics are then probed by the ESFG (Figure 3a). The ESFG
probe utilizes a mid
-
IR and near
-
IR pulses to interact with the
sample which creates
a signal at the sum of the two incoming
beam frequencies.
[72,73]
If the energy of the emitted signal
resonates with an electronic transition at interfaces, the emitted
signal is enhanced (inset of Figure 3a). We scanned the IR
frequency across a broad spectral range to cover the electronic
transitions.
The static ESFG spectrum in Figure3b of pristine monolayer
MoS
2
and Ru
-
MoS
2
show large peaks around 1.77
-
1.78 eV which
are assigned to be the ESFG resonances of trions, based on the
PL fitting in Figure 2 and UV
-
Vis absorption features in
supplement Figure S6b
. Interestingly, monolayer MoS
2
has a
higher ESFG peak intensity than Ru
-
MoS
2
sample, indicating a
decrease of transition from ground to trion states after doping.
This decrease suggests the interfacial electronic structures have
been modified, so tha
t the transition probabilities of excitons and
trions are changed. We note because monolayer MoS
2
emits
strong PL signals at 1.9eV, it interferes with ESFG signal of the
exciton A. For this reason, we focus on trion charge transfer
mechanism to infer the complete charge transfer picture.
T
he tr
-
ESFG spectra of pristine monolayer MoS
2
and Ru
-
MoS
2
show distinct time
-
dependent features. At first glance, pristine
TMD (see Figure 3c) exhibits a negative pump
-
probe signal right
after time zero and recovers back to zero with a lifetime of 16.4
ps before turning into a positive amplitude with a long life
time of
129.0ps. The negative transient signal has a center located at
1.79 eV, indicating a bleach of trion.
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RESEARCH ARTICLE
4
Figure 3.
transient ESFG measurements of Ru
-
MoS
2
heterojunction and pristine monolayer MoS
2
samples (a) illustration figure of Tr
-
ESFG; (b) static ESFG of
pristine monolayer MoS
2
and Ru
-
MoS
2
with no optical pumping; (c) Pseudo color 2D tr
-
ESFG spectrum pristine monolayer MoS
2
; (d) Pseudo color 2D tr
-
ESFG
spectrum Ru
–
MoS
2
. The color bar in (c) and (d)
represents the intensity of pump probe data.
The yellow
(blue)
color represents a positive
(negative)
tr
-
ESFG
signal (sample emits stronger
(weaker)
ESFG after being
pumped)
. The mathemstical description of how to calculate tr
-
ESFG is shown in supplement section 2
.
Interestingly, the positive signal redshifts to 1.76Ev. For the Ru
-
MoS
2
(Figure 3d), a negative signal persists over the entire
scanning range and only decays to a smaller amplitude. We note
the Ru(DPPZ)
2
L
-
pro monolayer alone shows negligible static and
transient ESFG signal (see supplement Figure S12).
Origins of Tr
-
ESFG Signals
To understand the tr
-
ESFG dynamics of pristine monolayer
MoS
2
and Ru
-
MoS
2
, it is necessary to dive into the origins of tr
-
ESFG, which, as shown below, has different spectral features
than the more commonly applied TA spectroscopy. For a full
theoretical treatment
[74
–
76]
, we refer to SI Section 1
1
. Briefly,
similar to
TA, there are three contributions to the tr
-
ESFG signals,
i.e. ground state bleach, stimulated emission and excited state
absorption, where the excited state absorption is always shifted
relative to the other two due to particle interactions. However,
dif
ferent from TA, whose ground state bleach and stimulated
emission signals have the same phase and thereby often add up
and dominate the TA features
–
masking other features, in tr
-
ESFG, ground state bleach and stimulated emission have
opposite phases and c
ancel each other, allowing other important
but subtle features to be analyzed.
The origin of the phase difference can be intuitively
understood by that tr
-
ESFG measures changes of ESFG
emission, whereas TA quantifies the modulations of absorptions.
As a result, stimulated emision enhances the ESFG emission, but
reduces the amount of
absorption in TA. Below, we show that this
cancelation allows us to better quantify the trion dynamics.
Charge
D
ynamics of
P
ristine
M
onolayer MoS
2
This opposite phase relationship between ground state bleach
and stimulated emission provides advantages in analyzing the
measured spectral dynamics. A typical monolayer MoS
2
photoexcitation and relaxation process
[77,78]
is described in
Figure4a. The electron is photoexcited from the valence band to
the conduction band and excitons are formed. An exciton and an
extra electron can further combine to form trions at a lower energy
state
(with a rate constant k
1
), which occurs at 10
-
20 ps according
to literatures
[79,80]
.
10.1002/anie.202405123
Accepted
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article
is protected
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rights
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RESEARCH ARTICLE
5
Figure 4.
tr
-
ESFG data analysis of monolayer MoS
2
. (a) Kinetic model for monolayer MoS
2
. (b) spectra components from global analysis of pristine
monolayer
MoS
2
.
(c) Population dynamics from global analysis of monolayer MoS
2
,
for trion GSB+SE, lifetime is 16.4
±
0.548 ps; for trion ESA, lifetime is 129.0
±
18.48 ps.
Solid line shows the best fitting result, while shaded area shows the confidence range of the fitting.
Lastly, trion recombination occurs with a rate constant of k
2
.
Based on this kinetic model, the transient population of each state
can be described as,
{
훥
[
퐸
]
(
푡
)
=
퐸
0
푒
−
푘
1
푡
훥
[
푇
]
(
푡
)
=
퐸
0
푘
1
푘
1
−
푘
2
푒
−
푘
2
푡
−
퐸
0
푘
1
푘
1
−
푘
2
푒
−
푘
1
푡
훥
[
퐺
]
(
푡
)
=
−
퐸
0
푘
1
푘
1
−
푘
2
푒
−
푘
2
푡
+
퐸
0
푘
2
푘
1
−
푘
2
푒
−
푘
1
푡
Eq1
where E
0
is the initial exciton population at t=0. The ground state
bleach and stimulated emission are at the same trion emission
frequency, so their signals need to be combined. Based on the
harmonic oscillator approximation, they emit at the same
intensity
[81]
, and the combined signal leads to a cancelation of the
dynamics related to k
2
,
퐼
퐺푆퐵
+
푆퐸
∝
훥
[
퐺
]
(
푡
)
+
훥
[
푇
]
(
푡
)
=
−
퐸
0
푒
−
푘
1
푡
Eq2
Thus, the ground state bleach and stimulated emission
spectra are only sensitive to the relaxation from excitons to trions.
For excited state absorption, it is expected that it emits at a lower
energy due to the binding energy between trions, and its dynam
ic
should reflect the trion population
.
퐼
퐸푆퐴
∝
훥
[
푇
]
(
푡
)
=
(
퐸
0
푘
1
푘
1
−
푘
2
푒
−
푘
2
푡
−
퐸
0
푘
1
푘
1
−
푘
2
푒
−
푘
1
푡
)
Eq3
The
tr
-
ESFG of pristine monolayer MoS
2
is therefore
composed of two components, one spectral component
corresponding to the combined ground state bleach and
stimulated emission signal, described by a single exponential,
and the other spectral component corresponding to excited state
absorption
, described by double exponentials. We therefore apply
a Global Analysis to the raw data of Figure 3b using the two
-
component model described by Eq. 2&3. The extracted earlier
negative signal represents the combination of
ground state bleach
and stimulated emission, described by the red spectrum in Figure
4b. It has a lifetime of 16.4
±
0.548 ps (
푘
1
=
0
.
061
푝푠
−
1
),
corresponding the exciton to trion relaxation timescale.
[82,83]
This
negative spectral feature can be intuitively viewed as a state
-
blocking effect from the trion, which commonly exists in TA.
The
positive feature
appears later is excited state absorption, which
has a biexponential dynamic, agreeing with our model. This signal
rises with the rate constant of k
1
and decays with a long lifetime
of 129.0
±
18.48 ps (
푘
2
=
0
.
0078
푝푠
−
1
, Figure 4c). Interestingly,
this excited state absorption signal peaks at 1.76 eV, indicating a
0.02 ~0.03 eV energy reduction due to the interaction between
trions. This binding energy is smaller than bi
-
excitons
[84]
.
Charge Transfer Dynamics of Ru
-
MoS
2
Heterojunction
We proceed to analyze the transient dynamics of Ru
-
MoS
2
monolayer. In
c
ompar
ison
to the pristine MoS
2
, a negative
transient signal emerges at an early time
, still indicative of the
ground state bleach and stimulated emission signal contributions
–
the state
-
blocking effect remains.
However, the lack of the
positive signal after 10 ps implies negligible excited state
absorption signal, indicating no trion formation, which agrees with
the suppression of PL signals from trions (Figure 1c). The band
alignment of the heterojunction sugge
sts that a hole transfer from
the valence band of MoS
2
to the HOMO of Ru(DPPZ)
2
L
-
Pro
is
energetically favorable, eliminating the
trion population.
A trion in a monolayer TMD forms when a photo
-
generated
electron
-
hole pair (or exciton) bonds with another free electron in
the TMD conduction band. At the molecule
-
TMD interface, after
the hole transfers to the molecule, two electrons remain in the
TMD. T
here are different potential pathways for these three
charge carriers to interact and relax after this interfacial charge
transfer: 1. The holes and electrons are separated into two free
electrons and one free hole. This process creates an excess
unpaired
electron in the conduction band of TMD due to
photoexcitation, which are now available to form additional trions
in TMDs. Thus, this scenario should enhance the ESFG signal of
trion transitions
–
creating a positive transient signal, which is
opposite from
the measured transient decrease of ESFG signals.
2. An electron and one transferred hole remain tightly bonded,
forming a CT exciton. This case leaves the other electron in the
original trion to be a free electron in the conduction band, no extra
free ele
ctron compared to the case without optical pumping, which
should lead to no tr
-
ESFG signals. 3. Two electrons in the
conduction band of monolayer MoS
2
and one hole in
Ru(DPPZ)
2
L
-
Pro bind into a CT
-
trion. This case leaves one less
free electron in the conduction band of monolayer MoS
2
, leading
to a transient negative ESFG signal. Only the last scenario agrees
with the experimental observation of a decreased transient ESFG
signals.
As a result, we assigned the long
-
lasting negative
10.1002/anie.202405123
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RESEARCH ARTICLE
6
transient ESFG signal to the bleaching of the regular trion due to
the CT trion formation, which retains the state
-
blocking effects
seen in the trion case in pure monlayer MoS
2
.
We note that to
measure the CT trions directly, it is necessary to shift the IR
spectral range to 9 μm, which is out of the practical range for our
instrument. To further examine this CT trion assignments, we
have conducted additional wavelength dependent
PL
measurements and control experiments of tr
-
ESFG of bare Ru
Ru(DPPZ)
2
L
-
Pro l
ayer (SI section
3 and section 10
).
We, therefore, add a competing pathway of CT trion formation
to the MoS
2
kinetic model. (see in Figure 5 and SI Section 1
3
)
Sequential model (exciton
-
> trion
-
> CT
-
trions
) is not chosen
because of the absence of trion signals in this sample. Using the
parallel kinetic model, the negative signal at 1.78eV can be
expressed as summation of ground state bleach and stimulated
emission:
퐼
푛푒푔푎푡푖푣푒
푠푖푔푛푎푙
=
퐼
퐺푆퐵
+
퐼
푆퐸
~
훥
[
퐺
]
(
푡
)
+
훥
[
푇
]
(
푡
)
=
퐸
0
(
푘
1
푘
2
−
푘
2
푘
4
+
푘
3
푘
4
−
푘
1
푘
3
+
푘
1
푘
4
−
푘
1
2
)
(
푘
1
−
푘
2
+
푘
3
)
(
푘
1
+
푘
3
−
푘
4
)
푒
−
(
푘
1
+
푘
3
)
푡
−
퐸
0
푘
3
푘
1
+
푘
3
−
푘
4
푒
−
푘
4
푡
Eq4
The described model suggests that the transient signal should
be a bi
-
exponential decay, with the dynamics independent of
frequency because only one spectral component should exist. We
confirm this prediction by fitting dynamics traces at various
frequenci
es into a biexponential decay. We find a fast decay with
(
푘
1
+
푘
3
)
−
1
= 5.6 ps and a long
-
lived decay with
푘
4
−
1
=
90.9 ps
regardless of the probing frequency (Figure 5b). The conclusion
is further supported by a global analysis (See Supplemental
Figure S1
5
)
: if we decompose the spectra into two
spectral/dynamical components, both components exhibit
identical spectra features. The results, therefore, indicate only a
single spectral component exists with a biexponential decay,
agreeing with our kinetic model.
With the validated kinetic model, we proceed to determine the
푘
1
a
nd
푘
3
from the spectra results. Because there is a negligible
excited state absorption signal in the tr
-
ESFG, it indicates no or
little population of trion in the measured time window, i.e.
,
훥
[
푇
]
≈
0
,
suggesting that
푘
1
is negligible comparing to k
3
. Thus,
푘
3
−
1
≈
(
푘
1
+
푘
3
)
−
1
=
5
.
6
ps
.The formation of CT
-
trion dominates the
exciton relaxation and quenches the original trion formation.
Figure 5.
tr
-
ESFG data analysis of Ru
-
MoS
2
heterojunctions. (a)
a
schematic
of kinetic
model for
the
Ru
-
MoS
2
heterojunction. The
horizontal bars
represent
energy
levels (esimation is based on the band alignment of heterojunction,
supplement Section 4 and
5);
(b) Bi
-
exponential fitting for pump
-
probe signal at
a certain frequency. A 0.01 offset is applied between different energies for a
clear visualization. At different probe photon energies, the bi
-
exponential fitting
has a similar result of fast decay with 5
.6ps and slow decay with 90.9ps.
Mechanism of CT
-
trion Formation
Based on control experiments in
Supplement
Section
9
, the
Ru(DPPZ)
2
L
-
pro prefers to bind to defect sites, forming a
localized hole acceptor.
Therefore, the mechanism of CT
-
trion
formation is:
After being
photoexcited, a hole from valence band
of MoS
2
transfers to the localized HOMO orbital. Meanwhile, the
electrons in conduction band of MoS
2
cool down and bind to the
transferred hole by the strong coulomb attraction to form a CT
-
trion. (Figure 6)
Figure 6
. Schematic of CT
-
trion formation and charge transfer process
Based on these insights, we discuss the conditions of forming
CT
-
trions at TMD/molecule heterojunctions. First, the appropriate
charge transfer needs to occur. For the current systems, the trion
in TMD is a negative trion (extra electrons on conduction ban
ds
plus an exciton). In this case, compared to an electron transfer, a
hole transfer maximally preserved the interactions for trion
formations. In contrast, should the adsorbed molecule facilitate
electron transfer, it may result in an exciton in TMD and a
n extra
free electron in molecules, as predicted by a theoretical study of
a MoS
2
/WS
2
[85]
. Thus, hole (electron) transfer is best to maintain
CT trion for negative (positive) trions.
Second, after charge transfer, the electron
-
hole attractions
should be preserved to form trions, which appears to be the case
here.
This result suggested that the screening from
TMD/molecular heterojunction interfaces is small enough to
maintain a large electron
-
hole attraction. We hypothesize that this
is due to the localized nature of the molecules
,
the TMD structures
are minimally disrupted, in sharp contrast to TMD/TMD or bulk
TMD, where large screening exists. Thus, the TMD/molecule
heterojunction lar
gely preserved the low screening character of
the 2D monolayer materials, allowing the electric fields between
10.1002/anie.202405123
Accepted
Manuscript
Angewandte Chemie International Edition
This
article
is protected
by
copyright.
All
rights
reserved.
15213773, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202405123 by California Inst of Technology, Wiley Online Library on [31/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License