Chemical
Physics
Letters
635
(2015)
40–44
Contents
lists
available
at
ScienceDirect
Chemical
Physics
Letters
jou
rn
al
h
om
epa
ge:
www.elsevier.com/locate/cplett
Enhancing
photoluminescence
of
trion
in
single-layer
MoS
2
using
p-type
aromatic
molecules
Weitao
Su
a,b,∗
,
Honglei
Dou
a
,
Dexuan
Huo
a,∗∗
,
Ning
Dai
b
,
Li
Yang
c
a
Institute
of
Materials
Physics,
Hangzhou
Dianzi
University,
310018
Hangzhou,
China
b
Ningbo
Institute
of
Materials
Technology
&
Engineering,
Chinese
Academy
of
Sciences,
315201
Ningbo,
China
c
Department
of
Chemistry,
Xi’an
Jiaotong-Liverpool
University,
215123
Suzhou,
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
3
May
2015
In
final
form
6
June
2015
Available
online
24
June
2015
a
b
s
t
r
a
c
t
Enhancing
photoluminescence
(PL)
of
single-layer
(1L)
MoS
2
is
essential
to
its
application
as
thinnest
and
high
efficient
light-emitting
devices
in
the
near
future.
Here,
we
report
an
unusual
enhanced
PL
intensity
of
charged
exciton
(trion)
using
p-type
aromatic
molecules.
Calculation
of
the
quantum
yield
using
exciton
dynamics
indicates
that
this
enhancement
of
trion
is
dominant
by
weakening
the
non-
radiative
combination
to
intrinsic
defects.
Our
research
further
extends
the
previous
theory
and
a
better
understanding
of
exciton
transition
in
1L
MoS
2
is
revealed.
©
2015
Elsevier
B.V.
All
rights
reserved.
1.
Introduction
Inspired
by
the
structural
analogue
of
graphene,
two-
dimensional
(2D)
MoS
2
with
a
layer
of
molybdenum
atoms
sandwiched
between
two
layers
of
sulphur
atoms
layer
material
[1–4],
has
attracted
much
attention
as
a
potential
material
for
future
electronic
and
photonic
applications.
When
the
layer
num-
ber
is
reduced
to
single
layer,
the
band
structure
of
MoS
2
transits
from
indirect
to
direct
band
with
band
gap
rising
from
∼1.1
to
∼2
eV
[1,3].
Single-layer
(1L)
MoS
2
exhibits
strong
PL
in
visible
light
[1,3]
,
high
charge
mobility
(>300
cm
2
/vs)
[2,4]
and
selective
optical
pumping
of
electrons
at
different
valleys
[5–7].
With
these
strik-
ing
features,
1L
MoS
2
has
potential
applications
in
light
emitting
devices
[8],
photon
resistor
[2,4,9,10],
light
harvesting
[11,12]
and
vallytronics
[5–7].
However,
PL
efficiency
of
pristine
1L
MoS
2
(∼0.5%)
is
still
too
low
to
meet
the
harsh
requirement
of
usable
light-emitting
material.
In
order
to
enhance
the
light
emission
properties,
p-type
molecules
such
as
H
2
O,
O
2
[13,14]
and
tetracyanoquino-dimethane
(TCNQ)
[15]
have
been
used
to
enhance
the
PL
yield
of
1L
MoS
2
by
sev-
eral
tens
times
of
magnitude.
However,
the
mechanism
of
the
PL
enhancement
is
still
debating.
Tongay
et
al.
[13],
Nan
et
al.
[14]
and
Mouri
et
al.
[15]
have
proposed
that
charge
transfer
is
the
main
factor
to
enhance
the
PL
yield.
In
this
theory,
it
was
claimed
∗
Corresponding
author
at:
Institute
of
Materials
Physics,
Hangzhou
Dianzi
Uni-
versity,
310018
Hangzhou,
China.
∗∗
Corresponding
author.
E-mail
addresses:
suweitao@hdu.edu.cn
(W.
Su),
dxhuo@hdu.edu.cn
(D.
Huo).
that
strong
Coulombic
screening
forces
most
of
the
free
excitons
to
recombine
non-radiatively
via
Auger
process,
and
consequently
diminishes
the
PL
intensity
of
1L
MoS
2
.
p-Type
dopant
molecules
on
1L
MoS
2
attract
electrons
from
MoS
2
and
reduce
the
density
of
excessive
electrons.
Thus
PL
enhancement
with
strong
intensity
of
neutral
exciton
is
finally
obtained.
Among
this
theory,
it
was
thought
to
be
no
enhanced
of
trion
and
only
neutral
exciton
can
be
enhanced
with
presence
of
p-type
dopants.
Lin
et
al.
[16]
have
pro-
posed
that
dielectric
screening
may
be
another
mechanism.
In
this
theory,
molecules
with
high
dielectric
constant
strongly
modify
the
dielectric
environment
of
MoS
2
and
consequently
induce
a
much
stronger
enhancement
of
neutral
exciton
than
trion.
Generally,
for
PL
enhancement
using
certain
doping
molecules,
both
the
charge
transfer
and
dielectric
screening
effects
need
to
be
considered
to
get
a
better
understanding.
In
current
study,
by
using
p-type
molecules
with
which
the
dielectric
screening
effect
can
be
neglected,
it
was
found
the
inten-
sity
of
trion
can
also
be
greatly
enhanced.
For
some
molecules
doped
1L
MoS
2
,
the
enhanced
intensity
of
trion
is
comparable
to
that
of
neutral
exciton.
This
unusual
enhancement
cannot
be
inter-
preted
either
by
charge
transfer
or
dielectric
screening
theory.
Previous
studies
[17–19]
have
pointed
out
that
intrinsic
defects,
probably
sulphur
vacancies
[20,21],
may
be
an
essential
factor
gov-
erning
the
light
emitting
of
1L
MoS
2
.
In
pristine
MoS
2
,
excitons
decay
into
such
defects
are
signified
by
very
long
life
time
measured
using
transient
absorption
[18].
Only
a
small
part
of
excitons
transit
to
ground
state
via
radiative
recombination
rates
and
produce
light
emission
we
observe.
However,
the
role
of
defect
in
PL
enhance-
ment
had
not
been
revealed
yet.
In
this
study
we
propose
that
defect
may
be
another
dominant
factor
that
governs
the
PL
yield
of
http://dx.doi.org/10.1016/j.cplett.2015.06.036
0009-2614/©
2015
Elsevier
B.V.
All
rights
reserved.