1. Researchers fabricated the first monolayer LED using tungsten diselenide (WSe2) monolayers through a dry transfer technique and electron beam lithography. 2. The WSe2 monolayer LED exhibited bright electroluminescence that was 1,000 times smaller in injection current and 10 times smaller in linewidth than molybdenum disulfide LEDs. 3. By increasing the injection bias, the researchers could tune the electroluminescence between regimes of impurity-bound, charged, and neutral excitons in the WSe2 monolayer.

1. 5 μm
Optical Image of BN
Xo
X+
X-
XI
1.5 1.6 1.7 1.8
Photon energy (eV)
Electroluminescenceintensity(a.u.)
I=24 nA
I=27 nA
I=31 nA
I=33 nA
x15
x5
x1.5
M
Bulk
M KK ΓΓ Γ Γ
Monolayer
Energy(eV)
44
-4 -4
00
-8-8
Background
Band Diagrams of WSe2
1
1.60 1.65 1.75
Gate(V)
Photon Energy (eV)
Xo
X+
X-
X -’
60
0
-60
40
20
-40
-20
0
1200
1.70
1Kumar et al. doi: 10.1140/epjb/e2012-30070-x
2Ross et al. doi:: 10.1038.ncomms2498
3Jones et al. doi: 10.1038/nnano.2013.151
References
Electrically tunable light-emitting diodes based on monolayer WSe2 p-n junctions
Marie E. Scott1, Jason S. Ross2, Xiaodong Xu1,2
1Department of Physics, University of Washington, 2Department of Material Science and Engineering, University of Washington
FabricationIntroduction Results
New two-dimensional (2D) semiconductors are exciting candidates for next-generation
optoelectronic devices because of their unique optical properties. Despite being
atomically thin, techniques have recently been developed to transfer and stack these
monolayers to produce arbitrarily complex heterostructures. Here, we present our
adaptation of these transfer techniques in conjunction with electron beam lithography
to produce the first monolayer LED using 2D crystals of tungsten diselendide (WSe2).
This structure allows effective injection of electrons and holes, and combined with the
high optical quality of WSe2, yields bright electroluminescence with 1,000 times smaller
injection current and 10 times smaller linewidth than in MoS2. By increasing the
injection bias we can tune the electroluminescence between regimes of impurity-
bound, charged, and neutral excitons.
Future Research
Future research includes a LED heterostructure and further development of a
new dry transfer technique that will allow for the production cleaner devices free
from debris and other contaminants.
Crystal Structure of WSe2
2
Mechanical exfoliation
Cartoon image of LEDFinished LED
Electron Beam Lithography
Gate-dependent
photoluminescence of monolayer
WSe2 – controlling exciton species
via electrostatic doping3:
1. No doping, or zero gate: only
neutral exciton (Xo) is present.
2. n-doping: only negative trion
species (X- and X-’).
3. p-doping: only positive trion (X+)
Funding
This work is supported by the US Department of Energy and National Science
Foundation. Device fabrication was preformed at the Washington
Nanofabrication Facility and NSF Nanotech User Facility.
Metal deposition Liftoff Final Structure
Spin-coated sample E-beam Exposure Chemical Development
Electroluminescence spectra at
different current.
Electroluminescence intensity plot as a
function of bias current and photon energy
compared to photoluminescence shows
the same exciton states.
Electroluminescence image.
W
Se
4 μm
Vg1 = Vg2 (V)
Current(nA)
0-8 -4 4 8
0
5
4
3
2
1
Vsd (V)
Current(nA)
5
0
-5
20-2
I(nA)
35
30
25
4
8
0
-4
-8
Vg(V)
1.58 1.62 1.66 1.70
Photon energy (eV)
Xo
X+
X-
XI
Energy (eV)
Vsd(V)
1.58 1.6 1.62 1.64 1.66 1.68 1.7 1.72
-8
-6
-4
-2
0
2
4
6
8
6
6.5
7
7.5
8
8.5
9
Energy (eV)
Vsd(V)
1.58 1.6 1.62 1.64 1.66 1.68 1.7 1.72
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
6
6.5
7
7.5
8
8.5
Xo
X+
X-
XI
Energy (eV)
Vsd(V)
1.58 1.6 1.62 1.64 1.66 1.68 1.7 1.72
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
6
6.5
7
7.5
8
8.5
6
7
8
ELintensity(logcounts)
Energy (eV)
Vsd(V)
1.58 1.6 1.62 1.64 1.66 1.68 1.7 1.72
-8
-6
-4
-2
0
2
4
6
8
6
6.5
7
7.5
8
8.5
9
6
7
8
9
PLintensity(logcounts)
Electroluminescence imageWSe2 Monolayer
2 μm
x40
1.68 1.71 1.74 1.77
Energy (eV)
1500
1000
500
0
PL(cts/sec)
Monolayer
Bilayer
Xo
X-
Photoluminescence of single
and bi-layer WSe2
Ambipolar transport curve (Black),
n-resistor bias curve (red), p-n
junction diode bias curve (blue).
- +
+
-
- +
- +
30 Kelvin
Transfer of WSe2 onto back gates
Optical Image of WSe2
10 μm