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Layer‐Number‐Dependent Electronic and Optoelectronic Properties of 2D WSe 2 ‐Organic Hybrid Heterojunction


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One of the most attractive features of 2D WSe2 is a tunability in its electronic and optoelectronic properties depending on the layer number. To harness such unique characteristics for device applications, high quality and easily processable heterojunctions are required and relevant layer‐number‐dependent properties must be understood. Herein, a study is reported on hybrid heterojunctions between 2D WSe2 and organic molecules from one‐step solution chemistry and their layer‐number‐dependent properties. Eosin Y (EY) dye is selected as a p‐dopant and uniformly stacked on mechanically exfoliated WSe2 flakes via van der Waals interaction, forming a hybrid heterojunction with a type II alignment. The EY‐WSe2 heterojunction shows significantly enhanced currents compared to pristine WSe2 with a lower barrier height and a longer effective screening length. The work function of the heterostructure is also lower than that of pristine WSe2. The efficient exciton dissociation and doping effect by EY are confirmed by photocurrent and photoluminescence measurements, where WSe2 emission is markedly quenched by EY and exciton contribution decreases with layer number. These findings shed critical insights into layer‐number‐dependent electronic and optoelectronic properties of organic‐WSe2 layers and also provide simple yet effective means to construct transition metal dichalcogenide‐based heterostructures, which should be valuable for developing layered 2D devices. 2D hybrid WSe2‐organic heterostructures are fabricated from facile solution chemistry. Eosin Y molecules self‐assemble to form a 0.7 nm thick uniform layer on WSe2 flakes via van der Waals interaction. Electronic and optoelectronic properties of the heterolayers are systematically studied as a function of layer number, including band alignment, carrier transport, work function, photoresponse, and doping effects.
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1900637 (1 of 8) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Layer-Number-Dependent Electronic and Optoelectronic
Properties of 2D WSe2-Organic Hybrid Heterojunction
Jaehoon Ji and Jong Hyun Choi*
DOI: 10.1002/admi.201900637
light-emitting devices,[9] photovoltaics,[10,11]
and phototransistors.[12]
To fully exploit the excellent proper-
ties of 2D WSe2 for nano- and opto-
electronics, it is important to establish
heterojunctions such as p–n, n–p–n, or
p–n–p structures[13,14] and their layer-
number-dependent properties at the
heterointerfaces must be understood.
However, such studies have been limited
due to the large difficulties in manufac-
turing. Conventional approaches require
multiple complex, laborious preparation
steps to construct heterostructures such
as TMDC-graphene,[10] TMDC–TMDC,[15]
TMDC-insulating 2D material (e.g., hex-
agonal boron nitride).[16] For example,
one of the heterolayers is manually trans-
ferred onto the other flake prepared in a
separate substrate by the “pick-up and
transfer” method using an elastomeric
stamp.[17] To overcome the difficulty and
cumbersomeness of the transfer pro-
cesses, direct synthesis of heterostructures
on a single substrate has been developed,
including serial chemical vapor deposition
(CVD) and stacking of dissimilar TMDC
layers[18] and deposition of organic layers onto TMDC flakes via
thermal evaporation.[19] However, these methods still require
multiple sublimation/deposition cycles at high temperatures
(e.g., 1000 °C) and low pressures (e.g., 0 Torr). In this work,
we demonstrate a hybrid heterostructure from organic mole-
cules stacked on mechanically exfoliated WSe2 flakes of mono-
layer (1L) through 6-layer (6L) via one-step solution-phase
chemistry and systematically investigate their layer-number-
dependent electronic and optoelectronic properties (Figure 1a).
Eosin Y (EY) dye molecules are selected in this work, as they
assemble a uniform layer on TMDC flakes via van der Waals
interaction from simple dipping of the flake/substrate into
EY-containing solution. The deposited EY layer forms a type II
band heterojunction with 2D WSe2 flakes. The energy levels
of 2D WSe2 and EY are illustrated in Figure 1b. As the layer
number of WSe2 increases, its conduction band minimum
(CBM) decreases, while the valence band maximum (VBM)
increases.[2,20,21] EY’s lowest unoccupied molecular orbital
(LUMO) level is positioned lower than the CBM of 6L WSe2,
and its highest occupied molecular orbital (HOMO) level
is lower than the VBM of 1L WSe2, forming a type II align-
ment.[22–24] Additionally, EY is a water-soluble dye molecule and
can provide a p-doping on WSe2.[25,26] In a previous report, we
One of the most attractive features of 2D WSe2 is a tunability in its elec-
tronic and optoelectronic properties depending on the layer number. To
harness such unique characteristics for device applications, high quality and
easily processable heterojunctions are required and relevant layer-number-
dependent properties must be understood. Herein, a study is reported
on hybrid heterojunctions between 2D WSe2 and organic molecules from
one-step solution chemistry and their layer-number-dependent properties.
Eosin Y (EY) dye is selected as a p-dopant and uniformly stacked on mechani-
cally exfoliated WSe2 flakes via van der Waals interaction, forming a hybrid
heterojunction with a type II alignment. The EY-WSe2 heterojunction shows
significantly enhanced currents compared to pristine WSe2 with a lower bar-
rier height and a longer effective screening length. The work function of the
heterostructure is also lower than that of pristine WSe2. The efficient exciton
dissociation and doping effect by EY are confirmed by photocurrent and pho-
toluminescence measurements, where WSe2 emission is markedly quenched
by EY and exciton contribution decreases with layer number. These findings
shed critical insights into layer-number-dependent electronic and optoelec-
tronic properties of organic-WSe2 layers and also provide simple yet effective
means to construct transition metal dichalcogenide-based heterostructures,
which should be valuable for developing layered 2D devices.
J. Ji, Prof. J. H. Choi
School of Mechanical Engineering
Purdue University
West Lafayette, IN 47907, USA
The ORCID identification number(s) for the author(s) of this article
can be found under
2D Hybrid Materials
1. Introduction
Atomically thin tungsten diselenide or WSe2 has garnered sub-
stantial interest due to its unique electrical and optical proper-
ties, which originate from the lattice structure and confined
dimensionality.[1] Its 2D layered structure provides sizable
bandgap depending on the thickness.[2,3] It exhibits inherent
p-type characteristics and relatively high hole mobility of
around 500–700 cm2 V1 s1.[4] These properties may be useful
for nanoelectronics including diodes,[5] field effect transis-
tors,[6] and logic circuits[7] with a high on/off ratio and high car-
rier mobility. Additionally, its optical properties such as strong
absorption and bright photoluminescence (PL) could be har-
nessed for optoelectronic applications such as photodetectors,[8]
Adv. Mater. Interfaces 2019, 6, 1900637
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1900637 (2 of 8)
found that not only organic dopants, but also solvents where
dopant molecules are dispersed can contribute to interlayer
charge transfer to/from 2D TMDC layers, depending on the
relative electronegativity.[27] In contrast, water exhibits no sig-
nificant doping effects on thermally annealed TMDCs. Thus, by
adopting EY molecules, it is possible to study pure doping pro-
cesses at the interface between EY and WSe2. The convenience
and easiness of this facile approach allows us to investigate the
layer-number-dependent characteristics of WSe2 heterojunc-
tions, which is otherwise a significant challenge due to difficul-
ties in fabrication of heterostructures.
In this work, the electronic and optoelectronic properties of
the EY-WSe2 heterojunction are studied as a function of layer
number with conductive and photoconductive atomic force
microscopy (C-AFM and PC-AFM), Kelvin probe force micros-
copy (KPFM), and PL spectroscopy.[28–30] We find that the barrier
height for electron transport in the heterojunction is lowered
than that of pristine WSe2 by 350 meV via type II alignment
and decreases monotonically with increasing layer number. The
work function of the heterostructure also drops by 114 meV
compared to pristine samples, as the work functions of WSe2
and EY layers become aligned. Under light illumination, the
EY-WSe2 layers produce greater photocurrents than pristine
layers due to facilitated exciton dissociation at the interface. The
charge transfer at the heterostructure is confirmed by strong
PL quenching in WSe2, which indicates a p-doing effect by EY.
These observations provide insight on layer-number-dependent
properties of the WSe2-organic heterojunctions, which could
form the basis for next-generation WSe2 based optoelectronics.
2. Results and Disccussion
2.1. Layer-Number-Dependent Electronic Properties
Various WSe2 flakes were prepared by mechanical exfoliation,
and their layer numbers were determined by Raman and AFM
measurements (Figure 1c and Figures S1–S3, Supporting Infor-
mation).[31,32] The WSe2 flakes were immersed in aqueous solu-
tion containing 1 × 103 m EY for 24 h, subsequently rinsed
with methanol and DI water, and blow dried with air (see
the Experimental Section). The EY-WSe2 heterolayers on the
Si/SiO2 substrate show a spatially uniform height difference of
about 0.70 ± 0.06 nm throughout the entire flakes compared
to the pristine WSe2. Even after harsh washing with DI water
for more than 2 h, the stacked EY layer was not dissolved or
peeled off from the flakes (Figure 1d,e). Importantly, similar
height difference is observed regardless of the WSe2 layer num-
bers (Figure S3, Supporting Information). The uniform layer of
EY molecules allows us to study the layer-number-dependent
Adv. Mater. Interfaces 2019, 6, 1900637
Figure 1. a) A schematic of the charge transfer between 2D WSe2 and EY. Charge carriers of WSe2 can be transferred to the stacked EY layer. b) Energy
band diagram of WSe2 flakes with different layer number (1L through 6L), EY, ITO, and Pt/Ir probe. c) Raman spectra of pristine WSe2 (dotted line) and
EY treated WSe2 (solid line). There are no significant changes in Raman signatures of the WSe2 flake before and after EY adsorption. d) AFM images
of pristine WSe2 (left) and EY treated WSe2 (right) on a Si/SiO2 substrate. e) Height profiles along the lines in (d), indicating that the WSe2 flake is 4L
and there is a uniform increase of 0.6 nm after EY functionalization. f) Schematic of PC-AFM. Electric potential is applied on the ITO substrate, while
the Pt/Ir probe is maintained as a ground. The current flow in the sample is spatially monitored by raster-scanning with the Pt/Ir-coated probe in dark
state and under photoirradiation from a 405 nm laser.
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properties of the hybrid heterostructure by varying the WSe2
thickness. It is also worth noting that EY molecules were selec-
tively adsorbed on WSe2 and there was no noticeable EY on the
bare Si substrate.
To investigate the current flow of pristine and hybrid WSe2,
the samples were prepared on indium tin oxide (ITO)-glass sub-
strates and measured with C-AFM (Figure 1f). When a positive
potential is applied on the ITO with the Pt/Ir probe grounded,
a diode-like behavior is observed in the pristine layers (also see
Figure S4, Supporting Information for current–voltage pro-
perties under negative potentials). For example, the current on
1L WSe2 does not change significantly under low bias voltage
and then increases drastically above 0.67 V (Figure 2a). The
threshold voltage (determined at 1.5 nA) increases as a func-
tion of layer number, from 0.95 V (2L) to 1.7 V (6L). The
electron affinity (i.e., CBM) of 1L WSe2 (Φ1L 3.70 eV)[2,21]
is relatively smaller than the work functions of the probe tip
(Φtip 5.05 eV)[33,34] and ITO (ΦITO 4.75 eV)[35] (Figure 1b and
Figure 2a inset), rendering a Schottky contact under equilibrium
conditions. We adopt the field-assisted Fowler– Nordheim (FN)
tunneling model to elucidate the current dynamics (shown as
fitted curves in Figure 2a; also see the Supporting Information
for details).[36] Two parameters that govern the current behav-
iors are barrier height (ΦB) and screening length (d), which are
shown as a function of layer number in Figure 2c. The barrier
height between 1L WSe2 and the probe in our experiment is
determined to be 1.10 eV (Figure 2c), which is lower than the
reported value based on the Schottky–Mott theory (1.35 eV).[37]
The difference may be attributed to the Bardeen pinning effect,
a screening from the interfacial dipole,[38–40] and similar off-
sets (i.e., 0.25 eV) were reported in previous experimental
As the layer number increases, the barrier height gradu-
ally decreases and reaches 0.80 eV for 6L WSe2 (Figure 2c).
This trend is consistent with conventional bandgap thinning
behavior.[3,21,43] The effective screening length of 1L WSe2 is
measured to be 2.5 nm (Figure 2c), which is in good agree-
ment with the theoretical value (3 nm).[44] As the layer number
increases, the screening length also increases from 3.8 nm
(2L) to 9.4 nm (6L). The barrier height and screening length
together (ΦB1.5d) yield a linear relationship with layer number
(Figure 2d), which is consistent with gradually increasing
Adv. Mater. Interfaces 2019, 6, 1900637
Figure 2. Layer-number-dependent carrier transport properties in WSe2 and its hybrid junction. a) Current–voltage characteristics of pristine WSe2 as
a function of layer number from 1L to 6L. Inset shows a schematic of ITO-WSe2-Pt/Ir probe junction under the forward bias applied on the ITO. The
fitted lines with corresponding colors of the experimental data (filled objects) are obtained from the Fowler–Nordheim tunneling model. b) Current
versus voltage plot of hybrid WSe2-EY (inset: a schematic of the heterojunction with the forward bias). c) Effective screening length d (black) and
barrier height ΦB (red) of pristine WSe2. d) Tunneling barrier coupled with effective screening length (ΦB3/2d) demonstrates a linear relationship with
layer number. e,f) Effective screen length, barrier height, and ΦB3/2d of the hybrid junctions as a function of layer number. g) Work functions of pristine
WSe2 and WSe2-EY.
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threshold voltage. A stacked layer of EY molecules forms
a uniform junction with WSe2 flakes as we observe no
apparent spatial current variations in the EY-WSe2 samples
(Figures S8–S13, Supporting Information). The hybrid het-
erostructure demonstrates a significantly increased level of
currents under the same potential window (Figure 2b). For
example, the heterostructure from EY and 1L WSe2 shows
11 nA under 500 mV, whereas pristine 1L WSe2 exhibits only
about 0.38 nA under the same potential. Similarly, other het-
erosamples (with 2L through 6L WSe2) also generate far greater
amount of currents than pristine layers at given voltage. As
illustrated in the inset in Figure 2b, the EY layer may serve as an
intermediate energy level,[26] facilitating electron transfer from
the probe to the WSe2 CBM and eventually to the ITO. Inter-
estingly, the threshold voltage of the heterosamples increases
gradually with layer number, yet it does not vary significantly at
thicker layers, e.g., from 0.80 V (5L) to 0.83 V (6L).
The measured barrier heights of the heterolayers range from
0.65 eV (1L) to 0.49 eV (6L) as shown in Figure 2e. The barrier
heights are consistently lower than those of the pristine WSe2
by 0.35 eV, which is similar to the energy difference between
the EY LUMO and the WSe2 CBM. The deposited EY also plays
a role in screening current flow. The effective screening lengths
of the heterostructures are greater than those of the pristine
WSe2 by 0.50 nm regardless of WSe2 layer number. These
results indicate that the organic layer forms a robust hetero-
junction on 2D WSe2, which is also consistent with the fact that
its thickness is uniform (0.70 nm) on all the WSe2 layers in
our experiment. When we consider the barrier height and the
screening length together, ΦB1.5d in Figure 2f shows a gradual
increase with WSe2 layer number, yet it levels off above 4L and
the increase is not significant in thicker samples. It is worth
noting that this trend corresponds to the insignificant variation
of threshold voltage in Figure 2b with thicker heterosamples
(i.e., 5L and 6L), and this may be attributed to the fact that the
CBM of thicker WSe2 layers approach the EY LUMO.
In EY-WSe2 samples, interlayer charge transfer occurs
between them and the work function is thus arranged. To deter-
mine the work function, the surface potential difference (Vme)
between the probe tip and the sample is recorded with KPFM.
The surface potential difference is correlated with work func-
tion: ΦS = Φtip + Vme, where ΦS and Φtip denotes the work func-
tions of the sample and the tip, respectively. The measured work
function of the 1L WSe2 is ≈−4.78 eV as shown in Figure 2h
(see also Figure S5, Supporting Information), which is con-
sistent with the value reported by Wang et al. (4.76 eV).[45] It
is noted that the work function is closer to the VBM of WSe2
(5.35 eV), which indicates that the pristine WSe2 has inherent
p-type characteristics. The flat band potential of the EY layer is
≈−5.04 eV.[25] In EY-WSe2 heterolayers, electron transfer will
take place from WSe2 to low-lying EY to compensate the energy
level difference, and their work function should be between
the levels of WSe2 and EY. Our KPFM confirms that the work
functions of EY-WSe2 hybrids differ from those of pristine sam-
ples by 114 meV on average, varying from 97 meV (1L) to
128 meV (8L). The greater adjustment of the work function in
thick layers may be induced by relatively large energy difference
between the deposited EY and thick WSe2 flake, which results
in stronger interlayer charge transfer.[46]
2.2. Layer-Number-Dependent Optoelectronic Properties
Figure 3a–f presents the current–voltage responses of the pris-
tine WSe2 and EY-WSe2 hybrids (WSe2 of 1L through 6L) with
and without light illumination. Given the limit in the CAFM
measurement (±13 nA), currents were recorded within the
limit. It is notable that asymmetric current–voltage responses
of the samples are observed depending on the direction of
applied potential. When the same magnitude of potential is
applied in different directions (e.g., +/ voltage), pristine WSe2
produces a higher level of currents under reverse bias. In con-
trast, hybrid samples exhibit an opposite behavior due to the
lowered barrier height, thus yielding more currents under for-
ward bias. For example, 2L WSe2 generates only 0.18 nA of cur-
rent under 0.75 V, but exhibits 3.6 nA under 0.75 V (blue
dots in Figure 3a–f), whereas EY-functionalized 2L WSe2 shows
12.27 and 0.64 nA under the identical potential (red dots).
Light illumination generates significantly more currents
in both pristine WSe2 and hetero layers compared to the
dark state. For example, photoirradiated 3L WSe2 produces
≈−3.6 nA under 0.5 V which is 16 times higher than that
without illumination. The illuminated EY-functionalized 3L
WSe2 demonstrates about 4.6 nA with the sample potential
which is higher than that in the dark state by a factor of more
than 30. In our experiment, a 405 nm diode laser is used for
irradiation, because this wavelength excites WSe2, but not EY
(see Figure S6, Supporting Information).[47] The photoinduced
excitons in WSe2 layers are dissociated into the current flow.
The stacked EY layer facilitates the dissociation of excitons
at the interface, generating a significantly greater amount of
The current enhancement by photoirradiation is not only
influenced by the presence of EY layer, but also depends on
the WSe2 layer number. To compare the photoresponse (E) of
the samples as a function of layer number, we define E as the
voltage difference needed to generate a fixed amount of current
(Ifix) with (VL) and without (VD) illumination, normalized by the
dark state voltage
This relation measures the ability of the sample to create photo-
currents relative to dark state. It is, however, difficult to directly
compare E of the pristine and hetero samples at identical cur-
rents. The pristine WSe2 shows strong currents at reverse bias,
whereas hetero layers exhibit an opposite trend. Thus, we com-
pare E of the pristine WSe2 under reverse bias with that of the
hybrid layers under forward bias, while maintaining the same
magnitude of currents. Figure 3g shows the photoresponse E
of both pristine and hybrid WSe2 as a function of layer number
at 10 nA for the pristine WSe2 and 10 nA for the hybrid layer.
The photoresponse of pristine WSe2 decreases with increasing
layer number from 0.57 (1L) to nearly 0.083 (4L), and then
slightly recovers in thicker layers (e.g., 0.27 with 6L). As the
layer number increases, the screening length of WSe2 also
expands. The majority of excitons in multilayer flakes may thus
be screened and cannot be dissociated to contribute to the cur-
rent flow.[48] Further, thicker flakes can absorb a greater amount
Adv. Mater. Interfaces 2019, 6, 1900637
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of photons than thinner layers,[42] which may result in a slight
recovery in E. The EY-WSe2 heterostructures show greater
photoresponses than the pristine samples, ranging between
0.74 (1L) and 0.58 (6L), yet the layer-number-dependence is
less prominent. The staggered bands between the EY layer and
WSe2 flake can promote the dissociation of excitons, thereby
leading to greater photoresponses.
We also measured the photoresponses of pristine and hetero
layers as functions of fixed current and layer number as shown
in Figure 3h,i. The pristine flakes show a less prominent layer-
number-dependent behavior at a low current (e.g., <−6 nA)
than at the high current (e.g., >−6 nA). At 1 nA, for example,
the photoresponse varies less than 0.20 between 1L and 6L.
At a high current (e.g., 10 nA), however, the photoresponse
decreases with increasing layer number and recovers moder-
ately at thicker layers, as discussed above (Figure 3g). Unlike
the low current conditions, two salient competing mechanisms
determine photoresponses at high currents. Thicker flakes
absorb more photons than thinner layers, yet their exciton
dissociation becomes less efficient due to the greater screening
effect. As a result, the photoresponse will decrease at first and
then increases with increasing layer number. On the other
hand, EY-functionalized WSe2 heterostructures show distinct
photoresponse behaviors, demonstrating not only significantly
enhanced E, but also nearly uniform responses at various cur-
rents for given layer number. The hetero samples thinner than
4L all maintain a similar level of photoresponses at 0.70 at var-
ious currents. The E values of thicker hetero layers (e.g., 5L and
6L) are slightly smaller, but still uniform at 0.50 regardless of
currents. The exciton dissociation in the hetero interfaces is
facilitated by the staggered band alignment, and may not be
severely interrupted by the screening, resulting in more uni-
form photoresponses regardless of currents and layer numbers.
2.3. Layer-Number-Dependent Doping Effect
We investigate the interlayer doping effect between the stacked
EY and WSe2 flakes by using PL spectroscopy. Figure 4a–f
Adv. Mater. Interfaces 2019, 6, 1900637
Figure 3. Photoconductivity of pristine WSe2 flakes and hybrid junctions for various layer numbers. a–f) Current–voltage characteristics of pristine
WSe2 (blue) and EY-WSe2 (red) in the dark (filled) and under illumination (hollow). g) A plot of photoresponse of pristine WSe2 (dark blue) and EY-
treated WSe2 (pink) to obtain 10 and 10 nA of current, respectively. To compare how effectively the sample transport photoexcited carriers, the photo-
response of the samples is defined as the required voltage difference to obtain a same amount of current (I ) with ( VL) and without (VD) illumination
normalized by the dark voltage itself. Under light illumination, even though only small amount of the voltage (0.15 V) is applied on the 1L heterojunc-
tion, it can generate 10 nA of current which requires 0.57 V in the dark. Mapping of photoresponse with respect to the layer number and current:
h) pristine WSe2 i) hybrid junction. The photoresponse of hybrid junction is uniform, even the current level varies.
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presents the PL spectra of the pristine (blue line) and the
hybrid (red line) samples for various WSe2 thickness. A 633 nm
laser is used for excitation which can only excite the WSe2, thus
the PL spectra are solely from the WSe2 emission. Pristine 1L
exhibits a dominant peak around 748 nm (1.66 eV). As the layer
number increases up to 4L, the PL spectra become broader,
with asymmetric tails at longer wavelength. Thicker flakes (5L
and 6L) display distinct two peaks around 768 nm (1.61 eV) and
848 nm (1.46 eV), which are much weaker in the intensity and
longer in the wavelength than those of 4L. EY-functionalized
WSe2 samples show drastically weaker PL spectra compared
to the pristine flakes. The strong PL quenching indicates that
p-doping take place on WSe2 by the EY layer, which results in
more positive trions and suppresses the excitonic recombina-
tion. The emission peaks of the hetero flakes are red-shifted
from those of pristine samples. For example, the 1L hybrid
has its maximum peak at 765 nm (1.62 eV) which is 17 nm
longer than that of pristine WSe2. Thicker hybrid samples
(5L and 6L) also show their emission features at longer wave-
lengths 774 nm (1.60 eV) and 862 nm (1.44 eV). The PL
shifts are caused by the dielectric screening by the EY layer (see
the Supporting Information for details).
To investigate the layer-number-dependent PL character-
istics of the samples, the PL spectra are deconvoluted with
multiple peaks (see Figure S7, Supporting Information for
details). The 1L WSe2 PL consists of two components: exciton
and positive trion.[49] Besides excitons (748 nm for pristine
1L WSe2 and 765 nm for EY-treated 1L sample), the hole-rich
characteristics of WSe2 causes positive trions to appear as the
shoulder at longer wavelengths around 757 nm (1.64 eV) for
the pristine 1L WSe2 and 772 nm (1.61 eV) for the hybrid 1L
samples (Figure 4g).[50] The PL signals of multi-layer samples
Adv. Mater. Interfaces 2019, 6, 1900637
Figure 4. a–f) PL spectra of pristine WSe2 (blue) and EY-WSe2 (red) for various layer numbers from 1L to 6L. The PL of each WSe2 flake is markedly
quenched by the EY treatment due to charge transfer. g) Spectral intensities and h) positions of excitons X (dark blue: pristine, red: hybrid junction),
trions X+ (cyan: pristine, magenta: hybrid junction), and indirect transitions i (purple: pristine, pink: hybrid junction) as a function of layer number.
i) Exciton contributions in PL signatures of pristine (dark blue) and EY (pink) treated WSe2 samples for various layer numbers. Exciton contribution
increases with layer number in both WSe2 and EY-WSe2 samples. Trion contribution is consistently greater in EY-WSe2 than pristine WSe2, due to a
p-doping effect by EY.
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are deconvoluted into direct band transitions (i.e., exciton and
trion) and also indirect band transition which arises from
interlayer interactions in multi-layer WSe2 (see the Supporting
Information for details).[43] The indirect transition becomes
prominent in thicker layers, displaying the distinct emission
peaks at long wavelengths at 848 nm for pristine WSe2 and
862 nm for EY-WSe2.
The PL intensity varies significantly with the layer number.
Pristine 1L shows very strong PL emission (Figure 4h). The
intensities of exciton (dark blue) and positive trion (cyan)
decreases monotonously about two orders of magnitude with
increasing the layer number up to 5L and then level off beyond
5L. Compared to exciton and trion transitions, the intensity of
the indirect peaks of few layers does not change significantly
(shown in purple).[42] The overall PL signal becomes weak and
the indirect transition becomes more prominent. EY-WSe2
heterolayers demonstrate more than an order of magnitude
smaller intensities than pristine samples (Figure 4h). Both
exciton (red) and trion (magenta) features decrease in their
intensities about tenfold with increasing layer number up to 4L.
Thicker layers (5L and 6L) do not show considerable difference
in the intensity. Similar to the pristine samples, the decrease of
the indirect peak intensity depending on the layer number is
relatively small.
The doping effect is manifested by comparing the intensity
ratio between exciton and trion peaks.[51] Figure 4i presents
the proportion of exciton (ΔXex) compared to positive trion,
, as a function of layer number. Here X and X+ denote
the intensity of exciton and trion recombination, respectively.
ΔXex of pristine WSe2 is 57% with 1L and gradually increases
to 75% with 6L. As the layer thickens, the hole-rich proper-
ties of WSe2 becomes more balanced with decreasing trion[52,53]
and increasing exciton contributions to the PL emission.[51] The
hetero flakes show considerably reduced exciton contributions,
ranging from 40% (1L) to 59% (6L). Nearly 20% of exciton
contribution is reduced compared to the pristine WSe2. The
reduced exciton contribution strongly supports the p-doping
effect of the stacked EY layer on WSe2 flakes.
3. Conlcusions
In summary, we have systemically investigated the layer-
number-dependent properties of the WSe2-organic hetero-
junctions from simple solution-based chemistry. Currents,
photocurrents, and work function all vary as a function of the
layer number of WSe2. EY serves as an intermediate layer to
facilitate the dissociation of the photoexcited energies into cur-
rents and is also effective in p-doping on WSe2 flakes. These
behaviors agree well with the designed type II alignment of het-
erojunction. It is evident that unique layer-number-dependent
properties of WSe2 are well retained after organic function-
alization. Given the simplicity and the effectiveness in the
preparation, our approach may be adopted to engineer the
properties of TMDCs for electronics and optoelectronics. This
work also offers a means to study the physics of 2D materials
and enhance the versatility and performance of the WSe2 based
4. Experimental Section
Sample Preparation: Microsized flakes were mechanically exfoliated
from WSe2 crystals (2D materials supplies) using the scotch tape
method.[54] The flakes were deposited onto the 285 nm thick Si/SiO2
substrates and their layer numbers were examined under a Renishaw
inVia confocal Raman microscope with a 100× objective lens. For the
electrical measurement, the exfoliated flakes were deposited on glass
substrates that had ITO with a thickness of 180 nm and resistivity of
8–10 Ω cm2. Before deposition, the substrates were cleaned by bath
sonication with acetone, methanol, and DI water, and also treated with
oxygen plasma for 10 min with 250 watts to improve the contact between
flakes and substrates. The deposited samples were subsequently
annealed at 250 °C for 1 h with Ar gas, which removed tape residues.
To construct hybrid heterojunctions, the deposited WSe2 flakes were
immersed in aqueous solution containing 1 × 103 m EY (Sigma-Aldrich)
at 4 °C for 24 h. The samples were then washed with methanol and DI
water to remove unbound and weakly bound EY molecules, and finally
dried with air.
Sample Characterization: Raman and PL signatures of WSe2 flakes
were measured under the Renishaw confocal microscope with a 633 nm
HeNe laser at room temperature. The low laser power (0.2 mW)
was used to avoid unexpected heating of the flakes. Both AFM and
PC-AFM measurements were conducted using a Bruker Dimension
Icon under the ambient conditions. AFM images were obtained using
a SCANASYST-AIR probe, while a SCM-PIC-V2 probe with a Pt/Ir coated
and antimony doped low resistive tip was used in a PF-TUNA module
for current measurement. In C-AFM, the current limit in the preamplifier
is ±13 nA and the current sensitivity is 1 nA V1. A 405 nm laser
(Laserglow Technologies) with 1 mW cm2 at the sample was used to
excite the flakes using a backside illumination optics assembly (Bruker).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
This work was supported by National Science Foundation.
Conflict of Interest
The authors declare no conflict of interest.
electronic properties, heterostructure, layer-number-dependence,
optoelectronics, transition metal dichalcogenide, WSe2
Received: April 10, 2019
Revised: June 9, 2019
Published online: July 3, 2019
[1] M. Zeng, Y. Xiao, J. Liu, K. Yang, L. Fu, Chem. Rev. 2018, 118, 6236.
[2] Y. Liu, P. Stradins, S.-H. Wei, Sci. Adv. 2016, 2, e1600069.
[3] A. Arora, M. Koperski, K. Nogajewski, J. Marcus, C. Faugeras,
M. Potemski, Nanoscale 2015, 7, 10421.
[4] H. C. P. Movva, A. Rai, S. Kang, K. Kim, B. Fallahazad, T. Taniguchi,
K. Watanabe, E. Tutuc, S. K. Banerjee, ACS Nano 2015, 9, 10402.
Adv. Mater. Interfaces 2019, 6, 1900637
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1900637 (8 of 8)
[5] A. Ahmet, M. Kolyo, M. E. Gonzalez, I. Giuseppe, W. Kenji,
T. Takashi, F. Gianluca, K. Andras, Adv. Mater. 2018, 30, 1707200.
[6] P. Pushpa Raj, G. S. Michael, O. Akinola, T. W. Anthony, N. H. Anna,
P. B. Dayrl, X. Kai, G. M. David, Z. W. Thomas, D. R. Philip, Nano-
technology 2017, 28, 475202.
[7] M. Tosun, S. Chuang, H. Fang, A. B. Sachid, M. Hettick, Y. Lin,
Y. Zeng, A. Javey, ACS Nano 2014, 8, 4948.
[8] S.-H. Jo, D.-H. Kang, J. Shim, J. Jeon, M. H. Jeon, G. Yoo, J. Kim,
J. Lee, G. Y. Yeom, S. Lee, H.-Y. Yu, C. Choi, J.-H. Park, Adv. Mater.
2016, 28, 4824.
[9] J. S. Ross, P. Klement, A. M. Jones, N. J. Ghimire, J. Yan,
D. G. Mandrus, T. Taniguchi, K. Watanabe, K. Kitamura, W. Yao,
D. H. Cobden, X. Xu, Nat. Nanotechnol. 2014, 9, 268.
[10] D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, M. C. Hersam,
ACS Nano 2014, 8, 1102.
[11] H. Zhang, J. Choi, A. Ramani, D. Voiry, S. N. Natoli, M. Chhowalla,
D. R. McMillin, J. H. Choi, ChemPhysChem 2016, 17, 2854.
[12] N. Guo, F. Gong, J. Liu, Y. Jia, S. Zhao, L. Liao, M. Su, Z. Fan,
X. Chen, W. Lu, L. Xiao, W. Hu, ACS Appl. Mater. Interfaces 2017, 9,
[13] a) H.-M. Li, D. Lee, D. Qu, X. Liu, J. Ryu, A. Seabaugh, W. J. Yoo,
Nat. Commun. 2015, 6, 6564; b) J. I. J. Wang, Y. Yang, Y.-A. Chen,
K. Watanabe, T. Taniguchi, H. O. H. Churchill, P. Jarillo-Herrero,
Nano Lett. 2015, 15, 1898.
[14] J. Choi, H. Chen, F. Li, L. Yang, S. S. Kim, R. R. Naik, P. D. Ye,
J. H. Choi, Small 2015, 11, 5520.
[15] R. Cheng, D. Li, H. Zhou, C. Wang, A. Yin, S. Jiang, Y. Liu, Y. Chen,
Y. Huang, X. Duan, Nano Lett. 2014, 14, 5590.
[16] J. Wierzbowski, J. Klein, F. Sigger, C. Straubinger, M. Kremser,
T. Taniguchi, K. Watanabe, U. Wurstbauer, A. W. Holleitner,
M. Kaniber, K. Müller, J. J. Finley, Sci. Rep. 2017, 7, 12383.
[17] F. Pizzocchero, L. Gammelgaard, B. S. Jessen, J. M. Caridad,
L. Wang, J. Hone, P. Bøggild, T. J. Booth, Nat. Commun. 2016, 7, 11894.
[18] Y. Gong, J. Lin, X. Wang, G. Shi, S. Lei, Z. Lin, X. Zou, G. Ye,
R. Vajtai, B. I. Yakobson, H. Terrones, M. Terrones, B. K. Tay, J. Lou,
S. T. Pantelides, Z. Liu, W. Zhou, P. M. Ajayan, Nat. Mater. 2014, 13,
[19] D. Jariwala, S. L. Howell, K.-S. Chen, J. Kang, V. K. Sangwan,
S. A. Filippone, R. Turrisi, T. J. Marks, L. J. Lauhon, M. C. Hersam,
Nano Lett. 2016, 16, 497.
[20] Z. Wenjun, H. Yi, M. Lianbo, Z. Guoyin, W. Yanrong, X. Xiaolan,
C. Renpeng, Y. Songyuan, J. Zhong, Adv. Sci. 2018, 5, 1700275.
[21] Z. Changjian, Z. Yuda, R. Salahuddin, W. Yi, L. Ziyuan, C. Mansun,
C. Yang, Adv. Funct. Mater. 2016, 26, 4223.
[22] V. S. Manikandan, A. K. Palai, S. Mohanty, S. K. Nayak, J. Photo-
chem. Photobiol., B 2018, 183, 397.
[23] E. L. Lim, C. C. Yap, M. Yahaya, M. M. Salleh, Semicond. Sci. Technol.
2013, 28, 045009.
[24] R. Sharma, A. Kamal, R. K. Mahajan, RSC Adv. 2016, 6, 71692.
[25] N. Zhang, J. Shi, F. Niu, J. Wang, L. Guo, Phys. Chem. Chem. Phys.
2015, 17, 21397.
[26] F. Zhang, F. Shi, W. Ma, F. Gao, Y. Jiao, H. Li, J. Wang, X. Shan,
X. Lu, S. Meng, J. Phys. Chem. C 2013, 117, 14659.
[27] J. Choi, H. Zhang, H. Du, J. H. Choi, ACS Appl. Mater. Interfaces
2016, 8, 8864.
[28] J. Choi, H. Zhang, J. H. Choi, ACS Nano 2016, 10, 1671.
[29] H. Zhang, J. Huang, Y. Wang, R. Liu, X. Huai, J. Jiang, C. Anfuso,
Opt. Commun. 2018, 406, 3.
[30] Y. Li, C.-Y. Xu, L. Zhen, Appl. Phys. Lett. 2013, 102, 143110.
[31] C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, S. Ryu, ACS Nano
2010, 4, 2695.
[32] P. Nemes-Incze, Z. Osváth, K. Kamarás, L. P. Biró, Carbon 2008, 46,
[33] Y. Lv, J. Cui, Z. M. Jiang, X. Yang, Nanoscale Res. Lett. 2012, 7, 659.
[34] R. B. Narsimha, K. P. Naresh, D. Melepurath, ChemPhysChem 2015,
16, 377.
[35] L. Yan, Y. Song, Y. Zhou, B. Song, Y. Li, Org. Electron. 2015, 17, 94.
[36] N. M. Ravindra, J. Zhao, Smart Mater. Struct. 1992, 1, 197.
[37] W. Jaegermann, C. Pettenkofer, B. A. Parkinson, Phys. Rev. B 1990,
42, 7487.
[38] W. Chen, E. J. G. Santos, W. Zhu, E. Kaxiras, Z. Zhang, Nano Lett.
2013, 13, 509.
[39] I. Popov, G. Seifert, D. Tománek, Phys. Rev. Lett. 2012, 108,
[40] S. G. Louie, M. L. Cohen, Phys. Rev. Lett. 1975, 35, 866.
[41] C. Gong, L. Colombo, R. M. Wallace, K. Cho, Nano Lett. 2014, 14,
[42] Y. Son, Q. H. Wang, J. A. Paulson, C.-J. Shih, A. G. Rajan, K. Tvrdy,
S. Kim, B. Alfeeli, R. D. Braatz, M. S. Strano, ACS Nano 2015, 9,
[43] Y. Li, X. Li, T. Yu, G. Yang, H. Chen, C. Zhang, Q. Feng, J. Ma,
W. Liu, H. Xu, Y. Liu, X. Liu, Nanotechnology 2018, 29, 124001.
[44] A. V. Stier, N. P. Wilson, G. Clark, X. Xu, S. A. Crooker, Nano Lett.
2016, 16, 7054.
[45] Z. Wang, Q. Li, Y. Chen, B. Cui, Y. Li, F. Besenbacher, M. Dong,
NPG Asia Mater. 2018, 10, 703.
[46] Q. Peng, Z. Wang, B. Sa, B. Wu, Z. Sun, Sci. Rep. 2016, 6, 31994.
[47] P. Chakraborty, B. Roy, P. Bairi, A. K. Nandi, J. Mater. Chem. 2012,
22, 20291.
[48] T. Akama, W. Okita, R. Nagai, C. Li, T. Kaneko, T. Kato, Sci. Rep.
2017, 7, 11967.
[49] S. B. Desai, G. Seol, J. S. Kang, H. Fang, C. Battaglia, R. Kapadia,
J. W. Ager, J. Guo, A. Javey, Nano Lett. 2014, 14, 4592.
[50] E. Courtade, M. Semina, M. Manca, M. M. Glazov, C. Robert,
F. Cadiz, G. Wang, T. Taniguchi, K. Watanabe, M. Pierre,
W. Escoffier, E. L. Ivchenko, P. Renucci, X. Marie, T. Amand,
B. Urbaszek, Phys. Rev. B 2017, 96, 085302.
[51] S. Mouri, Y. Miyauchi, K. Matsuda, Nano Lett. 2013, 13, 5944.
[52] W. Huang, X. Luo, C. K. Gan, S. Y. Quek, G. Liang, Phys. Chem.
Chem. Phys. 2014, 16, 10866.
[53] N. R. Wilson, P. V. Nguyen, K. Seyler, P. Rivera, A. J. Marsden,
Z. P. L. Laker, G. C. Constantinescu, V. Kandyba, A. Barinov,
N. D. M. Hine, X. Xu, D. H. Cobden, Sci. Adv. 2017, 3,
[54] H. Li, G. Lu, Y. Wang, Z. Yin, C. Cong, Q. He, L. Wang, F. Ding,
T. Yu, H. Zhang, Small 2013, 9, 1974.
Adv. Mater. Interfaces 2019, 6, 1900637
... 2,21 First, monolayer MoS 2 flakes were placed on a silicon (Si/SiO 2 ) substrate by mechanical exfoliation. 22 The MoS 2 samples were subsequently immersed in the solutions containing dye molecules to form organic layers on top of MoS 2 flakes. Atomic force microscopy (AFM) imaging reveals that the organic molecules, B3PyPB, EY, TCNQ, and CoPc, form uniform layers with an almost identical thickness of ∼0.9 nm (Figures S1 and S3; also see the SI for details). ...
... 52 Then, it was dried by blowing air and placed on a hot plate at 110°C for 2 min. 22 To fabricate the organic layers on top of the MoS 2 flakes, the samples were immersed in the solutions containing the dye molecules at room temperature for 8 h: B3PyPB (1 mg/mL in chloroform), EY (1 mg/mL in ethanol), TCNQ (0.6 mg/ mL in chloroform), and CoPc (0.4 mg/mL in dimethylformamide). These molecules are purchased from Sigma-Aldrich in a powder form. ...
... We kept in mind that the exfoliation process resulted in nanosheets having different thicknesses: monolayers, bilayers, trilayers, and also higher number of layers. We also kept in mind that the electronic properties of the TMDs depend on the layer number in the nanoflakes [31,32]. In STM, since individual nanosheets are probed in an extremely localized manner, we first determined their thickness. ...
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In this work, we form alloyed transition-metal dichalcogenides (TMDs), Mo1–xWxSe2, through a hydrothermal synthesis procedure. Due to the identical effective ionic radius of the cations, the common-anionic alloys did not experience any lattice strain. A complete series of the common-anionic TMD alloys could thereby be synthesized; their nanosheets were formed by a liquid exfoliation method. The electronic properties of the alloyed nanosheets in their monolayer, bilayer, and trilayer forms were recorded separately through scanning tunneling spectroscopy (STS) in an extremely localized manner. From the STS studies, which have a correspondence to the density of states of the semiconductors, we have deliberated on their electronic properties and commented on the band edges of the nanosheets in the alloy series. The transport gap of the alloys in their monolayer, bilayer, and trilayer forms exhibited band-gap bowing, and also a layer-number-dependent bowing coefficient. That is, the bowing phenomenon interestingly occurred only in the few-layered TMD alloys and diminished to finally vanish in thicker nanosheets. The conduction band has been found to be responsible for such a nonlinear behavior of the transport gap; the results could be explained in terms of the molecular orbitals which form the band. The results have established a coherent description of the band-gap tuning in atomically thin 2D TMD alloys.
... In general, the thinner, the larger. 13,36,37 Based the surface potential, the work function was calculated, the work function of the Mo 0.4 W 0.6 Se 2 monolayer on the ITO/glass is 4.71 eV. ...
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Abstract Atomically thin two‐dimensional (2D) alloys have attracted wide interests of study recently due to their potential in flexible electronic and optoelectronic applications. In particular, monolayer transition metal dichalcogenide (TMD) alloys have emerged as unique 2D semiconductors with tunable bandgaps, by means of alloying. However, response of surface electrical potential and barrier height to strain for 2D TMD alloys–electrode interface is rarely explored. Apparently, revealing such strain‐dependent evolution of electrical properties is crucial for developing advanced 2D TMD based flexible electronics and optoelectronics. Here we performed in situ strain Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C‐AFM) investigations of monolayer Mo0.4W0.6Se2 on Au coated flexible substrate, where controlled uniaxial tensile strain is applied. Both contact potential difference (CPD) and Schottky barrier heights (SBH) of monolayer Mo0.4W0.6Se2 show obvious decreases with the increase of strain, which is mainly due to the strain‐induced increment of TMD electron affinity. Our in situ strain photoluminescence (PL) measurements also indicate the changes of electronic band structures under strain. We further exploit the substrate effects on CPD by study the monolayer alloy on the mostly used substrates of SiO2/Si and indium tin oxide (ITO)/glass. Our findings could strengthen the foundation for the potential applications of 2D TMD and their alloys in the fields of strain sensors, flexible photodetectors, and other wearable electronic devices.
... Additionally, considering the valence band edge of −5.4 eV for WSe 2 (ref. 47 ) and the work function of 6.3 eV for CrSe 2 in the CrSe 2 /WSe 2 vdW heterostructure, electrons may transfer readily from WSe 2 to CrSe 2 . Indeed, the band profiles of CrSe 2 in the heterobilayer are shifted downward by 0.3 eV compared with the pristine CrSe 2 monolayer ( Supplementary Fig. 13), confirming electron transfer from the WSe 2 substrate to CrSe 2 . ...
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The discovery of intrinsic ferromagnetism in ultrathin two-dimensional van der Waals crystals opens up exciting prospects for exploring magnetism in the ultimate two-dimensional limit. Here, we show that environmentally stable CrSe2 nanosheets can be readily grown on a dangling-bond-free WSe2 substrate with systematically tunable thickness down to the monolayer limit. These CrSe2/WSe2 heterostructures display high-quality van der Waals interfaces with well-resolved moiré superlattices and ferromagnetic behaviour. We find no apparent change in surface roughness or magnetic properties after months of exposure in air. Our calculations suggest that charge transfer from the WSe2 substrate and interlayer coupling within CrSe2 play a critical role in the magnetic order in few-layer CrSe2 nanosheets. The highly controllable growth of environmentally stable CrSe2 nanosheets with tunable thickness defines a robust two-dimensional magnet for fundamental studies and potential applications in magnetoelectronic and spintronic devices.
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There is a critical need to develop high-performance supercapacitors that can complement and even rival batteries for energy storage. This work introduces a strategy to drastically enhance the energy storage performance of a supercapacitor by engineering electrode morphologies with ternary composites offering distinct benefits for the energy storage application. The electrodes were fabricated with conductive networks of carbon nanotubes (CNTs) coated with a zeolitic imidazole framework (ZIF) for high ion diffusivity and ion-accumulating molybdenum disulfide (MoS2) with various morphologies. These include flower-like (fMoS2), stacked-plate (pMoS2), and exfoliated-flake (eMoS2) structures from topochemical synthesis. CNT-ZIF-fMoS2 demonstrates an excellent energy density, reaching almost 80 Wh/kg, and a maximum power density of approximately 3000 W/kg in a half-cell. This is far superior to the electrodes containing pMoS2 and eMoS2 and attributed to the increased surface area and the faradaic reactivity offered by fMoS2. Additionally, the CNT-ZIF-fMoS2 electrode demonstrates exceptional stability with an ∼78% of capacitance retention over 10,000 cycles. This work suggests that the electrode morphologies can dominate the energy storage behaviors and that the heteromaterial approach may be crucial in designing next-generation supercapacitors.
An Ohmic contact is critical for achieving 2D material-based high performance electronic devices. Unfortunately, the formation of an intrinsic Ohmic contact for 2D materials is difficult; thus, current studies mostly stay in the Schottky regime. In this work, density functional calculations are performed for work function engineering for metal–semiconductor junctions involving 2D H-WSe2 and 2D metals of MX2 (M = Ti, V, Nb, Ta, Mo, and W and X = S and Se). We unambiguously identify a Schottky-to-Ohmic contact transition boundary, beyond which p-type Ohmic contacts are demonstrated to be stable. We show that the Fermi level pinning effect is relatively weak in the Schottky region, while similar pinning-like behavior is strong in the Ohmic region, creating a discontinuity near the contact transition boundary. The observed deviation from the ideal Schottky–Mott limit is directly related to the charge redistribution and interface dipole-induced potential step, reflected by metal work function modification due to contact formation. Our work not only provides a strategy to identify effective Ohmic contacts but also offers insights for prospection into the fundamental electronic properties of van der Waals-based heterojunctions.
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The worldwide unrestrained emission of carbon dioxide (CO2) has caused serious environmental pollution and climate change issues. For the sustainable development of human civilization, it is very desirable to convert CO2 to renewable fuels through clean and economical chemical processes. Recently, electrocatalytic CO2 conversion is regarded as a prospective pathway for the recycling of carbon resource and the generation of sustainable fuels. In this review, recent research advances in electrocatalytic CO2 reduction are summarized from both experimental and theoretical aspects. The referred electrocatalysts are divided into different classes, including metal–organic complexes, metals, metal alloys, inorganic metal compounds and carbon-based metal-free nanomaterials. Moreover, the selective formation processes of different reductive products, such as formic acid/formate (HCOOH/HCOO⁻), monoxide carbon (CO), formaldehyde (HCHO), methane (CH4), ethylene (C2H4), methanol (CH3OH), ethanol (CH3CH2OH), etc. are introduced in detail, respectively. Owing to the limited energy efficiency, unmanageable selectivity, low stability, and indeterminate mechanisms of electrocatalytic CO2 reduction, there are still many tough challenges need to be addressed. In view of this, the current research trends to overcome these obstacles in CO2 electroreduction field are summarized. We expect that this review will provide new insights into the further technique development and practical applications of CO2 electroreduction.
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Few-layered transition metal dichalcogenides (TMDs) are known as true two-dimensional materials, with excellent semiconducting properties and strong light–matter interaction. Thus, TMDs are attractive materials for semitransparent and flexible solar cells for use in various applications. Hoewver, despite the recent progress, the development of a scalable method to fabricate semitransparent and flexible solar cells with mono- or few-layered TMDs remains a crucial challenge. Here, we show easy and scalable fabrication of a few-layered TMD solar cell using a Schottky-type configuration to obtain a power conversion efficiency (PCE) of approximately 0.7%, which is the highest value reported with few-layered TMDs. Clear power generation was also observed for a device fabricated on a large SiO2 and flexible substrate, demonstrating that our method has high potential for scalable production. In addition, systematic investigation revealed that the PCE and external quantum efficiency (EQE) strongly depended on the type of photogenerated excitons (A, B, and C) because of different carrier dynamics. Because high solar cell performance along with excellent scalability can be achieved through the proposed process, our fabrication method will contribute to accelerating the industrial use of TMDs as semitransparent and flexible solar cells.
New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate‐tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic‐liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe2 transistor by only utilizing van der Waals bonded graphene and Co/h‐BN contacts. Toward this, first the charge injection efficiencies into WSe2 by graphene and Co/h‐BN contacts are characterized. While Co/h‐BN contact results in nearly Schottky‐barrier‐free charge injection, graphene/WSe2 interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work‐function values of graphene and Co/h‐BN, vertical devices are constructed where different gate‐tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts.
Transition metal dichalcogenides (TMDs) with typical layered structure are highly sensitive to their layer number in optical and electronic properties. Seeking a simple and effective method for layer number identification is very important to low-dimensional TMDs samples. Herein, a rapid and accurate layer number identification of few-layer WS2 and WSe2 is proposed via locking their photoluminescence (PL) peak-positions. As the layer number of WS2/WSe2 increases, it is found that indirect transition emission is more thickness-sensitive than direct transition emission, and the PL peak-position differences between the indirect and direct transitions can be regarded as fingerprints to identify their layer number. Theoretical calculation confirms that the notable thickness-sensitivity of indirect transition derives from the variations of electron density of states of W atom d-orbitals and chalcogen atom p-orbitals. Besides, the PL peak-position differences between the indirect and direct transitions are almost independent of different insulating substrates. This work not only proposes a new method for layer number identification via PL studies, but also provides a valuable insight into the thickness-dependent optical and electronic properties of W-based TMDs.
Photodetectors based on low-dimensional materials have attracted tremendous attention due to the high sensitivity and compatibility with conventional semiconductor technology. However, up-until-now, developing low-dimensional phototransistors with high responsivity and low dark currents over broadband spectra still remains a great challenge due to the trade-offs in the potential architectures. In this work, we report a hybrid phototransistor consisting of a single In2O3 nanowire as channel material and a multilayer WSe2 nanosheet as the decorating sensitizer for photodetection. Our devices show high responsivities of 7.5×10^5 A W^-1 and 3.5×10^4 A W^-1, and ultrahigh detectivities of 4.17×10^17 Jones and 1.95×10^16 Jones at the wavelengths of 637 nm and 940 nm, respectively. The superior detectivity of the hybrid architecture arise from the extremely low dark currents and the enhanced photogating effect in the depletion regime by the unique design of energy band alignment of the channel and sensitizer materials. Moreover, the visible to near-infrared absorption properties of the multilayer WSe2 nanosheet favor a broadband spectral response for the devices. Our results pave the way for developing ultrahigh sensitivity photodetectors based on low-dimensional hybrid architectures.
In this paper, high performance top-gated WSe2 field effect transistor (FET) devices are demonstrated via a two-step remote plasma assisted ALD process. High-quality, low-leakage aluminum oxide (Al2O3) gate dielectric layers are deposited onto the WSe2 channel using a remote plasma assisted ALD process with an ultrathin (~1 nm) titanium buffer layer. The first few nanometers (~2 nm) of the Al2O3 dielectric film is deposited at relatively low temperature (i.e. 50 ºC) and remainder of the film is deposited at 150 ºC to ensure the conformal coating of Al2O3 on the WSe2 surface. Additionally, an ultra-thin titanium buffer layer is introduced at the WSe2 channel surface prior to ALD process to mitigate oxygen plasma induced doping effects. Excellent device characteristics with current on-off ratio in excess of 106 and a field effect mobility as high as 70.1 cm2/V.s are achieved in a few-layer WSe2 FET device with a 30 nm Al2O3 top-gate dielectric. With further investigation and careful optimization, this method can play an important role for the realization of high performance top gated FETs for future optoelectronic device applications.
Low dimensional materials exhibit distinct properties compared to their bulk counterparts. A plethora of examples have been demonstrated in two-dimensional (2-D) materials, including graphene and transition metal dichalcogenides (TMDCs). These novel and intriguing properties at the nano-, molecular- and even monatomic scales have triggered tremendous interest and research, from fundamental studies to practical applications and even device fabrication. The unique behaviors of 2-D materials result from the special structure–property relationships that exist between surface topographical variations and mechanical responses, electronic structures, optical characteristics, and electrochemical properties. These relationships are generally convoluted and sensitive to ambient and external perturbations. Characterizing these systems thus requires techniques capable of providing multidimensional information under controlled environments, such as atomic force microscopy (AFM). Today, AFM plays a key role in exploring the basic principles underlying the functionality of 2-D materials. In this tutorial review, we provide a brief introduction to some of the unique properties of 2-D materials, followed by a summary of the basic principles of AFM and the various AFM modes most appropriate for studying these systems. Following that, we will focus on five important properties of 2-D materials and their characterization in more detail, including recent literature examples. These properties include nanomechanics, nanoelectromechanics, nanoelectrics, nanospectroscopy, and nanoelectrochemistry.