Large photoelectric-gating effect of two-dimensional van-der-waals organic/tungsten diselenide heterointerface

ABSTRACT Photo- or photoelectric-gating modulation is a promising strategy for high-performance photodetectors, which amplifies photoresponsivity by long-lived trapped charges at the

interface. However, the performance is normally limited by the uncontrollable trapping process. Here, we develop a large photoelectric-gating, which enhances interfacial charge trapping

process by a van-der-Waals interface with an electric-gating tunable energy barrier in the band alignment. By synergy of photo-gating and electric-gating effects, responsivity and

detectivity of 1,4-bis(4-methylstyryl)benzene/tungsten diselenide (WSe2) increase by 25-fold and 3-fold to 3.6 × 106 A/W and 8.6 × 1014 Jones. High-quality two-dimensional van-der-Waals

interface is of great importance. Sufficient supply of gas-phase molecules in physical vapor deposition is pivotal to obtain such interface between organic crystal and WSe2. As an

application, an electric-gating switchable photodetector has been developed, showing great potential of this strategy not only in high-performance photodetectors but also in new

photoelectrical devices. SIMILAR CONTENT BEING VIEWED BY OTHERS HIGH OSCILLATOR STRENGTH INTERLAYER EXCITONS IN TWO-DIMENSIONAL HETEROSTRUCTURES FOR MID-INFRARED PHOTODETECTION Article 29

June 2020 TWO-DIMENSIONAL PEROVSKITE OXIDE AS A PHOTOACTIVE HIGH-_Κ_ GATE DIELECTRIC Article 01 March 2024 STRAINED TWO-DIMENSIONAL TUNGSTEN DISELENIDE FOR MECHANICALLY TUNABLE EXCITON

TRANSPORT Article Open access 30 December 2024 INTRODUCTION Photo-gating modulation is normally regarded as one of the promising strategies for achieving high-performance

photodetection,1,2,3 which is realized by introducing another light absorption material to form a heterostructure. The heterostructure probably induces a photovoltaic effect under light

irradiation, resulting in long-lived trapped charges at the interface to amplify the photoresponsivity.4 By this strategy, photodetecting responsivity is greatly improved, especially for

two-dimensional (2D) materials. The interfacial charge trapping is of extreme significance for achieving efficient photo-gating modulation; however, until now effectively controlling the

trapping process is still lacking, leading to limited success in further increasing the photodetecting responsivity. For instance, 2D transition metal dichalcogenides (TMDs) such as

molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) have great potential in various photoelectrical applications,5,6,7 owing to their excellent physical,8,9 optical,5,6,7

electrical10,11,12 properties as well as advantages such as strong light-matter interaction in a wide range of wavelengths,13,14,15 high carrier mobility,14 thickness-modulated band gap,16

small dark current compared with graphene, etc. Recently, a photoresponsivity (_R_) (normally 103–104 A/W, the highest is 1.8 × 105 A/W) was gained in pristine 2D WSe2 photodetectors.5,17

Nonetheless, the sensitivity of photodetectors made of pristine 2D TMDs or other 2D materials is limited by the high transmittance of the atomically thin layers.18 To solve this problem,

materials like PbS colloidal quantum dots or other TMDs are introduced. The resulting photo-gating modulation improves the _R_ of MoS2 and WSe2 to 6 × 105 and 2 × 105 A/W, respectively.3,19

However, further increasing the photoresponsivity by normal photo-gating or photoelectric-gating modulation receives limited success, as a result of the absence of accumulated photogenerated

carriers at the interface depending on the band alignment. Organic semiconductors are promising materials for electrical or photoelectrical devices due to their high flexibility, ease of

processing,20 and tunable energy gap ranging from near infrared region to ultraviolet.21 In the case of organic/TMD heterostructures, Rubrene/MoS2 and C8BTBT/MoS2 only achieve a _R_ of 510

and 22 mA/W, respectively, by photo-gating modulation.22,23 Currently, the _R_ of organic/TMDs is still lower than 10 A/W. To solve the problem, an effective approach is required to enhance

the charge trapping process, thus achieving higher photodetection performance compared with devices under normal photo-gating or photoelectric-gating modulation. Moreover, the van-der-Waals

heterointerface quality is also of great importance. As a result of the poor interface quality, impurities in heterostructure interface produced via solution processes usually lead to

degradation of device performance.3,19 Epitaxial growth results in higher interface quality, especially for organic semiconductors. Owing to the ideal epitaxial interface, organic

crystal/graphene heterostructures achieve photoresponsivities (104 A/W) about six orders of magnitude higher than that of pristine graphene.24 In the case of 2D TMDs, a clear understanding

of the epitaxial growth of organic crystals on the surface is still absent, which results in uncontrollable interface quality and low performance of organic/TMDs heterostructures in

photoelectrical devices. Here, we demonstrate a large photoelectric-gating effect, which enhances the interfacial charge trapping process by the synergy of photo-gating and electric-gating

effects, leading to higher photoresponsivity compared with devices modulated by normal photo-gating or photoelectrical-gating effect. This strategy requires heterostructures with an

interfacial electric-gating tunable energy barrier in the band alignment, which allows unidirectional carrier injection. As an example, wide-band gap 1,4-bis(4-methylstyryl)benzene (p-MSB)

crystals are epitaxially grown on 2D WSe2 by physical vapor deposition (PVD). Under photo-gating modulation, the heterostructure exhibits _R_ of 1.4 × 105 A/W and detectivity (_D*_) of 2.5 ×

 1014 Jones (2 μW/cm2 365 nm light), more than 1 order higher than that of pristine 2D WSe2, respectively. The energy barrier can be tuned higher by electric-gating, thus trapping more

photogenerated electrons at the interface. Thus, the resulting gaint photoelectric-gating effect further increases the _R_ and _D*_ by about 1 order to 3.6 × 106 A/W and 8.6 × 1014 Jones (2 

μW/cm2 365 nm light), 1–9 orders of magnitude higher than that of existing photodetectors based on pristine WSe217 or TMDs-based heterostructures, such as PbS/WSe2 (2 × 105 A/W),3 PPh3/WSe2

(6.67 × 105 A/W),25 graphene/WSe2/graphene (0.01 A/W),26 MoS2/WSe2 (0.12 A/W),27 MoTe2 p–n junction (5 × 10-3 A/W),28 etc. Moreover, we find that sufficient supply of gas-phase molecules in

PVD is the pivotal factor to prepare ideal 2D van-der-Waals interface between organic crystal and TMDs for achieving large photoelectric-gating effect. As an application of the large

photoelectrical-gating modulation, we develop an electric-gating switchable photodetector based on p-MSB/WSe2. RESULTS EPITAXIAL GROWTH OF 2D VAN-DER-WAALS P-MSB/WSE2 INTERFACE Triangle 2D

WSe2 crystals were produced on SiO2 (300 nm)/Si substrates by chemical vapor deposition (CVD), and then epitaxial growth of p-MSB crystals on WSe2 was conducted by PVD at low pressure (Fig.

1a). The p-MSB molecules were evaporated from the center of the tubular furnace and were deposited on three substrates located 12 cm (p-MSB-1/WSe2), 13 cm (p-MSB-2/WSe2), and 14 cm

(p-MSB-3/WSe2) downstream. Figure 1b–d shows optical microscope images of the as-grown samples, which have different morphologies. Most p-MSB-3 crystals have a wire-like structure with

growth direction parallel to the edges of triangle WSe2 crystals. Neighbor wires form an angle of 60°, suggesting the epitaxial growth of p-MSB crystals on the WSe2. Most p-MSB-1 crystals

have a flake-like structure, covering the whole surface of the WSe2. In the p-MSB-2, both morphologies are observed. Figure 1e shows Raman spectra of WSe2 and p-MSB-1/WSe2. The WSe2 has two

characteristic peaks at 250.22 and 257.77 cm−1, corresponding to in-plane (E12g) and out-plane (A1g) vibrational modes, respectively.29 In the case of p-MSB-1/WSe2, these peaks shift to

higher wavelength by 1.43 cm−1 (E12g) and 0.73 cm−1 in (A1g), indicating p-type doping of WSe2 by the p-MSB molecules.30,31 The different shapes of as-grown p-MSB crystal grown on WSe2 are

attributed to the competition between thermodynamic and kinetic factors (Fig. 1f, g).32 At low deposition rate, thermodynamic factor dominates. According to Gibbs–Curie–Wulff theorem, the

equilibrium shape of a crystal minimizes the total surface energy, thus crystals tended to form wire-like shapes as a result of the strong π–π stacking along (100) direction. Higher

concentration of gas-phase molecules is supplied with decreasing the distance. As a result, kinetic factors lead to the extension of every lateral plane regardless of surface free energy

difference, forming large flake crystals on the WSe2. Therefore, sufficient supply of the molecules in PVD results in flake p-MSB crystal/WSe2 heterostructures with large-area high-quality

2D van-der-Waals interface. Atomic force microscope (AFM) image (Supplementary Fig. 1) shows that p-MSB molecules are stacked layer-by-layer with a single-layer thickness of 1.75 nm and a

plane parallel to the WSe2,33 indicating the 2D epitaxial growth mode of p-MSB on the WSe2. The p-MSB-1 crystal has a thickness up to 130 nm after 15 min growth. Thus, it has an adequate

thickness which enables high absorbance of UV light for sensitive photodetection. PHOTOELECTRICAL PROPERTIES OF P-MSB/WSE2 Devices (Fig. 2a, Supplementary Figs. 2–4) were fabricated on

p-MSB-1(130 nm thickness)/WSe2, p-MSB-3/WSe2, pristine WSe2 and pristine p-MSB with Au/Cr electrodes, a p++ Si back gate and 300 nm SiO2 as the gate dielectric by electron beam lithography.

According to the transfer curves (Supplementary Fig. 5), a positive shift of threshold voltage (_V_th) after p-MSB growth on WSe2 indicates p- type doping effect of the p-MSB molecules.30,31

The field effect mobilities are calculated up to 72, 57, and 10−5 cm2/V s for p-MSB-1/WSe2, WSe2, and p-MSB, respectively, indicating high quality of the as-grown samples and the fact that

the charge transport mainly takes place in 2D WSe2 instead of the p-MSB crystal. The devices were exposed under 365 or 405 nm UV light irradiation. The output curves (Fig. 2a, Supplementary

Fig. 6) of p-MSB-1/WSe2 exhibit remarkably increased current (_I__λ_) with light intensity increasing. The photocurrent (_I_ph = _I__λ_ − _I_dark) shows a nearly linear dependence with the

light intensity (Fig. 2b, Supplementary Fig. 7), indicating that there are low density of trap states derived from defects of WSe2 and p-MSB.34,35 Compared with p-MSB-3/WSe2 (Supplementary

Fig. 2), WSe2 (Supplementary Fig. 3) and p-MSB (Supplementary Fig. 4), p-MSB-1/WSe2 has much higher responsivity under the UV light, which can be evaluated by _R_ and _D*_. The _R_ is

calculated by _R_ = _I_ph/_SP_λ,36 where _S_ is the effective area and _P__λ_ is the light intensity. The _D*_ signifies the smallest detectable signal, which is calculated by _D*_ = 

_R_(_S_/2e_I_dark)1/2. At a 2 V bias (Fig. 2c, d, Supplementary Fig. 8), _R_ and _D*_ of p-MSB-1/WSe2 are 1135 A/W and 1014 Jones under 365 nm light (10.7 μW/cm2), and 987 A/W and 1013 Jones

under 405 nm light (22.3 μW/cm2), respectively. At a bias of 10 V (Fig. 2e, f), _R_ and _D*_ of p-MSB-1/WSe2 increase to 1.74 × 104 A/W and 1013 Jones under 365 nm light (3.6 μW/cm2), and

3.27 × 103 A/W and 1013 Jones under 405 nm light (31.6 μW/cm2). Under the same measurement condition, these values are about 1–2 or 3 orders of magnitude higher than that of pristine WSe2 or

p-MSB (Fig. 2c–f), respectively, and are about seven times higher than that of p-MSB-3/WSe2, indicating the importance of the heterostructure and its morphology in achieving high

photodetecting performance. The high _R_ and _D*_ of p-MSB-1/WSe2 are attributed to the photo-gating modulation effect at the heterostructure (Fig. 3a, b). The highest occupied molecular

orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of p-MSB are 5.6 and 2.7 eV, respectively.37 The valence band maximum (VBM) and conduction band minimum (CBM) of monolayer WSe2

are 5.10 and 3.53 eV, respectively.38 Under illumination, p-MSB serves as a light absorber; most photoexcited carriers are generated in p-MSB due to the strong absorption of UV light

(Supplementary Fig. 9). A built-in electric field across the junction separates the photoexcited electrons and holes. The HOMO of p-MSB is lower than the VBM of WSe2, which allows the

photoexcited holes transfer from p-MSB to WSe2 and then transport in WSe2 under a bias voltage. Owing to the high mobility, the WSe2 serves as the main conduction channel. At the same time,

a 0.83 eV electron-rejected barrier exists between LUMO of p-MSB and CBM of WSe2, thus the photoexcited electrons in WSe2 are trapped at the interface, resulting in unidirectional carrier

injection from p-MSB to WSe2.39 The trapped electrons modulate the conduction of the WSe2, introducing additional increase of photoresponsivity compared with normal photo-gating effect. As a

result of the photo-gating modulation, p-MSB-1/WSe2 has higher _R_ and _D*_ compared with pristine p-MSB and WSe2. The photo-gating effect usually leads to the shift of _V_th under

illumination.4 Fig. 3c shows that the _V_th shifts positively with the increasing incident power from 2 to 573 μW/cm2, which is similar with other photo-gating TMDs-based

photodetectors,4,40,41 proving the photo-gating modulation in p-MSB-1/WSe2. Moreover, the morphology of the interface is of great importance in this process. The high quality and large

interface area of 2D epitaxial p-MSB-1/WSe2 heterostructure prepared by PVD allows efficient charge transfer and trapping in photo-gating modulation, and results in higher photodetecting

performance compared with p-MSB-3/WSe2 (Fig. 2c, d, Supplementary Fig. 2). LARGE PHOTOELECTRIC-GATING EFFECT OF P-MSB/WSE2 The photodetecting performance of p-MSB-1/WSe2 can be further

improved by applying an electric gate (_V_g). The ratio of current at light/dark as a function of gate bias is shown in Supplementary Fig. 10. The transfer curves, _R_ vs. _V_g and _D*_ vs.

_V_g under different incident power density, are shown in Fig. 3c–e, respectively. Under 2 μW/cm2 365 nm light, the _R_ (_V_sd = 2 V) is 1.4 × 105 A/W when _V_g = 0 V, and it increases up to

3.6 × 106 A/W by more than 5 orders when _V_g varies from 20 V to −60 V. This value is 7 orders of magnitude higher than other reported organic-TMDs heterostructures.22,23 _D*_ (_V_sd = 2 

V) reaches 2.5 × 1014 Jones with the incident power density of 2 μW/cm2. The value increases to 8.6 × 1014 Jones when _V_g varies from 20 V to −60 V. Therefore, compared with the pristine

TMDs or TMDs-based heterostructures (Fig. 3f),42,43 although the _R_ and _D*_ of p-MSB-1/WSe2 are comparable or lower under photo-gating, the values dramatically increase by orders of

magnitude to a level among one of the highest values for TMDs-based heterostructure, to the best of our knowledge, when a negative _V_g is applied. Dramatic increase of photoresponse under a

negative _V_g is attributed to a new type of photo-gating effect, called large photoelectric-gating effect. Unlike normal photo-gating or photoelectric-gating modulation, the p-MSB-1/WSe2

has interfacial energy barrier between LUMO and CBM in band alignment, which allows unidirectional carrier injection. When a negative _V_g is applied, the Femi level of WSe2 shifts

downwards, leading to increase of the interfacial energy barrier. Thus, more and more photoexcited electrons are trapped at the interface, introducing dramatic increase of the photo-gating

modulation. Therefore, as a result of the tunable energy barrier in p-MSB-1/WSe2, the interfacial charge trapping processes are effectively modulated by the synergy of photo-gating and

electric-gating effects, leading to increase of the _R_ and _D*_ by several orders. This mechanism can be proved by the _V_g dependence of the response speed. The dynamic photoresponse of

normalized photocurrent at different _V_g is shown in Fig. 3g. The rise and decay time of photocurrent are 120 and 80 ms when _V_g = 0 V, about 1–2 orders of magnitude faster than that (rise

time: 2 s, decay time: 25 s) when _V_g = −60 V, which indicates that the electrical gating can effectively modulate the charge trapping process, and more electrons are accumulated when a

negative _V_g is applied.44,45 ELECTRIC-GATING SWITCHABLE PHOTODETECTORS As a result of electric-gating modulated charge trapping process (Fig. 4a, b), the photocurrent response of

p-MSB-1/WSe2 can be switched “ON” and “OFF”, or turned up by applying different _V_g. As shown in Fig. 4c, when alternating dark and light illumination (10.7 μW/cm2, 365 nm), the

p-MSB-1/WSe2 photodetector exhibits a good reversible detecting behavior. At _V_g = 0 V, the photocurrent response (at _V_sd = 2 V) remains a low value down to 5.5 nA, corresponding to the

“OFF” state. When applying a negative _V_g, the large photoelectric-gating effect increases the photocurrent response by about 1–3 orders to 200 nA (_V_g = −20 V), 700 nA (_V_g = −40 V), 2.5

 μA (_V_g = −60 V), corresponding to the “ON” state. In the “ON” state, the photocurrent response increases with increasing negative _V_g, implying that the photodetector can be turned up by

applying a larger negative _V_g. DISCUSSION In this article, we find that sufficient supply of gas-phase molecules in PVD is pivotal to obtain high-quality 2D epitaxial van-der-Waals

interface of p-MSB/WSe2, which is of great importance for achieving high-performance photodetectors. This finding extends the understanding of controllable growth of organic crystals on

TMDs, and will be valuable for the practical applications of organic/TMDs. More importantly, p-MSB/WSe2 has an electric-gating tunable barrier at the interface, which allows effective

modulation of the interfacial charge trapping process by _V_g. Thus, as a result of the synergy of photo-gating and electric-gating effects, the performance can be dramatically improved by

large photoelectric-gating modulation. At a _V_g of −60 V, _R_ and _D*_ of p-MSB/WSe2 increased to 3.6 × 106 A/W and 8.6 × 1014 Jones by 25- and 3-folds, which are among the highest values

for TMDs-based heterostructures, orders of magnitude higher than that of van-der-Waals heterostructures without interfacial electric-gating tunable barrier, i.e. graphene/WSe2/graphene (0.1 

A/W),26 WSe2/MoS2 p–n junction (0.12 A/W),27 etc. Besides p-MSB/WSe2, this new strategy also has potential for application in heterostructures made of other materials, opening up promising

avenues for designing photodetectors with higher performance. Moreover, we demonstrate an electric-gating switchable photodetector based on the large photoelectric-gating effect, which can

be switched “ON” and “OFF” or be turned up by _V_g, indicating the great potential of this strategy in developing new types of photoelectrical devices or applications. METHODS EPITAXIAL

GROWTH OF P-MSB ON WSE2 Se powder (400 mg, 99.5%, Sigma-Aldrich) and WO3 powder (40 mg, 99.9%, Sigma-Aldrich) were used as precursors for CVD growth of WSe2 crystals. Five milligrams p-MSB

(>98.0%) powder was placed in a quartz boat, locating in the center of the furnace. The p-MSB was purified at 190 oC for 1 h before epitaxial growth. WSe2 crystals were placed downstream

from the p-MSB source. The distance between WSe2 and p-MSB source ranged from 12 to 14 cm. The furnace was heated to 180 °C, and then epitaxial growth took place at a low pressure of 3.1 × 

10−1 Torr (Ar) for 15–25 min. After growth, the furnace was cooled to room temperature in Ar. CHARACTERIZATION The samples were measured by an optical microscope (Olympus), AFM (Multimode 8,

Bruker, noncontact mode), Raman (XploRA, HORIBA JobinYvon, laser: 532 nm), and a ultraviolet and visible spectrophotometer (Lambda 750, Perkin-Elmer). DEVICE FABRICATION AND MEASUREMENT The

drain and source electrodes (5 nm/50 nm Cr/Au) were patterned on the sample by e-beam lithography (FEI, NOVA NANOSEM450) and thermal evaporator (Kurt. J. Lesker, Nano 36). To obtain a

better contact between the sample and the Au/Cr electrodes, the devices were annealed in vacuum at 250 oC for 90 min. Electrical measurements were conducted in ambient using a semiconductor

analyzer (Keysight, B1500A) and a probe station (Everbeing, PE-4). DATA AVAILABILITY Data that support the findings of this study are available from the corresponding authors upon reasonable

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This work was supported by National Program for Thousand Young Talents of China, the National Natural Science Foundation of China (51773041, 21544001, 21603038), Shanghai Committee of

Science and Technology in China (Grant No. 18ZR1404900), State Key Laboratory of Molecular Engineering of Polymers and Fudan University. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * State

Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai, 200433, China Zhi Cai, Min Cao, Zhepeng Jin, Kongyang Yi, Xiaosong Chen

 & Dacheng Wei Authors * Zhi Cai View author publications You can also search for this author inPubMed Google Scholar * Min Cao View author publications You can also search for this

author inPubMed Google Scholar * Zhepeng Jin View author publications You can also search for this author inPubMed Google Scholar * Kongyang Yi View author publications You can also search

for this author inPubMed Google Scholar * Xiaosong Chen View author publications You can also search for this author inPubMed Google Scholar * Dacheng Wei View author publications You can

also search for this author inPubMed Google Scholar CONTRIBUTIONS D.W. designed research and supervised the project. Z.C. prepared the sample and did AFM, Raman, UV measurement. Z.C. and

M.C. did device measurement. J.Z. and Y.Y. prepared the WSe2. X.C. did thermal deposition. D.W. and Z.C. prepared the manuscript. All authors commented on the manuscript. CORRESPONDING

AUTHOR Correspondence to Dacheng Wei. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER'S NOTE: Springer Nature

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Large photoelectric-gating effect of two-dimensional van-der-Waals organic/tungsten diselenide heterointerface. _npj 2D Mater Appl_ 2, 21 (2018). https://doi.org/10.1038/s41699-018-0066-2

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