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Surfaces, Interfaces, and Applications

Charge Transfer Induced Photoluminescence Properties of WSe Monolayer-Bilayer Homojunction 2

Zhili Jia, Jia Shi, Qiuyu Shang, Wenna Du, Xinyan Shan, Binghui Ge, Jing Li, Xinyu Sui, Yangguang Zhong, Qi Wang, Lihong Bao, Qing Zhang, and Xinfeng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06017 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Materials & Interfaces

Charge Transfer Induced Photoluminescence Properties of WSe2 Monolayer-Bilayer Homojunction Zhili Jia,†,# Jia Shi,†,‡,# Qiuyu Shang,₤ Wenna Du,† Xinyan Shan,§ Binghui Ge,§,¶ Jing Li,|| Xinyu Sui,†,‡ Yangguang Zhong,† Qi Wang,† Lihong Bao,§ Qing Zhang,₤ and Xinfeng Liu†*

†CAS

Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center

for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ‡University

of Chinese Academy of Sciences 19 A Yuquan Rd, Shijingshan District, Beijing, P.

R. China 100049 ₤Department

of Materials Science and Engineering, College of Engineering, Peking

University, Beijing 100871, P. R. China §Beijing

National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, China ¶Institutes

of Physical Science and Information Technology, Anhui University, Hefei 230601,

China ||Key

Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

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ABSTRACT

Charge transfer process in transition metal dichalcogenides (TMDCs) lateral homojunction affects the electron-hole recombination process of in optoelectronic devices. However, the optical properties of homojunction reflecting charge transfer process has not been observed and studied. In this work, we have investigated the charge transfer induced emission properties based on monolayer (1L)-bilayer (2L) WSe2 lateral homojunction with dozens of nanometer monolayer region. On one hand, the photoluminescence (PL) emission of bilayer WSe2 from homojunction area blue shifts ~23 meV and ~31 meV for direct and indirect bandgap emission, compared with bare WSe2 bilayer region. The blue shift of emission spectrum in bilayer WSe2 is ascribed to the decrease of binding energy induced by charge transfer from monolayer to bilayer. On the other hand, the energy shift shows a tendency to increase as the temperature decreases. The energy blue shift is ~57 meV for direct bandgap emission at 80 K, which is larger than that (~23 meV) at room temperature. The larger energy blue shift at low temperature is derived from larger driving force under larger band offset. Our observations of the unique optical properties induced by efficient charge transfer are very helpful for exploring novel TMDCs based optoelectronic devices.

KEYWORDS: 2D materials, WSe2, homojunction, charge transfer, TMDCs

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INTRODUCTION Two-dimensional semiconductor materials such as TMDCs are served as promising and essential building blocks for optoelectronic device applications owing to their unique optical and electrical properties.1-4 The bandgap of TMDCs is mainly in the range of 1-2 eV and the corresponding emission wavelength is in the range of visible and near-infrared wavelengths.57

Combination of two TMDCs with different bandgaps, by means of vertically stacking or

laterally connecting, can construct TMDCs heterojunctions to realize the bandgap engineering.8-11 The formation of interlayer excitons with long valley lifetime10,

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and

ultrafast charge transfer8 make TMDCs heterojunctions good candidates for photovoltaics13 and photodetectors.14 However, the problem of lattice mismatch between two materials in TMDCs heterojunctions is an inherent challenge in synthesis process and it also influences the electronic features of heterojunction.15 The bandgap energy depends on the layer number, and the monolayer TMDCs is a direct bandgap semiconductor compared to bulk material with indirect bandgap.6,

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Changing layer number of TMDCs can construct lateral

homojunction easily and it belongs to type I band alignment.17-18 Layer-induced homojunctions can be prepared by chemical vapour deposition (CVD) method,19 which have sharp interface without lattice mismatch paving the road for novel optoelectronic devices. Moreover, the band offset, carrier confinement, and electron-hole recombination efficiency can also be modulated conveniently by changing the layer number. Therefore, these superior

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properties of homojunctions attract more and more attentions of researchers.20 TMDCs type I homojunction is an ideal structure for investigating current rectifying characteristics,21 conductivity at the interface,22 photocurrent,17, 23 and photodetection,24-25 etc. As charge transfer is an important process and underlies optoelectronic functions in TMDCs-based optoelectronic devices,8,

13, 26-27

how to detect electron transfer process by

monitoring the emission characteristics of TMDCs homojunctions is pivotal for optoelectronic applications. Previous studies have reported band alignment and confinement effect of WSe2 1L-2L homojunction.18,

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However, up to now, no fascinating optical

properties related to charge transfer process for WSe2 1L-2L homojunction have been observed and relevant research is urgently needed. Therefore, study on charge transfer induced emission properties is vitally important for applications of optoelectronic devices. In this study, based on WSe2 1L-2L lateral homojunction synthesized by CVD method, we investigated the charge transfer induced emission properties at the 1L-2L interface. The lateral size of monolayer region at the edge of WSe2 flake is about tens of nanometers, and the combination of monolayer and bilayer forms 1L-2L homojunction. We found that the emission peaks of bilayer at the junction region showed obvious blue shift compared to the bare bilayer region. The blue shift of emission spectrum is derived from the charge transfer induced decrease of exciton binding energy of bilayer WSe2. The energy shift of direct bandgap emission can be tuned under various temperatures. At 298 K, the energy shift is ~23 meV, while it reaches ~57 meV at 80 K. The temperature dependent energy shift originates

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from different charge transfer efficiency due to the temperature induced conduction band offset. The investigation for homojunction in our work reveals the charge transfer induced emission properties and is helpful for designing the optical devices based on TMDCs homojunctions. RESULTS AND DISCUSSION Figure 1a shows a schematic diagram of WSe2 flake of hexagonal shape on Si/SiO2/Al2O3 substrate, where the Al2O3 is deposited on SiO2 substrate by atomic layer deposition (ALD). The right side of Figure 1a are the zoomed-in pictures of side and top view of cross section of WSe2 1L-2L homojunction, respectively. In the zoomed-in images, the ultra-narrow monolayer WSe2 with size of several tens of nanometers only exists next to the short edges of bilayer.

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Figure 1. Schematic diagram of emission energy shift mechanism and transmission electron microscopy (TEM) characterizations of WSe2 flake. (a) The schematic diagram of the atomic structure of WSe2 flake, where the thickness of SiO2 and Al2O3 are 300 nm and 5 nm, respectively. WSe2 flake is placed on the top of Al2O3 layer. Inset shows the side view (top panel) and zoomed-in image (bottom panel) for the area marked by rectangular dashed line frame. (b) Typical low magnification TEM image for a corner of WSe2 flake. (c) High magnification TEM image for the area marked by rectangular dashed line frame in (b). (d) High resolution dark field STEM image for monolayer WSe2 marked in (c). The illustration in (d) is schematic diagram of WSe2 atomic structure. The in-plane lattice constant is 3.27 Å. (e) The schematic diagram of the mechanism for emission energy shift induced by charge transfer from WSe2 monolayer to bilayer. CBM and VBM represent conduction band minimum and valence band maximum, respectively. Eb stands for binding energy of exciton. The scale bars for (b), (c) and (d) are 500 nm, 20 nm and 1 nm, respectively. To explore the structural details of WSe2 flakes by TEM, we transferred the WSe2 flakes onto a TEM grid via the polystyrene (PS) mediated delamination process.28 Figure 1b shows a typical low magnification dark filed TEM image of WSe2 flake. As can be seen from Figure 1b and 1c, a narrow darker color region of tens of nanometers in lateral size exists at the short edge of the WSe2 flake. From the atomic force microscope (AFM) image in Figure S1a, the 6


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thickness of the WSe2 flake is about 1.6 nm, which corresponds to the thickness of bilayer WSe2. Combining with the TEM images, we can see that the regions of dark color and light color are monolayer and bilayer WSe2, respectively. Therefore, 1L-2L homojunction is formed at the short edges of the WSe2 flake. Moreover, 1L-2L homojunctions only exist at short edges rather than long edges of WSe2 flake (see Supporting Information, Figure S1b). Figure 1d presents a high-resolution scanning transmission electron microscopy (STEM) image for ultra-narrow monolayer WSe2 flake (STEM images of bilayer WSe2 are shown in

Supporting Information, Figure S1c and S1d). In the STEM image, the uniform white spots are the W atoms and the darker ones represent the Se atoms. The schematic diagram of WSe2 atom structure on the STEM image is used for guiding eyes. At the short edges, second layer with ~60° sawtooth edges was grown on the top of the first monolayer, and 1L-2L homojunctions were formed, as clearly shown in Figure 1b and 1c. It has been reported that the S-zigzag edges were not stable with higher formation energy and preferred to be decomposed into W-zigzag edges, leading the formation of 60° sawtooth edges.29 Moreover, the PL intensity for W-zigzag edges was larger than that of S-zigzag edges. Therefore, we infer that the sawtooth edges for WSe2 flake are Se-zigzag edges due to the ~60° sawtooth edges. At the short edge of WSe2 flake, type I homojunction is formed and carriers (electrons and holes) transfer from monolayer to bilayer at the interface.20 The mechanism associated with charge transfer process between monolayer and bilayer WSe2 is shown in Figure 1e. In

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previous report, band bending near the 1L-2L interface in type I band profile has been directly observed by scanning tunneling spectroscopy.18 For the charge transfer process, the electrons transfer from conduction band minimum (CBM) of the monolayer WSe2 to CBM of the bilayer. Meanwhile, the holes transfer from valence band maximum (VBM) of the monolayer WSe2 to VBM of the bilayer. The band offset value of CBM is larger than that of VBM for 1L-2L WSe2 homojunction.18 The values of band offsets of CBM and VBM are 1.06 eV and 0.27 eV, respectively.20 Under the larger band offset of CBM, electron transfer from monolayer to bilayer is more efficient than hole transfer. The injection of the electrons in bilayer induces the reduction of phase space due to Pauli blocking and further decreases the exciton binding energy.30 The exciton energy (Eex) can be calculated by the equation Eex = Eg-

Eb, where Eg and Eb are the bandgap energy and the exciton binding energy, respectively. The electron transfer leads to the decrease of exciton binding energy in bilayer WSe2, which results in the blue shift of exciton emission energy. In order to further study the modulation of emission properties for 1L-2L homojunction induced by charge transfer, PL mapping has been actualized, which visually displays the PL intensity and emission peak position distribution associated with the structure of WSe2 flake. Here, the excitation wavelength is 532 nm and the Al2O3 layer under WSe2 flakes helps to enhance the PL intensity for homojunction region (Supporting Information, Figure S2 and S3). As shown in Figure 2a, the PL intensity at the short edges is obviously stronger than that of other regions, which mainly comes from the emission of monolayer region. The PL spectra

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peak position mapping of WSe2 in Figure 2b clearly reveals the distinct contrast of peak positions. Obviously, the peak position at short edges exhibits relative blue shift compared with that of bilayer regions, which can be attributed to shorter wavelength emission of monolayer compared to bilayer WSe2.6, 31 STEM images from different domains (Supporting Information, Figure S1c and S1d) show that there are no obvious defects. Moreover, from the dark field TEM images in Figure 1b, no wrinkles are observed in the sample. Therefore, the different PL peak positions from different domains for the bilayer WSe2 do not come from defects32 or strain29. Distinct contrast of peak positions between two domains is most likely to come from different stacking configurations for bilayer WSe2 in the growth process. It is also reported that twist angle in MoS2 bilayer can affect the PL spectra.33 The bright spot in 2L WSe2 in Figure 2a comes from emission of IL WSe2 because the peak position in Figure 2b for this region is consistent with that of the 1L at short edge (homojunction). This 1L region is formed in 2L during the CVD growth process.

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Figure 2. PL mapping, PL spectra and power dependence PL spectra of WSe2 flakes at room temperature. (a) The PL intensity mapping of a typical WSe2 flake. (b) The PL peak position mapping of WSe2 flake. The thickness of Al2O3 is 5 nm. The wavelength range for PL intensity mapping and PL peak position mapping is 700 nm -780 nm under the 532 nm excitation. The scale bar for (a-b) is 2 m. (c) The multi-peak fittings of PL spectra from different positions (P1 and P2, as marked in the inset optical image). The thickness of Al2O3 is 10 nm. The scale bar for the inset optical image is 2 m. P1 is the position of homojunction. The excitation wavelength is 514 nm. Peak X, D1 (D2) and I1 (I2) represent neutral exciton, direct and indirect optical transition related excitons, respectively. (d)-(e) Plot of PL intensity as a function of the excitation power for peak X, D1 (D2) and I1 (I2). The symbols represent experimental data extracted from the PL spectra in Figure S4 by multi-peak fitting. The solid lines are the fitting results by using power law I ∝ Pk, where I, P and k are the PL peak intensity, excitation power and exponent factor, respectively. The values inset are the corresponding exponent factors. Different emission components of PL spectra of WSe2 flake can be analysed by multipeak fitting for further investigating the emission characteristics. Figure 2c shows the representative spectra extracted from two different positions (P1 and P2) and multi-peak fitting has been applied to analyse the different emission components of PL spectra. As can be seen from the fitted components, PL spectrum of 1L-2L homojunction region (P1) consists of 10


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three peaks which are named as peak X (neutral exciton), D1 (direct bandgap emission) and I1 (indirect bandgap emission), respectively. The PL spectrum of bilayer WSe2 region (P2) is composed of two peaks namely D2 (direct bandgap emission) and I2 (indirect bandgap emission) peaks. The energy for peak X, D1 and I1 in P1 spectrum is about ~1.72 eV, ~1.69 eV and ~1.62 eV, respectively. Whereas, the D2 and I2 peaks in the spectrum of P2 locate at ~1.67 eV and ~1.59 eV, respectively. The peak X mainly originates from the direct bandgap transition of monolayer WSe2, D1 and D2 peaks belong to the direct bandgap transition of bilayer, while the I1 and I2 peaks correspond to the indirect bandgap transition of bilayer. Obviously, blue shift occurs from peak D2 to D1 as well as from peak I2 to I1 and the energy shifts are ~23 meV and ~31 meV, respectively. The blue shifts of both direct and indirect PL peaks of bilayer WSe2 intuitively reflect the decrease of exciton binding energy of bilayer WSe2, which is caused by the injected electrons from the monolayer after the formation of type I homojunction.30 To further determine the accuracy of peaks source attribution in PL spectra, we carried out power-dependent PL spectra for deeper analysis. A plot of peak intensity versus excitation power from P1 and P2 are shown in Figure 2d and 2e, respectively, and the intensity of each peak is extracted from corresponding spectra (Supporting Information, Figure S4) by multi-peak fitting. The power law I ∝ Pk is used to fit the experimental data, where I, P and k are the PL peak intensity, excitation power and exponent factor, respectively. From the fitting results, we obtained the k values of peaks X, D1 and I1 for P1

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are 0.99, 1.00 and 1.00, respectively, and all the intensity present a linear dependence on the excitation power. Peak X comes from the emission of neutral excitons of monolayer WSe234 and peaks D1 and I1 are derived from the direct and indirect bandgap emission of bilayer WSe2, respectively. However, the k of peaks D2 and I2 for P2 are 0.78 and 0.83, which shows sublinear characteristics. It has been reported that the power-dependent emission of bilayer WS2 also shows sublinear (~0.9) characteristics.35 The sublinear power-dependent emission may come from the free-to-bound transitions.36 The recombination of electrons and holes in bilayer at homojunction becomes efficient because of charge transfer from monolayer to bilayer and confinement in it. Therefore, compared to D2 and I2, the peaks D1 and I1 exhibit linear power dependence. In addition, we note that the emission peak energy of our sample is significantly higher than that of the previous report for monolayer,6,

31, 34, 37-45

whereas the corresponding

emission peak energy is slightly blue shift for bilayer compared to other works.6, 31, 46-47 The higher emission peak energy may come from the growth parameters of the WSe2 flakes, such as growth temperature.42 Moreover, it is worth noting that the difference between neutral exciton of monolayer and direct bandgap emission of bilayer is larger than reported sample synthesized by CVD method.31 The notable emission blue shift of monolayer is attributed to the lateral confinement effect.48 As is known to all, the electronic structure can be tuned by changing the lateral size of monolayer and the exciton energy shows a blue shift with the decrease of lateral size of WSe2 monolayer.48 Here, the lateral size of monolayer in

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homojunction is larger than the exciton Bohr radius of WSe2 (about 1 nm)49 but exhibits a relatively ultra-narrow size with 30~40 nm compared to the large area monolayer obtained by CVD method. As a result, notable blue shift of peak X for monolayer WSe2 is attributed to the lateral confinement effect.48 The lateral size-dependent exciton energy of monolayer WSe2 can be described by the following equation:48 ħ2ρ20

Eex = E0 + 2M

exW

2

(1)

Where E0 is the exciton energy without lateral confinement, ħ is the reduced Planck’s constant, ρ0 is the first root of the zero-order Bessel function, Mex is effective mass of exciton, and W is the effective width (lateral size) of monolayer. The 1/W2 dependence of the exciton energy shift is the lateral confinement effects for WSe2 monolayer. Therefore, the exciton energy increases with the decrease of lateral size of monolayer WSe2. In addition to PL spectra, we also investigate Raman spectra of WSe2 flake. There is no obvious difference between the Raman spectra of 1L-2L homojunction and bare 2L WSe2 (Supporting

Information, Figure S5). In order to gain further insight into the properties of each exciton state, the temperaturedependent PL spectra from different positions (P1 and P2) of WSe2 flake have been investigated subsequently. The contour plots of PL spectra as a function of temperature from P1 and P2 positions of WSe2 flake are shown in Figure 3a and 3b, respectively, in which the dynamics of emission peaks are intuitionistic. The corresponding temperature dependent dynamic of each emission peaks extracted from Figure 3a and 3b are shown in Figure 3c.

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Form that, it is found that the peak energy of the peak X decreases markedly with increase of the temperature, which is consistent with previous reports for neutral exciton in monolayer WSe2.34 The temperature dependence of a semiconductor bandgap Eg(T) can be described by the following equation:50

[ ( ) - 1]

Eg(T) = Eg(0) - S〈ħω〉 coth

〈ħω〉

2KB T

(2)

Where Eg(0) is the bandgap at T = 0 K, S is a dimensionless electron-phonon coupling constant, and 〈ℏ𝜔〉 is an average acoustic phonon energy. From this equation, we can see that the bandgap becomes larger as the temperature decreases, and the energy of peak X from monolayer

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Figure 3. Temperature-dependent PL spectra of WSe2 flake. (a-b) The contour plots of PL spectra as a function of temperature (from 80 K to 298 K) from different positions of WSe2 flake. P1 and P2 correspond to the positions located at 1L-2L homojunction and bilayer region inside of the flake, respectively. The excitation wavelength is 532 nm. (c) The peak energy distributions of X, D1, I1, D2 and I2 with increasing temperature ranging from 80 to 298 K. The symbols represent experimental data extracted from the PL spectra used in (a) and (b) by multi-peak fitting. (d) The PL spectra and corresponding multi-peak fittings for spectra of P1 and P2 positions at 80 K. follows the law of this equation. However, for the peak D2, I1, and I2, the energy increases with the increase of temperature (80 K-200 K). This temperature dependence may come from

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both the lattice thermal expansion and electron-phonon interactions for bilayer TMDCs.51 As for peak D1, the peak energy does not change much with the increase of temperature. The difference trends for D1 and D2 indicates that the electrons injected into bilayer may change electron-phonon interactions and further influences the temperature-dependent exciton energy. Furthermore, the peak energy of D1 (I1) is higher than that of the peak D2 (I2) at each temperature, which indicates that charge transfer process still occurs. The energy difference (energy shift) of peaks D1 and D2 becomes larger with the decrease of temperature. For the spectra (P1) at temperature 80 K shown in Figure 3d, peaks at ~1.76 eV, ~1.69 eV and ~1.57 eV are recognized as X, D1 and I1, respectively. For the spectra from P2, the peak energies are ~1.63 eV and ~1.56 eV for D2 and I2, respectively. The binding energy of trion is around 30 meV for WSe2,41,

45, 52

and the energy of trion will show red shift under electron inject.

Therefore, ~1.69 eV is not from the emission of trion. The energy shift is about 57 meV for direct bandgap emission, from D2 to D1, which is larger than that at room temperature. The larger blue shift originates from effective electron transfer under larger driving force (-ΔG). The driving force for electron transfer is determined by the energy difference between conduction band energies of monolayer and bilayer.53 From the temperature-dependent PL data (peak energy distributions) in Figure 3e, the energy of peak X increases and the energy of peak D2 decreases with the temperature decreases. Due to the type I band alignment for 1L-2L homojunction, the increase of bandgap for monolayer and the decrease of direct bandgap for bilayer enlarge the band offset of CBM between monolayer and bilayer. Under the larger band offset, the driving force for charge transfer is enhanced. It has been reported 16


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that the size of quantum dots (QDs) influence the bandgap of the QDs and the charge transfer process.53-54 The charge transfer rate from CdSe QDs to TiO2 nanoparticles increases by nearly 3 orders of magnitude with the decrease of the QD’s size.53 Moreover, the charge transfer yields increase with the increase of the band offset in the polymer-QD system.54 Both the increasing of charge transfer rate and charge transfer yields come from the larger band offset. For the 1L-2L WSe2 homojunction, the electrons transfer from monolayer to bilayer is more efficient because of larger driving force under larger band offset at low temperature. The injection of electrons with high yields in bilayer WSe2 induces the decrease of exciton binding energy for bilayer WSe2. As a result, the emission energy of bilayer WSe2 shows larger blue shift in the PL spectra at low temperature. The difference electrostatic properties at different region of WSe2 flake related to the charge transfer process and corresponding emission properties at homojunction can be investigate by electric force microscope (EFM). Figure 4a exhibits a typical optical image of WSe2 flake where the white dashed lines represent for the edge of bilayer WSe2 and 1L-2L homojunction. The corresponding morphology and EFM phase images of WSe2 flake are shown in Figure 4b and 4c, respectively. Two-pass scan method is used for EFM measurement. For the first scan, tapping mode is used and morphology image of WSe2 flake is obtained. While for the second scan, the AFM tip is flake exhibit no apparent differences and the thickness of the flake is about 1.5 nm, which corresponds to the thickness of bilayer WSe2. However, at the region of 1L-2L homojunction, EMF phase image is darker than other

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regions, as shown in Figure 4c. The red and blue curves in Figure 4d are the intensities along the dashed lines shown in Figure 4c. It is found that there is a remarkable dip at the region of 1L-2L homojunction in the top panel of Figure 4d, whereas there is no dip in the bottom panel of Figure 4d. In dark colour region, attractive force between the AFM tip and monolayer is presented, which reduces the cantilever resonant frequency and causes a negative phase shift in the phase image. As the AFM tip is positively charged after applying bias voltage, it is certain that higher n-doping level in monolayer region is formed compared to bilayer. Eventually, when the 1L-2L homojunction is excited by laser, electrons and holes separate and transfer from monolayer to bilayer due to the type I band alignment. Efficient charge transfer from monolayer to bilayer at small homojunction reduces the ratio of trion emission,55 which results in the efficient emission of monolayer at homojunction. The PL intensity of monolayer WSe2 synthesized by CVD method is about twice that of the bilayer.42 In our sample, the area of a monolayer irradiated by laser is about one tenth of that of a bilayer at homojunction (Supporting Information, Figure S6). However, from the multi-peak fittings of PL spectra from P1 and P2 (Figure 2c) ， the intensity of peak X is about twice that of bilayer. Therefore, the monolayer at homojunction has high emission efficiency.

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Figure 4. Optical, AFM and EFM images of WSe2 flake. (a) Optical image of a typical WSe2 flake. The dashed lines show the boundary of WSe2 flake. (b) AFM morphology image of the WSe2 flake. The inset curve is the corresponding height profile of the flake. (c) EFM phase image of the WSe2 flake. The dark color and bright color represent the attraction and repulsion of the WSe2 flake to the AFM tip, respectively. The white dashed line shows the boundary of the WSe2 flake. (d) EFM phase along the red and blue dashed lines marked in (c). The scale bar for (a-c) is 1 m. CONCLUSIONS In conclusion, we investigate the charge transfer process induced emission properties of the 1L-2L WSe2 lateral homojunction synthesized by chemical vapor deposition. The 1L-2L WSe2 lateral homojunction has a monolayer region with several tens of nanometers. Because of the charge transfer from monolayer to bilayer, the doping of electrons for bilayer induces the decrease of binding energy and further induces the blue shift of emission. We observe ~23 meV blue shift for direct bandgap emission and ~31 meV for indirect bandgap emission compared to the WSe2 bilayer itself. Moreover, the energy shift of direct bandgap increases

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with the decrease of temperature, and we obtain ~57 meV energy shift at 80 K. Larger bandgap and conduction band offset for homojunction formed at low temperature cause efficient charge transfer from monolayer to bilayer. The study of homojunction emission properties induced by charge transfer is helpful for optical devices design based on TMDCs homojunctions.

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EXPERIMENTAL SECTION Sample Preparation. In this work, WSe2 flake was synthesized by CVD method on Si/SiO2 substrate (purchased from Nanjing NXNANO Tech. Co., Ltd.). Al2O3 layer (5 nm and 10 nm) was deposited on the Si/SiO2 (300 nm) substrate by ALD. Then, WSe2 flakes were transferred to the substrate by PS mediated delamination process. Morphology and Structure Characterization. The structure of WSe2 flake was characterized using a TEM (JEM ARM200F) under 200 kV. The morphology and electrostatic properties of WSe2 flake were characterized using an AFM/EFM (Nanoscope Ⅲa). Tapping mode was used for morphology image. For the EFM measurement, tip bias is 1.5 V. Optical Characterizations. The PL spectra and Raman spectra characterizations of WSe2 flakes were performed with a Raman spectroscope (Renishaw-1000, 100× objective, NA = 0.9). The excitation wavelength was 514 nm. The integral time was 10 s. The temperaturedependent PL spectra characterizations were carried out using Olympus microscope (BX51, 50× objective, NA = 0.5) combined with Princeton Instrument spectrometer (PI Acton, Spectra Pro 2500i). At different temperature controlled by a cryostat (Cryo Industries, CFM 2822 RS), 532 nm wavelength CW laser was used to excite the WSe2 flake. The emission signal was collected by the same microscope objective and analysed by Princeton Instrument spectrometer. PL mapping of WSe2 flake (WITec Alpha 300 access, 100× objective, NA = 0.9) were performed using 532 nm wavelength CW laser. The integral time for each spectrum was 0.05 s under ~100 W laser power and the step length for image scan was 50 nm.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The optical image, dark field TEM images, and STEM images of WSe2 flake. The optical image and PL spectra of WSe2 flake on TEM grid. The PL mapping and PL spectra of WSe2 flake for different Al2O3 layer thicknesses. The power-dependent PL spectra and corresponding multi-peak fittings of WSe2 flake (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions #These

authors contributed equally.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

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X.F.L thanks the support from the Ministry of Science and Technology (2016YFA0200700 and 2017YFA0205004), National Natural Science Foundation of China (21673054 and 11874130), Beijing Municipal Natural Science Foundation (4182076 and 4184109). Q.Z. also thanks funding support from the Ministry of Science and Technology (2017YFA0205700; 2017YFA0304600), National Natural Science Foundation of China (61774003 and 61521004). This work is also supported by the National Natural Science Foundation of China (61307120, 61704038 and 11474187).

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