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June‐Chul Shin, Jae Hwan Jeong, Junyoung Kwon, Yeon Ho Kim, Bumho Kim, Seung‐Je Woo, Kie Young Woo, Minhyun Cho, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Young Duck Kim, Yong‐Hoon Cho, Tae‐Woo Lee, James Hone, Chul‐Ho Lee, Gwan‐Hyoung Lee

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  This is the peer reviewed version of the following article: J.-C. Shin, J. H. Jeong, J. Kwon, Y. H. Kim, B. Kim, S.-J. Woo, K. Y. Woo, M. Cho, K. Watanabe, T. Taniguchi, Y. D. Kim, Y.-H. Cho, T.-W. Lee, J. Hone, C.-H. Lee, G.-H. Lee, Electrically Confined Electroluminescence of Neutral Excitons in WSe2 Light-Emitting Transistors. Adv. Mater. 2024, 36, 2310498, which has been published in final form at https://doi.org/10.1002/adma.202310498. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Electrically Confined Electroluminescence of Neutral Excitons in WSe<sub>2</sub> Light‐Emitting Transistors](https://mdr.nims.go.jp/datasets/026057c2-3729-4de6-8b87-0de3535c1e8b)

## Fulltext

Electrically Confined Electroluminescence of Neutral Excitons in WSe2 Light-emitting TransistorsJune-Chul Shin, Jae Hwan Jeong, Junyoung Kwon, Yeon Ho Kim, Bumho Kim, Seung-Je Woo, Kie Young Woo, Minhyun Cho, Kenji Watanabe, Takashi Taniguchi, Young Duck Kim, Yong-Hoon Cho, Tae-Woo Lee, James Hone, Chul-Ho Lee, Gwan-Hyoung Lee*Dr. J.-C. Shin, Dr. J.H. Jeong, Dr. S.-J. Woo, Prof. T.-W. Lee, Prof. G.H. LeeDepartment of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of KoreaE-mail: (EDr. J. KwonDepartment of Material Science and Engineering, Yonsei University, Seoul 03722, Republic of KoreaY.H. KimKU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of KoreaDr. B. Kim Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United StatesDr. K.Y. Woo, Prof. Y.-H. ChoDepartment of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of KoreaM. Cho, Prof. Y.D. KimDepartment of Physics and Department of Information Display, Kyung Hee University, Seoul 02447, Republic of KoreaDr. K. WatanabeResearch Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, JapanDr. T. TaniguchiResearch Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, JapanProf. J. HoneDepartment of Mechanical Engineering, Columbia University, New York, NY 10027, United StatesProf. C.-H. LeeDepartment of Electrical and Computer Engineering, Seoul National University, Seoul 08826, Republic of KoreaKeywords: Electrical confinement, neutral exciton, electroluminescence, van der Waals heterostructure, light-emitting transistorAbstract: Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential in optoelectronic applications due to their direct band gap and exceptional quantum yield. However, TMD-based light-emitting devices have shown low external quantum efficiencies as imbalanced free carrier injection often leads to the formation of non-radiative charged excitons, limiting practical applications. Here, we demonstrate electrically confined electroluminescence (EL) of neutral excitons in WSe2 light-emitting transistors (LETs) based on the van der Waals (vdW) heterostructure. The WSe2 channel is locally doped to simultaneously inject electrons and holes to the one-dimensional (1D) region by a local graphene gate. At balanced concentrations of injected electrons and holes, the WSe2 LETs exhibited strong EL with a high external quantum efficiency (EQE) of ~8.2% at room temperature. Our experimental and theoretical results consistently show that the enhanced EQE could be attributed to dominant exciton emission confined at the 1D region while expelling charged excitons from the active area by precise control of external electric fields. Our work shows a promising approach to enhancing the EQE of 2D light-emitting transistors and modulating the recombination of exciton complexes for excitonic devices.1. IntroductionTMDs and their vdW heterostructures have been considered promising platforms for next-generation optoelectronics because the TMDs have shown large binding energies of excitons and strong light-matter interactions with near-unity photoluminescence (PL) quantum yield (QY) in a monolayer limit due to broken-inversion symmetry.[1-7] The exceptional properties of the TMDs have facilitated the development of exotic optoelectronic quantum devices that exhibit unique capabilities, such as chiral light emission, excitonic complexes, and high-temperature exciton condensation, that have not been observed in conventional bulk materials.[8-12] Despite the great potential of the monolayer TMDs, the low EQE of TMD-based light-emitting devices and control of exciton complexes are major challenges to be addressed for practical applications.[13-18] Enhancing the EQE of TMD-based light-emitting devices necessitates the fulfillment of two main prerequisites: First, it is essential to elevate the efficacy of radiative electron-hole (e-h) recombination within the TMDs. Second, achieving efficient and balanced injection of charge carriers into the TMDs is of paramount importance for facilitating effective light emission.[19] However, it is challenging to achieve both conditions simultaneously. Facilitating effective carrier injection into the emission layer requires elevated doping concentrations using methods, such as electrostatic or chemical doping, aiming to decrease the Schottky barrier height.[20] However, these increased doping levels in TMDs lead to the creation of exciton complexes, like charged excitons, predominantly contributing to non-radiative recombination.[6] This compromise limits the feasible doping range as a viable approach for carrier injection, necessitating the exploration of alternative injection mechanisms that enable effective carrier injection without generating excessive charge populations.Here, we demonstrate electrically confined electroluminescence of neutral excitons in WSe2 LETs based on the vdW heterostructure consisting of tungsten diselenide (WSe2, channel), graphene (electrodes and gate), and hexagonal boron nitride (hBN, dielectric). To achieve highly efficient light emission in the gate-tunable LETs, we used WSe2 as an emitting layer due to its ambipolar transport properties, direct bandgap, and high QY at room temperature.[7,21-25] Through the utilization of monolayer graphene as the gate-tunable source and drain electrodes with van der Waals contacts, we effectively mitigated Fermi level pinning and successfully attained appropriate band alignment for ambipolar transport within the WSe2.[26-28] By electrically modulating gate and drain bias, our WSe2 LETs selectively injected charge carriers of electrons and holes into WSe2 from the respective graphene electrodes and effectively regulated the movement of these injected charges by modulating the barrier height at the WSe2 homojunction, resulting in reconfigurable electrical transport properties. By balancing the densities of injected electrons and holes, we achieved strong EL with a high EQE of ~8.2% at room temperature. The EL measurements revealed that neutral excitons dominantly contribute to the strong EL due to the confinement of neutral excitons within the 1D region facilitated by the in-plane electric field created by the local graphene gate and the expulsion of charged excitons from the 1D region by the charge interaction.2. Results and DiscussionFigure 1a illustrates the WSe2 LETs consisting entirely of 2D materials. We used double gates of graphene and Si to electrically confine the neutral excitons in the 1D region of WSe2. To fabricate the devices, exfoliated flakes were stacked layer-by-layer using a dry pick-up transfer method.[29] The hBN-encapsulated monolayer WSe2 FETs with the graphene contacts showed ambipolar transport characteristics (Figure S1), which is in contrast to the monolayer WSe2 FETs on a SiO2/Si substrate with metal electrodes that exhibit only p-type conductance.[30,31] For local gating, the graphene flake was aligned precisely to overlap half of monolayer WSe2. After transferring two monolayer graphene flakes for source and drain electrodes, the whole stack was encapsulated with the hBN because the hBN encapsulation can neutralize the doping level of TMDs and improve their optical and electrical properties.[32-34] Note that the WSe2 used in this work was grown by the flux method, which is known as a method to synthesize high-quality 2D crystals with low defect density (Figure S2).[35] After the transfer, the stack was annealed at 300 in a forming gas of hydrogen and argon for 3 h to enhance interfacial adhesion between the stacked layers.[36] Raman and PL maps of the stacked sample show that the WSe2 is in conformal contact with graphene electrodes (Figure S3). Since the stacked sample of WSe2 and graphene are embedded in the hBN, we used the graphene etch stop technique to form the via contacts to the embedded graphene electrodes, as described in our previous report.[37,38] The stacked sample was patterned by e-beam lithography and exposed to XeF2 gas to etch the top hBN (hBN1), followed by metal deposition. There were no contamination and cracks in the devices, indicating the clean interfaces in the vdW heterostructure (see Figure S4 for the fabrication process). As shown in the energy band diagram of Figure 1c, the Fermi energy levels of two graphene electrodes and the band offset of WSe2 can be separately modulated by double gates of graphene and Si. By controlling the contact barrier heights at graphene-WSe2 interfaces, two graphene electrodes can selectively inject electrons and holes into the WSe2, respectively. Then, injected electrons and holes emit light through e-h recombination in the 1D region of WSe2.The double gate structure in the WSe2 LET is crucial for the simultaneous modulation of injection and transport of both electrons and holes, unlike conventional ambipolar FETs with a single gate configuration that can transport only one type of charged carrier at a specific gate voltage. To validate gate-tunable carrier injection and transport in the WSe2 LETs, we manipulated gate voltages of graphene (VGG) and Si (VBG) gates, as shown in Figure 2. Depending on the applied gate voltages of VGG and VBG, the WSe2 LET can reconfigure the polarity of transfer characteristics between n-type and p-type modes within a single WSe2 channel, as shown in Figure 2a, b. When the Si back gate is biased at 60 V for n-type mode, the Fermi energy level of graphene source contact (Gr (S)) shifts close to the conduction band of WSe2. Simultaneously, the conduction band of WSe2 is bent down, resulting in efficient electron injection from Gr (S) to WSe2, as illustrated in Figure 2c. At VDS = 0.5 V, WSe2 LET exhibits n-type switching behavior with no noticeable hysteresis, consistent with previous reports.[39] This behavior can be attributed to the modulated transport of injected electrons by the energy barrier of WSe2 homojunction through sweeping VGG (see Figure 2c (i) – (ii)). Meanwhile, at a larger drain bias of VDS = 2 V, the transfer curve exhibited ambipolar transport behavior with a higher on-current compared to VDS = 0.5 V. Under positive voltage conditions of VGG, injected electrons from Gr (S) transport across the energy barrier at the WSe2 homojunction (Figure 2c (ii)). At VGG = -6 V, the current increases owing to the recombination of electrons and holes that are simultaneously injected from corresponding graphene contacts (Figure 2c (iii)). Conversely, when a negative gate voltage of VGG = -6 V is applied for p-type operation, holes can be injected from the Gr (D) into the WSe2 layer, leading to p-type transport (Figure S5). These results indicate that our device can efficiently control both the selective injection of charge carriers and the transport of injected carriers through WSe2.Because of modulation of charge injection and local doping, the pn homojunction in the WSe2 shows rectifying behaviors in the output curves of Figure 3a (green dots at VBG = 60 V and VGG = -6 V and blue dots at VBG = -60 V and VGG = 6 V). The increase in forward bias current is attributed to the elevated recombination probability of injected electrons and holes from corresponding graphene electrodes at the 1D region of WSe2. Therefore, when VDS was applied at 3 V at VBG = 60 V and VGG = -6 V, a strong EL peak was observed through the e-h recombination at the 1D region, which is similar to the PL spectrum of WSe2, as shown in Figure 3b. This correspondence indicates that the EL at 1.65 eV originates from the e-h recombination across the direct bandgap of WSe2. As estimated, the EL was clearly observed at the 1D region of WSe2, as shown in the optical microscopic image of Figure 3c. Our photocurrent measurements also show that the photocurrent predominantly generates at the 1D region (Figure S6). This indicates that the excitons generated by a laser can be dissociated into free carriers at the pn junction owing to a built-in electric field.[20,40] In addition, we observed long EL retention over ~104 s, indicating high stability of the WSe2 LETs (Figure S7). To investigate the electrical tunability of the WSe2 LETs, we measured the EL and EQE by varying drain bias and gate voltages in Figure 4. As the VDS increased (current density increases in the output curve in Figure 3a) at fixed gate voltages of VBG = 60 V and VGG = -8 V, the EL intensity increased with no EL peak shift as shown in Figure 4a. The EL intensity monotonically increased with the VDS owing to effective injection of electrons and holes over reduced contact barrier, reaching a high EQE of ~5.3% at VDS = 2.5 V at room temperature (Figure 4b and S8). The EQEs of the WSe2 LETs were extracted using the method described in other reports.[17,20] (see Supplementary Note 1 and Figure S9). At VDS > 2.5 V, the EQE decreased due to non-radiative recombination paths, such as exciton-exciton annihilation at high exciton concentration.[20] To modulate the density of injected electrons, we measured the EL and EQE by varying the VBG at fixed VGG = -8 V and VDS = 2.5 V (Figure 4c and d). In this condition, the density of injected holes is fixed and more electrons are injected into the WSe2 owing to reduced contact barrier by increasing the VBG. As a result, the EL increased with VBG, maintaining the EL peak position at 1.65 eV, as shown in Figure 4c. Similarly, we also observed an increase in the EL by varying VGG at fixed VBG and VDS due to the lowered contact barrier for hole injection (Figure S10). It is worth noting that the monochromic EL peaks in the contour plots of Figure 4a and c were consistently maintained at 1.65 eV, regardless of applied bias and gate voltages. It is because the EL of the WSe2 LETs occurs by e-h recombination through the intrinsic direct bandgap of WSe2. At fixed VGG = -8 V which observed the highest EQE in Figure S10, the EQE of the WSe2 LET extracted by varying the VBG increased as a function of the VBG, reaching a maximum EQE of ~8.2% at room temperature, as shown in Figure 4d. At the larger VBG, the EQE slightly decreased along with the saturation of EL intensity, which indicates that balanced concentrations of electrons and holes are required for high light emission efficiency. We measured the EQEs of several WSe2 LETs, where flux-grown and purchased samples of WSe2 were used. The flux-grown WSe2 showed higher EQEs than the purchased one, which is in agreement with the higher PL efficiency of the flux-grown samples (Figure S11).[35]Figure 5a and b show the PL and EL spectra of the WSe2 LET. All spectra can be deconvoluted into two peaks of neutral exciton (X0) and charged exciton (X*). When the WSe2 LET was partially p- and n-doped by applying VGG = -6 V and VBG = 60 V as shown in the inset of Figure 5a, the PL spectra of these regions exhibited two dominant PL peaks corresponding to X* and X0. Notably, the X* peak is stronger than the X0 peak for both p- and n-doped regions. In contrast, X0 is more dominant than X* in the EL spectrum of Figure 5b. As shown in Figure 5c, the EL intensity ratio of the neutral excitons to charged excitons (X0/X*) is fourteen times higher than PL intensity ratios of X0/X* in p- and n-doped regions. As shown in the schematic device geometry and energy band diagram of Figure 5d, the injected electrons and holes from n- and p-doped regions form exciton complexes at the 1D region. Notably, the in-plane electric field within the 1D region leads to the confinement of neutral excitons[41]. To investigate the effect of in-plane field and doping on the exciton behaviors, we calculated charge densities of electrons and holes and potentials of neutral excitons and charged excitons (X+ and X-) using the electrostatic simulation of the device as shown in Figure 5e. (See Experimental Section/Methods and Supplementary Note 2). The local doping by graphene and Si gates dopes the WSe2 separately into adjacent p- and n-doped regions with large charge densities and in-plane electric field along the edge of graphene gate.[40] The in-plane electric field confines neutral excitons at the 1D nanoscale region of WSe2 with minimum exciton potential. In addition, neutral excitons are pushed toward the 1D region by repulsive interaction that is caused by the charge density gradient.[41] This unique phenomenon is mainly observed in the ultrathin TMDs with large exciton binding energies, enabling excitons to withstand the in-plane electric fields.[42] Furthermore, the charged excitons drift away from the 1D region by potential gradients of charged excitons (bottom of Figure 5e).[43,44] As a result, the EL from neutral excitons is predominant owing to their electrical confinement in the 1D region of WSe2 LETs.3. ConclusionIn conclusion, we demonstrate the efficient light-emitting devices based on monolayer WSe2 that satisfy the prerequisites for high EQE: efficient injection of charge carriers and predominant emission of neutral excitons. Through precise band alignment of WSe2 and graphene and double gate geometry, the WSe2 LETs can achieve efficient charge injection and e-h recombination for strong EL with a high EQE of ~8.2%. Our results show that the strong EL peak of WSe2 LET is attributed to the electrical confinement of neutral excitons due to the in-plane electric field and strong charge interactions. Our work not only highlights the potential of 2D light-emitting devices but also provides a new avenue for modulating the recombination of neutral and charged excitons for excitonic devices.4. Experimental Section/Methods Device fabrication: All the flakes were mechanically exfoliated from a bulk crystal onto a SiO2/Si substrate with a thickness of 285 nm of SiO2. We used two types of WSe2 crystals, one grown by the self-flux method and the other purchased from SPI Supplies. A stack of flakes consisting of top hBN, graphene electrodes, WSe2, and bottom hBN was fabricated using the pick-up transfer technique with a polydimethylsiloxane (PDMS) stamp coated with a polycarbonate (PC) film.[29].\ The stacked heterostructure was transferred onto graphene as a gate on a SiO2 (285 nm)/Si substrate by releasing the PC film from the PDMS at 180 . The PC film was dissolved in chloroform overnight. The stacked heterostructure was annealed at 300 for 3 h in a 10-4 Torr vacuum to enhance adhesion between layers.[36] To form connections between the metal and embedded graphene, we patterned the sample using e-beam lithography (Raith pioneer 2) and then exposed it to XeF2 gas of 2 Torr for 200 s at room temperature using XeF2 etcher (VPE-4F, SAMCO).[37] The metals Cr (1 nm)/Pd (40 nm)/Au (50 nm) were deposited by an e-beam evaporator (Korea Vacuum Tech.) under ultrahigh vacuum conditions of ~10-7 Torr, followed by a lift-off process. Raman and PL measurements: Raman and PL spectra were measured using Raman spectroscopy (LabRAM HR Evolution) with 532 nm laser excitation under ambient conditions. To obtain Raman and photoluminescence mapping images, samples were scanned on an x-y stage with laser illumination.Electrical and optoelectrical measurements: For electrical measurement, a semiconductor parameter analyzer (Keithley 4200A-SCS) was used at room temperature under 10-3 Torr. EL spectra were measured using a customized optical measurement system with a photodetector (Si avalanche), and electrical bias and gate voltage were applied by source meters (Keithley 2400). All EL measurements were performed in a 10-2 Torr vacuum chamber, with light signals collected through ×100 objective lens. The use of vacuums enabled stable measurement of the devices.Electrostatic simulation for electrical confinement of exciton complexes: To obtain quantitative information on the in-plane electric field, charge density distributions, voltage, and excitonic confining potential of exciton and charged excitons in the device, we used commercial finite element analysis software (COMSOL Multiphysics). The simulated device geometry is depicted in Figure 5, including double gates of graphene gate and silicon back gate, 1L graphene electrodes, and hBN with 30nm thickness. The semiconductor behavior of WSe2 was calculated using the drift-diffusion model with the density-gradient theory.[45] The carrier density was determined based on the Fermi-Dirac distribution, considering the band structure, Fermi level, and density of states (DOS) of WSe2. The following material parameters were used in the calculation: bandgap of WSe2 = 1.65 eV, dielectric constant = 21, carrier mobility = 31 cm2/Vs, election affinity = 3.7 eV, effective density of states (DOS) = 6×1024 m-3. The dielectric constants for hBN were 3.76 and 6.93 along the out-of-plane and in-plane directions, respectively.[45-51] The excitonic confining potential was calculated using an equation derived from the previous studies, based on electric field and charge density obtained from calculation and measurement constants.[41] The constants used in this calculation were exciton polarizability (10.027×10-5 meVcm2/kV2), trion polarizability (411.367×10-5 meVcm2/kV2), trion hyperpolarizability (8.48×10-7 meVcm4/kV4), effective exciton-electron coupling strength (~0.7 μeVμm2), and trion-electron coupling strength (1.32 μeVμm2).[52-54].Supporting Information Supporting Information is available from the Wiley Online Library or from the author.AcknowledgmentsJ.-C.S. and J.H.J. contributed equally to this work. This work was supported by LG Display and the National Research Foundation (NRF) of Korea Grant funded by the Korean Government (2021R1A2C3014316, 2017R1A5A1014862 (SRC program: vdWMRC center). G.H.L. acknowledges the support from the Research Institute of Advanced Materials (RIAM), Institute of Engineering Research (IER), Institute of Applied Physics (IAP), and Inter-University Semiconductor Research Center (ISRC) at the Seoul National University. M.C and Y.D.K were supported by NRF of Korea, (2022R1A4A3030766, 2021R1A2C2093155, 2021R1A6C101A437). K.W. and T.T. acknowledge support from the JSPS KAKENHI (Grant Numbers 21H05233 and 23H02052) and World Premier International Research Center Initiative (WPI), MEXT, Japan. Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))Published online: ((will be filled in by the editorial staff))References[1] K. F. Mak, J. Shan, Nat. Photon. 2016, 10, 216.[2] T. Mueller, E. Malic, npj 2D Mater. Appl. 2018, 2, 29.[3] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, M. S. Strano, Nat. Nanotechnol. 2012, 7, 699.[4] A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, B. 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(c) Band diagrams of the device at electroluminescence operation. The double gates can modulate individual regions of the divided WSe2 layer, leading to injecting electrons or holes from graphene into the WSe2 layer and transporting charge carriers in the WSe2 layer. Light emission occurs in the 1D region between the p-n junction in a WSe2 layer due to the e-h recombination of transported carriers.Figure 2. Reconfigurable transport properties using electrical modulation of WSe2 LET. (a) Transfer curve (IDS-VGG) of the device at a fixed voltage of VBG = 60 V for n-type transport operation. (b) Transfer curve (IDS-VBG) of the device at a fixed voltage of VGG = -6 V for p-type transport operation. (c) Schematics of band diagrams of three conditions at the n-type operation of the device using electrical modulation. By adjusting the double gate and drain voltages, the carrier type injected into WSe2 can be selected, and the carrier transport can be switched by modulating the potential barrier at the WSe2 homojunction.Figure 3. Light emission of WSe2 LET. (a) Output curves (IDS-VDS) at opposite voltage using double gates. The output curves show a rectifying behavior due to the p-n junction formation at WSe2. (b) Normalized PL and EL spectra of the device. The EL spectrum is consistent with the PL spectrum, which indicates that EL originated from e-h recombination across the direct bandgap of WSe2. (c) Optical image of light emission of the device. Strong EL is observed at the p-n junction of WSe2. Figure 4. Electrical tunability of electroluminescence of WSe2 LET. (a) EL spectrum and contour plot of the device at varying VDS and fixed VGG = -8 V and VBG = 60 V. (b) EQE (purple) and integrated EL intensity (green) as a function of VDS. VGG and VBG were fixed at -8 V, and 60 V, respectively. (c) EL spectrum and contour plot of the device at varying VBG and fixed VDS = 2.5 V and VGG = -8 V. (d) EQE (purple) and integrated EL intensity (red) as a function of VBG. VDS and VGG were fixed at 2.5 V, and -8 V, respectively. By optimizing with VDS, VBG, and VGG, a maximum EQE of 8.2% was obtained with WSe2 LET and EL intensity exhibits no peak shift.Figure 5. Electroluminescence of electrically confined neutral exciton at the 1D region in the p-n junction of WSe2. (a) Normalized PL spectrum of hole-doped (blue) and electron-doped (red) WSe2 in the WSe2 LET measured at VGG = -6 V and VBG = 60 V, respectively. (b) Normalized EL spectrum of the device. All spectrums were deconvoluted into the two distinct peaks of the neutral exciton (X0, green) and charged exciton (X*, purple). (c) Comparison of X0/X* intensity ratio in PL and EL. Compared to PL, the EL intensity of the device primarily arises from the neutral exciton. (d) Schematic of the device structure and energy band diagram in WSe2 LET. (e) Electrostatic simulation of the spatial dependence of charge density of holes (blue dashed line) and electrons (red dashed line), exciton potential (green line), and potential of the positively charged exciton (blue line) and negatively charged exciton (red line), and voltage (black dashed line) in WSe2. Since neutral excitons are electrically confined to the 1D region in WSe2 and charged excitons are pushed away from the 1D region by the potential difference, WSe2 LET can emit predominantly neutral excitons.4image1.pngimage2.pngimage3.pngimage4.pngimage5.pngimage6.jpeg