Electrostatically defined p-n diodes in ambipolar monolayer WSe2 (Baugher, Churchill, Yang and Jarillo-Herrero, Nature Nanotechnology 9, 262-267, 2014) established a gate-tunable atomically thin diode with an ideality factor of about 2, photovoltaic external quantum efficiency of 0.2%, and electroluminescence, while explicitly leaving open the microscopic origins of its non-ideal behavior. Contact engineering, junction geometry, and demonstrations of electroluminescence efficiency well beyond the original ~1% estimate have since been substantially advanced in monolayer WSe2 and related two-dimensional semiconductors, so the surviving open core is the quantitative, mechanism-resolved identification of the recombination channels (Shockley-Read-Hall, Auger-Meitner, exciton-exciton annihilation, band-tail and trap-assisted processes) that set the near-2 ideality factor, and of the disorder-induced mid-gap states that mediate leakage, in electrostatically doped lateral p-n junctions of monolayer transition-metal dichalcogenides, together with the demonstration that suppressing these channels yields near-ideal, high-efficiency monolayer diodes.

Background

Monolayer WSe2 (tungsten diselenide) is a two-dimensional semiconductor of the transition-metal dichalcogenide (TMD) family: a three-atom-thick sheet in which tungsten and selenium atoms form a direct-gap, atomically thin crystal at optical energies near 1.6-1.7 eV. Because it is ambipolar, meaning a single sheet can transport both electrons and holes, a p-n junction, the basic rectifying building block of diodes, photodiodes, solar cells and light-emitting diodes (LEDs), can be created not by chemical doping but by electrostatic gating: two local gates beneath an insulator are biased to accumulate holes over one half of the sheet and electrons over the other, and the junction position and carrier densities remain reconfigurable in operation. Baugher, Churchill, Yang and Jarillo-Herrero (2014) demonstrated such split-gate monolayer WSe2 devices, realizing p-n and n-p diodes, photodetection with responsivity 210 mA W-1, photovoltaic power generation with a peak external quantum efficiency (EQE, the number of collected carriers per incident photon) of 0.2%, and electrically driven light emission; two companion papers published the same week (Pospischil, Furchi and Mueller; Ross et al.) reported the same junction concept with thin boron nitride supports and buried multi-gate architectures.

The electrical transport of a p-n diode is conventionally summarized by the Shockley diode equation I=Is(eqV/nkT−1)I = I_s(e^{qV/nkT}-1), where qq is the elementary charge, kTkT the thermal energy, and nn the ideality factor: n=1n=1 corresponds to the ideal case in which recombination is dominated by radiative band-to-band processes in the neutral, carrier-accumulated regions, while nn approaching 2 is the classical signature of recombination through defects with energy levels deep in the band gap, acting inside the depletion region (the interfacial zone depleted of free carriers). In bulk semiconductors the microscopic catalogue of such processes is classical: Shockley-Read-Hall (SRH) recombination, in which a mid-gap defect captures first one and then the other carrier type; Auger recombination, in which the recombination energy is transferred to a third carrier; and trap-assisted or band-tail-assisted processes arising from disorder. In a monolayer TMD this catalogue has to be rethought: Coulomb interactions are poorly screened so excitons (bound electron-hole pairs with binding energies of hundreds of meV) and nonradiative exciton-exciton annihilation dominate the optical physics, and the substrate, contacts, and intrinsic defects (chalcogen vacancies, charged impurities) strongly perturb the local band edges, producing exponential band tails, disorder-induced states extending into the gap from the band edges. The 2014 paper measured an ideality factor near 2 and estimated its electroluminescence efficiency at about 1%, and identified as open directions: identifying the recombination mechanisms behind the ideality factor; reducing contact resistance and contact asymmetry (poor hole injection at gold contacts); overcoming the geometric and optical-coupling limits of a three-atom-thick, laterally narrow junction; characterizing mid-gap, disorder-related conduction paths between the electrostatically doped regions; and quantitatively raising the electroluminescence efficiency. Later developments relevant to this question include: contact strategies for two-dimensional semiconductors (van der Waals contacts, electrodes transferred onto the semiconductor without direct metal deposition, which avoid deposition damage and Fermi-level pinning, the pinning of the contact Fermi level by interface states rather than by the metal work function; charge-transfer-doped p-type contacts); junction and optical designs that beat the absorption and outcoupling limits (suspended sheets, waveguide integration, vertical van der Waals heterostructure LEDs); and microscopic calculations of Auger-Meitner (free-carrier Auger) losses in monolayer TMDs, which quantified when those losses matter and found nontrivial density dependence due to bandgap renormalization, the density-dependent shift of the band gap itself.

Problem Statement

For an electrostatically defined lateral p-n junction in ambipolar monolayer WSe2, created by two local gates that accumulate hole and electron regions in a single sheet, three deliverables are jointly required:

  1. Identify, experimentally and theoretically, the microscopic recombination mechanisms that set the diode ideality factor of approximately 2 (about 1.9 in the original devices), including the relative contributions of Shockley-Read-Hall recombination through mid-gap defects, Auger (Auger-Meitner) processes, exciton-exciton annihilation, and disorder-induced band-tail or trap-assisted channels.

  2. Determine how disorder-related mid-gap states produce the thermally activated conduction and leakage paths observed between the differently doped regions, and how encapsulation, substrate choice, or material processing suppress them. This is a requirement distinct from deliverable 1: deliverable 1 concerns the band-tail and trap-assisted recombination channels within the junction's recombination zone, whereas this deliverable concerns the mid-gap conduction and shunt paths that connect the two electrostatically doped regions and their suppression.

  3. Demonstrate, from this mechanism-level understanding together with the contact and optical-coupling engineering already developed in the field, that suppressing the identified nonradiative and shunt channels translates directly into near-ideal, high-efficiency diodes (transport with an ideality factor approaching 1 and electroluminescence efficiency substantially exceeding the ~1% originally estimated) for a junction of the same electrostatically defined lateral split-gate type described above, rather than for other geometries such as vertical van der Waals heterostructure LEDs or waveguide-coupled heterojunctions.

The contact-resistance, junction-geometry, and electroluminescence-efficiency targets named in the source have since been largely met in monolayer WSe2 and related two-dimensional semiconductors and supply the available engineering building blocks, but they do not by themselves resolve the task.

References

  1. Baugher, B. W. H., Churchill, H. O. H., Yang, Y. & Jarillo-Herrero, P. Optoelectronic devices based on electrically tunable p-n diodes in a monolayer dichalcogenide. Nature Nanotechnology 9, 262-267 (2014). DOI: 10.1038/nnano.2014.25. https://doi.org/10.1038/nnano.2014.25
  2. Pospischil, A., Furchi, M. M. & Mueller, T. Solar-energy conversion and light emission in an atomic monolayer p-n diode. Nature Nanotechnology 9, 257-261 (2014). DOI: 10.1038/nnano.2014.14. https://doi.org/10.1038/nnano.2014.14
  3. Ross, J. S., Klement, P., Jones, A. M., Ghimire, N. J., Yan, J., Mandrus, D. G., Taniguchi, T., Watanabe, K., Kitamura, K., Yao, W., Cobden, D. H. & Xu, X.-D. Electrically tunable excitonic light-emitting diodes based on monolayer WSe2 p-n junctions. Nature Nanotechnology 9, 268-272 (2014). DOI: 10.1038/nnano.2014.26. https://doi.org/10.1038/nnano.2014.26
  4. Uddin, S. Z., Higashitarumizu, N., Kim, H., Rahman, I. K. M. R. & Javey, A. Efficiency Roll-Off Free Electroluminescence from Monolayer WSe2. Nano Letters 22, 5316-5321 (2022). DOI: 10.1021/acs.nanolett.2c01311. https://doi.org/10.1021/acs.nanolett.2c01311
  5. Hu, Z., Xia, W., Fu, Q., Wu, Z., Jiang, X., Han, X., Wang, S., Sun, X., Chen, J., Zhao, T., Liu, X., Duan, Z., Huang, Y., Wang, J., Lu, J., Zhong, F., Zhang, Q. & Ni, Z.-H. Highly Efficient Dual-Gated Light-Emitting Diodes Based on Suspended Monolayer WSe2. Nano Letters 26, 651-659 (2026; published online December 2025). DOI: 10.1021/acs.nanolett.5c06025. https://doi.org/10.1021/acs.nanolett.5c06025
  6. Hader, J. & Moloney, J. V. Free Carrier Auger-Meitner Recombination in Monolayer Transition Metal Dichalcogenides. Nano Letters 25, 284-290 (2025; published online December 2024). DOI: 10.1021/acs.nanolett.4c04963. https://doi.org/10.1021/acs.nanolett.4c04963
  7. Feng, J., Li, Y., Zhang, J., Tang, Y., Sun, H., Gan, L. & Ning, C.-Z. Injection-free multiwavelength electroluminescence devices based on monolayer semiconductors driven by an alternating field. Science Advances 8, eabl5134 (2022). DOI: 10.1126/sciadv.abl5134. https://doi.org/10.1126/sciadv.abl5134
  8. Sarangapani, P., Charles, J. & Kubis, T. Tuning Band Tails in Mono- and Multilayered Transition-Metal Dichalcogenides: A Detailed Assessment and a Quick-Reference Guide. Physical Review Applied 17, 024005 (2022). DOI: 10.1103/PhysRevApplied.17.024005. https://doi.org/10.1103/PhysRevApplied.17.024005
  9. Aftab, S., Akhtar, I., Seo, Y. & Eom, J. WSe2 Homojunction p-n Diode Formed by Photoinduced Activation of Mid-Gap Defect States in Boron Nitride. ACS Applied Materials & Interfaces 12, 42007-42015 (2020). DOI: 10.1021/acsami.0c12129. https://doi.org/10.1021/acsami.0c12129
  10. Zhou, C., Zhang, S., Lv, Z., Ma, Z., Yu, C., Feng, Z. & Chan, M. Self-driven WSe2 photodetectors enabled with asymmetrical van der Waals contact interfaces. npj 2D Materials and Applications 4, 46 (2020). DOI: 10.1038/s41699-020-00179-9. https://doi.org/10.1038/s41699-020-00179-9
  11. Allain, A., Kang, J., Banerjee, K. & Kis, A. Electrical contacts to two-dimensional semiconductors. Nature Materials 14, 1195-1205 (2015). DOI: 10.1038/nmat4452. https://doi.org/10.1038/nmat4452
  12. Xie, J., Zhang, Z., Zhang, H., Nagarajan, V., Zhao, W., Kim, H.-L., Sanborn, C., Qi, R., Chen, S., Kahn, S., Watanabe, K., Taniguchi, T., Zettl, A., Crommie, M. F., Analytis, J. & Wang, F. Low Resistance Contact to P-Type Monolayer WSe2. Nano Letters 24, 5937-5943 (2024). DOI: 10.1021/acs.nanolett.3c04195. https://doi.org/10.1021/acs.nanolett.3c04195
  13. Takano, K., Yamasue, K., Kato, T. & Cho, Y. Nanoscale carrier distribution and trap dynamics in supported and suspended WSe2 layers studied by scanning nonlinear dielectric microscopy. Applied Physics Letters 128, 052101 (2026). DOI: 10.1063/5.0309146. https://doi.org/10.1063/5.0309146
  14. Yang, X., Wu, R., Zheng, B., Luo, Z., You, W., Liu, H., Li, L., Zhang, Y., Tan, Q., Liang, D., Chen, Y., Qu, J., Yi, X., Wang, X., Zhou, J., Duan, H., Wang, S., Chen, S. & Pan, A. A Waveguide-Integrated Two-Dimensional Light-Emitting Diode Based on p-Type WSe2/n-Type CdS Nanoribbon Heterojunction. ACS Nano 16, 4371-4378 (2022). DOI: 10.1021/acsnano.1c10607. https://doi.org/10.1021/acsnano.1c10607
  15. Hu, Z., Fu, Q., Lu, J., Zhang, Y., Zhang, Q., Wang, S., Duan, Z., Zhang, Y., Liu, X., Pan, Q., Jiang, G., Yang, T., Han, X., Yang, Y., Liu, T., Tao, T., Wang, W., Zhao, B., Yuan, X., Wan, D., Liu, Y., You, Y., Zhou, P., Liu, H. & Ni, Z. Van der Waals integrated single-junction light-emitting diodes exceeding 10% quantum efficiency at room temperature. Science Advances 10, eadp8045 (2024). DOI: 10.1126/sciadv.adp8045. https://doi.org/10.1126/sciadv.adp8045
  16. Wang, S., Fu, Q., Zheng, T., Han, X., Wang, H., Zhou, T., Liu, J., Liu, T., Zhang, Y., Chen, K., Wang, Q., Duan, Z., Zhou, X., Watanabe, K., Taniguchi, T., Yan, J., Huang, Y., Xiong, Y., Yang, J. K. W., Hu, Z., Xu, T., Sun, L., Hong, J., Zheng, Y., You, Y., Zhang, Q., Lu, J. & Ni, Z. Light-emitting diodes based on intercalated transition metal dichalcogenides with suppressed efficiency roll-off at high generation rates. Nature Electronics 8, 56-65 (2025; published online October 2024). DOI: 10.1038/s41928-024-01264-3. https://doi.org/10.1038/s41928-024-01264-3
  17. Zhu, Y., Sun, X., Tang, Y., Fu, L. & Lu, Y. Two-dimensional materials for light emitting applications: Achievement, challenge and future perspectives. Nano Research 14, 1912-1936 (2021; published online November 2020). DOI: 10.1007/s12274-020-3126-9. https://doi.org/10.1007/s12274-020-3126-9