Abstract
Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron–hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10−6 S) and low power density regime (<102 W cm−2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.
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Data availability
All data are available in the article and the Supplementary Information, and are available from the corresponding authors on reasonable request.
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The source code for the calculations conducted in this study is available from the corresponding authors on reasonable request.
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Acknowledgements
We acknowledge the support from the Ministry of Education (MOE), Singapore, under Academic Research Fund (AcRF) Tier 3 (grant no. MOE2018-T3-1-005), and the National Research Foundation (NRF), under the Prime Minister’s Office, Singapore, under the Medium Sized Centre Programme and the Competitive Research Programme (CRP) (grant no. NRF-CRP17-2017-08). M.T. acknowledges Institute for Functional Intelligent Materials (I-FIM, grant no. EDUNC-33-18-279-V12). B.Ö. acknowledges the Singapore NRF Investigatorship (grant no. NRF-NRFI2018-8) and MOE-AcRF-Tier 2 (grant no. MOE-T2EP50220-0017). J.W. acknowledges the National Key R&D Program of China (grant no. 2021YFA1200804).
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G.E., Z.W. and V.K. conceived the project. Z.W. and J.L. fabricated the devices. Z.W. performed the measurements. Z.W. and V.K. analysed the experimental data. V.K., M.T. and Z.W. performed the theoretical analysis. G.E. and C.A.N. co-supervised the project. Z.W., V.K. and G.E. co-wrote the paper. G.E., C.A.N, Y.G. and B.Ö. provided equipment and laboratory facilities. All authors contributed to the scientific discussions and paper revisions.
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Wang, Z., Kalathingal, V., Trushin, M. et al. Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions. Nat. Nanotechnol. (2024). https://doi.org/10.1038/s41565-024-01650-0
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DOI: https://doi.org/10.1038/s41565-024-01650-0