Abstract
Based on ab initio screened configuration interaction calculations we find that TiI2 has a bright exciton ground state and identify two key mechanisms that lead to this unprecedented feature among transition metal dichalcogenides. First, the spin-orbit induced conduction band splitting results in optically allowed spin-alignment for electrons and holes across a significant portion of the Brillouin zone around the K-valley, avoiding band crossings seen in materials like monolayer MoSe2. Second, a sufficiently weak exchange interaction ensures that the bright exciton remains energetically below the dark exciton state. We further show that the bright exciton ground state is stable under various mechanical strains and that trion states (charged excitons) inherit this bright ground state. Our findings are expected to spark further investigation into related materials that bring along the two key features mentioned, as bright ground-state excitons are crucial for applications requiring fast radiative recombination.
Similar content being viewed by others
Data availability
The data used and analyzed during the current study is available from the corresponding author upon reasonable request. The underlying code for this study is not publicly available but may be made available upon reasonable request from the corresponding author.
Code availability
The underlying code for this study is not publicly available but may be made available upon reasonable request from the corresponding author.
References
Frenkel, J. On the Transformation of light into Heat in Solids. I. Phys. Rev. 37, 17–44 (1931).
Wannier, G. H. The structure of electronic excitation levels in insulating crystals. Phys. Rev. 52, 191–197 (1937).
Gu, J., Chakraborty, B., Khatoniar, M. & Menon, V. M. A room-temperature polariton light-emitting diode based on monolayer WS2. Nat. Nanotechnol. 14, 1024–1028 (2019).
Wen, W., Wu, L. & Yu, T. Excitonic lasers in atomically thin 2D semiconductors. ACS Mater. Lett. 2, 1328–1342 (2020).
Mak, K. F. & Shan, J. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nat. Photon 10, 216–226 (2016).
Mueller, T. & Malic, E. Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors. npj 2D Mater. Appl 2, 29 (2018).
Fang, H. H. et al. Control of the exciton radiative lifetime in van der Waals heterostructures. Phys. Rev. Lett. 123, 067401 (2019).
Mondal, N., Azam, N., Gartstein, Y. N., Mahjouri-Samani, M. & Malko, A. V. Photoexcitation dynamics and long-lived excitons in strain-engineered transition metal dichalcogenides. Adv. Mater. 34, 2110568 (2022).
Chen, H.-Y., Jhalani, V. A., Palummo, M. & Bernardi, M. Ab initio calculations of exciton radiative lifetimes in bulk crystals, nanostructures, and molecules. Phys. Rev. B 100, 075135 (2019).
Palummo, M., Bernardi, M. & Grossman, J. C. Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides. Nano Lett. 15, 2794–2800 (2015).
Bester, G., Nair, S. & Zunger, A. Pseudopotential calculation of the excitonic fine structure of million-atom self-assembled In1-xGaxAs/GaAs quantum dots. Phys. Rev. B 67, 161306 (2003).
Deilmann, T. & Thygesen, K. S. Dark excitations in monolayer transition metal dichalcogenides. Phys. Rev. B 96, 201113 (2017).
Deilmann, T. & Thygesen, K. S. Finite-momentum exciton landscape in mono- and bilayer transition metal dichalcogenides. 2D Mater. 6, 035003 (2019).
Malic, E. et al. Dark excitons in transition metal dichalcogenides. Phys. Rev. Mater. 2, 014002 (2018).
Torche, A. & Bester, G. First-principles many-body theory for charged and neutral excitations: trion fine structure splitting in transition metal dichalcogenides. Phys. Rev. B 100, 201403 (2019).
Robert, C. et al. Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers. Nat. Commun. 11, 4037 (2020).
Echeverry, J. P., Urbaszek, B., Amand, T., Marie, X. & Gerber, I. C. Splitting between bright and dark excitons in transition metal dichalcogenide monolayers. Phys. Rev. B 93, 121107 (2016).
Fu, H., Wang, L.-W. & Zunger, A. Excitonic exchange splitting in bulk semiconductors. Phys. Rev. B 59, 5568–5574 (1999).
Fadaly, E. M. T. et al. Direct-bandgap emission from hexagonal Ge and SiGe alloys. Nature 580, 205–209 (2020).
Yuan, L.-D., Li, S.-S. & Luo, J.-W. Direct bandgap emission from strain-doped germanium. Nat. Commun. 15, 618 (2024).
Molas, M. R. et al. Brightening of dark excitons in monolayers of semiconducting transition metal dichalcogenides. 2D Mater. 4, 021003 (2017).
Ren, L. et al. Control of the bright-dark exciton splitting using the Lamb shift in a two-dimensional semiconductor. Phys. Rev. Lett. 131, 116901 (2023).
Chowdhury, T. et al. Brightening of dark excitons in WS2 via tensile strain-induced excitonic valley convergence. Phys. Rev. B 110, L081405 (2024).
Zinkiewicz, M. et al. Neutral and charged dark excitons in monolayer WS2. Nanoscale 12, 18153–18159 (2020).
Becker, M. A. et al. Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189–193 (2018).
Tamarat, P. et al. The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state. Nat. Mater. 18, 717–724 (2019).
Swift, M. W., Sercel, P. C., Efros, A. L., Lyons, J. L. & Norris, D. J. Identification of semiconductor nanocrystals with bright ground-state excitons. ACS Nano 18, 19561–19567 (2024).
Li, X., Zhang, Z. & Zhang, H. High throughput study on magnetic ground states with hubbard u corrections in transition metal dihalide monolayers. Nanoscale Adv. 2, 495–501 (2020).
Li, W., Qian, X. & Li, J. Phase transitions in 2d materials. Nat. Rev. Mater. 6, 829–846 (2021).
Han, W. et al. Phase transition of 2D van der Waals ferroelectrics. 2D Mater. http://iopscience.iop.org/article/10.1088/2053-1583/add749 (2025).
Rasmussen, F. A. & Thygesen, K. S. Computational 2D materials database: electronic structure of transition-metal dichalcogenides and oxides. J. Phys. Chem. C. 119, 13169–13183 (2015).
Malyi, O. I., Sopiha, K. V. & Persson, C. Energy, phonon, and dynamic stability criteria of two-dimensional materials. ACS Appl. Mater. Interfaces 11, 24876–24884 (2019).
Kośmider, K., González, J. W. & Fernández-Rossier, J. Large spin splitting in the conduction band of transition metal dichalcogenide monolayers. Phys. Rev. B 88, 245436 (2013).
Liu, G.-B., Shan, W.-Y., Yao, Y., Yao, W. & Xiao, D. Three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides. Phys. Rev. B 88, 085433 (2013).
Kormányos, A., Zólyomi, V., Drummond, N. D. & Burkard, G. Spin-orbit coupling, quantum dots, and qubits in monolayer transition metal dichalcogenides. Phys. Rev. X 4, 011034 (2014).
Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: a new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010).
He, K. et al. Tightly bound excitons in monolayer WSe2. Phys. Rev. Lett. 113, 026803 (2014).
Haastrup, S. et al. The computational 2D materials database: high-throughput modeling and discovery of atomically thin crystals. 2D Mater. 5, 042002 (2018).
Liu, E. et al. Gate tunable dark trions in monolayer WSe2. Phys. Rev. Lett. 123, 027401 (2019).
Lyons, T. P. et al. The valley Zeeman effect in inter- and intra-valley trions in monolayer WSe2. Nat. Commun. 10, 2330 (2019).
Calman, E. V. et al. Indirect excitons and trions in MoSe2/WSe2 van der Waals heterostructures. Nano Lett. 20, 1869–1875 (2020).
Mørch Nielsen, C. E., Fischer, F. & Bester, G. Beyond the K-valley: exploring unique trion states in indirect band gap monolayer WSe2. npj 2D Mater. Appl. 9, 11 (2025).
Frisenda, R. et al. Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides. npj 2D Mater. Appl. 1, 10 (2017).
Giannozzi, P. et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys. Condens. Matter. 21, 395502 (2009).
Giannozzi, P. et al. Advanced capabilities for materials modelling with QUANTUM ESPRESSO. J. Phys. Condens. Matter. 29, 465901 (2017).
Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).
van Setten, M. J. et al. The PseudoDojo: training and grading a 85 element optimized norm-conserving pseudopotential table. Comput. Phys. Commun. 226, 39–54 (2018).
Sohier, T., Calandra, M. & Mauri, F. Density functional perturbation theory for gated two-dimensional heterostructures: theoretical developments and application to flexural phonons in graphene. Phys. Rev. B 96, 075448 (2017).
Franceschetti, A., Fu, H., Wang, L. W. & Zunger, A. Many-body pseudopotential theory of excitons in InP and CdSe quantum dots. Phys. Rev. B 60, 1819–1829 (1999).
Bester, G. Electronic excitations in nanostructures: an empirical pseudopotential based approach. J. Phys. Condens. Matter 21, 023202 (2008).
Onida, G., Reining, L. & Rubio, A. Electronic excitations: density-functional versus many-body Green’s-function approaches. Rev. Mod. Phys. 74, 601–659 (2002).
Marini, A., Hogan, C., Grüning, M. & Varsano, D. yambo: an ab initio tool for excited state calculations. Comput. Phys. Commun. 180, 1392–1403 (2009).
Sangalli, D. et al. Many-body perturbation theory calculations using the yambo code. J. Phys. Condens. Matter 31, 325902 (2019).
Torche, A. & Bester, G. Biexcitons fine structure and non-equilibrium effects in transition metal dichalcogenides monolayers from first principles. Commun. Phys. 4, 67 (2021).
Acknowledgements
The project is supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the Priority Program SPP2244 2DMP and by the Cluster of Excellence “Advanced Imaging of Matter” of the DFG -- EXC 2056 -- project ID 390715994. Calculations were carried out on Hummel funded by the DFG – 498394658. We acknowledge financial support from the Open Access Publication Fund of Universität Hamburg.
Funding
Open Access funding enabled and organized by Projekt DEAL.
Author information
Authors and Affiliations
Contributions
F.F. conceptualized the research, conducted all calculations, analyzed, and visualized the results. Method development was carried out by F.F. and C.E.M.N., all authors (F.F., C.E.M.N., M.P., and G.B.) contributed equally to the writing of the manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary information
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Fischer, F., Nielsen, C.E.M., Prada, M. et al. Unconventional bright ground-state excitons in monolayer TiI2 from first-principles calculations. npj 2D Mater Appl (2026). https://doi.org/10.1038/s41699-025-00656-z
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41699-025-00656-z
