Charged excitons, or trions, offering unique spin and charge degrees of freedom, have primarily been investigated in doped systems where charges are long considered indispensable. Besides the carriers that bind with excitons to form trions, the excess charges inevitably impact nearly every trion property. These charges limit trion emission through strong nonradiative Auger recombination and introduce many-body complexities that often necessitate describing trions within the exciton–polaron framework. Generating a “pure” trion flux in a charge- and trap-free emitter, which would avoid the above interactions and enable potential trion-based spin qubits, has remained elusive under the existing paradigm.

In this study, we observe trion transfer process in mixed-dimensional heterostructures, which are composed of carbon nanotubes (CNTs) and tungsten diselenide (WSe₂). The structure is prepared using the anthracene transfer technique developed in our laboratory. We first compare photoluminescence spectra at E₂₂ and X_WSe2 excitation peaks. When excited at the X_WSe2 peak, a prominent low-energy emission peak emerges at 0.817 eV. The energy separation between this subpeak and E₁₁, which is smaller than the ∼ 0.140 eV expected for CNT K-momentum excitons (K). We observe such low-energy emission peak in multiple samples. The diameter dependence of energy difference indicates this new subpeak comes from CNT trions T_CNT.

Spatially resolved PL excitation imaging can distinguish the exciton and trion transfer processes. Under X_WSe2 excitation, the E₁₁ emission produces a broadened PL excitation image due to the exciton reservoir, where A excitons diffuse to the CNT before transferring. Notably, under the same excitation energy, the T_CNT emission profile does not show such significant spatial broadening. The difference in the excitation images is consistent with the transfer picture where trions have much smaller diffusion length than excitons.

Unlike conventional trion formation which relies on free carriers in the emitter, trion transfer exploits the reservoir effect that promises brighter trion emission. To highlight the efficiency of transfer-based trion emission, we compare it with two standard trion-generation approaches: electrostatic doping and chemical doping. Both electrostatic doping and chemical doping samples show modest trion effieiency and a saturation behavior, suggesting an intrinsic efficiency limit from Auger recombination process. In contrast, transfer-based trion emission in heterostructure samples is more efficient with efficiencies more than two orders of magnitude above the limits of doping-based methods. Extending the trion transfer concept to other low-dimensional systems could open new opportunities in many-body excitonic physics, spin/valleytronics, and advanced optoelectronics, paving the way for novel trion-based devices.

To learn more about this work, please refer to:
N. Fang, U. Erkılıc, Y. R. Chang, S. Fujii, D. Yamashita, C. F. Fong, S. Morito, K. Kanahashi, T. Taniguchi, K. Watanabe, K. Ueno, K. Nagashio, Y. K. Kato Trion transfer in mixed-dimensional heterostructures ACS Nano 20, 10933 (2026).