Recently, a research team led by Prof. LIU Xinfeng from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS) reported the efficient phonon-assisted upconversion luminescence in a quasi-two-dimensional perovskite system. The research is published on Science Advances (Sci. Adv. 9, eadi9347, 2023)
Photon upconversion is a physical process where the emitted photon energy is greater than the absorbed photon energy. Upconversion (UC) can be achieved through various methods, including multi-photon nonlinear absorption, triplet-triplet annihilation in organic molecules, energy transfer in rare-earth metal doped materials, and thermal-assisted photon absorption. Among them, phonon-assisted upconversion is the theoretical and experimental basis for solid-state laser cooling. However, how phonons are involved in the anti-Stokes upconversion process and improve upconversion efficiency and energy gain has been a key scientific question in this field for a long time.
In the recent research work, Liu's group and co-workers have realized the self-trapped state emission by constructing nanoscale superlattice structures to enhance the electron-phonon strong coupling, and proposed that "organic"-"inorganic" soft lattice structures are expected to be a platform for the study of electron-phonon strong coupling effects (Nano Letters 21, 4137-4144, 2021).
On this basis, LIU’s group at the National Center for Nanoscience and Technology, together with Prof. BO Wu at South China Normal University and Associate Prof. WEI Ma at Ningxia University, successfully achieved efficient phonon-assisted upconversion luminescence in a quasi-two-dimensional perovskite system of organic-inorganic soft lattice by combining soft lattice with low dimensionality (Fig. 1).
The study reveals a phonon action time of about 1.2 ps and an anti-Stokes shift energy of more than 200 meV. Moreover, the team proposes to attribute the origin of high upconversion energy gains to strong lattice fluctuations, in contrast to the phonon absorption pictures in classical upconversion theory.
The research team carried out microscopic transient absorption (TA) measurements and density functional theory (DFT) calculations to explain efficient upconversion ultrafast process (Fig. 2). The motion of the organic cation at ps timescales causes the entire lattice to deform, resulting in rapid changes in the phonon-renormalized electron/exciton energy. This provides the low-energy excitons with sufficient energy to reach quasi-equilibrium states (free exciton) where they can recombine radiatively.
Hence, the efficient UC in quasi-2D perovskites is not directly attributed to the absorption of specific phonon modes in the perturbative theory. Instead, it arises from the alteration of electronic energy linked to the pronounced thermal-driven deformation of the entire lattice or, alternatively, the formation of dynamic polarons. The band energy fluctuation, reaching approximately ±180 meV at room temperature due to the strong non-perturbative interaction with lattice deformation, allows for a remarkable UC energy gain that cannot be attained in traditional semiconductors.
LIU said that the results clarify the time scale of the participation of chalcogenide phonons in upconversion and deepen the understanding of the electron-phonon coupling mechanism, and provide a new perspective for the design of high-efficiency upconversion. The optimized design of the material lattice structure to make full use of the energy of the lattice vibration is expected to take advantage of this physical mechanism in the field of luminescence of optical materials.
Figure 1. Schematics of the energy diagram and dynamics of UC in perovskites. (Image by LIU Xinfeng et al)
Figure 2. (a) Chemical structure of (PEA)2PbI4. (b) Transient absorption spectra of (PEA)2PbI4 nanosheets under upconversion excitation (565 nm). (c) Probing exciton bleaching kinetics for up-conversion and down-conversion at a wavelength of 522 nm. (Image by LIU Xinfeng et al)
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