Lytle Kylie M, Brass Emma L, Roman Benjamin J, Sheldon Matthew T
Department of Chemistry, Texas A&M University, College Station, Texas 77840-7896, United States.
Department of Chemistry, University of California Irvine, Irvine, California 92697, United States.
ACS Nano. 2024 Jul 16;18(28):18457-18464. doi: 10.1021/acsnano.4c03548. Epub 2024 Jul 5.
Optically driven cooling of a material, or optical refrigeration, is possible when optical up-conversion via anti-Stokes photoluminescence (ASPL) is achieved with near-unity quantum yield. The recent demonstration of optical cooling of CsPbBr perovskite nanocrystals (NCs) has provided a path forward in the development of semiconductor-based optical refrigeration strategies. However, the mechanism of ASPL in CsPbBr NCs is not yet settled, and the prospects for cooling technologies strongly depend on details of the mechanism. By analyzing the Arrhenius behavior of ASPL in CsPbBr NCs, we investigated the relationship between the average energy gained per photon during up conversion, Δ, and the thermal activation energy, . We find that is systematically larger than Δ, and that increases for larger Δ. We suggest that the additional energetic cost is due to a rearrangement of the crystal lattice as charge carriers pass from surface localized, structurally distinct sub-gap polaron states to the free exciton state during up-conversion. Our interpretation is further corroborated by quantifying the impact of ligand coverage on the NC surface. These findings help inform the development of CsPbBr NCs for applications in optical refrigeration.
当通过反斯托克斯光致发光（ASPL）实现近乎单位量子产率的光学上转换时，材料的光驱动冷却或光制冷是可行的。最近对CsPbBr钙钛矿纳米晶体（NCs）进行光制冷的演示为基于半导体的光制冷策略的发展提供了一条前进的道路。然而，CsPbBr NCs中ASPL的机制尚未确定，并且冷却技术的前景在很大程度上取决于该机制的细节。通过分析CsPbBr NCs中ASPL的阿仑尼乌斯行为，我们研究了上转换过程中每个光子获得的平均能量Δ与热活化能之间的关系。我们发现 系统地大于Δ，并且对于更大的Δ， 会增加。我们认为额外的能量成本是由于在向上转换过程中，当电荷载流子从表面局域化、结构不同的亚带隙极化子态跃迁到自由激子态时，晶格发生了重排。通过量化配体覆盖对NC表面的影响，我们的解释得到了进一步证实。这些发现有助于为CsPbBr NCs在光制冷中的应用开发提供参考。
