Boeije Yorrick, Hoang Anh Tuan, Lim Juhwan, Stranks Samuel D, Chhowalla Manish, Pop Eric, Mannix Andrew J, Rao Akshay
Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB3 0AS, U.K.
Department of Physics, Cavendish Laboratory, University of Cambridge; Cambridge CB3 0HE, U.K.
ACS Nano. 2025 Jul 29;19(29):26942-26952. doi: 10.1021/acsnano.5c08533. Epub 2025 Jul 21.
Defect engineering in transition metal dichalcogenide (TMD) monolayers enables applications in single-photon emission, sensing, and photocatalysis. These functionalities critically depend on defect type, density, spatial distribution, relative energy, and the dynamics of exciton trapping at the defect sites. The latter are mediated by coupling to optical phonons through mechanisms not yet fully understood. Traditionally, exciton or carrier trapping at defects in inorganic crystals has been described by incoherent multiphonon emission within the Born-Oppenheimer approximation─an approach that underpins the widely used Shockley-Read-Hall framework for nonradiative recombination. Here, we use impulsive vibrational spectroscopy to investigate exciton trapping in defect-modified monolayers of WS grown through metal-organic chemical vapor deposition. We find that the phonon coherences of the Raman-active A' and E' modes persist throughout the ultrafast (∼100 fs) exciton trapping process, indicating a continuous evolution of the excitonic wave function. This observation is consistent with a conical intersection-mediated trapping process, in which a potential energy surface crossing between the free and trapped excitonic states acts as a funnel to drive this nonadiabatic transition. Such a molecular-like, vibronically coherent mechanism lies beyond the Born-Oppenheimer approximation, in stark contrast to classical, incoherent trapping models in solids. Moreover, the faster dephasing of the E' mode in the trapped exciton state compared to the free exciton suggests it acts as a vibrational coordinate that promotes the trapping process. These findings provide mechanistic insights into exciton-phonon interactions at defects in TMD monolayers and inform strategies for engineering quantum and energy functionalities.
过渡金属二硫属化物（TMD）单层中的缺陷工程可实现单光子发射、传感和光催化等应用。这些功能严重依赖于缺陷类型、密度、空间分布、相对能量以及激子在缺陷位点处的捕获动力学。后者是通过尚未完全理解的机制与光学声子耦合来介导的。传统上，无机晶体中缺陷处的激子或载流子捕获是通过玻恩-奥本海默近似内的非相干多声子发射来描述的——这种方法支撑了广泛使用的用于非辐射复合的肖克利-里德-霍尔框架。在这里，我们使用脉冲振动光谱来研究通过金属有机化学气相沉积生长的缺陷修饰的WS单层中的激子捕获。我们发现拉曼活性A'和E'模式的声子相干在超快（约100飞秒）激子捕获过程中持续存在，这表明激子波函数在持续演化。这一观察结果与锥形交叉介导的捕获过程一致，在该过程中，自由激子态和捕获激子态之间的势能面交叉充当漏斗来驱动这种非绝热跃迁。这种类似分子的、振动电子相干机制超出了玻恩-奥本海默近似，与固体中的经典非相干捕获模型形成鲜明对比。此外，与自由激子相比，捕获激子态中E'模式更快的退相表明它充当促进捕获过程的振动坐标。这些发现为TMD单层中缺陷处的激子-声子相互作用提供了机理见解，并为工程量子和能量功能的策略提供了依据。
