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中国物理学会期刊

过渡金属硫族化合物中真空能级之上高能电子态的探测

Probing high-lying excited states above the vacuum level in transition metal dichalcogenides

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  • 光激发产生的非平衡载流子是光电转换过程中的重要能量载体。通常,当光激发电子的能量超过真空能级(above EV)时,电子将迅速逃逸出固体表面。然而,真空能级以上是否存在可被选择性布居的晶格调制高能终态,以及其激发机制如何,仍是亟需解决的重要问题。本文利用双光子光电子能谱技术,在2H-MoS2、2H-MoSe2、2H-WS2和2H-WSe2中观测到above EV的高能终态。实验通过系统调节入射光子能量、光强和偏振,发现终态信号仅在Γ点附近“导带→终态”跃迁达到能量阈值后出现,并呈现近二阶的非线性关系,表明其主要通过“价带→导带→终态”双光子跃迁获得布居。第一性原理计算进一步表明,终态波函数主要分布于晶格间隙区域,并通过面内轨道耦合与导带发生共振跃迁,表明该终态具有受晶格周期势调制的特征。这些结果揭示了高能终态选择性布居的微观机制及其在2H相过渡金属硫族化合物中的普遍性,为研究高能载流子的选择性激发与后续动力学、理解超快光电子发射过程以及提升光电转换效率提供了新的物理基础。

     

    Photoexcitation delivers energy into the electronic system, generating non-equilibrium carriers in conduction bands (CBs) below the vacuum level (EV). Once the electron energy exceeds EV, electrons are generally expected to escape rapidly from the solid. Whether such energetic electrons can couple to crystal-modulated final states (FSs) before photoemission, and how these states can be selectively excited, remain fundamental open questions. Here, using angle-resolved two-photon photoemission (2PPE) spectroscopy, we directly visualize crystal-modulated FSs in bulk 2H-MoS2 and three other 2H-phase transition-metal dichalcogenides (TMDs). By tuning the excitation photon energy, we identify an energy-resonant CB→FS transition at the Γ point, enabling efficient population of the FS and direct mapping of its electronic structure and excitation pathway. In 2H-MoS2, the FS exhibits a band minimum located approximately 0.60 eV above the EV. The FS signal emerges only when the photon energy exceeds ~3.58 eV, corresponding to the threshold for the resonant CB→FS transition. Power-dependent measurements exhibit a quadratic photoemission response, confirming a sequential VB→CB→FS two-photon excitation pathway. First-principles calculations reveal a striking contrast between the CB and FS wavefunctions: whereas the CB wavefunction is primarily localized around atomic sites, the FS wavefunction is distributed throughout the interstitial regions of the hexagonal lattice and possesses pronounced delocalized s-orbital character, providing a favorable transient reservoir for energetic electrons prior to photoemission. Calculated transition dipole moments, together with polarization-dependent measurements, further demonstrate that the CB→FS transition is dominated by in-plane optical coupling. Extending the measurements to 2H-MoSe2, 2H-WS2, and 2H-WSe2, we observe analogous dispersive FSs up to ~1 eV above their respective EV, establishing crystal-modulated FSs as a universal feature of the high-lying electronic structure of 2H-phase TMDs. Our results reveal previously uncharacterized high-energy electronic states above EV and establish their energy- and polarization-dependent excitation pathways, providing a microscopic basis for understanding high-energy carrier dynamics, nonlinear photoemission, and surface photochemical processes.

     

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