Shibuta Masahiro, Yamamoto Kazuo, Ohta Tsutomu, Inoue Tomoya, Mizoguchi Kaito, Nakaya Masato, Eguchi Toyoaki, Nakajima Atsushi
Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps-Universität, D-35032 Marburg, Germany.
Nakajima Designer Nanocluster Assembly Project, ERATO, Japan Science and Technology Agency (JST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan.
ACS Nano. 2021 Jan 26;15(1):1199-1209. doi: 10.1021/acsnano.0c08248. Epub 2021 Jan 7.
The plasmonic response of metallic nanostructures plays a key role in amplifying photocatalytic and photoelectric conversion. Since the plasmonic behavior of noble metal nanoparticles is known to generate energetic charge carriers such as hot electrons, it is expected that the hot electrons can enhance conversion efficiency if they are transferred into a neighboring molecule or semiconductor. However, the method of transferring the energized charge carriers from the plasmonically generated hot electrons to the neighboring species remains controversial. Herein, we fabricated a molecularly well-defined heterointerface between the size-selected plasmonic noble-metal nanoclusters (NCs) of Ag ( = 3-55)/Au ( = 21) and the organic C film to investigate hot electron generation and relaxation dynamics using time-resolved two-photon photoemission (2PPE) spectroscopy. By tuning the NC size and the polarization of the femtosecond excitation photons, the plasmonic behavior is characterized by 2PPE intensity enhancement by 10-100 times magnitude, which emerge at ≥ 9 for Ag NCs. The 2PPE spectra exhibit contributions from low-energy electrons forming coherent plasmonic currents and hot electrons with an excitation energy up to photon energy owing to two-photon excitation of an occupied state of the Ag NC below the Fermi level. The time-resolved pump-probe measurements demonstrate that plasmon dephasing generates hot electrons which undergo electron-electron scattering. However, no photoemission occurs the charge transfer state forming AgC located in the vicinity of the Fermi level. Thus, this study reveals the mechanism of ultrafast confined hot electron relaxation within plasmonic Ag NCs at the molecular heterointerface.
金属纳米结构的等离子体响应在放大光催化和光电转换中起着关键作用。由于已知贵金属纳米颗粒的等离子体行为会产生高能电荷载流子，如热电子，因此预计如果热电子转移到相邻分子或半导体中，它们可以提高转换效率。然而，将等离子体产生的热电子中的带电载流子转移到相邻物种的方法仍存在争议。在此，我们在尺寸选择的Ag（ = 3 - 55）/Au（ = 21）等离子体贵金属纳米团簇（NCs）与有机C膜之间制备了分子明确的异质界面，以使用时间分辨双光子光发射（2PPE）光谱研究热电子的产生和弛豫动力学。通过调整NC尺寸和飞秒激发光子的偏振，等离子体行为的特征是2PPE强度增强10 - 100倍，对于Ag NCs，在 ≥ 9时出现。2PPE光谱显示出低能电子形成相干等离子体电流以及由于费米能级以下Ag NC的占据态的双光子激发而产生的激发能高达光子能量的热电子的贡献。时间分辨泵浦 - 探测测量表明，等离子体退相产生经历电子 - 电子散射的热电子。然而，在费米能级附近形成AgC的电荷转移态处没有发生光发射。因此，本研究揭示了在分子异质界面处等离子体Ag NCs内超快受限热电子弛豫的机制。
