Browsing by Author "Guo, Xunmin"

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    Electron-phonon interactions in MoS2ﾠprobed with ultrafast two-dimensional visible/far-infrared spectroscopy
    (AIP Publishing LLC., 2015) Guo, Xunmin; Chen, Hailong; Wen, Xiewen; Zheng, Junrong
    An ultrafast two-dimensional visible/far-IR spectroscopy based on the IR/THz air biased coherent detection method and scanning the excitation frequencies is developed. The method allows the responses in the far-IR region caused by various electronic excitations in molecular or material systems to be observed in real time. Using the technique, the relaxation dynamics of the photo-excited carriers and electron/phonon coupling in bulk MoS2 are investigated. It is found that the photo-generation of excited carriers occurs within two hundred fs and the relaxation of the carriers is tens of ps. The electron-phonon coupling between the excitations of electrons and the phonon mode E1u of MoS2 is also directly observed. The electron excitation shifts the frequency of the phonon mode 9 cm−1 higher, resulting in an absorption peak at 391 cm−1 and a bleaching peak at 382 cm−1. The frequency shift diminishes with the relaxation of the carriers.
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    Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces
    (Springer Nature, 2016) Li, Jiebo; Qian, Huifeng; Chen, Hailong; Zhao, Zhun; Yuan, Kaijun; Chen, Guangxu; Miranda, Andrea; Guo, Xunmin; Chen, Yajing; Zheng, Nanfeng; Wong, Michael S.; Zheng, Junrong
    Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces.