Browsing by Author "Nakano, Aiichiro"

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    Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser
    (AIP Publishing, 2021) Britz, Alexander; Attar, Andrew R.; Zhang, Xiang; Chang, Hung-Tzu; Nyby, Clara; Krishnamoorthy, Aravind; Park, Sang Han; Kwon, Soonnam; Kim, Minseok; Nordlund, Dennis; Sainio, Sami; Heinz, Tony F.; Leone, Stephen R.; Lindenberg, Aaron M.; Nakano, Aiichiro; Ajayan, Pulickel; Vashishta, Priya; Fritz, David; Lin, Ming-Fu; Bergmann, Uwe
    Femtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d5/2 core level (M5-edge, 572–577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400 ± 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1–2 ps timescale, which is interpreted as electron–hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.
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    Molecular Simulation of MoS2 Exfoliation
    (Springer Nature, 2018) Zhou, Guoqing; Rajak, Pankaj; Susarla, Sandhya; Ajayan, Pulickel M.; Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya
    A wide variety of two-dimensional layered materials are synthesized by liquid-phase exfoliation. Here we examine exfoliation of MoS2 into nanosheets in a mixture of water and isopropanol (IPA) containing cavitation bubbles. Using force fields optimized with experimental data on interfacial energies between MoS2 and the solvent, multimillion-atom molecular dynamics simulations are performed in conjunction with experiments to examine shock-induced collapse of cavitation bubbles and the resulting exfoliation of MoS2. The collapse of cavitation bubbles generates high-speed nanojets and shock waves in the solvent. Large shear stresses due to the nanojet impact on MoS2 surfaces initiate exfoliation, and shock waves reflected from MoS2 surfaces enhance exfoliation. Structural correlations in the solvent indicate that shock induces an ice VII like motif in the first solvation shell of water.
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    Ultrafast non-radiative dynamics of atomically thin MoSe2
    (Springer Nature, 2017) Lin, Ming-Fu; Kochat, Vidya; Krishnamoorthy, Aravind; Bassman, Lindsay; Weninger, Clemens; Zheng, Qiang; Zhang, Xiang; Apte, Amey; Tiwary, Chandra Sekhar; Shen, Xiaozhe; Li, Renkai; Kalia, Rajiv; Ajayan, Pulickel; Nakano, Aiichiro; Vashishta, Priya; Shimojo, Fuyuki; Wang, Xijie; Fritz, David M.; Bergmann, Uwe
    Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.