Browsing by Author "Heinz, Tony F."

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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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    Graphene as an atomically thin interface for growth of vertically aligned carbon nanotubes
    (Macmillan Publishers Limited, 2013) Rao, Rahul; Chen, Gugang; Arava, Leela Mohana Reddy; Kalaga, Kaushik; Ishigami, Masahiro; Heinz, Tony F.; Ajayan, Pulickel M.; Harutyunyan, Avetik R.
    Growth of vertically aligned carbon nanotube (CNT) forests is highly sensitive to the nature of the substrate. This constraint narrows the range of available materials to just a few oxide-based dielectrics and presents a major obstacle for applications. Using a suspended monolayer, we show here that graphene is an excellent conductive substrate for CNT forest growth. Furthermore, graphene is shown to intermediate growth on key substrates, such as Cu, Pt, and diamond, which had not previously been compatible with nanotube forest growth. We find that growth depends on the degree of crystallinity of graphene and is best on mono- or few-layer graphene. The synergistic effects of graphene are revealed by its endurance after CNT growth and low contact resistances between the nanotubes and Cu. Our results establish graphene as a unique interface that extends the class of substrate materials for CNT growth and opens up important new prospects for applications.