Date/Time: 04-23-2019 - Tuesday - 05:00 PM - 07:00 PM
Zhiwei Wang1 Deshuai Li1 Zhong Lin Wang1

1, Chinese Academy of Sciences, Beijing, , China

The fabrication of high-performance thermal and electronic devices requires a deep and comprehensive understanding of electron-phonon coupling and lattice relaxation behaviors. Some theoretical models, such as the empirical two-temperature model, have been proposed to explaining for ultrafast dynamics processes, but the accurate mechanism description of structural transition phenomenon is far from trivial [1]. Ultrafast Transmission electron microscopy (UTEM) is one of the most important characterization techniques for investigating transient structural changes such as atomic movements and phase transitions, etc. with not only high spatial resolution but also ultrafast temporal resolution [2]. Herein, we present a combined static and ultrafast dynamic investigation of polycrystalline-like gold nanofilms on silicon nitride supports through in-situ heating and UTEM techniques.

Our experimental investigation was carried out on a 200 kV FEI G20 femto UEM coupled with a Clark Impulse fiber laser with pulse duration of ~ 210 fs and center wavelength of ~1.03um. Two laser beams were introduced into UTEM columns for pumping samples (frequency doubled) and producing ultrashort photoelectron packets (frequency-quadrupled). In situ heating observation was performed using a DENSsolutions heating specimen holder. The gold nanofilms were grown by way of electron beam evaporation.

The static, thermal-equilibrium investigation was firstly performed by acquiring the diffraction intensity and lattice spacing of {220} rings as a function of rising temperatures via in-situ heating. The systematic measurements enable a precise determination of the actual Debye temperature. The significantly smaller lattice expansion coefficient than the bulk value was also found, which indicates that a caution should be taken in using reported bulk parameters especially for nanoscale samples. We then applied the accurately calibrated diffraction intensity/spacing -temperature relationship from the in-situ heating study to the ultrafast pump-probe study. The electron-phonon coupling constant of the nanofilms was then determined. Most interestingly, we found that the temporal changes of lattice spacing and diffraction intensity feature significantly different maximum temperature rise at quasi-equilibrium states. This phenomenon may be only attributed to the effect of non-thermal equilibrium relaxation dynamics induced by femtosecond laser excitations. This study provides insights into quantitative understanding of electron-phonon coupling and lattice dynamics.

[1] Ono, S. Phys. Rev. B 2017, 96, 024301.
[2] Barwick, B.; Park, H. S.; Kwon, O. H.; Baskin, J. S.; Zewail, A. H. Science 2008, 322, 1227-1231.

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