Session 3

Dongseok Suh CINAP, IBS, Dept. of Energy Science, Sungkyunkwan University, Korea

In this presentation, the usage of graphene as an indicator to show the characteristics of contacting dielectric substrate is introduced. We employed a functional oxide material such as a ferroelectric single-crystal and a high-k epitaxial thin film as a substrate for graphene in the back-gate field-effect transistor device configuration, and examined the graphene’s conductance variation as a function of gate voltage. From the background knowledge about the intrinsic electrical transport of graphene, the properties of contacting functional oxide materials can be deduced by the careful examination about the movement of conductance minimum, the variation of quantum Hall conductance plateau region, and the tendency of hysteresis during the sweep of gate voltage. Because the graphene’s conductance can be sensitively affected by the selection of functional oxide materials, we can utilize the graphene transistor as a useful probe to study dielectrics as reported in the case of the graphene on Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ ferroelectric single-crystal substrate ([1]) and the graphene on epitaxial SrTiO₃ thin-film ([2,3]).

[1] N. Park, et al., ACS Nano 9 (11), 10729 (2015)

[2] J. Park, et al., Nano Lett. 16 (3), 1754 (2016)

[3] K. T. Kang, et al., Adv. Mater. 29 (18), 1700071 (2017)

K. Andre Mkhoyan Department of Chemical Engineering and Materials Science, U. of Minnesota

2D systems have been demonstrated to be excellent materials for charge and spin transport in devices. Their exceptional performances in these devices are afforded by their unique electronic structure in their single- or few-layer states. As such, it would be advantageous to characterize how their atomic and electronic structures change while embedded in actual devices as compared to their free-standing states. We present a method to study 2D materials fabricated in a device as well as their surrounding interfaces by preparing ultra-thin cross sectional TEM samples using a focused ion beam and measuring the atomic and electronic structure using analytical transmission electron microscopy (TEM). 2D material devices are first thinned using an FEI Helios G4 in order to minimize the ion beam damage incurred during TEM sample preparation. Subsequently, analytical TEM is performed on an aberration corrected FEI Titan G2 60-300 S/TEM. STEM is used simultaneously with energy dispersive x-ray spectroscopy at lower magnification to map the chemical structure of the device.

Ji-Hee Kim CINAP, IBS, Sungkyunkwan University, Suwon, Korea. E-mail: kimj@skku.edu

Two-dimensional (2D) van der Waals materials have attracted great interest due to their unique optical, electrical and structural properties [1,2,3], and have great potential for optoelectronic devices, such as light emitting diodes, photodetectors, and photovoltaics with efficient power conversion. The reduced dimensionality in 2D materials allows strong Coulomb coupling with large exciton binding energy and efficient light-matter interaction. These result in the enhanced excitonic absorption.

By using spectrally resolved ultrafast transient absorption spectroscopy, we examine the population dynamics of excitons in 2D materials. We probe the two excitonic transitions, A and B exciton at K-point as well as dynamics in indirect band, shown in Fig. 1(a) and (b). Formation of excitons, which arises in < 1 ps time scale [4], including the exciton absorption strength and the signatures of carrier multiplication, will be further discussed.

Figure (a) Spectrally- and temporally-resolved ultrafast response after resonant excitation at the B exciton in molebdenum ditellurides. (b) The corresponding temporal dynamics at different transition energies. (c) Coherent phonon oscillations with different pump fluence.

[1] K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich, S. Morozov, and A. Geim, Proc. Natl. Acad. Sci. 102, 10451 (2005)

[2] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Nano Lett. 10, 1271 (2010)

[3] D. H. Keum, S. Cho, J. H. Kim, D.-H. Choe, H.-J. Sung, M. Kan, H. Kang, J.-Y. Hwang, S. W. Kim, H. Yang, K. J. Chang & Y. H. Lee, Nat. Physics 11, 482 (2015).

[4] A. Singh et al., Phys. Rev. B 93, 041401 (2016)

David Flannigan Department of Chemical Engineering and Materials Science, U. of Minnesota

Conventional transmission electron microscopy (TEM) has become an indispensable tool for comprehensive nanoscale materials characterization. While TEM spatial and energy resolutions have reached half-angstrom and few-meV levels, respectively, state-of-the-art detectors are able to resolve dynamics occurring only on the order of milliseconds. Such temporal resolutions are insufficient for studying a wealth of charge-carrier, structural, and magnetic behaviors. To overcome this, stroboscopic pump/probe approaches have been developed by interfacing a conventional TEM with short-pulsed lasers. In this way, temporal resolutions can be improved by 10 orders of magnitude to sub-picosecond timescales. In this talk, I will describe our work on the development and the application of this approach – ultrafast electron microscopy (UEM). I will begin by providing an overview of the UEM methodology and technology specific to our lab. Following this, I will describe a selection of our results on resolving the influence of nanoscale structural discontinuities (e.g., interfaces and crystal terraces) on coherent, elastic strain-wave dynamics in layered materials. Among other behaviors, we find that wave-train emergence occurs at extended discontinuities, with propagation directions oriented normal to the interface, independent of in-plane crystallographic direction. Properties of the wave trains (GHz frequencies, speed-of-sound velocities, and single in-plane wave directions) suggest the generation of a single acoustic-phonon mode following photoexcitation, with observable interference effects occurring at vacuum/crystal interfaces. The main objective of the talk is to give a sense of the capabilities of the UEM lab at the University of Minnesota.