Direct identification of monolayer rhenium diselenide by an individual diffraction pattern

Abstract

In the current extensive studies of layered two-dimensional (2D) materials, compared to the hexagonal ones, like graphene, hBN and MoS2, low symmetry 2D materials have shown great potential for applications in anisotropic devices. Rhenium diselenide (ReSe2) has the bulk space group P1 � and belongs to triclinic crystal system with a deformed cadmium iodide type structure. Here we propose an electron diffraction based method to distinguish monolayer ReSe2 membrane from multilayer ReSe2, and its two different vertical orientations, our method could also be applicable to other low symmetry crystal systems, including both triclinic and monoclinic lattices, as long as their third unit-cell basis vectors are not perpendicular to their basal planes. Our experimental results are well explained by kinematical electron diffraction theory and corresponding simulations. The generalization of our method to other 2D materials, like graphene, is also discussed.Address correspondence to Chuanhong Jin, chhjin@zju.edu.cn Moreover, ReS2 and ReSe2 share similar crystal structures, they both have a distorted octahedral 1T structure and belong to triclinic crystal system, and Re atoms form zigzag chains in the basal plane, arising from the Peierls distortion. Therefore, ReSe2 and ReS2 flakes have both in-plane and out-of-plane anisotropy, and recently there have been numbers of applications based on their anisotropic properties [12][13][14][15][16][17].When materials are thinned down to atomically thin membranes, thickness begins to play an important role in tailoring their properties, and vice versa, there are a variety of techniques to determine their thicknesses, ranging from image contrast of reflected light microscopy [18][19][20][21][22][23][24], second harmonic microscopy [25][26][27][28], intuitive atomic force microscopy and cross-sectional imaging to Raman and photoluminescence spectroscopy [29][30][31][32]. In the field of electron microscopy, there are miscellaneous methods to identify thickness as well, which include peak shift in plasmon spectroscopy [33,34], direct high-resolution transmission electron microscopy (HRTEM) imaging combined with simulations [35] and linearity in annular dark field scanning transmission electron microscope (ADF-STEM) signals [36,37]. Apart from the above methods, electron diffraction analysis [34,[38][39][40][41][42][43] has also been demonstrated as an efficient tool to determine thickness in 2D materials, since it could be conducted on any commercial uncorrected TEM conveniently with negligible beam damages on a large area of pristine crystalline samples. Furthermore, diffraction based methods usually involve a series of sample tilting or use the relative ratio of the chosen diffraction spots, and some researchers noticed the intensity mismatch in a pair of crystallographically equivalent diffraction spots (Friedel pair) [41,44,45], here we demonstrate a method of identifying monolayer ReSe2 by the centrosymmetry with a single diffraction pattern.

Pressure dependence of direct optical transitions in ReS2 and ReSe2

“…Since the broadening parameter of the excitonic transitions at ambient pressure is around ≈ 20 meV, it is difficult to spectrally resolve and fit all the transitions that coexist in a range smaller than ≈ 40 meV with Eq. [1]. In order to evaluate all the excitonic transitions and their pressure dependence, polarization-resolved measurements have been performed at selected pressures (all spectra are shown in the S.I., Figs.…”

“…The energy of both transitions have been determined fitting Eq. [1] employing the same procedure used for ReS2. Fig.…”

“…These properties result from their particular band structure and reduced crystal symmetry (i.e. Td distorted octahedral structure, belonging to the triclinic crystal system 1 ) as well as their strong 2D character that has been attributed to weak van der Waals interlayer bonding even in their bulk form. 2,3 Besides the large fundamental interest, ReX2 has also shown to be a highly interesting technological material for many potential applications, including photodetectors, [4][5][6][7][8][9] solar cells, 10 photonics, 11 flexible electronics 12 and field-effect transistors.…”

Coherent Lattice Wobbling and Out-of-Phase Intensity Oscillations of Friedel Pairs Observed by Ultrafast Electron Diffraction

“…Unequal intensities of Friedel pairs can be observed. [8][9][10]13,14 This phenomenon breaks Friedel's law, when Friedel's law is stated directly by equal intensities of Friedel pairs (the first statement), 8,9 or the equal intensities of Friedel pairs can be deduced according to the crystal symmetry in the second statement. 6,13,14 Currently, most reports about such unequal Friedel pair intensities, or breaking of Friedel's law, are conducted in static diffractions, caused by either the curvature of the Ewald sphere 8−10 or the dynamic scattering effects.…”

“…Only by tilting the sample to a low-symmetry axis, such as the [212] direction in single-crystal TaSeTe (when the Friedel pair that breathes in opposite directions is cut by the Ewald sphere), 17 can out-ofphase intensity oscillations be experimentally observed, because of the breathing phonon and Ewald sphere curvature. [8][9][10]17 Distinctive from the previously studied 2D materials, monoclinic gallium telluride (GaTe) is a lowsymmetry crystal, 21,22 which belongs to the C2/m space group and has only four symmetry operations (i.e., E, C 2 , σ h , and i). It can be expected that distinct lattice dynamics and diffraction features can be found in monoclinic GaTe.…”