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We study pressure-induced structural evolution of vanadium diselenide (VSe 2 ), a 1T polymorphic member of the transition metal dichalcogenide (TMD) family, using synchrotron-based powder x-ray diffraction (XRD) and first-principles density functional theory (DFT). Our XRD results reveal anomalies at P ∼ 4 GPa in the c/a ratio, V-Se bond length, and Se-V-Se bond angle, signaling an isostructural transition. This transition is followed by a first-order structural transition from the 1T (space group P 3m1) phase to a 3R (space group R 3m) phase at P ∼ 11 GPa due to sliding of adjacent Se-V-Se layers. Both the transitions at ∼4 and 11 GPa are cognate with associated changes in the Debye-Waller factors not reported so far. We present various scenarios to understand the experimental results within DFT and find that the 1T to 3R transition is captured using spinpolarized calculations with Hubbard correction (U eff = U −J = 8 eV), giving a transition pressure of ∼9 GPa, close to the experimental value.

The recently discovered twisted graphene has attracted considerable interest. As imple chemical route was found to prepare twisted graphene by covalently linking layers of exfoliated graphene containing surfacec arboxyl groups with an amine-containingl inker (trans-1,4-diaminocyclohexane). The twisted graphene shows the expected selected area electron diffraction pattern with sets of diffraction spots out with different angular spacings, unlike graphene, which shows ah exagonal pattern. Twisted multilayerg raphene oxide could be prepared by the above procedure. Twisted boronn itride, prepared by cross-linking layers of boron nitride (BN) containing surface amino groups with oxalic acid linker,e xhibited ad iffraction pattern comparable to that of twisted graphene. First-principles DFT calculations threw light on the structures and the nature of interactions associated with twisted graphene/BN obtained by covalent linking of layers.Two-dimensional (2D) layered materials have become an importanta rea of research. [1][2][3][4][5] An important development in this area is the generation of van der Waals heterostructures formed by depositing am onolayero rafew layerso fa2D material on am onolayer or few layers of the same or another 2D material. [6,7] As an alternative to van der Waals heterostructures, superlattices of 2D materials have been generated by covalent cross-linking. [8, 9] These materials exhibit severaln ovel properties. An exciting recent discovery is that of 2D superlattices of graphene formed by twisted bilayers. [10,11] Twisted bilayer graphene (tBLG) superlattice is reported to be superconducting. The tBLG superlattice has been fabricated using drytransfer technique. [10] Mogerae tal. have synthesized superlattices of twisted multilayer graphene (tMLG) by drop coating so-lution of ah ydrocarbon such as naphthaleneo naN if oil followed by Joule heating. [11a] Selected area electron diffraction (SAED) patterns reveal angular relationsb etween the twisted layers of graphene. Twisted multilayer graphene exhibits split spots with definite angulars pacings in SAED pattern unlike graphite, which shows hexagonal diffraction spots, suggesting angular deviations from AB packing. Mosto ft he methods reportedf or the synthesis of twisted graphene require the use of substrates and are not amenable for large-scale synthesis. We considered it is worthwhile to explore whether such tMLG superlattices can be generated chemically by covalently linking the graphene 2D layers.W eh ave been successful in doing so with the exfoliated graphene (EG) by using trans-1,4-diaminocyclohexane (DACH)a st he linker (Scheme 1). Here, the layerby-layer assembly occurs with amide bonds between the layers. We have extended this coupling methodology to prepare twisted multilayer graphene oxide.M ore interestingly,w e have used amine-functionalized boronn itride (BN) layerst o link with oxalic acid to obtain twisted BN. Furthermore, we have simulatedt wisted graphene and BN structures by using first-principles density functional theory...

To cite this article: Subhadip Das et al 2021 Nanotechnology 32 045202 View the article online for updates and enhancements.

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