title: Topochemical Transformations of CaX2 (X=C, Si, Ge) to Form Free-Standing Two-Dimensional Materials.
abstract: Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high-yield synthesis of pristine, defect-free two-dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid-state dispersion-corrected calculations, mechanisms for such transformations are elucidated that provide an in-depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans-bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans-bent structure of acetylene in its first excited state (S1 ) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase-h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane.
authors.affiliation: Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur-700032, West Bengal (India).
authors.forename: Saied Md
authors.initials: SM
authors.lastname: Pratik
authors.affiliation: Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur-700032, West Bengal (India).
authors.forename: A
authors.initials: A
authors.lastname: Nijamudheen
authors.affiliation: Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur-700032, West Bengal (India). spad@iacs.res.in.
authors.forename: Ayan
authors.initials: A
authors.lastname: Datta
keywords: Group 14 elements; layered materials; metal carbides; phase transitions; tetrahedrane