File Name : figure-s1.tif Caption : phonon band structure for the chair conformation of sii calculated using the finite displacement method1 in a 3x3x1 supercell. File Name : figure-s2.tif Caption : contour plot of the total energy per si atom versus strain % applied along the a and b axes of the sii chair polymorph shown in figure 1(a). File Name : figure-s3.tif Caption : electronic band structure of the sii chair polymorph [figure 1(a)] for different strain levels applied along the a and b axes of the orthogonal conventional unit cell. the green arrows indicate the high symmetry points where the valence band maxima and conduction band minima are located in each case. the inset in the 0% strain diagram shows the corresponding brillouin zone and high symmetry points. File Name : figure-s4.tif Caption : electronic band structure of the type 1b si(mecn) sheet. the valence band maximum is set to 0 ev and the inset shows the first brillouin zone and the corresponding high symmetry points. File Name : figure-s5.tif Caption : electronic band structure of the type 1a si(mecn) polymorph (figure 3) for different strain levels applied along the a and b axes of the unit cell. the green arrows indicate the high symmetry points where the valence band maxima and conduction band minima are located in each case. the inset in the 0% strain diagram shows the corresponding brillouin zone and high symmetry points. File Name : figure-s6.tif Caption : contour plot of the total energy per si atom versus strain % applied along the a and b axes of the type 1 si(mecn) polymorph (shown in figure 3). File Name : figure-s7.tif Caption : top and side views of different polymorphs of 2d-si(mecn) sheets (si: orange, i: purple, n: blue, c: gray, h: white spheres). File Name : figure-s8.tif Caption : top and side views of different polymorphs of 2d-si(mecn) sheets (si: orange, i: purple, n: blue, c: gray, h: white spheres). File Name : figure-s9.tif Caption : top and side views of different polymorphs of 2d-si(mecn) sheets (si: orange, i: purple, n: blue, c: gray, h: white spheres). File Name : figure-s11.tif Caption : the supercell used to simulate a slab of casi2 crystal with exposed surfaces in the y and z directions with 20 å of vacuum between adjacent images. the reported lengths do not correspond to the scale of the figure. the slab is periodic and continuous along the x direction. the atoms in the bottom ca layer are kept fixed during structural relaxations (ca: green, si: orange spheres). File Name : figure-s12.tif Caption : reactions of iodine with a ca terminated casi2 crystal using the orthogonal cell of figure 6(a). δe is the change in total energy per cell relative to the bare surface of panel (a) calculated according to equation 2. all steps are exothermic and lead to the desorption of the top ca layer leaving exposed the si layer below. in this way the reaction cycle of figure 7 is reset. (ca: green, si: orange, i: purple spheres)