Stable line defects in silicene

Line defects in two-dimensional (2D) materials greatly modulate various properties of their pristine form. Using ab initio molecular dynamics (AIMD) simulations, we investigate the structural reconstructions of different kinds of grain boundaries in the silicene sheets. It is evident that depending upon the presence of silicon adatoms and edge shape of grain boundaries (i.e., armchair or zigzag), stable extended line defects (ELDs) can be introduced in a controlled way. Further studies show the stability of these line-defects in silicene, grown on Ag(111) surface at room-temperature. Importantly, unlike most of the 2D sheet materials such as graphene and hexagonal boron nitride, 5-5-8 line defects modify the nonmagnetic semimetallic pristine silicene sheet to spin-polarized metal. As ferromagnetically ordered magnetic moments remain strongly localized at the line defect, a one-dimensional spin channel gets created in silicene. Interestingly, these spin channels are quite stable because, unlike the edge of nanoribbons, structural reconstruction or contamination cannot destroy the ordering of magnetic moments here. Zigzag silicene nanoribbons with a 5-5-8 line defect also exhibit various interesting electronic and magnetic properties depending upon their width as well as the nature of the magnetic coupling between edge and defect spin states. Upon incorporation of other ELDs, such as 4-4-4 and 4-8 defects, 2D sheets and nanoribbons of silicene show a nonmagnetic metallic or semiconducting ground state. Highlighting the controlled formation of ELDs and consequent emergence of technologically important properties in silicene, we propose new routes to realize silicene-based nanoelectronic and spintronic devices.

Figure 1

Initial structures (i.e., before reconstruction) of grain boundaries of (a) ZSi-ZSi, (b) ASi-ASi, and (c) ZSi-KSi.

Figure 2

Snapshot of top and side views of ZSi-ZSi during AIMD simulations at constant temperature (300 K) and constant pressure (1 atm) after (a) 0, (b) 0.25, (c) 0.8, and (d) 5 ps.

Figure 3

Total potential energy during AIMD simulations of reconstruction of the ZSi-ZSi GB.

Figure 4

The top and side views of the equilibrated structure (i.e., after 10 ps) of (a) ASi-ASi and (b) ZSi-KSi. Simulated STM images of (c) 5-5-8 ELD, (d) 4-8 ELD, and (e) 4-4-4 ELD are shown. The isosurface is taken at 3.25 Å from the topmost Si atom in all these sheets.

Figure 5

The (a) side and (b) top views of the equilibrated structure (i.e., after 5 ps) of ZSi-ZSi on the surface of Ag(111). Green spheres represent Ag atoms.

Figure 6

(a) ZSi-ZSi structure with the 5-5-8 ELD (filled green) and (b) the rectangular BZ with high-symmetry $k$ points used here. Spin-polarized (c) band structure and (d) density of state for the 5-5-8-ELD-containing silicene sheet. Black (red) solid lines in (c) and (d) correspond to majority (minority) spin bands and majority (minority) total DOS, respectively. In (d), green (maroon) dashed lines represent the majority (minority) site-projected DOS where the contributions up to the second nearest neighbor of ${\mathrm{Si}}_{\mathrm{ad}}$ atoms are summed up. (e) Top and side views of spin densities for ZSi-ZSi, where ochre (red) isosurfaces signify majority (minority) spin. Isosurface value is considered to be 0.001 $e/{\AA }^3$. (f) Local magnetic moment with respect to distance from ${\mathrm{Si}}_{\mathrm{ad}}$ dimer atoms. The dimer line of ${\mathrm{Si}}_{\mathrm{ad}}$ is considered the origin.

Figure 7

Band-decomposed charge densities of ZSi-ZSi structure with the 5-5-8 ELD at high-symmetry $k$ points for (a) band 1, (b) band 2, and (c) band 3. The isosurface value is considered to be 0.001 $e/{\AA }^3$ for all charge densities.

Figure 8

(a), (d) Band structure and (b), (e) total and site-projected DOS for 4-8-ELD- and 4-4-4-ELD-containing silicene sheets, respectively. In the DOS plot, red dashed lines represent the site-projected DOS for which the contribution up to the second nearest neighbors of ${\mathrm{Si}}_{\mathrm{ad}}$ atoms are summed up. Band-decomposed charge densities at high-symmetry $k$ points of band 1 and band 2 for (c) ASi-ASi and (f) ZSi-KSi.

Figure 9

Band structures of 5-5-ZSi-ZSi-NR in (a) FM-1, (c) FM-2, and (e) AFM states. (b), (d), and (f) Top and side views of the spin-density plot for these states, respectively. Ochre and red isosurfaces signify majority and minority spins, respectively. The isosurface value is 0.01 e/${\AA }^3$.

Figure 10

Band-decomposed charge densities at high-symmetry $k$ points, i.e., $\mathrm{\Gamma }$ and $X$ points of 5-5-ZSi-ZSi-NR in the FM-1 and FM-2 states. The isosurface value is 0.0075 $e/{\AA }^3$.

Figure 11

(a) Band structure of ZSi-ZSi considering the spin-orbit-coupling effect. The band dispersion near the Fermi level only through the $\mathrm{\Gamma }-X$ direction is shown here. Band splitting at the $M$ and $K^{\prime \prime }$ points is shown. (b) Zoomed-in view of the band near the $M$ point and band-decomposed charge densities of SOC-split bands at particular $k$ points are shown.