Nanofilm Allotrope and Phase Transformation of Ultrathin Bi Film on $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}7\times 7$

Our scanning tunneling microscopy and electron diffraction experiments revealed that a new two-dimensional allotrope of Bi forms on the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}7\times 7$ surface. This pseudocubic $\{012\}$-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) phase, as the bulk contribution in the cohesion becomes dominant. Based on ab initio calculations, we propose that the new allotrope consists of black phosphorus-like puckered layers stabilized by saturating all the $p_z$ dangling bonds in the film.

Figure 1

Schematics of (a) top view and side view of the $\{012\}$-oriented phase and (b) top view and side view of the (001)-oriented phase, based on the so-called “distorted A7 structure” of the bulk Bi [5]. The orange arrow is the trigonal $\mathbf{c}$ axis ([001] axis). Blue and red arrows are [100] and [010] axes ($\mathbf{a}$ and $\mathbf{b}$ axes), respectively. The two-dimensional unit cell in the $\{012\}$ and (001) planes is indicated by each dotted white line.

Figure 2

Evolution of the SPA-LEED pattern (a)–(d), and the STM image (e)–(h) taken at RT as a function of Bi deposition with respect to the wetting layer; stage I ($<2\mathrm{M}\mathrm{L}$), stage II (2–4 ML), stage III (4–6 ML), and stage IV ($>6\mathrm{M}\mathrm{L}$). Primary energy of the electron beam in the LEED experiment is 65.0 eV. Insets in (a) and (b) are the magnification around the (00) and the (01) spots, with reciprocal space sizes of 0.26 and $0.32{\AA }^{-1}$, respectively. All the STM images are taken with $1600Å\times 1600\AA$ area. Tip bias is (e) 1.0 V, (f) 1.5 V (inset; 1.0 V), (g) 1.0 V, and (h) 1.0 V (inset; 0.1 V). The unit cell of the surface structure is indicated by the white dotted line in the insets in (f) and (h). (i) The RHEED pattern from a 15.2 ML film taken with 15 keV beam energy. (j),(k) STM profiles correspond to the white dotted lines in (f) and (h), respectively.

Figure 3

(a) Calculated layer adsorption energy $E_a(n)$ for the $\mathrm{B}\mathrm{i}\{012\}$ phase with $n$ layers as a function of film thickness for both Bi films with ${\mathrm{B}\mathrm{i}}_2{\mathrm{H}}_2$ (blue) and ${\mathrm{B}\mathrm{i}\mathrm{H}}_3$ (red) wetting layers. Energy is displayed with respect to the bulk cohesive energy. (b). The left-hand side is a side view of a four-layer $\mathrm{B}\mathrm{i}\{012\}$ phase with unrelaxed bulk structure viewed from the [100] direction. The right-hand side is the calculated atomic structure after structural optimization. The intrabilayer structure of this new nanofilm allotrope is analogous to the so-called “puckered-layer structure” of black phosphorus while their stacking sequence is different.

Figure 4

Cohesive energy $U(n)/n$ of the $\{012\}$ and (001) phases calculated as a function of layer thickness $n$ (red and blue curves, respectively). Stable regions are indicated by solid curves and unstable regions in dashed curves. The green arrow indicates the crossover thickness.