Energy-Gap Opening in a Bi(110) Nanoribbon Induced by Edge Reconstruction

Scanning tunnelling microscopy and spectroscopy experiments complemented by first-principles calculations have been conducted to study the electronic structure of 4 monolayer Bi(110) nanoribbons on epitaxial graphene on silicon carbide [$4H\mathrm{\text{−}}\mathrm{SiC}(0001)$]. In contrast with the semimetal property of elemental bismuth, an energy gap of 0.4 eV is measured at the centre of the Bi(110) nanoribbons. Edge reconstructions, which can facilitate the edge strain energy release, are found to be responsible for the band gap opening. The calculated density of states around the Fermi level are decreased quickly to zero from the terrace edge to the middle of a Bi(110) nanoribbon potentially signifying a spatial metal-to-semiconductor transition. This study opens new avenues for room-temperature bismuth nanoribbon-based electronic devices.

Figure 1

(a) Side view of 4 ML Bi film in a black phosphorouslike $A17$ structure (left) and bulk $A7$ structure (right). The lateral $[11\overline{2}]$ direction can be viewed from the left on both panels. (b) Atomic resolved STM image ($V_T=0.2\mathrm{V}$) of the Bi(110) nanoribbon terrace superimposed by the 1 ML Bi(110) atomic model.

Figure 2

(a) Large scale STM image ($V_T=-2.5\mathrm{V}$, $I_t=100\mathrm{pA}$) of the 4 ML Bi(110) nanoribbon on EG. The white arrow indicates the Bi stripe direction with respect to the notation of the epitaxial graphene on $\mathrm{SiC}(000\overline{1})$. (b) The differentiated STM image from the white square showing the atomic structure of step edges. (c) Line profiles along the edge (black line) and terrace (gray line) in panel (b).

Figure 3

(a) STM and (b) STS measurements on a typical 4 ML Bi(110) nanoribbon. The boundary between Bi on EG and Bi on the nanomesh (NM) is highlighted by a green dotted curve. The STS curves are shifted vertically for clarity. The characteristic peaks are denoted by labels.

Figure 4

Side view of symmetric 2 ML Bi(110) nanoribbon models with even edge (a) and odd edge (b) structure before (upper) and after (lower) structural relaxation. The fixed parts during structural optimization are shaded in gray. (c) The formation energy difference before (black square) and after (red dot) structural relaxation. (d) The per unit length edge energy in $A7$ and black phosphorous $A17$ structures with and without structural relaxation. The fitted exponential lines in both structures after relaxation are indicated by orange and blue colors as a guide to eye. (e) The per unit length edge energy variation upon the application of structural relaxation.

Figure 5

(a) Simulated STM images of a reconstructed $2\times 1$ 2 ML Bi(110) film. (b) Projected density of states (PDOS) of half-truncated 2 ML Bi(110) films as a function of energy. The shaded areas are a guide to eye, which correspond to characteristic peaks in experiments. The curves labeled in panel (a) are shifted for clarity. The Fermi levels as indicated by dashed lines in both figures are shifted to the zero energy. (c) The quantity of projected density of states at the Fermi level at different positions as indicated in panel (a).