Electronic coupling between Bi nanolines and the Si(001) substrate: An experimental and theoretical study

Atomic nanolines are one-dimensional systems realized by assembling many atoms on a substrate into long arrays. The electronic properties of the nanolines depend on those of the substrate. Here, we demonstrate that to fully understand the electronic properties of Bi nanolines on clean Si(001) several different contributions must be accounted for. Scanning tunneling microscopy reveals a variety of different patterns along the nanolines as the imaging bias is varied. We observe an electronic phase shift of the Bi dimers, associated with imaging atomic $p$ orbitals, and an electronic coupling between the Bi nanoline and neighboring Si dimers, which influences the appearance of both. Understanding the interplay between the Bi nanolines and Si substrate could open a novel route to modifying the electronic properties of the nanolines.

Figure 1

Appearance and structure of the Bi nanoline. $6.0\times 5.5{\mathrm{nm}}^2$ experimental STM micrographs of the same Bi nanoline at (a) $-2.0$ V (0.1 nA) and (b) $+2.0$ V (0.1 nA). (c) Ball and stick model of the Bi nanoline, showing the subsurface Haiku structure. Si atoms are in beige (light) and Bi in purple (dark).

Figure 2

(a) dI/dV maps of a Bi nanoline at different bias voltages. (b) Corresponding STM micrograph of the same location at $-3.0$ V (0.2 nA), with images aligned to defects in the nanoline (same image shown twice). (c) Averaged dI/dV spectra measured on the Bi nanoline (red) and on the bare Si (blue).

Figure 3

(a) $3.5\times 3.1{\mathrm{nm}}^2$ experimental STM micrograph of a Bi nanoline at 1.1 V (0.2 nA), compared against simulated STM images of the Bi nanoline at 0.7 V for various different Si surface arrangements. (b) $p(2\times 2)$, (c) $p(2\times 2)$ with a mirror plane through the center of the nanoline, (d) $c(4\times 2)$, (e) $c(4\times 2)$ with a mirror plane, (e) $p(2\times 2)$ and $c(4\times 2)$.

Figure 4

Simulated STM of the Bi nanoline, with a $p(2\times 2)$ Si reconstruction, showing regions inaccessible to experiment. (a) $-0.3$ to $-0.5$ V (left to right). (b) +0.7 V, provided for comparison. At negative bias the bright spots on the Bi nanoline neighbor up-buckled Si atoms, at positive bias they neighbor down-buckled Si atoms.

Figure 5

Simulated STM images for Bi nanolines with surface H passivation, at positive (left) and negative (right) bias. (a) Full H passivation at 0.8 and $-0.5$ V, (b) one DB at 0.7 and $-0.5$ V, (c) buckled clean Si dimer at 0.6 and $-0.5$ V. In (c) the up-buckled Si is on the left. Circles indicate depassivated regions. The link between a buckled Si substrate and the charge structure along the Bi nanoline is obvious.

Figure 6

$2.4\times 3.1{\mathrm{nm}}^2$ STM micrographs of a Bi nanoline at (a) 1.20 V, (b) 1.40 V, and (c) 1.45 V. DFT simulations of (d) the clean Bi nanoline at 0.3, 0.5 and 0.7 V, (e) the bare Haiku (no Bi) at 0.7 V, and (f) of a Bi dimer line without the Haiku structure at 0.4 and 0.6 V. In all cases, red lines indicate the position of the trench in the Si background.

Figure 7

(a) $2.6\times 3.1{\mathrm{nm}}^2$ experimental STM micrograph of a Bi nanoline at 1.5 V (1 nA), compared against simulated STM for a Bi nanoline on $c(4\times 2)$ Si at 1.1 V for isosurface values of (b) 0.1 and (c) 1 arbitrary units. (d) Simulated STM for a Bi nanoline on H:Si(001) at 0.88 V.