Controlling adsorption and spin configurations of Co atoms on $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$

Combining scanning tunneling microscopy and first-principles calculations, we have shown that single Co atoms adsorbed on a $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ surface have eight different configurations that possess different spin magnetic moments. Despite the large adsorption energy, we have demonstrated that both the position and the spin state of single adsorbed Co atoms can be well controlled through the vertical atomic manipulation and the “tip-touch” operation that converts Co atoms among various adsorption configurations. Our approach to construct atomic-scale magnetic structures on a semiconductor surface can provide a new pathway to realize spin-based devices, including a scalable solid-state quantum computer.

Figure 1

Schematic showing the basic steps for (a) vertical manipulation and (b) adsorption sites conversion, respectively, via the tip-touch method. The black arrow indicates the moving direction of the STM tip in each step. $\Delta Z$ is the approaching or retracting distance of the tip. $A-D$ represent different adsorption sites on the substrate.

Figure 2

Filled state and empty state STM topographic images, simulated STM image, and structural models (top view and side view) of (a) clean Si half unit cells and (b) eight identified adsorption configurations of a single Co atom on $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$. The filled and empty state images are indicated by “−” and “+,” respectively, in (a). (c) The structural model showing six equivalent adsorption sites for configuration $D$ and three equivalent adsorption sites for configuration $H$, respectively, in a basin (the area enclosed by a corner adatom and two nearby center adatoms in the same HUC [35]). The following images are the superposition of the simulated STM images of a Co atom at these equivalent adsorption sites. All the filled (empty) state STM images are taken at $V_{\mathrm{s}}=-0.5\mathrm{V}(V_{\mathrm{s}}=+1\mathrm{V})$ and $I=5\mathrm{pA}$. The simulated images are taken at $V_{\mathrm{s}}=-0.5\mathrm{V}$.

Figure 3

An example of vertical manipulation showing that configurations $G$ and $B$ contain only one single Co atom. (a) Image with configuration $G$ in the dashed triangle before the Co atom was picked up. (b) and (c) Images depicting the Co atom being picked up and after pick-up from the position marked by the green dot. (d) and (e) Images exhibiting the Co atom being dropped off and after drop off on the green spot to form configuration $B$. The two dotted lines in (b) and (d) indicate when the pick-up and drop-off operations were performed.

Figure 4

Calculated spin-polarized partial density of states for the $d$ orbitals of configuration $D$ at various $U_{\mathrm{eff}}$ values. For clarity, the majority and minority states are displayed as positive and negative values, respectively.

Figure 5

Calculated spin-polarized density of states of five $d$ suborbitals for configurations $A-H$ at $U_{\mathrm{eff}}=5.8\mathrm{eV}$.

Figure 6

(a)–(d) Four sets of STM images showing adsorption configuration conversions realized by the tip-touch operation. In each example, panels (i) and (ii) show the image before and after the conversion, respectively. The triangles mark the corresponding Co adsorbed HUCs. The green dots indicate the tip position in the tip-touch operation. The uppercase letters near the triangles in each panel indicate the adsorption configuration of the Co atom. The images are taken at $V_{\mathrm{s}}=-0.5\mathrm{V}$ and $I=5\mathrm{pA}$.

Figure 7

Schematic of all 14 types of realized adsorption configuration conversions. Black arrows indicate the directions of conversion with labeled numbers near the green dots for the tip-touch positions and near the arrow for the conversion type. The success rate of each conversion is shown in parentheses.

Figure 8

(a)–(d) Four examples of vertical manipulation of a single Co atom on $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$. Panels (i) and (ii) show the images before picking up and after dropping off the Co atom, respectively. The dotted blue triangles enclose the HUC where the Co atom stays before pick-up. The green dotted triangles enclose the HUC where the Co atom is put down. The uppercase letters near the triangles in each panel indicate the configuration of the Co atom. The green and pink circles represent the position of the tip in pick-up and drop-off processes, respectively. The images are taken at $V_{\mathrm{s}}=-0.5\mathrm{V}$ and $I=5\mathrm{pA}$.

Figure 9

An example of combined use of adsorption configurations conversion and vertical manipulation. (a) and (b) STM images of converting a Co atom (enclosed by the dotted blue triangle) from configuration $H$ to configuration $G$ by the tip-touch operation. The green circle indicates the tip position in the tip-touch operation. Panel (c) shows the image of picking up the Co atom in the dotted triangle. Panel (d) shows the image scanned by the STM tip within an extra Co atom on the tip apex. Panel (e) shows the image after dropping off the Co atom in the green dotted triangle. The green and pink circles in (c) and (d) mark the tip position in pick-up and drop-off processes, respectively.

Figure 10

Filled state STM images of the $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ surface (a) before and (b) after transferring five single Co atoms in the five neighboring clean FHUCs (marked by dotted blue triangles). All five Co atoms are converted into configuration $D$ as highlighted by the dashed pink triangles. The images are taken at $V_{\mathrm{s}}=-0.5\mathrm{V}$ and $I=5\mathrm{pA}$.