Initiating and Monitoring the Evolution of Single Electrons Within Atom-Defined Structures

Using a noncontact atomic force microscope, we track and manipulate the position of single electrons confined to atomic structures engineered from silicon dangling bonds on the hydrogen terminated silicon surface. An attractive tip surface interaction mechanically manipulates the equilibrium position of a surface silicon atom, causing rehybridization that stabilizes a negative charge at the dangling bond. This is applied to controllably switch the charge state of individual dangling bonds. Because this mechanism is based on short range interactions and can be performed without applied bias voltage, we maintain both site-specific selectivity and single-electron control. We extract the short range forces involved with this mechanism by subtracting the long range forces acquired on a dimer vacancy site. As a result of relaxation of the silicon lattice to accommodate negatively charged dangling bonds, we observe charge configurations of dangling bond structures that remain stable for many seconds at 4.5 K. Subsequently, we use charge manipulation to directly prepare the ground state and metastable charge configurations of dangling bond structures composed of up to six atoms.

Figure 1

Charge configurations of two closely-spaced DBs. (a) Constant current filled state STM image, $-1.8\mathrm{V}$, 50 pA. (b) Constant height $\mathrm{\Delta }f$ image, 0 V, $-300\mathrm{pm}$. (c) Two constant height $\mathrm{\Delta }f$ line scans (0 V, $-300\mathrm{pm}$) at the position indicated by the orange arrows in (b). (d) $\mathrm{\Delta }f(V)$ spectroscopy taken above an isolated DB ($-370\mathrm{pm}$). The two individual segments have been fitted by two parabolas (solid lines: fit, dashed lines: extrapolation) corresponding to the neutral and negatively charged states (${\mathrm{DB}}^0$ and ${\mathrm{DB}}^{-}$, respectively). (e) Combined map of 400 constant height $\mathrm{\Delta }f$ line scans (0 V, $-300\mathrm{pm}$) taken sequentially over a 4.8 minute period. (f) Time-dependent bistable signal for the two individual DBs extracted from (e). (g) Histograms of the signals in (e). Labels indicate the charge state assignment of each peak. Scale bar is 1 nm (a)–(c),(e).

Figure 2

Evolution of charge configurations of a symmetric six DB structure at different tip heights. (a) Constant height $\mathrm{\Delta }f$ image (0 V, $-300\mathrm{pm}$). (b) Maps of 800 constant height $\mathrm{\Delta }f$ line scans acquired over 18 minutes at $-320\mathrm{pm}$ (top panel) and $-270\mathrm{pm}$ (bottom panel). The scale bar in (a) is 3 nm and applies to (b). Color bars correspond to $\mathrm{\Delta }f$. Histograms of the $\mathrm{\Delta }f$ extracted over each DB in (b) are available in Supplemental Material, Fig. S(3) [29]. (c) The average occupation of the structure inferred from digitizing the charge configuration at different $\mathrm{\Delta }z$. Two interaction regimes: read and write are indicated. (d) Energetic shift of the ($0/-$) levels of each DB in the structure (1, 3, and 6 are negatively charged) with respect to the ($0/-$) level of an isolated DB (0 eV). The levels are shifted Coulombically by the negative charges confined to the structure and are calculated in the absence of the probe using an electrostatic approximation of point charges and a surface dielectric constant of 6.35. Because the exact energy of the ($0/-$) level is unknown, we indicate a range of energies (blue shaded area) over which the bulk Fermi level would give rise to the charge configurations observed in the lower panel of (b).

Figure 3

Mechanically induced charge switching of a DB in a pair. (a) $\mathrm{\Delta }f(z)$ at 0 V taken above a vacancy (i) and on a DB in a pair (ii—approach; iii—retract). Inset: the short range forces acting on the neutral DB [curve ii in (a)] up to the sharp step observed in $|\mathrm{\Delta }f|$. See SM for more details. (b) Top panel: sketch of the equilibrium position of the host atom of a neutral DB (left) and a negatively charged DB (right). Dark blue, light blue, and orange atoms represent bulk silicon, silicon dimers, and hydrogen, respectively. Bottom panel: Free energy diagram depicting the neutral to negative charge transition for a DB due to the mechanical displacement of the host atom by the tip. $\mathrm{\Delta }E$ corresponds to the lattice relaxation energy.

Figure 4

Controlled preparation of charge configurations in symmetric and asymmetric DB structures. Visualization of the scan modes: (a) all measurements are restricted to the read regime; (b) The tip is scanned from left to right (L to R) in the write regime, retracted 50 pm to the read regime, and scanned back across; (c) the same process as (b) with directions reversed (write R to L). (d) Constant height $\mathrm{\Delta }f$ image of the symmetric six-DB structure (0 V, $-300\mathrm{pm}$). (e)–(g) Maps of 200 line scans across structure in (d) corresponding to scheme (a) shown in (e), scheme (b) shown in (f), and scheme (c) shown in (g) (write regime: $-320\mathrm{pm}$, read regime: $-270\mathrm{pm}$). (h) Constant height $\mathrm{\Delta }f$ image of the asymmetric five-DB structure, $-350\mathrm{pm}$, 0 V. (i)–(k) Maps of 200 line scans across structure (i) corresponding to scheme (a) (i), scheme (b) (j), and scheme (c) (k) (write regime: $-370\mathrm{pm}$, read regime: $-320\mathrm{pm}$). The scale bars in (d) and (h) are 3 nm. Data displayed in (e)–(k) correspond to the read regime of each sequence.