Torque-Induced Change in Configuration of a Single NO Molecule on Cu(110)

We demonstrated that a nitric oxide (NO) molecule on Cu(110) acts as an “ON-OFF-ON toggle switch” that can be turned on and off by repulsive force and electron injection, respectively. On the surface, NO molecules exist in three configurations: flat along the [001] direction (ON), upright (OFF), and flat along $[00\overline{1}]$ (ON). An NO-functionalized tip, which was characterized by scanning tunneling microscopy and inelastic electron tunneling spectroscopy, can convert an upright NO adsorbate into a flat-lying NO. Atomic force microscopy and a simulation of the interactions between the NO molecules reveal that a repulsive force not aligned with the N—O bond provides the torque that detrudes the NO toggle; i.e., the upright NO adsorbate is tilted away from the tip. Therefore, the NO adsorbate behaves as a nonvolatile sensor for the detection of locally applied repulsive torque.

Figure 1

(a)–(c) STM images of upright NO (top), $(\mathrm{NO}{)}_2$ (middle), and flat-lying NO (bottom) on Cu(110) at 4.8 K with (a) Cu, (b) uNO, and (c) tNO tips. The blue dots in (a) represent the lateral positions of the N atoms. The insets depict line profiles measured over the upright NO along the yellow line in (a). (d) Atomic structure; the blue, red, and brown spheres represent N, O, and Cu atoms, respectively. The yellow lobes schematically represent the electron probability densities of the NO molecules near the Fermi level. (e)–(g) Side view schematic structures of an upright NO with (e) Cu, (f) uNO, and (g) tNO tips.

Figure 2

(a) $dI/dV$ (black) and $d^{2}I/d{V}^2$ (red) spectra recorded over the bare Cu(110) surface with a uNO tip. (b) $dI/dV$ spectrum recorded over the bare surface with a tNO tip. Set point: $V=30\mathrm{mV}$ and $I=200\mathrm{pA}$. The insets show side-view schematics of these systems.

Figure 3

Tip-induced conversion of upright NO into flat-lying NO. (a) STM images of two upright NO molecules on Cu(110) using a tNO tip. (b) The same area following approach of the tip to the surface over the red point in (a). (c) The same area after the tip approached the surface over the red point in (b). The green lines indicate the topmost Cu-atom lattice. (d) [(f)] $\mathrm{\Delta }f(z)$ and (e) [(g)] $U(z)$ for $(\mathrm{a})\to (\mathrm{b})$ [$(\mathrm{b})\to (\mathrm{c})$]. The tip approach and retraction are indicated in red and blue, respectively.

Figure 4

(a) $U(x,z)$ map recorded with a tNO tip near an upright NO on Cu(110). Inset shows an STM image of the target. The cyan line represents the $x$ region of the map. Black arrows represent force vectors $\mathit{F}(x,z)$ acting on the tip. Green line indicates the region where the NO configuration changes from upright to flat lying. (b) Depicting the side-view atomic structure immediately before conversion. (c) Simulated $U(x,\mathrm{\Delta }z)$ map for a flexible NO adsorbate with a fixed NO molecule at the tip apex. Black arrows represent $\mathit{F}(x,\mathrm{\Delta }z)$ acting on the tip. Inset displays the model structure and the definitions of $x$ and $\mathrm{\Delta }z$. Brown spheres represent particles that simulate the surface and the tip body. (d) Model structure corresponding to $(x,\mathrm{\Delta }z)=(-0.1\AA ,7.6\AA )$, which is located at the bottom-right edge of the map. (e) Simulated potential surface for the rotation of the NO adsorbate.