Diatomic Molecular Switches to Enable the Observation of Very-Low-Energy Vibrations

Using low-temperature scanning tunneling microscopy and spectroscopy, we found that the coadsorption of atomic hydrogen to single transition-metal and rare-earth-metal atoms on a Ag(100) surface gives rise to surprising phenomena, a bias dependent switching from a large to a small apparent size of the diatomic molecules and a concomitant appearance of very low-energy vibrational features of 3 to 7 meV in the differential conductance spectra. These phenomena, which have until now escaped observation, may be of general relevance for low-temperature adsorption.

Figure 1

(a) STM image of a $5\times {10}^{-4}\mathrm{\text{monolayer}}$ (ML) Ce on Ag(100) at a temperature of 4.9 K revealing two different adsorbate species at the surface, a stable one $A$ and an unstable switching object $B$. Scanning is horizontally line by line from bottom to top [(a),(b)]. Image size $40\times 40{\mathrm{nm}}^2$, $V_T=-100\mathrm{mV}$, $I_T=50\mathrm{pA}$. (b) Cross sections (as indicated by the vertical white line in the topography at $V_T=-20\mathrm{mV}$) and STM topographies of the two objects $A$ and $B$ at different bias voltages, showing the switching behavior in the apparent height of the object $B$.

Figure 2

(a) STM topography of a switching object $B$ and a nonswitching one $A$ recorded at $V_T=-100\mathrm{mV}$, $I_T=100\mathrm{pA}$. Image size $5.7\times 5.7{\mathrm{nm}}^2$. (b) Constant-current spectroscopy $z(V_T)$ on a nonswitching adsorbate $A$ and a switching one $B$. The vertical tip displacement at $V_T\cong -100\mathrm{mV}$ bias voltage corresponds to an apparent height difference of 0.1 nm. (c) One switching molecule, observed in its small state (2) at $V_T=-100\mathrm{mV}$ and in its large state (1, left) at $V_T=-20\mathrm{mV}$. The three other equivalent large configurations are revealed by displaying the height differences with respect to configuration (1). (d) Ball model (top and side views) showing the geometry of the CeH molecule adsorbed on Ag(100) in its small (2) and large (1) state.

Figure 3

$dI/dV$ spectra (black) and STM topographies of metal-monohydrides. CeH in its large (1) and small (2) state, as well as LaH and CrH in their small state recorded at $T=4.9\mathrm{K}$. Tunneling spectroscopy parameters $V_{\mathrm{mod}}=1\mathrm{mV}$, $I_T=500\mathrm{pA}$. Simulation of the data (red) using Eq. (1) yields an excitation energy $E_i\approx 3\mathrm{meV}$ for CeH and LaH and $\approx 7\mathrm{meV}$ for CrH.