A self-consistent model to link surface electronic band structure to the voltage dependence of hot electron induced molecular nanoprobe experiments.

Understanding the ultra-fast transport properties of hot charge carriers is of significant importance both fundamentally and technically in applications like solar cells and transistors. However, direct measurement of charge transport at the relevant nanometre length scales is challenging with only a few experimental methods demonstrated to date. Here we report on molecular nanoprobe experiments on the Si(111)-7 × 7 at room temperature where charge injected from the tip of a scanning tunnelling microscope (STM) travels laterally across a surface and induces single adsorbate toluene molecules to react over length scales of tens of nanometres. A simple model is developed for the fraction of the tunnelling current captured into each of the surface electronic bands with input from only high-resolution scanning tunnelling spectroscopy (STS) of the clean Si(111)-7 × 7 surface. This model is quantitatively linked to the voltage dependence of the molecular nanoprobe experiments through a single manipulation probability (i.e. fitting parameter) per state. This model fits the measured data and gives explanation to the measured voltage onsets, exponential increase in the measured manipulation probabilities and plateau at higher voltages. It also confirms an ultrafast relaxation to the bottom of a surface band for the injected charge after injection, but before the nonlocal spread across the surface.

Molecular and atomic manipulation mediated by electronic excitation of the underlying Si(111)-7x7 surface.

We report the local atomic manipulation properties of chemisorbed toluene molecules on the Si(111)-7x7 surface and of the silicon adatoms of the surface. Charge injected directly into the molecule, or into its underlying bonding silicon adatom, can induce the molecule to change bonding site. The voltage dependence of the rates of these processes match closely with scanning tunnelling spectroscopy of the toluene and adatom species. The branching ratio between toluene molecules which are moved to a neighbouring site, or those that travel further is invariant to voltage, suggesting a common final manipulation step for both injection into the molecule and into the bonding adatom site. At low temperatures the rate of silicon adatom manipulation matches that of toluene manipulation, further suggesting that all these manipulation processes are driven by electronic excitation of the underlying silicon surface. Our results therefore suggest that a common non-adiabatic process mediates atomic and molecular manipulation induced by the STM on the Si(111)-7x7 surface and may also mediate similar manipulation induced by the laser irradiation of the Si(111)-7x7 surface.

Non-local atomic manipulation on semiconductor surfaces in the STM: the case of chlorobenzene on Si(111)-7×7.

Control over individual atoms with the scanning tunnelling microscope (STM) holds the tantalising prospect of atomic-scale construction, but is limited by its "one atom at a time" serial nature. "Remote control" through non-local STM manipulation-as we have demonstrated in the case of chlorobenzene on Si(111)-7×7-offers a new avenue for future "bottom-up" nanofabrication, since hundreds of chemical reactions may be carried out in parallel. Thus a good understanding of the non-local manipulation process, as provided by recent experiments, is important. Comparison of scanning tunnelling spectroscopy (STS) measurements of the bare Si(111)-7×7 surface and chemisorbed chlorobenzene molecules with the voltage dependence of the non-local STM-induced desorption of chlorobenzene proves particularly instructive. For example, the chlorobenzene LUMO appears at +0.9 V with respect to the Fermi level, whereas non-local manipulation thresholds are found at +2.1 V and +2.7 V. This difference supports a picture in which the voltage thresholds for non-local electron-induced desorption depend principally on the energies of the electronic states of the surface. Furthermore, the demonstration that the non-local process is largely insensitive to surface steps up to five layers in height suggests that either the electron transport in this process is subsurface in character or surface charge transport is responsible but is in some way unaffected by the steps.

Concerted Thermal-Plus-Electronic Nonlocal Desorption of Chlorobenzene from Si(111)-7 × 7 in the STM.

The rate of desorption of chemisorbed chlorobenzene molecules from the Si(111)-7 × 7 surface, induced by nonlocal charge injection from an STM tip, depends on the surface temperature. Between 260 and 313 K, we find an Arrhenius thermal activation energy of 450 ± 170 meV, consistent with the binding energy of physisorbed chlorobenzene on the same surface. Injected electrons excite the chlorobenzene molecule from the chemisorption state to an intermediate physisorption state, followed by thermal desorption. We find a second thermal activation energy of 21 ± 4 meV in the lower temperature region between 77 and 260 K, assigned to surface phonon excitation.

A new mechanism of atomic manipulation: bond-selective molecular dissociation via thermally activated electron attachment.

We report a new mechanism of (bond-selective) atomic manipulation in the scanning tunneling microscope (STM). We demonstrate a channel for one-electron-induced C-Cl bond dissociation in chlorobenzene molecules chemisorbed on the Si(111)-7 × 7 surface, at room temperature and above, which is thermally activated. We find an Arrhenius thermal energy barrier to one-electron dissociation of 0.8 ± 0.2 eV, which we correlate explicitly with the barrier between chemisorbed and physisorbed precursor states of the molecule. Thermal excitation promotes the target molecule from a state where one-electron dissociation is suppressed to a transient state where efficient one-electron dissociation, analogous to the gas-phase negative-ion resonance process, occurs. We expect the mechanism will be obtained in many surface systems, and not just in STM manipulation, but in photon and electron beam stimulated (selective) chemistry.

Electronic switching of single silicon atoms by molecular field effects.

We have observed on-off switching of scanning tunneling microscope current flow to silicon adatoms of the Si(111)-(7 x 7) surface that are enclosed within a bistable dimeric corral of self-assembled chlorododecane molecules. These thermally activated oscillations amounted to an order of magnitude change in the current. Theory showed that small changes in molecular configuration could cause alterations in the corralled adatom's electronic energy by as much as 1 eV due to local field effects, accounting for the observed current switching.

Tip-state control of rates and branching ratios in atomic manipulation.

We report the atomic manipulation properties of two distinct, stable, and reproducible states of a scanning tunneling microscope tip applied to chlorobenzene/Si(111)-(7x7). We show that the tip state influences the rates of (current-driven) molecular desorption and C-Cl dissociation as well as the branching ratio between these processes, but does not change the mediating electronic channel or the required number of electrons. These manipulation properties combined with the imaging properties of the two tip-states suggest the major difference between tip-states is their coupling efficiency to the pi-states of the chlorobenzene molecule.