Nanoscale imaging of electronic surface transport probed by atom movements induced by scanning tunneling microscope current

Absence of Nonlocal Manipulation of Oxygen Atoms Inserted below the Si(111)-7×7 Surface

The injection of electrons from the scanning tunneling microscope tip can be used to perform nanoscale chemistry and study hot electron transport through surfaces. While nonlocal manipulation has been demonstrated primarily for aromatic adsorbates, here we confirm that oxygen atoms bonded to the Si(111) surface can also be nonlocally manipulated, and we fit the measured manipulation data to a single channel decay model. Unlike aromatic adsorption systems, oxygen atoms also insert below the surface of silicon. Although the inserted oxygen can be manipulated when the tip is directly over the relevant silicon adatom, it is not possible to induce nonlocal manipulation of inserted oxygen atoms at the same bias. We attribute this to the electrons injected at +4 eV initially relaxing to couple to the highest available surface state at +3.4 eV before laterally transporting through the surface. With a manipulation threshold of 3.8 eV for oxygen inserted into silicon, once carriers have undergone lateral transport, they do not possess enough energy to manipulate and remove oxygen atoms inserted beneath the surface of silicon. This result confirms that nonlocal nanoscale chemistry using the scanning tunneling microscope tip is dependent not only on the energy required for atomic manipulation, but also on the energy of the available surface states to carry the electrons to the manipulation site.

Atomically resolved real-space imaging of hot electron dynamics

The dynamics of hot electrons are central to understanding the properties of many electronic devices. But their ultra-short lifetime, typically 100 fs or less, and correspondingly short transport length-scale in the nanometre range constrain real-space investigations. Here we report variable temperature and voltage measurements of the nonlocal manipulation of adsorbed molecules on the Si(111)-7 × 7 surface in the scanning tunnelling microscope. The range of the nonlocal effect increases with temperature and, at constant temperature, is invariant over a wide range of electron energies. The measurements probe, in real space, the underlying hot electron dynamics on the 10 nm scale and are well described by a two-dimensional diffusive model with a single decay channel, consistent with 2-photon photo-emission (2PPE) measurements of the real time dynamics.

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.

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.

Hot carrier-selective chemical reactions on Ag(110)

Here, we show that the pathways, products, and efficiencies of reactions occurring on a metal surface can be spatially modulated by varying the type and energy of hot carriers produced by injecting tunneling electrons or holes from a scanning tunneling microscope tip into the metal surface. Control over the metal surface reactions was demonstrated for the large-scale dissociation reaction of O2 molecules on a Ag(110) surface. Hot electrons (or holes) transported through the metal surface to chemisorbed O2 selectively dissociated the molecule into two oxygen atoms separated along the [110] (or [001]) lattice direction. The reaction selectivity was enhanced compared to the selectivity of a direct reaction involving tunneling carriers.

Nonlocal desorption of chlorobenzene molecules from the Si(111)-(7×7) surface by charge injection from the tip of a scanning tunneling microscope: remote control of atomic manipulation

We report the nonlocal desorption of chlorobenzene molecules from the Si(111)-(7×7) surface by charge injection from the laterally distant tip of a scanning tunneling microscope and demonstrate remote control of the manipulation process by precise selection of the atomic site for injection. Nonlocal desorption decays exponentially as a function of radial distance (decay length ∼100  A) from the injection site. Electron injection at corner-hole and faulted middle adatoms sites couples preferentially to the desorption of distant adsorbate molecules. Molecules on the faulted half of the unit cell desorb with higher probability than those on the unfaulted half.

Nonlocal activation of a bistable atom through a surface state charge-transfer process on Si(100)-(2×1):H

The reversible hopping of a bistable atom on the Si(100)-(2×1):H surface is activated nonlocally by hole injection into Si-Si bond surface states with a low temperature (5 K) scanning tunneling microscope. In the contact region, at short distances (<1.5  nm) between the hole injection site and the bistable atom, the hopping yield of the bistable atom exhibits remarkable variations as a function of the hole injection site. It is explained by the density of state distribution along the silicon bond network that shows charge-transfer pathways between the injection sites and the bistable atom.

Nonlocal dissociative chemistry of adsorbed molecules induced by localized electron injection into metal surfaces

We present a novel approach to surface chemistry studies using scanning tunneling microscopy (STM), where dissociation of molecules adsorbed on metal surfaces is induced nonlocally in a 10-100 nm radius around the STM tip by hot electrons that originate from the STM tip and transport on the surface. Nonlocal molecular excitation eliminates the influence of the STM tip on the outcome of the electron-induced chemical reaction. The spatial attenuation of the nonlocal reaction is used as a direct measure of hot-electron transport on the surface.

Picometer-Scale Electronic Control of Molecular Dynamics Inside a Single Molecule

Tunneling electrons from a low-temperature (5 kelvin) scanning tunneling microscope were used to control, through resonant electronic excitation, the molecular dynamics of an individual biphenyl molecule adsorbed on a silicon(100) surface. Different reversible molecular movements were selectively activated by tuning the electron energy and by selecting precise locations for the excitation inside the molecule. Both the spatial selectivity and energy dependence of the electronic control are supported by spectroscopic measurements with the scanning tunneling microscope. These experiments demonstrate the feasibility of controlling the molecular dynamics of a single molecule through the localization of the electronic excitation inside the molecule.