Light emission from Ag(111) driven by inelastic tunneling in the field emission regime.

We study the light emission from a Ag(111) surface when the bias voltage on a scanning tunneling microscope (STM) junction is ramped into the field emission regime. Above the vacuum level, scanning tunneling spectroscopy (STS) shows a series of well defined resonances associated with the image states of the surface, which are Stark shifted due to the electric field provided by the STM tip. We present photon-energy resolved measurements that unambiguously show that the mechanism for light emission is the radiative decay of surface localized plasmons excited by the electrons that tunnel inelastically into the Stark shifted image states. Our work illustrates the effect of the tip radius both in the STS spectrum and the light emission maps by repeating the experiment with different tips.

Quantum dots with single-atom precision.

Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantum dots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. Electrostatic gates can be used to mitigate these variations by adjusting the electron energy levels, but the more ambitious goal of creating quantum dots with intrinsically digital fidelity by eliminating statistical variations in their size, shape and arrangement remains elusive. We used a scanning tunnelling microscope to create quantum dots with identical, deterministic sizes. By using the lattice of a reconstructed semiconductor surface to fix the position of each atom, we controlled the shape and location of the dots with effectively zero error. This allowed us to construct quantum dot molecules whose coupling has no intrinsic variation but could nonetheless be tuned with arbitrary precision over a wide range. Digital fidelity opens the door to quantum dot architectures free of intrinsic broadening-an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies of confined electrons.

Ge(001) as a template for long-range assembly of π-stacked coronene rows.

The adsorption of coronene molecules (C(24)H(12)) on the Ge(001) surface has been studied by means of scanning tunnelling microscopy (STM). Upon room temperature deposition, the coronene molecules adsorb in an upright geometry forming compact layers patterned in rows for coverages of one monolayer and less, being the only example investigated so far in which a pure aromatic hydrocarbon forms a well-ordered monolayer on a non-passivated semiconductor surface. At half monolayer, the molecular rows consist of long chains of π-stacked molecules and the distance between molecular planes is 8 Å. This configuration is maintained upon cooling the system below the transition temperature of Ge(001) (~220 K), but the molecular layer experiences also a transition from rows perpendicular to rows parallel to the Ge dimer rows. We interpret our observations in terms of a weak bonging between molecules and substrate, which facilitates the formation of large ordered domains of molecules, revealing Ge(001) as an ideal template for the growth of this and other aromatic hydrocarbons.