Differential conductance (dI/dV) images taken with a low-temperature scanning tunneling microscope enabled the first observation of the electron probability distribution of the molecular orbitals of a pentacene molecule directly adsorbed on a metal surface. The three highest occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) and the lowest unoccupied molecular orbital are imaged. Thus dI/dV imaging without any intervening insulating layer permits the visualization of a large variety of molecular orbitals in the electronic cloud of a wide-gap molecule physisorbed on a metal surface.

Gears are microfabricated down to diameters of a few micrometres. Natural macromolecular motors, of tens of nanometres in diameter, also show gear effects. At a smaller scale, the random rotation of a single-molecule rotor encaged in a molecular stator has been observed, demonstrating that a single molecule can be rotated with the tip of a scanning tunnelling microscope (STM). A self-assembled rack-and-pinion molecular machine where the STM tip apex is the rotation axis of the pinion was also tested. Here, we present the mechanics of an intentionally constructed molecule-gear on a Au(111) surface, mounting and centring one hexa-t-butyl-pyrimidopentaphenylbenzene molecule on one atom axis. The combination of molecular design, molecular manipulation and surface atomic structure selection leads to the construction of a fundamental component of a planar single-molecule mechanical machine. The rotation of our molecule-gear is step-by-step and totally under control, demonstrating nine stable stations in both directions.

A novel structure containing self-assembled, unstrained GaAs quantum dots is obtained by combining solid-source molecular beam epitaxy and atomic-layer precise in situ etching. Photo-luminescence (PL) spectroscopy reveals light emission with very narrow inhomogeneous broadening and clearly resolved excited states at high excitation intensity. The dot morphology is determined by scanning probe microscopy and, combined with single band and eight-band k.p theory calculations, is used to interpret PL and single-dot spectra with no adjustable structural parameter.

Quantum states of a trinaphthylene molecule were manipulated by putting its naphthyl branches in contact with single Au atoms. One Au atom carries 1-bit of classical information input that is converted into quantum information throughout the molecule. The Au-trinaphthylene electronic interactions give rise to measurable energy shifts of the molecular electronic states demonstrating a NOR logic gate functionality. The NOR truth table of the single molecule logic gate was characterized by means of scanning tunnelling spectroscopy.

A microscopic picture for the GaAs overgrowth of self-organized quantum dots is developed. Scanning tunneling microscopy measurements reveal two capping regimes: the first being characterized by a dot shrinking and a backward pyramid-to-dome shape transition. This regime is governed by fast dynamics resulting in island morphologies close to thermodynamic equilibrium. The second regime is marked by a true overgrowth and is controlled by kinetically limited surface diffusion processes. A simple model is developed to describe the observed structural changes which are rationalized in terms of energetic minimization driven by lattice mismatch and alloying.

By high resolution scanning tunneling microscopy, we investigate the morphological transition from pyramid to dome islands during the growth of Ge on Si(001). We show that pyramids grow from top to bottom and that, from a critical size on, incomplete facets are formed. We demonstrate that the bunching of the steps delimiting these facets evolves into the steeper dome facets. Based on first principles and Tersoff-potential calculations, we develop a microscopic model for the onset of the morphological transition, able to reproduce closely the experimentally observed behavior. DOI: 10.1103/PhysRevLett.93.216102 PACS numbers: 68.55.Ac, 68.35.Md, 68.37.Ef, 81.10.Aj Three-dimensional Ge islands coherently grown on Si(001) at high temperature are well known to show a bimodal behavior, with small, shallow f105g-faceted pyramids and larger domes, exhibiting steeper facets [1,2]. In the past few years some interpretations of the bimodal behavior have been provided, mostly relying on thermodynamic arguments. Based on volumetric strain relief, surface energies, and edge contributions [3], one explanation is that pyramids and domes correspond to two minima in the energy per atom, with an activated transition from one to the other morphology [1]. The second interpretation is grounded on the chemical potential of the island, which is argued to undergo an abrupt change at a certain critical volume, corresponding to the crossover between the energy per atom for a dome and the corresponding one for a pyramid [4][5][6]. Therefore, no energy minima are required to explain the bimodal behavior in this picture.When Ge is grown on Si(001) at relatively low temperature, only elongated {105}-faceted islands with narrow size distribution are observed. An interpretation of this phenomenon comes from kinetic models [7,8], where a self-limiting growth is explained in terms of kinetic slowing down occurring with increasing volume. This, in turn, should be provided by a size-dependent activation energy for adding a new monolayer to the f105g facets. In particular, Jesson et al. [7] suppose the additional layer to nucleate at a lower corner of the facet, where the strain is larger. Similarly, Kästner and Voigtländer [8] assume that the layer grows from bottom to top, founding their kinetic model on the stepped nature of the f105g facets. However, both the thermodynamic and kinetic models do not explain how the shape transition is microscopically accomplished. Recently, a description of how the shape transition occurs has been provided by Seifert and co-workers [9,10], suggesting a variation of the Jesson et al. model [7]. Here, at a critical pyramid size, new, steeper facets are supposed to nucleate close to the pyramid apex, where the lattice parameter is more relaxed. Yet, this hypothesis was based on qualitative arguments, with no modeling.In this Letter, we use high resolution scanning tunneling microscopy (STM), to investigate Ge islands grown on Si(001) with two different techniques. We show that the growth proceeds from top to bo...

Low-temperature epitaxial growth of Si–Ge heterostructures opens possibilities for synthesizing very small and abrupt low-dimensional structures due to the low adatom surface mobilities. We present photoluminescence from Ge quantum structures grown by molecular-beam epitaxy at low temperatures which reveals a transition from two-dimensional to three-dimensional growth. Phononless radiative recombination is observed from 〈105〉 faceted Ge quantum dots with height of approximately 0.9 nm and lateral width of 9 nm. Postgrowth annealing reveals a systematic blueshift of the Ge quantum dot’s luminescence and a reduction in nonradiative recombination channels. With increasing annealing temperatures Si–Ge intermixing smears out the three-dimensional carrier localization around the dot.

Whereas all 230 three-dimensional space groups occur in organic crystals, out of only 17 plane groups some highly symmetric ones such as p31m have not yet been observed in two-dimensional (2D) crystals of organic molecules. Here a kagome network with p31m symmetry is reported for cobalt phthalocyanine on Cu(111). This unusual structure results from substrate-induced reduction of molecular symmetry and substrate-mediated interaction via quantum interference of surface electrons. These interactions provide additional control over the symmetry of 2D crystals of phthalocyanines and lead to a variety of other symmetries in self-assembled arrays.