Low-pressure chemical vapor deposition allows one to grow high structural quality monolayer graphene on Ir(111). Using scanning tunneling microscopy, we show that graphene prepared this way exhibits remarkably large-scale continuity of its carbon rows over terraces and step edges. The graphene layer contains only a very low density of defects. These are zero-dimensional defects, edge dislocation cores consisting of heptagon−pentagon pairs of carbon atom rings, which we relate to small-angle in-plane tilt boundaries in the graphene. We quantitatively examined the bending of graphene across Ir step edges. The corresponding radius of curvature compares to typical radii of thin single-wall carbon nanotubes.

Structural Coherency of Graphene on Ir(111)

Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

Size-dependent valence and conduction band-edge energies of semiconductor nanocrystals.

Asthma is a common disease affecting an increasing number of children throughout the world. In asthma, pulmonary airways narrow in response to contraction of surrounding smooth muscle. The precise nature of functional changes during an acute asthma attack is unclear. The tree structure of the pulmonary airways has been linked to complex behaviour in sudden airway narrowing and avalanche-like reopening. Here we present experimental evidence that bronchoconstriction leads to patchiness in lung ventilation, as well as a computational model that provides interpretation of the experimental data. Using positron emission tomography, we observe that bronchoconstricted asthmatics develop regions of poorly ventilated lung. Using the computational model we show that, even for uniform smooth muscle activation of a symmetric bronchial tree, the presence of minimal heterogeneity breaks the symmetry and leads to large clusters of poorly ventilated lung units. These clusters are generated by interaction of short- and long-range feedback mechanisms, which lead to catastrophic shifts similar to those linked to self-organized patchiness in nature. This work might have implications for the treatment of asthma, and might provide a model for studying diseases of other distributed organs.

Self-organized patchiness in asthma as a prelude to catastrophic shifts

Ordered gold (Au) mono- and bilayer structures have been synthesized on a highly reduced titania surface. The Au bilayer exhibits a significantly higher catalytic activity for carbon monoxide oxidation than does the Au monolayer structure. This is the first report of Au completely wetting an oxide surface and demonstrates that ultrathin Au films on an oxide surface have exceptionally high catalytic activity, comparable to the activity observed for Au nanoparticles. This discovery is a key to understanding the nature of the active site of supported Au catalysts.

Catalytically Active Gold: From Nanoparticles to Ultrathin Films

The photochemistry of small molecules on well-defined metal surfaces has been the subject of intense research for more than three decades.1 This field is of interest because it rests on the superposition of two influences. On one hand, new reaction channels can become possible by electronic excitation, which are usually not accessible by thermal activation. On the other hand, compared to molecular photochemistry in the gas phase, interactions of molecules with solid substrates open unique pathways of photoexcitation and photoreaction not accessible in homogeneous reactions. This is primarily due to the fact that the bonding interactions with the substrate modify not only the ground state but also the electronically excited states of the adsorbates. In addition, the very rapid exchange of excitation energy between adsorbates and substrate, in particular on metal and semiconductor surfaces, can lead to fast quenching of excited adsorbate states by transfer of charge and/or energy from the adsorbates to the substrate. Furthermore, excitation of adsorbates by hot (excited) electrons produced by photoabsorption in the substrate plays an important role as it induces charge and energy transfer in the opposite direction, from the substrate to adsorbates. Well-defined metal surfaces covered by adsorbate layers under ultrahigh vacuum (UHV) conditions provide systems which are well characterized in all aspects, particularly regarding geometry and electronic structure. The fact that such layers usually consist of molecules which are naturally aligned on the surface2 makes surface photochemistry a powerful alternative to the stereodynamic control of chemical reactions by optically aligned molecular beams.3 Well-defined adsorbate systems thus provide unique playgrounds for surface photochemistry.

http://w0.rz-berlin.mpg.de/hjfdb/pdf/449e.pdf

Photochemistry on Metal Nanoparticles

We have measured ultraviolet photoemission spectra of quasi-size-selected silver clusters at T ­ 40 K grown in the nanopits of a graphite surface. The Fermi-level onset observed shows distinct deviations from the steplike shape typical for metals. A simple model that takes into account the finite lifetime of the photohole (corresponding to a charged cluster) can explain these deviations by the clustersubstrate interaction on a femtosecond time scale, and hence provide an explanation for the differences between photoemission spectra of free and deposited clusters in general. [S0031-9007(98)07729-1]

/pdf/cluster-substrate-interaction-on-a-femtosecond-time-scale-4y4txi7bsw.pdf

Cluster-Substrate Interaction on a Femtosecond Time Scale Revealed by a High-Resolution Photoemission Study of the Fermi-Level Onset

Modification of the Shockley-type surface state on Ag(111) by an adsorbed xenon layer

We have studied the structure of Xe adsorbed on a highly oriented pyrolytic graphite surface in the submonolayer regime using scanning tunneling microscopy (STM) at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5\mathrm{K}$. The Xe adlayer forms hexagonal domains in a honeycomblike superstructure. We observe domain walls tilted by roughly 12\ifmmode^\circ\else\textdegree\fi{} with respect to the atomic rows of the Xe layer, which explains the previously found rotation of the diffraction pattern of about 0.5\ifmmode^\circ\else\textdegree\fi{} relative to the ideal $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ layer. The STM signal exhibits a voltage-dependent contrast between Xe domains and domain walls, which can be understood by the different adsorption sites and the band structure of the Xe adlayer.

/pdf/physisorbed-rare-gas-monolayers-evidence-for-domain-wall-4lel7jh2de.pdf

Physisorbed Rare-Gas Monolayers: Evidence for Domain-Wall Tilting

Tunneling spectroscopy on silver clusters at T=5 K: size dependence and spatial energy shifts

We present a method for the preparation of single walled carbon nanotubes (SWNTs) on a highly oriented pyrolytic graphite (HOPG) surface in ultrahigh vacuum (UHV), for which the preparation parameters for the production of metal clusters, fixed to nanometer sized pits on the surface, and the subsequent deposition of carbon can be controlled separately. Using cobalt as the cluster metal we carried out a comprehensive study concerning the influence of the substrate temperature (up to 900 °C) and the effective film thickness for the carbon evaporation. With scanning tunneling microscopy in UHV at room temperature and at T=77 K we observed single, separated SWNTs of about 50 nm length, which frequently were angled or branched and included junctions between sections of different tube diameters. With a statistical evaluation of tube diameters, tube lengths, and cluster heights, we obtained new insights into the growth mechanisms. An increase of tube diameters with increasing substrate temperature and a strong c...

/pdf/growth-mechanisms-of-carbon-nanotubes-using-controlled-52li4docwh.pdf

B. Grimm

Papers

Cluster-Substrate Interaction on a Femtosecond Time Scale Revealed by a High-Resolution Photoemission Study of the Fermi-Level Onset

Modification of the Shockley-type surface state on Ag(111) by an adsorbed xenon layer

Physisorbed Rare-Gas Monolayers: Evidence for Domain-Wall Tilting

Tunneling spectroscopy on silver clusters at T=5 K: size dependence and spatial energy shifts

Growth mechanisms of carbon nanotubes using controlled production in ultrahigh vacuum