Shih-Hsin Chang

Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates

A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.

Molecular Kondo Chain

An important development in recent synthesis strategies is the formation of electronically coupled one and two-dimensional organic systems for potential applications in nanoscale molecule-based devices. Here, we assemble one-dimensional spin chains by covalently linking basic molecular building blocks on a Au(111) surface. Their structural properties are studied by scanning tunneling microscopy and the Kondo effect of the basic molecular blocks inside the chains is probed by scanning tunneling spectroscopy. Tunneling spectroscopic images reveal the existence of separate Kondo regions within the chains while density functional theory calculations unveil antiferromagnetic coupling between the spin centers.

Steering Two‐Dimensional Molecular Growth via Dipolar Interaction

The growth of self-organized molecular networks is recognized as a nature-given, potential bottom-up solution for further miniaturization of electronic devices into the nanometer regime. With self-assembling processes being fully parallelized rather than sequential as in conventional top-down approaches as e-beam writing, molecule-based devices gain their potential from a rational design of the fundamental molecular building blocks for controlled bond formation. Relevant aspects are bonding directions and bonding mechanisms. Several different bonding mechanisms are established for network formation as hydrogen bonding, covalent bonding, or even metal–organic coordination. Although the impact of dipole– dipole interaction on network formation is reported the design of molecular dipole fields is so far not further exploited. Here we report for the first time on such an approach and switch by synthetic means between dominating repulsive and attractive forces among metal–organic complexes. The effect is demonstrated for Co(5,5’-X2-Salen) complexes, X=H (1), Me (2), and Cl (3) locally monitored in ultra-high vacuum (UHV) by scanning tunneling microscopy (STM). Salen complexes of transition metals are versatile and easily modified to tailor electronic, magnetic, and structural bulk properties. Salen complexes are present in a variety of applications in material chemistry and catalysis. Moreover, most complexes are volatile in UHV which enable local studies in a well-defined nanoscopic environment. In Salen complexes the metal center is surrounded by two nitrogen and oxygen donor atoms, which can lead to complexes in quite high oxidization states. The hydrogen atoms at the 5,5’-positions can be substituted by CH3, F, Cl, Br, I, NO2, etc. via the respective salicylaldehyde precursor. Here, individual Co-salen complexes adsorbed on a surface are studied for the first time by STM and reveal an insight into the mechanism of intermolecular coupling. The experiments are performed in a variable temperature STM operated at ~25 K. Tips and Cu ACHTUNGTRENNUNG(111) surfaces are prepared by standard procedures with molecules sublimed from homebuilt Knudsen cells. Voltages refer to the potential of the tip relative to the sample. Positive voltages refer to tunneling into unoccupied sample states and negative voltages to tunneling out of occupied sample states. We focus on the analysis of STM images as acquired at 1.4 V, 0.1 V, and +1.4 V which are referred as ‘at large negative bias’, ‘at low bias’, and ‘at elevated positive bias’ throughout the text. Images at these biases show characteristic features as discussed below and are representative for images as acquired within a larger bias interval (~ 0.5 V) with only gradual changes in between. The chemical structures of the complexes used herein are presented in Figure 1 along with corresponding, equally scaled STM images at low bias of isolated molecules adsorbed on a Cu ACHTUNGTRENNUNG(111) surface. For visualization a scaled model of the structure is superimposed. For complex 1 the appearance of the complex in the STM image perfectly fits to the structure. Due to different substituents the apparent size of 2 and 3 varies in the topographic images. We will first focus on the appearance and adsorption of isolated molecules of complex 1, that is, after low temperature preparation which hinders thermally induced mobility and therefore self-assembling on Cu ACHTUNGTRENNUNG(111). Figures 2b–2d show the same isolated molecule of complex 1 but as imaged at different energies. Imaged at low bias (Figure 2c), the molecule nicely fits to the molecular structure and exhibits a maximum in the apparent height at a location between the cobalt center and the top C2H4 bridge. Imaged at a large negative energy a pronounced topographic maximum at the site of the Co ion dominates the overall molecular appearance (Figure 2b). We interpret this as resulting from occupied Co 3d orbitals perpendicular to the molecular plane similar to the case of other metal-organic complexes. A significant change can be observed when tunneling through unoccupied molecular states at elevated positive bias: molecules become asymmetric and the maximum in the apparent height is shifted towards one side of the C2H4 bridge (Figure 2d). A first interpretation is suggested by results of DFT calculations for the free complex. In Figure 2a an optimized structure of complex 1 is depicted. The side view clarifies a C2 symmetry due to a deformation of the C2H4-bridge which implies chirality. STM measurements identified complexes adsorbed in 12 different configurations on the Cu ACHTUNGTRENNUNG(111) surface. Figure 2e shows all 12 configurations as imaged at low bias. The observable asymmetry at elevated positive energies (Figure 2 f) unambiguously reveals two mirror symmetric (R vs L) sets of 6 molecules each rotated in steps of 608. With the exact crystallographic axes determined from atomically resolved images of the bare substrate, two mirror symmetric enantiomers can be attributed to each crystallographic direction. Molecules denoted with R are rotated clockwise by +118 38 relative to the substrate axes and with L anticlockwise by 88 38. This is schematically illustrated in Figure 2 g. [a] S. Kuck, Dr. S.-H. Chang, Dr. G. Hoffmann, Prof. Dr. R. Wiesendanger Institut f r Angewandte Physik Universit t Hamburg, Jungiusstrase 9, Hamburg (Germany) Fax: (+49)40-42838-2944 E-mail : skuck@physnet.uni-hamburg.de [b] J.-P. Klcckner, Prof. Dr. M. H. Prosenc Institut f r Anorganische und Angewandte Chemie Universit t Hamburg, Martin-Luther-King-Platz 6, Hamburg (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900281

High entropy alloy mediated growth of graphene

Pyrolysis of acetylene over thin films made of CuxFeCoNiMn yields graphene and its sheet dimensions are found to be controlled by x. A monolayer structure forms at x = 0.5 and the sheet size reaches a value as large as 600 μm2. The number of layers increases as x increases and turbostratic graphite forms at x = 1.5. The x controlled growth of graphene is confirmed by Raman mapping, atomic force microscopy (AFM) and transmission electron microscopy (TEM).

Adsorption Behavior of Asymmetric Pd Pincer Complexes on a Cu(111) Surface

We address the adsorption of asymmetric Pd pincer complexes on a Cu(111) surface by scanning tunneling microscopy. The structural asymmetry is manifested in the observation of two chiral enantiomers. To enable an unambiguous identification of individual constituents, three closely related complexes with small modifications are investigated in parallel. Thereby, methyl substituents determine attractive molecule-molecule interaction. Depending on their distribution, dimerization and tetramerization can be observed.

ZnO‐Coated Carbon Nanotubes: Inter‐Diffusion of Carboxyl Groups and Enhanced Photocurrent Generation

ZnO is a defect-governed oxide and emits light at both visible and UV regimes. This work employs atomic layer deposition to produce oxide particles on oxygenated carbon nanotubes, and the composites only show emission profiles at short wavelengths. The quenching of defect-related emissions at long wavelengths is verified, owing to carboxyl diffusion into oxygen vacancies, and doping is supported by ZnCO3 formation in oxide lattice. Fully coated tubes display an increased photocurrent and the quantum efficiency increases by 22 % relative to the bare nanotubes.

Scanning Tunneling Microscope Study of Structural Transformations on One Monolayer Pb/Si(111)

With a variable‐temperature scanning tunneling microscope (STM), we study structural phase transformations for one monolayer Pb/Si(111) from 170 K up to room temperature. On the 1×1 structure, Pb adatoms are located at the T1 site and the coverage is exactly 1 ML. The 1×1 phase undergoes a reversible phase transition into a 7 × 3 phase upon cooling below ∼260 K. Our study of the phase transition indicates that there is no coverage change across the transition and that the transition temperature decreases with the decreasing domain size. At room temperature, we find that some defects in the 1×1 phase can induce formation of a small region of alternating trimer domains around them. This may be a precursor to the formation of an entire region of a so‐called “incommensurate” phase, which is composed of alternating domains of two different trimer structures. The 7 × 3 and the “incommensurate” phases are basically the distorted 1×1 structure with Pb adatoms slightly displaced from the T1 site. The relationship ...