Hubertus Marbach

Adsorption of cobalt (II) octaethylporphyrin and 2H-octaethylporphyrin on Ag(111): new insight into the surface coordinative bond

The adsorption of cobalt (II) octaethylporphyrin (CoOEP) and 2H-octaethylporphyrin (2HOEP) on Ag(111) was investigated with scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS/UPS), in order to achieve a detailed mechanistic understanding of the surface chemical bond of coordinated metal ions. Previous studies of related systems, especially cobalt (II) tetraphenylporphyrin (CoTPP) on Ag(111), have revealed adsorption-induced changes of the oxidation state of the Co ion and the appearance of a new valence state. These effects were attributed to a covalent interaction of the Co ion with the silver substrate. However, recent studies show that the porphyrin ligand of adsorbed CoTPP undergoes a pronounced saddle-shape distortion, which could alter the electronic structure and thus provide an alternative explanation for the new valence state previously attributed to the formation of a surface coordinative bond. With the octaethylporphyrins investigated here, which were found to adsorb in a flat, undistorted conformation on Ag(111), the effects of geometric distortion can be separated from those of the electronic interaction with the substrate. The CoOEP monolayer gives rise to an adsorption-induced shift of the Co 2p signal (?1.9?eV relative to the multilayer), a new valence state at 0.6?eV below the Fermi energy, and a work-function shift of ?0.84?eV (2HOEP: ?0.44?eV) relative to the clean surface. Comparison with data for the distorted CoTPP confirms the existence of a covalent ion?surface interaction that is insensitive to the conformation of the ligand.

Surface-Confined Coordination Chemistry with Porphyrins and Phthalocyanines: Aspects of Formation, Electronic Structure, and Reactivity

Abstract Recent years have seen rapid progress in the field of surface-confined coordination chemistry. Adsorbed metal complexes of tetrapyrroles (porphyrins, phthalocyanines, corroles) are especially interesting in this context, since they combine a planar structure-determining element with an active site. While earlier studies of adsorbed metallo-tetrapyrroles mainly addressed aspects of molecular self-assembly, the focus of interest has shifted gradually to electronic structure and chemical reactivity. This article gives an overview of recent advances in the field of surface chemistry with tetrapyrroles. In particular, the following aspects will be discussed: intramolecular conformation and supramolecular ordering, electronic interaction with the substrate, surface-confined synthesis, and ligand-related effects such as the surface trans effect.

Electron-Beam-Induced Deposition in Ultrahigh Vacuum: Lithographic Fabrication of Clean Iron Nanostructures

The generation of nanostructures with arbitrary shapes and well-defined chemical composition is still a challenge and targets the core of the fast-growing field of nanotechnology. One approach is the maskless nanofabrication technique of electron-beam-induced deposition (EBID). Up to now, the purity of these EBID structures has been rather poor. Here we demonstrate that by performing the EBID process solely under ultrahigh vacuum conditions, the lithographic generation of iron nanostructures on Si(100) with an unprecedented purity of higher than 95% is possible. One particular new aspect is the formation of EBID deposits with reduced size in a strain-induced diffusive process, resulting in deposits significantly smaller than 10 nm.

Electrons as “Invisible Ink”: Fabrication of Nanostructures by Local Electron Beam Induced Activation of SiOx

The injection or removal of electrons can be used to trigger chemical processes, such as bond formation or dissociation. In this regard, electrons are an excellent and “clean” tool to modify or engineer the properties of different materials. The availability of localized electron probes, for example, in scanning electron microscopy (SEM), has made it possible to apply electron-induced processes on the nanometer and subnanometer scale. This approach can be used to target the generation of extremely small, pure nanostructures with lithographic control, which is one of the main goals in nanotechnology. The starting point of our study was the electron beam induced deposition (EBID) technique. The principle of EBID is outlined in Scheme 1a–c. A highly focused electron beam locally decomposes adsorbed precursor molecules to leave a deposit of nonvolatile fragments. The importance of EBID recently increased since it superseded focused ion beam processing as a method to repair lithographic masks in the semiconductor industry. The underlying physical and chemical principles of electron-induced bond making and breaking are in general also of great interest for important technological applications such as electron beam lithography (EBL), which is the standard method of generating the masks for UV lithography. As there is a large variety of precursor molecules and there are nearly no restrictions in regard to the substrate, EBID allows almost every combination of deposit material and substrate to be targeted. As a prototype example for conductive structures on an insulating material, our aim here was to generate clean iron nanostructures on a SiOx layer on Si(001). Scheme 1a–c depicts a schematic representation of

Substrate‐Mediated Phase Separation of Two Porphyrin Derivatives on Cu(111)

The self-assembly of molecular building blocks on flat surfaces opens up the perspective to engineer molecular architectures with tailored functionalities. Porphyrins appear as ideal candidates for such building blocks: they combine a rigid-structural theme, that is, the macrocycle, which often triggers long-range order, and a central metal atom as active site; this determines the intrinsic functionality. Indeed, the generation of multicomponent porphyrinoid adlayers has successfully been realized and characterized by scanning tunneling microscopy (STM) on different surfaces in ultrahigh vacuum (UHV) and in solution. The key for the fabrication of tailored supramolecular networks is detailed understanding of the adsorption behavior of the involved molecules, which is determined by the interplay of molecule–substrate and molecule–molecule interactions. Herein, we report the phase separation of CoTPP (TPP= tetraphenylporphyrin) and 2HTPP (2H-tetraphenylporphyrin) molecules on CuACHTUNGTRENNUNG(111) at RT. For comparison, also results on Ag ACHTUNGTRENNUNG(111) are discussed, for which the commonly observed behavior, namely the formation of ordered intermixed 2HTPP/CoTPP layers, is found. First, the adsorption of 2HTPP on AgACHTUNGTRENNUNG(111) and CuACHTUNGTRENNUNG(111) is discussed. For AgACHTUNGTRENNUNG(111), a square 2HTPP array with a lattice constant of 1.40 0.05 nm is observed as depicted in Figure 1 a. This arrangement and the specific appearance of individual 2HTPP molecules by STM (saddle-shape distortion) is well known and discussed in detail in Ref. [9]. Generally, on AgACHTUNGTRENNUNG(111) isolated TPP molecules are very mobile at RT; this allows for the formation of ordered islands with a square lattice. This type of incommensurate lateral arrangement was recently also reported for MTPPs (M=Co, Fe, Zn) on AgACHTUNGTRENNUNG(111)[9] and CoTPP on Cu ACHTUNGTRENNUNG(111)[10] and appears to be of general nature, not depending on the particular combination of adsorbate and substrate. For AgACHTUNGTRENNUNG(111), the specific arrangement within the square lattice is attributed to mutual stabilization by attractive, intermolecular, T-type interactions between the phenyl legs of adjacent porphyrins. Interestingly, on Ag ACHTUNGTRENNUNG(111) the square lattice is also observed for intermixed layers of different TPPs, as shown for 2HTPP and CoTPP in the STM image in Figure 1 b. This behavior clearly demonstrates that the arrangement in the square lattice is dominated by molecule–molecule interactions. In contrast to the well-ordered square lattice on AgACHTUNGTRENNUNG(111), a very different adsorption behavior is found for 2HTPP on [a] Dr. F. Buchner, E. Zillner, M. Rçckert, S. Gl ßel, Prof. Dr. H.-P. Steinr ck, Dr. H. Marbach Lehrstuhl f r Physikalische Chemie II Universit t Erlangen-N rnberg Egerlandstr. 3, 91058 Erlangen (Germany) Fax: (+49) 9131-85-28867 E-mail : marbach@chemie.uni-erlangen.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001008. Figure 1. Constant-current STM images: a) A monolayer of 2HTPP on Ag ACHTUNGTRENNUNG(111) (U= +0.45 V, I=25 pA), b) Intermixed 2HTPP/CoTPP monolayer with a ratio of 2:1 on Ag ACHTUNGTRENNUNG(111); the bright longish protrusions correspond to CoTPPs (U= 1.35 V, I=35 pA), c) 2HTPP on Cu ACHTUNGTRENNUNG(111) (U= 0.77 V, I=26 pA), d) CoTPP array on CuACHTUNGTRENNUNG(111) (U= 1.94 V, I= 27 pA). e) STM image of an individual 2HTPP with the corresponding, scaled, space-filling model (U= 1.49 V, I=30 pA). A closed-packed Cu atomic row is indicated, highlighting the coordinative bond between the iminic nitrogens of 2HTPP and the copper substrate atoms. f) STM image of a single CoTPP with the corresponding, scaled, space-filling model (U= 1.48 V, I= 27 pA).

Development and performance of the nanoworkbench: A four tip STM for conductivity measurements down to submicrometer scales

A multiple-tip ultrahigh vacuum (UHV) scanning tunneling microscope (MTSTM) with a scanning electron microscope (SEM) for imaging and molecular-beam epitaxy growth capabilities has been developed. This instrument (nanoworkbench) is used to perform four-point probe conductivity measurements at μm spatial dimension. The system is composed of four chambers, the multiple-tip STM∕SEM chamber, a surface analysis and preparation chamber, a molecular-beam epitaxy chamber, and a load–lock chamber for fast transfer of samples and probes. The four chambers are interconnected by a unique transfer system based on a sample box with integrated heating and temperature-measuring capabilities. We demonstrate the operation and the performance of the nanoworkbench with STM imaging on graphite and with four-point-probe conductivity measurements on a silicon-on-insulator (SOI) crystal. The creation of a local FET, whose dimension and localization are, respectively, determined by the spacing between the probes and their positio...

On the Energetics of Conformational Switching of Molecules at and Close to Room Temperature

We observe and induce conformational switching of individual molecules via scanning tunneling microscopy (STM) at and close to room temperature. 2H-5,10,15,20-Tetrakis-(3,5-di-tert-butyl)-phenylporphyrin adsorbed on Cu(111) forms a peculiar supramolecular ordered phase in which the molecules arrange in alternating rows, with two distinct appearances in STM which are assigned to concave and convex intramolecular conformations. Around room temperature, frequent bidirectional conformational switching of individual molecules from concave to convex and vice versa is observed. From the temperature dependence, detailed insights into the energy barriers and entropic contributions of the switching processes are deduced. At 200 K, controlled STM tip-induced unidirectional switching is possible, yielding an information storage density of 4.9 × 10(13) bit/inch(2). With this contribution we demonstrate that controlled switching of individual molecules at comparably high temperatures is possible and that entropic effects can be a decisive factor in potential molecular devices at these temperatures.

Coverage- and Temperature-Dependent Metalation and Dehydrogenation of Tetraphenylporphyrin on Cu(111)

Using temperature-programmed desorption, supported by X-ray photoelectron spectroscopy and scanning tunneling microscopy, a comprehensive overview of the main reactions of 5,10,15,20-tetraphenyl-21H,23H-porphyrin (2HTPP) on Cu(111) as a function of coverage and temperature is obtained. Three reactions were identified: metalation with Cu substrate atoms, stepwise partial dehydrogenation, and finally complete dehydrogenation. At low coverage the reactions are independent of coverage, but at higher coverage metalation becomes faster and partial dehydrogenation slower. This behavior is explained by a weaker interaction between the iminic nitrogen atoms and the Cu(111) surface in the high-coverage checkerboard structure, leading to faster metalation, and the stabilizing effect of T-type interactions in the CuTPP islands formed at high coverage after metalation, leading to slower dehydrogenation. Based on the amount of hydrogen released and the appearance in STM, a structure of the partially dehydrogenated molecule is suggested.

Generation of Clean Iron Structures by Electron-Beam-Induced Deposition and Selective Catalytic Decomposition of Iron Pentacarbonyl on Rh(110)

We explore the electron-beam-induced deposition (EBID) of iron pentacarbonyl, Fe(CO)5, in ultrahigh vacuum (UHV) on clean and modified Rh(110) surfaces by scanning electron microscopy (SEM), scanning Auger microscopy (SAM), and local Auger electron spectroscopy (AES). In EBID a highly focused electron beam is used to locally decompose the iron pentacarbonyl precursor molecules with the goal to generate pure iron nanostructures. It is demonstrated that the selectivity of the process strongly depends on the surface properties. On a perfect, clean Rh(110) surface almost no selectivity is observed; i.e., deposition of Fe is found on irradiated and nonirradiated surface regions due to catalytic decomposition of the Fe(CO)5. However, on a structurally nonperfect Rh(110) surface and on a Ti-precovered Rh(110) surface high selectivity is found; i.e., Fe deposits are primarily formed in irradiated regions. The role of catalytic and autocatalytic growth of iron on clean Rh respective iron deposits is discussed. The purity of the Fe deposits was always very high (>88%). It is demonstrated that the deposited Fe structures can be selectively oxidized to iron oxide by exposure to oxygen. Furthermore, attempts to write Fe line deposits were also successful, and line diameters smaller than 25 nm could be achieved.

Selforganization of alkali metal on a catalytic metal surface

The spatial distribution of potassium on an Rh(110) surface during the catalytic O2+H2 reaction is investigated employing photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM) as spatially resolving in situ methods. Depending on the reaction conditions, potassium condenses reversibly into macroscopic islands where it is coadsorbed with oxygen. Mass transport of potassium with the reaction fronts is observed. Differences in the mobility and in the bonding strength of potassium on the “reduced” and on the oxygen-covered surface areas are considered to be the key factors for the formation of the stationary concentration patterns.