Temperature-dependent chemical and structural changes of a submonolayer of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) were studied with photoelectron spectroscopy (XPS/UPS) and scanning tunneling microscopy (STM). 2HTPP reacts with Cu atoms from the substrate to form copper(II)-tetraphenylporphyrin (CuTPP). This metalation reaction starts at about 400 K and was investigated at various temperatures up to 500 K. At room temperature, adsorbed 2HTPP adopts an orientation with the molecular plane parallel to the substrate; the same holds for its reaction product CuTPP after annealing to 400 K. In contrast, annealing at 450 K yields a tilted orientation of CuTPP, as indicated by STM and supported by C 1s XPS shifts and changes in the Cu(111) surface state. Subsequent annealing at 500 K restores a flat-lying orientation; however, the appearance of the complex in STM images differs from the original appearance of CuTPP. In summary, 2HTPP undergoes three irreversible transformations upon annealing on Cu(111), a metalation reaction to CuTPP followed by two intramolecular structural changes.

Metalloporphyrins adsorbed on solid surfaces have received particular attention recently. [1] This high interest is due to their omnipresence in nature and in state-of-the art technology: The iron porphyrin in heme is responsible for oxygen transport in the bloodstream of mammals, and the magnesium porphyrin in chlorophyll is the key to photosynthesis; [2] in colorimetric sensors light-emitting properties of metalloporphyrins are utilized to identify chemical species adsorbed at the central metal atom. [3] The potential of porphyrins for the fabrication of tailor-made functional molecular architectures on well-defined substrates has stimulated significant activity in fundamental research. [4] One important observation was that metalloporphyrin layers can not only be prepared by direct deposition from the gas phase, but can also be synthesized in situ on the surface on demand by pre-or postdeposition of the respective metal atoms. This in situ metalation readily proceeds at room temperature (RT) or after moderate annealing. [5] It was studied in detail for 2Htetraphenylporphyrin (2HTPP) complexes with Co, [5a, 6] Fe, [5b,c, 7] Ni, [8] Cu, [9] Zn, [6,10] and Ce [1c, 11] on Ag(111) and Au(111). The surface-mediated reaction follows Equation (1),where M is the corresponding metal. The relevant elementary steps are: a) coordination of the metal atom by the intact freebase porphyrin, 2HTPP, b) successive transfer of the two hydrogen atoms from the respective nitrogen atoms to the metal atom, and c) formation and release of H 2 . Based on DFT calculations in the gas phase, the transfer of the first hydrogen to the metal was identified as the rate-limiting step and thus determines the activation energy for the metalation. [6,12] The experimental determination of the reaction kinetics and the activation barrier for in situ metalation on a surface, however, still presents a great challenge. In the liquid phase this is typically performed by isothermal studies at various temperatures, but to the best of our knowledge no such investigations are available for larger organic molecules adsorbed on surfaces. The only value for the activation energy of a metalation reaction reported so far was derived not from isothermal data, but from a temperature-programmed desorption (TPD) measurement of deuterium during the metalation of 2DTPP (deuterated analogue of 2HTPP) with Zn on Ag(111). [6] Redheads equation [13] was used to deduce the activation energy . The deduced value is in good agreement with the value obtained from DFT calculations for the free molecule, that is, neglecting the surface; this indicates that the influence of the surface is small. [6] The major weakness of the Redhead approximation is the fact that the prefactor k 0 cannot be determined but must be assumed; usually a value of 10 À13 s À1 is used in the corresponding analysis.Herein, we present a direct approach to study the kinetics and to determine the activation energy for the metalation of 2HTPP with Cu substrate atoms on a Cu(111) surface, by scanning tun...

SummaryThe autocatalytic growth of arbitrarily shaped nanostructures fabricated by electron beam-induced deposition (EBID) and electron beam-induced surface activation (EBISA) is studied for two precursors: iron pentacarbonyl, Fe(CO)5, and cobalt tricarbonyl nitrosyl, Co(CO)3NO. Different deposits are prepared on silicon nitride membranes and silicon wafers under ultrahigh vacuum conditions, and are studied by scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM), including near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It has previously been shown that Fe(CO)5 decomposes autocatalytically on Fe seed layers (EBID) and on certain electron beam-activated surfaces, yielding high purity, polycrystalline Fe nanostructures. In this contribution, we investigate the growth of structures from Co(CO)3NO and compare it to results obtained from Fe(CO)5. Co(CO)3NO exhibits autocatalytic growth on Co-containing seed layers prepared by EBID using the same precursor. The growth yields granular, oxygen-, carbon- and nitrogen-containing deposits. In contrast to Fe(CO)5 no decomposition on electron beam-activated surfaces is observed. In addition, we show that the autocatalytic growth of nanostructures from Co(CO)3NO can also be initiated by an Fe seed layer, which presents a novel approach to the fabrication of layered nanostructures.

In this study, we investigate the peculiar coverage dependent supramolecular arrangement of 2H-tetraphenylporhpyrin (2HTPP) on Cu(111) with scanning tunneling microscopy at room-temperature. At low coverage, "slow" diffusion of individual 2HTPP molecules along the close-packed atomic rows of the substrate is observed, and no supramolecular ordering occurs. However, at higher coverage, the formation of ordered, checkerboard-like domains is found, with two molecules per unit cell at different distances from the surface. This behavior is attributed to a complex interplay of site specific molecule-substrate interaction, mainly the strong interaction between the iminic N atoms and Cu substrate atoms, with intermolecular T-type and π-π interactions.

We demonstrate that surface-anchored metal-organic frameworks (SURMOFs) are extraordinary well-suited as resists for high-resolution focused electron beam induced processing (FEBIP) techniques. The combination of such powerful lithographic protocols with the huge versatility of MOF materials are investigated in respect to their potential in nanostructures fabrication. The applied FEBIP methods rely on the local decomposition of Fe(CO) and Co(CO)NO as precursors, either by the direct impact of the focused electron beam (electron beam induced deposition, EBID) or through the interaction of the precursor molecules with preirradiated/activated SURMOF areas (electron beam induced surface activation, EBISA). We demonstrate the huge potential of the approach for two different types of MOFs (HKUST-1 and Zn-DPDCPP). Our "surface science" approach to FEBIP, yields well-defined deposits with each investigated precursor/SURMOF combination. Local Auger electron spectroscopy reveals clean iron deposits from Fe(CO); deposits from Co(CO)NO contain cobalt, nitrogen, and oxygen. EBISA experiments were successful with Fe(CO). Remarkably EBISA with Co(CO)NO does not result in deposit formation on both resists, making the process chemically selective. Most importantly we demonstrate the fabrication of "nested-L" test structures with Fe(CO) on HKUST-1 with extremely narrow line widths of partially less than 8 nm, due to reduced electron proximity effects within the MOF-based resists. Considering that the actual diameter of the electron beam was larger than 6 nm, we see a huge potential for significant reduction of the structure sizes. In addition, the role and high potential of loading and transport of the precursor molecules within the porous SURMOF materials is discussed.