Hiroyuki Ozaki

Penning Ionization Electron Spectoscopy: Its Application to Surface Characterization of Organic Solids

In Penning ionization electron spectroscopy, the kinetic energy of electrons ejected by collisions between targets T (gas or solid) and metastable rare gas atoms A* is analyzed. This electron spectroscopy is selectively sensitive to the outermost surface layer of solids, since metastable atoms do not penetrate into inner layers. Furthermore, it provides information on the local electron distribution of individual orbitals exposed outside the outermost surface layer. The application of these unique features of Penning spectroscopy to the surface characterization of organic solids, including Langmuir-Blodgett films, is discussed. Both a Penning spectroscopy technique and the process of Penning ionization of gas-phase molecules are briefly described.

Growth of organic ultrathin films studied by Penning ionization electron and ultraviolet photoelectron spectroscopies: Pentacene

Penning ionization electron spectroscopy was applied to ultrathin pentacene films [monolayer (0.3 nm thick) to dozens of layers] prepared by vapor deposition under different conditions. Remarkable differences were found among the Penning ionization electron spectra (PIES). The local electron distribution of each molecular orbital (MO) protruding from the film surface was probed and the relation between the MO shape and the molecular orientation was investigated. Deposition onto a metal substrate without a crystallographical surface yields a crystalline film at room temperature. The molecules are oriented with the long axes almost perpendicular to the substrate and make the σ bands of the PIES by far stronger than the π bands. In the pure π region, the π9 and π7 MOs having large distribution at the long-axis end provide more intense bands than other π MOs. On the metal substrate held at 213 K, molecules form an amorphous film with the long axes inclined a little on average. The π and σ bands exhibit compar...

Observation of structural change in organic monolayer film by penning ionization electron spectroscopy: Fe-phthalocyanine on graphite

Abstract Structures of Fe-phthalocyanine adsorbed on a graphite substrate (cleavage plane) were studied by Penning ionization electron spectroscopy (PIES). The structural change, disordered layer → closely packed monolayer → island, was observed upon heating the film deposited on the substrate held at 123 K. For the monolayer and island structures, molecules were found to be oriented flat to the substrate. It was concluded that PIES was a useful technique to probe the change in molecular arrangement during order-disorder transformation in monolayer films.

Penning ionization electron spectroscopy of iron phthalocyanine thin films: study of electronic structure from local electron distribution of molecular orbitals

Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) were measured for two kinds of ultrathin films of iron phthalocyanine with different orientation of surface molecules. The local electron distribution of molecular orbitals (MO) at the different parts of the molecule was reflected in the PIES. From this the character of MOs responsible for each band in the PIES and UPS was revealed. The so‐called iron 3d bands were assigned to 3d⊥ (dxz , dyz, dz2)‐like MOs. The IP values for π MOs were found to be less than ∼12 eV in the solid phase. It was concluded that the type of MOs probed by PIES can be selected if the orientation of molecules is controlled appropriately, which will be of great help to elucidate the electronic structure of complex molecules.

Design and preparation of an organic monoatomic layer

Abstract Planar or chain molecules vapor-deposited onto a cleavage plane of cooled graphite form a monolayer in which they are gathered by van der Waals interaction with the carbon skeleton plane oriented parallel to the substrate surface. As a basis to utilize such a monolayer for material design, conditions that the molecules are combined one or two dimensionally with covalent bonds to form a single sheet of a planar carbon network are discussed. The virture of this idea is confirmed by Penning ionization electron spectroscopy. It is demonstrated that an organic monoatomic layer with a sashlike ( atomic sash ) or a clothlike structure ( atomic cloth ) can be prepared by intramonolayer polymerization of alkyldiyne or alkyltetrayne.

Formation of atomic cloth observed by Penning ionization electron spectroscopy

A single sheet of a clothlike macromolecule (atomic cloth; 4 A thick) comprising the columns of polydiacetylene and polyacetylene chains alternately crosslinked to the rows of alkyl chains was prepared by the photopolymerization of 1,15,17,31‐dotriacontatetrayne molecules laid flat in a monolayer. This process was monitored by Penning ionization electron spectroscopy.

Penning ionization electron and ultraviolet photoelectron spectroscopies of organic extra-thin films comprising alkyl chains: molecular aggregation, film growth and chemical reaction

Abstract The extra-thin evaporated films of long chain alkyl derivatives were characterized by Penning ionization electron and ultraviolet photoelectron spectroscopies. Two types of molecular aggregation were found in films prepared under different conditions. In particular, films made up of lying chains were studied in detail, the number of layers being controlled from 1 (4 A in thickness) to several, and the first stage of film growth was observed layer by layer. Based on these results, a monolayer composed of sash-like and flat-lying macromolecules was prepared by the photopolymerization of dialkyldiacetylene taking place in the outermost surface layer.

Characterization of Langmuir-Blodgett films of cadmium stearate by penning ionization electron spectroscopy

Abstract The characterization of cadmium stearate films prepared on silicon substrates by the conventional Langmuir-Blodgett method (for 1 layer) and those by the horizontal lifting (HL) method (for 2–4 layers) was carried out by means of Penning ionization electron spectroscopy and UV photoelectron spectroscopy. The analysis of the relative band intensities of the Penning spectrum indicates that the hydrocarbon end of the stearate ion is exposed outside the surface for all the films studied. The combination of this result and those of an X-ray diffraction study suggests that the films prepared by the HL method have a Y-type structure. The structural changes in the film due to increases in film thickness and to thermal annealing were also studied by Penning ionization and photoelectron spectroscopies.

Attempt to pile up extrathin sashlike macromolecules (atomic sashes)

Abstract Long-chain molecules vapor deposited onto cooled graphite lie flat and form a monolayer. When such a monolayer of 17,19-hexatriacontadiyne (HTDY) is irradiated with UV light, a surface topochemical reaction takes place to convert the molecules into a single sheet of a sashlike macromolecule ( atomic sash ) comprising the rows of alkyl chains bridged by a polydiacetylene chain. We attempted to pile up the atomic sashes by repeating the vapor deposition and UV irradiation, characterizing the products at all stages by Penning ionization electron spectroscopy. It is found that the aggregation of alkyl chains in the outermost layer (atomic sash or HTDY) is essentially unchanged up to approximately three layers independent of whether the underlying layer is graphite or the atomic sashes.

Monolayer and bilayer formation of 17,19-dotetracontadiyne at a liquid/solid interface

Monolayer and bilayer of 17,19-dotetracontadiyne (DTDY) on a graphite substrate were studied by scanning tunneling microscopy at a liquid solid interface of phenyloctane solution. The orientation of the layers was examined with respect to the highly oriented pyrolitic graphite. The first layer grew very quickly with many small domains some tens of nm in diameter, and the alkyl chains of the molecule in each domain align epitaxially along the a g -axis of graphite. When the solution remains at room temperature, the second layer of DTDY grew epitaxially on the first layer and the domain size was much larger than that of the first layer.