Ch. Wöll

Improved morphology and charge carrier injection in pentacene field-effect transistors with thiol-treated electrodes

The influence of chemical surface modifications of gold electrodes on the morphology and the electrical properties has been studied for pentacene based thin-film transistors with channel lengths of L⩽4μm. Self-assembled monolayers (SAMs) of various aliphatic and aromatic organothiols have been used to selectively modify the metallic source and drain electrodes and are further compared with reference samples with untreated electrodes. For all SAM-treated devices a reduced roughness of the pentacene film is observed which is accompanied by a reduction of the threshold voltage from about VT=2V for untreated transistors to −0.9V for transistors with SAM modified electrodes. Using aliphatic SAMs a poor on/off ratio of about 102 was obtained which is attributed to their low conductivity. In contrast, the on/off ratio is enhanced by four orders of magnitude if the surface is modified by an aromatic SAM. In this case a subthreshold swing as low as 0.55V/decade is achieved which corresponds to a trap density reduc...

Identification of transitions into Rydberg states in the X-ray absorption spectra of condensed long-chain alkanes

Abstract A detailed theoretical analysis of the high-resolution X-ray absorption spectra of condensed long-chain alkane molecules (hexatriacontane, C 36 H 74 and heptatriacontane, C 37 H 76 ) suggests that the resonance at 287.7 eV, which is the dominating feature in the near-edge X-ray absorption spectra of saturated long-chain hydrocarbons, is due to transitions into Rydberg states. These findings, which are at variance with the previous interpretation, are corroborated by the identification of a previously unresolved resonance at 288.2 eV in the high-resolution X-ray absorption spectra and of the observation of a strong quenching of the X-ray absorption resonances upon physisorption on a Cu(111)-metal surface.

Adsorption of acenes on flat and vicinal Cu(111) surfaces: Step induced formation of lateral order

The adsorption of benzene, naphthalene, and anthracene on Cu(111) and its vicinal Cu(221) and Cu(443) surfaces has been studied by means of thermal desorption spectra (TDS) and low energy electron diffraction. For each of the studied acenes (C4n+2H2n+4, n=1,2,3) two distinct binding states were observed in the TDS corresponding to adsorption on terraces and at step edges. The binding energies of both states are found to increase linearly with the number of aromatic rings n. In contrast to the Cu(111) surface where these acenes form disordered adlayers only, ordered films of naphthalene and anthracene with an alignment of the molecular axis along the steps could be prepared on the vicinal surfaces.

An orientation analysis of differently endgroup-functionalised alkanethiols adsorbed on Au substrates

The molecular orientation in self-assembled fims of methyl-, hydroxyl-, and carboxylic acid-terminated alkanethiols of different chain lengths (n-docosanethiol [DCT], 22-mercaptodocosanol [MDO], 22-mercaptodocosanoic acid [MDC], n-hexadecanethiol [HDT], 16-mercaptohexadecanol [MHO], 16-mercaptohexadecanoic acid [MHC]) on Au substrates has been investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). Whereas XPS-measurements do not reveal changes in thickness upon replacing the endgroups, the degree of orientation as determined with NEXAFS exhibits significant differences. The two methyl-terminated thiols show the same alkyl-chain tilt-angle α of 39°. For the short-chain OH-terminated films no significant difference could be detected, but the monolayers formed from the long-chain OH-terminated chains exhibit a slightly stronger anisotropy and thus a smaller tilt-angle α than the corresponding CH3-terminated thiol. NEXAFS-spectra recorded for films from COOH-terminated thiols reveal only a very small anisotropy which indicates the absence of significant molecular orientation resulting from a high degree of disorder.

Structure of self‐organizing organic films: A near edge x‐ray absorption fine structure investigation of thiol layers adsorbed on gold

Self‐assembled films of thiols on polycrystalline Au substrates have been investigated with near edge x‐ray absorption fine structure. From the angular dependence of two different resonances the orientation of the molecular axis of the alkyl chains with respect to the surface normal is determined and compared to results obtained by other techniques. By comparing data for thiol films and LB films of Cd and Ca arachidate it is found that thiol films are considerably better ordered.

Helium atom differential cross sections for scattering from single adsorbed CO molecules on a Pt(111) surface

Differential cross sections have been measured for the scattering of helium atoms from isolated CO molecules on a Pt(111) surface. The cross sections reveal an oscillatory structure as a function of scattering angle extending to large momentum transfer on both sides of the specular peak. Using a hard hemisphere model to approximate the interaction potential, the data can be well reproduced by a hard core of radius about 2.5 A. This result is compared to the charge density profile of the adsorbed molecule and also to the gas phase interaction potential, and in both cases good agreement with the experimentally predicted classical turning points is found.

Helium atom scattering studies of thermal energy vibrations on the clean and adsorbate-covered NI(100) surfaces

Abstract Vibrations at a Ni(100) surface have been studied by Helium atom scattering. Adsorption of various adsorbates (CO,H,O) leads to characteristic changes and new features in the phonon spectrum of the clean surface. The dispersion curves for the clean substrate are compared to recent EELS data. Experimentally determined values for the frequency of the hindered translation of adsorbed CO-molecules strongly deviate from theoretical predictions.

Rydberg transitions in X-ray absorption spectroscopy of alkanes: The importance of matrix effects

Precise ab initio electronic structure calculations on isolated alkanes (propane, butane) and on propane embedded in a matrix of several propane molecules were used in order to characterize the near-edge x-ray absorption fine structure (NEXAFS) resonances of condensed saturated hydrocarbons. The results demonstrate that upon condensation the dominating NEXAFS spectral features, i.e., the Rydberg resonances located between 287.4 and 288.1 eV, have a significant blue shift from the corresponding transition energies of an isolated alkane molecule. Furthermore, the theoretical results confirm the excitonic character of the final state. Additional calculations carried out for different conformations (trans/gauche) of isolated butane demonstrate that the transition energies of the corresponding NEXAFS resonances do not differ significantly. Previously observed changes in the electronic structure of disordered thin organic films of calciumarachidate are thus attributed to changes in the intermolecular spacing of...

Importance of dewetting in organic molecular-beam deposition: Pentacene on gold

Organic molecular-beam deposition of pentacene on gold substrates has been investigated using a multitechnique approach. The morphology of the organic thin films depends strongly on the substrate temperature. Pronounced dewetting and island formation are observed at room temperature. Whereas pentacene molecules adopt a planar monolayer structure, they continue to grow in an upright orientation in multilayer films as inferred from x-ray absorption spectroscopy and atomic force microscopy. These results are in pronounced contrast to a recent scanning tunneling microscopy (STM) study by Kang and Zhu [Appl. Phys. Lett. 82, 3248 (2003)] and indicate fundamental problems in the interpretation of STM measurements for organic thin films.

Spectroscopic evidence for the partial dissociation of H2O on ZnO(100)

The interaction of water with the non-polar ZnO(1010) surface has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Adsorption of water at room temperature leads to the partial dissociation of water molecules giving rise to a well defined (2x1) superstructure. This observation was confirmed by the HREELS data which show the water-induced O-H stretching modes at 396 and 460 meV (3193 and 3709 cm-1) as well as the peak at 456 meV (3677 cm-1) arising from the OH species. The large red shift of the loss at 396 meV indicates unusually strong hydrogen bonding interactions of water to both neighbouring adsorbate molecules and the surface O atoms which are responsible for the partial dissociation of water molecules on the perfect ZnO(1010) surface.