Substrate Rows Research Articles

Controlling the Self-Metalation Rate of Tetraphenylporphyrins on Cu(111) via Cyano Functionalization.

The reaction rate of the self-metalation of free-base tetraphenylporphyrins (TPPs) on Cu(111) increases with the number of cyano groups (n=0, 1, 2, 4) attached at the para positions of the phenyl rings. The findings are based on isothermal scanning tunneling microscopy (STM) measurements. At room temperature, all investigated free-base TPP derivatives adsorb as individual molecules and are aligned with respect to densely packed Cu substrate rows. Annealing at 400 K leads to the formation of linear dimers and/or multimers via CN-Cu-CN bonds, accompanied by self-metalation of the free-base porphyrins following a first-order rate equation. When comparing the non-cyano-functionalized and the tetracyano-functionalized molecules, we find a decrease of the reaction rate by a factor of more than 20, corresponding to an increase of the activation energy from 1.48 to 1.59 eV. Density functional theory (DFT) calculations give insights into the influence of the peripheral electron-withdrawing cyano groups and explain the experimentally observed effects.

The Au modified Ge(1 1 0) surface

The pristine Ge(1 1 0) surface is composed of Ge pentagons, which are arranged in relatively large (16 × 2) and c(8 × 10) unit cells. The deposition of sub-monolayer amounts of Au and mild annealing results into de-reconstructed Ge(1 1 0) regions completely free of Ge pentagons and regions composed of nanowires that are aligned along the high symmetry [11¯0] direction of the Ge(1 1 0) surface. The de-reconstructed Ge(1 1 0) regions consist of atomic rows that are aligned along the [11¯0] direction. A substantial fraction of these substrate rows are straight and resemble the atom rows of the unreconstructed, i.e. bulk terminated, Ge(1 1 0) surface, whereas the other substrate rows have a meandering appearance. These meandering atom rows are comprised of two types of atoms, one type that appears dim, whereas the other type appears bright in filled-state scanning tunneling microscopy images. Using density functional theory calculations, we have tested more than 20 different atomic models for the meandering atom rows. The density functional theory calculations reveal that it is energetically favorable for the deposited Au atoms to exchange position with Ge atoms in the first layer. Based on these findings we conclude that the bright atoms are Ge atoms, whereas the dim atoms are Au atoms.

Supramolecular order and structural dynamics: A STM study of 2H-tetraphenylporphycene on Cu(111).

The adsorption of 2H-tetraphenylporphycene (2HTPPc) on Cu(111) was investigated by scanning tunneling microscopy (STM). At medium coverages, supramolecular ordered islands are observed. The individual 2HTPPc molecules appear as two pairs of intense protrusions which are separated by an elongated depression. In the islands, the molecules are organized in rows oriented along one of the close packed Cu(111) substrate rows; the structure is stabilized by T-type interactions of the phenyl substituents of neighboring molecules. Two types of rows are observed, namely, highly ordered rows in which all molecules exhibit the same orientation, and less ordered rows in which the molecules exhibit two perpendicular orientations. Altogether, three different azimuthal orientations of 2HTPPc are observed within one domain, all of them rotated by 15° ± 1° relative to one closed packed Cu direction. The highly ordered rows are always separated by either one or two less ordered rows, with the latter structure being the thermodynamically more stable one. The situation in the islands is highly dynamic, such that molecules in the less ordered rows occasionally change orientation, also complete highly ordered rows can move. The supramolecular order and structural dynamics are discussed on the basis of the specific molecule-substrate and molecule-molecule interactions.

Molecular Growth Determined by Surface Domain Patterns

The growth of iron phthalocyanine (FePc) on InSb(001) c(8 x 2) at submonolayer coverage has been investigated with scanning tunneling microscopy (STM). FePc adsorbs flat centered on the In rows both at 70 K and at room temperature (RT). However, the shapes of the two-dimensional molecular islands are fundamentally different; while the RT growth results in chainlike structures along the [I 10] direction, as already observed for other Pc's adsorbed on the same surface, the islands are prolonged along [110], i.e., perpendicular to the substrate rows, at 70 K. These observations are explained on the basis of a recently observed new surface phase at low temperature, resulting in structural domains on the surface. The molecular growth front follows the propagating domain boundary that freezes at low temperature.

Self Assembly of Anisotropic Organic Molecules: Diffusion versus Sticking Anisotropy

AbstractThe molecular/crystal orientation and morphology of active molecular structures is a key determinant for the function of nanoscaled organic devices. In π-conjugated systems, both charge transport and optical properties will strongly depend on the molecular orientation due to the highly anisotropic charge carrier mobility in these organic crystals and the anisotropic absorption and luminescence behavior of the molecules. Although the importance of organic on inorganic interface formation and thin film growth is widely acknowledged, little is known regarding the growth kinetics. A better understanding of the processes driving molecular self-assembly is necessary if the self-assembly process is to be controlled. Moreover, it is interesting as the anisotropy of the molecular building blocks presents a fundamental difference from what is known from inorganic growth. Here we show that either sticking or diffusion anisotropy can control the growth depending on preparation conditions. This is illustrated by an investigation into the growth of sexiphenyl (6P) on the anisotropic TiO2(110)-(1×1) surface for temperatures between 80K and 400K using in-situ UHV photoemission, x-ray absorption spectroscopy, synchrotron x-ray diffraction and ex-situ atomic force microscopy. For 6P adsorption even at 80K we found that the molecules orient parallel to the TiO2 oxygen rows and form small crystallites. At 300K this molecular orientation is retained and large micrometer sized 6P(203) oriented needles running perpendicular to oxygen substrate rows are formed. In contrast, for growth at elevated temperatures the 6P molecular axis is near perpendicular to the surface and large islands elongated parallel to the substrate rows are formed. These differences in crystallite orientation and morphology can be explained by the domination of the growth kinetics by either sticking or diffusion anisotropy depending on growth temperature.

Adsorption geometry of Cu( [formula omitted])-(12 × 2)-14Xe

Low-energy electron diffraction was used to study the adsorption geometry of Xe on Cu(1 1 0). At low temperatures, a higher-order commensurate (12 × 2) structure can be formed that consists of rows of Xe atoms aligned along the direction of the rows on Cu(1 1 0), and having a spacing between the rows equal to the Cu row spacing. A dynamical LEED study of this structure at 20 K indicates that the rows of Xe atoms are localized on top of the Cu substrate rows. The lateral registry of the Xe rows along the Cu rows includes 2 Xe atoms per (12 × 2) unit cell in atop positions above Cu atoms, with the others in lower-symmetry sites. The average Xe–Cu interlayer spacing is 3.3 ± 0.1 Å and there is a small degree (0.1 Å) of vertical rumple in both the overlayer and the top substrate layer. The substrate surface with adsorbed Xe has essentially the same relaxation as the clean surface. The analysis of the adatom vibrations in this structure indicates that the isotropically-averaged rms vibration amplitude is 0.2 Å for Xe, while that of the top-layer Cu atoms is 0.04 Å.

Diffusion of Si and Ge dimers on Ge (001) surfaces

We have studied the various diffusion pathways of Si and Ge dimers on the Ge (001) surface using scanning tunneling microscopy. The adsorbed dimers can be classified into two categories: Dimers adsorbed on top of the substrate rows and dimers adsorbed in the troughs between the substrate rows. There are three different diffusion pathways for the dimers: Along the substrate rows, across the substrate rows, and in the troughs between the substrate rows. The activation barriers for diffusion of these three pathways have been determined for both Ge and Si dimers on Ge (001). The barriers for dimer diffusion of the system Ge/Ge (001) are slightly lower than for the Si/Ge (001) system. As compared to Si on Si (001) the activation barriers for dimer diffusion on Ge (001) are significantly lower.

STM investigation of the initial adsorption stage of Bi on Si(1 0 0)-(2×1) and Ge(1 0 0)-(2×1) surfaces

Scanning tunneling microscopy has been used to investigate the initial adsorption stage of Bi on Si(100) and Ge(100) surfaces at room temperature. The most favorable position for a Bi ad-dimer on both surfaces is the B-configuration (Bi ad-dimer positioned on-top of the substrate rows with its dimer bond aligned along the substrate dimer row direction). For Si(100) the A-type dimers (dimer bond aligned perpendicular to the substrate dimer row direction) occasionally rotate back and forth to a B-configuration. The diffusion rates of B-type and A-type dimers along the substrate row and the B–A, A–B rotations on Si(100) are extracted from an analysis of many sequences of STM images. Finally, it is shown that the presence of an attractive interaction between Bi ad-dimers (irrespective of their orientation) positioned on neighboring substrate dimer rows leads to the formation of local regions with a 2×2 reconstruction.

Origin of roughening in epitaxial growth of silicon on Si(0 0 1) and Ge(0 0 1) surfaces

It is shown that the formation of (2×1) reconstructed islands during the growth of Si on Si(001) and Ge(001) surfaces at moderate temperatures and the consequent anisotropic diffusion and sticking lead to roughening. As Si islands nucleate and expand, the dimer rows in neighboring rows need not, in general, align with each other. An anti-phase boundary (APB) will be found if two growing islands meet, but their internal dimer rows are not in the same registry. One type of APB runs perpendicular to the substrate rows, whereas the other runs along the substrate rows. As has been pointed out by Hamers et al. [J. Vac. Sci. Technol. A 8 (1990) 195] this first type of APB is a preferential center for nucleation of next layer islands and thus naturally leads to roughening. Here we show that the other type of APB leads, in combination with the anisotropy in sticking and diffusion of dimers, to the formation of long B-type double layer steps and narrow trenches. We argue that it is the kinetic suppression of filling of these trenches and not, as previously suggested the existence of step edge barrier for the double layer step edge that leads to roughening.

Low-energy electron diffraction study of krypton on Cu(110)

Low-energy electron diffraction (LEED) was used to study the adsorption of krypton on Cu(110). Adsorption isobars indicate that the isosteric heat of adsorption for one-half monolayer is 114±10 meV. A dynamical LEED study of the c(2×8) higher-order commensurate structure at 25 K indicates that rows of krypton atoms are localized on top of the copper substrate rows. The krypton rows are situated such that they have the lowest-coordinated position along the rows. The overlayer–substrate spacing is 3.36±0.07 Å and the top two interlayer spacings of the substrate are 1.18±0.04 Å and 1.31±0.02 Å, respectively.

Diffusion driven concerted motion of surface atoms: Ge on Ge(001)

The diffusion of Ge dimers on the Ge(001) surface has been studied with scanning tunneling microscopy. We have identified three different diffusion pathways for the dimers: diffusion of on-top dimers over the substrate rows, diffusion across the substrate rows, and diffusion of dimers in the trough. We report on a heretofore unknown phenomenon, namely, diffusion driven concerted motion of substrate atoms. This concerted motion is a direct consequence of the rearrangement of substrate atoms in the proximity of the trough dimer adsorption site.

THE ORIENTATION OF BENZENE ON BIMETALLIC SURFACES

Angle-resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation is used to determine the orientation of benzene molecules on two modifications of the pseudomorphic Ni/Cu(111) layer, the Ni-terminated adlayer and the Cu-terminated Ni sublayer. It is found that the molecules adsorb with their C–H bonds 30° off the close-packed substrate rows (σv orientation) on the Ni adlayer whereas no preferential orientation was found for the Cu-terminated Ni sublayer. For these and other close-packed mono- and bimetallic surfaces involving Cu, Ni and Ru, the correlation between adsorption geometry and reactivity of benzene is explored in connection with temperature-programmed desorption (TPD). Like for the Ni/Cu(111) sublayer, no preferential lateral orientation and a weak bond is found on most other Cu-terminated surfaces at 80 K. This goes along with the absence of any dissociation on these surfaces. Only on the stretched 1 ML Cu/Ru(0001) surface is the substrate–benzene bond strong enough to induce a clear preferential orientation (σd with C–H bonds parallel to the close-packed rows) but still too weak to induce dissociation. The same orientation was found for the saturated benzene layer on Ni(111) where the molecules also do not dissociate. Dissociation is, however, observed for the saturation coverage of benzene on the Ni/Cu(111) adlayer and for low benzene coverages on Ni(111). On these surfaces the azimuthal orientation of the benzene molecules is σv, i.e. rotated by 30° with respect to the close-packed rows.

One-dimensional band structures: Rare gases on Pt(110)1 x 2.

The formation of one-dimensional band structures has been observed for Kr and Xe adsorbed on the reconstructed Pt(110)1\ifmmode\times\else\texttimes\fi{}2 surface at submonolayer coverages: The ${\mathit{p}}_{3/2}$ and ${\mathit{p}}_{1/2}$ valence levels of both adsorbates exhibit significant dispersion for the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction, i.e., along the densely packed substrate rows, but no dispersion is observed for [001], i.e., perpendicular to the rows. The observed k-space periodicities and tight-binding band-structure calculations imply that both Kr and Xe are adsorbed in densely packed linear chains on the reconstructed Pt(110)1\ifmmode\times\else\texttimes\fi{}2 surface, with negligible interaction between neighboring rows.

Sulphur dioxide adsorption on the Ni(110) surface

The adsorption of SO 2 on Ni(110) has been studied in detail by temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and angle resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. At 100 K SO 2 is found to chemisorb in molecular form on Ni(110) with a saturation coverage of 0.5 ML (1 ML = 1 molecule Ni-atom ) . The saturated chemisorbed layer exhibits a c(2 × 2) LEED pattern. At initial coverages below 0.25 ML, SO 2 completely decomposes upon heating to 750 K; for higher coverages molecular SO 2 desorption is also observed in a single peak between 300 and 400 K. From the angle resolved UPS spectra and symmetry selection rules we deduce the symmetry of the adsorbed SO 2 molecules for low coverages as C s (σ yz ), with the molecular O-S-O plane oriented perpendicular to the surface and aligned along the [001] azimuth (perpendicular to the close-packed substrate rows). Within the plane the molecule is tilted, which is attributed to an additional Ni-O interaction. In the saturated layer the symmetry is further reduced to C 1 indicating an additional tilting and/or rotation of the molecular plane with respect to the substrate.

Adsorption and interaction of Sm on GaAs(110) studied by scanning tunneling microscopy.

Scanning-tunneling-microscopy studies of the interactions of Sm with GaAs(110) at 300 K show two growth structures. Low density zigzag chains form by 0.01 monolayer (ML) with (divalent) Sm atoms bound to Ga surface atoms along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction (Sm-Sm distances 6.9 and 7.9 \AA{}). These zigzag chains are limited in length but bridge two or three substrate rows. Higher-density linear chains of (divalent) Sm atoms form at the expense of the zigzag chains with increased deposition, and the adatoms are bonded beside two Ga atoms near an As atom (Sm-Sm distance \ensuremath{\sim}4 \AA{}). Only short single chains are observed, and they convert to reacted (trivalent) clusters that bridge substrate rows at higher coverage. By \ensuremath{\sim}0.33 ML, the surface structure is composed of linear chains and reacted clusters with a small number of residual zigzag chains. Synchrotron-radiation photoemission results provide a correlation of structure with Sm valence. Comparison to Cs/GaAs(110) offers insight into low-coverage nucleation.

Surface structures on spherical copper crystals after adsorption of oxygen

Spherical copper crystals were grown by solidification of a drop of melt. Immediately thereafter oxygen was adsorbed at 800 °C and 10 −4 Torr for several minutes. Diffraction of 60 kV electrons shows different structures on the main great circles of the crystal sphere. On higher indexed poles, distances of scattering centers were observed, which agree with the distances of substrate rows closely packed with copper atoms.