Scanning Tunneling Microscopy of Prochiral Anthracene Derivatives on Graphite: Chain Length Effects on Monolayer Morphology (original) (raw)
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The Journal of Physical Chemistry C, 2009
The monolayers self-assembled at the solution-graphite interface by 18 1,5-bis(2,X-dioxaalkyl) anthracene derivatives bearing odd length side chains are studied using scanning tunneling microscopy and molecular mechanics (MM) simulations. 1-D molecular tapes are the key and common building block of the many morphologies self-assembled by these compounds. The dominant monolayer morphology and the extent of monolayer polymorphism depend critically on the position of the X-ether group and side chain length. MM simulations of the compounds' different morphologies are useful tools with which to dissect, explain, and predict morphological variation and polymorphism as a function of side chain structure. The simulations reveal that the dependence on X-ether group position arises from dipolar repulsions between ether groups in adjacent tapes, dipolar attractions between ether and anthracene C-H groups in adjacent tapes, and geometric distortions of the side chains.
Scanning tunneling microscopy images of alkane derivatives on graphite: role of electronic effects
2008
Scanning tunneling microscopy (STM) images of self-assembled monolayers of close-packed alkane chains on highly oriented pyrolitic graphite often display an alternating bright and dark spot pattern. Classical simulations suggest that a tilt of the alkane backbone is unstable and, therefore, unlikely to account for the contrast variation. First principles calculations based on density functional theory show that an electronic effect can explain the observed alternation. Furthermore, the asymmetric spot pattern associated with the minimum energy alignment is modulated depending on the registry of the alkane adsorbate relative to the graphite surface, explaining the characteristic moiré pattern that is often observed in STM images with close packed alkyl assemblies.
Materialwissenschaft und Werkstofftechnik, 2011
Highly ordered monolayer films of an n-alkane and arachidic acid were deposited on graphite surface from their solutions in n-tetradecane, and their molecular structure was investigated by scanning tunneling microscopy. Close-packed lamella-like structures with long range order were observed. In alkane monolayers, each lamella is composed by single molecules, while in the case of acid, lamellas consist of dimers formed by acid molecules due to interaction between their carboxylic groups, probably via hydrogen bonds. An increased brightness of carboxyl head in each fifth acid molecule was observed, which is attributed to nonequivalent positions of acid molecules with respect to the substrate carbon atoms.
Direct observation of long chain alkane bilayer films on graphite by scanning tunneling microscopy
Surface Science, 1993
The interface between a solution of hexatriacontane (n-C36H74) in decane and the basal plane of graphite has been studied by scanning tunneling microscopy at room temperature. For the first time, we show that a second layer which is rotated by 60 ° with respect to the first layer may grow at the interface. We demonstrate that the carbon skeleton of the molecule is parallel to the graphite surface. We propose a model for the arrangement of the molecular layers and an explanation for the origin of the second layer rotation.
The Journal of Physical Chemistry C, 2008
Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (OMTS) are reported for N,N′-dioctyl-1,8:4,5-naphthalenediimide (diimide) adsorbed on highly ordered pyrolytic graphite (HOPG). The diimide forms well ordered monolayers either at the interface between HOPG and several phenylalkanes, or at the HOPG-air or HOPG-vacuum interface when adsorbed from toluene. Planar adsorption of the diimide ring on HOPG is observed. Hydrogen bonding, O and N interaction with HOPG, and π-π interactions appear to be the primary drivers for determining the monolayer structure which is stable and independent of the adsorption method. This is an unusual example since most alkane-substituted systems studied to date rely on alkane chain interactions (with HOPG and interdigitation) to drive the adsorbate structure on graphite. The observed unit cell has a) 2.0 (0.2 nm, b) 1.95 (0.2 nm, R) 67 (2°. The STM imaging is highly bias dependent and appears to be controlled (in the (2 V bias region) by an unoccupied orbital. Orbitalmediated tunneling spectra reveal a single strong electron affinity band near 3.5 eV below the vacuum level.
The Journal of Physical Chemistry B, 2005
Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to highand low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.
The Journal of Physical Chemistry B, 1997
Since the first report of the imaging of organic molecules by scanning tunneling microscopy (STM) (Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542-545) questions have arisen about the physical interaction between the scanning tip and the adsorbed monolayer. How much this interaction affects the adsorbed monolayer, and whether the monolayer structure observed by STM is the same structure which is present in the absence of STM observation have been difficult questions to address, since the monolayer typically cannot be seen at high resolution except by STM. Many reports of tip-induced substrate damage have appeared, but here we report nondestructive tip-induced orientation of an adsorbed monolayer confined in molecule corrals and on graphite terraces. We believe this is the first evidence that in some cases the scanning action of the STM tip can act to orient an adsorbed monolayer. For adsorbed monolayers of octadecanol and octatriacontane, the structures observed by STM are not the same structures which are present in the absence of STM observation. The ordering action of the STM tip removes information about the initial appearance of the monolayer but may lead to better understanding of the physical interactions between the tip and adsorbed monolayers and between the adsorbed monolayers and the substrate.
Scanning tunneling microscopy imaging of alkane bilayers adsorbed on graphite: mechanism of contrast
Surface Science, 1993
Thin films of insulating molecules (n-f36H74) absorbed on graphite have been studied using scanning tunneling microscopy (STM). By imaging the boundary of second layer islands in various conditions, we demonstrate that the contrast observed in STM images of these films is dominated by elastic deformations of substrate. The relief in these images may strongly differs from the real topography of the film.
Journal of The American Chemical Society, 1991
Monolayer films from ethanethiol (ET) and n-octadecanethiol (OT) spontaneously adsorbed onto epitaxially grown Au( 1 I I ) films on mica were examined by scanning tunneling microscopy (STM). The resulting atomically resolved images are the first reported for gold-adsorbed organothiolate molecules and reveal the packing arrangement of the overlayer. Tunneling is presumed to occur between the microscope tip and the gold-bound sulfur of the n-alkanethiolate head group. For both the ET and OT monolayers, an image that corresponds to a hexagonally packed array of adsorbates with respective nearest-neighbor and next-nearest-neighbor spacings of 0.50 f 0.02 and 0.87 f 0.04 nm was observed. This packing agrees well with the (d? X d/5)R3O0 structure determined for long-chain n-alkanethiolate monolayers on Au(l11) in recent helium diffraction' and clcctron diffraction2 studies. Furthermore, images with the above spacings were found to exhibit continuity over areas from a few square nanometers up to about 600 nm2, indicating the potential utility of STM for probing both the short-and long-range order of organic monolayer films. Structural interpretations of these images are presented and examined within the context of molecular level descriptions that have been recently developed from macroscopic characterization studies of these monolayers.
Physical Chemistry Chemical Physics, 2003
Static and dynamic structures in self-assembled monolayers containing 1-pyrenehexadecanoic acid (PHDA) at a liquid-solid interface were investigated with scanning tunneling microscopy. Uni-component adsorption layers made a specific structure having ring figures corresponding to pyrene groups, whereas the co-adsorption of PHDA and 4,4 0 -bipyridyl resulted in two different structures depending on their concentrations and the ambient temperature. When the concentration of 4,4 0 -bipyridyl was high and the temperature at around 290 K, a dynamic bistable structure was observed within a stacked row of molecules without causing any disorder to the assembly as a whole. The bistability is considered to be due to molecular desorption-adsorption process occurring at the anti-parallel configuration of two neighbouring PHDA molecules.