Formation of Two-Dimensional Mixed Crystals on Graphite Observed by Scanning Tunneling Microscopy

Stevens, Forrest; Dyer, Daniel J.; Walba, David M.

doi:10.1021/la950960c

Cited by 18 publications

(19 citation statements)

References 25 publications

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“…This is consistent with an earlier report that the double bonds in an unsaturated liquid crystal could not be reliably observed by STM. 7 The reason for this variability is not clear, although it could be tip-related. Because the double bond could not be consistently observed, no attempts were made to analyze the saturated/ unsaturated monolayer ratios quantitatively.…”

Section: Resultsmentioning

confidence: 99%

“…A mixed-monolayer liquid crystal was tentatively identified based on unit-cell dimensions, but the two different molecules could not be distinguished. 7 In a final case, protoporphyrin containing iron appeared bright by STM, whereas protoporphyrin without iron appeared relatively dark. Solutions of protoporphyrin with and without iron form mixed monolayers on graphite, and the two different molecules could be differentiated.…”

Section: Introductionmentioning

confidence: 86%

“…These molecules have been observed to change between bright and dark centers, but this is believed to be due to conformational changes rather than molecular exchange. A mixed-monolayer liquid crystal was tentatively identified based on unit-cell dimensions, but the two different molecules could not be distinguished . In a final case, protoporphyrin containing iron appeared bright by STM, whereas protoporphyrin without iron appeared relatively dark.…”

Section: Introductionmentioning

confidence: 94%

“…Mixed monolayers containing more than one organic compound are also of interest, but they greatly complicate the interpretation of STM images. However, several mixtures have been studied, including alkane/alcohol mixtures, 1 alcohols 2 or acids 3 of different chain lengths, n-alkylcyanobiphenyls, [4][5][6] unsaturated liquid crystals, 7 and others. [8][9][10] In most cases, the mixtures were observed to form separate domains, each composed of a pure component.…”

Section: Introductionmentioning

confidence: 99%

“…Mixed monolayers containing more than one organic compound are also of interest, but they greatly complicate the interpretation of STM images. However, several mixtures have been studied, including alkane/alcohol mixtures, alcohols or acids of different chain lengths, n -alkylcyanobiphenyls, − unsaturated liquid crystals, and others. − In most cases, the mixtures were observed to form separate domains, each composed of a pure component. The cyanobiphenyls form stoichiometric mixed-crystal domains, with the monolayer containing the two components in a fixed ratio and with a specific ordered structure in each unit cell.…”

Section: Introductionmentioning

confidence: 99%

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Dynamical Exchange Behavior in Organic Monolayers Studied by STM Analysis of Labeled Mixtures

and

Beebe

1999

Langmuir

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By using mixtures of molecules which are sterically similar but contain functional groups with differing electronic properties, we have obtained scanning tunneling microscopy (STM) images of domains of randomly mixed monolayers of organic molecules on graphite, and the different molecules could be distinguished by STM. It was determined that molecules in the centers of the domains could still exchange readily with the solution, something which could not be determined using monolayers with single-component domains. The typical residence time of a molecule in the monolayers appeared to be between 0.2 and 20 s, and some variation was observed for different mixtures. We have also shown one case in which the composition of the adsorbed monolayer appears to be nearly independent of the composition of the bulk solution over almost 3 orders of magnitude. Although the mixture components could often be distinguished by STM, other times little or no difference was observed, even when image resolution appeared to be quite good in general. This suggests that the factors leading to STM contrast in organic molecules are complex, and the quest for reliable functional group identification by STM may be more difficult than expected.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 86%

Section: Introductionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

“…Mixed monolayers containing more than one organic compound are also of interest, but they greatly complicate the interpretation of STM images. However, several mixtures have been studied, including alkane/alcohol mixtures, alcohols or acids of different chain lengths, n -alkylcyanobiphenyls, − unsaturated liquid crystals, and others. − In most cases, the mixtures were observed to form separate domains, each composed of a pure component. The cyanobiphenyls form stoichiometric mixed-crystal domains, with the monolayer containing the two components in a fixed ratio and with a specific ordered structure in each unit cell.…”

Section: Introductionmentioning

confidence: 99%

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Dynamical Exchange Behavior in Organic Monolayers Studied by STM Analysis of Labeled Mixtures

and

Beebe

1999

Langmuir

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Direkte Beobachtung von aus Enantiomeren aufgebauten enantiomorphen Monoschicht‐Kristallen mit der Rastertunnelmikroskopie

1996

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Die Bildung chiraler Strukturen aus achiralen oder racemischen Bestandteilen ist von großer Bedeutung z.B. hinsichtlich des Ursprungs der biologischen Chiralität. Monoschichten racemischer Moleküle können chirale Strukturen bilden, aber der Ursprung dieser Strukturen war nicht bekannt. Die kraftmikroskopische Abbildung der Oberflächenstrukturen von beiden Enantiomeren ((S)‐Enantiomer rechts gezeigt) und dem Racemat einer flüssigkristallinen Verbindung mit der Rastertunnelmikroskopie liefert deutliche Hinweise darauf, daß das Racemat Domänen aus enantiomerenreinen Molekülen bildet.

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Direct Observation of Enantiomorphous Monolayer Crystals from Enantiomers by Scanning Tunneling Microscopy

Stevens

Dyer

Walba

1996

Angew. Chem. Int. Ed. Engl.

116

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Pasteur's brilliant observation, using optical microscopy, that crystals of a salt of "racemic acid" actually existed in two enantiomorphous forms ['] not only uncovered the existence of enantiomers, but indeed stimulated the development of modern structural theory. The direct (without resorting to diffraction techniques) observation of supermolecular chirality and the relationship between supermolecular chirality and molecular structure have fascinated chemists ever since. Starting with early studies of the morphology of chiral crystals by optical microscopy, the scale for the observation of chiral supermolecular assemblies has been approaching that of the molecules themselves. McConneli and Weis first recorded the formation of chiral crystalline monolayer domains in Langmuir films of enantiomerically pure lipids by epifluorescence optical microscopy.[zl More recently, chiral monolayers have been observed at molecular resolution by scanning probe microscopy. Thus, chiral symmetry breaking from achiralc31 and racemicf4I molecules has been detected in Langmuir films by atomic force microscopy (AFM), and enantiomorphous monolayer domains from achiral liquid crystal (LC) molecules on graphite have been observed by scanning tunneling microscopy (STM) .Is1 In related work, amino acids have been spontaneously resolved by crystallization in two dimensions at the air-water interface.16]We report here on the molecular resolution STM observation of enantiomorphous images of crystal monolayers grown from pure enantiomers. Furthermore, a racemic mixture of the same molecules produced images of co-existing enantiomorphic domains indistinguishable from those obtained from the enantiomerically pure materials, providing strong evidence for chiral symmetry breaking to give a two-dimensional (2-D) conglomerate. These results suggest a complete 2-D analog of Pasteur's famous system.The experiments were performed on enantiomerically enriched and racemic biphenylbenzoates 1. These have one tetra-1 hedral stereogenic center (indicated by an asterisk). Both enantiomers were prepared in approximately 99 YO ee; the racemate was also prepared."] All three compounds were liquid crystalline with a smectic A phase at room temperature.Each STM sample was prepared by placing a small amount (< 1 mg) of the liquid crystal on a freshly cleaved surface of highly oriented pyrolytic graphite. Scanning tips were prepared from mechanically cut 0.025 cm platinum/iridium (80: 20) wire. Images were recorded in air at room temperature using a commercially available STM (Nanoscope 11, Digital Instruments, Inc.) with tip positive bias.

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Formation of Two-Dimensional Mixed Crystals on Graphite Observed by Scanning Tunneling Microscopy

Cited by 18 publications

References 25 publications

Dynamical Exchange Behavior in Organic Monolayers Studied by STM Analysis of Labeled Mixtures

Dynamical Exchange Behavior in Organic Monolayers Studied by STM Analysis of Labeled Mixtures

Direkte Beobachtung von aus Enantiomeren aufgebauten enantiomorphen Monoschicht‐Kristallen mit der Rastertunnelmikroskopie

Direct Observation of Enantiomorphous Monolayer Crystals from Enantiomers by Scanning Tunneling Microscopy

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