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Ordering of alkylcyanobiphenyl molecules at MoS2 and graphite surfaces studied by tunneling microscopy
A74
Dawn A. Bonnell Department of Materials Science and Engineering, Unit'ersity of Pennsyh'ania, Philadelphia, PA 10104, USA Received 21 February 1992; accepted for publication 4 July 1992 The surface structure of reduced TiO2 x(110) has been studied in ultrahigh-vacuum by scanning tunneling microscopy and tunneling spectroscopy. A variety of surface structure types with periodicities larger than the bulk TiO2ounit cell have been clearly resolved. These periodic structures, which have repeat distances ranging from 3.2 .~ to greater than 30 A, coexist with one another on the reduced surface and have domain sizes of < 1000 ,~. Tunneling spectra acquired from this surface show the bulk conduction band edge 0.5 eV above the Fermi level and a bandgap greater than 2.5 eV. These observations are compared to a surface structure model that assumes the presence of varying concentrations of bulk crystallographic shear plane defects that accommodate non-stoichiometry in this material.
Surface Science 278 (1992) 157-165 North-Holland
Observation of a stable methoxy intermediate on Cr(110) Neal D. Shinn ~ Sandia National Laboratories, Department 1! 14, Albuquerque, NM 87185, USA Received 21 April 1992; accepted for publication 7 July 1992 Ultraviolet photoelectron spectroscopy (UPS) and work function changes are used to identify a stable methoxy intermediate, CH30(ads), on C r ( l l 0 ) at 90 K following low exposures of methanol, CH~OH(g). Higher methanol exposures at 90 K add subsequent layers of physisorbed methanol which desorb above 120 K. Methoxy dissociation into atomic fragments occurs between 250 and 400 K. The absence of a stable CO(ads) dissociation intermediate indicates that methyl group dehydrogenation is rate limiting on Cr(ll0). As was previously found on Fe(100) and Mo(100), the stability of C H 3 0 ( a d s ) over CO(ads) on C r ( l l 0 ) is opposite of that proposed by Sexton based upon relative heats of adsorption. Consideration of the C H 3 0 : m e t a l and C O : m e t a l bonding mechanisms leads to the proposition that substrate electronic properties, rather than steric or substrate morphological factors, are responsible for the anomalous methoxy stability on these surfaces.
Surface Science 278 (1992) 166-174 North-Holland
Ordering of alkylcyanobiphenyl molecules at MoS 2 and graphite surfaces studied by tunneling microscopy D . P . E . S m i t h , W . M . H e c k l ~ a n d H . A . Klagges IBM Physics Group, Schellingstrasse 4, D-8000 Munich 40, Germany Received 23 April 1992; accepted for publication 23 July 1992 The interfacial ordering of the alkylcyanobiphenyl family of liquid crystal molecules is studied on the MoS 2 and graphite surfaces and found in both cases to arise from registry with the substrate. Aromatic and aliphatic groups are observed to have different contrasts in tunneling microscope images, with the aromatic/aliphatic contrast difference being much higher on graphite than on MoS 2.
Dawn A. Bonnell Department of Materials Science and Engineering, Unit'ersity of Pennsyh'ania, Philadelphia, PA 10104, USA Received 21 February 1992; accepted for publication 4 July 1992 The surface structure of reduced TiO2 x(110) has been studied in ultrahigh-vacuum by scanning tunneling microscopy and tunneling spectroscopy. A variety of surface structure types with periodicities larger than the bulk TiO2ounit cell have been clearly resolved. These periodic structures, which have repeat distances ranging from 3.2 .~ to greater than 30 A, coexist with one another on the reduced surface and have domain sizes of < 1000 ,~. Tunneling spectra acquired from this surface show the bulk conduction band edge 0.5 eV above the Fermi level and a bandgap greater than 2.5 eV. These observations are compared to a surface structure model that assumes the presence of varying concentrations of bulk crystallographic shear plane defects that accommodate non-stoichiometry in this material.
Surface Science 278 (1992) 157-165 North-Holland
Observation of a stable methoxy intermediate on Cr(110) Neal D. Shinn ~ Sandia National Laboratories, Department 1! 14, Albuquerque, NM 87185, USA Received 21 April 1992; accepted for publication 7 July 1992 Ultraviolet photoelectron spectroscopy (UPS) and work function changes are used to identify a stable methoxy intermediate, CH30(ads), on C r ( l l 0 ) at 90 K following low exposures of methanol, CH~OH(g). Higher methanol exposures at 90 K add subsequent layers of physisorbed methanol which desorb above 120 K. Methoxy dissociation into atomic fragments occurs between 250 and 400 K. The absence of a stable CO(ads) dissociation intermediate indicates that methyl group dehydrogenation is rate limiting on Cr(ll0). As was previously found on Fe(100) and Mo(100), the stability of C H 3 0 ( a d s ) over CO(ads) on C r ( l l 0 ) is opposite of that proposed by Sexton based upon relative heats of adsorption. Consideration of the C H 3 0 : m e t a l and C O : m e t a l bonding mechanisms leads to the proposition that substrate electronic properties, rather than steric or substrate morphological factors, are responsible for the anomalous methoxy stability on these surfaces.
Surface Science 278 (1992) 166-174 North-Holland
Ordering of alkylcyanobiphenyl molecules at MoS 2 and graphite surfaces studied by tunneling microscopy D . P . E . S m i t h , W . M . H e c k l ~ a n d H . A . Klagges IBM Physics Group, Schellingstrasse 4, D-8000 Munich 40, Germany Received 23 April 1992; accepted for publication 23 July 1992 The interfacial ordering of the alkylcyanobiphenyl family of liquid crystal molecules is studied on the MoS 2 and graphite surfaces and found in both cases to arise from registry with the substrate. Aromatic and aliphatic groups are observed to have different contrasts in tunneling microscope images, with the aromatic/aliphatic contrast difference being much higher on graphite than on MoS 2.