Yinying Wei - Academia.edu (original) (raw)

Papers by Yinying Wei

Research paper thumbnail of Boron Nitride Monolayers: Charge State Control of F 16 CoPc on h ‐BN/Cu(111) (Adv. Mater. Interfaces 15/2020)

Advanced Materials Interfaces

Research paper thumbnail of Charge State Control of F 16 CoPc on h ‐BN/Cu(111)

Advanced Materials Interfaces

Research paper thumbnail of Architecture-Electronic Property Relations Across Molecular Semiconductor Interfaces

Research paper thumbnail of Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

Scientific Reports, 2016

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental ... more Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photoactivated dehydrogenation of ethanol on reduced and oxidized rutile TiO 2 (110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO 2. Catalysis and photo-catalysis are fields of paramount importance both with a view on the demands on the chemical industry and the challenges in future renewable energy generation as well as to sustain our environment. To fulfil all these demands it is crucial to enhance the efficiency of designated chemical processes. One of the most promising materials in photo-catalysis is Titania (TiO 2) 1-6. Besides photo-catalysis, TiO 2 is used in thermal heterogeneous catalysis, solar cells, gas sensors, and biocompatible materials. An increasingly important field is the photo-catalytic and renewable generation of fuels, and particularly ethanol (CH 3 CH 2 OH or EtOH) holds great promise in this regard 5,6. In addition, EtOH is an essential solvent and it could be used as feedstock in a possible green chemistry in the future 7-9. These expectations, and the fact that alcohols serve as model molecules of catalytic and photo-catalytic processes 4,10-12 , have stimulated considerable research efforts towards the thermal and photo-catalytic oxidation of EtOH on TiO 2. Numerous studies have been conducted using TiO 2 powders, where TiO 2 nanoparticles (often a mix of the two most common polymorphs, anatase and rutile) were used to oxidize EtOH condensed as liquid or in the gas phase 13-21. While other work has addressed photo-catalytic reactions of methanol (MeOH) 4,6,11,22-26 it is worth indicating two fundamental differences between MeOH and EtOH. First, because of the carbon-carbon bond the chemistry of EtOH is akin to higher hydrocarbons and different from that of C1 hydrocarbons. Second, MeOH is made from syngas and therefore not a renewable feedstock. In surface science, where single crystalline surfaces can be studied at ultrahigh vacuum (UHV) conditions, particular focus has been on the interaction between EtOH and the rutile TiO 2 (110)-(1 × 1) surface 27-39 , because this surface is the most stable one of rutile and often serves as a model for transition metal surfaces 1,2,3,4,10-12. The TiO 2 (110)-(1 × 1) surface consists of alternating rows of fivefold-coordinated titanium (5f-Ti) atoms (the Ti troughs) and protruding, twofold coordinated bridge-bonded oxygen (O br) atoms. Following cycles of Ar + sputtering and vacuum-annealing, the TiO 2 (110) crystals are reduced, leading to the creation of bulk defects and O br vacancies on the surface 1,10,12,40,41. This leads to changes in electronic properties, where the empty Ti3d orbitals become populated, leading to a state within the ~3.1 eV wide band gap ~0.85 eV below the Fermi level (E F) 1,40-42. This Ti3d defect state can be removed upon oxygen adsorption 1,10,40-43. The adsorption of EtOH and the thermally activated chemistry of EtOH on rutile TiO 2 (110)-(1 × 1) have been studied previously by means of photoelectron spectroscopy (PES) 28,30,44 , temperature-programmed desorption (TPD) 27-29,32,33,36-39 , femtosecond two-photon photoemission spectroscopy (2PPE) 36 , and scanning tunnelling

Research paper thumbnail of A molecular view of TiOPc -- C70 interface formation

Research paper thumbnail of Atomic-Scale View on the H 2 O Formation Reaction from H 2 on O-Rich RuO 2 (110)

The Journal of Physical Chemistry C, 2014

The H2O formation reaction from H2 on O-rich RuO2(110) was studied by temperature-programmed deso... more The H2O formation reaction from H2 on O-rich RuO2(110) was studied by temperature-programmed desorption and reaction (TPD/TPR) and scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. On the one hand, following H2 adsorption at 270 K, our TPD/TPR measurements reveal that the on-top O species (Oot) enhances the sticking probability of H2, thus facilitating the H2 adsorption and dissociation on O-rich RuO2(110). On the other hand, for low H2 adsorption temperature (170 K), the limited mobility of Had species hinders H2 adsorption at a high coverage of preadsorbed Oot. To better understand the strong influence of the adsorption temperature and the interplay between coadsorbed species, we conducted DFT calculations and high-resolution STM measurements. Two distinct adsorbate configurations, Had–Oot and Oot–Had–Oot, are identified by STM. Mechanisms and molecular models for H2 dissociation and Had diffusion on O-rich RuO2(110) are proposed.

Research paper thumbnail of Dipole–Dipole Interactions in TiOPc Adlayers on Ag

The Journal of Physical Chemistry C, 2014

ABSTRACT

Research paper thumbnail of Role of Steps in the Dissociative Adsorption of Water on Rutile <span class="aps-inline-formula"><math display="inline"><msub><mi>TiO</mi><mn>2</mn></msub><mo stretchy="false">(</mo><mn>110</mn><mo stretchy="false">)</mo></math></span>

Physical Review Letters, 2013

The water-TiO(2) interaction is of paramount importance for many processes occurring on TiO(2), a... more The water-TiO(2) interaction is of paramount importance for many processes occurring on TiO(2), and the rutile TiO(2)(110)-(1×1) surface has often been considered as a test case. Yet, no consensus has been reached whether the well-studied surface O vacancies on the terraces are the only active sites for water dissociation on rutile TiO(2)(110)-(1 × 1), or whether another channel for the creation of H adatoms exists. Here we use high-resolution scanning tunneling microscopy and density functional theory calculations to tackle this long-standing question. Evidence is presented that a second water dissociation channel exists on the surfaces of vacuum-annealed TiO(2)(110) crystals that is associated with the ⟨111⟩ step edges. This second water dissociation channel can be suppressed by blocking of the ⟨111⟩ step edges using ethanol.

Research paper thumbnail of Direct Evidence for Ethanol Dissociation on Rutile TiO_{2}(110)

Physical Review Letters, 2011

We have studied the interaction of ethanol with reduced TiO(2)(110)-(1 × 1) by high-resolution sc... more We have studied the interaction of ethanol with reduced TiO(2)(110)-(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory calculations. The STM data revealed direct evidence for the coexistence of molecularly and dissociatively adsorbed ethanol species on surface Ti sites. In addition, we found evidence for dissociation of ethanol at bridge-bonded O vacancies. The density functional theory calculations support these findings and rationalize the distinct diffusion behaviors of molecularly and dissociatively adsorbed ethanol species, as revealed in time-lapsed STM images.

Research paper thumbnail of Ethanol Diffusion on Rutile TiO 2 (110) Mediated by H Adatoms

The Journal of Physical Chemistry Letters, 2012

We have studied the diffusion of ethanol on rutile TiO 2 (110)−(1 × 1) by high-resolution scannin... more We have studied the diffusion of ethanol on rutile TiO 2 (110)−(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Time-lapsed STM images recorded at ∼200 K revealed the diffusion of ethanol molecules both parallel and perpendicular to the rows of surface Ti atoms. The diffusion of ethanol molecules perpendicular to the rows of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (O br) atoms similarly to previous results obtained for water monomers. In contrast, the diffusion of H adatoms across the Ti rows, mediated by ethanol molecules, was observed only very rarely and exclusively on fully hydrogenated TiO 2 (110) surfaces. Possible reasons why the diffusion of H adatoms across the Ti rows mediated by ethanol molecules occurs less frequently than the cross-row diffusion of ethanol molecules mediated by H adatoms are discussed.

Research paper thumbnail of The Importance of Bulk Ti3+ Defects in the Oxygen Chemistry on Titania Surfaces

Research paper thumbnail of Effects of the crystal reduction state on the interaction of oxygen with rutile TiO2(110)

Catalysis Today, 2012

The interaction of O 2 with reduced rutile TiO 2 (1 1 0)-(1 × 1) has been studied by means of sca... more The interaction of O 2 with reduced rutile TiO 2 (1 1 0)-(1 × 1) has been studied by means of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and photoelectron spectroscopy (PES). It is found that the interaction of O 2 with TiO 2 (1 1 0) depends strongly on the reduction state of the TiO 2 (1 1 0) crystal. High-resolution STM studies revealed that the energy barrier for the non-vacancyassisted, 2nd O 2 dissociation channel decreases with increasing crystal reduction. Additionally, it is found in the STM studies that the Ti interstitial diffusion is slightly more facile in high-reduced TiO 2 (1 1 0) crystals compared to low-reduced ones. Accompanying TPD studies revealed that the line shape of the O 2-TPD peak occurring between ∼360 K and ∼450 K depends on the crystal reduction state. For high-reduced TiO 2 (1 1 0) crystals characterized by large terraces most O 2 molecules desorb at ∼386 K, whereas O 2 desorption is peaking at ∼410 K for low-and medium-reduced crystals. Furthermore, the O 2-TPD experiments revealed a highly non-linear behavior of the O 2 desorption peak integrals as function of the crystal reduction state. The presented results point to an ionosorption model where the adsorbates withdraw the excess charge (Ti 3+) from the near-surface region at temperatures < ∼360 K and where Ti interstitials react with oxygen species on the surface at temperatures ≥ ∼360 K.

Research paper thumbnail of Controlled Chemical Morphology in TiOPc-C $ _ {60} $ Films

Research paper thumbnail of Molecular Interface Formation in Titanyl Phthalocyanine-C60 Monolayer Films

Molecular interfaces have been prepared in titanyl phthalocyanine (TiOPc)−C 60 monolayer films on... more Molecular interfaces have been prepared in titanyl phthalocyanine (TiOPc)−C 60 monolayer films on Ag(111) and characterized with ultra high vacuum−scanning tunneling microscopy. Two distinct TiOPc monolayer phases, namely, a hexagonal phase and a honeycomb phase, were first generated as molecular-film substrates for C 60 growth. Both TiOPc monolayer structures rearrange upon C 60 adsorption, yielding films with nanophase-separated TiOPc and C 60 domains and a new cocrystalline TiOPc 2 (C 60) 1 honeycomb network. Kinetic access to the cocrystalline network is most facile from the hexagonal TiOPc monolayer, producing extended domains. Detailed models for emergent structural phases and molecular interfaces are presented. Mechanisms for C 60-induced rearrangements are developed from coverage-dependent measurements.

Research paper thumbnail of Directed Organization of C70 Kagome Lattice by Titanyl Phthalocyanine Monolayer Template

Controlled deposition of titanyl phthalocyanine (TiOPc) on Ag(111) produces a honeycomb monolayer... more Controlled deposition of titanyl phthalocyanine (TiOPc) on Ag(111) produces a honeycomb monolayer phase consisting of TiOPc molecules with two distinctive tilt angles. This periodic arrangement of polar molecules is used to direct C 70 growth into low-density 3D films with novel C 70 kagome lattice arrangements. Structural models for the C 70 kagome lattice are determined from layer-by-layer scanning tunneling microscopy images and related to the dipolar TiOPc template and C 70 's anisotropic polarizability. Molecular templates with designed electrostatic features offer a practical method to control 3D film organization on the nanoscale by harnessing anisotropic molecular interactions at the growth interface.

Research paper thumbnail of Flux-selected titanyl phthalocyanine monolayer architecture on Ag (111)

Research paper thumbnail of Potential Steps at C60–TiOPc–Ag (111) Interfaces: Ultrahigh-Vacuum–Noncontact Scanning Probe Metrology

ABSTRACT Nanoscale structure-electric potential relations in films of the organic molecular semic... more ABSTRACT Nanoscale structure-electric potential relations in films of the organic molecular semiconductors C(60) and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φ(step) = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C(60) islands. Potential steps in binary C(60)-TiOPc films, nanophase-separated into crystalline C(60) and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C(60)-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric potential across a material interface. The absence of potential asymmetry across this lateral interface sets the upper bound for the C(60)-TiOPc interface dipole moment as 0.012 e nm.

Research paper thumbnail of Polymorphism in Self-Assembled Structures of 9-Anthracene Carboxylic Acid on Ag (111)

Surface self-assembly process of 9-anthracene carboxylic acid (AnCA) on Ag(111) was investigated ... more Surface self-assembly process of 9-anthracene carboxylic acid (AnCA) on Ag(111) was investigated using STM. Depending on the molecular surface density, four spontaneously formed and one annealed AnCA ordered phases were observed, namely a straight belt phase, a zigzag double-belt phase, two simpler dimer phases, and a kagome phase. The two high-density belt phases possess large unit cells on the scale length of 10 nm, which are seldom observed in molecular self-assembled structures. This structural diversity stems from a complicated competition of different interactions of AnCA molecules on metal surface, including intermolecular and molecular-substrate interactions, as well as the steric demand from high molecular surface density.

Research paper thumbnail of Formation of metastable, heterolytic H-pairs on the RuO2(110) surface

ABSTRACT The interaction of molecular hydrogen (H2) with the stoichiometric RuO2(110) surface was... more ABSTRACT The interaction of molecular hydrogen (H2) with the stoichiometric RuO2(110) surface was studied at various temperatures and coverages by high-resolution scanning tunneling microscopy (STM) and temperature-programmed reaction (TPR). Metastable, heterolytic H-pairs of dissociated H2 consisting of one H adsorbed on a surface Ru atom and the other H at an adjacent bridging O atom are identified. This H-pair configuration is consistent with accompanying density functional theory (DFT) calculations and spectroscopic data. We propose that such metastable, heterolytic H-pairs are a typical feature of late transition metal oxide surfaces. Furthermore, the adsorption temperature was found to be decisive for the configuration of the adsorbed hydrogen species and the sticking of H2.

Research paper thumbnail of TiOPc Molecular Dislocation Networks as Nanotemplates for C60 Cluster Arrays

By controlled deposition, TiOPc, a molecular semiconductor with anisotropic interactions can gene... more By controlled deposition, TiOPc, a molecular semiconductor with anisotropic interactions can generate a molecular film with a characteristic pattern repeat size of 15 nm. This structure then served as a nanotemplate for a superlattice of C(60) clusters with characteristic diameters of 7 nm. As a result, C(60) deposition on the TiOPc film template forms a pattern of nanophase-separated C(60) and TiOPc domains with a characteristic domain size of 7 nm. This feature size is matched to the exciton diffusion length in photovoltaic materials composed of small organic molecules. A dislocation network in a molecular film provides a promising method for generating pattern features on the several-nanometer length scale, bridging the practical limits of &amp;amp;amp;amp;amp;amp;quot;bottom up&amp;amp;amp;amp;amp;amp;quot; and &amp;amp;amp;amp;amp;amp;quot;top down&amp;amp;amp;amp;amp;amp;quot; strategies.

Research paper thumbnail of Boron Nitride Monolayers: Charge State Control of F 16 CoPc on h ‐BN/Cu(111) (Adv. Mater. Interfaces 15/2020)

Advanced Materials Interfaces

Research paper thumbnail of Charge State Control of F 16 CoPc on h ‐BN/Cu(111)

Advanced Materials Interfaces

Research paper thumbnail of Architecture-Electronic Property Relations Across Molecular Semiconductor Interfaces

Research paper thumbnail of Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

Scientific Reports, 2016

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental ... more Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photoactivated dehydrogenation of ethanol on reduced and oxidized rutile TiO 2 (110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO 2. Catalysis and photo-catalysis are fields of paramount importance both with a view on the demands on the chemical industry and the challenges in future renewable energy generation as well as to sustain our environment. To fulfil all these demands it is crucial to enhance the efficiency of designated chemical processes. One of the most promising materials in photo-catalysis is Titania (TiO 2) 1-6. Besides photo-catalysis, TiO 2 is used in thermal heterogeneous catalysis, solar cells, gas sensors, and biocompatible materials. An increasingly important field is the photo-catalytic and renewable generation of fuels, and particularly ethanol (CH 3 CH 2 OH or EtOH) holds great promise in this regard 5,6. In addition, EtOH is an essential solvent and it could be used as feedstock in a possible green chemistry in the future 7-9. These expectations, and the fact that alcohols serve as model molecules of catalytic and photo-catalytic processes 4,10-12 , have stimulated considerable research efforts towards the thermal and photo-catalytic oxidation of EtOH on TiO 2. Numerous studies have been conducted using TiO 2 powders, where TiO 2 nanoparticles (often a mix of the two most common polymorphs, anatase and rutile) were used to oxidize EtOH condensed as liquid or in the gas phase 13-21. While other work has addressed photo-catalytic reactions of methanol (MeOH) 4,6,11,22-26 it is worth indicating two fundamental differences between MeOH and EtOH. First, because of the carbon-carbon bond the chemistry of EtOH is akin to higher hydrocarbons and different from that of C1 hydrocarbons. Second, MeOH is made from syngas and therefore not a renewable feedstock. In surface science, where single crystalline surfaces can be studied at ultrahigh vacuum (UHV) conditions, particular focus has been on the interaction between EtOH and the rutile TiO 2 (110)-(1 × 1) surface 27-39 , because this surface is the most stable one of rutile and often serves as a model for transition metal surfaces 1,2,3,4,10-12. The TiO 2 (110)-(1 × 1) surface consists of alternating rows of fivefold-coordinated titanium (5f-Ti) atoms (the Ti troughs) and protruding, twofold coordinated bridge-bonded oxygen (O br) atoms. Following cycles of Ar + sputtering and vacuum-annealing, the TiO 2 (110) crystals are reduced, leading to the creation of bulk defects and O br vacancies on the surface 1,10,12,40,41. This leads to changes in electronic properties, where the empty Ti3d orbitals become populated, leading to a state within the ~3.1 eV wide band gap ~0.85 eV below the Fermi level (E F) 1,40-42. This Ti3d defect state can be removed upon oxygen adsorption 1,10,40-43. The adsorption of EtOH and the thermally activated chemistry of EtOH on rutile TiO 2 (110)-(1 × 1) have been studied previously by means of photoelectron spectroscopy (PES) 28,30,44 , temperature-programmed desorption (TPD) 27-29,32,33,36-39 , femtosecond two-photon photoemission spectroscopy (2PPE) 36 , and scanning tunnelling

Research paper thumbnail of A molecular view of TiOPc -- C70 interface formation

Research paper thumbnail of Atomic-Scale View on the H 2 O Formation Reaction from H 2 on O-Rich RuO 2 (110)

The Journal of Physical Chemistry C, 2014

The H2O formation reaction from H2 on O-rich RuO2(110) was studied by temperature-programmed deso... more The H2O formation reaction from H2 on O-rich RuO2(110) was studied by temperature-programmed desorption and reaction (TPD/TPR) and scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. On the one hand, following H2 adsorption at 270 K, our TPD/TPR measurements reveal that the on-top O species (Oot) enhances the sticking probability of H2, thus facilitating the H2 adsorption and dissociation on O-rich RuO2(110). On the other hand, for low H2 adsorption temperature (170 K), the limited mobility of Had species hinders H2 adsorption at a high coverage of preadsorbed Oot. To better understand the strong influence of the adsorption temperature and the interplay between coadsorbed species, we conducted DFT calculations and high-resolution STM measurements. Two distinct adsorbate configurations, Had–Oot and Oot–Had–Oot, are identified by STM. Mechanisms and molecular models for H2 dissociation and Had diffusion on O-rich RuO2(110) are proposed.

Research paper thumbnail of Dipole–Dipole Interactions in TiOPc Adlayers on Ag

The Journal of Physical Chemistry C, 2014

ABSTRACT

Research paper thumbnail of Role of Steps in the Dissociative Adsorption of Water on Rutile <span class="aps-inline-formula"><math display="inline"><msub><mi>TiO</mi><mn>2</mn></msub><mo stretchy="false">(</mo><mn>110</mn><mo stretchy="false">)</mo></math></span>

Physical Review Letters, 2013

The water-TiO(2) interaction is of paramount importance for many processes occurring on TiO(2), a... more The water-TiO(2) interaction is of paramount importance for many processes occurring on TiO(2), and the rutile TiO(2)(110)-(1×1) surface has often been considered as a test case. Yet, no consensus has been reached whether the well-studied surface O vacancies on the terraces are the only active sites for water dissociation on rutile TiO(2)(110)-(1 × 1), or whether another channel for the creation of H adatoms exists. Here we use high-resolution scanning tunneling microscopy and density functional theory calculations to tackle this long-standing question. Evidence is presented that a second water dissociation channel exists on the surfaces of vacuum-annealed TiO(2)(110) crystals that is associated with the ⟨111⟩ step edges. This second water dissociation channel can be suppressed by blocking of the ⟨111⟩ step edges using ethanol.

Research paper thumbnail of Direct Evidence for Ethanol Dissociation on Rutile TiO_{2}(110)

Physical Review Letters, 2011

We have studied the interaction of ethanol with reduced TiO(2)(110)-(1 × 1) by high-resolution sc... more We have studied the interaction of ethanol with reduced TiO(2)(110)-(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory calculations. The STM data revealed direct evidence for the coexistence of molecularly and dissociatively adsorbed ethanol species on surface Ti sites. In addition, we found evidence for dissociation of ethanol at bridge-bonded O vacancies. The density functional theory calculations support these findings and rationalize the distinct diffusion behaviors of molecularly and dissociatively adsorbed ethanol species, as revealed in time-lapsed STM images.

Research paper thumbnail of Ethanol Diffusion on Rutile TiO 2 (110) Mediated by H Adatoms

The Journal of Physical Chemistry Letters, 2012

We have studied the diffusion of ethanol on rutile TiO 2 (110)−(1 × 1) by high-resolution scannin... more We have studied the diffusion of ethanol on rutile TiO 2 (110)−(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Time-lapsed STM images recorded at ∼200 K revealed the diffusion of ethanol molecules both parallel and perpendicular to the rows of surface Ti atoms. The diffusion of ethanol molecules perpendicular to the rows of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (O br) atoms similarly to previous results obtained for water monomers. In contrast, the diffusion of H adatoms across the Ti rows, mediated by ethanol molecules, was observed only very rarely and exclusively on fully hydrogenated TiO 2 (110) surfaces. Possible reasons why the diffusion of H adatoms across the Ti rows mediated by ethanol molecules occurs less frequently than the cross-row diffusion of ethanol molecules mediated by H adatoms are discussed.

Research paper thumbnail of The Importance of Bulk Ti3+ Defects in the Oxygen Chemistry on Titania Surfaces

Research paper thumbnail of Effects of the crystal reduction state on the interaction of oxygen with rutile TiO2(110)

Catalysis Today, 2012

The interaction of O 2 with reduced rutile TiO 2 (1 1 0)-(1 × 1) has been studied by means of sca... more The interaction of O 2 with reduced rutile TiO 2 (1 1 0)-(1 × 1) has been studied by means of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and photoelectron spectroscopy (PES). It is found that the interaction of O 2 with TiO 2 (1 1 0) depends strongly on the reduction state of the TiO 2 (1 1 0) crystal. High-resolution STM studies revealed that the energy barrier for the non-vacancyassisted, 2nd O 2 dissociation channel decreases with increasing crystal reduction. Additionally, it is found in the STM studies that the Ti interstitial diffusion is slightly more facile in high-reduced TiO 2 (1 1 0) crystals compared to low-reduced ones. Accompanying TPD studies revealed that the line shape of the O 2-TPD peak occurring between ∼360 K and ∼450 K depends on the crystal reduction state. For high-reduced TiO 2 (1 1 0) crystals characterized by large terraces most O 2 molecules desorb at ∼386 K, whereas O 2 desorption is peaking at ∼410 K for low-and medium-reduced crystals. Furthermore, the O 2-TPD experiments revealed a highly non-linear behavior of the O 2 desorption peak integrals as function of the crystal reduction state. The presented results point to an ionosorption model where the adsorbates withdraw the excess charge (Ti 3+) from the near-surface region at temperatures < ∼360 K and where Ti interstitials react with oxygen species on the surface at temperatures ≥ ∼360 K.

Research paper thumbnail of Controlled Chemical Morphology in TiOPc-C $ _ {60} $ Films

Research paper thumbnail of Molecular Interface Formation in Titanyl Phthalocyanine-C60 Monolayer Films

Molecular interfaces have been prepared in titanyl phthalocyanine (TiOPc)−C 60 monolayer films on... more Molecular interfaces have been prepared in titanyl phthalocyanine (TiOPc)−C 60 monolayer films on Ag(111) and characterized with ultra high vacuum−scanning tunneling microscopy. Two distinct TiOPc monolayer phases, namely, a hexagonal phase and a honeycomb phase, were first generated as molecular-film substrates for C 60 growth. Both TiOPc monolayer structures rearrange upon C 60 adsorption, yielding films with nanophase-separated TiOPc and C 60 domains and a new cocrystalline TiOPc 2 (C 60) 1 honeycomb network. Kinetic access to the cocrystalline network is most facile from the hexagonal TiOPc monolayer, producing extended domains. Detailed models for emergent structural phases and molecular interfaces are presented. Mechanisms for C 60-induced rearrangements are developed from coverage-dependent measurements.

Research paper thumbnail of Directed Organization of C70 Kagome Lattice by Titanyl Phthalocyanine Monolayer Template

Controlled deposition of titanyl phthalocyanine (TiOPc) on Ag(111) produces a honeycomb monolayer... more Controlled deposition of titanyl phthalocyanine (TiOPc) on Ag(111) produces a honeycomb monolayer phase consisting of TiOPc molecules with two distinctive tilt angles. This periodic arrangement of polar molecules is used to direct C 70 growth into low-density 3D films with novel C 70 kagome lattice arrangements. Structural models for the C 70 kagome lattice are determined from layer-by-layer scanning tunneling microscopy images and related to the dipolar TiOPc template and C 70 's anisotropic polarizability. Molecular templates with designed electrostatic features offer a practical method to control 3D film organization on the nanoscale by harnessing anisotropic molecular interactions at the growth interface.

Research paper thumbnail of Flux-selected titanyl phthalocyanine monolayer architecture on Ag (111)

Research paper thumbnail of Potential Steps at C60–TiOPc–Ag (111) Interfaces: Ultrahigh-Vacuum–Noncontact Scanning Probe Metrology

ABSTRACT Nanoscale structure-electric potential relations in films of the organic molecular semic... more ABSTRACT Nanoscale structure-electric potential relations in films of the organic molecular semiconductors C(60) and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φ(step) = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C(60) islands. Potential steps in binary C(60)-TiOPc films, nanophase-separated into crystalline C(60) and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C(60)-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric potential across a material interface. The absence of potential asymmetry across this lateral interface sets the upper bound for the C(60)-TiOPc interface dipole moment as 0.012 e nm.

Research paper thumbnail of Polymorphism in Self-Assembled Structures of 9-Anthracene Carboxylic Acid on Ag (111)

Surface self-assembly process of 9-anthracene carboxylic acid (AnCA) on Ag(111) was investigated ... more Surface self-assembly process of 9-anthracene carboxylic acid (AnCA) on Ag(111) was investigated using STM. Depending on the molecular surface density, four spontaneously formed and one annealed AnCA ordered phases were observed, namely a straight belt phase, a zigzag double-belt phase, two simpler dimer phases, and a kagome phase. The two high-density belt phases possess large unit cells on the scale length of 10 nm, which are seldom observed in molecular self-assembled structures. This structural diversity stems from a complicated competition of different interactions of AnCA molecules on metal surface, including intermolecular and molecular-substrate interactions, as well as the steric demand from high molecular surface density.

Research paper thumbnail of Formation of metastable, heterolytic H-pairs on the RuO2(110) surface

ABSTRACT The interaction of molecular hydrogen (H2) with the stoichiometric RuO2(110) surface was... more ABSTRACT The interaction of molecular hydrogen (H2) with the stoichiometric RuO2(110) surface was studied at various temperatures and coverages by high-resolution scanning tunneling microscopy (STM) and temperature-programmed reaction (TPR). Metastable, heterolytic H-pairs of dissociated H2 consisting of one H adsorbed on a surface Ru atom and the other H at an adjacent bridging O atom are identified. This H-pair configuration is consistent with accompanying density functional theory (DFT) calculations and spectroscopic data. We propose that such metastable, heterolytic H-pairs are a typical feature of late transition metal oxide surfaces. Furthermore, the adsorption temperature was found to be decisive for the configuration of the adsorbed hydrogen species and the sticking of H2.

Research paper thumbnail of TiOPc Molecular Dislocation Networks as Nanotemplates for C60 Cluster Arrays

By controlled deposition, TiOPc, a molecular semiconductor with anisotropic interactions can gene... more By controlled deposition, TiOPc, a molecular semiconductor with anisotropic interactions can generate a molecular film with a characteristic pattern repeat size of 15 nm. This structure then served as a nanotemplate for a superlattice of C(60) clusters with characteristic diameters of 7 nm. As a result, C(60) deposition on the TiOPc film template forms a pattern of nanophase-separated C(60) and TiOPc domains with a characteristic domain size of 7 nm. This feature size is matched to the exciton diffusion length in photovoltaic materials composed of small organic molecules. A dislocation network in a molecular film provides a promising method for generating pattern features on the several-nanometer length scale, bridging the practical limits of &amp;amp;amp;amp;amp;amp;quot;bottom up&amp;amp;amp;amp;amp;amp;quot; and &amp;amp;amp;amp;amp;amp;quot;top down&amp;amp;amp;amp;amp;amp;quot; strategies.