Ursula Mazur - Academia.edu (original) (raw)
Papers by Ursula Mazur
Chemischer Informationsdienst, Aug 11, 1981
ChemInform Abstract Die Titeluntersuchung zeigt, dass Co(III)-und Ni(II)-Glycin-Komplexe sich bev... more ChemInform Abstract Die Titeluntersuchung zeigt, dass Co(III)-und Ni(II)-Glycin-Komplexe sich bevorzugt im pH-Bereich 4-9 in verdünnten Lösungen an Al2O3 adsorbieren. Bevorzugter Oberflächenkomplex ist in allen Fällen einer, in dem Glycin als zweizähnigesGlycinat agiert. Die gegenwärtigen Daten sind jedoch ungenügend zur Bestimmung, ob es sich um Di-oder Monoglycinatkomplexe handelt.
Phthalocyanines are of interest to the materials research community because of their promising ph... more Phthalocyanines are of interest to the materials research community because of their promising photovoltaic and semiconducting properties. They have potential to form functional nanostructures that may allow energy harvesting from light. Previous work has shown acidic solvents induce TSPc to form aggregates. This project aims to investigate phthalocyanine tetrasulfonic acid (TSPc) aggregation in the presence of formic acid and acetic acid. We found the size and shape of the aggregates formed varies with the type of acid used. The use of ultraviolet-visible spectroscopy (UV-vis) was employed to give insight into aggregate formation in both solution and upon deposition. Atomic force microscopy was run to give more information about the morphology of the aggregate structures formed.
Bulletin of the American Physical Society, Mar 19, 2010
Tetra(sulfonatophenyl)porphyrin, TSPP, forms nanorods on Au(111) or HOPG. These nanorods were exa... more Tetra(sulfonatophenyl)porphyrin, TSPP, forms nanorods on Au(111) or HOPG. These nanorods were examined by XPS, STM, and STS. N1s XPS data demonstrate that the central four nitrogens of the porphyrin are completely protonated, generating a positive central core and a negative molecular periphery that plays a role in the rod formation process. STM studies in UHV also suggest the rods may be collapsed tubes. STS results show that while single rods are somewhat rectifying with primarily n-type conduction, bundles of rods show very strong rectification. This difference in bundle versus single rod conduction may be an artifact of the measurement method, with the single rod I-V curves containing both conduction and tunneling currents while thicker bundle data is dominated by direct conduction. Comparison of the STS of the nanorods with that of NiTPP molecules suggests that the lowest unoccupied molecular orbitals are those that participate in the conduction process. It is also interesting to note that the STS of the nanorods on gold is exactly the same as on HOPG, indicating that these rods have an effective Fermi energy that equilibrates with the substrate.
Journal of Applied Physics, Sep 15, 2005
We report on the growth-induced properties of 30 Å thick CrPt3(111) deposits on surfaces of WSe2(... more We report on the growth-induced properties of 30 Å thick CrPt3(111) deposits on surfaces of WSe2(0001) with a van der Waals interaction. Assemblies of (111)-oriented fcc grains are obtained with average lateral sizes ranging from 5 to 15 nm for growth temperatures increasing from 50 °C to 550 °C. The grains develop a partial L12-type chemical long-range order starting at a deposition temperature as low as 50 °C, much lower than reported for films grown on other suitable substrate surfaces. An ordering parameter of about 0.3 was found over a wide temperature range between 200 °C and 550 °C, for a deposition rate of 0.1 Å/s. Lowering the deposition rate by a factor of 5 results in a higher ordering parameter of about 0.4. Above 400 °C, ordering is accompanied by a pronounced segregation of Se atoms toward the free surface, forming a (4×4) superstructure. All samples exhibit superparamagnetic behavior with blocking temperatures that scale with the activation volume of the ferrimagnetic grains. This work shows the importance of substrate choice in reducing the ordering temperature of thin CrPt3 films, an effect that might also occur in other alloy systems.
Physical Chemistry Chemical Physics, 2019
S1. Preliminary calculations on the orientation of coronene molecules on Au(111) and HOPG. Previo... more S1. Preliminary calculations on the orientation of coronene molecules on Au(111) and HOPG. Previous STM studies of coronene on Au(111) 1 and HOPG 2 have determined that coronene adsorbs with a head-to-head coronene hydrogens configuration with its nearest neighbor on Au(111), while a staggered configuration on HOPG. To verify this, we performed preliminary DFT calculations using B88-vdW DFT functionals (see computational methods section in the manuscript for other calculation details) on head-to-head and staggered configurations of coronene on Au(111) and HOPG (Figure S3). In these calculations just one coronene molecule per unitcell is considered with 4x4x1 slab of Au(111) and 5x5x1 slab of HOPG. The gold slab has 3 layers of gold and HOPG has 2 layers of carbon. Isolated coronene molecule and substrates (with bottom layer frozen) were optimized prior to creating an interface. Upon complete optimization to SCF convergence of ~0.001 eV, we found that head-to-head orientation is energetically favorable by 0.34 eV on Au(111) and staggered orientation is favored on HOPG by 0.28 eV. The respective 4x4 supercells of optimized coronene/Au(111) (Figure S3-B) and coronene/HOPG (Figure S3-C) were used to develop the models (Figure 2, 3 in manuscript) for coverage dependent desorption studies. Fig. S1 Geometries of a) coronene/Au(111) in staggered configuration, b) coronene/Au(111) in head-tohead configuration, c) coronene/HOPG in staggered configuration, d) coronene/HOPG in head-to-head configuration. Colors: Au(111)-Orange, HOPG-cyan, grey-coronene and white line represents hexagonal lattice.
Chemical Physics Letters, 1981
ABSTRACT
 films grown on a low energy surface at room temperature](https://attachments.academia-assets.com/112509670/thumbnails/1.jpg)
](Journal of Applied Physics, May 13, 2002
Epitaxial ͑111͒-oriented CoPt 3 films were deposited on WSe 2 (0001) substrates at room temperatu... more Epitaxial ͑111͒-oriented CoPt 3 films were deposited on WSe 2 (0001) substrates at room temperature using molecular beam epitaxy. We observed strong growth induced uniaxial perpendicular magnetic anisotropy which has a maximum of 3.2ϫ10 6 erg/cm 3 and coercivity of about 200 Oe for films with thicknesses Ͻ6 nm. At a thickness larger than 6 nm the easy magnetization axis progressively rotates into the plane of the film as the film thickness is increased. The magnetic domain structure in films with perpendicular magnetic anisotropy was investigated by photoemission electron microscopy revealing a characteristic thickness dependence near the reorientation transition.
Journal of Physical Chemistry C, Dec 3, 2008
Scanning tunneling microscopy (STM) is employed to demonstrate that the presence of the vanadyl i... more Scanning tunneling microscopy (STM) is employed to demonstrate that the presence of the vanadyl ion (VO 2+) in the 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine complex (VOPcPhO) has a pronounced effect on the organization of the complex at the highly oriented pyrolytic graphite (HOPG)-n-alkyl benzene (n) 6, 7, 8, and 10) interface. VOPcPhO forms at least three different stable architectures, as compared to only one type of structure resulting from the adsorption of the metal-free tetraphenoxyphthalocyanine analog (H 2 PcPhO) on HOPG under conditions similar to VOPcPhO. All three observed VOPcPhO monolayer structures have high packing density and are commensurate with the underlying graphite lattice, whereas H 2 PcPhO forms a lower packing density incommensurate adlayer. The VOPcPhO structures display a small rectangular unit cell (1 molecule/cell) with lattice vector lengths of 1.28 and 1.48 nm, a larger rectangular unit cell (6 molecules/ cell) with lattice vector lengths of 4.43 nm 2.98 nm, and an oblique unit cell (2 molecules/cell and R) 84.7°) with lattice vector lengths of 1.48 and 2.99 nm. As a second layer begins to form, it preferentially adds to the large unit cell, keeping those lattice parameters, but with only four molecules per cell. The H 2 PcPhO adlayer achieves an oblique unit cell (R) 74°and 1 molecule/cell) with 1.58 and 1.68 nm length lattice vectors. On the basis of combined experimental results and theoretical calculations, we propose models for the observed molecular organizations.
Journal of Physical Chemistry C, Apr 15, 2015
ABSTRACT Temperature dependent desorption rates and desorption energies are determined from a mon... more ABSTRACT Temperature dependent desorption rates and desorption energies are determined from a monolayer assembly at the solution solid (SS) interface. Scanning tunneling microscopy (STM) was used to measure molecular scale temperature dependent desorption of cobalt(II) octaethylporphyrin (CoOEP) at the phenyloctane solution - highly ordered pyrolytic graphite (HOPG) interface. At lower temperatures, monolayer formation of metal(II) octaethylporphyrin (MOEP) on HOPG from solution was found to be completely controlled by kinetics and the adlayer formed was stable up to 70 °C. Significant desorption of CoOEP from the HOPG surface was observed above 80 °C on a time scale of hours. CoOEP desorbs from HOPG into phenyloctane at a rate of 0.0055 ± 0.0007 min-1 at 90 °C, 0.013 ± 0.001 min-1 at 100 °C, and 0.033 ± 0.003 min-1 at 110 °C. From these temperature and time dependent measurements, assuming an Arrhenius rate law, the activation energy of molecular desorption at the SS interface was determined using studies solely based on STM. The desorption energy of CoOEP from 2 HOPG into phenyloctane is determined to be 1.05 × 102 ± 0.03 × 102 kJ/mol. NiOEP desorption occurs at a slower rate and is homogeneous across HOPG terraces, unlike the inhomogeneous desorption observed on Au(111). A previous study performed on Au(111), reported that the rate of desorption of CoOEP is 0.004 min-1 at 135 °C. The calculated desorption rate on HOPG in this work is 0.22 min-1, making the rate of desorption of CoOEP from HOPG three orders of magnitude greater than from Au(111). On the other hand, for solution concentrations of the order of 120 μM, a dense monolayer is formed within seconds. For this fast adsorption process, where a full monolayer coverage occurs, the surface coverage of MOEP on both surfaces was determined by the relative concentration of each species in the phenyloctane solution. The rates of adsorption (for concentrations near 100 μM) are found to be within 20% of each other. The surface structures of both the NiOEP and CoOEP on HOPG and Au(111) are identical and can be described by A = 1.30 ± 0.02 nm, B = 1.40 ± 0.02 nm, and α = 57° ± 1° with an area of 1.82 ± 0.04 nm2/molecule.
Meeting abstracts, May 1, 2020
In biology, key molecules for life processes rely on cooperation between metal containing porphyr... more In biology, key molecules for life processes rely on cooperation between metal containing porphyrins to function effectively. In technology, (catalysis and sensing) cooperative interactions between the support and metal porphyrins play a key role in their efficiency. In this study, we present a molecular level study of cooperative surface reactions between nitrogen based ligands and Co(II) porphyrin adsorbates as a function of their spatial proximity at the solution/solid interface. Central metal ion reactivity as a function of surface coverage is measured at varying temperatures and ligand concentrations. Reaction rates and thermodynamic parameters are extracted from measurements made on single molecules.
MRS Proceedings, 1995
Direct ion beam sputter deposition of alN is studied using both pure nitrogen and a 75% N2/25% H2... more Direct ion beam sputter deposition of alN is studied using both pure nitrogen and a 75% N2/25% H2 mixture as the feed gas for the ion gun. the chemical characteristics of these films are probed using infrared spectroscopy and chemical etching. the presence of al-N2 species is associated with reactive and highly defective films. the presence of NHX species increases by several orders of magnitude the rate at which alN film are etched by base. However, stoichiometric and low defect alN films prepared by depositing alN:H onto substrates heated to 200 °C have reactivities similar to the best alN films produced in the absence of hydrogen.
Springer eBooks, 1982
The majority of tunneling studies to date have focused on the adsorption of organic alcohols, aci... more The majority of tunneling studies to date have focused on the adsorption of organic alcohols, acids, and amines on alumina and, less frequently, magnesia. This preoccupation with reactive organics is probably due to (a) the existence of a large body of ir, ESR, and chemical information concerning the adsorption of these materials on bulk oxides; (b) the desire of the experimenter to control the cleanliness of the surface by using vapor phase adsorption within the confines of the vacuum system whenever possible; (c) the high C-C and C-H bond energies of most of the systems studied preclude “backbone” reactions of the adsorbates and limit chemical modifications on adsorption to changes associated with the reactive group (OH, CO2H, NH2). This last factor, coupled with the rather large heats of formation of metal-oxygen and metal-amine bonds, limits the breadth of chemical and physical changes which can occur in adsorption and with deposition of the top metal electrode. For example, one does not usually observe a large (≥1%) change in C-C or C-H motions of adsorbed organics once the image-dipole correction has been made. The chemical specificity of reaction evidenced by these organic systems tends, therefore, to provide a somewhat simplistic picture of the processes which occur in the production of doped tunnel junction.
The Journal of Physical Chemistry, Oct 1, 1979
complete reduction. However, the observed experimental reaction was complete sulfidation of the a... more complete reduction. However, the observed experimental reaction was complete sulfidation of the active sites, the opposite of the reduction reaction of step iv. Furthermore, assuming "essentially all" means at least 90%, the H2/H2S ratio for complete sulfidation could be at least 4 X lo5 and the ratio of H2/H2S for complete reduction of the active surface could well be 106-107. Nonetheless these extrapolated values connected with experimental results indicate that the equilibrium values are lower bounds as one would expect. Also the thermodynamic properties for the bulk materials are likely somewhat different from those of the surface, particularly a catalytically active surface. Comparing the two cycles i + ii + iii and i + iv on the basis of the calculations presented here leads to the conclusion that the combination oxidation/reduction process is wasteful because of the necessity of carrying the unsulfidized nickel completely through the oxidation and subsequent reduction steps as discussed above. One would expect to lose the characteristic high surface area of the nickel in the process. The direct reduction of the Ni3S2 by H2 is seen to be thermodynamically possible. Such a reduction would not be expected to affect the unpoisoned Raney nickel. One would also have the option of pre-or post-reduction separation of the unaffected nickel from the poisoned/reduced material. It should be emphasized that the modeling summarized in this work is for batch processes only, with some gains to be made in driving the reduction reaction by removing the H2S as it is generated. The possibility of checking these ideas in a set of bench scale experiments seems attractive.
Tetrahedron, 1972
ABSTRACT
Journal of Materials Research, Jun 1, 1994
This work describes the formation of stoichiometric AIN films by single ion-beam sputtering of Al... more This work describes the formation of stoichiometric AIN films by single ion-beam sputtering of Al, using an ionized N2 (75%) + H2 (25%) mixture, onto substrates heated to 200 °C or above. The role of substrate temperature on film composition and properties is followed in the substrate temperature range between ambient and 250 °C. Infrared spectra of freshly prepared and 2 month old (aged in air) films demonstrate that substrate heating significantly affects the chemical nature of the resulting films. SEM and STM data, combined with IR and UV-visible spectral results, indicate that films formed at a substrate temperature of 5^200 CC are very smooth and highly resistant to attack by atmospheric gases. X-ray diffraction data show no diffraction peaks, indicating that the film is either amorphous or crystalline on a scale of less than 4 nm.
The Journal of Physical Chemistry, Jun 1, 1994
Tunneling spectra obtained from [Al-AIO,-pA-Pb] (where pA = anthracene, tetracene, pentacene, per... more Tunneling spectra obtained from [Al-AIO,-pA-Pb] (where pA = anthracene, tetracene, pentacene, perylene, and coronene) tunnel diodes are reported in the bias region extending from-2.3 to +2.3 V. Large intense features (&/a-0.1) are seen in the spectrum when P b is biased positively but are absent in the other bias. These features are not scan direction dependent and do not correlate with inelastic electronic transitions. The positions of the band maxima vary linearly with the Huckel LUMO energy and can be fit using a value of j3 = 67 kcal/mol. These novel bands are attributed to elastic tunneling mediated by the unoccupied T* orbitals of the polyacenes. While this type of tunneling has been previously postulated for the case of copper phthalocyanine, we believe that the present study is the first unequivocal demonstration of resonance-like elastic tunneling via unoccupied molecular orbitals. The implications of these results to molecular imaging in scanning tunneling microscopy are also discussed.
The Journal of Physical Chemistry, Nov 1, 1980
complete reduction. However, the observed experimental reaction was complete sulfidation of the a... more complete reduction. However, the observed experimental reaction was complete sulfidation of the active sites, the opposite of the reduction reaction of step iv. Furthermore, assuming "essentially all" means at least 90%, the H2/H2S ratio for complete sulfidation could be at least 4 X lo5 and the ratio of H2/H2S for complete reduction of the active surface could well be 106-107. Nonetheless these extrapolated values connected with experimental results indicate that the equilibrium values are lower bounds as one would expect. Also the thermodynamic properties for the bulk materials are likely somewhat different from those of the surface, particularly a catalytically active surface. Comparing the two cycles i + ii + iii and i + iv on the basis of the calculations presented here leads to the conclusion that the combination oxidation/reduction process is wasteful because of the necessity of carrying the unsulfidized nickel completely through the oxidation and subsequent reduction steps as discussed above. One would expect to lose the characteristic high surface area of the nickel in the process. The direct reduction of the Ni3S2 by H2 is seen to be thermodynamically possible. Such a reduction would not be expected to affect the unpoisoned Raney nickel. One would also have the option of pre-or post-reduction separation of the unaffected nickel from the poisoned/reduced material. It should be emphasized that the modeling summarized in this work is for batch processes only, with some gains to be made in driving the reduction reaction by removing the H2S as it is generated. The possibility of checking these ideas in a set of bench scale experiments seems attractive.
Chemistry of Materials, Feb 5, 2008
... Michael Halter † , Yi Liao † , Ryan M. Plocinik ‡ , David C. Coffey § , Sanchali Bhattacharje... more ... Michael Halter † , Yi Liao † , Ryan M. Plocinik ‡ , David C. Coffey § , Sanchali Bhattacharjee † , Ursula Mazur , Garth J. Simpson ‡ , Bruce H. Robinson † and Sarah L. Keller* †. Departments of Chemistry and Physics, University ...
Journal of Physical Chemistry C, Jul 29, 2020
Cooperative interactions play a critical role in the stability and reactivity of biological syste... more Cooperative interactions play a critical role in the stability and reactivity of biological systems and are an increasingly important consideration in the synthesis of functional materials, but quantitative single-molecule measurements of this phenomenon are rare. Many of these cooperative interactions necessarily occur on surfaces, making the study of cooperative effects at interfaces of particular importance. Here, we report a quantitative experimental and theoretical study of the cooperative binding of 1phenylimidazole (PhIm) to cobalt(II) octaethylporphyrin (CoOEP) on highly oriented pyrolytic graphite (HOPG) at the solution−solid interface. Scanning tunneling microscopy (STM) confirmed and monitored the binding of PhIm to HOPGsupported CoOEP with single-molecule resolution. Nearest-neighbor analysis of these STM images revealed positive cooperative binding behavior. Periodic plane-wave density functional theory (DFT) calculations of PhIm/CoOEP/HOPG and cobalt(II) porphine (CoP)/HOPG systems support the experimental observations of positive cooperativity. DFT calculations revealed that the binding energy of PhIm to Co-porphyrin increases as PhIm binds to more neighboring molecules. Calculations also suggest that the presence of HOPG is crucial to observe positive cooperativity in this system.
Journal of Molecular Spectroscopy, Apr 1, 1977
Rotational spectra of six isotopic species of ethylene ozonide-Da have been assigned. The 0,-O, d... more Rotational spectra of six isotopic species of ethylene ozonide-Da have been assigned. The 0,-O, distance was calculated six independent ways using Kraitchman's substitution equations, and values between 1.458 and 1.462 d were obtained. This is in contrast to a similar analysis for normal ethylene ozonide where the 0,-O, distance varied between 1.455 and 1.502 A. This difference is associated with the greater value of la-I. for the Dg species which markedly decreases the effect of axes rotations upon isotopic substitution. The structural parameters derived from the Da data set are in excellent agreement with those extracted from the Hp analyses after minimization of axes rotation effects.
Chemischer Informationsdienst, Aug 11, 1981
ChemInform Abstract Die Titeluntersuchung zeigt, dass Co(III)-und Ni(II)-Glycin-Komplexe sich bev... more ChemInform Abstract Die Titeluntersuchung zeigt, dass Co(III)-und Ni(II)-Glycin-Komplexe sich bevorzugt im pH-Bereich 4-9 in verdünnten Lösungen an Al2O3 adsorbieren. Bevorzugter Oberflächenkomplex ist in allen Fällen einer, in dem Glycin als zweizähnigesGlycinat agiert. Die gegenwärtigen Daten sind jedoch ungenügend zur Bestimmung, ob es sich um Di-oder Monoglycinatkomplexe handelt.
Phthalocyanines are of interest to the materials research community because of their promising ph... more Phthalocyanines are of interest to the materials research community because of their promising photovoltaic and semiconducting properties. They have potential to form functional nanostructures that may allow energy harvesting from light. Previous work has shown acidic solvents induce TSPc to form aggregates. This project aims to investigate phthalocyanine tetrasulfonic acid (TSPc) aggregation in the presence of formic acid and acetic acid. We found the size and shape of the aggregates formed varies with the type of acid used. The use of ultraviolet-visible spectroscopy (UV-vis) was employed to give insight into aggregate formation in both solution and upon deposition. Atomic force microscopy was run to give more information about the morphology of the aggregate structures formed.
Bulletin of the American Physical Society, Mar 19, 2010
Tetra(sulfonatophenyl)porphyrin, TSPP, forms nanorods on Au(111) or HOPG. These nanorods were exa... more Tetra(sulfonatophenyl)porphyrin, TSPP, forms nanorods on Au(111) or HOPG. These nanorods were examined by XPS, STM, and STS. N1s XPS data demonstrate that the central four nitrogens of the porphyrin are completely protonated, generating a positive central core and a negative molecular periphery that plays a role in the rod formation process. STM studies in UHV also suggest the rods may be collapsed tubes. STS results show that while single rods are somewhat rectifying with primarily n-type conduction, bundles of rods show very strong rectification. This difference in bundle versus single rod conduction may be an artifact of the measurement method, with the single rod I-V curves containing both conduction and tunneling currents while thicker bundle data is dominated by direct conduction. Comparison of the STS of the nanorods with that of NiTPP molecules suggests that the lowest unoccupied molecular orbitals are those that participate in the conduction process. It is also interesting to note that the STS of the nanorods on gold is exactly the same as on HOPG, indicating that these rods have an effective Fermi energy that equilibrates with the substrate.
Journal of Applied Physics, Sep 15, 2005
We report on the growth-induced properties of 30 Å thick CrPt3(111) deposits on surfaces of WSe2(... more We report on the growth-induced properties of 30 Å thick CrPt3(111) deposits on surfaces of WSe2(0001) with a van der Waals interaction. Assemblies of (111)-oriented fcc grains are obtained with average lateral sizes ranging from 5 to 15 nm for growth temperatures increasing from 50 °C to 550 °C. The grains develop a partial L12-type chemical long-range order starting at a deposition temperature as low as 50 °C, much lower than reported for films grown on other suitable substrate surfaces. An ordering parameter of about 0.3 was found over a wide temperature range between 200 °C and 550 °C, for a deposition rate of 0.1 Å/s. Lowering the deposition rate by a factor of 5 results in a higher ordering parameter of about 0.4. Above 400 °C, ordering is accompanied by a pronounced segregation of Se atoms toward the free surface, forming a (4×4) superstructure. All samples exhibit superparamagnetic behavior with blocking temperatures that scale with the activation volume of the ferrimagnetic grains. This work shows the importance of substrate choice in reducing the ordering temperature of thin CrPt3 films, an effect that might also occur in other alloy systems.
Physical Chemistry Chemical Physics, 2019
S1. Preliminary calculations on the orientation of coronene molecules on Au(111) and HOPG. Previo... more S1. Preliminary calculations on the orientation of coronene molecules on Au(111) and HOPG. Previous STM studies of coronene on Au(111) 1 and HOPG 2 have determined that coronene adsorbs with a head-to-head coronene hydrogens configuration with its nearest neighbor on Au(111), while a staggered configuration on HOPG. To verify this, we performed preliminary DFT calculations using B88-vdW DFT functionals (see computational methods section in the manuscript for other calculation details) on head-to-head and staggered configurations of coronene on Au(111) and HOPG (Figure S3). In these calculations just one coronene molecule per unitcell is considered with 4x4x1 slab of Au(111) and 5x5x1 slab of HOPG. The gold slab has 3 layers of gold and HOPG has 2 layers of carbon. Isolated coronene molecule and substrates (with bottom layer frozen) were optimized prior to creating an interface. Upon complete optimization to SCF convergence of ~0.001 eV, we found that head-to-head orientation is energetically favorable by 0.34 eV on Au(111) and staggered orientation is favored on HOPG by 0.28 eV. The respective 4x4 supercells of optimized coronene/Au(111) (Figure S3-B) and coronene/HOPG (Figure S3-C) were used to develop the models (Figure 2, 3 in manuscript) for coverage dependent desorption studies. Fig. S1 Geometries of a) coronene/Au(111) in staggered configuration, b) coronene/Au(111) in head-tohead configuration, c) coronene/HOPG in staggered configuration, d) coronene/HOPG in head-to-head configuration. Colors: Au(111)-Orange, HOPG-cyan, grey-coronene and white line represents hexagonal lattice.
Chemical Physics Letters, 1981
ABSTRACT
Journal of Applied Physics, May 13, 2002
Epitaxial ͑111͒-oriented CoPt 3 films were deposited on WSe 2 (0001) substrates at room temperatu... more Epitaxial ͑111͒-oriented CoPt 3 films were deposited on WSe 2 (0001) substrates at room temperature using molecular beam epitaxy. We observed strong growth induced uniaxial perpendicular magnetic anisotropy which has a maximum of 3.2ϫ10 6 erg/cm 3 and coercivity of about 200 Oe for films with thicknesses Ͻ6 nm. At a thickness larger than 6 nm the easy magnetization axis progressively rotates into the plane of the film as the film thickness is increased. The magnetic domain structure in films with perpendicular magnetic anisotropy was investigated by photoemission electron microscopy revealing a characteristic thickness dependence near the reorientation transition.
Journal of Physical Chemistry C, Dec 3, 2008
Scanning tunneling microscopy (STM) is employed to demonstrate that the presence of the vanadyl i... more Scanning tunneling microscopy (STM) is employed to demonstrate that the presence of the vanadyl ion (VO 2+) in the 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine complex (VOPcPhO) has a pronounced effect on the organization of the complex at the highly oriented pyrolytic graphite (HOPG)-n-alkyl benzene (n) 6, 7, 8, and 10) interface. VOPcPhO forms at least three different stable architectures, as compared to only one type of structure resulting from the adsorption of the metal-free tetraphenoxyphthalocyanine analog (H 2 PcPhO) on HOPG under conditions similar to VOPcPhO. All three observed VOPcPhO monolayer structures have high packing density and are commensurate with the underlying graphite lattice, whereas H 2 PcPhO forms a lower packing density incommensurate adlayer. The VOPcPhO structures display a small rectangular unit cell (1 molecule/cell) with lattice vector lengths of 1.28 and 1.48 nm, a larger rectangular unit cell (6 molecules/ cell) with lattice vector lengths of 4.43 nm 2.98 nm, and an oblique unit cell (2 molecules/cell and R) 84.7°) with lattice vector lengths of 1.48 and 2.99 nm. As a second layer begins to form, it preferentially adds to the large unit cell, keeping those lattice parameters, but with only four molecules per cell. The H 2 PcPhO adlayer achieves an oblique unit cell (R) 74°and 1 molecule/cell) with 1.58 and 1.68 nm length lattice vectors. On the basis of combined experimental results and theoretical calculations, we propose models for the observed molecular organizations.
Journal of Physical Chemistry C, Apr 15, 2015
ABSTRACT Temperature dependent desorption rates and desorption energies are determined from a mon... more ABSTRACT Temperature dependent desorption rates and desorption energies are determined from a monolayer assembly at the solution solid (SS) interface. Scanning tunneling microscopy (STM) was used to measure molecular scale temperature dependent desorption of cobalt(II) octaethylporphyrin (CoOEP) at the phenyloctane solution - highly ordered pyrolytic graphite (HOPG) interface. At lower temperatures, monolayer formation of metal(II) octaethylporphyrin (MOEP) on HOPG from solution was found to be completely controlled by kinetics and the adlayer formed was stable up to 70 °C. Significant desorption of CoOEP from the HOPG surface was observed above 80 °C on a time scale of hours. CoOEP desorbs from HOPG into phenyloctane at a rate of 0.0055 ± 0.0007 min-1 at 90 °C, 0.013 ± 0.001 min-1 at 100 °C, and 0.033 ± 0.003 min-1 at 110 °C. From these temperature and time dependent measurements, assuming an Arrhenius rate law, the activation energy of molecular desorption at the SS interface was determined using studies solely based on STM. The desorption energy of CoOEP from 2 HOPG into phenyloctane is determined to be 1.05 × 102 ± 0.03 × 102 kJ/mol. NiOEP desorption occurs at a slower rate and is homogeneous across HOPG terraces, unlike the inhomogeneous desorption observed on Au(111). A previous study performed on Au(111), reported that the rate of desorption of CoOEP is 0.004 min-1 at 135 °C. The calculated desorption rate on HOPG in this work is 0.22 min-1, making the rate of desorption of CoOEP from HOPG three orders of magnitude greater than from Au(111). On the other hand, for solution concentrations of the order of 120 μM, a dense monolayer is formed within seconds. For this fast adsorption process, where a full monolayer coverage occurs, the surface coverage of MOEP on both surfaces was determined by the relative concentration of each species in the phenyloctane solution. The rates of adsorption (for concentrations near 100 μM) are found to be within 20% of each other. The surface structures of both the NiOEP and CoOEP on HOPG and Au(111) are identical and can be described by A = 1.30 ± 0.02 nm, B = 1.40 ± 0.02 nm, and α = 57° ± 1° with an area of 1.82 ± 0.04 nm2/molecule.
Meeting abstracts, May 1, 2020
In biology, key molecules for life processes rely on cooperation between metal containing porphyr... more In biology, key molecules for life processes rely on cooperation between metal containing porphyrins to function effectively. In technology, (catalysis and sensing) cooperative interactions between the support and metal porphyrins play a key role in their efficiency. In this study, we present a molecular level study of cooperative surface reactions between nitrogen based ligands and Co(II) porphyrin adsorbates as a function of their spatial proximity at the solution/solid interface. Central metal ion reactivity as a function of surface coverage is measured at varying temperatures and ligand concentrations. Reaction rates and thermodynamic parameters are extracted from measurements made on single molecules.
MRS Proceedings, 1995
Direct ion beam sputter deposition of alN is studied using both pure nitrogen and a 75% N2/25% H2... more Direct ion beam sputter deposition of alN is studied using both pure nitrogen and a 75% N2/25% H2 mixture as the feed gas for the ion gun. the chemical characteristics of these films are probed using infrared spectroscopy and chemical etching. the presence of al-N2 species is associated with reactive and highly defective films. the presence of NHX species increases by several orders of magnitude the rate at which alN film are etched by base. However, stoichiometric and low defect alN films prepared by depositing alN:H onto substrates heated to 200 °C have reactivities similar to the best alN films produced in the absence of hydrogen.
Springer eBooks, 1982
The majority of tunneling studies to date have focused on the adsorption of organic alcohols, aci... more The majority of tunneling studies to date have focused on the adsorption of organic alcohols, acids, and amines on alumina and, less frequently, magnesia. This preoccupation with reactive organics is probably due to (a) the existence of a large body of ir, ESR, and chemical information concerning the adsorption of these materials on bulk oxides; (b) the desire of the experimenter to control the cleanliness of the surface by using vapor phase adsorption within the confines of the vacuum system whenever possible; (c) the high C-C and C-H bond energies of most of the systems studied preclude “backbone” reactions of the adsorbates and limit chemical modifications on adsorption to changes associated with the reactive group (OH, CO2H, NH2). This last factor, coupled with the rather large heats of formation of metal-oxygen and metal-amine bonds, limits the breadth of chemical and physical changes which can occur in adsorption and with deposition of the top metal electrode. For example, one does not usually observe a large (≥1%) change in C-C or C-H motions of adsorbed organics once the image-dipole correction has been made. The chemical specificity of reaction evidenced by these organic systems tends, therefore, to provide a somewhat simplistic picture of the processes which occur in the production of doped tunnel junction.
The Journal of Physical Chemistry, Oct 1, 1979
complete reduction. However, the observed experimental reaction was complete sulfidation of the a... more complete reduction. However, the observed experimental reaction was complete sulfidation of the active sites, the opposite of the reduction reaction of step iv. Furthermore, assuming "essentially all" means at least 90%, the H2/H2S ratio for complete sulfidation could be at least 4 X lo5 and the ratio of H2/H2S for complete reduction of the active surface could well be 106-107. Nonetheless these extrapolated values connected with experimental results indicate that the equilibrium values are lower bounds as one would expect. Also the thermodynamic properties for the bulk materials are likely somewhat different from those of the surface, particularly a catalytically active surface. Comparing the two cycles i + ii + iii and i + iv on the basis of the calculations presented here leads to the conclusion that the combination oxidation/reduction process is wasteful because of the necessity of carrying the unsulfidized nickel completely through the oxidation and subsequent reduction steps as discussed above. One would expect to lose the characteristic high surface area of the nickel in the process. The direct reduction of the Ni3S2 by H2 is seen to be thermodynamically possible. Such a reduction would not be expected to affect the unpoisoned Raney nickel. One would also have the option of pre-or post-reduction separation of the unaffected nickel from the poisoned/reduced material. It should be emphasized that the modeling summarized in this work is for batch processes only, with some gains to be made in driving the reduction reaction by removing the H2S as it is generated. The possibility of checking these ideas in a set of bench scale experiments seems attractive.
Tetrahedron, 1972
ABSTRACT
Journal of Materials Research, Jun 1, 1994
This work describes the formation of stoichiometric AIN films by single ion-beam sputtering of Al... more This work describes the formation of stoichiometric AIN films by single ion-beam sputtering of Al, using an ionized N2 (75%) + H2 (25%) mixture, onto substrates heated to 200 °C or above. The role of substrate temperature on film composition and properties is followed in the substrate temperature range between ambient and 250 °C. Infrared spectra of freshly prepared and 2 month old (aged in air) films demonstrate that substrate heating significantly affects the chemical nature of the resulting films. SEM and STM data, combined with IR and UV-visible spectral results, indicate that films formed at a substrate temperature of 5^200 CC are very smooth and highly resistant to attack by atmospheric gases. X-ray diffraction data show no diffraction peaks, indicating that the film is either amorphous or crystalline on a scale of less than 4 nm.
The Journal of Physical Chemistry, Jun 1, 1994
Tunneling spectra obtained from [Al-AIO,-pA-Pb] (where pA = anthracene, tetracene, pentacene, per... more Tunneling spectra obtained from [Al-AIO,-pA-Pb] (where pA = anthracene, tetracene, pentacene, perylene, and coronene) tunnel diodes are reported in the bias region extending from-2.3 to +2.3 V. Large intense features (&/a-0.1) are seen in the spectrum when P b is biased positively but are absent in the other bias. These features are not scan direction dependent and do not correlate with inelastic electronic transitions. The positions of the band maxima vary linearly with the Huckel LUMO energy and can be fit using a value of j3 = 67 kcal/mol. These novel bands are attributed to elastic tunneling mediated by the unoccupied T* orbitals of the polyacenes. While this type of tunneling has been previously postulated for the case of copper phthalocyanine, we believe that the present study is the first unequivocal demonstration of resonance-like elastic tunneling via unoccupied molecular orbitals. The implications of these results to molecular imaging in scanning tunneling microscopy are also discussed.
The Journal of Physical Chemistry, Nov 1, 1980
complete reduction. However, the observed experimental reaction was complete sulfidation of the a... more complete reduction. However, the observed experimental reaction was complete sulfidation of the active sites, the opposite of the reduction reaction of step iv. Furthermore, assuming "essentially all" means at least 90%, the H2/H2S ratio for complete sulfidation could be at least 4 X lo5 and the ratio of H2/H2S for complete reduction of the active surface could well be 106-107. Nonetheless these extrapolated values connected with experimental results indicate that the equilibrium values are lower bounds as one would expect. Also the thermodynamic properties for the bulk materials are likely somewhat different from those of the surface, particularly a catalytically active surface. Comparing the two cycles i + ii + iii and i + iv on the basis of the calculations presented here leads to the conclusion that the combination oxidation/reduction process is wasteful because of the necessity of carrying the unsulfidized nickel completely through the oxidation and subsequent reduction steps as discussed above. One would expect to lose the characteristic high surface area of the nickel in the process. The direct reduction of the Ni3S2 by H2 is seen to be thermodynamically possible. Such a reduction would not be expected to affect the unpoisoned Raney nickel. One would also have the option of pre-or post-reduction separation of the unaffected nickel from the poisoned/reduced material. It should be emphasized that the modeling summarized in this work is for batch processes only, with some gains to be made in driving the reduction reaction by removing the H2S as it is generated. The possibility of checking these ideas in a set of bench scale experiments seems attractive.
Chemistry of Materials, Feb 5, 2008
... Michael Halter † , Yi Liao † , Ryan M. Plocinik ‡ , David C. Coffey § , Sanchali Bhattacharje... more ... Michael Halter † , Yi Liao † , Ryan M. Plocinik ‡ , David C. Coffey § , Sanchali Bhattacharjee † , Ursula Mazur , Garth J. Simpson ‡ , Bruce H. Robinson † and Sarah L. Keller* †. Departments of Chemistry and Physics, University ...
Journal of Physical Chemistry C, Jul 29, 2020
Cooperative interactions play a critical role in the stability and reactivity of biological syste... more Cooperative interactions play a critical role in the stability and reactivity of biological systems and are an increasingly important consideration in the synthesis of functional materials, but quantitative single-molecule measurements of this phenomenon are rare. Many of these cooperative interactions necessarily occur on surfaces, making the study of cooperative effects at interfaces of particular importance. Here, we report a quantitative experimental and theoretical study of the cooperative binding of 1phenylimidazole (PhIm) to cobalt(II) octaethylporphyrin (CoOEP) on highly oriented pyrolytic graphite (HOPG) at the solution−solid interface. Scanning tunneling microscopy (STM) confirmed and monitored the binding of PhIm to HOPGsupported CoOEP with single-molecule resolution. Nearest-neighbor analysis of these STM images revealed positive cooperative binding behavior. Periodic plane-wave density functional theory (DFT) calculations of PhIm/CoOEP/HOPG and cobalt(II) porphine (CoP)/HOPG systems support the experimental observations of positive cooperativity. DFT calculations revealed that the binding energy of PhIm to Co-porphyrin increases as PhIm binds to more neighboring molecules. Calculations also suggest that the presence of HOPG is crucial to observe positive cooperativity in this system.
Journal of Molecular Spectroscopy, Apr 1, 1977
Rotational spectra of six isotopic species of ethylene ozonide-Da have been assigned. The 0,-O, d... more Rotational spectra of six isotopic species of ethylene ozonide-Da have been assigned. The 0,-O, distance was calculated six independent ways using Kraitchman's substitution equations, and values between 1.458 and 1.462 d were obtained. This is in contrast to a similar analysis for normal ethylene ozonide where the 0,-O, distance varied between 1.455 and 1.502 A. This difference is associated with the greater value of la-I. for the Dg species which markedly decreases the effect of axes rotations upon isotopic substitution. The structural parameters derived from the Da data set are in excellent agreement with those extracted from the Hp analyses after minimization of axes rotation effects.