Introduction The most common method for producing large area graphene films is by performing chem... more Introduction The most common method for producing large area graphene films is by performing chemical vapor deposition (CVD) on Cu foil substrates [1]. The reason for using Cu as a substrate is that it has a very low solubility for C at the temperature that the CVD is performed, which ensures a self-limited growth of a single monolayer of graphene. The goal of this research project is to determine the optimal procedure for producing graphene films with a low defect density on Cu foil substrates. The graphene films were grown on 99.8% pure Cu foils 0.001” thick, 0.25” wide, and 6” long. The Cu films were heated by passing a current through the film, and the temperature was monitored using a disappearing filament optical pyrometer. The precursor used was ethylene (C2H4).
Transparent conductive meshes were fabricated by ink-jet printing on flexible substrates using a ... more Transparent conductive meshes were fabricated by ink-jet printing on flexible substrates using a percolation pattern created by random removal of conducting bonds from a regular square two-dimensional lattice. With this approach, a higher gain in optical transmittance than electrical conductivity loss is achieved above the percolation threshold. As a result of this, a figure of merit for the percolation pattern is improved with respect to a regular square mesh. The transmittance (T), sheet resistance (R), and figure of merit (F) on percentage of removed bonds for square lattices printed were measured. The gain of the figure of merit was observed in the range of removed bonds from 5% to 15% .
Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and the... more Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and theoretically. Adsorption studies on model systems, where well ordered thin oxide films grown on a metal substrate to avoid sample charging in connection with electron spectroscopic measurements, were used, are reported. Two oxide systems are compared: (i) a reactive transition metal oxide surface of Cr,O, (111) where it is shown that the surface contains Cr'+ ions which trigger its reactivity; (ii) a non-reactive simple metal oxide surface of y-Al,Oa (111) which is used as a support model surface. The adsorption of various molecules on both surfaces has been examined, and how the properties of the surface are modified when metals are deposited on the oxide surface have been studied. The results of alkali metal deposits on CrsO,(111) and Pt deposits on y-Al,Oa (111) are presented. The applied methods include LEED, STM, TPD, ARUPS, ELS, XPS, HREELS and ISS.
Cu(111) in a UHV chamber by CVD using ethylene. The sample holder consisted of an oxygen series b... more Cu(111) in a UHV chamber by CVD using ethylene. The sample holder consisted of an oxygen series button heater with Ta heat shields to allow annealing the crystal to 900 • C at pressures as high as 100 mTorr. The crystal structure of the surface was determined using LEED. Growth attempts on the clean Cu(111) surface at ethylene pressures as high as 5 mTorr only resulted in trace amounts of graphene being grown on the surface. This is attributed to the low catalytic activity of the Cu(111) surface and the high vapor pressure of Cu at the growth temperature. To suppress the sublimation of Cu, an Ar overpressure was used. Ethylene partial pressures of 2, 5, 10, and 50 mTorr were used, keeping the total pressure at 50 mTorr. The films for 2 and 5 mTorr showed predominately single domain epitaxy. At 10 mTorr ethylene partial pressure, additional diffraction spots 30 • out of phase with the Cu(111) substrate were observed. At 50 mTorr of ethylene and no Ar overpressure, broad diffraction arcs were observed in LEED that were ±15 • out of phase with the substrate. Therefore, the carbon deposition rate, which depends on the ethylene partial pressure, has a large effect on the quality of the graphene film.
Ballistic-electron-emission microscopy measurements have been performed on n-type Au/Si͑100͒ inte... more Ballistic-electron-emission microscopy measurements have been performed on n-type Au/Si͑100͒ interfaces for injection energies up to 1.2 eV over a range of Au overlayer thicknesses from ϳ65 to ϳ340 Å at both room temperature and 77 K. Hot-electron attenuation lengths in the Au overlayer have been determined to be 133Ϯ2 Å at room temperature and 147Ϯ6 Å at 77 K over the energy range of 0.92-1.20 eV above the Fermi level. The lack of energy dependence and the relatively small temperature-dependent change in the attenuation lengths that have been measured indicate that electron scattering with defects is the dominant mechanism affecting hot-electron transport in these Au overlayers. The ratio of the zero-thickness collection current at 77 K to that at room temperature has been measured to be 1.79Ϯ0.09. This large increase in the collection efficiency at 77 K is attributed primarily to the large temperature dependence of the transverse acoustic-phonon population in Si. Images with significant reductions in the collection current at topographic locations that have a large surface gradient have been obtained at room temperature. Calculations, which assume that the probability of transmission across the interface is independent of the transverse momentum of the electron, correlate well with the experimentally observed reductions. This result indicates that the injected electrons remain forward focused with little broadening as they pass through the Au overlayer, which implies that elastic scattering at the Au/Si interface accounts for the observation from previous Au/Si ballistic-electron-emission microscopy studies that transverse momentum is not conserved.
Adsorption of sulfur on TiO 2 (110) at room temperature ͑RT͒ and 350°C has been studied with ultr... more Adsorption of sulfur on TiO 2 (110) at room temperature ͑RT͒ and 350°C has been studied with ultraviolet photoelectron spectroscopy. A TiO 2 (110) (1ϫ1) surface with a small amount of oxygen vacancies was prepared by sputtering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Sulfur adsorption at room temperature leads to the disappearance of this vacancy-related band-gap state, indicating that the surface oxygen vacancies are filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 eV. Adsorption of S at 350°C forms a (4ϫ1) superstructure at high coverages ͓Ϸ0.9 monolayer ͑ML͔͒ that is visible with low-energy electron diffraction. In a previously proposed model for this superstructure, sulfur replaces half of the in-plane oxygen atoms and all the bridging oxygen atoms are removed. In agreement with this model, the oxygen 2s peak is decreased significantly and the defect state is increased. Two additional valence features are observed: one at 2.7 eV and one at 3.9 eV. Due to those features the band gap vanishes. In resonant photoemission, these features show a similar, but weaker, resonance profile than the vacancy-related defect state. Hybridized Ti-derived states extend across the whole valence-band region. Generally, a higher resonant photon energy is found for valence-band states with lower binding energies, indicating mainly 3pϪ4s transitions in the upper valence band. Adsorption of sulfur reduces the strength of the resonances.
Journal of Photopolymer Science and Technology, 2013
Secondary electrons play critical roles in several imaging technologies, including extreme ultrav... more Secondary electrons play critical roles in several imaging technologies, including extreme ultraviolet (EUV) lithography. At longer wavelengths of light (e.g. 193 and 248 nm), the photons are directly involved in the photochemistry occurring during photolysis. EUV light (13.5 nm, 92 eV), however, first creates a photoelectron, and this electron, or its subsequent daughter electrons create most of the chemical changes that occur during exposure. Despite the importance of these electrons, the details surrounding the chemical events leading to acid production remain poorly understood. Previously reported experimental results using high PAG-loaded resists have demonstrated that up to five or six photoacids can be generated per incident photon. Until recently, only electron recombination events were thought to play a role in acid generation, requiring that at least as many secondary electrons are produced to yield a given number of acid molecules. However, the initial results we have obtained using a Monte Carlo-based modeling program, LESiS, demonstrate that only two to three secondary electrons are made per absorbed EUV photon. A more comprehensive understanding of EUVinduced acid generation is therefore needed for the development of higher performance resists.
Previous studies of the adsorption of CO on the catalytically active Cr(110) surface have found t... more Previous studies of the adsorption of CO on the catalytically active Cr(110) surface have found that the CO molecule dissociates upon adsorption at 300 K. One aspect of the CO adsorption process that has not been studied in detail is the temperature dependence of the ...
Understanding the influence that copper substrate surface symmetry and oxygen impurities have on ... more Understanding the influence that copper substrate surface symmetry and oxygen impurities have on the growth of graphene by chemical vapor deposition is important for developing techniques for producing high quality graphene. Therefore, we have studied the growth of graphene by catalytic decomposition of ethylene in an ultra-high vacuum chamber on both a clean Cu(100) surface and a Cu(100) surface pre-dosed with a layer of chemisorbed oxygen. The crystal structure of the graphene films was characterized with in-situ low energy electron diffraction. By heating the clean Cu(100) substrate from room temperature to the growth temperature in ethylene, epitaxial graphene films were formed. The crystal quality was found to depend strongly on the growth temperature. At 900 • C, well-ordered two-domain graphene
The adsorption of molecular sulfur on TiO 2 (1 1 0)(1 Â 1) has been studied with scanning tunneli... more The adsorption of molecular sulfur on TiO 2 (1 1 0)(1 Â 1) has been studied with scanning tunneling microscopy and photoelectron spectroscopy. At room temperature S binds dissociatively to 5-fold coordinated Ti atoms and oxygen vacancies. At elevated temperatures (120±440°C) sulfur replaces surface oxygen atoms. Evidence was found that the reduction state of TiO 2 crystals strongly aects the surface coverage of S at elevated temperatures. The rate of the O±S site exchange is kinetically limited by the arrival of diusing bulk defects at the surface.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2012
Graphene growth on Cu foils by catalytic decomposition of methane forms predominantly single-laye... more Graphene growth on Cu foils by catalytic decomposition of methane forms predominantly single-layer graphene films due to the low solubility of carbon in Cu. On the other hand, graphene growth on Cu–Ni foils can result in the controlled growth of few-layer graphene films because of the higher solubility of carbon in Ni. One of the key issues for the use of graphene grown by chemical vapor deposition for device applications is the influence of defects on the transport properties of the graphene. For instance, growth on metal foil substrates is expected to result in multidomain graphene growth because of the presence of grains within the foil that exhibit a variety of surface terminations. Therefore, the size and orientation of the grains within the metal foil should influence the defect density of the graphene. For this reason, we have studied the effect of total anneal time and temperature on the orientation and size of grains within Cu foils and Cu–Ni alloy foils with a nominal conc...
Journal of Electron Spectroscopy and Related Phenomena, 1994
We have investigated the electronic and geometric structure of sinfaces of transition metal oxide... more We have investigated the electronic and geometric structure of sinfaces of transition metal oxides and simple metal oxides applying electron spectroscopic methods. In order to avoid charging problems, we have resorted to the preparation of thin (5-50 A) metal oxide films grown on metallic substrates via several oxidation techniques. We have studied NiO, COO, Cr,O,, and A&O,. The thin films have the advantage that they may be easily cooled to liquid nitrogen and liquid helium temperatures. Another interesting feature of the thin films is the possibility to prepare thermodynamically unstable surfaces, such as (111) surfaces of ionic rock salt structures, and study the adsorption and reaction at such surfaces. Adsorption and reaction of molecules has not only been investigated on the clean oxide substrates but also on the surfaces modified through deposited ultrathin metal films. Such systems may be considered as models for heterogeneous catalysts.
The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has b... more The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O3<Cr3O4<Cr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a...
Introduction The most common method for producing large area graphene films is by performing chem... more Introduction The most common method for producing large area graphene films is by performing chemical vapor deposition (CVD) on Cu foil substrates [1]. The reason for using Cu as a substrate is that it has a very low solubility for C at the temperature that the CVD is performed, which ensures a self-limited growth of a single monolayer of graphene. The goal of this research project is to determine the optimal procedure for producing graphene films with a low defect density on Cu foil substrates. The graphene films were grown on 99.8% pure Cu foils 0.001” thick, 0.25” wide, and 6” long. The Cu films were heated by passing a current through the film, and the temperature was monitored using a disappearing filament optical pyrometer. The precursor used was ethylene (C2H4).
Transparent conductive meshes were fabricated by ink-jet printing on flexible substrates using a ... more Transparent conductive meshes were fabricated by ink-jet printing on flexible substrates using a percolation pattern created by random removal of conducting bonds from a regular square two-dimensional lattice. With this approach, a higher gain in optical transmittance than electrical conductivity loss is achieved above the percolation threshold. As a result of this, a figure of merit for the percolation pattern is improved with respect to a regular square mesh. The transmittance (T), sheet resistance (R), and figure of merit (F) on percentage of removed bonds for square lattices printed were measured. The gain of the figure of merit was observed in the range of removed bonds from 5% to 15% .
Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and the... more Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and theoretically. Adsorption studies on model systems, where well ordered thin oxide films grown on a metal substrate to avoid sample charging in connection with electron spectroscopic measurements, were used, are reported. Two oxide systems are compared: (i) a reactive transition metal oxide surface of Cr,O, (111) where it is shown that the surface contains Cr'+ ions which trigger its reactivity; (ii) a non-reactive simple metal oxide surface of y-Al,Oa (111) which is used as a support model surface. The adsorption of various molecules on both surfaces has been examined, and how the properties of the surface are modified when metals are deposited on the oxide surface have been studied. The results of alkali metal deposits on CrsO,(111) and Pt deposits on y-Al,Oa (111) are presented. The applied methods include LEED, STM, TPD, ARUPS, ELS, XPS, HREELS and ISS.
Cu(111) in a UHV chamber by CVD using ethylene. The sample holder consisted of an oxygen series b... more Cu(111) in a UHV chamber by CVD using ethylene. The sample holder consisted of an oxygen series button heater with Ta heat shields to allow annealing the crystal to 900 • C at pressures as high as 100 mTorr. The crystal structure of the surface was determined using LEED. Growth attempts on the clean Cu(111) surface at ethylene pressures as high as 5 mTorr only resulted in trace amounts of graphene being grown on the surface. This is attributed to the low catalytic activity of the Cu(111) surface and the high vapor pressure of Cu at the growth temperature. To suppress the sublimation of Cu, an Ar overpressure was used. Ethylene partial pressures of 2, 5, 10, and 50 mTorr were used, keeping the total pressure at 50 mTorr. The films for 2 and 5 mTorr showed predominately single domain epitaxy. At 10 mTorr ethylene partial pressure, additional diffraction spots 30 • out of phase with the Cu(111) substrate were observed. At 50 mTorr of ethylene and no Ar overpressure, broad diffraction arcs were observed in LEED that were ±15 • out of phase with the substrate. Therefore, the carbon deposition rate, which depends on the ethylene partial pressure, has a large effect on the quality of the graphene film.
Ballistic-electron-emission microscopy measurements have been performed on n-type Au/Si͑100͒ inte... more Ballistic-electron-emission microscopy measurements have been performed on n-type Au/Si͑100͒ interfaces for injection energies up to 1.2 eV over a range of Au overlayer thicknesses from ϳ65 to ϳ340 Å at both room temperature and 77 K. Hot-electron attenuation lengths in the Au overlayer have been determined to be 133Ϯ2 Å at room temperature and 147Ϯ6 Å at 77 K over the energy range of 0.92-1.20 eV above the Fermi level. The lack of energy dependence and the relatively small temperature-dependent change in the attenuation lengths that have been measured indicate that electron scattering with defects is the dominant mechanism affecting hot-electron transport in these Au overlayers. The ratio of the zero-thickness collection current at 77 K to that at room temperature has been measured to be 1.79Ϯ0.09. This large increase in the collection efficiency at 77 K is attributed primarily to the large temperature dependence of the transverse acoustic-phonon population in Si. Images with significant reductions in the collection current at topographic locations that have a large surface gradient have been obtained at room temperature. Calculations, which assume that the probability of transmission across the interface is independent of the transverse momentum of the electron, correlate well with the experimentally observed reductions. This result indicates that the injected electrons remain forward focused with little broadening as they pass through the Au overlayer, which implies that elastic scattering at the Au/Si interface accounts for the observation from previous Au/Si ballistic-electron-emission microscopy studies that transverse momentum is not conserved.
Adsorption of sulfur on TiO 2 (110) at room temperature ͑RT͒ and 350°C has been studied with ultr... more Adsorption of sulfur on TiO 2 (110) at room temperature ͑RT͒ and 350°C has been studied with ultraviolet photoelectron spectroscopy. A TiO 2 (110) (1ϫ1) surface with a small amount of oxygen vacancies was prepared by sputtering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Sulfur adsorption at room temperature leads to the disappearance of this vacancy-related band-gap state, indicating that the surface oxygen vacancies are filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 eV. Adsorption of S at 350°C forms a (4ϫ1) superstructure at high coverages ͓Ϸ0.9 monolayer ͑ML͔͒ that is visible with low-energy electron diffraction. In a previously proposed model for this superstructure, sulfur replaces half of the in-plane oxygen atoms and all the bridging oxygen atoms are removed. In agreement with this model, the oxygen 2s peak is decreased significantly and the defect state is increased. Two additional valence features are observed: one at 2.7 eV and one at 3.9 eV. Due to those features the band gap vanishes. In resonant photoemission, these features show a similar, but weaker, resonance profile than the vacancy-related defect state. Hybridized Ti-derived states extend across the whole valence-band region. Generally, a higher resonant photon energy is found for valence-band states with lower binding energies, indicating mainly 3pϪ4s transitions in the upper valence band. Adsorption of sulfur reduces the strength of the resonances.
Journal of Photopolymer Science and Technology, 2013
Secondary electrons play critical roles in several imaging technologies, including extreme ultrav... more Secondary electrons play critical roles in several imaging technologies, including extreme ultraviolet (EUV) lithography. At longer wavelengths of light (e.g. 193 and 248 nm), the photons are directly involved in the photochemistry occurring during photolysis. EUV light (13.5 nm, 92 eV), however, first creates a photoelectron, and this electron, or its subsequent daughter electrons create most of the chemical changes that occur during exposure. Despite the importance of these electrons, the details surrounding the chemical events leading to acid production remain poorly understood. Previously reported experimental results using high PAG-loaded resists have demonstrated that up to five or six photoacids can be generated per incident photon. Until recently, only electron recombination events were thought to play a role in acid generation, requiring that at least as many secondary electrons are produced to yield a given number of acid molecules. However, the initial results we have obtained using a Monte Carlo-based modeling program, LESiS, demonstrate that only two to three secondary electrons are made per absorbed EUV photon. A more comprehensive understanding of EUVinduced acid generation is therefore needed for the development of higher performance resists.
Previous studies of the adsorption of CO on the catalytically active Cr(110) surface have found t... more Previous studies of the adsorption of CO on the catalytically active Cr(110) surface have found that the CO molecule dissociates upon adsorption at 300 K. One aspect of the CO adsorption process that has not been studied in detail is the temperature dependence of the ...
Understanding the influence that copper substrate surface symmetry and oxygen impurities have on ... more Understanding the influence that copper substrate surface symmetry and oxygen impurities have on the growth of graphene by chemical vapor deposition is important for developing techniques for producing high quality graphene. Therefore, we have studied the growth of graphene by catalytic decomposition of ethylene in an ultra-high vacuum chamber on both a clean Cu(100) surface and a Cu(100) surface pre-dosed with a layer of chemisorbed oxygen. The crystal structure of the graphene films was characterized with in-situ low energy electron diffraction. By heating the clean Cu(100) substrate from room temperature to the growth temperature in ethylene, epitaxial graphene films were formed. The crystal quality was found to depend strongly on the growth temperature. At 900 • C, well-ordered two-domain graphene
The adsorption of molecular sulfur on TiO 2 (1 1 0)(1 Â 1) has been studied with scanning tunneli... more The adsorption of molecular sulfur on TiO 2 (1 1 0)(1 Â 1) has been studied with scanning tunneling microscopy and photoelectron spectroscopy. At room temperature S binds dissociatively to 5-fold coordinated Ti atoms and oxygen vacancies. At elevated temperatures (120±440°C) sulfur replaces surface oxygen atoms. Evidence was found that the reduction state of TiO 2 crystals strongly aects the surface coverage of S at elevated temperatures. The rate of the O±S site exchange is kinetically limited by the arrival of diusing bulk defects at the surface.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2012
Graphene growth on Cu foils by catalytic decomposition of methane forms predominantly single-laye... more Graphene growth on Cu foils by catalytic decomposition of methane forms predominantly single-layer graphene films due to the low solubility of carbon in Cu. On the other hand, graphene growth on Cu–Ni foils can result in the controlled growth of few-layer graphene films because of the higher solubility of carbon in Ni. One of the key issues for the use of graphene grown by chemical vapor deposition for device applications is the influence of defects on the transport properties of the graphene. For instance, growth on metal foil substrates is expected to result in multidomain graphene growth because of the presence of grains within the foil that exhibit a variety of surface terminations. Therefore, the size and orientation of the grains within the metal foil should influence the defect density of the graphene. For this reason, we have studied the effect of total anneal time and temperature on the orientation and size of grains within Cu foils and Cu–Ni alloy foils with a nominal conc...
Journal of Electron Spectroscopy and Related Phenomena, 1994
We have investigated the electronic and geometric structure of sinfaces of transition metal oxide... more We have investigated the electronic and geometric structure of sinfaces of transition metal oxides and simple metal oxides applying electron spectroscopic methods. In order to avoid charging problems, we have resorted to the preparation of thin (5-50 A) metal oxide films grown on metallic substrates via several oxidation techniques. We have studied NiO, COO, Cr,O,, and A&O,. The thin films have the advantage that they may be easily cooled to liquid nitrogen and liquid helium temperatures. Another interesting feature of the thin films is the possibility to prepare thermodynamically unstable surfaces, such as (111) surfaces of ionic rock salt structures, and study the adsorption and reaction at such surfaces. Adsorption and reaction of molecules has not only been investigated on the clean oxide substrates but also on the surfaces modified through deposited ultrathin metal films. Such systems may be considered as models for heterogeneous catalysts.
The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has b... more The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O3<Cr3O4<Cr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a...
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Papers by Carl Ventrice