Alex Laikhtman - Academia.edu (original) (raw)
Papers by Alex Laikhtman
Physical chemistry chemical physics : PCCP, Jan 20, 2018
Understanding the interaction of hydrogen with layered materials is crucial in the fields of sens... more Understanding the interaction of hydrogen with layered materials is crucial in the fields of sensors, catalysis, fuel cells and hydrogen storage, among others. Density functional theory, improved by the introduction of van der Waals dispersion forces, provides an efficient and practical workbench to investigate the interaction of molecular and atomic hydrogen with WS2 multilayers and nanotubes. We find that H2 physisorbs on the surface of those materials on top of W atoms, while atomic H chemisorbs on top of S atoms. In the case of nanotubes, the chemisorption strength is sensitive to the nanotube diameter. Diffusion of H2 on the surface of WS2 encounters quite small activation barriers whose magnitude helps to explain previous and new experimental results for the observed dependence of the hydrogen concentration with temperature. Intercalation of H2 between adjacent planar WS2 layers reveals an endothermic character. Intercalating H atoms is energetically favorable, but the interca...
The journal of physical chemistry. C, Nanomaterials and interfaces, 2017
The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanopartic... more The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanoparticles of WS2 are investigated. Our recent approaches of using hydrogen activated by either microwave or radiofrequency plasma dramatically increased the efficiency of its adsorption on the nanoparticles surface. In the current work we make an emphasis on elucidation of the chemical configuration of the adsorbed hydrogen. This configuration is of primary importance as it affects its adsorption stability and possibility of release. To get insight on the chemical configuration, we combined the experimental analysis methods with theoretical modeling based on the density functional theory (DFT). Micro-Raman spectroscopy was used as a primary tool to elucidate chemical bonding of hydrogen and to distinguish between chemi- and physisorption. Hydrogen adsorbed in molecular form (H2) was clearly identified in all the plasma-hydrogenated WS2 nanoparticles samples. It was shown that the adsorbed hydr...
Physical Review B, 2000
ABSTRACT In the present work we study the mechanism of positive-hydrogen-ion photon-stimulated de... more ABSTRACT In the present work we study the mechanism of positive-hydrogen-ion photon-stimulated desorption (PSID) in the 280–310 eV photon energy range from defective polycrystalline hydrogenated diamond film surfaces. Controlled defect levels within the near surface were introduced by ion bombardment of hydrogenated films. From a comparison between the PSID, the partial electron yield near-edge x-ray-absorption fine structure, and the low-energy secondary-electron emission, the mechanism of photodesorption from the ion-beam-damaged diamond surfaces is discussed. The main effect of ion implantation on the PSID of hydrogen ions is a change in the dominant mechanism of desorption. For ion-damaged diamond films, the desorption process promoted by direct C(1s)-σ* excitation of carbon atoms bonded to hydrogen, C-H (ads), becomes more efficient compared to the case of unimplanted diamond film. The indirect hydrogen desorption process proceeding through valence excitation of C-H(ads) bonds by secondary electrons, dominant for an unimplanted hydrogenated diamond sample, strongly decreases for the ion-beam-irradiated surfaces. This indirect process is the one that really characterizes H+ photodesorption from hydrogenated diamond surfaces. From the hydrogen PSID measurements we did not detect substantial removal of hydrogen from diamond films exposed to ion irradiation at the energies and doses applied in this work.
Physical Review B, 2001
In this work we report, a study of the mechanism of H- electron stimulated desorption (ESD) from ... more In this work we report, a study of the mechanism of H- electron stimulated desorption (ESD) from hydrogenated diamond films for incident electron energies in the 2-45 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes leading to desorption as a function of incident electron energy: (i) kinetic energy distribution (KED) of H- and (ii) H- anions yield at fixed ion energy (FIE) measurements. The KED measurements show that for incident electrons of up to ~11 eV the most probable kinetic energy of H- ions monotonically increases from about 1.7 to 3.3 eV. For higher incident electron energies, the ion energy distribution peaks at about 1.5 eV and is nearly constant. From these measurements it is derived that the H- ESD cross section has a resonance behavior displaying two well-defined peaks at 9 and 22 eV and a monotonic increase with a threshold at ~14 eV as a function of incident electron energy. From the KED and FIE spectra the 9- and 22-eV peaks are interpreted as due to dissociative electron attachment via a single Feshbach anion resonance state, albeit accessed directly and indirectly, respectively. A possible intermediate process involving a well-known electronic excitation of the hydrogenated diamond at 13 eV is suggested. For incident electron energies higher than ~14 eV, H- ESD proceeds also via dipolar dissociation processes.
In this report we elaborate on the effect of a low Earth orbit (LEO) space environment the fluoro... more In this report we elaborate on the effect of a low Earth orbit (LEO) space environment the fluorosilicone rubber polymers. In particular, we concentrate on the study of the influen of atomic oxygen (AO) and UV irradiation on their chemical, morphological, and mechani properties. The samples were exposed to the space environment in-flight and in a grou simulation radio frequency
International Journal of Hydrogen Energy, 2014
ABSTRACT In this work we investigate the possibility to use inorganic nanotubes (INT) and inorgan... more ABSTRACT In this work we investigate the possibility to use inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles, both composed of WS2, as hydrogen storage materials. These materials may allow hydrogen to be either chemi- or physisorbed inside their crystalline structure, inside hollow core of fullerenes/nanotubes or in the open interstitial pore spaces between the NP or nanotubes. While exposure to molecular hydrogen was found to have measurable but limited absorption rate: up to 1 weight % with the majority of hydrogen physisorbed, exposure of these materials to hydrogen activated by microwave (MW) plasma is supposed to be more effective: the flow of activated hydrogen interacts with the substrate which contains NP of IF and INT and substantial percentage of it may be chemisorbed. We report here the results of such exposures and analyze the absorption and diffusion of hydrogen by different methods ranging from simple weighting by analytical balances to absorption-desorption curves obtained by pressure temperature composition (PCT) apparatus, X-Ray Diffraction (XRD) investigation of interlayer distance changes induced by hydrogen penetration, and hydrogen depth profile measured by Secondary Ion Mass Spectroscopy (SIMS). We found that even very short exposure (5 min) to MW plasma at relatively mild conditions: low power and intermediate temperature (~ 400 °C) results in diffusion of hydrogen, though some etching of the substrate material may occur during the treatment. The ongoing research is aimed to determine the chemical state of the absorbed hydrogen as well as to optimize the parameters of MW hydrogen plasma as a hydrogen source.
In the present work we investigate friction and adhesion properties of microcrystalline diamond f... more In the present work we investigate friction and adhesion properties of microcrystalline diamond films grown on steel substrates with a Cr-N interlayer prepared at 500° C and 800° C. Scratch tests were performed at indentation loads of 1-4 N and sliding velocity of 0.08 mm/s. The friction coefficient and wear loss were assessed. The diamond film structure and composition are
Surface Review and Letters, 2000
In this study we report on absolute quantum photoyield (QPY) measurements from well-defined defec... more In this study we report on absolute quantum photoyield (QPY) measurements from well-defined defective diamond surfaces in the 140–200 nm spectral range. The effect of defects in polycrystalline diamond films on their photoemission properties is studied by intentionally introducing damage using room temperature 30 keV Xe + ion bombardment to doses ranging from 2×1013 to 2×1015 ions/cm 2. Ion bombardment results in a drastic degradation of the QPY, to less than 1% at 140 nm, even at the lowest implantation dose compared to ~11.5% measured for the unimplanted diamond film. Analysis of the ion-damaged diamond films is performed by photon-stimulated ion desorption (PSID) measurements of H +, high resolution C(KLL) Auger electron spectroscopy and X-ray photoelectron spectroscopy. These measurements reveal that the decay in photoemission is due to the gradual formation of nondiamond carbon in the near-surface region. This damage leads to a change of the electron affinity from negative to positive, as determined by secondary electron emission measurements. PSID measurements reveal that the ion-bombarded diamond films remain hydrogen-terminated. MW hydrogen plasma treatment results in complete regeneration of the photoemission properties for diamond films implanted to Xe + doses of up to 2×1014 cm -2; only partial recovery was obtained for films irradiated with a higher ion dose.
physica status solidi (a), 2002
ABSTRACT In the present work we study the processes involved in the interaction of molecular and ... more ABSTRACT In the present work we study the processes involved in the interaction of molecular and thermally activated oxygen with hydrogen-terminated and hydrogen-free polycrystalline diamond surfaces. As revealed by H+ photodesorption, near edge X-ray absorption fine structure and X-ray photoelectron spectroscopies, molecular oxygen does not adsorb onto the hydrogenated diamond surface; however, thermally activated oxygen does adsorb onto it. The dominant reaction pass leads to the formation of surface C–O–H bonds, although some abstraction of chemisorbed hydrogen happens as well. On the surface regions, where such an abstraction takes place, C=O (ads) bonds are mostly produced. The hydrogen-free diamond surface is considerably more reactive: the interaction of thermally activated oxygen with it produces C=O and C–O–C bonds, and unactivated oxygen molecules are adsorbed predominantly as C=O. The surface concentration of oxygen, however, is lower in the case of the exposure to unactivated molecules. As established by photon induced secondary electron emission (SEE) measurements, the hydrogen-terminated, oxygen-free diamond surface exhibits negative electron affinity and high SEE intensity. The adsorption of thermally activated oxygen results in a slightly positive electron affinity (PEA) (∼0.4 eV) and in a reduction in the intensity of SEE, whereas the electron emission properties of this sample exposed to molecular oxygen seem to be unaffected. The clean diamond surface has already a low SEE intensity along with a PEA of ∼1.2 eV. This is why the adsorption of molecular oxygen has little effect on its electron emission properties. A treatment of this surface with activated oxygen results in a further deterioration of the electron emission properties as revealed by a PEA of ∼1.7 eV and a poor SEE intensity.
Journal of Physics: Condensed Matter, 2005
The Journal of Chemical Physics, 2002
In this work we report on a study of the mechanism of O- electron stimulated desorption (ESD) fro... more In this work we report on a study of the mechanism of O- electron stimulated desorption (ESD) from hydrogenated and hydrogen-free polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4-22 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes: (i) total O- and H-
Journal of Applied Physics, 1995
ABSTRACT In the present work nucleation and growth of diamond by chemical vapor deposition (CVD) ... more ABSTRACT In the present work nucleation and growth of diamond by chemical vapor deposition (CVD) on highly oriented pyrolitic graphite (HOPG) and glassy carbon (GC) substrates have been investigated. These carbon substrates represent generic forms of well‐characterized ordered and disordered sp2 bonded carbon materials. The nature of the precursor to diamond CVD is assessed by studying nucleation and growth on substrates abraded with hard powders whose debris may act as initial growth centers, e.g., diamond and c‐BN, and hard powders onto which diamond CVD does not grow heteroepitaxially, e.g., alumina. Based on our experimental results it is concluded that the precursor to diamond nucleation may be debris left after the abrasion process and/or damage created preferentially on graphitic prism planes. A higher density of such damaged prism planes on GC than on HOPG resulted in a larger nucleation density on the former. Different morphologies of single particles deposited on HOPG and GC were found: well faceted on the former, and ball‐like on the later as determined by scanning electron microscopy. This difference is explained on the basis of a larger concentration of active carbon species present at the GC surface as compared to the HOPG surface. The additional source of carbon is from etching of the GC and HOPG substrates which, under the diamond CVD conditions used in the present study, is twice higher for GC. The thermal stability of continuous films deposited on HOPG is better than those deposited on GC. The presence of different carbon phases in the deposited material was investigated by micro‐Raman and scanning‐Auger electron spectroscopies. © 1995 American Institute of Physics.
Journal of Applied Physics, 2002
ABSTRACT In this work we investigate the influence of annealing and microwave (MW) hydrogen plasm... more ABSTRACT In this work we investigate the influence of annealing and microwave (MW) hydrogen plasma exposure of ion-beam-irradiated diamond film surfaces. In particular, we are interested in the recovery of secondary electron emission (SEE) and negative electron affinity (NEA) by removal of the damaged layer. To this aim, we correlate the SEE of variously treated Xe+ ion-damaged diamond films with their bonding structure in the near-surface region, as identified by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy. The 30 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of the near-surface region of a diamond film to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to MW hydrogen plasma treatment for 30 min produces NEA diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of positive electron affinity surfaces, and drastically degrades their electron emission properties. Prolonged (up to three hours) MW hydrogen plasma treatment of as-implanted diamond films gradually improves their crystal quality and results in a further increase of SEE intensity. The SEE intensity after three hours MW hydrogen plasma exposure of the ion-beam-irradiated films was found to be ∼50% above the value obtained for the as-deposited diamond films. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. Our results show that removal of damage from a highly disordered diamond surface and recovery of its electron emission properties are possible by MW hydrogen plasma. However, it is a slow process. This is most likely due to the very low etching rate of the low-level damage at the end of the ion beam range. © 2002 American Institute of Physics.
Journal of Applied Physics, 2001
Journal of Applied Physics, 2000
Journal of Applied Physics, 1997
ABSTRACT In the present work the effect of incident laser power on the Raman spectra of diamond i... more ABSTRACT In the present work the effect of incident laser power on the Raman spectra of diamond isolated particles and continuous films deposited on silicon and glassy carbon (GC) substrates by the chemical vapor deposition method is investigated. It is shown that the Raman line position measured for diamond particles shifts to lower wave numbers as a function of incident laser power. These shifts were most drastic for single particles deposited on GC that were examined using a Raman microprobe. In this case the diamond peak displayed a negative shift of ∼ 18 cm−1 when the laser power output was increased from 1 to 15 mW. The laser beam diameter was ∼ 2 μm and the diamond particle measured was 3–6 μm in diameter. Micro-Raman measurements of diamond particles deposited on a silicon substrate or continuous diamond films on GC display very small changes in the diamond Raman peak wavelength for the same laser power range. From our studies it is concluded that the negative shift of the Raman peak position is caused by laser-induced local heating of the irradiated diamond particles. The temperature under the laser spot was calculated from the intensity ratio of Stokes to anti-Stokes Raman lines measured as a function of laser power output. The Raman peak wavelength calculated for each temperature showed excellent agreement with our experimental results. The local temperature of an isolated diamond crystal on GC rises to ∼ 1000 K at 15 mW laser power output, whereas the temperature change of the continuous film on GC and of a single particle on silicon was in the 0–30 K range above room temperature for the same laser power output range. This difference in heating is explained on the basis of efficient heat dissipation through a large contact area between the deposited particles and the substrate surface in the case of single particles deposited on silicon or through grain boundaries in the case of the continuous film on GC. The inefficient heat dissipation from the isolated diamond particles on GC is related to the small contact area between the diamond crystals and the GC substrate as a result of etching during the deposition process and possibly to the presence of an amorphous component in the diamond crystals deposited. © 1997 American Institute of Physics.
Journal of Applied Physics, 1999
... A. Laikhtman, I. Gouzman, A. Hoffman, G. Comtet, L. Hellner, G. Dujardin. ... It is found tha... more ... A. Laikhtman, I. Gouzman, A. Hoffman, G. Comtet, L. Hellner, G. Dujardin. ... It is found that the binding energy in the implanted samples has a positive shift of 0.61 eV ... dose ion implantation was reflected by a sharp reduction in the intensity of the diamond core exciton peak and by ...
Physical chemistry chemical physics : PCCP, Jan 20, 2018
Understanding the interaction of hydrogen with layered materials is crucial in the fields of sens... more Understanding the interaction of hydrogen with layered materials is crucial in the fields of sensors, catalysis, fuel cells and hydrogen storage, among others. Density functional theory, improved by the introduction of van der Waals dispersion forces, provides an efficient and practical workbench to investigate the interaction of molecular and atomic hydrogen with WS2 multilayers and nanotubes. We find that H2 physisorbs on the surface of those materials on top of W atoms, while atomic H chemisorbs on top of S atoms. In the case of nanotubes, the chemisorption strength is sensitive to the nanotube diameter. Diffusion of H2 on the surface of WS2 encounters quite small activation barriers whose magnitude helps to explain previous and new experimental results for the observed dependence of the hydrogen concentration with temperature. Intercalation of H2 between adjacent planar WS2 layers reveals an endothermic character. Intercalating H atoms is energetically favorable, but the interca...
The journal of physical chemistry. C, Nanomaterials and interfaces, 2017
The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanopartic... more The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanoparticles of WS2 are investigated. Our recent approaches of using hydrogen activated by either microwave or radiofrequency plasma dramatically increased the efficiency of its adsorption on the nanoparticles surface. In the current work we make an emphasis on elucidation of the chemical configuration of the adsorbed hydrogen. This configuration is of primary importance as it affects its adsorption stability and possibility of release. To get insight on the chemical configuration, we combined the experimental analysis methods with theoretical modeling based on the density functional theory (DFT). Micro-Raman spectroscopy was used as a primary tool to elucidate chemical bonding of hydrogen and to distinguish between chemi- and physisorption. Hydrogen adsorbed in molecular form (H2) was clearly identified in all the plasma-hydrogenated WS2 nanoparticles samples. It was shown that the adsorbed hydr...
Physical Review B, 2000
ABSTRACT In the present work we study the mechanism of positive-hydrogen-ion photon-stimulated de... more ABSTRACT In the present work we study the mechanism of positive-hydrogen-ion photon-stimulated desorption (PSID) in the 280–310 eV photon energy range from defective polycrystalline hydrogenated diamond film surfaces. Controlled defect levels within the near surface were introduced by ion bombardment of hydrogenated films. From a comparison between the PSID, the partial electron yield near-edge x-ray-absorption fine structure, and the low-energy secondary-electron emission, the mechanism of photodesorption from the ion-beam-damaged diamond surfaces is discussed. The main effect of ion implantation on the PSID of hydrogen ions is a change in the dominant mechanism of desorption. For ion-damaged diamond films, the desorption process promoted by direct C(1s)-σ* excitation of carbon atoms bonded to hydrogen, C-H (ads), becomes more efficient compared to the case of unimplanted diamond film. The indirect hydrogen desorption process proceeding through valence excitation of C-H(ads) bonds by secondary electrons, dominant for an unimplanted hydrogenated diamond sample, strongly decreases for the ion-beam-irradiated surfaces. This indirect process is the one that really characterizes H+ photodesorption from hydrogenated diamond surfaces. From the hydrogen PSID measurements we did not detect substantial removal of hydrogen from diamond films exposed to ion irradiation at the energies and doses applied in this work.
Physical Review B, 2001
In this work we report, a study of the mechanism of H- electron stimulated desorption (ESD) from ... more In this work we report, a study of the mechanism of H- electron stimulated desorption (ESD) from hydrogenated diamond films for incident electron energies in the 2-45 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes leading to desorption as a function of incident electron energy: (i) kinetic energy distribution (KED) of H- and (ii) H- anions yield at fixed ion energy (FIE) measurements. The KED measurements show that for incident electrons of up to ~11 eV the most probable kinetic energy of H- ions monotonically increases from about 1.7 to 3.3 eV. For higher incident electron energies, the ion energy distribution peaks at about 1.5 eV and is nearly constant. From these measurements it is derived that the H- ESD cross section has a resonance behavior displaying two well-defined peaks at 9 and 22 eV and a monotonic increase with a threshold at ~14 eV as a function of incident electron energy. From the KED and FIE spectra the 9- and 22-eV peaks are interpreted as due to dissociative electron attachment via a single Feshbach anion resonance state, albeit accessed directly and indirectly, respectively. A possible intermediate process involving a well-known electronic excitation of the hydrogenated diamond at 13 eV is suggested. For incident electron energies higher than ~14 eV, H- ESD proceeds also via dipolar dissociation processes.
In this report we elaborate on the effect of a low Earth orbit (LEO) space environment the fluoro... more In this report we elaborate on the effect of a low Earth orbit (LEO) space environment the fluorosilicone rubber polymers. In particular, we concentrate on the study of the influen of atomic oxygen (AO) and UV irradiation on their chemical, morphological, and mechani properties. The samples were exposed to the space environment in-flight and in a grou simulation radio frequency
International Journal of Hydrogen Energy, 2014
ABSTRACT In this work we investigate the possibility to use inorganic nanotubes (INT) and inorgan... more ABSTRACT In this work we investigate the possibility to use inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles, both composed of WS2, as hydrogen storage materials. These materials may allow hydrogen to be either chemi- or physisorbed inside their crystalline structure, inside hollow core of fullerenes/nanotubes or in the open interstitial pore spaces between the NP or nanotubes. While exposure to molecular hydrogen was found to have measurable but limited absorption rate: up to 1 weight % with the majority of hydrogen physisorbed, exposure of these materials to hydrogen activated by microwave (MW) plasma is supposed to be more effective: the flow of activated hydrogen interacts with the substrate which contains NP of IF and INT and substantial percentage of it may be chemisorbed. We report here the results of such exposures and analyze the absorption and diffusion of hydrogen by different methods ranging from simple weighting by analytical balances to absorption-desorption curves obtained by pressure temperature composition (PCT) apparatus, X-Ray Diffraction (XRD) investigation of interlayer distance changes induced by hydrogen penetration, and hydrogen depth profile measured by Secondary Ion Mass Spectroscopy (SIMS). We found that even very short exposure (5 min) to MW plasma at relatively mild conditions: low power and intermediate temperature (~ 400 °C) results in diffusion of hydrogen, though some etching of the substrate material may occur during the treatment. The ongoing research is aimed to determine the chemical state of the absorbed hydrogen as well as to optimize the parameters of MW hydrogen plasma as a hydrogen source.
In the present work we investigate friction and adhesion properties of microcrystalline diamond f... more In the present work we investigate friction and adhesion properties of microcrystalline diamond films grown on steel substrates with a Cr-N interlayer prepared at 500° C and 800° C. Scratch tests were performed at indentation loads of 1-4 N and sliding velocity of 0.08 mm/s. The friction coefficient and wear loss were assessed. The diamond film structure and composition are
Surface Review and Letters, 2000
In this study we report on absolute quantum photoyield (QPY) measurements from well-defined defec... more In this study we report on absolute quantum photoyield (QPY) measurements from well-defined defective diamond surfaces in the 140–200 nm spectral range. The effect of defects in polycrystalline diamond films on their photoemission properties is studied by intentionally introducing damage using room temperature 30 keV Xe + ion bombardment to doses ranging from 2×1013 to 2×1015 ions/cm 2. Ion bombardment results in a drastic degradation of the QPY, to less than 1% at 140 nm, even at the lowest implantation dose compared to ~11.5% measured for the unimplanted diamond film. Analysis of the ion-damaged diamond films is performed by photon-stimulated ion desorption (PSID) measurements of H +, high resolution C(KLL) Auger electron spectroscopy and X-ray photoelectron spectroscopy. These measurements reveal that the decay in photoemission is due to the gradual formation of nondiamond carbon in the near-surface region. This damage leads to a change of the electron affinity from negative to positive, as determined by secondary electron emission measurements. PSID measurements reveal that the ion-bombarded diamond films remain hydrogen-terminated. MW hydrogen plasma treatment results in complete regeneration of the photoemission properties for diamond films implanted to Xe + doses of up to 2×1014 cm -2; only partial recovery was obtained for films irradiated with a higher ion dose.
physica status solidi (a), 2002
ABSTRACT In the present work we study the processes involved in the interaction of molecular and ... more ABSTRACT In the present work we study the processes involved in the interaction of molecular and thermally activated oxygen with hydrogen-terminated and hydrogen-free polycrystalline diamond surfaces. As revealed by H+ photodesorption, near edge X-ray absorption fine structure and X-ray photoelectron spectroscopies, molecular oxygen does not adsorb onto the hydrogenated diamond surface; however, thermally activated oxygen does adsorb onto it. The dominant reaction pass leads to the formation of surface C–O–H bonds, although some abstraction of chemisorbed hydrogen happens as well. On the surface regions, where such an abstraction takes place, C=O (ads) bonds are mostly produced. The hydrogen-free diamond surface is considerably more reactive: the interaction of thermally activated oxygen with it produces C=O and C–O–C bonds, and unactivated oxygen molecules are adsorbed predominantly as C=O. The surface concentration of oxygen, however, is lower in the case of the exposure to unactivated molecules. As established by photon induced secondary electron emission (SEE) measurements, the hydrogen-terminated, oxygen-free diamond surface exhibits negative electron affinity and high SEE intensity. The adsorption of thermally activated oxygen results in a slightly positive electron affinity (PEA) (∼0.4 eV) and in a reduction in the intensity of SEE, whereas the electron emission properties of this sample exposed to molecular oxygen seem to be unaffected. The clean diamond surface has already a low SEE intensity along with a PEA of ∼1.2 eV. This is why the adsorption of molecular oxygen has little effect on its electron emission properties. A treatment of this surface with activated oxygen results in a further deterioration of the electron emission properties as revealed by a PEA of ∼1.7 eV and a poor SEE intensity.
Journal of Physics: Condensed Matter, 2005
The Journal of Chemical Physics, 2002
In this work we report on a study of the mechanism of O- electron stimulated desorption (ESD) fro... more In this work we report on a study of the mechanism of O- electron stimulated desorption (ESD) from hydrogenated and hydrogen-free polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4-22 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes: (i) total O- and H-
Journal of Applied Physics, 1995
ABSTRACT In the present work nucleation and growth of diamond by chemical vapor deposition (CVD) ... more ABSTRACT In the present work nucleation and growth of diamond by chemical vapor deposition (CVD) on highly oriented pyrolitic graphite (HOPG) and glassy carbon (GC) substrates have been investigated. These carbon substrates represent generic forms of well‐characterized ordered and disordered sp2 bonded carbon materials. The nature of the precursor to diamond CVD is assessed by studying nucleation and growth on substrates abraded with hard powders whose debris may act as initial growth centers, e.g., diamond and c‐BN, and hard powders onto which diamond CVD does not grow heteroepitaxially, e.g., alumina. Based on our experimental results it is concluded that the precursor to diamond nucleation may be debris left after the abrasion process and/or damage created preferentially on graphitic prism planes. A higher density of such damaged prism planes on GC than on HOPG resulted in a larger nucleation density on the former. Different morphologies of single particles deposited on HOPG and GC were found: well faceted on the former, and ball‐like on the later as determined by scanning electron microscopy. This difference is explained on the basis of a larger concentration of active carbon species present at the GC surface as compared to the HOPG surface. The additional source of carbon is from etching of the GC and HOPG substrates which, under the diamond CVD conditions used in the present study, is twice higher for GC. The thermal stability of continuous films deposited on HOPG is better than those deposited on GC. The presence of different carbon phases in the deposited material was investigated by micro‐Raman and scanning‐Auger electron spectroscopies. © 1995 American Institute of Physics.
Journal of Applied Physics, 2002
ABSTRACT In this work we investigate the influence of annealing and microwave (MW) hydrogen plasm... more ABSTRACT In this work we investigate the influence of annealing and microwave (MW) hydrogen plasma exposure of ion-beam-irradiated diamond film surfaces. In particular, we are interested in the recovery of secondary electron emission (SEE) and negative electron affinity (NEA) by removal of the damaged layer. To this aim, we correlate the SEE of variously treated Xe+ ion-damaged diamond films with their bonding structure in the near-surface region, as identified by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy. The 30 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of the near-surface region of a diamond film to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to MW hydrogen plasma treatment for 30 min produces NEA diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of positive electron affinity surfaces, and drastically degrades their electron emission properties. Prolonged (up to three hours) MW hydrogen plasma treatment of as-implanted diamond films gradually improves their crystal quality and results in a further increase of SEE intensity. The SEE intensity after three hours MW hydrogen plasma exposure of the ion-beam-irradiated films was found to be ∼50% above the value obtained for the as-deposited diamond films. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. Our results show that removal of damage from a highly disordered diamond surface and recovery of its electron emission properties are possible by MW hydrogen plasma. However, it is a slow process. This is most likely due to the very low etching rate of the low-level damage at the end of the ion beam range. © 2002 American Institute of Physics.
Journal of Applied Physics, 2001
Journal of Applied Physics, 2000
Journal of Applied Physics, 1997
ABSTRACT In the present work the effect of incident laser power on the Raman spectra of diamond i... more ABSTRACT In the present work the effect of incident laser power on the Raman spectra of diamond isolated particles and continuous films deposited on silicon and glassy carbon (GC) substrates by the chemical vapor deposition method is investigated. It is shown that the Raman line position measured for diamond particles shifts to lower wave numbers as a function of incident laser power. These shifts were most drastic for single particles deposited on GC that were examined using a Raman microprobe. In this case the diamond peak displayed a negative shift of ∼ 18 cm−1 when the laser power output was increased from 1 to 15 mW. The laser beam diameter was ∼ 2 μm and the diamond particle measured was 3–6 μm in diameter. Micro-Raman measurements of diamond particles deposited on a silicon substrate or continuous diamond films on GC display very small changes in the diamond Raman peak wavelength for the same laser power range. From our studies it is concluded that the negative shift of the Raman peak position is caused by laser-induced local heating of the irradiated diamond particles. The temperature under the laser spot was calculated from the intensity ratio of Stokes to anti-Stokes Raman lines measured as a function of laser power output. The Raman peak wavelength calculated for each temperature showed excellent agreement with our experimental results. The local temperature of an isolated diamond crystal on GC rises to ∼ 1000 K at 15 mW laser power output, whereas the temperature change of the continuous film on GC and of a single particle on silicon was in the 0–30 K range above room temperature for the same laser power output range. This difference in heating is explained on the basis of efficient heat dissipation through a large contact area between the deposited particles and the substrate surface in the case of single particles deposited on silicon or through grain boundaries in the case of the continuous film on GC. The inefficient heat dissipation from the isolated diamond particles on GC is related to the small contact area between the diamond crystals and the GC substrate as a result of etching during the deposition process and possibly to the presence of an amorphous component in the diamond crystals deposited. © 1997 American Institute of Physics.
Journal of Applied Physics, 1999
... A. Laikhtman, I. Gouzman, A. Hoffman, G. Comtet, L. Hellner, G. Dujardin. ... It is found tha... more ... A. Laikhtman, I. Gouzman, A. Hoffman, G. Comtet, L. Hellner, G. Dujardin. ... It is found that the binding energy in the implanted samples has a positive shift of 0.61 eV ... dose ion implantation was reflected by a sharp reduction in the intensity of the diamond core exciton peak and by ...