Ahmed DKHISSI - Academia.edu (original) (raw)
Papers by Ahmed DKHISSI
Journal of Polymer Science Part A, Oct 13, 2008
This article reports the synthesis and characterization of a new polythiophene derivative phenoxy... more This article reports the synthesis and characterization of a new polythiophene derivative phenoxy‐substituted, the poly[3‐(4‐octylphenoxy)thiophene] (POPOT). The oxidative polymerization was found to yield low molecular weight material, whereas a modified Grignard metathesis (GRIM) yielded polymers of high molecular weights. One‐ and two‐dimensional NMR indicated the latter to be highly regioregular. POPOTs exhibited higher thermal stabilities than equivalent alkoxy‐substituted polythiophenes and exhibited red shifts in the absorption spectra with respect to equivalent. The absorption spectra showed a red shifted λmax at 540 nm in tetrahydrofuran solutions and 580 nm in spin‐coated films, with respect to poly(3‐alkylthiophene)s. A further red shift of 40 nm in going from solution (540 nm) to solid states (580 nm) is correlated with results from density functional theory electronic structure calculations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7505–7516, 2008
Physical Chemistry Chemical Physics, 2012
A combined theoretical and experimental investigation of the electronic structure and optical pro... more A combined theoretical and experimental investigation of the electronic structure and optical properties of poly(3-hexylthiophene) (P3HT), poly[3-(4-octylphenyl)thiophene] (POPT) and poly[3-(4-octylphenoxy)thiophene] (POPOT) is reported. In comparison with P3HT, POPT and POPOT exhibit better stabilities and the presence of an oxygen atom and/or a phenyl ring in the side chains enhances conjugation. Quantum chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT to the new derivative POPOT. The knowledge of the structure of these polymers is of utmost importance in understanding their optical properties in different phases (solution and condensed phase). The calculations indicate that, in opposition to P3HT and POPT polymers where the introduction of alkyl chains and the pendant phenyl disturbs the planarity of the backbone of the conjugated segment, POPOT has a better degree of organization in both states: the conjugated chain remains planar even in the presence of the phenoxy groups. Finally, the exciton binding energy is evaluated for these polymers and allows us to conclude that the POPOT is a promising polymer for photovoltaic applications when compared to P3HT and POPT.
Applied Physics Letters, 2009
Density functional theory calculations are used to identify preferential sites for oxygen adsorpt... more Density functional theory calculations are used to identify preferential sites for oxygen adsorption on SiGe. It is shown that Ge atoms hinder O incorporation in their vicinity. Additionally, the silanone structure, known to be the key intermediate prior to silicon dioxide formation, is shown to be not favorable when close to a Ge site on the surface.
Journal of Physical Chemistry A, Feb 18, 2000
ABSTRACT
Journal of Physical Chemistry A, Apr 12, 2002
The H-bonding interaction between the oxopurine base hypoxanthine and water is investigated using... more The H-bonding interaction between the oxopurine base hypoxanthine and water is investigated using matrixisolation FT-IR spectroscopy combined with theoretical density functional theory and ab initio methods. For vibrational frequency predictions, only the DFT/B3-LYP method is used, while for the prediction of relative energies and H-bond interaction energies of the complexes, four different theoretical methods are compared, i.e., RHF//RHF, MP2//RHF, DFT//DFT, and MP2//DFT. The oxo-N1HsN7H, the oxo-N1HsN9H, and the hydroxy-N9H (rotamer with the hydroxy-H atom pointed toward the N1 atom) are found to be the three most stable tautomeric forms of the free base. Different, stable complexes between these three tautomers and water have been predicted theoretically. The experimental FT-IR spectra agree well with this prediction, and most of the characteristic spectral data for the four most stable closed complexes, N7sH‚‚‚OsH‚‚‚OdC6 and N1sH‚‚‚OsH‚‚‚OdC6 of the O17 tautomer and N1sH‚‚‚OsH‚‚‚OdC6 and N9sH‚‚‚OsH‚‚‚N3 of the O19 tautomer, have been identified in the spectrum. The closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect. The obtained results allow to analyze the important relation between cooperativity and (non-)linearity of NsH‚‚‚OH-bonded systems.
Journal of Physical Chemistry A, Nov 19, 1999
The H-bond interaction of the cytosine model compound 2-hydroxypyridine and its tautomer 2-oxopyr... more The H-bond interaction of the cytosine model compound 2-hydroxypyridine and its tautomer 2-oxopyridine with HCl is investigated using the combined matrix-isolation FT-IR and theoretical density functional and ab initio methods. The theoretical calculations have been carried out at the B3-LYP/6-31++G** and RHF/6-31++G** levels of theory. Different types of hydrogen-bonding have been found: two closed complexes of the proton transfer type, each containing two hydrogen bonds, i.e., N + sH‚‚‚Cl-‚‚‚HsO and CdO + s H‚‚‚Cl-‚‚‚HsN; two open complexes of intermediate strength, N‚‚‚H-Cl and CdO‚‚‚H-Cl; and one weak complex, HsO‚‚‚HsCl. The theoretical results indicate that the closed H-bonded complexes are the most stable systems for both the hydroxy and the oxo tautomers. The increased stability of these complexes is due to a cooperative H-bonding effect. The experimental spectra are consistent with this prediction, but the weaker complexes are also identified. A comparison of the experimental and calculated IR frequencies demonstrates that the frequency shifts of the vibrational modes directly involved in the H-bond interactions, especially the X-H stretching modes, are better predicted by the DFT method than by the RHF method. For the other vibrational modes not directly involved in the H-bonds, the RHF methodology has a similar accuracy compared to the DFT method.
Thin Solid Films, 2012
Density functional theory calculations are used to highlight some basics of the densification mec... more Density functional theory calculations are used to highlight some basics of the densification mechanisms arising during atomic layer deposition of HfO 2 onto silicon dioxide. The obtained results are discussed at the light of a multi-model approach that enables process simulation at the atomic scale via Kinetic Monte Carlo simulations. The impact of the proposed densification mechanisms on the growth is demonstrated. We show that a complete coverage is possible thanks to these mechanisms at a slow rate after that all surface reactive sites (OH sites) have been consumed by precursor molecules.
Journal of Molecular Spectroscopy, Apr 1, 1999
ABSTRACT
Synthetic Metals, Jul 1, 2011
The exciton binding energy (E b) is one of the heavily debated issues in conjugated polymer liter... more The exciton binding energy (E b) is one of the heavily debated issues in conjugated polymer literature. Most of the experimental studies suggest values around 0.3-0.4 eV for PPV. On the other hand, binding energies ranging from 0.06 eV to more than 1 eV have been reported as well. Time Dependent Density Functional Theory (TDDFT) is employed to calculate E b for PPV and polythiophenes and their derivatives in order to (i) to clarify the controversial debate of the magnitude of the exciton in PPV and (ii) to study the influence of the substituents on the exciton binding energy of conjugated polymers.
International Journal of Quantum Chemistry, 2008
The temperature (T) and density (ρ) conditions at which hydrogen undergoes a molecular-to-atomic ... more The temperature (T) and density (ρ) conditions at which hydrogen undergoes a molecular-to-atomic (MA) transition is crucial to our understanding of the gas-giant planets such as Jupiter and Saturn. First-principles (FP) calculations suggest that this transition is coincident with metallization and acts as a catalyst for hydrogen-helium demixing, which has significant consequences for models of planetary interiors. Prediction of this transition boundary has proven to be difficult using FP methods. In particular, detailed comparisons of finite temperature density functional theory (FT-DFT) calculations of the MA transition in both the high-T , low-ρ regime, where the transition is largely T driven, and the low-T , high-ρ regime, where the transition is largely ρ driven, suggest that the transition is very sensitive to the exchange-correlation (xc) functional used in the calculation. Here we present a detailed comparison of previous multiple-shock electrical conductivity measurements with FT-DFT calculations employing various xc functionals to probe a regime where both T and ρ play an important role in the transition. The measurement results are found to be inconsistent with the semilocal xc functional PBE and are in much better agreement with the nonlocal xc functionals vdW-DF1 and vdW-DF2. Furthermore, we show that the inconsistency with PBE likely stems from pressure errors associated with the PBE xc functional, resulting in calculated pressures that are too low at these T and ρ conditions. Together with previous comparisons at high-T , low-ρ and low-T , high-ρ these results provide a consistent picture for the MA transition over a wide T and ρ range. This picture may also provide insight into differences in experimental observations of the metallization of liquid hydrogen and deuterium in the low-T regime.
Journal of Chemical Physics, Jun 7, 2002
Synthetic Metals, Mar 1, 2009
Theoretical calculations are performed in the framework of the interaction between the charged po... more Theoretical calculations are performed in the framework of the interaction between the charged poly(ethylenedioxythiophene) (PEDOT 2+) and two p-toluensulfonic acid (TSA −1). The influence of the counterion on the charge distribution in the PEDOT is investigated indicating that a strong influence of the interionic correlation on the stability of PEDOT by TSA. Further several configurations are studied for the interaction between PEDOT and PSS. The calculations indicate that the side assembly is the most stable configuration, however in the presence of the solvent both parallel and side assemblies have similar stability. These results give a new insight about the charge transport conduction in PEDOT/PSS interactions.
Journal of Chemical Physics, Jan 22, 2002
The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge stora... more The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge storage mechanism upon doping with lithium atoms have been studied using a combined experimental–theoretical approach. The density of states in the valence band region was measured using ultraviolet photoelectron spectroscopy, and the spectra interpreted with the help of the results of quantum-chemical calculations carried out at the Austin Model 1/Valence Effective Hamiltonian level. The valence band spectra as well as the calculated band structure and derived density of valence states of poly(9,9-dioctylfluorene) are compared to those of poly(p-phenylene) and a ladder-type derivative of poly(p-phenylene). The deposition of lithium atoms, in situ, onto the surface of the polymer films resulted in uniform (bulk) diffusion of the lithium atoms, charge transfer to the polymer (resulting in lithium ions), and the appearance of new electronic states in the gap. At low doping levels (up to about 0.1 Li-atom per repeat unit of the polymer), these states are polarons, and, as the doping level increases, they evolve into bipolarons.
Chemistry: A European Journal, May 17, 2002
The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n 1 ... more The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n 1 ± 4) and their b,b'-unsubstituted analogues (Tn, n 1 ± 4) are described. Both series are end capped with phenyl groups to prevent irreversible a-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E PA1 and E PA2) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E PA1 and E PA2 versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E PA1 and E PA2 is reached already at relatively short chain lengths (% 5 EDOT units) in contrast to the Tn series (> 10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2. , T3. , EDOT2. , and EDOT3. revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy.
Journal of Chemical Theory and Computation, Oct 15, 2008
An original integrated approach developed within a multiscale strategy, which combines first-prin... more An original integrated approach developed within a multiscale strategy, which combines first-principles quantum simulations and kinetic Monte Carlo (KMC), is presented to investigate the atomic layer deposition (ALD) of HfO 2 on Si(100) surface. Density functional theory within the hybrid functional is used to determine the detailed physicochemical mechanisms and associated energetics of the two half cycles taking place during the initial stage of film growth. A kinetic Monte Carlo model is then proposed that deals with the stochastic nature of the calculated DFT mechanisms and barriers. Beyond the chemical information emanating from DFT calculations, the lattice-based KMC approach requires preliminary physical considerations issued from the crystal structures that the system is intended to adopt. This is especially critical in the case of heterogeneous systems like oxides deposited on silicon. We also describe (i) how atomistic configuration changes are performed as a result of local events consisting in elementary reaction mechanisms occurring on specific lattice sites, (ii) the temporal dynamics, governed by transition probabilities, calculated for every event from DFT activation barriers, and (iii) the relation of KMC with the ALD experimental procedure. Some preliminary validation results of the whole multiscale strategy are given for illustration and pertinence with regard of the technological main issues.
MRS Proceedings, 2003
ABSTRACTThe present paper establishes some required elements from both Quantum calculations and K... more ABSTRACTThe present paper establishes some required elements from both Quantum calculations and Kinetic Monte Carlo Modeling to perform full atomic scale simulations of Zirconia and Hafnia Atomic Layer Deposition (ALD) on Silicon technology process. In this view, we present quantum cluster calculations that investigate reaction pathways being part of the chemical reactions taking place at the different stages of the ALD growth. In particular, we detail ongoing research effort on the hydrolysis of adsorbed HfCl3 and ZrCl3 on ultra-thin SiO2. At very low water dose, the hydrolysis appears to be un-favourable. The complete reaction pathways with their associated activation barrier are detailed. We then show that actual available mechanisms emanating from quantum calculations are not sufficient to give a coherent picture of the layer structuring through a Kinetic Monte Carlo technique with the hope of giving new directions for further quantum studies.
ECS Meeting Abstracts, 2009
Microelectronic Engineering, 2011
A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surfa... more A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surface in the presence of germanium and oxygen atoms is reported. The energies and structures of the defect structures (i.e. vacancy and relaxed tetrahedral Si interstitial) are calculated using supercell with up to 88 atoms. We present results obtained using the generalized gradient approximation (GGA) for the exchange-correlation energy. We examine the effect of the presence of germanium and oxygen atoms on the stability of Frenkel pairs generated near the Si(1 0 0) surface by comparing a number of individual cases, starting from vacancy interstitial pairs situated at various positions. The general tendency of the created interstitials is to climb towards the surface, but they generally remain in subsurface layers, ready to migrate into the layer. This tendency is enhanced by the presence of the Ge and/or O atoms. We show that the formation energy is lower and Si interstitials can be created with energies as low as 1.5 eV.
Thin Solid Films, 2010
Calculations based on Density Functional Theory are carried out to study interstitial generation ... more Calculations based on Density Functional Theory are carried out to study interstitial generation close to the Si(100) surface with further consideration of effects related to the presence of substitutional Ge atoms on the surface. Defect structures, vacancy and Si interstitial, and associated energies are calculated. We observe that germanium atoms tend to increase the stability of created defects, promote the
Abstract Electronic structure and optical properties of the GaP/AIP superlattice are studied theo... more Abstract Electronic structure and optical properties of the GaP/AIP superlattice are studied theoretically. This superlattice, which is made of indirect/indirect materials, is seen to exhibit features very different from those of the original materials. An improved sp 3 s ∗ tight-binding method is employed to obtain the oscillator strength at the Γ-point folded from X-point as well as the electronic structure of the superlattice. Furthermore, the structural dependence of the electronic structure throughout the entire Brillouin zone and the oscillator strength at the Γ-point is clarified for the first time.
Journal of Polymer Science Part A, Oct 13, 2008
This article reports the synthesis and characterization of a new polythiophene derivative phenoxy... more This article reports the synthesis and characterization of a new polythiophene derivative phenoxy‐substituted, the poly[3‐(4‐octylphenoxy)thiophene] (POPOT). The oxidative polymerization was found to yield low molecular weight material, whereas a modified Grignard metathesis (GRIM) yielded polymers of high molecular weights. One‐ and two‐dimensional NMR indicated the latter to be highly regioregular. POPOTs exhibited higher thermal stabilities than equivalent alkoxy‐substituted polythiophenes and exhibited red shifts in the absorption spectra with respect to equivalent. The absorption spectra showed a red shifted λmax at 540 nm in tetrahydrofuran solutions and 580 nm in spin‐coated films, with respect to poly(3‐alkylthiophene)s. A further red shift of 40 nm in going from solution (540 nm) to solid states (580 nm) is correlated with results from density functional theory electronic structure calculations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7505–7516, 2008
Physical Chemistry Chemical Physics, 2012
A combined theoretical and experimental investigation of the electronic structure and optical pro... more A combined theoretical and experimental investigation of the electronic structure and optical properties of poly(3-hexylthiophene) (P3HT), poly[3-(4-octylphenyl)thiophene] (POPT) and poly[3-(4-octylphenoxy)thiophene] (POPOT) is reported. In comparison with P3HT, POPT and POPOT exhibit better stabilities and the presence of an oxygen atom and/or a phenyl ring in the side chains enhances conjugation. Quantum chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT to the new derivative POPOT. The knowledge of the structure of these polymers is of utmost importance in understanding their optical properties in different phases (solution and condensed phase). The calculations indicate that, in opposition to P3HT and POPT polymers where the introduction of alkyl chains and the pendant phenyl disturbs the planarity of the backbone of the conjugated segment, POPOT has a better degree of organization in both states: the conjugated chain remains planar even in the presence of the phenoxy groups. Finally, the exciton binding energy is evaluated for these polymers and allows us to conclude that the POPOT is a promising polymer for photovoltaic applications when compared to P3HT and POPT.
Applied Physics Letters, 2009
Density functional theory calculations are used to identify preferential sites for oxygen adsorpt... more Density functional theory calculations are used to identify preferential sites for oxygen adsorption on SiGe. It is shown that Ge atoms hinder O incorporation in their vicinity. Additionally, the silanone structure, known to be the key intermediate prior to silicon dioxide formation, is shown to be not favorable when close to a Ge site on the surface.
Journal of Physical Chemistry A, Feb 18, 2000
ABSTRACT
Journal of Physical Chemistry A, Apr 12, 2002
The H-bonding interaction between the oxopurine base hypoxanthine and water is investigated using... more The H-bonding interaction between the oxopurine base hypoxanthine and water is investigated using matrixisolation FT-IR spectroscopy combined with theoretical density functional theory and ab initio methods. For vibrational frequency predictions, only the DFT/B3-LYP method is used, while for the prediction of relative energies and H-bond interaction energies of the complexes, four different theoretical methods are compared, i.e., RHF//RHF, MP2//RHF, DFT//DFT, and MP2//DFT. The oxo-N1HsN7H, the oxo-N1HsN9H, and the hydroxy-N9H (rotamer with the hydroxy-H atom pointed toward the N1 atom) are found to be the three most stable tautomeric forms of the free base. Different, stable complexes between these three tautomers and water have been predicted theoretically. The experimental FT-IR spectra agree well with this prediction, and most of the characteristic spectral data for the four most stable closed complexes, N7sH‚‚‚OsH‚‚‚OdC6 and N1sH‚‚‚OsH‚‚‚OdC6 of the O17 tautomer and N1sH‚‚‚OsH‚‚‚OdC6 and N9sH‚‚‚OsH‚‚‚N3 of the O19 tautomer, have been identified in the spectrum. The closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect. The obtained results allow to analyze the important relation between cooperativity and (non-)linearity of NsH‚‚‚OH-bonded systems.
Journal of Physical Chemistry A, Nov 19, 1999
The H-bond interaction of the cytosine model compound 2-hydroxypyridine and its tautomer 2-oxopyr... more The H-bond interaction of the cytosine model compound 2-hydroxypyridine and its tautomer 2-oxopyridine with HCl is investigated using the combined matrix-isolation FT-IR and theoretical density functional and ab initio methods. The theoretical calculations have been carried out at the B3-LYP/6-31++G** and RHF/6-31++G** levels of theory. Different types of hydrogen-bonding have been found: two closed complexes of the proton transfer type, each containing two hydrogen bonds, i.e., N + sH‚‚‚Cl-‚‚‚HsO and CdO + s H‚‚‚Cl-‚‚‚HsN; two open complexes of intermediate strength, N‚‚‚H-Cl and CdO‚‚‚H-Cl; and one weak complex, HsO‚‚‚HsCl. The theoretical results indicate that the closed H-bonded complexes are the most stable systems for both the hydroxy and the oxo tautomers. The increased stability of these complexes is due to a cooperative H-bonding effect. The experimental spectra are consistent with this prediction, but the weaker complexes are also identified. A comparison of the experimental and calculated IR frequencies demonstrates that the frequency shifts of the vibrational modes directly involved in the H-bond interactions, especially the X-H stretching modes, are better predicted by the DFT method than by the RHF method. For the other vibrational modes not directly involved in the H-bonds, the RHF methodology has a similar accuracy compared to the DFT method.
Thin Solid Films, 2012
Density functional theory calculations are used to highlight some basics of the densification mec... more Density functional theory calculations are used to highlight some basics of the densification mechanisms arising during atomic layer deposition of HfO 2 onto silicon dioxide. The obtained results are discussed at the light of a multi-model approach that enables process simulation at the atomic scale via Kinetic Monte Carlo simulations. The impact of the proposed densification mechanisms on the growth is demonstrated. We show that a complete coverage is possible thanks to these mechanisms at a slow rate after that all surface reactive sites (OH sites) have been consumed by precursor molecules.
Journal of Molecular Spectroscopy, Apr 1, 1999
ABSTRACT
Synthetic Metals, Jul 1, 2011
The exciton binding energy (E b) is one of the heavily debated issues in conjugated polymer liter... more The exciton binding energy (E b) is one of the heavily debated issues in conjugated polymer literature. Most of the experimental studies suggest values around 0.3-0.4 eV for PPV. On the other hand, binding energies ranging from 0.06 eV to more than 1 eV have been reported as well. Time Dependent Density Functional Theory (TDDFT) is employed to calculate E b for PPV and polythiophenes and their derivatives in order to (i) to clarify the controversial debate of the magnitude of the exciton in PPV and (ii) to study the influence of the substituents on the exciton binding energy of conjugated polymers.
International Journal of Quantum Chemistry, 2008
The temperature (T) and density (ρ) conditions at which hydrogen undergoes a molecular-to-atomic ... more The temperature (T) and density (ρ) conditions at which hydrogen undergoes a molecular-to-atomic (MA) transition is crucial to our understanding of the gas-giant planets such as Jupiter and Saturn. First-principles (FP) calculations suggest that this transition is coincident with metallization and acts as a catalyst for hydrogen-helium demixing, which has significant consequences for models of planetary interiors. Prediction of this transition boundary has proven to be difficult using FP methods. In particular, detailed comparisons of finite temperature density functional theory (FT-DFT) calculations of the MA transition in both the high-T , low-ρ regime, where the transition is largely T driven, and the low-T , high-ρ regime, where the transition is largely ρ driven, suggest that the transition is very sensitive to the exchange-correlation (xc) functional used in the calculation. Here we present a detailed comparison of previous multiple-shock electrical conductivity measurements with FT-DFT calculations employing various xc functionals to probe a regime where both T and ρ play an important role in the transition. The measurement results are found to be inconsistent with the semilocal xc functional PBE and are in much better agreement with the nonlocal xc functionals vdW-DF1 and vdW-DF2. Furthermore, we show that the inconsistency with PBE likely stems from pressure errors associated with the PBE xc functional, resulting in calculated pressures that are too low at these T and ρ conditions. Together with previous comparisons at high-T , low-ρ and low-T , high-ρ these results provide a consistent picture for the MA transition over a wide T and ρ range. This picture may also provide insight into differences in experimental observations of the metallization of liquid hydrogen and deuterium in the low-T regime.
Journal of Chemical Physics, Jun 7, 2002
Synthetic Metals, Mar 1, 2009
Theoretical calculations are performed in the framework of the interaction between the charged po... more Theoretical calculations are performed in the framework of the interaction between the charged poly(ethylenedioxythiophene) (PEDOT 2+) and two p-toluensulfonic acid (TSA −1). The influence of the counterion on the charge distribution in the PEDOT is investigated indicating that a strong influence of the interionic correlation on the stability of PEDOT by TSA. Further several configurations are studied for the interaction between PEDOT and PSS. The calculations indicate that the side assembly is the most stable configuration, however in the presence of the solvent both parallel and side assemblies have similar stability. These results give a new insight about the charge transport conduction in PEDOT/PSS interactions.
Journal of Chemical Physics, Jan 22, 2002
The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge stora... more The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge storage mechanism upon doping with lithium atoms have been studied using a combined experimental–theoretical approach. The density of states in the valence band region was measured using ultraviolet photoelectron spectroscopy, and the spectra interpreted with the help of the results of quantum-chemical calculations carried out at the Austin Model 1/Valence Effective Hamiltonian level. The valence band spectra as well as the calculated band structure and derived density of valence states of poly(9,9-dioctylfluorene) are compared to those of poly(p-phenylene) and a ladder-type derivative of poly(p-phenylene). The deposition of lithium atoms, in situ, onto the surface of the polymer films resulted in uniform (bulk) diffusion of the lithium atoms, charge transfer to the polymer (resulting in lithium ions), and the appearance of new electronic states in the gap. At low doping levels (up to about 0.1 Li-atom per repeat unit of the polymer), these states are polarons, and, as the doping level increases, they evolve into bipolarons.
Chemistry: A European Journal, May 17, 2002
The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n 1 ... more The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n 1 ± 4) and their b,b'-unsubstituted analogues (Tn, n 1 ± 4) are described. Both series are end capped with phenyl groups to prevent irreversible a-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E PA1 and E PA2) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E PA1 and E PA2 versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E PA1 and E PA2 is reached already at relatively short chain lengths (% 5 EDOT units) in contrast to the Tn series (> 10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2. , T3. , EDOT2. , and EDOT3. revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy.
Journal of Chemical Theory and Computation, Oct 15, 2008
An original integrated approach developed within a multiscale strategy, which combines first-prin... more An original integrated approach developed within a multiscale strategy, which combines first-principles quantum simulations and kinetic Monte Carlo (KMC), is presented to investigate the atomic layer deposition (ALD) of HfO 2 on Si(100) surface. Density functional theory within the hybrid functional is used to determine the detailed physicochemical mechanisms and associated energetics of the two half cycles taking place during the initial stage of film growth. A kinetic Monte Carlo model is then proposed that deals with the stochastic nature of the calculated DFT mechanisms and barriers. Beyond the chemical information emanating from DFT calculations, the lattice-based KMC approach requires preliminary physical considerations issued from the crystal structures that the system is intended to adopt. This is especially critical in the case of heterogeneous systems like oxides deposited on silicon. We also describe (i) how atomistic configuration changes are performed as a result of local events consisting in elementary reaction mechanisms occurring on specific lattice sites, (ii) the temporal dynamics, governed by transition probabilities, calculated for every event from DFT activation barriers, and (iii) the relation of KMC with the ALD experimental procedure. Some preliminary validation results of the whole multiscale strategy are given for illustration and pertinence with regard of the technological main issues.
MRS Proceedings, 2003
ABSTRACTThe present paper establishes some required elements from both Quantum calculations and K... more ABSTRACTThe present paper establishes some required elements from both Quantum calculations and Kinetic Monte Carlo Modeling to perform full atomic scale simulations of Zirconia and Hafnia Atomic Layer Deposition (ALD) on Silicon technology process. In this view, we present quantum cluster calculations that investigate reaction pathways being part of the chemical reactions taking place at the different stages of the ALD growth. In particular, we detail ongoing research effort on the hydrolysis of adsorbed HfCl3 and ZrCl3 on ultra-thin SiO2. At very low water dose, the hydrolysis appears to be un-favourable. The complete reaction pathways with their associated activation barrier are detailed. We then show that actual available mechanisms emanating from quantum calculations are not sufficient to give a coherent picture of the layer structuring through a Kinetic Monte Carlo technique with the hope of giving new directions for further quantum studies.
ECS Meeting Abstracts, 2009
Microelectronic Engineering, 2011
A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surfa... more A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surface in the presence of germanium and oxygen atoms is reported. The energies and structures of the defect structures (i.e. vacancy and relaxed tetrahedral Si interstitial) are calculated using supercell with up to 88 atoms. We present results obtained using the generalized gradient approximation (GGA) for the exchange-correlation energy. We examine the effect of the presence of germanium and oxygen atoms on the stability of Frenkel pairs generated near the Si(1 0 0) surface by comparing a number of individual cases, starting from vacancy interstitial pairs situated at various positions. The general tendency of the created interstitials is to climb towards the surface, but they generally remain in subsurface layers, ready to migrate into the layer. This tendency is enhanced by the presence of the Ge and/or O atoms. We show that the formation energy is lower and Si interstitials can be created with energies as low as 1.5 eV.
Thin Solid Films, 2010
Calculations based on Density Functional Theory are carried out to study interstitial generation ... more Calculations based on Density Functional Theory are carried out to study interstitial generation close to the Si(100) surface with further consideration of effects related to the presence of substitutional Ge atoms on the surface. Defect structures, vacancy and Si interstitial, and associated energies are calculated. We observe that germanium atoms tend to increase the stability of created defects, promote the
Abstract Electronic structure and optical properties of the GaP/AIP superlattice are studied theo... more Abstract Electronic structure and optical properties of the GaP/AIP superlattice are studied theoretically. This superlattice, which is made of indirect/indirect materials, is seen to exhibit features very different from those of the original materials. An improved sp 3 s ∗ tight-binding method is employed to obtain the oscillator strength at the Γ-point folded from X-point as well as the electronic structure of the superlattice. Furthermore, the structural dependence of the electronic structure throughout the entire Brillouin zone and the oscillator strength at the Γ-point is clarified for the first time.