Determining the Fermi level position for a given material is important to understand many of its ... more Determining the Fermi level position for a given material is important to understand many of its electronic and chemical properties. Ab initio methods are effective in computing Fermi levels when using charge neutral supercells. However, in the case where charges are explicitly included, the compensating homogeneous background charge, which is necessary to maintain charge neutrality in periodic models, causes the vacuum potential to be ill-defined, which would otherwise have been a reliable reference potential. Here, we develop a method based on recursively integrating the density of states to determine shifts in the Fermi level with charge. By introducing incremental charges, one can compute the density of states profile and determine the shift in the Fermi level that corresponds to adding or removing a given increment of charge δq, which allows the evaluation of the Fermi level for any arbitrary charge q. We test this method for a range of materials (graphene, h-BN, C 3 N 4 , Cu and MoS 2), and demonstrate the that this method can produce a reasonable agreement with models that rely on localized compensating background charges. Graphical TOC Entry Evaluating Fermi level shifts for charged slabs is problematic within periodic DFT framework. Here, a new method based on integrating the density of states is used to mitigate problems associated with ill-defined electrostatic potentials. 2
The creation of multiple emission pathways in quantum dots (QDs) is an exciting prospect with fun... more The creation of multiple emission pathways in quantum dots (QDs) is an exciting prospect with fundamental interest and optoelectronic potential.
The interface at metal oxide-carbon hybrid heterojunction is the source to 'synergistic effect' i... more The interface at metal oxide-carbon hybrid heterojunction is the source to 'synergistic effect' in catalysis. Understanding the structure-function property is key for designing more advanced catalyst-support systems. Using a model Mn III-O x single-layer catalyst on carbon, we herein report a full elucidation to the catalytic synergism at hybrid heterojunction in the oxygen reduction reaction (ORR). A successful fabrication of the single-layer catalyst from bottom-up is fully characterized by X-ray absorption fine structure (XAFS) and high-resolution transmission electron microscopy (HR-TEM). For oxygen electrocatalysis over this model hybrid heterostructure, our results, from both theory and experiment, show that the synergistic ORR truly undergoes a cooperated two-step electrocatalysis with catalytic promotion (∆E onset = 60 mV) near the heterojunction and over the single-layer catalyst through an interfacial electronic interplay, rather than an abstruse transition towards a one-step dissociative pathway. Finally, we report a superior peroxide-reducing activity of 432.5 mA cm-2 mg (M)-1 over our Mn III-O x single-layer catalyst.
Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin... more Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin degrees of freedom, can host various phase transitions, which is particularly interesting if these transitions can be tuned by external fields. Recently, ferromagnetism was found together with a seemingly insulating state in superlattices of manganites and titanates. We therefore study the (111) oriented superlattices by means of ab initio calculations, predicting a ferromagnetic ground state due to double exchange in all cases. We shed light on the ferromagnetic coupling in the LaMnO 3 region and at the interfaces. The insulating states of specific superlattices can be understood on the basis of Jahn-Teller modes and electron/hole doping.
Versatile superstructures composed of nanoparticles have recently been prepared using various dis... more Versatile superstructures composed of nanoparticles have recently been prepared using various disassembly methods. However, little information is known on how the structural disassembly influences the catalytic performance of the materials. Here we show how the disassembly of an ordered porous La0.6Sr0.4MnO3 perovskite array, to give hexapod mesostructured nanoparticles, exposes a new crystal facet which is more active for catalytic methane combustion. On fragmenting three-dimensionally ordered macroporous (3DOM) structures in a controlled manner, via a process that has been likened to retrosynthesis, hexapod-shaped building blocks can be harvested which possess a mesostructured architecture. The hexapod-shaped perovskite catalyst exhibits excellent low temperature methane oxidation activity (T90%=438 °C; reaction rate=4.84 × 10(-7) mol m(-2) s(-1)). First principle calculations suggest the fractures, which occur at weak joints within the 3DOM architecture, afford a large area of (0...
Using ab initio calculations, we investigate the effect of hydrostatic pressure on the electronic... more Using ab initio calculations, we investigate the effect of hydrostatic pressure on the electronic structure of LaMnO 3 (111) bilayers sandwiched between LaAlO 3. In the ideal heterostructure we observe Dirac cones at the Fermi energy. However, octahedral tiltings open a band gap and thus destroy the Dirac nature. We show that the effect of the tiltings can be suppressed by hydrostatic pressure from 40 GPa to 60 GPa. At higher pressure further phase transitions are encountered.
International Journal of New Computer Architectures and their Applications, 2014
Artificial muscles are one of the most important applications of the technology of flexible struc... more Artificial muscles are one of the most important applications of the technology of flexible structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of modeling and usage of artificial muscles. Our application is based on three IPMC muscles used to simulate a three fingers grasp. The magnitude of the voltage is utilized to compute the buildup columbic load using the capacitor block in the model.
Although the equilibrium phase diagram predicts that alloys in the central part of the V-Zr syste... more Although the equilibrium phase diagram predicts that alloys in the central part of the V-Zr system should consist of V 2 Zr Laves phase with partial segregation of one element, it is known that under non-equilibrium conditions these materials can form amorphous structures. Here we examine the structures and stabilities of thin film V-Zr alloys deposited at room temperature by magnetron sputtering. The films were characterized by X-ray diffraction, transmission electron microscopy and computational methods. Atomic scale modelling was used to investigate the enthalpies of formation of the various competing structures. The calculations confirmed that an amorphous solid solution would be significantly more stable than a random body-centred solid solution of the elements, in agreement with the experimental results. In addition, the modelling effort provided insight into the probable atomic configurations of the amorphous structures allowing predictions of the average distance to first and second nearest neighbours in the system.
Density functional based simulation, corrected for finite size effects, is used to investigate sy... more Density functional based simulation, corrected for finite size effects, is used to investigate systematically the formation of antisite defects in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb). Different charge states are modelled as a function of the Fermi level and under different growth conditions. The formation energies of group III antisites (IIIVq) decrease with increasing covalent radius of the group V atom though not group III radius, whereas group V antisites (VIIIq) show a consistent decrease in formation energies with increase in group III and group V covalent radii. In general, IIIVq defects dominate under III-rich conditions and VIIIq under V-rich conditions. Comparison with equivalent vacancy formation energy simulations shows that while antisite concentrations are always dominant under stoichiometric conditions, modest variation in growth or doping conditions can lead to a significantly higher concentration of vacancies.
The interstitial diffusion mechanism of Pd in Ge that has a migration energy barrier of only 0.03... more The interstitial diffusion mechanism of Pd in Ge that has a migration energy barrier of only 0.03 eV.
The diffusion of phosphorous in germanium is very fast, requiring point defect engineering strate... more The diffusion of phosphorous in germanium is very fast, requiring point defect engineering strategies to retard it in support of technological application. Density functional theory corroborated with hybrid density functional calculations are used to investigate the influence of the isovalent codopants tin and hafnium in the migration of phosphorous via the vacancy-mediated diffusion process. The migration energy barriers for phosphorous are increased significantly in the presence of oversized isovalent codopants. Therefore, it is proposed that tin and in particular hafnium codoping are efficient point defect engineering strategies to retard phosphorous migration.
Density functional theory calculations (DFT) are used to investigate the strain-induced changes t... more Density functional theory calculations (DFT) are used to investigate the strain-induced changes to the electronic structure of biaxially strained (parallel to the (001), (110) and (111) planes) and uniaxially strained (along the [001], [110] and [111] directions) germanium (Ge). It is calculated that a moderate uniaxial strain parallel to the [111] direction can efficiently transform Ge to a direct bandgap material with a bandgap energy useful for technological applications.
In germanium, phosphorous and antimony diffuse quickly and as such their transport must be contro... more In germanium, phosphorous and antimony diffuse quickly and as such their transport must be controlled in order to design efficient n-typed doped regions. Here, density functional theory based calculations are used to predict the influence of double donor co-doping on the migration activation energies of vacancy-mediated diffusion processes. The migration energy barriers for phosphorous and antimony were found to be increased significantly when larger clusters involving two donor atoms and a vacancy were formed. These clusters are energetically stable and can lead to the formation of even larger clusters involving a number of donor atoms around a vacancy, thereby affecting the properties of devices.
Using electronic structure calculations, we systematically investigate the formation of vacancies... more Using electronic structure calculations, we systematically investigate the formation of vacancies in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb), for a range of charges (−3≤q≤3) as a function of the Fermi level and under different growth conditions. The formation energies were corrected using the scheme due to Freysoldt et al. [Phys. Rev. Lett. 102, 016402 (2009)] to account for finite size effects. Vacancy formation energies were found to decrease as the size of the group V atom increased. This trend was maintained for Al-V, Ga-V, and In-V compounds. The negative-U effect was only observed for the arsenic vacancy in GaAs, which makes a charge state transition from +1 to –1. It is also found that even under group III rich conditions, group III vacancies dominate in AlSb and GaSb. For InSb, group V vacancies are favoured even under group V rich conditions.
Density functional theory calculations are used to investigate the formation and diffusion of tin... more Density functional theory calculations are used to investigate the formation and diffusion of tin-vacancy pairs (SnV) in germanium (Ge). Depending upon the Fermi energy, SnV pairs can form in neutral, singly negative, or doubly negative charged states. The activation energies of diffusion, also as function of the Fermi energy, are calculated to lie between 2.48-3.65 eV, in agreement with and providing an interpretation of available experimental work.
The significant diffusion of Ga under Ga-rich conditions in GaAs and GaSb is counter intuitive as... more The significant diffusion of Ga under Ga-rich conditions in GaAs and GaSb is counter intuitive as the concentration of Ga vacancies should be depressed although Ga vacancies are necessary to interpret the experimental evidence for Ga transport. To reconcile the existence of Ga vacancies under Ga-rich conditions, transformation reactions have been proposed. Here, density functional theory is employed to calculate the formation energies of vacancies on both sublattices and the migration energy barriers to overcome the formation of the vacancy-antisite defect. Transformation reactions enhance the vacancy concentration in both materials and migration energy barriers indicate that Ga vacancies will dominate.
The Third International Conference on e-Technologies and Networks for Development (ICeND2014), 2014
Artificial muscles are one of the most important applications of the technology of flexible elect... more Artificial muscles are one of the most important applications of the technology of flexible electronic structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of using and modeling artificial muscles.
International Journal of New Computer Architectures and their Applications (IJNCAA), 2014
Artificial muscles are one of the most important applications of the technology of flexible struc... more Artificial muscles are one of the most important applications of the technology of flexible structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of modeling and usage of artificial muscles. Our application is based on three IPMC muscles used to simulate a three fingers grasp. The magnitude of the voltage is utilized to compute the buildup columbic load using the capacitor block in the model.
International Journal of New Computer Architectures and their Applications, 2014
Artificial muscles are one of the most important applications of the technology of flexible struc... more Artificial muscles are one of the most important applications of the technology of flexible structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of modeling and usage of artificial muscles. Our application is based on three IPMC muscles used to simulate a three fingers grasp. The magnitude of the voltage is utilized to compute the buildup columbic load using the capacitor block in the model.
Determining the Fermi level position for a given material is important to understand many of its ... more Determining the Fermi level position for a given material is important to understand many of its electronic and chemical properties. Ab initio methods are effective in computing Fermi levels when using charge neutral supercells. However, in the case where charges are explicitly included, the compensating homogeneous background charge, which is necessary to maintain charge neutrality in periodic models, causes the vacuum potential to be ill-defined, which would otherwise have been a reliable reference potential. Here, we develop a method based on recursively integrating the density of states to determine shifts in the Fermi level with charge. By introducing incremental charges, one can compute the density of states profile and determine the shift in the Fermi level that corresponds to adding or removing a given increment of charge δq, which allows the evaluation of the Fermi level for any arbitrary charge q. We test this method for a range of materials (graphene, h-BN, C 3 N 4 , Cu and MoS 2), and demonstrate the that this method can produce a reasonable agreement with models that rely on localized compensating background charges. Graphical TOC Entry Evaluating Fermi level shifts for charged slabs is problematic within periodic DFT framework. Here, a new method based on integrating the density of states is used to mitigate problems associated with ill-defined electrostatic potentials. 2
The creation of multiple emission pathways in quantum dots (QDs) is an exciting prospect with fun... more The creation of multiple emission pathways in quantum dots (QDs) is an exciting prospect with fundamental interest and optoelectronic potential.
The interface at metal oxide-carbon hybrid heterojunction is the source to 'synergistic effect' i... more The interface at metal oxide-carbon hybrid heterojunction is the source to 'synergistic effect' in catalysis. Understanding the structure-function property is key for designing more advanced catalyst-support systems. Using a model Mn III-O x single-layer catalyst on carbon, we herein report a full elucidation to the catalytic synergism at hybrid heterojunction in the oxygen reduction reaction (ORR). A successful fabrication of the single-layer catalyst from bottom-up is fully characterized by X-ray absorption fine structure (XAFS) and high-resolution transmission electron microscopy (HR-TEM). For oxygen electrocatalysis over this model hybrid heterostructure, our results, from both theory and experiment, show that the synergistic ORR truly undergoes a cooperated two-step electrocatalysis with catalytic promotion (∆E onset = 60 mV) near the heterojunction and over the single-layer catalyst through an interfacial electronic interplay, rather than an abstruse transition towards a one-step dissociative pathway. Finally, we report a superior peroxide-reducing activity of 432.5 mA cm-2 mg (M)-1 over our Mn III-O x single-layer catalyst.
Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin... more Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin degrees of freedom, can host various phase transitions, which is particularly interesting if these transitions can be tuned by external fields. Recently, ferromagnetism was found together with a seemingly insulating state in superlattices of manganites and titanates. We therefore study the (111) oriented superlattices by means of ab initio calculations, predicting a ferromagnetic ground state due to double exchange in all cases. We shed light on the ferromagnetic coupling in the LaMnO 3 region and at the interfaces. The insulating states of specific superlattices can be understood on the basis of Jahn-Teller modes and electron/hole doping.
Versatile superstructures composed of nanoparticles have recently been prepared using various dis... more Versatile superstructures composed of nanoparticles have recently been prepared using various disassembly methods. However, little information is known on how the structural disassembly influences the catalytic performance of the materials. Here we show how the disassembly of an ordered porous La0.6Sr0.4MnO3 perovskite array, to give hexapod mesostructured nanoparticles, exposes a new crystal facet which is more active for catalytic methane combustion. On fragmenting three-dimensionally ordered macroporous (3DOM) structures in a controlled manner, via a process that has been likened to retrosynthesis, hexapod-shaped building blocks can be harvested which possess a mesostructured architecture. The hexapod-shaped perovskite catalyst exhibits excellent low temperature methane oxidation activity (T90%=438 °C; reaction rate=4.84 × 10(-7) mol m(-2) s(-1)). First principle calculations suggest the fractures, which occur at weak joints within the 3DOM architecture, afford a large area of (0...
Using ab initio calculations, we investigate the effect of hydrostatic pressure on the electronic... more Using ab initio calculations, we investigate the effect of hydrostatic pressure on the electronic structure of LaMnO 3 (111) bilayers sandwiched between LaAlO 3. In the ideal heterostructure we observe Dirac cones at the Fermi energy. However, octahedral tiltings open a band gap and thus destroy the Dirac nature. We show that the effect of the tiltings can be suppressed by hydrostatic pressure from 40 GPa to 60 GPa. At higher pressure further phase transitions are encountered.
International Journal of New Computer Architectures and their Applications, 2014
Artificial muscles are one of the most important applications of the technology of flexible struc... more Artificial muscles are one of the most important applications of the technology of flexible structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of modeling and usage of artificial muscles. Our application is based on three IPMC muscles used to simulate a three fingers grasp. The magnitude of the voltage is utilized to compute the buildup columbic load using the capacitor block in the model.
Although the equilibrium phase diagram predicts that alloys in the central part of the V-Zr syste... more Although the equilibrium phase diagram predicts that alloys in the central part of the V-Zr system should consist of V 2 Zr Laves phase with partial segregation of one element, it is known that under non-equilibrium conditions these materials can form amorphous structures. Here we examine the structures and stabilities of thin film V-Zr alloys deposited at room temperature by magnetron sputtering. The films were characterized by X-ray diffraction, transmission electron microscopy and computational methods. Atomic scale modelling was used to investigate the enthalpies of formation of the various competing structures. The calculations confirmed that an amorphous solid solution would be significantly more stable than a random body-centred solid solution of the elements, in agreement with the experimental results. In addition, the modelling effort provided insight into the probable atomic configurations of the amorphous structures allowing predictions of the average distance to first and second nearest neighbours in the system.
Density functional based simulation, corrected for finite size effects, is used to investigate sy... more Density functional based simulation, corrected for finite size effects, is used to investigate systematically the formation of antisite defects in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb). Different charge states are modelled as a function of the Fermi level and under different growth conditions. The formation energies of group III antisites (IIIVq) decrease with increasing covalent radius of the group V atom though not group III radius, whereas group V antisites (VIIIq) show a consistent decrease in formation energies with increase in group III and group V covalent radii. In general, IIIVq defects dominate under III-rich conditions and VIIIq under V-rich conditions. Comparison with equivalent vacancy formation energy simulations shows that while antisite concentrations are always dominant under stoichiometric conditions, modest variation in growth or doping conditions can lead to a significantly higher concentration of vacancies.
The interstitial diffusion mechanism of Pd in Ge that has a migration energy barrier of only 0.03... more The interstitial diffusion mechanism of Pd in Ge that has a migration energy barrier of only 0.03 eV.
The diffusion of phosphorous in germanium is very fast, requiring point defect engineering strate... more The diffusion of phosphorous in germanium is very fast, requiring point defect engineering strategies to retard it in support of technological application. Density functional theory corroborated with hybrid density functional calculations are used to investigate the influence of the isovalent codopants tin and hafnium in the migration of phosphorous via the vacancy-mediated diffusion process. The migration energy barriers for phosphorous are increased significantly in the presence of oversized isovalent codopants. Therefore, it is proposed that tin and in particular hafnium codoping are efficient point defect engineering strategies to retard phosphorous migration.
Density functional theory calculations (DFT) are used to investigate the strain-induced changes t... more Density functional theory calculations (DFT) are used to investigate the strain-induced changes to the electronic structure of biaxially strained (parallel to the (001), (110) and (111) planes) and uniaxially strained (along the [001], [110] and [111] directions) germanium (Ge). It is calculated that a moderate uniaxial strain parallel to the [111] direction can efficiently transform Ge to a direct bandgap material with a bandgap energy useful for technological applications.
In germanium, phosphorous and antimony diffuse quickly and as such their transport must be contro... more In germanium, phosphorous and antimony diffuse quickly and as such their transport must be controlled in order to design efficient n-typed doped regions. Here, density functional theory based calculations are used to predict the influence of double donor co-doping on the migration activation energies of vacancy-mediated diffusion processes. The migration energy barriers for phosphorous and antimony were found to be increased significantly when larger clusters involving two donor atoms and a vacancy were formed. These clusters are energetically stable and can lead to the formation of even larger clusters involving a number of donor atoms around a vacancy, thereby affecting the properties of devices.
Using electronic structure calculations, we systematically investigate the formation of vacancies... more Using electronic structure calculations, we systematically investigate the formation of vacancies in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb), for a range of charges (−3≤q≤3) as a function of the Fermi level and under different growth conditions. The formation energies were corrected using the scheme due to Freysoldt et al. [Phys. Rev. Lett. 102, 016402 (2009)] to account for finite size effects. Vacancy formation energies were found to decrease as the size of the group V atom increased. This trend was maintained for Al-V, Ga-V, and In-V compounds. The negative-U effect was only observed for the arsenic vacancy in GaAs, which makes a charge state transition from +1 to –1. It is also found that even under group III rich conditions, group III vacancies dominate in AlSb and GaSb. For InSb, group V vacancies are favoured even under group V rich conditions.
Density functional theory calculations are used to investigate the formation and diffusion of tin... more Density functional theory calculations are used to investigate the formation and diffusion of tin-vacancy pairs (SnV) in germanium (Ge). Depending upon the Fermi energy, SnV pairs can form in neutral, singly negative, or doubly negative charged states. The activation energies of diffusion, also as function of the Fermi energy, are calculated to lie between 2.48-3.65 eV, in agreement with and providing an interpretation of available experimental work.
The significant diffusion of Ga under Ga-rich conditions in GaAs and GaSb is counter intuitive as... more The significant diffusion of Ga under Ga-rich conditions in GaAs and GaSb is counter intuitive as the concentration of Ga vacancies should be depressed although Ga vacancies are necessary to interpret the experimental evidence for Ga transport. To reconcile the existence of Ga vacancies under Ga-rich conditions, transformation reactions have been proposed. Here, density functional theory is employed to calculate the formation energies of vacancies on both sublattices and the migration energy barriers to overcome the formation of the vacancy-antisite defect. Transformation reactions enhance the vacancy concentration in both materials and migration energy barriers indicate that Ga vacancies will dominate.
The Third International Conference on e-Technologies and Networks for Development (ICeND2014), 2014
Artificial muscles are one of the most important applications of the technology of flexible elect... more Artificial muscles are one of the most important applications of the technology of flexible electronic structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of using and modeling artificial muscles.
International Journal of New Computer Architectures and their Applications (IJNCAA), 2014
Artificial muscles are one of the most important applications of the technology of flexible struc... more Artificial muscles are one of the most important applications of the technology of flexible structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of modeling and usage of artificial muscles. Our application is based on three IPMC muscles used to simulate a three fingers grasp. The magnitude of the voltage is utilized to compute the buildup columbic load using the capacitor block in the model.
International Journal of New Computer Architectures and their Applications, 2014
Artificial muscles are one of the most important applications of the technology of flexible struc... more Artificial muscles are one of the most important applications of the technology of flexible structure, designed to imitate human or animal muscles and to monitor its activities and conditions. The aim of our paper is to study the different possibilities of modeling and usage of artificial muscles. Our application is based on three IPMC muscles used to simulate a three fingers grasp. The magnitude of the voltage is utilized to compute the buildup columbic load using the capacitor block in the model.
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Papers by H. Tahini