We have studied eight collinear and non-collinear magnetic orientations of GdB4 using the GGA + U... more We have studied eight collinear and non-collinear magnetic orientations of GdB4 using the GGA + U method, without and with spin-orbit coupling, for values of U - J between 0 and 6. For U -- J = 6, the value which had been found to yield the correct Gd lattice constants, we obtain GdB4 lattice constants within 0.26% of experiment. We find the magnetization lies in-plane but is collinear, in disagreement with the most recent experimental determination.
We have studied eight collinear and noncollinear magnetic orientations of GdB4 using the GGA+U me... more We have studied eight collinear and noncollinear magnetic orientations of GdB4 using the GGA+U method, without and with spin-orbit coupling, for values of U-J between 0 and 6. For U-J=6 , the value which had been found to yield the correct Gd lattice constants, we obtain GdB4 lattice constants within 0.26% of experiment. We find that the magnetization lies in
Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectr... more Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes to have certain electronic properties. Unfortunately, at present available semiconductors do not meet all these criteria. So, a thorough understanding of band-engineering for mixed alloys is necessary to successfully design these photoelectrodes. Among the semiconductors, transition metal oxides are of particular interest due to their low cost and relatively high stability in aqueous media. Here, we will present a theoretical study of delafossite-alloys for PEC photo-electrodes. Previous studies have indicated that the group IIIA delafossite family (CuMO2, M = Al, Ga, In) do not exhibit direct band gaps. Their fundamental band gaps are significantly smaller than their reported optical band gaps. On the other hand group IIIB delafossite family (CuMO2, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO2, band gaps are above 3.00 eV. However, both of these two families exhibit p-type conductivity. We will show that by appropriate alloying of these two delafossite-families we can tune their band gaps and other opto-electronic properties. These types of alloying are desirable, as these introduce no localized impurity states in the band gap due to isovalent alloying. Also, the electronic effective masses can be lowered by selective doping of main group elements. Finally, it will be discussed that, lowering the symmetry constraints of these alloys would enhance their optical absorption properties. We'll also discuss that alloying with other 3d metal elements may decrease the band gap, but would increase the effective masses of the photo-electrons.
Efficient photoelectrochemical H2 production by solar irradiation depends not only on the photoca... more Efficient photoelectrochemical H2 production by solar irradiation depends not only on the photocatalyst's band gap and its band-edge positions but also on the detailed electronic nature of the bands, such as the localization or delocalization of the band edges and their orbital characteristics. These determine the carrier transport properties, reactivity, light absorption strength, etc. and significantly impact the material's efficiency as a photoconverter. The localization or delocalization of the band edges may arise either due to the orbital nature of the bands or the structural morphology of the material. A recent experimental report on a photocatalyst based on s/p orbitals showed very poor performance for H2 production despite the delocalized nature of the s/p bands as compared to the d-bands of transition metal oxides. It is then important to examine whether this poor performance is inherent to these materials or rather arises from some experimental limitations. A theoretical analysis by first-principle methods is well suited to shed light on this question.
Electronic structure of ZnO:GaN compounds: Asymmetric bandgap engineering. Muhammad N. Huda * , Y... more Electronic structure of ZnO:GaN compounds: Asymmetric bandgap engineering. Muhammad N. Huda * , Yanfa Yan, Su-Huai Wei, and Mowafak M. Al-Jassim National Renewable Energy Laboratory, Golden, Colorado 80401, USA. ...
... T. Bak, J. Nowotny, M. Rekas, and CC Sorrell, Int. J. Hydrogen Energy 27, 991 (2002). E. Frad... more ... T. Bak, J. Nowotny, M. Rekas, and CC Sorrell, Int. J. Hydrogen Energy 27, 991 (2002). E. Fradkin, SA Kivelson, and V. Oganesyan, Science 315, 196 (2007). M. Gräzel, Nature 414, 338 (2001). MN Huda, Y. Yan, S.-H. Wei, and MM Al-Jassim, Phys. Rev. B 78, 195204 (2008). ...
The prospect of solar energy as a renewable resource is ever-increasing. Density functional theor... more The prospect of solar energy as a renewable resource is ever-increasing. Density functional theory (DFT) calculations can elicit reliable behavior predictions in energy conversion materials to achieve higher efficiencies. Chemical stability of the photo-catalysts in aqueous solution is of particular interest for its long term performances. The bulk modulus is a mechanical property that is a good indicator of material stability. GaP has a low band gap and is a good candidate for use as a photocatalyst for hydrogen evolution by splitting water. Unfortunately, it is not stable and highly susceptible to corrosion over a very short time period, making it unfeasible for long-term use. GaN has too high of a band gap but a good stability factor. While these materials both possess desirable qualities, they cannot be used solitarily. We will report electronic properties and bulk moduli from the total energy calculations of the zincblende and wurtzite species using DFT-GGA and DFT+U as a function of doping concentration x. We will also present the density of states and charge density distribution of the alloy materials to study the localization/delocalization effects of N defects levels and their impact on the alloys' stability.
Among iron oxides, α-Fe2O3 is the most abundant on earth. Because it has a band gap of approximat... more Among iron oxides, α-Fe2O3 is the most abundant on earth. Because it has a band gap of approximately 2 eV, it is stable and inexpensive to process, α-Fe2O3 has been considered as a potential photoelectrocatalyst for solar driven photoelectrochemical (PEC) water splitting to make hydrogen. However, as α-Fe2O3 is a charge-transfer type insulator, the poor conduction properties have limited its efficiency as a PEC material. We will present our study on the doping of group IIIA elements in α-Fe2O3 to improve its performance. All the calculations were done with DFT+U. The main electronic features of α-Fe2O3 remained almost unchanged for group IIIA doping. While for Al-doping, the band gap remained almost the same, for Ga and In substitution the band gap marginally increased. However, increased conduction and PEC efficiency has been experimentally reported for Al-doped α-Fe2O3. It will be shown that the change in volume plays an important role in this behavior. A dramatic increase in photo-response cannot be expected for this type of doping in α-Fe2O3.
Silicon nano-structures can have important roles in many useful applications, such as in nano-sca... more Silicon nano-structures can have important roles in many useful applications, such as in nano-scale energy conversion materials, as nano-detectors of gas particles or as thermoelectric materials. To achieve efficient performance of these nano-devices, electronically tailored nano-materials are needed. For this a thorough understanding of both doped and undoped nano-structures is essential. Here we will present results of our first principles spin polarized electronic structure calculations of noble metal atom doped silicon nanocrystals using a hybrid density functional theory method (B3LYP-DFT) and a LanL2DZ basis set. The nanocrystals are used here as a test group, and are based on three different isomers of bulk silicon: diamond, wurtzite, and BC8. Geometry optimizations of the pure Sin nanocrystals were performed for spin magnetic moments of s=0 μB and s=2 μB for each isomer. Then the substitutional doping of M atom was done separately at the inside and at the surface of the nanocrystals. The doped nanocrystals' geometries were also optimized for spin magnetic moments s=1 μB and s=3 μB. For the bigger nanocrystals, the energy differences between the two spin states are very small. Binding energies and HOMO-LUMO gaps were calculated and a comparative analysis of the pure and doped silicon nanocrystals will be presented.
The quest for a efficient energy conversion material has necessitated a detail study of semicondu... more The quest for a efficient energy conversion material has necessitated a detail study of semiconductors. Silicon is already playing important roles in many useful nano-applications. To optimize these nano-applications, electronically tailored nano-materials are needed. A number of semiconductor nanomaterials are synthesized using metal as catalysts contributing to various impurities into the nanomaterials. The solubility of a metal in nanomaterials is significantly higher than that in bulk materials. In this presentation, electronic and structural properties of noble metal atoms doping in silicon nano-crystals will be explored using density functional theory. The pristine nanocrystals are based on three different isomers of bulk silicon. We have identified the lower energy isomer and doped it with noble metals. Characterization of the structural changes is accomplished by studying the bonding near the impurity as a function of dopant site. Furthermore, energetic of these nano-structures, both doped and un-doped, such as binding energies, formation energies, and HOMO LUMO gaps will be compared along with their charge densities to identify localizations with respect to impurity site. Magnetism and surface terminations will also be addressed.
... DE-AC36-99-GO10337. AKR gratefully acknowledges the partial support from the Welch Foundation... more ... DE-AC36-99-GO10337. AKR gratefully acknowledges the partial support from the Welch Foundation (Grant No. Y-1525). References. [1] XL Wu, JY Fan, T. Qiu, X. Yang, GG Siu and PK Chu, Phys. Rev. Lett. 94 (2005), p. 026102. ...
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Formation of SiC fullerene type structures is still an unsolved issue as there is no experimental... more Formation of SiC fullerene type structures is still an unsolved issue as there is no experimental confirmation reported so far regarding their existence. However, theoretical research results are available in the literature where carbon fullerenes were taken as a base model for SiC fullerene type structures. In this presentation, we show by a systematic study on C20 fullerene based SiC structures that this approach may not always reproduce the ground state structures; rather, the energetically favorable structure based on C20 could be highly distorted and open structures. In general, Si atoms tend to cluster on the C20 base structures. We observed a very systematic linear trend in the evolution of binding energies of SiC clusters from carbon fullerene and a detailed set of results on the electronic and geometric structures of these clusters will be presented.
We have studied eight collinear and non-collinear magnetic orientations of GdB4 using the GGA + U... more We have studied eight collinear and non-collinear magnetic orientations of GdB4 using the GGA + U method, without and with spin-orbit coupling, for values of U - J between 0 and 6. For U -- J = 6, the value which had been found to yield the correct Gd lattice constants, we obtain GdB4 lattice constants within 0.26% of experiment. We find the magnetization lies in-plane but is collinear, in disagreement with the most recent experimental determination.
We have studied eight collinear and noncollinear magnetic orientations of GdB4 using the GGA+U me... more We have studied eight collinear and noncollinear magnetic orientations of GdB4 using the GGA+U method, without and with spin-orbit coupling, for values of U-J between 0 and 6. For U-J=6 , the value which had been found to yield the correct Gd lattice constants, we obtain GdB4 lattice constants within 0.26% of experiment. We find that the magnetization lies in
Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectr... more Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes to have certain electronic properties. Unfortunately, at present available semiconductors do not meet all these criteria. So, a thorough understanding of band-engineering for mixed alloys is necessary to successfully design these photoelectrodes. Among the semiconductors, transition metal oxides are of particular interest due to their low cost and relatively high stability in aqueous media. Here, we will present a theoretical study of delafossite-alloys for PEC photo-electrodes. Previous studies have indicated that the group IIIA delafossite family (CuMO2, M = Al, Ga, In) do not exhibit direct band gaps. Their fundamental band gaps are significantly smaller than their reported optical band gaps. On the other hand group IIIB delafossite family (CuMO2, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO2, band gaps are above 3.00 eV. However, both of these two families exhibit p-type conductivity. We will show that by appropriate alloying of these two delafossite-families we can tune their band gaps and other opto-electronic properties. These types of alloying are desirable, as these introduce no localized impurity states in the band gap due to isovalent alloying. Also, the electronic effective masses can be lowered by selective doping of main group elements. Finally, it will be discussed that, lowering the symmetry constraints of these alloys would enhance their optical absorption properties. We'll also discuss that alloying with other 3d metal elements may decrease the band gap, but would increase the effective masses of the photo-electrons.
Efficient photoelectrochemical H2 production by solar irradiation depends not only on the photoca... more Efficient photoelectrochemical H2 production by solar irradiation depends not only on the photocatalyst's band gap and its band-edge positions but also on the detailed electronic nature of the bands, such as the localization or delocalization of the band edges and their orbital characteristics. These determine the carrier transport properties, reactivity, light absorption strength, etc. and significantly impact the material's efficiency as a photoconverter. The localization or delocalization of the band edges may arise either due to the orbital nature of the bands or the structural morphology of the material. A recent experimental report on a photocatalyst based on s/p orbitals showed very poor performance for H2 production despite the delocalized nature of the s/p bands as compared to the d-bands of transition metal oxides. It is then important to examine whether this poor performance is inherent to these materials or rather arises from some experimental limitations. A theoretical analysis by first-principle methods is well suited to shed light on this question.
Electronic structure of ZnO:GaN compounds: Asymmetric bandgap engineering. Muhammad N. Huda * , Y... more Electronic structure of ZnO:GaN compounds: Asymmetric bandgap engineering. Muhammad N. Huda * , Yanfa Yan, Su-Huai Wei, and Mowafak M. Al-Jassim National Renewable Energy Laboratory, Golden, Colorado 80401, USA. ...
... T. Bak, J. Nowotny, M. Rekas, and CC Sorrell, Int. J. Hydrogen Energy 27, 991 (2002). E. Frad... more ... T. Bak, J. Nowotny, M. Rekas, and CC Sorrell, Int. J. Hydrogen Energy 27, 991 (2002). E. Fradkin, SA Kivelson, and V. Oganesyan, Science 315, 196 (2007). M. Gräzel, Nature 414, 338 (2001). MN Huda, Y. Yan, S.-H. Wei, and MM Al-Jassim, Phys. Rev. B 78, 195204 (2008). ...
The prospect of solar energy as a renewable resource is ever-increasing. Density functional theor... more The prospect of solar energy as a renewable resource is ever-increasing. Density functional theory (DFT) calculations can elicit reliable behavior predictions in energy conversion materials to achieve higher efficiencies. Chemical stability of the photo-catalysts in aqueous solution is of particular interest for its long term performances. The bulk modulus is a mechanical property that is a good indicator of material stability. GaP has a low band gap and is a good candidate for use as a photocatalyst for hydrogen evolution by splitting water. Unfortunately, it is not stable and highly susceptible to corrosion over a very short time period, making it unfeasible for long-term use. GaN has too high of a band gap but a good stability factor. While these materials both possess desirable qualities, they cannot be used solitarily. We will report electronic properties and bulk moduli from the total energy calculations of the zincblende and wurtzite species using DFT-GGA and DFT+U as a function of doping concentration x. We will also present the density of states and charge density distribution of the alloy materials to study the localization/delocalization effects of N defects levels and their impact on the alloys' stability.
Among iron oxides, α-Fe2O3 is the most abundant on earth. Because it has a band gap of approximat... more Among iron oxides, α-Fe2O3 is the most abundant on earth. Because it has a band gap of approximately 2 eV, it is stable and inexpensive to process, α-Fe2O3 has been considered as a potential photoelectrocatalyst for solar driven photoelectrochemical (PEC) water splitting to make hydrogen. However, as α-Fe2O3 is a charge-transfer type insulator, the poor conduction properties have limited its efficiency as a PEC material. We will present our study on the doping of group IIIA elements in α-Fe2O3 to improve its performance. All the calculations were done with DFT+U. The main electronic features of α-Fe2O3 remained almost unchanged for group IIIA doping. While for Al-doping, the band gap remained almost the same, for Ga and In substitution the band gap marginally increased. However, increased conduction and PEC efficiency has been experimentally reported for Al-doped α-Fe2O3. It will be shown that the change in volume plays an important role in this behavior. A dramatic increase in photo-response cannot be expected for this type of doping in α-Fe2O3.
Silicon nano-structures can have important roles in many useful applications, such as in nano-sca... more Silicon nano-structures can have important roles in many useful applications, such as in nano-scale energy conversion materials, as nano-detectors of gas particles or as thermoelectric materials. To achieve efficient performance of these nano-devices, electronically tailored nano-materials are needed. For this a thorough understanding of both doped and undoped nano-structures is essential. Here we will present results of our first principles spin polarized electronic structure calculations of noble metal atom doped silicon nanocrystals using a hybrid density functional theory method (B3LYP-DFT) and a LanL2DZ basis set. The nanocrystals are used here as a test group, and are based on three different isomers of bulk silicon: diamond, wurtzite, and BC8. Geometry optimizations of the pure Sin nanocrystals were performed for spin magnetic moments of s=0 μB and s=2 μB for each isomer. Then the substitutional doping of M atom was done separately at the inside and at the surface of the nanocrystals. The doped nanocrystals' geometries were also optimized for spin magnetic moments s=1 μB and s=3 μB. For the bigger nanocrystals, the energy differences between the two spin states are very small. Binding energies and HOMO-LUMO gaps were calculated and a comparative analysis of the pure and doped silicon nanocrystals will be presented.
The quest for a efficient energy conversion material has necessitated a detail study of semicondu... more The quest for a efficient energy conversion material has necessitated a detail study of semiconductors. Silicon is already playing important roles in many useful nano-applications. To optimize these nano-applications, electronically tailored nano-materials are needed. A number of semiconductor nanomaterials are synthesized using metal as catalysts contributing to various impurities into the nanomaterials. The solubility of a metal in nanomaterials is significantly higher than that in bulk materials. In this presentation, electronic and structural properties of noble metal atoms doping in silicon nano-crystals will be explored using density functional theory. The pristine nanocrystals are based on three different isomers of bulk silicon. We have identified the lower energy isomer and doped it with noble metals. Characterization of the structural changes is accomplished by studying the bonding near the impurity as a function of dopant site. Furthermore, energetic of these nano-structures, both doped and un-doped, such as binding energies, formation energies, and HOMO LUMO gaps will be compared along with their charge densities to identify localizations with respect to impurity site. Magnetism and surface terminations will also be addressed.
... DE-AC36-99-GO10337. AKR gratefully acknowledges the partial support from the Welch Foundation... more ... DE-AC36-99-GO10337. AKR gratefully acknowledges the partial support from the Welch Foundation (Grant No. Y-1525). References. [1] XL Wu, JY Fan, T. Qiu, X. Yang, GG Siu and PK Chu, Phys. Rev. Lett. 94 (2005), p. 026102. ...
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Formation of SiC fullerene type structures is still an unsolved issue as there is no experimental... more Formation of SiC fullerene type structures is still an unsolved issue as there is no experimental confirmation reported so far regarding their existence. However, theoretical research results are available in the literature where carbon fullerenes were taken as a base model for SiC fullerene type structures. In this presentation, we show by a systematic study on C20 fullerene based SiC structures that this approach may not always reproduce the ground state structures; rather, the energetically favorable structure based on C20 could be highly distorted and open structures. In general, Si atoms tend to cluster on the C20 base structures. We observed a very systematic linear trend in the evolution of binding energies of SiC clusters from carbon fullerene and a detailed set of results on the electronic and geometric structures of these clusters will be presented.
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