The surface segregation of indium atoms in InGaAs is investigated using first-principles calculat... more The surface segregation of indium atoms in InGaAs is investigated using first-principles calculations based on density functional theory. Through the calculation of segregation energies for (100), (110), and (111) surfaces of GaAs we analyze the decisive role of surface orientation on indium segregation. Further, our calculations reveal that the variation of segregation energy as a function of applied strain strongly depends on surface reconstruction. Obtained segregation energy trends are discussed in light of atomic bonding probed via integrated crystal orbital Hamilton population. Results presented in this paper are anticipated to guide experimental efforts to achieve control on In segregation by managing As-flux along with the application of strain.
Zirconium trisulphide (ZrS 3), a member of the layered transition metal trichalcogenides (TMTCs) ... more Zirconium trisulphide (ZrS 3), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS 3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS 3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS 3 are highly aniso-tropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D (planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.
The effect of oxygen vacancies on the elastic properties of zinc oxide (ZnO) is examined using fi... more The effect of oxygen vacancies on the elastic properties of zinc oxide (ZnO) is examined using first-principles calculations based on density functional theory. Formation energies of vacancies in different types of oxygen deficient structures were analyzed to ascertain their stability. This analysis reveals that the doubly-charged oxygen vacancy under zinc-rich growth conditions is the most stable. Results show considerable degradation of some of the elastic moduli due to the presence of oxygen vacancies, which is in agreement with recent experiments. The decrease observed in elastic constants is more pronounced with increase in vacancy concentration. Further, the charge state of the defect structure was found to influence the shear elastic constants. Evaluation of elastic anisotropy of stoichiometric and oxygen deficient ZnO indicates the significant anisotropy in elastic properties and stiff c-axis orientation.
Among the panoply of exciting properties that metal–organic frameworks (MOFs) exhibit, fully reve... more Among the panoply of exciting properties that metal–organic frameworks (MOFs) exhibit, fully reversible pressure-induced phase transformations (PIPTs) are particularly interesting as they intrinsically relate to the flexibility of MOFs. Recently, a number of MOFs have been reported to exhibit this feature, which is attributed to bond rearrangement with applied pressure. However, the experimental assessment of whether a given MOF exhibits PIPT or not requires sophisticated instruments as well as detailed structural investigations. Can we capture such low pressure transformations through simulations is the question we seek to answer in this paper. For this, we have performed first-principles calculations based on the density functional theory, on a MOF, [tmenH 2 ][Y(HCOO) 4 ] 2 (tmenH 2 2+ = N,N,N 0 ,N 0-tetramethylethylenediammonium). The estimated lattice constants for both the parent and product phases of the PIPT agree well with the earlier experimental results available for the same MOF with erbium. Importantly, the results confirm the observed PIPT, and thus provide theoretical corroborative evidence for the experimental findings. Our calculations offer insights into the energetics involved and reveal that the less dense phase is energetically more stable than the denser phase. From detailed analyses of the two phases, we correlate the changes in bonding and electronic structure across the PIPT with elastic and electronic conduction behavior that can be verified experimentally, to develop a deeper understanding of the PIPT in MOFs.
Hydroxyapatite (HAp), a primary constituent of human bone, is usually nonstoichiometric with vary... more Hydroxyapatite (HAp), a primary constituent of human bone, is usually nonstoichiometric with varying Ca/P molar ratios, with the well-known fact that Ca deficiency can cause marked reductions in its mechanical properties. To gain insights into the mechanism of this degradation, we employ first-principles calculations based on density functional theory and determine the effects of Ca deficiency on structure, vibrational, and elastic properties of HAp. Our simulation results confirm a considerable reduction in the elastic constants of HAp due to Ca deficiency, which was experimentally reported earlier. Stress-induced transformation of the Ca-deficient defected structure into a metastable state upon the application of stress could be a reason for this. Local structural stability of HAp and Ca-deficient HAp structures is assessed with full phonon dispersion studies. Further, specific signatures in the computed vibrational spectra for Ca deficiency in HAp can be utilized in experimental characterization of different types of defected HAp.
Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-i... more Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at ∼7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-à-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally.
The surface segregation of indium atoms in InGaAs is investigated using first-principles calculat... more The surface segregation of indium atoms in InGaAs is investigated using first-principles calculations based on density functional theory. Through the calculation of segregation energies for (100), (110), and (111) surfaces of GaAs we analyze the decisive role of surface orientation on indium segregation. Further, our calculations reveal that the variation of segregation energy as a function of applied strain strongly depends on surface reconstruction. Obtained segregation energy trends are discussed in light of atomic bonding probed via integrated crystal orbital Hamilton population. Results presented in this paper are anticipated to guide experimental efforts to achieve control on In segregation by managing As-flux along with the application of strain.
Zirconium trisulphide (ZrS 3), a member of the layered transition metal trichalcogenides (TMTCs) ... more Zirconium trisulphide (ZrS 3), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS 3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS 3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS 3 are highly aniso-tropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D (planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.
The effect of oxygen vacancies on the elastic properties of zinc oxide (ZnO) is examined using fi... more The effect of oxygen vacancies on the elastic properties of zinc oxide (ZnO) is examined using first-principles calculations based on density functional theory. Formation energies of vacancies in different types of oxygen deficient structures were analyzed to ascertain their stability. This analysis reveals that the doubly-charged oxygen vacancy under zinc-rich growth conditions is the most stable. Results show considerable degradation of some of the elastic moduli due to the presence of oxygen vacancies, which is in agreement with recent experiments. The decrease observed in elastic constants is more pronounced with increase in vacancy concentration. Further, the charge state of the defect structure was found to influence the shear elastic constants. Evaluation of elastic anisotropy of stoichiometric and oxygen deficient ZnO indicates the significant anisotropy in elastic properties and stiff c-axis orientation.
Among the panoply of exciting properties that metal–organic frameworks (MOFs) exhibit, fully reve... more Among the panoply of exciting properties that metal–organic frameworks (MOFs) exhibit, fully reversible pressure-induced phase transformations (PIPTs) are particularly interesting as they intrinsically relate to the flexibility of MOFs. Recently, a number of MOFs have been reported to exhibit this feature, which is attributed to bond rearrangement with applied pressure. However, the experimental assessment of whether a given MOF exhibits PIPT or not requires sophisticated instruments as well as detailed structural investigations. Can we capture such low pressure transformations through simulations is the question we seek to answer in this paper. For this, we have performed first-principles calculations based on the density functional theory, on a MOF, [tmenH 2 ][Y(HCOO) 4 ] 2 (tmenH 2 2+ = N,N,N 0 ,N 0-tetramethylethylenediammonium). The estimated lattice constants for both the parent and product phases of the PIPT agree well with the earlier experimental results available for the same MOF with erbium. Importantly, the results confirm the observed PIPT, and thus provide theoretical corroborative evidence for the experimental findings. Our calculations offer insights into the energetics involved and reveal that the less dense phase is energetically more stable than the denser phase. From detailed analyses of the two phases, we correlate the changes in bonding and electronic structure across the PIPT with elastic and electronic conduction behavior that can be verified experimentally, to develop a deeper understanding of the PIPT in MOFs.
Hydroxyapatite (HAp), a primary constituent of human bone, is usually nonstoichiometric with vary... more Hydroxyapatite (HAp), a primary constituent of human bone, is usually nonstoichiometric with varying Ca/P molar ratios, with the well-known fact that Ca deficiency can cause marked reductions in its mechanical properties. To gain insights into the mechanism of this degradation, we employ first-principles calculations based on density functional theory and determine the effects of Ca deficiency on structure, vibrational, and elastic properties of HAp. Our simulation results confirm a considerable reduction in the elastic constants of HAp due to Ca deficiency, which was experimentally reported earlier. Stress-induced transformation of the Ca-deficient defected structure into a metastable state upon the application of stress could be a reason for this. Local structural stability of HAp and Ca-deficient HAp structures is assessed with full phonon dispersion studies. Further, specific signatures in the computed vibrational spectra for Ca deficiency in HAp can be utilized in experimental characterization of different types of defected HAp.
Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-i... more Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at ∼7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-à-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally.
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Papers by Soumya S Bhat