Narrow Results

We present a theoretical study on the point defects in ZrO₂–silicon interfaces using molecular dynamics (MD) calculations. A super-cell model that contains 9 atomic layers of silicon and 9 atomic layers of ZrO₂ was used for the simulation. Three atomic layers containing 17 oxygen atoms, eight silicon atoms, and nine Zr atoms were used to simulate the ZrO₂–silicon interface. We then performed density functional theory (DFT) with plane-wave basis to calculate the interface band structure. Results demonstrate that the stretched Zr–O bonds at the interface would produce some defect levels in the band gap. Particularly, the defect levels originated from the interstitial oxygen atoms are located close to the bottom of the ZrO₂ conduction band and hence it will affect the electrical properties of the gate dielectrics.

The electronic band structure, density of states, optical absorption, phonon spectrum, stability, and thermodynamic properties of 1T’-phase RuOsSe₂ hybrid monolayer were systematically studied using ab initio calculations based on Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) within the generalized gradient approximation (GGA) and the HSE06 functional of hybrid correlation–exchange. Indirect bandgaps $E_g=0.68$eV and $E_g=1.23$eV were obtained within the calculation level GGA-PBE and HSE06. The investigation of optical absorption shows that the RuOsSe₂ monolayer exhibits a significant absorption in the ultraviolet and visible regions of the electromagnetic spectrum. Thermodynamic potentials and specific heat at constant volume were calculated, of which dependence on the temperature is discussed. We predict a new RuOsSe₂ monolayer from the 1T’ phase that could potentially be synthesized for future electronic devices and bring potential technological applications.

First-principles calculations have been performed to study the structure, elastic and lattice dynamical properties of C40 XSi₂ (X=Cr, Mo, W) under hydrostatic pressure. The obtained structural parameters are in line with existing experimental and theoretical data. The evolutions of fundamental bandgap energies, elastic moduli, IR absorption spectra with pressure have been investigated in detail. Our results indicate that the energy gaps of XSi₂ (X=Cr, Mo, W) show different trends as the pressure increases. Larger B_H/G_H ratio and Poisson’s ratio are achieved with pressure, suggesting an improved ductility for XSi₂ (X=Cr, Mo, W). Moreover, a large elastic anisotropy under pressure is exhibited in Young’s anisotropic factors. The infrared-active phonon frequencies exhibit substantial blueshifts under pressure.

The aluminum-based intermetallic compounds AlZr₃, AlCu₃ and AlCu₂Zr are studied for elastic and mechanical properties by a DFT embodied in WIEN2k code with Generalized Gradient Approximation (GGA) as an exchange correlation functional. The cubic elastic parameters $C_{11}$, $C_{12}$ and $C_{44}$ are found keeping bulk modulus value the same as in the structural optimization. The mechanical properties such as Young’s modulus, shear modulus, anisotropic factor and Poisson’s ratio are then found using these fundamental parameters. It is found that our calculated results are in good agreement with available theoretical and experimental results.

In this paper, we have explored the physical, mechanical, chemical bonding, dialectical and thermodynamic properties of ARh₂Ge₂ (A = Ca, Sr, Y and Ba) theoretically for the first time. This investigation has been completed by density functional theory (DFT) calculations with the help of CASTEP code. The structural optimized factors of ARh₂Ge₂ (A = Ca, Sr, Y and Ba) are in excellent concurrence with the existing experimental data. The observed elastic constants are positive and prove the mechanical constancy for all these compounds. The calculated Pugh’s ratio and Poisson’s ratio show the ductile behaviors of Ca/YRh₂Ge₂ and brittleness behaviors of Sr/BaRh₂Ge₂, whereas the Cauchy pressure indicates the ductility for all these phases. The anisotropic factors, universal anisotropy indicator and fraction of anisotropy in compression and shear ensure the elastically anisotropic nature for all these phases. Bulk modulus and hardness values indicate that Sr/BaRh₂Ge₂ are soft and easily machinable in comparison with Y/CaRh₂Ge₂. The analysis of the band structure diagrams as well as density of states (total density of states and partial density of states) evidence the metallic behavior for all the compounds. The analysis of Mulliken bond populations and charge density maps give the existence of covalent and metallic bonding in these compounds. The optical properties point out that all phases can be used as coating materials at low energies. For all the phases the Debye temperatures have been calculated via elastic constant data. We have also evaluated the minimum thermal conductivity for these compounds. All compounds possess the relatively low minimum thermal conductivity with the low value of Debye temperatures which also evidence that all compounds could be applied like thermal fence covering material.

Current research is a computational study in which we focus on calculating optical properties of Eu-doped Cadmium Sulfide (CdS) system. We employ Perdew–Burke–Ernzerhof (PBE)–generalized gradient approximation (GGA) for accomplishment of our study and we assume various Eu concentrations (3.12%, 6.25% and 9.37%) for doping into host CdS lattice. We substitute Cd atoms with Eu atoms while supercell size is kept fixed ($1\times 2\times 2)$. We present a detailed comparison among optical properties of pure CdS and various Eu concentrations. Partial density of states (PDOS) plots reveal hybridization among Cd $s$-states, S $p$-states and Eu $d\text{–}f$-states and because of it, material (CdS:Eu) shows exceptional energy transfer which influences optical spectra and electronic properties. A considerable change in absorption spectra is noted, where in comparison to pure CdS, absorption shows blueshift with increasing Eu concentrations. Our DFT results were found to have great resemblance with existing literature. Addition of Eu into the CdS lattice originates novelty in CdS:Eu system and number of potential applications related to the field of biomedical physics, amperometric biosensors, quantum dots (QDs) sensors, photonics, bioprinting, biosensing luminophores, solar cells and optoelectronics industry may be explored in technological perspectives.

Density Functional Theory (DFT) is used to investigate the phonon properties of CoFeCrZ (Z=Al, Si, Ga, Ge) equiatomic Quaternary Heusler Alloys. These alloys crystallize in face centered cubic (FCC) structure and have three crystal structures Y-Type I, Y-Type II and Y-Type III on the basis of their atomic positions. For CoFeCrZ (Z=Al, Si, Ga, Ge), Y-Type I is the most stable structure found in the literature, so phonon dispersion curves for this structure are obtained with the help of norm-conserving pseudo potentials in Quantum ESPRESSO. Absence of negative frequencies in phonon dispersion curves proves the dynamical stability of all these alloys. Phonon dispersion curves are further used to obtain Reststrahlen band, a region where light reflects 100%. The calculated Reststrahlen bands for CoFeCrAl, CoFeCrSi, CoFeCrGa and CoFeCrGe are 4.179 THz ($\mathrm{\Delta }\lambda =71.73\mu$m), 4.30 THz ($\mathrm{\Delta }\lambda =69.71\mu$m), 3.35 THz ($\mathrm{\Delta }\lambda =89.49\mu$m) and 3.05 THz ($\mathrm{\Delta }\lambda =98.99\mu$m), respectively. These obtained values of Reststrahlen bands for CoFeCrZ (Z=Al, Si, Ga, Ge) lie within the far infra-red (FIR) region, and can be used in sensing, imaging and optoelectronic devices.

We constructed ZnO quantum dots (QDs) with a diameter of 1.2nm of wurtzite structure, hydrogenated the surface and doped single Mn atoms using Zn-site substitution. We present an investigation of the structural and electronic properties of ZnO QDs of three doping sites: inner, intermediate and outer. Based on the density functional theory and the plane-wave pseudo potential method, the structural characteristics, charge distribution, conductivity and formation energy of doped ZnO QDs are analyzed. We obtained the effect of doping site depth on the properties above with the nearly optimal nanoparticle doping concentration (2.222%). The internal doping (inner and intermedium) configuration results in higher electrical conductivity and stronger bonds than the outer doping configuration. In particular, intermediate doping is highly efficient and valuable for obtaining Mn-doped ZnO nanoparticles with good structural and electronic properties. This study provides a theoretical reference for the survey of zero-dimensional dilute magnetic semiconductors.

We report ab initio molecular dynamics simulations of carbon clusters in free space and inside C₆₀ using SIESTA. We have studied the stability and geometries of small carbon clusters consisting of 2–12 carbon atoms inside the C₆₀ molecule and in free space by optimizing the atomic geometries. We have found that the C–C bond length is in agreement with the 1.40 and 1.45 values reported earlier. We find that the clusters inside the C₆₀ are more stable than clusters in free space. Binding energy per carbon atom initially increases with number of carbon atoms in the cluster and then decreases after maximizing for the 9-atom cluster. For more than 9 carbon atoms in the cluster inside C₆₀, C atoms of the cluster start forming bond with the C₆₀ cage and the C₆₀ structure gets distorted. We have done calculations for charge transfer and chemical reactivity. The calculations of ionization potential and electron affinity show that clusters in free space are less reactive compared with C_n@C₆₀. Charge transfer calculations show that the bond formation of C atoms with the C₆₀ cage is accompanied with a transfer of charge from carbon cluster to C₆₀.

The crystal structure, electronic structure and optical properties of N-doped ${\mathrm{\text{SiO}}}_2$ with different N impurity concentrations were calculated by density function theory within GGA+U method. The crystal distortion, impurity formation energy, band gap, band width and optical parameter of N-doped ${\mathrm{\text{SiO}}}_2$ are closely related with N impurity concentration. Based on the calculated results, there are three new impurity energy levels emerging in the band gap of N-doped ${\mathrm{\text{SiO}}}_2$, which determine the electronic structure and optical properties. The variations of optical properties induced by N doping are predominately determined by the unsaturated impurity states, which are more obvious at higher N impurity concentration. In addition, all the doping effects of N in both $\alpha$-quartz ${\mathrm{\text{SiO}}}_2$ and $\beta$-quartz ${\mathrm{\text{SiO}}}_2$ are very similar. According to these findings, one could understand the relationship between nitrogen concentration and optical parameter of ${\mathrm{\text{SiO}}}_x{\mathrm{\text{N}}}_y$ materials, and design new optoelectrionic Si–O–N compounds.

We present an ab initio study of surface supported Au–Mn nanowires. Three different substrates are discussed: Cu(110), stepped Cu(111) and Si(001) surface. The emergence of stable antiferromagnetic (AFM) solutions in Au–Mn nanowires was found in all three cases. We found the nonzero magnetic moments of Mn atoms, however, the bulk of manganese is paramagnetic. The critical temperature of the Au–Mn wires is calculated by means of kinetic Monte Carlo simulation. The strong size-effect of the critical temperature is demonstrated.

The CuCoMnX (X = Si, Sn, Sb) equiatomic quaternary Heusler alloys (QHAs) are studied for phonon spectra by using density functional theory. The crystals exist in three possible structures Y-type I, Y-type II and Y-type III on the basis of their crystallographic positions. Y-type III structural arrangements proved to be the most stable and phonon density of states and phonon dispersion curves are obtained by using nonconserving pseudo-potentials for this type. There are no negative vibrational phonon modes in phonon density of states and phonon dispersion curves, so the alloys are dynamically stable. From the phonon dispersion curves, reststrahlen bands are calculated for which these crystals behave as reflectors for incoming light. The calculated reststrahlen bands are 1.470 THz $(\lambda =203.94\mu \mathrm{\text{m}})$, 0.357 THz $(\lambda =839.74\mu \mathrm{\text{m}})$ and 0.220 THz $(\lambda =1362.69\mu \mathrm{\text{m}})$ for CuCoMnSi, CuCoMnSn and CuCoMnSb alloys, respectively. These values correspond to far infra-red (FIR) spectral region so these alloys can be used for manufacturing FIR-devices.

Optical properties of semiconductor materials have been intensively studied for potential applications in perovskite solar cells. HfO₂ is recently substituting the conventional semiconductor materials due to excellent photovoltaic characteristics. The electronic and optical properties of Ti- and Zr-incorporated HfO₂ were investigated in this work using density functional theory. Ab-initio calculations were performed using Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA). Electronic properties were studied by analyzing the band structure and density of states. Refractive index, attenuation coefficient, dielectric function, loss factor, energy loss spectra and absorption coefficient were calculated for the detailed study of optical response. A significant increase in absorption of HfO₂ in the visible region with the incorporation of Ti revealed its important practicability in photovoltaic devices.

Using the density functional theory (DFT) computations implemented in WIEN2K package and Boltzmann theory with the BoltzTrap code, we study certain physical properties of a new class of Cu₂SrXSe₄ (X = Ge, Si, Sn) materials. Concretely, we investigate the electronic, the optical and the thermoelectric aspects of such materials from the generalized gradient approximation (GGA) corrected by the Tran Balaha modified Becke–Johnson exchange potential (GGA+TB-mBJ). Analyzing the computed electronic properties, we show the semiconducting nature of these compounds. Precisely, we reveal that these compounds involve an indirect band gap with p type. This electronic aspect is very important for the absorber solar cell layers due to the fact that the length diffusion of the electron is larger than the one of the hole. We find that the obtained gap values of such a new family inspired by CZTS materials can be compared with various absorber layers. For the Cu₂SrSnSe₄ model, we observe similar behaviors compared to the silicon solar cell absorber layers. Using the scalar dielectric function, we investigate the optical properties of the studied materials. Precisely, we obtain that such a family of materials involve higher values of the absorption coefficient in the visible and the ultra violet light spectrum. For each material, we observe that the absorption starts at the corresponding band gap energies. Exploiting Boltzmann theory with the BoltzTrap code, we compute and examine the thermoelectric properties such as the Seebeck coefficient, the thermal conductivity, the electrical conductivity, and the figure of merit as a function of the temperature. As a result, we show that the studied materials involve a high absorption spectra and a good figure of merit (ZT) showing relevant features for photovoltaic and thermoelectric applications.

Superacids are systems that exhibit higher acidity compared to extremely strong mineral acid, i.e. 100% pure sulphuric acid (H₂SO₄). They can be designed by the protonation of appropriate superhalogen anions. Superhalogens possess higher electron affinity (EA) than halogens. Most recent examples include CF${}_{4-n}{({\mathrm{\text{SO}}}_3)}_n$, which behave as superhalogens for $n=1-4$. In this work, we studied the protonation of CF${}_{4-n}{({\mathrm{\text{SO}}}_3)}_n^{-}$ using density functional theory calculations at the B3LYP/6-311$++G(d,p)$ level. The Gibbs’ free energies of deprotonation ($\mathrm{\Delta }{G}_{\mathrm{\text{acid}}}$) values of the resulting protonated complexes, HCF${}_{4-n}{({\mathrm{\text{SO}}}_3)}_n$ for $n=1$–4; are either lower or nearly equivalent to that of H₂SO₄ ($\mathrm{\Delta }{G}_{\mathrm{\text{acid}}}=302.2$kcal/mol). This suggests that all these protonated complexes exhibit superacidic behavior. Notably, the HC${({\mathrm{\text{SO}}}_3)}_4$ compound undergoes significant structural relaxation, leading to a remarkably less $\mathrm{\Delta }{G}_{\mathrm{\text{acid}}}$ value. Our findings should enrich the literature with a new series of superacids and motivate further studies to explore novel organic superacids.

Nitrogen is an important impurity in Czochralski grown silicon (Cz–Si) as it enhances oxygen precipitation through the formation of vacancy–nitrogen–oxygen clusters and in particular the ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ complexes. Here, we employ density functional theory (DFT) to predict the structure of ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ ($m,n=1,2$). We report that the lowest energy ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ ($m,n=1,2$) defects are very strongly bound. These results are consistent, and support the previously reported theoretical and experimental conclusions that ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ structures could form.

A comprehensive comparative study utilizing HSE06 and GGA density functional calculations was conducted to investigate the impact of Li and Na doping, as well as their co-doping, on the physical properties of cuprous oxide (Cu₂O). This study examined three possible structures, including substitution of Li, Na, and Li/Na for Cu, and interstitial Li, Na, and Li/Na in both tetrahedral and octahedral sites. The results of the study revealed that the introduction of alkaline atoms leads to structural changes in Cu₂O, and the degree of lattice parameter extension or compression varies across different doping sites. Additionally, the study provided an estimation of the enthalpies of formation for pure and doped-Cu₂O, which is useful in understanding the stability of the systems. Notably, the study found that Li, Na, and Li/Na-doped-Cu₂O were more readily formed in substitutional sites rather than in interstitial sites. The findings also indicate that substitutional doping and co-doping exhibit a large band gap while maintaining the properties of a p-type semiconductor, while interstitial doping and co-doping of Cu₂O led to significant absorption enhancement and n-type conductivity characteristics. These results provide new insights into the structural and electronic properties of Cu₂O, with the findings suggesting that interstitial doping of Li and Na could be a promising approach for improving the absorption of visible light in Cu₂O-based solar cells, thus contributing to the development of more efficient and cost-effective photovoltaic devices.

Selenium selenide (SnSe) has attracted widespread attention because of its environmental friendliness and ultra-low lattice thermal conductivity. Single-crystal SnSe has been discovered to exhibit a high ZT value, but its mechanical qualities are weak and its manufacturing process is complicated, rendering it unsuitable for commercial usage. Polycrystalline SnSe is facile to synthesis; however, due to its weakened electrical performance, it has a poor thermoelectric property. In this study, polycrystalline SnSe samples are prepared using hydrothermal synthesis combined with vacuum sintering, and their thermoelectric properties are modulated using alkali metal element doping.

Adsorbed hydrogen layers on the Mo(110) surface have been investigated both experimentally by temperature programmed desorption (TPD) method and theoretically by means of DFT-based optimization of surface structures. We suggest a novel microscopic model of the associative hydrogen desorption, which explains essential features of the process. In this model, the process of hydrogen desorption can be described as association of hydrogen atoms on the surface, but molecular formation is actually accomplished while the molecule moves away from the surface. We also suggest a new algorithm for realistic Monte Carlo simulations of associative desorption, which implements the microscopic description of the association of hydrogen adatoms into a molecule with activation energy, found from the DFT calculations. Good agreement between simulated and experimental TPD spectra gives insight into different behavior of the spectra, obtained for low and high hydrogen coverages on the Mo(110) surface.

The addition of hydrogen to the carbon–carbon double bond of 2-butenes adsorbed on Pd(111) was studied within the density functional theory (DFT) and using a periodic slab model. For that purpose, the Horiuti–Polanyi mechanisms for both complete hydrogenation and isomerization were considered. The hydrogenation of cis and trans-2-butene to produce butane proceeds via the formation of eclipsed and staggered-2-butyl intermediates, respectively. In both cases, a relatively high energy barrier to produce the half-hydrogenated intermediate makes the first hydrogen addition the slowest step of the reaction. The competitive production of trans-2-butene from cis-2-butene requires the conversion from the eclipsed-2-butyl to the staggered-2-butyl isomer. As the corresponding energy barrier is relatively small and because the first of these isomers is less stable than the second, an easy conversion is predicted.