Principles Method Research Articles

First principles calculation of interface interactions and photoelectric properties of ZnSe/SnSe heterostructure.

Heterostructure engineering is an effective technology to improve photo-electronic properties of two dimensional layered semiconductors. In this paper, based on first principles method, we studied the structure, stability, energy band, and optical properties of ZnSe/SnSe heterostructure change with film layer. Results show that all heterostructures are the type-II band arrangement, and the interlayer interaction is characterized by van der Waals. The electron concentration and charge density difference implies the electron (holes) transition from SnSe to monolayer ZnSe. By increasing the layer of SnSe films, the quantum effects are weakened leading to the band gap reduced, and eventually show metal properties. The optical properties also have obvious change, the excellent absorption ability of ZnSe/SnSe heterostructures mainly near the infrared spectroscopy. These works suggest that ZnSe/SnSe heterostructure has significant potential for future optoelectronic applications.

Effect of F− doping on structural, elastic, and electronic properties of SmO1−xFxBiS2: A first principles investigation

Structural, elastic, and electronic properties of SmO1−xFxBiS2 were studied using first principles method, and effects of F− doping were clearly observed. By increasing x, the increase of lattice parameter ‘a’ and decrease of ‘c’ are mainly due to the displacement of S1 and Sm atoms respectively. For x = 0.3, new (Bi–S1)2 bonds formation was observed. The gap between Bi- and S1-planes decreases and disappeared at x = 0.9. Cauchy pressures, B/G, and ν indicate ductile nature having metallic bonds with small ionic contribution. B, G, and E are highly anisotropic and show anomaly at x = 0.3, 0.5, and 0.8. Sm 4f states are mainly responsible for metallicity. For x = 0.3, total DOS decreases due to new bonds formation. Anomalous displacement of Sm-sites by doping at O- or Sm-sites induces anomalous displacement of S1 atoms which may be the main reason for change of different properties of SmO1-xFxBiS2.

Insights into the formation of titanium hydrides from first principles calculations

Hydride induced embrittlement is one of the main causes for the stress corrosion crack of titanium alloys. The formation process of the hydride at atomic level remains controversial as it is not accessible directly by experiments. In the present work, the formation of titanium hydride is investigated by using a first principles method. By calculating the formation energies of TiHm with various types of H occupation state in HCP and FCC Ti matrix, we show that there exists phase equilibrium between α-Ti-H solid solution and δ/ε hydrides with χ and γ hydrides appearing as metastable phases. The energy barrier for the HCP → FCC structure transformation for the formation of the hydrides decreases with increasing H:Ti ratio m. The structure transformation is accompanied with spontaneous shift of half amounts of the H atoms from the octahedral to the tetrahedral interstices. The energy barrier for the migration of the remaining half of the H atoms from the octahedral to the tetrahedral interstices reduces with increasing m and approaches to a constant value. The hydrostatic compressive stress acting on TiHm, resulted from the H-induced lattice dilation tendency and the constraint of the surrounding matrix, does not facilitate the HCP → FCC transformation and the migration of the H atoms. The present work sheds light on the hydride formation process at atomic level.

Effect of lattice defects on electronic structure and thermoelectric properties of 2D monolayer MoS2

Molybdenum disulfide (MoS2) is considered as a promising economical and non-toxic thermoelectric material due to its low thermal conductivity and high Seebeck coefficient. However, its power factor is not ideal, which limits its energy conversion efficiency in thermoelectric applications. In this paper, the electronic structure of pure and lattice-defect MoS2 2D monolayer is studied by using the first principles method and non-equilibrium Green's function (DFT-NEGF). The effects of atom substitution and vacancy defect on the thermoelectric properties of MoS2 2D monolayer are also investigated. The result shows that the S-vacancy changes the bandgap by introducing donor and acceptor levels, while the substitution of Se and Te atoms slightly increases the bandgap value. In the case of n-type doping, the ZT value of the S atom substituted MoS2 reaches 1.19 at 300 K, while the Te atom substitution MoS2 has a higher ZT value of almost 2.26 at 800 K. The analysis shows that the thermal conductivity of MoS2 decreases due to the presence of S-vacancy, while the atomic substitution of Se and Te increases the power factor of MoS2. This work may provide a new method and theoretical guidance for achieving high performance of thermoelectric devices in the energy industry.

Regulation of photoelectric properties of multi-element (Sn, Fe, Ce) co-doped anatase TiO2: The first principles study

The energy band structure, density of states and optical properties of multi-element (Sn, Fe, Ce) co-doped anatase titanium dioxide have been calculated using the first principles method. The results show that two-element (Sn, Fe, Ce) doping affects the energy band structure more than the single doping behaviour. Co-doping of two of the three elements (Sn, Fe, Ce) reduced the energy band gap values of TiO2 compared to pure anatase TiO2.The forbidden bandwidths of Sn and Ce were smaller than those of mono-doped Sn, but the co-doped system had higher bandwidth narrowing than that of the mono-doped system. The co-doping of Sn and Ce is 0.206 eV smaller than the single doping of Sn, and the co-doping of Sn and Fe is 0.56 eV smaller than the single doping of Sn. The utilization of the regional light absorption is increased, and the absorption spectra are red-shifted. It can be seen that the co-doping method is more favourable to the improvement of visible light response and visible photocatalytic activity of TiO2 than the single doping method.

Robust type-III C3N/Ga2O3 van der Waals heterostructures

By combining two-dimensional (2D) materials with different band alignments at the contact interface, van der Waals heterostructures (vdWHs) can achieve multifunctional device applications. Here, through first principles methods, we constructed two C3N/Ga2O3 vdWHs named P↑ and P↓ because of the intrinsic polarization of Ga2O3 to study electronic, interfacial and optical characteristics. We find that the P↑ and P↓ both display a unique kind of band alignment known as broken-gap or type-III. This feature makes them highly suitable for tunnel field effect transistors (TFETs) applications. To the best of our knowledge, the tunnel window of the P↓ is the largest among the theoretical studies, reaching 0.993 eV. The electronic structure and band edge alignment of the C3N/Ga2O3 vdWHs show a weak dependence on strain engineering or external electric field. Normal-to-plane compressive strains effectively enhance the tunneling probability. We can acquire essential insights into this novel material system and its promise for future TFETs by analyzing the electrical characteristics of the C3N/Ga2O3 vdWHs.

Insights into heterogeneous nucleation interface of Y2O3//NbN via first principles calculation

The heterogeneous nucleation interface between Y2O3 and NbN was investigated by first principles method in this paper. The different low index interfaces mismatch degrees of Y2O3//NbN were calculated. Then, the interface models of Y2O3//NbN were constructed, and their interface properties were analyzed. The results show that the mismatch degree of Y2O3(111)//NbN(110) interface is the smallest, which is 7.61%. It reveals the effectiveness of Y2O3 as the heterogeneous nucleation nucleus of NbN is medium. Four interface models of Y2O3(111)//NbN(110) interface are constructed, namely Y-NbNⅠ, Y-NbNII, O-NbNⅠ and O-NbNII interfaces. Among them, the adhesive work of O-NbNII interface is the largest, which is 6.26 J/m2, and its interface energy is the smallest, which is −3.08 J/m2. The O-NbNII interface mainly involves the binding of Nb-O ionic bonds, and N-O covalent bonds. Therefore, Y2O3 can serve as the heterogeneous nucleation nucleus of NbN, and the O-NbNII interface is the most stable one.

Tailoring intrinsic properties for tetrahedral carbon/copper composites construction via electronic interface engineering

The electronic structure induced interface properties are important for active control of the advanced equipment performance. Nevertheless, it is difficult to reveal the intrinsic mechanism by experiments. In this paper, the interfacial properties and electronic characteristics of eight different stacking ta-C/Cu interfaces were studied via first principles method. The work of adhesion of ta-C/Cu(110)-2nd interface the largest (6.19 J/m2) and its interfacial energy is the smallest (3.74 J/m2), which indicates that it is the most stable structure with the smallest interface formation resistance. In addition, the polar covalent bonds dominated by electronic structures are major contributor of the ta-C/Cu interface properties. During tensile process, the critical strain of ta-C/Cu(110)-2nd is the largest (17.0 %), and the tensile strength of ta-C/Cu(100)-2nd is the largest (61.59 GPa). When all interfaces are fractured under tensile force, the atoms on Cu surfaces are transferred to ta-C surfaces and separated into two free parts. When the energy of one of the wave valleys first reaches the threshold of the lowest plane-averaged potential, the interfacial structure breaks at the interfacial gap corresponding to the wave valley. Changes in interfacial bonding properties can therefore be judged by changes in the magnitude of the electrostatic potential.

Optimized InGaAs growth on GaP/Si(1 0 0) templates with different low-temperature layers

In this paper, the effect of low-temperature buffer layer on the quality of InGaAs grown on GaP/Si templates is investigated. Three different materials, InP, GaAs/InAs and InAs/InP, were grown on GaP/Si templates by molecular beam epitaxy (MBE). The interfacial states between the above materials and the GaP layer were calculated using first principles methods. It was found that the interfacial structure formed with the InAs/InP material was the most robust due to the shorter bond lengths and stronger interaction forces between the interfaces. Theoretical calculations are in agreement with the experimental results that the InGaAs grown using the InAs/InP digital alloy structure as a low temperature buffer layer has the best material quality.

First-principles investigations of the interaction between alloying atom and dislocation and its implication to the rafting of Ni-based superalloys

The rafting of the microstructure of Ni-based superalloys significantly deteriorates the mechanical properties. According to the plastic rafting model based on the dislocation mechanism, the directional atomic diffusion induced by the interaction between the alloying atom and the interfacial dislocation is critical to the evolution of the rafting microstructure. In the present work, the interactions between the alloying atom and dislocation core in both matrix γ phase and the precipitate γ′ phase are investigated by using a first principles method in combination with the semi-discrete variational Peierls-Nabarro dislocation model. We show that the alloying atoms Al, Cr, Co, Mo, Ru, Ta, W, Re lower the (edge for Cr and Co) dislocation core energy in the γ phase, indicating that the alloying atoms are attractive to the dislocation core. For the γ′ phase, Cr, Mo, Ta, W, Re raise whereas Co and Ru lower the dislocation core energy, i.e., Cr, Mo, Ta, W, Re are repulsive but Co and Ru are attractive to the dislocation core. With the calculated interaction between the alloying atoms and the dislocation core, the influence of these alloying elements on the rafting of the superalloy is discussed.

Prediction of high temperature mechanical and thermodynamic properties of W-Mn alloys based on first principles method

In this paper, four kinds of W-Mn alloy structure models are constructed, namely W15Mn1, W14Mn2, W12Mn4 and W8Mn8 alloys. By investigating the phonon spectra of these alloys, it is found that W8Mn8 alloy is thermodynamically unstable. Therefore, the mechanical and thermodynamic properties of W15Mn1, W14Mn2 and W12Mn4 alloys are calculated based on first principles method. Pure W is also introduced as a comparison object. The investigation demonstrates that the bulk modulus of W-Mn alloys decreases with increasing temperature. In particular, W14Mn2 and W12Mn4 alloys decay rapidly with the increase of temperature, which indicates that excessive doping of Mn in W leads to instability of W-Mn alloys at high temperatures. For W15Mn1 alloy, the coefficient of thermal expansion is greater than that of pure W, and increases with increasing temperature, showing reasonable thermal expansion behavior. For W14Mn2 and W12Mn4 alloys, the coefficient of thermal expansion increases rapidly with increasing temperature may lead to the deformation or destruction of W-Mn alloys at high temperatures. However, all alloys have lower thermal conductivity than pure W. The thermal conductivity follows the order: pure W˃W14Mn2˃ W15Mn1˃W12Mn4. At 0 K, doping Mn in W decreases the mechanical strength of pure W, but improves its ductility. Other parameters such as anisotropy, Debye temperature, melting point, hardness, free energy, entropy, heat capacity, and equilibrium volume are also investigated. The improvement of the above performance is more conducive to the application of W-Mn alloys in the aviation field and as plasma facing first wall materials in nuclear fusion.

Physical characteristics of Cs2XTlCl6(X = Sc, Y) double perovskites for energy harvesting applications

To compute the structural, electronic, optical, elastic, and thermoelectric (TE) characteristics of Cs2ScTlCl6 and Cs2YTlCl6 halide double perovskites, the first principles method is employed by using density functional theory (DFT) based full potential linearized augmented plane wave (FP-LAPW) scheme. Band structure (BS) and density of states (DOS) revealed the direct band gap (Eg) of 4.44/5.22 eV for Cs2ScTlCl6/Cs2YTlCl6, correspondingly. To explore the thermodynamic and structural stability of the Cs2ScTlCl6 and Cs2YTlCl6, the formation enthalpy (ΔH) and tolerance factor (τ) are computed. The optical absorption edges are located at 3.8 for Cs2ScTlCl6 and 5.1 eV for Cs2YTlCl6. The distinct absorption characteristics observed, particularly in the UV spectrum, suggest that both materials hold promise for use in advanced optoelectronic devices. Elastic constants for both compounds are predicted to determine mechanical features. The Poisson's ratio and Cauchy pressure (CP) depicted brittle nature of both perovskites. Furthermore, BoltzTraP program is utilized to examine TE features and the analysis of ZT spectrum reveals an increase in ZT, rising from 0.747 to 0.800 for Cs2ScTlCl6 and from 0.740 to 0.802 for Cs2YTlCl6, as temperatures increase from 300 to 800 K. TE materials, with enhanced ZT values, hold promise for diverse applications, including efficient power generation, wearable technology, automotive systems, space exploration, and industrial processes.

Defects in <i>h</i>-BN: computer simulation of size effects

The influence of size effects on the properties of a hexagonal boron nitride (h-BN) monolayer containing CBVN, NBVN, and OBOBVN defects has been studied by first principles methods. These defects are potentially capable of generating single photons in quantum optics and quantum information devices. Size effects here mean the dependence of the studied model properties on the simulated fragment size of the 2D structure under periodic boundary conditions. Physically, this means that the properties of a monolayer depend on the distance between defects. This dependence allows us to judge how strongly the defects interact with each other and whether they interact at all. For technical applications, the characteristics of the band structure (band gap, spectrum and density of electron states induced by the defect in the band gap) and the atomic structure of the defect (defect formation energy, geometry in the equilibrium configuration), which form this band pattern, are important. In this work, these properties are studied by using the density functional theory with the basis of atom-like functions (SIESTA package) and plane waves (VASP package). The results obtained using both packages are consistent with each other. It has been established that the defects can be considered non-interacting, when the distance between them is ten unit cell parameters.

Adsorption behaviors of organic molecules on Nb/Zr–modified bismuthene: A first–principles study

In this work, the geometric and electronic properties of the adsorption of volatile organic molecules (VOCs) such as CH2O, C2H4O, C2H6O on bismuthene monolayer with transition metal (TM = Nb, Zr) atoms adsorbed are investigated by using first–principles methods, aiming to explore the potential applications of TM–modified bismuthene monolayer in the field of organic molecular gas sensors. In contrast to the adsorption of organic molecules on Zr–bismuthene, Nb–bismuthene appears the chemisorption with multiple organic molecules, and it can be found that Nb–bismuthene could stably adsorb target molecules and exhibit outstanding charge transfer. The longer desorption time of organic molecules on Nb–bismuthene and the higher organic molecular adsorption capacity of the substrate at most temperatures, which suggests that Nb–bismuthene is suitable as a disposable sorbent for organic molecules, providing a viable direction for the application of Nb–bismuthene monolayer as a sensitive material for the removal of organic molecules.

Theoretical insights into the structural, electronic and thermoelectric properties of the inorganic biphenylene monolayer.

Being motivated by a recently synthesized biphenylene carbon monolayer (BPN), using first principles methods, we have studied its inorganic analogue (B-N analogue) named I-BPN. A comparative study of structural, electronic and mechanical properties between BPN and I-BPN was carried out. Like BPN, the stability of I-BPN was verified in terms of formation energy, phonon dispersion calculations, and mechanical parameters (Young's modulus and Poisson's ratio). The chemical inertness of I-BPN was also investigated by adsorbing an oxygen molecule in an oxygen-rich environment. It has been found that the B-B bond favours the oxygen molecule to be adsorbed through chemisorption. The lattice transport properties reveal that the phonon thermal conductivity of I-BPN is ten times lower than that of BPN. The electronic band structure reveals that I-BPN is a semiconductor with an indirect bandgap of 1.88 eV, while BPN shows metallic behaviour. In addition, we investigated various thermoelectric properties of I-BPN for possible thermoelectric applications. The thermoelectric parameters, such as the Seebeck coefficient, show the highest peak value of 0.00289 V K-1 at 300 K. Electronic transport properties reveal that I-BPN is highly anisotropic along the x and y-axes. Furthermore, the thermoelectric power factor as a function of chemical potential shows a peak value of 0.057 W m-1 K-2 along the x-axis in the p-type doping region. The electronic figure of merit shows a peak value of approximately unity. However, considering lattice thermal conductivity, the peak value of the total figure of merit (ZT) reduces to 0.68(0.46) for p-type and 0.56(0.48) for n-type doping regions along the x(y) direction at 900 K. It is worth noting that our calculated ZT value of I-BPN is higher than that of many other reported B-N composite materials.

Electric field enhances the electronic and diffusion properties of penta-graphene nanoribbon anodes in lithium-ion batteries.

Enhancement of the ionic conductivity and reduction of diffusion barriers of lithium-ion batteries are crucial for improving the performance of the fast-growing energy storage devices. Recently, the fast-charging capability of commercial-like lithium-ion anodes with the smallest modification of the current manufacturing technology has been of great interest. We used first principles methods computations with density functional theory and the climbing image-nudged elastic band method to evaluate the impact of an external electric field on the stability, electronic band gap, ionic conductivity, and lithium-ion diffusion coefficient of penta-graphene nanoribbons upon lithium adsorption. By adsorbing a lithium atom, these semiconductor nanoribbons become metal with a formation energy of -0.22 eV, and an applied electric field perpendicular to the surface of these nanoribbons further stabilizes the structure of these lithium-ion systems. Using the Nernst-Einstein relation, in the absence of an electric field, the ionic conductivity of these penta-graphene nanoribbons amounts to 1.24 × 10-4 S cm-1. In the presence of an electric field, this conductivity can reach a maximum value of 8.89 × 10-2 S cm-1, emphasizing the promising role of an electric field for supporting fast-charging capability. Our results highlight the role of an external electric field as a novel switch to improve the efficiency of lithium-ion batteries with penta-graphene nanoribbon electrodes and open a new horizon for the use of pentagonal materials as anode materials in the lithium-ion battery industry.

Self-energy correction and numerical simulation for efficient lead-free double perovskite solar cells.

Inorganic double perovskites have garnered significant attention due to their desirable characteristics, such as low-toxicity, stability and long charge-carrier lifetimes. However, most double perovskites, especially Cs2AgBiBr6, have wide bandgaps, which limits power conversion efficiencies. In this work, through the first principles method corrected by self-energy, we investigate the mechanical, electric and optical properties of Cs2B'B''Br6 (B' = Ag, Au, Cu; B'' = Bi, Al, Sb, In). Based on performance screening, three kinds of materials with good toughness, high carrier mobility and wide visible-light absorption (around 105 cm-1) are obtained, which are compared with Cs2AgBiBr6. Meanwhile, we use a SACPS-1D simulation to design lead-free double perovskites with excellent properties suitable for photovoltaic solar cell devices, which are made into a planar perovskite heterojunction. Ultimately, the optimal structure is determined to be FTO/WS2/Cs2CuBiBr6/spiro-OMeTAD/Ag, which achieves a power conversion efficiency of 14.08%, surpassing the conventional structure efficiency of 6.1%. It provides valuable guidance for the structure design of a lead-free double perovskite device and offers new insights into the development of optoelectronic devices for solar energy utilization.

Computational analysis of electronic structure and optical properties of monocrystalline silicon-vacancy defect system based on density functional theory

Monocrystalline silicon is an important substrate or base material for manufacturing integrated circuit chips, mirrors, etc. However, it is a typical hard and brittle material, which is very prone to produce defects during processing. This paper investigates the electronic structure and optical properties of sphalerite monocrystalline silicon with vacancy concentrations of 1.5625%, 3.125%, 4.6875%, and 6.25% using the first principles method based on density functional theory. The results demonstrate that the introduction of impurities results in the formation of impurity bands (IB) within the band structure, thereby facilitating electronic transitions. However, vacancy defects do not alter the properties of monocrystalline silicon as an indirect semiconductor. Regarding optical properties, an increase in defect concentration leads to a decrease in the macroscopic symmetry of the system, resulting in noticeable anisotropy in each property. Also found that the main spectral peaks of the defect system are red-shifted, and the vacancy defects introduce new peaks in the near-infrared region and the visible region with increasing intensity, but the peaks in the ultraviolet region gradually decrease with the increase of the defect concentration. The refractive index peak gradually decreases with the increase of defect concentration, and the calculated average refractive index is around 1.2. In the high-energy region, the refractive index takes a very small value, and the reflection of electromagnetic waves from the doped system is also weak at this time, which indicates that the system has a high transmittance property. Importantly, there is a higher intensity of energy loss at photon energies around 16.5 eV. Consequently, these calculations are deemed valuable for advancing research and applications involving monocrystalline silicon optical mirrors.

First-principles investigation of structural, electronic, and optical properties of cubic halide perovskite RbSrI3

In this article, the structural, electronic, and optical properties of lead-free cubic halide perovskite RbSrI3 are investigated by the first principles method that employs the density functional theory (DFT). This study implements the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE-GGA) as the exchange-correlation potential. The structural stability of the compound is calculated using the Murnaghan equation of state, and the band structure calculations show the presence of a wide indirect bandgap of 3.32 eV. The optical response of the compound over the energy spectrum of 0–12 eV is studied and the compound exhibits good reflectance of 30.73 % at 7.66 eV and absorption of 112.45 × 104 cm−1 at 11.74 eV in the ultraviolet region. The values of reflectivity R(ω) and absorption coefficient α(ω) indicate that the compound shows potential in the fabrication of UV-related optoelectronic devices.

COMPARATIVE ANALYSIS OF THE AIR FLOW VELOCITY DETERMINATION METHODS

Study subject. The article presents the comparative analysis results of air flow speed measuring methods by using various physical principles (methods) of obtaining primary information. The task is to perform a comparative analysis of air flow speed measuring methods that based on pressure drop, as well as mechanical, thermoelectric, ultrasonic, laser and vortex methods. The goal is to assess the considered methods prospects for aircraft use. The obtained results: A comparative analysis of the main methods for airspeed measuring has been performed, and the obtained results have allowed to evaluate advantages and disadvantages of the considered methods. Conclusions: The performed comparative analysis results provided the conclusion that search for an optimal method remains actual task. The search of optimal method that will enhance the accuracy of airspeed determination with minimal implementation costs requires further research in this direction