Search Results (51)

Two−dimensional halide perovskites have emerged as promising optoelectronic materials, yet the uncontrolled defect formation during synthesis remains a critical challenge for their practical applications. In this work, we systematically investigate the structural, electronic, and optical properties of monolayer CsPb₂Br₅ in two representative configurations: ds−CsPb₂Br₅ and ss−CsPb₂Br₅. By introducing four types of vacancy defects—V_Br−c, V_Br−b, V_Cs, and V_Pb, we analyze their structural distortions, formation energies, and their impact on band structure and optical response using first−principles calculations. Our results reveal that Br−related vacancies are energetically most favorable and induce shallow defect levels and absorption edge redshifts in the ds−CsPb₂Br₅ structure, while in the ss−CsPb₂Br₅ configuration, only V_Br−b forms a defect state. V_Pb and V_Cs lead to significant sub−bandgap absorption enhancement and dielectric response due to band−edge reorganization, despite not introducing in−gap states. Notably, V_Br−c exhibits distinct infrared absorption in the ss−CsPb₂Br₅ model without electronic trap formation. These findings underscore the critical influence of defect type and slab asymmetry on the optoelectronic behavior of CsPb₂Br₅, providing guidance for defect engineering in perovskite−based optoelectronic applications. Full article

During the thermal treatment of kaolinite, the main mineral phase in kaolin rock, dehydroxylation occurs, forming metakaolin through a process that has significant industrial applications. This study experimentally analyzed dehydroxylation in two kaolinite samples: a well-crystallized reference sample from the Clay Mineral Society and a locally sourced, industrial kaolin sample. The mechanism and kinetic parameters were evaluated from a series of thermogravimetric measurements. Non-isothermal kinetic analysis revealed that dehydroxylation followed a third-order (F3) reaction mechanism, with activation energies ($E_a$) ranging from 35 to 60 kcal/mol. Additionally, theoretical calculations based on Density Functional Theory were performed on four systems in which a water molecule was removed by combining OH group and H atom vacancies in the kaolinite unit cell. These models represented the onset of dehydroxylation and provided values for the reaction energy Q from first-principles calculations, which served as reference values for $E_a$. The results confirm that water molecule formation involving both OH at the kaolinite outer surface and inner surface are energetically competitive and highlight the crucial role of structural relaxations following water removal to determine Q values in the range of 30–50 kcal/mol, in very good agreement with the experiments. Full article

Magnesium silicate (MgSiO₃) is widely utilized in glass manufacturing, with its performance influenced by structural modifications. In this study, we employ classical and density functional theory (DFT) simulations to investigate the defect and dopant characteristics of MgSiO₃. Our results indicate that a small amount of Mg-Si anti-site defects can exist in the material. Additionally, MgO Schottky defects are viable, requiring only slightly more energy to form than anti-site defects. Regarding the solubility of alkaline earth dopant elements, Ca preferentially incorporates into the Mg site without generating charge-compensating defects, while Zn exhibits a similar behavior among the 3D block elements. Al and Sc are promising dopants for substitution at the Si site, promoting the formation of Mg interstitials or oxygen vacancies, with the latter being the more energetically favorable process. The solution of isovalent dopants at the Si site is preferred by Ge and Ti. Furthermore, we analyze the electronic structures of the most favorable doped configurations. Full article

Using density functional theory (DFT) and scanning tunneling microscopy (STM), the intrinsic point defects, formation energy, and electronic structure of 1T-TiS₂ were investigated. Defect systems include single-atom vacancies, interstitial and adatom additions, and direct atomic substitution. Using a collective approach for analyzing realistic systems for point defect investigation, we provide a more straightforward comparison to the experimental measurements, reproducing more realistic environmental conditions related to thin film growth. STM images are compared to computationally simulated electron density images to identify specific geometries that result from favorable point defects. DFT suggests that titanium interstitials are the most energetically favorable intrinsic defect, and sulfur vacancies are more likely to form than titanium vacancies within this realistic analysis, which is in agreement with STM data. A pristine, stoichiometric monolayer system is calculated to have a direct band gap of 0.422 eV, which varies based on local point defects. Local semiconducting-to-metallic electronic transitions are predicted to occur based on the presence of Ti interstitials. Full article

To provide insight into the interface structure in Ti particle-reinforced Mg matrix composites, this study investigates the inherent Mg/Ti interface structure formed during the solidification of supercooled Mg melt on a (0001)Ti substrate using ab initio molecular dynamics (AIMD) simulations and density function theory (DFT) calculation. The resulting interface exhibits an orientation relationship of ${\left(0001\right)}_{\mathrm{Mg}}{//\left(0001\right)}_{\mathrm{Ti}}$ with a lattice mismatch of approximately 8%. Detailed characterizations reveal the occurrences of ${\left(0001\right)}_{\mathrm{Mg}}$ plane rotation and vacancy formation to overcome the lattice mismatch at the inherent Mg/Ti interface while allowing Mg atoms to occupy the energetically favorable hollow sites above the Ti atomic layer. The atomic diffusion behaviors of rare-earth elements Gd and Y at the Mg/Ti interface was examined using the climbing image nudged elastic band (CI-NEB) method, demonstrating a strong segregation tendency towards the interface promoted by the inherent interface structure. Additionally, the calculated Griffith work indicates enhanced interfacial adhesion due to the segregation of Gd and Y, which is beneficial for the mechanical properties of the composite. Full article

Animals utilize their well-evolved dynamic vision systems to perceive and evade collision threats. Driven by biological research, bio-inspired models based on lobula giant movement detectors (LGMDs) address certain gaps in constructing artificial collision-detecting vision systems with robust selectivity, offering reliable, low-cost, and miniaturized collision sensors across various scenes. Recent progress in neuroscience has revealed the energetic advantages of dendritic arrangements presynaptic to the LGMDs, which receive contrast polarity-specific signals on separate dendritic fields. Specifically, feed-forward inhibitory inputs arise from parallel ON/OFF pathways interacting with excitation. However, none of the previous research has investigated the evolution of a computational LGMD model with feed-forward inhibition (FFI) separated by opposite polarity. This study fills this vacancy by presenting an optimized neuronal model where FFI is divided into ON/OFF channels, each with distinct synaptic connections. To align with the energy efficiency of biological systems, we introduce an activation function associated with neural computation of FFI and interactions between local excitation and lateral inhibition within ON/OFF channels, ignoring non-active signal processing. This approach significantly improves the time efficiency of the LGMD model, focusing only on substantial luminance changes in image streams. The proposed neuronal model not only accelerates visual processing in relatively stationary scenes but also maintains robust selectivity to ON/OFF-contrast looming stimuli. Additionally, it can suppress translational motion to a moderate extent. Comparative testing with state-of-the-art based on ON/OFF channels was conducted systematically using a range of visual stimuli, including indoor structured and complex outdoor scenes. The results demonstrated significant time savings in silico while retaining original collision selectivity. Furthermore, the optimized model was implemented in the embedded vision system of a micro-mobile robot, achieving the highest success ratio of collision avoidance at 97.51% while nearly halving the processing time compared with previous models. This highlights a robust and parsimonious collision-sensing mode that effectively addresses real-world challenges. Full article

CO₂ photoreduction into valuable hydrocarbons, such as CO, CH₄, and C₂H₄, delivers a promising approach to address both environmental and energy challenges. Transition metal chalcogenides, particularly cadmium sulfide (CdS), have emerged as prominent candidates due to their tunable electronic properties and availability. This study delves into a comprehensive investigation of how CdS crystalline facets and sulfur-deficient surfaces modulate the product selectivity. Through employing density functional theory (DFT), we unravel the catalytic performance of various CdS crystal orientations and sulfur vacancy configurations. The results have shown that different CdS facets exhibit unique electronic characteristics and surface energetics, which influence the adsorption dynamics and reaction pathways. The introduction of sulfur vacancies further modulates the nature of active sites, leading to substantial shifts in product selectivity. A detailed investigation on the reaction mechanisms unveils that specific facets preferentially facilitate the formation of CO, while others are more conducive to the generation of hydrocarbons such as CH₄ and C₂H₄, due to the variations in activation barriers and intermediate stabilities. These findings underscore the importance of crystal facet engineering and defect manipulation in tailoring catalyst performance thus providing valuable insights for the rational design of efficient and selective CO₂ reduction metal catalysts. Full article

Recent studies revealed anomalous underscreening in concentrated electrolytes, and we suggest that the underscreened electrostatic forces between membrane proteins play a significant role in the process of self-assembly. In this work, we assumed that the underscreened electrostatic forces compete with the thermodynamic Casimir forces induced by concentration fluctuations in the lipid bilayer, and developed a simplified model for a binary mixture of oppositely charged membrane proteins with different preference to liquid-ordered and liquid-disordered domains in the membrane. In the model, like macromolecules interact with short-range Casimir attraction and long-range electrostatic repulsion, and the cross-interaction is of the opposite sign. We determine energetically favored patterns in a system in equilibrium with a bulk reservoir of the macromolecules. Different patterns consisting of clusters and stripes of the two components and of vacancies are energetically favorable for different values of the chemical potentials. Effects of thermal flutuations at low temperature are studied using Monte Carlo simulations in grand canonical and canonical ensembles. For fixed numbers of the macromolecules, a single two-component cluster with a regular pattern coexists with dispersed small one-component clusters, and the number of small clusters depends on the ratio of the numbers of the molecules of the two components. Our results show that the pattern formation is controlled by the shape of the interactions, the density of the proteins, and the proportion of the components. Full article

Tetrasodium silicate (Na₄SiO₄) has emerged as a promising candidate for battery applications due to its favorable ionic transport properties. Atomic-scale simulations employing classical pair potentials have elucidated the defect mechanisms and ion migration dynamics in Na₄SiO₄. The Na Frenkel defect, characterized by the creation of a Na vacancy and an interstitial Na⁺ ion, is identified as the most energetically favorable defect process, facilitating efficient vacancy-assisted Na⁺ ion migration. This process results in three-dimensional ion diffusion with a low activation energy of 0.55 eV, indicating rapid ion movement within the material. Among monovalent dopants (Li⁺, K⁺, and Rb⁺), K⁺ was found to be the most advantageous for substitution on the Na site. For trivalent doping, Al is the most favorable on the Si site, generating additional Na⁺ ions and potentially enhancing ionic conductivity. Ge was identified as a promising isovalent dopant for the Si site. These theoretical findings suggest that Na₄SiO₄ could offer high ionic conductivity and stability when optimized through appropriate doping. Experimental validation of these predictions could lead to the development of advanced battery materials with improved performance and durability. Full article

Nitrogen-doped graphene has been increasingly utilized in a variety of energy-related applications, serving as a catalyst or support material for fuel cells, and as an anode material for lithium-ion batteries, among others. The thermal reduction of graphene oxide (GO) in nitrogenous sources to incorporate nitrogen, producing nitrogen-doped reduced graphene oxide (NRGO), is the most favored method. Controlling atomic configurations of nitrogen-doped sites is the key factor for tailoring the physico-chemical properties of NRGO, but major challenges remain in identifying detailed atomic arrangements at nitrogen binding sites on highly defective and chemically functionalized GO surfaces. In this paper, we present atomistic-scale modeling of the nitrogen doping process of GO with different types of vacancy defects. Molecular dynamics simulations using a reactive force field indicate that the edge carbon atoms on defect sites are the dominant initiation location for nitrogen doping. Further, first-principles calculations using density functional theory present energetically favorable chemical transition pathways for nitrogen doping. The significance of this work lies in providing important chemical insights for the effective control of the desired properties of NRGO by suggesting a detailed mechanism of the nitrogen doping process of GO. Full article

The possibility of using graphane monolayer crystals as an electrode material is becoming popular. Graphane is stable at room temperature and has a large surface area, but its chemical inertness hinders its direct interactions with Li ions. In this study, we performed density functional theory calculations to study the energetic stability and structural and electronic properties of Li on graphane with various CH divacancy configurations (v₁₂, v₁₃, and v₁₄). The results show that the adsorption of the Li atom reduces the formation energy of the CH divacancy configurations. The Li-v₁₂ is most stable with the highest binding energy of 3.25 eV/Li and relaxes to in-plane with other C atoms. Altering the Li charge state to have Li⁻¹-v₁₂ or Li⁺¹-v₁₂ affects the energetic stability and electronic characters of Li-v₁₂. The Li⁻¹-v₁₂ (Li⁺¹-v₁₂) slightly (greatly) reduces the binding force between the Li and v₁₂ configuration, and furthermore it improves (deteriorates) the conductivity of the structure. Further investigation of graphane with vacancies is encouraged due to these intriguing observations, as it holds promise for potential utilization as an electrode material. Full article

Silicon, as the basic material of biochips and electronic devices, is often exposed to irradiation environments, and its radiation resistance has attracted much attention in recent decades. We calculated collision cascade in a silicon-based device under energetic Ar ions irradiation by using Monte Carlo and molecular dynamics simulations. The difference in vacancy probability density under different energetic incident ion irradiation is caused by the penetrating power and the straggling power of incident ions. The kinetic energy of an incident ion determines the size of local collision cascade density; a high energy incident ion can induce greater local collision cascade density. The efficiency of transferring energy from incident ions to target electrons at the silicon surface is more than in silicon, and the recoil atoms dissipate most of their energy at the lattice sites where they are stopping. These results provide more insight into the radiation resistance of silicon-based devices. Full article

Atomic diffusion by the vacancy defect of L1₂-Al₃M (M = Sc, Zr, Er, Y) was investigated based on a first-principles calculation. The point defect formation energies were firstly evaluated. Then, the migration energy for different diffusion paths was obtained by the climbing-image nudged elastic band (CI-NEB) method. The results showed that Al atomic and M atomic diffusions through nearest-neighbor jump (NNJ) mediated by Al vacancy (V_Al) were, respectively, the preferred diffusion paths in Al₃M phases under both Al-rich and M-rich conditions. The other mechanisms, such as six-jump cycle (6JC) and next-nearest-neighbor jump (NNNJ), were energetically inhibited. The order of activation barriers for NNJ(Al-V_Al) was Al₃Zr < Al₃Y < Al₃Er < Al₃Sc. The Al₃Sc phase had high stability with a high self-diffusion activation barrier, while the Al₃Zr and Al₃Y phases were relatively unstable with a low self-diffusion activation energy. Moreover, the atomic-diffusion behavior between the core and shell layers of L1₂-Al₃M was also further investigated. Zr atoms were prone to diffusion into the Al₃Y core layer, resulting in no stable core-shelled Al₃(Y,Zr), which well agreed with experimental observation. Full article

Nanostructured Guinier–Preston (GP) zones are critical for the strength of Al-Mg-Si(-Cu) aluminum alloys. However, there are controversial reports about the structure and growth mechanism of GP zones. In this study, we construct several atomic configurations of GP zones according to the previous research. Then first-principles calculations based on density functional theory were used to investigate the relatively stable atomic structure and GP-zones growth mechanism. The results show that on the (100) plane, GP zones consist of {MgSi} atomic layers without Al atoms, and the size tends to grow up to 2 nm. Along the (100) growth direction, even numbers of {MgSi} atomic layers are more energetically favorable and there exist Al atomic layers to relieve the lattice strain. {MgSi}₂Al₄ is the most energetically favorable GP-zones configuration, and the substitution sequence of Cu atoms in {MgSi}₂Al₄ during the aging process is Al → Si → Mg. The growth of GP zones is accompanied by the increase in Mg and Si solute atoms and the decrease in Al atoms. Point defects, such as Cu atoms and vacancies, exhibit different occupation tendencies in GP zones: Cu atoms tend to segregate in the Al layer near the GP zones, while vacancies tend to be captured by the GP zones. Full article

Cu/W composites are widely used in various industrial fields as they show thermomechanical properties suitable for a wide range of applications. Additionally, in semiconductor products, WTi in contact with Cu acts as a barrier material between Cu and Si/SiO₂. Therefore, the bonding behavior of both Cu/W and Cu/WTi is of great economical interest, also with respect to the effects that impurities could have on the behaviour of the Cu/W(Ti) interface. The segregation behavior of relevant impurities has not been studied in detail before. In this work, we create atomistic models of the Cu/W and Cu/WTi interfaces, compare their energetics to previously known interfaces and study the effect of segregation on the interface cohesion. We find that all investigated segregants, i.e. Ti, Cl, S, Al, H, O, and vacancies weaken the cohesion of the interface. Full article