Materials Science (cond-mat.mtrl-sci)

Abstract: Ferroelectric memristors have attracted much attention as a type of nonvolatile resistance switching memories in neuromorphic computing, image recognition, and information storage. Their resistance switching mechanisms have been studied several times in perovskite and complicated materials systems. It was interpreted as the modulation of carrier transport by polarization control over Schottky barriers. Here, we experimentally report the isothermal resistive switching across a CoPt/MgZnO Schottky barrier using a simple binary semiconductor. The crystal and texture properties showed high-quality and single-crystal Co$_{0.30}$Pt$_{0.70}$/Mg$_{0.20}$Zn$_{0.80}$O hetero-junctions. The resistive switching was examined by an electric-field cooling method that exhibited a ferroelectric T$_C$ of MgZnO close to the bulk value. The resistive switching across CoPt/MgZnO Schottky barrier was accompanied by a change in the Schottky barrier height of 26.5 meV due to an interfacial charge increase and/or orbital hybridization induced reversal of MgZnO polarization. The magnitude of the reversed polarization was estimated to be a reasonable value of 3.0 (8.25) $\mu$ C/cm$^2$ at 300 K (2 K). These findings demonstrated the utilities of CoPt/MgZnO interface as a potential candidate for ferroelectric memristors and can be extended to probe the resistive switching of other hexagonal ferroelectric materials.

Abstract: Neutron and $\gamma$-ray irradiation damages to transistors are found to be non-additive, and this is denoted as the irradiation synergistic effect (ISE). Its mechanism is not well-understood. The recent defect-based model [ACS Appl. Electron. Mater. 2, 3783 (2020)] for Silicon bipolar junction transistors (BJT) achieve quantitative agreement with experiments, but it remains phenomenological and its assumptions on the defect reactions are unverified. Going beyond the phenomenological model requires directly representing the effect of $\gamma$-ray irradiation in first-principles calculations, which is not feasible previously. In this work, we examine the defect-based model of the ISE by developing a multiscale method for the simulation of the $\gamma$-ray irradiation, where the $\gamma$-ray-induced electronic excitations are treated explicitly in excited-state first-principles calculations. We find the calculations agree with experiments, and the effect of the $\gamma$-ray-induced excitation is significantly different from the effects of defect charge state and temperature. We propose a diffusion-based qualitative explanation of the mechanism of positive/negative ISE in NPN/PNP BJTs in the end.

Abstract: Topotactic transition is a structural phase change in a matrix crystal lattice mediated by the ordered loss/gain and rearrangement of atoms, leading to unusual coordination environments and metal atoms with rare valent states. As early as in 1990s, low temperature hydride reduction was utilized to realize the topotactic transition. Since then, topological transformations have been developed via multiple approaches. Especially, the recent discovery of the Ni-based superconductivity in infinite-layer nickelates has greatly boosted the topotactic transition mean to synthesizing new oxides for exploring exotic functional properties. In this review, we have provided a detailed and generalized introduction to oxygen-related topotactic transition. The main body of our review include four parts: the structure-facilitated effects, the mechanism of the topotactic transition, some examples of topotactic transition methods adopted in different metal oxides (V, Mn, Fe, Co, Ni) and the related applications. This work is to provide timely and thorough strategies to successfully realize topotactic transitions for researchers who are eager to create new oxide phases or new oxide materials with desired functions.

Abstract: First-principles calculations were carried out to study the segregation behavior of Mg, and Cu and their effect on the energy and mechanical properties of different Al grain boundaries (GBs). Four symmetrical tilt GBs were selected for study, namely {\Sigma}5[001](210) GB, {\Sigma}5[001](310) GB, {\Sigma}9[110](221) GB, and {\Sigma}11[110](332) GB. The results show that both Mg and Cu have a segregation tendency at the GBs, and the segregation tendency of Cu is stronger than Mg. Mg is prone to form substitutional segregation at the GBs, but Cu is more likely to segregate at the interstitial sites. The segregation of Mg and Cu can reduce GB energy, and the GB energy continues to decrease with the increase of the segregation concentration. First-principles calculation tensile test shows that the segregation of Mg has a negative effect on the strength of GBs, and the GB strength decreases with the increase of the Mg concentration, while the GB strength was gradually enhanced with the increase of the Cu concentration. The strength of {\Sigma}5(210) GB and {\Sigma}9(221) GB are more sensitive to the segregation of solute atoms than the other two GBs. By calculating the charge density and the density of states of the pristine and the segregated GBs, it was found that the segregation of Mg caused charge depletion and structure expansion at the GBs, while the segregation of Cu increases the charge density of GBs and form new bonds with the surrounding Al atoms. The results provide useful information for improving the mechanical properties of materials by using the concept of GB segregation engineering.

Abstract: Exciton binding energy and lifetime are the two most important parameters controlling exciton dynamics, and the general consensus is that the larger the former the larger the latter. However our first-principles study of monolayer ferroelectric TiOCl$_2$ shows that this is not always the case. We find that ferroelectric polarization tends to weaken exciton binding but enhance exciton lifetime. This stems from the different effects of the induced built-in electric field and structural distortion by the spontaneous polarization: the former always destabilizes or even dissociates the exciton while the latter leads to a relaxation of the selection rule and activates excitons that are otherwise not optically active. Their combined effect leads to a halving of the exciton binding energy but a substantial increase in lifetime by 40 times. Our results deepen the understanding of the interaction of light with ferroelectric materials and provide new insights into the use of ferroelectricity to control exciton dynamics.

Abstract: To understand the interfacial bond formation between polycarbonate (PC) and magnetron-sputtered metal nitride thin films, PC | X interfaces (X = AlN, TiN, TiAlN) are comparatively investigated by ab initio simulations as well as X-ray photoelectron spectroscopy. The simulations predict significant differences at the interface, as N and Ti form bonds with all functional groups of the polymer, while Al reacts selectively only with the carbonate group of pristine PC. In good agreement with simulations, experimental data reveal that the PC | AlN and the PC | TiAlN interfaces are mainly defined by interfacial C-N bonds, whereas for PC | TiN, the interface formation is also characterized by numerous C-Ti and (C-O)-Ti bonds. Bond strength calculations combined with the measured interfacial bond density indicate the strongest interface for PC | TiAlN followed by PC | AlN, whereas the weakest is predicted for PC | TiN due to its lower density of strong interfacial C-N bonds. This study shows that the employed computational strategy enables prediction of the interfacial bond formation between PC and metal nitrides and that it is reasonable to assume that the research strategy proposed herein can be readily adapted to other organic | inorganic interfaces.

Abstract: Grain boundary (GB) segregation of solute atoms plays an important role in the microstructure and macroscopic mechanical properties of materials. The study of GB segregation of solute atoms using computational simulation has become one of the hot spots in recent years. However, most studies mainly focus on ground-state GB structures with the lowest energy, and the impact of GB metastability with higher energy on solute segregation remains poorly understood. In this work, the first-principles method based on the density functional theory was adopted to investigate the effect of solute atoms Mg and Cu segregation on ground-state {\Sigma}5(210) GB (GB-I) and metastable GBs(GB-II, GB-III) in Al. GB energy, segregation energy, and theoretical tensile strength of Mg and Cu segregation at three GBs were calculated. The results show that both Mg and Cu have a large driving force to segregate to Al GBs, which reduces the GB energy and improves improve GB stability. The segregation of Mg and Cu on GB-III induces the transformation of the GB structural unit and the GB structural phase transformations. For the above three GBs, Cu segregation increases the theoretical tensile strength of GBs to varying degrees. The segregation of Mg would reduce the resistance of GB-I and GB-II, but enhances the strength of GB-III. The effect of solute atoms segregation on the mechanical properties of GBs was investigated by charge density distribution and density of states.

Abstract: Many stiff biological materials exhibiting outstanding compressive strength/weight ratio are characterized by high porosity, spanning different size-scales, typical examples being bone and wood. A successful bio-mimicking of these materials is provided by a recently-obtained apatite, directly produced through a biomorphic transformation of natural wood and thus inheriting its highly hierarchical structure. This unique apatite (but also wood and bone) is characterized by two major distinct populations of differently-sized cylindrical voids, a porosity shown in the present paper to influence failure, both in terms of damage growth and fracture nucleation and propagation. This statement follows from failure analysis, developed through in-silico generation of artificial samples (reproducing the two-scale porosity of the material) and subsequent finite element modelling of damage, implemented with phase-field treatment for fracture growth. It is found that small voids promote damage nucleation and enhance bridging of macro-pores by micro-crack formation, while macro-pores influence the overall material response and drive the propagation of large fractures. Our results explain the important role of multiscale porosity characterizing stiff biological materials and lead to a new design paradigm, by introducing an in-silico tool to implement bio-mimicking in new artificial materials with brittle behaviour, such as carbide or ceramic foams.

Abstract: Atomistic spin dynamics (ASD) is a standard tool to model the magnetization dynamics of a variety of materials. The fundamental dynamical model underlying ASD is entirely classical. In this letter, we present two approaches to effectively incorporate quantum effects into ASD simulations, thus enhancing their low temperature predictions. The first allows to simulate the magnetic behavior of a quantum spin system by solving the equations of motions of a classical spin system at an effective temperature. This effective temperature is determined a priori from the microscopic properties of the system. The second approach is based on a semi-classical model where classical spins interact with an environment with a quantum-like power spectrum. The parameters that characterize this model can be calculated ab initio or extracted from experiments. This semi-classical model quantitatively reproduces the low-temperature behavior of a magnetic system, thus accounting for the quantum mechanical aspects of its dynamics. The methods presented here can be readily implemented in current ASD simulations with no additional complexity cost.

Abstract: We investigate the electronic properties of a graphene and $\alpha$-ruthenium trichloride (hereafter RuCl$_3$) heterostructure, using a combination of experimental and theoretical techniques. RuCl$_3$ is a Mott insulator and a Kitaev material, and its combination with graphene has gained increasing attention due to its potential applicability in novel electronic and optoelectronic devices. By using a combination of spatially resolved photoemission spectroscopy, low energy electron microscopy, and density functional theory (DFT) calculations we are able to provide a first direct visualization of the massive charge transfer from graphene to RuCl$_3$, which can modify the electronic properties of both materials, leading to novel electronic phenomena at their interface. The electronic band structure is compared to DFT calculations that confirm the occurrence of a Mott transition for RuCl$_3$. Finally, a measurement of spatially resolved work function allows for a direct estimate of the interface dipole between graphene and RuCl$_3$. The strong coupling between graphene and RuCl$_3$ could lead to new ways of manipulating electronic properties of two-dimensional lateral heterojunction. Understanding the electronic properties of this structure is pivotal for designing next generation low-power opto-electronics devices.

Abstract: Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Here we report predictions from an advanced self-consistent many body perturbative theory and show that with respect to one-particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses . With respect to two-particle properties, significant differences with Si appear. We report the excitonic spectrum for both q=0 and finite q, and show that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about $\sim$1.5 eV, which may play a crucial role in application of BAs in optoelectronics.

Abstract: In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a first crucial step to evaluate the reliability of such computations. We discuss here general recommendations for verification studies aiming at further testing precision and transferability of density-functional-theory computational approaches and codes. We illustrate such recommendations using a greatly expanded protocol covering the whole periodic table from Z=1 to 96 and characterizing 10 prototypical cubic compounds for each element: 4 unaries and 6 oxides, spanning a wide range of coordination numbers and oxidation states. The primary outcome is a reference dataset of 960 equations of state cross-checked between two all-electron codes, then used to verify and improve nine pseudopotential-based approaches. Such effort is facilitated by deploying AiiDA common workflows that perform automatic input parameter selection, provide identical input/output interfaces across codes, and ensure full reproducibility. Finally, we discuss the extent to which the current results for total energies can be reused for different goals (e.g., obtaining formation energies).