The electronic properties of epitaxial heterojunctions consisting of the prototypical perovskite oxide semiconductor, $n-{\mathrm{SrTiO}}_3$, and the high-mobility Group IV semiconductor $p$-Ge have been investigated. Hard x-ray photoelectron spectroscopy with a new method of analysis has been used to determine band alignment while at the same time quantifying a large built-in potential found to be present within the Ge. Accordingly, the built-in potential within the Ge has been mapped in a layer-resolved fashion. Electron transfer from donors in the $n-\mathrm{SrTi}{\mathrm{O}}_3$ to the $p$-Ge creates a space-charge region in the Ge resulting in downward band bending, which spans most of the Ge gap. This strong downward band bending facilitates visible light, photogenerated electron transfer from Ge to STO, favorable to drive the hydrogen evolution reaction associated with water splitting. Ti 2p and $\mathrm{Sr}3d$ core-level line shapes reveal that the STO bands are flat despite the space-charge layer therein. Inclusion of the effect of Ge band bending on band alignment is significant, amounting to a $\sim 0.4\mathrm{eV}$ reduction in valence band offset compared to the value resulting from using spectra averaged over all layers. Density functional theory allows candidate interface structural models deduced from scanning transmission electron microscopy images to be simulated and structurally optimized. These structures are used to generate multislice simulations that reproduce the experimental images quite well. The calculated band offsets for these structures are in good agreement with experiment.

The TiN/$\mathrm{Si}{\mathrm{N}}_x$ nanocomposite and nanolaminate systems are the archetype for super if not ultrahard materials. Yet, the nature of the $\mathrm{Si}{\mathrm{N}}_x$ tissue phase is debated. Here, we show by atomically resolved electron microscopy methods that $\mathrm{Si}{\mathrm{N}}_x$ is epitaxially stabilized in a NaCl structure on the adjacent TiN(001) surfaces. Additionally, electron energy loss spectroscopy, supported by first-principles density functional theory calculations infer that $\mathrm{Si}{\mathrm{N}}_x$ hosts Si vacancies.

Tape-mediated mechanical exfoliation of layered materials is known to require multiple exfoliation steps and to produce small, irregularly shaped monolayer samples. Recently, it has been demonstrated experimentally that deposition of a thin layer of Au on the layer to be exfoliated prior to application of the tape can yield large monolayer samples of transition metal dichalcogenides in one exfoliation step. Here the mechanism underlying these improvements is explored using atomic scale total energy calculations to study thin-film-assisted mechanical exfoliation of both ${\mathrm{MoS}}_2$ and graphene. This study focuses on the influence of epitaxially induced biaxial strain on the exfoliated layer and how this impacts monolayer selectivity during exfoliation. For graphite modeled with a reactive empirical bond order (REBO) plus a Lennard-Jones potential, tensile biaxial strain favors monolayer exfoliation, whereas compressive strain favors multilayer exfoliation. For graphite modeled with a more accurate registry-dependent potential and for ${\mathrm{MoS}}_2$ modeled with a REBO plus a Lennard-Jones potential, both compressive and tensile biaxial strains favor monolayer exfoliation. A simple model based on the interlayer potential is introduced to explain these observations, and a linear stability analysis of this model shows that the differences between the thin-film-assisted exfoliation of the considered materials arise from the stacking structure of the layered material and the details of the interlayer potential. The potential for using this simple model to screen suitable metal films for exfoliation of other 2D materials is discussed.

Large-scale atomistic simulations with classical potentials can provide valuable insights into microscopic deformation mechanisms and defect-defect interactions in materials. Unfortunately, these assets often come with the uncertainty of whether the observed mechanisms are based on realistic physical phenomena or whether they are artifacts of the employed material models. One such example is the often reported occurrence of stable planar faults (PFs) in body-centered cubic (bcc) metals subjected to high strains, e.g., at crack tips or in strained nano-objects. In this paper, we study the strain dependence of the generalized stacking fault energy (GSFE) of $\left\{110\right\}$ planes in various bcc metals with material models of increasing sophistication, i.e., (modified) embedded atom method, angular-dependent, Tersoff, and bond-order potentials as well as density functional theory. We show that under applied tensile strains the GSFE curves of many classical potentials exhibit a local minimum which gives rise to the formation of stable PFs. These PFs do not appear when more sophisticated material models are used and have thus to be regarded as artifacts of the potentials. We demonstrate that the local GSFE minimum is not formed for reasons of symmetry and we recommend including the determination of the strain-dependent $\left(110\right)$ GSFE as a benchmark for newly developed potentials.

Understanding the energetics of radiation-induced defects is critical to the development of next-generation materials for nuclear and other energy systems and for aerospace applications. However, it remains a challenge to experimentally characterize defect morphologies and energies, especially in regard to anion defects in irradiated oxides. Here, using a combination of advanced structural and calorimetric characterization techniques, we show that the energetic response of defects in ${\mathrm{CeO}}_2$ is strongly coupled to atomic disordering on the oxygen sublattice induced by energetic heavy ions. Fitting of calorimetric data yields an estimate of stored energy in the form of defects and microstrain. These results provide a means to calculate the efficiency of structural destabilization and aid in a better understanding of defect formation and annealing mechanisms in fluorite-structured materials subject to extreme conditions.

Methane is the simplest hydrocarbon and its aggregation structures are determined merely by van der Waals (vdW) interaction. Here we present the direct observation of a vdW two-dimensional crystal and small clusters of methane on Cu surfaces using a low-temperature scanning tunneling microscope. Methane shows a hexagonal molecular-packing structure on the Cu(110) and Cu(111) surfaces. However, methane preferentially forms a dimer and a trimer on the $\mathrm{Cu}\left(110\right)-(2\times 1)\text{-O}$ added-row structure, indicating different aggregation behavior from the bare Cu surfaces. The adsorption structures are examined in detail by density functional theory calculations, providing atomistic insights into the adsorption geometry and aggregation mechanism of methane on the Cu surfaces, which are governed by a subtle balance between the intermolecular and molecule–surface interactions.

Nanoscale membranes have emerged as a new class of vertical nanostructures that enable the integration of horizontal networks of III-V nanowires on a chip. To generalize this method to the whole family of III-Vs, progress in the understanding of the membrane formation by selective area epitaxy in oxide slits is needed, in particular for different slit orientations. Here, it is demonstrated that the shape is primarily driven by the growth kinetics rather than determined by surface energy minimization as commonly occurs for faceted nanostructures. To this end, a phase-field model simulating the shape evolution during growth is devised, in agreement with the experimental findings for any slit orientations, even when the vertical membranes turn into multifaceted fins. This makes it possible to reverseengineer the facet-dependent incorporation times, which were so far unknown, even for common low-index facets. The compelling reproduction of the experimental morphologies demonstrates the reliability of the growth model and offers a general method to determine microscopic kinetic parameters governing out-of-equilibrium three-dimensional growth.

We report on a laser annealing procedure which greatly improves the quality of suspended monolayers of chemical vapor deposition grown ${\mathrm{MoSe}}_2$. Annealing with a green laser locally heats the suspended flake to approximately 600 K, which both removes contaminants and reduces strain gradients. At 4 K, we observe linewidths as narrow as 3.5 meV (1.6 nm) full width at half maximum for both photoluminescence and reflection. Large peak reflectances up to 47% are also observed. These values are comparable to those of the highest quality hexagonal boron nitride encapsulated samples. We demonstrate that this laser annealing process can yield highly spatially homogeneous samples, with the length scale of the homogeneity limited primarily by the size of the suspended area. Annealed regions are very stable, exhibiting negligible deterioration over 24 h at cryogenic temperatures. The annealing method is also very repeatable, with substantial improvements of sample quality on every spot ($>40$) tested.

The magnetic behavior of Sn-substituted ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ has been examined through magnetization measurements and single-crystal neutron diffraction on ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_4$ for $0.05\lesssim x\lesssim 0.35$. For all $x$ examined, the magnetic ground state is similar to that induced in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ by application of a magnetic field. This magnetic state is characterized by a small net moment due to the alignment of canted moments within an otherwise antiferromagnetic structure. The weak ferromagnetic behavior is observed in the magnetization data for both unique crystallographic orientations. Diffraction data reveal an anisotropic response in the lattice parameters, with an overall lattice expansion, as well as a small reduction in the rotation of the ${\mathrm{IrO}}_6$ octahedra. The magnetic ordering temperature is continually suppressed with increasing $x$, and long-range magnetic order persists to the highest Sn concentrations synthesized.

Fundamental theories predict that reductions in thermal conductivity from point and extended defects can arise due to phonon scattering with localized strain fields. To experimentally determine how these strain fields impact phonon scattering mechanisms, we employ ion irradiation as a controlled means of introducing strain and assorted defects into the lattice. In particular, we observe the reduction in thermal conductivity of intrinsic natural silicon after self-irradiation with two different silicon isotopes, ${}^{28}\mathrm{Si}^{+}$ and ${}^{29}\mathrm{Si}^{+}$. Irradiating with an isotope with a nearly identical atomic mass as the majority of the host lattice produces a damage profile lacking mass impurities and allows us to assess the role of phonon scattering with local strain fields on the thermal conductivity. Our results demonstrate that point defects will decrease the thermal conductivity more so than spatially extended defect structures assuming the same volumetric defect concentrations due to the larger strain per defect that arises in spatially separated point defects. With thermal conductivity models using density functional theory, we show that for a given defect concentration, the type of defect (i.e., point vs extended) plays a negligible role in reducing the thermal conductivity compared to the strain per defect in a given volume.

We present first-principles calculations of the solid-state phase diagram of PbTe-SrTe that include both configurational and vibrational contributions to the free energy. These configurational and vibrational free energies were calculated using the cluster expansion (CE) approach and quasiharmonic phonon calculations, respectively. The coupled configurational and vibrational degrees of freedom were taken into account by including temperature-dependent CE interactions with the $T=0\mathrm{K}$ values and temperature dependence of the interactions determined from total energies and phonon free energies, both calculated using density functional theory. This Hamiltonian was then employed in Monte Carlo simulations to obtain a quantitative prediction of the PbTe-SrTe phase diagram. The phase diagram consists of a miscibility gap with a maximum temperature of ∼500 K, with the inclusion of the vibrational entropy resulting in a more than fourfold increase in the calculated solubility limits of SrTe in PbTe. Our results suggest that the equilibrium solubility of this system is likely much larger than previously thought and deserves further experimental investigation.

Rare-earth iron garnet thin films with perpendicular magnetic anisotropy (PMA) have recently attracted a great deal of attention for spintronic applications. Thulium iron garnet (TmIG) has been successfully grown and TmIG/Pt heterostructures have been characterized. However, TmIG is not the only rare-earth iron garnet that can be grown with PMA. We report the growth, magnetic, and spintronic properties of epitaxial terbium iron garnet (TbIG) and europium iron garnet (EuIG) thin films with PMA. Reciprocal space mapping shows the films are lattice matched to the substrate without strain relaxation, even for films up to 56 nm thick. The lattice strain and magnetostriction coefficient produce PMA in certain cases. TbIG grows on (111) gadolinium gallium garnet (GGG) with PMA due to the in-plane compressive strain, whereas TbIG on (111) substituted GGG (SGGG) is in tension and has an in-plane easy axis. EuIG grows with PMA on (100) and (111) GGG substrates, which facilitates the investigation of spintronic properties as a function of orientation. Both garnets have excess rare earth, which is believed to occupy Fe octahedral sites and in the case of TbIG is associated with an increase in the compensation temperature to 330 K, higher than the bulk value. Anomalous Hall effect (AHE) measurements of Pt/EuIG Hall crosses show that the spin mixing conductance of Pt/ (111) and (100) EuIG is similar. AHE measurements of Pt/TbIG Hall crosses reveal a sign change in the AHE amplitude at the compensation point analogous to all-metallic systems.

Since Mohs devised his prominent scale for mineral classification, the scratching behavior of inorganic solids has been intuitively related to material hardness. However, lateral deformation testing by instrumented indentation reveals a large variability in the resistance to scratch deformation relative to hardness, caused by the extent to which scratching requires higher work of deformation at given hardness due to material pileup and friction. Across a broad variety of glassy materials (with covalent, ionic, and metallic bonding), there is a strong correlation between scratch hardness and bulk modulus. Other than in crystalline materials, however, the spatial distribution of bond energy is heterogeneous on a molecular scale, so that no simple correlation exists with the mean-field average of bond energy density. Instead, inherent heterogeneity in the spatial distribution of bond energy and associated fluctuations in rigidity on a superstructural scale suggest an analogy between glasses and granular media.

Using the strongly constrained and appropriately normed (SCAN) and SCAN+$U$ approximations for describing electron exchange correlation (XC) within density functional theory, we investigate the oxidation energetics, lattice constants, and electronic structure of binary Ce, Mn, and Fe oxides, which are crucial ingredients for generating renewable fuels using two-step, oxide-based, solar thermochemical reactors. Unlike other common XC functionals, we find that SCAN does not overbind the ${\mathrm{O}}_2$ molecule, based on direct calculations of its bond energy and robust agreement between calculated formation enthalpies of main group oxides versus experiments. However, in the case of transition-metal oxides, SCAN systematically overestimates (i.e., yields too negative) oxidation enthalpies due to remaining self-interaction errors in the description of their ground-state electronic structure. Adding a Hubbard $U$ term to the transition-metal centers, where the magnitude of $U$ is determined from experimental oxidation enthalpies, significantly improves the qualitative agreement and marginally improves the quantitative agreement of SCAN+$U$-calculated electronic structure and lattice parameters, respectively, with experiments. Importantly, SCAN predicts the wrong ground-state structure for a few oxides, namely, $\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3, \mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_3$, and $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$, while SCAN+$U$ predicts the right polymorph for all systems considered in this paper. Hence, the SCAN+$U$ framework, with an appropriately determined $U$, will be required to accurately describe ground-state properties and yield qualitatively consistent electronic properties for most transition-metal and rare-earth oxides.

We present a method that makes it possible to determine an electrode potential in an electrode/electrolyte solution system. We consider the electrode potential of the standard hydrogen electrode (SHE) reaction $\mathrm{R}:1/2{\mathrm{H}}_2\left(\mathrm{gas}\right)+{\mathrm{H}}_2\mathrm{O}\left(1\mathrm{M}\mathrm{HCl}\mathrm{aq}.\right)\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}\left(1\mathrm{M}\mathrm{HCl}\mathrm{aq}.\right)+e^{-}$ (electrode M), and conduct density functional theory (DFT) calculations combined with the effective screening medium (ESM) method and the reference interaction site model (RISM). The electrostatic field from a charged slab described by DFT with ESM is screened by that from the charge distribution in an electrolyte solution. This screening enables us to define the inner potential ${\mathrm{\Phi }}_{\mathrm{S}}$ at the bulk solution region, which is the reference potential for the electrode potential, that is, the chemical potential of electrons (${\mu }_{\mathrm{e}}$). Grand potentials of the left and right sides in reaction R at the equilibrium point derive the corresponding SHE potential of ${\mu }_{\mathrm{e}}^{\mathrm{SHE}}=-5.27\mathrm{eV}$ vs ${\mathrm{\Phi }}_{\mathrm{S}}$ for a Pt(111) electrode. Another pathway that uses the free energy difference gives the same SHE potential; the equivalence of the electrode potentials from chemical potential and from free energy difference is validated within ESM-RISM. Even using a different electrode of Al yields the value of ${\mu }_{\mathrm{e}}^{\mathrm{SHE}}=-5.22\mathrm{eV}$ vs ${\mathrm{\Phi }}_{\mathrm{S}}$, which indicates that the electrode potential is independent of the electrodes. Finally, the potential energy profile in a vacuum/metal/solution/vacuum region shows that a difference between the inner and outer potentials is necessary to compare an absolute SHE potential and the SHE potential vs ${\mathrm{\Phi }}_{\mathrm{S}}$.

Mechanical behavior of polycrystalline materials, such as creep and microstructural evolution, is dramatically impacted by the mobility of grain boundaries. Existing methods for extracting mobility that combine atomistic simulations with conventional Brownian-like random walk model for grain boundary motion miss critical insights and are reliable only above very high (and unrealistic) temperatures. In this paper, we present a computational method based on the classical Green-Kubo relation for computing interface mobility over a wide range of temperatures. Our study makes an intriguing revelation that the severe time limitation of molecular dynamics simulations warrants the use of a generalized diffusion equation for Brownian particles not accounted for in current studies. Moreover, the method furnishes analytical expressions for the interface mobility in terms of the velocity autocorrelation functions. Taken together, our results possibly provide the first reliable estimates for interface mobility in the limit of zero driving forces. This is in sharp contrast to studies based on the widely used interface random walk approach, which extracts mobility by fitting the conventional Brownian motion equation to atomistic simulations. The efficiency of the method and applicability over a range of temperatures should open avenues for integration of computations and experiments to understand and engineer material systems with desired properties.

We investigate nanolamination as a means of increasing the refractive index of materials, with a focus on [amorphous InGaZnO (a-IGZO)/${\mathrm{TiO}}_2{]}_i$ nanolaminates as composite transparent conducting oxides with a tunable index. We demonstrate that by periodic layering with ${\mathrm{TiO}}_2$ the refractive index of a-IGZO can be increased by up to $\mathrm{\Delta }n\approx 0.39$ while the carrier mobility and conductivity retain 70% of their original values. This is shown to be a result of the alignment of the conduction bands of a-IGZO and ${\mathrm{TiO}}_2$. We also use optical modeling to verify the integral refractive index behavior of the nanolaminates and outline its range of applicability.

The spiral growth of crystals, mediated by their screw dislocations, is a general crystal-growth mechanism observed over a large variety of crystalline solids. Despite its general nature, direct observations of the spiral growth of ice have been rarely reported so far. Here, with the aid of advanced optical microscopy, we succeed in making in situ observations of the perfect spiral growth during the vapor growth of ice. We find that the spiral steps observed are well described by the classical Burton–Cabrera–Frank theory, taking into account surface diffusion of water admolecules between adjacent steps. This is distinct from the dominant growth mode that we have assigned as spiral in our recent studies, which accompanies fluctuations of the adjacent step interval. We also successfully capture the birth of a screw dislocation and the ensuing spiral growth, originating in the lattice mismatch arising from the coalescence of single ice crystals. Furthermore, we demonstrate that the nucleation of quasi-liquid layers (QLLs) takes place at the spiral center immediately after the birth of the dislocation, which suggests a link between screw dislocations and the generation of QLLs near the ice melting point.

Atomistic simulations are employed to demonstrate the existence of a well-defined thermodynamic phase transformation between grain boundary (GB) phases with different atomic structures. The free energy of different interface structures for an embedded-atom-method model of the $\mathrm{\Sigma }5\left(310\right)\left[001\right]$ symmetric tilt boundary in elemental Cu is computed using the nonequilibrium Frenkel-Ladd thermodynamic integration method through molecular dynamics simulations. It is shown that the free-energy curves predict a temperature-induced first-order interfacial phase transition in the GB structure in agreement with computational studies of the same model system. Moreover, the role of vibrational entropy in the stabilization of the high-temperature GB phase is clarified. The calculated results are able to determine the GB phase stability at homologous temperatures less than 0.5, a temperature range particularly important given the limitation of the methods available hitherto in modeling GB phase transitions at low temperatures. The calculation of GB free energies complements currently available 0 K GB structure search methods, making feasible the characterization of GB phase diagrams.