Understanding and engineering the properties of crystalline surfaces has been critical in achieving functional electronics at the nanoscale. Employing scanning tunneling microscopy, surface x-ray diffraction, and high-resolution x-ray reflectivity experiments, we present a thorough study of epitaxial graphene (EG)/Ge(110) and report a Ge(110) “6 × 2” reconstruction stabilized by the presence of epitaxial graphene unseen in group-IV semiconductor surfaces. X-ray studies reveal that graphene resides atop the surface reconstruction with a 0.34 nm van der Waals (vdW) gap and provides protection from ambient degradation.

${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$ presents a system whose ${\mathrm{Co}}^{2+}$ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the $S=1/2$ TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend toward zero-temperature order. Below 0.1 K, however, our low-field measurements show an unexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the $(H,T)$ phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low- and high-field phase boundaries, and an ordered regime apparently dominated by the collinear “up-up-down” state. ${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$, therefore, offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the $S=1/2$ Heisenberg TLAFM.

We propose a general approach to design broadband and wide-angle impedance-matched acoustic metamaterials. Such an unusual acoustic impedance matching characteristic can be well explained by using a spatially dispersive effective medium theory. For demonstrations, we used silicone rubber, which has a huge impedance contrast with water, to design one- and two-dimensional acoustic structures which are almost perfectly impedance matched to water for a wide range of incident angles and in a broad frequency band. Our work opens up an approach to realize extraordinary acoustic impedance matching properties via metamaterial-design techniques.

Halide substitution gives rise to a tunable band gap as a function of composition in halide perovskite materials. However, photoinduced phase segregation, observed at room temperature in mixed halide $A\text{Pb}{\left({\mathrm{I}}_x{\mathrm{Br}}_{1-x}\right)}_3$ systems, limits open circuit voltages and decreases photovoltaic device efficiencies. We investigate equilibrium phase stability of orthorhombic $Pnma\gamma$-phase $\text{Cs}M{\left(X_x{Y}_{1-x}\right)}_3$ perovskites where $M$ is Pb or Sn, and $X$ and $Y$ are Br, Cl, or I. Finite-temperature phase diagrams are constructed using a cluster expansion effective Hamiltonian parameterized from first-principles density-functional-theory calculations. Solid solution phases for $\text{Cs}M{\left({\mathrm{I}}_x{\mathrm{Br}}_{1-x}\right)}_3$ and $\text{Cs}M{\left({\mathrm{Br}}_x{\mathrm{Cl}}_{1-x}\right)}_3$ are predicted to be stable well below room temperature while $\text{Cs}M{\left({\mathrm{I}}_x{\mathrm{Cl}}_{1-x}\right)}_3$ systems have miscibility gaps that extend above 400 K. The height of the miscibility gap correlates with the difference in volume between end members. Also layered ground states are found on the convex hull at $x=2/3$ for ${\mathrm{CsSnBr}}_2\mathrm{Cl},{\mathrm{CsPbI}}_2\mathrm{Br}$, and ${\mathrm{CsPbBrCl}}_2$. The impact of these ground states on the finite temperature phase diagram is discussed in the context of the experimentally observed photoinduced phase segregation.

We apply machine learning (ML) methods to a database of 390 experimentally reported $AB{\mathrm{O}}_3$ compounds to construct two statistical models that predict possible new perovskite materials and possible new cubic perovskites. The first ML model classified the 390 compounds into 254 perovskites and 136 that are not perovskites with a 90% average cross-validation (CV) accuracy; the second ML model further classified the perovskites into 22 known cubic perovskites and 232 known noncubic perovskites with a 94% average CV accuracy. We find that the most effective chemical descriptors affecting our classification include largely geometric constructs such as the $A$ and $B$ Shannon ionic radii, the tolerance and octahedral factors, the $A$-O and $B$-O bond length, and the $A$ and $B$ Villars' Mendeleev numbers. We then construct an additional list of $625AB{\mathrm{O}}_3$ compounds assembled from charge conserving combinations of $A$ and $B$ atoms absent from our list of known compounds. Then, using the two ML models constructed on the known compounds, we predict that 235 of the 625 exist in a perovskite structure with a confidence greater than 50% and among them that 20 exist in the cubic structure (albeit, the latter with only $\sim 50\%$ confidence). We find that the new perovskites are most likely to occur when the $A$ and $B$ atoms are a lanthanide or actinide, when the $A$ atom is an alkali, alkali earth, or late transition metal atom, or when the $B$ atom is a $p$-block atom. We also compare the ML findings with the density functional theory calculations and convex hull analyses in the Open Quantum Materials Database (OQMD), which predicts the $T=0$ K ground-state stability of all the $AB{\mathrm{O}}_3$ compounds. We find that OQMD predicts 186 of 254 of the perovskites in the experimental database to be thermodynamically stable within 100 meV/atom of the convex hull and predicts 87 of the 235 ML-predicted perovskite compounds to be thermodynamically stable within 100 meV/atom of the convex hull, including 6 of these to be in cubic structures. We suggest these 87 as the most promising candidates for future experimental synthesis of novel perovskites.

Linear scaling relations of adsorbate energies across a range of catalytic surfaces have emerged as a central interpretive paradigm in heterogeneous catalysis. They are, however, typically developed for low adsorbate coverages which are not always representative of realistic heterogeneous catalytic environments. Herein, we present generalized linear scaling relations on transition metals that explicitly consider adsorbate-coadsorbate interactions at variable coverages. The slopes of these scaling relations do not follow the simple bond counting principles that govern scaling on transition metals at lower coverages. The deviations from bond counting are explained using a pairwise interaction model wherein the interaction parameter determines the slope of the scaling relationship on a given metal at variable coadsorbate coverages, and the slope across different metals at fixed coadsorbate coverage is approximated by adding a coverage-dependent correction to the standard bond counting contribution. The analysis provides a compact explanation for coverage-dependent deviations from bond counting in scaling relationships and suggests a useful strategy for incorporation of coverage effects into catalytic trends studies.

Bismide semiconductor materials and heterostructures are considered a promising candidate for the design and implementation of photonic, thermoelectric, photovoltaic, and spintronic devices. This work presents a detailed theoretical study of the electronic and optical properties of strongly coupled ${\mathrm{GaBi}}_x{\mathrm{As}}_{1-x}$/GaAs multiple quantum well (MQW) structures. Based on a systematic set of large-scale atomistic tight-binding calculations, our results reveal that the impact of atomic-scale fluctuations in alloy composition is stronger than the interwell coupling effect, and plays an important role in the electronic and optical properties of the investigated MQW structures. Independent of QW geometry parameters, alloy disorder leads to a strong confinement of charge carriers, a large broadening of the hole energies, and a red-shift in the ground-state transition wavelength. Polarization-resolved optical transition strengths exhibit a striking effect of disorder, where the inhomogeneous broadening could exceed an order of magnitude for MQWs, in comparison to a factor of about 3 for single QWs. The strong influence of alloy disorder effects persists when small variations in the size and composition of MQWs typically expected in a realistic experimental environment are considered. The presented results highlight the limited scope of continuum methods and emphasize on the need for large-scale atomistic approaches to design devices with tailored functionalities based on the novel properties of bismide materials.

We have investigated the properties and conduction limitations of spray pyrolysis grown, low-cost transparent conducting oxide ZnO thin films doped with indium. We analyze the optical, electrical, and crystallographic properties as functions of In content with a specific focus on postgrowth heat treatment of these thin films at $320{}^{\circ }\mathrm{C}$ in an inert, nitrogen atmosphere, which improves the films electrical properties considerably. The effect was found to be dominated by nitrogen-induced grain-boundary passivation, identified by a combined study using $in$ situ resistance measurement upon annealing, x-ray photoelectron spectroscopy, photoluminescence, and x-ray diffraction studies. We also highlight the chemical mechanism of morphologic and crystallographic changes found in films with high indium content. By optimizing growth conditions according to these findings, ZnO:In with a resistivity as low as $2\times {10}^{-3}\mathrm{\Omega }\mathrm{cm}$, high optical quality $\left(T\approx 90\%\right)$, and sheet resistance of $32\mathrm{\Omega }/\square$ has been obtained without any need for postgrowth treatments.

Rectangular iron nitride monatomic layers are fabricated on the threefold symmetric Cu(111) substrate by taking advantage of the stability of the square nitride film. Two different ordered structures are observed on the Cu(111) substrate by scanning tunneling microscopy after annealing at 510 K and 580 K. Their chemical composition and lattice symmetry are investigated by x-ray photoelectron spectroscopy and low energy electron diffraction. The monolayer film prepared at 580 K is a distorted ${\mathrm{Fe}}_2\mathrm{N}$ monolayer analogous to a ferromagnetic square ${\mathrm{Fe}}_2\mathrm{N}$ monolayer with a clock reconstruction on the Cu(001) substrate. The lattice deformation of the square ${\mathrm{Fe}}_2\mathrm{N}$ monolayer is induced by using Cu(111) with threefold symmetry.

The layered narrow-band-gap semiconductor ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is composed of heavy elements with strong spin-orbital coupling, which has been identified both as a good candidate for a thermoelectric material with high thermoelectric figure of merit $\left(ZT\right)$ and as a topological insulator of the $Z_2$ type with a gapless surface band in a Dirac-cone shape. The existence of a conjugated $\pi$-bond system on the surface of each ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ quintuple layer is proposed based on an extended valence bond model with valence electrons distributed in the hybridized orbitals. Supporting experimental evidence of a two-dimensional (2D) conjugated $\pi$-bond system on each quintuple layer of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is provided using electron energy-loss spectroscopy and electron density mapping through inverse Fourier transform of x-ray diffraction data. Quantum chemistry calculations support the $\pi$-bond existence between partially filled $4p_z$ orbitals of Se via side-to-side orbital overlap positively. The conjugated $\pi$-bond system on the surface of each quintuple ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layer is proposed to be similar to that found in graphite (graphene) and responsible for the unique 2D conduction mechanism. The van der Waals (vdW) attractive force between quintuple layers is interpreted to be coming from the antiferroelectrically ordered effective electric dipoles, which are constructed with $\pi$-bond trimer pairs on Se layers across the vdW gap of minimized Coulomb repulsion.

The linear optical response of metastable $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ is investigated by spectroscopic ellipsometry. We determine the ordinary dielectric function from lattice vibrations up to the vacuum ultraviolet spectral range at room temperature for a sample with a $\left(0001\right)$ surface. Three out of four $E_{\mathrm{u}}$ infrared-active phonon modes are unambiguously determined, and their frequencies are in good agreement with density functional theory calculations. The dispersion of the refractive index in the visible and ultraviolet part of the spectrum is determined. High-energy interband transitions are characterized up to $20\mathrm{eV}$. By comparison with the optical response of $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ and with theoretical results, a tentative assignment of interband transitions is proposed.

The complete process of amorphization and crystallization of the phase-change material $\mathrm{G}{\mathrm{e}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_5$ is investigated using nanosecond ab initio molecular dynamics simulations. Varying the quench rate during the amorphization phase of the cycle results in the generation of a variety of structures from entirely crystallized (−0.45 K/ps) to entirely amorphized (−16 K/ps). The 1.5-ns annealing simulations indicate that the crystallization process depends strongly on both the annealing temperature and the initial amorphous structure. The presence of crystal precursors (square rings) in the amorphous matrix enhances nucleation/crystallization kinetics. The simulation data are used to construct a combined continuous-cooling-transformation (CCT) and temperature-time-transformation (TTT) diagram. The nose of the CCT-TTT diagram corresponds to the minimum time for the onset of homogenous crystallization and is located at 600 K and 70 ps. That corresponds to a critical cooling rate for amorphization of −4.5 K/ps. The results, in excellent agreement with experimental observations, suggest that a strategy that utilizes multiple quench rates and annealing temperatures may be used to effectively optimize the reversible switching speed and enable fast and energy-efficient phase-change memories.

We report on the physical properties of the intermetallic series $R{\mathrm{NiSi}}_3(R=\text{Y}$, Gd-Tm, Lu). High quality single crystals with platelike morphology were grown using the Sn flux method. X-ray powder diffraction data show that this series crystallizes in the orthorhombic space group Cmmm, and Laue patterns indicate that the b axis remains perpendicular to the plane of the plates. Magnetization measurements show anisotropic antiferromagnetic ground states for $R$ = Gd-Tm with Néel temperatures ranging from $T_N=2.6$ K $\left({\mathrm{TmNiSi}}_3\right)$ up to 32.2 K $\left({\mathrm{TbNiSi}}_3\right)$, as well as metamagnetic transitions that in some cases appear together with hysteresis $({\mathrm{TbNiSi}}_3,{\mathrm{DyNiSi}}_3$, and ${\mathrm{HoNiSi}}_3)$. The easy axis changes from a axis to b axis on going from $R$ = Gd-Ho to $R$ = Er-Tm. All transitions from antiferromagnetic to paramagnetic states are clearly marked by sharp peaks in specific heat as well as in the derivative of resistivity measurements, which show metallic temperature dependence for all compounds and residual values in the range of $1\mu \mathrm{\Omega }\mathrm{cm}$. ${\mathrm{DyNiSi}}_3$ has two close phase transitions, while ${\mathrm{HoNiSi}}_3$ presents distinct critical temperatures for applied fields in the $a$ or $c$ directions (10.4 and 6.3 K, respectively), pointing to possible component-specific ordering of the local magnetic moments.

Resistive random-access memory (ReRAM) is a promising class of nonvolatile memory capable of storing information via its resistance state. In the case of hafnium oxide-based devices, experimental evidence shows that a conductive oxygen-deficient filament is formed and broken inside of the device by oxygen migration, leading to switching of its resistance state. However, little is known about the nature of this conductive phase, its interface with the host oxide, or the associated interdiffusion of oxygen, presenting a challenge to understanding the switching mechanism and device properties. To address these problems, we present atomic-scale first-principles simulations of a prototypical conductive phase (HfO), the electronic properties of its interface with ${\mathrm{HfO}}_2$, as well as stability with respect to oxygen diffusion across the interface. We show that the conduction-band offset between HfO and ${\mathrm{HfO}}_2$ is 1.3 eV, smaller than typical electrode-${\mathrm{HfO}}_2$ band offsets, suggesting that positive charging and band bending should occur at the conductive filament-${\mathrm{HfO}}_2$ interface. We also show that transfer of oxygen across the interface, from ${\mathrm{HfO}}_2$ into HfO, costs around 1.2 eV per atom and leads to a gradual opening of the HfO band gap, and hence disruption of the electrical conductivity. These results provide invaluable insights into understanding the switching mechanism for ${\mathrm{HfO}}_2$-based ReRAM.

Mobility is an important parameter to gauge the performance of graphene devices, which is usually measured by FET or Hall methods relying on the use of insulating substrates. However, these methods are not applicable for the case of graphene on semiconductors, because some current will inevitably cross their junctions and flow through the semiconductors except directly traversing the graphene surface. Here we demonstrate a method for measuring the local mobility of graphene on gallium nitrides combining Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM). The carrier density related to Fermi level shifts in graphene can be acquired from KPFM. The local mobility of graphene is calculated from the carrier mean free path available from the effective contact area, which can be fitted from the local I-V curves in graphene/GaN junctions by C-AFM. Our method can be used to investigate an arbitrary region in graphene and also be applied to other semiconductor substrates and do not introduce damages. These results will benefit recent topical application researches for graphene integration in various semiconductor devices.

The size and shape of a large variety of polymeric particles, including biological cells, star polymers, dendrimes, and microgels, depend on the applied stresses as the particles are extremely soft. In high-density suspensions these particles deform as stressed by their neighbors, which implies that the interparticle interaction becomes of many-body type. Investigating a two-dimensional model of cell tissue, where the single particle shear modulus is related to the cell adhesion strength, here we show that the particle deformability affects the melting scenario. On increasing the temperature, stiff particles undergo a first-order solid/liquid transition, while soft ones undergo a continuous solid/hexatic transition followed by a discontinuous hexatic/liquid transition. At zero temperature the melting transition driven by the decrease of the adhesion strength occurs through two continuous transitions as in the Kosterlitz, Thouless, Halperin, Nelson, and Young scenario. Thus, there is a range of adhesion strength values where the hexatic phase is stable at zero temperature, which suggests that the intermediate phase of the epithelial-to-mesenchymal transition could be hexatic type.

To overcome the limitations of graphene due to lack of intrinsic band gap, it is generally functionalized with hydrogen or halogen atoms such as fluorine and chlorine. Generally, such functionalization yields a moderate- to high-band-gap material in case of 100% coverage, for example $\approx 1.5$ eV in graphene functionalized with chlorine atoms or chlorographene. In this paper, using ab initio calculations, we report very interesting transformations observed in chlorographene under external strain, driving it to a state with nearly vanishing band gap (under tensile strain) and even converting it to a metal (under compressive strain). We also show the importance of spin-orbit coupling, responsible for the few meV band gap of chlorographene observed under high tensile strain, which would have been a gapless semimetal otherwise.

We experimentally quantify the molecular bending of a partially fluorinated pentacene (PEN) compound, namely 2,3,9,10-tetrafluoropentacene (F4PEN), adsorbed on Cu(111). By means of the x-ray standing wave (XSW) technique, we directly measure the adsorption distance of three inequivalent carbon sites, the fluorine atoms as well as the total and backbone carbon average adsorption distances. The precise positioning of different sites within the carbon core allows us to resolve two adsorption behaviors, namely a PEN-like strong coupling between the backbone and the substrate, and a repulsive interaction involving the fluorinated short molecular edges, which are $0.91\pm 0.09\AA$ above the central benzene ring. This finding is further supported by additional electronic and in-plane-structure measurements, thus showing that the selective fluorination of a PEN molecule has only a local conformational effect and it is not sufficient to modify its interface properties. Yet, in the multilayer regime, the electronic and growth properties of the film differ completely from those of PEN and its perfluorinated derivative.

We report polycrystalline residual patterns with dendritic micromorphologies upon fast evaporation of a mixed-solvent sessile drop containing reactive ZnO nanoparticles. The molecular and particulate species generated in situ upon evaporative drying collude with and modify the Marangoni solvent flows and Bénard-Marangoni instabilities, as they undergo self-assembly and self-organization under conditions far from equilibrium, leading to the ultimate hierarchical central cellular patterns surrounded by a peripheral coffee ring upon drying.