Supported core-shell Ni/NiO/CoO nanoparticle (NP) films were obtained by deposition of preformed and mass-selected Ni NPs on a buffer layer of CoO, followed by a top CoO layer. The resulting NPs have core/shell morphology, with a McKay icosahedral Ni core and a partially crystalline CoO shell. X-ray photoelectron spectroscopy evidenced the presence of a thin NiO layer, which was shown to be between the Ni core and the CoO shell by elemental TEM mapping. CoO and NiO shells with different thickness values were obtained, allowing us to investigate the evolution of the magnetic properties of the NP assemblies as a function of the oxide shell thickness. Both exchange-coupling and magnetostatic interactions significantly contribute to the magnetic behavior of Ni/NiO/CoO NP films. After the Ni/NiO/CoO NPs are cooled in a weak magnetic field, they have blocking temperature higher than room temperature because of strong magnetostatic interactions, which support the formation of a spin-glass-like state below $\sim 250\mathrm{K}$. Exchange coupling dominates the magnetic behavior after the NPs are cooled in a strong magnetic field. The exchange bias (EB) is in the 0.17–2.35 kOe range and strongly depends on the CoO thickness (0.4–2.7 nm), showing the onset of the EB at the few-nanometer scale. The switching field distribution showed that the EB opposes the magnetization reversal from the direction along the cooling field but it does not significantly ease the opposite process. The EB depends on $t_{\mathrm{CoO}}$ only for $t_{\mathrm{NiO}}\le 0.5\mathrm{nm}$, but when NiO is 0.7 nm thick it strongly interacts with CoO and a large increase of the EB and coercivity is observed.

We investigate crystal-growth kinetics in the presence of strong shear flow in the liquid, using molecular-dynamics simulations of a binary-alloy model. Close to the equilibrium melting point, shear flow always suppresses the growth of the crystal-liquid interface. For lower temperatures, we find that the growth velocity of the crystal depends nonmonotonically on the shear rate. Slow enough flow enhances the crystal growth, due to an increased particle mobility in the liquid. Stronger flow causes a growth regime that is nearly temperature-independent, in striking contrast to what one expects from the thermodynamic and equilibrium kinetic properties of the system, which both depend strongly on temperature. We rationalize these effects of flow on crystal growth as resulting from the nonlinear response of the fluid to strong shearing forces.

We designed a class of layered microribbons self-assembled from perylene diimide (PDI) molecules that exhibited a fast electrically driven twisting response. By increasing the length of the side chains of the PDIs, the microribbons maintained the same intralayer molecular orientation but exhibited enlarged interlayer distances, and the elasticity moduli of the resulting layered microribbons were effectively reduced from ∼1.5 GPa to $\sim 75\mathrm{MPa}$. The electrically driven twisting response of the microribbons was inversely proportional to the elasticity modulus, indicating that the electroresponse of the resulting materials can be controlled through the interlayer distance. Furthermore, we demonstrated that the influence of the elastic modulus of the microribbons on the electrically driven twisting follows a screw dislocation mechanism. Our work provides a way to design and develop soft materials with a fast mechanical response to external stimuli.

III-nitride nanostructures possess ideal attributes for harvesting solar energy and generating solar fuel through natural water splitting. The most basic requirement of the latter is to engineer the band gap of the semiconductor to straddle the redox potential of water molecules. To this end, using first principles method we predict that GaN engineered with Sb doping at the dilute limit of 0.3% and/or slightly less is suitable for photochemical water splitting applications. The valence band edge is very significantly enhanced by dilute Sb doping while the conduction band edge is not. The microscopic physics behind the strong band bowing by such a small impurity concentration, not seen in other III-V semiconductors, is revealed by investigating the quantum interaction between Sb impurity states and the host GaN states. The dilute doping limit dictates very large systems to be calculated at the hybrid exchange-correlation level which is made possible by our newly developed first principles approach.

The exact intrinsic carrier effective mass $m^{*}$ of a well-studied transparent oxide semiconductor $\mathrm{BaSn}{\mathrm{O}}_3$ is unknown because the reported $m^{*}$ values are scattered from $0.06m_0$ to $3.7m_0$. This paper identifies the intrinsic $m^{*}$ of $\mathrm{BaSn}{\mathrm{O}}_3,m^{*}=0.40\pm 0.01m_0$, by the thermopower modulation clarification method and determines the threshold of the degenerate/nondegenerate semiconductor. At the threshold, the thermopower values of both the La-doped $\mathrm{BaSn}{\mathrm{O}}_3$ and $\mathrm{BaSn}{\mathrm{O}}_3$ thin-film transistor structures are $240\mu \mathrm{V}{\mathrm{K}}^{-1}$, the bulk carrier concentration is $1.4\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$, and the two-dimensional sheet carrier concentration is $1.8\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$. When the Fermi energy $E_{\mathrm{F}}$ is located above the parabolic shaped conduction band bottom, the mobility is rather high. In contrast, $E_{\mathrm{F}}$ below the threshold exhibits a very low carrier mobility, most likely because the tail states suppress the carrier mobility. The present results are useful to further develop $\mathrm{BaSn}{\mathrm{O}}_3$-based oxide electronics.

We characterized the interface between fcc Cu and various single-layer transition metal dichalcogenides (TMDs) using density functional theory calculations. We found that monolayer Mo, W, Nb, Ti, and V disulfides, diselenides, and ditellurides are stable on Cu(111) with binding energies higher than those of $h$-BN and graphene. An analysis of the electronic structure of the interfaces indicates partial covalent bonding and a complex redistribution of electronic density, consisting of electron accumulation in the gap region, depletion near the Cu and TMD surfaces, and charge density oscillations within both materials. The resulting net electric dipoles significantly alter the electron work function of the Cu surface. Interestingly, capping Cu(111) surfaces with group-IV and -V TMDs leads to an increase in the work function of up to 1 eV, while group-VI TMDs can decrease the work function by up to 1 eV. Finally, the complex charge distributions at the Cu/TMD interfaces include opposing dipoles and explain the fact that net dipoles associated with Cu/TMD interfaces are comparable to or smaller than those of Cu/graphene and Cu/$h$-BN, even though the Cu/TMD binding energies are significantly higher.

3C-SiC is a promising semiconductor for many applications where doping and heat dissipation are fundamental parameters in the device design. However, the variation of thermal conductivity with carrier concentration remains to be explored. Using density functional theory, we computed the lattice thermal conductivity in intrinsic and doped 3C-SiC with charge carrier concentrations in the range of ${10}^{17}$ to $10{}^{21}{\mathrm{cm}}^{-3}$. From the calculated phonon dispersion, group velocities, and phonon-phonon scattering rates for undoped bulk 3C-SiC, we obtain a thermal conductivity of 491 W/m K at 300 K. In the case of doped 3C-SiC, we find that the lattice thermal conductivity is strongly reduced at high carrier concentrations. We also predict the effects of electron-phonon interaction (EPI) to be much stronger for hole- than electron-doped material, which is explained by the features of the electronic band structure near the band edges. In the limit of high carrier concentration of $10{}^{21}{\mathrm{cm}}^{-3}$, the thermal conductivity drops by 57% for hole and 32% for electron doping. Our results and analysis provide an in-depth understanding of phonon transport for the design of novel SiC-based electronics.

Molecular simulations are used to study the effect of synthesis conditions on the tensile response of liquid-crystalline elastomers formed by block copolymer chains. Remarkably, it is found that despite the significant presence of trapped entanglements, these networks can exhibit the sawtooth tensile response previously predicted for ideal unentangled networks. It is also found that the monomer concentration during crosslinking can be tuned to limit the extent of entanglements and inhomogeneities while also maximizing network extensibility. It is predicted that networks synthesized at a “critical” concentration will have the greatest toughness.

The optical properties of pristine and titanium-doped ${\mathrm{LiNbO}}_3$ are modeled from first principles. The dielectric functions are calculated within time-dependent density-functional theory, and a model long-range contribution is employed for the exchange-correlation kernel in order to account for the electron-hole binding. Our study focuses on the influence of substitutional titanium atoms on lithium sites. We show that an increasing titanium concentration enhances the values of the refractive indices and the reflectivity.

We present an investigation into the intrinsic magnetic properties of the compounds ${\mathrm{YCo}}_5$ and ${\mathrm{GdCo}}_5$, members of the ${\mathrm{RETM}}_5$ class of permanent magnets (RE = rare earth, TM = transition metal). Focusing on Y and Gd provides direct insight into both the TM magnetization and RE-TM interactions without the complication of strong crystal field effects. We synthesize single crystals of ${\mathrm{YCo}}_5$ and ${\mathrm{GdCo}}_5$ using the optical floating zone technique and measure the magnetization from liquid helium temperatures up to 800 K. These measurements are interpreted through calculations based on a Green's function formulation of density-functional theory, treating the thermal disorder of the local magnetic moments within the coherent potential approximation. The rise in the magnetization of ${\mathrm{GdCo}}_5$ with temperature is shown to arise from a faster disordering of the Gd magnetic moments compared to the antiferromagnetically aligned Co sublattice. We use the calculations to analyze the different Curie temperatures of the compounds and also compare the molecular (Weiss) fields at the RE site with previously published neutron scattering experiments. To gain further insight into the RE-TM interactions, we perform substitutional doping on the TM site, studying the compounds ${\mathrm{RECo}}_{4.5}{\mathrm{Ni}}_{0.5}$, ${\mathrm{RECo}}_4\mathrm{Ni}$, and ${\mathrm{RECo}}_{4.5}{\mathrm{Fe}}_{0.5}$. Both our calculations and experiments on powdered samples find an increased/decreased magnetization with Fe/Ni doping, respectively. The calculations further reveal a pronounced dependence on the location of the dopant atoms of both the Curie temperatures and the Weiss field at the RE site.

Quasiparticle self-consistent $GW$ calculations of the electronic band structures of ${\text{Cd-IV-N}}_2$ with group-IV elements, Si, Ge, and Sn, are reported. The lattice parameters in the $Pna{2}_1$ crystal structure are obtained both in the local density approximation and generalized gradient approximation (GGA) and provide respectively an underestimate and overestimate of the lattice constants for these until now not synthesized materials. The Wyckoff positions are obtained by structural minimization. At the GGA lattice constant, which is expected to be the most reliable, ${\mathrm{CdSiN}}_2$ is found to have an indirect gap $(U-\mathrm{\Gamma })$ of 2.72 eV, which is 0.55 eV lower than the direct gap at $\mathrm{\Gamma }$ of 3.27 eV. In ${\mathrm{CdGeN}}_2$, the indirect gap of 2.01 eV is only 0.1 eV smaller than the direct gap of 2.11 eV, and in ${\mathrm{CdSnN}}_2$ the gap is direct and equals 0.64 eV close to that of InN, as expected. The direct/indirect nature of ${\mathrm{CdGeN}}_2$ is found to be sensitive to shear strain. The conduction band effective masses decrease from ${\mathrm{CdSiN}}_2$ to ${\mathrm{CdGeN}}_2$ to ${\mathrm{CdSnSn}}_2$. The energies of formation indicate these materials to be stable with the possible exception of ${\mathrm{CdSnN}}_2$.

Here, we investigate single crystals of ${\mathrm{CaCo}}_5{\mathrm{As}}_3$ by means of single-crystal x-ray diffraction, microprobe, magnetic susceptibility, heat capacity, and pressure-dependent transport measurements. ${\mathrm{CaCo}}_5{\mathrm{As}}_3$ shares the same structure of ${\mathrm{CaFe}}_4{\mathrm{As}}_3$ with an additional Co atom filling a lattice vacancy and undergoes a magnetic transition at $T_M=16$ K associated with a frustrated magnetic order. ${\mathrm{CaCo}}_5{\mathrm{As}}_3$ displays metallic behavior and its Sommerfeld coefficient ($\gamma =70$ mJ/mol ${\mathrm{K}}^2$) indicates a moderate enhancement of electron-electron correlations. Transport data under pressures to 2.5 GPa reveal a suppression of $T_M$ at a rate of $-0.008$ K/GPa. First-principles electronic structure calculations show a complex three-dimensional band structure and magnetic moments that depend on the local environment at each Co site. Our results are compared with previous data on ${\mathrm{CaFe}}_4{\mathrm{As}}_3$ and provide a scenario for a magnetically frustrated ground state in this family of compounds.

The improvement of Li-ion battery performance requires development of models that capture the essential physics and chemistry in Li-ion battery electrode materials. Phase-field modeling has recently been shown to have this ability, providing new opportunities to gain understanding of these complex systems. In this paper, a novel electrochemical phase-field model is presented that captures the thermodynamic and kinetic properties of lithium insertion in $\mathrm{Ti}{\mathrm{O}}_2$-anatase, a well-known and intensively studied Li-ion battery electrode material. Using a linear combination of two regular solution models, the two phase transitions during lithiation are described as lithiation of two separate lattices with different physical properties. Previous elaborate experimental work on lithiated anatase $\mathrm{Ti}{\mathrm{O}}_2$ provides all parameters necessary for the phase-field simulations, giving the opportunity to gain fundamental insight in the lithiation of anatase and validate this phase-field model. The phase-field model captures the essential experimentally observed phenomena, rationalizing the impact of C rate, particle size, surface area, and the memory effect on the performance of anatase as a Li-ion battery electrode. Thereby a comprehensive physical picture of the lithiation of anatase $\mathrm{Ti}{\mathrm{O}}_2$ is provided. The results of the simulations demonstrate that the performance of anatase is limited by the formation of the poor Li-ion diffusion in the ${\mathrm{Li}}_1{\mathrm{TiO}}_2$ phase at the surface of the particles. Unlike other electrode materials, the kinetic limitations of individual anatase particles limit the performance of full electrodes. Hence, rather than improving the ionic and electronic network in electrodes, improving the performance of anatase $\mathrm{Ti}{\mathrm{O}}_2$ electrodes requires preventing the formation of a blocking ${\mathrm{Li}}_1{\mathrm{TiO}}_2$ phase at the surface of particles. Additionally, the qualitative agreement of the phase-field model, containing only parameters from literature, with a broad spectrum of experiments demonstrates the capabilities of phase-field models for understanding Li-ion electrode materials, and its promise for guiding the design of electrodes through a thorough understanding of material properties and their interactions.

Methylcellulose is a thermoresponsive polymer that undergoes a morphological transition at elevated temperature, forming uniform diameter fibrils. However, the gelation mechanism is still unclear, in particular, at higher polymer concentrations. We use Langevin dynamics simulations to investigate a coarse-grained model for methylcellulose that produces collapsed ringlike structures in dilute solution with a radius close to the fibrils observed in experiments. We show that the competition between the dihedral potential and self-attraction causes these collapsed states to undergo a rapid conformational change, which helps the chain to avoid kinetic traps by permitting a transition between collapsed states. If the dihedral potential is removed, the chains do not escape from their collapsed configuration, whereas at high dihedral potentials, the chains cannot stabilize the collapsed state. We provide systematic data on the effect of the dihedral potential in a model of methylcellulose, and discuss the implication of these previously overlooked rapid conformational fluctuations on the spontaneous formation of high-aspect-ratio fibrils.

While bismuth-related states are known to localize carriers in $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ alloys, the localization behavior of distinct Bi pair, triplet, and cluster states bound above the valence band is less well understood. We probe localization at three different Bi complex states in dilute $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ alloys using magnetophotoluminescence and time-resolved photoluminescence spectroscopy. The mass of electrons Coulomb-bound to holes trapped at Bi pair states is found to increase relative to the average electron mass in the alloy. This increase is attributed to enhanced local compressive strain in the immediate vicinity of the pairs. The dependence of energy transfer between these states on composition is also explored.

Surface defects created and probed with scanning tunneling microscopes are a promising platform for atomic-scale electronics and quantum information technology applications. Using first-principles calculations we demonstrate how to engineer dangling bond (DB) defects on hydrogenated Si(100) surfaces, which give rise to isolated impurity states that can be used in atomic-scale devices. In particular, we show that sample thickness and biaxial strain can serve as control parameters to design the electronic properties of DB defects. While in thick Si samples the neutral DB state is resonant with bulk valence bands, ultrathin samples (1–2 nm) lead to an isolated impurity state in the gap; similar behavior is seen for DB pairs and DB wires. Strain further isolates the DB from the valence band, with the response to strain heavily dependent on sample thickness. These findings suggest new methods for tuning the properties of defects on surfaces for electronic and quantum information applications. Finally, we present a consistent and unifying interpretation of many results presented in the literature for DB defects on hydrogenated silicon surfaces, rationalizing apparent discrepancies between different experiments and simulations.

The conversion processes of electron-hole (e-h) pairs into carriers and excitons in organic light emitting diodes (OLEDs) have been explored by simultaneous photoluminescence- and photocurrent-detected magnetic resonance (PLDMR and PCDMR) techniques as a function of bias. Both PLDMR and PCDMR signals are shown to disappear or extremely reduce with increasing the field strength, whereas photoluminescence intensity is unchanged and photocurrent rather increases. This is clear evidence that e-h pairs are dissociated by the field. A simple model for pair dissociation is proposed and explains the field dependence of PLDMR intensity.