Physicochemical Properties of Organic Molecular Ferroelectric Diisopropylammonium Chloride Thin Films.

We fabricated ferroelectric films of the organic molecular diisopropylammonium chloride (DIPAC) using the dip-coating technique and characterized their properties using various methods. Fourier-transform infrared, scanning electron microscopy, and X-ray diffraction analysis revealed the structural features of the films. We also performed ab-initio calculations to investigate the electronic and polar properties of the DIPAC crystal, which were found to be consistent with the experimental results. In particular, the optical band gap of the DIPAC crystal was estimated to be around 4.5 eV from the band structure total density-of-states obtained by HSE06 hybrid functional methods, in good agreement with the value derived from the Tauc plot analysis (4.05 ± 0.16 eV). The films displayed an island-like morphology on the surface and showed increasing electrical conductivity with temperature, with a calculated thermal activation energy of 2.24 ± 0.03 eV. Our findings suggest that DIPAC films could be a promising alternative to lead-based perovskites for various applications such as piezoelectric devices, optoelectronics, sensors, data storage, and microelectromechanical systems.

Dative Epitaxy of Commensurate Monocrystalline Covalent van der Waals Moiré Supercrystal.

Realizing van der Waals (vdW) epitaxy in the 1980s represents a breakthrough that circumvents the stringent lattice matching and processing compatibility requirements in conventional covalent heteroepitaxy. However, due to the weak vdW interactions, there is little control over film qualities by the substrate. Typically, discrete domains with a spread of misorientation angles are formed, limiting the applicability of vdW epitaxy. Here, the epitaxial growth of monocrystalline, covalent Cr5 Te8 2D crystals on monolayer vdW WSe2 by chemical vapor deposition is reported, driven by interfacial dative bond formation. The lattice of Cr5 Te8 , with a lateral dimension of a few tens of micrometers, is fully commensurate with that of WSe2 via 3 × 3 (Cr5 Te8 )/7 × 7 (WSe2 ) supercell matching, forming a single-crystalline moiré superlattice. This work establishes a conceptually distinct paradigm of thin-film epitaxy, termed "dative epitaxy", which takes full advantage of covalent epitaxy with chemical bonding for fixing the atomic registry and crystal orientation, while circumventing its stringent lattice matching and processing compatibility requirements; conversely, it ensures the full flexibility of vdW epitaxy, while avoiding its poor orientation control. Cr5 Te8 2D crystals grown by dative epitaxy exhibit square magnetic hysteresis, suggesting minimized interfacial defects that can serve as pinning sites.

Ferromagnetic Cr2Te3 nanorods with ultrahigh coercivity.

Ferromagnetic Cr2Te3 nanorods were synthesized by a one-pot high-temperature organic-solution-phase method. The crystalline phases and magnetic properties can be systematically tuned by varying the molar ratio of the Cr and Te precursors. A magnetically hard phase, identified as chemically ordered Cr2Te3, is the dominating one at the precursor ratio between Cr : Te = 1 : 1.2 and 1 : 1.8. A magnetically soft phase, attributed to chemical disorder due to composition inhomogeneity and stacking faults, is present under either Cr-rich or Te-rich synthesis conditions. A large coercivity of 9.6 kOe is obtained for a Cr : Te precursor ratio of 1 : 1.8, which is attributed to the large magnetocrystalline anisotropy of ordered Cr2Te3 nanorods, and verified by density-functional theory calculations. The hard and soft phases sharing coherent interfaces co-exist in a seemingly single-crystalline nanorod, showing an unusual transition from exchange-coupled behavior at higher temperatures to two-phase behavior as the temperature is lowered.

Crystallographic, vibrational modes and optical properties data of α-DIPAB crystal.

The Crystallographic data of the α-DIPAB sample was measured using powder X-ray diffraction (PXRD). The crystal structure was also optimized using density functional based method. The calculations were performed both including van der Waals (vdW) interactions and excluding them to quantify the effects of vdW interaction on the crystal formation. The vibrational modes of DIPAB crystal corresponding to the Bromine deficient samples are calculated and presented. These show the origin of drastic change in dielectric response in Br deficient samples as compared to the ideal stoichiometric DIPAB crystal (Alsaad et al. 2018) [4]. Optical properties of an idealα-DIPAB were calculated using GGA and HSE06 hybrid functional methods implemented in VASP package. We mainly calculated the real and imaginary parts of the frequency-dependent linear dielectric function, as well as the related quantities such as the absorption, reflectivity, energy-loss function, and refractive index of α-DIPAB in the energy window of (0-12) eV.

Deciphering chemical order/disorder and material properties at the single-atom level.

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Effects of the dielectric properties of the ceramic-solvent interface on the binding of proteins to oxide ceramics: a non-local electrostatic approach.

The rapid development of nanoscience and nanotechnology has raised many fundamental questions that significantly impede progress in these fields. In particular, understanding the physicochemical processes at the interface in aqueous solvents requires the development and application of efficient and accurate methods. In the present work we evaluate the electrostatic contribution to the energy of model protein-ceramic complex formation in an aqueous solvent. We apply a non-local (NL) electrostatic approach that accounts for the effects of the short-range structure of the solvent on the electrostatic interactions of the interfacial systems. In this approach the aqueous solvent is considered as a non-ionic liquid, with the rigid and strongly correlated dipoles of the water molecules. We have found that an ordered interfacial aqueous solvent layer at the protein- and ceramic-solvent interfaces reduces the charging energy of both the ceramic and the protein in the solvent, and significantly increases the electrostatic contribution to their association into a complex. This contribution in the presented NL approach was found to be significantly shifted with respect to the classical model at any dielectric constant value of the ceramics. This implies a significant increase of the adsorption energy in the protein-ceramic complex formation for any ceramic material. We show that for several biocompatible ceramics (for example HfO2, ZrO2, and Ta2O5) the above effect predicts electrostatically induced protein-ceramic complex formation. However, in the framework of the classical continuum electrostatic model (the aqueous solvent as a uniform dielectric medium with a high dielectric constant ∼80) the above ceramics cannot be considered as suitable for electrostatically induced complex formation. Our results also show that the protein-ceramic electrostatic interactions can be strong enough to compensate for the unfavorable desolvation effect in the process of protein-ceramic complex formation.

X-ray absorption study of ceria nanorods promoting the disproportionation of hydrogen peroxide.

A quasi in situ X-ray absorption study demonstrated that the disproportionation of hydrogen peroxide (H2O2) promoted by ceria nanorods was associated with a reversible Ce(3+)/Ce(4+) reaction and structural transformations in ceria. The direction of this reversible reaction was postulated to depend on the H2O2 concentration and the fraction of Ce(3+) species in ceria nanorods.

Spin density waves in periodically strained graphene nanoribbons.

Zigzag graphene nanoribbons (ZGNRs) are antiferromagnetic in the ground state with zero net magnetization due to the compensation of contributions from opposite edges. Uniform deformations (both shear and axial) do not produce magnetization due to symmetry restrictions. However, we report the results of first-principles calculations that predict the induction of spin density waves (SDWs) in ZGNRs under non-uniform periodic strain. Using the density functional theory (DFT) method, we show that a sinusoidal magnetization variation along the axis of the ribbon occurs under a sinusoidal transversal shear strain. SDWs appear due to the presence of a strain gradient that induced asymmetry of magnetization on opposite edges of ZGNRs which do not compensate each other. The amplitude of SDWs is estimated at ∼0.066 µB when deformations transverse to the ZGNR axis have a sinusoidal profile with a period of 88.6 Å and an amplitude of 1 Å. Our study suggests that the periodic lattice deformations strongly affect the magnetic structure of ZGNRs in the case of acoustic phonons or mechanical waves.

Magnetic phases of cobalt atomic clusters on tungsten.

First-principle calculations are employed to show that the magnetic structure of small atomic clusters of Co, formed on a crystalline W(110) surface and containing 3-12 atoms, strongly deviates from the usual stable ferromagnetism of Co in other systems. The clusters are ferri-, ferro- or non-magnetic, depending on cluster size and geometry. We determine the atomic Co moments and their relative alignment, and show that antiferromagnetic spin alignment in the Co clusters is caused by hybridization with the tungsten substrate and band filling. This is in contrast with the typical strong ferromagnetism of bulk Co alloys, and ferromagnetic coupling in Fe/W(110) clusters.

Phase stabilization in nitrogen-implanted nanocrystalline cubic zirconia.

The phase stability of nanocrystallites with metastable crystal structures under ambient conditions is usually attributed to their small grain size. It remains a challenging problem to maintain such phase integrity of these nanomaterials when their crystallite sizes become larger. Here we report an experimental-modelling approach to study the roles of nitrogen dopants in the formation and stabilization of cubic ZrO(2) nanocrystalline films. Mixed nitrogen and argon ion beam assisted deposition (IBAD) was applied to produce nitrogen-implanted cubic ZrO(2) nanocrystallites with grain sizes of 8-13 nm. Upon thermal annealing, the atomic structure of these ZrO(2) films was observed to evolve from a cubic phase, to a tetragonal phase and then a monoclinic phase. Our X-ray absorption near edge structure study on the annealed samples together with first-principle modelling revealed the significance of the interstitial nitrogen in the phase stabilization of nitrogen implanted cubic ZrO(2) crystallites via the soft mode hardening mechanism.

Steric and electrostatic complementarity in the assembly of two-dimensional virus arrays.

A highly ordered assembly of biological molecules provides a powerful means to study the organizational principles of objects at the nanoscale. Two-dimensional cowpea mosaic virus arrays were assembled in an ordered manner on mica using osmotic depletion effects and a drop-and-dry method. The packing of the virus array was controlled systematically from rhombic packing to hexagonal packing by modulating the concentrations of poly(ethylene glycol) surfactant in the virus solutions. The orientation and packing symmetry of the virus arrays were found to be tuned by the concentrations of surfactants in the sample solutions. A phenomenological model for the present system is proposed to explain the assembly array morphology under the influence of the surfactant. Steric and electrostatic complementarity of neighboring virus capsids is found to be the key factors in controlling the symmetry of packing.

Dependence of exchange coupling on interfacial conditions in SmCo5/Co system: a first-principles study.

We have performed first-principles calculations to study the interfacial exchange coupling in a SmCo5/Co multilayer model system. The hard phase hcp SmCo5 and the soft phase hcp Co (or Co(1-x)Fe(x)) stacking along (1010) direction are structurally well matched. The atomic structure, including the alignment and the separation between layers, were optimized first. Then the non-collinear magnetic structures were calculated to explore the exchange coupling dependence on the variation of the atomic composition across the interface. We found that the inter-phase exchange coupling strength is strongly dependent on the interface condition between the hard and soft phase by comparing the exchange coupling strengths in different interface conditions. The findings were further confirmed by the calculated site-to-site exchange parameters across the interface.

Lotus effect in engineered zirconia.

Patterned micro- and nanostructured surfaces have received increasing attention because of their ability to tune the hydrophobicity and hydrophilicity of their surfaces. However, the mechanical properties of these studied surfaces are not sufficiently robust for load-bearing applications. Here we report transparent nanocrystalline ZrO 2 films possessing combined properties of hardness and complete wetting behavior, which are expected to benefit tribology, wear reduction, and biomedical applications where ultrahydrophilic surfaces are required. This ultrahydrophilic behavior may be explained by the Wenzel model.

Structural study of titanium oxide films synthesized by ion beam-assisted deposition.

The application of titanium dioxide (TiO(2)) films as surgical implant coatings for antibiotic attachment depends crucially on their available surface area and thus their surface morphology and crystallinity. Here, we report our fabrication of high Wenzel ratio TiO(2) films targeted to increase the film surface area using the ion beam-assisted deposition (IBAD) technique at high-deposition temperatures (approximately 610 degrees C). The modulation of the films' surface morphology was accomplished by varying the chemical identity of the concurrent ion beams bombarded on the films during the e-beam evaporation process. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed to investigate the surface morphology of the as-deposited films. X-ray diffractometry (XRD) revealed that these nanocrystalline films primarily consist of anatase phase TiO(2). Wenzel ratio, the ratio of the actual surface area to the projected area, of IBAD films prepared with argon, oxygen, and nitrogen ion beams was measured to be 1.52, 1.31 and 1.49, respectively. The effect of the differences in chemical reactivity and ion size of these three type ion beams are discussed to explain the present results.

Structural and magnetic properties of Gd3N@C80.

Using relativistic and on-site correlation-corrected density functional theory, we have investigated the structural and magnetic properties of recently synthesized Gd3N@C80. The most stable structure of Gd3N@C80 has the three magnetic Gd ions pointing to the centers of hexagons in C80. The magnetic ground state of this structure has the three coplanar spins (S = 7/2) offset by 120 degrees angles. At the same time, the state with the highest multiplicity, where all the spins are parallel aligned, is found only about 4.5 meV higher in energy. Therefore, at room temperature, we expect Gd3N@C80 to be paramagnetic with the spin fluctuating between different multiplicities. As a result, Gd3N@C80 may exhibit greater proton relaxivity than Gd@C60 and Gd@C82 and serve as a possible candidate for the next generation of commercially available magnetic resonance imaging contrast agents.

Interface effect on ferroelectricity at the nanoscale.

Interfaces play a critical role in nanoscale ferroelectricity. We perform a first-principles study of ultrathin KNbO(3) ferroelectric films placed between two metal electrodes, either SrRuO(3) or Pt. We show that bonding at the ferroelectric-metal interfaces imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode. If the interface bonding is sufficiently strong, the ground-state represents a ferroelectric domain with an interface domain wall, driven by the intrinsic oppositely oriented dipole moments at the two interfaces. The critical thickness for the net polarization of the KNbO(3) film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO(3) electrodes.