Abstract Van der Waals (vdW) stacking is a powerful technique to achieve desired properties in condensed matter systems through layer-by-layer crystal engineering. A remarkable example is the control over the twist angle between artificially-stacked vdW crystals, enabling the realization of unconventional phenomena in moiré structures ranging from superconductivity to strongly correlated magnetism. Here, we report the appearance of unusual 120° twisted faults in vdW magnet CrI3 crystals. In exfoliated samples, we observe vertical twisted domains with a thickness below 10 nm. The size and distribution of twisted domains strongly depend on the sample preparation methods, with as-synthesized unexfoliated samples showing tenfold thicker domains than exfoliated samples. Cooling induces changes in the relative populations among different twisting domains, rather than the previously assumed structural phase transition to the rhombohedral stacking. The stacking disorder induced by sample fabrication processes may explain the unresolved thickness-dependent magnetic coupling observed in CrI3.

Crystallization plays a critical role in determining crystal size, purity and morphology. Therefore, uncovering the growth dynamics of nanoparticles (NPs) atomically is important for the controllable fabrication of nanocrystals with desired geometry and properties. Herein, we conducted in situ atomic-scale observations on the growth of Au nanorods (NRs) by particle attachment within an aberration-corrected transmission electron microscope (AC-TEM). The results show that the attachment of spherical colloidal Au NPs with a size of about 10 nm involves the formation and growth of neck-like (NL) structures, followed by five-fold twin intermediate states and total atomic rearrangement. The statistical analyses show that the length and diameter of Au NRs can be well regulated by the number of tip-to-tip Au NPs and the size of colloidal Au NPs, respectively. The results highlight five-fold twin-involved particle attachment in spherical Au NPs with a size of 3-14 nm, and provide insights into the fabrication of Au NRs using irradiation chemistry.

This study reports on the synthesis of highly oriented chromium triiodide (CrI3) magnetic inclusions inside nano/microfibres with a polyethylene oxide matrix, prepared by the electrospinning technique. The structural, microstructural and spectroscopic analysis shows uniformly dispersed CrI3 nanosized inclusions inside the fibres, presenting a C2/m monoclinic structure at room temperature, where their c-axis is perpendicular to the fibre mat plane and the ab layers are in-plane. Analysis of the magnetic properties show that the samples have a ferromagnetic-paramagnetic phase transition at ∼55-56 K, lower than that of bulk CrI3. Noticeably, a field-driven metamagnetic transition is observed below ∼45 K, from M versus H curves, when the applied magnetic field is perpendicular to the fibre mat plane, while it is strongly reduced when the field is in-plane. This anisotropic behaviour is attributed to the field-induced changes from antiferromagnetic to ferromagnetic interlayer magnetic moment alignment along the CrI3 c-axis stacked layers. These CrI3 electrospun fibres then show an efficient cost-effective route to synthesize magnetic composite fibres with highly oriented van der Walls inclusions, for spintronic applications, taking advantage of their anisotropic 2D layered materials properties.

Although liquid mercury (Hg) has been known since antiquity, the formation of stable solid nano forms of Hg at room temperature has not been reported so far. Here, for the first time, we report a simple sonochemical route to obtain solid mercury nanoparticles, stabilized by reduced graphene oxide at ambient conditions. The as-formed solid Hg nanoparticles were found to exhibit remarkable rhombohedral morphology and crystallinity at room temperature. Extensive characterization using various physicochemical techniques revealed the unique properties of the solid nanoparticles of Hg compared to its bulk liquid metal phase. Furthermore, the solid nature of the Hg nanoparticles was studied electrochemically, revealing distinctive properties. We believe that solid Hg nanoparticles have the potential for important applications in the fields of electroanalytical chemistry and electrocatalysis.

The intercalation of layered compounds opens up a vast space of new host–guest hybrids, providing new routes for tuning the properties of materials. Here, it is shown that uniform and continuous layers of copper can be intercalated within the van der Waals gap of bulk MoS 2 resulting in a unique Cu–MoS 2 hybrid. The new Cu–MoS 2 hybrid, which remains semiconducting, possesses a unique plasmon resonance at an energy of ≈1eV, giving rise to enhanced optoelectronic activity. Compared with high‐performance MoS 2 photodetectors, copper‐enhanced devices are superior in their spectral response, which extends into the infrared, and also in their total responsivity, which exceeds 10 4 A W −1 . The Cu–MoS 2 hybrids hold promise for supplanting current night‐vision technology with compact, advanced multicolor night vision.

High-throughput flame spray pyrolysis affords a low-loading Pt catalyst supported on cerium oxide, which is an excellent material for selective C2H2 semihydrogenation at 180 °C, with near-complete conversion and high selectivity towards C2H4 (87.1%).

The origin of alternating wavy dark-bright stripe-like contrast in strain contrast transmission electron microscopy images of NiCo2O4 (NCO) epitaxial thin films grown by pulsed laser deposition has been investigated. The nanoscale stripe-like pattern is determined to be associated with coexisting rock salt (RS) and inverse spinel crystal phases. The presence of two different phases, not addressed in previous reports, is experimentally confirmed by both electron diffraction and high resolution transmission electron microscopy imaging. First principles based calculations, together with compressive strain present in the films, support the formation of such coexisting crystallographic phases in NCO. Similar microstructural patterns and RS structure are not observed in epitaxial films of two other oxides of the spinel family, namely, NiFe2O4 and CoFe2O4. A correlation between the coexisting structures and the macroscopic physical properties of NCO is discussed.

In general, crystallite size reduction is known to suppress the ferroic order (polarization or magnetization) in ferroelectric and magnetic systems. Using free particles of a giant tetragonality $(c/a--1=0.18)$ ferroelectric alloy $\mathrm{BiFe}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ as the model system, here we show that the intrinsic size effect in ferroelectrics can as well manifest in terms of switching the ground state from one ferroelectric phase (tetragonal with polarization along [001]) to another ferroelectric phase (rhombohedral with polarization along [111]). In this particular case, because of the strong coupling of the magnetic and structural degrees of freedom, a magnetic order also sets in below the critical size, making it a size induced magnetoferroelectric transformation. The driving force for this unusual transformation is argued to be the large depolarizing and domain wall energies in the tetragonal phase.

We report on detailed microstructural and atomic ordering characterization by transmission electron microscopy in epitaxial LiFe5O8 (LFO) thin films grown by chemical vapor deposition (CVD) on MgO (001) substrates. The experimental results of LFO thin films are compared with those for bulk LFO single crystal. Electron diffraction studies indicate weak long-range ordering in LFO (α-phase) thin films in comparison to bulk crystal where strong ordering is observed in optimally annealed samples. The degree of long-range ordering depends on the growth conditions and the thickness of the film. Annealing experiment along with diffraction study confirms the formation of α-Fe2O3 phase in some regions of the films. This suggests that under certain growth conditions γ-Fe2O3-like phase forms in some pockets in the as-grown LFO thin films that then convert to α-Fe2O3 on annealing.

Facile and efficient ball milling and polyol methods were employed for the synthesis of nickel selenide (NiSe) nanoparticle. The particle size of the NiSe nanoparticle has been controlled mechanically by varying the ball size in the milling process. The role of the surfactants in the formation of various morphologies was studied. The compounds were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS). The efficiency of the NiSe nanoparticle as a catalyst was tested for the reduction of para-nitroaniline (PNA) to para-phenyldiamine (PPD) and para-nitrophenol (PNP) to para-aminophenol (PAP) using NaBH4 as the reducing agent. Particle size, morphology and the presence of surfactant played a crucial role in the reduction process.

We have utilized spatially resolved high resolution electron energy loss spectroscopy to quantify the relative percentage of ferromagnetic order in the core and the surface regions of CuCr2S4 nanoparticles with nanocube and nanocluster morphology. The organic capping layer is found to play a significant role in restoring magnetic order at the surface. The technique is based on recording the fine features of the Cr L-3 absorption edge and matching them with the theoretical spectra. The nanoscale probing technique we have developed is quite versatile and can be extended to understand magnetic ordering in a number of nanodimensional magnetic materials. (C) 2015 AIP Publishing LLC.

For decades it has been a well-known fact that among the few ferroelectric compounds in the perovskite family namely BaTiO3, KNbO3, PbTiO3 Na1/2Bi1/2TiO3 the dielectric and piezoelectric properties of BaTiO3 is considerably higher than the others in polycrystalline form at room temperature. Further, similar to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals of BaTiO3 exhibits anomalously large piezoelectric response when poled away from the direction of spontaneous polarization at room temperature. These anomalous features in BaTiO3 remained unexplained so far from the structural stand point. In this work we have used high resolution synchrotron X-ray powder diffraction, atomic resolution aberration corrected transmission electron microscopy, in conjunction with a novel powder poling technique, to reveal that (i) the equilibrium state of BaTiO3 is characterized by coexistence of a subtle monoclinic (Mc) phase and tetragonal phase, and (ii) strong electric field induces an orthorhombic phase at 300 K. These results suggest that BaTiO3 at room temperature is within an instability regime, and that this instability is therefore the fundamental factor responsible for the anomalous dielectric and piezoelectric properties of BaTiO3 as compared to the other homologous ferroelectric perovskite compounds. The results demonstrate that pure BaTiO3 at room temperature more akin to lead-based ferroelectric alloys close to the morphotropic phase boundary where polarization rotation and field induced ferroelectric-ferroelectric phase transformations play fundamental role in influencing the piezoelectric behavior.

Electron magnetic chiral dichroism (EMCD) in a transmission electron microscope is an element-specific magnetic characterization technique and is extremely powerful for understanding magnetism of materials at the nanoscale. However, quantitative EMCD remains a challenge. In the present paper, we have highlighted and overcome major difficulties associated with the technique. For example, the experimentally observed low dichroic signal and imbalance between the L-3 and L-2 edge have been explained based on the oscillatory nature of electron propagation through the crystal thickness and specific momentum resolved signal detection, respectively. With this advancement in understanding, site-specific quantitative EMCD has been accomplished in epitaxial thin films of two important ferrimagnetic spinel oxides, NiFe2O4 (NFO) and CoFe2O4 (CFO), with varying degree of cation mixing and A site cation defects. A simple model based on phenomenological absorption has been developed for different site-specific signal contributions for the inverse spinel structure. The experimental moment values for NFO and CFO obtained using EMCD are in good agreement with first principle based theoretical calculations and the results strengthen the promise of utilizing EMCD as a routine nanoscale magnetic characterization technique.

Unlike other shapes, the design of tetrahedral Pt nanocrystals (Pt-NTd), which have the largest number of Pt(111) surface atoms and highest catalytic activities toward the electron transfer reactions, has widely been considered a synthetic challenge due to their thermodynamic instability. Here, we show that, by inducing their nucleation on functionalized carbon, Pt NTds can be obtained with tunable sizes and high yields. The carbon support anchors the nanocrystals early and prevents their oriented attachment leading to nanowire formation. Therein, an in situ generated amine is crucial for stabilization of Pt-NTds, which can later be removed to expose the Pt(111) facets for higher catalytic efficiency. The bare nanocrystals exhibit much improved stability and electrocatalytic activity characteristic of Pt(111) toward oxygen reduction reaction (ORR) and methanol and formic acid oxidation reactions. For example, ∼90% of their activity was retained after 5000 potential cycles, while the ORR onset potential was recorded to be very high, 1.01 V vs reversible hydrogen electrode (RHE).

The effect of B-site cation ordering on the room temperature structural and ferromagnetic resonance (FMR) properties of single crystal spinel lithium ferrite (LiFe5O8, LFO) have been investigated. A detailed microstructural analysis is done through X-ray diffraction, polarized Raman spectroscopy, and transmission electron microscopy (TEM) to examine the effect of post-annealing on the B-site cation ordering. The X-ray diffraction pattern of the as-grown crystal indicates a disordered state of the crystal. However, the annealed sample shows additional superlattice reflections corresponding to the ordered phase. This ordering is further confirmed by Raman spectra and TEM images, which reveal ordering of Li and Fe ions at the octahedral sites contrasting with the relatively high degree of octahedral site disorder in the as-grown crystal. To study the effect of B-site ordering on the magnetic properties and FMR linewidth, vibrating sample magnetometry and broadband FMR measurements have been performed for both the ordered and disordered phases of lithium ferrite. The value of saturation magnetization for both phases is ∼290 emu/cm3. A single mode FMR profile is observed for both phases with little distortion. The linewidth characteristics of the ordered and disordered phases of lithium ferrite phases are compared, and it is observed that the linewidth is independent of the cation ordering. Both the phases exhibit a low linewidth (∼26 Oe at 30 GHz) and the effective damping parameter for the as-grown and annealed samples is determined to be 0.0021 ± 0.0001.

We report on the observation of Co vacancy (VCo) induced charge ordering and ferromagnetism in CoO epitaxial thin film. The ordering is associated with the coexistence of commensurate, incommensurate, and discommensurate electronic phases. Density functional theory calculation indicates the origin of ordering in Co atoms undergoing high spin to low spin transition immediately surrounding the VCo(16.6 at. %). Electron magnetic chiral dichroism experiment confirms the ferromagnetic signal at uncompensated Co spins. Such a native defects induced coexistence of different electronic phases at room temperature in a simple compound CoO is unique and adds to the richness of the field with the possibility of practical device application.

Robust, 26 nm thick free-standing platinum nanosheets, an extremely rare morphology for metal nanostructures, are obtained by employing fluid induced shearing force of the order of 1.8 N and differential shear-stress of 0.5 kPa across the diameter of a Te template nanorod undergoing galvanic displacement by Pt4+ . Corrugation leads to their large surface area and much improved electrocatalytic properties when compared with conventional Pt catalysts.

In this article, we report on the Co atom incorporation, secondary phase formation and composition-dependent magnetic and optical properties of mixed valence Co(II, III)xZn1−xO epitaxial thin films grown by pulsed laser deposition. The intended total Co concentration is varied between ~6–60at.% with relatively higher concentration of +3 over +2 charge state. Mixed valence Co(II, III) shows high solubility in ZnO (up to 38 at.%) and ferromagnetism is observed in samples with total Co incorporation of ~29 and 38at.%. Electron diffraction pattern and high resolution transmission electron microscopy images reveal single crystalline nature of the thin films with wurtzite structure. Co oxide interlayer, with both rock salt and spinel structure, are observed to be formed between the substrate and wurtzite film for total Co concentration at ~17at.% and above. Magnetization shows composition dependence with a saturation moment value of ~93emucm−3 and a coercive field of ~285Oe observed for ~38at.% Co:ZnO films. Ferromagnetism was not observed for films with Co concentration 17 and 9at.%. The Co oxide interlayer does not show any ferromagnetism. All the films are n-type with carrier concentration ~1019cm−3. The observed magnetism is probably resulting from direct antiferromagntic exchange interaction between Co2+ and Co3+ ions favored by heavy Co alloying giving rise to ferrimagnetism in the system.

We report here the growth of epitaxial Co-metal thin film on c-plane sapphire by pulsed laser deposition (PLD) using Co:ZnO target utilizing the composition inhomogeneity of the corresponding plasma. Two distinct plasma composition regions have been observed using heavily alloyed Co0.6Zn0.4O target. The central and intense region of the plasma grows Co:ZnO film; the extreme tail grows only Co metal with no trace of either ZnO or Co-oxide. In between the two extremes, mixed phases (Co+Co-oxides+Co:ZnO) were observed. The Co metal thin film grown in this way shows room temperature ferromagnetism with large in-plane magnetization ~1288 emu cm−3 and a coercivity of ~230 Oe with applied field parallel to the film–substrate interface. Carrier density of the film is ~1022 cm−3. The film is epitaxial single phase Co metal which is confirmed by both X-ray diffraction and transmission electron microscopy characterizations. Planar Hall Effect (PHE) and Magneto Optic Kerr Effect (MOKE) measurements confirm that the film possesses similar attributes of Co metal. The result shows that the epitaxial Co metal thin film can be grown from its oxides in the PLD.