Aberration-corrected Transmission Electron Microscopy Images Research Articles

Probing magnetism in atomically thin semiconducting PtSe2

Atomic-scale disorder in two-dimensional transition metal dichalcogenides is often accompanied by local magnetic moments, which can conceivably induce long-range magnetic ordering into intrinsically non-magnetic materials. Here, we demonstrate the signature of long-range magnetic orderings in defective mono- and bi-layer semiconducting PtSe2 by performing magnetoresistance measurements under both lateral and vertical measurement configurations. As the material is thinned down from bi- to mono-layer thickness, we observe a ferromagnetic-to-antiferromagnetic crossover, a behavior which is opposite to the one observed in the prototypical 2D magnet CrI3. Our first-principles calculations, supported by aberration-corrected transmission electron microscopy imaging of point defects, associate this transition to the interplay between the defect-induced magnetism and the interlayer interactions in PtSe2. Furthermore, we show that graphene can be effectively used to probe the magnetization of adjacent semiconducting PtSe2. Our findings in an ultimately scaled monolayer system lay the foundation for atom-by-atom engineering of magnetism in otherwise non-magnetic 2D materials.

Cooperative and fully reversible color switching activation in hybrid graphene decorated nanocages and copper-TiO2 nanoparticles

Nanostructured systems showing reversible color switching are envisaged to play a significant role in photo-switches, photo-optical sensors, smart windows, displays, optical storage memories. Most of the materials exhibiting reversible color switching are organic molecules. However, their UV-light activation, low thermal and chemical stability, as well as harmful synthesis methods, limit their extensive use. In this research, we have created an inorganic switchable photochromic material exploiting TiO2 ability of creating an exciton upon excitation, copper as the chromophore, and graphene's extraordinarily high electron mobility. Spatially-resolved electron energy-loss spectroscopy and aberration-corrected transmission electron microscopy imaging highlight the high sp2 content of the graphene flakes as well as the presence of few-layers nanocage graphene decorating the surface of the flakes. Our material showed itself to be able to work under visible-light, its photochromic property being three times faster than conventional titania based photochromic materials, reaching a stable change in coloration after only 30 min of visible-light irradiation (vs. > 120 min in conventional Cu–TiO2). With the addition of just 1 wt% graphene, the material exhibited a staggeringly stable photochromic switching over repeated cycles. These results relate to the best previously reported values for any form of TiO2-based photochromic material. This is therefore an excellent candidate for smart-windows, light-sensitive information and energy storage devices, and other chromic devices and applications.

Atomic Layer Deposition of Titanium Oxide on Single-Layer Graphene: An Atomic-Scale Study toward Understanding Nucleation and Growth.

Controlled synthesis of a hybrid nanomaterial based on titanium oxide and single-layer graphene (SLG) using atomic layer deposition (ALD) is reported here. The morphology and crystallinity of the oxide layer on SLG can be tuned mainly with the deposition temperature, achieving either a uniform amorphous layer at 60 °C or ∼2 nm individual nanocrystals on the SLG at 200 °C after only 20 ALD cycles. A continuous and uniform amorphous layer formed on the SLG after 180 cycles at 60 °C can be converted to a polycrystalline layer containing domains of anatase TiO2 after a postdeposition annealing at 400 °C under vacuum. Using aberration-corrected transmission electron microscopy (AC-TEM), characterization of the structure and chemistry was performed on an atomic scale and provided insight into understanding the nucleation and growth. AC-TEM imaging and electron energy loss spectroscopy revealed that rocksalt TiO nanocrystals were occasionally formed at the early stage of nucleation after only 20 ALD cycles. Understanding and controlling nucleation and growth of the hybrid nanomaterial are crucial to achieving novel properties and enhanced performance for a wide range of applications that exploit the synergetic functionalities of the ensemble.

Novel near-infrared emission from crystal defects in MoS2 multilayer flakes.

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission.

BiFeO3Domain Wall Energies and Structures: A Combined Experimental and Density FunctionalTheory+UStudy

We determined the atomic structures and energies of 109°, 180°, and 71° domain walls in BiFeO3, combining density functional theory+U calculations and aberration-corrected transmission electron microscopy images. We find a substantial Bi sublattice shift and a rather uniform Fe sublattice across the walls. The calculated wall energies (γ) follow the sequence γ109<γ180<γ71 for the 109°, 180°, and 71° walls. We attribute the high 71° wall energy to an opposite tilting rotation of the oxygen octahedra and the low 109° wall energy to the opposite twisting rotation of the oxygen octahedra across the domain walls.

A Study of Commercial Nanoparticulate γ‐Al2O3 Catalyst Supports

AbstractThis study investigates a range of commercially available γ‐Al2O3 powders by using a combination of integrated experimental techniques. These included general measurements of powder properties by using helium density, BET surface area, and scanning electron microscopy (SEM) analyses. In addition, dynamic light scattering and zeta potential measurements were used to investigate nanoparticle dispersions. Bulk crystal structures were analysed by using comparative X‐ray and neutron powder diffraction (XRD and NPD) analyses. Conventional transmission electron microscopy (TEM) was used to determine particle morphology, particle size, composition, and structure. Aberration‐corrected TEM was used to investigate the crystallinity of nanoparticles including the existence of any surface reconstruction on commonly observed facetted, cubeoctahedral γ‐Al2O3 nanoparticles. From the observation of peak splittings in diffraction data, we favour a description of the γ‐Al2O3 structure based on a distortion of the conventionally accepted face‐centred cubic (Fd$\bar 3$m) structure into a tetragonal I41/amd structure. Distinct differences between TEM, XRD, and NPD data indicate the presence of some cation disorder within a rigid close‐packed oxygen framework. The Rietveld refinement of the NPD data suggests a high level of microstrain of 1.2 %. An improvement to the model is achieved by reducing the aluminium content in the unit cell, which is commensurate with the migration of aluminium ions to the surface and some degree of nonstoichiometry in the particle core. Aberration‐corrected TEM imaging and exit wave reconstruction confirm previous evidence for the presence of enhanced surface contrast at {1 1 1} surface facets, which we associate with the presence of excess cation termination. In addition, these {1 1 1} facets are observed to be heavily stepped. These results may have important implications for the thermal stability of metal catalyst nanoparticles on these high‐surface area supports; the migration of aluminium ions to the surface provides clear evidence of why these materials perform so well as catalyst supports.