[Show abstract][Hide abstract] ABSTRACT: Researchers have demonstrated that BiFeO3 exhibits ferroelectric hysteresis but none have shown a strong ferromagnetic response in either bulk or thin film without significant structural or compositional modification. When remanent magnetisations are observed in BiFeO3 based thin films, iron oxide second phases are often detected. Using aberration-corrected scanning transmission electron microscopy, atomic resolution electron energy loss spectrum-mapping and quantitative energy dispersive X-ray spectroscopy analysis, we reveal the existence of a new Fe2O3-rich perovskite nanophase, with an approximate formula (Fe0.6Bi0.25Nd0.15)3+Fe3+O3, formed within epitaxial Ti and Nd doped BiFeO3 perovskite films grown by pulsed laser deposition. The incorporation of Nd and Bi ions on the A-site and coherent growth with the matrix stabilise the Fe2O3-rich perovskite phase and preliminary density functional theory calculations suggest that it should have a ferrimagnetic response. Perovskite-structured Fe2O3 has been reported previously but never conclusively proven when fabricated at high-pressure high-temperature. This work suggests the incorporation of large A-site species may help stabilise perovskite-structured Fe2O3. This finding is therefore significant not only to the thin film but also to the high-pressure community.
[Show abstract][Hide abstract] ABSTRACT: The ability to control the properties of electrical contacts to nanostructures is essential to realize operational nanodevices. Here, we show that the electrical behavior of the nanocontacts between free-standing ZnO nanowires and the catalytic Au particle used for their growth can switch from Schottky to Ohmic depending on the size of the Au particles in relation to the cross-sectional width of the ZnO nanowires. We observe a distinct Schottky-to-Ohmic transition in transport behavior at an Au-to-nanowire diameter ratio of 0.6. The current-voltage electrical measurements performed with a multi-probe instrument are explained using 3-D self-consistent electrostatic and transport simulations revealing that tunneling at the contact edge is the dominant carrier transport mechanism for these nanoscale contacts. The results are applicable to other nanowire materials such as Si, GaAs, and InAs when the effects of surface charge and contact size are considered.
[Show abstract][Hide abstract] ABSTRACT: Transition metal dichalcogenides have a laminar structure, with strongly covalently bonded layers weakly interacting through van der Waals forces. They are of special interest also because of their unique properties once exfoliated in nanoflakes. We analyse the microstructure of oxidised TiS2 nanoflakes with atomically resolved scanning transmission electron microscopy and propose a comprehensive model for their reactivity by means of first principles simulations. In particular we find that reaction to water proceeds from the edges of the flake, while it is thermodynamically possible but kinetically hindered in the middle, unless it is initiated by the presence of a surface vacancy. Importantly O substitution for S allows fine tuning control of the flake bandgap, paving the way for the use of TiS(2-x)Ox alloys as surface catalysts and photovoltaic materials.
The Journal of Physical Chemistry C 06/2015; 119(27):150602115423002. DOI:10.1021/acs.jpcc.5b03212 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here we demonstrate the production of large quantities of gallium sulfide (GaS) nanosheets by liquid exfoliation of layered GaS powder. The exfoliation was achieved by sonication of the powder in suitable solvents. The variation of dispersed concentration with solvent was consistent with classical solution thermodynamics and showed successful solvents to be those with Hildebrand solubility parameters close to 21.5 MPa1/2. In this way, nanosheets could be produced at concentrations of up to ~0.2 mg/ml with lateral sizes and thicknesses of 50-1000 nm and 3-80 layers, respectively. The nanosheets appeared to be relatively defect free although oxygen was observed in the vicinity of the edges. Using controlled centrifugation techniques, it was possible to prepare dispersions containing size-selected nanosheets. Spectroscopic measurements showed the optical properties of the dispersions to vary strongly with nanosheet size, allowing the elucidation of spectroscopic metrics for in-situ estimation of nanosheet size and thickness. These techniques allow the production of nanosheets with controlled sizes which will be important for certain applications. To demonstrate this, we prepared films of GaS nanosheets of three different sizes for use as hydrogen evolution electrocatalysts. We found a clear correlation between performance and size showing small nanosheets to be more effective. This is consistent with the catalytically active sites residing on the nanosheet edges.
Chemistry of Materials 04/2015; 27(9):150421100338004. DOI:10.1021/acs.chemmater.5b00910 · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Thin-film oxide heterostructures show great potential for use in spintronic memories, where electronic charge and spin are coupled to transport information. Here we use a La0.7Sr0.3MnO3 (LSMO)/PbZr0.2Ti0.8O3 (PZT) model system to explore how local variations in electronic and magnetic phases mediate this coupling. We present direct, local measurements of valence, ferroelectric polarization and magnetization, from which we map the phases at the LSMO/PZT interface. We combine these experimental results with electronic structure calculations to elucidate the microscopic interactions governing the interfacial response of this system. We observe a magnetic asymmetry at the LSMO/PZT interface that depends on the local PZT polarization and gives rise to gradients in local magnetic moments; this is associated with a metal-insulator transition at the interface, which results in significantly different charge-transfer screening lengths. This study establishes a framework to understand the fundamental asymmetries of magnetoelectric coupling in oxide heterostructures.
[Show abstract][Hide abstract] ABSTRACT: Low-dimensional carbon materials, i.e., graphene and its functionalization with a number of semiconductor or conductor materials, such as noble metal nanostructures, have primary importance for their potential exploitation as electro-active materials, i.e., as new generation catalysts. Here, low-cost, solution chemistry-based, two-step functionalization of an individual, free-standing, chemical vapor-deposited graphene monolayer is reported, with noble metal (Au, Pt, Pd) nanoparticles to build up two-side decorated graphene-based metal nanoclusters. Either the same metal (symmetric decoration) or different metals (asymmetric decoration) are used for the preparation of bimetal graphene sandwiches, which are adsorbed at the liquid/liquid (organic/water) interface. The successful fabrication of such dual-decorated graphene-based metal nanocomposites is confirmed using various microscopic techniques (scanning electron and atomic force microscopies) and several spectroscopic methods (x-ray photoelectron, energy dispersive x-ray, mapping mode Raman spectroscopy, and electron energy loss spectroscopy). Taken together, it is inferred from these techniques that the location of deposited metal nanoparticles is on opposite sides of the graphene.
[Show abstract][Hide abstract] ABSTRACT: Copper on gold forms a monolayer deposit via underpotential deposition. For gold particles adsorbed at a liquid-liquid interface this results in a uniform one monolayer thick shell. This approach offers a new route for the uniform functionalisation of nanoparticles and presents a way to probe fundamental processes that underlie nanoparticle synthesis.
[Show abstract][Hide abstract] ABSTRACT: A simple method for the deposition of noble metal (Pd, Au) nanoparticles on a free-standing chemical vapour deposited graphene (CVD GR) monolayer is reported. The method consists of assembling the high purity CVD GR, by transfer from poly (methyl methacrylate) (PMMA), at the organic/water interface. Metal deposition can then proceed using either spontaneous or electrochemically-controlled processes. The resultant graphene-based metal nanoclusters are characterized using atomic force and electron microscopy techniques, and the location of the nanostructures underneath the graphene layer is determined from the position and the intensity changes of the Raman bands (D, G, 2D). This novel process for decoration of a single-layer graphene sheet with metal nanoparticles using liquid/liquid interfaces opens an alternative and useful way to prepare low dimensional carbon-based nanocomposites and electrode materials.
Chemical Science 01/2015; 6(2):1316-1323. DOI:10.1039/C4SC03504F · 8.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A-site deficient Nd2/3TiO3 ceramics stabilized with CaTiO3, with an overall composition of 0.9 Nd2/3TiO3-0.1 CaTiO3, were synthesized by the mixed oxide route. Synchrotron X-ray diffraction was used to identify the basic perovskite structure and revealed cross-type superlattice reflections. An incommensurate superlattice structure with dimensions of a ≈ b ≈ 20ap and c = 2ap (where ap is the cell parameter for the parent perovskite phase) was identified, giving rise to contrast features resembling a nanochessboard pattern in electron microscopy images. The superlattice was further characterized by aberration-corrected scanning transmission electron microscopy (STEM): atomically resolved lattice images were obtained along ⟨100⟩ orientations to visualize the A-site (Ca, Nd, and vacancies) and B-site (Ti) cation column intensities, in correlation with observations of the nanochessboard superlattice. Electron energy loss spectroscopy (EELS) was used to precisely determine the distribution of Nd and Ca across the structure, confirming the absence of long-range elemental segregation or phase separation across the nanochessboard superstructure. Closer inspection of the chemical maps in two orthogonal directions, however, suggests the presence of localized ordering of cations and vacancies. The chessboard pattern superlattice is thus likely to be caused by periodic octahedral tilt distortions of the O sublattice, possibly induced by these short-range chemical variations, as a result of a complex interplay between cation and vacancy ordering in three dimensions.
Chemistry of Materials 01/2015; 27(2):150113155829001. DOI:10.1021/cm5036985 · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Few layer black phosphorus is a new two-dimensional material which is of
great interest for applications, mainly in electronics. However, its lack of
stability severely limits our ability to synthesise and process this material.
Here we demonstrate that high-quality, few-layer black phosphorus nanosheets
can be produced in large quantities by liquid phase exfoliation in the solvent
N-cyclohexyl-2-pyrrolidone (CHP). We can control nanosheet dimensions and have
developed metrics to estimate both nanosheet size and thickness
spectroscopically. When exfoliated in CHP, the nanosheets are remarkably stable
unless water is intentionally introduced. Computational studies show the
degradation to occur by reaction with water molecules only at the nanosheet
edge, leading to the removal of phosphorus atoms and the formation of phosphine
and phosphorous acid. We demonstrate that liquid exfoliated black phosphorus
nanosheets are potentially useful in a range of applications from optical
switches to gas sensors to fillers for composite reinforcement.
[Show abstract][Hide abstract] ABSTRACT: Strain engineering of epitaxial ferroelectrics has emerged as a powerful method to tailor the electromechanical response of these materials, although the effect of strain at the atomic scale and the interplay between lattice displacements and electronic structure changes are not yet fully understood. Here, using a combination of scanning transmission electron microscopy (STEM) and density functional theory (DFT), we systematically probe the role of epitaxial strain in mixed phase bismuth ferrite thin films. Electron energy loss O K and Fe L2,3 edge spectra acquired across the rhombohedral (R)-tetragonal (T) phase boundary reveal progressive, and systematic changes in electronic structure going from one phase to the other. The comparison of the acquired spectra with theoretical simulations using DFT suggests a breakage in the structural symmetry across the boundary due to the simultaneous presence of increasing epitaxial strain and off- axial symmetry in the T phase. This implies that the imposed epitaxial strain plays a significant role in not only changing the crystal-field geometry, but also the bonding environment surrounding the central iron cation at the interface thus providing new insights and a possible link to understand how the imposed strain could perturb magnetic ordering in the T phase BFO.
[Show abstract][Hide abstract] ABSTRACT: The passage of an electric current through graphite or few-layer graphene can result in a striking structural transformation, but there is disagreement about the precise nature of this process. Some workers have interpreted the phenomenon in terms of the sublimation and edge reconstruction of essentially flat graphitic structures. An alternative explanation is that the transformation actually involves a change from a flat to a three-dimensional structure. Here we describe detailed studies of carbon produced by the passage of a current through graphite which provide strong evidence that the transformed carbon is indeed three-dimensional. The evidence comes primarily from images obtained in the scanning transmission electron microscope using the technique of high-angle annular dark-field imaging, and from a detailed analysis of electron energy loss spectra. We discuss the possible mechanism of the transformation, and consider potential applications of 'three-dimensional bilayer graphene'.
[Show abstract][Hide abstract] ABSTRACT: One of the most desirable goals of graphene research is to produce ordered two-dimensional (2D) chemical derivatives of suitable quality for monolayer device fabrication. Here we reveal, by focal series exit wave reconstruction (EWR), that C2F chair is a stable graphene derivative and demonstrates pristine long-range order limited only by the size of a functionalized domain. Focal series of images of graphene and C2F chair formed by reaction with XeF2 were obtained at 80 kV in an aberration-corrected transmission electron microscope. EWR images reveal that single carbon atoms and carbon-fluorine pairs in C2F chair alternate strictly over domain sizes of at least 150 nm(2) with electron diffraction indicating ordered domains ≥0.16 μm(2). Our results also indicate that, within an ordered domain, functionalization occurs on one side only as theory predicts. In addition, we show that electron diffraction provides a quick and easy method for distinguishing between graphene, C2F chair and fully fluorinated stoichiometric CF 2D phases.
[Show abstract][Hide abstract] ABSTRACT: Ion irradiation has been observed to induce a macroscopic flattening and in-plane shrinkage of graphene sheets without a complete loss of crystallinity. Electron diffraction studies performed during simultaneous in-situ ion irradiation have allowed identification of the fluence at which the graphene sheet loses long-range order. This approach has facilitated complementary ex-situ investigations, allowing the first atomic resolution scanning transmission electron microscopy images of ion-irradiation induced graphene defect structures together with quantitative analysis of defect densities using Raman spectroscopy.
[Show abstract][Hide abstract] ABSTRACT: Isotopically anomalous carbonaceous grains in extraterrestrial samples represent the most pristine organics that were delivered to the early Earth. Here we report on gentle aberration-corrected scanning transmission electron microscopy investigations of eight (15)N-rich or D-rich organic grains within two carbonaceous Renazzo-type (CR) chondrites and two interplanetary dust particles (IDPs) originating from comets. Organic matter in the IDP samples is less aromatic than that in the CR chondrites, and its functional group chemistry is mainly characterized by C-O bonding and aliphatic C. Organic grains in CR chondrites are associated with carbonates and elemental Ca, which originate either from aqueous fluids or possibly an indigenous organic source. One distinct grain from the CR chondrite NWA 852 exhibits a rim structure only visible in chemical maps. The outer part is nanoglobular in shape, highly aromatic, and enriched in anomalous nitrogen. Functional group chemistry of the inner part is similar to spectra from IDP organic grains and less aromatic with nitrogen below the detection limit. The boundary between these two areas is very sharp. The direct association of both IDP-like organic matter with dominant C-O bonding environments and nanoglobular organics with dominant aromatic and C-N functionality within one unique grain provides for the first time to our knowledge strong evidence for organic synthesis in the early solar system activated by an anomalous nitrogen-containing parent body fluid.
Proceedings of the National Academy of Sciences 10/2014; 111(43). DOI:10.1073/pnas.1408206111 · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Highly strained films of BiFe0.5Mn0.5O3 (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (TC ∼ 600K), with a room temperature saturation moment (MS) of up to 90 emu/cc (∼ 0.58 μB/f.u) on high quality (001) SrTiO3. X-ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe3+ and Mn3+. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.
[Show abstract][Hide abstract] ABSTRACT: The functional properties of transition metal dichalcogenides (TMDs) may be promoted by the inclusion of other elements. Here, we studied the local stoichiometry of single cobalt promoter atoms in an industrial-style MoS2-based hydrotreating catalyst. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy show that the Co atoms occupy sites at the (−100) S edge terminations of the graphite-supported MoS2 nanocrystals in the catalyst. Specifically, each Co atom has four neighboring S atoms that are arranged in a reconstructed geometry, which reflects an equilibrium state. The structure agrees with complementary studies of catalysts that were prepared under vastly different conditions and on other supports. In contrast, a small amount of residual Fe in the graphite is found to compete for the S edge sites, so that promotion by Co is strongly sensitive to the purity of the raw materials. The present single-atom-sensitive analytical method therefore offers a guide for advancing preparative methods for promoted TMD nanomaterials.