Marta D. Rossell

Graphene at the Edge: Stability and Dynamics

Although the physics of materials at surfaces and edges has been extensively studied, the movement of individual atoms at an isolated edge has not been directly observed in real time. With a transmission electron aberration–corrected microscope capable of simultaneous atomic spatial resolution and 1-second temporal resolution, we produced movies of the dynamics of carbon atoms at the edge of a hole in a suspended, single atomic layer of graphene. The rearrangement of bonds and beam-induced ejection of carbon atoms are recorded as the hole grows. We investigated the mechanism of edge reconstruction and demonstrated the stability of the “zigzag” edge configuration. This study of an ideal low-dimensional interface, a hole in graphene, exhibits the complex behavior of atoms at a boundary.

Direct imaging of lattice atoms and topological defects in graphene membranes.

We present a transmission electron microscopy investigation of graphene membranes, crystalline foils with a thickness of only 1 atom. By using aberration-correction in combination with a monochromator, 1-A resolution is achieved at an acceleration voltage of only 80 kV. The low voltage is crucial for the stability of these membranes. As a result, every individual carbon atom in the field of view is detected and resolved. We observe a highly crystalline lattice along with occasional point defects. The formation and annealing of Stone-Wales defects is observed in situ. Multiple five- and seven-membered rings appear exclusively in combinations that avoid dislocations and disclinations, in contrast to previous observations on highly curved (tube- or fullerene-like) graphene surfaces.

Reversible electric control of exchange bias in a multiferroic field-effect device

Electric-field control of magnetization has many potential applications in magnetic memory storage, sensors and spintronics. One approach to obtain this control is through multiferroic materials. Instead of using direct coupling between ferroelectric and ferromagnetic order parameters in a single-phase multiferroic material, which only shows a weak magnetoelectric effect, a unique method using indirect coupling through an intermediate antiferromagnetic order parameter can be used. In this article, we demonstrate electrical control of exchange bias using a field-effect device employing multiferroic (ferroelectric/antiferromagnetic) BiFeO(3) as the dielectric and ferromagnetic La(0.7)Sr(0.3)MnO(3) as the conducting channel; we can reversibly switch between two distinct exchange-bias states by switching the ferroelectric polarization of BiFeO(3). This is an important step towards controlling magnetization with electric fields, which may enable a new class of electrically controllable spintronic devices and provide a new basis for producing electrically controllable spin-polarized currents.

Three-dimensional atomic imaging of crystalline nanoparticles

Determining the three-dimensional (3D) arrangement of atoms in crystalline nanoparticles is important for nanometre-scale device engineering and also for applications involving nanoparticles, such as optoelectronics or catalysis. A nanoparticle’s physical and chemical properties are controlled by its exact 3D morphology, structure and composition. Electron tomography enables the recovery of the shape of a nanoparticle from a series of projection images. Although atomic-resolution electron microscopy has been feasible for nearly four decades, neither electron tomography nor any other experimental technique has yet demonstrated atomic resolution in three dimensions. Here we report the 3D reconstruction of a complex crystalline nanoparticle at atomic resolution. To achieve this, we combined aberration-corrected scanning transmission electron microscopy, statistical parameter estimation theory and discrete tomography. Unlike conventional electron tomography, only two images of the target—a silver nanoparticle embedded in an aluminium matrix—are sufficient for the reconstruction when combined with available knowledge about the particle’s crystallographic structure. Additional projections confirm the reliability of the result. The results we present help close the gap between the atomic resolution achievable in two-dimensional electron micrographs and the coarser resolution that has hitherto been obtained by conventional electron tomography.

A two-dimensional polymer prepared by organic synthesis

Synthetic polymers are widely used materials, as attested by a production of more than 200 millions of tons per year, and are typically composed of linear repeat units. They may also be branched or irregularly crosslinked. Here, we introduce a two-dimensional polymer with internal periodicity composed of areal repeat units. This is an extension of Staudingers polymerization concept (to form macromolecules by covalently linking repeat units together), but in two dimensions. A well-known example of such a two-dimensional polymer is graphene, but its thermolytic synthesis precludes molecular design on demand. Here, we have rationally synthesized an ordered, non-equilibrium two-dimensional polymer far beyond molecular dimensions. The procedure includes the crystallization of a specifically designed photoreactive monomer into a layered structure, a photo-polymerization step within the crystal and a solvent-induced delamination step that isolates individual two-dimensional polymers as free-standing, monolayered molecular sheets.

A strong electro-optically active lead-free ferroelectric integrated on silicon

The development of silicon photonics could greatly benefit from the linear electro-optical properties, absent in bulk silicon, of ferroelectric oxides, as a novel way to seamlessly connect the electrical and optical domain. Of all oxides, barium titanate exhibits one of the largest linear electro-optical coefficients, which has however not yet been explored for thin films on silicon. Here we report on the electro-optical properties of thin barium titanate films epitaxially grown on silicon substrates. We extract a large effective Pockels coefficient of r(eff) = 148 pm V(-1), which is five times larger than in the current standard material for electro-optical devices, lithium niobate. We also reveal the tensor nature of the electro-optical properties, as necessary for properly designing future devices, and furthermore unambiguously demonstrate the presence of ferroelectricity. The integration of electro-optical active films on silicon could pave the way towards power-efficient, ultra-compact integrated devices, such as modulators, tuning elements and bistable switches.

Microwave-assisted solution synthesis of doped LiFePO4 with high specific charge and outstanding cycling performance

A microwave-assisted liquid-phase synthesis route to LiFePO4 doped with divalent (Mn, Ni, Zn), trivalent (Al) and tetravalent (Ti) metal ions in varying concentrations is presented. In spite of the low synthesis temperature of 180 °C all the as-synthesized powders are highly crystalline. The short reaction times of just a few minutes represent the basis for an efficient and time-saving screening of different types of dopants with respect to optimized electrochemical performance in lithium-ion batteries. The Ni- and Zn-doped LiFePO4 with nominal dopant concentrations of 7 and 2 mol%, respectively, outperformed all the other samples, offering initial specific charge of 168 A h kg−1 and excellent capacity retention of 97% after 300 full cycles. A discharge rate of 8 C still resulted in 152 A h kg−1 after 50 cycles. The electrochemical investigations are accompanied by a detailed structural and morphological characterization. Whereas the elemental composition, obtained from quantitative energy dispersive X-ray (EDX) analysis, and the electric conductivity could not directly be correlated to the electrochemical performance, the Rietveld analysis showed that the better the fit the better the electrochemical performance. This observation points to a relation between the phase-purity of a sample and its electrochemical properties.

Interplay Between Size and Crystal Structure of Molybdenum Dioxide Nanoparticles—Synthesis, Growth Mechanism, and Electrochemical Performance

A detailed study is presented on the formation of MoO(2) nanoparticles from the dissolution of the precursor to the final rodlike product, with a focus on the exploration of the inorganic reaction occurring ahead of the nucleation step, and interplay between size and crystal structure of MoO(2). In situ X-ray absorption spectroscopy experiments show that the crystallization and the growth process of MoO(2) nanorods is initiated by rapid reduction of the MoO(2) Cl(2) precursor in benzyl alcohol and acetophenone. This reaction triggers the nucleation of 2 nm MoO(2) particles with spherical shape and hexagonal crystal structure. The transformation from spheres into rods emerges as a complex process driven by oriented attachment. High-resolution transmission electron microscopy and X-ray diffraction results provide evidence that the 2 nm particles first aggregate into 5-20 nm-large oriented assemblies. The increase in particle size induces the phase transition from hexagonal to the less symmetrical monoclinic crystal structure, and finally the transformation into rods. Is it shown that electrodes for lithium-ion batteries based on MoO(2) nanorods have a long-term cycling life. The specific discharge capacity even after 200 cycles at a discharge rate of 1 C is about 300 Ah kg(-1) .

Stabilization of the cubic phase of HfO2 by Y addition in films grown by metal organic chemical vapor deposition

Addition of yttrium in HfO2 thin films prepared on silicon by metal organic chemical vapor deposition is investigated in a wide compositional range (2.0–99.5at.%). The cubic structure of HfO2 is stabilized for 6.5at.%. The permittivity is maximum for yttrium content of 6.5–10at.%; in this range, the effective permittivity, which results from the contribution of both the cubic phase and silicate phase, is of 22. These films exhibit low leakage current density (5×10−7A∕cm2 at −1V for a 6.4nm film). The cubic phase is stable upon postdeposition high temperature annealing at 900°C under NH3.

Template-free co-assembly of preformed Au and TiO2 nanoparticles into multicomponent 3D aerogels

The self-assembly properties of titania nanoparticles, which are strong enough to bridge several lengths scales, make it possible to use them as building blocks for aerogels. Controlled by their surface functionalization, the nanoparticles undergo an oriented attachment process during gelation, building up a 3-dimensional macroporous network. The presence of other nanoparticles in the initial reaction mixture results in the formation of multicomponent aerogels, offering a flexible tool to vary the composition of the gels and thus the chemical properties. Exemplarily this is shown by the co-assembly of titania with gold nanoparticles, leading to a Au–TiO2 composite aerogel with enhanced photocatalytic activity under visible light.