Long-Range Alignments of Single Fullerenes by Site-Selective Inclusion into a Double-Cavity 2D Open Network

We show by means of STM that C60 molecules can be trapped into specific sites of a 2D double-cavity open network, thus forming long-range alignments of single molecules. Since only one of the two cavities has the right size to host C60, the smallest cavity remains empty and is thus available to trap additional species of smaller size. This novel 2D supramolecular network opens new perspectives in the design of multicomponent guest?host architectures with electronic functionalities

Interaction of low-energy electrons with surface polarity near ferroelastic domain boundaries

We derive surface polarity at and near ferroelastic domain boundaries from molecular dynamics simulations based on an ionic spring model. Interatomic gradient forces lead to flexoelectricity which, in turn, generates polarity at the surface and in twin boundaries. We then derive generic properties of electron scattering spectra equivalent to those observed in low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) experiments. Negatively (positively) charged surfaces reflect (attract) incident electrons with low kinetic energy. The electron images reveal the valley and ridge surface structures near the intersection of the twin boundary and the surface. Polarity in surface layers is predicted to be visible in LEEM and MEM spectra at neutral surfaces, but much less when surfaces are charged. Inward polarity reflects electrons similar to negative surface charges, and outward polarity backscatters electrons like positive surface charges. Both the polarity in the twin boundary and the physical topography scatter electrons, consistent with experimental LEEM and MEM experiments on CaTi O 3 with (001) and (111) surface terminations.EPSR

Ferroelectricity in a quasiamorphous ultrathin BaTiO3 film

Until now, the quasiamorphous (QA) phase in BaTiO3 (BTO), SrTiO3 (STO), and BaZrO3 was achieved by pulling a thick film through a steep temperature gradient. Here, we show that a room-temperature deposited ultrathin film, subsequently annealed in O-2 can also produce a QA phase. The atomic, electronic, and ferroelectric (FE) structure of a QA, ultrathin BTO grown on STO were studied by x-ray diffraction (XRD), x-ray photoelectron diffraction (XPD), x-ray photoelectron spectroscopy (XPS), and piezoforce microscopy (PFM). The absence of long-range order is confirmed by in-and out-of-plane XRD as well as Ti 2p XPD. FE polarized domains with good retention have been successfully written into the QA film and exhibit a clear P-E hysteresis loop. Substrate clamping frustrates volume expansion during annealing leading to a QA film. Photoelectron spectroscopy confirms a similar overall electronic structure as for thicker films but with some significant differences. Simple charge-transfer arguments are not sufficient to explain the high-resolution core-level spectra. Ba, Ti, and O all show components associated with a surface region. We suggest that the observation of such a component in the Ti 2p spectrum is linked with the high dynamic charge tensor induced by the large off-center displacement of the Ti ion.8420French National Research Agency (ANR) [ANR-10-BLAN-1012]CEAFrench National Research Agency (ANR) [ANR-10-BLAN-1012

Control of surface potential at polar domain walls in a nonpolar oxide

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential

Ferroelastic Twin Angles at the Surface of CaTiO 3 Quantified by Photoemission Electron Microscopy

International audienc

Anti-ferromagnetic coupling in hybrid magnetic tunnel junctions mediated by monomolecular layers of α-sexithiophene

International audienceWe report here on the magnetic coupling taking place between Fe3O4 and Co layers across an organic monolayer of α-sexithiophene (6T). The controlled growth of 6T ultrathin films on epitaxial Fe3O4 surfaces allows to prepare highly homogeneous insulating layers with thicknesses in the range 0.5–2.0 monolayers (ML). A combined study using vibrating sample magnetometry and polarized neutron reflectivity reveals that hybrid Fe3O4/6T/Co tunnel junctions show different magnetic couplings depending on the 6T thickness. In particular, magnetic coupling between Fe3O4 and Co layers separated by 1 ML of 6T is consistent with anti-ferromagnetic coupling, opening new perspectives for controlling magnetization in organic spintronic devices

Towards ferroelectric control of topological insulators and surface states

May 11-15, 2015.International audienceThe ferroelectric control of the electronic properties of topological or surface states is a very promising approach for future electronic applications such as in spintronics. Recently, first principle calculations have predicted that bismuth (Bi) spin Rashba splitting can be manipulated by the spontaneous electronic polarization of a ferroelectric material [1]. The purpose of our work is thus to determine directly the effect of the electric polarization of written ferroelectric domains on the Rashba spin splitting of a thin Bi layer using spatially resolved photoemission spectroscopy. Towards this goal, on the one hand, ferrolectric domains were polarized in (Pb,Zr)TiO3 thin film using piezoresponse force microscopy (PFM). Then, we have characterized these domains using energy filtered photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM). On the other hand, we have optimized and characterized the growth of Bi by e-beam evaporation on an oxide substrate. Finally, we performed PEEM experiments in the momentum space to measure the electronic band structure of the Bi thin film deposited on the oxide substrate and the ferroelectric material. This approach will show the control of electronic properties, while measuring in situ the band structure by photoemission spectroscopy, and it will illustrate the direct response of electric manipulation of the spin degree of freedom for spintronics application. [1] H. Mirhosseini et al., PRB 81, 073406 (2010)

Physical chemistry of the TiN/Hf 0.5 Zr 0.5 O 2 interface

International audienceFerroelectric hafnia-based thin films are promising candidates for emerging high-density embedded nonvolatile memory technologies, thanks to their compatibility with silicon technology and the possibility of 3D integration. The electrode–ferroelectric interface and the crystallization annealing temperature may play an important role in such memory cells. The top interface in a TiN/Hf0.5Zr0.5O2/TiN metal–ferroelectric–metal stack annealed at different temperatures was investigated with X-ray photoelectron spectroscopy. The uniformity and continuity of the 2 nm TiN top electrode was verified by photoemission electron microscopy and conductive atomic force microscopy. Partial oxidation of the electrode at the interface is identified. Hf is reduced near the top interface due to oxygen scavenging by the top electrode. The oxygen vacancy (VO) profile showed a maximum at the top interface (0.71%) and a sharp decrease into the film, giving rise to an internal field. Annealing at higher temperatures did not affect the VO concentration at the top interface but causes the generation of additional VO in the film, leading to a decrease of the Schottky Barrier Height for electrons. The interface chemistry and n-type film doping are believed to be at the origin of several phenomena, including wake-up, imprint, and fatigue. Our results give insights into the physical chemistry of the top interface with the accumulation of defective charges acting as electronic traps, causing a local imprint effect. This may explain the wake-up behavior as well and also can be a possible reason of the weaker endurance observed in these systems when increasing the annealing temperatur

Towards ferroelectric control of topological insulators and surface states

May 11-15, 2015.International audienceThe ferroelectric control of the electronic properties of topological or surface states is a very promising approach for future electronic applications such as in spintronics. Recently, first principle calculations have predicted that bismuth (Bi) spin Rashba splitting can be manipulated by the spontaneous electronic polarization of a ferroelectric material [1]. The purpose of our work is thus to determine directly the effect of the electric polarization of written ferroelectric domains on the Rashba spin splitting of a thin Bi layer using spatially resolved photoemission spectroscopy. Towards this goal, on the one hand, ferrolectric domains were polarized in (Pb,Zr)TiO3 thin film using piezoresponse force microscopy (PFM). Then, we have characterized these domains using energy filtered photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM). On the other hand, we have optimized and characterized the growth of Bi by e-beam evaporation on an oxide substrate. Finally, we performed PEEM experiments in the momentum space to measure the electronic band structure of the Bi thin film deposited on the oxide substrate and the ferroelectric material. This approach will show the control of electronic properties, while measuring in situ the band structure by photoemission spectroscopy, and it will illustrate the direct response of electric manipulation of the spin degree of freedom for spintronics application. [1] H. Mirhosseini et al., PRB 81, 073406 (2010)