Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m (Sb2 Te3 )n Lamellae.

The possibility to engineer (GeTe)m (Sb2 Te3 )n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)m (Sb2 Te3 )n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe)m (Sb2 Te3 )1 blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m (Sb2 Te3 )1 lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative device architectures.

Geometry of tellurene adsorbed on the Si(111)-R30°-Sb surface from first principles calculations.

The 2D form of tellurium, named tellurene, is one of the latest discoveries in the family of 2D mono-elemental materials. In a trilayer configuration, free-standing tellurene was predicted theoretically to acquire two crystallographic forms, the α and ß phases, corresponding to either a 1T-MoS2-like geometry or a trilayer slab exposing the Te(101̄0) surface of bulk Te with helical chains lying in-plane and further reconstructed due to the formation of interchain bonds. Either one or the other of the two phases was observed experimentally to prevail depending on the substrate they were grown onto. In the perspective to integrate tellurene on silicon, we here report an ab initio study of the adsorption of tellurene on the Si(111)-R30° surface passivated by antinomy. According to the literature, this substrate is chosen for the growth of several tellurides by molecular beam epitaxy. The calculations reveal that on this substrate the adsorption energy mostly compensates the energy difference between the α and ß phases in the free-standing configuration which suggests that the prevalence of one phase over the other might in this case strongly depend on the kinetics effects and deposition conditions.

Analyzing the Perceived Utility of Covid-19 Countermeasures: The Role of Pronominalization, Moral Foundations, Moral Disengagement, Fake News Embracing, and Health Anxiety.

An online survey (N = 210) is presented on how the perceived utility of correct and exaggerated countermeasures against Covid-19 is affected by different pronominalization strategies (impersonal form, you, we). In evaluating the pronominalization effect, we have statistically controlled for the roles of several personal characteristics: Moral Disengagement, Moral Foundations, Health Anxiety, and Embracing of Fake News. Results indicate that, net of personal proclivities, the you form decreases the perceived utility of exaggerated countermeasures, possibly due to simulation processes. As a second point, through a Structural Equation Model, we show that binding moral values (Authority, Ingroup, and Purity) positively predict both fake news embracing and perceived utility of exaggerated countermeasures, while individualizing moral values (Harm and Fairness) negatively predict fake news embracing and positively predict the perceived utility of correct countermeasures. Lastly, fake news embracing showed a doubly bad effect: not only does it lead people to judge exaggerated countermeasures as more useful; but, more dangerously, it brings them to consider correct countermeasures as less useful in the struggle against the pandemic.

Mechanism of amorphous phase stabilization in ultrathin films of monoatomic phase change material.

Elemental antimony has been recently proposed as a promising material for phase change memories with improved performances with respect to the most used ternary chalcogenide alloys. The compositional simplification prevents reliability problems due to demixing of the alloy during memory operation. This is made possible by the dramatic stabilization of the amorphous phase once Sb is confined in an ultrathin film 3-5 nm thick. In this work, we shed light on the microscopic origin of this effect by means of large scale molecular dynamics simulations based on an interatomic potential generated with a machine learning technique. The simulations suggest that the dramatic reduction of the crystal growth velocity in the film with respect to the bulk is due to the effect of nanoconfinement on the fast ß relaxation dynamics while the slow α relaxation is essentially unaffected.

A first-principles study of the switching mechanism in GeTe/InSbTe superlattices.

Interfacial Phase Change Memories (iPCMs) based on (GeTe)2/Sb2Te3 superlattices have been proposed as an alternative candidate to conventional PCMs for the realization of memory devices with superior switching properties. The switching mechanism was proposed to involve a crystalline-to-crystalline structural transition associated with a rearrangement of the stacking sequence of the GeTe bilayers. Density functional theory (DFT) calculations showed that such rearrangement could be achieved by means of a two-step process with an activation barrier for the flipping of Ge and Te atoms which is sensitive to the biaxial strain acting on GeTe bilayers. Within this picture, strain-engineering of GeTe bilayers in the GeTe-chalcogenide superlattice can be exploited to further improve the iPCM switching performance. In this work, we study GeTe-InSbTe superlattices with different compositions by means of DFT, aiming at exploiting the large mismatch (3.8%) in the in-plane lattice parameter between GeTe and In3SbTe2 to reduce the activation barrier for the switching with respect to the (GeTe)2-Sb2Te3 superlattice.

Structural and electronic properties of liquid, amorphous, and supercooled liquid phases of In2Te5 from first-principles.

In2Te5 is a stoichiometric compound in the In-Te system of interest for applications in phase change electronic memories and thermoelectrics. Here, we perform a computational study of the structural, dynamical, and electronic properties of the liquid, supercooled liquid, and amorphous phases of this compound by means of density functional molecular dynamics simulations. Models of the supercooled liquid and amorphous phases have been generated by quenching from the melt. The structure of the liquid phase is characterized by a mixture of defective octahedral and tetrahedral local environments of In atoms, while the amorphous phase displays a mostly tetrahedral local geometry for In atoms with corner and edge sharing tetrahedra similar to those found in the crystalline phases of the In2Te5, InTe, and In2Te3 compounds. Comparison with our previous results on liquid and amorphous In2Te3 and further data on the structural properties of liquid In2Te3 are also discussed. The analysis of the electronic properties highlights the opening of a mobility gap in In2Te5 at about 150 K below the liquidus temperature.

Interfacial layering of a room-temperature ionic liquid thin film on mica: a computational investigation.

The structure of a thin (4 nm) [bmim][Tf(2)N] film on mica was studied by molecular dynamics simulations using an empirical force field. Interfacial layering at T=300 K and at T=350 K is investigated by determining the number- and charge-density profiles of [bmim][Tf(2)N] as a function of distance from mica, and by computing the normal force F(z) opposing the penetration of the ionic liquid film by a spherical nanometric tip interacting with [bmim][Tf(2)N] atoms by a short-range potential. The results show that layering is important but localised within ~1 nm from the interface. The addition of a surface charge on mica, globally neutralised by an opposite charge on the [bmim][Tf(2)N] side, gives rise to low-amplitude charge oscillations extending through the entire film. However, outside a narrow interfacial region, the resistance of the [bmim][Tf(2)N] film to penetration by the mesoscopic tip is only marginally affected by the charge at the interface. The results obtained here for [bmim][Tf(2)N]/mica are similar to those obtained using the same method for the [bmim][Tf(2)N]/silica interface, and agree well with experimental force-distance profiles measured on the latter interface at ambient conditions.