3 research outputs found
Prebiotic NH<sub>3</sub> Formation: Insights from Simulations
Simulations of prebiotic NH<sub>3</sub> synthesis from NO<sub>3</sub><sup>ā</sup> and NO<sub>2</sub><sup>ā</sup> on pyrite surfaces under hydrothermal conditions are
reported. Ab initio metadynamics calculations have successfully explored
the full reaction path which explains earlier experimental observations.
We have found that the reaction mechanism can be constructed from
stepwise single atom transfers which are compatible with the expected
reaction time scales. The roles of the hot-pressurized water and of
the pyrite surfaces have been addressed. The mechanistic picture that
emerged from the simulations strengthens the theory of chemoautotrophic
origin of life by providing plausible reaction pathways for the formation
of ammonia within the ironāsulfur-world scenario
Dynamical Heterogeneity in the Supercooled Liquid State of the Phase Change Material GeTe
A contending
technology for nonvolatile memories of the next generation
is based on a remarkable property of chalcogenide alloys known as
phase change materials, namely their ability to undergo a fast and
reversible transition between the amorphous and crystalline phases
upon heating. The fast crystallization has been ascribed to the persistence
of a high atomic mobility in the supercooled liquid phase, down to
temperatures close to the glass transition. In this work we unravel
the atomistic, structural origin of this feature in the supercooled
liquid state of GeTe, a prototypical phase change compound, by means
of molecular dynamic simulations. To this end, we employed an interatomic
potential based on a neural network framework, which allows simulating
thousands of atoms for tens of ns by keeping an accuracy close to
that of the underlying first-principles framework. Our findings demonstrate
that the high atomic mobility is related to the presence of clusters
of slow and fast moving atoms. The latter contain a large fraction
of chains of homopolar GeāGe bonds, which at low temperatures
have a tendency to move by discontinuous cage-jump rearrangements.
This structural fingerprint of dynamical heterogeneity provides an
explanation of the breakdown of the StokesāEinstein relation
in GeTe, which is the ultimate origin of the fast crystallization
of phase change materials exploited in the devices
A Novel Sb<sub>2</sub>Te<sub>3</sub> Polymorph Stable at the Nanoscale
We report on the MOCVD synthesis
of Sb<sub>2</sub>Te<sub>3</sub> nanowires that self-assemble in a
novel metastable polymorph. The
nanowires crystallize in a primitive trigonal lattice (<i>P</i>3Ģ
<i>m</i>1 SG #164) with lattice parameters <i>a</i> = <i>b</i> = 0.422 nm, and <i>c</i> = 1.06 nm. The stability of the polymorph has been studied by first
principle calculations: it has been demonstrated that the stabilization
is due to the particular side-wall faceting, finding excellent agreement
with the experimental observations