arXiv subjects

Nicolas Onofrio

Publications and source records attributed to Nicolas Onofrio.

5 recordsLinked to original sources

arXiv2018

Prediction of low energy phase transition in metal doped MoTe$_2$ from first principle calculations

Metal-insulator transitions in two dimensional materials represent a great opportunity for fast, low energy and ultra-dense switching devices. Due to the small energy difference between its semimetallic and semiconducting crystal phases, phase transition in MoTe$_2$ can occur with an unprecedented small amount of external perturbations. In this work, we used density functional theory to predict critical strain and electrostatic voltage required to control the phase transition of 3d and 4d metal doped MoTe$_2$. We found that small doping contents dramatically affect the relative energies of MoTe$_2$ crystal phases and can largely reduced the energy input to trigger the transition, compared to pristine case. Moreover, the kinetics corresponding to the phase transition in the proposed doped materials are several order of magnitude faster than in MoTe$_2$. For example, we predict 6.3 \% Mn doped MoTe$_2$ to switch phase under 1.19 V gate voltage in less than 1 $μ$s with an input energy of 0.048 aJ/nm$^3$. Due to the presence of dopant, the controlled change of phase is often complemented with a change in magnetic moment leading to multi-functional phase transition.

cond-mat.mtrl-sci↗

arXiv2017

Novel doping alternatives for single-layer transition metal dichalcogenides

Successful doping of single-layer transition metal dichalcogenides (TMDs) remains a formidable barrier to their incorporation into a range of technologies. We use density functional theory to study doping of molybdenum and tungsten dichalcogenides with a large fraction of the periodic table. An automated analysis of the energetics, atomic and electronic structure of thousands of calculations results in insightful trends across the periodic table and points out promising dopants to be pursued experimentally. Beyond previously studied cases, our predictions suggest promising substitutional dopants that result in p-type transport and reveal interesting physics behind the substitution of the metal site. Doping with early transition metals (TMs) leads to tensile strain and a significant reduction in the bandgap. The bandgap increases and strain is reduced as the d-states are filled into the mid TMs; these trends reverse are we move into the late TMs. Additionally, the Fermi energy increases monotonously as the d-shell is filled from the early to mid TMs and we observe few to no gap states indicating the possibility of both p- (early TMs) and n- (mid TMs) type doping. Quite surprisingly, the simulations indicate the possibility of interstitial doping of TMDs; the energetics reveal that a significant number of dopants, increasing in number from molybdenum disulfide to diselenide and to ditelluride, favor the interstitial sites over adsorbed ones. Furthermore, calculations of the activation energy associated with capturing the dopants into the interstitial site indicate that the process is kinetically possible. This suggets that interstitial impurities in TMDs are more common than thought to date and we propose a series of potential interstitial dopants for TMDs relevant for application in nanoelectronics based on a detailed analysis of the predicted electronic structures.

cond-mat.mtrl-sci↗

arXiv2016

The dynamics of copper intercalated molybdenum ditelluride

Layered transition metal dichalcogenides are emerging as key materials in nanoelectronics and energy applications. Predictive models to understand their growth, thermomechanical properties and interactions with metals are needed in order to accelerate their incorporation into commercial products. Interatomic potentials enable large-scale atomistic simulations at the device level, beyond the range of applications of first principle methods. We present a ReaxFF reactive force field to describe molybdenum ditelluride and its interactions with copper. We optimized the force field parameters to describe the properties of layered MoTe2 in various phases, the intercalation of Cu atoms and clusters within its van der Waals gap, including a proper description of energetics, charges and mechanical properties. The training set consists of an extensive set of first principle calculations computed from density functional theory. We use the force field to study the adhesion of a single layer MoTe2 on a Cu(111) surface and the results are in good agreement with density functional theory, even though such structures were not part of the training set. We characterized the mobility of the Cu ions intercalated into MoTe2 under the presence of an external electric fields via molecular dynamics simulations. The results show a significant increase in drift velocity for electric fields of approximately 0.4 V/A and that mobility increases with Cu ion concentration.

cond-mat.mtrl-sci↗

arXiv2015

Voltage equilibration for reactive atomistic simulations of electrochemical processes

We introduce EChemDID, a model to describe electrochemical driving force in reactive molecular dynamics simulations. The method describes the equilibration of external electrochemical potentials (voltage) within metallic structures and their effect on the self consistent partial atomic charges used in reactive molecular dynamics. An additional variable assigned to each atom denotes the local potential in its vicinity and we use fictitious, but computationally convenient, dynamics to describe its equilibration within not-simply connected metallic structures on-the-fly during the molecular dynamics simulation. This local electrostatic potential is used to dynamically modify the atomic electronegativities used to compute partial atomic changes via charge equilibration. Validation tests show that the method provides an accurate description of the electric fields generated by the applied voltage and the driving force for electrochemical reactions. We demonstrate EChemDID via simulations of the operation of electrochemical metallization cells. The simulations predict the switching of the device between a high-resistance to a low-resistance state as a conductive metallic bridge is formed and resistive currents that can be compared with experimental measurements. In addition to applications in nanoelectronics, EChemDID could be useful to model electrochemical energy conversion devices.

cond-mat.mtrl-sci↗

arXiv2015

Stability and Migration of Small Copper Clusters in Amorphous Dielectrics

We use density functional theory (DFT) to study the thermodynamic stability and migration of copper ions and small clusters embedded in amorphous silicon dioxide. We perform the calculations over an ensemble of statistically independent structures to quantify the role of the intrinsic atomic-level variability in the amorphous matrix affect the properties. The predicted formation energy of a Cu ion in the silica matrix is 2.7+/-2.4 eV, significantly lower the value for crystalline SiO2. Interestingly, we find that Cu clusters of any size are energetically favorable as compared to isolated ions; showing that the formation of metallic clusters does not require overcoming a nucleation barrier as is often assumed. We also find a broad distribution of activation energies for Cu migration, from 0.4 to 1.1 eV. This study provides insights into the stability of nanoscale metallic clusters in silica of interest in electrochemical metallization cell memories and optoelectronics.

cond-mat.mtrl-sci↗