20 research outputs found
Thermal transport in MoS2 from molecular dynamics using different empirical potentials
Thermal properties of molybdenum disulfide (MoS2) have recently attracted attention related to fundamentals
of heat propagation in strongly anisotropic materials, and in the context of potential applications to optoelectronics and thermoelectrics. Multiple empirical potentials have been developed for classical molecular dynamics
(MD) simulations of this material, but it has been unclear which provides the most realistic results. Here, we
calculate lattice thermal conductivity of single- and multilayer pristine MoS2 by employing three different
thermal transport MD methods: equilibrium, nonequilibrium, and homogeneous nonequilibrium ones. We mainly
use the Graphics Processing Units Molecular Dynamics code for numerical calculations, and the Large-scale
Atomic/Molecular Massively Parallel Simulator code for crosschecks. Using different methods and computer
codes allows us to verify the consistency of our results and facilitate comparisons with previous studies, where
different schemes have been adopted. Our results using variants of the Stillinger-Weber potential are at odds
with some previous ones and we analyze the possible origins of the discrepancies in detail. We show that, among
the potentials considered here, the reactive empirical bond order (REBO) potential gives the most reasonable
predictions of thermal transport properties as compared to experimental data. With the REBO potential, we
further find that isotope scattering has only a small effect on thermal conduction in MoS2 and the in-plane thermal
conductivity decreases with increasing layer number and saturates beyond about three layers. We identify the
REBO potential as a transferable empirical potential for MD simulations of MoS2 which can be used to study
thermal transport properties in more complicated situations such as in systems containing defects or engineered
nanoscale features. This work establishes a firm foundation for understanding heat transport properties of MoS2
using MD simulations
Single- and Multilayers of Alkali Metal Atoms inside Graphene/MoS<sub>2</sub> Heterostructures: A Systematic First-Principles Study
Stacking
various 2D materials in van der Waals heterostructures
is a novel approach to design new systems, which can host alkali metal
(AM) atoms to tune their electronic properties or store energy. Using
state-of-the-art first-principles calculations, we systematically
study the intercalation of the most widespread AMs (Li, Na, and K)
into a graphene/MoS2 heterostructure. Contrary to the previous
work on the intercalation of AMs into various heterostructures based
on 2D materials, we consider not only single-, but also multi-layer
configurations of AM atoms. We assess the intercalation energetics
for various concentrations of AM atoms, calculate charge transfer
from AM atoms to the host system, and show that although intercalation
of AMs as a single layer is energetically preferable, multi-layer
configurations can exist at high concentrations of AM atoms. We further
demonstrate that the transition of the MoS2 layer from
the H to T′ phase is possible upon Li intercalation, but not
for Na or K. Our findings should help to better understand the behavior
of heterostructures upon AM atom intercalation and may stimulate further
experiments aimed at the tailoring of heterostructure properties and
increasing the capacity of anode materials in AM ion batteries
Semiconductor to Metal to Half-Metal Transition in Pt-Embedded Zigzag Graphene Nanoribbons
The electronic and magnetic properties
of Pt-embedded zigzag graphene nanoribbons (Pt–ZGNRs) are investigated
using density-functional theory calculations. It is found that Pt–ZGNRs
exhibit a semiconductor–metal–half-metal transition
as the position of Pt substitutional impurities in the ribbon changes
from the center to edge sites. This behavior can be attributed to
the interaction between Pt impurities and edge states of ZGNRs, which
governs the electron occupation of the edge states. The transition
always occurs independent of ribbon width. However, Pt impurity concentration
is important for obtaining this transition. Our results demonstrate
that Pt–ZGNRs can be used as versatile electronic devices
Solid-State Growth of One- and Two-Dimensional Silica Structures on Metal Surfaces
Crystalline
or vitreous silica layers are new two-dimensional (2D) nanomaterials,
which have shown surprising structural similarities with graphene
and promise interesting properties. In this study, one-dimensional
(1D) silica structures are formed on metal surfaces. In an in situ
electron microscopy experiment it is demonstrated that lines of silica
grow along step edges on metal surfaces. The growth of 1D silica occurs
in competition with the formation of 2D networks and adopts the crystalline
symmetry of the metal surface. Transformations between 1D and 2D structures
are observed. Density functional theory calculations show that 1D
silica is energetically favorable over 2D structures if surface steps
prevail on the substrate. Our results indicate that lateral heterostructures
with interesting properties may be developed on metal substrates
Magnetic State Control of Non-van der Waals 2D Materials by Hydrogenation
Controlling the magnetic
state of two-dimensional (2D) materials
is crucial for spintronics. By employing data-mining and autonomous
density functional theory calculations, we demonstrate the switching
of magnetic properties of 2D non-van der Waals materials upon hydrogen
passivation. The magnetic configurations are tuned to states with
flipped and enhanced moments. For 2D CdTiO3a diamagnetic
compound in the pristine casewe observe an onset of ferromagnetism
upon hydrogenation. Further investigation of the magnetization density
of the pristine and passivated systems provides a detailed analysis
of modified local spin symmetries and the emergence of ferromagnetism.
Our results indicate that selective surface passivation is a powerful
tool for tailoring magnetic properties of nanomaterials, such as non-vdW
2D compounds
Solubility of Boron, Carbon, and Nitrogen in Transition Metals: Getting Insight into Trends from First-Principles Calculations
Efficient chemical vapor deposition
synthesis of two-dimensional
(2D) materials such as graphene, boron nitride, and mixed BCN systems
with tunable band gaps requires precise knowledge of the solubility
and mobility of B/C/N atoms in the transition metals (TMs) used as
substrates for the growth. Yet, surprisingly little is known about
these quantities either from experiments or simulations. Using first-principles
calculations, we systematically study the behavior of B/C/N impurity
atoms in a wide range of TMs. We compute formation energies of B/C/N
interstitials and demonstrate that they exhibit a peculiar but common
behavior for TMs in different rows of the periodic table, as experimentally
observed for C. Our simulations indicate that this behavior originates
from an interplay between the unit cell volume and filling of the
d-shell electronic states of the metals. We further assess the vibrational
and electronic entropic contributions to the solubility, as well as
the role of anharmonic effects. Finally, we calculate the migration
barriers, an important parameter in the growth kinetics. Our results
not only unravel the fundamental behavior of interstitials in TMs
but also provide a large body of reference data, which can be used
for optimizing the growth of 2D BCN materials
<i>In Situ</i> Growth of Cellular Two-Dimensional Silicon Oxide on Metal Substrates
Crystalline hexagonally ordered silicon oxide layers with a thickness of less than a nanometer are grown on transition metal surfaces in an <i>in situ</i> electron microscopy experiment. The nucleation and growth of silica bilayers and monolayers, which represent the thinnest possible ordered structures of silicon oxide, are monitored in real time. The emerging layers show structural defects reminiscent of those in graphene and can also be vitreous. First-principles calculations provide atomistic insight into the energetics of the growth process. The interplay between the gain in silica–metal interaction energy due to their epitaxial match and energy loss associated with the mechanical strain of the silica network is addressed. The results of calculations indicate that both ordered and vitreous mono/bilayer structures are possible, so that the actual morphology of the layer is defined by the kinetics of the growth process
Experimental Observation of Boron Nitride Chains
We report the formation and characterization of boron nitride atomic chains. The chains were made from hexagonal boron nitride sheets using the electron beam inside a transmission electron microscope. We find that the stability and lifetime of the chains are significantly improved when they are supported by another boron nitride layer. With the help of first-principles calculations, we prove the heteroatomic structure of the chains and determine their mechanical and electronic properties. Our study completes the analogy between various boron nitride and carbon polymorphs, in accordance with earlier theoretical predictions
<i>In Situ</i> Growth of Cellular Two-Dimensional Silicon Oxide on Metal Substrates
Crystalline hexagonally ordered silicon oxide layers with a thickness of less than a nanometer are grown on transition metal surfaces in an <i>in situ</i> electron microscopy experiment. The nucleation and growth of silica bilayers and monolayers, which represent the thinnest possible ordered structures of silicon oxide, are monitored in real time. The emerging layers show structural defects reminiscent of those in graphene and can also be vitreous. First-principles calculations provide atomistic insight into the energetics of the growth process. The interplay between the gain in silica–metal interaction energy due to their epitaxial match and energy loss associated with the mechanical strain of the silica network is addressed. The results of calculations indicate that both ordered and vitreous mono/bilayer structures are possible, so that the actual morphology of the layer is defined by the kinetics of the growth process
Formation of In-Plane Semiconductor–Metal Contacts in 2D Platinum Telluride by Converting PtTe<sub>2</sub> to Pt<sub>2</sub>Te<sub>2</sub>
Monolayer PtTe2 is a narrow gap semiconductor
while
Pt2Te2 is a metal. Here we show that the former
can be transformed into the latter by reaction with vapor-deposited
Pt atoms. The transformation occurs by nucleating the Pt2Te2 phase within PtTe2 islands, so that a metal–semiconductor
junction is formed. A flat band structure is found with the Fermi
level of the metal aligning with that of the intrinsically p-doped
PtTe2. This is achieved by an interface dipole that accommodates
the ∼0.2 eV shift in the work functions of the two materials.
First-principles calculations indicate that the origin of the interface
dipole is the atomic scale charge redistributions at the heterojunction.
The demonstrated compositional phase transformation of a 2D semiconductor
into a 2D metal is a promising approach for making in-plane metal
contacts that are required for efficient charge injection and is of
particular interest for semiconductors with large spin–orbit
coupling, like PtTe2