100 research outputs found
Phonon transport in single-layer Boron nanoribbons
Inspired by the successful synthesis of several allotropes, boron sheets have
been one of the hottest spot areas of focus in various fields. Here, we study
phonon transport in three types of boron nanoribbons with zigzag and armchair
edges by using a non-equilibrium Green's function combined with first
principles methods. Diverse transport properties are found in the nanoribbons.
At the room temperature, their highest thermal conductance can be comparable
with that of graphene, while the lowest thermal conductance is less than half
of graphene's. The three boron sheets exhibit different anisotropic transport
characteristics. Two of these sheets have stronger phonon transport abilities
along the zigzag edges than the armchair edges, while in the case of the third,
the results are reversed. With the analysis of phonon dispersion, bonding
charge density, and simplified models of atomic chains, the mechanisms of the
diverse phonon properties are discussed. Because all boron allotropes consists
of hexagonal and triangular rings, many hybrid patterns can be constructed
naturally without doping, adsorption, and defects. Our results are useful in
materials and devices design using boron sheets for thermal management
Coexistence of flat bands and Dirac bands in a carbon-Kagome-lattice family
The Dirac bands and flat bands are difficult to coexist because they
represent two extreme ends of electronic properties. However, in this paper, we
propose a carbon-Kagome-lattice (CKL) family based on first-principles
calculations, and the coexistence of Dirac and flat bands are observed in this
series of three-dimensional carbon structures. The flat bands are originated
from the orbital interactions of the Kagome lattices, while the Dirac bands are
related to the carbon zigzag chains. A tight-binding model is used to explain
the various band structures in different CKLs. The coexistence of flat and
Dirac bands around the Fermi level implies that CKL structures maybe can serve
as superconductors. In addition, electronic properties of the thinnest CKL
slabs, only consisting of benzene rings, are studied. Flat bands are found in
the band spectra of the two-dimensional structures, and split into spin-up and
spin-down bands because of strong correlated effect in the case of hole doping
Transition of thermal rectification in silicon nanocones
Current understanding of thermal rectification asserts that the rectification
ratio (R), which measures the relative heat flux between two ends of a
nanostructure, is determined by its geometric asymmetry. The higher the
asymmetry, the higher the R. However, by using nonequilibrium molecular
dynamics method we have calculated thermal transport in Si nanocones as an
example, the results show that such an understanding may be incorrect and R may
not increase monotonically with geometric asymmetry. Rather, R exhibits a sharp
reverse when the vertex angle ({\theta}) of the nanocone is approximately
90{\deg}. In other words, when {\theta} > 90{\deg}, R decreases, rather than
increasing. We show that this abnormal behavior is originated from a change in
the thermal transport mechanism. At small {\theta}s, phonon transport is
dominated by localized modes, especially for transport from tip to bottom. At
large {\theta}s, however, these localized modes disappear, leading to R
decrease.Comment: in Applied Thermal Engineering 201
Electron and phonon properties and gas storage in carbon honeycomb
A new kind of three-dimensional carbon allotropes, termed carbon honeycomb
(CHC), has recently been synthesized [PRL 116, 055501 (2016)]. Based on the
experimental results, a family of graphene networks are constructed, and their
electronic and phonon properties are calculated by using first principles
methods. All networks are porous metal with two types of electron transport
channels along the honeycomb axis and they are isolated from each other: one
type of channels is originated from the orbital interactions of the carbon
zigzag chains and is topologically protected, while the other type of channels
is from the straight lines of the carbon atoms that link the zigzag chains and
is topologically trivial. The velocity of the electrons can reach ~10^6 m/s.
Phonon transport in these allotropes is strongly anisotropic, and the thermal
conductivities can be very low when compared with graphite by at least a factor
of 15. Our calculations further indicate that these porous carbon networks
possess high storage capacity for gaseous atoms and molecules in agreement with
experiment.Comment: Nanoscale, 201
Nodal-chain network, intersecting nodal rings and triple points coexisting in nonsymmorphic Ba3Si4
Coexistence of topological elements in a topological metal/semimetal (TM) has
gradually attracted attentions. However, the non-topological factors always
mess up the Fermi surface and cover interesting topological properties. Here,
we find that Ba3Si4 is a "clean" TM in which coexists nodal-chain network,
intersecting nodal rings (INRs) and triple points, in the absence of spin-orbit
coupling (SOC). Moreover, the nodal rings in the topological phase exhibit
diverse types: from type-I, type-II to type-III rings according to band
dispersions. All the topological elements are generated by crossings of three
energy bands, and thus they are correlated rather than mutual independence.
When some structural symmetries are eliminated by an external strain, the
topological phase evolves into another phase including Hopf link,
one-dimensional nodal chain and new INRs
Thermal transport in MoS2/Graphene hybrid nanosheets
Heat dissipation is a very critical problem for designing nano-functional
devices, including MoS2/Graphene heterojunctions. In this paper we investigate
thermal transport in MoS2/Graphene hybrid nanosheets under various heating
conditions, by using molecular dynamics simulation. Diverse transport processes
and characteristics, depending on the conducting layers, are found in these
structures. The thermal conductivities can be tuned by interlayer coupling,
environment temperature and interlayer overlap. The highest thermal
conductivity at room temperature is achieved as more than 5 times of that of
single layer MoS2 when both layers are heated and 100% overlapped. Different
transport mechanisms in the hybrid nanosheets are explained by phonon density
of states, temperature distribution, and ITR. Our results not only could
provide clues to master the heat transport in functional devices based on
MoS2/Graphene heterojunctions, but also are useful to analyze thermal transport
in other van der Waals hybrid nanosheets
Double Kagome bands in a two-dimensional phosphorus carbide P2C3
The interesting properties of Kagome bands, consisting of Dirac bands and a
flat band, have attracted extensive attention. However, the materials with only
one Kagome band around the Fermi level cannot possess physical properties of
Dirac fermions and strong correlated fermions simultaneously. Here, we propose
a new type of band structure --- double Kagome bands, which can realize
coexistence of the two kinds of fermions. Moreover, the new band structure is
found to exist in a new two-dimensional material, phosphorus carbide P2C3. The
carbide material shows good stability and unusual electronic properties. Strong
magnetism appears in the structure by hole doping of the flat band, which
results in spin splitting of the Dirac bands. The edge states induced by Dirac
and flat bands coexist on the Fermi level, indicating outstanding transport
characteristics. In addition, a possible route to experimentally grow P2C3 on
some suitable substrates such as the Ag (111) surface is also discussed
Design triple points, nexus points and related topological phases by stacking monolayers
Triple points and nexus points are two interesting topological phases, which
have been reported in some three-dimensional (3D) materials. Here, we propose
that triple points, nexus points and related phases, such as topological tangle
nodal lines, can be obtained by alternatively stacking two types of monolayers.
Two conditions for the stacking monolayers are required: the first condition is
that they have a three-fold (C3) rotation symmetry and three mirror planes
along the C3 axis; the second condition is that one of the monolayers should be
insulating while the other one should be metallic (or semiconducting) and has a
double degenerate band and a nondegenerate band at {\Gamma}point around the
Fermi level. Hexagonal boron nitride (HBN) and {\alpha}/{\alpha}^'-boron sheets
({\alpha}/{\alpha}^'-BS) are suggested as candidate materials. Even if HBN is a
wide-gap insulator, the interactions between layers lead to crossings of the
nondegenerate and double degenerate bands along the direction normal to the
nanosheets, and thus form triple/nexus points or related phases. A
tight-binding model is adopted to explain the phase transition between triple
points, nexus points and other related phases
Geometry, stability and thermal transport of hydrogenated graphene nanoquilts
Geometry, stability, and thermal transport of graphene nanoquilts folded by
hydrogenation are studied using molecular dynamics simulations. The
hydrogenated graphene nanoquilts show increased thermodynamic stability and
better transport properties than folded graphene structures without
hydrogenation. For the two-fold graphene nanoquilt, both geometry and thermal
conductivity are very sensitive to the adsorbed hydrogen chains, which is
interpreted by the red-shift of PDOS. For the multi-fold nanoquilts, their
thermal conductivities can be tuned from 100% to 15% of pristine graphene, by
varying the periodic number or length. Our results demonstrated that the
hydrogenated graphene nanoquilts are quite suitable to be thermal management
devices
Versatile electronic properties and exotic edge states in single-layer tetragonal silicon carbides
Three single-layer tetragonal silicon carbides (SiC), termed as T1,T2 and T3,
are proposed by density functional theory (DFT) computations. Although the
three structures have the same topological geometry, they show versatile
electronic properties from semiconductor (T1), semimetal (T2) to metal (T3).The
versatile properties are originated from the rich bonds between Si and C atoms.
The nanoribbons of the three SiC also show interesting electronic properties.
Especially, T1 nanoribbons possess exotic edge states, where electrons only
distribute on one edge's silicon or carbon atoms. The band gaps of the T1
nanoribbons are constant because of no interaction between the edge states
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