Hexagonal BC$_{2}$N is a superhard material recently identified to be comparable to or even harder than cubic boron nitride (c-BN) due the full $sp^3$ bonding character and the higher number of C-C and B-N bonds compared to C-N and B-C.Using a first-principles approach to calculate force constants and an exact numerical solution to the phonon Boltzmann equation, we show that BC$_{2}$N has a high lattice thermal conductivity exceeding that of c-BN owing to the strong C-C and B-N bonds, which produce high phonon frequencies as well as high acoustic velocities. The existence of large group velocities in the optical branches is responsible for its large thermal conductivity. Its coefficient of thermal expansion (CTE) is found to vary from 2.6$\times$10$^{-6}$/K at room temperature to almost 5$\times$ 10$^{-6}$/K at 1000K. The combination of large thermal conductivity and a good CTE match with that of Si, makes BC$_2$N a promising material for use in thermal management and high-power electronics applications. We show that the application of compressive strain increases the thermal conductivity significantly. This enhancement results from the overall increased frequency scale with pressure, which makes acoustic and optic velocities higher, and weaker phonon-phonon scattering rates.
Superstructures of cubic and hexagonal diamonds (h- and c-diamond) comprising a family of stable diamond-like $sp^3$ hybridized novel carbon allotropes are proposed, which are referred to as U$_n$-carbon where $n \geq 2$ denotes the number of structural layers in a unit cell. The conventional h- and c-diamond are included in this family as members with $n=2$ and 3, respectively. U$_n$-carbon ($n=4-6$), which are unveiled energetically and thermodynamically more stable than h-diamond and possess remarkable kinetic stabilities, are shown to be insulators with indirect gaps of $5.6 \sim 5.8$ eV, …
A novel stable carbon allotrope is predicted by using first-principles calculations. This allotrope is obtained by replacing one of the two atoms in the primitive cell of diamond with a carbon tetrahedron, thus it contains five atoms in one primitive cell, termed T5-carbon. The stabilities of T5-carbon are checked in structural, thermal, vibrational and energy calculations. The calculations on electronic, thermal, and mechanical properties reveal that T5-carbon is a semiconductor with an indirect band gap of 3.18 eV, and has a lattice thermal conductivity of 409 W/mK. More importantly, the Vic…
The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B3N5) containing hexagonal B3N3 layers sandwiched with intercalated freely rotating N2 molecules. At 15 GPa, a three-dimensional C2221 structure …
Understanding the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. Silicon clathrates have the archetype structures that can serve an existant model compounds for superhard clathrate frameworks "Si-B", "Si-C", "B-C" and "C" with intercalated atoms (e.g. alkali metals or even halogenes) that can assure the metalic properties. Here we report the in situ and ex situ studies of high-pressure formation and stability of clathrates Na8Si46 (structure I) and Na24+xSi136 (structure II). Experime…
Although enhanced conductivity at ferroelectric domain boundaries has been found in BiFeO$_3$ films, Pb(Zr,Ti)O$_3$ films, and hexagonal rare-earth manganite single crystals, the mechanism of the domain wall conductivity is still under debate. Using conductive atomic force microscopy, we observe enhanced conductance at the electrically-neutral domain walls in semiconducting hexagonal ferroelectric TbMnO$_3$ thin films where the structure and polarization direction are strongly constrained along the c-axis. This result indicates that domain wall conductivity in ferroelectric rare-earth manganit…
The phonon dispersion, density of states, Grüneisen parameters, and the lattice thermal conductivity of single- and multi-layered boron nitride were calculated using first-principles methods. For the bulk {\it h}-BN we also report the two-phonon density of states. We also present simple analytical solutions to the acoustic vibrational mode-dependent lattice thermal conductivity. Moreover, computations based on the elaborate Callaway-Klemens and the real space super cell methods are presented to calculate the sample length and temperature dependent lattice thermal conductivity of single- and mu…
We report the nanoscale electrical imaging results in hexagonal $Lu_{0.6}Sc_{0.4}FeO_3$ single crystals using conductive atomic force microscopy (C-AFM) and scanning microwave impedance microscopy (MIM). While the dc and ac response of the ferroelectric domains can be explained by the surface band bending, the drastic enhancement of domain wall (DW) ac conductivity is clearly dominated by the dielectric loss due to DW vibration rather than mobile-carrier conduction. Our work provides a unified physical picture to describe the local conductivity of ferroelectric domains and domain walls, which …
The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a micro-bridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11-12 atomic layer h-BN samples with suspended length ranging between 3 and 7.5 um, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 Wm-1K-1, approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 …
Designing and searching for a high thermal conductivity material in both bulk and nanoscale is highly demanding for electronics cooling. In this work, we studied the thermal conductivity of 2H-Germanium Carbide(2H-GeC) using first principles calculations. At 300 K, we are reporting a high thermal conductivity of 1350 Wm-1K-1 and 1050 Wm-1K-1 along a-axis and c-axis respectively for pure 2H-GeC. These values are 130% higher than the thermal conductivity of 2H-silicon carbide and 20% lower than cubic germanium carbide(c-GeC). We analyzed the phonon group velocities, phonon scattering rates and m…
We report a study of magnetism and magnetic transitions of hexagonal ErMnO$_3$ single crystals by magnetization, specific heat and heat transport measurements. Magnetization data show that the $c$-axis magnetic field induces three magnetic transitions at 0.8, 12 and 28 T. The specific heat shows a peak at 2.2 K, which is due to a magnetic transition of Er$^{3+}$ moments. For low-$T$ thermal conductivity ($κ$), a clear dip-like feature appears in $κ(H)$ isotherm at 1--1.25 T for $H \parallel ab$; while in the case of $H \parallel c$, a step-like increase is observed at 0.5--0.8 T. The transitio…
Two-dimensional materials are characterised by a number of unique physical properties which can potentially make them useful to a wide diversity of applications. In particular, the large thermal conductivity of graphene and hexagonal boron nitride has already been acknowledged and these materials have been suggested as novel core materials for thermal management in electronics. However, it was not clear if mass produced flakes of hexagonal boron nitride would allow one to achieve an industrially-relevant value of thermal conductivity. Here we demonstrate that laminates of hexagonal boron nitri…
We calculate the lattice thermal conductivity ($κ$) for cubic (zinc-blende) and hexagonal (wurtzite) phases for 8 semiconductors using $\textit{ab initio}$ calculations and solving the Phonon Boltzmann Transport Equation, explaining the different behavior of the ratio $κ_{\rm hex}/κ_{\rm cub}$ between the two phases. We show that this behavior depends on the relative importance of two antagonistic factors: anharmonicity, which we find to be always higher in the cubic phase; and the accessible phase space, which is higher for the less symmetric hexagonal phase. Based on that, we develop a metho…
Engineering materials with high thermal conductivity are of fundamental interest for efficiently dissipating heat in micro/nanoelectronics. Using first principles computations we report an ultra-high thermal conductivity of 2090 Wm-1K-1 (1395 Wm-1K-1) for hexagonal pure (natural) BC6N(h-BC6N). This value is among the highest thermal conductivities known after diamond and cubic boron arsenide. This ultra-high lattice thermal conductivity (k) is mainly attributed with high phonon group velocities of both acoustic and optical phonons arising from strong C-C and B-N bonds as well as the light atom…
We present a thorough dielectric investigation of the hexagonal perovskites La1.2Sr2.7IrO7.33 and La1.2Sr2.7RuO7.33 in a broad frequency and temperature range, supplemented by additional infrared measurements. The occurrence of giant dielectric constants up to 10^5 is revealed to be due to electrode polarization. Aside of dc and ac conductivity contributions, we detect two intrinsic relaxation processes that can be ascribed to ionic hopping between different off-center positions. In both materials we find evidence for charge transport via hopping of localized charge carriers. In the infrared r…
We use a phase field crystal model to generate large-scale bicrystalline and polycrystalline single-layer hexagonal boron nitride (h-BN) samples and employ molecular dynamics (MD) simulations with the Tersoff many-body potential to study their heat transport properties. The Kapitza thermal resistance across individual h-BN grain boundaries is calculated using the inhomogeneous nonequilibrium MD method. The resistance displays strong dependence on the tilt angle, the line tension and the defect density of the grain boundaries. We also calculate the thermal conductivity of pristine h-BN and poly…
Designing and searching for high lattice thermal conductivity materials in both bulk and nanoscale level is highly demanding for electronics cooling. Boron phosphide is a III-V compound semiconductor with superior structural and thermal properties. In this work, we studied the lattice thermal conductivity of hexagonal boron phosphide(h-BP) using first principles calculations. For pure h-BP, we found a high lattice thermal conductivity (at 300K) of 561.2 Wm-1K-1 and 427.4 Wm-1K-1 along a-axis and c-axis respectively. These values are almost equal to hexagonal silicon carbide(2H-SiC) and cubic b…
Recently a novel two-dimensional (2D) C$_{60}$ based crystal called quasi-hexagonal-phase fullerene (QHPF) has been fabricated and demonstrated to be a promising candidate for 2D electronic devices [Hou et al. Nature 606, 507-510 (2022)]. We construct an accurate and transferable machine-learned potential to study heat transport and related properties of this material, with a comparison to the face-centered-cubic bulk-phase fullerene (BPF). Using the homogeneous nonequilibrium molecular dynamics and the related spectral decomposition methods, we show that the thermal conductivity in QHPF is an…
We deduce the intrinsic conductivity properties of the ferroelectric domain walls around the topologically protected domain vortex cores in multiferroic YMnO3. This is achieved by performing a careful equivalent-circuit analysis of dielectric spectra measured in single-crystalline samples with different vortex densities. The conductivity contrast between the bulk domains and the less conducting domain boundaries is revealed to reach up to a factor 500 at room temperature, depending on sample preparation. Tunneling of localized defect charge carriers is the dominant charge-transport process in …
Covalent functionalization is a way to tune the electrochemical properties of hexagonal boron nitride (h-BN) monolayers. The wide band gap insulator h-BN may become metallic conductor upon functionalization with strong oxidants, such as fluorosulfonyl radicals ($\cdot$OSO$_2$F), as known since 1978 [N. Bartlett et al., J. Chem. Soc. Chem. Comm. {\bf 5}, 200 (1978)], with electrical conductivity of 1.5 S/cm [C. Shen et al., J. Solid State Chem. {\bf 147}, 74 (1999)] that greatly surpasses commercial cathode material Li$_{x}$CoO$_{2}$ while retaining excellent ionic conductivity. Functionalized …
Advanced Technology Research Institute of Taihu Photon Center, School of Physical Science and Technology, Suzhou University of Science and Technology（太湖光子中心先进技术研究院，苏州科技大学物理科学与技术学院）
Thermal management materials are of critical importance for engineering miniaturized electronic devices, where theoretical design of such materials demands the evaluation of thermal conductivities which are numerically expensive. In this work, we applied the recently developed machine learning interatomic potential (MLIP) to evaluate the thermal conductivity of hexagonal boron nitride monolayers. The MLIP is obtained using the Gaussian approximation potential (GAP) method, and the resulting lattice dynamical properties and thermal conductivity are compared with those obtained from explicit fro…
The highly anisotropic thermal conductivity in layered materials is crucial for a broad range of applications such as thermal management of electronic devices, thermal insulation, and thermoelectrics. Understanding of anisotropic thermal transport in layered materials largely depends on atomistic simulations based on density functional theory (DFT) or empirical potentials, which however suffer either low computational efficiency or accuracy. Recently, machine learning interatomic potentials (MLIPs) are emerging as a powerful tool to bridge the gap. Despite the recent progress in developing MLI…
$β$- Eucryptite (LiAlSiO4) is a potential electrolyte for Li- ion battery due to its high Li- ion conductivity and very small volume thermal expansion coefficient. We have performed ab-initio molecular dynamics simulations of $β$- Eucryptite to study the origin of high temperature superionic phase transition in this material. The simulations are able to provide the microscopic understanding of the one -dimensional superionicity that occurs along the hexagonal c-axis and is associated with the order-disorder nature of the phase transition. The Li ionic conductivity is found to enhance due to th…
We present a comprehensive computational study on the properties of face-centered cubic and hexagonal chalcogenide Ge2Sb2Te5. We calculate the electronic structure using density functional theory (DFT); the obtained density of states (DOS) compares favorably with experiments, also looking suitable for transport analysis. Optical constants including refraction index and absorption coefficient capture major experimental features, aside from an energy shift owed to an underestimate of the band gap that is typical of DFT calculations. We also compute the phonon DOS for the hexagonal phase, obtaini…
An increasing number of two-dimensional (2D) materials have already been achieved experimentally or predicted theoretically, which have potential applications in nano- and opto-electronics. Various applications for electronic devices are closely related to their thermal transport properties. In this work, the strain dependence of phonon transport in monolayer SiC with a perfect planar hexagonal honeycomb structure is investigated by solving the linearized phonon Boltzmann equation. It is found that room-temperature lattice thermal conductivity ($κ_L$) of monolayer SiC is two orders of magnitud…
Silicon carbide (SiC) is a wide bandgap (WBG) semiconductor with promising applications in high-power and high-frequency electronics. Among its many useful properties, the high thermal conductivity is crucial. In this letter, the anisotropic thermal conductivity of three SiC samples: n-type 4H-SiC (N-doped 1x10^19 cm-3), unintentionally doped (UID) semi-insulating (SI) 4H-SiC, and SI 6H-SiC (V-doped 1x10^17 cm-3), is measured using femtosecond laser based time-domain thermoreflectance (TDTR) over a temperature range from 250 K to 450 K. We simultaneously measure the thermal conductivity parall…
Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning tr…
Atomically thin monolayers of graphene show excellent electronic properties which have led to a great deal of research on their use in nanoscale devices. However, heat management of such nanoscale devices is essential in order to improve their performance. Graphene supported on hexagonal boron nitride (h-BN) substrate has been reported to show enhanced (opto)electronic and thermal properties as compared to extensively used SiO2/Si supported graphene. Motivated by this, we have performed temperature- and power-dependent Raman Spectroscopic measurements on four different types of (hetero)structu…
We perform molecular dynamics (MD) simulations to investigate the effect of polar surfaces on the thermal transport in zinc oxide (ZnO) nanowires. We find that the thermal conductivity in nanowires with free polar (0001) surfaces is much higher than in nanowires that have been stabilized with reduced charges on the polar (0001) surfaces, and also hexagonal nanowires without any transverse polar surfaces. From normal mode analysis, we show that the higher thermal conductivity is due to a shell-like reconstruction that occurs for the free polar surfaces. This shell-like reconstruction suppresses…
We design a hybrid graphene/hexagonal boron nitride superlattice monolayer and investigate its thermoelectric properties using density functional theory and Boltzmann transport equations with the relaxation time accurately treated by electron-phonon coupling calculations. Compared with that of pristine graphene, the lattice thermal conductivity of the superlattice structure is more than two orders of magnitude lower due to the enhanced three-phonon scattering process originated from the mixed-bond characteristics. Besides, the coexistence of light and heavy bands around the Fermi level leads t…
Combining first-principles density functional method and crystal structure prediction techniques, we report a series of hexagonal two-dimensional (2D) transition metal borides (TMBs) including Sc2B2, Ti2B2, V2B2, Cr2B2, Y2B2, Zr2B2, and Mo2B2. Their dynamic and thermal stabilities are testified by phonon and molecular dynamics simulations. We investigate the potential of 2D Ti2B2 monolayer as the anode material for Li-ion batteries (LIBs) and Na-ion batteries (NIBs). The Ti2B2 monolayer possesses high theoretical specific capacities of 456 and 1027 mAhg-1 for Li and Na, respectively. The very …
Thermal conduction was explored and discussed through a combined theoretical and simulation approach in this work. The thermal conductivity k of polycrystalline graphene was calculated by molecular dynamics simulations based on a hexagonal patch model in close consistence with microstructural characterization in experiments. The effects of grain size, alignment, and temperature were identified with discussion on the microscopic phonon scattering mechanisms. The effective thermal conductivity is found to increase with the grain size and decrease with the mismatch angle and dislocation density a…
We study the low-temperature heat transport, as well as the magnetization and the specific heat, of TmMnO_3 single crystals to probe the transitions of magnetic structure induced by magnetic field. It is found that the low-T thermal conductivity (κ) shows strong magnetic-field dependence and the overall behaviors can be understood in the scenario of magnetic scattering on phonons. In addition, a strong "dip"-like feature shows up in κ(H) isotherms at 3.5--4 T for H \parallel c, which is related to a known spin re-orientation of Mn^{3+} moments. The absence of this phenomenon for H \parallel a …
Phagraphene is a novel 2D carbon allotrope with interesting electronic properties which has been recently theoretically proposed. Phagraphene is similar to a defective graphene structure with an arrangement of pentagonal, heptagonal and hexagonal rings. In this study we investigate thermal conductivity and mechanical properties of phagraphene using molecular dynamics simulations. Using the non-equilibrium molecular dynamics method, we found the thermal conductivity of phagraphene to be anisotropic, with room temperature values of $218 \pm 20$ W/m-K along the armchair direction and $285 \pm 29$…
Lattice dynamics in low-dimensional materials and, in particular, the quadratic behaviour of the flexural acoustic modes play a fundamental role in their thermomechanical properties. A first-principles evaluation of these can be very demanding, and can be affected by numerical noise that breaks translational or rotational invariance. In order to overcome these challenges, we study the Gartstein internal-coordinate potential and tune its 13 parameters on the first-principles interatomic force constants for graphene. We show that the resulting potential not only reproduces very well the phonon d…
Gate-tunable electronic properties of epitaxial Bi (111) films using a printable hexagonal boron nitride ionogel
Argonne National Laboratory（阿贡国家实验室）；Northwestern University（西北大学）；Case Western Reserve University（凯斯西储大学）；University of California, Riverside（加州大学河滨分校）
We have used scanning tunneling microscopy and spectroscopy to resolve the spatial variation of the density of states of twisted graphene layers on top of a highly oriented pyrolytic graphite substrate. Owing to the twist a moire pattern develops with a periodicity that is substantially larger than the periodicity of a single layer graphene. The twisted graphene layer has electronic properties that are distinctly different from that of a single layer graphene due to the nonzero interlayer coupling. For small twist angles (about 1-3.5 degree) the integrated differential conductivity spectrum ex…
It is challenging to characterize thermal conductivity of materials with strong anisotropy. In this work, we extend the time-domain thermoreflectance (TDTR) method with a variable spot size approach to simultaneously measure the in-plane (Kr) and the through-plane (Kz) thermal conductivity of materials with strong anisotropy. We first determine Kz from the measurement using a larger spot size, when the heat flow is mainly one-dimensional along the through-plane direction, and the measured signals are sensitive to only Kz. We then extract the in-plane thermal conductivity Kr from a second measu…
Germanium selenide (GeSe) is a unique two-dimensional (2D) material showing various polymorphs stable at ambient condition. Recently, a new phase with a layered hexagonal lattice (γ-GeSe) was synthesized with ambient stability and extraordinary electronic conductivity even higher than graphite while its monolayer is semiconducting. In this work, via using first-principles derived force constants and Boltzmann transport theory we explore the lattice thermal conductivity (κ_l) of the monolayer γ-GeSe, together with a comparison with monolayer α-GeSe and β-GeSe. The κ_l of γ-phase is relatively l…
Ever since the high thermal conductivity in cubic boron arsenide (c-BAs) was predicted theoretically by Lindsay et. al in 2013, countless studies have zeroed in on this particular material. Most recently, c-BAs has been confirmed experimentally to have a thermal conductivity of around 1,100 W/m-K. In this study, we investigate the seldom studied two dimensional hexagonal form of boron arsenide (h-BAs) using a first-principles approach and by solving the Boltzmann Transport Equation for phonons. Traditionally, a good indicator of a high thermal conductivity material is its high Debye temperatur…
The substrate material of monolayer graphene influences the charge carrier mobility by various mechanisms. At room temperature, the scattering of conduction electrons by phonon modes localized at the substrate surface can severely limit the charge carrier mobility. We here show that for substrates made of the piezoelectric hexagonal boron nitride (hBN), in comparison to the widely used SiO$_2$, this mechanism of remote phonon scattering is --at room temperature-- weaker by almost an order of magnitude, and causes a resistivity of approximately 3\,$Ω$. This makes hBN an excellent candidate mate…
Due to potential applications in nano- and opto-electronics, two-dimensional (2D) materials have attracted tremendous interest. Their thermal transport properties are closely related to the performance of 2D materials-based devices. Here, the phonon transports of monolayer GeC with a perfect planar hexagonal honeycomb structure are investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Without inclusion of Born effective charges ($Z^*$) and dielectric constants ($\varepsilon$), the lattice thermal conductivity ($κ_L$) almost…
Using topological band theory analysis we show that the nonsymmorphic symmetry operations in hexagonal lattices enforce Weyl points at the screw-invariant high-symmetry lines of the band structure. The corepresentation theory and connectivity group theory show that Weyl points are generated by band crossings in accordion-like and hourglass-like dispersion relations. These Weyl points are stable against weak perturbations and are protected by the screw rotation symmetry. Based on first-principles calculations we found a complete agreement between the topological predicted energy dispersion rela…
A radical anion based functionalization of the basal plane of two-dimensional (2D) materials is proposed in the present study. Simple charge neutral radical functionalizations typically detach from the basal planes upon reduction. For example, epoxy oxygens irreversibly detach from graphene when reduced by an alkali metal. The radical anion functionalization of 2D materials results in a stable reduced state that can reversibly be oxidized and has high ionic conductivity due to the great mobility of the cations between the negatively charged functional groups on the surface. Depending on the ox…
