Search

Using a density functional theory‐based thermal transport model, which includes the effects of temperature (T)‐dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher‐order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κL) and a non‐standard κL∝T−0.491 dependence. At high T, the departure from the standard κL∝T−1 law originates in the dual particle‐wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κL=2.98 W mK‐1 arises mainly from the conventional particle‐like propagation of phonons, there is also a significant atypical wave‐like phonon tunneling effect, leading to a 20% glass‐like heat transport contribution. The phonon broadening effect lowers the particle‐like contribution but increases the glass‐like one. Upon T increase, the glass‐like contribution increases and dominates above T = 850 K. Overall, the low κL with a weak T‐dependence points to a new utility for LaWN3 in energy technology applications, and motivates synthesis and exploration of nitride perovskites. [ABSTRACT FROM AUTHOR]

Although isolated nonhexagonal carbon rings in graphene are associated with strain relaxation and curvature, dense and ordered arrangements of four‐, five‐, and eight‐membered rings with strained carbon–carbon bonds can tile 2D planar layers. Using the Boltzmann transport equation formalism in combination with density functional theory calculations, how the presence of nonhexagonal rings impacts the thermal conductivity of three 2D carbon allotropes: T‐graphene (four and eight rings), biphenylene (four, six, and eight rings), and net‐graphene (four, six, and eight rings), is investigated. The phonon thermal conductivity (κph), which captures three‐phonon, four‐phonon, and phonon–electron interactions, is significantly lowered with respect to pristine graphene. In compensation, the electron thermal conductivity (κe), which captures electron–phonon interactions, is enhanced to record high values, such that the room‐temperature total thermal conductivity κtotal = κph + κe approaches the values of pristine graphene. 2D carbon allotropes could be of interest for applications requiring thermal energy transfer by a combination of diffusion of electrons and phonon vibrations. [ABSTRACT FROM AUTHOR]

2D beryllium polynitrides or beryllonitrene is a newly synthesized layered material displaying anisotropic Dirac cones and van Hove singularity (VHS) located only ≈0.5 eV above the Fermi level. Using the Boltzmann transport equation with many‐body effects and first‐principles calculations, it is uncovered that beryllonitrene has an in‐plane anisotropic room‐temperature phonon thermal conductivity (κph) of 78.6 and 98.8 W mK−1, and an electron thermal conductivity (κe) of 23.0 and 60.7 W mK−1, along the in‐plane directions. κph is dominated by the large heat capacity flexural acoustic (ZA) modes, which are susceptible to three‐phonon and four‐phonon scatterings but rather immune to scattering onto electrons. Filling the Dirac cones till VHS and above gradually enhances the phonon–electron coupling and monotonically decreases κph by up to 55%. Instead, κe displays unusual nonmonotonic variations with the increase in the carrier density and follows the electron density of states at corresponding Fermi levels. The results shed light on the thermal and electrical transport properties in beryllonitrene and reveal a thermal conductivity modulation mechanism that includes a 60% increase of κe upon filling of the Dirac cones until VHS. [ABSTRACT FROM AUTHOR]

While graphene grain boundaries (GBs) are well characterized experimentally, their influence on transport properties is less understood. As revealed here, phononic thermal transport is vulnerable to GBs even when they are ultra‐narrow and aligned along the temperature gradient direction. Non‐equilibrium molecular dynamics simulations uncover large reductions in the phononic thermal conductivity (κp) along linear GBs comprising periodically repeating pentagon‐heptagon dislocations. Green's function calculations and spectral energy density analysis indicate that the origin of the κp reduction is hidden in the periodic GB strain field, which behaves as a reflective diffraction grating with either diffuse or specular phonon reflections, and represents a source of anharmonic phonon–phonon scattering. The non‐monotonic dependence with dislocation density of κp uncovered here is unaccounted for by the classical Klemens theory. It can help identify GB structures that can best preserve the integrity of the phononic transport. [ABSTRACT FROM AUTHOR]

MXenes are a recent family of 2D materials with very interesting electronic properties for device applications. One very appealing feature is the wide range of work functions shown by these materials, depending on their composition and surface terminations, that can be exploited to adjust band alignments between different material layers. In this work, based on density functional theory calculations, how mixed terminations of F, OH, and/or O affect the work function of Ti3C2 MXene is analyzed in detail, covering the whole phase‐space of mixtures. The Ti3C2/CH3NH3PbI3 (MAPbI3) perovskite coupled system for solar cell applications is also analyzed. A strong nonlinear behavior is found when varying the relative concentrations of OH, O, and F terminations, with the strongest effect of the OH groups in lowering the work function, already at a relative amount of 25%. A surprising minimum work function is found for relative OH:O fraction of 75:25, explained in terms of the nonlinear electronic response in screening the surface dipoles. [ABSTRACT FROM AUTHOR]

In this work we use the multi-scale software tool tiberCAD to study the electronic and optical properties of a InGaN quantum disk (QD) in a GaN nanocolumn p-i-n diode structure, both with a macroscopical and an atomistic approach. Results show that the strain relaxation close to the lateral surface is evident for smaller nanocolumns but its effect tends to vanish for higher values of the column width. Self-consistent Schrödinger-Poisson/drift-diffusion calculations show that particle density and therefore radiative recombination rate are focused in the center of the QD (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]