Tin selenide (SnSe) and tin sulfide (SnS) have recently attracted particular interest due to their great potential for large-scale thermoelectric applications. A complete prediction of the thermoelectric performance and the understanding of underlying heat and charge transport details are the key to further improvement of their thermoelectric efficiency. We conduct comprehensive investigations of both thermal and electrical transport properties of SnSe and SnS using first-principles calculations combined with the Boltzmann transport theory. Due to the distinct layered lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and electrical behaviors. The cross-plane lattice thermal conductivity  L is 40-60% lower than the in-2 plane values. Extremely low  L is found for both materials because of high anharmonicity while the average  L of SnS is ~ 8% higher than that of SnSe from 300 to 750 K. It is suggested that nanostructuring would be difficult to further decrease  L because of the short mean free paths of dominant phonon modes (1-30 nm at 300 K) while alloying would be efficient in reducing  L considering that the relative  L contribution (~ 65%) of optical phonons is remarkably large. On the electrical side, the anisotropic electrical conductivities are mainly due to the different effective masses of holes and electrons along the a, b and c axes. This leads to the highest optimal ZT values along the b axis and lowest ones along the a axis in both p-type materials.However, the n-type ones exhibit the highest ZTs along the a axis due to the enhancement of power factor when the chemical potential gradually approaches the secondary conduction band valley that causes significant increase in electron mobility and density of states. Owing to the larger mobility and smaller  L along the given direction, SnSe exhibits larger optimal ZTs compared with SnS in both p-and n-type materials. For both materials, the peak ZTs of n-type materials are much higher than those of p-type ones along the same direction. The predicted highest ZT values at 750 K are 1.0 in SnSe and 0.6 in SnS along the b axis for the p-type doping while those for the n-type doping reach 2.7 in SnSe and 1.5 in SnS along the a axis, rendering them among the best bulk thermoelectric materials for large-scale applications. Our calculations show reasonable agreements with the experimental results and quantitatively predict the great potential in further enhancing the thermoelectric performance of SnSe and SnS, especially for the n-type materials.

Based on first principles calculations and self-consistent solution of the linearized Boltzmann-Peierls equation for phonon transport approach within a three-phonon scattering framework, we characterize lattice thermal conductivities k of freestanding silicene, germanene and stanene under different isotropic tensile strains and temperatures. We find a strong size dependence of k for silicene with tensile strain, i.e., divergent k with increasing system size; however, the intrinsic room temperature k for unstrained silicene converges with system size to 19.34 W m(-1) K(-1) at 178 nm. The room temperature k of strained silicene becomes as large as that of bulk silicon at 84 μm, indicating the possibility of using strain in silicene to manipulate k for thermal management. The relative contribution to the intrinsic k from out-of-plane acoustic modes is largest for unstrained silicene, ∼39% at room temperature. The single mode relaxation time approximation, which works reasonably well for bulk silicon, fails to appropriately describe phonon thermal transport in silicene, germanene and stanene within the temperature range considered. For large samples of silicene, k increases with tensile strain, peaks at ∼7% strain and then decreases with further strain. In germanene and stanene, increasing strain hardens and stabilizes long wavelength out-of-plane acoustic phonons, and leads to similar k behaviors to those of silicene. These findings further our understanding of phonon dynamics in group-IV buckled monolayers and may guide transfer and fabrication techniques for these freestanding samples and engineering of k by size and strain for applications of thermal management and thermoelectricity.

Abstract. Aerosol hygroscopicity is crucial for understanding roles of aerosol particles in atmospheric chemistry and aerosol climate effects. Light-scattering enhancement factor f (RH, λ) is one of the parameters describing aerosol hygroscopicity, which is defined as f (RH, λ) = σ sp (RH, λ)/σ sp (dry, λ), where σ sp (RH, λ) or σ sp (dry, λ) represents σ sp at wavelength λ under certain relative humidity (RH) or dry conditions. Traditionally, an overall hygroscopicity parameter κ can be retrieved from measured f (RH, λ), hereinafter referred to as κ f (RH) , by combining concurrently measured particle number size distribution (PNSD) and mass concentration of black carbon. In this paper, a new method is proposed to directly derive κ f (RH) based only on measurements from a three-wavelength humidified nephelometer system. The advantage of this newly proposed approach is that κ f (RH) can be estimated without any additional information about PNSD and black carbon. This method is verified with measurements from two different field campaigns. Values of κ f (RH) estimated from this new method agree very well with those retrieved by using the traditional method: all points lie near the 1 : 1 line and the square of correlation coefficient between them is 0.99. The verification results demonstrate that this newly proposed method of deriving κ f (RH) is applicable at different sites and in seasons of the North China Plain and might also be applicable in other regions around the world.

Using the Boltzmann-Peierls equation for phonon transport approach with the inputs of interatomic force constants from the self-consistent charge density functional tight binding method, we calculate the room-temperature in-plane lattice thermal conductivities k of multilayer graphene (up to four layers) and graphite under different isotropic tensile strains. The calculated in-plane k of graphite, finite monolayer graphene and 3-layer graphene agree well with previous experiments. For unstrained graphene systems, both the intrinsic k and the extent of the diffusive transport regime present a drastic dimensional transition in going from monolayer to 2-layer graphene and thereafter a gradual transition to the graphite limit. We find a peak enhancement of intrinsic k for multilayer graphene and graphite with increasing strain with the largest enhancement amplitude ∼40%. Competition between the decreased mode heat capacities and the increased lifetimes of flexural phonons with increasing strain contribute to this k behavior. Similar k behavior is observed for 2-layer hexagonal boron nitride systems. This study provides insights into engineering k of multilayer graphene and boron nitride by strain and into the nature of thermal transport in quasi-two-dimensional and highly anisotropic systems.