Atomistic Insights into the Origin of High-Performance Thermoelectric Response in Hybrid Perovskites.

Due to their tantalizing prospect of heat-electricity interconversion, hybrid organic-inorganic perovskites have sparked considerable research interests recently. Nevertheless, understanding their complex interplay between the macroscopic properties, nonintuitive transport processes, and basic chemical structures still remains far from completion, although it plays a fundamental role in systematic materials development. On the basis of multiscale first-principles calculations, this understanding is herein advanced by establishing a comprehensive picture consisting of atomic and charge dynamics. It is unveiled that the ultralow room-temperature lattice thermal conductivity (≈0.20 W m-1 K-1 ) of hybrid perovskites is critical to their decent thermoelectric figure of merit (≈0.34), and such phonon-glass behavior stems from not only the inherent softness but also the strong anharmonicity. It is identified that the 3D electrostatic interaction and hydrogen-bonded networks between the PbI3- cage and embedded cations result in the strongly coupled motions of inorganic framework and cation, giving rise to their high degree of anharmonicity. Furthermore, such coupled motions bring about low-frequency optical vibrational modes, which leads to the dominant role of electron scattering with optical phonons in charge transport. It is expected that these new atomistic-level insights offer a standing point where the performance of thermoelectric perovskites can be further enhanced.

Functional-Unit-Based Material Design: Ultralow Thermal Conductivity in Thermoelectrics with Linear Triatomic Resonant Bonds.

We demonstrate the use of functional-unit-based material design for thermoelectrics. This is an efficient approach for identifying high-performance thermoelectric materials, based on the use of combinations of functional fragments relevant to desired properties. Here, we reveal that linear triatomic resonant bonds (LTRBs) found in some Zintl compounds provide strong anisotropy both structurally and electronically, along with strong anharmonic phonon scattering. An LTRB is thus introduced as a functional unit, and compounds are then screened as potential thermoelectric materials. We identify 17 semiconducting candidates from the MatHub-3d database that contain LTRBs. Detailed transport calculations demonstrate that the LTRB-containing compounds not only have considerably lower lattice thermal conductivities than other compounds with similar average atomic masses, but also exhibit remarkable band anisotropy near the valence band maximums due to the LTRB. K5CuSb2 is adopted as an example to elucidate the fundamental correlation between the LTRB and thermoelectric properties. The [Sb-Cu-Sb]5- resonant structures demonstrate the delocalized Sb-Sb interaction within each LTRB, resulting in the softening of TA phonons and leading to large anharmonicity. The low lattice thermal conductivity (0.39 W/m·K at 300 K) combined with the band anisotropy results in a high thermoelectric figure of merit (ZT) for K5CuSb2 of 1.3 at 800 K. This work is a case study of the functional-unit-based material design for the development of novel thermoelectric materials.

A mammalian methylation array for profiling methylation levels at conserved sequences.

Infinium methylation arrays are not available for the vast majority of non-human mammals. Moreover, even if species-specific arrays were available, probe differences between them would confound cross-species comparisons. To address these challenges, we developed the mammalian methylation array, a single custom array that measures up to 36k CpGs per species that are well conserved across many mammalian species. We designed a set of probes that can tolerate specific cross-species mutations. We annotate the array in over 200 species and report CpG island status and chromatin states in select species. Calibration experiments demonstrate the high fidelity in humans, rats, and mice. The mammalian methylation array has several strengths: it applies to all mammalian species even those that have not yet been sequenced, it provides deep coverage of conserved cytosines facilitating the development of epigenetic biomarkers, and it increases the probability that biological insights gained in one species will translate to others.

Metallic VS2/graphene heterostructure as an ultra-high rate and high-specific capacity anode material for Li/Na-ion batteries.

Graphene has been used as a conductive substrate to improve the electrochemical performance of layered VS2 as an anode material for lithium-ion batteries. However, there is still a lack of in-depth understanding of the synergistic effect between the layered VS2 and graphene, which contributes to the enhanced performance of Li/Na-ion batteries. In this work, using first-principles calculations, we have systematically studied the VS2/graphene heterostructure as an anode material for Li/Na-ion batteries. Our results show that the VS2/graphene heterostructure is a promising anode material with good structural stability, high adsorption strength, high stiffness, intrinsic metallic characteristic after Li/Na adsorption, high theoretical specific capacity, shallow averaged open-circuit voltage and ultra-low ion diffusion barriers. The diffusion barriers are found to be 0.03 eV (Li) and 0.08 eV (Na), superior to that of the widely studied heterostructure materials, which guarantees an extremely fast Li/Na diffusion rate during charge/discharge cycling. The anode overall open-circuit voltages for the Li/Na-ion batteries are calculated to be as low as 0.65 and 0.46 V, and the maximum theoretical storage capacity is 771 and 578 mA h g-1, respectively. The above results provide valuable insights into the experimental set-up of the VS2/graphene nanocomposite anode material for ultra-high rate and high-specific capacity Li/Na-ion batteries.

Materials informatics platform with three dimensional structures, workflow and thermoelectric applications.

Since the proposal of the "Materials Genome Initiative", several material databases have emerged and advanced many materials fields. In this work, we present the Materials Informatics Platform with Three-Dimensional Structures (MIP-3d). More than 80,000 structural entries, mainly from the inorganic crystal structural database, are included in MIP-3d. Density functional theory calculations are carried out for over 30,000 entries in the database, which contain the relaxed crystal structures, density of states, and band structures. The calculation of the equations of state and sound velocities is performed for over 12,000 entries. Notably, for entries with band gap values larger than 0.3 eV, the band degeneracies for the valence band maxima and the conduction band minima are analysed. The electrical transport properties for approximately 4,400 entries are also calculated and presented in MIP-3d under the constant electron-phonon coupling approximation. The calculations of the band degeneracies and electrical transport properties make MIP-3d a database specifically designed for thermoelectric applications.

Epigenetic clock and methylation studies in elephants.

Age-associated DNA-methylation profiles have been used successfully to develop highly accurate biomarkers of age ("epigenetic clocks") in humans, mice, dogs, and other species. Here we present epigenetic clocks for African and Asian elephants. These clocks were developed using novel DNA methylation profiles of 140 elephant blood samples of known age, at loci that are highly conserved between mammalian species, using a custom Infinium array (HorvathMammalMethylChip40). We present epigenetic clocks for Asian elephants (Elephas maximus), African elephants (Loxodonta africana), and both elephant species combined. Two additional human-elephant clocks were constructed by combining human and elephant samples. Epigenome-wide association studies identified elephant age-related CpGs and their proximal genes. The products of these genes play important roles in cellular differentiation, organismal development, metabolism, and circadian rhythms. Intracellular events observed to change with age included the methylation of bivalent chromatin domains, and targets of polycomb repressive complexes. These readily available epigenetic clocks can be used for elephant conservation efforts where accurate estimates of age are needed to predict demographic trends.