Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe2.

Research on 2D materials originally focused on the highly symmetrical materials like graphene, h-BN. Recently, 2D materials with low-symmetry lattice such as PdSe2 have drawn extensive attention, due to the interesting layer-dependent bandgap, promising mechanical properties and excellent thermoelectric performance, etc. In this work, the phonon thermal transport is studied in PdSe2 with a pentagonal fold structure. The thermal conductivity of PdSe2 flakes with different thicknesses ranging from few nanometers to several tens of nanometers is measured through the thermal bridge method, where the thermal conductivity increases from 5.04 W mk-1 for 60 nm PdSe2 to 34.51 W mk-1 for the few-layer one. The atomistic modelings uncover that with the thickness thinning down, the lattice of PdSe2 becomes contracted and the phonon group velocity is enhanced, leading to the abnormal increase in the thermal conductivity. And the upshift of the optical phonon modes contributes to the increase of the thermal conductivity as well by creating less acoustic phonon scattering as the thickness reduces. This study probes the interesting abnormal thickness-dependent thermal transport in 2D materials, which promotes the potential thermal management at nanoscale.

Low voltage-driven high-performance thermal switching in antiferroelectric PbZrO3 thin films.

Effective control of heat transfer is vital for energy saving and carbon emission reduction. In contrast to achievements in electrical conduction, active control of heat transfer is much more challenging. Ferroelectrics are promising candidates for thermal switching as a result of their tunable domain structures. However, switching ratios in ferroelectrics are low (<1.2). We report that high-quality antiferroelectric PbZrO3 epitaxial thin films exhibit high-contrast (>2.2), fast-speed (<150 nanoseconds), and long-lifetime (>107) thermal switching under a small voltage (<10 V). In situ reciprocal space mapping and atomistic modelings reveal that the field-driven antiferroelectric-ferroelectric phase transition induces a substantial change of primitive cell size, which modulates phonon-phonon scattering phase space drastically and results in high switching ratio. These results advance the concept of thermal transport control in ferroic materials.

Phonon thermal transport in ferroelectricα-In2Se3 via first-principles calculations.

Two-dimensional (2D) ferroelectrics are promising candidates in the field of microelectronics due to their unique properties such as excellent photoelectric responsiveness. However, the thermal properties of 2D ferroelectrics are less investigated. Here, the thickness dependent thermal conductivity in ferroelectricα-In2Se3is systematically investigated by the first-principles method combined with the phonon Boltzmann transport equation. On this basis, the strain and oxidation effects on the thermal conductivity of monolayerα-In2Se3is further studied. The calculation results show that the thermal conductivity has a significant reduction with decreasing film thickness or increasing tensile strain, and the anharmonic phonon-phonon scattering rate is the intrinsic mechanism for the reduction in thermal conductivity. On the other hand, the replacement of Se atoms by O atoms can achieve a bidirectional and wide-range (12×) tuning of thermal conductivity. The increase in specific heat and phonon group velocity is responsible for the thermal conductivity enhancement at high doping levels while that in phonon-phonon scattering rate is responsible for the thermal conductivity reduction at low doping levels. In all cases, acoustic phonons dominate the in-plane thermal transport behavior. These findings broaden our understanding of phonon transport and its control in ferroelectric semiconductorα-In2Se3.

Unexpected doping effects on phonon transport in quasi-one-dimensional van der Waals crystal TiS3 nanoribbons.

Doping usually reduces lattice thermal conductivity because of enhanced phonon-impurity scattering. Here, we report unexpected doping effects on the lattice thermal conductivity of quasi-one-dimensional (quasi-1D) van der Waals (vdW) TiS3 nanoribbons. As the nanoribbon thickness reduces from ~80 to ~19 nm, the concentration of oxygen atoms has a monotonic increase along with a 7.4-fold enhancement in the thermal conductivity at room temperature. Through material characterizations and atomistic modellings, we find oxygen atoms diffuse more readily into thinner nanoribbons and more sulfur atoms are substituted. The doped oxygen atoms induce significant lattice contraction and coupling strength enhancement along the molecular chain direction while have little effect on vdW interactions, different from that doping atoms induce potential and structural distortions along all three-dimensional directions in 3D materials. With the enhancement of coupling strength, Young's modulus is enhanced while phonon-impurity scattering strength is suppressed, significantly improving the phonon thermal transport.

Hexagonal Close-Packed Polar-Skyrmion Lattice in Ultrathin Ferroelectric PbTiO_{3} Films.

Polar skyrmions are topologically stable, swirling polarization textures with particlelike characteristics, which hold promise for next-generation, nanoscale logic and memory. However, the understanding of how to create ordered polar skyrmion lattice structures and how such structures respond to applied electric fields, temperature, and film thickness remains elusive. Here, using phase-field simulations, the evolution of polar topology and the emergence of a phase transition to a hexagonal close-packed skyrmion lattice is explored through the construction of a temperature-electric field phase diagram for ultrathin ferroelectric PbTiO_{3} films. The hexagonal-lattice skyrmion crystal can be stabilized under application of an external, out-of-plane electric field which carefully adjusts the delicate interplay of elastic, electrostatic, and gradient energies. In addition, the lattice constants of the polar skyrmion crystals are found to increase with film thickness, consistent with expectation from Kittel's law. Our studies pave the way for the development of novel ordered condensed matter phases assembled from topological polar textures and related emergent properties in nanoscale ferroelectrics.

Food volume estimation by multi-layer superpixel.

Estimating the volume of food plays an important role in diet monitoring. However, it is difficult to perform this estimation automatically and accurately. A new method based on the multi-layer superpixel technique is proposed in this paper to avoid tedious human-computer interaction and improve estimation accuracy. Our method includes the following steps: 1) obtain a pair of food images along with the depth information using a stereo camera; 2) reconstruct the plate plane from the disparity map; 3) warp the input image and the disparity map to form a new direction of view parallel to the plate plane; 4) cut the warped image into a series of slices according to the depth information and estimate the occluded part of the food; and 5) rescale superpixels for each slice and estimate the food volume by accumulating all available slices in the segmented food region. Through a combination of image data and disparity map, the influences of noise and visual error in existing interactive food volume estimation methods are reduced, and the estimation accuracy is improved. Our experiments show that our method is effective, accurate and convenient, providing a new tool for promoting a balanced diet and maintaining health.

Giant thermal switching in ferromagnetic VSe2 with programmable switching temperature.

Active and reversible modulation of thermal conductivity can realize efficient heat energy management in many applications such as thermoelectrics. Using first-principles calculations, this study reports a giant thermal switching ratio of 12, much higher than previously reported values, in monolayer 2H-VSe2 above room temperature. Detailed analysis indicates that the high thermal switching ratio is dominated by the ferromagnetic ordering induced phonon bandgap, which significantly suppresses the phonon-phonon scattering phase space across the entire vibration spectrum. The thermal switching in bulk 2H-VSe2 is also investigated and the thermal switching ratio reaches 9.2 at the magnetic transition temperature. Both the phonon-phonon scattering space phase and phonon anharmonicity are responsible for the 9.2-fold thermal switching. This study advances the understanding of heat energy transport in two-dimensional ferromagnets, and also provides new insight into heat energy control and conversion.

Phase Competition in High-Quality Epitaxial Antiferroelectric PbZrO3 Thin Films.

Antiferroelectric PbZrO3 has attracted renewed interest in recent years because of its unique properties and wide range of potential applications. However, the nature of antiferroelectricity and its evolution with the electric field and temperature remain controversial, mostly due to the difficulty of obtaining high-quality single-crystal samples. The lack of consensus regarding the phase transition in PbZrO3 is not only important on a fundamental side but also greatly hinders further applications. Herein, high-quality PbZrO3 epitaxial thin films are successfully fabricated by pulsed laser deposition. The structural and physical properties of the films are systematically studied via a combination of electric property measurements, X-ray diffraction, scanning transmission electron microscopy imaging, and second-harmonic generation studies. Our studies unveil the noncentrosymmetric nature of PbZrO3 films at room temperature. Moreover, the Curie temperature increased to 270°, ∼40° higher than that in the bulk, and no intermediate ferroelectric phase was observed. Besides, an incipient ferroelectric with relaxor-like behavior above the Curie temperature due to the existence of a local polar cluster in the high-temperature paraelectric phase is experimentally observed for the first time. Our studies provide a better understanding of PbZrO3 thin films and pave the way for practical applications of antiferroelectric material in modern electronic devices.

Large Thermal Conductivity Switching in Ferroelectrics by Electric Field-Triggered Crystal Symmetry Engineering.

A convenient, reversible, fast, and wide-range switching of thermal conductivity is desired for efficient heat energy management. However, traditional methods, such as temperature-induced phase transition and chemical doping, have many limitations, e.g., the lack of continuous tunability over a wide temperature range and low switching speed. In this work, a strategy of electric field-driven crystal symmetry engineering to efficiently modulate thermal conductivity is reported with first-principles calculations. By simply changing the direction of an external electric field loaded in ferroelectric PbZr0.5Ti0.5O3, near the morphotropic phase boundary composition, we obtain the largest switching of thermal conductivity for ferroelectric materials at room temperature based on the dual-phonon theory, i.e., normal and diffuson-like phonons, with three different criteria. The calculation results indicate that with decreasing crystal symmetry, the degeneracy of phonon modes reduces and the avoid-crossing behavior of phonon branches enhances, leading to the increase of diffuson-like phonons and weighted phonon-phonon scattering phase space. A thermal switch prototype based on PbZr0.5Ti0.5O3 is further shown that can protect the Li-ion battery by modulating its temperature up to 17.5 °C. Our studies would pave the way for designing next-generation thermal switch with high speed, a wide temperature range, and a large switching ratio.

Phonon transport in graphene based materials.

Graphene, due to its atomic layer structure, has the highest room temperature thermal conductivity k for all known materials. Thus, it is expected that graphene based materials are the best candidates for thermal management in next generation electronic devices. In this perspective, we first review the in-plane k of monolayer graphene and multilayer graphene obtained using experimental measurements, theoretical calculations and molecular dynamics (MD) simulations. Considering the importance of four-phonon scattering in graphene, we also compare the effects of three-phonon and four-phonon scattering on phonon transport in graphene. Then, we review phonon transport along the cross-plane direction of multilayer graphene and highlight that the cross-plane phonon mean free path is several hundreds of nanometers instead of a few nanometers as predicted using classical kinetic theory. Recently, hydrodynamic phonon transport has been observed experimentally in graphitic materials. The criteria for distinguishing the hydrodynamic from ballistic and diffusive regimes are discussed, from which we conclude that graphene based materials with a high Debye temperature and high anharmonicity (due to ZA modes) are excellent candidates to observe the hydrodynamic phonon transport. In the fourth part, we review how to actively control phonon transport in graphene. Graphene and graphite are often adopted as additives in thermal management materials such as polymer nanocomposites and thermal interface materials due to their high k. However, the enhancement of the composite's k is not so high as expected because of the large thermal resistance between graphene sheets as well as between the graphene sheet and matrix. In the fifth part, we discuss the interfacial thermal resistance and analyze its effect on the thermal conductivity of graphene based materials. In the sixth part, we give a brief introduction to the applications of graphene based materials in thermal management. Finally, we conclude our review with some perspectives for future research.

The effects of contact atom distribution at the interface on the phonon transport.

At the nanometer scale, heat (phonon) transport is sensitive to the contact details at the interface due to the phonon wave property. However, the effects of contact atom distribution are ignored. In this work, the atomic Green's function (AGF) method and molecular dynamics (MD) simulation are applied to explore those effects. A parameter named as the average distance d[combining macron] is raised here to measure the distribution of contacted atoms at the interface. Based on the AGF method, phonon transmission profiles at different d[combining macron] (distribution) with the same number of contacted atoms have a coincident point, the reverse frequency fr. If the phonon frequency f is smaller (larger) than fr, smaller d[combining macron] has smaller (larger) phonon transmission. The overlap of the vibrational density of states from the MD simulation and the local density of states from the AGF method indicate that the reverse frequency is caused by the match degree of vibration modes across the interface. The existence of reverse frequency leads to the reverse temperature Tr. Increasing the contact area or the interfacial coupling strength can cause the blue shift of fr and the increase of Tr. The MD simulations observe a larger temperature jump at the interface for larger d[combining macron], similar to that from the AGF method at temperatures higher than Tr due to the high-temperature limit property in MD. The results are independent of the choice of cutoff distance in potential and interfacial coupling strength, indicating that the conclusion here is applicable for the general interface.

The reservoir area dependent thermal transport at the nanoscale interface.

The reservoir area dependent thermal transport at nanoscale two-dimensional and one-dimensional interfaces is investigated by the non-equilibrium Green's function method. For the two-dimensional nanoscale interface composed of graphene sheets, the reservoir area is identical to the contact area S at the interface. As S increases from few atoms, the interfacial thermal conductance σ per S (Λ = σ/S) is negatively dependent on S due to the decrease of phonon transmission per S. With S increasing to several square nanometers, Λ converges to a constant value. However, for the one-dimensional nanoscale interface composed of nested carbon nanotubes (NCNTs), it is σ instead of Λ that converges to a constant value because the reservoir in one-dimensional nanoscale NCNTs has a fixed area, which can only provide finite transport channels. There are two competitive factors influencing the thermal transport at the interface in the NCNT model. One is phonon mode coupling and the other is phonon scattering. These two factors lead to an interesting trend of σ that as the overlap between NCNTs increases, σ increases at first and then decreases and converges to a constant value. These findings indicate that the thermal transport behavior has a strong dependence on the contact details and reservoir area at the nanoscale interface.

The enhancement of heat conduction across the metal/graphite interface treated with a focused ion beam.

In this work, we report a convenient and efficient approach to improve heat conduction across the metal/graphite interface. It is demonstrated that the interfacial thermal conductance between Al and graphite can be enhanced by a factor of ∼5 after milling the graphite with a focused ion beam. Such enhancement is attributed to the decreased Fermi level of the milled graphite compared with the pristine counterpart. Once graphite is milled with the focused ion beam, surface defects are formed that induce the redistribution of electrons at the interface between Al and graphite. The formation of enormous dipoles on the milled graphite/Al interface leads to the conversion of the interfacial interaction from physisorption to chemisorption, which is beneficial for phonon transmission across the interface. Based on the measured Fermi level difference, the non-equilibrium Green's function method predicts that the interfacial interaction strength in the Al/milled graphite is increased 4-fold compared with Al/pristine graphite, which causes the increase of the interfacial thermal conductance. Our theoretical model also predicts that the interfacial thermal conductance does not increase monotonically with the interaction strength. Once the interaction strength exceeds a critical value, the interface thermal conductance will decrease.

Computational modeling of ionic currents through difform graphene nanopores with consistent cross-sectional areas.

Understanding the mechanism of ion transport and the related ionic current through a nanopore is significant for improving the sensing accuracy of biophysical and diagnostic applications using the nanopore technology. Here, systematic theoretical studies of ionic current dependence on the geometry of a nanopore were performed. Surprisingly, it was found that the ionic current through a nanopore with a smaller perimeter was obviously larger than that through a nanopore with a larger perimeter although all the nanopores had consistent cross-sectional areas; this was also found for nanopores with different hydrophobicities. This interesting result originates from the decrease in ion concentration, mobility and conductivity in proximity to the nanopore surface. Besides, an obvious ionic current enhancement was observed for hydrophobic nanopores compared to that for the hydrophilic nanopores, which was caused by the increased ion mobility through the hydrophobic nanopores. A simple model that combined the distribution of ion conductivity as well as the traditional Ohm's law was successfully applied to predict the ionic current through difform nanopores with different hydrophobicities. This work will aid the development of high-resolution nanopore sensors in the near future.

Distinct Signatures of Electron-Phonon Coupling Observed in the Lattice Thermal Conductivity of NbSe3 Nanowires.

The last two decades have seen tremendous progress in quantitative understanding of several major phonon scattering mechanisms (phonon-phonon, phonon-boundary, phonon-defects), as they are the determinant factors in lattice thermal transport, which is critical for the proper functioning of various electronic and energy conversion devices. However, the roles of another major scattering mechanism, electron-phonon (e-ph) interactions, remain elusive. This is largely due to the lack of solid experimental evidence for the effects of e-ph scattering in the lattice thermal conductivity for the material systems studied thus far. Here we show distinct signatures in the lattice thermal conductivity observed below the charge density wave transition temperatures in NbSe3 nanowires, which cannot be recaptured without considering e-ph scattering. Our findings can serve as the cornerstone for quantitative understanding of the e-ph scattering effects on lattice thermal transport in many technologically important materials.

Thermal Transport in Quasi-1D van der Waals Crystal Ta2Pd3Se8 Nanowires: Size and Length Dependence.

Van der Waals (vdW) crystals with covalently bonded building blocks assembled together through vdW interactions have attracted tremendous attention recently because of their interesting properties and promising applications. Compared to the explosive research on two-dimensional (2D) vdW materials, quasi-one-dimensional (quasi-1D) vdW crystals have received considerably less attention, while they also present rich physics and engineering implications. Here we report on the thermal conductivity of exfoliated quasi-1D Ta2Pd3Se8 vdW nanowires. Interestingly, even though the interatomic interactions along each molecular chain are much stronger than the interchain vdW interactions, the measured thermal conductivity still demonstrates a clear dependence on the cross-sectional size up to >110 nm. The results also reveal that partial ballistic phonon transport can persist over 13 µm at room temperature along the molecular chain direction, the longest experimentally observed ballistic transport distance with observable effects on thermal conductivity so far. First-principles calculations suggest that the ultralong ballistic phonon transport arises from the highly focused longitudinal phonons propagating along the molecular chains. These data help to understand phonon transport through quasi-1D vdW crystals, facilitating various applications of this class of materials.

Stimulation of intramembranous bone repair in rats by ghrelin.

Researchers in our laboratory have previously shown that ghrelin, a gastric peptide hormone, may regulate mesenchymal cell differentiation into adipocytes and myocytes. Here we show that ghrelin promotes osteogenesis of intramembranous bone and improves the repair of calvarial bone defects in rats. Rats with a 9 mm full-thickness calvarial bone defect received either Bio-Oss (control group) or Bio-Oss mixed with 20 mug ghrelin (treatment group), followed by local administration of saline or ghrelin (10 microg), respectively, on days 5, 10 and 15. After 6 and 12 weeks, new bone formation was assessed. Animals treated with ghrelin showed a significant increase in new bone formation as demonstrated by an increment in bone mineral density and fluorescence labelling of tetracycline relative to the control group. At 6 weeks, bone mineral density increased from 54 +/- 7 (control group) to 78 +/- 9 mg cm(-2) in the treatment group, while the tetracycline fluorescence labelling increased by 61 +/- 15%. A similar increment was observed at 12 weeks. Quantitative reverse transcriptase-polymerase chain reaction showed that expression of alkaline phosphatase (ALP), osteocalcin and collagen type I was elevated. Relative to the control animals, mRNAs for ALP, osteocalcin and collagen type I increased 2.4 +/- 0.4-, 4.7 +/- 1.9- and 4.0 +/- 1.7-fold, respectively, in animals treated with ghrelin for 6 weeks (P < 0.05). At 12 weeks, mRNA levels of ALP, osteocalcin and collagen type I showed a decline relative to levels at 6 weeks but still remained significantly higher than in the control group, with fold changes of 2.4 +/- 0.8, 2.4 +/- 1.2 and 2.1 +/- 0.7, respectively (P < 0.05). This study demonstrated that ghrelin stimulates intramembranous osteogenesis.