Machine Learning Aided Design and Optimization of Thermal Metamaterials.

Artificial Intelligence (AI) has advanced material research that were previously intractable, for example, the machine learning (ML) has been able to predict some unprecedented thermal properties. In this review, we first elucidate the methodologies underpinning discriminative and generative models, as well as the paradigm of optimization approaches. Then, we present a series of case studies showcasing the application of machine learning in thermal metamaterial design. Finally, we give a brief discussion on the challenges and opportunities in this fast developing field. In particular, this review provides: (1) Optimization of thermal metamaterials using optimization algorithms to achieve specific target properties. (2) Integration of discriminative models with optimization algorithms to enhance computational efficiency. (3) Generative models for the structural design and optimization of thermal metamaterials.

High-Temperature Superlubricity in MoS2/Graphene van der Waals Heterostructures.

Achieving high-temperature superlubricity is essential for modern extreme tribosystems. Solid lubrication is the sole viable alternative due to the degradation of liquid ones but currently suffers from notable wear, instability, and high friction coefficient. Here, we report robust superlubricity in MoS2/graphene van der Waals heterostructures at high temperatures up to ∼850 K, achieved through localized heating to enable reliable friction testing. The ultralow friction of the MoS2/graphene heterostructure is found to be notably further reduced at elevated temperature and dominantly contributed by the MoS2 edge. The observation can be well described by a multi-contact model, wherein the thermally activated rupture of edge-contacts facilitates the sliding. Our results should be applicable to other van der Waals heterostructures and shed light on their applications for superlubricity at elevated temperature.

Fully Printed Multilayer Ceramic Capacitors Based on High-k Perovskite Nanosheets.

Printing technology enables the integration of chemically exfoliated perovskite nanosheets into high-performance microcapacitors. Theoretically, the capacitance value can be further enhanced by designing and constructing multilayer structures without increasing the device size. Yet, issues such as interlayer penetration in multilayer heterojunctions constructed using inkjet printing technology further limit the realization of this potential. Herein, a series of multilayer configurations, including Ag/(Ca2NaNb4O13/Ag)n and graphene/(Ca2NaNb4O13/graphene)n (n = 1-3), are successfully inkjet-printed onto diverse rigid and flexible substrates through optimized ink formulations, inkjet printing parameters, thermal treatment conditions, and rational multilayer structural design using high-k perovskite nanosheets, graphene nanosheets and silver. The dielectric performance is optimized by fine-tuning the number of dielectric layers and modifying the electrode/dielectric interface. As a result, the graphene/(Ca2NaNb4O13/graphene)3 multilayer ceramic capacitors exhibit a remarkable capacitance density of 346 ± 12 nF cm-2 and a high dielectric constant of 193 ± 18. Additionally, these devices demonstrate moderate insulation properties, flexibility, thermal stability, and chemical sensitivity. This work shed light on the potential of multilayer structural design in additive manufacturing of high-performance 2D material-based ceramic capacitors.

Universal Behavior of Highly Confined Heat Flow in Semiconductor Nanosystems: From Nanomeshes to Metalattices.

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible to engineer their thermal properties. However, the influence of boundaries limits the validity of bulk models, while first-principles calculations are too computationally expensive to model real devices. Here we use extreme ultraviolet beams to study phonon transport dynamics in a 3D nanostructured silicon metalattice with deep nanoscale feature size and observe dramatically reduced thermal conductivity relative to bulk. To explain this behavior, we develop a predictive theory wherein thermal conduction separates into a geometric permeability component and an intrinsic viscous contribution, arising from a new and universal effect of nanoscale confinement on phonon flow. Using experiments and atomistic simulations, we show that our theory applies to a general set of highly confined silicon nanosystems, from metalattices, nanomeshes, porous nanowires, to nanowire networks, of great interest for next-generation energy-efficient devices.

High Thermoelectric Performance Related to PVDF Ferroelectric Domains in P-Type Flexible PVDF-Bi0.5 Sb1.5 Te3 Composite Film.

There is increasing demand to power Internet of Things devices using ambient energy sources. Flexible, low-temperature, organic/inorganic thermoelectric devices are a breakthrough next-generation approach to meet this challenge. However, these systems suffer from poor performance and expensive processing preventing wide application of the technology. In this study, by combining a ferroelectric polymer (Polyvinylidene fluoride (PVDF, ß phase)) with p-type Bi0.5 Sb1.5 Te3 (BST) a thermoelectric composite film with maximum is produced power factor. Energy filter from ferroelectric-thermoelectric junction also leads to high Seebeck voltage ≈242 µV K-1 . For the first time, compelling evidence is provided that the dipole of a ferroelectric material is helping decouple electron transport related to carrier mobility and the Seebeck coefficient, to provide 5× or more improvement in thermoelectric power factor. The best composition, PVDF/BST film with BST 95 wt.% has a power factor of 712 µW•m-1  K-2 . A thermoelectric generator fabricated from a PVDF/BST film demonstrated Pmax T 12.02 µW and Pdensity 40.8 W m-2 under 50 K temperature difference. This development also provides a new insight into a physical technique, applicable to both flexible and non-flexible thermoelectrics, to obtain comprehensive thermoelectric performance.

Laser CVD Growth of Uniquely <010>-oriented ß-Ga2 O3 Films on Quartz Substrate with Ultrafast Photoelectric Response.

The oriented growth of ß-Ga2 O3 films has triggered extensive interest due to the remarkable and complex anisotropy found in the ß-Ga2 O3 bulks. Remarkable properties, including stronger solar-blind ultraviolet (SBUV) absorption, better mobility, and higher thermal conductivity, are usually observed along <010> direction as compared to other low-index axes. So far, <010>-oriented ß-Ga2 O3 film growth has been hindered by the lack of suitable substrates and higher surface energy of the (010) crystal plane. Herein, the first growth of uniquely <010>-oriented ß-Ga2 O3 films on quartz substrates by laser chemical vapor deposition (LCVD) are reported. By investigating the effects of deposition temperature (Tdep ) and O2 flow rate (RO2 ) on the growth of ß-Ga2 O3 films, it is found that the formation of <010> orientation is closely related to the higher stability of oxygen close-packed planes under O-rich condition. As a result, a grain size of up to ≈2 µm and a deposition rate of up to ≈ 40 µm h-1 are obtained. Metal-semiconductor-metal (MSM) type detector based on <010>-oriented ß-Ga2 O3 film exhibits ultra-fast response speed, 1-2 orders of magnitude higher than those of most detectors based on ß-Ga2 O3 films with other orientations.

Aqueous MXene/Xanthan Gum Hybrid Inks for Screen-Printing Electromagnetic Shielding, Joule Heater, and Piezoresistive Sensor.

MXenes have exhibited potential for application in flexible devices owing to their remarkable electronic, optical, and mechanical properties. Printing strategies have emerged as a facile route for additive manufacturing of MXene-based devices, which relies on the rational design of functional inks with appropriate rheological properties. Herein, aqueous MXene/xanthan gum hybrid inks with tunable viscosity, excellent printability, and long-term stability are designed. Screen-printed flexible MXene films using such hybrid inks exhibit a high conductivity up to 4.8 × 104  S m-1 , which is suitable to construct multifunctional devices mainly including electromagnetic shielding, Joule heaters, and piezoresistive sensors. The average electromagnetic interference (EMI) shielding value can reach to 40.1 dB. In the Joule heater, the heating rate of printed MXene film can reach 20 °C s-1 under a driving voltage of 4 V, with a highest steady-state temperature of 130.8 °C. An MXene-based piezoresistive sensor prepared by the printing interdigital electrode also presents good sensing performance with a short response time of 130 ms and wide pressure region up to 30 kPa. As a result, screen-printed MXene film exhibits reinforced multifunctional performance, which is promising for application in the next-generation of intelligent and wearable devices.

Thermal conductivity of one-dimensional organic nanowires: effect of mass difference phonon scattering.

We report the thermal conductivity of π-stacked metallophthalocyanine nanowires using the thermal bridge method. In the temperature range of 20-300 K, the thermal conductivity of copper phthalocyanine nanowires (CuPc NWs) and iron phthalocyanine nanowires (FePc NWs) increases with temperature and reaches a peak value at around T = 40 K, then decreases at a higher temperature following T -1 behavior. For three FePc NWs, the peak values are 7.1 ± 1.21, 8.3 ± 1.33, and 7.6 ± 1.42 Wm-1 K-1, respectively. The peak thermal conductivity is 6.6 ± 0.67 and 6.6 ± 0.51 Wm-1 K-1 for the two CuPc nanowires. The thermal conductivity of FePc NWs is slightly larger than that of CuPc NWs, which is believed to result from the different mass of metal atoms in the phthalocyanine centers, indicating a phonon mass-difference scattering effect. Meanwhile, the thermal contact conductance of the FePc-Pt interface is measured, which will benefit from a better understanding of the thermal transport across dissimilar interfaces.

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface-Enhanced Raman Spectroscopy Substrates with High Sensitivity.

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface-enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO2 ) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10-8 m and enhancement factor up to 2.1 × 105 , exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO2 substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle-based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV-vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO2 nanosheets. The overall superior performance makes the MoO2 nanosheets an ideal substrate for practical SERS applications.

Tailoring the Thermal and Mechanical Properties of Graphene Film by Structural Engineering.

Due to substantial phonon scattering induced by various structural defects, the in-plane thermal conductivity (K) of graphene films (GFs) is still inferior to the commercial pyrolytic graphite sheet (PGS). Here, the problem is solved by engineering the structures of GFs in the aspects of grain size, film alignment, and thickness, and interlayer binding energy. The maximum K of GFs reaches to 3200 W m-1 K-1 and outperforms PGS by 60%. The superior K of GFs is strongly related to its large and intact grains, which are over four times larger than the best PGS. The large smooth features about 11 µm and good layer alignment of GFs also benefit on reducing phonon scattering induced by wrinkles/defects. In addition, the presence of substantial turbostratic-stacking graphene is found up to 37% in thin GFs. The lacking of order in turbostratic-stacking graphene leads to very weak interlayer binding energy, which can significantly decrease the phonon interfacial scattering. The GFs also demonstrate excellent flexibility and high tensile strength, which is about three times higher than PGS. Therefore, GFs with optimized structures and properties show great potentials in thermal management of form-factor-driven electronics and other high-power-driven systems.

Raman Spectral Band Oscillations in Large Graphene Bubbles.

Raman spectra of large graphene bubbles showed size-dependent oscillations in spectral intensity and frequency, which originate from optical standing waves formed in the vicinity of the graphene surface. At a high laser power, local heating can lead to oscillations in the Raman frequency and also create a temperature gradient in the bubble. Based on Raman data, the temperature distribution within the graphene bubble was calculated, and it is shown that the heating effect of the laser is reduced when moving from the center of a bubble to its edge. By studying graphene bubbles, both the thermal conductivity and chemical reactivity of graphene were assessed. When exposed to hydrogen plasma, areas with bubbles are found to be more reactive than flat graphene.

Atomic Layer Engineering of High-κ Ferroelectricity in 2D Perovskites.

Complex perovskite oxides offer tremendous potential for controlling their rich variety of electronic properties, including high-TC superconductivity, high-κ ferroelectricity, and quantum magnetism. Atomic-scale control of these intriguing properties in ultrathin perovskites is an important challenge for exploring new physics and device functionality at atomic dimensions. Here, we demonstrate atomic-scale engineering of dielectric responses using two-dimensional (2D) homologous perovskite nanosheets (Ca2Nam-3NbmO3m+1; m = 3-6). In this homologous 2D material, the thickness of the perovskite layers can be incrementally controlled by changing m, and such atomic layer engineering enhances the high-κ dielectric response and local ferroelectric instability. The end member (m = 6) attains a high dielectric constant of ∼470, which is the highest among all known dielectrics in the ultrathin region (<10 nm). These results provide a new strategy for achieving high-κ ferroelectrics for use in ultrascaled high-density capacitors and post-graphene technology.

A Series Circuit of Thermal Rectifiers: An Effective Way to Enhance Rectification Ratio.

A novel approachis proposed to enhance the thermal rectification ratio, namely, arranging two thermal rectifiers in series. Through theoretical analysis and molecular dynamics simulations on graphene/phononic crystal structures, the results show that the series thermal rectifiers enhance thermal rectification ratio significantly, compared to a single rectifier. Meanwhile, the results of theoretical prediction match well with simulation results.

Endogenous controls of gene expression in N-methyl-N-nitrosourea-induced T-cell lymphoma in p53-deficient mice.

BACKGROUND: Real-time polymerase chain reaction (PCR) has become an increasingly important technique for gene expression profiling because it can provide insights into complex biological and pathological processes and be used to predict disease or treatment outcomes. Although normalized data are necessary for an accurate estimation of mRNA expression levels, several pieces of evidence suggest that the expression of so-called housekeeping genes is not stable. This study aimed to validate reference genes for the normalization of real-time PCR in an N-methyl-N-nitrosourea (MNU)-induced T-cell lymphoma mouse model. METHODS: T-cell lymphomas were generated in p53-deficient mice by treatment with 37.5 mg/kg MNU. Thymus and spleen were identified as the primary target organs with the highest incidences of lymphomas. We analyzed the RNA expression levels of eight potential endogenous reference genes (Gapdh, Rn18s, Actb, Hprt, B2M, Rplp0, Gusb, Ctbp1). The expression stabilities of these reference genes were tested at different time points after MNU treatment using geNorm and NormFinder algorithms. RESULTS: A total of 65% of MNU-treated mice developed T-cell lymphomas, with the spleen and thymus as the major target organs. All candidate reference genes were amplified efficiently by quantitative reverse-transcription polymerase chain reaction (RT-qPCR). Gene stability evaluation after MNU treatment and during lymphomagenesis revealed that Ctbp1 and Rplp0 were the most stably expressed genes in the thymus and spleen, respectively. RT-PCR of thymus RNA using two additional sets of primer confirmed that Ctbp1 was the most stable of all the candidate reference genes. CONCLUSIONS: We provided suitable endogenous controls for gene expression studies in the T-cell lymphoma model.

Diffusion of active dimers in a Couette flow.

We study the 3D dynamics of an elastic dimer consisting of an active swimmer bound to a passive cargo, both suspended in a Couette flow. Using numerical simulations, we determine the diffusivity of such an active dimer in the presence of long-range hydrodynamic interactions for different values of its self-propulsion speed and the Couette flow. We observe that the effect of hydrodynamic interactions is greatly enhanced under the condition that self-propulsion is strong enough to contrast the shear flow. The magnitude of the effect grows with the size of the dimer's constituents relative to their distance, which makes it appreciable under experimental conditions.

Hexagonal boron nitride: a promising substrate for graphene with high heat dissipation.

Supported graphene on a standard SiO2 substrate exhibits unsatisfactory heat dissipation performance that is far inferior to the intrinsic ultrahigh thermal conductivity of a suspended sample. A suitable substrate for enhancing thermal transport in supported graphene is highly desirable for the development of graphene devices for thermal management. By using molecular dynamics simulations, here we demonstrate that bulk hexagonal boron nitride (h-BN) is a more appealing substrate to achieve high performance heat dissipation in supported graphene. Notable length dependence and high thermal conductivity are observed in h-BN-supported single-layer graphene (SLG), suggesting that the thermal transport characteristics are close to that of suspended SLG. At room temperature, the thermal conductivity of h-BN-supported SLG is as high as 1347.3 ± 20.5 Wm-1 K-1, which is about 77% of that for the suspended case, and is more than twice that of the SiO2-supported SLG. Furthermore, we find that the smooth and atomically flat h-BN substrate gives rise to a regular and weak stress distribution in graphene, resulting in a less affected phonon relaxation time and dominant phonon mean free path. We also find that stacking and rotation significantly impacts the thermal transport in h-BN-supported graphene. Our study provides valuable insights towards the design of graphene devices on realistic substrate for high performance heat dissipation applications.

Coexistence of Magnetic Order and Ferroelectricity at 2D Nanosheet Interfaces.

Multiferroic materials, in which the electronic polarization can be switched by a magnetic field and vice versa, are of fundamental importance for new electronic technologies. However, there exist very few single-phase materials that exhibit such cross-coupling properties at room temperature, and heterostructures with a strong magnetoelectric coupling have only been made with complex techniques. Here, we present a rational design for multiferroic materials by use of a layer-by-layer engineering of 2D nanosheets. Our approach to new multiferroic materials is the artificial construction of high-quality superlattices by interleaving ferromagnetic Ti0.8Co0.2O2 nanosheets with dielectric perovskite-structured Ca2Nb3O10 nanosheets. Such an artificial structuring allows us to engineer the interlayer coupling, and the (Ti0.8Co0.2O2/Ca2Nb3O10/Ti0.8Co0.2O2) superlattices induce room-temperature ferroelectricity in the presence of the ferromagnetic order. Our technique provides a new route for tailoring artificial multiferroic materials in a highly controllable manner.

Birch-Type Hydrogenation of Few-Layer Graphenes: Products and Mechanistic Implications.

Few-layer graphenes, supported on Si with a superficial oxide layer, were subjected to a Birch-type reduction using Li and H2O as the electron and proton donors, respectively. The extent of hydrogenation for bilayer graphene was estimated at 1.6-24.1% according to Raman and X-ray photoelectron spectroscopic data. While single-layer graphene reacts uniformly, few-layer graphenes were hydrogenated inward from the edges and/or defects. The role of these reactive sites was reflected in the inertness of pristine few-layer graphenes whose edges were sealed. Hydrogenation of labeled bilayer (12C/13C) and trilayer (12C/13C/12C) graphenes afforded products whose sheets were hydrogenated to the same extent, implicating passage of reagents between the graphene layers and equal decoration of each graphene face. The reduction of few-layer graphenes introduces strain, allows tuning of optical transmission and fluorescence, and opens synthetic routes to long sought-after films containing sp3-hybridized carbon.

Diffusion of eccentric microswimmers.

We model the two-dimensional diffusive dynamics of an eccentric artificial microswimmer in a highly viscous medium. We assume that the swimmer's propulsion results from an effective force applied to a center distinct from its center of mass, both centers resting on a body's axis parallel to its average self-propulsion velocity. Moreover, we allow for angular fluctuations of the velocity about the body's axis. We prove, both analytically and numerically, that the ensuing active diffusion of the swimmer is suppressed to an extent that strongly depends on the model parameters. In particular, the active diffusion constant undergoes a transition from a quadratic to a linear dependence on the self-propulsion speed, with practical consequences on the interpretation of the experimental data. Finally, we extend our model to describe the diffusion of chiral eccentric swimmers.