Polar Nanostructures Research Articles

Phonon polariton-mediated heat conduction: Perspectives from recent progress

AbstractIt has been well-accepted that heat conduction in solids is mainly mediated by electrons and phonons. Recently, there has been a strong emerging interest in the contribution of various polaritons, quasi-particles resulting from the coupling between electromagnetic waves and different excitations in solids, to heat conduction. Traditionally, the polaritonic effect on conduction has been largely neglected because of the low number density of polaritons. However, it has been recently predicted and experimentally confirmed that polaritons could play significant roles in heat conduction in polar nanostructures. Since the transport characteristics of polaritons are very different from those of electrons and phonons, polariton-mediated heat conduction provides new opportunities for manipulating heat flow in solid-state devices for more efficient heat dissipation or energy conversion. In view of the rapid growth of polariton-mediated heat conduction, especially by phonon polaritons, here we review the recent progress in this field and provide perspectives for challenges and opportunities. Graphical abstract

Long Propagating Polaritonic Heat Transfer: Shaping Radiation to Conduction.

Surface phonon polaritons (SPhPs) originate from the coupling of mid-IR photons and optical phonons, generating evanescent waves along the polar dielectric surface. The emergence of SPhPs gives rise to a phase of quantum matter that facilitates long-range energy transfer (100s μm-scale). Albeit of the recent experimental progress to observe the enhanced thermal conductivity of polar dielectric nanostructures mediated by SPhPs, the potential mechanism to present the high thermal conductivity beyond the Landauer limit has not been addressed. Here, we revisit the comprehensive theoretical framework to unify the distinct pictures of two heat transfer mechanisms by conduction and radiation. We first designed our experimental platform to distinguish contributions of two distinct fundamental modes of SPhPs, resulting in far-field radiation and long propagating conduction, respectively, by tuning the configuration of a nanostructured heat channel integrated into the thermometer. We could effectively control the transmission of long-propagating SPhPs to influence the apparent thermal conductivity of the nanostructure. This study reveals the high thermal conductance of 1.63 nW/K by a fast SPhP mode comparable to that by classical phonons, with measurements showing apparent conductivity values of up to 2 W/m·K at 515 K. The origin of the enhanced thermal conductivity was exploited by observing the interference of dispersive evanescent waves by double heat channels. Furthermore, our experimental observations of length-dependent thermal conductance by SPhPs are in good agreement with the revisited Landauer formula to illustrate a polaritonic mode of heat conduction, considering the dispersive nature of radiation not limited to the physical boundaries of a solid object yet directionally guided along the surface.

Phase transition and polar cluster behavior above Curie temperature in ferroelectric BaTi0.8Zr0.2O3

We study the phase transition behavior of the ferroelectric BaTi0.8Zr0.2O3 in the paraelectric region. The temperature dependencies of thermal, polar, elastic, and dielectric properties indicate the presence of local structures above the paraelectric–ferroelectric transition temperature Tc = 292 K. The non-zero remnant polarization is measured up to a characteristic temperature T* ∼ 350 K, which coincides with the temperature where the dielectric constant deviates from the Curie–Weiss law. Resonant piezoelectric spectroscopy shows that DC field cooling above Tc using fields smaller than the coercive field leads to an elastic response and remnant piezoelectricity below T*, which likely corresponds to the coherence temperature associated with polar nanostructures in ferroelectrics. The observed remnant effect is attributed to the reorientation of polar nanostructures above Tc.

Enhanced bending electromechanical properties of maleic anhydride-grafted SEBS gels by blending with acrylonitrile-styrene copolymer

The poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) gel which a type of electroactive thermoplastic elastomer has significant intrinsic electromechanical coupling effect owing to its unique microphase separated structure. However, the relationship between the structure and electromechanical properties of SEBS gel remains incompletely understood. This study investigates the effects of polar groups and nanostructure parameters on the bending electromechanical property of SEBS gel for improving its bending electromechanical properties, using maleic anhydride-grafted SEBS/white oil (WO) gel (SEBS-M gel) and blends of SEBS-M gel with acrylonitrile-styrene copolymer (SAN). The SEBS-M gel/SAN films exhibited a hexagonally packed cylinder (HEX) morphology, and the radius of polystyrene domains (rPS) as well as the thickness of poly(ethylene-co-butylene) (PEB) domains (dPEB) in the SEBS-M gel/SAN slightly decreased with increasing SAN content up to 15 phr, followed by an increase. The bending displacement and actuation stress of SEBS-M gel/SAN increased with increasing SAN content up to 15 phr. The bending displacement of SEBS-M gel at 1.4 kV is approximately 74 % of that of SEBS gel, while the bending displacement of SEBS-M gel/SAN-15 at 1.4 kV is approximately 195 % and 266 % higher than that of SEBS gel and SEBS-M gel when the SAN content reaches 15 phr, respectively. The grafted maleic anhydride has a counteractive effect on the bending electromechanical properties of SEBS gel, while blending with a certain amount of SAN in SEBS-M gel has a synergistic effect on the bending electromechanical properties of SEBS gel. It is demonstrated that the enchantment of bending electromechanical properties of SEBS gel is achieved not only by adjusting the parameters of its microphase-separated structure but also by inducing segment deformation through electric field-induced dipole orientations.

A novel lead‐free relaxor with endotaxial nanostructures for capacitive energy storage

AbstractDielectric capacitors with a fast charging/discharging rate, high power density, and long‐term stability are essential components in modern electrical devices. However, miniaturizing and integrating capacitors face a persistent challenge in improving their energy density (Wrec) to satisfy the specifications of advanced electronic systems and applications. In this work, leveraging phase‐field simulations, we judiciously designed a novel lead‐free relaxor ferroelectric material for enhanced energy storage performance, featuring flexible distributed weakly polar endotaxial nanostructures (ENs) embedded within a strongly polar fluctuation matrix. The matrix contributes to substantially enhanced polarization under an external electric field, and the randomly dispersed ENs effectively optimize breakdown phase proportion and provide a strong restoring force, which are advantageous in bolstering breakdown strength and minimizing hysteresis. Remarkably, this relaxor ferroelectric system incorporating ENs achieves an exceptionally high Wrec value of 10.3 J/cm3, accompanied by a large energy storage efficiency (η) of 85.4%. This work introduces a promising avenue for designing new relaxor materials capable of capacitive energy storage with exceptional performance characteristics.

Topological phases in polar oxide nanostructures

Topology provides a framework for an understanding of emergent phenomena in various fields such as the appearance of new results in the field of ferroelectrics. A historical introduction and a primer on the concepts of topology are followed by a discussion of experimental and theoretical methods that elucidate the exotic textures that appear in polar oxide nanostructures and superlattices. The complex interplay of several fundamental processes occurring on different energy scales leads to ground states that compete with one another and have large and/or novel responses with respect to external stimuli.

Rice husk-derived mesoporous biogenic silica nanoparticles for gravity chromatography

Biogenic silica nanoparticle is a superb alternative to synthetic silica because of their highly active, polar, and porous nanostructure with a large interior area. Among the available agricultural bioresources, biogenic silica extracted from rice husks could be a simple, easily available, and cost-effective resource to use as the stationary phase for the column chromatographic technique. In the present study, highly pure amorphous biogenic silica nanoparticles (bSNPs) were synthesized using rice husk by a controlled combustion route followed by the sol-gel method. The bSNPs show better performance for the separation and isolation of ortho- and para-nitrophenol and nitroaniline. The outstanding performance of the as-synthesized bSNPs is attributed to the high surface area, high porosity, and presence of Si–OH polar bonds. These preliminary findings imply that rice husk, an agricultural waste, could be an alternative source of silica and applicable as a stationary phase in column chromatography.

Ultra-high energy density integrated polymer dielectric capacitors

A novel one-step Roll & Press process for fabricating capacitors with embedded electrodes and with highly reversible polar nanostructures and superior energy storage performance (energy density: 50 J cm−1; efficiency: 80%).

Condensation of collective polar vortex modes

The dynamics of extended objects such as domain walls, domain bubbles, vortex structures, etc., can be described by their equations of motion associated with their effective mass and spring constant. Here we analytically derive the equations of motion for the polarization dynamics and elastodynamics for the structural responses of ferroelectric polar vortices, and theoretically extract their effective mass, spring constant, and mode frequencies. We demonstrate two subterahertz phonon modes and predicted their frequencies, both consistent with our recent experimental measurements and phase-field simulations. We show that elastic modulation of the energy function and spring constants leads to a condensation of a collective mode upon a second-order structural transition from symmetric to asymmetric vortices at a critical strain, analogous to the ferroelectric soft phonon mode at a ferroelectric transition. The present work offers a theoretical framework for predicting and manipulating the ultrafast collective dynamics of polar nanostructures.

Polarity-dependent nonlinear optics of nanowires under electric field

Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. Among these properties are piezoelectricity, ferroelectricity, pyroelectricity, and the bulk photovoltaic effect. Here we report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively. This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. EFM-SHG may be observed in any type of polar nanostructures and used to map the absolute polarity of materials at the nanoscale.

Lamellar-like nanostructure in a relaxor ferroelectrics Pb(Mg1/3Nb2/3)O3

The nanostructure of relaxor ferroelectric materials has been a central focus for investigating the microscopic origin of their intriguing physical properties. While it is believed that relaxor ferroelectricity is governed by polar nanostructures, such as polar nanoregions or nanodomains, recent studies have indicated the importance of additional mechanisms, such as the competition of ferroelectric/anti-ferroelectric order and the formation of hierarchical nanodomains. This calls for further investigation on the nanostructure. Here, we used conventional, in situ, and atomic-scale electron microscopy to study prototypic relaxor ferroelectrics, Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT). We found that a lamellar-like nanostructure was present in pure PMN, which had been overlooked in past studies and did not have a strong correlation with the polar nanostructure and the chemically ordered region. Unlike the lamellar-like nanodomains in PMN-PT, the lamellar-like nanostructure in PMN was not coupled with Pb-ion displacement and was not reoriented by the presence of an electric field. The results suggested that the formation of a lamellar-like structure occurs prior to the formation of larger-scale polar order in relaxor ferroelectrics.

High Surface Phonon-Polariton in-Plane Thermal Conductance along Coupled Films.

Surface phonon-polaritons (SPhPs) are evanescent electromagnetic waves that can propagate distances orders of magnitude longer than the typical mean free paths of phonons and electrons. Therefore, they are expected to be powerful heat carriers capable of significantly enhancing the in-plane thermal conductance of polar nanostructures. In this work, we show that a SiO/Si (10 m thick)/SiO layered structure efficiently enhances the SPhP heat transport, such that its in-plane thermal conductance is ten times higher than the corresponding one of a single SiO film, due to the coupling of SPhPs propagating along both of its polar SiO nanolayers. The obtained results thus show that the proposed three-layer structure can outperform the in-plane thermal performance of a single suspended film while improving significantly its mechanical stability.

Giant energy storage density in PVDF with internal stress engineered polar nanostructures

Abstract High power dielectric capacitors with high energy density are needed in order to develop modern electronic and electrical systems, including hybrid vehicles, telecommunication infrastructures and portable electronic devices. Relaxor ferroelectric polymers (RFP) are considered to be the most promising candidates for the next generation of capacitors owing to their relatively high energy storage density. However, the commercialization of RFP capacitors in power systems is hindered by their high cost and low dielectric breakdown strength. In this study, inexpensive, free-standing nano-crystalline (~3.3 nm) poly (vinylidene fluoride) (PVDF) films with high β phase content (~98%), “relaxor-like” ferroelectric behaviour and high breakdown strength (880 kV/mm) were fabricated using the facile Press & Folding (P&F) technique. An internal stress dominated polarization switching model is proposed to explain the origin of the relaxor-like ferroelectric behaviour. The internal stress generated during pressing alters the intermolecular chain distance of the (200) plane of β-PVDF from 4.24 A in internal stress free films to 4.54 A in P&F films, corresponding to a tensile strain and residual stress of 7.11% and 142 MPa, respectively. The internal stress acts to partially reverse the polarization on reversal of the applied electric field. This, combined with preferred in-plane orientation of the crystallites, results in a polar nanostructure with high polarization reversibility at high electric fields. A giant discharged energy storage density of 39.8 J/cm3 at 880 kV/mm was achieved for P&F films, which surpasses all previously reported polymer-based materials.

Wavelength tuning in the purple wavelengths using strain-controlled AlxGa1–xN/GaN disk-in-wire structures

AlxGa1–xN/GaN disk-in-wire polar nanostructures were fabricated, and their optical properties were studied. Wavelength tuning was observed by locally controlling the strain in each nanopillar via its diameter. The measured wavelength shift was in an excellent agreement with a one-dimensional strain relaxation model considering only the elastic and piezoelectric properties of the material. The inhomogeneous broadening decreases and internal quantum efficiency increases with a decreasing nanopillar diameter. The potential extension of strain-induced wavelength tuning across ultraviolet through near infrared was also discussed.

An Empirical Model for GaN Light Emitters with Dot-in-Wire Polar Nanostructures.

A set of empirical equations were developed to describe the optical properties of III-nitride dot-in-wire nanostructures. These equations depend only on the geometric properties of the structures, enabling the design process of a III-nitride light emitter comprised of dot-in-wire polar nanostructures, to be greatly simplified without first-principle calculations. Results from the empirical model were compared to experimental measurements and reasonably good agreements were observed. Strain relaxation was found to be the dominant effect in determining the optical properties of dot-in-wire nanostructures.

Mesoscale origin of dielectric relaxation with superior electrostrictive strain in bismuth ferrite-based ceramics

A strategy to optimize electrostrictive strain in BFO-based relaxor ferroelectrics was proposed. By operating the domain configuration via ion engineering the B-site order/disorder and random fields, two kinds of polar nano-structures can be formed.

Electric-field control of the ferro-paraelectric phase transition in Cu:KTN crystals

We report an efficient approach for ferro-paraelectric phase transition control in Cu:KTN crystals by using AC electric fields. Based on a dielectric loss mechanism, the thermal effect is induced directly inside the crystal without using extra thermal sources or complicated thermostat systems. By adjusting either the amplitude or the frequency of an AC electric field, we can easily and precisely control the ferro-paraelectric phase transition process which concurs with dynamic development of polar nanostructures of KTN crystals. In addition, we use a laser beam to probe in real time the dynamic process of crystalline phase transition. The study is important for developing novel electro-optic and ferroelectric devices for a wide range of photonic applications.

MBE growth and morphology control of ZnO nanobelts with polar axis perpendicular to growth direction

In quasi-one-dimensional polar nanostructures the relative orientation of the long and polar axes will determine how the polarity of the nanostructure may be exploited for applications. Here we present the growth by molecular-beam epitaxy of quasi-1d ZnO nanostructures (specifically nanobelts) with the polar axis perpendicular to the growth axis. We demonstrate the control of nanostructure morphology by growth temperature. Our work represents a key milestone towards the development of future polarization-engineered oxide heterostructures embedded in quasi-1d nanodevices.

Microwave-assisted synthesis and characterization of BiOIO3 nanoplates for photocatalysis

New polar BiOIO3 nanostructures were successfully synthesized by 90 and 180W microwave for 20–100min. X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible spectroscopy revealed the presence of pure orthorhombic BiOIO3 nanoplates with 2.5–2.9eV energy gap. The photodegradation of rhodamine B (RhB) by BiOIO3 photocatalyst was tested under visible radiation for 60min. In this research, BiOIO3 photocatalyst synthesized by 180W microwave for 60min has the highest activity of 96%.

Low temperature non radiative process's effects on the optical emission of GaN/Al0.5Ga0.5N nanostructures

The optical properties of self-assembled (0001) polar and (11–22) semipolar GaN nanostructures embedded in Al0.5Ga0.5N matrix and grown by molecular beam epitaxy are reported. A statistical analysis of the nanostructure's height dispersion is done by transmission electron microscopy (TEM) in order to have a good estimation of the electric field inside the structure and also to be able to give an explanation for the saturation of the emission energy for polar orientation when the height exceed some values. It is found that the non-radiative processes are not negligible at low temperature and should be taken into account as the radiative ones. The combined comparison between experimental results and our model has allowed us to determine the non-radiative lifetimes at low temperature. We show that the use of epitaxial lateral overgrowth, in addition to the growth of GaN nanostructures, will improve the radiative efficiency that can reach 60%.