Using Landau-Ginzburg-Devonshire theory and effective medium approximation, we analytically calculate typical dependences of the pyroelectric and electrocaloric coefficients on external electric field, temperature and radius for spherical single-domain ferroelectric nanoparticles. The considered physical model corresponds to the nanocomposite with small fraction of ferroelectric nanoparticles.Within the framework of the analytical model we establish how the size changes determine the ferroelectric thin films [31, 32, 33, 34], multilayers [35, 36, 37] and other low-dimensional materials [38] can be very different from those of single crystals [39].As it is known [40,41], the polar materials in adiabatic conditions are characterized by the PEE (charge or electric field generation under temperature change) and by the inverse ECE (temperature change under application or removal of an electric field). The vivid manifestation of PEE and ECE in ferroelectrics is a consequence of the strong temperature dependence of the spontaneous polarization [42,43,44], especially in the vicinity of phase transitions [45,46] or near the morphotropic phase boundary [47]. This property is the basis for the widespread applications of ferroelectric materials for pyroelectric detectors and energy converters, as well as for realizing their potentiality in modern electrocaloric converters [48,49,50].At present, ECE and PEE in ferroelectric crystals, ceramics and polymers, thin films and multilayer structures are the objects of intensive theoretical, experimental, and applied studies.Nevertheless, ECE and PEE in ferroelectric nanoparticles are relatively poor studied. The possible reason is the strong influence of size effects via depolarization field [24] and polarization-strain coupling [20, 25] on the polarization distribution, ferroelectric transition temperature, dielectric, PE, and EC properties. There are several studies directed on the elucidation of the features of the PEE and ECE in nanowires, nanotubes [51,52,53,54], and nanoparticles [55]. However, the analytical description of ECE and PEE in the most "technological" spherical nanoparticles and nanocomposites, allowing for depolarization and incomplete screening effect, is still missing.Using the LGD theory and effective medium approximation, this work analyzes typical dependences of the polarization, dielectric permittivity, PE and EC coefficients on external electric field, temperature, and radius for spherical ferroelectric nanoparticles covered by a semiconducting shell and placed in a dielectric medium. The considered physical model corresponds to a nanocomposite "nanoparticles-matrix" with a small fraction (less than 10%) of the ferroelectric nanoparticles.The manuscript has the following structure. Problem statement containing free energy and basic equations with boundary conditions is formulated in Section II. Section III introduces approximate analytical expressions for the transition temperature, EC temperature change, heat capacity, and related physical quantities. Siz...
Tunability of polar and semiconducting properties of low-dimensional transition metal dichalcogenides (TMDs) have propelled them to the forefront of fundamental and applied physical research. These materials can vary from non-polar to ferroelectric, and from direct-band semiconductor to metallic.However, in addition to classical controls such as composition, doping, and field effect in TMDs the additional degrees of freedom emerge due to the curvature-induced electron redistribution and associated changes in electronic properties. Here we numerically explore the elastic and electric fields, flexoelectric polarization and free charge density for a TMD nanoflake placed on a rough substrate with a sinusoidal profile of the corrugation using finite element modelling (FEM). Numerical results for different flake thickness and corrugation depth yield insight into the flexoelectric nature of the out-of-plane electric polarization and establish the unambiguous correlation between the polarization and static conductivity modulation caused by inhomogeneous elastic strains coupled with deformation potential and strain gradients, which evolve in TMD nanoflake due to the adhesion between the flake surface and corrugated substrate. We revealed a pronounced maximum at the thickness dependences of the electron and hole conductivity of MoS2 and MoTe2 nanoflakes placed on a corrugated substrate, which opens the way for their geometry optimization towards significant improvement their polar and electronic properties, necessary for their advanced applications in nanoelectronics and memory devices. Specifically, obtained results can be useful for elaboration of nanoscale straintronic devices based on the bended MoS2, MoTe2 and MoSTe nanoflakes, such as diodes and bipolar transistors with a bending-controllable sharpness of p-n junctions.
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