A novel nonorganic wet route for direct synthesis of uniform hexagonal β-NaYF4:Ln(3+) (Ln = Eu, Tb, Ce/Tb, Yb/Er, and Yb/Tm) microcrystals with various morphologies has been developed wherein the intermediate routine cubic-hexagonal (α → β) phase transfer process was avoided. The morphology can be effectively tuned into hexagonal disc, prism, and novel hierarchical architectures by systematically fine manipulating the Na2CO3/F(-) feeding ratio. It has been found that the routine α → β phase transfer for NaYF4 was not detected during the growth, while NaY(CO3)F2 emerged in the initial reaction stage and fast transformed into β-NaYF4 via a novel topotactic transformation behavior. Detailed structural analysis showed that β-NaYF4 preferred the [001] epitaxial growth direction of NaY(CO3)F2 due to the structural matching of [001]NaY(CO3)F2//[0001]β-NaYF4. Besides, the potential application of the as-prepared products as phosphors is emphasized by demonstrating multicolor emissions including downconversion, upconversion, and energy transfer (Ce-Tb) process by lanthanides doping.
The influence of strains on the channel current–gate voltage behaviors and memory windows of ferroelectric memory field-effect transistors (FeMFETs) were studied using an improved model based on the Landau–Devonshire theory. ‘Channel potential–gate voltage’ ferroelectric polarization and silicon surface potential diagrams were constructed for strained single-domain BaTiO3 FeMFETs. The compressive strains can increase (or decrease) the amplitude of transistor currents and enlarge memory windows. However, tensile strains only decrease the maximum value of transistor currents and compress memory windows. Mismatch strains were found to have a significant influence on the electrical behaviors of the devices, therefore, they must be considered in FeMFET device designing. (Some figures may appear in colour only in the online journal)
Ferroelectric diodes hold significant promise for potential applications in nonvolatile memories and logic devices. The nondestructive readout of binary information can be achieved by using a bipolar switching with two different conductances under two opposite polarizations in a ferroelectric diode, which exhibits ultrahigh density and ultrafast operating speed. However, the diode current is limited because most ferroelectrics have wide band gaps. Therefore, in modern micromemory circuits, obtaining a sufficient ferroresistive diode current to detect the status of memory stably is a major challenge. Herein, a high current-intensity resistive switching behavior in nanometer-thick BiFe0.9Co0.1O3 films is reported. Epitaxial films were prepared on a (00l) Nb/SrTiO3 single-crystal substrate via the chemical solution epitaxial deposition method. The conductance of the BiFeO3 diode improved by up to 200 times that of the original. This improvement can be attributed to the bandgap decrease in ferroelectric film induced by Co doping, as confirmed by spectrophotometry and first-principles calculations. This device shows a stable bipolar resistive switching feature, a satisfactory switching ratio of ∼103, good data retention, and antifatigue characteristics for up to 107 cycles. The results are useful in exploring the potential applications of a ferroelectric diode in RRAM.
In this paper, the hydroxyl radical yield of a cavitation bubble and its influencing factors in the process of chitosan degradation with hydrodynamic cavitation in a single-hole orifice plate was investigated by a numerical simulation method. The hydroxyl radical yield of the cavitation bubble was calculated and analyzed by the Gilmore equation as the dynamic equation combined with the mass transfer equation, heat transfer equation, energy balance equation, and the principle of Gibbs free energy minimization. The influence of geometric parameters of the orifice plate and operating parameters on the formation of hydroxyl radicals was investigated. The results showed that the hydroxyl radicals produced at the moment of cavitation bubble collapse increased with the increase of the initial radius (R0), upstream inlet pressure (P1), downstream recovery pressure (P2), downstream pipe diameter (dp), and the ratio of the orifice diameter to the pipe diameter (β). The simulation results provide a certain basis for the regulation of hydrodynamic cavitation degradation of chitosan.
Abstract Inorganic halide perovskites CsPb X 3 ( X = I, Br) have attracted tremendous attention in solar cell applications. However, the bulk form of the cubic phase CsPb X 3 , which offers moderate direct bandgaps, is metastable at room temperature and tends to transform into a tetragonal or orthorhombic phase. Here, our density functional theory calculation results found that the surface energies of the cubic phase are smaller than those of the orthorhombic phase, although the bulk counterpart of the cubic phase is less stable than that of the orthorhombic phase. These results suggest a surface stabilization strategy to maintain the stability of the cubic phase at room temperature that an enlarged portion of surfaces shall change the relative stability of the two phases in nanostructured CsPb X 3 . This strategy, which may potentially solve the long-standing stability issue of cubic CsPb X 3 , was demonstrated to be feasible by our calculations in zero-, one-, and two-dimensional nanostructures. In particular, confined sizes from few to tens of nanometers could keep the cubic phase as the most thermally favored form at room temperature. Our predicted values in particular cases, such as the zero-dimensional form of CsPbI 3 , are highly consistent with experimental values, suggesting that our model is reasonable and our results are reliable. These predicted critical sizes give the upper and lower limits of the confined sizes, which may guide experimentalists to synthesize these nanostructures and promote likely practical applications such as solar cells and flexible displays using CsPb X 3 nanostructures.
By adjusting different polarization status in ferroelectric thin films, effect of saturation polarization, bias voltage, dielectric barrier width, and ferroelectric barrier width on electrical properties of a metal-dielectric-ferroelectric-metal structure are investigated in detail. Our simulation results show that the electrical properties of the tunnel junction can be effectively improved by utilizing the inverse piezoelectric effect of ferroelectric thin films. When the strain effect is taken into account, by controlling the polarization of the ferroelectric layer, the number of logic values could be modulated flexibly.
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