Graphene on cubic-SiC

The outstanding properties of graphene make it a top candidate for replacing silicon in future electronic devices. However, for technological applications, graphene must be synthesized on the surface of wide-gap semiconductors. In this review, we focus on graphene synthesized on single-crystalline cubic-SiC thin films epitaxially grown on standard silicon wafers. These low-cost substrates are commercially available and fully compatible with existing silicon technologies. The results obtained in recent years demonstrate that few-layer graphene synthesized on cubic-SiC substrates possesses the atomic structure and electronic properties of quasi-free-standing graphene. However, according to data obtained by various techniques, few-layer graphene on cubic-SiC consists of nanodomains connected to one another through nanodomain boundaries. After optimization of the preparation procedures, such a nanostructured graphene overlayer can represent a very promising system for the development of new graphene-based electronic devices. In particular, recent works demonstrate that continuous few-layer graphene with self-aligned nanodomain boundaries can be synthesized on vicinal SiC(0 0 1) substrates. Electrical measurements show the opening of a transport gap in nanostructured trilayer graphene synthesized on SiC/2°-off Si(0 0 1) wafers. This development may lead to new tunable electronic nanostructures made from graphene on cubic-SiC, opening up opportunities for a wide range of applications.     

Nanoscale origins of small hysteresis and remnant strain in Bi0.5Na0.5TiO3-based lead-free ceramics

BiAlO₃-doped Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃ (BA-doped BNT-BKT) ceramics are greatly concerned due to their sufficient electric-field-induced strain with small hysteresis and remnant strain for high precision positioning devices and other actuators. In this paper, the structural analysis especially the high-resolution transmission electron microscope (HRTEM) is used to reveal the origin of excellent properties obtained in 0.96(0.75BNT-0.25BKT)-0.04BA, which exhibits a large strain of 0.21% at ～70 kV/cm, a small strain hysteresis of only 24% and a near-zero remnant strain. Using HRTEM, the antiferroelectric nano-domains composited by three variants of in-phase a⁰a⁰c⁺ octahedral tilting coexisted with the remnant ferroelectric nano-domains of anti-phase a^?a^?a^? octahedral tilting are directly identified. Then a continuous tilting model is proposed to interpret the gradually transitional tilting involving nano-domains leading to the small hysteresis and near-zero remnant strain. The findings may pave a way for further optimizing the properties through creating stable antiferroelectric nano-domains in BNT-based ceramics and the analogues.     

Enhanced strains of Nb-doped BNKT-4ST piezoelectric ceramics via phase boundary and domain design

A strategy that constructs the morphotropic phase boundary and manipulates the domain structure has been used to design the component of 0.96[Bi_0.5(Na_0.84K_0.16)_0.5Ti_(1-x)Nb_xO₃]-0.04SrTiO₃ (BNKT-4ST-100xNb) to enhance the strain properties for actuator application. Non-equivalent Nb⁵⁺ donor doping modulates the phase transition from the mixture of rhombohedral and tetragonal phases to the pseudocubic phase and results in the coexistence of multiple phases. Moreover, the high-resolution TEM confirms the existence of polar nano regions that contribute to the macroscopic relaxor behaviour. The size of the domains is reduced with increasing Nb⁵⁺, resulting in an enhanced relaxor behaviour. The ferroelectric-relaxor transition temperature decreases from 85 to below 30 °C, implying a non-ergodic to ergodic relaxor transition. An improved strain of 0.56% and a giant normalized strain of 1120 pm/V were achieved for BNKT-4ST-1.5Nb, which were attributed to the unique domain structure in which nanodomains are embedded in an undistorted cubic matrix. Ferroelectric, antiferroelectric, and relaxor phases coexist. As the electric field is large enough, a reversible phase transition occurs. Furthermore, good temperature stability was obtained due to the stability of the nanodomains, and no degradation in strains was observed even after 10⁴ cycles, which may originate from the reversible phase transition and dynamic domain wall. The results show that this design strategy offers a reference way to improve the strain behaviour and that BNKT-4ST-100xNb ceramics could be a potential material for high-displacement actuator applications.     

The effects of sulfur doping on the performance of O3-Li0.7[Li1/12Ni1/12Mn5/6]O2 powder

Li_0.7[Li_1/12Ni_1/12Mn_5/6]O₂ and Li_0.7[Li_1/12Ni_1/12Mn_5/6]O_2-yS_y (y=0.1, 0.2, 0.3) powders were synthesized by using a sol-gel method. As-prepared samples showed typical rhombohedral O3 layered structure. The shape of the initial discharge curve for the samples was almost equal to that of the layered structure. However, the electrode materials were transferred from layered to spinel structures with cycling. At the first cycle, Li_0.7[Li_1/12Ni_1/122Mn₂5/6]O₂ and Li_0.7[Li_1/12Ni_1/12 Mn_5/6]O_1.9S_0.1, Li_0.7[Li_1/12Ni_1/12Mn_5/6]O_1.8S_0.2, and Li_0.7[Li_1/12Ni_1/12Mn_5/6]O_1.7S_0.3 delivered the discharge capacities of 238, 230,224, and 226 mAh/g, respectively, with their capacity fading rates of 0.34, 0.21, 0.12, 0.25%/cycle, respectively. The partial substitutions of Ni and S for Mn and O in Li_0.7[Li_1/12Ni_1/12Mn_1/12]O₂ significantly enhanced the electrochemical properties of the lithium manganese oxide materials.     

Feasible acid-etching method for investigating temperature-dependent domain configurations of ferroelectric ceramics

Domain structure often plays significantly important roles in determining both the piezoelectric properties and the piezoelectric temperature stabilities of ferroelectric ceramics. However, a convenient and reliable technique for studying temperature-dependent domain configurations is greatly desired. Inspired by “burning CDs”, we proposed here a temperature-variable acid-etching method (abbreviated hereafter as VTAE) for such purpose. Utilizing this method, domain configurations of the poled 0.96(K_0.48Na_0.52)(Nb_0.96Sb_0.04)O₃–0.04(Bi_0.50Na_0.50)ZrO₃ (abbreviated hereafter as KNNS-4%BNZ)lead-free ceramic were investigated at different temperatures. Diversified domain patterns that consist of parallel-stripe domains, herringbone domains and wedge-shaped domains are recognized in different phases, respectively. It is found from the evolution of domain configurations with temperature that nanodomain structure is closely related to the high piezoelectric responses and temperature instabilities in the KNNS-4%BNZ ceramic. The result proves that this VTAE method is a feasible method for studying the temperature-dependent domain configurations of ferroelectric ceramics.     

Nanoscale origins of small hysteresis and remnant strain in Bi0.5Na0.5TiO3-based lead-free ceramics

BiAlO₃-doped Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃ (BA-doped BNT-BKT) ceramics are greatly concerned due to their sufficient electric-field-induced strain with small hysteresis and remnant strain for high precision positioning devices and other actuators. In this paper, the structural analysis especially the high-resolution transmission electron microscope (HRTEM) is used to reveal the origin of excellent properties obtained in 0.96(0.75BNT-0.25BKT)-0.04BA, which exhibits a large strain of 0.21% at ∼70 kV/cm, a small strain hysteresis of only 24% and a near-zero remnant strain. Using HRTEM, the antiferroelectric nano-domains composited by three variants of in-phase a⁰a⁰c⁺ octahedral tilting coexisted with the remnant ferroelectric nano-domains of anti-phase a⁻a⁻a⁻ octahedral tilting are directly identified. Then a continuous tilting model is proposed to interpret the gradually transitional tilting involving nano-domains leading to the small hysteresis and near-zero remnant strain. The findings may pave a way for further optimizing the properties through creating stable antiferroelectric nano-domains in BNT-based ceramics and the analogues.     

Construction of multi-domain coexistence enhanced piezoelectric properties of Bi0.5Na0.5TiO3-based thin films

Although the multi-phase coexistence makes Bi_0.5Na_0.5TiO₃-based piezoelectric thin films possess stronger piezoelectric properties and more spacious application prospects in electronic devices, the domain reversal mechanism of Bi_0.5Na_0.5TiO₃-based thin films cannot be accurately understood due to the size effect. In this study, the relationship between domain structure and piezoelectric properties of the (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xBi(Fe_0.95Mn_0.03Ti_0.02)O₃ thin films are studied by using visualization technology PFM, structure and electrical properties characterizations. The results show that the addition of Bi(Fe_0.95Mn_0.03Ti_0.02)O₃ creates a long-range ordered/short-range disordered nanodomain coexisting structure. This kind of coexisting domain structure can realize the long-range reversal driven by disordered nanodomains under the external electric field, reduce the potential barrier and the hysteresis, and significantly enhance the piezoelectric properties of the thin films. Under the same conditions, the piezoelectric properties of the 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ thin films are enhanced nearly 2.3 times. This provides a reference for exploring the physical mechanism of high performance lead-free piezoelectric thin films.     

The structural origin of enhanced energy harvesting performance in piezoelectric perovskite

Energy harvesting, which can translate the wasted vibration energy into electric energy, is now a hot topic in the field of new energy, and the key point is to design high power piezoelectric ceramic according with the requirements of low-frequency vibration energy harvesting. In this study, high quality Co-modified 0.2Pb(Zn_1/3Nb_2/3)O₃–0·8Pb(Zr_0·50Ti_0·50)O₃ (PZN–PZT+Co) ceramics have been prepared by the two-stage method, and the energy harvesting characteristics were investigated. The results showed that the hierarchical nanodomain structure boosts the strong piezoelectric activity, leading to the high energy harvesting performance. The PZN–PZT+Co ceramic sintered at 1000 °C exhibits an excellent d₃₃ × g₃₃ value of 14080 × 10^?15 m²/N, which are much larger than that of commercial PZT-based ceramics. In the mode of the cantilever-type energy harvester, the output voltage and energy density of 33 V, 4.4 μW/mm³ were obtained at a low resonance frequency of 85 Hz and acceleration of 10 m/s², showing potential application in piezoelectric energy harvester.