Enhanced piezoelectric and energy-harvesting characteristics of Mn-doped (Na0.5K0.5)NbO3 (NKN) nanofibers have been investigated with actual fabrication of potential flexible nanogenerators. The electrospinning process of nanofibers has been initially optimized with the proper level of chelating agent and annealing temperature. High quality nanofibers are successfully obtained only by means of a certain level of doped-Mn, which incorporates into the NKN perovskite structure and facilitates significant grain growth. A single-particle-stacked structure along the direction of fiber length becomes more evident with increasing Mn content. An XPS analysis confirms that Mn exists in multivalent states of Mn(2+)/Mn(3+). The effective piezoelectric coefficient of the nanofibers is found to be enhanced by 5 times with Mn-doping up to 3 mol % as characterized by piezoelectric force microscopy. The resultant flexible nanogenerators on PES films have exhibited ∼0.3 V output voltage and ∼50 nA output current under a bending strain.

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MXenes, an emerging class of 2D transition metal carbides and nitrides with the general formula Mn+1XnTx (n = 1–4), have potential for application as floating gates in memory devices because of their intrinsic properties of a 2D structure, high density‐of‐states, and high work function. In this study, a series of MXene–TiO2 core–shell nanosheets are synthesized by deterministic control of the surface oxidation of MXene. The floating gate (multilayer MXene) and tunneling layer (TiO2) in a nano‐floating‐gate transistor memory (NFGTM) device are prepared simultaneously by a facile, low‐cost, and water‐based process. The memory performance is optimized via adjustment of the thickness of the oxidation layer formed on the MXene surface. The fabricated MXene NFGTMs exhibit excellent nonvolatile memory characteristics, including a large memory window (>35.2 V), high programming/erasing current ratio (≈106), low off‐current (<1 pA), long retention (>104 s), and cyclic endurance (300 cycles). Furthermore, synaptic functions, including the excitatory postsynaptic current/inhibitory postsynaptic current, paired‐pulse facilitation, and synaptic plasticity (long‐term potentiation/depression), are successfully emulated using the MXene NFGTMs. The successful control of MXene oxidation and its application to NFGTMs are expected to inspire the application of MXene as a data‐storage medium in future memory devices.

The rapid enhancement of the thermoelectric (TE) figure‐of‐merit (zT) in the past decade has opened opportunities for developing and transitioning solid state waste heat recovery systems. Here, a segmented TE device architecture is demonstrated in conjunction with heterogeneous material integration that results in high unicouple‐level conversion efficiency of 12% under a temperature difference of 584 K. This breakthrough is the result of success in fabricating bismuth telluride/half‐Heusler segmented TE unicouple modules using a “hot‐to‐cold” fabrication technique that provides significantly reduced electrical and thermal contact resistance. Extensive analytical and finite element modeling is conducted to provide an understanding of the nature of thermal transport and contributions arising from various thermal and physical parameters. Bismuth telluride/half‐Heusler based segmented thermoelectric generators (TEGs) can provide higher practical temperature difference with optimum average zT across the whole operating range. These results will have immediate impact on the design and development of TEGs and in the general design of devices based upon heterostructures that take advantage of gradients in the figure of merit.

The filling fraction limitation (FFL) in n-type CoSb3 skutterudites is far below that of p-type (Fe,Co)­Sb3-based skutterudites, and it is critical to increase FFL for accomplishing high thermoelectric figure of merit (ZT max). Here, a series of Yb x Co4–y Fe y Sb12 alloys with x = 0.25–0.5 and y = 0.1–0.5 were synthesized, which demonstrate a clear increase of the FFL of Yb from ∼0.3 in CoSb3 to 0.5. Ultralow thermal conductivities of 2.0–2.5 W/m·K at 300 K and 1.75 W/m·K at ∼600 K have been achieved, which are the lowest values reported among skutterudite materials and comparable with p-type skutterudites. These ultralow thermal conductivities result from the combination of secondary phase scattering and phonon scattering from dynamic electron exchange between Fe2+ and Co3+. High ZT max values of 1.28 at 740 K and 1.34 at 780 K are obtained, which are among the best values reported in the temperature range of 740–800 K. The temperature at which maximum ZT max appears is shifted below 850 K. These results are highly exciting toward the development of multistage segmented and cascade thermoelectric power generators for in-air operations.