The reported thermal conductivity (kappa) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that kappa of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to kappa in suspended graphene according to a theoretical calculation.

For an electrodeposited bismuth telluride ͑Bi x Te 1−x ͒ nanowire from one batch with x found to be about 0.46, the Seebeck coefficient ͑S͒ was measured to be 15%-60% larger than the bulk values at temperature 300 K. For four other nanowires from a different batch with x Ϸ 0.54, S was much smaller than the bulk values. The electrical conductivity of the nanowires showed unusually weak temperature dependence and the values at 300 K were close to the bulk values. Below 300 K, phonon-boundary scattering dominated phonon-phonon Umklapp scattering in the nanowires, reducing the lattice thermal conductivity.

We present a method to develop high performance flexible piezoelectric nanogenerators (NGs) by employing Li-doped ZnO nanowires (NWs). We synthesized Li-doped ZnO NWs and adopted them to replace intrinsic ZnO NWs with a relatively low piezoelectric coefficient. When we exploited the ferroelectric phase transition induced in Li-doped ZnO NWs, the performance of the NGs was significantly improved and the NG fabrication process was greatly simplified. In addition, our approach can be easily expanded for large-scale NG fabrication. Consequently, the NGs fabricated by our simple method exhibit the excelling output voltage and current, which are stable and reproducible during periodic bending/releasing measurement over extended cycles. In addition, output voltage and current up to ∼ 180 V and ∼ 50 μA, respectively, were obtained in the large-scale NG. The approach introduced here extends the performance limits of ZnO-based NGs and their potentials in practical applications.

We report a study of the effect of the growth base pressure on the thermoelectric (TE) properties of indium antimonide (InSb) nanowires (NWs) synthesized using a vapour-liquid-solid method at different base pressures varying from ambient to high vacuum.A suspended device was used to characterize the TE properties of the NWs, which are zinc-blende structure with 1 1 0 growth direction based on transmission electron microscopy (TEM) characterization of the same NWs assembled on the suspended device. The obtained Seebeck coefficient is negative, with the magnitude being smaller than the literature bulk values and increasing with decreasing growth base pressure. These results are attributed to the loss of In from the source materials due to oxidation by residual oxygen in the growth environment and the consequent formation of Sb-doped NWs. The electron mobility and lattice thermal conductivity in the NWs are lower than the corresponding bulk values because of both surface scattering and stronger dopant scattering in the Sb-doped NWs. Based on these findings, it is suggested that growth from In-rich source materials can be used to achieve composition stoichiometry in the NWs so as to increase the Seebeck coefficient and TE figure of merit.

It has been suggested by theoretical calculation that indium antimonide (InSb) nanowires can possess improved thermoelectric properties compared to the corresponding bulk crystal. Here we fabricated a device using electron beam lithography to measure the thermopower and electrical conductivity of an individual InSb nanowire grown using a vapor-liquid-solid method. The comparison between the measurement results and transport simulations reveals that the nanowire was unintentionally degenerately doped with donors. Better control of the impurity doping concentration can improve the thermoelectric properties.

Recent advances in soft materials and mechanics activate development of many new types of electrical medical implants. Electronic implants that provide exceptional functions, however, usually require more electrical power, resulting in shorter period of usages although many approaches have been suggested to harvest electrical power in human bodies by resolving the issues related to power density, biocompatibility, tissue damage, and others. Here, we report an active photonic power transfer approach at the level of a full system to secure sustainable electrical power in human bodies. The active photonic power transfer system consists of a pair of the skin-attachable photon source patch and the photovoltaic device array integrated in a flexible medical implant. The skin-attachable patch actively emits photons that can penetrate through live tissues to be captured by the photovoltaic devices in a medical implant. The wireless power transfer system is very simple, e.g., active power transfer in direct current (DC) to DC without extra circuits, and can be used for implantable medical electronics regardless of weather, covering by clothes, in indoor or outdoor at day and night. We demonstrate feasibility of the approach by presenting thermal and mechanical compatibility with soft live tissues while generating enough electrical power in live bodies through in vivo animal experiments. We expect that the results enable long-term use of currently available implants in addition to accelerating emerging types of electrical implants that require higher power to provide diverse convenient diagnostic and therapeutic functions in human bodies.

The piezoelectric potential screening by large excess electrons in nominally undoped ZnO has limited the energy conversion efficiency of the ZnO nanogenerators (NGs). In this study, we report a simple and effective approach to enhance the piezoelectric output performance of the ZnO NGs by forming a CuO-ZnO heterostructure. By depositing a ZnO thin film on the pre-deposited CuO thin film, which forms a p-n junction, excess electrons in ZnO can be effectively reduced. Thus, the piezoelectric potential generated in ZnO by an applied force can be less affected. Using this approach, we obtained an output voltage up to $7.5 V and a maximum current of 4.5 mA cm À2 measured under the forward connection, which is a 7-fold higher output voltage and an approximately one order of magnitude higher current density by comparison to the ZnO NGs without a CuO layer. Our results clearly demonstrate the effectiveness of a CuO-ZnO heterostructure for realizing high performance flexible energy harvesting devices.

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