Mechanical energy conversion technologies such as piezoelectric or triboelectric nanogenerators are able to harvest environmental energy (e.g., vibration, wind, tidal wave) and human body motion for powering electric vehicles, sensor networks, and wearable devices. [1][2][3] Traditional triboelectric nanogenerators (TENGs) may generate high voltage but with extremely low AC current ( J ≈ 0.01-0.1 A m −2 ) density. [1] The performance of TENGs is optimal only at high frequency due to the dielectric displacement current mechanism, while the environmental mechanical sources usually have frequencies lower than 10 Hz. [4] In contrast, the non-equilibrium tribo-tunneling phenomenon, recently discovered in the semiconductor-based Schottky moving contacts, is capable of generating a continuous DC current as high as 100 A m −2 regardless of the motion direction, and not limited by the mechanical source frequency. [3,[5][6][7][8][9][10] The tribo-tunneling transport tip-enhanced current generation, as reported by Liu et al. [3,6] using conductive-atomic force microscope (C-AFM), show that the tribo-tunneling current density ( J) output can be boosted by the nano-sized contact (tip radius R ≈ 30 nm) up to 10 6 A m −2 due to the enhanced electronic excitation and strong localized electric field E. It has been reported that a micro-tip (tip radius R ≈ 30 µm) sliding system produces a current density of 35 A m −2 while larger tip radius (R ≈ 100-300 µm) yields a current density of 10 A m −2 in the test probe sliding system. [5,6] However, scaling up the concept with micro-electromechanical systems (MEMS)-fabricated tip array is time-consuming and costly. The metal micro-tips also cause substrate surface scratching, which impacts the sustainability of the power generation. Moreover, the relatively low open-circuit voltage (V oc , 300-600 mV) of the single metal/Si sliding unit is insufficient for practical applications in electronics. To address those issues, we developed a carbon aerogel-based system in this work, which scales up the DC output and enhances the Voc output by one order via naturally formed Schottky nanocontacts.Carbon aerogel is electrically conductive, synthetic ultralight material composed of 3D network structures of interconnected amorphous carbon nanoparticles. [11] It has been widely used for nanocomposite, electrodes, desalination filters, and heterogeneous catalysis due to its large surface area. [12] In this work, Although tip-enhanced tribo-tunneling in metal/semiconductor point nanocontact is capable of producing DC with high current density, scaling up the process for power harvesting for practical applications is challenging due to the complexity of tip array fabrication and insufficient voltage output. Here, it is demonstrated that mechanical contact between a carbon aerogel and silicon (SiO 2 /Si) interface naturally forms multiple nanocontacts for tribo-tunneling current generation with an open-circuit voltage output (V OC ) reaching 2 V, and short-circuit DC current output (I SC ) of ≈15 µA. It h...

Boosting the photovoltaic power output is the key to large-scale implementation of solar cell technologies for renewable energy applications. Traditional solar energy harvesting is limited by the costly fabrication of p-n junctions, the duration of sunlight irradiation, and theoretical output limit. In this work, Liu et al. demonstrate that the photovoltaic power output can be dramatically enhanced by mechanical friction between a metal and a semiconductor, leading to the development of a new power generation approach called tribo-photovoltaic generator. It enables highly efficient solar-mechanical energy co-harvesting in the daytime as well as mechanical energy at night. The tribo-photovoltaic effect may be utilized for co-harvesting of solar and mechanical energy in various scenarios such as ocean tidal energy harvesters, wind turbines, and aerospace energy collectors.

Techniques for scaling-up the direct-current (dc) triboelectricity generation in MoS 2 multilayer-based Schottky nanocontacts are vital for exploiting the nanoscale phenomenon for real-world applications of energy harvesting and sensing. Here, we show that scaling-up the dc output can be realized by using various MoS 2 multilayer-based heterojunctions including metal/semiconductor (MS), metal/insulator (tens of nanometers)/semiconductor (MIS), and semiconductor/insulator (a few nanometers)/semiconductor (SIS) moving structures. It is shown that the tribo-excited energetic charge carriers can overcome the interfacial potential barrier by different mechanisms, such as thermionic emission, defect conduction, and quantum tunneling in the case of MS, MIS, and SIS moving structures. By tailoring the interface structure, it is possible to trigger electrical conduction resulting in optimized power output. We also show that the band bending in the surfacecharged region of MoS 2 determines the direction of the dc power output. Our experimental results show that engineering the interface structure opens up new avenues for developing next-generation semiconductor-based mechanical energy conversion with high performance.

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