Direct-current Triboelectricity Generation by Electron Tunneling Transport

  • Author / Creator
    Liu, Jun
  • The direct conversion of mechanical energy into electricity by nanomaterial-based devices offers potential for green energy harvesting. One of the main challenges for mechanical energy harvesting technologies, such as piezoelectric or triboelectric generators, is to provide sufficient current density for powering electronics, which is generally limited by the high impedance of material systems. A conventional triboelectric nanogenerator converts frictional energy into electricity by producing alternating current (a.c.) triboelectricity. However, this approach is limited by low current density and the need for rectification. In this thesis, we show that continuous direct-current (d.c.) with a maximum density of 10^6 A m−2 can be directly generated by a sliding Schottky nanocontact without the application of an external voltage. We demonstrate this by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS2). Finite element simulation reveals that the anomalously high current density can be attributed to the non-equilibrium carrier transport phenomenon enhanced by the strong local electrical field (10^5−10^6 V m−2) at the conductive nanoscale tip. We hypothesize that the charge transport may be induced by electronic excitation under friction, and the nanoscale current−voltage spectra analysis indicates that the rectifying Schottky barrier at the tip–sample interface plays a critical role in efficient d.c. energy harvesting. Furthermore, we have observed sustainable tunneling current generation at an unbiased, triboelectrically charged metal-insulator-semiconductor (MIS) point contact consisting of p-type silicon, silicon oxide and a metal tip. It is found that the native thin oxide (~1.6 nm) on the silicon surface provides a natural pathway for the quantum mechanical tunneling of triboelectric charges. The charges are then collected by the semiconducting substrate, thereby generating direct current (d.c.) with very high current density. The measured direct current (d.c.) shows an exponential decay with the thickness of oxide layer deposited with atomic layer deposition (ALD), confirming the quantum mechanical tunneling mechanism. It is proposed that the contact potential difference enhanced by triboelectric charging provides the potential difference between metal point contact and the substrate. With single metallic micro probe sliding on a moderately doped p-type silicon, an open circuit voltage (Voc) of 300-400 mV and a short-circuit direct current (Isc) of 3-5 μA (a corresponding high current density, J, in the order of 1-10 A/m2) have been observed. It is predicted by conductive-atomic force microscopy (C-AFM) experiment that the theoretical J can be as high as 10^4 A/m2. It has been proved that the d.c. triboelectricity generation by MIS frictional contact can be directly stored by charging capacitors. This concept has the potential as a green energy harvesting technique where a broad range of material candidates and device configurations could be used.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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