  Direct-current (d.c.) electricity generation using moving Schottky contact is
emerging as a new strategy for mechanical energy conversion. Here, we
demonstrate the generation of d.c.tunneling current with a density of ~35 A/m2
at a metal-insulator-semiconductor (MIS) sliding system using micro-tips. The
measured current densities were found to be three to four orders of magnitude
higher than that observed with the conventional polymer-based triboelectric
nanogenerators (TENGs). The electromotive force for the tribo-tunneling
transport comes from the dynamic electronic excitation at the frictional
interface rather than from the electrostatically trapped surface charges as in
the case of conventional TENGs. The strong electronic excitation can give rise
to a non-equilibrium interfacial charge variation. Depending on the energy
distribution of the excited electrons/holes, the charge variation subsequently
dissipates non-adiabatically into tunneling current and trapped surface
charges, or adiabatically into heat. These fundamental results not only enhance
our understanding of the triboelectric phenomenon, but also open up new paths
for the development of next-generation mechanical energy harvesting and sensing
techniques.
