Nanoscale triboelectrification of two-dimensional chemical vapor deposited transition metal dichalcogenides

Abstract

Triboelectrification, a contact-induced electrification where a material becomes electrically charged after brought into contact with a dissimilar one via friction [1], is an available, stable and efficient method to realize the mechanical-to-electrical energy conversion. Until now, this phenomenon has been investigated for insulators like polymers and silicon dioxide, and zero-bandgap semiconductor like graphene. For semiconductors with non-zero bandgaps, transition metal dichalcogenides have been paid much attention due to their distinctive optical, electrical and mechanical properties, and it has been reported that the performance of triboelectric nanogenerators can be dramatically enhanced by introducing single-layer MoS2 into the friction layer as the triboelectric electron-accepter layer [2], but the theories behind are still poorly understood, so the aim of this investigation is to fill this research gap.

With this aim in mind, high-quality MoS2, WS2, MoSe2, and WSe2 nanoflakes were synthesized by chemical vapor deposition method and their triboelectric properties were characterized with atomic force microscopy and Kelvin force microscopy. Due to the various work functions of these four materials in reference to the conductive Pt-coated tip, the electrons are transferred from the samples to the tip during triboelectrification in the case of WS2, MoSe2, and WSe2 whereas MoS2 exhibits an opposite transfer direction. The densities of tribo-charges are different for these four materials but they can be modified by applying diverse biases to the conductive tip during the rubbing process. Impressively, these tribocharges, tunneling to the interlayer between the nanoflakes and the underlying insulating substrate, show more than two orders of magnitude longer lifetime than conventional triboelectrification, and MoS2 owns the longest lifetime while WSe2 has the shortest among these four materials. In addition, the diffusion processes of WS2 and MoSe2 are alike thanks to their similar work functions. The surface-adsorbed water molecules from the atmosphere can act as carrier trappers to affect the surface potential and charge distribution of 2D materials given the interaction with the environment nearby, but heat treatment can efficiently solve this problem. Besides, a positive correlation between the layer number and resistance to charge dissipation was observed, and there also exists the transfer of materials during the frictional process, which can be utilized for tuning the triboelectric properties of the rubbed material.

On these basis, applicable fields consisting of bandgap modification as well as tunable antenna have been investigated. For the bandgap modification, the in situ effect of strain was investigated and it exhibits that the bandgap decreases monotonically with the increase of applied force. The reason behind is the reduction of orbital overlapping and hybridization due to the weakened atomic bonds when localized strain appears. In addition, while the bandgap of MoS2 is almost unchanged when an external perpendicular electric field is applied because of the very subtle band structure deformation, the bandgap of bilayer MoS2 can be modified via the electric field generated by the underlying tribo-charges, and applying positive and negative biases has different effects on the bandgap thanks to the spontaneous polarization along the direction perpendicular to the nanoflakes plane. Meanwhile, a MoSe2- based triboelectrically-controlled tunable antenna was proposed to apply to the upcoming 5G base station. As the tribo-charges can tunnel through the MoSe2 nanoflakes and localize at the interlayer to act as a bias voltage, the conductive/insulating states MoSe2 can be controlled, which results in the adjustable working frequency band from 28 GHz to around 38 GHz. In the meantime, a stable gain (less than 0.1 dBi variation in the whole range) is achieved for this omnidirectional antenna. Additionally, a unidirectional radiation can be reached with a metallic reflector placed at the back of the tunable antenna, and a broad beamwidth can still remain. Due to the long-term preservation of the tribo-charges, no extra bias is needed in this case and the energy can be saved to a large extent.

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Identifiers

ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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