[Paper Review] Band bending and ratcheting explain triboelectricity in a flexoelectric contact diode

Karl Olson, Christopher A. Mizzi+1 more|arXiv (Cornell University)|Jan 12, 2022

Advanced Sensor and Energy Harvesting Materials90 references28 citations

TL;DR

This study proposes that flexoelectric band bending—driven by strain gradients at nanoscale asperity contacts—explains triboelectric charge transfer in non-metallic materials. Using conductive atomic force microscopy on Nb-doped SrTiO3, the authors quantitatively demonstrate that a model combining flexoelectric effects, Schottky barrier formation, and semiconductor band bending accurately predicts force-dependent current-voltage behavior with only two adjustable parameters, providing strong evidence that flexoelectricity is a fundamental mechanism in triboelectricity.

ABSTRACT

Triboelectricity was recognized millennia ago, but the fundamental mechanism of charge transfer is still not understood. We have recently proposed a model where flexoelectric band bending due to local asperity contacts drives triboelectric charge transfer in non-metals. While this ab-initio model is consistent with a wide range of observed phenomena, to date there have been no quantitative analyses of the proposed band bending. In this work we use a Pt$_{\mathrm{0.8}}$Ir$_{\mathrm{0.2}}$ conductive atomic force microscope probe to simultaneously deform a Nb-doped SrTiO$_{\mathrm{3}}$ sample and collect current-bias data. The current that one expects based upon an analysis including the relevant flexoelectric band-bending for a deformed semiconductor quantitively agrees with the experiments. The analysis indicates a general ratcheting mechanism for triboelectric transfer and strong experimental evidence that flexoelectric band-bending is of fundamental importance for triboelectric contacts.

Motivation & Objective

• To establish a quantitative link between flexoelectric effects and triboelectric charge transfer in non-metals.
• To test the hypothesis that flexoelectric band bending drives electron transfer during asperity contacts.
• To develop a predictive model for current-voltage behavior under mechanical loading in metal/semiconductor Schottky diodes.
• To resolve long-standing inconsistencies in triboelectricity theories by incorporating flexoelectricity, strain gradients, and semiconductor physics.
• To provide experimental validation of an ab-initio model of triboelectricity based on flexoelectricity and band bending.

Proposed method

• Conducted force-dependent current-bias measurements using a Pt0.8Ir0.2 conductive atomic force microscope (CAFM) probe on Nb-doped SrTiO3.
• Applied a Hertzian contact model to calculate strain and strain gradients at the tip-sample interface.
• Incorporated flexoelectric polarization and its contribution to band bending using ab-initio-derived flexoelectric coefficients.
• Modelled the Schottky barrier using semiconductor physics, including depletion regions and image force effects.
• Used a variational calculus approach to determine electron tunneling and transport paths through the spatially varying conduction band.
• Fitted experimental I-V curves with a diode equation incorporating flexoelectric band bending, using only two adjustable parameters: ideality factor and Schottky barrier height.

Experimental results

Research questions

• RQ1 Does flexoelectric band bending quantitatively explain the observed force-dependent current-voltage response in triboelectric contacts?
• RQ2 How do strain gradients at nanoscale asperities influence electron transfer in semiconductor-metal junctions?
• RQ3 To what extent do flexoelectric effects dominate over work function differences and Schottky barrier effects in triboelectric charge transfer?
• RQ4 What is the role of band bending and ratcheting in the net charge transfer during cyclic loading and unloading?
• RQ5 Why is the current response in sliding Schottky generators weakly dependent on the sign of the flexoelectric coefficient?

Key findings

• The model including flexoelectric band bending quantitatively reproduces the experimental I-V curves across multiple forces with only two adjustable parameters.
• The fitted Schottky barrier height was 0.85 eV and the ideality factor was 2.04 ± 0.09, consistent with semiconductor diode behavior.
• The depth of the conduction band saddle point scaled with force as z ∝ F^(1/3), confirming Hertzian contact mechanics.
• The ratcheting mechanism—where electrons transfer to the metal during unloading due to asymmetric potential wells—was directly visualized in simulations.
• The model remains robust even when the signs of flexoelectric coefficients were flipped, indicating insensitivity to coefficient sign and shear/tensile contributions.
• The analysis provides strong experimental evidence that flexoelectric band bending is a fundamental driver of triboelectric charge transfer in non-metals.