Evidence of length scale effect in contact electrification in conducting thin film heterostructures

Technical Summary: Evidence of Length Scale Effect in Contact Electrification in Conducting Thin Film Heterostructures

Problem Statement
Contact electrification (CE) between conducting materials is traditionally understood as a surface phenomenon where charge accumulation occurs until Fermi levels coincide, with the interior of bulk conductors remaining unaffected due to electrostatic screening. However, recent studies suggest that at the nanoscale, charge accumulation may diffuse into the interior of conducting materials, leading to phenomena such as electronic dynamical multiferroicity and magnetic behavior in non-magnetic silicon. The central hypothesis of this work is that CE in conducting thin films exhibits a length scale effect: as material dimensions decrease, the electrostatic screening capability of free charge carriers diminishes. Consequently, interfacial charge generated by CE is expected to diffuse deeper into the material, potentially penetrating the entire thickness of sufficiently thin films, whereas in thicker films, the diffusion is limited to a critical penetration depth near the interface.

Methodology
The authors experimentally investigated this hypothesis using flexoelectricity-mediated contact electrification in freestanding heterostructures. The experimental design involved fabricating two distinct sets of samples with varying p-Si (degenerately doped) thicknesses while keeping the permalloy (Py) layer thickness constant:

1. Thin Films (Samples 1 & 2): Pd (1 nm)/Py (25 nm)/MgO (1.8 nm)/SiO₂ (3.8 nm)/p-Si (400 nm).
2. Thicker Films (Sample 3): Pd (1 nm)/Py (25 nm)/MgO (1.8 nm)/SiO₂ (3.8 nm)/p-Si (2 µm).

The samples were made freestanding to induce buckling due to residual stresses, creating a strain gradient that mediates flexoelectricity and, subsequently, CE without requiring physical separation of the contacting surfaces. The researchers employed lock-in resistance measurements across a temperature range (400 K to 5 K) and utilized magnetoresistance (MR) and second-harmonic voltage response measurements to characterize the electrical and magnetic properties. By comparing the heterostructure responses against control samples (isolated Py and p-Si layers), the authors extracted the specific resistance contributions of the individual layers to analyze charge transfer and phase transitions.

Key Results
The study yielded distinct behaviors based on the thickness of the p-Si layer, confirming the length scale effect:

• 400 nm p-Si Layers (Samples 1 & 2):

  • Full Thickness Penetration: In these samples, the charge carrier transfer from the Py layer to the p-Si layer occurred across the entire 400 nm thickness.
  • Resistance Changes: The p-Si layer resistance decreased significantly (by ~45% in Sample 1) compared to control samples, indicating a higher charge carrier concentration. Conversely, the Py layer resistance increased, suggesting charge transfer from Py to p-Si.
  • Metal-Insulator Transition (MIT): Both samples exhibited a sharp change in resistance slope and a plateau below ~52 K, identified as a Mott MIT. This transition occurred across the full thickness of the p-Si layer, attributed to the lack of electrostatic screening at this length scale.

• 2 µm p-Si Layer (Sample 3):

  • Limited Penetration Depth: In the thicker sample, the screening effect was stronger. The charge transfer from CE was found to diffuse only to a depth of approximately 51 nm from the interface.
  • Partial MIT: A metal-insulator transition was observed at 252.5 K, but it was confined to a **21 nm layer** adjacent to the interface where the injected charge carrier density was highest. The remaining bulk of the p-Si layer remained unaffected.
  • Current Bias Dependence: Increasing the current bias (from 100 µA to 900 µA) enhanced the strain gradient via Joule heating-induced thermal mismatch, thereby increasing the flexoelectric CE response and lowering the overall sample resistance.
  • Transport Phenomena: Second-harmonic measurements revealed a transition from anisotropic magneto-thermopower (AMTP) at 300 K (indicating interlayer coupling) to a Nernst effect at 200 K (indicating decoupling due to the insulating MIT layer).

Key Contributions

1. Experimental Demonstration of Length Scale Effect: The paper provides direct experimental evidence that the depth of charge diffusion in conducting materials during contact electrification is inversely related to the material's thickness.
2. Quantification of Penetration Depth: The study quantifies the penetration depth of CE-induced charge carriers in degenerately doped p-Si, establishing a limit of ~51 nm for 2 µm thick films and full penetration for 400 nm films.
3. Mechanism Elucidation: The work clarifies that the observed phenomena are driven by flexoelectricity-mediated CE, where strain gradients in freestanding films facilitate charge transfer, leading to significant modifications in carrier concentration and phase transitions (MIT).

Significance and Claims
The authors claim that these results demonstrate a new mechanism for tailoring the physical properties of conducting materials. By exploiting the length scale effect in flexoelectricity-mediated CE, it is possible to control charge distribution and induce metal-insulator transitions in conducting thin films. The paper posits that this approach offers a novel method to manipulate material properties similar to electrostatic gating, but driven by mechanical strain and contact electrification rather than external electric fields. The findings suggest that in sufficiently thin conducting films, electrostatic screening becomes negligible, rendering the material "electrostatically transparent" to interfacial charge transfer.