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Polarization switching on the open surfaces of the wurtzite ferroelectric nitrides: ferroelectric subsystems and electrochemical reactivity

Abstract

Piezoresponse force microscopy (PFM) and spectroscopy on wurtzite ferroelectric nitrides resolve polarization switching, electromechanical response, and surface deformation during first-order reversal curves; complementary ReaxFF MD on AlN with applied electric fields and DFT adsorption energies address how surface adsorbates and hydroxyl chemistry couple to switching.

Summary

The study addresses polarization switching in binary ferroelectric nitrides (e.g., AlScN-type systems in the graphical abstract), where reversal ties to N- versus M-terminated surface polarity and electrochemical coupling. Multidimensional high-resolution PFM (including band-excitation PFM workflows) maps domains, switching, and topography under varying DC pulse magnitude and duration. ReaxFF molecular dynamics of AlN thin films under external electric fields probes water dissociation, hydroxyl chemisorption versus molecular water physisorption, and field-driven desorption; first-principles calculations (details in Supporting Information) report adsorption energies for water and hydroxyl on Al- versus N-terminated surfaces.

Methods

Experiments (PFM)

  • Platform: Automated / high-throughput PFM workflows on a band-excitation (BE) PFM platform: DC pulses of varying magnitude and duration nucleate or grow domains, followed by BE PFM imaging of amplitude and phase; topography was recorded in parallel.
  • Protocols: First-order reversal curve (FORC) measurements capture evolution of ferroelectric domains, electromechanical response, and surface deformation during bias ramps; vacuum PFM measurements are noted for part of the work (author roles in the proof).
  • Sample context: Wurtzite nitride thin films (e.g., AlScN in the graphical abstract text); film growth and basic structural/ferroelectric characterization by collaborators (see author list).

Atomistic modeling

  • ReaxFF MD: LAMMPS Molecular dynamics with the ReaxFF potential for AlN thin films at a fixed isothermal temperature in K (thermostat and K-value in SI) under applied electric fields (full ReaxFF development in Supporting Information). Polarization sign conventions: P > 0 toward the free Al-terminated surface; P < 0 toward the substrate (N-terminated). MD emphasizes Al-terminated surfaces as a low-field screening model.
  • DFT: First-principles adsorption energies for H₂O and OH on Al- and N-terminated surfaces support interpretation (values stated in the main text include approximately −1.50 eV and −0.46 eV for most favorable H₂O adsorption on Al- vs N-terminated surfaces, and about −2.43 eV for OH on the Al-terminated surface in the reported comparison).

N/A on this wiki page: molecular dynamics code (e.g. LAMMPS version), NVE/NVT/NPT label, time step (fs), ps-scale trajectory length, full slab atom list, thermostat/barostat settings, and hydrostatic pressure in GPa/bar (see SI). PBC in-plane periodic AlN thin-film cells for ReaxFF (details in SI). The study reports reactive ReaxFF MD of AlN films under applied electric field; N/A in this short summary for replica exchange-style rare-event sampling.

Findings

  • PFM: FORC measurements resolve multiple switching stages (partial switching, nested domains, back-switching, complete switching under the probe) consistent with complex domain topology (“shark-teeth” / fringing-ridge behavior) discussed in the text.
  • Subsystems: Evidence supports two weakly coupled ferroelectric subsystems and bias-induced electrochemical reactivity at surfaces.
  • ReaxFF / DFT: MD shows hydroxyl groups from water dissociation chemisorb on AlN while molecular water remains physisorbed; increasing electric field can strip hydroxyls, leaving a cleaner surface for switching. DFT adsorption energies support stronger binding on Al-terminated vs N-terminated surfaces in the reported comparisons. The Wiley author proof pdf_path may differ from the final VOR PDF; confirm field and kinetic numbers in the published issue.

Limitations

The corpus PDF is a Wiley author proof (online proofing overlay); final pagination and copyediting may differ from the version of record. Full numerical protocols appear in Supporting Information, not in the local PDF alone.

Relevance to group

Adri C. T. van Duin is credited with ReaxFF training; co-authors performed ReaxFF simulations and DFT alongside ORNL-led PFM.

Citations and evidence anchors

  • DOI: 10.1002/adma.202511001 — Advanced Materials (galley PDF in corpus).