Ambient Adsorbates Obscure Sliding Ferroelectric Polarization in Bilayer Hexagonal Boron Nitride

Sliding ferroelectricity in van der Waals layered materials offers a new route to ultrathin, low-energy, nonvolatile memories, but experimental characterization of polarization remains challenging. Surface contamination during the preparation and measurement can obscure intrinsic signals. Here we use first-principles calculations to quantify how airborne adsorbates affect polarization in bilayer hexagonal boron nitride (h-BN). We find that typical contaminants induce substantial errors. Polarization can be incorrectly estimated by up to 36% (H₂O), 24% (O₂), and less than 15% (NH₃, CO₂, N₂, and CH₄). Remarkably, these large variations arise despite negligible adsorption energy differences (<2 meV per molecule) between polarized and nonpolarized surfaces, pointing to interfacial charge transfer as the dominant mechanism, rather than interlayer charge redistribution with the bilayer. Our results reveal that even weak physisorption can obscure intrinsic ferroelectric behavior, underscoring the urgent need for stringent environmental control in probing sliding ferroelectricity and guiding its integration into next-generation nanoelectronic devices.