Surface dipole chaos: Mixed SAM-engineered Bi2S3 for unclonable hardware security

Abstract

Physically unclonable functions (PUFs) provide device-level randomness for secure hardware applications. Here we report a Bi2S3 thin-film PUF based on mixed self-assembled monolayer (SAM) doping. Phenyltrichlorosilane (PTS) and octadecyltrichlorosilane (ODTS) were co-deposited to introduce interfacial dipole variations on the bismuth sulfide (Bi2S3) surface. The resulting polarity disorder modulates charge injection barriers and generates random conductivity across two-terminal devices. Structural and surface analyses, including XPS, contact angle, and Kelvin probe force microscopy, offer the coexistence of distinct SAM dipoles and their effect on work function distribution. The mixed SAM-doped PUF exhibits uniformity (∼51.5%), inter-Hamming distance (∼43.42%), and entropy (∼0.94). Using these random responses, we demonstrate pixel-level image encryption that can only be decrypted with the same device. This approach highlights interfacial dipole engineering as an effective route to stable and unclonable hardware security.