Multiscale tribomechanical analysis of wear mechanisms of monocrystalline silicon probe on functional KDP laser optics during ink-free DPN processes

Ink-free dip-pen nanolithography (DPN) enables surface modification via chemo-mechanical interactions to achieve nano-defect control in KDP (KH₂PO₄) optics. However, probe wear mechanisms on soft-brittle KDP substrates remain unexplored. This study investigated underlying mechanisms through multi-scale experiments and simulations under low-humidity conditions. Post-cleaning characterization revealed removable KDP debris adhesion and direct evidence of environmental oxidative corrosion on probe surfaces. During penetration stage, soft KDP crystal enabled gradual probe penetration under increasing loads without irreversible probe damage. Stress relaxation fully restored initial cubic diamond phase, where hydrostatic stress governed structural evolution. Etching stage identified two critical force thresholds (50 nN and 75 nN) for a 71.3 Å tip diameter probe: the lower threshold prevented wear at efficiency’s expense, while the higher balanced efficiency with minimal edge damage. Excessive loads (over 200 nN) caused brittle fracture via shear stress concentration. Tip diameter increased through fracture-induced blunting and progressive debris accumulation during reciprocating etching, which also imposed cyclic stresses leading to cumulative fatigue damage, irreversible material degradation, and crystalline-to-amorphous transition. Charge transfer analysis demonstrated redox reactions between silicon in probe and oxygen from KDP substate, forming metastable Si-O bonds (1.76 Å length, 0.32 population) with high dissociation tendency. This study advanced fundamental understanding of AFM probe wear mechanisms in ink-free DPN processes.