Lateral force microscopy reveals subsurface atomic vacancies in MoS₂, overturning a long-held assumption
By Marie, science journalist for 1ban.news
For decades, lateral force microscopy (LFM), a scanning probe technique that measures frictional forces at the nanoscale, was thought to be sensitive only to the very topmost atomic layer. Subsurface features, it was assumed, were invisible to the tip sliding across the surface.
A team led by Cristina Gómez-Navarro at the Universidad Autónoma de Madrid and J. G. Vilhena at the Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) has proven that assumption wrong. In a study published July 9 in Nature Communications, they demonstrate that LFM can not only detect atomic vacancies beneath the surface of molybdenum disulfide (MoS₂), but can also classify them by depth based on distinctive frictional fingerprints.
Two signatures, two depths
The researchers used LFM in contact mode on monolayer and few-layer MoS₂, a transition metal dichalcogenide widely studied for its electronic and optical properties. As the sharp tip scans across the surface, torsional deflection of the cantilever records the lateral (frictional) force at each point.
When the tip encountered a surface vacancy, a missing sulfur atom in the topmost layer, the frictional signal showed a characteristic pattern the team calls “drop-and-rise”: the lateral force drops suddenly as the tip dips into the missing-atom site, then rises sharply as it climbs out.
When the tip passed over a subsurface vacancy, a missing atom one layer below the surface, the signature was entirely different. There was no entry drop (the surface layer is intact, so the tip does not fall into a hole), but a pronounced frictional peak appeared at the exit side. This exit barrier, the team explains, arises because the missing atom below creates a local lattice distortion that “pinches” the tip asymmetrically as it moves across.
Verified by simulation
The experimental observations were confirmed by molecular dynamics simulations and quantitatively captured by a Prandtl-Tomlinson model, a classic framework for nanoscale friction. Critically, the simulations revealed that the mechanism is geometric rather than chemical: the lattice disruption caused by the missing atom affects the tip’s trajectory regardless of the specific material.
This means the technique should generalize across the entire family of layered materials, graphene, hexagonal boron nitride, other transition metal dichalcogenides, and beyond.
A practical tool for 2D material quality control
The finding has immediate practical implications. Atomic vacancies are among the most common defects in two-dimensional materials, and their presence, and depth, directly affects electronic, optical, and mechanical properties. Until now, identifying subsurface vacancies required transmission electron microscopy (which can damage the sample) or scanning tunnelling microscopy (which requires vacuum and conductive samples). LFM offers a non-destructive, high-resolution, and relatively high-throughput alternative that works in ambient conditions.
The team demonstrated the method’s utility by comparing defect density and type between MoS₂ grown by chemical vapour deposition (CVD) and mechanically exfoliated samples. CVD-grown films showed higher overall vacancy density but a different surface-to-subsurface ratio, information that could feed back into synthesis optimisation.
Broader implications
The work also reshapes how researchers should interpret existing LFM data. If subsurface features, not just surface topography, influence friction on MoS₂, the same may be true for other materials where nanoscale friction has been measured. Some frictional signals previously attributed to surface chemistry may, in retrospect, have been caused by hidden subsurface defects.
For the burgeoning field of 2D material devices, the ability to non-destructively map subsurface vacancies at scale could become a standard metrology step, one that requires no specialised instrumentation beyond the atomic force microscopes already common in nanoscience laboratories.
The work was supported by the Spanish Ministry of Science, the ERC Starting Grant HeaT2Defects, the Comunidad de Madrid, and the Polish National Science Center, among others.
Sources:
1. Gutiérrez-Varela, O., Zambudio, A., Ares, P. et al. “Unveiling surface and subsurface atomic vacancies in MoS₂ with lateral force microscopy.” Nature Communications (2026). DOI: 10.1038/s41467-026-75151-0
2. ICMM-CSIC / UAM / IFIMAC press materials, July 2026.