  Atomic force microscope (AFM) generally works on the basis of manipulating
absolute magnitude of van der Waals (vdW) force between the tip and specimen.
The force is, however, less sensitive to alternation of atom species than to
tip-sample separations, resulting in the difficulty of compositional
identification, even under multi-modal strategies and other AFM variations.
Here, we report a phenomenon of light enhancement of van der Waals force (LvF),
and the enhancement factor is found specific to materials. The force difference
prior and after illumination, instead of the tip-specimen force itself, is
employed for discriminating heterogeneous phases. The corresponding LvF
microscopy (LvFM) demonstrates not only a ultra-high compositional resolution
represented by 20 dB enhancement factor and 150 times of the detection limit,
but also a sub-10 nm lateral spatial resolution much smaller than the tip size
of 20 nm. The simplicity of the opto-thermal mechanism, minuteness of
excitation light power and wide availability of boosting lasers at various
wavelengths imply broad applications of LvFM on nano-materials
characterization, particularly on two-dimensional semiconductors that are
promising as new generation of chip materials.
