  A method for subsurface visualization and characterization of hidden
subsurface nano-structures based on Scanning Tuneling Microscopy/Spectroscopy
(STM/STS) has been developed. The nano-objects buried under a metal surface up
to several tens of nanometers can be visualized through the metal surface and
characterized with STM without destroying the sample. This non-destructive
method exploits quantum well (QW) states formed by partial electron confinement
between the surface and buried nano-objects. The specificity of STM allows for
nano-objects to be singled out and easily accessed. Then, their shape, size and
burial depth can be determined by analyzing the spatial distribution and
oscillatory behavior of the electron density at the surface of the sample. The
proof of concept was demonstrated by fabricating argon nanoclusters embedded
into a single-crystalline Cu matrix. Taking advantage of the specific
electronic band structure Cu and inner electron focusing, we experimentally
demonstrated that noble-gas nanoclusters of several nanometers large buried as
deep as 80 nm can be detected, characterized and imaged. The ultime depth of
this ability is estimated as 110 nm. This approach using QW states paves the
way for an enhanced 3D characterization of nanostructures hidden well below a
metallic surface.
