How thin can a material be made? Down to one layer of atoms seems crazy but this is the answer. The intuitive ways we have of understanding a crystal, which usually assume an infinite structure of periodic repeating units, start to break down in this limit. This means that usually an ultrathin film cannot be considered as just a slice of a large "bulk-like" piece of material and a bulk-like material cannot be considered as a stack of ultrathin films. The properties of the two can be completely different.
Part of the work I did for my PhD thesis in Professor J.-M. Triscone's group at the University of Geneva was to address this question. We grew thin films of a metallic oxide, lanthanum nickelate, and studied their electronic and vibrational properties.
In our 2017 Advanced Materials paper we showed that the conductivity is modulated with the thickness, reaching a maximum at around 3 nm. Combining both computational and experimental studied of the crystal structure we proposed that this behaviour was coming from a differentiated local structure where the surface and the interface with the substrate play increasingly dominant roles as the thickness is reduced.
In our 2020 APL Materials paper we pushed the limits of what is possible by Raman spectroscopy, a technique that provides information on the vibrational properties of a material, by acquiring spectra on oxide films almost as thin as 1 nm. We discovered that the vibrational spectra are significantly altered when the films get so thin, behaviour that is reminiscent of the vibrational properties of surfaces. We therefore believe that our ultrathin films must be considered as hybrid entities of the bulk-like and surface properties.