Why is the dielectric constant smaller in few-layer materials than in bulk materials?
I believe this is an open research topic, at least for materials such as SrTiO3 known as incipient ferroelectrics . For SrTiO3 an optical phonon mode is soft within harmonic approximation in bulk samples and is responsible for the high dielectric constant . Somehow (several mechanisms exist in literature) , this phonon mode gets hardened in thin films and as such the dielectric constant is much smaller for the thing film. I would interested to hear any insights on this topic.
See this article as an example:
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