Metal-insulator transition in a semiconductor nanocrystal network

In bulk semiconductors, the metal-insulator transition is described by the well-known Mott criterion. A recent theory proposes a more stringent criterion condition in nanocrystal (NC) networks, dependent on the electron density n and the inter-NC facet radius ρ: nρ³ ≈ 0.3. Here we use plasma-synthesized ZnO NCs coated with Al₂O₃ via ALD to study the electronic properties of a NC network as a function of n and ρ. Through a xenon-flashlamp annealing process, we selectively sinter/dope the NC network by flashing the film before/after the ALD infill, allowing independent control of ρ and n. While we observe large changes in the mobility and n, we do not see finite conductivity (σ) as T→0K. To cross the transition, prior to the Al₂O₃ ALD infill we coat the NCs with 8 cycles of ZnO ALD. This changes ρ from ~1.5 nm to ~2.8 nm and increases the packing density from 33% to 47%. By then tuning n we achieve a transition from the semiconducting to metallic state, with finite σ as T→0K. At the transition we see power law conductivity of the unusual form σ(T) ∝ T^1/5, and observe critical scaling behavior when scaling σ by the nρ³ criterion. This is the first conclusive evidence for metallic behavior in a NC network.