  In bulk semiconductor materials, the insulator-metal transition (IMT) is
governed by the concentration of conduction electrons. Meanwhile, even when
fabricated from metallic building blocks, nanocrystal films are often
insulating with inter-nanocrystal contacts acting as electron transport
bottlenecks. Using a library of transparent conducting tin-doped indium oxide
nanocrystal films with varied electron concentration, size, and contact area,
we test candidate criteria for the IMT and establish a phase diagram for
electron transport behavior. From variable temperature conductivity
measurements, we learn that both the IMT and a subsequent crossover to
conventional metallic behavior near room temperature are governed by the
conductance of the inter-nanocrystal contacts. To cross the IMT,
inter-nanocrystal coupling must be sufficient to overcome the charging energy
of a nanocrystal, while conventional metallic behavior requires contact
conductance to reach the conductance of a nanocrystal. This understanding can
enable the design and fabrication of metallic conducting materials from
nanocrystal building blocks.
