  Rock-salt lead selenide nanocrystals can be used as building blocks for large
scale square superlattices via two-dimensional assembly of nanocrystals at a
liquid-air interface followed by oriented attachment. Here we report
measurements of the local density of states of an atomically coherent
superlattice with square geometry made from PbSe nanocrystals. Controlled
annealing of the sample permits the imaging of a clean structure and to
reproducibly probe the band gap and the valence hole and conduction electron
states. The measured band gap and peak positions are compared to the results of
optical spectroscopy and atomistic tight-binding calculations of the square
superlattice band structure. In spite of the crystalline connections between
nanocrystals that induce significant electronic couplings, the electronic
structure of the superlattices remains very strongly influenced by the effects
of disorder and variability.
