Nanocrystal
(NC) solids are commonly prepared from nonpolar organic
NC suspensions. In many cases, the capping on the NC surface is preserved
and forms a barrier between the NCs. More recently, superstructures
with crystalline connections between the NCs, implying the removal
of the capping, have been reported, too. Here, we present large-scale
uniform superstructures of attached PbSe NCs with a silicene-type
honeycomb geometry, resulting from solvent evaporation under nearly
reversible conditions. We also prepared multilayered silicene honeycomb
structures by using larger amounts of PbSe NCs. We show that the two-dimensional
silicene superstructures can be seen as a crystallographic slice from
a 3-D simple cubic structure. We describe the disorder in the silicene
lattices in terms of the nanocrystals position and their atomic alignment.
The silicene honeycomb sheets are large enough to be used in transistors
and optoelectronic devices.
It
has been shown recently that atomically coherent superstructures of
a nanocrystal monolayer in thickness can be prepared by self-assembly
of monodisperse PbSe nanocrystals, followed by oriented attachment.
Superstructures with a honeycomb nanogeometry are of special interest,
as theory has shown that they are regular 2-D semiconductors, but
with the highest valence and lowest conduction bands being Dirac-type,
that is, with a linear energy-momentum relation around the K-points
in the zone. Experimental validation will require cryogenic measurements
on single sheets of these nanocrystal monolayer superstructures. Here,
we show that we can incorporate these fragile superstructures into
a transistor device with electrolyte gating, control the electron
density, and measure the electron transport characteristics at room
temperature. The electron mobility is 1.5 ± 0.5 cm2 V–1 s–1, similar to the mobility
observed with terahertz spectroscopy on freestanding superstructures.
The terahertz spectroscopic data point to pronounced carrier scattering
on crystallographic imperfections in the superstructure, explaining
the limited mobility.
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