Two-Dimensional and Subnanometer-Thin Quasi-Copper-Sulfide Semiconductor Formed upon Copper–Copper Bonding

Ultrathin
two-dimensional (2D) semiconductors exhibit outstanding
properties, but it remains challenging to obtain monolayer-structured
inorganic semiconductors naturally occurring as nonlayered crystals.
Copper sulfides are a class of widely studied nonlayered metal chalcogenide
semiconductors. Although 2D copper sulfides can provide extraordinary
physical and chemical applications, investigations of 2D copper sulfides
in the extreme quantum limit are constrained by the difficulty in
preparing monolayered copper sulfides. Here, we report a subnanometer-thin
quasi-copper-sulfide (q-CS) semiconductor formed upon self-assembly
of copper­(I)–dodecanethiol complexes. Extended X-ray absorption
fine structure analysis revealed that the existence of Cu–Cu
bonding endowed the layer-structured q-CS with semiconductor properties,
such as appreciable interband photoluminescence. Theoretical studies
on the band structure demonstrated that the optical properties of
copper–dodecanethiol assemblies were dominated by the q-CS
layer and the photoluminescence originated from exciton radiative
recombination across an indirect band gap, borne out by experimental
observation at higher temperatures, but with the onset of a direct
emission process at cryogenic temperatures. The following studies
revealed that the metal–metal bonding occurred not only in
copper–alkanethiolate complex assemblies with variable alkyl
chain length but also in silver–alkanethiolate and cadmium–alkanethiolate
assemblies. Therefore, the current studies may herald a class of 2D
semiconductors with extremely thin thickness out of nonlayered metal
sulfides to bridge the gap between conventional inorganic semiconductors
and organic semiconductors.