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Two-dimensional (2D) materials with natural layer structures have been proven to provide extraordinary physical
and chemical properties. Bismuth chalcogenides are examples of such two-dimensional materials. They are strongly
bonded within layers and weak van der Waals interaction ties those layers together. Such naturally layered structure
allows chemical intercalation of foreign atoms into the van der Waals gaps. Here, we show that by adding large
number of copper atoms into van der Waals gaps of bismuth chalcogenides, we observed counter-intuitive
enhancement of optical transparency together with improved electrical conductivity, which is on the contrary to
most bulk materials in which doping reduces the light transmission. This surprising behavior is caused by substantial
tuning of material optical property and nanophotonic anti-reflection effect unique to ultra-thin nanoplates. With the
intercalation of copper atoms, large number of electrons have been added into the semiconducting material system
and effectively lift the Fermi level of the resulting material to its conduction band, as proved by our densityfunctional-
theory computations. Occupied lower states in the conduction band do not allow the optical excitation of
electrons in the valence band to the bottom of the conduction band, leading to an effective widening of optical band
gap. Optical transmission is further enhanced by constructive interference of reflected beams as bismuth
chalcogenides have large permittivity than the environment. The synergy of these two effects in two-dimensional
nanostructures can be exploited for various optoelectronic applications including transparent electrode. The
reversible intercalation process allows potential dynamic tuning capability.
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