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Approaching a Two-Dimensional (2D) Metallic State on the Surface of the Organic Semiconductor Rubrene

Whether metallic behavior
can exist in 2D materials is a question that has troubled condensed matter
physics for decades. Although originally thought impossible, evidence for such
in ultra-clean high-purity doped inorganic semiconductor heterostructures based
on materials such as Si and GaAs eventually changed the prevailing view.
Research performed in IRG-1 using an approach to doping known as electrolyte
gating has now shown that highly conductive (close to metallic) behavior can
also be seen in 2D in an organic semiconductor, rubrene. This was
enabled by techniques, based on the use of ionic liquids, that increase the
density of holes on the surface by a thousand times over prior work. The
mobility of the holes in rubrene remains far lower than inorganic semiconductors,
however, raising perplexing questions about the fundamental origin of the
conductive state.