  The excitonic insulator is an elusive electronic phase exhibiting a
correlated excitonic ground state. Materials with such a phase are expected to
have intriguing properties such as excitonic high-temperature
superconductivity. However, compelling evidence on the experimental realization
is still missing. Here, we theoretically propose hybrids of two-dimensional
semiconductors functionalized by organic molecules as prototypes of excitonic
insulators, with the exemplary candidate WS$_2$-F6TCNNQ. This material system
exhibits an excitonic insulating phase at room temperature with a ground state
formed by a condensate of interlayer excitons. To address an experimentally
relevant situation, we calculate the corresponding phase diagram for the
important parameters: temperature, gap energy, and dielectric environment.
Further, to guide future experimental detection, we show how to optically
characterize the different electronic phases via far-infrared to terahertz
(THz) spectroscopy.
