  The key feature of a thermophotovoltaic (TPV) emitter is the enhancement of
thermal emission corresponding to energies just above the bandgap of the
absorbing photovoltaic cell and simultaneous suppression of thermal emission
below the bandgap. We show here that a single layer plasmonic coating can
perform this task with high efficiency. Our key design principle involves
tuning the epsilon-near-zero frequency (plasma frequency) of the metal acting
as a thermal emitter to the electronic bandgap of the semiconducting cell. This
approach utilizes the change in reflectivity of a metal near its plasma
frequency (epsilon-near-zero frequency) to lead to spectrally selective thermal
emission and can be adapted to large area coatings using high temperature
plasmonic materials. We provide a detailed analysis of the spectral and angular
performance of high temperature plasmonic coatings as TPV emitters. We show the
potential of such high temperature plasmonic thermal emitter coatings (p-TECs)
for narrowband near-field thermal emission. We also show the enhancement of
near-surface energy density in graphene-multilayer thermal metamaterials due to
a topological transition at an effective epsilon-near-zero frequency. This
opens up spectrally selective thermal emission from graphene multilayers in the
infrared frequency regime. Our design paves the way for the development of
single layer p-TECs and graphene multilayers for spectrally selective radiative
heat transfer applications.
