  The ability to actively regulate heat flow at the nanoscale could be a game
changer for applications in thermal management and energy harvesting. Such a
breakthrough could also enable the control of heat flow using thermal circuits,
in a manner analogous to electronic circuits. Here we demonstrate switchable
thermal transistors with an order of magnitude thermal on/off ratio, based on
reversible electrochemical lithium intercalation in MoS2 thin films. We use
spatially-resolved time-domain thermoreflectance to map the lithium ion
distribution during device operation, and atomic force microscopy to show that
the lithiated state correlates with increased thickness and surface roughness.
First principles calculations reveal that the thermal conductance modulation is
due to phonon scattering by lithium rattler modes, c-axis strain, and stacking
disorder. This study lays the foundation for electrochemically-driven nanoscale
thermal regulators, and establishes thermal metrology as a useful probe of
spatio-temporal intercalant dynamics in nanomaterials.
