  There is currently substantial effort being invested into creating efficient
thermoelectric nanowires based on topological insulator chalcogenide-type
materials. A key premise of these efforts is the assumption that the generally
good thermoelectric properties that these materials exhibit in bulk form will
translate into similarly good or even better thermoelectric performance of the
same materials in nanowire form. Here, we calculate thermoelectric performance
of topological insulator nanowires based on Bi2Te3, Sb2Te3 and Bi2Se3 as a
function of diameter and Fermi level. We show that the thermoelectric
performance of topological insulator nanowires does not derive from the
properties of the bulk material in a straightforward way. For all investigated
systems the competition between surface states and bulk channel causes a
significant modification of the thermoelectric transport coefficients if the
diameter is reduced into the sub-10 um range. Key aspects are that the surface
and bulk states are optimized at different Fermi levels or have different
polarity as well as the high surface to volume ratio of the nanowires. This
limits the maximum thermoelectric performance of topological insulator
nanowires and thus their application in efficient thermoelectric devices.
