The
application of carbon-based nanomaterials in the field of thermoelectric
is enormously restricted for its high thermal conductivity. Graphyne,
as the allotrope of graphene, expresses low thermal conductivity;
therefore, it can be expected to break the limitation of carbon-based
materials in thermoelectric field. By using the first-principles calculation
combined with the Boltzmann theory, the thermoelectric properties
of types of graphynes, i.e., 6,6,6-, 6,6,12-, 14,14,18-, and 18,18,18-graphyne,
are systematically investigated. It is interesting to find that 14,14,18-graphyne
presents excellent thermoelectric properties, whose maximum value
of ZT can reach 1.6 at room temperature; therefore, it can be seen
as an excellent carbon-based thermoelectric material. More importantly,
its ZT at low temperature is abnormally higher than that at high temperature,
which breaks the traditional view that the higher ZT exists in a high
temperature range. Furthermore, it can also be noticed that the other
three types of graphynes exhibit unexpected special thermoelectric
properties: (1) 6,6,6-graphyne shows the highest Seebeck coefficient;
however, its ZT value is the lowest. Moreover, its ZT value for n-type
doping is twice as that for p-type doping. (2) The ZT of 6,6,12-graphyne
expresses enormous anisotropy, whose value along the zigzag direction
is even 1–2 orders of magnitude higher than that along the
armchair direction. (3) The ZT of 18,18,18-graphyne shows completely
opposite anisotropy compared with the other three types of graphynes,
and its ZT value along the armchair direction is higher. Our research
provides some meaningful results for enhancing the thermoelectric
performance of carbon-based nanomaterials, especially for the development
of high-performance thermoelectric materials at room temperature.