Nanotechnology

A flexible feature for the long-reigning thermoelectric champion bismuth telluride


Considering the IoT applications such as wearable sensors, making a conformable flexible thermoelectric generator (f-TEG) is obviously advantageous, which is why much research has been performed in recent years on flexible materials such as polymers7,8. However, despite advantages such as flexibility and cost-efficiency, the polymers developed up to now have lagged behind their inorganic counterparts in terms of thermoelectric performance. On the other hand, despite being the only thermoelectric material to be widely applied, albeit so far solely as Peltier coolers, bismuth telluride is particularly famous for being rather brittle. Lu et al. demonstrate that centimetre-scale flexible films of both p- and n- type bismuth telluride can be obtained by peeling them from single crystals. Bismuth telluride has a layered crystal structure with weak van der Waals bonding between tellurium terminated layers, so such exfoliation in itself is not so surprising. What Lu et al. find is that exfoliated films form staggered atomic layers — encompassing slip and exchange of atoms therein — that can absorb bending stress, leading to excellent flexibility and high durability. They also observe that these staggered layers act as dopants, which are not detrimental to the thermoelectric properties, as supported by band structure calculations. As a result, flexible films with comparative thermoelectric performance to single crystals of Bi2Te3 can be obtained. Namely, exhibiting power factors of 4.2 mW m−1 K−2 and 4.6 mW m−1 K−2 for p- and n- type films, respectively.