Obtaining thermoelectric materials with high figure of merit ZT is an exacting challenge because it requires the independent control of electrical conductivity, thermal conductivity and Seebeck coefficient, which are often unfavourably coupled. Recent works have devised strategies based on nanostructuring and alloying to address this challenge in thin films, and to obtain bulk p-type alloys with ZT>1. Here, we demonstrate a new class of both p- and n-type bulk nanomaterials with room-temperature ZT as high as 1.1 using a combination of sub-atomic-per-cent doping and nanostructuring. Our nanomaterials were fabricated by bottom-up assembly of sulphur-doped pnictogen chalcogenide nanoplates sculpted by a scalable microwave-stimulated wet-chemical method. Bulk nanomaterials from single-component assemblies or nanoplate mixtures of different materials exhibit 25–250% higher ZT than their non-nanostructured bulk counterparts and state-of-the-art alloys. Adapting our synthesis and assembly approach should enable nanobulk thermoelectrics with further increases in ZT for transforming thermoelectric refrigeration and power harvesting technologies.
Figures at a glance
Figure 1: Schematic representation of the scalable synthesis used to obtain both n- and p-type bulk thermoelectric nanomaterials with high figures of merit. 
Rapid (2–10 g min−1) microwave synthesis of sulphur-doped nanoplates followed by cold-compaction and sintering yields up to 92±3% dense bulk pellets with nanostructured grains. Sulphur doping provides control over the electrical conductivity, Seebeck coefficient and majority carrier type, while nanostructuring results in very low thermal conductivity. The combined effect is applicable to multiple pnictogen chalcogenide systems and their combinations. The graph compares the best ZT of our p- and n-type nanomaterials with those of the best p- and n-bulk materials, denoted as p- or n-bulk1, 2, nanoparticle-dispersed n-bulk, referred to as n-nano33, and a p-type ball-milled alloy, denoted as p-nano6.
Figure 2: Figures of merit ZT for single-component and multicomponent bulk nanostructured pnictogen chalcogenides. 
a, Room-temperature ZT of the best pellets of each pnictogen chalcogenide system. The ZT values of the corresponding bulk material of the same stoichiometry are indicated for comparison. b, Temperature-dependent ZT of an n-type Bi2Te3, p-type Sb2Te3 and p-type Bi0.5Sb1.5Te3 sintered pellet. c,d, Seebeck coefficient (c) and room-temperature ZT (d) plotted as a function of Sb2Te3 nanoplate mole fraction in Bi2Te3–Sb2Te3 nanoplate mixtures. The dotted lines are to guide the eye. The error bars denote the experimental measurement uncertainties for each sample; the spread in data points connotes sample-to-sample variations.
Figure 3: Single-crystal hexagonal pnictogen chalcogenide nanoplates and mercaptan-mediated sulphur injection. 
a, Scanning electron micrograph showing Bi2Te3 nanoplates; more than 95% are hexagonal and less than 5% are triangular (see arrow). b, Bright-field TEM image of Bi2Te3 nanoplate. Inset: A high-resolution lattice image of the (0001) plane; the scale bar is 2 nm. c, Electron diffraction pattern from a nanoplate down the [0001] zone. The faint spots (for an example see the arrow) are kinematically forbidden reflections arising from antisite defects. d, Bright-field TEM image from a Bi2Se3 nanoplate with faceted islands (black arrows) that grow parallel to the parent plate face (white arrows). e, Edge-on bright-field image of a Bi2Te3 nanoplate showing ~1 nm steps of (0003) planes. f, Moiré fringes and rotated spot patterns due to twist boundaries between overlapping Bi2Se3 nanoplate crystals. g,h, C 1s (g) and S 2s (h) core-level bands from annealed and unannealed pnictogen chalcogenide nanoplate films. A baseline spectrum from unligated mercaptan groups is also shown for comparison.
Figure 4: Thermoelectric characterization of bulk-nanostructured pnictogen chalcogenides. 
a, Thermal conductivity κ of nanoplate pellets plotted as a function of electrical conductivity σ. The slope is the Lorenz number L and the lattice thermal conductivity κL is the ordinate intercept. The error bars denote the experimental uncertainties in the thermal conductivity measurements. b, Absolute value of the Seebeck coefficient α plotted as a function of σ. Dotted curves denote different power factors α2σ. c, Seebeck coefficient versus the sulphur content for sulphur-doped and undoped Bi2Te3; the error bars for the sulphur content are the standard deviations for 10–12 wavelength-dispersive X-ray spectroscopy measurements from 10- to 15-μm-size regions on each sample. d, Sulphur 1s core-level bands obtained using synchrotron radiation from sintered pellets of Sb2Te3 and Bi0.5Sb1.5Te3 alloys, indicating a correlation between high power factors and low-energy sulphur states.
Figure 5: Diminution of lattice thermal conductivity in bulk-nanostructured chalcogenides due to nanoscale grain size and porosity.
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