Thermopower enhancement in lead telluride nanostructures

We demonstrate an enhancement in the thermopower of PbTe nanostructures with grain sizes on the order of $30–50\mathrm{nm}$, relative to bulk. The enhancement is similar in magnitude to that reported in the literature for $\mathrm{Pb}\mathrm{Te}∕\mathrm{Pb}{\mathrm{Se}}_x{\mathrm{Te}}_{1-x}$ quantum dot superlattices. We provide proof, based on measurements of the transverse Nernst effect, that the enhancement has its origin in electron energy filtering induced by an alteration of the scattering mechanism.

Figure 1

Thermopower versus hole concentration for p-type PbTe. The bottom solid line represents a fit to literature data on bulk samples using (Ref. 12) a hole effective mass (Ref. 13) $m_d*\approx 0.16$ and a scattering parameter $\lambda \approx 0.7$. The middle and upper solid lines correspond to the results of the model calculated by incrementing $\lambda$ by amounts of 0.5 ($\lambda =1.2$ and 1.7, respectively). The open circles represent data on our bulk samples. (▴) are thermopower data for nanogranular PbTe samples, which exhibit an enhancement similar to that observed in quantum dot PbSeTe superlattices (Ref. 6) indicated by (◆). Thermopower data on coarse-grained (+) and nanogranular (x) PbTe samples containing inclusions of EuTe are also shown. The nanogranular EuTe-containing sample exhibits an enhanced thermopower; the coarse grained sample does not.

Figure 2

Electrical resistivity (top panel) and thermopower (bottom panel) versus temperature for two nanogranular PbTe samples with hole concentrations of $2\times {10}^{17}{\mathrm{cm}}^{-3}$ (∎) and $8\times {10}^{18}{\mathrm{cm}}^{-3}$ (▴; 2 samples). Data on bulk PbTe (엯) with a hole concentration of $2\times {10}^{18}{\mathrm{cm}}^{-3}$ are shown for comparison.

Figure 3

Hall coefficient (top panel) and isothermal Nernst coefficient (bottom panel) for the same set of samples as in Fig. 2. For bulk PbTe the Nernst coefficient is negative over the entire temperature range. Relative to this bulk sample, for nanogranular samples the Nernst coefficient increases and eventually becomes positive.

Figure 4

Electron scattering parameter $\lambda$ as a function of temperature, for two nanogranular PbTe samples with hole concentrations of $2\times {10}^{17}{\mathrm{cm}}^{-3}$ (∎) and $8\times {10}^{18}{\mathrm{cm}}^{-3}$ (▴). Data on bulk PbTe samples (dashed lines) with a hole concentration of $1\times {10}^{18}{\mathrm{cm}}^{-3}$ (▿), $2\times {10}^{18}{\mathrm{cm}}^{-3}$ (엯), and $7\times {10}^{18}{\mathrm{cm}}^{-3}$ (▷) are shown for comparison.

Figure 5

Electron scattering parameter $\lambda$ at $300\mathrm{K}$ as a function of hole concentration in bulk (엯) and nanogranular (▴) PbTe. For nanogranular samples $\lambda$ is enhanced relative to bulk PbTe of the same hole concentration.