Large Enhancements of Thermopower and Carrier Mobility in Quantum Dot Engineered Bulk Semiconductors

Abstract

The thermopower (S) and electrical conductivity (σ) in conventional semiconductors are coupled adversely through the carriers' density (n) making it difficult to achieve meaningful simultaneous improvements in both electronic properties through doping and/or substitutional chemistry. Here, we demonstrate the effectiveness of coherently embedded full-Heusler (FH) quantum dots (QDs) in tailoring the density, mobility, and effective mass of charge carriers in the n-type Ti(0.1)Zr(0.9)NiSn half-Heusler matrix. We propose that the embedded FH QD forms a potential barrier at the interface with the matrix due to the offset of their conduction band minima. This potential barrier discriminates existing charge carriers from the conduction band of the matrix with respect to their relative energy leading to simultaneous large enhancements of the thermopower (up to 200%) and carrier mobility (up to 43%) of the resulting Ti(0.1)Zr(0.9)Ni(1+x)Sn nanocomposites. The improvement in S with increasing mole fraction of the FH-QDs arises from a drastic reduction (up to 250%) in the effective carrier density coupled with an increase in the carrier's effective mass (m*), whereas the surprising enhancement in the mobility (μ) is attributed to an increase in the carrier's relaxation time (τ). This strategy to manipulate the transport behavior of existing ensembles of charge carriers within a bulk semiconductor using QDs is very promising and could pave the way to a new generation of high figure of merit thermoelectric materials.

Phase separation of full-Heusler nanostructures in half-Heusler thermoelectrics and vibrational properties from first-principles calculations

“…If precipitation occurs at the nano-scale, the FH phase may be difficult to observe experimentally by X-ray diffraction (XRD) techniques. 13 However, the authors of Ref. 13 argue that the measured HH bulk lattice parameter remains constant for all compositions with x < 3%, suggesting two-phase coexistence.…”

“…13 However, the authors of Ref. 13 argue that the measured HH bulk lattice parameter remains constant for all compositions with x < 3%, suggesting two-phase coexistence.…”

“…The excess Ni creates FH nano-structures that make coherent or semi-coherent boundaries with the HH matrix. [12][13][14] Coherent nano-structures are thought to be less detrimental to the electronic transport and thus are beneficial for TE materials. 15 Makongo and coworkers 12 found that adding 2 to 3% excess Ni created semi-coherent FH nanostructures which simultaneously increased the Seebeck coefficient and the electrical conductivity while slightly decreased the lattice thermal conductivity.…”

On the tuning of electrical and thermal transport in thermoelectrics: an integrated theory–experiment perspective

“…The beneficial effect of energy barriers can be found in the systems having a semiconducting host matrix and metallic composites or nano-inclusions, such as the pristine PbTe matrix containing Ag-doped PbTe nanoparticles, 57 semimetallic ErAs particles in InGaAs/InGaAlAs superlattices, 58,59 and half-Heusler compounds with full-Heusler metallic inclusions (Figure 3a). 60,61 Another possible beneficial scheme relating to the interface between matrix and nanostructure is the modulation doping and δ-doping techniques. In principle, it is different from the barrier scattering mentioned above since the dopants are only incorporated into certain areas, e.g., spatially separated nanograins, leading to reduced electron scattering and thus higher mobility.…”

Enhanced power factor and reduced thermal conductivity of a half-Heusler derivative Ti9Ni7Sn8: A bulk nanocomposite thermoelectric material

“…23 In the recent years, several strategies such as doping, 9,15 solid solution alloying, [12][13][14] and nanostructuring 10,16 in HH compound have been adopted to disrupt heat carrying phonons to significantly reduce their j. Recently, full-Heusler (FH) inclusions within the p and n type HH compounds have been produced by several groups [24][25][26][27][28][29][30] by adding excess Coconcentration in p-type MCoSb (where M ¼ Ti, Zr, Hf) and Ni-concentration in n-type MNiSn (where M ¼ Ti, Zr, Hf). A significant decrease in thermal conductivities of these materials was noted.…”