Sequential Organic−Inorganic Templating and Thermoelectric Properties of High-Aspect-Ratio Single-Crystal Lead Telluride Nanorods

Abstract

Chalcogenides such as bismuth telluride and lead telluride exhibit high thermoelectric figures of merit ZT 1-3 in the bulk because of a combination of high electrical conductivity σ and Seebeck coefficient S, and low thermal conductivity κ, making them attractive for refrigeration and generating electrical power from heat. 4-6 Nanostructuring these materials along one dimension (e.g., nanolayers) has been shown to increase ZT because of size and quantum effects on S, σ, and κ. 7,8 Greater degrees of confinement (e.g., PbTe nanowires with characteristic dimensions <5 nm) 9 are expected to provide higher ZT increases and open up entirely new types of possibilities for novel applications such as cooling nanoscale hotspots in nanodevice interconnections comprising nanowires or nanotubes and creating nanoscale power generators.Growing one-dimensional nanocrystals of chalcogenides with a cubic crystal structure (e.g., PbTe, rocksalt structure) is a challenge because of the necessity of a symmetrybreaking pathway for enabling anisotropic growth. 10 Solvothermal decomposition or polyol reduction yield spherical 11 or cube-shaped 12,13 nanoparticles. Incorporating surfactants provides particle size control, but is ineffective in inducing shape anisotropy because of the high crystallographic degeneracy in cubic crystals. Alternative methods based on oriented attachment of nanoparticles in solution, 14-16 catalystmediated vapor-liquid-solid reactions, 17,18 and inorganic templating have been devised. 19,20 However, nanowire polycrystallinity and template removal issues limit the utility of these routes for fundamental studies and for reaping the maximal benefit of the properties of single crystals for applications. Recently devised sonochemical and autoclavebased techniques, 21-23 e.g., <10 nm diameter nanorods 12 with aspect ratios j7, and formation of single-crystal nanowires through nanoparticle coalescence and ripening, however, are promising approaches. 31 Here, we report a completely new two-step organicinorganic templating method to obtain single-crystal PbTe nanorods with aspect ratios up to 27, at moderate temperatures, without using an autoclave. We exploit anisotropic nanotube growth through organic surfactant templating and transform the nanotubes into single-crystal PbTe nanorods via reaction with a lead salt obviating any subsequent template removal. Nanorod to nanotube conversion has been reported, 25 but the reverse process reported here is new and can be applied to producing anisotropic nanostructures of materials with a cubic structure.In the first step, Te nanotubes were synthesized by solvothermal reduction of TeO 2 in the presence of cetyl ether,

Synthesis and growth mechanism of rough PbTe polycrystalline thermoelectric nanorods

“…The method is superior in simplicity, rapidity, and high yield. Different from the previously reported single-crystalline PbTe nanowires and nanorods [19][20][21], the morphology analyses reveal that the nanorods with rough surfaces are composed of PbTe nanoparticles, whose sizes range from 30 to 50 nm. Such rough polycrystalline nanorods are promising for high-efficiency TE micro-devices.…”

“…Harman et al [8] have fabricated PbTe/PbSe quantum dot superlattices and estimated a ZT value of $2.0 at 300 K. The Seebeck coefficient enhancement in nanostructured PbTe was also found when the grain sizes were comparable with the average excitonic Bohr radius (about 46 nm) [12,13]. Many of the previous studies on nanostructured PbTe were mainly focused on the controlled synthesis of different morphologies, including PbTe nanoparticles [14,15], nanospheres [16], nanotube [16][17][18], nanowires [19,20], nanorods [21], nanoboxes [22], and hierarchical dendrites [23]. Among them, 1D PbTe nanostructures are very promising for highefficiency TE micro-devices.…”

Physical properties of elongated inorganic nanoparticles

“…Films of cross-linked Bi 2 S 3 nanorods have been fabricated on Si substrates and showed a high Seebeck coefficient of −755 µV/K [240]. High aspect ratio PbTe nanorods were assembled into thin films onto glass substrates prepatterned with Au electrodes by dip coating, and a subsequent hydrazine treatment was applied in order to enhance the film conductivity [241]. Such films showed a band gap of 0.27 eV, and a Seebeck coefficient of 263 µV/K, which is comparable to bulk PbTe.…”

Synthesis and characterisation of PbTe thermoelectric nanopowders via a gas induced reduction route

“…Purkayastha et al 15 demonstrated a new two-step organic-inorganic templating route to synthesise high aspect ratio PbTe nanorods from Te nanotubes and pointed out that the aspect ratio of the nanorod could be tailored by changing reaction temperature and cetylether concentration. Wan et al 16 prepared PbTe nanorods via the composite hydroxide mediated.…”