In this paper, we review recent progress on colloidal growth of 2D nanocrystals. We identify the four main sources of anisotropy which lead to the formation of plate- and sheet-like colloidal nanomaterials. Defect-induced anisotropy is a growth method which relies on the presence of topological defects at the nanoscale to induce 2D shapes objects. Such a method is particularly important in the growth of metallic nano-objects. Another way to induce anisotropy is based on ligand engineering. The availability of some nanocrystal facets can be tuned by selectively covering the surface with ligands of tunable thickness. Cadmium chalcogenides nanoplatelets (NPLs) strongly rely on this method which offers atomic control in the thinner direction, down to a few monolayers. Two-dimensional objects can also be obtained by post or in situ self-assembly of nanocrystals. This growth method differs from the previous ones in the sense that the elementary objects are not molecular precursors and is a common method for lea...

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Two-Dimensional Colloidal Nanocrystals

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60-year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up-to-date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)-model and applied to elucidating the donor-like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201600004

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

Energy formation of antisite defects in doped Sb2Te3 and Bi2Te3 crystals

Antisite defects in Sb2-xBixTe3 mixed crystals

Antimony telluride has a low thermoelectric figure of merit (ZT < ∼0.3) because of a low Seebeck coefficient α arising from high degenerate hole concentrations generated by antimony antisite defects. Here, we mitigate this key problem by suppressing antisite defect formation using subatomic percent sulfur doping. The resultant 10–25% higher α in bulk nanocrystalline antimony telluride leads to ZT ∼ 0.95 at 423 K, which is superior to the best non-nanostructured antimony telluride alloys. Density functional theory calculations indicate that sulfur increases the antisite formation activation energy and presage further improvements leading to ZT ∼ 2 through optimized doping. Our findings are promising for designing novel thermoelectric materials for refrigeration, waste heat recovery, and solar thermal applications.

Seebeck and Figure of Merit Enhancement in Nanostructured Antimony Telluride by Antisite Defect Suppression through Sulfur Doping

A single crystal of antimony telluride, grown from elements of semiconductor purity by means of the Bridgman method, is characterised by the departure from stoichiometry as determined from energy dispersive X-ray spectroscopy, by the concentration of free charge carriers p as obtained from the interpretation of the reflectivity spectra in the plasma resonance frequency range, by its density d, and by lattice parameters a and c. The investigated crystal of stoichiometry Sb 2 Te 2.948 has the following values: p = 6.714 x 10 19 cm -3 , d = (6.50 ± 0.01) g cm -3 , a = 0.42643(5) nm, c = 3.0427(4) nm. The departure from stoichiometry is explained using a model taking into account the existence of antisite Sb' Te defects as well as tellurium and antimony vacancies in the crystal lattice: this model in connection with the experimental data allows us to calculate the concentration of Sb' Te defects and to determine their formation energy E AS = (0.32 ± 0.01) eV.

R. Novotný

Papers

Non-stoichiometry of the crystal lattice of antimony telluride

Properties of Sb2Te3, Single Crystals Doped with Tl Atoms