Defects in Layered van der Waals Heterostructures: Implications for Thermoelectrics

Abstract

Layered van der Waals heterostructures provide extraordinary opportunities for applications such as thermoelectrics and allow for tunability of optical and electronic properties. The performance of devices made from these heterostructures will depend on their properties, which are sensitive to the nanoarchitecture (constituent layer thicknesses, layer sequence, etc.). However, performance will also be impacted by defects, which will vary in concentration and identity with the nanoarchitecture and preparation conditions. Here, we identify several types of defects and propose mechanisms for their formation, focusing on compounds in the ([SnSe] 1+δ ) m (TiSe 2 ) n system prepared using the modulated elemental reactants method. The defects were observed by atomic resolution high-angle annular dark-field scanning transmission electron microscopy and can be broadly categorized into those that form domain boundaries as a result of rotational disorder from the self-assembly process and those that are layer-thickness-related and result from local or global deviations in the amount of material deposited. Defect type and density were found to depend on the nanoarchitecture of the heterostructure. Categorizing the defects provides insights into defect formation in these van der Waals layered heterostructures and suggests strategies for controlling their concentrations. Strategies for controlling defect type and concentration are proposed, which would have implications for transport properties for applications in thermoelectrics.

Recent Progress of Two-Dimensional Transition Metal Dichalcogenides for Thermoelectric Applications

“…The misfit compounds show a phononglass like behavior due to the lattice mismatch between the MX and TX 2 layers. These attractive properties make (MX) 1+x (TX 2 ) m an ideal class of compounds for TE energy conversion applications [112][113][114][115]. The powder of misfit compounds is often prepared by using a solid-liquid-vapor reaction method.…”

“…To improve the TE performance in (MX) 1+x (TX 2 ) m materials, various strategies have been proposed to promote their electrical transport properties [114,[119][120][121], including improving carrier mobility, optimizing carrier concentration, and enhancing DOS distortion. Alternatively, thermal conductivity could be minimized by softening lattice or introducing planar defects (such as translational displacement and stacking faults) [110,115,122]. Among all the misfit compounds (SnS) 1.2 (TiS 2 ) 2 showed the highest ZT of 0.5 at 625 K [123].…”

Microstructural evolution of layered K‐doped RuCl3 during annealing traced by thermogravimetric analysis and 3D electron diffraction

“…For example, increased dislocation densities and decreasing grain size in thermoelectric materials may lead to reduced lattice thermal conductivity. [11,12] In semiconductors, such as GaAs, GaN, ZnTe, or elemental semiconductors, e. g., Ge doped with transition metal ions (Cr, Mn, or Fe), the formation of self-assembled quantum dots or nano-islands with the composition differing from that of the matrix is strictly correlated with the presence of high-T C ferromagnetism, and has been extensively studied for potential applications in lowcurrent semiconductor lasers, solar cells, single-photon emitters and detectors, quantum processors, and memories. [13] Spinodal decomposition is proposed for stabilizing self-assembled interfaces between topological insulators by combining layers of iso-structural TlBiX 2 (X = S, Se, Te) materials.…”

“…Moreover, creating boundaries between chemically distinct but structurally related phases may remarkably affect physical properties. For example, increased dislocation densities and decreasing grain size in thermoelectric materials may lead to reduced lattice thermal conductivity [11,12] . In semiconductors, such as GaAs, GaN, ZnTe, or elemental semiconductors, e. g., Ge doped with transition metal ions (Cr, Mn, or Fe), the formation of self‐assembled quantum dots or nano‐islands with the composition differing from that of the matrix is strictly correlated with the presence of high‐ T C ferromagnetism, and has been extensively studied for potential applications in low‐current semiconductor lasers, solar cells, single‐photon emitters and detectors, quantum processors, and memories [13] .…”