Electronic structure of the misfit-layer compound (SnS)1.17NbS2 deduced from band-structure calculations and photoelectron spectra

Giant Third-Order Nonlinear Hall Effect in Misfit Layer Compound (SnS)1.17(NbS2)3

The nonlinear Hall effect (NLHE) holds immense significance in recognizing the band geometry and its potential applications in current rectification. Recent discoveries have expanded the study from second-order to third-order nonlinear Hall effect (THE), which is governed by an intrinsic band geometric quantity called the Berry Connection Polarizability tensor. Here we demonstrate a giant THE in a misfit layer compound, (SnS)1.17(NbS2)3. While the THE is prohibited in individual NbS2 and SnS due to the constraints imposed by the crystal symmetry and their band structures, a remarkable THE emerges when a superlattice is formed by introducing a monolayer of SnS. The angular-dependent THE and its scaling relationship indicate that the phenomenon could be correlated to the band geometry modulation, concurrently with the symmetry breaking. The resulting strength of THE is orders of magnitude higher compared to recent studies. Our work illuminates the modulation of structural and electronic geometries for novel quantum phenomena through interface engineering.

Monolayer Behavior of NbS2 in Natural van der Waals Heterostructures

Monolayer transition metal dichalcogenides (TMDs) constitute an important family of materials with many intriguing properties and applications. The ability to achieve large-size and high-quality monolayer TMDs is the key prerequisite toward a deep understanding and practical application of TMDs in electronics and optoelectronics. Here, on the basis of high-resolution angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we find a monolayer NbS₂-dominated Fermi-level feature in a misfit compound, which is a type of natural heterostructures. Considering the infrequency of the synthesis approach and electronic properties of the NbS₂ monolayer, our results cannot only provide direct insight into the electronic structure of van der Waals heterostructures (VDWHs) but also shed light on the way toward rationally investigating  the monolayer TMDs, which are hardly obtained and studied.

Charge transfer in (PbSe)1+δ (NbSe2)2 and (SnSe)1+δ (NbSe2)2 ferecrystals investigated by photoelectron spectroscopy

Rotationally disordered, layered (PbSe)[Formula: see text](NbSe₂)₂ and (SnSe)[Formula: see text](NbSe₂)₂ ferecrystal heterostructures, consisting of stacked two-dimensional bilayers of either PbSe or SnSe alternating with two planes of NbSe₂, were synthesized from modulated elemental reactants. The electronic structure of these ternary systems was investigated using x-ray photoelectron spectroscopy and compared to the binary bulk compounds PbSe, SnSe and NbSe₂. The Pb and Sn core level spectra show a significant shift towards lower binding energies and the peak shape becomes asymmetric in the ferecrystals, while the electronic structure of the NbSe₂ layers does not change compared to the bulk. This is interpreted in terms of an interlayer interaction in the form of a charge transfer of electrons from PbSe or SnSe into the NbSe₂ layers, which is supported by valence band spectra and is consistent with prior results from transport measurements.

Misfit Layer Compounds and Ferecrystals: Model Systems for Thermoelectric Nanocomposites

A basic summary of thermoelectric principles is presented in a historical context, following the evolution of the field from initial discovery to modern day high-zT materials. A specific focus is placed on nanocomposite materials as a means to solve the challenges presented by the contradictory material requirements necessary for efficient thermal energy harvest. Misfit layer compounds are highlighted as an example of a highly ordered anisotropic nanocomposite system. Their layered structure provides the opportunity to use multiple constituents for improved thermoelectric performance, through both enhanced phonon scattering at interfaces and through electronic interactions between the constituents. Recently, a class of metastable, turbostratically-disordered misfit layer compounds has been synthesized using a kinetically controlled approach with low reaction temperatures. The kinetically stabilized structures can be prepared with a variety of constituent ratios and layering schemes, providing an avenue to systematically understand structure-function relationships not possible in the thermodynamic compounds. We summarize the work that has been done to date on these materials. The observed turbostratic disorder has been shown to result in extremely low cross plane thermal conductivity and in plane thermal conductivities that are also very small, suggesting the structural motif could be attractive as thermoelectric materials if the power factor could be improved. The first 10 compounds in the [(PbSe)_1+δ]_m(TiSe₂)_n family (m, n ≤ 3) are reported as a case study. As n increases, the magnitude of the Seebeck coefficient is significantly increased without a simultaneous decrease in the in-plane electrical conductivity, resulting in an improved thermoelectric power factor.

Hierarchical Architecturing for Layered Thermoelectric Sulfides and Chalcogenides

Sulfides are promising candidates for environment-friendly and cost-effective thermoelectric materials. In this article, we review the recent progress in all-length-scale hierarchical architecturing for sulfides and chalcogenides, highlighting the key strategies used to enhance their thermoelectric performance. We primarily focus on TiS₂-based layered sulfides, misfit layered sulfides, homologous chalcogenides, accordion-like layered Sn chalcogenides, and thermoelectric minerals. CS₂ sulfurization is an appropriate method for preparing sulfide thermoelectric materials. At the atomic scale, the intercalation of guest atoms/layers into host crystal layers, crystal-structural evolution enabled by the homologous series, and low-energy atomic vibration effectively scatter phonons, resulting in a reduced lattice thermal conductivity. At the nanoscale, stacking faults further reduce the lattice thermal conductivity. At the microscale, the highly oriented microtexture allows high carrier mobility in the in-plane direction, leading to a high thermoelectric power factor.

Two-dimensional and tubular structures of misfit compounds: Structural and electronic properties

Misfit layer compounds are structures that consist of two sublattices differing in at least one of their lattice constants. The two different layers are stacked either an alternating or in a more complex series resulting in mono- or multi-layer misfit compounds. To date, planar and bent misfit structures, such as tubes, scrolls or nanoparticles, have been synthesized and interesting magnetic and physical properties have been observed as a result of their special structures. Based on these observations, we present an overview of such misfit systems and summarize and discuss their electronic structure as well as the interlayer bonding behaviour, which is not completely understood yet. Furthermore, a more detailed insight into the SnS-SnS2 system is given, which was the first tubular misfit compound that has been synthesized and extensively investigated.

Nanotubes from Misfit Layered Compounds: A New Family of Materials with Low Dimensionality

Nanotubes that are formed from layered materials have emerged to be exciting one-dimensional materials in the last two decades due to their remarkable structures and properties. Misfit layered compounds (MLC) can be produced from alternating assemblies of two different molecular slabs with different periodicities with the general formula [(MX)1+x]m[TX2]n (or more simply MS-TS2), where M is Sn, Pb, Bi, Sb, rare earths, T is Sn, Nb, Ta, Ti, V, Cr, and so on, and X is S, Se. The presence of misfit stresses between adjacent layers in MLC provides a driving force for curling of the layers that acts in addition to the elimination of dangling bonds. The combination of these two independent forces leads to the synthesis of misfit layered nanotubes, which are newcomers to the broad field of one-dimensional nanostructures and nanotubes. The synthesis, characterization, and microscopic details of misfit layered nanotubes are discussed, and directions for future research are presented.

The electronic structure and optical properties of Mn and B, C, N co-doped MoS2 monolayers

The electronic structure and optical properties of Mn and B, C, N co-doped molybdenum disulfide (MoS2) monolayers have been investigated through first-principles calculations. It is shown that the MoS2 monolayer reflects magnetism with a magnetic moment of 0.87 μB when co-doped with Mn-C. However, the systems co-doped with Mn-B and Mn-N atoms exhibit semiconducting behavior and their energy bandgaps are 1.03 and 0.81 eV, respectively. The bandgaps of the co-doped systems are smaller than those of the corresponding pristine forms, due to effective charge compensation between Mn and B (N) atoms. The optical properties of Mn-B (C, N) co-doped systems all reflect the redshift phenomenon. The absorption edge of the pure molybdenum disulfide monolayer is 0.8 eV, while the absorption edges of the Mn-B, Mn-C, and Mn-N co-doped systems become 0.45, 0.5, and 0 eV, respectively. As a potential material, MoS2 is widely used in many fields such as the production of optoelectronic devices, military devices, and civil devices.

Stabilization of the misfit layer compound (PbS)1.13TaS2 by metal cross substitution

Photoemission microspectroscopy on the layered misfit compound (PbS)1.13TaS2 provides direct evidence for Ta substitution into PbS layers as well as for Pb substitution into TaS2 layers. This metal cross substitution alters the charge balance between alternating layers and can explain the remarkable stability of (PbS)1.13TaS2 and, possibly, of analogous misfit compounds. It is suggested that even formally stoichiometric misfit compounds can be stabilized by this mechanism.