Preparation, Formation, and Structure of [(SnSe)1.04]m(MoSe2)n Intergrowth Compounds (0 < m and n < 32) from Designed Precursors

Synthesis of Metastable Inorganic Solids with Extended Structures

The number of known inorganic compounds is dramatically less than predicted due to synthetic challenges, which often constrains products to only the thermodynamically most stable compounds. Consequently, a mechanism-based approach to inorganic solids with designed structures is the holy grail of solid state synthesis. This article discusses a number of synthetic approaches using the concept of an energy landscape, which describes the complex relationship between the energy of different atomic configurations as a function of a variety of parameters such as initial structure, temperature, pressure, and composition. Nucleation limited synthesis approaches with high diffusion rates are contrasted with diffusion limited synthesis approaches. One challenge to the synthesis of new compounds is the inability to accurately predict what structures might be local free energy minima in the free energy landscape. Approaches to this challenge include predicting potentially stable compounds thorough the use of structural homologies and/or theoretical calculations. A second challenge to the synthesis of metastable inorganic solids is developing approaches to move across the energy landscape to a desired local free energy minimum while avoiding deeper free energy minima, such as stable binary compounds, as reaction intermediates. An approach using amorphous intermediates is presented, where local composition can be used to prepare metastable compounds. Designed nanoarchitecture built into a precursor can be preserved at low reaction temperatures and used to direct the reaction to specific structural homologs.

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.

Confined lattice dynamics of single and quadruple SnSe bilayers in [(SnSe)(1.04)](m)[MoSe2](n) ferecrystals

The Sn specific densities of phonon states in the SnSe subunits of [(SnSe)1.04]m[MoSe2]n ferecrystals with (m,n) = (1,1), (4,1) and in bulk SnSe were derived from nuclear inelastic scattering by the (119)Sn Mössbauer resonance. Using different measurement configurations, phonons with polarization parallel and perpendicular to the ferecrystal plane were specifically probed. Vibrational properties and phonon spectral weight are found to strongly depend on the phonon polarization and layer count m. A highly peculiar feature of these ferecrystal densities of phonon states is the emergence of rather sharp high energy vibrational modes polarized perpendicular to the ferecrystal plane, which contrasts with usual findings in thin layered structures and nanostructured materials in general, and a depletion of modes with a gap appearing between acoustic and high energy modes. The spectral weight of these phonons depends on the overall SnSe content, m, but cannot be unambiguously attributed to SnSe-MoSe2 interfaces. Considering the low energy part of lattice dynamics, ferecrystals exhibit rather low average phonon group velocities as compared to the speed of sound in the long wavelength limit. For the (1,1) ferecrystal, this effect is most pronounced for vibrations polarized in the ferecrystal plane. Thus, an experimental microscopic origin for the vibrational and bonding anisotropy in subunits of ferecrystals is provided.

Synthesis and Thermal Properties of Solid-State Structural Isomers: Ordered Intergrowths of SnSe and MoSe2

A family of structural isomers [(SnSe)1.05]m(MoSe2)n were prepared using the modulated elemental reactant method by varying the layer sequence and layer thicknesses in the precursor. By varying the sequence of Sn-Se and Mo-Se layer pairs deposited and annealing the precursors to self-assemble the targeted compound, all six possible isomers [(SnSe)1.05]4(MoSe2)4, [(SnSe)1.05]3(MoSe2)3[(SnSe)1.05]1(MoSe2)1, [(SnSe)1.05]3(MoSe2)2[(SnSe)1.05]1(MoSe2)2, [(SnSe)1.05]2(MoSe2)3[(SnSe)1.05]2(MoSe2)1, [(SnSe)1.05]2(MoSe2)1[(SnSe)1.05]1(MoSe2)2[(SnSe)1.05]1(MoSe2)1, and [(SnSe)1.05]2(MoSe2)2[(SnSe)1.05]1(MoSe2)1[(SnSe)1.05]1(MoSe2)1 were prepared. The structures were characterized by X-ray diffraction and electron microscopy which showed that all of the compounds have very similar c-axis lattice parameters and in-plane constituent lattice parameters yet distinct isomeric structures. These studies confirm that the structure, order, and thickness of the constituent layers match that of the precursors. The cross-plane thermal conductivity is found to be very low (∼0.08 Wm(-1) K(-1)) and independent of the number of SnSe-MoSe2 interfaces within uncertainty. The poor thermal transport in these layered isomers is attributed to a large cross-plane thermal resistance created by SnSe-MoSe2 and MoSe2-MoSe2 turbostratically disordered van der Waals interfaces, the density of which has less variation among the different compounds than the SnSe-MoSe2 interface density alone.