Mechanochemical Synthesis of Sustainable Ternary and Quaternary Nanostructured Cu2SnS3, Cu2ZnSnS4, and Cu2ZnSnSe4 Chalcogenides for Thermoelectric Applications.

Copper-based chalcogenides have emerged as promising thermoelectric materials due to their high thermoelectric performance, tunable transport properties, earth abundance and low toxicity. We have presented an overview of experimental results and first-principal calculations investigating the thermoelectric properties of various polymorphs of Cu2SnS3 (CTS), Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe) synthesized by high-energy reactive mechanical alloying (ball milling). Of particular interest are the disordered polymorphs of these materials, which exhibit phonon-glass-electron-crystal behavior-a decoupling of electron and phonon transport properties. The interplay of cationic disorder and nanostructuring leads to ultra-low thermal conductivities while enhancing electronic transport. These beneficial transport properties are the consequence of a plethora of features, including trap states, anharmonicity, rattling, and conductive surface states, both topologically trivial and non-trivial. Based on experimental results and computational methods, this report aims to elucidate the details of the electronic and lattice transport properties, thereby confirming that the higher thermoelectric (TE) performance of disordered polymorphs is essentially due to their complex crystallographic structures. In addition, we have presented synchrotron X-ray diffraction (SR-XRD) measurements and ab initio molecular dynamics (AIMD) simulations of the root-mean-square displacement (RMSD) in these materials, confirming anharmonicity and bond inhomogeneity for disordered polymorphs.

Water Uptake in an Anion Exchange Membrane Based on Polyamine: A First-Principles Study.

An atomistic level study of a single monomer of polyamine interacting with water molecules and hydroxide anions (OH-) was carried out to investigate the role of the polyamine structure in the hydrated morphology of anion exchange membranes (AEMs) for alkaline fuel cells and its influence on ionic conductivity and chemical stability. DFT calculations were performed to find the ground state of the system, studying the interactions of the solvent species with three different regions of the polymer─the amine functional group, the backbone, and the carbonyl group. The hydrophilic/hydrophobic behavior of each segment was determined, with calculated binding energies and Bader charge analysis providing a more quantitative analysis of the interactions and activation and reaction energies computed to investigate the chemical degradation mechanism. The results show the tendency of both OH- and water molecules to form water clusters in the proximity of the ionized amine group. As such, these regions constitute the preferential pathway for ionic conductivity. Besides, the essential role of the water content is pointed out, not only to enhance conductivity but also to reduce degradation in an alkaline environment. The present work provides a baseline to assess the impact of polymer chemistry on the ionic conductivity of the membrane and acts as the first step for the development of high-performance AEMs and for an improvement of the overall performance of the fuel cell.

Static and dynamic components of Debye-Waller coefficients in the novel cubic polymorph of low-temperature disordered Cu2ZnSnS4.

Cu2ZnSnS4 (CZTS) is an attractive material for sustainable photovoltaics and thermoelectrics, and several properties originate from its marked polymorphism. High-energy mechanical alloying is found to lead to a disordered phase that possesses a sphalerite-like cubic structure. This is investigated in detail with the aid of laboratory and synchrotron radiation X-ray diffraction, Raman spectroscopy, electron microscopy and ab initio molecular dynamics. The disordered cubic polymorph is preserved below 663 K. With thermal treatments above 663 K, the tetragonal kesterite phase forms, used here as a reference for structural and microstructural features. Particular attention is paid to the stacking arrangement: a significant fraction of twin faults was found in the disordered cubic samples, which then progressively annealed with domain growth and with the transition to the ordered tetragonal phase. This study also focuses on Debye-Waller coefficients, which were found to be considerably larger for the disordered cubic than the tetragonal sample. Indeed, disorder leads to an ∼1 Å2 upward shift through the temperature range 100-700 K, a feature confirmed by ab initio calculations, which points to a particularly high contribution from disordered Sn cations. This supports the general understanding that structural disorder introduces a temperature-independent static contribution to the atomic mean-square displacement. Debye-Waller coefficients are found to be a good measure of this disorder, known to have a critical effect on transport properties.

Topological Anderson Insulator in Cation-Disordered Cu2ZnSnS4.

We present the first candidate for the realization of a disorder-induced Topological Anderson Insulator in a real material system. High-energy reactive mechanical alloying produces a polymorph of Cu2ZnSnS4 with high cation disorder. Density functional theory calculations show an inverted ordering of bands at the Brillouin zone center for this polymorph, which is in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu2ZnSnS4 can be connected to the closely related Cu2ZnSnSe4, which was previously predicted to be a 3D topological insulator, while band structure calculations with a slab geometry reveal the presence of robust surface states. This evidence makes a strong case in favor of a novel topological phase. As such, the study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily-synthesized multinary, disordered compounds.

Surface softening in palladium nanoparticles: effects of a capping agent on vibrational properties.

The presence of a capping agent (CTAB) on Pd nanoparticles produces a strong static disorder in the surface region. This results in a surface softening, which contributes to an overall increase in the Debye-Waller coefficient measured by X-ray powder diffraction. Molecular dynamics and density functional theory simulations show that the adsorption-induced surface disorder is strong enough to overcome the effects of nanoparticle size and shape.

Reflection of Molecular Twist in Unoccupied Molecular Orbitals in PTCDI Derivatives: A Density Functional Study.

In this work, the structural and electronic properties of perylene tetracarboxylic diimide (PTCDI) derivative molecules have been calculated using density functional theory simulations. Here, we have obtained the equilibrium geometry for certain PTCDI derivatives and calculated their occupied and unoccupied density of states separately for molecular orbitals lying in-plane (σ type) and orthogonal to the plane (π type) of the molecules. We have also simulated the X-ray absorption spectroscopy (XAS) spectra for these molecules separately for π- and σ-type orbitals. A comparison between the unoccupied density of states and XAS data has been made because both provide a description of the molecular orbitals above the Fermi level. We have observed the presence of shallow-lying σ orbitals in twisted molecules and have obtained an almost linear relationship between the abundance of these orbitals and the degree of molecular twist. Additionally, we have shown the possibility of an experimentally viable stereoisomerism in PTCDI-C3.