Two-dimensional iron chalcogenide intercalates display a remarkable correlation of the interlayer spacing with enhancement of the superconducting critical temperature (Tc). In this work, synchrotron X-ray absorption (XAS; at the Fe and Se K-edges) and emission (XES; at the Fe Κβ) spectroscopies allow one to discuss how the important rise of Tc (∼44 K) in the molecule-intercalated Lix(C5H5N)yFe2-zSe2 relates to the electronic and local structural changes felt by the inorganic host upon doping (x). XES shows that widely separated layers of edge-sharing FeSe4 tetrahedra carry low-spin moieties, with a local Fe magnetic moment slightly reduced compared to the parent β-Fe2-zSe2. Pre-edge XAS expresses the progressively reduced mixing of metal 3d-4p states upon lithiation. Doping-mediated local lattice modifications, probed by conventional Tc optimization measures (cf. the anion height and FeSe4 tetrahedra regularity), become less relevant when layers are spaced far away. On the basis of extended X-ray absorption fine structure, such distortions are compensated by a softer Fe network that relates to Fe-site vacancies, alleviating electron-lattice correlations and superconductivity. Density functional theory (DFT) guided modification of the isolated Fe2-zSe2 (z, vacant sites) planes, resembling the host layers, identify that Fe-site deficiency occurs at low energy cost, giving rise to stretched Fe sheets, in accordance with experiments. The robust high-Tc in Lix(C5H5N)yFe2-zSe2, arises from the interplay of electron-donating spacers and the iron selenide layer's tolerance to defect chemistry, a tool to favorably tune its Fermi surface properties.

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Lix(C5H5N)yFe2-zSe2: A Defect-Resilient Expanded-Lattice High-Temperature Superconductor.

Layer-by-layer stacking of two-dimensional materials results in van der Waals heterostructures (vdWH) of nanometer thickness and clean interfaces that often exhibit exceptional properties and present novel challenges. We perform first-principles calculations using density functional theory to examine the electronic properties of vdWH consisting of graphene (Gr) and semiconducting transition metal dichalcogenide (TMD) monolayers at several twist angles. We describe in detail our methodology for the creation of simulation cells which are almost free of strain due to lattice mismatch (less than 1%) and for unfolding the electronic bands of the vdWH in a way that allows for a straightforward comparison with the electronic structure of the constituent monolayers. The weak interlayer interactions in Gr/${\mathrm{MoS}}_{2}$ and Gr/${\mathrm{WS}}_{2}$ heterobilayers leave the Gr and TMD band structures almost unaffected but move the Fermi level closer to the TMD conduction band minimum. Careful examination of transitions from direct to indirect band gaps for some twist angles reveals that these are due to very small strain remaining in the simulation cells. In ${\mathrm{WS}}_{2}/{\mathrm{MoS}}_{2}$ and ${\mathrm{WSe}}_{2}/{\mathrm{MoSe}}_{2}$ heterobilayers, interlayer interactions do not affect the conduction band minimum at $\mathrm{K}$ but lead to eigenenergy splitting and eigenstate hybridization at the valence band around the $\mathrm{\ensuremath{\Gamma}}$ point, which is very sensitive to interlayer distance and determines whether the valence band maximum is at the $\mathrm{\ensuremath{\Gamma}}$ or $\mathrm{K}$ point. Our results unveil a small redshift of the intralayer electronic transitions of TMDs when interacting with either Gr or other TMD monolayers. 

Unfolding the band structure of van der Waals heterostructures

Imidazole represents a fascinating class of explosophoric units with exciting structures and unique properties. As compared to other nitrogen-rich heterocycles, imidazole demonstrates great potential applications due to economic effectiveness and superior energetic performances. The field of traditional chemistry has been extensively explored for imidazole, and thus established bond-building methods and functionalization strategies promote further development as high-energy density materials (HEDMs). This review addresses the development of energetic imidazole compounds in the past decade, summarizes their physiochemical properties, and is divided into three parts (explosives, propellants, and energetic biocides) according to application requirements. Various synthetic strategies for these energetic molecules are highlighted, including the construction of heterocyclic frameworks and following functionalization. The selected and discussed reactions illustrate the versatility of imidazole in energetic applications as building blocks for the future design of new HEDMs.

Imidazole-Based Energetic Materials: A Promising Family of N-Heterocyclic Framework.

Modulation of the Fermi level using an ultraviolet (UV)-assisted photochemical method is demonstrated in tungsten diselenide monolayers. Systematic shifts and relative intensities between charged and neutral exciton species indicate a progressive and controllable decrease of the electron density and switch tungsten diselenide from n-type to a p-type semiconductor. The presence of chlorine in the 2D crystal shifts the Fermi level closer to the valence band while the effect can be only partially reversible via continuous wave laser rastering process. The presence of chlorine species in the lattice is validated by X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations predict that adsorption of chlorine on the selenium vacancy sites leads to p-type doping. The results of our study indicate that photochemical techniques have the potential to enhance the performance of various 2D materials, making them suitable for potential applications in optoelectronics.

Electron density control in tungsten diselenide monolayers via photochlorination

A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron-selenide host by organic molecular donors, aiming at the formation of the expanded-lattice Lix(C5H5N)yFe2-zSe2 hybrid superconductor. The intercalates are found to crystallize in the tetragonal ThCr2Si2-type structure at the average level, however, with an enhanced interlayer iron-selenide spacing (d = 16.2 Å) that accommodates the heterocyclic molecular spacers. Quantitative atomic pair distribution function (PDF) analysis at variable times suggests distorted FeSe4 tetrahedral local environments that appear swollen with respect to those in the parent β-FeSe. Simultaneously acquired in situ synchrotron X-ray powder diffraction data disclose that secondary phases (α-Fe and Li2Se) grow significantly when a higher lithium concentration is used in the solvothermal reaction or when the solution is aged. These observations are in line with the strongly reducing character of the intercalation medium's solvated electrons that mediate the defect chemistry of the expanded-lattice superconductor. In the latter, intralayer correlated local distortions indicate electron-donating aspects that reflect in somewhat enlarged Fe-Se bonds. They also reveal a degree of relief of chemical pressure associated with a large distance between Fe and Se sheets ("taller" anion height) and a stretched Fe-Fe square planar topology. The elongation of the latter, derived from the in situ PDF study, speaks for a plausible increase in the Fe-site vacancy concentration. The evolution of the local structural parameters suggests an optimum reaction window where kinetically stabilized phases resemble the distortions of the edge-sharing Fe-Se tetrahedra, required for a high-Tc in expanded-lattice iron-chalcogenides.

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In Situ Visualization of Local Distortions in the High-Tc Molecule-Intercalated Lix(C5H5N)yFe2-zSe2 Superconductor.

In-depth studies elaborating on hybrid iron chalcogenide superconductors are of a great interest to the condensed matter physics community due to their intriguing behaviors arising from structural modifications and electronic correlations upon intercalation. Intercalation of the 8 K FeSe superconductor with organic molecules leads to a substantial enhancement of the critical temperature (T c ) up to 42 K [1]. In effect, in such hyper-expanded lattice superconductors subtle local distortions in their nanoscale environment are introduced (Fig.1). Our team has developed solvothermal synthesis pathways for Li x (C 5 H 5 N) y Fe 2-z Se 2 intercalated samples with the purpose to investigate them with state-of-the-art synchrotron science tools. In one hand, high Q-resolution X-ray diffraction, over a broad temperature range (10-300 K) is utilized to shed light on possible structural phase transitions, taking place at the average level when the materials cross the T c . Rietveld analysis suggests that their high-T c relates to the tetragonal ThCr 2 Si 2 -type structure. On the other hand, complementary high-energy synchrotron X-rays enable a wide-Q field of view, appropriate for total scattering insights [2]. When combined with pair distribution function (PDF) analysis, correlated local distortions, pertaining to the electronic active Fe-selenide layers, are identified as the outcomes of the electron donor moieties, accommodated in the

Alexandros Deltsidis

Papers

In Situ Visualization of Local Distortions in the High-Tc Molecule-Intercalated Lix(C5H5N)yFe2-zSe2 Superconductor.

Lix(C5H5N)yFe2-zSe2: A Defect-Resilient Expanded-Lattice High-Temperature Superconductor.

Fe-based Superconductors: Structural Phase Transitions Tuned by Electronic Fluctuations