Structural Properties and Phase Transition of Exfoliated-Restacked Molybdenum Disulfide

The product of exfoliation and restacking of MoS₂ in acidic conditions is studied in detail using X-ray powder diffraction, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The temperature dependence of powder patterns reveals that the heating of exfoliated-restacked MoS₂ is a way to a new nanostructured MoS₂-based layered material that remains nanosized even upon heating to 850 °C. Previously this material has been described as 2H-MoS₂, but according to the X-ray diffraction (XRD) data, its structure cannot be correctly described by any of the “usual” MoS₂ polytypes. A model of the structure of the material describing its XRD patterns and thermal behavior is discussed in detail

Highly Flexible Molecule “Chameleon”: Reversible Thermochromism and Phase Transitions in Solid Copper(II) Diiminate Cu[CF₃C(NH)CFC(NH)CF₃]₂

Three thermochromic phases (α, green; β, red; γ, yellow) and six polymorphic modifications (α₁, monoclinic, <i>P</i>2₁/<i>n</i>, <i>Z</i> = 2; β₁, monoclinic, <i>P</i>2₁/<i>c</i>, <i>Z</i> = 4; β₂, triclinic, <i>P</i>1̅, <i>Z</i> = 4; β₃, monoclinic, <i>P</i>2₁/<i>n</i>, <i>Z</i> = 4; γ₁ and γ₂, tetragonal, <i>P</i>4₂/<i>n</i>, <i>Z</i> = 4) have been found and structurally characterized for copper­(II) diiminate Cu­[CF₃C­(NH)CFC­(NH)CF₃]₂ (<b>1</b>). The α phase is stable under normal conditions, whereas the high-temperature β and γ phases are metastable at room temperature and transform slowly into the more stable α phase over several days or even weeks. X-ray diffraction study revealed that the title molecules adopt different conformations in the α, β, and γ phases, namely, staircase-like, twisted, and planar, respectively. The investigation of the α, β, and γ phases by differential scanning calorimetry showed that the three endothermic peaks in the range 283, 360, and 438 K are present on their thermograms upon heating/cooling. The two peaks at 283 and 360 K correspond to the solid–solid phase transitions, and the high-temperature peak at 438 K belongs to the melting process of <b>1</b>. The temperature and thermal effect of all the observed transitions depend on the prehistory of the crystalline sample obtained. A reversible thermochromic single-crystal-to-single-crystal α₁⇌β₁ phase transition occurring within a temperature interval of 353–358 K can be directly observed using a CCD video camera of the X-ray diffractometer. A series of other solid–solid α₁→γ₁, β₂→γ₁, β₃→γ₁, and γ₁⇌γ₂ phase transitions can be triggered in <b>1</b> by temperature. It has been suggested that, under equilibrium conditions, the α₁→γ₁ and β₂→γ₁ phase transitions should proceed stepwise through the α₁→β₁→β₂→β₃→γ₁ and β₂→β₃→γ₁ stages, respectively. The mechanism of the phase transitions is discussed on the basis of experimental and theoretical data

Cu(II)-Silsesquioxanes as Secondary Building Units for Construction of Coordination Polymers: A Case Study of Cesium-Containing Compounds

Five new bi- and trimetallic copper-organosilsesquioxanes {[VinSiO₂]₁₂­Cu₄Cs₄­(BuOH)₂­(EtOH)₂­(MeOH)}­·2BuOH (<b>1</b>), {[PhSiO₂]₁₂­Cu₄Cs₂K₂­(1,4-dioxane)₉­(H₂O)₂}­·3.4­(1,4-dioxane) (<b>2</b>), {[PhSiO₂]₁₂­Cu₄Cs₄­(DMF)₆}­·2DMF (<b>3</b>), {[MeSiO₂]₁₂­Cu₄Cs₄(THF)_4.5­(MeOH)₂­(H₂O)_0.25} (<b>4</b>), and {[MeSiO₂]₂₄­Cu₁₀Cs₆­(OH)₂­(THF)_4.2­(MeOH)_4.1­(H₂O)_0.7} (<b>5</b>) have been synthesized by an exchange reaction between discrete cage alkali,copper-siloxane and cesium chloride (<b>1</b>,<b> 2</b>) or cesium carbonate (<b>4</b>,<b> 5</b>) or by interaction of copper-phenylsiloxane with cesium phenylsiloxanolate (<b>3</b>). While in <b>1</b>–<b>4</b> the alkali,copper-silsesquioxane cage remains stable during reaction procedures, complex <b>5</b> was obtained by unexpected dimerization of two cages. The neutral cages act with solvent molecules and neighboring cages as square (<b>1</b>,<b> 3</b>,<b> 5</b>), tetrahedral (<b>4</b>), or octahedral (<b>2</b>) nodes giving, respectively, the two-periodic (2D) <b>sql</b> net, and the three-periodic (3D) <b>dia</b> or <b>pcu</b> nets. The roles of the cage structure, nature of metal atoms, and organic coating in the formation of one-, two-, and three-periodic coordination polymers are discussed in the example of newly synthesized and previously obtained alkali,copper-organosiloxanes and copper-organosiloxanes with sandwich or globular cage structures. What’s more, the charge distribution in crystals of <b>1</b>–<b>3</b> was analyzed by means of Bader’s Quantum Theory of Atoms-in-Molecules approach giving evidence of relatively strong bonding between neighboring cages

Stabilization of 1T-MoS₂ Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals

We report a facile, room-temperature assembly of MoS₂-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methyl­imid­azolium (BuMeIm), 2-phenyl­imid­azolium, and 2-methyl­benz­imid­azolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS₂ sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5–20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS₂ sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS₂ sheets of the NCs are of the nonconventional 1T-MoS₂ (metallically conducting) structure. The sheets’ puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS₂ modification and raises the temperature for its transition to the conventional 2H-MoS₂ (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS₂ (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS₂

Stabilization of 1T-MoS₂ Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals

We report a facile, room-temperature assembly of MoS₂-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methyl­imid­azolium (BuMeIm), 2-phenyl­imid­azolium, and 2-methyl­benz­imid­azolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS₂ sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5–20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS₂ sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS₂ sheets of the NCs are of the nonconventional 1T-MoS₂ (metallically conducting) structure. The sheets’ puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS₂ modification and raises the temperature for its transition to the conventional 2H-MoS₂ (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS₂ (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS₂