Low Temperature Solution-Phase Deposition of SnS Thin Films

The solution-phase deposition of inorganic semiconductors is a promising, scalable method for the manufacture of thin film photovoltaics. Deposition of photovoltaic materials from molecular or colloidal inks offers the possibility of inexpensive, rapid, high-throughput thin film fabrication through processes such as spray coating. For example, CdTe, Cu(In,Ga)(S,Se)_2 (CIGS), and CH_3NH_3Pb(Cl,I)_3 perovskite-based thin film solar cells have been previously deposited using solution-based processes. Inks have also recently been developed for the solution deposition of Cu_2ZnSn(S,Se)_4 (CZTS) and FeS_2 (iron pyrite) absorber layers for thin film solar applications, in order to provide sustainable alternatives to materials that contain environmentally harmful heavy metals (e.g., Cd, Pb) and/or scarce elements (e.g., Te, In)

n-Type PbSe Quantum Dots via Post-Synthetic Indium Doping

We developed a postsynthetic treatment to produce impurity n-type doped PbSe QDs with In^(3+) as the substitutional dopant. Increasing the incorporated In content is accompanied by a gradual bleaching of the interband first-exciton transition and concurrently the appearance of a size-dependent, intraband absorption, suggesting the controlled introduction of delocalized electrons into the QD band edge states under equilibrium conditions. We compare the optical properties of our In-doped PbSe QDs to cobaltocene treated QDs, where the n-type dopant arises from remote reduction of the PbSe QDs and observe similar behavior. Spectroelectrochemical measurements also demonstrate characteristic n-type signatures, including both an induced absorption within the electrochemical bandgap and a shift of the Fermi-level toward the conduction band. Finally, we demonstrate that the In^(3+) dopants can be reversibly removed from the PbSe QDs, whereupon the first exciton bleach is recovered. Our results demonstrate that PbSe QDs can be controllably n-type doped via impurity aliovalent substitutional doping

Synthesis-Dependent First-Order Raman Scattering in SrTiO 3 Nanocubes at Room Temperature

Raman spectroscopy was used to demonstrate that the lattice dynamics of SrTiO 3 (STO) nanoparticles strongly depends on their microstructure, which is in turn determined by the synthetic approach employed. First-order Raman modes are observed at room temperature in STO single-crystalline nanocubes with average edge lengths of 60 and 120 nm, obtained via sol-precipitation coupled with hydrothermal synthesis and a molten salt procedure, respectively. First-order Raman scattering arises from local loss of inversion symmetry caused by surface frozen dipoles, oxygen vacancies, and impurities incorporated into the host lattice. The presence of polar domains is suggested by the pronounced Fano asymmetry of the peak corresponding to the TO2 polar phonon, which does not vanish at room temperature. These noncentrosymmetric domains will likely influence the dielectric response of these nanoparticles

Structural Evolution of BaTiO₃ Nanocrystals Synthesized at Room Temperature

Sub-10 nm BaTiO₃ nanocrystals were synthesized at room temperature via the vapor diffusion sol–gel method, and their structural evolution during nucleation and growth stages was followed using a series of techniques that probe the atomic structure on different length and time scales. Special emphasis was placed on assessing the evolution of the local symmetry and structural coherence of the resulting nanocrystals, as these are the structural bases for cooperative properties such as ferroelectricity. Although the room-temperature crystal structure of the fully grown nanocrystals appears cubic to Rietveld analysis of synchrotron X-ray diffraction data, Raman spectroscopy and pair distribution function analysis demonstrate the presence of non-centrosymmetric regions arising from the off-centering of the titanium atoms. This finding demonstrates that accounting for diffuse scattering is critical when attempting the structural characterization of nanocrystals with X-ray diffraction. The local symmetry of acentric regions present in BaTiO₃ nanocrystals, particularly structural correlations within an individual unit cell <i>and</i> between two adjacent unit cells, is best described by a tetragonal <i>P</i>4<i>mm</i> space group. The orthorhombic <i>Amm</i>2 space group also provides an adequate description, suggesting both types of local symmetry can coexist at room temperature. The average magnitude of the local off-center displacements of the titanium atoms along the polar axis is comparable to that observed in bulk BaTiO₃, and their coherence length is on the order of 16 Å. The presence of local dipoles suggests that a large amount of macroscopic polarization can be achieved in nanocrystalline BaTiO₃ if the coherence of their ferroelectric coupling is further increased

Low Temperature Synthesis of Complex Ba_1–<i>x</i>Sr_<i>x</i>Ti_1–<i>y</i>Zr_<i>y</i>O₃ Perovskite Nanocrystals

Low Temperature Synthesis of Complex Ba_1–<i>x</i>Sr_<i>x</i>Ti_1–<i>y</i>Zr_<i>y</i>O₃ Perovskite Nanocrystal

Bimetallic Trifluoroacetates as Single-Source Precursors for Alkali–Manganese Fluoroperovskites

Alkali–manganese­(II) trifluoroacetates were synthesized, and their potential as single-source precursors for the solid-state and solution-phase synthesis of AMnF₃ fluoroperovskites (A = Na, K, Rb, Cs) was demonstrated. Crystals of Na₂Mn₂(tfa)₆(tfaH), K₂Mn₂(tfa)₆(tfaH)₂·H₂O, Rb₂Mn₂(tfa)₆·H₂O, and CsMn­(tfa)₃ (tfa = trifluoroacetato) were grown via solvent evaporation and their crystal structures solved using single-crystal X-ray diffraction (XRD). Chemical purity was confirmed using thermal analyses (TGA/DTA) and Rietveld analysis of powder XRD patterns. Thermal decomposition of Na₂Mn₂(tfa)₆(tfaH), K₂Mn₂(tfa)₆(tfaH)₂·H₂O, Rb₂Mn₂(tfa)₆·H₂O, and CsMn­(tfa)₃ in both the solid state and solution phase yielded crystalline, single-phase NaMnF₃, KMnF₃, RbMnF₃, and CsMnF₃ fluoroperovskites, respectively. Nanocrystals (<100 nm) and submicrocrystals (<500 nm) were obtained in a mixture of high-boiling-point organic solvents. Crystal structures of bimetallic trifluoroacetates displayed a variety of building blocks, coordination environments of the alkali atoms, and coordination modes of the trifluoroacetato ligand. Alkali–fluorine interactions ranging from chemical bonds to short contacts were observed throughout the series. The coordination flexibility of the trifluoroacetato ligand was attributed to the ability of the −CF₃ groups to interact with alkali atoms over a broad range of distances. The synthetic approach described in this investigation provides a starting point to expand the library of fluorinated single-source precursors suitable for solution-phase routes to mixed-metal fluorides

Structural Disorder in AMoO₄ (A = Ca, Sr, Ba) Scheelite Nanocrystals

The crystal structure of sub-15 nm AMoO₄ (A = Ca, Sr, Ba) scheelite nanocrystals has been investigated using a dual-space approach that combines Rietveld and pair distribution function (PDF) analysis of synchrotron X-ray diffraction data. Rietveld analysis yields an average crystal structure in which the Mo–O bond distance exhibits an anomalously large contraction (2.8%) upon chemical substitution of Ba²⁺ for Ca²⁺. Such a dependence on chemical composition contradicts the well-known rigid character of Mo^VI–O bonds and the resulting rigidity of MoO₄ tetrahedra in scheelites. Unlike Rietveld, PDF analysis yields a local crystal structure in which the Mo–O bond distance shows a negligible contraction (0.4%) upon going from Ba²⁺ to Ca²⁺ and, therefore, appears independent of the chemical composition. Analysis of the anisotropic displacement parameters of the oxygen atom reveals that the disagreement between the average and local structural models arises from the presence of static orientational disorder of the MoO₄ tetrahedra. Rietveld analysis averages the random rotations of the MoO₄ tetrahedra across the scheelite lattice yielding an apparent Mo–O bond distance that is shorter than the true bond distance. In contrast, PDF analysis demonstrates that the structural integrity of the MoO₄ tetrahedra remains unchanged upon chemical substitution of the alkaline-earth cation, and that their orientational disorder is accommodated through geometric distortions of the AO₈ dodecahedra

Average and Local Crystal Structure of β‑Er:Yb:NaYF₄ Upconverting Nanocrystals Probed by X‑ray Total Scattering

The average and local structures of ∼22 nm β-Er:Yb:NaYF₄ upconverting nanocrystals were probed using a dual-space approach combining Rietveld and pair distribution function analysis of X-ray total scattering. Comparison of the fits provided by the structural models derived from <i>P</i>6̅2<i>m</i>, <i>P</i>6̅, and <i>P</i>6₃/<i>m</i> space groups demonstrates that the latter yields a crystallochemically meaningful description of the nanocrystals’ average and local structures. This result is in line with those previously reported for bulk β-Na_3<i>x</i>RE_2–<i>x</i>F₆ (<i>x</i> ∼ 0.45; RE = Y, Er, Tm, Yb) using powder X-ray diffraction, and for β-NaREF₄ (RE = Y, Lu) nanocrystals using solid-state NMR; however, it differs from those reported in studies of β-NaREF₄ (RE = La, Gd, Er) single crystals, which favored the structural model derived from the <i>P</i>6̅ space group. The proposed structural model features sodium cations distributed in four translation-equivalent chains, each shifted relative to the others along the <i>c</i> axis. Interestingly, the structural model derived for the mineral gagarinite (NaCaREF₆, <i>P</i>6₃/<i>m</i>), in which sodium cations are distributed in eight chains, also provides an adequate fit to the X-ray scattering data. The potential implications of this finding are discussed from the perspective of the size dependence of the crystal structure of β-NaREF₄