18 research outputs found
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 BaTiO<sub>3</sub> Nanocrystals Synthesized at Room Temperature
Sub-10 nm BaTiO<sub>3</sub> 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 BaTiO<sub>3</sub> 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 BaTiO<sub>3</sub>, and their coherence length is on the order of 16 Ã…. The presence
of local dipoles suggests that a large amount of macroscopic polarization
can be achieved in nanocrystalline BaTiO<sub>3</sub> if the coherence
of their ferroelectric coupling is further increased
Low Temperature Synthesis of Complex Ba<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>Ti<sub>1–<i>y</i></sub>Zr<sub><i>y</i></sub>O<sub>3</sub> Perovskite Nanocrystals
Low Temperature Synthesis
of Complex Ba<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>Ti<sub>1–<i>y</i></sub>Zr<sub><i>y</i></sub>O<sub>3</sub> Perovskite
Nanocrystal
Bimetallic Trifluoroacetates as Single-Source Precursors for Alkali–Manganese Fluoroperovskites
Alkali–manganeseÂ(II) trifluoroacetates
were synthesized, and their potential as single-source precursors
for the solid-state and solution-phase synthesis of AMnF<sub>3</sub> fluoroperovskites (A = Na, K, Rb, Cs) was demonstrated. Crystals
of Na<sub>2</sub>Mn<sub>2</sub>(tfa)<sub>6</sub>(tfaH), K<sub>2</sub>Mn<sub>2</sub>(tfa)<sub>6</sub>(tfaH)<sub>2</sub>·H<sub>2</sub>O, Rb<sub>2</sub>Mn<sub>2</sub>(tfa)<sub>6</sub>·H<sub>2</sub>O, and CsMnÂ(tfa)<sub>3</sub> (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 Na<sub>2</sub>Mn<sub>2</sub>(tfa)<sub>6</sub>(tfaH), K<sub>2</sub>Mn<sub>2</sub>(tfa)<sub>6</sub>(tfaH)<sub>2</sub>·H<sub>2</sub>O, Rb<sub>2</sub>Mn<sub>2</sub>(tfa)<sub>6</sub>·H<sub>2</sub>O, and CsMnÂ(tfa)<sub>3</sub> in both the
solid state and solution phase yielded crystalline, single-phase NaMnF<sub>3</sub>, KMnF<sub>3</sub>, RbMnF<sub>3</sub>, and CsMnF<sub>3</sub> 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 −CF<sub>3</sub> 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 AMoO<sub>4</sub> (A = Ca, Sr, Ba) Scheelite Nanocrystals
The crystal structure of sub-15 nm
AMoO<sub>4</sub> (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 Ba<sup>2+</sup> for Ca<sup>2+</sup>. Such a dependence
on chemical composition contradicts the well-known rigid character
of Mo<sup>VI</sup>–O bonds and the resulting rigidity of MoO<sub>4</sub> 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 Ba<sup>2+</sup> to
Ca<sup>2+</sup> 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 MoO<sub>4</sub> tetrahedra. Rietveld analysis averages
the random rotations of the MoO<sub>4</sub> 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 MoO<sub>4</sub> tetrahedra
remains unchanged upon chemical substitution of the alkaline-earth
cation, and that their orientational disorder is accommodated through
geometric distortions of the AO<sub>8</sub> dodecahedra
Synthesis and Characterization of Ternary Sn<sub><i>x</i></sub>Ge<sub>1–<i>x</i></sub>Se Nanocrystals
Synthesis and Characterization
of Ternary Sn<sub><i>x</i></sub>Ge<sub>1–<i>x</i></sub>Se Nanocrystal
Average and Local Crystal Structure of β‑Er:Yb:NaYF<sub>4</sub> Upconverting Nanocrystals Probed by X‑ray Total Scattering
The
average and local structures of ∼22 nm β-Er:Yb:NaYF<sub>4</sub> 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>6Ì…2<i>m</i>, <i>P</i>6Ì…, and <i>P</i>6<sub>3</sub>/<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<sub>3<i>x</i></sub>RE<sub>2–<i>x</i></sub>F<sub>6</sub> (<i>x</i> ∼ 0.45;
RE = Y, Er, Tm, Yb) using powder X-ray diffraction, and for β-NaREF<sub>4</sub> (RE = Y, Lu) nanocrystals
using solid-state NMR; however, it differs
from those reported in studies of β-NaREF<sub>4</sub> (RE =
La, Gd, Er) single crystals, which favored the structural model derived
from the <i>P</i>6Ì… 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 (NaCaREF<sub>6</sub>, <i>P</i>6<sub>3</sub>/<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-NaREF<sub>4</sub>