36 research outputs found
Dichroism in helicoidal crystals
Accounting for the interactions of light with heterogeneous, anisotropic, absorbing, optically active media is part of the characterization of complex, transparent materials. Stained biological structures in thin tissue sections share many of these features, but systematic optical analyses beyond the employ of the simple petrographic microscopes have not be established. Here, this accounting is made for polycrystalline, spherulitic bundles of twisted d-mannitol lamellae grown from melts containing light-absorbing molecules. It has long been known that a significant percentage of molecular crystals readily grow as helicoidal ribbons with mesoscale pitches, but a general appreciation of the commonality of these non-classical crystal forms has been lost. Helicoidal crystal twisting was typically assayed by analyzing refractivity modulation in the petrographic microscope. However, by growing twisted crystals from melts in the presence of dissolved, light-absorbing molecules, crystal twisting can be assayed by analyzing the dichroism, both linear and circular. The term "helicoidal dichroism" is used here to describe the optical consequences of anisotropic absorbers precessing around radii of twisted crystalline fibrils or lamellae. d-Mannitol twists in two polymorphic forms, α and δ. The two polymorphs, when grown from supercooled melts in the presence of a variety of histochemical stains and textile dyes, are strongly dichroic in linearly polarized white light. The bis-azo dye Chicago sky blue is modeled because it is most absorbing when parallel and perpendicular to the radial axes in the respective spherulitic polymorphs. Optical properties were measured using Mueller matrix imaging polarimetry and simulated by taking into account the microstructure of the lamellae. The optical analysis of the dyed, patterned polycrystals clarifies aspects of the mesostructure that can be difficult to extract from bundles of tightly packed fibrils
Punin Ripening and the Classification of Solution-Mediated Recrystallization Mechanisms
Ripening (also called recrystallization) is a process that occurs commonly in nature and industry that shifts the size distribution of an ensemble of crystals toward a smaller number of larger crystals. Ostwald ripening is by far the best known recrystallization mechanism and sometimes is mistakenly considered as the only mechanism for shifting the crystal size distribution. Ostwald ripening accounts for recrystallization under thermodynamic control and is driven only by the well-known size dependence of solubility. There are, however, other recrystallization mechanisms that can be observed on laboratory timescales for crystals of any size under certain conditions. Internal stress dispersion is a thermodynamic ripening mechanism that depends not on surface energies but rather on crystal defects. In addition, there are two other mechanisms that are kinetic in nature. The most efficient is driven by the size dependence of growth and dissolution rates at low supersaturation. Finally, a mechanism proposed by Punin is driven by the difference between growth and dissolution rates due to crystal defects. All the four mechanisms can be at work simultaneously. The efficiency of ripening can be enhanced by temperature oscillations, but only the thermodynamic mechanisms can work at constant temperature. In this paper, we discuss the fundamentals of these four ripening mechanisms and revisit in detail Punin's mechanism because it is the least well articulated in the literature.This research was supported by the ACS Petroleum Research Fund (award #61270-ND10). This work was supported partially by the MRSEC Program of the National Science Foundation under award number DMR-1420073. J.M.G.R. acknowledges a grant of the program Salvador de Madariaga (Ministry of Economy and Competitiveness of Spain) and the Molecular Design Institute of NYU for hospitality
Nanoscale observations of the epitaxial growth of hashemite on barite (001)
The heteroepitaxial growth of hashemite BaCrO4 on barite BaSO4(001) from supersaturated aqueous solutions
was observed in situ using an atomic force microscope (AFM). It was shown that the first hashemite layer grows
via two-dimensional nucleation easily forming a complete epitaxial layer,which is likely to have a low level of
intrinsic stress. Two-dimensional nucleation of the second and subsequent layers proceeds with significantly lower
rates,and growth occurs with lower step velocities. These layers seem to have significant level of intrinsic stress and
tend to reduce it via the formation of free surface normal to the growth layer (holes in the layer,dendrite-like shape
of nuclei and steps,preferable formation of nuclei at the step edges). As a result,the initially flat surface becomes
rough. The process described corresponds to the Stranski-Krastanov epitaxial growth mode, which is well known
for growth of semiconductor and metal films but not previously recognised for crystals grown from aqueous
solutions.Depto. de MineralogÃa y PetrologÃaFac. de Ciencias GeológicasTRUEpu
Mechanics of twisted hippuric acid crystals untwisting as they grow
Spontaneous twisting of single crystals is a common growth induced deformation. But as twisted crystals thicken they can untwist, restoring a straight form. The mechanics of this process was studied for vapor grown needle-like crystals of N-benzoylglycine (hippuric acid) and N-(2-thienylcarbonyl)glycine, and analyzed by phenomenological models. The elastic stress at the crystal tip undergoes plastic relaxation leading to the twisting deformations. As the crystal grows and thickens it partially untwists showing linear increases of the twist period with crystal thickness. Such behavior was simulated with a model that assumes the constant density of defects in successive growth layers. However, transmission electron microscopy does not reveal any dislocations or other extended defects typically associated with plastic deformation. Published data on other materials show the linear dependencies of pitch on thickness suggesting comparable untwisting mechanisms for different materials.publishe
Effect of Step Anisotropy on Crystal Growth Inhibition by Immobile Impurity Stoppers
Step pinning by immobile
stoppers is the most important crystal
growth inhibition mechanism. It was first studied by Cabrera and Vermilyea
in 1958, who considered the macroscopic effect of a periodic array
of pinning sites. However, their analysis (and others since) involved
uncontrolled approximations and did not consider what happens when
step anisotropy induces faceting. Here we revisit the motion of a
step past a periodic array of pinning sites, simulating the evolution
numerically using a semi-implicit front-tracking scheme for anisotropic
surface energies and kinetic coefficients. We also provide exact formulas
for the average step velocity when the anisotropy is such that the
interface is fully faceted. We compare the average step velocities
obtained numerically to the estimates derived in the isotropic setting
by Cabrera & Vermilyea (1958) and Potapenko (1993), and to the
exact results obtained in the fully faceted setting. Our results show
that while the local geometry of the propagating step varies considerably
with anisotropy, the average step velocity is surprisingly insensitive
to anisotropy. The behavior starts changing only when the ratio between
minimum and maximum values of the surface energy is roughly less than
0.1
Non-Topotactic Phase Transformations in Single Crystals of β‑Glycine
The
metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal
transformation (SCSCT) to either the α or γ phase with
retention of the crystal habit of the parent β phase. X-ray
diffraction and optical microscopy reveal that of 51 single crystals
of the β parent phase, 24 form single domain crystals of α
or γ with a single orientation matrix, confirming an SCSCT.
The remaining 27 β parent crystals transform to crystals with
a few domains of either α and/or γ, each domain arising
from a single nucleation event. In three cases the β →
α and β → γ transformations occurred within
the same glycine single crystal. In all cases, regardless of the number
of domains, the transformation occurred with retention of the original
habit of the parent phase. Both β → α and β
→ γ transformations proceed in a non-topotactic manner,
as evident from the random orientations of the daughter phases in
the crystallographic reference frame of the parent β phase.
The transformation rates, as measured by advancement of the growth
front with polarized optical microscopy, vary from crystal to crystal
as well as for different regions within the same crystal. Moreover,
the transformation rate increases substantially with relative humidity,
and at a relative humidity of 90% α-glycine was observed as
the only product
Coherence in Polycrystalline Thin Films of Twisted Molecular Crystals
Helicoidal crystallites in rhythmically banded spherulites manifest spectacular optical patterns in small molecules and polymers. It is shown that concentric optical bands indicating crystallographic orientations typically lose coherence (in-phase twisting) with growth from the center of nucleation. Here, coherence is shown to increase as the twist period decreases for seven molecular crystals grown from the melt. This dependence was correlated to crystallite fiber thickness and length, as well as crystallite branching frequency, a parameter that was extracted from scanning electron micrographs, and supported by numerical simulations. Hole mobilities for 2,5-didodecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-C12) measured by using organic field-effect transistors demonstrated that more incoherent boundaries between optical bands in spherulites lead to higher charge transport for films with the same twist period. This was rationalized by combining our growth model with electrodynamic simulations. This work illustrates the emergence of complexity in crystallization processes (spherulite formation) that arises in the extra variable of helicoidal radial twisting. The details of the patterns analyzed here link the added complexity in crystal growth to the electronic and optical properties of the thin films.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Non-Topotactic Phase Transformations in Single Crystals of β‑Glycine
The
metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal
transformation (SCSCT) to either the α or γ phase with
retention of the crystal habit of the parent β phase. X-ray
diffraction and optical microscopy reveal that of 51 single crystals
of the β parent phase, 24 form single domain crystals of α
or γ with a single orientation matrix, confirming an SCSCT.
The remaining 27 β parent crystals transform to crystals with
a few domains of either α and/or γ, each domain arising
from a single nucleation event. In three cases the β →
α and β → γ transformations occurred within
the same glycine single crystal. In all cases, regardless of the number
of domains, the transformation occurred with retention of the original
habit of the parent phase. Both β → α and β
→ γ transformations proceed in a non-topotactic manner,
as evident from the random orientations of the daughter phases in
the crystallographic reference frame of the parent β phase.
The transformation rates, as measured by advancement of the growth
front with polarized optical microscopy, vary from crystal to crystal
as well as for different regions within the same crystal. Moreover,
the transformation rate increases substantially with relative humidity,
and at a relative humidity of 90% α-glycine was observed as
the only product