Scanning Pyroelectric Microscopy Research Articles

Hydroxypropyl methylcellulose-controlled in vitro calcium phosphate biomineralization

Scanning pyroelectric microscopy of DCPD single crystals.

Vapour growth, morphology, absolute structure and pyroelectric coefficient ofmeta-nitroaniline single crystals

The growth speed of (hkl) faces in the vapour phase, the absolute structure obtained by X-ray crystallography, and the value and the sign of the pyroelectric coefficient ofmeta-nitroaniline (mNA) were analysed in detail. Thein situobservation of morphologically well developed faces of several mNA crystals growing in evacuated ampoules reveals no pronounced growth speed anisotropy for polar faces defining the unique axis 2 of themm2 group. Scanning pyroelectric microscopy confirms mono-domain mNA crystals. X-ray measurements in the space groupPca21show that the molecular planes coincide with the {\bar 211} and {\bar 2\bar 1\bar 1} faces, and the nitro groups cover the {201} face in the opposite direction to the crystal tip, characterizing the polar habitus studied here. At room temperature, the sign of the pyroelectric coefficient is positive for a measured effective value of 6.3 µC m−2 K−1, in good agreement with values reported by other authors. From previous elastic and piezoelectric published data, the secondary pyroelectric effect was calculated to be positive and far greater than the effective one, yielding a negative value for the primary pyroelectric coefficient.

Polar Ordering of Macromolecular Chains in Composite Materials: Calcium Carbonate and Calcium Oxalate

The formation of composite materials is a fundamental process in nature. Fluorapatite (FAp) and calcium carbonate (CaCO3) grown in gelatin can be used as in vitro substitutes for biomimetic composite formation. Here we present our findings about polarity formation in CaCO3 composites grown in macromolecular gels such as gelatin, agar, and carrageenan, extending the analysis to another calcium mineral: calcium oxalate (CaC2O4). The grown spherical composites are investigated by scanning pyroelectric microscopy (SPEM), which allows a mapping of the polarity of a surface. SPEM measurements revealed bipolar structures for all measured composites, indicative of an underlying mechanism of molecular recognition leading to polar ordering of macromolecules. The process of polarity formation inside such composites can be explained by a Markov-type mechanism. The present results are compared to previously published data on FAp/gelatin composites, showing that they are similar. A new group of biomimetic materials was...

Scanning Pyroelectric Microscopy of Subsurface Dielectric Failures

Dielectric breakdown is a very important factor determining the reliability of multilayer ceramic capacitors, and its microscopy imaging is still a pending work so far. Here, a high resolution scanning pyroelectric microscopy (SPeM) is developed for direct imaging dielectric failure behavior of multilayer ceramic capacitors (MLCCs) due to the combination of pyroelectric effect and scanning thermal microscopy. Subsurface dielectric‐breakdown regions along the depth profile inside BaTiO3 capacitors are successfully visualized by measuring 2ω‐pyroelectric current via the frequency modulated SPeM. It provides a new approach for exploring subsurface dielectric breakdown microstructures, and for further understanding the underlying physical phenomena inside the MLCCs.

Peculiar orientational disorder in 4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP) leading to bipolar crystals.

180° orientational disorder of molecular building blocks can lead to a peculiar spatial distribution of polar properties in molecular crystals. Here we present two examples [4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP)] which develop into a bipolar final growth state. This means orientational disorder taking place at the crystal/nutrient interface produces domains of opposite average polarity for as-grown crystals. The spatial inhomogeneous distribution of polarity was investigated by scanning pyroelectric microscopy (SPEM), phase-sensitive second harmonic microscopy (PS-SHM) and selected volume X-ray diffraction (SVXD). As a result, the acceptor groups (NO2 or CN) are predominantly present at crystal surfaces. However, the stochastic process of polarity formation can be influenced by adding a symmetrical biphenyl to a growing system. For this case, Monte Carlo simulations predict an inverted net polarity compared with the growth of pure BNBP and BCNBP. SPEM results clearly demonstrate that 4,4'-dibromobiphenyl (DBBP) can invert the polarity for both crystals. Phenomena reported in this paper belong to the most striking processes seen for molecular crystals, demonstrated by a stochastic process giving rise to symmetry breaking. We encounter here further examples supporting the general thesis that monodomain polar molecular crystals for fundamental reasons cannot exist.

Polarization domain information on insect wing chitin using the scanning pyroelectric microscope (SPEM)

Scanning Pyroelectric Microscopy (SPEM) at micrometer resolutions has been employed to study permanent polarity and its spatial organization in various insect wings containing chitin, one of the most abundant biopolymers. Both measurements on wings of houseflies and wasps revealed a well defined pyroelectric response, showing a multidomain polarization pattern at several length scales, from millimeter sized domains in the wing veins, to 20 micrometer sized domains in the membranes. The size of the smaller domains correlates well with the average chitin fiber length of 18 micrometers.

Spontaneous Polarization in Bio-organic Materials Studied by Scanning Pyroelectric Microscopy (SPEM) and Second Harmonic Generation Microscopy (SHGM)

Bio-organic materials such as bones, teeth, and tendon generally show nonlinear optical (Masters and So in Handbook of Biomedical Nonlinear Optical Microscopy, 2008), pyro- and piezoelectric (Fukada and Yasuda in J Phys Soc Jpn 12:1158, 1957) properties, implying a permanent polarization, the presence of which can be rationalized by describing the growth of the sample and the creation of a polar axis according to Markov’s theory of stochastic processes (Hulliger in Biophys J 84:3501, 2003; Batagiannis et al. in Curr Opin Solid State Mater Sci 17:107, 2010). Two proven, versatile techniques for probing spontaneous polarization distributions in solids are scanning pyroelectric microscopy (SPEM) and second harmonic generation microscopy (SHGM). The combination of pyroelectric scanning with SHG-microscopy in a single experimental setup leading to complementary pyroelectric and nonlinear optical data is demonstrated, providing us with a more complete image of the polarization in organic materials. Crystals consisting of a known polar and hyperpolarizable material, CNS (4-chloro-4\('\)-nitrostilbene) are used as a reference sample, to verify the functionality of the setup, with both SPEM and SHGM images revealing the same polarization domain information. In contrast, feline and human nails exhibit a pyroelectric response, but a second harmonic response is absent for both keratin containing materials, implying that there may be symmetry-allowed SHG, but with very inefficient second harmonophores. This new approach to polarity detection provides additional information on the polar and hyperpolar nature in a variety of (bio) materials.

Barium hydrogen phosphate/gelatin composites versus gelatin-free barium hydrogen phosphate: Synthesis and characterization of properties

Recently, attention has been spent on crystal growth of phosphate compounds in gels for studying the mechanism of in vitro crystallization processes.Here, we present a gel-based approach for the synthesis of barium hydrogen phosphate (BHP) crystals using single and double diffusion techniques in gelatin. The composite crystals were compared with analytical grade BHP powder, single and polycrystalline BHP materials using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), scanning pyroelectric microscopy (SPEM), optical microscopy (OM), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD). FTIR spectra showed surface adsorption of gelatin molecules by using BHP stacked sheets due to CH2 stretching, CH2 bending and amide I vibrations are found in a gelatin content of about 2% determined by dissolution. SEM shows various crystal morphologies of the BHP/gelatin composites forming bundled micro-flakes to irregular bundled needles and spheres different from gel-free crystals. The variety in morphology depends on the ion concentration, pH of gel as well as the method of crystal growth. SPEM investigation of BHP/gelatin aggregates revealed polar domains showing alteration of the polarization. Moreover, BHP/gelatin composite crystals showed a higher thermal stability in comparison with analytical grade BHP or/and BHP single crystals due to strong interactions between gelatin and BHP. The XRD diffraction analysis demonstrated that the single and double diffusion techniques in gelatin led to the formation of orthorhombic BHP. This study demonstrates that gelatin is a useful high molecular weight biomacromolecule for controlling the crystallization of a composite material by producing a variety of morphological forms.

Symmetry and the polar state of condensed molecular matter

Polar molecular crystals seem to contradict a quantum mechanical statement, according to which no stationary state of a system features a permanent electrical polarization. By stationary we understand here an ensemble for which thermal averaging applies. In the language of statistical mechanics we have thus to ask for the thermal expectation value of the polarization in molecular crystals. Nucleation aggregates and growing crystal surfaces can provide a single degree of freedom for polar molecules required to average the polarization. By means of group theoretical reasoning and Monte Carlo simulations we show that such systems thermalize into a bi-polar state featuring zero bulk polarity. A two domain, i.e. bipolar state is obtained because boundaries are setting up opposing effective electrical fields. Described phenomena can be understood as a process of partial ergodicity-restoring. Experimentally, a bi-polar state of molecular crystals was demonstrated using phase sensitive second harmonic generation and scanning pyroelectric microscopy

Orientational disorder: A key to understand polarity of molecular crystals

Polarity of molecular crystals is understood here as a result of 180° orientational disorder of asymmetrical building blocks. Symmetry arguments based on (i) a single rotational degree of freedom, (ii) the finite size of crystals and (iii) interactions in between asymmetrical molecules lead to the conclusion that such crystals should express a bi-polar (∞/∞m) average state of zero polarity. This basic property of molecular crystals is exemplified by forming solid solutions of 4-iodo-4′-nitro-biphenyl (INBP) and symmetrical bi-phenyls (BP: A-π-A, D-π-D; A: acceptor; D: donor). Monte Carlo simulations based on a layer-by-layer growth model predict a reversal of the bi-polar state of pure INBP by forming a solid solution of (INBP)1−x(D-π-D)x. In the case of the addition of A-π-A reversal as found for pure INBP is promoted, i.e. needs less growth steps (layers) to be accomplished. Real crystals representing solid solutions were grown from 2-butanon solutions using symmetrical BPs. Scanning pyroelectric microscopy confirmed the qualitative behavior seen in Monte Carlo simulations. These findings represent an experimentum crucis supporting the general validity of the theory of stochastic polarity formation applied to single component or solid solution molecular crystals.

Pyroelectric and piezoelectric scanning microscopy applied to reveal the bipolar state of 4-iodo-4′-nitrobiphenyl (INBP)

Two recent scanning probe techniques were applied to investigate the bipolar twin state of 4-iodo-4′-nitrobiphenyl (INBP) crystals. Solution grown crystals of INBP show typically a morphology which does not express that of a mono-domain polar structure (Fdd2, mm2). From previous X-ray diffraction a twinning volume ratio of ∼70 : 30 is now explained by two unipolar domains (Flack parameter: 0.075(29)) of opposite orientation of the molecular dipoles, joined by a transition zone showing a width of ∼140 μm. Scanning pyroelectric microscopy (SPEM) demonstrates a continuous transition of the polarization P from +P into −P across the zone. Application of piezoelectric force microscopy (PFM) confirms unipolar alignment of INBP molecules down to a resolution of ∼20 nm. A previously proposed real structure for INBP crystals built from lamellae with antiparallel alignment is thus rejected. Anomalous X-ray scattering was used to determine the absolute molecular orientation in the two domains. End faces of the polar axis 2 are thus made up by NO2 groups. Using a previously determined negative pyroelectric coefficient pc leads to a confirmation also by a SPEM analysis. Calculated values for functional group interactions (D⋯A), (A⋯A), (D⋯D) and the stochastic theory of polarity formation allow us to predict that NO2 groups should terminate corresponding faces. Following the present analysis, INBP may represent a first example undergoing dipole reversal upon growth to end up in a bipolar state.

Symmetry and the polar state of condensed molecular matter

Polar molecular crystals seem to contradict a quantum mechanical statement, according to which no stationary state of a system features a permanent electrical polarization. By stationary we understand here an ensemble for which thermal averaging applies. In the language of statistical mechanics we have thus to ask for the thermal expectation value of the polarization in molecular crystals. Nucleation aggregates and growing crystal surfaces can provide a single degree of freedom for polar molecules required to average the polarization. By means of group theoretical reasoning and Monte Carlo simulations we show that such systems thermalize into a bi-polar state featuring zero bulk polarity. A two domain, i.e. bipolar state is obtained because boundaries are setting up opposing effective electrical fields. Described phenomena can be understood as a process of partial ergodicity-restoring. Experimentally, a bi-polar state of molecular crystals was demonstrated using phase sensitive second harmonic generation and scanning pyroelectric microscopy

Polar alignment of dye molecules in sectors of host lattices revealed by phase sensitive second harmonic generation and scanning pyroelectric microscopy

Phase sensitive second harmonic generation (PS-SHG) was applied to investigate the effect of polar surface recognition of dye molecules (amaranth and sunset yellow, as well as dimethylaminostyryl-pyridine) in symmetry related sectors of host lattices such as KH2PO4 and phthalic acid, respectively. PS-SHG revealed bi-polar states in all cases, meaning that sectors growing in opposite directions feature opposite absolute polarities. Scanning pyroelectric microscopy demonstrated that the dipole moments of amaranth in KH2PO4 are pointing towards the growing crystal face. The present work reveals symmetry breaking through anomalous mixed crystal growth to produce physical properties associated with polar axes.

Scanning pyroelectric microscopy revealing the spatial polarity distribution in topologically centric crystals of trans-4-chloro-4′-nitrostilbene

Scanning pyroelectric microscopy (SPEM) was applied to investigate grown-in polarity in trans-4-chloro-4'-nitrostilbene (CNS) single crystals. Sectors {011} and {011} were scanned for cross sections perpendicular to the a-direction. A rotational analysis for {011}, {011} faces was performed for a needle like crystal turned around the a-axis. All pyroelectric experiments confirm a bipolar grown-in state of polarity for sector involving the twofold axis b. These measurements agree with theoretical force field and stochastic calculations, predicting a bipolar state and a nearly identical extent of polarity for two different sectors.

New Physical Methods for a Space Resolved Mapping of the Macroscopic Polarisation in Molecular Crystals and a Stochastic Theory for Understanding

The spatial distribution of polarity in molecular crystals built up by dipolar entities was imaged by two new techniques: Scanning pyroelectric microscopy (SPEM) and phase-sensitive second harmonic microscopy (PS-SHM). SPEM maps the variation in electrical polarisation due to a change in temperature (pyroelectric effect). The PS-SHM contrast is caused by a change of the sign of the molecular hyperpolarisability (nonlinear optical effect). A spatial inhomogeneity of polar properties in molecular crystals is predicted by a 2D Ising-type model describing 180° orientational disorder at the crystal-nutrient interface. As a result of (slow) growth, centric and acentric seed crystals can continuously transform into bipolar (twinned) crystals showing an inhomogeneous spatial distribution of polarity in the as-grown state.

Spatially resolved heat conduction in polar perhydrotriphenylene inclusion compounds studied by means of thermal waves

Scanning pyroelectric microscopy was applied to study the local thermal diffusivity in polar perhydrotriphenylene (PHTP) inclusion compounds. A thermal wave technique with special heat flow and pyroelectric detection principles was used to measure the longitudinal (K∥) and transversal (K⊥) thermal diffusivity of needle shaped crystals. As expected, we have found a higher thermal conductivity along the channel direction: K∥>K⊥. K∥ also showed a variation along the needle axis, the K∥ values near the capping faces being higher than in the center region. Based on these observations we suggest a relation between the longitudinal thermal conductivity and the local density of orientational defects of the dipolar guest molecules which reaches a maximum in the area around the seed. These findings are in agreement with previous pyroelectric studies, revealing a macrodomain structure and a spatially dependent pyroelectric activity for two different PHTP inclusion compounds.

3D Imaging and Simulation of the Polarisation Distribution in Molecular Crystals

The spatial polarisation distribution in inhomogeneously polar molecular crystals has been imaged by scanning pyroelectric tomography and simulated by a Markov process. The experimental technique combines scanning pyroelectric microscopy (SPEM) with layerwise thinning of crystals. The SPEM probes the local spontaneous polarisation by its temperature dependence (pyroelectric effect). A focused and intensity modulated laser beam scans the surface of a polar sample and induces temperature changes in a volume depending on the laser spot size and the thermal diffusion length λth. Since high lateral resolution of the SPEM is only available for small values of λth. depth information to a resolution of ∼ 10 μ is achieved by repeated scanning and stepwise thinning of crystals. A layer by layer technique can provide 3D imaging of the polar ordering with a lateral resolution of ∼ 20 μ at a probed layer thickness of ∼ 10 μ. Applied to perhydrotriphenylene (PHTP) co-crystallised with 1–(4-nitrophenyl)piperazine (NPP) two conical macro-domains of opposite and nearly constant polarisation were found. The SPEM results are in good agreement with a homogeneous Markov chain model driving dipolar molecules into a parallel state within channels of PHTP. The new tomographic view reveals that the opposing cones are partially overlapping at the tips. Lateral growth processes are assumed to be responsible for this. A theoretical section covers aspects of the convergency into the polar state and the length of polar chains discussed in terms of intermolecular interaction energies.

Spontaneous polarization and orientational dynamics of polar rod-like molecules in host/guest materials

The polar domain structure and dynamics of host/guest materials which include polar rodlike molecules were investigated by pyroelectric and dielectric techniques. Using scanning pyroelectric microscopy (SPEM), spatially resolved information on a /spl mu/m scale was obtained of the 3-D domain structure of the polarization caused by an acentric host lattice and/or the polar arrangement of dipolar guest molecules. Two classes of pyroelectric host/guest materials were examined with SPEM: (1) an inorganic host (zeolite AlPO/sub 4/-5) which was loaded with p-nitroaniline (PNA) guest molecules by an adsorption process, and (2) an organic channel-type inclusion material, which establishes its macroscopic polarity by cocrystallization of perhydrotriphenylene (PHTP) with the polar guest 1-(4-nitrophenyl)piperazine) (NPP). Despite great differences in the chemical composition and morphology, both types of host/guest systems revealed 180/spl deg/ macro-domains, of which the particular shape and sign of the polarization were in accordance with models about crystal growth and/or directed adsorption of polar molecules. In order to assess the molecular dynamics of the included guest molecules, frequency dependent local pyroelectric measurement, based on SPEM, and broad-band dielectric relaxation spectroscopy were performed. Whereas guest molecules included in PHTP channels show no rotational dynamics, we found several relaxation processes in PNA-loaded AlPO/sub 4/-5 crystals, which were assigned to local and cooperative relaxation modes of molecular chains of the hydrogen-bonded PNA guest molecules.

A New Organic Nanoporous Architecture: Dumb-Bell-Shaped Molecules with Guests in Parallel Channels

A new type of dumb-bell-shaped host molecule (6-8) has been synthesised, of which 1,8-bis((1)-adamantyl)-1,3,5,7-octatetrayne (8 = BAOT) forms an open porous architecture when cocrystallised with a number of typical solvent molecules. Adamantyl substituents attached to a tetraalkyne spacer build up the walls of parallel channels wherein guest molecules are aligned. Surprisingly, the tetraalkyne unit is significantly bent. Desolvation experiments provide evidence for a reversible inclusion of guests. In the case of the inclusion of 2-butanone, a partial substitution by symmetrical and asymmetrical long-chain chromophores during crystallisation was possible. Stained crystals showed optical frequency doubling. The crystal structure analysis revealed a centric space group, although considerable translational and orientational disorder was present. Application of scanning pyroelectric microscopy revealed that the growth of inclusion compounds with 2-butanone produced polar ordering of guest molecules, which were aligned in two macro-domains of opposing polarity. The resulting orientation of the carbonyl dipoles is in agreement with the theoretical prediction of a Markov model of spontaneous polarity formation based on molecular recognition processes on growing crystal faces. The present case represents a new example of a property-driven supramolecular synthesis.

Polar Growth and Directional Adsorption of Large AlPO4-5 Crystals Determined by Scanning Pyroelectric Microscopy

Scanning pyroelectric microscopy on large crystals reveals the three-dimensional polarization distribution in AlPO4-5 crystals. AlPO4-5 probably grows in such a way that one particular absolute configuration is formed. In general twinned crystals are formed. The twin domains exhibit an opposite polarization. A single domain is found when crystals are grown from a substrate. In some cases the crystals show a more complex domain structure, which can be attributed to specific growth conditions. From the polarization distribution the history of the crystal growth can be retrieved. A mechanism for the adsorption of p-nitroaniline molecules is proposed on the basis of the polarization distribution in AlPO4-5 crystals and earlier observed preferential orientation of this guest molecule in the host structure.