Near-field scattering techniques: Novel instrumentation and results from time and spatially resolved investigations of soft matter systems

Recently the investigation of non-stationary systems exhibiting spatial and dynamic heterogeneities has propelled the development of innovative optical techniques providing the missing link between the scattering and imaging approaches. The novel techniques are characterized by the fact that the scattered radiation is measured close to the sample. They allow to recover information on the structure and dynamics of the system under investigation equivalent to that available with scattering techniques, with the great advantage that they simultaneously allow to achieve a local characterization of the sample, which is lost in traditional scattering measurements.     

A significant enhancement in thermoelectric power factor of In2Te3 thin films by Se doping and Te composition tuning

Thermoelectric power factor of a material significantly relies on its electrical conductivity, thermal conductivity, and Seebeck coefficient. Herein, an attempt has been made to enhance the thermoelectric power factor of In₂Te₃ thin films by tuning their Te composition and via Se doping. The optimum Se-doping concentration and Te composition enhanced the power factor of pristine In₂Te₃ films by 14 and 7.4 times, respectively. The modified chemical composition, structural characteristics, and surface morphological features of In₂Te₃ films are observed to be pivotal in improving their thermoelectric power factor. Overall, this study offers a facile approach to control the thermoelectric power factor of In₂Te₃ thin films which is significant for their futuristic applications.     

Solving inverse scattering problems in biological samples by quantitative phase imaging

Quantitative phase imaging (QPI), a method that precisely recovers the wavefront of an electromagnetic field scattered by a transparent, weakly scattering object, is a rapidly growing field of study. By solving the inverse scattering problem, the structure of the scattering object can be reconstructed from QPI data. In the past decade, 3D optical tomographic reconstruction methods based on QPI techniques to solve inverse scattering problems have made significant progress. In this review, we highlight a number of these advances and developments. In particular, we cover in depth Fourier transform light scattering (FTLS), optical diffraction tomography (ODT), and white‐light diffraction tomography (WDT).

     

Remote control of extended depth of field focusing

We propose a novel system allowing remote control of the location of the extended focal depth region. This system may find applications in situations where controllable optical access to an enclosure is needed and it is desirable to minimize the movement of the focusing lens. Our scheme uses three axicons, two external ones generating a ring and an internal one producing the focal region. We present a proof of concept of this system, consisting of measurements of the longitudinal and transverse intensity distributions, for a reference configuration and one with a displaced position of the extended depth of field focus.     

Characterization of the morphology of co-extruded, thermoplastic/rubber multi-layer tapes

Tapes with alternating semi-crystalline thermoplastic/rubber layers with thicknesses varying from 100 nm up to several μm were prepared by multi-layer co-extrusion. The variation in layer thickness was obtained by varying the thermoplastic/rubber feed ratio. A systematic study on the use of various microscopy techniques to visualize the morphology of the layered systems is presented. The relatively large length scales and the sample preparation make optical microscopy (OM) unsuitable to study the morphology of the multi-layer tapes. Although excellent contrast between the thermoplastic and rubber layers can be obtained, the usually applied, relatively large magnifications limit the use of transmission electron microscopy (TEM) and atomic force microscopy (AFM) to small sample areas. The large range of applicable magnifications makes scanning electron microscopy (SEM) the most suitable technique to study the morphology of the multi-layer tapes. The sample preparation for SEM with a secondary electron (SE) detector is often based on the removal of one of the components, which may induce changes in the morphology. SEM with a back-scattered electron (BSE) detector is a very convenient method to study the morphology over a wide range of length scales, where the contrast between the different layers can be enhanced by chemical staining. Finally, the nucleation behavior (homogeneous versus heterogeneous) of the semi-crystalline layers, as probed by differential scanning calorimetry (DSC), provides valuable information on the layered morphology. The use of relatively straightforward DSC measurements shows a clear advantage with respect to the discussed microscopy techniques, since no sample preparation is required and relatively large samples can be studied, which are more representative for the bulk.     

Crystallization and Melting Behavior of Polypropylene in Presence of Hydrogenated Dicyclopentadiene

A study of miscibility, morphology, crystallization, and dynamic mechanical properties of polypropylene in the presence of a low-molecular-weight compound, hydrogenated dicyclopentadiene (DCPD), is described. Differential scanning calorimetry (DSC), optical microscopy (OM), and dynamic mechanical analysis (DMA) were used for this purpose. The blends exhibit composition-dependent melting point depression and reduced degree of crystallinity. Isothermal crystallization measurements show that the crystallization takes place at a lower temperature with an increase in crystallization half time with respect to pristine PP. This indicates retardation in crystallization of PP in the blends. The observed changes in the melting and crystallization behaviors are ascribed to the miscibility of the two components. The interaction parameter estimated using the Flory–Huggins equation shows negative values, suggesting the presence of strong specific interaction between PP and DCPD.     

Micro-Raman analysis of coloured lithographs

Raman micro-spectroscopy was chosen for analysis and identification of the pigments present in four nineteenth-century hand-coloured lithographs, as this technique has several advantages over others for this purpose. The possibility of performing completely non-destructive analysis without any sampling is probably one of its most favourable qualities for art analysis. Raman spectroscopy can also be used to determine some pigments that cannot be detected using FTIR, such as vermilion, carbon blacks, cadmium pigments, etc. Among others, Prussian blue, ultramarine blue, carbon black, chrome yellow, yellow ochre, red lead, red iron oxide, burnt Sienna, indigo blue, chrome orange, phthalocyanine green, and some other organic pigments, were determined in the specimens. The results obtained have led to doubts about the age of the lithographs.     

Microscopic in situ diffuse reflectance spectroelectrochemistry of solid state electrochemical reactions of particles immobilized on electrodes

An instrumental setup is described which allows electrochemical measurements to be performed with solid particles immobilized on the surface of a graphite electrode with in situ recording of diffuse reflectance spectra under an incident light microscope. The instrument used includes a special electrochemical cell and a microscope which is interfaced by a light␣guide to a transputer-integrated photodiode-array spectrometer allowing measurements ranging from 320 to 950 nm with a resolution of 3.2 nm/pixel and a PC-interfaced potentiostat. The ₀R₀ geometry of the optical arrangement and the use of crossed polarization filters for blocking specular reflectance makes it possible to use the Kubelka-Munk function for quantifying the optical measurements. The above instrument has been used for the study of the electrochromic system of solid silver octacyanomolybdate(IV/V) adjacent to a silver nitrate solution. The in situ diffuse reflec tance spectroelectrochemical measurements prove that the electrochemical reaction starts at the graphite/sample interface and then advances into the bulk of the sample towards the sample/electrolyte interface. The ratios Red:Ox determined by spectrometry and chronocoulometry as a function of electrode potentials are identical.     

Measurement of surface orientation in uniaxial poly(ethylene terephthalate) films using polarised specular reflectance Fourier transform infrared microscopy

Polarised Fourier transform infrared (FT-IR) external-reflection spectroscopy has been used to measure molecular orientation at the surface of uniaxially drawn poly(ethylene terephthalate) (PET) films as a function of draw ratio. The spectra, which were obtained using an FT-IR microscope operating in the reflectance mode, were of high signal-to-noise (S/N) ratio, and contained only the specular component of the reflected radiation. The dichroic ratio of the 1019 cm⁻¹ ring stretching band was used to measure the PET orientation function, P₂₀₀. Prior to quantifying band intensities, the reflectance spectra were either transformed using the Kramers-Kronig algorithm, or simply differentiated; it was found that the dichroic ratios obtained using either pretreatment were similar. It was shown that the P₂₀₀ values obtained using the FT-IR method compared well with those obtained using surface refractive index measurements, provided that the intensity of the 1019 cm⁻¹ band was normalised relative to a non-dichroic band in the spectrum prior to computing dichroic ratios. The use of a reflecting microscope to perform the analysis of surface orientation not only simplifies sample alignment compared with “macro” reflectance accessories, but also allows awkward-shaped or small samples to be examined with relative ease. Unfortunately, a significant current limitation is that samples must be sufficiently “optically thick” to prevent double-pass (“transflectance”) radiation reaching the detector and yielding non-specular features in the reflectance spectrum. For polyaromatics, this probably means thicknesses in excess of 50 μm to obtain good spectra in the fingerprint region. Polarised attenuated total reflectance microscopy may present an alternate approach for examining thinner films.     

Colloidal chemistry as a guide to design intended dispersions of carbon nanomaterials

In this review, some established concepts from Colloidal Science and their application to graphene and carbon nanotubes dispersions in organic or aqueous media are highlighted to rationalize alternatives for some issues in terms of colloidal properties. Recent applications for carbon-based dispersions are presented, as well as van der Waals interactions in carbon materials and strategies to overcome these interactions, such as increasing electrostatic repulsion between dispersed particles, surface functionalization, or adsorption of passivation agents such as macromolecules, which are the basis of many dispersion and exfoliation procedures. The demonstration of how knowledge and fine control of colloidal interactions have been used to overcome several limitations, such as the preparation of stable and concentrated dispersions of carbon materials and keeping appreciable electrical conductivity, is presented. It is also showed that the same knowledge can help the development of more environmentally friendly carbon-based colloids as well as the improvement of similar systems as dispersions of two-dimensional materials.