Spherulitic and rotational crystal growth of Quartz thin films

To obtain crystalline thin films of alpha-Quartz represents a challenge due to the tendency for the material towards spherulitic growth. Thus, understanding the mechanisms that give rise to spherulitic growth can help regulate the growth process. Here the spherulitic type of 2D crystal growth in thin amorphous Quartz films was analyzed by electron back-scatter diffraction (EBSD). EBSD was used to measure the size, orientation, and rotation of crystallographic grains in polycrystalline SiO2 and GeO2 thin films with high spatial resolution. Individual spherulitic Quartz crystal colonies contain primary and secondary single crystal fibers, which grow radially from the colony center towards its edge, and fill a near circular crystalline area completely. During their growth, individual fibers form so-called rotational crystals, when some lattice planes are continuously bent. The directions of the lattice rotation axes in the fibers were determined by an enhanced analysis of EBSD data. A possible mechanism, including the generation of the particular type of dislocation(s), is suggested.

The use of single-crystal cuvettes with the properties of an optical shutter in X-ray diffractometers

The atomic structure of substances can be studied using X-ray diffraction methods. X-ray diffractometers contains X-ray source and goniometer with a detector of scattered radiation. A sample holder (a cuvette) with the material under study is placed in the center of the goniometer. The diffraction spectrum which represents the structure of the sample under study is recorded upon angular scanning of the sample and detector. Study of crystalline powders, amorphous substances, nanocrystalline and partially crystalline objects is often based on the Bragg-Brentano scheme (θ – 2θ-scanning) with divergent X-ray beams irradiating the entire sample surface. Scattered radiation from the cuvette can also enter the detector and affect the recorded diffraction spectrum. The aforementioned distortion is rather weak for polycrystalline samples due to strong intensity of crystalline reflections. However, when studying amorphous substances, nanocrystalline and partially crystalline objects, contribution of the scattered radiation can be compared with the radiation from the sample and thus interfere with an unambiguous determination of the sample structure. The results of using standard cuvettes for X-ray diffractometers D500, D5000 (Siemens), D8 ADVANCE (Bruker) and DRON are analyzed with a special attention paid to distortion of X-ray spectra attributed to the use of cuvettes made of plastic or amorphous quartz when studying amorphous, nanocrystalline and partially crystalline samples. Special cuvettes, being a kind of optical shutters for background scattering, which do not distort the diffraction spectra of the objects under study are developed: single crystal plates of a special orientation with cylindrical cavities for the samples. The advantages of a single-crystal silicon cuvette compared to plastic cuvette usually supplied for D500 diffractometer are estimated when studying an amorphous sample.

First Principles Investigation on the Structural, Electronic and Optical Properties of Amorphous Silica Glass

The structural, electronic, and optical properties of an amorphous SiO2 (a-SiO2) model is investigated by using first-principles calculation. Most research works used beta-cristobalite glass structure as a reference to amorphous silica structure. However, only the electronic properties were been presented without any link towards the optical properties. Here, we demonstrate simultaneous electronic and optical properties, which closely matched to a-SiO2 properties by generating small sample of amorphous quartz glass. Using the Rietveld refinement, amorphous silica structure was generated and optimized using density functional theory in CASTEP computer code. A thorough analysis of the amorphous quartz structure obtained from different thermal treatment was carried out. The structure of amorphous silica was validated with previous theoretical and experimental works. It is shown that small sample of amorphous silica have similar structural, electronic and optical properties with a larger sample. The calculated optical and electronic properties from the a-SiO2 glass match closely to previous theoretical and experimental data from others. The a-SiO2 band gap of 5.853 eV is found to be smaller than the experimental value of ~9 eV. This is due to the underestimation and assumption made in DFT. However, the band gap value is in good agreement with the other theoretical works. Apart from the absorption edge at around 6.5 eV, the refractive index is 1.5 at 0eV. Therefore, this atomic structure can served as a reference model for future research works on amorphous structures.

Sum frequency generation vibrational spectroscopy of methacrylate-based functional monomers at the hydrophilic solid–liquid interface

An SFGVS study showed H-bonding interactions between the carbonyl groups of methacrylate liquid monomers and surface silanol groups of amorphous quartz.