Somnath Bhattacharyya

Aspects regarding measurement of thickness of intergranular glassy films

Materials such as Si3N4, SiC and SrTiO3 can have grain boundaries characterized by the presence of a thin intergranular amorphous film of nearly constant thickness, in some cases (e.g. Si3N4) almost independent of the orientation of the bounding grains, but dependent on the composition of the ceramic. Microscopy techniques such as high‐resolution lattice fringe imaging, Fresnel fringe imaging and diffuse dark field imaging have been applied to the study of intergranular glassy films. The theme of the current investigation is the use of Fresnel fringes and Fourier filtering for the measurement of the thickness of intergranular glassy films. Fresnel fringes hidden in high‐resolution micrographs can be used to objectively demarcate the glass–crystal interface and to measure the thickness of intergranular glassy films. Image line profiles obtained from Fourier filtering the high‐resolution micrographs can yield better estimates of the thickness. Using image simulation, various kinds of deviation from an ideal square‐well potential profile and their effects on the Fresnel image contrast are considered. A method is also put forth to objectively retrieve Fresnel fringe spacing data by Fourier filtering Fresnel contrast images. Difficulties arising from the use of the standard Fresnel fringe extrapolation technique are outlined and an alternative method for the measurement of the thickness of intergranular glassy films, based on zero‐defocus (in‐focus) Fresnel contrast images is suggested. The experimental work is from two ceramic systems: Lu‐Mg‐doped Si3N4 and SrTiO3 (stoichiometric and nonstoichiometric). Further, a comparison is made between the standard high‐resolution lattice fringe technique, the standard Fresnel fringe extrapolation technique and the methods of analyses introduced in the current work, to illustrate their utility and merits. Taking experimental difficulties into account, this work is intended to be a practical tool kit for the study of intergranular glassy films.

Structure and chemistry across interfaces at nanoscale of a Ge quantum well embedded within rare earth oxide layers.

Structure and chemistry across the rare earth oxide-Ge interfaces of a Gd2O3-Ge-Gd2O3 heterostructure grown on p-Si (111) substrate using encapsulated solid phase epitaxy method have been studied at nanoscale using various transmission electron microscopy methods. The structure across both the interfaces was investigated using reconstructed phase and amplitude at exit plane. Chemistry across the interfaces was explored using elemental mapping, high-angle annular dark-field imaging, electron energy loss spectroscopy, and energy dispersive X-ray spectrometry. Results demonstrate the structural and chemical abruptness of both the interfaces, which is most essential to maintain the desired quantum barrier structure.

Measuring Electrostatic Potential Profiles across Amorphous Intergranular Films by Electron Diffraction

Amorphous 1-2-nm-wide intergranular films in ceramics dictate many of their properties. The detailed investigation of structure and chemistry of these films pushes the limits of todays transmission electron microscopy. We report on the reconstruction of the one-dimensional potential profile across the film from an experimentally acquired tilt series of energy-filtered electron diffraction patterns. Along with the potential profile, the specimen thickness, film orientation with respect to the grain lattice and specimen surface, and the absolute specimen orientation with respect to the laboratory frame of reference are retrieved.

Assessing thermodynamic properties of amorphous nanostructures by energy-filtered electron diffraction

Since the discovery of the existence of 0.5 – 5nm wide amorphous intergranular films (IGF) in many ceramic materials almost 30 years ago [1] a lot of effort has been put into understanding why they exist as well as into the development of new experimental techniques to probe their properties. A recently developed phase field model [2] which includes energetic contributions from chemical, structural, as well as electrostatic effects able to describe the structural transition at the crystal/amorphous interfaces as well as the role of dopant cations as glass network modifiers relies on a set of thermodynamic parameters (e.g. gradient energy penalties for various fields) which are not accessible in bulk structures. It is therefore important to obtain as much experimental information from these structures under investigation, as possible. However, the small width of these films of 1-2nm and the even narrower interface regions are pushing the limits of today’s high resolution (TEM) imaging techniques. Even the width of these glassy films, one of only a few observables which the theory may be compared against cannot be determined without ambiguity [3,4], a fact which can be attributed to the unavoidable imperfections of electron lenses as well as the TEM image formation process itself. Electron diffraction is the perfect tool to identify the crystallinity of a structure and is also unaffected by phase shifts produced by lens aberrations. Using the Koehler illumination system provided in LEO instruments such as the LEO912, or the SESAMe, which has recently been installed in our lab, in conjunction with an in-column energy filter, we can record the small angle scattering produced by an almost parallel electron beam with a diameter of 100-200nm diameter incident on a specimen area including the IGF (see figure 1a). Because of the low signal produced by the scattering off the amorphous IGF we use imaging plates as the recording medium (fig. 1b). While taking account of dynamical scattering effects a functional form inspired by diffuse interface theory [5,6] of the cross-sectional density α(x) of the amorphous IGF could be fitted to the diffraction data with high precision (fig.1c), thus providing the crystallinity η(x)=1-α(x), one of the phase fields included in the model mentioned above[2]. Using energy-loss spectroscopic profiling (ELSP) [7] and electron holography, additional fields, such as chemical concentration profiles and charge density distribution can be determined. This allows thermodynamic parameters in the model to be fitted very precisely and thus the theory to be extended to new systems. This work was done in collaboration with the other partners in the EU/NSF funded NANOAM project. We would like to particularly thank W.C. Carter and C.M. Bishop at MIT for developing the relevant phase field model. We kindly acknowledge financial support through the EU/NSF NANOAM project (EU project Nr. GRD2-CT-2000-30351).

Stabilization of coherent precipitates in nanoscale thin films

Coherent precipitates, on growth beyond a critical size (), become semi-coherent by the presence of interfacial misfit edge dislocations. In the case of precipitation in ‘small’ domains (matrix), the strain energy of the precipitate is altered with respect to its value in bulk domains, due to a lesser volume of strained material and domain deformations, resulting from the proximity of free surfaces. This results in a change in the value of critical size for the coherent to semi-coherent transition of the interface. In the current work, the Cu-2wt.%Fe system is used as model system to study the coherent to semi-coherent transition of precipitates. In this system, spherical Fe-2wt.%Cu precipitates form, which become cuboidal on growth beyond the critical size. Transmission electron microscopy is used to show experimentally for the first time that a coherent precipitate can be stable beyond , in nanoscale free-standing thin films (i.e. critical size for coherent to semi-coherent transition for thin films () > ). Electron energy loss spectroscopy has been used to determine the thickness of the sample. In parallel, finite element simulations are used to compute the critical size in representative domains and to comprehend the energetic basis for the observed phenomenon. Eigenstrains are used to simulate the precipitation process and the stress state due to an interfacial misfit dislocation in the finite element model.

Studying nanocrystallization behaviour of different inorganic glasses using transmission electron microscopy

Nano technology and nano materials are considered to be the key technologies of the 21st century. Among the nano materials, glass ceramics are expected to play a major role, especially for optical applications. Glass-ceramics containing metal fluorides crystals with crystalline sizes 5 to 50 nm are considered to be materials with high potential for numerous photonic applications [1,2,3]. Crystals in this size range and narrow size distribution can only be obtained in multicomponent systems when the interface formed during nucleation and crystal growth acts as diffusional barrier that restricts crystal growth. In this present work, using transmission electron microscopy (TEM) we studied the nanocrystallization behaviour of two glass systems of silicate compositions in one of which (1) crystalline barium fluoride and in other (2) lanthanum fluoride precipitated.