Rodney A. Herring

Surface Eu3+ ions are different than “bulk” Eu3+ ions in crystalline doped LaF3 nanoparticles

Distinct surface effects on the luminescence properties of highly crystalline LaF3:Eu nanoparticles have been demonstrated, by incorporating different ligands on their surface as well as forming core–shell nanoparticles (i.e. doped LaF3:Eu core with an undoped LaF3 shell). These studies have established that the surface Eu3+ ions have a less symmetric crystal field than the “bulk”, and that they are responsible for the ligand induced changes in the asymmetric ratio, which is determined from the relative intensity ratio of the 5D0 → 7F2 (612 and 618 nm) and 5D0 → 7F1 (591 nm) transitions. This surface effect can very effectively be reduced by forming core–shell particles. The multi-exponential luminescent decay curves observed for these nanoparticles have been fitted using a three-parameter model involving the radiative decay constant of the nine inner shells (kR), the radiative decay constant of the outermost shell (kR10), and a parameter C, which describes the quenching, and is a function of the particular luminescent lanthanide ion, the type and amount of quenchers on or near the surface. The model accounts well for the observed changes in the asymmetric ratio of luminescence of these nanoparticles brought about by the ligands used to stabilise them.

Interferometry using convergent electron diffracted beams plus an electron biprism (CBED + EBI)

Abstract A method of interferometry which interferes convergent electron beams by means of an electron biprism, CBED + EBI, is presented. The method requires an electron biprism which is placed below the specimen and in between any two or more convergent beams. The biprism compensates the convergent beams deviation angle by means of an applied potential. When overlaid the diffracted beams interfere to produce an interferogram. Theoretical and practical descriptions of the CBED + EBI method are presented, as well as some of its special features such as its ability to interfere high spatial frequency beams, to produce high contrast fringes and to measure the electron beams coherency.

Realization of a mixed type of interferometry using convergent-beam electron diffraction and an electron biprism

Abstract A new method of interferometry has been realized using a transmission electron microscope with a stable field emission gun, electron biprism and specimen holder. The method involves interfering diffracted beams from a crystal by use of an electron biprism. The interferograms produced provide information about the crystal. Use of a small electron probe affords structural information about the crystal at the atomic level.

Design of a confocal holography microscope for three-dimensional temperature measurements of fluids in microgravity

The design of a Confocal Scanning Laser Holography (CSLH) microscope applied to microgravity studies of fluids is described. This microscope generates a hologram for each three-dimensional point describing an object and offers a new, non-intrusive means to determine the three-dimensional temperature and composition of objects, which is useful information for heat and mass transfer studies. The holograms are created from the interference of a ‘known’ reference beam to an ‘unknown’ object beam, which contains the phase information from which the object’s index of refraction is determined. The key feature of the microscope for microgravity experimentation is the object remains stationary as the beam is rastered through the object, ensuring a quiescent environment. Additional vibration disturbances due to the motion of optical components are minimized by applying counter balances and by using the Motion-vibration Isolation System (MIM).

Determination of three-dimensional strain state in crystals using self-interfered split HOLZ lines

An experimental method to measure the strain through the thickness of a crystal is demonstrated. This enables the full three-dimensional stress-strain state of a crystal at the nanoscale to be determined taking the current practice from two-dimensional strain state determination. Knowing the 3D strain state is desired by crystal growers in order to improve their crystals quality. This method involves combining electron diffraction with electron interferometry in a transmission electron microscope. The electron diffraction uses a split higher order Laue zone (HOLZ) line and the electron interferometry uses an electron biprism.

Coherent electron interference from amorphous TEM specimens.

For the first time, the electron intensity on the diffraction plane from amorphous transmission electron microscope (TEM) specimens has been found to have sufficient coherence to produce fringes in interferograms that were created using a wavefront splitting method of diffracted beam interferometry. The fringes were found to exist from low to high electron-scattering angles. Their spatial frequency depended on the angular overlap of the interfering beams, which was controlled by an electron biprism. From these interferograms, phase information of amorphous materials, which is information now lacking and required for determining their atomic structures, was obtained. An immediate application of this interference is a new method to determine the spatial resolution of the TEM that occurs at the shear angle for fringe disappearance.

Imaging and diffraction of protein crystallization using TEM.

Structural biology relies on good-quality protein crystals in order for structure determination. Many factors affect the growth process of a protein crystal including the way it nucleates and the types of damage and contamination during its growth. Although the nucleation process and quality of a crystal is vital to structure determination, they are both under-studied areas of research. Our research begins to explore ways of measuring the quality of protein crystals, using TEM, thus overcoming the problems associated with viewing wet specimens in a vacuum. Our current understanding of nucleation is that it is a two-step mechanism involving the formation of nuclei from dense liquid clusters; however; it is still unclear whether nuclei first start as amorphous aggregates or as crystalline lattices. Potentially, electron diffraction may be capable of uncovering this process. Using TEM imaging and diffraction of lysozyme as a model protein crystal, we report the internal two-dimensional strain and the density of crystallites in a protein crystal, at a resolution never seen before. The TEM diffraction shows unique features of crystal mosaicity that can be directly correlated to TEM images.

Developing a Confocal Acoustic Holography Microscope for non-invasive 3D temperature and composition measurements

A confocal acoustic holography microscope (CAHM) has been designed, simulated and partially verified experimentally to take holograms for non-invasive, three-dimensional measurements of a specimens refractive indices from one view point. The designed and simulated prototype CAHM used a frequency of 2.25 MHz and measured sound speed changes of 16 m/s, temperature changes of 5 degrees C and had a spatial resolution of 660 microm. With future improvements utilizing the latest technologies such as two-dimensional array detectors, Micro-Electro-Mechanical Systems (MEMS), and acoustic lenses, resolutions of 1m/s, 0.5 degrees C, and 150 microm are expected. The CAHM is expected to have many useful applications, including non-invasive mass and heat transfer measurements in fluids and materials and as a medical diagnostic tool to non-intrusively visualize compositions and temperatures within the human body.

Examining Protein Crystallization Using Scanning Electron Microscopy

Elucidation of protein structure using X-ray crystallography relies on the quality of the crystal. Crystals suffer from many different types of disorder, some of which occur during crystal nucleation and early crystal growth. To date, there are few studies surrounding the quality and nucleation of protein crystals partly due to difficulties surrounding viewing biological samples at high resolution. Recent research has led our current understanding of nucleation to be a two-step mechanism involving the formation of nuclei from dense liquid clusters; it is still unclear whether nuclei first start as amorphous aggregate or as crystalline lattices. Our research examines this mechanism through the use of electron microscopy. Using scanning electron microscopy imaging of the protein crystal growth process, a stacking, spiraling manner of growth is observed. The tops of the pyramid-like tetragonal protein crystal structures measure ~0.2 μm across and contain ~125,000 lysozyme units. This noncrystalline area experiences strain due to growth of the protein crystal. Our work shows that it is possible to view detailed early stage protein crystal growth using a wet scanning electron microscopy technique, thereby overcoming the problem of viewing liquids in a vacuum.

A New Twist for Electron Beams

Passing an electron beam through carefully prepared holograms creates electron vortex beams that improve resolution and allow samples to be manipulated. The transmission electron microscope (TEM) has primarily been used by physical and life scientists for imaging structures and compositions ranging in size from atoms to cells. New applications are likely to emerge from recent demonstrations that it is possible to change the nature of the primary electron source used to create images. Normally, an electron is emitted from its source in a TEM as a plane wave. However, as shown on page 192 of this issue by McMorran et al. (1) as well in recent studies by Verbeeck et al. (2), passing the electron plane wave through a hologram that contains a dislocation causes it to undergo diffraction and split into an electron vortex beam. This type of electron beam can be used to create higher-resolution images and to manipulate the structure and properties of the sample.