Coupling of strain, stress, and oxygen non-stoichiometry in thin film Pr0.1Ce0.9O2-δ

Stress and strain in thin films of Pr0.1Ce0.9O2-δ, supported on yttria stabilized zirconia (YSZ) and sapphire substrates, induced by large deviations from oxygen stoichiometry (δ = 0) were investigated by in situ high temperature X-ray diffraction and wafer curvature studies. The measured stresses and strains were correlated with change in δ, measured in situ using optical transmission spectroscopy of defect centers in the films and compared with prior chemical capacitance studies. The coefficient of chemical expansion and elastic modulus values for the films were found to be 18% less than, and 16% greater than in the bulk, respectively. Irreproducible stress and strain during cycling on YSZ substrates was observed and related to microstructural changes as observed by TEM. The enthalpy of defect formation was found to be similar for films supported on sapphire and YSZ, and appeared to decrease with tensile stress, and increase with compressive stress. Larger stresses observed for YSZ supported films as compared to sapphire supported films were found and accounted for by the difference in film orientations.

Current status and future directions for in situ transmission electron microscopy

This review article discusses the current and future possibilities for the application of in situ transmission electron microscopy to reveal synthesis pathways and functional mechanisms in complex and nanoscale materials. The findings of a group of scientists, representing academia, government labs and private sector entities (predominantly commercial vendors) during a workshop, held at the Center for Nanoscale Science and Technology- National Institute of Science and Technology (CNST-NIST), are discussed. We provide a comprehensive review of the scientific needs and future instrument and technique developments required to meet them.

Growth behavior near the ultimate resolution of nanometer-scale focused electron beam-induced deposition

An attempt has been made to reach the ultimate spatial resolution for electron beam-induced deposition (EBID) using W(CO)(6) as a precursor. The smallest dots that have been written have an average diameter of 0.72 nm at full width at half maximum (FWHM). A study of the nucleation stage revealed that the growth is different for each dot, despite identical growth conditions. The center of mass of each dot is not exactly on the position irradiated by the e-beam but on a random spot close to the irradiated spot. Also, the growth rate is not constant during deposition and the final deposited volume varies from dot to dot. The annular dark field signal was recorded during growth in the hope to find discrete steps in the signal which would be evidence of the one-by-one deposition of single molecules. Discrete steps were not observed, but by combining atomic force microscope measurements and a statistical analysis of the deposited volumes, it was possible to estimate the average volume of the units of which the deposits consist. It is concluded that the volume per unit is as small as 0.4 nm(3), less than twice the volume of a single W(CO)(6) molecule in the solid phase.

Diffraction patterns of artificial two-dimensional crystals synthesized in situ in an environmental scanning transmission electron microscope

In this study, we demonstrated the use of electron-beam-induced deposition for synthesis of artificial two-dimensional crystals with an in situ scanning transmission electron microscope. The structures were deposited from W(CO)6 in an environmental scanning transmission electron microscope on a 30-nm-thick Si3N4 substrate. We present clear electron beam diffraction patterns taken from those structures. The distance between the diffraction peaks corresponded to the dot spacing in the self-made surface crystal. We propose using these arrays of dots as anchor points for making artificial crystals for diffraction analysis of weakly scattering or beam-sensitive molecules such as proteins.

The MIDAS project at ASU: John Cowley's vision and practical results

An overview of the conception and development of the MIDAS system at Arizona State University is given: a Microscope for Imaging, Diffraction and Analysis of Surfaces. John Cowley's vision in the early 1980s was ambitious and far-reaching, and it was because of him the authors came to ASU. We were centrally involved in the design and implementation of MIDAS from the mid 1980s onwards; the novel design features are briefly reviewed. Practical results obtained using this instrument are listed, and the scope for future development and applications are indicated. While it is clear that many new results have been demonstrated, even more possibilities still remain to be explored. Some comments are made about the feasibility of such developments in the light of competing instrumentation.

New magnetic order in buried native iron oxide layers

The properties of a magnetically ordered buried Fe oxide layer are presented. This oxide has a room-temperature magnetization exceeding that of Fe3O4 by 42% and of gamma-Fe2O3 by 89%. The oxide consists of a component (70%) with a net moment of 2.0 micro(B)/Fe ion, while the remaining spins yield no net moment. The oxide magnetization is stabilized in part by the proximate Fe metal.

Lattice measurement and alloy compositions in metal and bimetallic nanoparticles

A new reliable method for determining the lattice spacings of metallic and bimetallic nanoparticles in phase contrast high resolution electron microscopy (HREM) images was developed. In this study, we discuss problems in applying HREM techniques to single metal (Pt and Au) and bimetallic (AuPd) nanoparticles of unknown shapes and random orientations. Errors arising from particle tilt and edge effects are discussed and analysis criteria are presented to reduce these errors in measuring the lattice parameters of nanoparticles. The accuracy of an individual particle lattice measurement is limited by an effective standard deviation which depends on the size of the individual nanoparticle. For example, the standard deviation for 20-30 A Pt or Au nanoparticles is about 1.5%. To increase the accuracy in determining the lattice spacings of nanoparticles, statistical methods have to be used to obtain the average lattice spacing of an ensemble of nanoparticles. We measured approximately 100 nanoparticles with sizes in the range of 20-30 A and found that the mean lattice spacing can be determined to within 0.2%. By applying Vegard's law to the AuPd bimetallic systems we successfully detected the presence of alloying. For 30 A nanoparticles, the estimated ultimate error in determining the composition of the AuPd alloy is about 3% provided that at least 100 particles are measured. Finally, the challenges in determining the presence of more than one alloy phases in bimetallic nanoparticle systems were also discussed.

Low-temperature epitaxial growth of the quaternary wide band gap semiconductor SiCAlN

Two compounds SiC and AlN, normally insoluble in each other below approximately 2000 degrees C, are synthesized as a single-phase solid-solution thin film by molecular beam epitaxy at 750 degrees C. The growth of epitaxial SiCAlN films with hexagonal structure takes place on 6H-SiC(0001) substrates. Two structural models for the hexagonal SiCAlN films are constructed based on first-principles total-energy density functional theory calculations, each showing agreement with the experimental microstructures observed in cross-sectional transmission electron microscopy images. The predicted fundamental band gap is 3.2 eV for the stoichiometric SiCAlN film.

In Situ Electron Microscopy: Introduction

This issue of Microscopy and Microanalysis contains invited and contributed papers from the 1998 Arizona State University (ASU) Workshop on In Situ Electron Microscopy, held January 7-10 at the Embassy Suites in Scottsdale, Arizona. This was the eighteenth in a series of Workshops organized annually by the Facility for High Resolution Electron Microscopy in the Center for Solid State Science at ASU. The Workshops are part of an educational program, and are intended to foster interaction and collaboration between researchers in electron microscopy. The workshop was attended by over 80 participants from universities, industries, and government laboratories around the world. Many of the contributions to the workshop are included in this issue.

Oxidation and Reduction of Small Palladium Particles on Silica

: We have used the technique of in situ electron microscopy to study the oxidation and reduction of the palladium (Pd) catalysts. In this study, we have subjected a Pd catalyst to oxidation and reduction cycles and studied the changes in particle structure and morphology with in situ electron diffraction and imaging. The PdO particles can be reduced to Pd metal in situ at temperatures as low as 200 degreesC in an atmosphere of a few Torr of both H2 and O2. We also found that essentially the same reduction occurred in the vacuums of 10(-6) to 10(-7) Torr in two different electron microscopes. Our in situ reduction studies show that many of the oxide particles form voids when reduced to Pd metal. The decrease in volume that occurs during reduction is often accommodated by a combination of particle shrinkage and void formation. The production of voids does not seem to depend on either the reducing atmosphere or the rate of reduction, although the voids appear to be unstable above 500 degreesC.

SSZ-26 and SSZ-33: Two Molecular Sieves with Intersecting 10- and 12-Ring Pores

The framework structures of two closely related molecular sieves, SSZ-26 and SSZ-33, are described. These materials possess a previously missing but desired structural feature in a group of industrially significant zeolites. They contain a three-dimensional pore system that provides access to the crystal interior through both 10- and 12-rings. This property is a consequence of the organic structure-directing agents used in the synthesis of these materials. These materials are examples of the purposeful design of a micropore architecture. Both SSZ-26 and SSZ-33 contain the 4=4-1 building unit that had been previously found only in natural zeolites.

Preparation and characterization of MgO surfaces by reflection electron microscopy

We have employed several different methods to prepare (100) and (111) surfaces of MgO crystals. (100) surfaces prepared by simple cleaving give good reflection high energy electron diffraction (RHEED) patterns and surfaces with a high density of coarse steps. Chemical polishing of this surface results in a roughening of the topography whilst annealing in oxygen considerably smoothens the surfaces although they appear to be contaminated. Under certain conditions we find that the MgO crystals will cleave along the (111) plane. Both cleaved and mechanically polished (111) surfaces are atomically flat and reconstructed after oxygen annealing.