Search

By analyzing the angular correlations in scanning electron nanodiffraction patterns from a melt-spun Zr(36)Cu(64) glass, the dominant local order was identified as icosahedral clusters. Mapping the extent of this icosahedral short-range order demonstrates that the medium-range order in this material is consistent with a face-sharing or interpenetrating configuration. These conclusions support results from atomistic modeling and a structural basis for the glass formability of this system.

It has long been recognised that spatial fluctuations in local order in disordered assemblies of particles can be probed using limited-volume diffraction measurements. These measurements have unique advantages over broad-beam diffraction experiments that isotropically average over many structural configurations and result in one-dimensional intensity curves, requiring modelling to interpret. Despite the advantages of limiting illumination to a low number of particle configurations, obtaining quantitative measurements of local order from such experiments remains a challenge. The effects on the diffraction pattern of changing the beam energy, lateral size, aberrations and coherence and the specimen thickness have only recently been clarified. We review theoretical and experimental efforts in this direction in the fields of both electron and x-ray diffraction and identify promising areas of future development.

We employ an electron phase retrieval technique in the transmission electron microscope to reconstruct the projected thickness maps of metallic glass specimens and measure the void distribution at a microscopic level. We examine an as-spun melt-spun ${\text{Zr}}_{66.7}{\text{Cu}}_{33.3}$ glass and the shear bands formed in this glass from inhomogeneous deformation in tension and compression. Both as-spun and deformed glasses show no variation in projected thickness indicative of voids down to the limit of this medium-resolution technique ($0.32\phantom{\rule{0.28em}{0ex}}\text{nm}$). This demonstrates that the free volume generated in deformation does not condense into stable voids larger than $0.32\phantom{\rule{0.28em}{0ex}}\phantom{\rule{4pt}{0ex}}\text{nm}$ in radius, but is distributed diffusely in shear bands.

We investigate the structure and mechanical properties of pressure-induced (PI) amorphous silicon $(a\text{-Si})$ and compare this to the more extensively characterized case of $a\text{-Si}$ created by ion implantation. To study the effect of thermal history we also examine the structure of both PI and ion-implanted $a\text{-Si}$ after a low-temperature ``relaxation'' anneal $(450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$. Indentation testing suggests that structural changes are induced by thermal annealing. As-prepared forms of $a\text{-Si}$ deform via plastic flow, while relaxed forms of $a\text{-Si}$ transform to high-pressure crystalline phases. These structural changes are confirmed by more explicit measurements. Raman microspectroscopy shows that the short-range order as expressed by the average bond-angle distortion of the as-prepared amorphous phases is the same and reduced by the same amount following the low-temperature anneal. Fluctuation electron microscopy demonstrates that the as-prepared PI $a\text{-Si}$ displays a much lower variance of the diffracted intensity, a feature directly correlated with the medium-range order, than the as-prepared ion-implanted $a\text{-Si}$. However, relaxation brings this variance of the two networks to the same intermediate level. The mechanical tests and structural probes indicate that annealing the amorphous silicon network can bring it to a common state with the same structure and properties regardless of the initial state. This final state might be the closest attainable to the continuous random network model.

The intermediate voltage electron microscope-tandem user facility in the Electron Microscopy Center at Argonne National Laboratory is described. The primary purpose of this facility is electron microscopy with in situ ion irradiation at controlled sample temperatures. To illustrate its capabilities and advantages a few results of two outside user projects are presented. The motion of dislocation loops formed during ion irradiation is illustrated in video data that reveals a striking reduction of motion in Fe-8%Cr over that in pure Fe. The development of extended defect structure is then shown to depend on this motion and the influence of nearby surfaces in the transmission electron microscopy thin samples. In a second project, the damage microstructure is followed to high dose (200 dpa) in an oxide dispersion strengthened ferritic alloy at 500 degrees C, and found to be qualitatively similar to that observed in the same alloy neutron irradiated at 420 degrees C.

Nanocrystalline silicon (nc-Si:H) is an important material for solar cell applications. Thin films of nc-Si:H can be deposited cost-effectively using chemical vapor deposition (CVD) and sputter deposition. While CVD method offer higher deposition rate and is commonly used in PV industry, microstructural properties (such as hydrogen incorporation, crystallinity, surface morphology, dopant incorporation) can be efficiently controlled using sputter deposition. While all the previous studies on sputter deposited nc-Si used crystalline Si (c-Si) sputter target, the current study compares the properties of nc:Si-H films using single and polycrystalline Si (p-Si) targets. P-Si is relatively inexpensive and easy to fabricate in bulk quantities. Several sets of nc-Si:H films were deposited using both c-Si and p-Si targets under similar conditions on glass substrates and characterized using TEM, XRD, AFM, UV-VIS spectroscopy and DC conductivity. Results indicate that surface morphology, size of nanocrystals, crystalline fraction, optical bandgap, coefficient of optical absorption and activation energy of samples were independent of sputter target used. Preferred orientation of nanocrystallites for all the films was along (111) direction with a crystallite size of 4–10 nm. All the samples have wide optical band gap ranging between 2 and 2.1 eV. Comparable activation energies at low (≪370K) and high temperature (≫370K) regions was obtained and was attributed to carrier transport through nanocrystalline and amorphous phase of the films. These results indicate that nc-Si:H thin films for solar cell applications can deposited using low-cost p-Si targets in-lieu of conventional c-Si targets.

We compare fluctuation electron microscopy data to simulations from phenomenological atomic models and demonstrate a strong correspondence between some features in the experimental data and certain atomic configurations. This allows the nature of atomic clustering in a highly hydrogenated diamondlike carbon to be determined more closely. We compare the structural information garnered from fluctuation electron microscopy and Raman spectroscopy for a particular diamondlike carbon and find consistency between the two techniques in the region where their information overlaps.

A series of hydrogenated diamondlike carbon films grown using plasma-enhanced chemical-vapor deposition is systematically studied as a function of source gas composition using transmission electron microscopy. The structure of the films is examined at three distinct length scales. Both plan-view and cross-sectional studies are undertaken to reveal any large-scale inhomogeneities or anisotropy in the films. The degree of medium-range order in the films is measured by performing fluctuation electron microscopy on the plan-view and cross-sectional specimens. Electron-energy-loss spectroscopy is employed to measure the mass density and $s{p}^{2}:s{p}^{3}$ carbon bonding ratios of the samples. Thus, inhomogeneity as a function of depth in the film is revealed by the measurements of the short- and medium-range orders in the two different sample geometries. Soft, low-density diamondlike carbon films with low coefficients of friction are found to be more homogeneous as a function of depth in the film and possess reduced medium-range order in the surface layer. We find that these properties are promoted by employing a high hydrogen content methane and hydrogen admixture as the growth ambient. In contrast, harder, denser films with higher coefficients of friction possess a distinct surface layer with a relatively elevated level of carbon $s{p}^{3}$ bonding and a higher degree of medium-range order. The structure of the films is examined in the light of the energetics of the growth process. It appears that a high flux of penetrating hydrogen ions modifies the surface layer containing the remnant damage from the carbon ions, homogenizing it and contributing to a lowering of the coefficient of friction.

In the last decade, the synthesis and characterization of nanometer sized carbon clusters have attracted growing interest within the scientific community. This is due to both scientific interest in the process of diamond nucleation and growth, and to the promising technological applications in nanoelectronics and quantum communications and computing. Our research group has demonstrated that MeV carbon ion implantation in fused silica followed by thermal annealing in the presence of hydrogen leads to the formation of nanocrystalline diamond, with cluster size ranging from 5 to 40 nm. In the present paper, we report the synthesis of carbon nanoclusters by the implantation into fused silica of keV carbon ions using the Plasma Immersion Ion Implantation (PIII) technique, followed by thermal annealing in forming gas (4% 2 H in Ar). The present study is aimed at evaluating this implantation technique that has the advantage of allowing high fluence-rates on large substrates. The carbon nanostructures have been characterized with optical absorption and Raman spectroscopies, cross sectional Transmission Electron Microscopy (TEM), and Parallel Electron Energy Loss Spectroscopy (PEELS). Nuclear Reaction Analysis (NRA) has been employed to evaluate the deuterium incorporation during the annealing process, as a key mechanism to stabilize the formation of the clusters.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Phase retrieval is widely used in phase contrast microscopy. Here we present an autofocus algorithm that allows the phase of the exit wave function, from a single-material object, to be reconstructed at medium resolution from a single phase contrast image without any a priori knowledge of the imaging system or object. The algorithm is demonstrated on coherent out-of-focus electron micrographs of 30 nm latex sphere calibration standards, giving

Multiwavelength Raman spectroscopy has been used to study the structure of hydrogenated amorphous carbon films synthesized by plasma-enhanced chemical vapor deposition. The analysis of different parameters from the Raman spectra demonstrates that the diameter, structural, and topological orders of the sp2-bonded ring clusters increase with increasing fractions of hydrogen in the source gas. We report the existence of an unusual peak at 867cm−1 that in such extended clusters of aromatic rings, could correspond to a graphitic mode activated by a relaxation of the phonon selection-rule resulting from defects.

The quality of a crystallized potassium-doped amorphous silicon film was investigated using transmission electron microscopy. It was discovered that the planar epitaxial growth of the amorphous layer was upset after a certain concentration of potassium was encountered by the interface. The crystal recovered subsequent to this point was twinned. The twins lie on {111} planes. The results from the transmission electron microscope analyses were correlated with the rate of solid-phase epitaxy as deduced using time resolved reflectivity and the potassium distribution measured using secondary ion mass spectroscopy. The roughening of the interface due to the growth of twins was determined as a function of depth from the extinction of reflectivity amplitude in the time resolved reflectivity trace.

The crystallisation of deep amorphous wells is studied. These model systems are formed by high energy self implantation through a mask into silicon. The amorphised regions have an aspect ratio opposite to that employed in previous experiments. At elevated temperatures crystallisation proceeds inwards with the amorphous-phase being transformed through both lateral and vertical solid-phase epitaxy (SPE). Complementary information is obtained from performing plan view and cross-sectional transmission electron microscopy analyses. It is discovered that the recovery of the amorphous material is governed by the dynamics of solid-phase epitaxy and that unique secondary structures result from the heat treatment.