P. Fraundorf

Discovery of nuclear tracks in interplanetary dust

Nuclear tracks have been identified in interplanetary dust particles (IDPs) collected from the stratosphere. The presence of tracks unambiguously confirms the extraterrestrial nature of IDPs, and the high track densities (1010 to 1011 per square centimeter) suggest an exposure age of approximately 104 years within the inner solar system. Tracks also provide an upper temperature limit for the heating of IDPs during atmospheric entry, thereby making it possible to distinguish between pristine and thermally modified micrometeorites.

Interplanetary dust in the transmission electron microscope: diverse materials from the early solar system

Abstract An analytical electron microscope study of dispersed interplanetary dust aggregates collected in the earths stratosphere shows that, in spite of their similarities, the aggregates exhibit significant differences in composition, internal morphology, and mineralogy. Of 11 chondritic particles examined, two consist mostly of a noncrystalline chondritic material with atomic ( S Fe ) ≥ 2 in places, one consists of submicron metal and reduced silicate ‘microchondrules’ and sulfide grains embedded in a carbonaceous matrix, and another consists of submicron magnetite-decorated unequilibrated silicate and sulfide grains with thick low-Z coatings. Although the particles are unmetamorphosed by criteria commonly applied for chondritic meteorites, the presence of reduced chemistries and the ubiquity of mafic, instead of hydrated, silicates confirm that they are not simply C1 or C2 chondrite matrix material. The observations indicate that portions of some particles have not been significantly altered by thermal or radiation processes since their assembly, and that the particles probably contain fine debris from diverse processes in the early solar system.

Carbon compounds in interplanetary dust: evidence for formation by heterogeneous catalysis.

Associations of carbonaceous material with iron-nickel alloy, carbides, and oxides were identified by analytical electron microscopy in ten unmelted chondritic porous micrometeorites from the earths stratosphere. These associations, which may be interpreted in terms of reactions between a carbon-containing gas and catalytically active dust grains, suggest that some of the carbon in the chondritic porous subset of interplanetary dust was emplaced through heterogeneous catalysis.

Ultrafine sputter-deposited Pt nanoparticles for triiodide reduction in dye-sensitized solar cells: impact of nanoparticle size, crystallinity and surface coverage on catalytic activity

This paper presents a detailed electrochemical impedance spectroscopy and cyclic voltammetry (CV) investigation into the electrocatalytic activity of ultrafine (i.e., smaller than 2 nm) platinum (Pt) nanoparticles generated on a fluorine-doped tin oxide (FTO) surface via room temperature tilted target sputter deposition. In particular, the Pt-decorated FTO electrode surfaces were tested as counter electrode candidates for triiodide (I3(-)) reduction in dye-sensitized solar cells (DSSCs). We observed a direct correlation between size-dependent Pt nanoparticle crystallinity and the I3(-) reduction activity underlying DSSC performance. CV analysis confirmed the higher electrocatalytic activities of sputter-deposited crystalline Pt nanoparticles (1-2 nm) compared with either sub-nanometre Pt clusters or a continuous Pt thin film. While the low catalytic activity and DSSC performance of Pt clusters smaller in size than 1 nm is believed to arise from their non-crystalline nature and charge-trapping attributes, we attribute the high catalytic performance of larger Pt nanoparticles in the 1-2 nm regime to their well-defined crystallinity and fast electron transfer kinetics. For DSSC applications, the optimized Pt loading was calculated to be ~2.54 × 10(-7) g cm(-2), which corresponds to surface coverage by ~1.6 nm sized Pt nanoparticles.

Stardust in the TEM

Abstract We report here high resolution TEM and diffraction studies of two phases, diamond and silicon carbide, recently discovered in dissolution residues from two carbonaceous meteorites (Allende and Murray). Isotopic anomalies of Xe, Ne, C, N, and Si in the residues indicate a presolar origin for these phases in carbon-rich circumstellar environments. Lattice column images and azimuthally-averaged diffraction ring profiles confirm previous reports of the fcc diamond structure, but ring broadening indicates that the number-average crystallite size is ≈ 1 nm. The diamond crystals were therefore likely quenched from the solid phase, or nucleated from the vapor phase under high supersaturation conditions, and given little chance to grow thereafter. The silicon carbide crystals, ranging in size typically from tens of nm to several μm, are fcc β-SiC crystals. High resolution images show frequent 111; twins in the form of 2 nm laths on the twin plane, and occasional “bulk” twins with boundaries near perpendicular to the plane of twinning. Neither dislocations or facets associated with growth, nor structural damage associated with exposure to radiation, are evident from examinations do far. Size distributions for SiC and diamond show little overlap, and thereby cluster only at opposite ends of the 1 μm to 5 nm range observationally associated with dust between stars.

Stereo analysis of single crystal electron diffraction data

In the analysis of single crystal electron diffraction data, full advantage of information on sample geometry can be taken if one first determines sample coordinates for a crystallographic basis triplet. Using this strategy, a unified approach is possible for (i) computer indexing of diffraction spots, (ii) testing of diffraction data for consistency with known structures, (iii) measuring the lattice parameters of an unknown crystal, (iv) determining the crystallographic orientation of micrograph features, and (v) calculating goniometer coordinates to access a desired crystallographic orientation. The approach, which can be implemented with only modest microprocessor support, increases the usefulness of diffraction “patterns” with only one diffraction spot. It therefore extends the versality of a single-tilt goniometer stage, and allows the above tasks to be performed on smaller crystals and more beam-sensitive samples than could otherwise be considered.

TEM study of B- and Er-containing dispersoids in rapidly solidified dispersion-strengthened titanium and titanium aluminide alloys

Abstract The structure of B- and Er-containing dispersoids in rapidly solidified, dispersion-strengthened Ti-0.5at%Er, Ti-6at%B, Ti-25at%Al-0.5at%Er, Ti-25at%Al-4at%B, Ti-48at%Al-0.5at%Er, and Ti-48at%Al-5at%B was examined by TEM. Rapid solidification was achieved using the electron beam melting/splat quenching technique, and the specimens were annealed at 800°C, 1000°C and 1200°C in vacuo for 1 hour. Boron added to Ti and Ti-25at%Al reacted with the matrix to form faceted TiB dispersoids with well-defined orientation relationships. The dispersoids had a semi-ordered sequence of planar faults parallel to TiB(020) planes. Boron in Ti-48at%Al reacted to form faceted TiB 2 dispersoids which are randomly oriented with respect to the matrix. The erbium-containing alloys displayed 100–500 nm diameter, spheroidal, incoherent precipitates of Er 2 O 3 . However, in Ti-48at%Al-0.5at%Er, Er 2 O 3 precipitates smaller than 50 nm were observed by HRTEM to form with a distinct orientation relationship, and in some cases a semi-ordered fault structure. When Ti-48at%Al-0.5at%Er was heated above 1000°C, a new Er-Ti-O phase formed, with a distinct habit and morphology.

Erosion of mylar and protection by thin metal films

Mylar strips, 2.5 microns thick, uncoated and coated with 50A, 100A and 200A of Al, Pd, and Au/Pd were exposed on STS-5 in order to measure the erosion of mylar and to test means of protecting thin plastic foils commonly used for space experiments in low earth orbit. Analysis by optical microscopy, SEM and STEM investigation, EDX measurements, FTIR spectroscopy and weight loss measurements showed that while up to 75 percent of the uncoated mylar was eroded during exposure, thin coatings of the above metals can protect mylar for integrated oxygen-fluxes of at least 10 to the 21st atoms/sq cm.

The instrument response function in air-based scanning tunneling microscopy

Abstract The distinction between point and line resolution in transmission electron microscopy (TEM) arises because an ability to image sub-0.2 nm fringes is a necessary, but not a sufficient, condition for imaging individual atoms. In scanned tip microscopy, as in TEM, empirical data on instrument response should precede assertions about point resolution. In the “slow scan limit”, time-domain noise and geometry effects decouple, and tip shape can take on the role of a two-dimensional impulse response function. We show here that nuclear track pits can be used to quantitatively measure tip geometry with nanometer-scale resolution in three dimensions , that stationary tip images provide a robust measure of time-domain instabilities, and that when these data are taken before and after imaging an unknown, images with instrument response quantitatively constrained by experiment are possible. Specimen-induced tip effects also become measurable in situ.

Single-Slice Nanoworlds Online

Although general multi-slice calculations remain too slow, web-browsers on many platforms now make possible real-time single-slice (strong phase/amplitude object) simulation, with live image, diffraction, image power-spectrum, and darkfield-image modes, including specimen rotation e.g. for atomic-resolution images with specimens having several tens of thousands of atoms. Moreover, a wide range of qualitative phenomena emerge that include diffraction-contrast effects associated with thickness, orientation changes, and defect strain. Hence students with no math background can get a visceral feel for the way 2-D lattice-projections, diffraction-patterns, image power-spectra, aperture size/position, and darkfield images relate to a specimens structure & orientation, as well as microscope contrast-transfer, well before access to a real electron microscope is available.