Aberration Corrected STEM Analysis of Electrophoresis Separates of Allende Nanodiamond
Introduction: Nanodiamond is an historically important phase for cosmochemical studies because it was the first phase identified as the carrier of nucleosynthetic anomalies indicative of formation in a star other than the Sun [1] Present in the matrix of primitive chondrites at levels up to 2000 ppm, nanodiamonds are also arguably the most abundant presolar phase that survived incorporation into the Solar System [2] The difficulty with this assertion is that, at present, there is no method for measuring isotope compositions, other than possibly for C, of individual 2 nm nanodiamonds. Measured as an ensemble, the average C and N isotope compositions are within the range of Solar System values. Furthermore, the nucleosynthetic anomalies that provide signatures of preserved nucleosynthesis of stars pre dating the Sun are present in very low abundance, e.g. 1 Xe atom per 105 nanodiamonds. Thus, the vast majority of diamonds could have an origin in the early Solar System [3] The existence of two or more sub populations of nanodiamonds has been inferred from multiple lines of evidence, e.g. variations in N content and release patterns [4] bi modal noble gas release patterns [5] variation in the noble gas content of ultrafine density [6] and electrophoresis separates [7] which contain different quantities of noble gases that are indicative of an extra solar origin [8] These sub populations in the nanodiamond residues have yet to be fully characterized, but aberration corrected scanning transmission electron microscopy (STEM) is a promising tool for doing so [9] In particular, aberration corrected STEM, at low voltage and in UHV, is optimal for analysis of nanodiamond, and other nanoscale phases of carbon, because of the single atom sentivity for imaging and spectroscopy, as well as the greatly reduced beam etching. Samples and Methods: Nanodiamond separates extracted from the Allende (CV3) meteorite at Washington University by established methods, were further separated into 12 fine fractions with electrophoresis methods described previously [7] The electrophoresis technique involves application of high voltage to separate components of the colloidal suspension by competition of sedimentation and electrostatic forces. The total Xe and Xe HL components of the 12 fractions were measured by single step pyrolysis; additional noble gas release studies are reported at this meeting [10] The powders were re suspended in an ultrapure water isopropanol mixture and pipetted as aliquots onto lacey carbon film coated Cu TEM grids, and allowed to dry in air. The TEM grids were first cleaned with an Ar+ plasma, to remove any volatile hydrocarbons. The grids were subsequently stored in plasmacleaned glass vials. Aberration corrected STEM measurements were performed at the Naval Research Laboratory with PRISM, a custom Nion UltraSTEM200, which incorporates a 0.7 sr, windowless Bruker SDD energy dispersive x ray (EDX) system, and a Gatan Enfinium ER electron energy loss (EEL) spectrometer. STEM analyses of fractions 1, 2, 4, 8 and 10, representing a range of electrophoretic separation were carried out at 60kV, with a nominal probe size of 150 pm, and probe current of 50 pA. Results: The STEM analyses confirm that all of the analyzed fractions do contain nanodiamond. However, the abundance of nanodiamond, and amounts of other phases, present on the TEM grids varied significantly. Nanodiamond was most concentrated in the #1 (Fig. 1) and #4 fractions (Fig. 2) with 10 or more clusters of nanodiamonds per grid square 104 mm2) compared to less than 1 cluster per square for fractions 2, 8, and 10. Fractions 8 and 10 contain silicate nanoparticles in addition to diamond.