AVS2013 Session SA-TuP: Synchrotron Analysis Poster Session

HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) is a relatively insensitive high explosive at room temperature. Mesoscale voids are thought to influence sensitivity and detonation properties in polymer bound explosive compositions, where HMX crystals are mixed with, for example, ~5% Viton. HMX molecular crystals undergo a solid-solid phase transition from the so-called beta- to delta- phases at elevated temperatures around 170 Celsius, an prior to this study, little was known about how this phase transition affected mesoscale voids and microstructure. We have measured the ultra-small angle x-ray scattering (USAXS) as the explosive was heated through this phase transition. The USAXS is sensitive to structure from about 10 nm to about 5, and shows how the porosity in these size regimes evolves during the phase change. X-ray computed microtomography was also performed before and after temperature cycling to observe changes on length scales larger than a micron. These results enable studies to determine how the mesocale porosity affects detonation properties in heated HMX-based explosives.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Synchrotron radiation based electron and ionic spectroscopy is a group of very powerful experimental techniques, which have been very successfully used in the study of free atoms and molecules, surfaces and solid-state samples. The main point with this work is extends the possible targets for these techniques to liquids with high vapor pressure, including systems and phenomena of fundamental interest to physics, chemistry and biology. To achieve this we are developing a source for ionic and electron spectroscopy studies of liquid samples. As spectroscopy techniques require vacuum, they have been very difficult to apply to liquid systems. Recent developments of micro-jet techniques, however, have made such studies feasible [1-4].

The main experimental chamber consist in a time-of-flight spectrometer positioned in 54.7⁰ or a Scienta electron spectrometer positioned perpendicular to the plane formed by the micro-jet system, an ultraviolet excitation source, and an efficient high vacuum system allowing to keep a work pressure of 10^-6 mbar in the spectrometer chamber. A secondary chamber is used to store and freeze the liquid jet, it is called trap chamber and it is in fact a differential pumping chamber.

1. M. Faubel, B. Steiner and J.P. Toennies, Z. Phys. D, 10, 269 (1988); J. Chem. Phys. 106, 9013 (1997); J. Electron Spectrosc. Relat. Phenom. 95, 159 (1998).

2. L. Wendy et al., Intern. J. of Mass Spectr. 207, 1 (2001).

3. N. Ottosson, K. J. Børve, D. Spangberg, H. Bergersen, L. J. Sæthre, M. Faubel, W. Pokapanich, G. Ohrwall, O. Bjorneholm, B. Winter, J. Am. Chem. Soc., 133, 3120 (2011)

4. B. M. Messer,C. D. Cappa, J. D. Smith, W. S. Drisdell,C. P. Schwartz,R. C. Cohen, and R. J. Saykally, J. Phys. Chem. B, Vol. 109, No. 46, (2005)

This work was supported financially by FAP-DF, CNPq and DPP-UnB.

For many applications involving chemisorption of monolayers onto Si, for example, very rare higher-diamondoids, the fabrication will be limited by the available quantities of alkene-functionalized precursors. As a model system, hexadecene has been used as a surrogate for rare materials. A simple method has been proposed for fabricating large-area molecular monolayers on silicon surfaces. Two nearly atomically flat, ~1 cm² hydrogen-terminated Si wafers are brought face-to-face, with an extremely small (~100 nL) hexadecene film between them, and are gently rubbed against each other. We posit hydrosilylation chain reactions are activated at friction-induced “hot spots”, resulting in formation of chemisorbed molecular monolayers on the Si surface. Near-edge x-ray absorption fine structure (NEXAFS) and photoemission spectroscopy (PES) were used to characterize composition and electronic structure of the friction-activated monolayer of hexadecene on Si(111). The results are compared to those of the thermally- and sonochemically activated monolayers to demonstrate that this friction technique could be used for a facile and efficient chemical functionalization of Si.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Crystalline GeSn alloys have triggered enormous research efforts in the last decade for future optoelectronic devices. The Ge_1-xSn_x material system exhibits a tunable direct energy gap in the composition range 0 < x < 0.15, enabling its use in light emitting/absorbing components. In addition, GeSn alloys are predicted to show enhanced carrier mobility when compared to elemental Ge, a necessary prerequisite for high-speed semiconductor devices.

The growth of single crystalline GeSn thin films is very challenging due to the limited solubility of Sn in Ge and the large lattice mismatch. Progresses made in the last years using epitaxial growth techniques such as molecular beam epitaxy (MBE), GeSn thin films can be grown with high crystal quality and Sn concentrations above 1 at.% [1]. However, as these films are highly metastable with respect to their equilibrium conditions, sustaining their quality, and hence their electrical and optical properties upon further processing will be as demanding as their growth. As such, the applicability of GeSn in electronic devices will depend on their stability upon e.g. thermal treatment.

We report on the thermal stability of strained GeSn (4-6 at.% Sn) thin films grown by low-temperature MBE on Ge. We discuss degradation mechanisms observed in these layers and the thermal budget that can be derived from this. The samples were characterized using synchrotron-based, reference-free X-ray fluorescence analysis [2] in grazing incidence mode (GIXRF). This technique is based on the in-depth intensity variations within the X-ray Standing Wave (XSW) field which arises between primary and reflected beam. During a GIXRF measurement, the depth distribution of Sn is combined with the intensity distribution of the XSW field, resulting in a distribution specific angular fluo­res­cence curve [3]. This method enables us to gain information about the in depth and the integral changes of the Sn concentration in the layer.

A relative comparison of GIXRF profiles recorded on pristine and annealed GeSn indicates significant diffusion of Sn and compositional changes in the GeSn layer for high annealing temperatures. Complementary analysis reveal morphological changes on the surface of the annealed films, e.g. the formation of large islands. IR-based scattering type scanning-near-field optical microscopy (s-SNOM) [4] measurements show that these islands exhibit distinctively different optical properties than the GeSn layer.

[1] Shimura et al., Thin Solid Films 518 2010

[2] Beckhoff et al., Anal. Chem. 79, 7873 2007

[3] Hönicke et al., Anal. Bioanal. Chem. 396, 2825 2010

[4] Hermann et al., Optics Express 21, 2913 2013