Exploring Phonon Signals by High Energy / High Spatial Resolution EELS
O.L. Krivanek, N. Dellby, T.C. Lovejoy, N.J. Bacon, G.J. Corbin, P. Hrncirik, Z.S. Szilagyi, T. Aoki, R.W. Carpenter, P.A. Crozier, J. Zhu, P. Rez
(+2 others)
2014
Microscopy and Microanalysis
Using the High Energy Resolution Monochromated EELS-STEM (HERMES) instrument we have developed [1] together with a Gatan Enfinium spectrometer retrofitted with extra-stable power supplies [2] , we now achieve sub-20 meV resolution in electron energy loss spectra (EELS) acquired in >1 s. We have also been able to improve the rate of the decay of the zero loss peak (ZLP) such that tail intensities of <4x10-4 of the ZLP maximum have been recorded at energy losses as small as 100 meV. Besides the
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... gh energy resolution and minimized tail ZLP, HERMES provides several new capabilities, including: 1) spatial resolutionwe can record full-resolution spectra from sample areas as small as a few Ǻ; 2) flexibilitywe can also record spectra from areas as large as 10 μm, and attain an angular resolution of ~1 μrad (using a camera length setting of ~1 km); 3) angular acceptancewe can attain a good energy resolution with EELS acceptance angles of ±30 mrad and greater; and 4) brightnesswe use a bright cold field emission (CFE) gun, and can therefore extract appreciable signals from Ǻ-sized areas. Taken together, these capabilities promise that signals not accessible in electron microscopy before are about to become available. Here we explore phonon spectroscopy; in a related contribution at this meeting, we explore detecting H and other light elements by energy-filtered HADF imaging [3]. Fig. 1 illustrates how important signals have up to now been "hidden in plain sight"obscured by a broad zero loss peak. The solid green spectrum was recorded in 0.13 s at 60 keV, with no sample, and with the beam passing through the monochromator but the energy-selecting slit retracted. The ZLP FWHM is ~250 meV, typical of tungsten CFE operated with a total emission current <1 μA. The red (line) spectrum was recorded with the slit in, in 0.1 s, and shows FWHM of 14 meV. It demonstrates the ~20x improvement in energy resolution that's readily available with our system. The blue spectrum in Fig. 1 was recorded from a ~2 nm Ø area in SiO 2 , in a single acquisition of 10 s duration, with a beam current of ~10 pA, probe convergence angle of ±12 mrad and collection angle also ±12 mrad. Its ZLP (not shown) had a FWHM of ~17 meV. The optical phonon peak visible at 140 meV energy loss had a peak intensity of ~1x10 -4 (above background) relative to the zero loss peak, a level the tail of the unmonochromated ZLP only decays to at about 1.7 eV. The energy of the phonon peak is in good agreement with the energy of the strongest feature normally observed in infrared spectra of SiO2, at 1100 cm -1 . (To convert cm -1 to meV, divide by 8.) The phonon signal is due to two types of interactions: a short-range and a long-range one. Fig. 2 demonstrates that both types of signals can appear in a single line scan. Fig. 2(a) shows a high-angle 66
doi:10.1017/s1431927614002050
fatcat:h3p33k7onzgjlg36q4tk6u4sli