Vibrational Spectroscopy in the Electron Microscope

Vibrational spectroscopy in the scanning transmission electron microscope (STEM) was introduced two years ago [1, 2], and it has made much progress since. It has opened a new window on the world of materials, in which nothing is quite like it was before. The main vibrational modes occur at energies of 0‐500 meV, and exploring them requires a monochromated STEM‐EELS system with an energy resolution The energy of vibrational modes is given by ΔE = ħ √(k/m) , where k is the force constant of the atomic bond and m the effective mass of the vibrating nucleus. Strongly bonded light atoms give the highest vibrational energies, starting with hydrogen, an element that is nearly invisible in traditional electron microscopy. Fig. 1(a) shows a vibrational spectrum of Ca(OH) 2 [3], in which the peak at 452 meV is due to O‐H stretch, and Fig. 1(b) shows the particle from which the spectrum was recorded. Fig. 1(c) shows how the strength of the vibrational peak varied with the distance from the particle: the signal decayed only gradually outside the particle, and was still 50% strong 35 nm away. Fig. 2 shows an EEL spectrum of guanine compared to an IR spectrum from the same specimen [4]. The agreement between the two types of spectra is very good. EELS has worse energy resolution (~10 meV), but much better spatial resolution than regular IR. As is typical of vibrational spectroscopies, the different peaks can be assigned to different types of bonds and vibration modes (see the inset in Fig. 1). In order to minimize radiation damage, both the OH and guanine spectra were acquired in an “aloof” mode, with the electron beam parked just outside the sample [1, 3‐5]. Aloof spectroscopy makes it possible to select the maximum energy of the beam‐sample interaction, simply by adjusting the beam‐sample distance [4,5]. Its great import to vibrational EELS is that the vibrational signal can be excited even when the interaction energy is limited so that ionization damage of the sample cannot occur. It may even be possible to spatially map the vibrational features of a beam‐sensitive sample by “coarse step (leapfrog) scanning”: scanning with a discrete pixel increment of 10‐100 nm, so that even though the area that the beam traverses in each new position is essentially destroyed, large parts of the sample are not touched by the beam and remain in a pristine state [6].

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