Advances in Ultra-High Energy Resolution STEM-EELS

The Ultra-High Energy Resolution Monochromated EELS-STEM (U-HERMES ™ ) system developed by Nion combines a dispersing-undispersing ground-potential monochromator [1], a bright cold-field-emission gun, an advanced aberration corrector, and a new EEL spectrometer. The latest version of the system allows 5 meV energy resolution EELS and has achieved 1.07 Å spatial resolution at the sample at 30kV when monochromating [2], and it greatly extends the capabilities of vibrational spectroscopy in the EM, introduced 4 years ago [3]. U-HERMES ™ has so far been used for: damage-free identification of different bonds including hydrogen bonds in guanine [4]; probing atomic vibrations at surfaces and edges of nano-objects with nm-level spatial resolution [5]; achieving sub-nm spatial resolution in images obtained with dark-field EELS vibrational signals [6]; nanoscale mapping of phonon dispersion curves [7]; nanoscale temperature determination by electron energy gain spectroscopy [8]; identification of different isotopes by vibrational spectroscopy in the EM [9]; and vibrational spectroscopy of ice [10]. A key challenge for EELS in the EM is to improve the energy resolution to 5 meV and better, as needed for vibrational spectroscopy of organic compounds in which different C-C, C-N etc. vibrational modes are resolved. This required a new electron energy loss spectrometer, with an energy resolution that matches the Nion ultra-high energy resolution monochromator. The monochromator produces a diffraction-limited spectrum at the energy-selecting slit