LACBED patterns provide a map of scattered intensity as a function of incident angle and specimen position for a specific scattering vector, s [1]. The pattern is generated by focusing the probe on the specimen, raising the specimen by an amount, z, and then selecting a scattering vector, s, using a selected area aperture. The aperture then selects those electrons scattered with a specific range of transverse momenta. The smaller the aperture and the greater the defocus, the smaller the range of momenta selected.
Normally, the scattering vector is chosen to be a Bragg vector, g, and the LACBED patterns then map the intensity scattered into g. This intensity is due primarily to elastic scattering processes [2]. In this work, we consider zero-loss energy-filtered LACBED patterns where the scattering vector, s, is chosen not to equal a Bragg vector. In this case, the intensity scattered into the pattern derives almost entirely from electrons that have scattered from phonons because of the energy filtering action of the SA aperture and the imaging filter. Specifically, these patterns map the angular and spatial dependence of scattering from all phonon modes with wave vectors, q, such that s=g+q, where g can be any Bragg vector. By using a 1micron selected area aperture and a large z, a scattering vector can be selected with an angular resolution within about 0.01A-1. We apply this approach to map the scattering from the low energy transverse acoustic mode in silicon with wave vectors in the planes {011} [3] and to map scattering in spatially inhomogeneous materials. If this approach were executed in an ultrahigh resolution STEM, such as have recently become available, it may be possible to resolve the currently elusive low energy transverse acoustic phonon modes.
Acknowledgements: Instrumentation and staff at the Monash Centre for Electron Microscopy