Content deleted Content added
Citation bot (talk | contribs) m Alter: isbn, first1. Add: pmid, issue. Removed parameters. Formatted dashes. | You can use this bot yourself. Report bugs here. | User-activated. |
Citation bot (talk | contribs) m Add: pmid. | You can use this bot yourself. Report bugs here.| Activated by User:Nemo bis | via #UCB_webform |
||
Line 9:
The contrast of a HRTEM image arises from the [[Interference (wave propagation)|interference]] in the image plane of the [[Wave-particle duality|electron wave]] with itself. Due to our inability to record the phase of an electron wave, only the amplitude in the image plane is recorded. However, a large part of the structure information of the sample is contained in the phase of the electron wave. In order to detect it, the aberrations of the microscope (like defocus) have to be tuned in a way that converts the phase of the wave at the specimen exit plane into amplitudes in the image plane.
The interaction of the electron wave with the crystallographic structure of the sample is complex, but a qualitative idea of the interaction can readily be obtained. Each imaging electron interacts independently with the sample. Above the sample, the wave of an electron can be approximated as a plane wave incident on the sample surface. As it penetrates the sample, it is attracted by the positive atomic potentials of the atom cores, and channels along the atom columns of the crystallographic lattice (s-state model<ref>{{cite journal|last=Geuens|first=P|author2=van Dyck, D|title=The S-state model: a work horse for HRTEM.|journal=Ultramicroscopy|date=Dec 2002|volume=3-4|issue=3–4|pages=179–98|doi=10.1016/s0304-3991(02)00276-0|pmid=12492230}}</ref>). At the same time, the interaction between the electron wave in different atom columns leads to [[Bragg diffraction]]. The exact description of dynamical scattering of electrons in a sample not satisfying the [[weak phase object approximation]] (WPOA), which is almost all real samples, still remains the holy grail of electron microscopy. However, the physics of electron scattering and electron microscope image formation are sufficiently well known to allow accurate simulation of electron microscope images.<ref>{{cite journal |title=Computed crystal structure images for high resolution electron microscopy|author=O'Keefe, M. A., Buseck, P. R. and S. Iijima|volume=274|year=1978|pages=322–324| doi= 10.1038/274322a0 | journal=Nature | issue=5669 | bibcode=1978Natur.274..322O}}</ref>
As a result of the interaction with a crystalline sample, the '''electron exit wave''' right below the sample ''φ<sub>e</sub>('''x''','''u''')'' as a function of the spatial coordinate '''''x''''' is a superposition of a plane wave and a multitude of diffracted beams with different in plane [[Spatial frequency|spatial frequencies]] '''''u''''' (spatial frequencies correspond to scattering angles, or distances of rays from the optical axis in a diffraction plane). The phase change ''φ<sub>e</sub>('''x''','''u''')'' relative to the incident wave peaks at the location of the atom columns. The exit wave now passes through the imaging system of the microscope where it undergoes further phase change and interferes as the '''image wave''' in the imaging plane (mostly a digital pixel detector like a CCD camera). It is important to realize, that the recorded image is NOT a direct representation of the samples crystallographic structure. For instance, high intensity might or might not indicate the presence of an atom column in that precise location (see simulation). The relationship between the exit wave and the image wave is a highly nonlinear one and is a function of the aberrations of the microscope. It is described by the ''contrast transfer function''.
| |||