Mass contrast

In neutron reflectivity, neutrons strike the surface of a specimen at small angles and the percentage of neutrons reflected at the corresponding angle are measured. The an jular dependence of the reflectivity is related to the variation in concentration of a labeled component as a function of distance from the surface. Typically the component of interest is labeled with deuterium to provide mass contrast against hydrogen. Use of polarized neutrons permits the determination of the variation in the magnetic moment as a function of depth. In all cases the optical transform of the concentration profiles is obtained experimentally. 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

As mentioned above, cardiac CT is somewhat limited as it only allows the cHnical application of CTA techniques, possibly in combination with plain scans for identification of calcifications or delayed scanning to assess mass contrast agent uptake. MW provides a much more sophisticated range of applications that may rule out or confirm clinical suspected pathologies or may narrow the list of differential diagnosis in some cases, MW may even be able to demonstrate features specific for a certain pathology and therefore yield a final diagnosis. 

TEM image contrast arises from the interaction of the incident electron beam with the specimen. Electrons travel inside specimen as a wave, and the electron wave can change both amplitude and phase. Both amplitude contrast and phase contrast contribute to TEM imaging. TEM imaging has several major imaging contrast mechanisms thickness-mass contrast, diffraction contrast, and phase contrast. 

Thickness-mass contrast is an amplitude contrast resulting from the absorption of electrons traveling inside the specimen. Both mass and thickness variations can produce contrast, because electrons interact with more material. Thickness-mass contrast is the most important one for amorphous materials. Although we will discuss below, the thickness-mass contrast always coexists with other contrast mechanisms. 

The mode of action of EM is based on the interaction of an electron beam with the atoms of the material under study, with contrast generally arising from the production of secondary or back-scattered electrons (in SEM) or the scattering of electrons by the spedmen (in TEM). Thus, it is necessary to have a sufifident intrinsic atomic mass contrast and an appropriate surface topography for TEM and SEM investigations, respectively. 

Polymers are comprised mainly of low-molar-mass elements such as carbon, hydrogen, and nitrogen, and consequently the atomic mass contrast is very low. 

In order to basically interpret the aforementioned seed-promoted core-shell NC growth mechanism, Tsuji et al. examined the two-step synthesis of Au-Ag core-shell nanostructures [207, 253]. Based on the fact that Au and Ag show the same fee crystal structure with similar lattice constants, nanostructures exhibiting comparable shapes and sizes are expected to evolve. Furthermore Au-Ag core-shell S5 ems are ideal candidates to be investigated at TEM structural analyses as they own different mass contrast therefore easily distinguishable in TEM images [33,253]. 

The earliest work on imaging block copolymer microdomains relied heavily upon transmission electron microscopy (TEM), and it still proves to be a useful tool to this day [19]. Samples are either microtomed or solvent cast to produce thin (ca. 100 nm) sections. PS-PI or PS-PB samples can be stained with osmium tetroxide to increase contrast. Osmium tetroxide reacts selectively with unsaturated double bonds such as found in PI or PB microdomains so as to provide mass contrast [21]. Unfortunately, TEM requires that the samples be freestanding or transferred to a transparent support (e.g. carbon), a cumbersome and time-consuming process that is largely incompatible with silicon or GaAs wafers. While silicon nitride membranes can be employed for TEM, these expensive and delicate structures are not easily accessible to all researchers [22]. 

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