Double-tilt

Compounds crystallized directly onto the carbon grid or with a defined orientation, due to other preparation methods, normally exhibit a suitable initial zone close to 0°. Samples from insoluble compounds are almost statistically oriented only biased by the particle shape. In this case, it is difficult to find a single crystalline part of appropriate thickness oriented with a suitable zone parallel to the surface. The best flexibility, and therefore the best possibility to orient a zone correctly, is given by a recently developed rotation-double tilt holder (Gatan Inc.). Through the combination of rotation and additional tilt (beta tilt) it is possible to orient the tilt axis exactly even if the crystal is not sitting flat on the support film (see Fig. 4). The tilt range, dependent on the pole piece distance of the objective lens, should be at least 40°. 

Figure 4 Movement steps using a double-tilt rotation holder 1) Rotate the crystal until the chosen axis is oriented along the goniometer axis 2) Tilt the crystal perpendicular to the goniometer axis until it is perpendicular to the electron beam 3) Adjust eucentric height 4) Tilt around the goniometer axis to reach different zones.

In the case of P Cu-phthalocyanine (space group P2i/c) only 8 different zones were found at 100 kV via SAED using a rotation-tilt holder (a=14.50 A, b= 4.70 A, c= 19.32 A, b=123°). With a rotation-double-tilt holder and in nano diffraction mode it was possible to detect 17 different zones at 300 kV (initial zone [001] and tilt about a and b ) from one crystal at room temperature (a=14.50A, b=4.84A, c=19.4lA, p=120.3°). In Fig. 5 the initial zone (middle) and two other zones obtained through different tilt series (a left hand side b right hand side) are shown exemplarily. 

Figure 5 Selected electron diffraction patterns collected with CCD from p copper-phthalocyanine taken with a rotation double-tilt holder at 300 kV with FEITECNAIF30 ST zone [012] from tilt about a (left hand side), initial zone [001] (middle), zone [-101] from tilt about b (right hand side)...

Instrumentation. For many mineralogical studies, the minimum requirement is a 100-kV microscope with a point-to-point resolution of 0.4 nm, fitted with a double-tilt (or tilt-rotate) specimen holder. 

Figure 3.6 A specimen holder for a TEM (JEM-2010F) with a double-tilting function. The insert shows the enlarged part of the holder end where the 3-mm disc of the specimen is installed.

The specimen holder is a sophisticated, rod-like device that not only holds the specimen but also is able to tilt it for better viewing inside the TEM column. Types of specimen holders used include single-tilt and double-tilt. The single-tilt holder allows a specimen to be titled along one axis in the disc plane. The double-tilt holder allows the specimen be tilted independently along two axes in the disc plane. The double-tilt holder is necessary for analytical studies of crystalline structures. 

Figure 1. A modem high-resolution TEM operating at 200 kV (JEOL JEM-2010). The spherical aberration of the objective lens is 0.5 mm and the resultant point resolution is about 0.20 nm. The double-tilt specimen holder can rotate to +20°.

Fig. 3.4. (a) Two-dimensional illustrations of the eight possible double-tilted bent-core... 

Clearly, if a rubber modified blend contains particles larger than the film thickness (about 1 /xm), the film cannot be representative of the bulk. For the study of craze tips and craze growth, a double tilting stage was used to obtain stereo pairs of the craze tip [396]. 

To tilt specimen to the right orientation using a double-tilt holder is always the difficult part for a TEM beginner. Different users have different ways to double-tilt specimen. Here is just one way that can be used. 

DOWN positions which correspond to the LEFT and RIGHT positions for the director that moves along the surface of a cone, the axis of which is normal to the layers and parallel to the cell substrates. In the LEFT and RIGHT positions the director remains parallel to the substrates and the SSFLC cell behaves as a uniaxial phase plate. The total angle of switching equals the double tilt angle 6. 

The double-tilted structure, which has a triclinic configuration with chiral Q symmetry. It corresponds to the SmCg phase proposed by de Gennes (G stands for "general"). Experimental evidences of this phase were foimd recently both in free-standing filmsii and in bulk samples. ... 

The B7.11 materials listed in Table 1.2, and that form telephone wire type structures (see Figure 6.16) when cooling from the isotropic phase, can be switched under high (E>10 V/fim) electric fields that decrease with decreasing temperatures. materials can be both ferroelectrici and antiferroelectric, and based on polarization current and electro-optical observations, it has been suggested tiiat tiiey have out-of-plane polarization components, i.e., they correspond to the double-tilted SttiCq phase. 

On the same grounds one also can imagine that both tilt directions are possible simultaneously [7]. Those double - tilted situations are labeled both with C and... 

Figure 2-4. (a) Possible double - tilted bent-core smectic structures. Top row Illustration of the fluid in plane orders of the layer polarization (either uniform or modulated), and 3 - dimensional explanation of tilt. Middle and bottom rows 2 dimensional illustrations of the eight possible situations (b) Graphical explanation of the SmCLP structures in term of two rotation angles 0 and <()... 

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