Amorphous carbon layer

The non-diamond carbon phase in polycrystalline diamond films (often referred to as graphite, although this conclusion is far from accurate [23]) is first and foremost the disordered carbon in the intercrystallite boundaries. Their exposure to the film surface can be visualized by using a high-resolution SEM techniques [24] the intercrystallite boundaries thickness comes to a few nanometers. In addition to the intercrystallite boundaries, various defects in the diamond crystal lattice contribute to the non-diamond carbon phase, not to mention a thin (a few nanometers in thickness) amorphous carbon layer on top of diamond. This layer would form during the latest, poorly controlled stage of the diamond deposition process, when the gas phase activation has ceased. The non-diamond layer affects the diamond surface conduc-... 

Figure 4.15 An in situ electron microscopic video of the migration of fluidized Fe-C particles In the amorphous carbon layer at temperature 650°C [4]. (Courtesy of K. I. Zamaraev )...

The HRTEM study of diamond nucleation and growth on copper TEM grids in HFCVD by Singh provides direct evidence for the formation of a diamond-like amorphous carbon layer. The intermediate layer is 8-14 nm thick, in which small diamond microcrystallites approximately 2-5 nm across were embedded (Fig. 6a), and large diamond crystallites were observed to grow from these microcrystallites (Figs. 6b-c). 

Figure 4. STEM images for Iwo Fe-FTS catalyst grains. The nanozone areas (Zones 1-4) are marked by the arrows and the dotted lines show the location of the amorphous carbon layer which is not clearly visible in STF.M mode.

The observation of a layer of amorphous carbon at the film surface, in combination with the observation of liquid crystal alignment on this surface, suggested the breakthrough idea to replace the polyimide polymer film with an amorphous carbon layer [34]. The essential requirement for liquid crystal alignment (as stated by our model), namely the presence of an anisotropic distribution of directional bonds, can be fulfilled by an ion beam irradiated amorphous carbon layer. This is demonstrated by the presence of the resonance associated with tt orbitals at 285 eV in the absorption spectrum of amorphous carbon (bottom of Fig, 6.12). Its presence indicates that amorphous carbon contains unsaturated sp2 and sp hybridized carbon atoms. While sps hybridization does not lead to any anisotropy, the directional nature of carbon double and triple bonds formed by sp2 and sp hybridized carbon atoms can lead to a breaking of the isotropy of the molecular distribution. It therefore mainly remains the question whether a statistically significant anisotropy in these carbon bonds can be achieved by ion beam irradiation of an amorphous carbon layer. 

Comparing the spectra in the right panel, which characterize the molecular tilt angle, one finds the same polarization dependence for the two ion beam irradiated materials, which is opposite to the one of the rubbed polyimide film. Hence, a downwards liquid crystal pretilt angle is expected for both ion beam treated surfaces. Again, since the overall shape and the tt intensities and their dichroism is comparable for the two ion beam irradiated films, liquid crystals ai e expected to exhibit a technologically sufficient pretilt angle on an ion beam irradiated amorphous carbon layer. 

In line with these expectations, liquid crystal alignment has indeed been observed on ion beam irradiated amorphous carbon layers deposited either by sputtering or CVD [35]. In fact, also a wide variety of other materials showed liquid crystal alignment upon ion beam irradiation, for example, SiNa, hydro-... 

Our study of ion beam irradiated polyimide films revealed that in this case the aligning surface consists of an amorphous carbon layer, which is caused by degradation of the molecular polymer structure in the ion beam. This observation led to the idea to replace the polyimide film with an amorphous carbon film. Indeed, an ion beam irradiated amorphous carbon layer appears to align liquid crystals as well as an ion beam irradiated polymer film. These results convinced IBM to develop a new manufacturing technology for liquid crystal displays, which successfully produced displays of highest quality. 

The fact that this new manufacturing technology was developed by IBM, was also due to the circumstance, that there, the deposition of amorphous carbon layers was a well-known and well-established process, since these layers are used as protective coatings on magnetic hard disks. Based on the developed know-how and the experience in depositing these as homogeneous and ultra-smooth thin coatings, there was confidence that amorphous car-... 

Fig. 7.7 SEM image (a-b) showing the shape of the secondary particles. There are slight agglomeration and small quantity of fragments. Values of the grain size are given in nm. TEM images (c-d) showing the amorphous carbon layer deposited onto the LiFeP04 crystallite...

The width of the G-line at 1569 cm is 99.3 cm characteristic of hydrogen-free amorphous carbon layers and markedly larger than the width of this line in the hydrogenated amorphous carbon(a-C H) [128]. This result gives evidence that although there is some hydrogen in the carbon deposited on LiFeP04 the H/C ratio is very small. This is actually consistent with the fact that the dramatic increase in the electronic conductivity after pyrolysis at temperatures above... 

Ti02-B nanowires encapsulated inside and an amorphous carbon layer coating the outside were obtained by hydrothermal process [349]. These carbon-Ti02-B nanowires exhibited a high reversible capacity of 560 mAh g after 100 cycles at the current density of 30 mA g good cycling stability, and rate capability... 

---