3.3 Tem imaging of carbon nanomaterials  (Page 2/3)

The family of carbon allotropes and carbon nanomaterials

Common carbon allotropes include diamond, graphite, amorphrous C (a-C), fullerene (also known as buckyball), carbon nanotube (CNT, including single wall CNT and multi wall CNT), graphene. Most of them are chemically inert and have been found in nature. We can also define carbon as sp ² carbon (which is graphite), sp ³ carbon (which is diamond) or hybrids of sp ² and sp ³ carbon. As shown in [link] , (a) is the structure of diamond, (b) is the structure of graphite, (c) graphene is a single sheet of graphite, (d) is amorphous carbon, (e) is C ₆₀ , and (f) is single wall nanotube. As for carbon nanomaterials, fullerene, CNT and graphene are the three most well investigated, due to their unique properties in both mechanics and electronics. Under TEM, these carbon nanomaterials will display three different projected images.

Atomic structure of carbon nanomaterials under tem

All carbon naomaterials can be investigated under TEM. Howerver, because of their difference in structure and shape, specific parts should be focused in order to obtain their atomic structure.

For C ₆₀ , which has a diameter of only 1 nm, it is relatively difficult to suspend a sample over a lacey carbon grid (a common kind of TEM grid usually used for nanoparticles). Even if the C ₆₀ sits on a thin a-C film, it also has some focus problems since the surface profile variation might be larger than 1 nm. One way to solve this problem is to encapsulate the C ₆₀ into single wall CNTs, which is known as nano peapods. This method has two benefits:

1. CNT helps focus on C ₆₀ . Single wall is aligned in a long distance (relative to C ₆₀ ). Once it is suspended on lacey carbon film, it is much easier to focus on it. Therefore, the C ₆₀ inside can also be caught by minor focus changes.
2. The CNT can protect C ₆₀ from electron irradiation. Intense high energy electrons can permanently change the structure of the CNT. For C ₆₀ , which is more reactive than CNTs, it can not survive after exposing to high dose fast electrons.

In studying CNT cages, C ₉₂ is observed as a small circle inside the walls of the CNT. While a majority of electron energy is absorbed by the CNT, the sample is still not irradiation-proof. Thus, as is seen in [link] , after a 123 s exposure, defects can be generated and two C ₉₂ fused into one new larger fullerene.

Although, the discovery of C ₆₀ was first confirmed by mass spectra rather than TEM. When it came to the discovery of CNTs, mass spectra was no longer useful because CNTs shows no individual peak in mass spectra since any sample contains a range of CNTs with different lengths and diameters. On the other hand, HRTEM can provide a clear image evidence of their existence. An example is shown in [link] .