Graphite atomic resolution

Because STM measures a quantum-mechanical tunneling current, the tip must be within a few A of a conducting surface. Therefore any surface oxide or other contaminant will complicate operation under ambient conditions. Nevertheless, a great deal of work has been done in air, liquid, or at low temperatures on inert surfaces. Studies of adsorbed molecules on these surfaces (for example, liquid crystals on highly oriented, pyrolytic graphite ) have shown that STM is capable of even atomic resolution on organic materials. 

In Fig. 4 we show an atomic resolution image of a carbon tube. The structure imaged at the upper right corner of the picture comes from another tube. Both of them were —1000 A long. A perfect honeycomb surface structure is observed. By taking into account the curvature of the tube surface and the STM imaging profile, we find the same lattice parameter as that of. graphite (1.42 A). This directly proves that the tubu-... 

Figure 7.12 Images at atomic resolution of graphite obtained with scanning tunneling (left) and atomic force microscopy (middle). The graphite lattice contains two types of sites A-sites with a carbon atom neighbor in the second layer and B-sites without a neighbor in the next layer. STM detects the B-sites, whereas the A-sites show up better in AFM. (STM image courtesy of TopoMetrix AFM image courtesy of M.W.G.M. Verhoeven, Eindhoven).

The STM has successfully resolved a number of organic molecules adsorbed on various conducting substrates (Sleator and Tycko, 1988 Ohtani et al., 1988 Foster and Frommer, 1988). A commonly used substrate is graphite, which is easy to prepare, defect free on large areas (several micrometers), and inert. Under favorable conditions, nearly atomic or atomic resolution can be achieved. In many cases, the organic molecules adsorbed on crystalline substrates form regular patterns, which are of scientific interest in and of themselves. 

Tip-state effects 19—20, 126, 297 atomic resolution, and 32 corrugation enhancement 125 corrugation inversion, and 137 graphite, and 146 layered materials, on 20 metal surfaces, on 19 scanning tunneling spectroscopy, and 24, 308 Tip-sample distance 53... 

The study of organic molecules on surfaces has been an early application of STM. Smith [10] has found that liquid crystal molecules can be adsorbed on graphite and imaged with atomic resolution by STM in ambient conditions. Organic molecules were also studied by STM in vacuum [11-13]. After true atomic resolution by AFM became feasible, several groups imaged molecules in vacuum by AFM [14-16]. 

Fullerene C(,o adsorbed onto STM tips has been reported to enhance atomic resolution images of highly oriented pyrolytic graphite [110]. Recently, Dai et al. [Ill] have demonstrated that multiwalled carbon nanotubes (MWNTs) attached to the silicon cantilever of a conventional atomic force microscope (AFM) can be used as well-defined tips with exceptionally high resistance to damage from tip crashes. The MWNT were attached by first coating the bottom 1 -2 mm section of the silicon tip with an acrylic adhesive by inserting them... 

Fig. 2. (A) Atomic resolution STM image of a carbon tube, 35 A in diameter. In addition to the atomic honeycomb structure, a zigzag superpattern along the tube axis can be seen. (B) "Ball-and-stick" structural model of a Cg(,-based carbon tube. The upper part is closed by a Cgo hemisphere cap. (C) Structural model of a giant superpattern produced by two misoriented graphitic sheets. The carbon atoms in the first layer are shaded, and the second layer atoms are open. Between the two dashed lines, we highlight those first layer atoms with white that do not overlap with second layer atoms. Because of their higher local density of states at the Fermi level, these atoms (p-type atoms) appear particularly bright in STM images (16,21). [ results in a zigzag superpattern along the tube axis within the two white dashed lines as indicated.

Figure 20.8 Ex situ atomic resolution scanning tunneling microscopy (STM) images of sulfur atoms adsorbed on highly oriented pyrolytic graphite (HOPG) (a) 3x3nm (b) 6.32x6.32nm. ...

---