Graphite monolayer

This leaves us with a computed resuit in iess than satisfactory agreement with the experimental value of about 170 kcal/mol(57). The neglect of electron correlation and the limited basis set used are the most important sources of the discrepancy. In a previous study on monolayer graphite(56), basis set effects were found to lead to a significant underestimation of the cohesive energy. 

Y. Gamo, A. Nagashima, M. Wakabayashi, M. Terai, C. Oshima, Atomic structure of monolayer graphite formed on Ni(lll), Surface Science, 374 (1997) 61-64. 

Nagashima A, Nuka K, Itoh H, Ichinokawa T, Oshima C, Otani S (1993) Electronic states of monolayer graphite framed on TiC(l 11) sinface. Surf Sci 291 93-98... 

Aizawa, T., Souda, R., Otani, S., Ishizawa, Y., Oshima, C., 1990. Bond softening in monolayer graphite formed on transistion metal carbide surfaces. Physical Review B 42,11469—11478. 

III. FIELD EMISSION PROPERTIES OF MONOLAYER GRAPHITE/TMC EMITTERS... 

Table 2 Fonning Conditions and Lattice Constants of Monolayer Graphite on Transition Metal Carbide Surfaces...

It has to be noted that monolayer graphite (MLG) can be synthesized on the tip surface due to the surface processing using ethylene. This was confirmed by forming monolayer graphite on the surfaces of carbide single-crystal disks under the same condition as the surface processing of the tip (39-41). 

Monolayer graphite can be formed on the flat surfaces by heating the single-crystal carbide disks in an ethylene atmosphere. It has been found that the (111) surface is much more reactive than the (001) surface. The forming conditions on carbide surfaces (40-46) are summarized in Table 2. In the case of the TiC( 111) surface, minimal ethylene exposure is 100-200 L at 1100°C. It is very difficult to form MLG on a TiC(OOl) surface, indicating that exposures over 1 million L using ethylene gas are needed (45). In the case of the NbC(lll) surface, minimal ethylene exposure is 100 L at 1000-1100°C, and more than about 25,000 L exposure at 1100-1250°C is needed to form graphite on an NbC(lOO) substrate. 

Figure 9 LEED pattern of monolayer graphite on NbC single-crystal surfaces, (a) Monolayer graphite on the NbC(lll) surface, E = 117.3 eV. (b) Monolayer graphite on the NbC(lOO) surface, E = 170.1 eV. (From Ref. 41.)...

It is evident Irom Fig. 10 that there are six distinct phonon modes in the monolayer graphite. The LO branch is a longitudinal optical mode. The LA branch is a longitudinal acousticlike mode. The ZO branch is a vertically vibrating transverse optical mode. The ZA branch is a vertically vibrating acoustic-like mode. The SHO branch is a shear horizontal optical mode. The SHA is a shear horizontal acoustic-like mode. The last two SH modes appear because of the lack of mirror symmetry in these experiments (39). 

The phonon dispersion relation of the monolayer graphite on the NbC(OOl) surface is almost same as that of pristine graphite, whereas that of monolayer graphite on NbC(l 11) shows large softening as shown in Fig. 10. This tendency is observed in other TMCs, indicating that... 

B. Field Emission Characteristics of Monolayer Graphite/TMC Emitters... 

Figure 16 shows the stable emission current versus pressure relation for the surface-processed TiC<110> and NbC<110> tips (10). Data represented by open circles (a) are for the TiC<110> tip and data shown by full circles (b) and a solid line (c) are for the NbC<110> tips. The data of (a) and (b) show that log I is proportional to -log P. The current fluctuations are proportional to the product of I and P. The solid line in Fig. 16c is a calculated stable emission current for the 25,000 L ethylene-processed NbC<110> tip using data for the stable emission current of 24 pA at 2.1 X 10 Pa. The stable emission current of the 25,000 L ethylene-processed NbC<110> tip is about 50 pA at 1 x 10 Pa. Figure 16 indicates that the stable emission current for the ethylene-processed NbC tip is larger than that for the ethylene-processed TiC<110> tip. That is, the NbC tip is more stable than the TiC tip. This difference in emission stability is attributable to lack or existence of monolayer graphite on the (100) surface. Monolayer graphite can be formed on the NbC(lOO) surface over 25,000 L ethylene exposure, but it is quite difficult to form monolayer graphite on the TiC(lOO) surface. 

The author would like to thank Dr. Takashi Aizawa for numerous helpful discussions and for supplying phonon dispersion figures for monolayer graphite and photographs of their LEED... 

T Aizawa, R Souda, S Otani, Y Ishizawa, C Oshima. Anomalous bond of monolayer graphite on transition-metal carbide surfaces. Phys Rev Lett 64 768, 1990. 

Y Hwang, T Aizawa, W Hayami, S Otani, Y Ishizawa, SJ Park. Surface phonon and electronic structure of a graphite monolayer formed on ZtC(lll) and (001) surfaces. Surf Sci 271 299, 1992. B Tilley, T Aizawa, R Souda, W Hayami, S Otani, Y Ishizawa. Monolayer graphite on a tungsten-segregated TiC(lOO) surface. Sohd State Commun 94 685. 1995. 

A Nagashima, K Nuka, K Satoh, H Itoh, T Ichinokawa, C Oshima, S Otani. Electronic structure of monolayer graphite on some transition metal carbide surfaces. Surf Sci 287/288 609, 1993. 

T Aizawa, Y Ishizawa. Monolayer graphite on transition-metal carbides and application to field emitter. TANSO [No 155] 335, 1992 (in Japanese). 

Y Hwang, T Aizawa, W Hayami, S Otani, Y Ishizawa, SJ Park. Charge transfer between monolayer graphite and NbC single crystal substrates. Solid State Commun 81 397, 1992. 

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