Field Emission And Field Evaporation Properties Of Single Carbon Nanotube

The Low-Energy Electron Point Source (LEEPS) microscope projects a beam of electrons, ~102-103 eV in energy, on an object to create an enlarged shadow image on a screen. When a coherent electron beam illuminates a nanometer-scale material, interference pattern or holographic image is available, so that the topography and structure of the sample can be analyzed. In search for a high-intensity coherent electron source, a facility has been developed that can function as a LEEPS microscope or a field-emission/field ion microscope (FEM/FIM). During this process, the following problems were tackled: the obtainment of ultrahigh vacuum (UHV, 3.1×10-8Pa), the isolation of external vibration, the design of the inch-worm and its controlling circuit, automatic measurement of the curves of field emission current versus applied voltage (I-V curves) and sample transfer in UHV, etc. Also, a platform was developed for measuring field emission current, which precision could be as small as of the 10-1 nA order, based on the fact that a field emitter was actually a constant current source. Its cost was negligibly low except for the data acquisition card. Automatic control of the facility was attained using the virtual-instrument technology. So far, the smallest tip-sample separation is less than 5μm, the largest magnification of the projection image is over 4×104, and the best resolution is smaller than 10 nm. The factors that may affect the resolution of the projection image have also been analyzed theoretically and investigated experimentally. During the process of imaging, the multi-walled carbon nanotubes (MWCNTs) and carbon membrane on TEM grid were sometimes damaged or even disintegrated under the irradiation of electrons whose energy was as low as ~102 eV. The mean free path of these low-energy electrons in carbonaceous materials was only 1-2 nm and the MWCNTs and carbon membrane were completely opaque to them. We think the damage and disintegration were caused by the electrified molecules in the residual gas, which collided with the C atoms and broke the C-C bonds.An individual MWCNT was assembled onto a W tip under in situ observation in a transmission electron microscope (TEM), and then the sample was transferred into the FEM. Large field emission currents could blunt the MWCNT, so that its end structure underwent change and its I-V behavior was stabilized. Interestingly, a bistability in field emission was observed. That is, the I-V behavior jumped between two states, which both follow the Fowler-Nordheim (F-N) formula. The emission currents in the two states could differ by an order under the same voltage. The initial analysis has arrived at the conclusion that the bistability essentially originated from the adsorption and desorption. The solid angle of the electron beam was smaller than 10-2. The maximum brightness obtained was 1.3×1012 A?m2?sr-1, corresponding to a reduced brightness of 1.6×109 A?m2?sr-1?V-1.The FEM pattern of a Y-shaped MWCNT consisted of parallel streaks. The streaks are well interpreted as the result of a Young's double-slit interference with the ends of the two MWCNT branches treated as two secondary sources of the electron wave. The simulation result based on the Fraunhofer's diffraction theory indicates that the virtual source of Y-shaped MWCNT had a"∞"shape.It has been found experimentally that the evaporation fields of nanometer-scale materials are much lower than those of bulk materials. The evaporation field of an MWCNT is about 10V/nm. As a contrast, the evaporation field of graphite is as high as 103V/nm. A strong field could remove some atoms from an MWCNT, flatten an MWCNT end or open a capped MWCNT. The MWCNT could also be shortened if the evaporation time was long enough. Under a field of approximate 10 V/nm, the evaporation rate of the MWCNTs under investigation was several nanometers per minute. MWCNTs in an array could also be evaporated at a relatively low field, indicating an alternative to the post-fabrication treatment of arrays of nanometer-scale materials. The physical origin of the fact that the evaporation field of the MWCNT was much lower than the theoretical value for carbon is also qualitatively discussed. Firstly, the clean end of the MWCNT, which resulted from field desorption, contained a large number of dangling bonds and the C atoms on it had small co-ordination number, thus the heat of sublimation of these C atoms was low. Secondly, H atoms that possibly existed in the MWCNT could collide with the C atoms under strong electric field and make the latter's evaporation easier. Thirdly, the MWCNT itself was not perfect in structure and was susceptible to the strong mechanical stress under high fields. Thus, the MWCNT was prone to experience a reconstruction and part of it could be directly removed by the high tensile force. As disclosed by previous researchers, the major cluster that leaves the MWCNT due to field evaporation was C20+. A C20 tends to reconstruct into a dodecahedron. The heat of sublimation and the energy released during the reconstruction are assumed to be quite similar, so that they cancel each other. The calculated evaporation field based on this simple model is approximately in agreement with the experimental result.