B. Cho

Highly efficient electron gun with a single-atom electron source

The authors have demonstrated highly collimated electron-beam emission from a practical electron gun with a single-atom electron source; ∼80% of the total emission current entered the electron optics. This ratio was two or three orders of magnitude higher than those of the conventional electron sources such as a cold field emission gun and a Zr∕O∕W Schottky gun. At the pressure of less than 1×10−9Pa, the authors observed stable emission of 20nA, which generates the specimen current of 5pA required for scanning electron microscopes.

Measurement of pressures in 10−10Pa range from the damping speed of field emission current

The damping behavior of field emission (FE) currents was monitored with W(111) tips under pressures down to 5×10−10Pa. Semilogarithmic damping curves of FE currents were linear for 0.3L hydrogen exposure in the thoroughly degassed extreme high vacuum (XHV) FE system. The slope of semilogarithmic damping curves was linearly proportional to the operation pressure, suggesting a method of measuring pressure in an XHV range. Electron stimulated desorption was not observed.

Thermodynamically stable nanotips of Au–Mo alloy

The authors propose a simple new method for fabricating stable nanotips using sharpened Au–Mo alloy tips. By annealing at 1000–1200K in UHV, Au atoms segregate to the alloy surface to form a Au film of one physical monolayer thickness, resulting in formation of nanopyramids on the (111) surface. Although field ion microscopy images of the nanotips show no single-atom termination similar to the previous cases of Au-deposited W nanotips, energy spectra of the emitted electrons strongly suggest the realization of a single-atom termination just after annealing.

Direct Confirmation of the High Coherency of the Electron Beam from a Nanotip

The coherency of an electron beam from a nanotip was evaluated and compared to that from a tungsten (110) oriented tip by using nano-biprisms in a field-emission projection microscope (FPM). The FPM images were consistent with the corresponding field emission patterns of the employed tip. The nanotip generated much sharper biprism interference patterns than the W(110) tip, and comparison of the visibilities of interference patterns demonstrated directly that a highly coherent electron beam was emitted from the nanotip.

Coherent and intense multibeam generation by the apex of sharp nano-objects: Electron half-circular prism

Mutually coherent multiple electron beams (multibeam) were generated from diffraction at the apex of sharp nano-objects, especially carbon nanotubes, in a field emission projection microscope. Simulation using a simple scalar diffraction model showed that the apex of sharp nano-objects played the role of an electron half-circular prism, deflecting an electron beam through a constant angle toward the prism axis. The multibeam diffraction pattern gave a high visibility of ∼0.8 and high intensity.Mutually coherent multiple electron beams (multibeam) were generated from diffraction at the apex of sharp nano-objects, especially carbon nanotubes, in a field emission projection microscope. Simulation using a simple scalar diffraction model showed that the apex of sharp nano-objects played the role of an electron half-circular prism, deflecting an electron beam through a constant angle toward the prism axis. The multibeam diffraction pattern gave a high visibility of ∼0.8 and high intensity.

Record of the lowest frequency shot noise measurement below 10Hz

The noise of field emission (FE) currents ranging from 10pAto100μA was measured under ∼7×10−10Pa. Stepwise features were routinely observed in damping curves of the FE current, indicating remarkable sensitivity of the FE current measuring system. The lowest frequency measurement of shot noise was recorded below 10Hz. Space-charge suppression of noise was not observed for FE currents up to 100μA. Adsorption of residual gas increased the intensity of noise and made the noise of field emission current be dominated by the flicker noise.

Extreme high vacuum field emission microscope for study on the inherent fluctuation of field emission

An extreme high vacuum field emission microscope (XHV FEM) was constructed for the study of inherent fluctuations of field emission (FE) current. The damping and fluctuation behaviors of FE current from clean W(111) tips at 90K were observed using the XHV FEM. Exposure to XHV of 7.5×10−10Pa continued to damp FE current for more than 2000min. The fluctuation (∼0.01%) of FE current (1nA) for the clean W(111) tips was comparable to the corresponding shot noise fluctuation (0.005%), demonstrating the suitability of the XHV FEM for the inherent fluctuation study of the FE process. After saturating the tip surface with hydrogen, the FE current showed fluctuations of <0.1%

Evaluation of the coherence of electron beam from low temperature field emitter

Summary form only given. Multi-walled carbon nanotubes (MWCNT) have been known to function as nano-biprism in projection microscope. Here we measure the transverse coherence length (Lt) of electron beam (e-beam) from tungsten field emitter by using the nano-biprism. A MWCNT is irradiated by an e-beam radially propagating from a field emitter and its magnified image is projected on the screen. Tip approach enlarges the image and beautiful interference patterns always show up as shown. With decreasing the source temperature from RT to 78K, the visibility of the interference fringe increases by a factor of 3, and the bandwidth of interference pattern widens by a factor of 5. This enhancement of coherence with temperature fall strongly suggests the direct relationship between the coherences of electronic state inside the field emitter and e-beam. Now we are preparing experiments at liquid helium temperature with conventional tungsten tips and superconducting niobium tips, which will provide further enhancement of the coherence of e-beam.

Field electron emission from a high-T/sub c/ superconductor Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// whisker

In this study, field emission (FE) properties of a high-T/sub c/ superconductor Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// (Bi-2212) whisker as well as the optimization of the sample preparation methods were explored. Either chemical etching or mechanical cutting was undertaken at first in order to sharpen one end of the whisker. Both of the treatments successfully yielded a sharply pointed tip with its radius less than 500 nm. Subsequent preparation treatments in a UHV chamber provided layered patterns in FTM observation, which were very characteristic of those of other layered superconductors. More importantly, the treatments inside the UHV chamber stabilized FE currents to a significant level. The resulting fluctuation was subtle enough to be analyzed the energy distribution curves of the FE electron beams issuing from the superconducting Bi-2212.