Akihiro Takakura

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.

Field emission from multiwall carbon nanotubes in controlled ambient gases, H2, CO, N2 and O2.

Adsorption and desorption on clean pentagons at a tip of multiwall carbon nanotube (MWNT) have been investigated by field emission microscopy (FEM) in an atmosphere of various gases, i.e., hydrogen, carbon monoxide, nitrogen and oxygen. A MWNT with clean surface which is obtained by heat treatment gives FEM patterns consisting of six bright pentagonal rings. Adsorbates are recognized as bright spots in the FEM pattern. They reside preferentially on the pentagonal sites where the strong electric field is concentrated, and bring about stepwise increase in the emission current. Heat treatment of the MWNT emitter at about 1300K allows adsorbates to desorb. After the desorption of hydrogen and nitrogen, the original clean surface with pentagons is recovered, while the tip structure is destroyed after the desorption of oxygen.

Interference fringes observed in electron emission patterns of a multiwalled carbon nanotube

Field electron emission patterns from a MWNT with clean surface exhibit fine structures originating from pentagons located at the tip end. The field emission microscopy (FEM) has resolved six pentagons each of which has a small dark spot in its center. Moreover, the bright streaks have been observed at the boundary regions between adjacent pentagons. By calculations based on the formula of Youngs interference of two beams, Oshima et al. (2002) reported that the observed streaks are interference fringes. In this paper, we show experimental results to prove that the streaks are Youngs interference fringes.

Energy distributions of field emitted electrons from a multi-wall carbon nanotube.

Field emission energy distributions of electrons from one of the six pentagons located at the end of a multi-wall carbon nanotube have been measured by means of a high-resolution cylindrical energy analyzer. In a clean pentagon, the sub-peak was obtained at about 500 meV below the main peak, exhibiting a shift with increasing applied voltage. For electrons emitted from an adsorbate onto the pentagon, no fine structure was observed in the spectra. The broadening of the leading edge was also observed for both clean and adsorbed pentagon, indicating the field penetration into the nanotube due to its semimetallic nature. The full-width at half-maximum was 280 meV at the applied voltage of 660 V and increased linearly with applied voltage.

Energy spectra of field emission electrons from multiwalled carbon nanotubes

We have measured energy spectra of field emission (FE) electrons from multiwalled carbon nanotubes in various conditions such as a tip temperature, electric fields, the emission direction, and adsorption. The observed spectra indicated that peak near EF originated from the FE electrons from the inner walls, while FE electrons forming the shoulder at deeper energy came from a surface wall. Both the position and the intensity of the shoulder changed from one specimen to the others; no spectra were the same as the others. The Fermi level of the surface wall is different from that of the inner walls under the FE condition.

Electron Emission Sites on Carbon Nanotubes and the Energy Spectra.

Two kinds of electron emission sites on carbon nanotubes have been clarified; one is a nanoprotrusion exhibiting deformed honeycomb structures composed of carbon hexagons,pentagons and possibly heptagons. The other is either an edged species or adsorbates. The emission spectra show two characteristic features; a broad main peak as compared with theoretical curves based on Fowler-Nordheim theory, and an additional shoulder at about 0.5 eV from EF, of which the features are observed independent of the emission direction. The broad main peak may indicate that energy band bending occurs near the emission sites.

Field emission from carbon nanotubes with clean surface and adsorbed molecules

Adsorption and desorption on pentagons at a tip of multiwall carbon nanotube (MWNT) have been investigated by field emission microscopy (FEM) in an ultra-high vacuum and in an atmosphere of nitrogen or oxygen. MWNTs with clean surface which are obtained by heat treatment give FEM patterns consisting of six bright pentagonal rings. Adsorbates (nitrogen and oxygen) are recognized as bright spots in the FEM pattern. They reside preferentially on the pentagonal sites where the strong electric field is concentrated, and bring about stepwise increase in the emission current. Heat treatment of the MWNT emitter at about 1300 K allows nitrogen and oxygen to desorb. After the desorption of nitrogen, the original clean surface with pentagons is recovered, while the tip structure is destroyed after the desorption of oxygen.

Electron Emission from Pentagons on a Carbon Nanotube Tip Revealed by Field Emission Microscopy

Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10 -10 Torr, residual gas molecules, attracted by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for stepwise increases and decreases in the emission current, respectively.

Interference fringes observed in electron emission patterns of a multi-wall carbon nanotube

Field electron emission patterns from a MWNT with clean surface exhibit fine structures originating from pentagons located at the tip end. The field emission microscopy (FEM) has resolved six pentagons each of which has a small dark spot in its center. Moreover, the bright streaks have been observed at the boundary regions between adjacent pentagons. By calculations based on the formula of Youngs interference of two beams, Oshima et al. (2002) reported that the observed streaks are interference fringes. In this paper, we show experimental results to prove that the streaks are Youngs interference fringes.

Field Emission Microscopy of Carbon Nanotubes

Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10−10 Torr, residual gas molecules, coming from the nanotube shank through field-enhanced migration or directly from the gas phase by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for step-wise increases and decreases in the emission current, respectively. Energy spectra of electrons from a clean pentagon and through an adsorbed molecule are measured individually. For a clean surface a subpeak is observed at about 0.5 eV lower than the main peak, while the subpeak disappeared for the adsorbed surface.