Jungna Heo

Thickness effect on secondary electron emission of MgO layers

Two series of MgO thin layers having various thicknesses were prepared on the Si substrate by electron-beam evaporation and by spin coating of MgO precursor solutions. We found that the magnitude of the secondary electron emission (SEE) yield of the MgO films strongly depends on the film thickness and the sample bias voltage. We ascribed it to the electric field through the insulating MgO layer, which allowed fast supply of electrons from the Si substrate to the surface. The mechanism of electron supply can be explained either as an acceleration through the MgO layer that becomes partially conductive upon primary electrons bombardment (radiation induced conductivity), or as a tunneling through the non-irradiated region of the insulating layer where the primary electrons cannot reach deeply into the sample with a certain penetration depth. The maximum SEE yield of the each MgO film on the Si substrate was observed when the penetration depth of primary electrons was close to the thickness of the MgO film, if the applied electric potential to the sample was low. Under a strong electric potential, the relationship between the penetration depth of primary electrons and the thickness of MgO films is not observed. It suggests the existence of the non-irradiated region, where electron supply is allowed by electron tunneling. Therefore, the magnitude of SEE yield for the thin insulating layer is strongly related to the detailed mechanism of electron supply, which is determined by the thickness of the insulating layer and the applied bias voltage to the sample during the SEE process.

Secondary electron emission yields from MgO deposited on carbon nanotubes

Enormously high secondary electron emission yields under electric field are observed from MgO deposited on carbon nanotubes. The yields reach a value as high as 15 000 and are strongly dependent upon the bias voltage applied to the sample. The creation of the electric field across the MgO film after bombardment of primary electrons is considered as one of key features, since positive charges are generated at the surface by departure of secondary electrons. Subsequent bombarding electrons produce other secondary electrons inside the MgO film, then the liberated secondaries are accelerated towards the surface under the strong field. Under this condition, the secondary electrons gain sufficient energy to create further electrons by impact ionization. The process continues until an equilibrium avalanche is established. To elucidate the earlier explanations, the kinetic energy spectra of secondary electrons are measured by an energy analyzer at various bias voltages in MgO/carbon nanotube samples. The analysis...

Secondary electron emission from magnesium oxide on multiwalled carbon nanotubes

We have investigated effects of electric fields on the yield of secondary electron emission (SEE) from the primary electron bombardment on magnesium oxide (MgO) covering vertically grown multiwalled carbon nanotubes (MWCNTs). We observe that the yield of SEE increases up to at least 22 000 at a special condition. The strong local field generated by the sharp tip of vertically grown MWCNTs accelerates secondary electrons generated by primary electrons. This eventually gives rise to so called Townsend avalanche effect, generating huge number of secondary electrons in a MgO film. Emission mechanism for such a high SEE will be further discussed with energy spectrum analysis.

Current degradation mechanism of single wall carbon nanotube emitters during field emission

Electron emission current degradation is often observed from printed single wall carbon nanotube emitters during field emission process. After a highly imposed emission, structural deformation of emitters from thin crystalline nanotube bundle to thick amorphous-type carbon fiber was observed. This deformation seems to relate to the current degradation, deteriorating the efficiency of field emission either by increasing the resistance of emitters or by decreasing the field enhancement factor of emitter tips. Two possible mechanisms of structural deformation are internal structural transformation by Joule heating under excessively imposed emission current and continuous adsorption of carbon particles on actively working emitters.

Energy distribution for undergate-type triode carbon nanotube field emitters

Field emission energy distribution (FEED) has been measured for undergate-type triode carbon nanotube (CNT) field emitters where the gate electrodes are located underneath the cathode electrodes. The diode-type emission for these CNT emitters was found to follow the Fowler–Nordheim relation, whereas the triode-type emission exhibited the deviation from this relation. The FEED peaks for the undergate CNT emitters under the triode-type emission shifted to lower energy as the gate voltage increased, indicating nonmetallic behavior for the CNT emitters. There exist two different characteristic FEED peaks, where their peak energy shifts as a function of the gate voltage belong to two different slopes. From the difference in the position and intensity of the peaks, it was found that one was field emission directly from CNTs and the other might be emitted from CNTs through glass powders which were added during the CNT field emitter fabrication process.

Effect of Randomly Networked Carbon Nanotubes in Silicon-Based Anodes for Lithium-Ion Batteries

Despite the advantageous characteristics of a carbon nanotube (CNT), commercially reliable Si-CNT nanocomposites have not yet been developed due to the difficulties in effective dispersion of CNTs and low initial coulombic efficiency arising from the high surface area of CNTs. In this study, we present a facile wet-type milling process, providing Si-CNT nanocomposites with excellent electrochemical performances (initial capacity > 2000 mAh g ―1 , initial coulombic efficiency ∼80%, and improved lifetime) through a proper selection of a liquid medium and a post-thermal treatment. The improved performance is largely contributed from a suppressed oxidation of silicon nanograins as well as a stronger linkage between silicon and CNTs. The comparison of a series of Si-carbon nanocomposites prepared by an identical method but using different types of carbons indicates that only the one-dimensional CNT achieved effective percolation through the randomly networked electrical conduction in silicon nanograin aggregates.

Undergate-type Triode Carbon Nanotube Field Emission Display with a Microchannel Plate

The characteristics of a field emission display (FED), which is based on an undergate-type triode carbon nanotube (CNT), have been examined by incorporating an electron-multiplying microchannel plate (MCP) between the anode and cathode plates of a FED. The MCP was fabricated by electroless plating and the sol–gel process on punched alumina. By applying appropriate voltages between the two faces of an MCP within a FED, the current at the anode plate of a FED was found to be enhanced more than three to five times, leading to higher brightness. The focusing of field emitted electrons was also improved by adjusting the bottom voltage of the MCP, which resulted in a clear image. Therefore, the incorporation of the MCP improved the performance of an undergate-type CNT FED, which can now be considered as one of the key candidates for flat panel displays.

Uniformity measurement of electron emission from carbon nanotubes using electron-beam resist

The field-emission sites’ distribution was measured to monitor the emission uniformity from randomly oriented carbon-nanotube (CNT) emitters using electron-beam resists (ER). The dot-patterned CNT emitters were fabricated by screen-printing a photoimageable CNT paste on an indium doped tin oxide (ITO) coated glass plate. An ER-coated Si substrate used as an anode provides the detection of the location and amount of the electron emission from the partial number of active emission sites among many existing CNTs. The measurements were carried out with the variation of electrical fields through continuous- or pulsed-voltage applications on a diode-type configuration. Developed ER images after a similar dosage of field-emission current flow indicate that emission uniformity is improved as the electrical field is increased. This method suggests that the emission uniformity could be estimated for various conditions of emitter preparation, such as CNT type, paste composition, and dispersion process, as well as th...

Improvement of field emission characteristics of carbon nanotubes through metal layer intermediation

A method of fabricating carbon nanotube (CNT)-based field emitters has been studied to improve field emission characteristics. From the supplementary substrate coated with CNTs, CNTs were transferred to the objective substrate through the metal intermediation (MI) layer where the heat and pressure were applied. CNTs were vertically aligned on the objective substrate after removing the supplementary substrate. The field enhancement effect of emitters can be increased by the formation of the sharp edges through CNT transfer process. This MI process allows one to lower the processing temperature below 300 °C and form the patterned CNT emitter arrays.

Study of the secondary-electron emission from thermally grown SiO2 films on Si

Abstract The secondary-electron emission (SEE) coefficient, δ, was measured for thermally grown SiO 2 films on a Si wafer. As the thickness of SiO 2 film becomes greater than 40∼50 nm, the SEE curve changes to the double-humped shape having a low δ value from the typical ‘universal curve’ shown at ordinary SEE measurements. We conclude, from a comparison of the SiO 2 thickness and the penetration depth of primary electrons, that this is due to the surface charging effect of an insulating SiO 2 film. To overcome the charging effect, an electric field is introduced inside a thick SiO 2 film (55 nm) by applying a negative bias potential to the sample. When the bias potential is increased, δ of thick SiO 2 is increases constantly up to value similar to that for thin SiO 2 (2 nm), and the SEE curve recovers the original, universal curve form. The role of the bias potential is believed to increase the tunneling probability between the bulk Si and the SiO 2 surface, and thus electrons from Si can easily be supplied to the SiO 2 surface, where holes are generated upon the departure of secondary electrons.