Vu Thien Binh

Freestanding vertically aligned arrays of individual carbon nanotubes on metallic substrates for field emission cathodes

Direct growth of individual and vertically aligned carbon nanotubes (CNTs) onto a metallic tip apex using a two-chamber radio-frequency plasma-enhanced chemical vapor deposition is reported. Individual Ni nanocatalysts, obtained by a sol–gel combustion technique, were dots for the nucleation of individual CNTs that were freestanding, clean, and vertically aligned by the presence of a controlled applied field. The arrays of CNTs obtained, having a low-density spatial distribution to avoid mutual electrostatic field screening, gave uniform stable overall field emission patterns after a conditioning process. Effective total current densities up to 1A∕cm2 can be extracted.

Sharpening of metal tips by heat treatment in vacuum

It is well known that a metal tip blunts if it is heated in vacuum. Sharpening may occur in the case of an evaporation of the tip material. To test this, molybdenum tips were heated in vacuum. A typical result: at 2400K, the tip radius decreases from 8 mu m to 0.3 mu m thus confirming the hypothesis. Measured and calculated final radii agree roughly. The pure material evaporation can be replaced by a surface reaction followed by the evaporation of the reaction products. This is shown on tungsten tips heated (1750K) in the presence of oxygen (0.5 mTorr). The evaporation of tungsten oxides results in a sharpening to a radius to 0.05 mu m. Under special conditions, in connection with the formation of solid drops, radii down to 0.01 mu m are obtained.

Planar Cold Cathodes

Publisher Summary A planar cathode is a thin-film emitter deposited on a conducting surface. It emits electrons when an electric field is applied by an anode separated from the film front surface by a vacuum gap. This chapter discusses thin film and ultra-thin film planar cathodes with effective low work function. It focuses on basic principles underlying the electron emission to avoid the misinterpretation of experimental results. The chapter describes the two basic mechanisms for extracting electrons from solids to determine the precise limits of the present theoretical approaches—both by analytical and numerical simulations. In the chapter, scanning anode field emission microscopy (SAFEM) is described and experimental results with flat cathodes are presented in a later section with their field emission characteristic analyzed by SAFEM. Spindt field emission arrays (FEAs), which are the heart of field emission displays (FEDs), are basically microfabricated molybdenum tips in gated configuration that use the local field enhancement at the apex of each microtip in front of a gate located at a microscale distance to lower the threshold voltage (100 V), thus enabling field emission.

Magnetic nanotips: the realisation of an atomic-scale beam-splitter

Abstract We present the first realisation of ferromagnetic Fe nanotips. Studies of the field emitted beams of electrons and metallic ions above and under the Curie temperature T c were made. We have observed a reversible splitting of the electron beams when the temperature crossed through T c and multiple spot patterns for the emitted metallic ion beams. These phenomena were not observed with Fe microtips or non-magnetic nanotips and thus were correlated to a strong magnetic interaction at the atomic scale apex of the nanotips. The Fe nanotips constitute an atomic scale beam-splitter.

Field Emission from Low Barrier Surface Cathodes

A systematic study was done in order to relate the J-F characteristic variation with the three emission mechanisms, tunnelling, thermionic and ballistic. All three are now effective during the field emission from surface with low work function. Empirical relations will be given to extract some cathode properties from the measured J-F characteristics

Solid-State Field-Controlled Electron Emission: An alternative to thermionic and field-emission

In the solid-state field-controlled emitter (SSE), the emission barrier, which is the factor of utmost importance for surface electron emission, is tailored by a controlled extrinsic parameter like the injected space charge located near the surface. This is done by depositing an ultra-thin wide band-gap semiconductor layer on a metallic surface. It is an alternative approach to the thermionic or field emission for which the work function value is intrinsic to the material used. The emission current measurements from the SSE cold cathodes show stable emission, at low applied field (≈50 V/μm) and in poor vacuum (≈10 −7 Torr). The new emission mechanism has been modeled, the calculations and the theoretical analysis confirm the experimental results. The fabrication of the SSE, either by a sputter deposition in vacuum or by a sol-gel technique, meets most of the demands specific to high throughput fabrication of cold cathodes with large emitting area dedicated to applications in vacuum microelectronics.

Temperature measurement of microcrystals heated by electron bombardment: W tips from 2800K to 3650K

For controlled heating of metal microcrystals up to the melting point, the end of the field emitter tips are bombarded by an electron beam (8 keV, 400 mu A, 200 mu m beam diameter). To measure local temperatures on small areas down to 2 mu m diameter a special one-wavelength micropyrometer was built. Using an optical microscope, an image of the glowing tip was projected on to a screen outside the vacuum chamber. The screen has a movable hole behind which a one-wavelength filter and a photomultiplier are placed to measure local radiant intensities. The relative temperatures are obtained from Wiens law, and are absolutely calibrated by a thermocouple spot-welded on the emitter shank. Local relative temperatures and temperature gradients for a tungsten emitter are measured in the range between 2800K and 3650K, with an error and a stability better than 10K. For absolute temperatures the error is 50K when no evaporation occurs.

Field emission properties of carbon nanosphere chain arrays self-assembled on porous alumina templates

A new type of nanoscale field emitter array, consisting of carbon nanonecklaces and nanotentacles, has been produced by a novel multi-level self assembly process employing flexible porous alumina films. The field emission characteristics of the carbon nanostructures were measured using a scanning field emission microscope (SAFEM) and they exhibited strong Fowler-Nordheim emission. This new synthetic approach could find potential applications in flexible and inexpensive arrays of nanoscale cold cathode emitters.

Vertically Aligned Carbon Nanotubes for Field Emission Cathodes

Systematic field emission (FE) studies of cathodes made from arrays of individual vertically-aligned carbon nanotubes (va-CNTs) and arrays of isolated tufts of va-CNTs were performed in order to assess the most advantageous spatial configuration for FE stability and lifetime in a poor vacuum working environment (~10-7 torr), which is specific for industrial applications. We conclude that under large FE currents the back-ion sputtering, due to the ionization of the residual gas under the e-beam, destroyed the individual CNTs. This led to a gradual destruction of the CNTs in the arrays then to the failure of the cathode when most of them have disappeared. In the case of arrays of tufts of CNTs, results indicate that the consequence of the e-beam induced ion sputtering only pruned out the tuft. A tuft is still a FE site as long as one CNT exists. Under ion sputtering, the collective behavior of the tufts is the main reason for the observed extension of the FE lifetime by a factor in the range of 5 to 10 compared to isolated individual va-CNTs

Characterization and Field Emission Properties of Lanthanum Monosulfide Nanodot and Nanowire Arrays Deposited by Pulsed Laser Deposition on Self-assembled Nanoporous Al2O3 Matrix

Successful growth of lanthanum monosulfide (LaS) nanodot and nanowire arrays has been performed by pulsed laser deposition (PLD) using self-assembled nanoporous alumina templates containing pores about 50 nm wide and 200-300 nm deep. The arrays were characterized by X-ray diffraction, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM) and scanning anode field emission microscopy (SAFEM). The array of LaS nanodots grows selectively at boundaries between different regions of the alumina film with local hexagonal symmetry of pores. The density of these dots is about 109 /cm2. Cross sectional FE-SEM showed that LaS nanowires also grow inside the pores with a density of 1010/cm2 , which is equal to the pore density of the templates. The field emission properties of the LaS nanodots grown on top of the alumina templates were measured using SAFEM. A typical current-voltage characteristic is shown. Using the conventional Fowler-Nordheim relation, the work function of the LaS nanodots has been extracted from the slope of the plot ln(J/F2) vs 1/F, where J is the field emission current density and F is the local applied field. This leads to an outstanding, reproducible effective work function value of ~1 eV for the LaS nanodots. The threshold for an emission current density of 1 mA/cm2 occurs around a 150 V/mum which is considerably lower than the 230 V/mum threshold value reported recently for LaS thin films