Field emission from metal particles in a vacuum gap

Carbon nanotubes (CNT) are being considered for field emission applications because of their low turn-on voltage and ability to support large current densities. The localization of emission and large currents from CNTs result in significant anode heating. The present work investigates the electron energy distribution at the anode surface through simulation of the field emission process and the trajectory of electrons across the vacuum gap. Field emission is modeled by Fowler-Nordheim-like expressions where the emission site is assumed to be a ring with the diameter of a nanotube. The electron trajectory is determined through a Monte Carlo simulation including Coulomb interactions between electrons. Results indicate that the electron beam spreads due to Coulomb interaction, but that the initial ring is preserved. In fact, the ring diameter at the anode spreads to 3μ per 10μ of vacuum gap in a field of 10 Vμm. This estimate matches well with reported observations. Further, the spreading becomes more significant with increased fields due to the higher current density of field emitted electrons.

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Field Emission TEM and STEM of Ultrafine Metal Particles

Microscopy and Microanalysis ◽

10.1017/s1431927600008837 ◽

1997 ◽

Vol 3 (S2) ◽

pp. 389-390

Author(s):

M. M. Disko ◽

A. Orchowski

Keyword(s):

Field Emission ◽

High Performance ◽

High Current Density ◽

High Sensitivity ◽

Ease Of Use ◽

Atomic Scale ◽

Metal Particles ◽

Vacuum System ◽

Supported Metal Catalysts ◽

Performance Factors

Accurate descriptions of dispersed metals are of central importance in optimizing the activity, selec-tivity and stability of a diverse set of catalysts and chemistries. Improved methods for quantifying metal dispersion, alloying and support interactions help form mechanistic understandings of catalyst function at the atomic scale. In this paper we discuss ideal capabilities of a transmission electron microscope for catalysis, and give an update on the use of a next-generation field-emission TEM/STEM for structural and analytical studies of supported metal catalysts.Characterizing nanometer-sized metal particles requires high performance imaging, high sensitivity analysis, and minimal disruption of the physical state of the particles. No single microscope combines all key performance factors with ease of use. Schottky field-emitter instruments provide high current-density probes with excellent stage stability (nm drifts in > 10 min. observed), < 0.7 eV energy reso-lution for EELS, and a turbo-pumped vacuum system for minimal carbon contamination while using nanometer probes and nanoamps of current. Scanning TEM performance is also key.

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Observation of a supported metal catalyst in an ultra-high-resolution field-emission SEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100149751 ◽

1993 ◽

Vol 51 ◽

pp. 784-785

Author(s):

J. Liu ◽

G. E. Spinnler

Keyword(s):

High Resolution ◽

Field Emission ◽

Carbon Film ◽

Incident Beam ◽

Metal Particles ◽

Metal Catalyst ◽

Sem Images ◽

Catalytic Systems ◽

Conducting Materials ◽

Small Metal Particles

Zeolites loaded with noble-metal particles are interesting catalytic systems. The relative positions of small metal particles on and within the support structure may have significant consequences for catalytic activity. Small metal particles dispersed on model and commercial catalyst supports have been observed by using secondary and Auger electrons in a UHV STEM. In this paper we further report preliminary results of observing a metal/zeolite catalyst in a commercial high-resolution field-emission SEM and investigate the contrast variations of SEM images with the change of incident beam energies.Metal-loaded zeolite samples were ground to fine powders and were dispersed onto a thin holey carbon film coated on a copper grid. The samples were directly observed without being coated with conducting materials. Such coating would obscure fine-scale surface details and make imaging of small metal particles impossible. The experiments were performed in a field-emission Hitachi S-5000 SEM, operating from 0.5 to 30 kV.

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