Nanotips and nanomagnetism

https://doi.org/10.1016/S0169-4332(98)00158-5Get rights and content

Abstract

Nanotips are nanoscale protrusions ending in one atom fabricated in vacuum by 2D surface melting and field gradient. They are field emission sources of electrons and ions, producing self-collimated and strongly coherent beams with high brightness. Using the nanotip as the source in a projection microscope results in the Fresnel projection microscope. It allows imaging at energies less than 300 eV of fragile nanoscale structures which are either amplitude or phase nano-objects. Experimental Fresnel images of magnetic nanoparticles show the potential for mapping very weak magnetic stray fields with the Fresnel projection microscope.

Introduction

In general, the size of the source is an important parameter for applications involving particles beams, be they electrons, ions or photons. For example, the resolution in electron microscopes is ultimately related to the source size. In current electron microscopies, the size of the source used for imaging is reduced to atomic scale by using electron optical lenses and diaphragms. Another procedure is also conceivable; it is to physically reduce the emitting region of the source to one atom. This can be actually achieved by fashioning a nanoprotrusion with the apex ending in one atom to give a nanotip or a teton-tip [1],1 by controlled surface diffusion mechanism and then extracting the electrons or ions by field emission [2]. The atomic size of nanotips results in beams of high coherence and brightness. Moreover, using extraction lenses located at micron distances from the source permits the field emission to be performed at voltages in the 100 to 500 V range, without loss of the coherence and brightness at these low energies.

Immediate applications of atomic size sources emitting low-energy electrons and ions with high coherence can be foreseen in low-energy and high resolution microscopies. For example, projection microscopy [3]takes advantage of the nanotip characteristics to create a lensless low-energy and high magnification observation tool [4]. This is of particular importance for the observation of fragile nanoscale structures which are either amplitude or phase nano-objects.

Three aspects of the nanotips and its applications are developed in this article. Section 2.2describes the mechanism for the nanotip fabrication. In Section 2.3, the measured field emission properties of nanotips are reported. The nanotip formation and field emission analysis were experimentally performed inside one UHV chamber (base vacuum ∼10−10 Torr). The chamber contained facilities for field electron microscopy (FEM), field ion microscopy (FIM) and field electron energy spectroscopy (FEES). This permits transfer among the nanotip formation, FEM, FEES and FIM measurements at will and within the same environment by a mechanical movement coupled with electrostatic and magnetic deflection systems. In Section 3, one application which shows a breakthrough in electron microscopy by the use of nanotips is presented. It is the mapping of the magnetic leakage fields from nanoparticles with the Fresnel Projection microscopy (FPM), which is a projection microscope using the nanotip as a radial electron projection source.

Section snippets

Nanotips vs. build-up (BU) tips

The guiding line in the elaboration of nanotips is the setting up of a technique to obtain, in a straightforward and reproducible way, stable field emission sources with an unique emitting zone limited to one atom. In general, there are three different possible approaches: (i) to decrease the whole tip radius, i.e., to produce ultra-sharp tips; (ii) to confine the emission over a small area by modifying the atomic structure and/or the work function; and (iii) to confine the field over a small

Fresnel projection microscopy [24]

The interest in nanotips depends essentially on the new possibilities that they can open due to the specific field emission properties attached to the atomic size of their emitting area. We describe in this section the new possibility for observation of magnetic leakage fields by the use of a nanotip as radial projection electron source in a projection microscope.

The projection microscope was originally introduced by Morton and Ramberg [3]. It is essentially a lensless microscope with

Conclusions

The size reduction to one atom of the field emission area, which is the main characteristic of the nanotips, is obtained by taking advantage of the protrusion effect to enhance locally the field over the topmost atom of the nanoprotrusion. The field emission beam, which comes exclusively from this atom, manifests specific properties that are attached to the atomic size of these nanosources. The nanotip is a coherent, monochromatic e-beam source and could also be used as ion point sources.

One

Acknowledgements

This work is supported by European Union Contracts (BRITE and ESPRIT) and by DRET. The contributions of the Service Central d'Analyse CNRS and of D. Guillot are highly appreciated.

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