Field ion microscope

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Field ion microscopy (FIM) is an analytical technique used in materials science. The field ion microscope is a type of microscope that can be used to image the arrangement of atoms at the surface of a sharp metal tip. It was the first technique by which individual atoms could be spatially resolved. The technique was pioneered by Erwin Wilhelm Müller. Images of atomic structures of tungsten were first published in 1951 in the journal Zeitschrift für Physik.

Contents

Introduction

In FIM, a sharp (<50 nm tip radius) metal tip is produced and placed in an ultra high vacuum chamber, which is backfilled with an imaging gas such as helium or neon. The tip is cooled to cryogenic temperatures (20–100 K). A positive voltage of 5 to 10 kilovolts is applied to the tip. Gas atoms adsorbed on the tip are ionized by the strong electric field in the vicinity of the tip (thus, "field ionization"), becoming positively charged and being repelled from the tip. The curvature of the surface near the tip causes a natural magnification — ions are repelled in a direction roughly perpendicular to the surface (a "point projection" effect). A detector is placed so as to collect these repelled ions; the image formed from all the collected ions can be of sufficient resolution to image individual atoms on the tip surface.

Unlike conventional microscopes, where the spatial resolution is limited by the wavelength of the particles which are used for imaging, the FIM is a projection type microscope with atomic resolution and an approximate magnification of a few million times.

Design, limitations and applications

FIM like Field Emission Microscopy (FEM) consists of a sharp sample tip and a fluorescent screen (now replaced by a multichannel plate) as the key elements. However, there are some essential differences as follows:

Like FEM, the field strength at the tip apex is typically a few V/Å. The experimental set-up and image formation in FIM is illustrated in the accompanying figures.

In FIM the presence of a strong field is critical. The imaging gas atoms (He, Ne) near the tip are polarized by the field and since the field is non-uniform the polarized atoms are attracted towards the tip surface. The imaging atoms then lose their kinetic energy performing a series of hops and accommodate to the tip temperature. Eventually, the imaging atoms are ionized by tunneling electrons into the surface and the resulting positive ions are accelerated along the field lines to the screen to form a highly magnified image of the sample tip.

In FIM, the ionization takes place close to the tip, where the field is strongest. The electron that tunnels from the atom is picked up by the tip. If we look into the theory of the process in detail, there is a critical distance, xc, at which the tunneling probability is a maximum. This distance is typically about 0.4nm. The very high spatial resolution and high contrast for features on the atomic scale arises from the fact that the electric field is enhanced in the vicinity of the surface atoms because of the higher local curvature. The resolution of FIM is limited by the thermal velocity of the imaging ion. Resolution of the order of 1Å (atomic resolution) can be achieved by effective cooling of the tip.

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