Field emission microscope
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Field-emission microscopy (FEM) is an analytical technique used in materials science to investigate molecular surface structures and their electronic properties. Invented by
Erwin Wilhelm Müller Erwin Wilhelm Müller (or ''Mueller'') (June 13, 1911 – May 17, 1977) was a German physicist who invented the Field Emission Electron Microscope (FEEM), the Field Ion Microscope (FIM), and the Atom-Probe Field Ion Microscope. He and his st ...
in 1936, the FEM was one of the first surface-analysis instruments that approached near-
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
ic
resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...
.


Introduction

Microscopy techniques are used to produce real-space magnified images of a surface showing what it looks like. In general, microscopy information concerns surface
crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics (condensed matter physics). The wor ...
(i.e. how the atoms are arranged at the surface), surface morphology (i.e. the shape and size of topographic features making the surface), and surface composition (the elements and compounds the surface is composed of). Field-emission microscopy (FEM) was invented by Erwin Müller in 1936. In FEM, the phenomenon of
field electron emission Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field. The most common context is field emission from a solid surface into a vacuum. However, field emissio ...
was used to obtain an image on the detector on the basis of the difference in work function of the various crystallographic planes on the surface.


Design

A field-emission microscope consists of a metallic sample in the form of a sharp tip and a conducting fluorescent screen enclosed in ultrahigh vacuum. The tip radius used is typically of the order of 100 nm. It is composed of a metal with a high
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
, such as
tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
. The sample is held at a large negative potential (1–10 kV) relative to the fluorescent screen. This gives the electric field near the tip apex to be the order of 1010 V/m, which is high enough for
field emission Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field An electric field (sometimes E-field) is the physical field that surrounds electrically charged ...
of electrons to take place. The field-emitted electrons travel along the field lines and produce bright and dark patches on the fluorescent screen, giving a one-to-one correspondence with the crystal planes of the hemispherical emitter. The emission current varies strongly with the local
work function In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" m ...
in accordance with the Fowler–Nordheim equation; hence, the FEM image displays the projected work function map of the emitter surface. The closely packed faces have higher work functions than atomically rough regions, and thus they show up in the image as dark spots on the brighter background. In short, the work-function anisotropy of the crystal planes is mapped onto the screen as intensity variations. The magnification is given by the ratio M = L/R, where R is the tip apex radius, and L is the tip–screen distance. Linear magnifications of about 105 to 106 are attained. The spatial resolution of this technique is of the order of 2 nm and is limited by the momentum of the emitted electrons parallel to the tip surface, which is of the order of the
Fermi velocity The Fermi energy is a concept in quantum mechanics usually referring to the energy difference between the highest and lowest occupied single-particle states in a quantum system of non-interacting fermions at absolute zero temperature. In a Fermi ga ...
of the electron in metal. It is possible to set up an FEM with a probe hole in the phosphor screen and a
Faraday cup A Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. The Faraday cup was named after Michael Faraday w ...
collector behind it to collect the current emitted from a single plane. This technique allows the measurement of the variation of work function with orientation for a wide variety of orientations on a single sample. The FEM has also been used to study
adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...
and
surface diffusion Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters ( adparticles) at solid material surfaces.Oura, Lifshits, Saranin, Zotov, and Katayama 2003, p. 325 The process can generally be thought of in t ...
processes, making use of the work-function change associated with the adsorption process. Field emission requires a very good vacuum, and often even in
ultra-high vacuum Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...
(UHV), emission is not due to the clean surface. A typical field emitter needs to be "flashed" to clean it, usually by passing a current through a loop on which it is mounted. After flashing the emission current is high but unstable. The current decays with time and in the process becomes more stable due to the contamination of the tip, either from the vacuum, or more often from diffusion of adsorbed surface species to the tip. Thus the real nature of the FEM tips during use is somewhat unknown. Application of FEM is limited by the materials that can be fabricated in the shape of a sharp tip, can be used in a UHV environment, and can tolerate the high electrostatic fields. For these reasons,
refractory metals Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group diff ...
with high melting temperature (e.g. W, Mo, Pt, Ir) are conventional objects for FEM experiments.


See also

*
Atom probe The atom probe was introduced at th14th Field Emission Symposium in 1967by Erwin Wilhelm Müller and J. A. Panitz. It combined a field ion microscope with a mass spectrometer having a single particle detection capability and, for the first time, ...
*
Electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
*
Field ion microscope The Field ion microscope (FIM) was invented by Müller in 1951. It is a type of microscope that can be used to image the arrangement of atoms at the surface of a sharp metal tip. On October 11, 1955, Erwin Müller and his Ph.D. student, Kanwar ...
*
List of surface analysis methods This is a list of analysis methods used in materials science. Analysis methods are listed by their acronym, if one exists. Symbols * μSR – see muon spin spectroscopy * χ – see magnetic susceptibility A * AAS – Atomic absorption spec ...


References

* 2. K.Oura, V.G.Lifshits, A.ASaranin, A.V.Zotov and M.Katayama, Surface Science – An Introduction, (Springer-Verlag Berlin Heidelberg 2003). * 3. John B. Hudson, Surface Science – An Introduction, (BUTTERWORTH-Heinemann 1992). {{refend Electron microscopy techniques Microscopy Scientific techniques