Field emission microscopy
Encyclopedia
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
in 1936, the FEM was one of the first surface analysis instruments that approached near-atomic
resolution
.
(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 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.
, such as tungsten
. 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 of electrons to take place. Fig.1 shows the experimental set up for FEM.
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 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
, where 
is the tip apex radius and 
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 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
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
and surface diffusion
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
(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 which 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
with high melting temperature (for e.g. W, Mo, Pt, Ir) are conventional objects for FEM experiments.
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
to investigate molecular surface structures and their electronic properties. Invented by Erwin Wilhelm Müller
Erwin Wilhelm Müller
Erwin Wilhelm Müller was a German physicist who invented the Field Emission Electron Microscope , the Field Ion Microscope , and the Atom-Probe Field Ion Microscope...
in 1936, the FEM was one of the first surface analysis instruments that approached near-atomic
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
.
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 crystallographyCrystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
(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 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 pointMelting point
The melting point of a solid 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 on pressure and is usually specified at standard atmospheric pressure...
, such as tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
. 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 of electrons to take place. Fig.1 shows the experimental set up for FEM.
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
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
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
, where is the tip apex radius and 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 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
Faraday cup
A Faraday cup is a metal 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...
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
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
and surface diffusion
Surface diffusion
Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters at solid material surfaces. The process can generally be thought of in terms of particles jumping between adjacent adsorption sites on a surface, as in figure 1...
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
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...
(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 which 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
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 differs...
with high melting temperature (for e.g. W, Mo, Pt, Ir) are conventional objects for FEM experiments.
See also
- Atom ProbeAtom probeThe atom probe is a microscope used in material science that was invented in 1967 by Erwin Wilhelm Müller, J. A. Panitz, and S. Brooks McLane. The atom probe is closely related to the method of Field Ion Microscopy, which is the first microscopic method to achieve atomic resolution, occurring in...
- Electron microscopeElectron microscopeAn electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
- Field ion microscopeField ion microscopeField ion microscopy 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....
- List of surface analysis methods