Field ion microscope

Field ion microscope image of the end of a sharp platinum needle. Each bright spot is a platinum atom.

The field-ion microscope (FIM) was invented by Müller in 1951.^[1] 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 Bahadur (Pennsylvania State University) observed individual tungsten atoms on the surface of a sharply pointed tungsten tip by cooling it to 21 K and employing helium as the imaging gas. Müller & Bahadur were the first persons to observe individual atoms directly.^[2]

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Hi my name is Tomasz Olewicz; I am a graduate student in Electrical and Computer Engineering department, former grad student of Professor Gert Ehrlich. I would like to explain to you how the microscope which we are using to investigate surface Atom Diffusion in atomic scale resolution works. This microscope was designed and built in Gert Ehrlich's research group here at the University of Illinois Urbana-Champaign. In our microscope, the sample is a very sharp tip which is about 500 angstroms in radius. The tip is placed in the front of the screen. We are using imaging gas - in our case, helium. The helium is delivered into the system via vycor. The vycor is nothing else than just a piece of quartz which is surrounded by the heater. The helium diffuses through out the vycor into the gas chamber where it's purified by titanium which spread from the titanium filaments from the gutter. After the helium is purified then it's delivered into the microscope chamber. The pressure of helium is regulated by this closing valve. A base pressure in the microscope is around 10 to minus 11 tor however during imaging the pressure is about 10 to minus 4 tor. After the gas is delivered, we apply high voltage between the tip which is our sample, and the screen. The voltage is about 10 kilovolts, and the polarity of the voltage is positive on the tip and negative on the screen. The electric field polarized the imaging gas atoms since the electric field is not uniform and the dialectic constant of the gas is higher than the vacuum, the atoms of the gas are attracted towards the sample surface. Where they are ionized within 4 angstroms from the surface. When they are above the sharpest spots on the surface which in our case are steppage atoms or single atoms. Straight afterwards, helium ions are repelled from the surface towards the screen where they cause brightening. The bright spots observed on the screen correspond to relative position of atoms on the tip. The sample is cooled down with liquid helium which is delivered to the cold finger by transfer tube. The cold finger is the element which is responsible for transferring heat through the sample and from the sample. The atoms are deposit on the surface from the evaporator. In our case the evaporator is the tungsten filament which is heated by the current. The filament is heated to the point where the atoms are terminally absorbed from the filament and deposit on the tip surface. The experimental procedure is as follows: After we image the surface with current position of the ad-atom we turn off the electric field and we heat the sample. For certain amount of time at certain temperature. Afterwards we abruptly cool down the sample and we apply the field again to observe new position of the ad-atom. We repeat the cycle of imaging, heating, and imaging again several times, about seventy to a hundred times for each temperature to obtain proper statistics. From that we can determine miniscular displacement and other properties. The sequence is controlled by the sequencer. In sequencer we can set the time of imaging and time of delay after we turn off the voltage. We can also control the heating time and also delay for the cooling down of the sample. The temperature of the sample is controlled by heat controller we dial desired resitivity corresponds to the temperature that we wish to heat the sample. The heat controller applies a current through the hairpin until we reach the resistivity that we dial.

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:

The tip potential is positive.
The chamber is filled with an imaging gas (typically, He or Ne at 10⁻⁵ to 10⁻³ Torr).
The tip is cooled to low temperatures (~20-80K).

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. There is a critical distance, xc, at which the tunneling probability is a maximum. This distance is typically about 0.4 nm. 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.

Application of FIM, like FEM, is limited by the materials which can be fabricated in the shape of a sharp tip, can be used in an ultra high vacuum (UHV) environment, and can tolerate the high electrostatic fields. For these reasons, refractory metals with high melting temperature (e.g. W, Mo, Pt, Ir) are conventional objects for FIM experiments. Metal tips for FEM and FIM are prepared by electropolishing (electrochemical polishing) of thin wires. However, these tips usually contain many asperities. The final preparation procedure involves the in situ removal of these asperities by field evaporation just by raising the tip voltage. Field evaporation is a field induced process which involves the removal of atoms from the surface itself at very high field strengths and typically occurs in the range 2-5 V/Å. The effect of the field in this case is to reduce the effective binding energy of the atom to the surface and to give, in effect, a greatly increased evaporation rate relative to that expected at that temperature at zero fields. This process is self-regulating since the atoms that are at positions of high local curvature, such as adatoms or ledge atoms, are removed preferentially. The tips used in FIM is sharper (tip radius is 100~300 Å) compared to those used in FEM experiments (tip radius ~1000 Å).

FIM has been used to study dynamical behavior of surfaces and the behavior of adatoms on surfaces. The problems studied include adsorption-desorption phenomena, surface diffusion of adatoms and clusters, adatom-adatom interactions, step motion, equilibrium crystal shape, etc. However, there is the possibility of the results being affected by the limited surface area (i.e. edge effects) and by the presence of large electric field.

In a recent study from Günther Rupprechter laboratory examined a rhodium nanocrystal surface using field emission microscopy consisting of different nanometer-sized nanofacets as a model of a compartmentalized reaction nanosystem. Different reaction modes were observed, including a transition to spatio-temporal chaos. The transitions between different modes were caused by variations of the hydrogen pressure modifying the strength of diffusive coupling between individual nanofacets.^[3]

References

^ Müller, Erwin W. (1951). "Das Feldionenmikroskop". Zeitschrift für Physik. 131 (8): 136–142. Bibcode:1951ZPhy..131..136M. doi:10.1007/BF01329651.
^ Müller, Erwin W.; Bahadur, Kanwar (1956). "Field Ionization of gases at a metal surface and the resolution of the field ion microscope". Phys. Rev. 102 (3): 624–631. Bibcode:1956PhRv..102..624M. doi:10.1103/physrev.102.624.
^ Raab, Maximilian; Zeininger, Johannes; Suchorski, Yuri; Tokuda, Keita; Rupprechter, Günther (2023-02-10). "Emergence of chaos in a compartmentalized catalytic reaction nanosystem". Nature Communications. 14 (1): 736. doi:10.1038/s41467-023-36434-y. ISSN 2041-1723. PMC 9911747. PMID 36759520.

K.Oura, V.G.Lifshits, A.ASaranin, A.V.Zotov and M.Katayama, Surface Science – An Introduction, (Springer-Verlag Berlin Heidelberg 2003).
John B. Hudson, Surface Science – An Introduction, BUTTERWORTH-Heinemann 1992.

External links

Wikimedia Commons has media related to Field-ion microscopy.

Northwestern University Center for Atom-Probe Tomography
Photograph of tungsten needle tip imaged through FIM at the Wayback Machine (archived November 22, 2013)
Microscope Parts need to know.

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