Field evaporation

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

In field evaporative units utilizing natural gas as the fuel source, the primary driving force is the heat supplied to the water. The theoretical evaporation rate for these units may be expressed as... 

Sanchez CG, Lozovoi AY, Alavi A, 2004. Field-evaporation from first-principles. Mol Phys 102 (9-10) 1045-1055. 

An FIM may be modified so that the imaged atom chosen for analysis can be positioned over a small aperture in the phosphor-coated screen. If the electric field is raised to a sufficiently high value, material may be removed from the surface by field evaporation. The specimen is subjected to a high-voltage pulse, which causes a number of atoms on the specimen surface to field evaporate as positive ions. Only the atom that was imaged over the aperture (or probe hole ) passes into a time-of-flight mass spectrometer, all the other atoms being blocked off by the screen. The applied... 

The PLAP applies a short duration (100 ps 10 ns) laser pulse to the apex of the specimen. The heat generated is sufficient to promote the field evaporation at the standing voltage of the specimen. The specimens need only to be sufficiently conductive to permit field ion imaging. The peak temperature in the PLAP is only 300 K for a period of a few nanoseconds, which is not sufficiently high for surface diffusion on semiconductor materials, and so the spatial resolution is not downgraded. 

Atom probe techniques have been used to investigate adsorption processes and surface reactions on metals. The FIM specimen is first cleaned by the application of a high-voltage field evaporation pulse, and then exposed to the gas of interest. The progress of adsorption and surface reaction is monitored by the application of a second high-voltage desorption pulse and a controlled time delay. 

The most frequently studied samples with FIM are refractory metal tips, such as W, Mo, Pt, Ir, etc. The field evaporation threshold for refractory metals is appreciably higher than the field to ionize helium atoms, which is 4.5 V/A. Field evaporation is also used for forming and cleaning the FIM sample, which is the tip end, to make it a sharp end and to remove adsorbed exotic atoms. A typical FIM image is shown in Fig. 1.34. 

The surfaces prepared for FIM reflect the field evaporation process itself, whereas the surfaces studied by STM are the thermal equilibrium surfaces. 

Fig. 13.6. Tip formation by field evaporation. Top left, FIM image of a (I I l)-oriented W tip the (111) apex plane contains 18 atoms. Top right, the field evaporation process. Bottom, tip with pyramidal apex with one, three, and seven W atoms at the apex plane. (Reproduced from Fink, 1986, with permission.)...

Field evaporation 287 Field-emission microscopy 43 Field-ion microscopy 39—43 comparison with STM 41 directional walk of single atoms 42 resolution 40 Force and deformation 37 Force in tunneling experiments 49, 53, 171... 

Tip treatment 281—293, 301 annealing 286 annealing with a field 288 atomic metallic ion emission 289 controlled collision 293 controlled deposition 288 field evaporation 287 for scanning tunneling spectroscopy 301 high-field treatment 291 Tip wavefunctions 76—81 explicit forms 77 Green s functions, and 78 Tip-state characterization 306, 308 ex situ 306 in situ 308... 

Fig. 1.4 Diagram showing the principle of operation of a time-of-flight atom-probe. The tip is mounted on either an internal or an external gimbal system. The tip orientation is adjusted so that atoms of one s choice for chemical analysis will have their images falling into the small probe-hole at the screen assembly. By pulse field evaporating surface atoms, these atoms, in the form of ions, will pass through the probe hole into the flight tube, and be detected by the ion detector. From their times of flight, their mass-to-charge ratios are calculated, and thus their chemical species identified.

Fig. 2.3 Magnetic sector mass spectrometer combined with a retarding potential ion energy filter used by Ernst et al 6 for measuring ion energy distributions in field ionization and field evaporation.

When the applied electric field reaches a few volts per angstrom range, atoms on a surface, irrespective of whether they are lattice atoms or adsorbed atoms and of whether the surface temperature is high or low, may start to emit out of the surface in the form of ions. This high electric field produced evaporation phenomenon is usually called field evaporation if the surface atoms are lattice atoms, and is called field desorption if they are adsorbed atoms. From a theoretical point of view there are no fundamental differences. We will use the term field desorption for general purposes, especially for theoretical discussions, since desorption is the term used in many other adsorption-desorption phenomena. When we specifically mean removal of lattice atoms by electric field the term field evaporation will be used. Sometimes field evaporation is used where it may mean both field evaporation and field desorption. 

As field evaporation is a thermally activated process, the field evaporation rate is given by... 

Table 2.2 Field evaporation parameters for various elements... 

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