Field emission spectroscopy

Extension of the tunnelling problem to three dimensions was first attempted by Itskovich who showed that the band structure of the emitter would affect both the FN plot and TED. Gadzuk and Politzer and Cutler have argued that the spatial extent of the wave-function is important in determining the yield of field emitted electrons, and the latter authors conclude that the Fowler-Nordheim model succeeds for 2 d metals only because the contribution from the 3d band to the total field emitted current is small so that the current reflects the nearly-free-electron character of the 4s-p bands. 

The contribution to the TED by electrons tunnelling from surface states has been 

Some Developments in Field Emission Techniques and their Application 

Although a very compact version of this form of analyser has been devised for 

The dispersion analyser yields the energy distribution directly. Electrons at the pass energy are detected using an electron multiplier in the counting mode and the counts stored in either a computer or in a multichannel analyser, whose channel 

After the first theoretical work of Tamm (1932), a series of theoretical papers on surface states were published (for example, Shockley, 1939 Goodwin, 1939 Heine, 1963). However, there has been no experimental evidence of the surface states for more than three decades. In 1966, Swanson and Grouser (1966, 1967) found a substantial deviation of the observed fie Id-emission spectroscopy on W(IOO) and Mo(lOO) from the theoretical prediction based on the Sommerfeld theory of metals. This experimental discovery has motivated a large amount of theoretical and subsequent experimental work in an attempt to explain its nature. After a few years, it became clear that the observed deviation from free-electron behavior of the W and Mo surfaces is an unambiguous exhibition of the surface states, which were predicted some three decades earlier. 

The theory of field-emission spectroscopy for free-electron metals was developed by Young (1959). We present here a simplified version of Young s theory, which includes all the essential physics related to the experimental observation of surface states. 

According to the Fowler-Nordheim equation, Eq. (1.34), the field-emission current from electrons at energy j E below the vacuum level is proportional to 

Gonsider now energy levels not far from the Fermi level, that is, by a small energy deviation e -= cj) 

Expand the exponent in Eq. (4.17) into power series, we obtain 

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Field emission is a tunneling phenomenon in solids and is quantitatively explained by quantum mechanics. Also, field emission is often used as an auxiliary technique in STM experiments (see Part II). Furthermore, field-emission spectroscopy, as a vacuum-tunneling spectroscopy method (Plummer et al., 1975a), provides information about the electronic states of the tunneling tip. Details will be discussed in Chapter 4. For an understanding of the field-emission phenomenon, the article of Good and Muller (1956) in Handhuch der Physik is still useful. The following is a simplified analysis of the field-emission phenomenon based on a semiclassical method, or the Wentzel-Kramers-Brillouin (WKB) approximation (see Landau and Lifshitz, 1977). 

The surface states observed by field-emission spectroscopy have a direct relation to the process in STM. As we have discussed in the Introduction, field emission is a tunneling phenomenon. The Bardeen theory of tunneling (1960) is also applicable (Penn and Plummer, 1974). Because the outgoing wave is a structureless plane wave, as a direct consequence of the Bardeen theory, the tunneling current is proportional to the density of states near the emitter surface. The observed enhancement factor on W(IOO), W(110), and Mo(IOO) over the free-electron Fermi-gas behavior implies that at those surfaces, near the Fermi level, the LDOS at the surface is dominated by surface states. In other words, most of the surface densities of states are from the surface states rather than from the bulk wavefunctions. This point is further verified by photoemission experiments and first-principles calculations of the electronic structure of these surfaces. 

In general, the tip DOS does not resemble that of a free-electron metal. To have meaningful STS measurements, the tip DOS must be determined independently. A well-established experimental method for determining the energy spectmm a sharp metal tip is field emission spectroscopy (FES), which was described in Section 4.4. For free-electron metal tips, the FES is described by the Young formula, Eq. (4.20). A deviation from the Young formula indicates a deviation from a free-electron metal behavior. 

Binh, V. T., Purcell, S. T., Garcia, N., and Doglini, J. (1992). Field emission spectroscopy of single-atom tips. Phys. Rev. Lett. 69, 2527-2530. 

Plummer, E. W. (1975). Photoemission and field emission spectroscopy. Interactions on Metal Surfaces, edited by R. Gomer, Springer-Verlag, New York, 143-223. 

Fowler-Nordham equation 45 Field emission spectroscopy 44,... 

In field emission spectroscopy (FEM), a refactory metal, such as tungsten, is fabricated to give a very fine hemispherical tip of radius of curvature about 1CT7 m. The tip is located at the centre of curvature of a hemispherical fluorescent screen and a potential difference of about 10 kV is applied, with the fluorescent screen as... 

Introduction.—Without doubt the most important recent advance in field emission techniques has been the development of field emission spectroscopy. It has led to a critical re-examination of the theory of field emission and contributed to the recent considerable advance in understanding of the electronic structure of surfaces and of chemisorbed species. A comprehensive review of the theory and practice has been... 

Using an electric field as a probe can cause emission of electrons from a suitable sample. Measurement of the distribution of kinetic energy of the emitted electrons is called field-emission spectroscopy (FES). 

Field emission spectroscopy FES Electric field II Electron emission... 

FES field emission spectroscopy flame emission spectrometry... 

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