Spectroscopies tunneling

Classically, electrons still need to have an extra energy ( above the Fermi energy to move from the specimen to the tip or vice versa, even if the tip and the specimen come in close proximity (provided there is no conductive connection). 

29) Here and in the following discussion it is assumed that the positive pole of the voltage is applied to the tip  

This approximation shows that the tunneling current depends linearly on the bias voltage t/Bias and exponentially on the distance d. The other quantities in the equation, namely the work function / , the electron charge e and mass nte, and Planck s constant ft, are constant for a given material. For a typical value of the work function of 4 electron Volts (eV), the tunneling current reduces by a factor 

7 for every 0.1 nm increase in d. This means that over a typical atomic diameter of 0.3 nm, the tunneling current changes by a factor 456 The tunneling current depends on the distance so strongly that it is dominated by the contribution flowing between the last atom of the tip and the nearest atom in the specimen. As mentioned previously this is the primary reason for the high resolution of the STM. 

In order to observe images in atomic or sub-molecular resolution with STM, a high sensitivity is essential and, in particular, the tip must be held in a stable position in close vicinity to the surface. To remain as sharp as possible, the tip must approach to the surface without uncontrolled contact. This can be realized in various experimental STM set-ups (see Tutorial 4 on STM designs). 

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E. L. Wolf, ed.. Principles of Electron Tunneling Spectroscopy, Oxford, New York, 1985. 

Liu K, Brown M G and Saykally R J 1997 Terahertz laser vibration rotation tunneling spectroscopy and dipole moment of a cage form of the water hexamer J. Phys. Chem. A 101 8995-9010... 

Hamers R 1993 Methods of tunnelling spectroscopy with the STM Scanning Tunnelling Microscopy and Spectroscopy ed D A Bonnell (Weinheim VCH) ch 4... 

Hamers R J and Kohler U K 1989 Determination of the local electronic structure of atomic-sized defects on Si(OOI) by tunnelling spectroscopy J. Vac. Sc/. Technol. A 7 2854... 

Hasegawa Y and Avouris Ph 1993 Direct observation of standing wave formation at surface steps using scanning tunnelling spectroscopy Rhys. Rev. Lett. 71 1071... 

Another technique that has proved useful in establishing chemical bonding of coupling agents at interfaces is inelastic electron tunneling spectroscopy (ITES). For example. Van Velzen [16] examined 3-(trimethoxysilyl)propanethiol by this technique. Approximately monolayer quantities of this silane were adsorbed on the barrier oxide of an aluminum-aluminum oxide-metal tunneling junction two metals were investigated, lead and silver. It was concluded that the silane is... 

Scanning tunneling spectroscopy (STS) can, in principle, probe the electronic density of states of a singlewall nanotube, or the outermost cylinder of a multi-wall tubule, or of a bundle of tubules. With this technique, it is further possible to carry out both STS and scanning tunneling microscopy (STM) measurements at the same location on the same tubule and, therefore, to measure the tubule diameter concurrently with the STS spectrum. No reports have yet been made of a determination of the chiral angle of a tubule with the STM technique. Several groups have, thus far, attempted STS studies of individual tubules. 

Ogawa, S., Fan, F.-R.F. and Bard, A. J. (1995) Scanning tunneling microscopy, tunneling spectroscopy, and photoelectrochemistry of a film of Q-CdS particles incorporated in a self-assembled monolayer on a gold surface. J. Phys. Chem., 99, 11182-11189. 

The results discussed in Sections 3.1-3.3 turned out as very valuable with respect to the knowledge about the transition from bulk metal to molecule. The most important method, however, to gain direct information from individual metal nanoparticles on their inner electronic life is the tunnelling spectroscopy. The method is based on the single-electron tunnelling (SET) through an intermediate island between two metal electrodes as is indicated in Figure 10. 

The next smaller ligand-protected nanocluster that was investigated by scanning tunneling spectroscopy (STS) was the four-shell cluster Pt309phen 36O20 [20,21]. The diameter of the Pt core is 1.8 nm, about a tenth of the former example. However, even here a Coulomb blockade could only be observed at 4.2 K, i.e. at room temperature the particle still has metallic behaviour. Since... 

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