Spectroscopy scanning tunneling

A scanning tunneling microscope is the principal piece of equipment. The microscope should be placed in an UHV chamber in order to ensure clean surfaces. A lock-in-amplifier can be used to acquire df/dV or dl/dz (where z is the height of the tip above the surface) curves. The experiments are often carried out at cryogenic temperatures. 

Samples should have reasonably flat surfaces and preferably be metaUic or semiconducting. However, it has been shown that very thin flhns of insulators can also be analysed. 

The electrons tunnel from flUed states to empty states. The flUed states can be in the tip or in the sample. Hence by varying the sample tip voltage both filled and empty states in the sample can be probed. A good understanding of the electronic stracture of the tip is necessary to understand the data. The data is often displayed in the form (d//dV)/(//V) which is known as the normaHzed conductance which approximates the density of states of the sample. 

The technique is sensitive to surface contamination. The nature of the tip can affect both the spatial resolution and the I/V curves of the acquired STS data. 

STS spectra are used to acquire information regarding the electronic structure at atomic resolution. 

The parameters controlled in an STM experiment are current, J the energy of the tunneling electrons, and the width of the barrier, that is, the height x of the tip 

The matrix element for the tunneling process is described by perturbation theory by an integral over an energy surface composed of the tip and sample wave functions Pand their spatial derivatives  

For a low temperature, not too high a voltage, and a constant matrix element. Equation 2.51 reduces to 

Finally, for a constant DOS of the tip in the considered energy range, the tunneUng conductance measures the DOS of the sample  

Fermi level of the semiconductor Ej, Fermi level of the tip , donor levels (a) negative voltage applied to sample, 4 0 (tip grounded) (b) V, 0. 

STM can locally investigate electronic states at the atomic and molecular scale, for instance, the electronic differences within an individual molecule [9]. As one can 

---

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

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. 

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... 

The ESTM experiment provides actually five measurable quantities tunnelling current, / at the applied voltage, U, and three dimensions, x, y, z. The standard STM can therefore easily be modified by recording the local l-Uy U-zy or /-z characteristics (z is vertical distance of the tip from the electrode surface). Plot of dl/dU or d//dz versus x and y brings additional information on the electronic and chemical surface properties (local work functions, density-of-states effects, etc.), since these manifest themselves primarily as l-U dependences. The mentioned plots are basis of the scanning tunnelling spectroscopy (STS). 

---