Preparation Tunneling current

The STM uses this eflFect to obtain a measurement of the surface by raster scanning over the sample in a manner similar to AFM while measuring the tunneling current. The probe tip is typically a few tenths of a nanometer from the sample. Individual atoms and atomic-scale surface structure can be measured in a field size that is usually less than 1 pm x 1 pm, but field sizes of 10 pm x 10 pm can also be imaged. STM can provide better resolution than AFM. Conductive samples are required, but insulators can be analyzed if coated with a conductive layer. No other sample preparation is required. 

STM can be operated in a wide range of environments a stable tunnel current can be maintained in almost any nonconducting medium, including air, liquid, or vacuum. It is also relatively forgiving for an STM operation to prepare a sample the main requirement is that the sample conduct... 

Without the need for complicated magnetic lenses and electron beams, the STM is far less complex than the electron microscope. The tiiiy tunneling current can be simply amplified through electronic circuitry similar to that which is used in other electronic equipment, such as a stereo. In addition, the sample preparation is usually less tedious. Many samples can be im ed in air with essentially no preparation. For more sensitive samples which react with air, imaging is done in vacuum. A requirement for the STM is that the samples be electrically conductir, such as a metal. 

Fig, 10.1. Self-organized monolayer prepared by solution casting of octyl-decorated Frechet dendritic wedges functionalized by a methyl ester group (30 nm x 30 nm, bias voltage tJbias = 500 mV, tunnelling current /t = 70 pA 0.2 mM in hexane), C. Rohr, LMU... 

Fig. 5.37 Plot of the spin polarization of the tunneling current at bias voltages that correspond to the binding energies of the majority and minority part of the Gd(OOOl) surface state (the ordinate is on a logarithmic scale) as obtained by measuring the same sample in three subsequent scans during a time period of approximately 24 h. The data points marked 1, 2, and 3 denote three different locations of the sample. Apparently, the spin polarization decreases with time eltqtsed from surface preparation (reprinted with permission from [131]. Copyright 1999, American Institute of Physics)...

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