Scanning tunneling scan measuring height

Measurements. Absorption spectra measurements on the film and solutions of the metal complexes were measured on a modified Cary 14 spectrometer. Luminescence spectra were recorded with a custom photon counting spectrometer or a PTI (Deer Park Drive, South Brunswick, NJ 08852) luminescence spectrometer system. The FT-IR spectra were recorded with a PE-1600 spectrometer or a Bio-Rad FTS-40 spectrometer (Professor R. Crooks, Chemistry Department, University of New Mexico). RA spectra for LB films were measured on the FTS-40 with a Hanick gra2dng angle attachment and MCT detection system. Typically 256 scans were requir to obtain adequate intensity for the monolayer films. The scanning tunneling microscopy measurements employed a Nanoscope II system (Digital Instruments, Inc.). Etched Pt-Ir tips were used with the standard, 0.6 micron head. The system can be operated in either the constant current or constant height mode but the film contours were similar for either. 

Powerful methods that have been developed more recently, and are currently used to observe surface micro topographs of crystal faces, include scanning tunnel microscopy (STM), atomic force microscopy (AFM), and phase shifting microscopy (PSM). Both STM and AFM use microscopes that (i) are able to detect and measure the differences in levels of nanometer order (ii) can increase two-dimensional magnification, and (iii) will increase the detection of the horizontal limit beyond that achievable with phase contrast or differential interference contrast microscopy. The presence of two-dimensional nuclei on terraced surfaces between steps, which were not observable under optical microscopes, has been successfully detected by these methods [8], [9]. In situ observation of the movement of steps of nanometer order in height is also made possible by these techniques. However, it is possible to observe step movement in situ, and to measure the surface driving force using optical microscopy. The latter measurement is not possible by STM and AFM. 

The constant height mode of operation results in a faster measurement. In this analysis, the tip height is maintained at a constant level above the surface and differences in tunneling current are measured as the tip is scanned across the surface. This approach is not as sensitive to surface irregularities as the constant current mode, but it does work well for relatively smooth surfaces. 

In scanning tunnelling microscopy a typical barrier height (V - E in eqn 3.6) is 2 eV. If the measured current i is proportional to the tunnelling probability for electrons, show that it should vary with distance d from the surface (expressed in nanometres) according to... 

Q.29.12 A scanning tunneling microscope is set to operate in constant height mode at 0.9 nm above the surface with a tip bias of 0.01 V. A scan is performed in the +x direction. What topography do the following measured currents (in picoamperes) suggest ... 

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