Scanning Tunneling Microscopy (STM)

Scanning Tunneling Microscopy (STM) uses a tunneling current between the probe and the sample to sense the topography of the sample. The STM probe, a sharp metal tip (ideally, atomically sharp), is positioned a few atomic diameters above a conducting sample that is electrically biased with respect to the tip. At a distance under 1 nanometer (0.001 µm), a tunneling current will flow from sample to tip. In operation, the bias voltages typically range from 10 to 1000 mV while the tunneling currents typically vary from 0.2 to 10 nA. The tunneling current changes exponentially with the tip-sample separation, typically decreasing by a factor of 2 as the separation is increased by 0.2 nm. The exponential relationship between the tip separation and the tunneling current makes the tunneling current an excellent parameter for sensing the tip-to-sample separation. These sensitive changes in tunneling current as the tip scans over the sample surface are used to gather data and construct a nanoscale or atomic scale image.

STM Applications:

• Imaging at atomic/molecular resolution
• Samples with deeply relieved features, where AFM probes may not be able to penetrate, and/or where feature verticality is very close to 90 degrees
• Polished samples, where it is necessary to image different layers with similar topography but different electrical conductivities
• Under conditions where contact with the sample surface is prohibited

STM relies on a precise scanning technique to produce very high-resolution, three-dimensional images of sample surfaces. The STM scans the tip over the sample surface in a raster pattern while sensing and outputting the tunneling current to the NanoScope control station. The digital signal processor (DSP) in the workstation controls the Z position of the piezo based on the tunneling current error signal. The STM operates in both “constant height” and “constant current” data modes, depending on a parameter selection in the Feedback panel. The DSP always adjusts the height of the tip based on the tunneling current error signal, but if the feedback gainsThe gains are scaling factors for the error signal to determine how quickly corrections should be applied in the feedback loop. There are two important gain factors: the proportional gain scales to the current value of the error signal, and the integral gain scales the aggregate of the past values of the error signal (the "area under the curve"). are low, the piezo remains at a nearly “constant height” and tunneling current data is collected. With the gains high, the piezo height changes to keep the tunneling current nearly constant, and the change in piezo height is collected by the system. The exponential relationship between the tip separation and the tunneling current allows the tip height to be controlled very well. For example, if the tunneling current stays within 20 percent of the setpointOperator-selected threshold used as the target of the feedback control loop. value (in STM, this is the current to be maintained by the feedback system), the variation in the tip-sample separation is less than 0.02 nm.

The STM tip is held using a dedicated STM probe holder that plugs into the end of the Dimension Icon Dimension Icon SPM Scanner’s tube:

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