The tunneling current changes exponentially with the tip-sample separation, typically decreasing by a factor of two as the separation is increased 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. In essence, a reproduction of the sample surface is produced by scanning the tip over the sample surface and sensing the tunneling current.
STM relies on a precise scanning technique to produce very high-resolution, three-dimensional images of sample surfaces. The STM scans the sample surface beneath the tip in a raster pattern while sensing and outputting the tunneling current to the SPM Controller. The digital signal processor (DSP) in the Controller controls the Z position of the Piezo Scanner based on the tunneling current error signal. The STM operates in both “constant height” and “constant current” data modes, depending on the Feedback Gain settings. The DSP always adjusts the height of the tip based on the tunneling current error signal, but if the feedback gains are set extremely low (e.g., Integral Gain < 15 and Proportional Gain < 15), the piezo remains at a nearly constant height while tunneling current data is collected. With the Feedback Gains high (e.g., Integral Gain >15 and Proportional Gain >15), the Scanners Piezo height changes to keep the tunneling current nearly constant, and changes in piezo height are used to construct the image. The exponential relationship between tip-sample separation and tunneling current allows the tip height to be controlled very precisely. Angstrom Advanced AFM systems are available with multi-function setups to cover the broadest range of laboratory and academic applications.
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In Tapping Mode, a cantilever is oscillating in free air at its resonant frequency. A piezo stack excites the cantilever’s substrate vertically, causing the tip to bounce up and down. As the cantilever bounces vertically, the reflected laser beam is deflected in a regular pattern over a photodiode array, generating a sinusoidal electronic signal. And this signal is converted to a root mean square (RMS) amplitude value.
When the same cantilever is oscillating at the sample surface, Although the piezo stack continues to excite the cantilever’s substrate with the same energy, the tip is deflected in its encounter with the surface. The reflected laser beam reveals information about the vertical height of the sample surface.
The feedback system controls the scanner’s Z voltage to maintain the tip-sample force constant, which leads the Up-Down signal equals to the Setpoint. The Z voltage is recorded for calculating the sample topography. Figure 1: No force between tip and sample, no cantilever deflection. Figure 2: Repulsion between tip and sample, cantilever deflects upside. Figure 3: Repulsion between tip and sample, cantilever deflects upside:
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