Non-Contact Mode AFM
Figure 1 : AFM cantilever oscillation amplitude (upper) and phase (lower) vs. tip-sample separation (d) known as the A-d curve for constant z-actuator driving power. The red curves denote the tip approaching the sample and the blue curves show the response when the tip is retracted from the sample surface. [ 8 ] (Figure used with permission from Nanoscientifi c.)
is undesirable for imaging purposes [ 9 ]. In these conditions, the feedback system becomes bistable. SmartScan™ Auto Mode . SmartScan™ is the name for new
operating soſt ware developed by Park Systems for research AFM systems ( Figure 2 ). Auto Mode is one of the major innovations provided by SmartScan™ and is used for performing measurements. T ere are two automation steps aſt er placing the tip and sample,
positioning and imaging. In the positioning step, the location of sample surface is found by detecting tip-sample van der Waals interactions. In this step, the tip approaches the sample at a rate of 0.5 mm/sec to fi nd the sample surface. Although this is a fairly rapid approach speed, the tip does not come into physical contact with the sample surface. Instead, the tip-sample attractive state and its eff ect on cantilever oscillation amplitude are used to detect the sample surface. As a result, tip sharpness and the original state of the sample surface are preserved. Aſt er detecting the surface, the tip is liſt ed to a safe distance (about 200 μ m) to allow navigating the sample surface and locating the area of interest via a light optical microscope. Because the location of the sample surface is known, the light microscope is brought into focus automatically. Aſt er locating the area of interest, the imaging step begins. During imaging, oscillation frequency, free air and set point amplitudes, feedback control parameters, and scan rate are optimized automatically according to the desired image quality. T e user only needs to specify scan size, number of scan lines, and desired image quality. As described above, it is important to maintain accurate tip-sample separation to maintain tip-sample interaction in the attractive state. Accurate tip-sample separation is determined by SmartScan™ Auto Mode based on the A-d curves shown in Figure 1 .
Results T e graphene/hBN samples were imaged using standard non-contact mode silicon AFM probe with a nominal tip radius of 7 nm and force constant of 42 N/m. T e tip resonance frequency was set at 316 kHz by the soſt ware. T e sample surface
Figure 2 : The newly developed SmartScan™ Auto Mode, which allows automated sample surface detection and imaging with minimal user interaction. On the left side, the automated buttons including “position” and “image” are shown with arrow A. The software is collecting a 150 nm wide image of the graphene/hBN sample in non-contact mode in this screenshot. The user only needs to select scan size, image resolution, and quality/speed from the menu showed by arrow B.
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www.microscopy-today.com • 2015 November
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