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The most commonly used non-contact or partial contact technique, developed to improve the resolution of soft materials, is known as “intermittent contact” (IC) imaging or tapping mode imaging or “dynamic contact mode”. In this mode, the cantilever is modulated on or near its natural resonance frequency (f0) and at a desired amplitude . When the tip is far above the surface, the modulation amplitude and phase remain constant. When the tip is brought into the attractive regime, the modulation amplitude decays sharply due to the shift in its natural resonance. The magnitude of the decay depends on the tip-surface interaction, which in turn, is determined by the modulation amplitude , tip-surface separation, and functionalities of both the tip and surface. The feedback circuit keeps the modulation amplitude constant during image acquisition. In IC or tapping mode, the average distance between the tip and the surface is an amount corresponding to no contact; however, the tip may touch the surface periodically during modulation. The advantages of IC mode are (1) the brief contact time between the tip and the surface, which effectively removes the stick-and-slip behavior; and (2) the average tip-surface distance remains in the non-contact regime, which reduces the surface deformation when there is brief contact. The disadvantage of this mode is that the images are acquired in the attractive regime, which is not as sensitive as the repulsive region used during contact mode imaging. In other words, the intrinsic resolution of IC mode is not as high as the contact mode for hard and non adhesive surfaces.

For tapping mode or IC mode the RHK SPM100 is extended with Stanford Research Systems model 830dsp lockin amplifier. This type of lockin amplifier can apply an offset to the output (amplitude) signal. The sin wave output of the lockin needs to be connected to the piezo that excites the cantilever. It is applied to the ribbon cable connector going to the head via BNC connector B on the rear side of the AIM-MI. The sine wave signal coming from the PS detector in the scan head can be found on BNC connector A on the rear of the AIM-MI. This signal is lead to the input signal-in A of the lockin amplifier. The channel A output signal of the lockin amplifier should be connected to the rear panel of the SPM100 unit , the BNC on the right-hand side of the DB9 connector in the Preamp Input section. Set the switch towards the BNC connector. The amplitude output signal from the lockin to the BNC on the back of the SPM100 will become the feedback signal and can be imaged by turning on channel that is now labeled Error Signal if you are in AFM mode. You should change the imaging mode from AFM to User Defined and then enter Amplitude as the Feedback Signal so the channel is labeled correctly. The phase, output channel B on the lockin amplifier, can be connected to any of the rear panel Aux BNC.

On the front side of the SPM 100 the polarity switch should be changed from ” +” to “ –“. Do NOT switch the polarity switch on the rear side of the SPM100, it should stay at the “-“position.

The cantilever should be mounted as described in the contact mode manual. The cantilever holder should be suited for tapping mode. Before beginning, verify that the mode switch on the MultiMode SPM’s base is switched to AFM & LFM , not to TM! If the switch is toggled to STM, the laser will be turned off.

a. Find the edge of the substrate (X-axis adjustment knob).
(Note: Ensure that you have reached the front edge of the substrate, which is different to the tip of the cantilever. You may need to follow along the edge of the substrate to reach the correct position.)
b. Find the base of the cantilever (Y-axis adjustment knob).
c. Move to the tip of the cantilever (X-axis adjustment knob).

First set the driving amplitude to 0.5-1.0 Volts. The most straightforward method to determine the natural resonance frequency of a cantilever is to monitor the amplitude of the cantilever vibration with an oscilloscope while scanning the driving frequency (the maximum frequency the SR830 lockin amplifier can deal with is 100 kHz). When the cantilever is excited with a frequency near its resonance, the amplitude will start to increase. A typical cantilever, with k = 2.2 N/m typically exhibits a resonance at 82 kHz (see cantilever box). The frequency spectra provide a good guide for intermittent contact conditions. The modulation frequency (f) should very near the peak of the spectrum which is the natural frequency of the cantilever, a bit lower so that the amplitude is like 90% of the maximum . The amplitude should be sufficiently high (especially under ambient conditions) in order to overcome the capillary forces and pull the cantilever off the surface after brief contact (~1nm).

Before approaching the tip:

1. Adjust the offset in such a way that the offset is about half the value of the amplitude.

2. On the scan head, adjust the PSD in such a way that both in both directions the signal is about 0, so no offset.

3. Set the time constant of the lockin as fast as possible to achieve a stable output. Then the dominant time constant in the system will be the feedback loop bandwidth and not the lockin which could cause delays in responding.

In the measurement a relatively high time constant on the RHK box (upto 10.0) might be needed. However, in the approaching procedure the time constant should be a low value, about 1.0. Otherwise the tip might crash.

software: line time 1 sec 500×500 nm,aux feedback enable, time constant 10 sec, output 0

spm100: Z-gain position 128 (flat sample surface), gain 0.0 time constant 10.0,feedback linear, scan range X,Y: x0.1 (max. 800×800 nm for the EV scanner).

lockin: signal input A, AC float, channel A , R, display; channel B, theta, display, amplitude 0.5 V, frequency 75 kHz, offset 40%, time constant 100 microsec, 24 dB, sensitivity 200 mV

* AFM tapping mode
xpmpro_tapping_prm.txt