Who is in the driver’s seat of your cantilever? Self-driving cantilevers and more….
Most atomic force microscopy (AFM) methods require driving or actuating the cantilever at some frequency. Typically, you want to drive the cantilever at its resonance frequency to obtain a good signal to noise ratio on the vibration amplitude of the cantilever. This approach is behind the operation of tapping mode AFM, the most common imaging mode (tapping mode is also called dynamic mode, vibrating mode, AC mode depending on the vendor.) There are even some methods where you drive the cantilever off-resonance (such as force modulation) or at a frequency that is higher than the natural frequency of the cantilever (e.g. contact resonance.) But in all cases, you apply a voltage to the shake piezo, which sits right under your cantilever, in order to drive it at a particular frequency. When you “auto-tune” a cantilever, typically the first step in tapping mode operation, the shake piezo is simply oscillating the cantilever through a range of frequencies in order to find its resonance frequency.
Shake piezos have been the driver behind frequency tuning of cantilevers since the early days of AFM. Until recently. Although they are very effective at vibrating the cantilever, parasitic resonances from the shake piezo can affect your tuning curve resulting in very ugly cantilever tunes. These tunes can be so ugly that it can be hard to identify which peak is associated with the vibration of your cantilever? (figure 1)
Figure 1: Piezo tune
“Ugly” tunes where the signal on your actual resonance peak cannot easily be differentiated from the noisy peaks around it are especially problematic for AFM work done in fluids. Fluids significantly dampen cantilever oscillations making it especially difficult to identify the actual resonance frequency of your cantilever.
As a result, different ways to drive the cantilever have entered the market over the past few years to give much cleaner, shake piezo parasitic-resonance-free tunes, which also gives a lot of hope especially to those doing AFM fluid imaging!
Magnetic actuation was the first alternative way to drive the cantilever. One of the earliest implementations of magnetic actuation came from Agilent AFM’s called MAC (magnetic AC) mode, in which a magnetically coated cantilever was driven by an oscillating magnetic field to provide clean cantilever tunes. Other implementation of this cleaner form of actuation included iDrive (Asylum Research) and using the Lorentz force to drive the cantilever (Anasys Instruments, now Bruker).
The most currently popular implementation of non-piezo actuation of cantilever is photothermal actuation where a second laser is now used to oscillate the cantilever. The laser power of this second laser is modulated to induce a clean vibration of the cantilever. Asylum Research has commercialized this with a blue (405nm) laser with BlueDrive™. The results are shown below where in figure 2 is a cantilever tune in red with piezo actuation (notice the asymmetric peak and slight shoulder on the right hand side) with the clean, symmetric peak in blue obtained with BlueDrive. ™ The results are more dramatic for those pesky cantilever tunes in fluid, shown in figre 3. The red is again the piezo-actuated tune, which is so messy that it is very difficult to identify peak associated with the cantilever vibration. The cantilever peak in blue from BlueDrive™ easily identifies the cantilever frequency at approximately 750kHz.
Figure 2: A Cantilever tune in air, (courtesy of Asylum Research, Oxford Instruments)
Figure 3: A cantilever tune in water, (courtesy of Asylum Research, Oxford Instruments)
Finally, AFM has its own version of self-driving cantilevers! Turn off all the drivers – piezo, magnetic, photothermal – and the cantilever will still vibrate on its own due to the thermal motion. These thermal fluctuations can be measured! Lo and behold, the “thermal tune” of a cantilever is also very clean and beautiful – see below an example (figure 4) of a piezo-actuated tune (black) with the corresponding thermal tune (pink). These thermal tunes can be very helpful for resolving which peak is the “right” one (i.e. the one associated with the motion of cantilever vibration) in air.
Figure 4: piezo + thermal tune
So if thermal tunes are so great and provide us with clean tunes, why did we ever get involved with driven cantilevers – i.e. piezo, magnetic, or photothermal drivers? Why don’t we just stick with the thermal tunes? Send me your answer at email@example.com... I will answer in a future blog!