AFM and Piezoresponsive Materials

To visualize domains in piezoresponse materials, methods based on optics and domain-selective etching are used. These methods have limitations. For example: the optical methods are non-destructive, and the analysis can be performed in situ—but they tend to have poor lateral resolution. To achieve domain selective etching, harsh chemicals such as HF are required but good lateral resolution is achieved. Measurements from the AFM tend to have very high lateral resolution (dependent only on the end radius of the tip) and are non-destructive. These are the primary reasons for which there is an increasing use of scanning probe microscopy-based methods for understanding the behavior of piezoresponse materials.

To measure the deformations of a piezoresponse (PR) material in the vertical or lateral direction, the sample is actuated using an AC electric field. The AFM is operated in contact mode where the tip touches the surface of the PR material and maintains a constant average deflection delta A. Gruverman, O. Auciello and H. Tokumoto, “Imaging and control of domain structures in ferroelectric thin films via scanning force microscopy,” Annu. Rev. Mater. Sci. 1998. 28:101-123.1. In response to the applied AC electric field, the sample expands and contracts. The vertical deflection of the cantilever measured by the four-quadrant photodiode detector is the input source to a lock-in amplifier. The vertical deflection is modulated at the same frequency as the applied electric field. An important point is that the changes in the vertical deflection, when translated into physical units, are of the order of a few picometers to a few tens of picometers. This necessitates the use of a lock-in amplifier to measure such small signals. The applied electric field is the reference input to the lock-in amplifier.

The NanoScope V Controller has three internal lock-ins. Lock-ins 1 and 2 are capable of operating up to 5 MHz (denoted as high-speed) while lock-in 3 is capable of operating up to 50 kHz (denoted as low-speed). A schematic of a contact mode AFM with a piezoresponse measurement set-up is shown:

The AFM tip is maintained with a constant deflection delta on the surface of a PR material. The vertical (or lateral) deflection of the cantilever signal is the first input (source) to the NanoScope Controller internal lock-in. AC bias is applied to the sample (or tip) causing the PR material to expand and contract. This is the second input (reference) to the NanoScope Controller internal lock-in.

At tip-position A, suppose that the domains of the PR material oscillate in-phase with the AC drive signal. The lock-in amplifier measures the component of the deflection signal that oscillates in-phase with the AC drive signal and the component of the deflection signal that oscillates perpendicular (at 90 degrees) to the AC drive signal. From these two signals, the amplitude and the phase angles of the deflection signal are calculated and displayed in the software. When the tip moves over to position-B, the domains of the PR material oscillate out-of-phase with respect to the AC drive signal and similar information is collected via the lock-in amplifier.

At the domain wall, ideally the amplitude drops to zero and the phase measured by the lock-in amplifier changes by 180 degrees. To translate the amplitude of the PR oscillations from volts to picometers, you must calibrate the deflection sensitivity of the cantilever. Similar measurements may be performed to image the lateral domains in a PR material by setting the lock-in source for the amplifier to lateral deflection.

Piezoresponse microscopy is also known as Piezoresponse force microscopy (PFM).

The NanoScope V Controller and software allows a user to perform piezoresponse measurements in various configurations.

The first configuration is Piezoresponse microscope mode. In Piezoresponse mode, you can image vertical domains with both the low-speed (up to 50 kHz) and the high-speed lock-ins (up to 5 MHz). You can apply 16 times gain to the vertical signal to boost the deflection signal on both or either of the lock-ins. The low-speed and the high-speed measurements may be performed simultaneously, with one measurement on the main line and another on the interleave line. The minimum lock-in bandwidth for the low-speed lock-in is 5 Hz while that for the high-speed lock-in is 500 Hz. See PFM Optimized Vertical Domains Operation for details.

The second configuration is the use of the Generic Lock-In interface in contact mode operation. This mode allows the user to perform mapping of the vertical and lateral domains simultaneously. You can also lock-in on a harmonic signal of the applied frequency, which makes the use of Generic Lock-In interface attractive for certain applications. See PFM Vertical and Horizontal Domains Operation for details.

In both of the above modes, you may operate near the contact resonance of the system, which has the advantage of a very good signal-to-noise ratio compared to operating away from contact resonance. However, interpretation of the data becomes more complex.

Both the Piezoresponse microscope mode and the Generic Lock-In interface in contact mode can be used in conjunction with the NanoMan V software and the Point and Shoot feature. The NanoMan V interface enables you with capabilities to write and switch domains on thin-film piezoresponse materials. The Point and Shoot interface allows you to perform sample bias ramps while simultaneously measuring the piezoresponse properties of the material.