CleanDrive: Photothermal Excitation of the Cantilever

Nanosurf introduces CleanDrive - a clean and reliable way to oscillate an AFM cantilever using photothermal excitation. CleanDrive uses a near IR 785 nm laser aimed at the base of the cantilever to deliver stable and robust excitation of the AFM cantilever. This is in addition to the 840 nm superluminescent diode light source used for detecting the cantilever deflection. The photothermal excitation of the cantilever provides significant improvement in AFM performance, stability, resolution, and usability, and is especially effective in liquid environments.

CleanDrive

Most widely used AFM modes rely on actuating the cantilever and vibrating it at a particular frequency. Dynamic mode imaging and phase imaging, as well as some of the main electrical modes such as electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) all rely on oscillating cantilevers for gentle interaction with the sample. Dynamic mode presents two main experimental challenges:

The first challenge is to obtain a “clean” frequency sweep with a peak exhibiting good signal to noise. Often the background in the frequency spectrum is noisy because the piezo-acoustic excitation vibrates not only the cantilever, but also the whole cantilever chip, cantilever holder assembly and the surrounding environment. This untargeted excitation of significant mass beyond just the cantilever of interest is a messy way to actuate the cantilever and results in reduced performance for many of the most popular AFM modes. The frequency sweep thus detects other peaks associated with instrument resonance or cantilever coupling to the cantilever holder. These other peaks can have stronger signal to noise than even the peak associated with the cantilever’s resonance frequency. This problem exists in all environments and is particularly worse in liquids.

The second challenge of dynamic mode is to maintain a stable interaction between the tip and sample. The piezo-acoustic driven frequency spectrum can change with time resulting in shifts in frequency, amplitude, and phase. This is especially the case in liquid imaging, where the additional coupling between the cantilever and its surrounding environment causes a changing forest of peaks that superimposes the highly damped cantilever resonance peak. As a result, the excitation efficiency at a given frequency can change significantly with time.

Photothermal excitation offers significant improvement over conventional “piezo-acoustic” excitation of AFM cantilevers. CleanDrive provides focused actuation of just the cantilever by using light to induce a bimorph bending effect. Bimorph bending occurs due to different coefficients of thermal expansions for the two materials on the AFM cantilever - the silicon underside and the metal coating (typically Au or Al) on the top side, whose purpose is to enhance the reflectivity of the cantilever for monitoring and detection. CleanDrive works with all commercial cantilevers in both air and fluid and is also compatible with small, high-frequency cantilevers. Coated cantilevers are preferred as they respond more strongly; gold-coated cantilevers are strongly recommended for fluid environments because of the limited stability of Al coatings in aqueous solutions.

Frequency sweep

CleanDrive offers reliable automatic tuning and clean excitation at a wide range of frequencies. Figure 1a shows a comparison of the traditional shaker piezo-driven frequency sweep (in black) and the improved photothermal CleanDrive sweep (in red) of a NCSTAuD cantilever in air.

The first eigenmode is clearly seen at approximately 160 kHz in the CleanDrive spectrum. Detection of this peak is not so clear in the piezo-driven spectrum, which also shows significant structure in the background. CleanDrive also provides clear peak detection for the higher order eigenmodes, for example the second mode at approximately 900 kHz. This is in strong contrast to the piezo-driven actuation, where the various eigenmode peaks are confounded by the background structure. A clean background that enables straightforward and quick cantilever resonance frequency determination is an even more pronounced problem in liquid. Figure 1b and 1c show respectively the amplitude and phase plots as a function of frequency for an AC 40 cantilever in a buffer solution. The CleanDrive frequency spectrum (in red) quickly identifies the damped peak at approximately 32 kHz in Figure1b. In contrast, the shaker piezo-driven sweep shows multiple peak possibilities for this cantilever such as the doublet at approximately 34 kHz and 38 kHz, and peaks at 48 kHz and even 60 kHz. This “rich” structure, the so-called forest of peaks, in the piezo-excited frequency sweep background is common in liquids and makes peak identification very challenging. The phase plot (1c) also shows significant advantage of CleanDrive over piezo excitation. This phase plot with a clear transition through 0 degrees phase at resonance is required for stable phase imaging and frequency tracking, and it is observed only when using CleanDrive for cantilever excitation in liquid.

CleanDrive shows textbook-like amplitude response and no “forest of peaks
Fig. 1: CleanDrive shows textbook-like amplitude response and no “forest of peaks

Stable amplitude

Another challenge with oscillating the cantilever in a liquid environment is that changes in the local environment affect the frequency spectrum stability and thus the effective amplitude of the cantilever.

Stable amplitude in a changing environment
Fig. 2: Stable amplitude in a changing environment.

Figure 2 shows the unprecedented quality of CleanDrive in such a changing liquid environment with a cantilever immersed in a 100 µl droplet of buffer and excited with an amplitude of 3.6 nm. Using CleanDrive, the cantilever amplitude stays constant at 3.6 nm over the course of approximately 3 hours as the droplet evaporates. The amplitude of the cantilever oscillation did not change significantly, despite the fact that the surrounding environment is changing over time. The amplitude changed only after the droplet dried enough and the cantilever was no longer immersed in liquid.

Stable imaging of a silicon surface.
Fig. 3: Stable imaging of a silicon surface.

Stable imaging

One of CleanDrive’s most significant benefits is long-term stability in imaging. An example of stable imaging is shown in Figure 3 where the same 1 µm x 1 µm area of a silicon wafer was scanned for over 50 times. The surface roughness was calculated over the set of images finding no significant change, indicative of both stable imaging and preservation of tip sharpness, thanks to CleanDrive.

Stable imaging of the TipChecker sample
Fig. 4: Stable imaging of the TipChecker sample

Similarly, 100 scans of a notoriously sharp sample known as TipCheck is shown in Figure 4. TipCheck is typically used to measure the radius and shape of an AFM tip, but it is also known to quickly wear down a tip and as such, needs to be used sparingly. CleanDrive allowed continous scanning of TipCheck sample with no signs of tip degradation.

Long-term, stable imaging has important implications for operation. Typically, dynamic mode operation needs the constant attention of the user to adjust for continual changes in frequency and amplitude that occur with a piezo-driven oscillation. Due to the exceptional reliability and stability of the frequency and amplitude with CleanDrive, long-term unattended operation is now possible.

Long-term stable imaging of clock copolymer samples.
Fig. 5: Long-term stable imaging of block co-polymer samples.

Figure 5 shows 500 nm x 500 nm topography (top) and phase (bottom) images of a polystyrene-polyethylene glycol (PS-PEG) block copolymer film with. Initially, an NCST cantilever was set to oscillate with 17 nm amplitude and a setpoint of 86% resulting the images on the left. The setpoint was chosen to be 2% below making initial contact with the surface. During ~14 hours of continuous overnight measurement, the cantilever always maintained contact with the surface and resulted in the images on the right where the high quality and resolution are preserved. The cantilever could then be brought out of contact with the surface by increasing the setpoint by the before-mentioned 2%.

Stable high-resolution imaging in liquids

While all the examples thus far have shown the remarkable improvements in stability and quality of dynamic mode operation in air, the most significant benefits of CleanDrive are for liquid operation. Figure 6 shows two images of DNA on mica in buffer solution. The image in the top is the first scan and the one on the bottom is the 20th consecutive scan. These images display impressive resolution showing the major and minor grooves of DNA, without any compromise to stability or resolution after even 20 scans without any readjustments. Additionally, the near IR wavelength of 785 nm for CleanDrive avoids interference with (live) biological samples or fluorescence imaging, making it an excellent choice for photothermal excitation for imaging biological systems.

table high resolution imaging of DNA in liquids
Fig. 6: Stable high-resolution imaging of DNA in liquids

Summary

CleanDrive uses a near IR laser to provide a clean and stable targeted excitation of the AFM cantilever in both air and liquid environments. This photothermal excitation leads to several important advantages including easier imaging in liquids, improved resolution, preservation of tip shape, and more stable imaging over long periods of time. These improvements will lead to enhanced results and a significantly improved user experience for AFM measurements.

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DriveAFM — Performance without compromise

The DriveAFM, Nanosurf‘s new flagship instrument, utilizes the latest technology to deliver stable, high-end performance. It was designed to fulfill the needs of top notch research, today and in the future: CleanDrive: stable excitation in air and liquid | Ultra-low noise | Direct drive: high-resolution imaging and large scan area | Fully motorized system: full control via software