Oxford Instruments Asylum Research became the pioneer in the use of photothermal excitation in commercially available AFMs when blueDrive™ was first introduced in 2013. With a current installed base of hundreds of blueDrive AFM systems, no one understands the advantages of photothermal excitation better than Asylum Research and our customers.
No matter what your field of research or industry may be, blueDrive makes all tapping mode techniques simpler, more stable, and more accurate. For any AFM applications where the sample is investigated in liquid, including biomolecules and biomembranes, batteries and other electrochemical systems, and other surface chemistry studies at a solid-liquid interface, blueDrive dramatically improves both overall ease of use and long-term imaging stability. These benefits become even more critical for video-rate AFM applications where the Cypher VRS1250 is used to visualize nanoscale dynamic processes in biomolecules, etch and dissolution, and self-assembly processes. But the benefits of blueDrive go beyond just applications in liquid. In air, blueDrive helps enable fast, stable imaging that protects both the sample integrity and tip sharpness for consistent ultra-high-resolution results, whether one is researching 2D materials or measuring the roughness of semiconductor or data storage materials. The benefits of blueDrive are not limited to just basic topographic imaging, either. The clean, stable tapping mode tunes improve the consistency of quantitative nanomechanical imaging techniques like AM-FM Viscoelastic Mapping Mode for applications including polymer science and nanoelectrical imaging techniques like Kelvin Probe Force Microscopy for applications including photovoltaic materials.
This page describes blueDrive photothermal excitation, its capabilities, and benefits to the AFM users across all of these research fields and industries.Get information from an AFM expert
Tapping mode is by far the dominant imaging mode in the world of AFM, measuring not just topography, but also mechanical, electrical, and magnetic properties of samples. Typically, piezoacoustic excitation is used to drive the cantilever oscillation required for tapping mode imaging. Though piezoacoustic drive is favored for design simplicity, the response is often far from ideal. Since the piezo shakes the whole AFM, it excites spurious mechanical resonances that couple into the cantilever response, resulting in a “forest of peaks”. By replacing the piezo by a laser beam, Asylum’s blueDrive excitation mechanism produces an almost perfect response by directly exciting the cantilever photothermally.
[Left] Schematic representation of the blueDrive laser (blue) and the detection laser (red) placement on the cantilever. [Right] The animation shows how modulating the blueDrive laser results in cantilever deflection.
Photothermal mechanism directly excites the cantilever and does not couple to other system resonances, so the forest of peaks is avoided in both air and water, as shown in the blueDrive vs piezo drive tunes below. In fact, the photothermally excited response (blue curves) closely matches theoretical predictions (dashed lines).
One of the biggest advantages of atomic force microscopy is that samples and processes can be visualized in a wide variety of environments, including liquids. For biological samples this usually means water or aqueous buffer solutions. The benefits of using blueDrive photothermal excitation in liquid environment are immense. Those with experience using conventional AFMs will know that cantilever tuning in liquid is normally much more complex than in air. It can be difficult to determine which peak to tune and use. blueDrive eliminates this problem by directly exciting only the cantilever so the cantilever resonance peak in liquid is just a clean, clear, and easy to tune as what you see in air. Not only does blueDrive make setup effortless, it also makes imaging in liquid simple and provides the highest quality images.
Lipid bilayers consisting of 50:50 DOPC and DPPC were deposited on mica and imaged in water using blueDrive photothermal excitation and tapping mode on a Cypher S AFM. Image size 3 µm.
DNA sample molecules on mica, in buffer. Acquired in tapping mode on a Jupiter XR using blueDrive. Image size: 500 nm.
DNA molecule on mica, in buffer. Red arrows show the spacing between three turns of the DNA helix; the green and gold arrows indicate the major and minor grooves, respectively. Acquired in tapping mode on Cypher ES using blueDrive. Image size: 65 nm.
Unlike the shifting forest of peaks produced by piezoelectric excitation, the photothermal drive response remains constant with time irrespective of the fluid volume. This enables uninterrupted imaging for the entire length of experiments, even during continuous fluid perfusion such as the calcite screw dislocation experiment shown below.
Furthermore, combining blueDrive with an electrochemistry cell makes the AFM a valuable tool for real-time monitoring of electrochemically driven processes. The figure below shows an example of in situ monitoring of copper crystal growth on a Cypher ES AFM. Crystals were first deposited from an acidic aqueous solution of copper sulfate and were then stripped away by changing the potential of a gold electrode.
A key aspect of fast-scanning AFMs is the use of small cantilevers, roughly 10 times smaller than conventional cantilevers with resonance frequencies greater than 1 MHz in air. These cantilevers not only enable faster imaging but also higher image resolution because of lower noise. Unfortunately, small cantilevers only exacerbate the problems of piezoelectric excitation. Tunes are more likely to show severe distortions and to be highly variable in time, making setup and stable operation more complicated. In contrast, blueDrive photothermal excitation provides a clean, stable response at high frequencies, making fast imaging much easier and quicker. The response remains constant with time in liquid, even during perfusion experiments. This feature is valuable for many experiments, such as the ones features below: interactions between biomolecules, molecular self-assembly, and dynamics in surfactant films.
Several techniques involving dynamic modes have been recently developed for mapping of both elastic and viscoelastic properties. Although often adequate for basic imaging in air, piezoelectric excitation is more problematic for nanomechanical mapping. Even in air, the cantilever response with piezoelectric excitation contains both on- and off-resonance distortions. An insufficiently flat drive response makes it difficult to evaluate whether changes in the cantilever response are due to sample properties or drive artifacts. This can cause errors in the results from model-based quantitative analysis and create tracking instabilities. Using photothermal excitation, however, the cantilever response possesses a nearly perfect resonance peak and is otherwise flat in frequency. As a result, measurements agree more closely with theoretical predictions, leading to less uncertainty in modulus calculations and improved accuracy.
Nanomechanical map of a multilayer polymer composite viewed in cross section. Elastic modulus measurement is shown overlaid on a topography image: polyethylene terephthalate (green), polyethylene (blue), and ethylene vinyl alcohol (yellow). Acquired in AM-FM Viscoelastic Mapping mode on the Cypher S using blueDrive. Image size: 9 μm.
Elastic modulus overlay on topography of a polymer blend sample composed of polyethylene (darkest), polypropylene (matrix) and polystyrene (brightest). Acquired in AM-FM Viscoelastic Mapping mode on the Jupiter XR using blueDrive. Image size: 25 μm.
Comparison of cantilever amplitude stability over time using piezo drive (red) and blueDrive (blue). Performed on a Cypher AFM, 2 hours long experiment.
Piezo-driven cantilever response can be distorted, additionally, it can also vary with time. As the cantilever response drifts relative to the chosen setpoint, the tip-sample forces change. It can be difficult to distinguish true sample dynamics from artifacts of this drift. The direct excitation provided by blueDrive is highly immune to drift in the cantilever response as shown in the graph below. Slight temperature variations or changes in the fluid volume cause the amplitude response to change over time when using piezo excitation. However, blueDrive keeps the amplitude stable even during fluid heating and fluid perfusion, which maintains high quality imaging without any user adjustments.
Since the cantilever amplitude remains stable throughout the experiment, the imaging force is kept constant which, in turn, preserves tip sharpness and sample integrity. Imaging is stable over extended durations with no need to readjust the amplitude setpoint such as demonstrated in the atomic resolution data of mica and silicon wafer surface roughness data shown below.
This application note focuses on examples that demonstrate the key benefits of blueDrive photothermal excitation such as simplified control, stable operation that's gentle on tips and samples, and improved quantitative results of nanomechanical AFM measurements.
Download the application note to learn:
View more AFM images that have benefitted from blueDrive photothermal excitation in our image gallery!
See How blueDrive Can Improve Your Research
blueDrive is available on both the Cypher AFM Family and Jupiter XR AFM platforms. It is included on all Jupiter XR and Cypher VRS1250 systems and is available as an option on Cypher S and Cypher ES systems for both new and existing systems.
Contact us to discuss how blueDrive can help drive your research forward and to request pricing for either a new blueDrive-equipped AFM or a blueDrive upgrade.