Atomic force microscopy is a powerful technique for measurement and mapping of nanoscale topography and electrical and mechanical sample properties. The Nanosurf DriveAFM is a new generation instrument that combines ease of use and high performance through full motorization, CleanDrive photothermal excitation, and a mechanical and electrical design that allows for both high-resolution and large-range imaging.
Since its invention in 1986,1 atomic force microscopy (AFM) has evolved from a technology for surface topography imaging with sub-nanometer resolution into a multifunctional toolbox, which is also capable of measuring electrical and mechanical sample properties with nanometer spatial resolution. Recent technological developments provide new AFM systems with unprecedented ease of use, flexibility, and performance. In particular, the Nanosurf DriveAFM, Fig. 1(a), represents a new generation of the atomic force microscope, combining a highly versatile tip-scanning design with a compact scanner, full motorization, and CleanDrive photothermal excitation. In this work, we cover key aspects of the DriveAFM design and show how these features combine and unite both ease of use and high performance to make the DriveAFM a powerful and multifunctional AFM.
II. NEW OPTICAL BEAM TRACKING CONCEPT
Tip-scanning AFMs excel in several ways over sample-scanning AFMs. With all the essential components of an AFM system incorporated into a single unit, the DriveAFM, Fig. 1(a), is a truly portable and configurable multifunctional device. It can be operated as a benchtop system with a small footprint for opaque samples or easily transferred to an inverted optical microscope within minutes to allow for simultaneous AFM and unperturbed optical observation, thanks to a stationary sample. Furthermore, a tip-scanning AFM such as the DriveAFM can readily investigate large and heavy samples without compromising the AFM scanner’s performance. Nevertheless, tip-scanning AFMs are intrinsically more difficult to design compared to sample-scanning AFMs.2 The DriveAFM introduces a new concept to track the cantilever’s scanning motion with the laser beam of the deflection detection unit. This new design avoids mounting a heavy deflection detection unit onto the scanner that negatively impacts its performance, instead introducing only low-weight passive scanner-mounted elements, Fig. 1(b). This new patented beam tracking design3 allows for placing the photodetector and light source off of the scanning unit and even enables adding an additional light source for photothermal excitation and robust motorization of all adjustments. Thanks to the full motorization, Fig. 1(c), the DriveAFM can be operated fully remotely, allowing the user to adjust and optimize the system without disturbing the controlled measurement environment.
III. CLEANDRIVE PHOTOTHERMAL EXCITATION
Many popular AFM modes require oscillating the cantilever near, or at, its resonance frequency. The oscillation is usually indirectly driven by using a shaker piezo placed near the cantilever. This drive mechanism typically leads to many spurious signals.4 The CleanDrive option of the DriveAFM represents an alternate approach, also known as photothermal excitation, that uses an additional intensity-modulated laser that can be independently positioned, Fig. 2(a), to directly excite cantilever oscillations, typically through a local bimorph effect.5
CleanDrive photothermal excitation offers significant advantages over the conventional piezo-based excitation. Particularly in liquids, CleanDrive excels with a reliable textbook-like amplitude and phase response, Fig. 2(b). Piezo-based excitation typically produces a so-called forest of peaks as not only the cantilever but also the cantilever holder, and its surroundings are excited and eventually coupled back into its motion, Fig. 2(b).4,6 The peak distribution in such a forest of peaks depends on many factors (e.g., tip–surface distance) and may vary with time. CleanDrive also allows for exciting the cantilever at frequencies of up to several MHz, where piezo-based excitation suffers from significant loss in efficiency, Fig. 2(c). Without the limitations of the piezo-based excitation, the oscillation amplitude of the cantilever can be maintained over hours even in liquid environments with drastically changing volume, Fig. 2(d).
IV. HIGH-RESOLUTION LARGE-RANGE SCANNER
Rapid and accurate scanning, a critical AFM functionality, requires a combination of high resonance frequency scanner mechanics and precise low-noise electronics. Most large-range AFMs mechanically amplify a small piezo motion to achieve a larger scan range. This amplification, however, causes parasitic force losses in the amplifying structure and requires softening of the scanner structure. In contrast, the DriveAFM uses a direct drive scanner architecture with large-range piezo actuators. This 1:1 piezo motion to scanner motion coupling, Fig. 3(a), provides more driving force and allows for maximizing the scanner stiffness. Combining this approach with the new beam tracking concept results in a very compact scanner design with minimized moving mass and high stiffness that yields a large 100 × 100 × 20 μm3 scanner with high resonance frequencies. The primary system resonance at 3.5 kHz, Fig. 3(b), represents an excellent figure for such a large-range scanner.
The CX controller provides high 28-bit resolution output signals to the DriveAFM scanner, allowing very small yet smooth and accurate scan signals in comparison to the 20-bit resolution of most competitor systems, Fig. 3(c). Furthermore, the electronic design of the CX controller ensures outstanding output noise performance over the whole +/−10 V output range, not only at 0 V, Fig. 3(d). Above 10 Hz, the noise floor of <20 nV/√Hz is equal at both output values, with only a slight increase in the 1/f noise floor of the output at 9 V. Such noise performance allows the user to exploit the full performance of the DriveAFM system over the entire scan range.
V. APPLICATION EXAMPLES
A. High-resolution DNA imaging
CleanDrive excitation provides reliable cantilever excitation, even at small oscillation amplitudes of a few nm and in liquid environments. In this example, these capabilities were used to image double-stranded DNA (dsDNA) at high resolution in buffer solution using a 20 μm long and 10 µm wide cantilever (USC-F0.3-k0.3, Nanoworld). The detection optics of the DriveAFM permits using such small, high resonance frequency cantilevers for faster imaging than with traditional AFM systems. The DNA molecules shown in Fig. 4(a) clearly show a banding pattern all over the molecules that originates from the major and minor groves of the DNA double helix. Thanks to the stable CleanDrive excitation, the banding pattern can be stably imaged without the need for imaging parameter adjustments. The inset of Fig. 4(a) shows a surface profile of a stretch of dsDNA double helix. The profile is characterized by a wavy pattern with a periodicity of 3.42 nm for a motif of two valleys.
B. Indentation of virus capsids
With its low noise and high force resolution, the DriveAFM can be used as a tool for investigation of the mechanical properties of a variety of different samples. Figure 4(b) shows an example of imaging and mechanically probing virus capsids, which are the protein shells of viruses. The stable mechanical loop of the DriveAFM allows for imaging the virus capsids in buffer solution at high resolution, revealing the icosahedral structure and even the protein complex arrays that form the capsid’s protein cage. In this experiment that was performed on an inverted optical microscope, the virus capsids were first imaged [Fig. 4(b) (before)], mechanically indented with the AFM tip [Fig. 4(b) (force curve)], and then reimaged [Fig. 4(b) (after)] to observe the structural changes induced by the indentation. The force–distance curve reveals that before perforation, the capsid shows a linear force response with a spring constant of ∼0.3 N/m [Fig. 4(b), force curve, orange line]. Perforation of the protein layer on the opposite side of the capsid is characterized by a steep force increase followed by a small drop in force. Upon reimaging the capsids, a hole is visible in the capsid, caused by the indenting tip.
C. Moiré pattern on twisted graphene
In this application, CleanDrive photothermal excitation was used to excite the cantilever on the contact resonance frequency peak during contact mode imaging (force modulation) of graphene layers twisted by 0.5°. The Moiré superlattice, Fig. 4(c), in the phase image results from variations of the contact resonance frequency when scanning the sample. The photothermal excitation and the stiff scanner design allow for zooming in to reveal both Moiré and the atomic lattice in a single image, Figs. 4(d)–4(f).
The DriveAFM is a new generation AFM with a unique combination of features that are enabled by a novel patented design. Full motorization provides ease of use and the ability to adjust the system without disturbing a controlled environment. The stable mechanical design, direct drive scanner, and low-noise electronics of the CX controller yield high-performance imaging from the atomic lattice resolution up to 100 × 100 µm2 scan sizes. CleanDrive photothermal excitation and readout optics for cantilevers down to 10 μm in width provide straightforward and stable cantilever excitation, even in liquid environments, and imaging at faster rates than traditionally achievable in AFM. These features, together with the broad range of available AFM modes, accessories, and options, make the DriveAFM a powerful and truly multifunctional AFM.
Conflict of Interest
The authors have no conflict to disclose.
The data that support the findings of this study are available from the corresponding author upon reasonable request.