Focus on Microscopy: One Button and Two Heads—Mainstreaming AFM

The past decade has seen dramatic growth in scanning probe microscopy (SPM) technology, both in functionality and in broader acceptance. Market research conducted in mid-2007 by The Microscopy and Imaging Place, Inc.1 (McKinney, TX) indicates that approximately 20% of microscopists now use atomic force microscopy (AFM) or scanning tunneling microscopy (STM), with numbers reaching nearly 25% for industrial laboratories. Because of their ability to measure a wide assortment of physical and chemical parameters with nanoscale resolution, AFM and STM have become the instruments of choice for nanotechnology.

Figure 1 - SolverNext dual-head system.

Interestingly, this technology has taken a dual development path. On one side, these instruments have evolved into ultrahigh-end, exotic AFM/Raman hybrids2–5 or amalgams of AFM and ultramicrotomy.6 On the other, they have emerged as very simple instruments for classroom use or limited routine analysis. The challenge with mainstreaming an instrument as diverse as an SPM is loss of functionality, especially in resolution and choice of imaging mode. As seen in Figure 1, the unique dual-head approach of SolverNext (NT-MDT, Zelenograd, Russia) meets this challenge, offering a full range of conventional SPM measuring techniques in one sleek, easy-to-use system, making SPM available to the mainstream.

SolverNext’s novel architecture offers AFM and STM under one hood. The sample is placed on the scanning stage, located in the center between the two heads. With the click of a mouse, the head of choice automatically moves into position; the cantilever, laser, and photodiode are automatically aligned; and the image is acquired. Figure 1 shows the AFM head in position. With another click of the mouse, that head automatically retracts and the STM moves into place, ready for a variety of electrical measurements. A third location is available for special heads to image in liquid or for nanoindentation.

The next-generation SPM

Figure 2 - In SolverNext, AFM and STM heads can be augmented with special heads for a) imaging in liquid, b) nanoindentation, or c) amplitude:frequency dependencies for nanoindentation.

The core of this device is the integrated availability of multiple heads. As explained in the side bar and illustrated in Figure 1, SolverNext is delivered with both AFM and STM, with a third position for easy insertion for specialized heads for imaging in liquid or nanoindentation. Figure 2a illustrates imaging in liquid using protein deposited on mica, still in buffer solution (scan size: 320 × 320 nm), an important application for biological and pharmaceutical laboratories. For more industrial situations, Figure 2b demonstrates multiple nanoindentations on a sapphire surface, with the accompanying approach curves showing the amplitude and frequency dependencies (Figure 2c).

Unlike most AFM systems, that operate in open air, the SolverNext heads sit within an environmental chamber. Sliding the door down encases the sample in a temperature- and humidity-controlled chamber. The enclosure eliminates the drafts and thermal currents as well as the parasitic electromagnetic fields and electrostatic-free staging that can disturb the cantilever as it scans, causing erroneous images. Temperature within the chamber can be controlled between ambient conditions and 150 °C, and both the chamber temperature and relative humidity can be read directly from an on-board liquid crystal display panel. The door is also deeply tinted, providing additional safety against stray laser reflections. Although extremely versatile, SolverNext uses space wisely, taking up relatively little bench space.

Easy operation boosts accessibility to a broader population

Well-executed alignment is always the first step to good microscopy. However, alignment has been a tedious process in conventional scanning probe microscopy. First, the area to be measured must be found and carefully placed under the scanning area of the cantilever. Next, a laser is centered on the back of the cantilever. Finally, the beam from the laser is centered onto the photodiode, which will record the measurement. Each step requires a delicate touch on precisely machined centration screws.

SolverNext dramatically simplifies the entire alignment process. First, the sample is viewed through an integrated optical microscope. The user sees the image directly and can readily identify the region of interest. A mouse click on that area causes the stage to automatically move that region under the cantilever. Stepper motors, driven by a proprietary NT-MDT algorithm, automatically locate the cantilever position, then align the laser to the cantilever, and, finally, center the beam on the photodiode. The whole process is literally click-and-play.

Even scan area can be automatically selected. On conventional research-level systems, one scanner head must be exchanged for another to move from a routine scan size to the smaller areas used for high-resolution imaging. However, SolverNext uses a simple voltage system. High-voltage mode activates a capacitance, closed-loop scanner for 100 × 100 × 10 μm scans. The closed-loop sensors compensate for inherent imperfections in the piezoelectric scanners such as scan nonlinearity, creep, and hysteresis. A click of the mouse activates a low-voltage, open-loop scanner for high-resolution scans over a 3 × 3 × 2 μm area.