An Optical ‘Tripod’ for Atomic-Force Microscopes

Gavin King, a physicist out of University of Missouri, together with Allison Churnside and Thomas Perkins from Department of Molecular, Cellular, and Developmental Biology at the University of Colorado at Boulder, have published an interesting paper in SPIE that takes a look at the future of scanning-probe microscopes, and possible ways to overcome some of the problems that are still not resolved in this technology. Scanning Probe Microscopy (SPM) obtains an image of the surface by mechanically moving a probe, but it suffers from intrinsic mechanical drift. The authors believe they can overcome this detrimental effect that occurs between the scanning-probe tip and sample with some quantum trickery.
From the paper, here’s how they propose to do it:

Drift occurs in all scanning-probe instruments owing to environmental perturbations. When operating such instruments at liquid-helium temperatures, the gold-standard conditions for high-precision SPM work, tip-sample drift rates are reduced to ~0.01Å/min. This extreme instrumental stability facilitates detailed dynamic studies and enables atomic-scale patterning of matter. In recent work,2 we showed that it is possible to approach similar levels of tip-sample stability in ambient, ‘real-world’ operating conditions, where instrumental drift rates are typically 1000-fold higher.
Mechanical drift between an SPM tip and a sample limits many aspects of SPM performance. For example, atomic-force microscopes (AFMs), the most prominent type of SPM instrument, would benefit from the ability to enhance image resolution by scanning slowly and averaging cantilever response, return the tip to a precise feature in an image (e.g., a region of a protein), hover the tip over a feature for long time periods to study local dynamics (e.g., conformational fluctuations), and precisely control the tip’s 3D position when disengaged from the surface (e.g., force spectroscopy). Unfortunately, none of these important tasks can be achieved with current AFMs in real-world conditions because of drift. Long-term atomic-scale stability between tip and sample is needed to fully exploit the advantages of AFM across a broad array of disciplines.
To surmount the limitations imposed by drift, we have developed a unique, ultrastable AFM measurement platform. Our approach, inspired by precision optical-trapping techniques,3 establishes a local differential reference frame to control the tip/sample displacement (see Figure 1). Briefly, focused lasers of different wavelengths (red and green) locally report tip and sample position by scattering off both the tip’s apex and a fiducial mark (a nanoscale silicon disk) affixed to the sample plane. Backscattered light is separated by wavelength and collected to yield the 3D position of each object with atomic precision. This data is used as feedback to piezo stages (not shown) to actively stabilize the tip’s position with respect to the sample. The technique’s precision hinges on maintaining extreme 3D differential pointing stability between the two laser focal volumes. The method requires low laser power (1mW), is independent of tip/sample interactions, and negligibly perturbs the tip. A third laser (gold) is reflected off the backside of the cantilever to report tip/sample force in a standard optical lever-arm arrangement.
Our nascent, ultrastable AFM has achieved an unprecedented level of tip/sample control in ambient conditions.2 With an ultrastable AFM, images can be obtained at high resolution while maintaining atomic-scale registration between tip and sample during and after scans. Hence, regions of interest can be identified in a scan and later interrogated in detail. The instrument has the newfound ability to hover an AFM tip over specific regions of interest with single-Ångstrom stability for time periods on the order of tens of minutes.

Full story: An optical ‘tripod’ enhances the use of atomic-force microscopy …