High-Speed Quantitative Nanomechanical Mapping by Photothermal Off-Resonance Atomic Force Microscopy

By Bart Hoogenboom.

Hans Gunstheimer^a,b, Gotthold Fläschner^c, Jonathan D. Adams^b, Hendrik Hölscher^a, and Bart W. Hoogenboom^b

Link to publication: High-Speed Quantitative Nanomechanical Mapping by Photothermal Off-Resonance Atomic Force Microscopy,

#Done with a DriveAFM: Performance without compromise

One of the great assets of atomic force microscopy (AFM) is its ability to inspect mechanical properties of materials at nanometer resolution. Consequently, AFM has been widely applied for the characterization of materials including metals, polymeric coatings, and living cells and tissues. However, since this requires the AFM probe to move up and down at least once for every pixel in an image, such nanomechanical mapping is time-consuming, taking up to tens of minutes for high-resolution maps. 

As described in our recent publication, the throughput of such measurements can be enhanced by 1 to 3 orders of magnitude by generating the fast up-and-down movements via an oscillatory laser beam that locally heats the AFM probe. Acting as a bimetal, the probe rapidly moves with the modulated laser intensity, creating the down-and-up movement required to transiently indent the material under study, and hence to determine its mechanical response. This approach was implemented in a DriveAFM with WaveMode NMA (Nanosurf AG, Liestal, Switzerland). 

When this approach was first tested, rather unexpected and rich physics emerged, in that the probe motion due to such photothermal actuation strongly depends on the propagation of heat along the probe. Most of these probes are essentially miniature diving boards, called cantilevers; variations in heat propagation along the cantilever translate into different bending shapes and hence different up-and-down movements as a function of time and of position along the cantilever. 

To understand this motion and its detection, we established a thermomechanical model of the cantilever response. Next, building on this understanding, we defined a calibration procedure that enabled us to obtain accurate nanomechanical information of sample materials despite the thermomechanical complexities of the probe/cantilever itself. 

We tested this method first by measuring the mechanics of a well-defined elastic microfabricated structure (in fact: another cantilever), and next by applying it to more complex materials such as metal alloys and polymer blends. The acquired elasticity data aligned closely and quantitively with expected results, demonstrating the validity of our approach. 

In this first study, high-quality data was obtained at up to 25 kHz, which sets the upper limit for the pixel rate at which nanomechanical maps can be acquired. This compares very favorably with the 1~100 Hz pixel rates using traditional nanomechanical mapping, and also with the few kHz pixel rates of other, more recent methods without photothermal actuation. 

Ever since the invention of AFM, there has been a push to enhance the throughput of what originally was a rather slow technique. Exploiting the thermomechanical response of AFM probes, we now do nanomechanical mapping at speeds that were previously reserved solely for topographic imaging.

To read more:

WaveMode NMA: High-Speed Nanomechanical Mapping