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. 2016 Nov 11:6:36972.
doi: 10.1038/srep36972.

Controlled tip wear on high roughness surfaces yields gradual broadening and rounding of cantilever tips

Daan Vorselen  1   2 , Ernst S Kooreman  1 , Gijs J L Wuite  1 , Wouter H Roos  1   3
Affiliations

Controlled tip wear on high roughness surfaces yields gradual broadening and rounding of cantilever tips

Daan Vorselen et al. Sci Rep. .

Abstract

Tip size in atomic force microscopy (AFM) has a major impact on the resolution of images and on the results of nanoindentation experiments. Tip wear is therefore a key limitation in the application of AFM. Here we show, however, how wear can be turned into an advantage as it allows for directed tip shaping. We studied tip wear on high roughness polycrystalline titanium and diamond surfaces and show that tip wear on these surfaces leads to an increased tip size with a rounded shape of the apex. Next, we fitted single peaks from AFM images in order to track the changes in tip radius over time. This method is in excellent agreement with the conventional blind tip reconstruction method with the additional advantage that we could use it to demonstrate that the increase in tip size is gradual. Moreover, with our approach we can shape and control the tip size, while retaining identical chemical and cantilever properties. This significantly expands the reproducibility of AFM force spectroscopy data and is therefore expected to find a wide applicability.

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Figures

Figure 1
Figure 1. AFM images of polycrystalline surfaces.
2.5 × 2.5 μm images made at 1,024 × 1,024 pixel resolution. (a–c) Titanium surface. (d–f) UNCD surface. (a,d) Images made at 5 nN normal imaging force. (b,e) Images made at 25 nN normal imaging force. (c,f) Images made at 5 nN imaging force after wear experiments. Images (c,f) are made with the same tip as in (b,e) respectively. (d) corresponds to the same tip as (e,f) whereas a is a different tip than (b,c). Notice the decrease in resolution when comparing panels (a,d) with (c,f). The decrease in resolution is also visible from top to bottom in panel (b,e). Upper colorbar marks the height scale of (a–c). Lower color bar marks the height scale of (d–f).
Figure 2
Figure 2. AFM tip images of a single tip created using blind tip reconstructions.
(a) Reconstruction before and (b) after a wear experiment on a UNCD surface.
Figure 3
Figure 3. Radius and sphericity Ψo of new and worn silicon nitride tips.
(a) Radius of curvature obtained from reconstructed tip images using blind tip estimation for both titanium (Ti) and UNCD surfaces before and after the wear experiment. (b) Quantification of the sphericity of the tips. An adjusted sphericity measure (open sphericity) was used which takes into account the open shape (see main text). Errorbars indicate standard error of the mean with N = 12 for the tips worn on the Titanium surface and N = 5 for the tips worn on the UNCD surface. The sphericity increased significantly for tips worn on both Ti and UNCD surfaces.
Figure 4
Figure 4. Tip radius determination from line profiles allows tip wear tracking over time.
(a–c) Histograms showing typical distribution of peak radii from images. Data from UNCD surfaces. In black: normal-lognormal fits, for which the log-normal distribution parameters (attributed to the surface) were fit globally and are the same for all 3 histograms. Histograms were made using 1/6 image, which corresponds to a scanning area of 0.4 × 2.5 μm. (d) Comparison of radii obtained using blind tip reconstruction and subsequent fitting of the obtained tip image (RBTR) and parabolic fitting of individual peaks in line profiles along the fast scanning axes and subsequent fitting using a normal-lognormal distribution (RNLN). Both from UNCD surfaces (N = 10). Errorbars in x-direction represent the standard deviation of the tip radius measurements from reconstructed images in both scan directions (forward and reverse scan direction). Errorbars in y-direction represent 95% fitting parameter confidence intervals, assuming accurate determination of surface properties. Most errorbars are smaller than the marker size. The blue line represents y = x. (e) Tip radius versus sliding distance when scanning on a UNCD surface with 25 nN normal imaging force. 5 individual tips are visualized using various line styles and colors. Errorbars correspond to 95% confidence interval of the fitted mean, under the assumption that the surface properties were determined accurately. Average tip radius of the 5 tips increased from 22 ± 2 nm (s.e.m.) to 32 ± 3 nm (s.e.m.). (a–c) correspond to the first, second and fourth data point in black in panel (e) respectively.
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