High-resolution dynamic atomic force microscopy in liquids with different feedback architectures

Abstract

SummaryThe recent achievement of atomic resolution with dynamic atomic force microscopy (dAFM) [Fukuma et al., Appl. Phys. Lett. 2005, 87, 034101], where quality factors of the oscillating probe are inherently low, challenges some accepted beliefs concerning sensitivity and resolution in dAFM imaging modes. Through analysis and experiment we study the performance metrics for high-resolution imaging with dAFM in liquid media with amplitude modulation (AM), frequency modulation (FM) and drive-amplitude modulation (DAM) imaging modes. We find that while the quality factors of dAFM probes may deviate by several orders of magnitude between vacuum and liquid media, their sensitivity to tip–sample forces can be remarkable similar. Furthermore, the reduction in noncontact forces and quality factors in liquids diminishes the role of feedback control in achieving high-resolution images. The theoretical findings are supported by atomic-resolution images of mica in water acquired with AM, FM and DAM under similar operating conditions.

High viscosity environments: an unexpected route to obtain true atomic resolution with atomic force microscopy

“…Small cantilevers, with increased resonance frequencies without a corresponding increase in spring constant or [44] may yield increased performance relative to conventional cantilevers. In low Q environments, we observe that 〈 〉 may be reduced by further lowering by increasing the viscosity of the environment (lower limit ≳ 1), thereby facilitating experiments at small amplitudes [14,28].…”

“…The AM signal reflects a convolution of conservative and dissipative tip-sample interactions. In FM imaging, those contributions are decoupled [14]: the frequency shift reflecting the conservative part while the dissipation (or drive) signal represents the dissipative part of the tip sample interaction [54]. In figure 5 we compare the dissipation images for AM (calculated from the measured phase and average imaging amplitude) [55] and FM (measured), which both show an increased energy dissipation of the cantilever when it is between lattice centres (H sites in figure 4).…”

“…Under these conditions, equation 1 predicts an increase of ′ with decreasing Q, implying that operation in high viscosity environments results in reduced performance (i.e., lower SNR) for the same force gradient [14,29]. However, in liquid environments both assumptions may be violated by i)…”

“…Taking this into account and using the low Q approximation (equation 4), the 〈 〉 within B can be written as [14]:…”

“…A reduction in the quality factor (Q) of a cantilever in fluid is associated with a higher contribution of thermal noise resulting in a reduced SNR and is generally expected to impede highresolution imaging [12][13][14]. Despite these limitations, high lateral resolution AFM investigations in aqueous environments have been the subject of extensive studies and are indispensable for gaining insight into interactions at the solid-liquid interface [15][16][17][18][19][20][21].…”