Atomic Resolution of Calcium and Oxygen Sublattices of Calcite in Ambient Conditions by Atomic Force Microscopy Using qPlus Sensors with Sapphire Tips

Abstract

Characterization and imaging at the atomic scale with atomic force microscopy in biocompatible environments is an ongoing challenge. We demonstrate atomically resolved imaging of the calcite (101̅4) surface plane using stiff quartz cantilevers ("qPlus sensors", stiffness k = 1280 N/m) equipped with sapphire tips in ambient conditions without any surface preparation. With 10 atoms in one surface unit cell, calcite has a highly complex surface structure comprising three different chemical elements (Ca, C, and O). We obtain true atomic resolution of calcite in air at relative humidity ranging from 20% to 40%, imaging atomic steps and single atomic defects. We observe a great durability of sapphire tips with their Mohs hardness of 9, only one step below diamond. Depending on the state of the sapphire tip, we resolve either the calcium or the oxygen sublattice. We determine the tip termination by comparing the experimental images with simulations and discuss the possibility of chemical tip identification in air. The main challenges for imaging arise from the presence of water layers, which form on almost all surfaces and have the potential to dissolve the crystal surface. Frequency shift versus distance spectra show the presence of at least three ordered hydration layers. The measured height of the first hydration layer corresponds well to X-ray diffraction data and molecular dynamic simulations, namely, ∼220 pm. For the following hydration layers we measure ∼380 pm for the second and third layer, ending up in a total hydration layer thickness of at least 1 nm. Understanding the influence of water layers and their structure is important for surface segregation, surface reactions including reconstructions, healing of defects, and corrosion.

Advances in AFM: Seeing Atoms in Ambient Conditions

“…Because the water layer plays such a large role, there are strong differences in acquiring data with a hydrophillic versus a hydrophobic tip [10]. Finally, we could show observations of a single-atom defect and an atomic step with atomic resolution on either terrace [11].…”

Amplitude dependence of image quality in atomically-resolved bimodal atomic force microscopy

“…24,25,[31][32][33][34][35][36][37] In the previous work, the ideal amplitude of oscillation was determined for single-mode FM-AFM measurements in ambient conditions. 24,25,31 On the KBr(100) surface, the highest SNR was observed with an amplitude of A $ 75 pm. 24 With smaller amplitudes, the signal becomes noisier due to instrumental noise.…”

Ambient atomic resolution atomic force microscopy with qPlus sensors: Part 1

“…The influences of ambient conditions on an experiment are often hard to predict in contrast to UHV conditions where a defined environment eases experiments on the atomic scale (Giessibl, ). Using new methods atomic resolution was recently shown in air on several surfaces, where resolution is comparable with UHV experiments (Wastl et al, , , ).…”

“…Obtaining atomic resolution in ambient conditions and liquids, has proven to be more difficult than in UHV as changes of oscillation and damping characteristics, pose high stability requirements to the measurement electronics while approaching to the sample (Fukuma, Onishi, Kobayashi, Matsuki, & Asakawa, ; Wastl et al, ). Several reasons playing a role in the stability could be related to fundamental factors like sensor and detection properties (material, stiffness, sensitivity, noise) (Giessibl, Hembacher, Herz, Schiller, & Mannhart, ; Wastl et al, , ), combination of tip and sample materials and environmental effects (Giessibl, ; Wastl et al, , ).…”