Sebastian Rode

True Atomic-Resolution Imaging of (101̅4) Calcite in Aqueous Solution by Frequency Modulation Atomic Force Microscopy

Calcite (CaCO3) is one of the most abundant minerals on earth and plays an important role in a wide range of different fields including, for example, biomineralization and environmental geochemistry. Consequently, surface processes and reactions such as dissolution and growth as well as (macro)molecule adsorption are of greatest interest for both applied as well as fundamental research. An in-depth understanding of these processes requires knowledge about the detailed surface structure in its natural state which is quite often a liquid environment. We have studied the most stable cleavage plane of calcite under liquid conditions using frequency modulation atomic force microscopy. Using this technique, we achieved true atomic-resolution imaging, demonstrating the high-resolution capability of frequency modulation atomic force microscopy in liquids. We could reproduce contrast features reported before using contact mode atomic force microscopy, originating from the protruding oxygen atom of the carbonate groups. Besides this contrast, however, our results, indeed, indicate that we obtain more detailed structural information, revealing the calcium sublattice of the (1014) cleavage plane.

Clear Signature of the (2 × 1) Reconstruction of Calcite (101̅4)

Calcite is a mineral of fundamental importance that plays a crucial role in many fields of research such as biomineralization, biomolecule adsorption, and reactivity as well as industrial and daily life applications. Consequently, the most stable cleavage plane of calcite has been studied extensively using both direct imaging techniques such as atomic force microscopy as well as spectroscopic and diffraction techniques. Several surface structures have been reported for the (1014) cleavage plane of calcite differing from the simple bulk-truncated structure and an ongoing controversy exists in literature whether the cleavage plane exhibits a (2 x 1) reconstruction or not. We study the (1014) cleavage plane using high-resolution noncontact atomic force microscopy (NC-AFM) under ultrahigh vacuum conditions and obtain a clear signature of the (2 x 1) reconstruction. This reconstruction is observed in very narrow tip-surface distance ranges only, explaining why in some experiments the reconstruction has been observed and in others not. Moreover, as all sample preparation is performed in ultrahigh vacuum, the possibility of the (2 x 1) reconstruction being adsorbate-induced appears rather unlikely. Additionally, tip-induced surface changes are ruled out as origin for the observed reconstruction either. In conclusion, our study suggests that the (2 x 1) reconstruction is a true surface property of the (1014) cleavage plane of calcite.

Modification of a commercial atomic force microscopy for low-noise, high-resolution frequency-modulation imaging in liquid environment

A key issue for high-resolution frequency-modulation atomic force microscopy imaging in liquids is minimizing the frequency noise, which requires a detailed analysis of the corresponding noise contributions. In this paper, we present a detailed description for modifying a commercial atomic force microscope (Bruker MultiMode V with Nanoscope V controller), aiming at atomic-resolution frequency-modulation imaging in ambient and in liquid environment. Care was taken to maintain the AFMs original stability and ease of operation. The new system builds upon an optimized light source, a new photodiode and an entirely new amplifier. Moreover, we introduce a home-built liquid cell and sample holder as well as a temperature-stabilized isolation chamber dedicated to low-noise imaging in liquids. The success of these modifications is measured by the reduction in the deflection sensor noise density from initially 100 fm/√Hz to around 10 fm/√Hz after modification. The performance of our instrument is demonstrated by atomically resolved images of calcite taken under liquid conditions.

Thermal noise limit for ultra-high vacuum noncontact atomic force microscopy

Summary The noise of the frequency-shift signal Δf in noncontact atomic force microscopy (NC-AFM) consists of cantilever thermal noise, tip–surface-interaction noise and instrumental noise from the detection and signal processing systems. We investigate how the displacement-noise spectral density d z at the input of the frequency demodulator propagates to the frequency-shift-noise spectral density d Δ f at the demodulator output in dependence of cantilever properties and settings of the signal processing electronics in the limit of a negligible tip–surface interaction and a measurement under ultrahigh-vacuum conditions. For a quantification of the noise figures, we calibrate the cantilever displacement signal and determine the transfer function of the signal-processing electronics. From the transfer function and the measured d z, we predict d Δ f for specific filter settings, a given level of detection-system noise spectral density d z ds and the cantilever-thermal-noise spectral density d z th. We find an excellent agreement between the calculated and measured values for d Δ f. Furthermore, we demonstrate that thermal noise in d Δ f, defining the ultimate limit in NC-AFM signal detection, can be kept low by a proper choice of the cantilever whereby its Q-factor should be given most attention. A system with a low-noise signal detection and a suitable cantilever, operated with appropriate filter and feedback-loop settings allows room temperature NC-AFM measurements at a low thermal-noise limit with a significant bandwidth.

Photothermal excitation setup for a modified commercial atomic force microscope.

High-resolution imaging in liquids using frequency modulation atomic force microscopy is known to suffer from additional peaks in the resonance spectrum that are unrelated to the cantilever resonance. These unwanted peaks are caused by acoustic modes of the liquid and the setup arising from the indirect oscillation excitation by a piezoelectric transducer. Photothermal excitation has been identified as a suitable method for exciting the cantilever in a direct manner. Here, we present a simple design for implementing photothermal excitation in a modified Multimode scan head from Bruker. Our approach is based on adding a few components only to keep the modifications as simple as possible and to maintain the low noise level of the original setup with a typical deflection noise density of about 15 fm/√[Hz] measured in aqueous solution. The success of the modification is illustrated by a comparison of the resonance spectra obtained with piezoelectric and photothermal excitation. The performance of the systems is demonstrated by presenting high-resolution images on bare calcite in liquid as well as organic adsorbates (Alizarin Red S) on calcite with simultaneous atomic resolution of the underlying calcite substrate.

Frequency-modulated atomic force microscopy operation by imaging at the frequency shift minimum: The dip-df mode

In frequency modulated non-contact atomic force microscopy, the change of the cantilever frequency (Δf) is used as the input signal for the topography feedback loop. Around the Δf(z) minimum, however, stable feedback operation is challenging using a standard proportional-integral-derivative (PID) feedback design due to the change of sign in the slope. When operated under liquid conditions, it is furthermore difficult to address the attractive interaction regime due to its often moderate peakedness. Additionally, the Δf signal level changes severely with time in this environment due to drift of the cantilever frequency f0 and, thus, requires constant adjustment. Here, we present an approach overcoming these obstacles by using the derivative of Δf with respect to z as the input signal for the topography feedback loop. Rather than regulating the absolute value to a preset setpoint, the slope of the Δf with respect to z is regulated to zero. This new measurement mode not only makes the minimum of the Δf(z) curve directly accessible, but it also benefits from greatly increased operation stability due to its immunity against f0 drift. We present isosurfaces of the Δf minimum acquired on the calcite CaCO3(101̅4) surface in liquid environment, demonstrating the capability of our method to image in the attractive tip-sample interaction regime.