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

In bimodal FM-AFM, two flexural modes are excited simultaneously. The total vertical oscillation deflection range of the tip is the sum of the peak-to-peak amplitudes of both flexural modes (sum amplitude). We show atomically resolved images of KBr(100) in ambient conditions in bimodal AFM that display a strong correlation between image quality and sum amplitude. When the sum amplitude becomes larger than about 200 pm, the signal-to-noise ratio (SNR) is drastically decreased. We propose this is caused by the temporary presence of one or more water layers in the tip-sample gap. These water layers screen the short range interaction and must be displaced with each oscillation cycle. Further decreasing the sum amplitude, however, causes a decrease in SNR. Therefore, the highest SNR in ambient conditions is achieved when the sum amplitude is slightly less than the thickness of the primary hydration layer.Comment: 3000 words, 3 Figures, 3 supplimentary figure

Atomar aufgelöste Rasterkraftmikroskopie an Luft: Aufbau, Technik, Optimierung und Anwendung auf Graphit, Graphen, Kaliumbromid, Calcit und Molekülfilmen

Diese Arbeit zeigt die Weltneuheit Rasterkraftmikroskopie mit wahrer atomarer Auflösung direkt an Luft und in Flüssigkeiten. Es wurde ein Quarzstimmgabel basierender Sensor -der sogenannte qPlus Sensor- der beliebige Spitzenkristalle tragen kann, verwendet. Wir befassen uns mit dem zugehörigen Aufbau, der Technik und Optimierung, sowie der Anwendung der wahren atomaren Auflösung zur Untersuchung von Graphit, Graphen, Kaliumbromid, Calcit und Molekülfilmen. In diesem Zusammenhang werden einzelne Spitzenmaterialien, wie Silizium, Wolfram, Platin-Iridium, Wolframcarbit und Saphir diskutiert und ihre Eigenschaften und Anwendungen erklärt und demonstriert. Während der Untersuchung von Monolagen- und Bilagengraphen und letztlich Graphit konnte eine selbstorganisierte Streifenstruktur auf den beiden inerteren Oberflächen abgebildet werden. Diese bildet sich scheinbar auf beliebigen, geringreaktiven und mit Graphen bedeckten Substraten an Luft. Die Optimierung der atomaren Auflösung wird mittels eines neu entwickelten Verfahrens, der sogenannten „Q –Spektroskopie“, welche für jede beliebige Spitzen-Proben-Kombination anwendbar ist und die optimalen Abbildungsparameter liefert demonstriert. Das Verfahren wird auf Kaliumbromid in aller Ausführlichkeit gezeigt und schrittweise erarbeitet. Anschließend wird das Verfahren zur Optimierung der Auflösung auf Calcit verwendet und wahre atomare Auflösung an Luft durch das Abbilden von atomaren Fehlstellen im Sauerstoffuntergittern und atomaren Stufen des Kristalles demonstriert. Außer auf Ionenkristallen wird die neue Technik auch auf Graphit und Graphen zur atomaren Auflösung verwendet und in diesem Zusammenhang das Verhalten und die Eigenschaften hydrophiler Siliziumspitzen und hydrophober, neu eingeführter Saphirspitzen gezeigt und diese gegenüber gestellt. Besonderes Augenmerk liegt hierbei auf den stark voneinander abweichenden Abstandsspektren der verschiedenen Spitzentypen. Es wurde entdeckt, dass die neue Technik an Luft und in Flüssigkeiten (z. B. auf Mica) uneingeschränkt anwendbar und auch für hochviskose Flüssigkeiten kein Hindernis darstellt. Durch die Neuerungen in dieser Arbeit werden nicht nur die Untersuchungen von Oberflächen mit höchster Auflösung direkt an Luft und in Flüssigkeiten möglich, sondern auch die Untersuchung von biologischen Proben oder chemischen Reaktionen in natürlichen Bedingungen. In dieser Arbeit wird somit eine revolutionäre Technik für die Studie von einzelnen Atomen und Moleküle, bis hin zu ganzen lebenden Organismen in kleinsten Dimensionen eingeführt

qPlus magnetic force microscopy in frequency-modulation mode with millihertz resolution

Magnetic force microscopy (MFM) allows one to image the domain structure of ferromagnetic samples by probing the dipole forces between a magnetic probe tip and a magnetic sample. The magnetic domain structure of the sample depends on the alignment of the individual atomic magnetic moments. It is desirable to be able to image both individual atoms and domain structures with a single probe. However, the force gradients of the interactions responsible for atomic contrast and those causing domain contrast are orders of magnitude apart, ranging from up to 100 Nm−1 for atomic interactions down to 0.0001 Nm−1 for magnetic dipole interactions. Here, we show that this gap can be bridged with a qPlus sensor, with a stiffness of 1800 Nm−1 (optimized for atomic interaction), which is sensitive enough to measure millihertz frequency contrast caused by magnetic dipole–dipole interactions. Thus we have succeeded in establishing a sensing technique that performs scanning tunneling microscopy, atomic force microscopy and MFM with a single probe

Optimizing atomic resolution of force microscopy in ambient conditions

Ambient operation poses a challenge to atomic force microscopy because in contrast to operation in vacuum or liquid environments, the cantilever dynamics change dramatically from oscillating in air to oscillating in a hydration layer when probing the sample. We demonstrate atomic resolution by imaging of the KBr(001) surface in ambient conditions by frequency-modulation atomic force microscopy with a cantilever based on a quartz tuning fork (qPlus sensor) and analyze both long- and short-range contributions to the damping. The thickness of the hydration layer increases with relative humidity; thus varying humidity enables us to study the influence of the hydration layer thickness on cantilever damping. Starting with measurements of damping versus amplitude, we analyzed the signal and the noise characteristics at the atomic scale. We then determined the optimal amplitude which enabled us to acquire high-quality atomically resolved images

Advances in AFM: Seeing Atoms in Ambient Conditions

It's hard to imagine that we can take a splinter and sharpen it down to the atomic level. It's even more impressive that we can bring this sharp tip close to a surface, scan it over the surface, and be sensitive to the tiny forces between the apex atom and individual atoms on the surface. Measuring and interpreting these forces is the goal of high-resolution atomic force microscopy (AFM). We perform frequency-modulation AFM (FM-AFM), in which we oscillate the tip and record the change in frequency as a measure of the interaction with the surface. FM-AFM performed in vacuum with stiff sensors has lead to amazing discoveries. Now, we are returning to the challenge of imaging samples in device- and biologically-relevant conditions. This contribution summarizes work that was performed in the Giessibl group to image with atomic resolution in ambient and liquid environments. We demonstrated atomic resolution with the qPlus sensor on KBr, and followed this with investigations on graphitic surfaces. We have also shown single-atomic defects and steps on the calcite surface

Atomically Resolved Graphitic Surfaces in Air by Atomic Force Microscopy

Imaging at the atomic scale using atomic force microscopy in biocompatible environments is an ongoing challenge. We demonstrate atomic resolution of graphite and hydrogen-intercalated graphene on SiC in air. The main challenges arise from the overall surface cleanliness and the water layers which form on almost all surfaces. To further investigate the influence of the water layers, we compare data taken with a hydrophilic bulk-silicon tip to a hydrophobic bulk-sapphire tip. While atomic resolution can be achieved with both tip materials at moderate interaction forces, there are strong differences in force versus distance spectra which relate to the water layers on the tips and samples. Imaging at very low tip–sample interaction forces results in the observation of large terraces of a naturally occurring stripe structure on the hydrogen-intercalated graphene. This structure has been previously reported on graphitic surfaces that are not covered with disordered adsorbates in ambient conditions (i.e., on graphite and bilayer graphene on SiC, but not on monolayer graphene on SiC). Both these observations indicate that hydrogen-intercalated graphene is close to an ideal graphene sample in ambient environments

Influence of PEGylation on nanoparticle mobility in different models of the extracellular matrix

Nanoparticle transport inside the extracellular matrix (ECM) is a crucial factor affecting the therapeutic success. In this work, two in vitro ECM models - a neutrally charged collagen I network with an effective pore size of 0.47 mu m and Matrigel, a basement membrane matrix with strong negative charge and effective pore size of 0.14 mu m - were assessed for barrier function in the context of diffusing nanoparticles. Nanoparticles with a size of 120 nm were coated with poly(ethylene glycol) (PEG) of different molecular weights - 2, 5 and 20 kDa - over a range of gradually increasing coating densities - precisely 0.2, 2, 8 and 20 PEG/nm(2). The PEG corona was imaged in its native state without any drying process by atomic force microscopy, revealing that the experimentally determined arrangement of PEG at the surface did not match with what was theoretically expected. In a systematic investigation of nanoparticle mobility via fluorescence recovery after photobleaching, increasing both PEG MW and PEGylation density gradually improved diffusion properties predominately in collagen. Due to its smaller pore size and electrostatic obstruction, diffusion coefficients were about ten times lower in Matrigel than in the collagen network and an extension of the PEG MW and density did not necessarily lead to better diffusing particles. Consequently, collagen gels were revealed to be a poor model for nanoparticle mobility assessment, as neither their pore size nor their electrostatic properties reflect the expected in vivo conditions. In Matrigel, diffusion proceeded according to a sigmoidal increase with gradually increasing PEG densities showing threshold zeta potentials of 11.6 mV (PEG(2kDa)) and 13.8 mV (PEG(5kDa)) below which particles were, regarded as mobile. Irrespective of the molecular weight particles with a PEGylation density lower than 2 PEG/nm(2) were defined as immobile and those with a PEG coverage of more than 8 PEG/nm(2) as mobile. (C) 2016 Elsevier B.V. All rights reserved

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

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

Influence of matrix and filler fraction on biofilm formation on the surface of experimental resin-based composites

The aim of this study was to investigate the impact of resin matrix chemistry and filler fraction on biofilm formation on the surface of experimental resinbased composites (RBCs). Specimens were prepared from eight experimental RBC formulations differing in resin matrix blend (BisGMA/TEGDMA in a 7:3 wt% ratio or UDMA/aliphatic dimethacrylate in a 1:1 wt% ratio) and filler fraction (no fillers; 65 wt% dental glass with an average diameter of 7 or 0.7 lm or 65 wt% SiO2 with an average diameter of 20 nm). Surface roughness, surface free energy, and chemical surface composition were determined; surface topography was visualized using atomic force microscopy. Biofilm formation was simulated under continuous flow conditions for a 48 h period using a monospecies Streptococcus mutans and a multispecies biofilm model. In the monospecies biofilm model, the impact of the filler fraction overruled the influence of the resin matrix, indicating lowest biofilm formation on RBCs with nano-scaled filler particles and those manufactured from the neat resin blends. The multispecies model suggested a more pronounced effect of the resin matrix blend, as significantly higher biofilm formation was identified on RBCs with a UDMA/dimethacrylate matrix blend than on those including a BisGMA/TEGDMA matrix blend but analogous filler fractions. Although significant differences in surface properties between the various materials were identified, correlations between the surface properties and biofilm formation were poor, which highlights the relevance of surface topography and chemistry. These results may help to tailor novel RBC formulations which feature reduced biofilm formation on their surface