In situ characterization of nanoscale contaminations adsorbed in air using atomic force microscopy

Hydration Dynamics and the Future of Small-Amplitude AFM Imaging in Air

Here, we discuss the effects that the dynamics of the hydration layer and other variables, such as the tip radius, have on the availability of imaging regimes in dynamic AFM-including multifrequency AFM. Since small amplitudes are required for high-resolution imaging, we focus on these cases. It is possible to fully immerse a sharp tip under the hydration layer and image with amplitudes similar to or smaller than the height of the hydration layer, i.e., ~1 nm. When mica or HOPG surfaces are only cleaved, molecules adhere to their surfaces, and reaching a thermodynamically stable state for imaging might take hours. During these first hours, different possibilities for imaging emerge and change, implying that these conditions must be considered and reported when imaging.

Charge Calibration Standard for Atomic Force Microscope Tips in Liquids

An electric charge standard with nanoscale resolution is created using the known charge distribution of a single tobacco mosaic virus coat protein combined with the known packing of these proteins in the virus capsid. This advances the ability to measure charge on nanometric samples. Experimental atomic force microscope (AFM) force-distance curves are collected under aqueous conditions with controlled pH and ion concentration. A mathematical model that considers a polarizable dielectric tip immersed in an electrolyte is used to obtain charge density from the AFM measurements. Interactions between the tip and the sample are modeled using theory that includes monopolar electrostatic interactions, dipolar interactions, screening from both the dielectric nature of ambient water and solvated ions as described by the linear Poisson-Boltzmann equation, and hard-core repulsion. It is found that the tip charge density changes on a timescale of hours requiring recalibration of the tip for experiments lasting more than an hour. As an example of how a charge-calibrated tip may be used, the surface charge densities on 20 individual carboxylate-modified polystyrene (PS) beads are measured. The average of these AFM-measured bead charge densities is compared with the value obtained from conventional titration combined with electron microscopy. The two values are found to agree within 20%. While the comparison demonstrates similarity of the two charge measurements, hypotheses are put forward as to why the two techniques might be expected not to provide identical mean charge densities. The considerations used to build these hypotheses thus underscore the relevance of the method performed here if charge information is required on individual nanoparticles.

Removal of Surface Oxygen Vacancies Increases Conductance Through TiO2 Thin Films for Perovskite Solar Cells

We report that UV-ozone treatment of TiO₂ anatase thin films is an efficient method to increase the conductance through the film by more than 2 orders of magnitude. The increase in conductance is quantified via conductive scanning force microscopy on freshly annealed and UV-ozone-treated TiO₂ anatase thin films on fluorine-doped tin oxide substrates. The increased conductance of TiO₂ anatase thin films results in a 2% increase of the average power conversion efficiency (PCE) of methylammonium lead iodide-based perovskite solar cells. PCE values up to 19.5% for mesoporous solar cells are realized. The additional UV-ozone treatment results in a reduced number of oxygen vacancies at the surface, inferred from X-ray photoelectron spectroscopy. These oxygen vacancies at the surface act as charge carrier traps and hinder charge extraction from the adjacent material. Terahertz measurements indicate only minor changes of the bulk conductance, which underlines the importance of UV-ozone treatment to control surface-based defects.