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Atomic Force Microscopy
Published in Arthur T. Hubbard, The Handbook of Surface Imaging and Visualization, 2022
Andrew A. Gewirth, John R. LaGraff
The AFM possesses a number of advantages. In contrast to the STM, almost any type of surface can be imaged with the AFM. The insulating tip and the lack of specific electrochemical phenomena occurring between sample and tip allow stable imaging even when substantial faradic currents are being passed at the sample surface. Thus, the AFM operates well in the electrochemical environment. The AFM imaging mechanism makes only a few demands on the types of samples suitable for imaging. Insulators, semiconductors, and metals have all been imaged with high resolution. Softer materials, such as organics adsorbed onto metal surfaces, are substantially more difficult to image, especially in air. However, molecularly resolved images of alkanethiolates,17 DNA plasmids,8,18,19 and Langmuir-Blodgett films have been reported.20–27 The AFM also seems to work particularly well when the sample is immersed in liquid, in part because the liquid (which can be water or an organic solvent) minimizes the often large and deleterious capillary forces between sample and tip that develop from adsorbed moisture.
Introduction — Measurement Techniques and Applications
Published in Bharat Bhushan, Handbook of Micro/Nano Tribology, 2020
STM is ideal for atomic-scale imaging. To obtain atomic resolution with AFM, the spring constant of the cantilever should be weaker than the equivalent spring between atoms. For example, the vibration frequencies ω of atoms bound in a molecule or in a crystalline solid are typically 1013 Hz or higher. Combining this with the mass of the atoms m, on the order of 10−25 kg, gives interatomic spring constants k, given by ω2m, on the order of 10 N/m (Rugar and Hansma, 1990). (For comparison, the spring constant of a piece of household aluminum foil that is 4 mm long and 1 mm wide is about 1 N/m.) Therefore, a cantilever beam with a spring constant of about 1 N/m or lower is desirable. Tips have to be as sharp as possible. Tips with a radius ranging from 20 to 50 nm are commonly available.
Post-Process Surface Metrology
Published in Richard Leach, Simone Carmignato, Precision Metal Additive Manufacturing, 2020
Nicola Senin, Francois Blateyron
SPMs are classified based on the nature of the interactions between the probe and the surface. In atomic force microscopy (AFM), the tip of the probe (typical height less than 20 µm, radius less than 10 nm) is placed so close to the surface that the attractive and repulsive interatomic forces can be sensed. As the probe is translated across the surface, interatomic forces and cantilever displacement are monitored and used to reconstruct the reliefs underneath. The stylus vertical displacements are measured and recorded, for example by optical means, through measuring the deflection angle of a laser beam incident onto the stylus shaft. In scanning tunnelling microscopy (STM), a similar mechanism is used where the tunnelling current created between the stylus tip and the surface (which must be conductive) is measured and kept constant by raising or lowering the tip so that it keeps a constant distance from the surface.
Understanding the fundamentals of TiO2 surfaces
Published in Surface Engineering, 2022
Both STM and NC-AFM techniques have been used to establish the relationship between surface structure and chemistry on the atomic scale. A number of STM studies have highlighted the difficulty of distinguishing between oxygen vacancy defects and hydroxyl groups resulting from the dissociation of water at the TiO2 surface. However, the authors have found suitable solutions for this problem [e.g. 2,5,62,155,161,162,174,248,249,324,335]. The OHb species due to the water dissociation on VO’s, were STM imaged for the first time by Suzuki et al. [324].
A fresh study for dynamic behaviour of atomic force microscope cantilever by considering different immersion environments
Published in Journal of Experimental Nanoscience, 2020
Ali Hossein Gholizadeh Pasha, Ali Sadeghi
Binning et al. [1] invented a powerful device for studying the surface samples, which they called scanning tunnelling microscope (STM). STM scans the sample through a very fine metallic tip. The tip is mechanically connected to the scanner, an XYZ positioning device realised by means of piezoelectric materials. The sample is positively or negatively biased so that a small current, the ‘tunnelling current’ flows if the tip is in contact with the sample. This feeble tunnelling current is amplified and measured. The tunnelling current helps the electronic feedback keep the distance between tip and sample constant. If the tunnelling current exceeds its present value, the distance between tip and sample increases. However, if it falls below this value, the feedback reduces the distance. The tip is scanned line by line above the sample surface following the topography of the sample. The tunnelling current flows across the small gap that separates the tip from the sample and in fact very small changes in the tip–sample separation induce large changes in the tunnelling current, so the tip–sample separation can be precisely controlled. However, the disadvantage of STM lies in the fact that it can only be applied for conductive materials; insulting materials can therefore not be imaged by STM. The schematic of a STM is shown in Figure 1. In order to overcome the above-stated disadvantage of STM, Quate and Gerber [3] developed atomic force microscope (AFM) for analysing all types of materials. In an AFM, the tip is attached near to one end and the radius of the contact size is only several nanometers or even hundreds of Angstroms. As the contact load can be several nano Newtons, the damage to the material is relatively small. Tips must be carefully selected for specific applications. Very sharp tips may be used to measure samples with high variations in topography and less sharp tips may be employed to image flatter surfaces. The most common tip shapes are pyramidal and conical. In most cases, the radius of an AFM tip ranges between 40 and 250 nm based on the sample shape. According to Hertzian contact theory, it is possible to consider both the tip and the sample radii.