Probing the Nature of Chemical Bonds by Atomic Force Microscopy

Abstract

The nature of the chemical bond is important in all natural sciences, ranging from biology to chemistry, physics and materials science. The atomic force microscope (AFM) allows to put a single chemical bond on the test bench, probing its strength and angular dependence. We review experimental AFM data, covering precise studies of van-der-Waals-, covalent-, ionic-, metallic- and hydrogen bonds as well as bonds between artificial and natural atoms. Further, we discuss some of the density functional theory calculations that are related to the experimental studies of the chemical bonds. A description of frequency modulation AFM, the most precise AFM method, discusses some of the experimental challenges in measuring bonding forces. In frequency modulation AFM, forces between the tip of an oscillating cantilever change its frequency. Initially, cantilevers were made mainly from silicon. Most of the high precision measurements of bonding strengths by AFM became possible with a technology transfer from the quartz watch technology to AFM by using quartz-based cantilevers (“qPlus force sensors”), briefly described here.

Real-Space Identification of Non-Noble Single Atomic Catalytic Sites within Metal-Coordinated Supramolecular Networks

“…For the DFT-simulated, intact molecule, the charge accumulation between the molecule and surface is much less pronounced, whereby the intramolecular contrast of the molecular structure is more distinct, as known for physisorbed molecules (Figure S3). − …”

“…In the present study, we investigate the adsorption and heat-induced dehydrogenation reactions of melamine and melem molecules on a Cu(111) surface (Figure a–c). For the structural characterization, we performed ultrahigh-resolution nc-AFM experiments using an O-terminated copper tip (CuOx-tip). , Such an atomically defined functionalized probe tip effectively passivates the tip apex, which allows imaging in the repulsive force regime where a drastically increased resolution can be obtained. − The application of this powerful technique to functional organic layers relevant in heterogeneous catalysis could help to identify local reactive sites. These experiments are complemented by STM imaging, XPS analyses and density functional theory (DFT)-based simulations.…”

The applications of the optical tweezer and atomic force microscope at the molecular level

“…To test a single chemical bond, AFM is operated in the condition of an ultrahigh vacuum with a low temperature which is commonly at a liquid helium temperature of 4 K [14]. In the experiment, frequency modulation (FM-) AFM whose cantilever oscillates at frequency f with a fixed amplitude A is chosen.…”

“…A series of calculations and the experimental demonstrated that highly stiff cantilevers and amplitudes help to stabilize oscillations and maximize the short-range force contribution respectively to optimize the signal-to-noise ratio [14].…”

“…Specifically, the experiment carried out in 1995 used a cantilever made of silicon with a stiffness of 17 N/m and an amplitude of 34 nm to investigate the Si-Si covalent bond [14]. Even though the stiffness and amplitude selected are not within the range of what was previously determined to be ideal, the AFM image of Si ( 111) -(7*7) was successfully obtained due to the strong covalent bonds with magnitude on the order of 2 nN.…”