Contrast Atomic Force Microscopy Research Articles

Simulations of Subnanometer Scale Image Contrast in Atomic Force Microscopy of Self-Assembled Monolayers in Water.

Achieving high-resolution images using dynamic atomic force microscopy (AFM) requires understanding how chemical and structural features of the surface affect image contrast. This understanding is particularly challenging when imaging samples in water. An initial step is to determine how well-characterized surface features interact with the AFM tip in wet environments. Here, we use molecular dynamics simulations of a model AFM tip apex oscillating in water above self-assembled monolayers (SAMs) with different chain lengths and functional groups. The amplitude response of the tip is characterized across a range of vertical distances and amplitude set points. Then relative image contrast is quantified as the difference of the amplitude response of the tip when it is positioned directly above a SAM functional group vs positioned between two functional groups. Differences in contrast between SAMs with different lengths and functional groups are explained in terms of the vertical deflection of the SAMs due to interactions with the tip and water during dynamic imaging. The knowledge gained from simulations of these simple model systems may ultimately be used to guide selection of imaging parameters for more complex surfaces.

Atomic force microscopy contrast with CO functionalized tips in hydrogen-bonded molecular layers: Does the real tip charge distribution play a role?

The interplay of van der Waals (vdW), electrostatic (ES), and short-range (SR) interactions on both the intra- and intermolecular contrast observed in high-resolution atomic force microscopy (HR-AFM) is explored in a hydrogen-bonded monolayer of triazine molecules. Our efficient model to simulate AFM images uses the three-dimensional (3D) charge distribution of both tip and sample to calculate the ES interaction, takes into account the tilting of the CO molecule, and reproduces with high accuracy density functional theory calculations. In spite of triazine's hexagonal structure, the intramolecular contrast has triangular symmetry, reflecting the charge density of the molecule. Stripelike intermolecular features, which join the molecules in the H-bond directions, originate from the overlap of the charge density of the atoms in neighboring molecules and are sharpened by the CO tilt. We demonstrate the existence of different potential energy surface minima for the CO tilt and discuss its influence on imaging. Our results clearly show that the ES interaction maps represent a local 3D average of the ES potential of the sample weighted by the tip's charge density, while the SR interaction resembles a local 3D average of the charge density of the sample. However, the strong cancellation of both contributions results in a net interaction dominated by the ES and vdW far from the molecules, and by the SR at short distance. This cancellation, which essentially removes the dependence on the detailed charge distribution of the tip, explains why AFM images can be reproduced using only sample properties such as the $z$ component of the electric field and the charge density of the molecule, and the success of simple models that only incorporate pairwise, point-charge interactions.

Modeling atomic force microscopy at LiNbO3 surfaces from first-principles

Atomic force microscopy (AFM) at LiNbO3 surfaces is simulated from first-principles. The forces acting on different atomic tip models interacting with x-cut and z-cut LiNbO3 surfaces are calculated within density functional theory. The AFM simulations are compared with atomic resolved AFM measurements. The species-dependent tip-surface interaction found in the simulation allows for the verification of microscopic models for different surfaces. In particular, it is shown that the atomic force microscopy contrast is governed by the charge accumulation at the oxygen ions. Van der Waals forces cloud the scans, without qualitatively modifying the AFM pattern. The microscopic surface models proposed here explain both the origin of the measured AFM patterns and the observed spatial resolution difference corresponding to different surface orientations.

Unraveling theLiNbO3X-cut surface by atomic force microscopy and density functional theory

The ${\text{LiNbO}}_{3}$(2$\overline{1}\overline{1}0$) surface, commonly referred to as X-cut, is investigated by means of atomic force microscopy and first-principles calculations. Atomically resolved atomic force microscopy images show geometrical patterns not compatible with truncated bulk terminations. Fast Fourier transformation of the real-space images shows an oblique surface unit cell with lattice parameters of $a=0.75\ifmmode\pm\else\textpm\fi{}0.02$ nm, $b=0.54\ifmmode\pm\else\textpm\fi{}0.02$ nm, and $\ensuremath{\alpha}=94.{8}^{\ensuremath{\circ}}$. Comparing these experimental results with the theoretical models of stable surface terminations provides clear evidence for the formation of a -${\text{Li}}_{12}$ termination. The LN X-cut is, thus, characterized by a nonstoichiometric and Li-enriched composition. An analysis of the surface electronic charge indicates that the atomic force microscopy contrast is governed by the charge accumulation at the oxygen ions.

10.1007/s11447-008-2003-2

The mechanism of the formation of phase contrast in atomic force microscopy (AFM) is studied for various conditions of an oscillating tip interacting with the surface. A phase shift is detected in oscillations of the resonating AFM tip during its interaction with the substrate surface when the AFM tip moves over the surface. We substantiate kinetic mechanism of the formation of phase contrast in AFM, which is initiated when the velocity of the AFM tip moving over the substrate surface increases as a result of increasing friction force. A dependence of the kinetic contrast in AFM on the effective roughness of the surface is discovered. Images of the distribution of copper impurity over the silicon surface under atmospheric conditions are obtained using the method of kinetic phase contrast in AFM.

Hydrogen-related contrast in atomic force microscopy

We study the effect of hydrogen adsorption on gadolinium islands epitaxiallygrown on W(110) utilizing atomic force microscopy operated in the non-contactregime. In constant force images, gadolinium islands exhibit two height levels,corresponding to hydrogen covered and clean gadolinium areas, respectively. Theexperimentally measured height differences are strongly bias dependent, showing that thecontrast pattern is dominated by electrostatic tip–sample forces. We interpretour experimental findings in terms of a local reduction of the work function andthe presence of localized charges on hydrogen covered areas. Both effects leadto a variation of the contact potential difference between tip and surface areas,which are clean or hydrogen covered gadolinium. After clarifying the electrostaticcontrast formation, we can unambiguously identify regions of clean gadoliniumon the islands. These results are important for further magnetic exchange forcemicroscopy based studies, because hydrogen also alters the magnetic properties locally.

Structure and stability of semiconductor tip apexes for atomic force microscopy

The short range force between the tip and the surface atoms, that is responsible foratomic-scale contrast in atomic force microscopy (AFM), is mainly controlled by the tipapex. Thus, the ability to image, manipulate and chemically identify single atoms insemiconductor surfaces is ultimately determined by the apex structure and its composition.Here we present a detailed and systematic study of the most common structures that canbe expected at the apex of the Si tips used in experiments. We tackle the determination ofthe structure and stability of Si tips with three different approaches: (i) first principlessimulations of small tip apexes; (ii) simulated annealing of a Si cluster; and (iii) aminima hopping study of large Si tips. We have probed the tip apexes by makingatomic contacts between the tips and then compared force–distance curves with theexperimental short range forces obtained with dynamic force spectroscopy. The mainconclusion is that although there are multiple stable solutions for the atomicallysharp tip apexes, they can be grouped into a few types with characteristic atomicstructures and properties. We also show that the structure of the last atomiclayers in a tip apex can be both crystalline and amorphous. We corroborate thatthe atomically sharp tips are thermodynamically stable and that the tip–surfaceinteraction helps to produce the atomic protrusion needed to get atomic resolution.

Phase noise induced due to amplitude fluctuations in dynamic force microscopy

In dynamic force microscopy, the force sensor is driven on its resonance frequency and the amplitude of the cantilever is sustained at a constant value. The amplitude typically ranges between 0.1 and 30 nm. If a large amplitude is set, the cantilever tip senses both long-range and short-range interaction forces provided that the tip is close to the sample surface. The short-range interactions are decisive for the atomic contrast in atomic force microscopy (AFM) images. They can be separated from the long-range interactions by setting an amplitude which encompasses the typical range of the interaction force, i.e., the subangstrom regime for van der Waals contribution. It is distinctive for cantilevers operated at small driving amplitudes that the cantilever deflection can be considered as a sinusoidal signal superimposed with a quasimonochromatic random signal originating from fluctuations. If one measures experimentally the standard deviation of the phase ${\ensuremath{\sigma}}_{\ensuremath{\varphi}}$ of the signal with respect to a monochromatic reference signal, a universal relationship between the standard deviation of the phase ${\ensuremath{\sigma}}_{\ensuremath{\varphi}}$ and the cantilever amplitude ${x}_{0}$ is found. The smaller the ratio of rms amplitude of the sinusoidal signal and the rms value of random signal is, the larger the phase fluctuations are. Phase fluctuations are of importance for measurements at small amplitudes, since they determine the limit of phase-sensitive measurements or the lateral imaging resolution in the so-called pendulum mode of AFM operation. In this paper we develop a heuristic model, which provides an analytical formula for the probability density of phase noise of a sinusoidal signal superimposed by a quasimonochromatic one with respect to a reference oscillator. The variance of the phase noise can be deduced from the distribution functions. The suggested model is verified experimentally and is compared with theoretical predictions. The amplitude-dependent phase fluctuations are a powerful tool to determine the spring constant of the beam or to calibrate its oscillation amplitude.

Kinetic contrast in atomic force microscopy

The mechanism of the formation of phase contrast in atomic force microscopy (AFM) is studied for various conditions of an oscillating tip interacting with the surface. A phase shift is detected in oscillations of the resonating AFM tip during its interaction with the substrate surface when the AFM tip moves over the surface. We substantiate kinetic mechanism of the formation of phase contrast in AFM, which is initiated when the velocity of the AFM tip moving over the substrate surface increases as a result of increasing friction force. A dependence of the kinetic contrast in AFM on the effective roughness of the surface is discovered. Images of the distribution of copper impurity over the silicon surface under atmospheric conditions are obtained using the method of kinetic phase contrast in AFM.

Force- and bias-dependent contrast in atomic force microscopy based photocurrent imaging on GaAs–AlAs heterostructures

In this work, photocurrent images of lithographically processed GaAs/AlAs heterostructures are recorded by an atomic force microscope. It is found that the AFM tip sample contact is strongly dependent on the thickness of the native oxide layer on the sample surface. Therefore, the photocurrents increase if a successively increasing tip sample force is applied, which leads to a gradual penetration of the surface oxide layer. Due to the complex behavior of the photocurrent as a function of tip-bias and tip-force, the contrast in photocurrent images is non-monotonic and can be reversed under appropriate bias and force conditions.

Observation of antiphase domains in CdxHg1−x Te films on silicon by the phase contrast method in atomic force microscopy

It has been shown that phase contrast in atomic force microscopy (AFM) can be used to obtain adequate information on the density and distribution of antiphase domains on the surface of CdHgTe films grown by molecular beam epitaxy on a Si(301) substrate. By comparing the AFM phase images of the film surface with TEM images of structural defects in the near-surface region, the relation between microstructure and micromorphology of the films is revealed.

The height dependence of image contrast when imaging by non-contact AFM

Using the pair-wise summation of the Lennard–Jones potential, we have analyzed the height dependence of the image contrast in atomic force microscopy at atomic resolution when scanning in non-contact mode. When we study the images obtained as the surfaces of constant force gradient (fast feedback on frequency shift), we find that images remain qualitatively similar for the range of tip–sample distances analyzed. However, for a slow feedback mode, which is more commonly used because of mechanical instabilities that exist in the fast feedback regime, we have found a spectrum of artefacts that are dependent on the tip–sample distance.

Atomic Force Microscope Image Contrast Mechanisms on Supported Lipid Bilayers

This work presents a methodology to measure and quantitatively interpret force curves on supported lipid bilayers in water. We then use this method to correlate topographic imaging contrast in atomic force microscopy (AFM) images of phase-separated Langmuir-Blodgett bilayers with imaging load. Force curves collected on pure monolayers of both distearoylphosphatidylethanolamine (DSPE) and monogalactosylethanolamine (MGDG) and dioleoylethanolamine (DOPE) deposited at similar surface pressures onto a monolayer of DSPE show an abrupt breakthrough event at a repeatable, material-dependent force. The breakthrough force for DSPE and MGDG is sizable, whereas the breakthrough force for DOPE is too small to measure accurately. Contact-mode AFM images on 1:1 mixed monolayers of DSPE/DOPE and MGDG/DOPE have a high topographic contrast at loads between the breakthrough force of each phase, and a low topographic contrast at loads above the breakthrough force of both phases. Frictional contrast is inverted and magnified at loads above the breakthrough force of both phases. These results emphasize the important role that surface forces and mechanics can play in imaging multicomponent biomembranes with AFM.

Investigation of protein partnerships using atomic force microscopy.

The origin of contrast in atomic force microscopy (AFM) lies in the probe's response to forces between itself and the sample. These forces most commonly result from changes in height as the tip is scanned over the surface, but can also originate in properties inherent in the sample. These have been exploited as further means of contrast and have spawned an array of similar imaging techniques, such as chemical force microscopy, magnetic force microscopy, and frictional force microscopy. All of these techniques use AFM as an extremely sensitive gauge to map forces at discrete sites on the surface. A natural extension of this approach is to map forces in an array, in order to create a force map. AFM can be used in aqueous or fluid environments, thus allowing the exploration of forces in biological systems under physiologically relevant conditions. By immobilizing one half of an interacting pair of proteins onto the tip and the other half onto the substrate, it is possible to investigate the electrostatic and hydrophobic interactions between them. We employed these techniques to examine the interaction between a pair of proteins of known affinity that are involved in exocytosis (NSF and alpha-SNAP) and separately to demonstrate how two-dimensional force mapping can be applied to the nuclear envelope to identify nuclear pore complexes.

Investigation of protein partnerships using atomic force microscopy

The origin of contrast in atomic force microscopy (AFM) lies in the probe's response to forces between itself and the sample. These forces most commonly result from changes in height as the tip is scanned over the surface, but can also originate in properties inherent in the sample. These have been exploited as further means of contrast and have spawned an array of similar imaging techniques, such as chemical force microscopy, magnetic force microscopy, and frictional force microscopy. All of these techniques use AFM as an extremely sensitive gauge to map forces at discrete sites on the surface. A natural extension of this approach is to map forces in an array, in order to create a force map. AFM can be used in aqueous or fluid environments, thus allowing the exploration of forces in biological systems under physiologically relevant conditions. By immobilizing one half of an interacting pair of proteins onto the tip and the other half onto the substrate, it is possible to investigate the electrostatic and hydrophobic interactions between them. We employed these techniques to examine the interaction between a pair of proteins of known affinity that are involved in exocytosis (NSF and α-SNAP) and separately to demonstrate how two-dimensional force mapping can be applied to the nuclear envelope to identify nuclear pore complexes. Microsc. Res. Tech. 44:368–377, 1999. © 1999 Wiley-Liss, Inc.

Atomic force microscopy for transition metals and tip structure effects

We study the influence of tip morphology on the contrast in atomic force microscopy (AFM) for transition metals tips and samples (T/S). For this, we consider a model based on the real space tight binding approach taking into account the electronic structure of the T/S system. Images of an W(001) surface obtained with atomically sharp or blunt tips are considered. The resolution of AFM images, in the attractive force range, is discussed for monatomic (multiatomic) apex(es) tips. Images of clusters deposited on the W(001) surface are also obtained.

Atomic force microscopy for transition metals and tip structure effects

We study the influence of tip morphology on the contrast in atomic force microscopy (AFM) for transition metals tips and samples (T/S). For this, we consider a model based on the real space tight binding approach taking into account the electronic structure of the T/S system. Images of an W(001) surface obtained with atomically sharp or blunt tips are considered. The resolution of AFM images, in the attractive force range, is discussed for monatomic (multiatomic) apex(es) tips. Images of clusters deposited on the W(001) surface are also obtained.