Noncontact Atomic Force Microscopy Images Research Articles

Polarization effects in noncontact atomic force microscopy: A key to model the tip-sample interaction above charged adatoms

We discuss the influence of short-range electrostatic forces, so-called dipolar forces, between the tip of an atomic force microscope (AFM) and a surface carrying charged adatoms. Dipolar forces are of microscopic character and have their origin in the polarizability of the foremost atoms on tip and surface. In most experiments performed by noncontact AFM, other forces such as binding forces dominate the interaction. However, in the experiments presented by Gross et al. [Science 324, 1428 (2009)], where the charge state of individual gold atoms adsorbed on a thin dielectric layer was determined, binding forces are negligible as the tip-sample distance is relatively large. We develop a model which mimics the experimental tip-sample geometry of the aforementioned experiments. The model includes van der Waals and long-range electrostatic interactions, as well as the short-range electrostatic interaction based on the self-consistent description of electronic polarization effects on neutral and charged adatoms. The model is based on a calculation of the electrostatic energy of the tip-sample geometry. Our calculations of noncontact AFM imaging as well as of bias spectroscopic curves are in good agreement with the experimental ones presented by Gross et al. It is demonstrated that the short-range dipolar force is mainly responsible for the contrast observed in topography imaging above charged species. However, it is the long-range capacitive force which is responsible for the detection of the charge state in bias spectroscopy. We discuss implications of our findings on future experiments which aim to detect single charges by means of Kelvin probe force microscopy.

Atomic resolution force microscopy imaging on a strongly ionic surface with differently functionalized tips

Three types of tips for noncontact atomic force microscopy imaging, namely, a silicon nanopillar tip, a carbon nanopillar tip, and a fluoride cluster tip, are prepared for atomic resolution imaging on the CaF2(111) surface. The most enhanced atomic corrugation is obtained with the fluoride cluster tip prepared by gently touching the fluorite surface. Atom resolved images are much harder to obtain with the other tips. This demonstrates the importance of having a polar tip for atomic resolution imaging of an ionic surface and supports the general notion that a surface is best imaged with a tip of the same material.

A force-matching method for quantitative hardness measurements by atomic force microscopy with diamond-tipped sapphire cantilevers

We present a new method to improve the accuracy of force application and hardness measurements in hard surfaces by using low-force (<50 μN) nanoindentation technique with a cube-corner diamond tip mounted on an atomic force microscopy (AFM) sapphire cantilever. A force calibration procedure based on the force-matching method, which explicitly includes the tip geometry and the tip-substrate deformation during calibration, is proposed. A computer algorithm to automate this calibration procedure is also made available. The proposed methodology is verified experimentally by conducting AFM nanoindentations on fused quartz, Si(1 0 0) and a 100-nm-thick film of gold deposited on Si(1 0 0). Comparison of experimental results with finite element simulations and literature data yields excellent agreement. In particular, hardness measurements using AFM nanoindentation in fused quartz show a systematic error less than 2% when applying the force-matching method, as opposed to 37% with the standard protocol. Furthermore, the residual impressions left in the different substrates are examined in detail using non-contact AFM imaging with the same diamond probe. The uncertainty of method to measure the projected area of contact at maximum force due to elastic recovery effects is also discussed.

Defect Structure of Ultrathin Ceria Films on Pt(111): Atomic Views from Scanning Tunnelling Microscopy

Atomically resolved scanning tunnelling microscopy (STM) images have been obtained on ultrathin films of CeO2(111) supported on Pt(111). The ultrathin films were grown in two ways, by reactive deposition in an oxygen atmosphere and by postoxidation of Ce/Pt surface alloys. STM results are compared with previously reported high-temperature STM and noncontact atomic force microscopy (NC-AFM) images of the native CeO2(111) surface. The similarity between these images is striking and allows a number of defects and adsorbates in our ultrathin film to be assigned. Moreover, the similarity in structure between the native oxide and the ceria ultrathin film indicates that it is an excellent topographic mimic of the native oxide.

Application of the KolibriSensor® to combined atomic-resolution scanning tunneling microscopy and noncontact atomic-force microscopy imaging

Combined atomic-resolution scanning tunneling microscopy (STM) and noncontact atomic-force microscopy (NC-AFM) studies are carried out with the piezoelectric KolibriSensor in ultrahigh vacuum at room temperature. The sensor exhibits a very low spectral deflection noise density of only 6.5 fm/Hz which favors in combination with its high spring constant of 540 000 N/m stable NC-AFM operation at subnanometer oscillation amplitudes. The authors present atomic-resolution imaging on the Si(111)(7×7) surface recorded in STM and NC-AFM feedback mode. They find that the tip surface distance during atomic-resolution imaging on silicon is much smaller for NC-AFM compared to STM. It is shown that atomic-resolution NC-AFM and dynamic STM images of the same area on the Si(111)(7×7) surface enable a discrimination of vacancies and adsorbates. Furthermore, the topography of graphite imaged in dynamic STM and NC-AFM feedback mode is compared.

COATING THICKNESS STUDY OF BIOPOLYMER-MAGNETITE CORE–SHELL NANOPARTICLES

Magnetite and biopolymer-magnetite nanoparticles coated with polyethylene glycol (PEG) and chitosan have been synthesized. The adsorption of the biopolymers on the magnetite nanoparticles is confirmed using Fourier Transform Infrared (FTIR) Spectroscopy. Atomic Force Microscopy (AFM) imaging revealed magnetite-biopolymer core–shell nanoparticles of typical size range 25–80 nm. We report a novel way of determining the thickness of the biopolymer coating using noncontact AFM imaging. AFM has been used to study the variation of the biopolymer coating thickness as a function of the magnetite core diameter, biopolymer type, and its concentration. The thickness of the chitosan coating varies in the range of 4–11 nm and increases linearly with increase in magnetite core size. PEG coating thickness has similar values as for the chitosan coating.

The role of tip size and orientation, tip–surface relaxations andsurface impurities in simultaneous AFM and STM studies on theTiO2(110) surface

In this work we investigate some of the key factors in simultaneously recorded scanningtunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) images of theTiO2(110) surface, particularly the role of tip size and orientation in the obtained contrast pattern,and the importance of tip–surface relaxations and surface impurities in measured currents.We show that, while using multi-channel scanning modes provides an increase inphysical data from a given measurement and greatly aids in interpretation, it alsodemands much greater rigor in simulations to provide a complete comparison.

Thin film evolution: Dewetting dynamics of a bimodal molecular system

Deposition of submonolayer coverages of 3,4,9,10-perylenetetracarboxylic diimide onto NaCl(001) results in the bimodal growth of coexisting square and needle-shaped islands. The epitaxy of each island type was determined from molecular and atomic resolution noncontact atomic force microscopy images. Postdeposition, the needle islands grow while the square islands disappear completely within days. This is characterized as a dewetting transition, the dynamics of which has been described and empirically fit to the monomolecular growth model, a model typically applied to biological growth.

Efficient First-Principles Simulation of Noncontact Atomic Force Microscopy for Structural Analysis

We propose an efficient scheme to simulate noncontact atomic force microscopy images by using first-principles self-consistent potential from the sample as input without explicit modeling of the atomic force microscopy tip. Our method is applied to various types of semiconductor surfaces including Si(111)-(7x7), TiO2(110)-(1x1), Ag/Si(111)-(sqrt[3]xsqrt[3])R30 degrees, and Ge/Si(105)-(1x2) surfaces. We obtain good agreement with experimental results and previous theoretical studies, and our method can aid in identifying different structural models for surface reconstruction.

Imaging of the Hydrogen Subsurface Site in RutileTiO2

From an interplay between simultaneously recorded noncontact atomic force microscopy and scanning tunneling microscopy images and simulations based on density functional theory, we reveal the location of single hydrogen species in the surface and subsurface layers of rutile TiO2. Subsurface hydrogen atoms (H(sub)) are found to reside in a stable interstitial site as subsurface OH groups detectable in scanning tunneling microscopy as a characteristic electronic state but imperceptible to atomic force microscopy. The combined atomic force microscopy, scanning tunneling microscopy, and density functional theory study demonstrates a general scheme to reveal near surface defects and interstitials in poorly conducting materials.

Evidence for Vacancy Creation by Chromium Doping of Rutile Titanium Dioxide (110)

Chromium doped rutile TiO2(110) is studied using noncontact atomic force microscopy (NC-AFM) at room temperature. High-resolution NC-AFM images allow for investigating the effect of chromium doping on the surface structure at the atomic level. Besides added rows that are ascribed to Cr2O3 structures, we observe a considerable amount of surface hydroxyl defects, which are known to originate from oxygen vacancies. A careful analysis of the surface oxygen vacancy density is presented, providing strong experimental evidence for enhanced formation of surface oxygen vacancies upon chromium doping. We can explain these experimental results by a simple model based on maintaining charge neutrality while at the same time minimizing lattice stress. This simple model accounts for an increased oxygen vacancy density, in good agreement with our experimental findings.

Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of theTiO2(110)surface

The atomic-scale contrast in noncontact atomic force microscopy (nc-AFM) images is determined by the geometry and exact atomic structure of the tip apex. However, the tip state is an experimentally unknown parameter, and the lack of insight into the tip apex often limits the possibilities of extracting precise quantitative and qualitative atomistic information on the surface under inspection. From an interplay between simultaneously recorded nc-AFM and scanning tunneling microscopy (STM) data, and atomistic STM simulations based on multiple scattering theory, we demonstrate how the state of the scanning probe microscopy (SPM) tip in the experiments may be determined. The analysis of a large number of experimental SPM images recorded with different tips reveals that no general correlation exists between the contrast observed in the nc-AFM and the tunneling current $({I}_{t})$ images on ${\text{TiO}}_{2}(110)$ surface. The exact state of the SPM tip must, therefore, be determined for each specific case, which is normally a very difficult endeavor. However, our analysis of the AFM contrast on ${\text{TiO}}_{2}(110)$ surface allows us to considerably reduce the number of tips to be considered in a full simulation. By carefully evaluating the contrast of a handpicked library of SPM tips, we manage to determine a very accurate model of the SPM tip used in an experiment for the first time. It is envisioned that the approach presented here may eventually be used in future studies to screen for and select a SPM tip with a special functionalization prior to imaging an unknown sample, and in that way facilitate precise modeling and chemical identification of surface species.

High-Spatial-Resolution Topographic Imaging and Dimer Distance Analysis of Si(100)-(2×1) Using Noncontact Atomic Force Microscopy

The distance between bright spot in a dimer on the Si(100)-(2×1) surface imaged by noncontact atomic force microscopy (NC-AFM) is analyzed. We used in the analysis only high-spatial-resolution topographic NC-AFM images obtained using different tips at room temperature. Atoms in the dimer was clearly resolved, which enabled us to analyze the distance between two bright spots in the dimer statistically. The average distance was similar to that obtained in a previous study and the theoretical predition. The results indicated that dimer flipping was induced by the AFM tip during scanning and upper–upper atoms are imaged at room temperature.

Repulsive interaction and contrast inversion in noncontact atomic force microscopy imaging of adsorbates

To understand contrast formation in atomic resolution noncontact atomic force microscopy (NC-AFM), we investigate whether or not repulsive tip-sample interaction contributes to contrast formation. We relate attractive and repulsive interactions to contrast features depending on both oscillating amplitude and measured detuning. Simulations based on a Morse potential illustrate the mechanism behind contrast inversion due to repulsive interactions above an adsorbate on the surface. Experimental NC-AFM images of adsorbates on mica and ${\text{TiO}}_{2}$ surfaces confirm our simulations. Furthermore, we discuss the influence of the topography feedback loop on contrast formation above adsorbates, which illustrates that data interpretation can become rather delicate for constant-detuning images.

Noncontact atomic force microscopy imaging mechanism on Ag(110): Experiment and first-principles theory

In this study, we analyze the noncontact atomic force microscopy (NC-AFM) imaging mechanism on the Ag(110) surface by experiment and ab initio theory. The experimental NC-AFM images exhibit atomic-scale resolution in the topography and dissipation signal. Interestingly, the maximum of the damping signal is between the maxima of the topography image. Comparing the geometry of the Ag(110) surface with the topography of a simulated NC-AFM image, we found that the first surface layer silver atoms are imaged as maxima in the topographic NC-AFM images. The overall structure and the corrugation height of the theoretical NC-AFM image are in good agreement with the experimental ones. Furthermore, the analysis of the short-range tip-sample interactions calculated at specific lattice sites revealed strong hysteresis effects. Our simulations also indicate that clean silicon tips might become contaminated with silver atoms during a NC-AFM experiment. Indeed, the NC-AFM experiments showed that the Ag(110) surface is difficult to image probably due to contamination of the silicon tip during the imaging process.

Dynamical processes at oxide surfaces studied with the virtual atomic force microscope

We present the results of calculations performed to simulate the process of atomic scale imaging with the noncontact atomic force microscope that explicitly take into account thermally induced dynamical processes occurring at the surface in real time, using a different multiscale method. Two model defect systems are studied atomistically: a Pd atom adsorbed on MgO (001) which can diffuse across the surface, and a water molecule adsorbed on the $\mathrm{Ce}{\mathrm{O}}_{2}$ (111) surface which can rotate about an oxygen atom. Each of these processes is evolved in real time using a kinetic Monte Carlo method, while simultaneously being imaged using a virtual atomic force microscope which performs a simulation of the entire instrumentation. The results of these simulations show how dynamical processes can significantly change the contrast seen in noncontact atomic force microscope images and that mobile defects present on the surface may not be visible in the image.

Non-contact Atomic Force Microscopy Simulations of Hydrogen-terminated Si(100) Surfaces with a Methyl

Toward the development of a simulator of scanning probe microscope for systems with adsorbed organic molecules, we simulate non-contact atomic force microscope images of hydrogen-terminated Si(001)2×1 surfaces with an isolated methyl using a Si tip with and without a hydrogen atom at the apex, using a density-functional based tight-binding method. We examine the constant height, constant frequency and energy dissipation images. In addition to normal images of constant height and constant frequency modes, we obtain the exotic energy dissipation images, where a low dissipation spot is surrounded by a ring of high dissipation region. These images can be understood from the site-dependence of force hysteresis.

Noncontact Atomic Force Microscopy Investigations of Au55 Thin Films Deposited on Gold and Graphite Substrates

Thin films of ligand-stabilized Au55 clusters deposited on Au(111) and highly oriented pyrolytic graphite (HOPG) substrates have been investigated by noncontact atomic force microscopy (NC-AFM) and spectroscopy. The NC-AFM images show small size islands (consisting of one up to 20 clusters) on the Au(111) substrate which are locally ordered. In contrast, on HOPG, the clusters stick together in disordered monolayer islands of larger size. Conservative and dissipative interactions between the AFM probe and individual Au55 clusters have been investigated for the two systems. The force as well as the dissipation curves show different behavior for the two systems.

Simulated non-contact AFM images of an alcohol molecule in an alkanethiol self-assembledmonolayer

Based on all-atom empirical force field calculations, non-contact atomic force microscopy(nc-AFM) images are simulated of a self-assembled monolayer (SAM) of 1-hexanethiol(C6) in which a single4-mercapto-1-butanol (C4OH) molecule is inserted. The tip is modelled by a carbon nanoconeapex and the temperature is assumed to be 0 K. It is found that theC6 molecules can be recognized individually in the topographic, frequencyshift and energy dissipation images. In addition, the position of aH2O molecule, which is adsorbed by forming a hydrogen bond with the hydroxyl head group of theC4OH molecule,is observed more clearly in the energy dissipation image than in the topographic image. Furthermore,this H2O molecule is observed to be higher than the surroundingC6 matrix in the topographic image, although the former is slightly lower than the latter. Thiscan be explained by the enhancement of the normal force by an attractive electrostaticinteraction between the water molecule and the tip apex.

Noncontact atomic force microscopy imaging of water dissociation products onTiO2(110)

We have imaged the products of water dissociation on TiO2(110) with noncontact atomic force microscopy (NC-AFM). Scanning tunneling microscopy (STM) experiments have revealed that water splits into a hydroxyl group that sits in an O vacancy and an H adatom which lies on a bridging O atom. With this knowledge, STM images acquired in the same area allow an unambiguous identification of these H adatoms in the corresponding NC-AFM images.