Noncontact Atomic Force Microscopy Images Research Articles

Submolecular-Resolution Studies on Metal-Phthalocyanines by Noncontact Atomic Force Microscopy

Copper phthalocyanine (CuPc) thin films deposited on Au (111) surfaces were investigated by noncontact atomic force microscopy (NC-AFM). Structures with a periodic modulation of the spacing between the molecular rows of CuPc multilayer films were found. Submolecular features of each individual molecule on a film were successfully imaged. The obtained contrast was interpreted by the chemical interactions between a tip and the spatial electron density distribution of chemically active molecular orbitals such as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). In addition, the energy dissipation in the NC-AFM imaging of monolayer films was studied on a molecular scale. The obtained high-resolution contrast in the dissipation images was discussed in connection with the random fluctuation of molecules.

Noncontact atomic force microscopy study of copper-phthalocyanines: Submolecular-scale contrasts in topography and energy dissipation

Copper-phthalocyanine (CuPc) thin films on MoS2 surfaces were investigated by noncontact atomic force microscopy (NC-AFM). Submolecular resolution was successfully obtained in both topographic and dissipation images of CuPc monolayers. For topographic contrasts, the influence of short-range chemical interactions is particularly considered while the dissipation contrasts are discussed in relation to the tip-induced molecular fluctuations. Molecularly-resolved NC-AFM image was also obtained on CuPc multilayer, which revealed the structural difference between the monolayer and multilayer surfaces. The energy dissipation measured on these surfaces showed distinctive difference reflecting the different structural stabilities in the films. Furthermore, local surface modification of CuPc monolayer was demonstrated by NC-AFM. This is a direct evidence for the existence of energy transfer from the vibrating cantilever to the molecules through dissipative tip–sample interactions.

Atom-selective imaging and mechanical atom manipulation using the non-contact atomic force microscope.

We succeeded in distinguishing between oxygen and silicon atoms on an oxygen-adsorbed Si(111)7 x 7 surface, and also distinguished between silicon and tin atoms on Si(111)7 x 7-Sn intermixed and Si(111) square root(3) x square root(3)-Sn mosaic-phase surfaces using non-contact atomic force microscopy (NC-AFM) at room temperature. Atom species of individual atoms are specified from the number of each atom in NC-AFM images, the tip-sample distance dependence of NC-AFM images and/or the surface distribution of each atom. Further, based on the NC-AFM method but using soft nanoindentation, we achieved two kinds of mechanical vertical manipulation of individual atoms: removal of a selected Si adatom and deposition of a Si atom into a selected Si adatom vacancy on the Si(111)7 x 7 surface at 78 K. Here, we carefully and slowly indented a Si atom on top of a clean Si tip apex onto a predetermined Si adatom to remove the targeted Si adatom and onto a predetermined Si adatom vacancy to deposit a Si atom, i.e. to repair the targeted Si adatom vacancy. By combining the atom-selective imaging method with two kinds of mechanical atom manipulation, i.e. by picking up a selected atom species and by depositing that atom one by one at the assigned site, we hope to construct nanomaterials and nanodevices made from more than two kinds of atom species in the near future.

Tip–sample distance dependency of non-contact atomic force microscopy images on aGaAs(110) surface

Tip–sample interactions are at the origin of atomic resolution in non-contact atomic force microscopy(NC-AFM) but also make it difficult to understand the meaning of atomic features observed asNC-AFM images. (110) surfaces of III–V semiconductors such as GaAs or InP have relaxed(1 × 1) surfaces with both anions and cations of different dangling bond states. Since these atomsare located at the surface where an atom attached to the tip apex can reach to interactwith them and show significant relaxation via tip–sample interaction, NC-AFM images ofthe surface are expected to strongly depend upon the atomic species or the electronic stateof the topmost atom at the tip apex. We have taken a number of NC-AFM images ofGaAs(110) surfaces with a Si tip by means of room-temperature ultra-high vacuumNC-AFM and categorized them into two types of image which have differenttip–sample distance dependencies. In comparison with a theoretical prediction oftip–surface interaction on the GaAs(110) surface, we have found that one of theseimages is due to a Si atom attached to the apex which reveals the tip–sampleinteraction with As and Ga surface atoms as individual peaks in the NC-AFM image.

Theoretical simulation of non-contact atomic force microscopy imaging of theα-alumina(0001) surface

We have studied the interactions responsible for the contrast formation in nc-AFM imaging of theα-alumina(0001) surface using the oxide and Si dangling bond tip models. The full imagesimulation performed using the rigid oxide tip and common cantilever parameters suggeststhe possibility of atomic and chemical resolution on the plane surface in a narrow range ofexperimental parameters. The interaction of both the oxide and Si dangling bond tips withthe surface is stronger at the centres of triangles formed by three oxygen ions. This makesthe image interpretation for the Si dangling bond tip unique—the bright spots shouldcorrespond to the centres of O triangles and centres of dark spots are located above thesurface Al ions. In the case of the oxide tip, the image pattern depends on thetip polarity and is different, but easily recognizable. However, only Al ions ortriangles formed by three oxygen ions could be resolved in the simulated images.

The dynamic behaviour of CH3OH and NO2 adsorbed on CeO2(111) studied by noncontact atomic force microscopy

The adsorption and reaction of methanol(CH3OH) and nitrogendioxide (NO2) on CeO2(111) surfaces in different oxidation states were studied by noncontact atomic force microscopy (NC-AFM). AfterCH3OH exposure,surface oxygen defects without reconstruction of the surrounding oxygen atoms were healed by methoxyspecies (CH3O).Under a CH3OH atmosphere, a heterogeneous feature in the reactivity of surface oxygen atoms toCH3OH was observed by successive NC-AFM observation: new line defects ofoxygen were produced as a result of a chemical event. Successive NC-AFMmeasurements also revealed that oxygen vacancies on a slightly reducedCeO2(111) surface were healedby exposure to NO2. The contrast of healed oxygen defects in NC-AFM images was similar to that ofother surface oxygen atoms. This indicates that dissociative adsorption ofNO2 occurred at the defects. Nitrogen trioxide(NO3) was also observedon a slightly reduced CeO2(111) surface after NO2 exposure. On a CeO2(111) surface without surface oxygen defects, neither adsorption ofNO2 norformation of NO3 was observed. The reaction mechanisms for the formation of line defects andNO3 are discussed briefly.

A Study of Nano-scale Mechanics-Basic Principle for Nano-fabrication and -machines-

We discuss recent studies of nano-size mechanics from the stand points of nano-fabrication and nano-machine. First, a nano-mechanical simulator developed by us is explained. Theoretical simulations of noncontact atomic force microscopy images by taking thermal fluctuation into consideration are in very good agreement with experimental results. Next, a new nano-size super-lubricated system that is not subject to friction, “C60 molecular bearing,” is described. This sandwiched structure which confines a C60 monolayer between graphite plates, exhibits superlubrication with zero dynamical frictional forces. Observed results and a theoretical concept of, “step rotation model” to interpret the mechanism of super lubrication, are explained by using a picture of “single molecular bearing.” This research is the first proposal as an artificial nano-machine using fullerene, which can be called the smallest bearing in the world.

Simulating atomic force microscopy imaging of the ideal and defectedTiO2(110) surface

In this study we simulate noncontact atomic force microscopy imaging of the ${\mathrm{TiO}}_{2}$ (110) surface using first-principles and atomistic methods. We use three different tip models to investigate the tip-surface interaction on the ideal surface, and find that agreement with experiment is found for either a silicon tip or a tip with a net positive electrostatic potential from the apex. Both predict bright contrast over the bridging oxygen rows. We then study the interaction of this tip with a bridging oxygen vacancy on the surface, and find that the much weaker interaction observed would result in vacancies appearing as dark contrast along the bright rows in images.

Atom-resolved noncontact atomic force microscopic and scanning tunneling microscopic observations of the structure and dynamic behavior of CeO 2(1 1 1) surfaces

Atomic-scale structures and dynamic behaviors of CeO 2(1 1 1) surfaces were imaged by noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). Hexagonally arranged oxygen atoms, oxygen point vacancies, multiple oxygen vacancies, and hydrogen adatoms at the surfaces were visualized by atom-resolved NC-AFM observations. Multiple defects were stabilized by displacement of the surrounding oxygen atoms around the multiple defects, which gave enhanced brightness in the NC-AFM image due to a geometric reason. Multiple defects without reconstruction of the surrounding oxygen atoms were reactive and were healed by exposure to O 2 gas and methanol at RT. Successive NC-AFM and STM measurements of slightly reduced CeO 2(1 1 1) surfaces revealed that hopping of surface oxygen atoms faced to the metastable multiple defects was thermally activated even at room temperature (RT) and more promoted at higher temperatures. Heterogeneous feature of the reactivity of surface oxygen atoms with methanol was imaged by successive NC-AFM observations. These observations gave a new insight for understanding the surface structures and behavior of CeO 2− x with the facile oxygen reservoir and oxidation–reduction properties related to the unique catalysis.

KPFM imaging of Si(1 1 1) [formula omitted]-Sb surface for atom distinction using NC-AFM

We investigate the possibility of distinction of individual atom species using noncontact atomic force microscopy (NC-AFM) combined with the electrostatic force (ESF) measurement. We simultaneously measured the topography and the surface potential on the Si(1 1 1) 5 3 ×5 3 -Sb surface by Kelvin probe force microscopy (KPFM). The surface potential image indicates the existence of two kinds of adatoms while the difference is not clear in topography. Atom species of each adatom are specified from other NC-AFM images with different experimental conditions and therefore spots with a lower surface potential are specified as Si adatoms. It is found that, in addition to adatoms, Si rest atoms can be specified from the surface potential image. The result indicate that KPFM has the ability to distinguish the individual atom species on intermixing surfaces.

Formic Acid Adsorption on Anatase TiO2(001)−(1 × 4) Thin Films Studied by NC-AFM and STM†

Atomic resolution scanning tunneling microscopy (STM), noncontact atomic force microscopy (NC-AFM), X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED), and formic acid adsorption experiments were used to characterize the (001) surface of anatase TiO2. A combination of NC-AFM and STM imaging was used to distinguish features due to geometrical and electronic effects. The contrast in images of the bare (1 × 4) surface and the formate-covered surface is dominated by the surface topography in both NC-AFM and STM, although electronic effects in the troughs contribute features to the STM images that are absent from NC-AFM images. High-resolution imaging by STM and NC-AFM revealed that the highest point of the ridge of the (1 × 4) structure consists of a single row of atoms. Formate adsorbs at under-coordinated Ti sites in the added rows with a minimum separation of 2a0 and never adsorbs in the trenches even though the trenches also likely expose under-coordinated Ti atoms. The stickin...

Atomically Resolved Imaging of Si(100)2×1, 2×1:H and 1×1:2H Surfaces with Noncontact Atomic Force Microscopy

Atomically resolved imaging of Si(100)2×1-clean, Si(100)2×1:H-monohydride and Si(100)1×1:2H-dihydride surfaces using a noncontact atomic force microscope (NC-AFM) is reviewed. These experimental results clarify that our original NC-AFM had lateral resolution of about 0.01 nm and could image tilted dangling bonds of the Si(100)2×1-clean surface, individual hydrogen atoms of the Si(100)2×1:H-monohydride surface, and atomic strain around missing dimers of the Si(100)2×1-clean surface. In addition, the NC-AFM image pattern of the Si(100)1×1:2H-dihydride surface can be changed from 1×1 through 2×1 to 1×1 by decreasing the tip-sample distance. From these results, it was concluded that the NC-AFM could mechanically induce phase transition of surface reconstruction from the Si(100)1×1:2H to Si(100)2×1:2H-dihydride surface due to self-organization, and could also pull up the nearest hydrogen atoms one by one by increasing the attractive force between the tip apex atom and sample surface atom.

Atomic resolution imaging of Si(1 0 0)1×1:2H dihydride surface with noncontact atomic force microscopy (NC-AFM)

We investigated the Si(1 0 0)1×1 :2H dihydride surface using the noncontact atomic force microscopy (NC-AFM). NC-AFM images showed that the pattern of the dihydride surface changed depending on the distance between the tip and the sample surface. The image with 1×1 structure appeared when the tip was a little far from the sample surface. When the tip became close to the surface, 2×1 structure where the bright lines and the dark lines were alternately located appeared. This 2×1 image was stable for retracting the tip from the surface. Furthermore, when the tip became close to the surface, 1×1 structure reappeared. It turned out that the structures of the dihydride surface changed due to the attractive force between the tip and the sample under NC-AFM measurement.

Visualization of thermally fluctuating surface structure in noncontact atomic-force microscopy and tip effects on fluctuation: theoretical study of Si(111)-(square root[3] x square root[3])-Ag surface.

We investigated noncontact atomic-force microscopy (NC-AFM) images of a thermally fluctuating surface structure together with tip effects based on the first-principles electronic state calculation. As an example the Si(111)-(square root[3] x square root[3])-Ag (square root[3]-Ag) surface is studied. We have succeeded in theoretically visualizing the thermal fluctuation of the square root[3]-Ag surface at room temperature, and in reproducing the observed NC-AFM image for the first time. Further, the pinning effect of the thermal fluctuation of the square root[3]-Ag surface by the tip is clarified, which shows a novel ability of NC-AFM to modify the surface structure.

非接触原子間力顕微鏡 なにがどこまで見えるか？ 吸着有機分子の非接触型原子間力顕微鏡観察 分子像の定量的理解に向けて

Noncontact atomic force microscopy (NC-AFM) images of the mixed monolayers of carboxylates prepared on a TiO2(110)-(1×1) surface are presented. The order of the height of formate, acetate, pivalate, and propiolate molecules is reproduced in the NC-AFM images, while the difference in the height is smaller than that expected from their true heights. The shrinkage of the height difference is ascribed to the contribution of several molecules to the tip-molecule force. On the other hand, trifluoroacetate is imaged as a spot shorter than the acetate though the two ions have an equal true height. This indicates that the image contrast reflects the permanent dipole moment of the molecules. The experimental results show that the organic molecules can be individually identified by the NC-AFM and the experimentally obtained topography is interpreted referring to physical and chemical characteristics of the molecules.

Theoretical Analysis of Tip Effect on Noncontact Atomic Force Microscopy Image of Si(100) 2×1: H Surface

The influence of the tip structure on the noncontact atomic force microscopy (nc-AFM) image of a hydrogenated Si(100) 2×1 surface is revealed by a theoretical simulation with the first-principles density functional approach. Two different Si tips, i.e., a clean Si(111) tip and a hydrogen terminated Si(111) tip, are modeled by SiH3 and SiH4 clusters, respectively, and the simulated nc-AFM images are compared for the respective tip models. The clean Si tip produced a much larger corrugation amplitude than the hydrogen-terminated Si tip. For the latter tip, a marked change in the image is observed with the approach of the tip turning point towards the surface. When the tip turning point becomes very close to the surface, the inversion of the brightness of the image is found to appear.

Noncontact Atomic Force Microscopy. Nano-scale Structure and Electronic Properties of DNA Molecules.

Non-contact atomic force microscopy (NC-AFM) has been employed to observe double-stranded DNA. Cu(111) surface is found to be useful to realize high-resolution imaging of DNA molecules without the effect of water-layer and to improve the tip shape during the scan for imaging. NC-AFM images reveal the double-helix structure whose averaged repeat distance agrees with Watson-Crick model. However, the observed height of DNA molecules is only 1 nm that is the half of natural diameter of double-stranded DNA suggesting the strong deformation due to the surface adsorption. The simultaneous measurements of frequency-shift and tunneling current indicate that DNA molecules show tunneling conductivity across the strand with the attenuation factor β = 1.1.

Noncontact Atomic Force Microscopy. Possibility of NC-AFM Imaging of Surface Magnetic Structure.

Many kinds of scanning probe microscopes have been proposed for imaging surface magnetic structures. Theoretical calculations predict that the detection of the short-range magnetic interaction such as exchange interaction reveals the magnetic structures on an atomic scale. The non-contact atomic force microscopy (NC-AFM) detecting the short-range magnetic interaction is called exchange force microscopy (EFM). In order to detect the exchange interaction between a magnetic tip and a sample, we performed NC-AFM imaging of an antiferromagnet NiO(001) surface using a ferromagnetic Fe-coated tip. To discuss the interaction atomically resolved images obtained are analyzed by the superposition method. The results of the analysis demonstrates that the spin configuration revealed from the superimposed images coincides with the expected one of the NiO(001) surface. To evaluate the corrugation amplitude of the atoms, we introduced the topographic asymmetry, which indicates that the short-range magnetic interaction between the Fe-coated tip and sample can be detected by NC-AFM.

非接触原子間力顕微鏡 なにがどこまで見えるか？ 非接触原子間力顕微鏡で半導体の何がどこまで見えるか？

In this review article, we clarified what we can image on semiconductor surfaces using noncontact atomic force microscopy (NC-AFM): 1. We can image covalent bonding force on Si(111)7×7 surface. 2. We can discriminate Si adatoms from Sb adatoms on Si(111)5√3×5√3-Sb surface with a Si tip and a Sb adsorbed tip. 3. We can image tilted dangling bond on Si(100)2×1 surface. 4. We can image individual hydrogen atom on Si(100)2×1:H monohydride surface. 5. We can measure atomic strain around missing dimer. 6. We can image a potential map on Si(111)√3×√3-Ag surface by measuring NC-AFM image as a function of tip-sample distance. 7. We can selectively control atomic force by placing an adequate atom on the tip apex. 8. We can pull up lower Si atoms constituting buckled Si dimer by increasing attractive force between tip and sample surface.

A needle-like organic molecule imaged by noncontact atomic force microscopy

We present a noncontact atomic force microscopy (NC-AFM) image of a mixed monolayer composed of propiolate (HCCCOO −) and formate (HCOO −) molecules prepared on a TiO 2(1 1 0)-(1×1) surface. An isolated former molecule can be regarded as a needle with a single-atom diameter perpendicular to the surface. The height difference between the propiolate and formate in the NC-AFM image was smaller than the physical height difference. A simple simulation that employed van der Waals forces between the tip and monolayer-covered surface reproduced the experimental topography.