Yoshiaki Sugimoto

Chemical identification of individual surface atoms by atomic force microscopy

Scanning probe microscopy is a versatile and powerful method that uses sharp tips to image, measure and manipulate matter at surfaces with atomic resolution. At cryogenic temperatures, scanning probe microscopy can even provide electron tunnelling spectra that serve as fingerprints of the vibrational properties of adsorbed molecules and of the electronic properties of magnetic impurity atoms, thereby allowing chemical identification. But in many instances, and particularly for insulating systems, determining the exact chemical composition of surfaces or nanostructures remains a considerable challenge. In principle, dynamic force microscopy should make it possible to overcome this problem: it can image insulator, semiconductor and metal surfaces with true atomic resolution, by detecting and precisely measuring the short-range forces that arise with the onset of chemical bonding between the tip and surface atoms and that depend sensitively on the chemical identity of the atoms involved. Here we report precise measurements of such short-range chemical forces, and show that their dependence on the force microscope tip used can be overcome through a normalization procedure. This allows us to use the chemical force measurements as the basis for atomic recognition, even at room temperature. We illustrate the performance of this approach by imaging the surface of a particularly challenging alloy system and successfully identifying the three constituent atomic species silicon, tin and lead, even though these exhibit very similar chemical properties and identical surface position preferences that render any discrimination attempt based on topographic measurements impossible.

Complex Patterning by Vertical Interchange Atom Manipulation Using Atomic Force Microscopy

The ability to incorporate individual atoms in a surface following predetermined arrangements may bring future atom-based technological enterprises closer to reality. Here, we report the assembling of complex atomic patterns at room temperature by the vertical interchange of atoms between the tip apex of an atomic force microscope and a semiconductor surface. At variance with previous methods, these manipulations were produced by exploring the repulsive part of the short-range chemical interaction between the closest tip-surface atoms. By using first-principles calculations, we clarified the basic mechanisms behind the vertical interchange of atoms, characterizing the key atomistic processes involved and estimating the magnitude of the energy barriers between the relevant atomic configurations that leads to these manipulations.

Room-temperature reproducible spatial force spectroscopy using atom-tracking technique

A method for reproducible site-specific force spectroscopic measurements using frequency modulation atomic force microscopy at room temperature is presented. The stability and reproducibility requirements, fulfilled so far only in cryogenic environment, are provided through the compensation of the thermal drift using the atom-tracking technique. The method has been tested performing spectroscopic measurements on atomic positions of the Si(111)-(7×7) surface with Si tips. The room-temperature results presented here compare in quality to previously reported quantitative force spectroscopic data obtained at cryogenic temperatures.

Drift-compensated data acquisition performed at room temperature with frequency modulation atomic force microscopy

The authors have performed distortionless atom imaging and force mapping experiments, under a large thermal drift condition at room temperature (RT), using frequency modulation atomic force microscopy (FM-AFM) that had been done previously only at low temperature. In the authors’ experimental scheme, three-dimensional position feedback with atom tracking detects the thermal drift velocity that is constant for a period of time at RT. The detected velocity is then used as the model for implementing the feedforward in order to compensate for the thermal drift. This technique can be expected to be used for precise positioning of the tip-sample in atom manipulation experiments using the FM-AFM at RT.

Lateral manipulation of single atoms at semiconductor surfaces using atomic force microscopy

Experimental results on the lateral manipulation of single atoms at semiconductor surfaces using non-contact atomic force microscopy (NC-AFM) are presented. These experiments prove that deposited adsorbates on top of a surface, as well as intrinsic adatoms of semiconductor surfaces, are suitable for being manipulated using the short-range interaction force acting between the outermost atoms of a semiconductor tip and the atoms at the surface. The analysis of the data from some of the experiments presented here indicates a pulling process of the tip on the manipulated atoms. The atom-by-atom creation, at room temperature, of patterns composed by a few inherent atoms of a heterogeneous surface is also presented.

Simultaneous measurement of force and tunneling current at room temperature

We have performed simultaneous scanning tunneling microscopy and atomic force microscopy measurements in the dynamic mode using metal-coated Si cantilevers at room temperature. Frequency shift (Δf) and time-average tunneling current (⟨It⟩) images are obtained by tip scanning on the Si(111)-(7×7) surface at constant height mode. By measuring site-specific Δf(⟨It⟩) versus tip-surface distance curves, we derive the force (tunneling current) at the closest separation between the sample surface and the oscillating tip. We observe the drop in the tunneling current due to the chemical interaction between the tip apex atom and the surface adatom, which was found recently, and estimate the value of the chemical bonding force. Scanning tunneling spectroscopy using the same tip shows that the tip is metallic enough to measure local density of states of electrons on the surface.

Atom tracking for reproducible force spectroscopy at room temperature with non-contact atomic force microscopy

A method for reproducible site-specific force spectroscopic measurements at room temperature by combining frequency modulation atomic force microscopy and the atom tracking technique is proposed. The atom tracking enables us to compensate the change in the tip–sample relative position induced by the thermal drift as well as to precisely position the tip over the same spot of the surface within sub-Angstrom stability. Here, we describe our atom-tracking implementation and the protocol we have followed for the reproducible room-temperature acquisition of series of frequency shift versus tip–sample distance (Δf–Z) curves using this technique. With this acquisition protocol, a large number of equivalent Δf–Z curves can be averaged, resulting in a considerable noise reduction, and therefore avoiding its propagation to the corresponding calculated force curve.

Chemical structure imaging of a single molecule by atomic force microscopy at room temperature

Atomic force microscopy is capable of resolving the chemical structure of a single molecule on a surface. In previous research, such high resolution has only been obtained at low temperatures. Here we demonstrate that the chemical structure of a single molecule can be clearly revealed even at room temperature. 3,4,9,10-perylene tetracarboxylic dianhydride, which is strongly adsorbed onto a corner-hole site of a Si(111)–(7 × 7) surface in a bridge-like configuration is used for demonstration. Force spectroscopy combined with first-principle calculations clarifies that chemical structures can be resolved independent of tip reactivity. We show that the submolecular contrast over a central part of the molecule is achieved in the repulsive regime due to differences in the attractive van der Waals interaction and the Pauli repulsive interaction between different sites of the molecule.

Dynamic force spectroscopy using cantilever higher flexural modes

By means of force spectroscopy measurements performed with the cantilever first and second flexural modes under the frequency modulation detection method, the authors corroborate the validity of the relation between tip-surface interaction force and frequency shift for force spectroscopy acquisition using higher cantilever eigenmodes. They estimate a cantilever effective stiffness for the second eigenmode 73 times larger than the static stiffness. This large effective stiffness enables them to perform force spectroscopy with a cantilever oscillation amplitude (A0) as small as 3.6A. The authors provide experimental evidence that, at such small A0 values, normalized frequency shift curves deviate from a A03∕2 scaling and the signal-to-noise ratio is considerably enhanced.

Ultrahigh-resolution imaging of water networks by atomic force microscopy

Local defects in water layers growing on metal surfaces have a key influence on the wetting process at the surfaces; however, such minor structures are undetectable by macroscopic methods. Here, we demonstrate ultrahigh-resolution imaging of single water layers on a copper(110) surface by using non-contact atomic force microscopy (AFM) with molecular functionalized tips at 4.8 K. AFM with a probe tip terminated by carbon monoxide predominantly images oxygen atoms, whereas the contribution of hydrogen atoms is modest. Oxygen skeletons in the AFM images reveal that the water networks containing local defects and edges are composed of pentagonal and hexagonal rings. The results reinforce the applicability of AFM to characterize atomic structures of weakly bonded molecular assemblies.