Ayhan Yurtsever

Kelvin probe force microscopy characterization of TiO2 (110)-supported Au clusters

We present a combined non-contact atomic force microscopy and Kelvin probe force microscopy (KPFM) investigation of the structural and electrical properties of Au nanoclusters on a TiO2 model-oxide surface at room temperature. The KPFM images reveal an increase in the local contact potential difference (LCPD) of Au clusters with respect to the supporting TiO2 surface. Variation of the LCPD shift with a broad range of tip-cluster distances verifies that the obtained KPFM images are not a cause of the topographic artifacts. Based on the LCPD shift, both in the distance-dependent bias spectroscopy and KPFM images, we provide new evidence supporting the charge transfer formation from the surface to the Au cluster. We attribute the increment of the LCPD over an Au cluster site to the presence of surface dipoles, created by the charge transfer, pointing towards the substrate.

Mechanical gate control for atom-by-atom cluster assembly with scanning probe microscopy

Nanoclusters supported on substrates are of great importance in physics and chemistry as well as in technical applications, such as single-electron transistors and nanocatalysts. The properties of nanoclusters differ significantly from those of either the constituent atoms or the bulk solid, and are highly sensitive to size and chemical composition. Here we propose a novel atom gating technique to assemble various atom clusters composed of a defined number of atoms at room temperature. The present gating operation is based on the transfer of single diffusing atoms among nanospaces governed by gates, which can be opened in response to the chemical interaction force with a scanning probe microscope tip. This method provides an alternative way to create pre-designed atom clusters with different chemical compositions and to evaluate their chemical stabilities, thus enabling investigation into the influence that a single dopant atom incorporated into the host clusters has on a given cluster stability.

Role of tip chemical reactivity on atom manipulation process in dynamic force microscopy.

The effect of tip chemical reactivity on the lateral manipulation of intrinsic Si adatoms toward a vacancy site on a Si(111)-(7 × 7) surface has been investigated by noncontact atomic force microscopy at room temperature. Here we measure the atom-hopping probabilities associated with different manipulation processes as a function of the tip-surface distance by means of constant height scans with chemically different types of tips. The interactions between different tips and Si atoms are evaluated by force spectroscopic measurements. Our results demonstrate that the ability to manipulate Si adatoms depends extremely on the chemical nature of the tip apex and is correlated with the maximal attractive force measured over Si adatoms. We rationalize the observed dependence of the atom manipulation process on tip-apex chemical reactivity by means of density functional theory calculations. The results of these calculations suggest that the ability to reduce the energy barrier associated with the Si adatom movement depends profoundly on tip chemical reactivity and that the level of energy barrier reduction is higher with tips that exhibit high chemical reactivity with Si adatoms. The results of this study provide a better way to control the efficiency of the atomic manipulation process for chemisorption systems.

Frequency modulated torsional resonance mode atomic force microscopy on polymers

In-plane mechanics of polymers can be probed by integrating frequency modulation and torsional resonance mode atomic force microscopy. We investigated a thin film of polystyrene-block-polybutadiene diblock copolymer. To gain more insight into image contrast formation, we examined displacement curves on polystyrene homopolymer surfaces of different molecular weights focusing on energy dissipation and frequency shift. Data suggest that the transition from a highly motile surface layer to the bulk material depends on the molecular weight of the polymer. This, in turn, indicates that the tip is slightly oscillating within the sample surface during imaging.

Response of a laterally vibrating nanotip to surface forces

The torsional eigenmodes of atomic force microscope (AFM) cantilevers are highly sensitive toward in-plane material properties of the sample. We studied the effect of viscosity and lateral contact stiffness on the detuning, amplitude, and phase response numerically. To verify the theoretical considerations, a torsion mode AFM was operated in frequency modulation. During approach and retract cycles, we observed a negative detuning of the torsional resonant frequency close to the sample surface depending on the tilt angle between the tip and the sample. Thus, the tilt has a significant effect on the imaging process in torsional resonance mode.

Amplitude and frequency modulation torsional resonance mode atomic force microscopy of a mineral surface

Scanning probe imaging in a shear force mode allows for the characterization of in-plane surface properties. In a standard AFM, shear force imaging can be realized by the torsional resonance mode. In order to investigate the imaging conditions on mineral surfaces, a torsional resonance mode atomic force microscope was operated in amplitude (AM) and frequency modulation (FM) feedback. Freshly cleaved chlorite was investigated, which showed brucite-like and talc-like surface areas. In constant amplitude FM mode, a slight variation in energy dissipation was observed between both surfaces. Amplitude and frequency vs. distance curves revealed that the tip was in repulsive contact with the specimen during imaging.

Many-body effects in the Coulomb drag between low density electron layers

Abstract Recent Coulomb drag experiments in low-density double-layer electron systems have the power of distinguishing various many-body formulations of the effective interactions. In this work we theoretically study the correlation effects on the drag resistivity in these systems within various models. The effective inter-layer interactions are best described by the generalization to the double-layer case of the Kukkonen–Overhauser approach which differs significantly from the self-consistent field approach of Singwi et al. [Phys. Rev. 176 (1968) 589]. Following the formulation of Vignale and Singwi [Phys. Rev. B 32 (1985) 2156] we derive an expression for the effective inter-layer interaction which embodies the many-body correlations through the local-field corrections. The drag resistivity is calculated within this approach together with the Hubbard approximation for the intra-layer local-field factor and a simple model for the inter-layer correlations. Comparison with the recent measurements of Kellogg et al. [Solid State Commun. 123 (2002) 515] yields very good agreement. Our results are also contrasted with the corresponding drag resistivities given by the Singwi et al. theory, the dynamic random-phase approximation and the Hubbard approximation. The significant differences found between these theories emphasize the strong sensitivity of the drag resistivity to the effective inter-layer interactions.

(2n × 1) Reconstructions of TiO2(011) Revealed by Noncontact Atomic Force Microscopy and Scanning Tunneling Microscopy.

We have used noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM) to study the rutile TiO2(011) surface. A series of (2n × 1) reconstructions were observed, including two types of (4 × 1) reconstruction. High-resolution NC-AFM and STM images indicate that the (4 × 1)-α phase has the same structural elements as the more widely reported (2 × 1) reconstruction. An array of analogous higher-order (2n × 1) reconstructions were also observed where n = 3–5. On the other hand, the (4 × 1)-β reconstruction seems to be a unique structure without higher-order analogues. A model is proposed for this structure that is also based on the (2 × 1) reconstruction but with additional microfacets of {111} character.

Effect of tip polarity on Kelvin probe force microscopy images of thin insulator CaF2 films on Si(111)

We investigate thin insulating CaF2 films on a Si (111) surface using a combination of noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). Atomic-scale NC-AFM and KPFM images are obtained in different imaging modes by employing two different tip polarities. The KPFM image contrast and the distance-dependent variation of the local contact potential difference (LCPD) give rise to a tip-polarity-dependent contrast inversion. Ca2+ cations had a higher LCPD contrast than F− anions for a positively terminated tip, while the LCPD provided by a negatively charged tip gave a higher contrast for F− anions. Thus, this result implies that it is essential to determine the tip apex polarity to correctly interpret LCPD signals acquired by KPFM.

Initial and secondary oxidation products on the Si(111)-(7 × 7) surface identified by atomic force microscopy and first principles calculations

We investigate the initial and secondary oxidation products on the Si(111)-(7 × 7) surface at room-temperature using atomic force microscopy (AFM) and density functional theory calculations. At the initial oxidation stages, we find that there are two types of bright spots in AFM images. One of them is identified as a Si adatom with one O atom inserted into one of the backbonds, while the other is ascribed to a Si adatom with two inserted O atoms. We observe that the latter one turns into the secondary oxidation product by a further coming O2 molecule, which appears as a more protruded bright spot. The atomic configuration of this product is identified as Si adatom whose top and all three backbonds make bonds with O atoms. The appearances of initial and secondary oxidation products are imaged as bright and dark sites by scanning tunneling microscopy, respectively. It is revealed that AFM gives us the topographic information close to the real atomic corrugation of adsorbed structures on the semiconductor su...