Constant Height Mode Research Articles

Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)

We propose an atom-superposition-based method for simulating spin-polarized scanning tunneling microscopy (SP-STM) from first principles. Our approach provides bias-dependent STM images in high spatial resolution, with the capability of using either constant current or constant height modes of STM. In addition, topographic and magnetic contributions can clearly be distinguished, which are directly comparable to results of SP-STM experiments in the differential magnetic mode. Advantages of the proposed method are that it is computationally cheap, it is easy to parallelize, and it can employ the results of any ab initio electronic structure code. Its capabilities are illustrated for the prototype frustrated hexagonal antiferromagnetic system, Cr monolayer on Ag(111) in a noncollinear magnetic ${120}^{\ensuremath{\circ}}$ N\'eel state. We show evidence that the magnetic contrast is sensitive to the tip electronic structure, and this contrast can be reversed depending on the bias voltage.

Simultaneous Imaging and Chemical Attack of a Single Living Cell within a Confluent Cell Monolayer by Means of Scanning Electrochemical Microscopy

Epithelial cell monolayers from rat kidney were imaged by scanning electrochemical microscopy (SECM) with sub-micrometer resolution in both lateral and vertical direction. Platinum disk ultra-microelectrodes (UMEs) with effective electrode radii between 200 and 600 nm were operated in the constant-height mode. The quality of the recorded SECM images compare favorably with those of phase contrast and confocal laser scanning microscopy. Besides the acquisition of SECM images, the UME was used to selectively attack a single living cell within the monolayer ensemble. Hydroxide ions were locally generated in the vicinity of a single target cell by the UME. The increase in pH induced cell necrosis that was subsequently imaged by SECM. It could be clearly demonstrated that the single target cell was selectively affected, whereas the adjacent reference cells remained unchanged.

Deconvoluting Topography and Spatial Physiological Activity of Live Macrophage Cells by Scanning Electrochemical Microscopy in Constant-Distance Mode

Detection of reactive oxygen species (ROS) released from live macrophage cells (RAW264.7) without any addition of external redox mediators using constant-height and constant-distance mode scanning electrochemical microscopy (SECM) was presented in this Letter. The successful separation of the ROS profile from the topography of cells in the physiological condition was demonstrated by recording the amperometric current and probing position in the z-direction along with lateral coordinates at each pixel where an alternating current (AC) was kept constant. It was discovered that the nucleus region of the cell releases more ROS than other organelle regions and the height of the cell is approximately 4.8 μm. To our best knowledge, this work reports the first spatially monitored ROS release without the influence of cell morphology using SECM.

Scanning Electrochemical Microscopy in Neuroscience

This article reviews recent work involving the application of scanning electrochemical microscopy (SECM) to the study of individual cultured living cells, with an emphasis on topographical and functional imaging of neuronal and secretory cells of the nervous and endocrine system. The basic principles of biological SECM and associated negative amperometric-feedback and generator/collector-mode SECM imaging are discussed, and successful use of the methodology for screening soft and fragile membranous objects is outlined. The drawbacks of the constant-height mode of probe movement and the benefits of the constant-distance mode of SECM operation are described. Finally, representative examples of constant-height and constant-distance mode SECM on a variety of live cells are highlighted to demonstrate the current status of single-cell SECM in general and of SECM in neuroscience in particular.

Simultaneous atomic force and scanning tunneling microscopy study of the Ge(111)-c(2×8) surface

In this article, the authors present the simultaneous noncontact atomic force microscopy and scanning tunneling microscopy measurement of the Ge(111)-c(2×8) surface using PtIr-coated Si cantilevers at room temperature. In both frequency shift and time-averaged tunneling current images at constant-height mode, each atom was clearly resolved. The image contrasts differ because the time-averaged tunneling current image is more directly coupled with local density of states than the frequency shift image. They demonstrate the measurement of the site-specific scanning tunneling spectroscopy (STS) spectra, which are in good agreement with typical STS measurements. Moreover, they demonstrate the simultaneous measurements of site-specific frequency shift and tunneling current as a function of tip-sample distance curves. On the Ge(111)-c(2×8) surface, tunneling current dropped at the near-contact region where a strong tip-sample interaction force is observed.

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.

Simulation of Atomic-Scale Wear of Graphite - Nanotip Induced Graphene Formation

The atomic-scale wear, the formation process of the graphene during the lateral line scan process of the nanoscale tip on the multi-layered graphene substrate is studied by using molecular relaxation method. The nanotip is scanned in line forward (along [1 2 -3 0] direction) and backward under the constant-height mode. Analysis of the effect of the tip height on the relative motion of the nanotip, the 1st graphene layer, and the 2nd graphene layer, reveals the transition from the nanotip state to the graphene tip state. During the nanotip state the mean lateral force rapidly increases as the mean loading force increases. Here the friction between the tip and the surface occurs. However, during the graphene tip state, takes nearly the constant value independent of . Here the internal friction among the 1st, the 2nd and the 3rd graphene layers occurs. The marked scan directional dependence and the increase of near the graphene edge appears. The irreversible shift of the graphene layer after all the scan processes can explain the mechanism of the elementary process of the atomic-scale wear. [DOI: 10.1380/ejssnt.2009.173]

Near-field radiative heat transfer for structured surfaces

We apply an analytical approach for determining the near-field radiative heat transfer between a metallic nanosphere and a planar semi-infinite medium with some given surface structure. This approach is based on a perturbative expansion, and evaluated to first order in the surface profile. With the help of numerical results obtained for some simple model geometries we discuss typical signatures that should be obtainable with a near-field scanning thermal microscope operated in either constant-height or constant-distance mode.

AFM with the Slope Compensation Technique for High-Speed Precision Measurement of Micro-Structured Surfaces

In this paper, we applied the contact constant-height mode together with the pre-compensation technique which can realize the capability of high speed as well as faithful topographical image. Before scanning, the slope variation of the micro-structured surface was measured by the capacitance sensor and then stored in a PC. During the surface profile scanning, a piezoelectric actuator is applied which can provide the inconsecutive motion that corresponds to the pre-measured slope variation. As a result, the precision measurement can also be achieved. The validity of the proposed method and its performance are verified by compare the topographical images that were gained by the contact constant-force mode with feedback control. However, the scanning speed of our method is obviously high.

Imaging the real shape of nanoclusters in scanning force microscopy

A quantitative comparison between experiment and theory is given for the constant height mode imaging of metal nanoclusters in dynamic scanning force microscopy. We explain the fundamental mechanisms in the contrast formation with the help of the system Pd/MgO(001). The comparison shows that the shape and size of nanoclusters are precisely imaged due to the sharpness of the tip’s last nanometer. This quantitative comparison proves our previously proposed model for the contrast formation.

Direct real-space imaging of thec(2×8)∕(2×4)GaAs (001) surface structure

We have performed frequency-modulated atomic-force microscopy (FM-AFM) on the $c(2\ifmmode\times\else\texttimes\fi{}8)∕(2\ifmmode\times\else\texttimes\fi{}4)$ GaAs (001) surface obtained from the $c(8\ifmmode\times\else\texttimes\fi{}2)∕(4\ifmmode\times\else\texttimes\fi{}6)$ surface by exposing it to ${\mathrm{As}}_{2}$ gas and annealing. Highly resolved interaction patterns reflect prevailing surface dimer pairs consistent with a so-called $\ensuremath{\beta}2$ structure, but more rare motifs characteristic of $\ensuremath{\alpha}2$ and $\ensuremath{\beta}$ structures are also seen. Atoms of the dimers interact with the atomic force microscope tip repulsively and appear as sharp features on a smooth background when imaged in constant-height mode. An analysis of the interaction decay length and lateral size of the atomic features indicates that the surface atoms are visualized through a core-core repulsion mechanism. In this imaging mode, the FM-AFM can be regarded as a true surface structure tool, since the observed features are, in the absence of significant lateral relaxation, associated with surface atoms directly.

Probing the transverse magneto-optical Kerr effect at the nanoscale

This article presents a theoretical formalism for a magneto-optical scanning near-field microscope (MO-SNOM) in the transverse magneto-optical Kerr effect configuration. Polarized light emitted by a nanosource is scattered by the magnetic object towards the detector. The nanosource is modelled as a circular aperture in an infinitely thin, perfectly conducting screen following the Bethe–Bouwkamp model. The theorem of reciprocity is used for calculating the detected signal in a constant-height scanning mode for a Co thin film pattern. Two magnetic distributions are investigated: the single domain and the Bloch wall. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface nanostructure of pure iron after combined electrochemical passivation and thermal annealing treatments

The properties of the surface oxide film on pure iron after electrochemical passivation and thermal annealing treatments were investigated using a variety of techniques. Passivation was carried out with an applied potential of 800 mV (vs Ag/AgCl) for 15 min in a pH 8.4 borate buffer solution at 30 °C, whilst annealing was carried out in air in an electric furnace at temperatures up to 300 °C. Analysis of the surface properties was then carried out using X-ray diffraction to determine oxide composition, a spectroscopic ellipsometer to measure the optical properties and oxide thickness, and a scanning probe microscope to measure the surface roughness using tapping mode AFM and to observe the nanoscale structure using constant height mode STM.

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.

Channel selective scanning tunneling spectroscopy

In this paper we simulate STM and STS experiments for CO monomers and dimers on Cu(1 1 1) surface. We show that the contrast of STM images can be attributed to interference effects between tunneling channels, and suggest that functionalizing the microscope tip improves the channel selectivity of STM. Furthermore, we show that voltage and position dependent tunneling spectra also reflect the same interference effects, but adds the energy resolution to the channel analysis. Especially in the case of nonresonant tunneling, STS measures local density of states only indirectly. The present study suggests that STS in constant height mode can be used in investigating the phase and energy sensitivity of tunneling channels in adsorbate molecules and nanostructures.

Length Dependence of Tunneling Current Through Single Phenylene Oligomers Measured by Scanning Tunneling Microscopy at Low Temperature

The length dependence of tunneling current through single phenylene oligomers, i.e., benzenemethanethiol, 4-biphenylmethanethiol and [1,1':4',1''-terphenyl]-4-methanethiol, was determined experimentally by scanning tunneling microscopy (STM). The single phenylene oligomers were isolated in a self-assembled monolayer (SAM) matrix of a disulfide with two spherical bicyclo[2.2.2]octane moieties on Au(111). The STM measurement was conducted at a low temperature to prevent the thermal motions of the isolated molecules that occur at room temperature. The isolated single molecules inserted in the SAM matrix were observed as protrusions in an STM topography using a constant-current mode owing to their higher tunneling ability. The decay constant, β, of the tunneling current through single phenylene oligomers was estimated from the STM heights of the protrusions corresponding to the single phenylene oligomers using a bilayer tunnel junction model. The value of β for tunneling current through single phenylene oligomers was 5.5 ±0.2 nm-1. In a constant-height mode, we measured the conductance of the isolated single molecules. We estimated an averaged single-molecular conductance of the isolated molecules of a biphenyl derivative to be ca. 2 nS. Here, one side of each isolated molecule was bound via an Au–S bond and the other phenyl end was physically in contact with an Au STM tip.

Imaging nanoclusters in the constant height mode of the dynamic SFM

For the first time, high quality images of metal nanoclusters which were recordedin the constant height mode of a dynamic scanning force microscope (dynamicSFM) are shown. Surfaces of highly ordered pyrolytic graphite (HOPG) were usedas a test substrate since metal nanoclusters with well defined and symmetricshapes can be created by epitaxial growth. We performed imaging of goldclusters with sizes between 5 and 15 nm in both scanning modes, constantΔf mode and constant height mode, and compared the image contrast. We noticethat clusters in constant height images appear much sharper, and exhibit morereasonable lateral shapes and sizes in comparison to images recorded in the constantΔf mode. With the help of numerical simulations we show that only a microscopically smallpart of the tip apex (nanotip) is probably the main contributor for the imagecontrast formation. In principle, the constant height mode can be used for imagingsurfaces of any material, e.g. ionic crystals, as shown for the system Au/NaCl(001).

Phase-sensitive lock-in imaging of surface densities of states

A new way of imaging the local density of states has been devised through a combination ofthe constant-height scanning tunnelling microscopy operational mode and lock-in techniques.We have obtained current images simultaneously with real space dynamical conductance maps(d I/d V) for energies around the Fermi level, on theSi(111)-(7 × 7) surface. We reconstructed the normalized dynamical conductancespectra—(d I/d V)/(I/V). Since the (d I/d V)/(I/V) curves are closely related to the local densities of states, we compared their sum over theunit cell to photoelectron spectra and theoretical calculations. We find that the results arein good agreement. Consequently, the extent of localization of surface electronic states atlattice positions was determined.

Optical activity of a single MnAs cluster: Birefringence or Kerr effect

We have grown In 0.54Ga 0.46As:Mn/MnAs granular paramagnetic–ferromagnetic hybrid structures by metal–organic vapor-phase epitaxy. The MnAs clusters have a Curie temperature of about 320 K. We have studied the optical activity of individual ferromagnetic MnAs clusters embedded in the paramagnetic In 0.54Ga 0.46As:Mn matrix at room temperature by far-field depolarization measurements. A scanning near-field optical microscopy set-up in constant height mode ( ≈ 100 nm above the sample surface) was used to achieve a high spatial resolution. Individual MnAs clusters rotate the linear polarization of the incoming light by almost 2 ∘ in this reflection geometry. This optical activity was analyzed in terms of birefringence and polar Kerr effect and correlated with the structural and magnetic properties of the MnAs clusters as determined by ferromagnetic resonance measurements. The optical activity of the MnAs clusters turns out to be dominated by linear birefringence caused by the uniaxial symmetry of the hexagonal crystal structure of MnAs. The polar Kerr effect plays a minor role in this experiment.

Model Analysis of Anomalies in Highly Oriented Pyrolytic Graphite Images by Scanning Tunneling Microscope in Air

The atomic corrugations of highly oriented pyrolytic graphite (HOPG) were measured in the constant current mode using a scanning tunneling microscope (STM) in air. The apparent corrugations depend on the angular frequency of the vertical vibrational motion of the tip. The tip motion resulted in a forced vibration of the sample surface through the aid of the contact of the tip with the sample surface. In our previous work, we managed to analyze constant height mode images with a simple model of the tunneling current, and reported that the apparent current modulations corresponding to the atomic corrugation are strongly influenced by the elastic deformation of the sample surface [Jpn. J. Appl. Phys. 34 (1995) 578]. In the present paper, a model for the constant current mode is presented taking additionally into consideration the one-dimensional spring deformation model to explain the forced vibration. From this model, well-known anomalies of atomic corrugations in HOPG images can be explained.