Self-doping of ultrathin insulating films by transition metal atoms.

Single magnetic Co atoms are deposited on atomically thin NaCl films on Au(111). Two different adsorption sites are revealed by high-resolution scanning tunneling microscopy (STM), i.e., at Na and at Cl locations. Using density functional based simulations of the STM images, we show that the Co atoms substitute with either a Na or Cl atom of the NaCl surface, resulting in cationic and anionic Co dopants with a high thermal stability. The dependence of the magnetic coupling between neighboring Co atoms on their separation is investigated via spatially resolved measurement of the local density of states.

Tuning quantum corrections and magnetoresistance in ZnO nanowires by ion implantation.

Using ion implantation, the electrical as well as the magnetotransport properties of individual ZnO nanowires (NWs) can be tuned. The virgin NWs are configured as field-effect transistors which are in the enhancement mode. Al-implanted NWs reveal a three-dimensional metallic-like behavior, for which the magnetoresistance is well described by a semiempirical model that takes into account the presence of doping induced local magnetic moments and of two conduction bands. On the other hand, one-dimensional electron transport is observed in Co-implanted NWs. At low magnetic fields, the anisotropic magnetoresistance can be described in the framework of weak electron localization in the presence of strong spin-orbit scattering. From the weak localization, a large phase coherence length is inferred that reaches up to 800 nm at 2.5 K. The temperature-dependent dephasing is shown to result from a one-dimensional Nyquist noise-related mechanism. At the lowest temperatures, the phase coherence length becomes limited by magnetic scattering.

Lateral quantization of two-dimensional electron states by embedded Ag nanocrystals.

We show that quantization of image-potential state (IS) electrons above the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking-fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy, the n=1 IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons in the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect-induced scattering above nanostructured surfaces.

Formation of Co/Ge intermixing layers after Co deposition on Ge(111)2 × 1 surfaces.

The formation of a novel surface reconstruction upon Co deposition on freshly cleaved Ge(111)2 × 1 surfaces is studied by means of scanning tunneling microscopy (STM) at 4.5 K. Previously we demonstrated that at this low substrate temperature the deposited Co atoms remain immobile after they become embedded underneath the Ge(111)2 × 1 surface. We now demonstrate that at higher substrate temperatures the embedded Co atoms are able to diffuse below the surface in a direction parallel to the upper π-bonded chain rows. This one-dimensional temperature-induced mobility results in subsurface accumulation of Co atoms at atomic steps, at domain boundaries and on atomically flat Ge terraces at, e.g., vacancies or adatoms, where reconstructed Co/Ge intermixing layers are formed. Voltage dependent STM images reveal that the Co related surface reconstruction locally exhibits an ordered atomic structure with the same inter-atomic distance as that of the initial 2 × 1 reconstructed pure Ge(111) surface. On the other hand, the presence of the Co results in a doubling of the periodicity along the [21[overline]1[overline]] direction in the STM images, which can be related to the modified electronic properties of the π-bonded chains.

Unexpected optical response of single ZnO nanowires probed using controllable electrical contacts.

Relying on combined electron-beam lithography and lift-off methods Au/Ti bilayer electrical contacts were attached to individual ZnO nanowires (NWs) that were grown by a vapor phase deposition method. Reliable Schottky-type as well as ohmic contacts were obtained depending on whether or not an ion milling process was used. The response of the ZnO NWs to ultraviolet light was found to be sensitive to the type of contacts. The intrinsic electronic properties of the ZnO NWs were studied in a field-effect transistor configuration. The transfer characteristics, including gate threshold voltage, hysteresis and operational mode, were demonstrated to unexpectedly respond to visible light. The origin of this effect could be accounted for by the presence of point defects in the ZnO NWs.

Probing quantized image-potential states at supported carbon nanotubes.

Discrete image-potential states (ISs) are revealed at double-walled carbon nanotubes by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in the distance-voltage z(V) spectroscopy mode. The nanotubes are supported by flat Au(111) substrates. Due to the high sensitivity of the hot IS electrons to local variations of the surface potential, they can be considered as a sensitive probe to investigate interactions with the supporting substrate and impurities or defects at the nanotube surface. ISs provide information on the local electronic structure as well as on the electron dynamics at supported nanotubes.

Quantum confinement of hot image-potential state electrons.

Discrete image-potential state (IS) resonances at Co nanoislands on Au(111) are probed using scanning tunneling microscopy and spectroscopy. We observe particle-in-box-type standing wave patterns, which is surprising in view of the high energy of the IS electrons when compared to the confining potential imposed by the island edges. The weak confining potential experienced by the IS electrons results in electronic interaction effects between closely spaced islands. Probing high-energy ISs hence provides a novel route to investigate electronic coupling between nanoislands on surfaces.

A study of the electronic properties of Au nanowires and Au nanoislands on Au(111) surfaces.

By means of ion bombardment of clean Au(111) films, atomically flat nanoparticles of various shapes and sizes were created, ranging from several tens of nm(2) down to only a few nm(2). Both two-dimensional Au islands as well as one-dimensional Au nanowire-like structures have been investigated by means of low-temperature scanning tunneling microscopy and spectroscopy. We were able to probe their local electronic structure in a broad energy range, which was found to be dominated by pronounced size-dependent confinement effects. Mapping of the local density of states revealed complex standing wave patterns that arise due to interference of scattered Au surface state electrons at the edges of the Au nanoparticles. The observed phenomena could be modeled with high accuracy by theoretical particle-in-a-box calculations based on a variational method that can be applied to '2D boxes' of arbitrary polygonal shape and that we have previously successfully applied to explain the electronic wave patterns on Co islands on Au(111). Our findings support the general validity of this particle-in-a-box model.

Low-temperature scanning tunneling microscopy and spectroscopy investigation of the electronic surface state of self-organized Cr islands on Au(111).

We have investigated the morphologic and electronic properties of self-organized nanosized Cr islands grown on atomically flat Au(111) surfaces by means of low-temperature scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. We observe the existence of an electronic surface state on the Cr islands, which manifests itself through the formation of standing wave patterns within the interior of the atomically flat islands that gain in complexity for increasing bias voltages. The patterns are only weakly observable when compared to similar standing waves formed within self-organized Co islands on Au(111). This difference is attributed to the more irregular shape of the Cr islands. Furthermore, we have found that the presence of the Cr surface state is reflected by the appearance of a distinct electronic state in the -200 mV to -150 mV range, resembling to previous findings for self-organized Co islands on Au(111).

Low-temperature scanning tunneling microscopy of ring-like surface electronic structures around Co islands on InAs(110) surfaces.

We report on the experimental observation by scanning tunneling microscopy at low temperature of ring-like features that appear around Co metal islands deposited on a clean (110) oriented surface of cleaved p-type InAs crystals. These features are visible in spectroscopic images within a certain range of negative tunneling bias voltages due to the presence of a negative differential conductance in the current-voltage dependence. A theoretical model is introduced, which takes into account non-equilibrium effects in the small tunneling junction area. In the framework of this model the appearance of the ring-like features is explained in terms of interference effects between electrons tunneling directly and indirectly (via a Co island) between the tip and the InAs surface.

On the interface magnetism of thin oxidized Co films: orbital and spin moments.

An x-ray magnetic circular dichroism study of a polycrystalline Co/CoO bilayer is presented. Using both the chemical specificity and surface sensitivity in the core level techniques, we find that uncompensated Co(2+) spin moments participate in the remanent ferromagnetic response of the bilayer that has oxygen nearest neighbors. These are likely located at the Co/CoO interface. As intermixing of magnetic species is not present in Co/CoO, it is concluded that the observed interface moments are due to interface roughness. Given their direction, these moments appear to not directly correlate to the exchange bias in these bilayers.

Spin-polarized scanning tunneling spectroscopy of self-organized nanoscale Co islands on Au(111) surfaces.

Magnetic monolayer and bilayer Co islands of only a few nanometer in size were grown by atomic deposition on atomically flat Au(111) films. The islands were studied in situ by scanning tunneling microscopy (STM) and spectroscopy at low temperatures. Spin-resolved tunneling spectroscopy, using an STM tip with a magnetic coating, revealed that the Co islands exhibit a net magnetization perpendicular to the substrate surface due to the presence of spin-polarized d-states. A random distribution of islands with either upward or downward pointing magnetization was observed, without any specific correlation of magnetization orientation with island size or island height.

The work function of few-layer graphene.

A theoretical and experimental study of the work function of few-layer graphene is reported. The influence of the number of layers on the work function is investigated in the presence of a substrate, a molecular dipole layer, and combinations of the two. The work function of few-layer graphene is almost independent of the number of layers with only a difference between monolayer and multilayer graphene of about 60 meV. In the presence of a charge-donating substrate the charge distribution is found to decay exponentially away from the substrate and this is directly reflected in the work function of few-layer graphene. A dipole layer changes the work function only when placed in between the substrate and few-layer graphene through a change of the charge transfer between the two.

Roughness and hydrophobicity studies of nanofiltration membranes using different modes of AFM.

Determination of the surface roughness by AFM is crucial to the study of particle fouling in nanofiltration. It is, however, very difficult to compare the different roughness values reported in the literature because of a lack in uniformity in the methods applied to determine surface roughness. AFM is used in both noncontact mode and tapping mode; moreover, the size of the scan area is highly variable. This study compares, for six different nanofiltration membranes (UTC-20, N30F, Desal 51HL, Desal 5DL, NTR7450, NF-PES-10), noncontact mode AFM with tapping mode AFM for several sizes of the scan area. Although the absolute roughness values are different for noncontact AFM and tapping mode AFM, no difference is found between the two modes of AFM in ranking the nanofiltration membranes with respect to their surface roughness. NTR 7450 and NF-PES-10 are the smoothest membranes, while the roughest surface can be found with Desal 51HL and Desal 5DL. UTC-20 and N30F are characterized by an intermediate roughness value. An increase in roughness with increasing scan area is observed for both AFM modes. Larger differences between the roughnesses of the membranes are obtained with tapping mode AFM because of the tapping of the tip on the surface. Phase imaging is an extension of tapping mode AFM, measuring the phase shift between the cantilever oscillation and the oscillation of the piezo driver. This phase shift reflects the interaction between the cantilever and the membrane surface. A comparison with contact angle measurements proves that a small phase shift corresponds to a large contact angle, representing a hydrophobic membrane surface.

Probing Magnetism in 2D Molecular Networks after in Situ Metalation by Transition Metal Atoms.

Metalated molecules are the ideal building blocks for the bottom-up fabrication of, e.g., two-dimensional arrays of magnetic particles for spintronics applications. Compared to chemical synthesis, metalation after network formation by an atom beam can yield a higher degree of control and flexibility and allows for mixing of different types of magnetic atoms. We report on successful metalation of tetrapyridyl-porphyrins (TPyP) by Co and Cr atoms, as demonstrated by scanning tunneling microscopy experiments. For the metalation, large periodic networks formed by the TPyP molecules on a Ag(111) substrate are exposed in situ to an atom beam. Voltage-induced dehydrogenation experiments support the conclusion that the porphyrin macrocycle of the TPyP molecule incorporates one transition metal atom. The newly synthesized Co-TPyP and Cr-TPyP complexes exhibit striking differences in their electronic behavior, leading to a magnetic character for Cr-TPyP only as evidenced by Kondo resonance measurements.

Evolution of magnetism on a curved nano-surface.

To design custom magnetic nanostructures, it is indispensable to acquire precise knowledge about the systems in the nanoscale range where the magnetism forms. In this paper we present the effect of a curved surface on the evolution of magnetism in ultrathin iron films. Nominally 70 Å thick iron films were deposited in 9 steps on 3 different types of templates: (a) a monolayer of silica spheres with 25 nm diameter, (b) a monolayer of silica spheres with 400 nm diameter and (c) for comparison a flat silicon substrate. In situ iron evaporation took place in an ultrahigh vacuum chamber using the molecular beam epitaxy technique. After the evaporation steps, time differential nuclear forward scattering spectra, grazing incidence small angle X-ray scattering images and X-ray reflectivity curves were recorded. In order to reconstruct and visualize the magnetic moment configuration in the iron cap formed on top of the silica spheres, micromagnetic simulations were performed for all iron thicknesses. We found a great influence of the template topography on the onset of magnetism and on the developed magnetic nanostructure. We observed an individual magnetic behaviour for the 400 nm spheres which was modelled by vortex formation and a collective magnetic structure for the 25 nm spheres where magnetic domains spread over several particles. Depth selective nuclear forward scattering measurements showed that the formation of magnetism begins at the top region of the 400 nm spheres in contrast to the 25 nm particles where the magnetism first appears in the region where the spheres are in contact with each other.

Imaging the elastic properties of coiled carbon nanotubes with atomic force microscopy

Coiled carbon nanotubes were produced catalytically by thermal decomposition of hydrocarbon gas. After deposition on a silicon substrate, the three-dimensional structure of the helix-shaped multiwalled nanotubes can be visualized with atomic force microscopy. Helical structures of both chiralities are present in the nanotube deposits. For larger coil diameters ( >170 nm), force modulation microscopy allows one to probe the local elasticity along the length of the coil. Our results agree with the classical theory of elasticity. Similar to the case of straight nanotubes, the Young modulus of coiled multiwalled nanotubes remains comparable to the very high Young modulus of hexagonal graphene sheets.

Impurity spin magnetization of thin Fe doped Au films

In order to probe the influence of the surface-induced anisotropy on the impurity spin magnetization, we measure the anomalous Hall effect in thin AuFe films at magnetic fields up to 15 T. The observed suppression of the anomalous Hall resistivity at low fields as well as the appearance of a minimum in the differential Hall resistivity at higher fields can be explained by our theoretical model, which takes into account the influence of a polycrystalline film structure on the surface-induced anisotropy. Our results imply that the apparent discrepancy between different experimental results for the size effects in dilute magnetic alloys can be linked to a different microstructure of the samples.

Immobilizing and imaging microtubules by atomic force microscopy.

Microtubules isolated from pig brains have been immobilized on an inorganic substrate for use in AFM studies. The method employs 4-aminobutyldimethylmethoxysilane and glutaraldehyde to activate a silicon wafer for binding the biopolymer. The covalent bond ensures the positional stability of the tubules on the substrate, and allows reproducible scanning probe experiments. Microtubules have been imaged both by atomic force and scanning tunneling microscopy, yielding results very similar to electron microscopy. The average apparent height of the tubules is smaller than observed with transmission electron microscopy (25 nm) and is smaller in buffer solution (10 nm) than in air (15 nm). The biopolymer surface is softer under buffer than in air. The highest resolution was obtained with the tapping mode where surface features as small as 10 nm in X and Y have been resolved. Gold-coated tubules bound on silicon have been successfully imaged by STM, while images of uncertain origin were generated for tubules deposited on graphite. It is shown that artefacts imaged on a blank graphite surface can easily be confounded with collapsed tubules.

Biophysical and electrical aspects of radiofrequency catheter ablation.

UNLABELLED: Radiofrequency (RF) catheter ablation is becoming increasingly accepted as the treatment of choice for definitive management of a variety of supraventricular arrhythmias in selected patients. Many interdependent variables may affect the amount of tissue heating at the target site. Power controlled RF devices may fail to ablate the targeted tissue due to a mismatch between the actual resistance of the electrode-tissue interface and the internal resistance of the RF generator. CONCLUSION: the use of temperature controlled RF devices may optimize power delivery to the tissue and therefore facilitate radiofrequency catheter ablation procedures.