Direct near-field optical imaging of higher order plasmonic resonances

We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions.

Positioning of strained islands by interaction with surface nanogrooves

When strained Stranski-Krastanow islands are used as "self-assembled quantum dots," a key goal is to control the island position. Here we show that nanoscale grooves can control the nucleation of epitaxial Ge islands on Si(001), and can drive lateral motion of existing islands onto the grooves, even when the grooves are very narrow and shallow compared to the islands. A position centered on the groove minimizes energy. We use as prototype grooves the trenches which form naturally around islands. During coarsening, the shrinking islands move laterally to sit directly astride that trench. In subsequent growth, we demonstrate that islands nucleate on the "empty trenches" which remain on the surface after complete dissolution of the original islands.

Chiral kagomé lattice from simple ditopic molecular bricks

Self-assembly techniques allow for the fabrication of highly organized architectures with atomic-level precision. Here, we report on molecular-level scanning tunneling microscopy observations demonstrating the supramolecular engineering of complex, regular, and long-range ordered periodic networks on a surface atomic lattice using simple linear molecular bricks. The length variation of the employed de novo synthesized linear dicarbonitrile polyphenyl molecules translates to distinct changes of the bonding motifs that lead to hierarchic order phenomena and unexpected changes of the surface tessellations. The achieved 2D organic networks range from a close-packed chevron pattern via a rhombic network to a hitherto unobserved supramolecular chiral kagomé lattice.

Amplitude- and phase-resolved optical near fields of split-ring-resonator-based metamaterials

We investigate the local optical response of split-ring resonator-(SRR)-based metamaterials with an apertureless scanning near-field optical microscope. By mapping the near fields of suitably resonant micrometer-sized SRRs in the near-infrared spectral region with an uncoated silicon tip, we obtain a spatial resolution of better than lambda/50. The experimental results confirm numerical predictions of the near-field excitations of SRRs. Combining experimental near-field optical studies with near- and far-field optical simulations provides a detailed understanding of resonance mechanisms in subwavelength structures and will facilitate an efficient approach to improved designs.

Polymer nanofibers via nozzle-free centrifugal spinning

We report on the unexpected finding of nanoscale fibers with a diameter down to 25 nm that emerge from a polymer solution during a standard spin-coating process. The fiber formation relies upon the Raleigh-Taylor instability of the spin-coated liquid film that arises due to a competition of the centrifugal force and the Laplace force induced by the surface curvature. This procedure offers an attractive alternative to electrospinning for the efficient, simple, and nozzle-free fabrication of nanoscale fibers from a variety of polymer solutions.

Background removal in scanning tunneling spectroscopy of single atoms and molecules on metal surfaces

Scanning tunneling spectroscopy has developed into a powerful spectroscopic technique that has found wide application in the atomic scale characterization of the electronic properties of clean surfaces as well as adsorbates and defects at surfaces. However, it still lacks the standard methods for data treatment and removal of artifacts in spectra as they are, e.g., common in photoemission spectroscopy. The properties of the atomic scale tip apex--the probe of the instrument--tend to introduce spurious background signals into tunneling spectra. We present and discuss two methods which permit to extract tip-independent information from low temperature tunneling spectra acquired on single atoms and molecules on single crystal surfaces by background subtraction. The methods rely on a characterization of the tip on the clean metal surface. The performance of both methods is demonstrated and compared for simulated and experimental tunneling spectra.

Metal-organic honeycomb nanomeshes with tunable cavity size

We present a systematic study of metal-organic honeycomb lattices assembled from simple ditopic molecular bricks and Co atoms on Ag(111). This approach enables us to fabricate size- and shape-controlled open nanomeshes with pore dimensions up to 5.7 nm. The networks are thermally robust while extending over microm2 large areas as single domains. They are shape resistant in the presence of further deposited materials and represent templates to organize guest species and realize molecular rotary systems.

Exchange interaction between single magnetic adatoms

The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 A we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.

Magic alkali-fullerene compound clusters of extreme thermal stability

The thermal stability of free pure C60-, as well as C60-alkali, and -alkaline-earth metal compound clusters is investigated. We find that small (C60)m-clusters (m<or=6) decay at comparatively low temperatures below 400 K, as a consequence of weak intermolecular van der Waals interaction. Adding barium or potassium to the clusters dramatically increases the decay temperatures for "magic" configurations of (C60)mBa(2m-1) and (C60)mK(2m), which reach values as high as 1780 K. Contrary to common belief, the superstable compound clusters are not characterized by filled geometrical or electronic shells. Density functional calculations show that the delicate interplay of ionic (K, Ba) and covalent (Ba) interaction between C60 and the metal atoms, on the one hand, and entropic contributions to the Gibbs free energy, on the other hand, determine the unusual stability.

Coexistence of one- and two-dimensional supramolecular assemblies of terephthalic acid on Pd(111) due to self-limiting deprotonation

The adsorption of terephthalic acid [C(6)H(4)(COOH)(2), TPA] on a Pd(111) surface has been investigated by means of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure spectroscopy under ultrahigh vacuum conditions at room temperature. We find the coexistence of one- (1D) and two-dimensional (2D) molecular ordering. Our analysis indicates that the 1D phase consists of intact TPA chains stabilized by a dimerization of the self-complementary carboxyl groups, whereas in the 2D phase, consisting of deprotonated entities, the molecules form lateral ionic hydrogen bonds. The supramolecular growth dynamics and the resulting structures are explained by a self-limiting deprotonation process mediated by the catalytic activity of the Pd surface. Our models for the molecular ordering are supported by molecular mechanics calculations and a simulation of high resolution STM images.

Impact of a functional polymorphism in the IL12B promoter region on survival in patients with malignant melanoma

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Background: Interleukin-12 (IL-12), a heterodimeric cytokine, is important in the generation of a Th1-biased immune response. Several polymorphisms have been described in IL12B, the gene encoding the p40 subunit of IL-12. A bi-allelic polymorphism within the IL12B promoter region has been reported to show association with diseases as diverse as severe childhood asthma and fatal cerebral malaria. Methods: In order to define the molecular basis for these disease associations we investigated the secretion of IL-12 by human monocyte-derived dendritic cells. The amount of IL-12p40 and p70 produced by DCs from normal blood donors was determined following CD40L activation. Subsequently, we therefore sought to investigate the frequency of IL12B genotypes and their impact on the outcome of patients with malignant melanoma. Genomic DNA was isolated from 75 normal blood donors and 80 melanoma cell lines and used for genotyping the IL12B polymorphism. Corresponding clinical data were retrieved from the in-house melanoma database. Results: Allele frequency for IL12Bpro-1 is 0.45 and for IL12Bpro-2 0.55. Homozygotes for the IL12B promoter polymorphism showed a 10-fold difference in median p70 secretion in response to CD40 ligation. Remarkably, this difference resulted from the inability of most allele 1 homozygotes to secrete heterodimeric IL-12. In contrast, most of the donors homozygous for allele 2 had detectable secretion. Distribution of genotypes in melanoma patients was not significantly different from normal blood donors. Kaplan-Meier survival analysis showed significantly decreased survival of patients homozygous for the longer allele, previously defined as being associated with decreased IL-12 secretion (log-rank p=0.015). Subgroup analysis indicated that this survival advantage was seen exclusively in female patients. Conclusions: The recently described IL12B promoter polymorphism is highly correlated with secretion of bioactive IL-12 and has a significant impact on the clinical course of disease in patients with melanoma and may therefore be useful for prognostic stratification.

No significant financial relationships to disclose.

Interplay between thermodynamics and kinetics in the capping of InAs/GaAs(001) quantum dots

A microscopic picture for the GaAs overgrowth of self-organized quantum dots is developed. Scanning tunneling microscopy measurements reveal two capping regimes: the first being characterized by a dot shrinking and a backward pyramid-to-dome shape transition. This regime is governed by fast dynamics resulting in island morphologies close to thermodynamic equilibrium. The second regime is marked by a true overgrowth and is controlled by kinetically limited surface diffusion processes. A simple model is developed to describe the observed structural changes which are rationalized in terms of energetic minimization driven by lattice mismatch and alloying.

Local pressure-induced metallization of a semiconducting carbon nanotube in a crossed junction

The electronic and vibrational density of states of a semiconducting carbon nanotube in a crossed junction was investigated by elastic and inelastic scanning tunneling spectroscopy. The strong radial compression of the nanotube at the junction induces local metallization spatially confined to a few nanometers. The local electronic modifications are correlated with the observed changes in the radial breathing and G band phonon modes, which react very sensitively to local mechanical deformation. In addition, the experiments reveal the crucial contribution of the image charges to the contact potential at nanotube-metal interfaces.

Kondo effect of molecular complexes at surfaces: ligand control of the local spin coupling

The spin state of single magnetic atoms and molecules at surfaces is of fundamental interest and may play an important role in future atomic-scale technologies. We demonstrate the ability to tune the coupling between the spin of individual cobalt adatoms with their surroundings by controlled attachment of molecular ligands. The strength of the coupling is determined via the Kondo resonance by low-temperature scanning tunneling spectroscopy. Spatial Kondo resonance mapping is introduced as a novel imaging tool to localize spin centers in magnetic molecules with atomic precision.

Lateral motion of SiGe islands driven by surface-mediated alloying

SiGe islands move laterally on a Si(001) substrate during in situ postgrowth annealing. This surprising behavior is revealed by an analysis of the substrate surface morphology after island removal using wet chemical etching. We explain the island motion by asymmetric surface-mediated alloying. Material leaves one side of the island by surface diffusion, and mixes with additional Si from the surrounding surface as it redeposits on the other side. Thus the island moves laterally while becoming larger and more dilute.

Deficient IL-12p70 secretion by dendritic cells based on IL12B promoter genotype

Interleukin-12 (IL-12), a heterodimeric cytokine, is important in the generation of a Th1-biased immune response. Several polymorphisms have been described in IL12B, the gene encoding the p40 subunit of IL-12. A bi-allelic polymorphism within the IL12B promoter region has been reported to show association with diseases as diverse as severe childhood asthma and fatal cerebral malaria. In order to define the molecular basis for these disease associations, we investigated the secretion of IL-12 by human monocyte-derived dendritic cells. Homozygotes for the IL12B promoter polymorphism showed a 10-fold difference in median p70 secretion in response to CD40 ligation. Remarkably, this difference resulted from the inability of most allele 1 homozygotes to secrete heterodimeric IL-12. In contrast, most of the donors homozygous for allele 2 had detectable secretion. These findings are important for the understanding of the highly complex regulation of IL-12 secretion, and its consequent impact on disease susceptibility, in humans.

Synthesis, Photoluminescence, and Adsorption of CdS/Dendrimer Nanocomposites

CdS/dendrimer nanocomposites can be synthesized from methanolic Cd(2+) and S(2-) with amine-terminated polyamidoamine dendrimers of generation 8 (G8NH(2)) as stabilizers. By controlling the preparation conditions, nanoparticles with diameters < or = 2 nm can be obtained with a narrow size distribution. They show blue photoluminescence at approximately 450 nm. We studied the effects of various additives on the photoluminescence and elucidated its mechanism. Stable aggregates of two to three G8NH(2) molecules with several CdS nanoparticles form; the particles are located at the surface of the G8NH(2) molecules. The adsorption of the CdS/G8NH(2) nanocomposites on flat substrate surfaces is determined by the substrate chemistry. The hydrophilic nature of G8NH(2) results in weak affinity to graphite but strong affinity to hydroxy-terminated substrates such as mica, oxidized silicon wafers, and carboxylate-terminated monolayers. Patterning of nanocomposites on these hydrophilic substrates is achieved by the microcontact printing method. We propose to use only one molecule, a large dendrimer, to control the nanoparticle formation and also the immobilization of the synthesized nanoparticle/dendrimer composites.

Atomic-scale pathway of the pyramid-to-dome transition during ge growth on Si(001)

By high resolution scanning tunneling microscopy, we investigate the morphological transition from pyramid to dome islands during the growth of Ge on Si(001). We show that pyramids grow from top to bottom and that, from a critical size on, incomplete facets are formed. We demonstrate that the bunching of the steps delimiting these facets evolves into the steeper dome facets. Based on first principles and Tersoff-potential calculations, we develop a microscopic model for the onset of the morphological transition, able to reproduce closely the experimentally observed behavior.

Kondo temperature of magnetic impurities at surfaces

Based on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.

Phonon spectromicroscopy of carbon nanostructures with atomic resolution

The vibrational properties of single-wall carbon nanotubes have been probed locally with atomic-scale resolution by inelastic electron tunneling spectroscopy with a low-temperature scanning tunneling microscope. The high spatial resolution has allowed the unraveling of changes in the local phonon spectrum related to topological defects. We demonstrated that the radial breathing mode is suppressed within tube segments of lengths below approximately 3 nm, and that in the cap region phonon modes characteristic of the fullerene hemisphere are emerging. Phonon spectromicroscopy should lead to a better understanding of the mechanisms that limit the transport of heat or electrical charge inside nanostructured carbon materials.

Oscillatory magnetic anisotropy in one-dimensional atomic wires

One-dimensional Co atomic wires grown on Pt(997) have been investigated by x-ray magnetic circular dichroism. Strong changes of the magnetic properties are observed as the system evolves from 1D- to 2D-like. The easy axis of magnetization, the magnetic anisotropy energy, and the coercive field oscillate as a function of the transverse width of the wires, in agreement with theoretical predictions for 1D metal systems.